KR101620738B1 - Wire rod having high strength and impact toughness and method for manufacturing thereof - Google Patents

Abstract

The present invention provides a wire rod having a high strength and impact toughness, and a method to manufacture the same. The wire rod comprises: 0.05-0.15 wt% of carbon (C); 0.1 wt% or less of silicon (Si); 3.0-5.0 wt% of manganese (Mg); 0.5-2.0 wt% of chromium (Cr); 0.010-0.030 wt% of niobium (Nb); 0.020 wt% or less of phosphorus (P); 0.020 wt% or less of sulfur (S); and the remaining consisting of residual iron (Fe) and inevitable foreign substances. The method to manufacture the wire rod comprises: a step of preparing steel having the composition described above, and reheating the steel at a temperature in the range of Ae3 + 150°C to Ae3 + 250°C; a step of starting hot rolling to the reheated steel, and finishing the hot rolling at temperatures in a range of Ar3 + 200°C to Ar3 + 300°C; a step of cooling the rolled steel which has undergone hot rolling at a cooling speed of 0.2 °C/s or more up to a temperature in a range of Mf to Mf - 50°C; and a step of performing air cooling to the cooled steel at room temperature.

Description

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

More particularly, the present invention relates to a steel material for use in industrial machines or machine parts such as automobiles, which are exposed to various external load environments. And to a method of manufacturing the same.

Recently, efforts to reduce the emission of carbon dioxide, which is regarded as the main cause of environmental pollution, have become global issues. As a part of this, there is also an act of regulating automobile exhaust gas. As a countermeasure, automakers are trying to solve this problem by improving fuel efficiency. In order to improve the fuel efficiency, it is required to reduce the weight of the automobile and to improve the performance thereof, and accordingly, the necessity of high strength of automobile materials or parts is increasing. In addition, the demand for stability against external impact increases, and impact toughness is also recognized as an important property of the material or parts.

There is a limit 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 both 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 the high strength and high impact toughness can be ensured without the above-mentioned additional heat treatment, a number of processes from the material to the part production can be omitted or simplified, thereby improving the productivity and lowering the manufacturing cost .

However, since wire rods capable of stably obtaining bainite or martensite structure having high strength and impact toughness by using hot rolling and continuous cooling processes have not been developed yet, there is a demand for development of such wire rods.

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

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

According to an aspect of the present invention,

(C): 0.05 to 0.15%, silicon (Si): 0.1% or less, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0%, niobium (Nb): 0.010 To 0.030%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, balance Fe and unavoidable impurities.

In the present invention, manganese (Mn), chromium (Cr) and carbon (C) are preferably contained so as to satisfy the following relational expression (1).

[Relation 1]

3.5 ≤ C (Mn + Cr) 5/50 ≤ 9.0

Note that manganese (Mn), chromium (Cr), and carbon (C) in the above formula (1) mean the content by weight of the corresponding element, respectively.

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

In the present invention, the wire rod may have a tensile strength of 1100 to 1300 MPa and an impact value of 100 J or more.

Further, according to the present invention,

(C): 0.05 to 0.15%, silicon (Si): 0.1% or less, manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0%, niobium (Nb): 0.010 (S): 0.020% or less, the balance Fe and unavoidable impurities, and then reheating the steel material in a temperature range of Ae3 + 150 ° C to Ae3 + 250 ° C fair;

Subjecting the reheated steel material to hot rolling to finish hot rolling in a temperature range of Ar 3 + 200 ° C to Ar 3 + 300 ° C;

Cooling the hot-rolled steel material to a temperature range of Mf to Mf - 50 ° C at a cooling rate of 0.2 ° C / s or more; And

And a step of air-cooling the cooled steel material to a normal temperature. The present invention also relates to a method of producing a wire material excellent in tensile strength and impact toughness.

In the present invention, it is preferable that the manganese (Mn), chromium (Cr) and carbon (C) are contained so as to satisfy the relational expression (1).

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

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

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

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

The wire rod of the present invention is characterized by containing 0.05 to 0.15% of carbon (C), 0.1% or less of silicon (Si), 3.0 to 5.0% of manganese (Mn), 0.5 to 2.0% of chromium (Cr) 0.020-0.030% of O3 (Nb), 0.020% or less of phosphorus (P), 0.020% or less of sulfur (S), and the balance Fe and unavoidable impurities.

Hereinafter, the reasons for limiting the composition and composition range of the steel wire rod according to the present invention will be described in detail.

Carbon (C): 0.05 to 0.15%

Carbon is an indispensable element for ensuring strength and is either dissolved in steel or in the form of carbides or cementites. The easiest way to increase the strength is to increase the carbon content to form carbide or cementite. On the contrary, it is necessary to limit the addition of carbon to a certain extent because the ductility and impact toughness are reduced. In the present invention, it is preferable to limit the content of carbon (C) in the range of 0.05-0.15% 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%, the 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 5.0%

Manganese increases the strength of the steel and improves the hardenability, facilitating the formation of low temperature structures such as bainite or martensite at a wide range of cooling rates. However, when the manganese content is less than 3.0%, the hardenability is not sufficient, and it is difficult to stably secure the low-temperature structure by the continuous cooling process after the hot rolling. When the manganese content is more than 5.0%, segregation of Mn during the solidification tends to be facilitated. In consideration of this, it is preferable to limit the manganese content to 3.0 to 5.0% in the present invention.

Chromium (Cr): 0.5 to 2.0%

Chromium increases the strength and hardenability of steel similar to manganese and improves impact toughness, especially when added with manganese. However, when the content of chromium is less than 0.5%, the effect of improving the strength, hardenability and impact properties is not significant. When the content of chromium exceeds 2.0%, the impact and the impact properties may be deteriorated. In view of this, in the present invention, the content of chromium is preferably limited to a range of 0.5 to 2.0%.

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 is more than 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 a range of 0.010 to 0.030%.

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%.

In the present invention, it is preferable that the manganese (Mn), chromium (Cr) and carbon (C) are contained so as to satisfy the following relational expression (1).

[Relation 1]

3.5 ≤ C (Mn + Cr) 5/50 ≤ 9.0

Note that manganese (Mn), chromium (Cr), and carbon (C) in the above formula (1) mean the content by weight of the corresponding element, respectively.

By controlling the contents of manganese, chromium and carbon as in the above-mentioned relational expression 1, a martensitic high-manganese steel wire rod having excellent impact toughness can be produced. That is, manganese and chromium increase the hardenability, so that martensite can be easily produced even when the cooling rate is relatively small, and low carbon and chromium can contribute greatly to improvement of impact toughness of martensite.

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

In the present invention, the grain size of the martensite is preferably 10 탆 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 producing a wire according to the present invention comprises the steps of: preparing a steel having the above composition and reheating the steel in a temperature range of Ae3 + 150 deg. C to Ae3 + 250 deg. Subjecting the reheated steel material to hot rolling to finish hot rolling in a temperature range of Ar 3 + 200 ° C to Ar 3 + 300 ° C; Cooling the hot-rolled steel material to a temperature range of Mf to Mf - 50 ° C at a cooling rate of 0.2 ° C / s or more; And a step of air-cooling the cooled steel material to a normal temperature.

First, in the present invention, a steel material having the above-mentioned composition components is prepared and reheated. In the present invention, the reheating temperature range is preferably 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 in a range of Ar3 + 200 ° C to Ar3 + 300 ° C. If the final hot rolling temperature is lower than Ar3 + 200 deg. C, there is a high possibility that surface defects are caused in the steel material. If the final hot rolling temperature is higher than Ar3 + 300 deg. C, 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 a temperature range of Mf to Mf - 50 ° C. If the cooling end temperature exceeds Mf, it is difficult to obtain a sufficient amount of martensite structure. If it is lower than Mf-50 ° C, the steel material is sufficiently cooled to facilitate handling. However, since the productivity is lowered, the cooling end temperature is preferably from Mf to Mf - It is preferable to control it in the range.

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

Also, the high manganese steel wire of the present invention manufactured by the above-described manufacturing process may have a tensile strength of 1100 to 1300 MPa and an impact value of 100 J or more.

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

(Example)

Molten steel having the composition components shown in Table 1 were each cast into an ingot, and homogenized at 1200 ° C for 12 hours. The homogenized steel material was reheated under the conditions shown in Table 2 and subjected to hot rolling at a final hot rolling temperature of 15 mm as shown in Table 2, followed by air cooling.

Then, each of the steel materials prepared as described above was subjected to solution treatment at 900 캜 and then cooled at a cooling rate shown in Table 2. The grain size of the martensite crystals was measured for each of the cooled steels, and the results are shown in Table 2. The tensile strength and impact value were measured and are shown in Table 2.

In Table 2, the graininess of the martensite of the steel material was measured using an image analyzer. The room temperature tensile test 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 Cr Nb P S One 0.09 0.04 4.3 1.0 0.015 0.013 0.006 7.53 2 0.12 0.08 3.2 1.4 0.023 0.014 0.007 4.94 3 0.07 0.05 3.8 1.6 0.019 0.010 0.011 6.43 4 0.13 0.06 4.7 0.9 0.027 0.011 0.010 14.32 5 0.08 0.03 4.0 0.6 0.026 0.009 0.008 3.30 6 0.11 0.09 3.3 1.5 0.012 0.013 0.012 5.61 7 0.07 0.02 3.5 1.8 0.020 0.015 0.006 5.85 8 0.13 0.07 4.2 0.9 0.017 0.015 0.011 8.97 9 0.09 0.05 3.3 1.5 0.052 0.010 0.009 4.59 10 0.13 0.06 3.9 2.7 0.011 0.015 0.014 32.56 11 0.15 0.06 5.3 1.0 0.018 0.012 0.011 29.77 12 0.27 0.04 4.0 0.8 0.026 0.008 0.005 13.76 13 0.08 0.03 2.4 0.3 0.023 0.010 0.011 0.23 14 0.10 0.09 3.5 1.6 0.029 0.012 0.014 6.90 15 0.09 0.48 3.2 1.4 0.015 0.011 0.012 3.71 16 0.11 0.06 3.5 1.6 0.013 0.012 0.005 7.59 17 0.10 0.05 3.8 1.4 0.015 0.008 0.008 7.60

* Equation in Table 1 1 ^{C (Mn + Cr) 5/} 50

division Specimen No Reheat temperature
(° C) Finishing hot rolling temperature (캜) Cooling rate
(° C / s) Martensite fraction
(%) Martensite crystal grain size (탆) The tensile strength
(MPa) Shock
(J)


Honor



One 1030 820 5 98 9 1210 165 2 980 770 10 99 7 1185 180 3 1020 850 2 98 10 1231 152 4 1050 830 One 97 10 1292 105 5 960 750 20 99 5 1193 149 6 980 800 3 98 9 1146 189 7 990 790 7 98 8 1175 174 8 960 770 15 99 6 1220 155


Comparative Example




9 1010 840 10 99 28 1054 87 10 1020 800 6 98 9 1305 98 11 970 770 13 99 6 1334 79 12 990 830 2 99 10 1358 73 13 1000 810 3 5 9 783 186 14 980 790 0.05 10 13 1089 168 15 1040 820 One 97 10 1340 47 16 1160 840 5 98 28 1084 82 17 1040 960 9 99 25 1145 70

As shown in Tables 1 and 2, in the case of Inventive 1-8 in which the steel composition component and the manufacturing process conditions satisfied the range of the present invention, all the martensite structures were obtained and showed a high tensile strength of 1100-1300 MPa Giving. In addition, it can be seen that, even though the tempering treatment is not carried out, high impact strength and high impact toughness of 100 J or more are exhibited.

In particular, the invention example in the Inventive Example (1-3, 6-8) is satisfied, and to which the content of manganese, chromium and carbon relation 1 [3.5 ≤ C (Mn + Cr) 5/50 ≤ 9.0] honor It can be seen that the impact toughness is better than Examples 4 and 5 which do not satisfy the above-mentioned relational expression 1. [

On the other hand, in Comparative Example 9, when the addition amount of the niobium exceeds the component range of the present invention, the size of the niobium carbonitride became too large to effectively suppress the grain growth, .

Comparative Examples 10 and 11, in which the chromium and manganese components exceeded the range of the present invention, show increased strength and decreased ductility, resulting in poor impact toughness.

In addition, Comparative Example 12 shows that when the content of carbon is added in excess of the content range of the present invention, the strength is greatly increased and the impact toughness is decreased due to an increase in the effect of strengthening the solid solution of martensite base of carbon.

Comparative Example 13 is a case where manganese and chromium are added in a smaller amount than the component range of the present invention. However, since the curing ability is relatively small, the cooling rate is satisfied, but the bainite forms a matrix structure instead of martensite, .

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

Further, in Comparative Example 15, it was confirmed that the silicone contained in the composition exceeded the range of the present invention, and the tensile strength was greatly increased, and the impact toughness thereof was drastically reduced.

In Comparative Examples 16 and 17, when the steel composition of the present invention is satisfied but each of the reheating temperature range and the final hot rolling temperature range exceeds the range of the present invention, since austenite is not sufficiently miniaturized, And the impact toughness is decreased.

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)

(C): 0.05 to 0.15%, silicon (Si): 0.1% or less (0% is not included), manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0% 0.020 to 0.030% of phosphorus (Nb), 0.020% or less of phosphorus (P), 0.020% or less of sulfur (S), the balance Fe and unavoidable impurities,
Wherein said manganese (Mn), chromium (Cr) and carbon (C) are contained so as to satisfy the following relational expression (1).
[Relation 1]
3.5 ≤ C (Mn + Cr) 5/50 ≤ 9.0
Note that manganese (Mn), chromium (Cr), and carbon (C) in the above formula (1) mean the content by weight of the corresponding element, respectively.
delete The wire rod according to claim 1, wherein the wire rod has a microstructure composed of martensite and residual austenite (?) Of 95% or more by area, and is excellent in tensile strength and impact toughness.
The wire according to claim 1, wherein the wire has a tensile strength of 1100 to 1300 MPa and an impact value of 100 J or more.
(C): 0.05 to 0.15%, silicon (Si): 0.1% or less (0% is not included), manganese (Mn): 3.0 to 5.0%, chromium (Cr): 0.5 to 2.0% 150 ° C to Ae3 + 0.03% or less after preparing a steel material comprising 0.010 to 0.030% of niobium (Nb), 0.020% or less of phosphorus (P), 0.020% or less of sulfur (S), the remainder Fe and unavoidable impurities, Reheating in a temperature range of 250 占 폚;
Subjecting the reheated steel material to hot rolling to finish hot rolling in a temperature range of Ar 3 + 200 ° C to Ar 3 + 300 ° C;
Cooling the hot-rolled steel material to a temperature range of Mf to Mf - 50 ° C at a cooling rate of 0.2 ° C / s or more; And
And air-cooling the cooled steel material to a normal temperature,
Wherein said manganese (Mn), chromium (Cr) and carbon (C) are contained so as to satisfy the following relational expression (1).
[Relation 1]
3.5 ≤ C (Mn + Cr) 5/50 ≤ 9.0
Note that manganese (Mn), chromium (Cr), and carbon (C) in the above formula (1) mean the content by weight of the corresponding element, respectively.
delete The method according to claim 5, wherein the wire rod has a microstructure composed of martensite and residual austenite (?) Of 95% or more by area, and is excellent in tensile strength and impact toughness.