KR20160063564A - Wire rod having high strength, and method for manufacturing thereof - Google Patents

Abstract

Provided are a wire rod having high strength, and a manufacturing method thereof. The present invention relates to a wire rod 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); 0.5-1.5 wt% of copper (Cu); 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. The present invention provides a wire rod, which is able to only have high strength with hot rolling and a continuous cooling process without an additional thermal treatment process, and a manufacturing method thereof.

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 which can be used for industrial machinery or machine parts such as automobiles, which are exposed to various external load environments, .

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 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 limit in ensuring a high strength of ferrite or pearlite structure in the wire rods. 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.

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

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), 0.5 to 1.5% of copper (Cu), 0.010 to 0.030% of titanium (Ti), 0.0050% of nitrogen (N) or less and the balance Fe and unavoidable impurities , The fine structure of which is composed of bainite of 90% or more by area and residual martensite (MA).

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

The wire rod may have a fine copper precipitate having a diameter of 50 nm or less distributed in the matrix.

The wire rod may have a tensile strength of 700 to 900 MPa and a ductility of 15% or more.

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% , The balance Fe and unavoidable impurities, in the range of 0.001 to 0.0030% of boron (B), 0.5 to 1.5% of copper (Cu), 0.010 to 0.030% of titanium (Ti) And then reheating the same;

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 air-cooling the cooled steel material.

It is preferable that the microstructure of the wire rod is composed of 90% by area or more of bainite and residual martensite (MA).

The present invention according to the above-described structure can provide a wire rod having excellent strength characteristics required for industrial machinery 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.

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

First, the wire material of the present invention having high strength characteristics will be described.

The wire material of the present invention preferably contains 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) ): Not more than 0.020%, boron (B): 0.0010 to 0.0030%, copper (Cu): 0.5 to 1.5%, titanium (Ti): 0.010 to 0.030%, nitrogen (N) .

Hereinafter, the reasons for limiting the composition and composition range of the wire rod steel of the present invention 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. However, in order to secure the bainite structure, it is necessary to limit the addition amount 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 to 0.15%. If the carbon content is less than 0.05%, it is difficult to obtain the target strength, and if it exceeds 0.15% It is because.

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 silicon 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, 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%.

Copper (Cu): 0.5 to 1.5%

Copper contributes to strength improvement by being dissolved or precipitated in the matrix. The employment strengthening effect is relatively small and it is necessary to utilize the precipitation strengthening effect in order to increase the strength. If the addition amount of copper is less than 0.5%, it is not enough to obtain an effect of increasing the strength by precipitation strengthening. If the amount exceeds 1.5%, surface defects may easily occur during hot rolling. 1.5%.

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 wire material 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. If the fraction is high, the strength of the steel may be increased but ductility may deteriorate. Therefore, it is desirable to manage 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.

In the present invention, fine copper precipitate having a diameter of 50 nm or less can be formed in the wire base structure during cooling after hot rolling, thereby enhancing the strength of the steel.

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.

Next, a method of manufacturing a wire rod having high 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 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 air cooling the cooled steel material.

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, .

By controlling the cooling rate in the cooling section as described above, a bainite microstructure excellent in strength characteristics with an area fraction of 90% or more can be obtained. As a result, it is possible to effectively produce a wire having a tensile strength of 700 to 900 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 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 and then cooled to the cooling rate shown in Table 2. 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.
Composition Component (% by weight) C Si Mn P S Cu Ti B N One 0.10 0.03 3.2 0.013 0.010 0.5 0.016 0.0012 0.0037 2 0.07 0.07 3.4 0.012 0.016 1.3 0.017 0.0021 0.0041 3 0.08 0.05 3.5 0.015 0.013 0.8 0.013 0.0022 0.0040 4 0.13 0.09 3.9 0.016 0.009 1.2 0.025 0.0026 0.0039 5 0.11 0.04 3.8 0.014 0.008 0.7 0.023 0.0019 0.0041 6 0.06 0.06 3.1 0.015 0.005 1.0 0.021 0.0018 0.0045 7 0.23 0.08 3.6 0.011 0.011 1.2 0.020 0.0014 0.0040 8 0.06 0.10 3.3 0.012 0.010 0.1 0.023 0.0017 0.0049 9 0.09 0.08 1.9 0.013 0.009 0.7 0.017 0.0006 0.0035 10 0.10 0.06 3.2 0.013 0.014 0.6 0.019 0.0024 0.0043 11 0.06 0.05 3.6 0.012 0.008 1.4 0.022 0.0017 0.0042 12 0.07 0.05 3.5 0.011 0.010 1.1 0.007 0.0028 0.0036 13 0.05 0.09 4.3 0.008 0.011 0.9 0.014 0.0014 0.0037 14 0.10 0.07 3.4 0.012 0.008 2.0 0.020 0.0020 0.0036 15 0.09 0.5 3.5 0.014 0.009 1.0 0.019 0.0023 0.0041

division Psalm NO. Cooling rate
(° C / s) MA fraction
(%) MA crystal grain size
(탆) Copper precipitate
Size (nm) The tensile strength
(MPa) ductility
(%)


Honor

One 0.6 7 3.9 40 720 25 2 1.4 9 2.5 31 880 18 3 1.0 8 3.3 34 785 22 4 1.9 10 2.3 26 893 17 5 0.6 7 3.7 40 782 21 6 0.2 5 4.5 45 774 23



Comparative Example



7 0.4 5 4.0 43 956 13 8 0.9 8 3.5 35 689 27 9 0.6 6 3.6 41 650 28 10 0.05 3 5.9 80 667 27 11 3 13 1.9 19 973 12 12 0.4 6 3.9 44 692 25 13 1.4 8 2.8 33 960 13 14 0.7 6 3.5 38 920 15 15 0.8 7 3.5 37 944 14

As shown in Tables 1 and 2, in the case of Inventive Examples 1-6 in which the stressed component is within the range of the present invention and the cooling rate of 0.1-2 DEG C / s is satisfied, all of 90% or more of bainite microstructure is obtained The tensile strength of 700-900 MPa and the ductility of 15% or more can be seen.

On the contrary, in Comparative Example 7, it is confirmed that the carbon content is increased and the tensile strength is improved while the ductility is decreased, because carbon increases cementite which is a hard phase.

Comparative Example 8 shows that when the copper content is less than the range of the present invention, the precipitation strengthening effect by copper is small, so that the tensile strength is small and the ductility is improved.

Comparative Example 9 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 bainite structure are mixed with each other, so that tensile strength decreases and ductility increases.

Further, in Comparative Example 10, the ferrite component satisfies the range of the present invention but has a slow cooling rate in the manufacturing process, showing that the ferrite is formed, the strength is decreased, and the ductility is increased.

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

In Comparative Example 12, the addition amount of titanium was small, and the solute boron amount was decreased, so that the hardenability decreased. When the cooling rate was small, 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 13, manganese is added, and martensite is produced even when cooled at the cooling rate proposed in the present invention because the hardening ability is too large, and the strength is increased and the ductility is lowered.

Comparative Example 14 shows a case where a large amount of copper is added, and the effect of precipitation strengthening by copper is so large that tensile strength is increased but ductility is decreased.

In Comparative Example 15, when a large amount of silicon was added, the tensile strength was increased and the ductility was decreased because the effect of strengthening the solid solution by silicon was too large.

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 (7)

(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), 0.5 to 1.5% of copper (Cu), 0.010 to 0.030% of titanium (Ti), 0.0050% of nitrogen (N) or less and the balance Fe and unavoidable impurities , And the microstructure is composed of bainite of 90% or more by area and residual martensite (MA).
The wire rod according to claim 1, wherein the residual grained martensite (MA) has a grain size of 5 탆 or less.
The wire according to claim 1, wherein the wire has fine copper precipitates of 50 nm or less in diameter distributed in the matrix.
The wire according to claim 1, wherein the wire has a tensile strength of 700 to 900 MPa and a ductility of 15% or more.
(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% , The balance Fe and unavoidable impurities, in the range of 0.001 to 0.0030% of boron (B), 0.5 to 1.5% of copper (Cu), 0.010 to 0.030% of titanium (Ti) And then reheating the same;
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 air-cooling the cooled steel material.
The method according to claim 5, wherein the microstructure of the wire comprises at least 90% by area of bainite and the remaining martensite (MA)
The method for producing a wire rod according to claim 6, wherein the residual grained martensite (MA) has a grain size of 5 탆 or less.