KR20160063553A - Wire having high strength, and method for manufacturing thereof - Google Patents

Abstract

Provided are a steel wire having high strength, and a manufacturing method thereof. The present invention relates to a steel wire having excellent strength, comprising: 0.05-0.15 wt% of carbon (C): 0.1 wt% or lower of silicon (Si); 3.0-4.0 wt% of manganese (Mn); 0.020 wt% or lower of phosphorus (P): 0.020 wt% or lower of sulfur (S); 0.0010-0.0030 wt% of boron (B); 0.010-0.030 wt% of titanium (Ti); 0.0050 wt% or lower of nitrogen (N); and the remaining consisting of iron (Fe) and unavoidable impurities, whose minute tissue includes 90 area% or higher of bainite, and the remaining consisting of martensite-austenite (MA) constituent whose tensile strength is 700-900 MPa and whose ductility is 10% or higher. The present invention provides a steel wire, which is able to only have high strength with hot rolling and continuous cooling processes and cold rolling processing without an additional thermal treatment process, and a manufacturing method thereof.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength steel wire and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 > WIRE HAVING HIGH STRENGTH, AND METHOD FOR MANUFACTURING THEREOF &

More particularly, the present invention relates to a steel wire which can be used for industrial machines or machine parts such as automobiles, which are exposed to various external load environments, and a steel wire having excellent strength and a method for manufacturing the steel wire .

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 secured without 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.

SUMMARY OF THE INVENTION 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 by hot rolling and continuous cooling process and cold drawing without any additional heat treatment process 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,

(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% 0.0010 to 0.0030% of boron (B), 0.010 to 0.030% of titanium (Ti), 0.0050% or less of nitrogen (N), the balance of Fe and unavoidable impurities, (MA) having a tensile strength of 700 to 900 MPa and a ductility of 10% or more.

It is preferable that the grain size of the amorphous martensite (MA) is 5 占 퐉 or less.

Further, according to the present invention,

(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% (B): 0.0010 to 0.0030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0050% or less, the remainder Fe and unavoidable impurities, and then reheating the steel material;

Subjecting the reheated steel material to a final hot rolling and then cooling the material to a temperature range of Bf to Bf - 50 占 폚 at a cooling rate of 0.1 to 2 占 폚 / s; And

And a step of cold-drawing the cooled steel material after cooling the steel material, and a method of manufacturing a steel wire having excellent strength characteristics.

The microstructure of the steel wire is preferably composed of 90% or more of bainite and residual martensite (MA).

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

INDUSTRIAL APPLICABILITY The present invention according to the above-described constitution can provide a steel wire excellent in strength and impact toughness 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 steel wire of the present invention having high strength characteristics will be described.

The steel wire according to the present invention comprises 0.05 to 0.15% of carbon (C), 0.1% or less of silicon (Si), 3.0 to 4.0% of manganese (Mn), 0.020% or less of phosphorus (P) ): 0.020% or less, boron (B): 0.0010 to 0.0030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0050% or less, balance Fe and unavoidable impurities.

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

Carbon (C): 0.05 to 0.15%

Carbon is an indispensable element for securing strength, which 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. On the contrary, the ductility and impact toughness decrease, so it is necessary to limit the addition amount of carbon within a certain range. In the present invention, it is preferable to limit the content of carbon (C) in the range of 0.05 to 0.15% because 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%, impact toughness can be drastically reduced .

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, the addition of silicon greatly increases the strength, but the ductility and impact toughness decrease sharply, so that the addition of silicon is very limited for cold forging parts that require sufficient ductility. In the present invention, it is preferable to limit the content of silicon to 0.1% or less, because if the silicon content exceeds 0.1%, securing the target impact toughness may be difficult.

Manganese (Mn): 3.0 to 4.0%

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, if the manganese content is less than 3.0%, the hardenability is not sufficient, and it becomes difficult to stably obtain the low-temperature structure by the continuous cooling process after the hot rolling. On the other hand, if it exceeds 4.0%, the curing ability is too high, which makes it impossible to obtain martensite structure even during air cooling. In consideration of this, in the present invention, the content of manganese is preferably limited to 3.0 to 4.0%.

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%.

Boron (B): 0.0010 to 0.0030%

Boron diffuses into the austenite grain boundary as an element for improving the hardenability and inhibits ferrite formation upon cooling and facilitates the formation of bainite or martensite. However, if the addition amount is less than 0.0010%, the effect of the addition can not be expected. If the addition amount exceeds 0.0030%, the effect increase can not be expected any more, and the boron nitride is precipitated in the grain boundaries, . Accordingly, in the present invention, it is preferable to limit the addition range of boron to 0.0010 to 0.0030%.

Titanium (Ti): 0.010 to 0.030%

Titanium has the greatest reactivity with nitrogen and forms nitrides first. When TiN is formed by adding titanium, most of the nitrogen in the steel is exhausted, boron can be prevented from being precipitated and the boron can be present in a soluble state, thereby improving the hardenability. However, when the addition amount is less than 0.010%, the effect of the addition is insufficient, and when the addition amount is more than 0.030%, a coarse nitride is formed and the mechanical properties may be degraded. In consideration of this, in the present invention, the addition amount of the titanium is preferably limited to a range of 0.010 to 0.030%.

Nitrogen (N): Not more than 0.0050%

It is preferable that the upper limit of nitrogen is limited to 0.0050% in order to keep the boron soluble state so that the effect of improving the hardenability can be sufficiently exhibited.

In addition, the steel wire of the present invention preferably has a steel microstructure composed of 90% or more of bainite and martensite Austenite Constituent (MA). The residual martensite (MA) is formed along the main bainite grain boundaries. When the fraction is high, the strength of the steel is increased and the impact toughness is deteriorated. Therefore, it is desirable to control the fraction as low as possible.

Considering this, in the present invention, it is desirable that the area fraction of the above-mentioned residual martensite (MA) is controlled to 10% or less (in other words, 90% or more of the main phase bainite structure). In the present invention, the area fraction of graphite martensite (MA), which is the residual structure, can be effectively achieved by controlling the cooling rate during cooling after hot rolling the steel material.

Also, in the present invention, it is preferable that the residual grains of amorphous martensite (MA) have a grain size of 5 탆 or less.

The steel wire of the present invention having a steel composition component and a microstructure as described above may have a tensile strength of 700 to 900 MPa and a ductility of 10% or more.

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 a final hot rolling and then cooling the material to a temperature range of Bf to Bf - 50 占 폚 at a cooling rate of 0.1 to 2 占 폚 / s; And a step of cold-drawing the cooled steel material, 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 the temperature range of Bf to Bf - 50 ° C. If the cooling end temperature exceeds Bf, it is difficult to obtain a sufficient amount of bainite structure. If Bf is less than 50 ° C, the steel is sufficiently cooled to facilitate handling, but the productivity is lowered. It is preferable to control it in the range.

Further, in the present invention, it is preferable to cool the zone from the finish hot rolling to the cooling end temperature at a cooling rate of 0.1 to 2 占 폚 / s. If the cooling rate is less than 0.1 ° C / s, the formation of pro-eutectoid ferrite is increased. If the cooling rate exceeds 2 ° C / s, the formation of martensite increases, .

It is possible to produce a steel material capable of obtaining bainite microstructure with an area fraction of 90% or more through cooling rate control in the cooling section as described above.

Subsequently, in the present invention, the produced steel material is air-cooled and subjected to cold drawing processing. Specifically, in the present invention, cold working is applied to a work through a die for cold drawing by using the steel material. In this case, it is preferable that the cold reduction ratio is 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 decreased.

The steel wire of the present invention manufactured through the above-described manufacturing process has a tensile strength of 700 to 900 MPa and a ductility of 10% or more.

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

(Example)

Molten steel having the compositional ingredients shown in Table 1 were each cast into an ingot, and homogenized at 1250 占 폚 for 12 hours. The homogenized steel material was hot-rolled to a thickness of 25 mm and then air-cooled.

Then, the steel materials prepared as described above were subjected to solution treatment at 900 캜, followed by cooling at the cooling rate shown in Table 2, followed by air cooling. The cold-rolled steels were subjected to cold drawing at a reduction ratio as shown in Table 2, and then the fraction and grain size of the ground martensite (MA), which is the remainder of the steel, were measured and shown in Table 2 , And tensile strength and ductility were measured and shown in Table 2.

In Table 2, the area fraction and grain size of the on-road martensite (MA) of the steel were measured using an image analyzer. The tensile test at room temperature was carried out at a crosshead speed of 0.9 mm / min until the yield point and then at a rate of 6 mm / min.

Pseudo-No.
                           Steel composition (% by weight) C Si Mn P S Ti B N One 0.12 0.05 3.1 0.018 0.019 0.015 0.0025 0.0044 2 0.10 0.03 3.6 0.014 0.017 0.017 0.0028 0.0042 3 0.11 0.10 3.5 0.016 0.013 0.030 0.0023 0.0039 4 0.05 0.09 3.8 0.015 0.015 0.011 0.0024 0.0044 5 0.06 0.06 3.0 0.013 0.011 0.027 0.0018 0.0048 6 0.13 0.07 3.3 0.016 0.018 0.013 0.0018 0.0045 7 0.11 0.08 4.0 0.009 0.020 0.019 0.0022 0.0040 8 0.25 0.06 3.4 0.014 0.013 0.030 0.0025 0.0037 9 0.15 0.50 3.3 0.011 0.015 0.021 0.0020 0.0050 10 0.11 0.03 2.0 0.018 0.014 0.018 0.0005 0.0043 11 0.09 0.01 3.6 0.016 0.017 0.021 0.0025 0.0041 12 0.08 0.06 3.2 0.011 0.016 0.020 0.0021 0.0047 13 0.07 0.04 3.2 0.014 0.016 0.023 0.0017 0.0050 14 0.10 0.05 3.9 0.020 0.014 0.017 0.0027 0.0037 15 0.06 0.05 3.5 0.012 0.011 0.005 0.0027 0.0035 16 0.07 0.08 4.3 0.010 0.012 0.016 0.0018 0.0048

division Psalter
No. Cooling rate
(° C / s) MA fraction (%) MA crystal grain size
(탆) Cold reduction ratio
(%) The tensile strength
(MPa) ductility(%)

Honor



One 0.5 7 3.9 11 709 16 2 0.2 5 4.7 25 796 14 3 1.3 9 2.4 21 775 15 4 1.9 9 2.1 30 877 12 5 0.3 5 4.6 16 723 16 6 0.7 7 3.8 28 853 13 7 1.1 8 3.3 19 802 14



Comparative Example



8 2 15 2.5 28 915 9 9 One 11 3.5 19 1012 5 10 0.7 9 2.4 12 603 26 11 3 12 1.7 30 903 9 12 0.05 4 6.1 23 687 22 13 1.5 8 2.3 38 927 9 14 0.8 7 3.5 5 679 22 15 One 2 8.6 15 645 23 16 1.8 8 3.2 21 925 9

As shown in Tables 1 and 2, in the case of the present invention 1-7 in which the stressed component is within the range of the present invention and the cooling rate of 0.1-2 캜 / s and the cold reduction ratio of 10-30% % Or more of bainite microstructure is obtained, and the mechanical properties also show a tensile strength of 700 to 900 MPa and a ductility of 10% or more.

On the other hand, in Comparative Example 8, since the carbon content is increased to increase cementite, which is a hard phase, the tensile strength is increased and the ductility is weakened.

Comparative Example 9 is a case where the silicon content is out of the range of the present invention, and silicon is also added to the carbon in a similar manner to carbon, so that it is employed at the base and ultimately exhibits the effect of strengthening employment. That is, even when the amount of silicon added is 0.5%, the tensile strength becomes very large and the ductility decreases sharply.

Comparative Example 10 shows that the addition of manganese and boron decreases the curing ability of the steel, so that even when the cooling condition is satisfied, the ferrite and the bainite structure are mixed together to decrease tensile strength and increase ductility.

In Comparative Example 11, the stressed component satisfies the range of the present invention, but as the cooling rate is increased in the manufacturing process, martensite is formed and the strength is increased and ductility is degraded.

In Comparative Example 12, the ferrite component satisfies the range of the present invention, but the ferrite is formed in a case where the cooling rate is slow in the manufacturing process, so that the strength decreases and the ductility increases.

Comparative Example 13 shows that the stress-extinguishing component satisfies the range of the present invention, but when the cold reduction rate in the cold drawing process exceeds the range of the present invention, the work hardening becomes severe, the strength increases, and the ductility decreases.

In Comparative Example 14, the stress-extinguishing component satisfied the range of the present invention, but the cold cut-off rate in the cold drawing process was less than the range of the present invention and the work hardening effect was not obtained, .

In Comparative Example 15, the amount of titanium added was small, and the solute boron amount was decreased, so that the hardening ability was decreased. When the cooling rate was low, the amount of pro-eutectoid ferrite precipitated was increased and the tensile strength was decreased and the ductility was relatively increased .

In addition, in Comparative Example 16, manganese is added, and martensite is generated even when the martensite is cooled at the cooling rate proposed in the present invention because the martensite has a relatively high hardenability. This shows that the strength is increased and the ductility is lowered.

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 (6)

(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% 0.0010 to 0.0030% of boron (B), 0.010 to 0.030% of titanium (Ti), 0.0050% or less of nitrogen (N), the balance of Fe and unavoidable impurities, (MA) having a tensile strength of 700 to 900 MPa and a ductility of 10% or more.
The steel wire according to claim 1, wherein the residual grained martensite (MA) has a grain size of 5 탆 or less.
(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% (B): 0.0010 to 0.0030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0050% or less, the remainder Fe and unavoidable impurities, and then reheating the steel material;
Subjecting the reheated steel material to a final hot rolling and then cooling the material to a temperature range of Bf to Bf - 50 占 폚 at a cooling rate of 0.1 to 2 占 폚 / s; And
A step of cold-drawing the cooled steel material and then cold drawing the steel material; and a method of manufacturing a steel wire having excellent strength characteristics
The steel wire manufacturing method according to claim 3, wherein the microstructure of the steel wire comprises bainite of 90% or more by area and residual martensite (MA)
The method of manufacturing a steel wire according to claim 4, wherein the residual grained martensite (MA) has a grain size of 5 탆 or less.
The method of manufacturing a steel wire according to claim 3, wherein the reduction rate in cold drawing is in the range of 10 to 30%.