KR20160063563A - Wire rod having high strength and impact toughness, and method for manufacturing thereof - Google Patents

Abstract

Provided are a wire rod having excellent strength and impact toughness, and a manufacturing method thereof. The present invention relates to the wire rod having excellent tensile strength and impact toughness, 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 niobium (Nb); 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 is to provide a wire rod, which is able to only have high strength and high impact toughness with hot rolling and continuous cooling processes without an additional thermal treatment process, and a manufacturing method thereof.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod having excellent strength and impact resistance,

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

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. In addition, since the stability against external impact is also increasing, impact toughness is also recognized as an important physical property of the material or parts.

There is a limitation in securing a high strength and high impact toughness in a ferrite or pearlite structure in a wire rod. The materials having these structures usually have a high impact toughness and a relatively low strength. In order to increase the strength, cold drawing can obtain high strength, while the impact toughness decreases rapidly in proportion to the increase in strength have.

Therefore, in order to realize high strength and high impact toughness at the same time, bainite structure 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 a high strength and high impact 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, since wire rods capable of stably obtaining bainite or martensite structure by using hot rolling and continuous cooling processes have not yet been developed, there is a demand for development of such wire rods.

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 wire rod which can have high strength and high 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,

(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% (N): 0.0010 to 0.0030%, Nb: 0.010 to 0.030%, Ti: 0.010 to 0.030%, nitrogen (N): 0.0050% or less, the balance Fe and unavoidable impurities And the microstructure thereof is composed of bainite of 90% or more by area and residual martensite (MA), which is excellent in tensile strength and impact toughness.

It is preferable that the contents of manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) satisfy the following relational expression (1).

[Relation 1]

Mn + 5 (Ti-3.5N) / B? 5.0

However, manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) in the above relational expression 1 means the content by weight of the corresponding element, respectively.

It is preferable that the bainite has a grain size of 10 mu m or less and the amorphous martensite (MA) has a grain size of 5 mu m or less.

The wire rod may have a tensile strength of 700 to 800 MPa and an impact value of 170 to 220J.

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% (N): 0.010 to 0.030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0050% or less, the balance Fe and unavoidable impurities And then reheating it to a temperature range of Ae3 + 150 ° C to Ae3 + 250 ° C;

Starting rolling with the reheated steel material to finish hot rolling in a temperature range of Ar 3 + 100 ° C to Ar 3 + 200 ° C;

Cooling the hot-rolled steel material to a temperature range of Bf to Bf - 50 ° C at a cooling rate of 0.1 to 2 ° C / s; And

And air cooling the cooled steel material. The present invention also relates to a method of manufacturing a wire rod excellent in tensile strength and impact toughness.

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

It is preferable that the bainite has a grain size of 10 mu m or less and the amorphous martensite (MA) has a grain size of 5 mu m or less.

It is preferable that the content of manganese (Mn), titanium (Ti), boron (B) and nitrogen (N)

The present invention according to the above-described structure can provide a wire rod excellent in strength and impact toughness required in 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 of the present invention having high strength and high impact toughness 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%, niobium: 0.010 to 0.030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N) It is made of impurities.

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. 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%.

Niobium (Nb): 0.010 to 0.030%

Niobium reacts with carbon and nitrogen during hot rolling to form fine carbonitride. Since these carbonitrides fix the austenite grain boundary system, grain growth can be suppressed. Also, the carbonitride that is finely distributed in the steel plays a role of increasing the strength by precipitation strengthening effect. If the addition amount of sodium niobium is less than 0.010%, the precipitation amount of the niobium carbonitride is small and the grain growth inhibition and strength enhancement effect is insufficient. If the addition amount of niobium exceeds 0.030%, the size of the carbonitride becomes large and the crystal grain inhibiting effect may be lost. In view of this, in the present invention, it is preferable to limit the addition amount of the niobium to 0.010-0.030%.

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 the present invention, it is preferable that the manganese, titanium, boron and nitrogen are contained so as to satisfy the following relational expression (1).

[Relation 1]

Mn + 5 (Ti-3.5N) / B? 5.0

However, manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) in the above relational expression 1 means the content by weight of the corresponding element, respectively.

In the present invention, manganese improves the hardenability and thus facilitates the production of bainite even when the cooling rate is relatively small. And, the titanium is combined with nitrogen to form a nitride, and the boron is sufficiently dissolved in the steel, thereby suppressing the ferrite formation and allowing the bainite to be easily produced.

As a result of extensive research and experimentation, the present inventors have found that when the relationship between manganese, titanium, boron, and nitrogen satisfies Mn + 5 (Ti-3.5N) / B? And a wire material of a bainite structure having impact toughness.

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. 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.

Further, in the present invention, it is preferable that the bainite has a grain size of 10 mu m or less, and the residual martensite (MA) has a grain size of 5 mu m or less.

Next, a method for manufacturing a wire rod having high strength and high impact toughness according to the present invention will be described.

The method for manufacturing a wire according to the present invention comprises the steps of: preparing a steel material having the above composition and then reheating the steel material to a temperature range of Ae3 + 150 DEG C to Ae3 + 250 DEG C; Starting rolling with the reheated steel material to finish hot rolling in a temperature range of Ar 3 + 100 ° C to Ar 3 + 200 ° C; Cooling the hot-rolled steel material to a temperature range of Bf to Bf - 50 ° C at a cooling rate of 0.1 to 2 ° C / 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. In the present invention, it is preferable to limit the reheating temperature to a range of Ae3 + 150 ° C to Ae3 + 250 ° C. If the reheating temperature is lower than Ae3 + 150 deg. C, the temperature of the steel is too high during hot rolling to cause surface defects, and when Ae3 + 250 deg. C is exceeded, the austenite grains grow to a great extent, Because.

In the present invention, the reheated steel is hot-rolled, and the finish hot rolling temperature is preferably limited to a range of Ar3 + 100 ° C to Ar3 + 200 ° C. If the final hot rolling temperature is lower than Ar3 + 100 占 폚, there is a high possibility that surface defects are caused in the steel material. If the final hot rolling temperature is higher than Ar3 + 200 占 폚, the crystal grains do not become finer and desired mechanical properties can not be obtained.

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 increases. If the cooling rate exceeds 2 ° C / s, the formation of martensite increases and the strength and impact toughness are lowered. .

By controlling the cooling rate in the cooling section as described above, a bainite microstructure excellent in strength and impact toughness of 90% or more in area fraction can be obtained. The wire rod thus produced may have a tensile strength of 700 to 800 MPa and an impact value of 170 to 220J.

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 reheated under the same conditions as shown in Table 2, hot rolled to a final thickness of 15 mm under the conditions of Table 2, 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 of crystalline martensite (MA), the crystal grain size and the crystal grain size of bainite were measured for each of the cooled steels, and the results are shown in Table 2, and the tensile strength and impact value were measured and shown in Table 2 .

In Table 2, the area fraction and grain size of the martensite (MA) on the steel and the grain size of the bainite 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. The impact test was carried out at room temperature using an impact tester with an edge curvature of 2 mm and a test capacity of 500 J of the striker impacting the specimen.

Pseudo-No.
Composition Component (% by weight) Relationship 1
C Si Mn P S Nb Ti B N One 0.10 0.06 3.3 0.015 0.014 0.010 0.014 0.0023 0.0050 -4.3 2 0.09 0.03 3.4 0.014 0.009 0.021 0.023 0.0015 0.0048 24.1 3 0.11 0.04 3.5 0.011 0.016 0.014 0.021 0.0024 0.0037 20.3 4 0.05 0.05 3.5 0.012 0.014 0.028 0.017 0.0027 0.0040 9.1 5 0.12 0.09 3.4 0.013 0.011 0.025 0.028 0.0030 0.0039 27.3 6 0.07 0.10 3.9 0.015 0.014 0.017 0.014 0.0023 0.0041 3.1 7 0.27 0.06 3.5 0.013 0.008 0.023 0.027 0.0026 0.0032 33.9 8 0.14 0.45 3.6 0.010 0.005 0.012 0.018 0.0020 0.0031 21.5 9 0.13 0.02 2.2 0.016 0.010 0.020 0.017 0.0007 0.0042 18.6 10 0.10 0.03 3.5 0.014 0.013 0.030 0.020 0.0021 0.0045 13.6 11 0.05 0.05 3.1 0.013 0.011 0.019 0.024 0.0025 0.0039 23.8 12 0.08 0.07 3.6 0.012 0.013 0.026 0.006 0.0017 0.0034 -13.8 13 0.07 0.04 4.4 0.011 0.007 0.022 0.017 0.0028 0.0046 6.0 14 0.06 0.09 3.5 0.014 0.006 0.050 0.023 0.0023 0.0043 20.8 15 0.11 0.005 3.1 0.011 0.005 0.021 0.021 0.0018 0.0040 22.5 16 0.10 0.06 3.4 0.008 0.007 0.016 0.019 0.0019 0.0043 13.8

* In Table 1, the relational expression 1 is Mn + 5 (Ti-3.5N) / B

division Psalter
No. Reheating
Temperature (℃) Finishing hot rolling temperature
(° C) Cooling rate (° C / s) MA fraction
(%) MA
Crystal grain size (탆) Bainite crystal grain size (탆)
The tensile strength
(MPa) Shock
(J)

Honor


One 1010 810 0.8 7 3.4 8 740 175 2 990 780 1.6 9 2.9 7 775 198 3 1000 820 0.9 7 3.2 8 772 189 4 970 770 1.9 10 2.5 6 736 200 5 1040 800 0.5 6 3.8 9 780 211 6 1020 810 0.3 4 4.3 9 730 180



Comparative Example




7 1000 830 0.4 5 4.0 9 843 130 8 1010 810 0.9 8 3.3 8 976 110 9 1020 820 0.6 6 3.7 9 675 186 10 990 800 0.06 2 5.7 16 680 183 11 960 770 3.2 12 2.0 5 810 95 12 1040 790 0.5 5 3.8 8 662 192 13 980 790 1.5 8 3.0 7 850 80 14 1000 780 0.7 7 3.6 34 678 185 15 1150 850 1.1 7 3.3 23 689 167 16 1040 950 1.6 8 3.0 29 725 161

As shown in Tables 1 and 2, in the case of Inventive 1-6 in which the steel composition and the manufacturing process conditions are in the range of the present invention, it can be seen that 90% or more of bainite microstructure is obtained, Mechanical properties also show tensile strength of 700-800 MPa and excellent impact toughness of 170-220J.

Particularly, among the inventions, Inventive Example 2-5 is an example satisfying the relational expression (Mn + 5 (Ti-3.5N) / B = 5.0) of manganese, titanium, boron and nitrogen, It can be seen that impact toughness is further improved as compared with Examples 1 and 6.

On the contrary, in Comparative Example 7, it is confirmed that the tensile strength is improved due to the higher carbon content, while the impact toughness is lowered because carbon increases cementite, which is a hard phase.

Comparative Example 8 is a case where the silicon content is out of the range of the present invention, and as the amount of silicon to be added increases similarly to that of carbon, the amount of silicon in the base is increased and finally the effect of strengthening the employment is exhibited. That is, even when the amount of silicon added is 0.45%, the tensile strength becomes very large, and the impact toughness decreases sharply.

Comparative Example 9 shows that the addition of manganese and boron decreases the hardenability of the steel, so that even when the cooling conditions are satisfied, the ferrite and bainite structure are mixed with each other, so that the tensile strength is decreased and the impact toughness is increased.

In addition, Comparative Example 10 shows that the steel composition component satisfies the range of the present invention but the cooling rate is slow in the manufacturing process, and the ferrite is formed to decrease the strength and increase the impact toughness.

In Comparative Example 11, the steel composition satisfies the range of the present invention. However, as the cooling rate is increased in the manufacturing process, martensite is formed to increase strength and impact toughness.

In Comparative Example 12, the amount of titanium added was small. As the amount of solute boron decreased, the hardenability decreased. When the cooling rate was low, the amount of pro-eutectoid ferrite precipitated increased, and the tensile strength decreased and the impact toughness increased relatively have.

In addition, in Comparative Example 13, manganese is added, and martensite is generated even when cooled at the cooling rate as described in the present invention because the hardening ability is too large. This shows that the strength is increased and the impact toughness is lowered.

Comparative Example 14 shows that when a large amount of niobium is added, since the coarse niobium carbonitride is formed during the hot deformation, the effect of grain refinement is reduced, the strength is decreased, and the impact toughness is relatively increased.

In Comparative Examples 15 and 16, the steel composition of the present invention is satisfied but each exceeds the reheating temperature range and the final hot rolling temperature range. Since the austenite is not sufficiently refined, the final bainite grain size becomes 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 (8)

(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% (N): 0.0010 to 0.0030%, Nb: 0.010 to 0.030%, Ti: 0.010 to 0.030%, nitrogen (N): 0.0050% or less, the balance Fe and unavoidable impurities And the microstructure is composed of bainite of 90% or more by area and residual martensite (MA), and is excellent in tensile strength and impact toughness.
The wire according to claim 1, wherein the contents of manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) satisfy the following relational expression (1).
[Relation 1]
Mn + 5 (Ti-3.5N) / B? 5.0
However, manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) in the above relational expression 1 means the content by weight of the corresponding element, respectively.
The wire according to claim 1, wherein the bainite has a grain size of 10 탆 or less and the grain martensite (MA) has a grain size of 5 탆 or less.
The wire rod according to claim 1, wherein the wire rod has a tensile strength of 700 to 800 MPa and an impact value of 170 to 220 J.
(P): 0.020% or less, S: not more than 0.020% (by mass), carbon (C): 0.05 to 0.15% (N): 0.010 to 0.030%, titanium (Ti): 0.010 to 0.030%, nitrogen (N): 0.0050% or less, the balance Fe and unavoidable impurities And then reheating it to a temperature range of Ae3 + 150 ° C to Ae3 + 250 ° C;
Starting rolling with the reheated steel material to finish hot rolling in a temperature range of Ar 3 + 100 ° C to Ar 3 + 200 ° C;
Cooling the hot-rolled steel material to a temperature range of Bf to Bf - 50 ° C at a cooling rate of 0.1 to 2 ° C / 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 5, wherein the content of manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) satisfies the following relational expression
[Relation 1]
Mn + 5 (Ti-3.5N) / B? 5.0
However, manganese (Mn), titanium (Ti), boron (B) and nitrogen (N) in the above relational expression 1 means the content by weight of the corresponding element, respectively.
The method for producing a wire rod according to claim 6, wherein the bainite has a grain size of 10 탆 or less, and the residual grain martensite (MA) has a grain size of 5 탆 or less.