US20170298471A1

US20170298471A1 - Wire rod having enhanced strength and impact toughness and preparation method for same

Info

Publication number: US20170298471A1
Application number: US15/516,783
Authority: US
Inventors: Hyong-Jik Lee; Geun-Soo Ryu
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2014-11-03
Filing date: 2015-11-02
Publication date: 2017-10-19
Also published as: MX2017005038A; CN107075648A; JP6488008B2; DE112015004992T5; JP2017538034A; KR101714903B1; KR20160053776A; CN107075648B

Abstract

Provided is a wire rod having enhanced strength and impact toughness comprising, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (5):0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities. A microstructure includes bainitic ferrite in an area fraction of 90% or more, and a martensite/austenite (M/A) constituent as a residual component thereof.

Description

TECHNICAL FIELD

The present disclosure relates to a wire rod having enhanced strength and impact toughness and a preparation method for the same. The wire rod may be used for components of industrial machines, vehicles, and the like, exposed to various external load environments.

BACKGROUND ART

Recently, efforts to reduce emissions of carbon dioxide, a main cause of environmental pollution, have become a global issue. In line with this, active movements to regulate vehicle exhaust gas emissions exist. As a measure to comply with such regulations, automakers are attempting to reduce emissions through improvement of fuel efficiency. However, in order to improve fuel efficiency, vehicles are required to be lightweight while having high performance. Thus, the requirement for high strength in materials for vehicles and components thereof is increased. In addition, since demand for resistance to the shock of external impacts is also increased, impact resistance is also recognized as an important material property of a material or a component.
A wire rod having a ferrite or perlite structure is limited in terms of securing excellent strength and impact toughness. In a material having a structure described above according to the related art, impact toughness is high, but strength is relatively low. When cold drawing is performed in order to increase strength, high strength may be obtained. However, there may be a disadvantage in that impact toughness may rapidly decrease in proportion to an increase in strength.
Thus, in general, in order to simultaneously realize excellent strength and impact toughness, a bainite structure or a tempered martensite structure are used. A bainite structure may be obtained through a constant temperature transformation heat treatment using a steel material having been hot rolled, and a tempered martensite structure may be obtained through a quenching and tempering heat treatment. However, since there are limitations on obtaining the structures described above through only using hot rolling and continuous cooling processes according to the related art, it is necessary to perform an additional heat treatment process described above using a hot rolled steel material.
If high strength and excellent impact toughness are secured without an additional heat treatment being performed, a portion of a process from a material to a component having been manufacturing may be omitted or may be simplified. Thus, there is an advantage in which productivity is improved and manufacturing costs are lowered.
However, a wire rod, capable of being provided with a stable bainitic or martensitic structure using hot rolling and continuous cooling processes without an additional heat treatment process being required, has not yet been developed, so demand for the development of such a wire rod has emerged.

DISCLOSURE

Technical Problem

An aspect of the present disclosure may provide a wire rod having high strength and excellent impact toughness with hot rolling and continuous cooling processes without an additional heat treatment process, and a preparation method for same.
An objective to be solved in the present disclosure is not limited to the above-mentioned objective, and other objectives not mentioned can be clearly understood by those skilled in the art from the following description.

Technical Solution

According to an aspect of the present disclosure, a wire rod having enhanced strength and impact toughness includes, by wt %: carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities, wherein a microstructure includes bainitic ferrite in an area fraction of 90% or more, and a martensite/austenite (M/A) constituent as a residual component thereof.
According to another aspect of the present disclosure, a method of preparing a wire rod having enhanced strength and impact toughness includes: reheating a steel material including, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities; hot rolling the steel material having been reheated; cooling the steel material at a rate of 0.1° C./s to 2° C./s in a temperature within a range of Bf° C. to Bf−50° C., after the hot rolling; and air cooling the steel material having been cooled.

Advantageous Effects

According to an exemplary embodiment in the present disclosure, only using hot rolling and continuous cooling processes, a wire rod having enhanced strength and impact toughness, required for a material and a component for an industrial machine or a vehicle, may be provided.
In addition, an additional heat treatment process according to the related art may be omitted, whereby there is an advantageous in reducing total manufacturing costs.

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail.
First, a wire rod according to the present disclosure will be described in detail. The wire rod according to the present disclosure includes, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities.
Hereinafter, a steel component of a wire rod and a limitation reason of a composition range will be described in detail (hereinafter, wt %).
Carbon (C): 0.05% to 0.15%
Carbon is an essential element for ensuring strength, and is dissolved in steel or exists in the form of carbide or cementite. The simplest method for increasing strength is to form carbide or cementite by increasing the content of carbon. However, ductility and impact toughness are reduced in this case. Thus, it is required to adjust an addition amount of carbon within a certain range. In the present disclosure, it is preferable to add the content of C in the range of 0.05% to 0.15%. In a case in which the content of carbon is less than 0.05%, it may be difficult to achieve target strength. In a case in which the content of carbon exceeds 0.15%, impact toughness may be significantly reduced.
Silicon (Si): 0.2% or Less
Silicon is known as a deoxidizing element, along with aluminum, and is an element for improving strength. Silicon is known as an element highly effective in increasing strength through solid solution strengthening of a steel material through being dissolved in ferrite when added to the steel material. However, when silicon is added thereto, strength may be significantly increased, but ductility and impact toughness are significantly reduced. In a case of a cold forging component requiring sufficient ductility, the addition of silicon should be limited. In the present disclosure, to secure excellent impact toughness while significantly reducing a decrease in strength, a content of silicon of 0.2% or less is included. In a case in which the content of silicon exceeds 0.2%, there may be limitations on securing target impact toughness. More preferably, a content of silicon of 0.1% or less is included.
Manganese (Mn): 3.0% to 4.0%
Manganese allows strength of a steel material to be increased and hardenability thereof to be improved, thereby allowing a low temperature structure such as bainite or martensite to be easily formed in a wide range of cooling rates. However, in a case in which the content of manganese is lower than 3.0%, hardenability is insufficient, and thus, it is difficult to stably secure a low temperature structure in a continuous cooling process after hot rolling. In addition, in a case in which the content of manganese exceeds 4.0%, hardenability is significantly high, and thus, such a case is inappropriate since a martensite structure may be obtained even in air cooling. In this regard, in the present disclosure, it is preferable to include a content of manganese of 3.0% to 4.0%.
Phosphorus (P): 0.020% or Less
Phosphorus is a main cause of a reduction in toughness and a reduction in delayed fracture resistance, as being segregated in a grain boundary, and thus, it is preferable not to include phosphorous. For this reason, an upper limit thereof in the present disclosure is limited to 0.020%.
Sulfur (S): 0.020% or Less
Sulfur is segregated in a grain boundary to reduce toughness and allows a low melting point emulsion to be formed so as to inhibit hot rolling, and thus, it is preferable not to include sulfur. For this reason, an upper limit thereof in the present disclosure is limited to 0.020%.
Boron (B): 0.0010% to 0.0030%
Boron is an element for improving hardenability, and is an element suppressing formation of ferrite in cooling through being diffused at an austenite grain boundary, and allowing bainite or martensite to be easily formed. However, in a case in which an addition amount thereof is less than 0.0010%, it is difficult to expect an effect due to addition. In a case in which an addition amount thereof exceeds 0.0030%, it is difficult to expect an increase in an effect, while grain boundary strength is reduced due to the precipitation of boron-based nitride in a grain boundary, thereby decreasing hot workability. Thus, in this regard, a range of addition of boron in the present disclosure is 0.0010% to 0.0030%.
Titanium (Ti): 0.010% to 0.030%
Titanium has the highest reactivity with nitrogen, thereby forming nitride first. When most of nitrogen in steel is exhausted by forming titanium nitride (TiN) due to the addition of titanium, titanium allows boron to exist in a soluble state by preventing precipitation of BN, thereby obtaining an effect of improving hardenability. However, in a case in which an addition amount thereof is less than 0.010%, an effect due to addition is insufficient. In a case in which an addition amount thereof exceeds 0.030%, coarse nitride is formed, thereby reducing mechanical properties. In this regard, the content of titanium in the present disclosure is 0.010% to 0.030%.
Nitrogen (N): 0.0050% or Less
Nitrogen is maintained in a state of being soluble with boron. To sufficiently exhibit an effect of improving hardenability, it is preferable not include nitrogen. Thus, in the present disclosure, it is preferable that the content thereof be 0.0050% or less.
Aluminum (Al): 0.010% to 0.050%
Aluminum is a powerful deoxidizing element, and allows oxygen in steel to be removed so as to improve cleanliness and is combined with nitrogen dissolved in steel so as to form aluminum nitride (AlN), thereby improving impact toughness. In the present disclosure, aluminum is actively added. In a case in which a content thereof is less than 0.010%, it is difficult to expect an addition effect thereof. In a case in which a content thereof exceeds 0.050%, a large amount of alumina inclusion is generated, thereby significantly reducing mechanical properties. In this regard, in the present disclosure, it is preferable that the content of aluminum be in the range of 0.010% to 0.050%.
In addition to compositions described above, chromium (Cr) of 0.3% less than may be additionally included. Chromium increases strength and hardenability of a steel material, in a manner similar to manganese. In a case in which the content of chromium is 0.3% or more, hardenability may be improved and strength may be increased due to a solid solution strengthening effect, but impact toughness may be reduced. In this regard, in the present disclosure, it is preferable to include the content of chromium in the range of less than 0.3%.
In addition to composition described above, a residual component thereof includes Fe and unavoidable impurities. In the present disclosure, addition of other alloys in addition to an alloy composition described above is not excluded.
Meanwhile, in the present disclosure, it is preferable that the content of manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) is contained to satisfy Relational Expression 1,
Mn+5(Ti−3.5N)/B≧5.0. [Relational Expression 1]
Here, in Relational Expression 1, manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) refer to the contents by weight of elements, respectively.
In the present disclosure, manganese increases hardenability, even when a cooling rate is relatively low, manganese allows bainitic ferrite to be easily generated. In addition, titanium is combined with nitrogen to form a nitride and allows boron to be sufficiently dissolved in steel, thereby suppressing the generation of ferrite and allowing bainitic ferrite to be easily generated.
The inventors of the present disclosure have repeatedly conducted research and experiments based on the description above. As a result, when the relationship among manganese, titanium, boron, and nitrogen satisfies Mn+5(Ti−3.5N)/B≧5.0 based on weight %, it is recognized that a wire rod having a bainitic ferrite structure with more excellent strength and impact toughness is provided and Relational Expression 1 is derived.
In addition, in the present disclosure, it is preferable that the content of manganese (Mn) and silicon (Si) satisfies Relational Expression 2,
Mn/Si≧18. [Relational Expression 2]
Here, in Relational Expression 2, manganese (Mn) and silicon (Si) refer to the contents by weight of elements, respectively.
In the present disclosure, manganese increases hardenability. Thus, even when a cooling rate is relatively low, manganese allows bainite to be easily generated. In addition, silicon is dissolved in steel, and thus, strength may be increased, whereas impact toughness may be reduced.
The inventors have repeatedly conducted research and experiments based on the description above. As a result, when the relationship between manganese and silicon satisfies Mn/Si≧18 based on weight o, it is confirmed that a wire rod having a bainitic ferrite structure with more excellent strength and impact toughness is provided, and thus, a Relational Expression of a composition component is proposed.
Meanwhile, it is preferable that a wire rod according to the present disclosure be provided with an arbitrary cross sectional area in which a ratio of a maximum concentration [Mn_max] and a minimum concentration [Mn_min] of manganese satisfies Relational Expression 3,
[Mn_max]/[Mn_min]≦3. [Relational Expression 3]
In the present disclosure, manganese increases hardenability. Even when a cooling rate is relatively low, manganese allows bainitic ferrite to be easily generated. However, when manganese is locally segregated, martensite may be easily generated. In addition, in an area in which manganese is depleted, ferrite may be formed. Thus, a microstructure may be non-uniform, and impact toughness may be reduced.
The inventors have repeatedly conducted researches and experiments based on the description above. As a result, when a ratio of a maximum concentration and a minimum concentration of manganese is 3 or less in an arbitrary cross sectional area of the wire rod, it is confirmed that a wire rod having a bainitic ferrite structure with excellent strength and impact toughness is provided, and thus, Relational Expression is proposed.
Hereinafter, a microstructure according to the present disclosure will be described in detail.
It is preferable that a microstructure of a wire rod according to the present disclosure includes bainitic ferrite of 90 area % or more and a residual martensite-austenite (M/A) constituent. Meanwhile, bainite may be referred to as various terms depending to the content of carbon or morphology. According to the related art, bainite is referred to as upper/lower bainite above the range of medium carbon (about 0.2 wt % to 0.45 wt %). However, within the range of low carbon of 0.2% or less, bainite is referred to as bainitic ferrite, acicular ferrite, granular ferrite, or the like, depending on a temperature range. In the present disclosure, due to a low carbon region, a bainitic ferrite structure is included.
Since a microstructure of a wire rod according to the present disclosure includes bainitic ferrite of 90 area % or more, excellent strength and impact toughness may be secured. When a phase fraction, not of bainitic ferrite, but of ferrite according to the related art is increased, it may be advantageous in terms of impact toughness. However, since it is limited to preventing strength from being reduced, it is not preferable.
Meanwhile, the martensite-austenite constituent is formed along a bainitic ferrite grain boundary which is columnar. When a fraction thereof is high, strength of a steel material may be increased. However, since impact toughness may be degraded, it is preferable to manage a fraction thereof to be as low as possible. In this regard, in the present disclosure, it is preferable that a fraction of the martensite-austenite constituent is managed to be, by area %, 10% or less, (that is, a bainitic ferrite structure, which is columnar, of 90% or more). To obtain a microstructure of a wire rod according to the present disclosure described above, in the present disclosure, after a steel material is hot rolled, a cooling end temperature and a cooling rate when cooling the steel material are adjusted, thereby effectively achieving obtainment.
Meanwhile, it is preferable that a grain size of the martensite/austenite (M/A) constituent be 5 μm or less. When a grain size of the martensite/austenite (M/A) constituent exceeds 5 μm, an area of an interface in contact with a bainitic ferrite base is increased, impact toughness may be reduced.
Next, a method of manufacturing a wire rod according to the present disclosure will be described in detail.
A method of manufacturing a wire rod according to the present disclosure may include: reheating steel having a composition described above after preparing the steel; hot rolling a steel material having been reheated; cooling the steel material at a rate of 0.1° C./s to 2° C./s to a temperature within a range of Bf° C. to Bf−50° C. after the hot rolling; and air cooling the steel material having been cooled.
First, in the present disclosure, after a steel material having a composition component described above is prepared, the steel material is reheated. A reheating temperature applied in the present disclosure is preferably in the range of 1000° C. to 1100° C.
A form of the steel material is not particularly limited, but it is preferably a bloom or a billet according to the related art.
Next, the steel material having been reheated is hot rolled to manufacture a wire rod. A finish hot rolling temperature of the hot rolling is not particularly limited, but it is preferable to be in the range of 850° C. to 950° C.
The steel material having been hot rolled is cooled, and it is preferable that cooling is performed at a cooling rate of 0.1° C./s to 2° C./s to a temperature within a range of Bf° C. to Bf−50° C. When a cooling end temperature exceeds Bf, it is difficult to secure a sufficient amount of a bainitic ferrite structure. When the cooling end temperature is less than Bf−50° C., a steel material is sufficiently cooled to be easily handled. However, since productivity is lowered, it is preferable that the cooling end temperature be in a temperature within a range of Bf° C. to Bf−50° C. Bf refers to a temperature in which phase transformation from austenite to bainite or bainitic ferrite ends.
In the present disclosure, since a bainitic ferrite structure is secured by performing continuous cooling after hot rolling, excellent strength and impact toughness may be secured. Here, since a heat treatment such as quenching and tempering, performed according to the related art, may be omitted, an additional process is not required. Thus, it is advantageous in terms of manufacturing costs.
In addition, in the present disclosure, it is preferable that a section from a cooling start temperature to a cooling end temperature is cooled at a cooling rate of 0.1° C./s to 2° C./s. When the cooling rate is less than 0.1° C./s, formation of pro-eutectoid ferrite increases. When the cooling rate exceeds 2° C./s, formation of martensite increases. Thus, strength and impact toughness may be reduced. In the present disclosure, it is preferable that the cooling rate be managed to 0.1° C./s to 2° C./s.
As described above, as a cooling rate is secured in a cooling section, a wire rod having enhanced strength and impact toughness, having bainitic ferrite of 90% or more by area fraction may be obtained.

[Mode for Invention]

Hereinafter, an exemplary embodiment according to the present disclosure will be described in detail. The exemplary embodiment described below according to the present disclosure is provided for the purpose of understanding the present disclosure, and should not be construed as limiting the disclosure thereto.

Exemplary Embodiment

After a molten steel having a composition component of Table 1 was cast, the casted steel was reheated at 1100° C., the casted steel was wire-rod rolled to have a diameter of 15 mm, the wire rod was cooled to 300° C., below a temperature, Bf, at a cooling rate of Table 2, and the wire rod was air cooled, thereby manufacturing a wire rod. Meanwhile, Bf, a bainite phase transformation end temperature, was measured using a dilatometer, slightly varies depending on a chemical composition, and exists in the range of 300° C. to 350° C.
In the wire rod manufactured described above, a microstructure was analyzed and analysis thereof was illustrated in Table 2. In addition, tensile strength and impact toughness thereof were measured, and illustrated in Table 2. In a microstructure of the wire rod, an area fraction and a grain size of a martensite/austenite (M/A) constituent were measured using an image analyzer, and a concentration of manganese was measured using electron probe micro-analysis (EPMA).
In addition, a room temperature tensile test was carried out for measurement, in which a crosshead speed was a rate of 0.9 mm/min to a yield point and was a rate of 6 mm/min thereafter. In addition, an impact test was carried out at room temperature for measurement, by using an impact tester in which curvature of an edge portion of a striker for impacting a specimen was 2 mm and test capacity was 500 J.

TABLE 1

	Rela-	Rela-
	tional	tional
	Expres-	Expres-

Composition component (weight %)

sion

No.

C

Si

Mn

Cr

P

S

Ti

B

N

Al

1

2

1	0.12	0.19	3.1	0.15	0.018	0.019	0.015	0.0025	0.0044	0.023	2.3	16.3
2	0.08	0.18	3.7	—	0.017	0.020	0.020	0.0016	0.0049	0.015	12.6	20.6
3	0.10	0.13	3.6	0.18	0.014	0.017	0.017	0.0028	0.0042	0.040	7.7	27.7
4	0.07	0.20	3.4	0.07	0.011	0.015	0.025	0.0030	0.0036	0.033	24.1	17.0
5	0.11	0.18	3.5	0.24	0.016	0.013	0.030	0.0023	0.0039	0.038	39.0	19.4
6	0.05	0.16	3.8	0.22	0.015	0.015	0.011	0.0024	0.0044	0.043	−5.4	23.8
7	0.07	0.16	3.2	0.11	0.014	0.016	0.023	0.0017	0.0050	0.026	19.4	20.0
8	0.06	0.09	3	—	0.013	0.011	0.027	0.0018	0.0048	0.020	31.3	33.3
9	0.10	0.15	3.9	0.10	0.020	0.014	0.017	0.0027	0.0037	0.030	11.4	26.0
10	0.13	0.19	3.3	0.16	0.016	0.018	0.013	0.0018	0.0045	0.035	−4.3	17.4
11	0.11	0.18	4	0.05	0.009	0.020	0.019	0.0022	0.0040	0.044	15.4	22.2
12	0.25	0.16	3.4	—	0.014	0.013	0.030	0.0025	0.0037	0.019	37.5	21.3
13	0.15	0.25	3.3	0.13	0.011	0.015	0.021	0.0020	0.0050	0.022	12.1	13.2
14	0.11	0.15	2	0.07	0.018	0.014	0.018	0.0005	0.0043	0.031	31.5	13.3
15	0.09	0.17	3.6	—	0.016	0.017	0.021	0.0025	0.0041	0.028	16.9	21.2
16	0.08	0.16	3.2	0.21	0.011	0.016	0.02	0.0021	0.0047	0.017	11.7	20.0
17	0.06	0.15	3.5	0.17	0.012	0.011	0.005	0.0027	0.0035	0.034	−9.9	23.3
18	0.07	0.18	4.3	0.12	0.010	0.012	0.016	0.0018	0.0048	0.026	2.1	23.9

(In Table 1, Relational Expression 1 is Mn + 5 (Ti − 3.5N)/B, Relational Expression 2 is Mn/Si, and a residual component thereof is Fe and unavoidable impurities)

TABLE 2

							Rela-
		Cool-		M/A		Impact	tional
		ing	M/A	grain	Tensile	tough-	Expres-
Classifi-		rate	fraction	size	strength	ness	sion
cation	No.	(° C./s)	(%)	(μm)	(MPa)	(J)	3

Inventive	1	0.5	7	3.9	659	158	2.1
example	2	1	8	3.3	660	163	2.6
	3	0.2	5	4.7	652	180	2.3
	4	2	10	2.0	680	159	2.4
	5	1.3	9	2.4	664	160	2.2
	6	1.9	9	2.1	670	152	2.8
	7	1.5	8	2.3	665	168	2.3
	8	0.3	5	4.6	635	199	2.0
	9	0.8	7	3.5	657	172	2.7
	10	0.7	7	3.8	650	155	2.2
	11	1.1	8	3.3	663	165	2.9
Compara-	12	2	15	2.5	730	100	2.4
tive	13	1	11	3.5	754	87	2.4
example	14	0.7	9	2.4	543	172	1.6
	15	3	12	1.7	700	94	2.6
	16	0.05	4	6.1	557	157	2.3
	17	1	2	8.6	560	151	2.5
	18	1.8	8	3.2	825	80	3.3

(In Table 2, Relational Expression 3 is [Mn_max]/[Mn_min])

As illustrated in Tables 1 and 2, in the cases of Inventive examples 1 to 11 satisfying a steel composition and a manufacturing method thereof according to the present disclosure, bainitic ferrite of 90 area % or more may be obtained therefrom. For mechanical properties thereof, it is confirmed that tensile strength of 600 MPa to 700 MPa and excellent impact toughness of 150 J to 200 J were shown.
In the case of Inventive example 8, the content of silicon was 0.1 wt % or less, and thus, it is confirmed that impact toughness was further improved. Among Inventive examples, in the cases of Inventive examples 2, 3, 5, 7, 6, 9, and 11, satisfying Relational Expression 1 (Mn+5(Ti−3.5N)/B 5.0) of manganese, titanium, boron, and nitrogen, in addition to Relational Expression 2 (Mn/Si≧18) of manganese and silicon, it is confirmed that impact toughness was further excellent, as compared to different cases.
In other words, among Inventive examples, in the cases of Inventive examples 1, 4, 6, and 10, not satisfying Relational Expression 1(Mn+5(Ti−3.5N)/B≧5.0) and/or Relational Expression 2(Mn/Si≧18), it is confirmed that impact toughness was somewhat reduced.
Meanwhile, in the case of Comparative example 12, the content of carbon was higher. Thus, it is confirmed that tensile strength was excellent, but impact toughness was reduced. In this regard, because carbon was dissolved in an M/A phase, a stable M/A phase was increased. In the case of Comparative example 13, the content of silicon was outside of a range according to the present disclosure. In a manner similar to carbon, as an addition amount of silicon increases, an addition amount of silicon in a base increases. Thus, silicon has an effect of solid solution strengthening. In other words, when an addition amount of silicon was about 0.25%, while tensile strength may be significantly high, impact toughness may be significantly reduced. In the case of Comparative example 14, since the hardenability of a steel material was reduced due to an insignificant addition amount of manganese and boron, even when a cooling condition was satisfied, ferrite and a bainitic ferrite structure were mixed, and thus, it is confirmed that tensile strength was reduced.
Meanwhile, in the case of Comparative example 15, a steel composition component thereof satisfies a range according to the present disclosure. As a cooling rate in a manufacturing process increases, martensite was formed. Thus, it is confirmed that strength was increased, but impact toughness was reduced. In the case of Comparative example 16, a steel composition component thereof satisfies a range according to the present disclosure, but a cooling rate in a manufacturing process was slow. Thus, it is confirmed that strength was reduced as ferrite was formed.
In addition, in the case of Comparative example 17, an addition amount of titanium was low. Since an amount of solute boron was reduced, hardenability was also reduced. When a cooling rate was low, a precipitation amount of pro-eutectoid ferrite increases. Thus, it is confirmed that tensile strength was reduced.
In addition, in the case of Comparative example 18, when a large amount of manganese was added thereto, relatively hardenability was significant. Even when cooling was performed at a cooling rate presented in the present disclosure, martensite was formed. Thus, it is confirmed that strength increases while impact toughness was reduced. In addition, since manganese was segregated in steel, due to formation of a locally uneven structure, it is confirmed that impact toughness was reduced.
While the present disclosure has been particularly shown and described with reference to exemplary embodiments thereof, but is not limited thereto. It will be apparent to those skilled in the art that various changes and modifications thereof may be made within the spirit and scope of the present disclosure, and therefore, it is to be understood that such changes and modifications belong to the scope of the appended claims.

Claims

1. A wire rod having enhanced strength and impact toughness comprising, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities,

wherein a microstructure includes bainitic ferrite in an area fraction of 90% or more, and a martensite/austenite (M/A) constituent as a residual component thereof.

2. The wire rod having enhanced strength and impact toughness of claim 1, wherein the wire rod additionally comprises chromium (Cr): less than 0.3%.

3. The wire rod having enhanced strength and impact toughness of claim 1, wherein the content of manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) satisfies Relational Expression 1,

Mn+5(Ti−3.5N)/B≧5.0. [Relational Expression 1]

4. The wire rod having enhanced strength and impact toughness of claim 1, wherein the content of manganese (Mn) and silicon (Si) satisfies Relational Expression 2,

Mn/Si≧18. [Relational Expression 2]

5. The wire rod having enhanced strength and impact toughness of claim 1, wherein the wire rod is provided with an arbitrary cross section in which a ratio of a maximum concentration [Mn_max] and a minimum concentration [Mn_min] of manganese satisfies Relational Expression 3,

[Mn_max]/[Mn_min]≦3. [Relational Expression 3]

6. The wire rod having enhanced strength and impact toughness of claim 1, wherein a grain size of the martensite/austenite (M/A) constituent is 5 μm or less.

7. A method of preparing a wire rod having enhanced strength and impact toughness comprising:

reheating a steel material including, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities;

hot rolling the steel material having been reheated;

cooling the steel material at a rate of 0.1° C./s to 2° C./s in a temperature within a range of Bf° C. to Bf−50° C., after the hot rolling; and

air cooling the steel material having been cooled.

8. The method of preparing a wire rod having enhanced strength and impact toughness of claim 7, wherein the steel material additionally includes chromium (Cr): less than 0.3%.

9. The method of preparing a wire rod having enhanced strength and impact toughness of claim 7, wherein the content of manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) satisfies Relational Expression 1,

Mn+5(Ti−3.5N)/B≧5.0. [Relational Expression 1]

10. The method of preparing a wire rod having enhanced strength and impact toughness of claim 7, wherein the content of manganese (Mn) and silicon (Si) satisfies Relational Expression 2,

Mn/Si≧18. [Relational Expression 2]

11. The method of preparing a wire rod having enhanced strength and impact toughness of claim 7, wherein the reheating is performed at a temperature of 1000° C. to 1100° C.

12. The method of preparing a wire rod having enhanced strength and impact toughness of claim 7, wherein finish hot rolling, of the hot rolling, is performed at a temperature within a range of 850° C. to 950° C.