KR101271937B1 - Steel having excellent strength and impact toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel having excellent strength and impact toughness and a method of manufacturing the same, and more particularly, to a steel having excellent strength and impact toughness and a method of manufacturing the same that can be used for welding structures such as offshore wind towers and foundation structures. will be.
In the present invention, by weight%, C: 0.04-0.20%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Nb: 0.005-0.10%, Mo: 0.01-1.0%, Cr: 0.05-1.0%, Ti : 0.005 to 0.1%, Al: 0.005 to 0.5%, N: 15 to 150 ppm, P: 0.02% or less, S: 0.01% or less, further Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Carbide index represented by the following relation 1, containing at least one member selected from the group consisting of V: 0.005 to 0.3%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%, balance Fe and other unavoidable impurities Provided is a steel having an excellent strength and impact toughness (CI) of 0.04 to 0.15 and containing 5 × 10 ⁵ to 20 × 10 ⁵ carbides per 1 mm ² , and a method of manufacturing the same.
[Relationship 1] = 0.146 x Nb + 0.272 x Ti + 0.261 x V + 0.071 x Mo + 0.081 x W + 0.167 x Cr + 0.037 x Co

Description

Steel material with excellent strength and impact toughness and its manufacturing method {STEEL HAVING EXCELLENT STRENGTH AND IMPACT TOUGHNESS AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel having excellent strength and impact toughness and a method of manufacturing the same, and more particularly, to a steel having excellent strength and impact toughness and a method of manufacturing the same that can be used for welding structures such as offshore wind towers and foundation structures. will be.

Recently, development of alternative energy such as solar power and wind power is being actively conducted as an environmentally friendly energy resource. In the case of wind power generation, wind power generators, which were mainly built on land, have been recently constructed on the sea, which is advantageous for high efficiency and reduction of pollution such as noise. In particular, the expansion of installation in cryogenic offshore areas such as North America and Northern Europe is expected. Accordingly, the development of normalized heat-treated steel for low temperature impact toughness is required.

In the case of steel materials used in wind power generators and the like, they are mainly produced by a method called normalizing heat treatment. However, during the conventional normalizing heat treatment process, inversely transformed austenite is formed mainly at grain boundaries and grows to form coarse austenite, and ferrite is formed from the coarse austenite during cooling.

In other words, such normalizing heat treatment has many problems in forming coarse ferrite to impart important steel properties such as lowering strength and guaranteeing low temperature impact toughness.

One aspect of the present invention is to provide a steel and a method for producing the same by improving the tensile strength and impact toughness by appropriately controlling the composition components and manufacturing conditions to distribute the carbide evenly in the steel.

In the present invention, by weight%, C: 0.04-0.20%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Nb: 0.005-0.10%, Mo: 0.01-1.0%, Cr: 0.05-1.0%, Ti : 0.005 to 0.1%, Al: 0.005 to 0.5%, N: 15 to 150 ppm, P: 0.02% or less, S: 0.01% or less, further Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Carbide index represented by the following relation 1, containing at least one member selected from the group consisting of V: 0.005 to 0.3%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%, balance Fe and other unavoidable impurities Provided is a steel having an excellent strength and impact toughness (CI) of 0.04 to 0.15 and containing 5 × 10 ⁵ to 20 × 10 ⁵ carbides per 1 mm ² .

[Relationship 1] = 0.146 x Nb + 0.272 x Ti + 0.261 x V + 0.071 x Mo + 0.081 x W + 0.167 x Cr + 0.037 x Co

The steel may further comprise Ca: 0.0005 ~ 0.0060%, the tensile strength is 500MPa or more, Charpy impact energy at -50 ℃ is preferably 150J or more.

In the present invention, by weight%, C: 0.04-0.20%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Nb: 0.005-0.10%, Mo: 0.01-1.0%, Cr: 0.05-1.0%, Ti : 0.005 to 0.1%, Al: 0.005 to 0.5%, N: 15 to 150 ppm, P: 0.02% or less, S: 0.01% or less, further Cu: 0.010 to 1.0%, Ni: 0.01 to 2.0%, Carbide index represented by the following relation 1, containing at least one member selected from the group consisting of V: 0.005 to 0.3%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%, balance Fe and other unavoidable impurities A reheating step of reheating the steel having a (CI) of 0.04 to 0.15 at 1050 to 1250 ° C; Rolling the reheated steel at 1250 ° C. to Ar 3; Heating the rolled steel at a temperature of Ac 3 to 1000 ° C., and maintaining a heating (1.3 × t + 10 to 30) minute (where t denotes the thickness of the steel (mm)); And it provides a method of producing a steel having excellent strength and impact toughness comprising the step of air-cooling the heated and maintained steel.

[Relationship 1] = 0.146 x Nb + 0.272 x Ti + 0.261 x V + 0.071 x Mo + 0.081 x W + 0.167 x Cr + 0.037 x Co

The steel may further comprise Ca: 0.0005 to 0.0060%.

According to one aspect of the present invention, it is possible to provide a steel having a tensile strength of 500MPa or more and Charpy impact energy of 150J or more at -50 ° C.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows that reverse transformation austenite is formed at the time of normalizing heat processing from carbide.
Figure 2 is a graph showing the carbide density according to the carbide index of the invention examples in accordance with the present invention.

Hereinafter, the present invention will be described.

C: 0.04-0.20 wt%

If the content of C is less than 0.04%, carbides cannot be formed sufficiently. However, when the content of C exceeds 0.2%, the pearlite fraction increases, so that the number of effective carbides decreases. Therefore, it is preferable to limit the range of C to 0.04 to 0.20%. In the case of a plate used as a steel structure for welding, it is preferable to limit the range of C to 0.04 to 0.15% for weldability.

Mn: 0.3 ~ 2.5%

Mn is a useful element that improves strength by solid solution strengthening, so it is necessary to add 0.3% or more. However, when the content exceeds 2.5%, the toughness of the weld portion is greatly reduced due to excessive increase in the hardenability, so that the content of Mn is preferably limited to 0.3 to 2.5%.

Si: 0.01 ~ 0.6%

Si is used as a deoxidizer and is useful because of its strength-improving effect, but when added in excess of 0.6%, Si lowers toughness at the same time and deteriorates weldability. On the other hand, when added in less than 0.01% deoxidation effect is insufficient, it is preferable to limit the content of Si to 0.01 ~ 0.6%.

Nb: 0.005 to 0.10%

Nb is the most important element in the production of TMCP steel and precipitates in the form of NbC or NbCN to greatly improve the strength of the base metal and the welded portion. In addition, when reheated at a high temperature, the dissolved Nb has an effect of suppressing recrystallization of austenite and suppressing transformation of ferrite or bainite to refine the structure. In addition, the present invention not only allows bainite to be formed even at low cooling rate when the slab is cooled after rough rolling, but also greatly improves the stability of austenite during cooling after the final rolling, thereby promoting the generation of martensite at low speed. It also plays a role. For the above effect, Nb is preferably added at 0.005% or more, but when it exceeds 0.10%, the possibility of causing brittle cracks in the corners of the steel increases, which is not preferable. Accordingly, the content of Nb is preferably limited to the range of 0.005 ~ 0.10%.

Mo: 0.01 ~ 1.0%

Mo needs to be added at 0.01% or more because Mo can greatly improve the hardenability and suppress the formation of ferrite, and the strength can be greatly improved even with a small amount of addition. However, if it exceeds 1.0%, the hardness of the weld is excessive. May increase and inhibit toughness. Therefore, the Mo content is preferably limited to the range of 0.01 to 1.0%, and in the present invention, in order to form the island martensite in an appropriate range for securing the tensile strength, it is more limited to the range of 0.02 to 0.2%. desirable.

Cr: 0.05 to 1.0%

Since Cr has a great effect on increasing the strength by increasing the hardenability, it is necessary to add 0.05% or more in order to obtain such effects, but when it exceeds 1.0%, the weldability can be greatly reduced, so the range of 0.05 to 1.0% It is preferred to be added. Moreover, in order to obtain stable phase martensite even at a comparatively low cooling rate, it is more preferable to add in 0.2 to 0.5% of range.

Ti: 0.005 to 0.1%

Addition of Ti can greatly improve low-temperature toughness by inhibiting grain growth upon reheating, so 0.005% or more must be added to express the effect. However, if the content exceeds 0.1%, there is a problem in that low temperature toughness due to clogging of the playing nozzle or crystallization of the center part is reduced. Therefore, the Ti content is preferably limited to 0.005 to 0.1%.

Al: 0.005-0.5%

Since Al is an element capable of cheaply deoxidizing molten steel, it is preferable to add 0.005% or more, but if it exceeds 0.5%, it causes nozzle clogging during continuous casting. Therefore, the Al content is limited to 0.005 to 0.5%. It is desirable to.

N: 15 ~ 150ppm

Since the addition of N increases the strength while greatly reducing the toughness, it is necessary to limit the content to 150 ppm or less. However, since the N content control of 15 ppm or less increases the steelmaking load, the lower limit of the N content is preferably 15 ppm.

P: 0.02% or less

Although P is an element that is advantageous in improving the strength and corrosion resistance, it is preferable to be contained as low as possible because it is an element that greatly impairs impact toughness. However, since P is an inevitable impurity in the manufacturing process, it is preferable to limit the upper limit to 0.02%.

S: 0.01% or less

Since S is an element which forms MnS or the like and greatly impairs the impact toughness, S is preferably contained as low as possible. However, since S is an impurity inevitably contained in the manufacturing process, it is preferable to limit the upper limit to 0.01%.

Cu: 0.010 ~ 1.0%

Since Cu is an element capable of increasing the strength while minimizing the toughness of the base metal, at least 0.01% must be added in order for the effect to appear. On the other hand, if it exceeds 1.0%, since the surface quality of the product is greatly inhibited, the content of Cu is preferably limited to 0.010 to 1.0 %%.

Ni: 0.01 to 2.0%

Ni is an element that can improve the strength and toughness of the base material at the same time, and 0.01% or more must be added for the effect to appear. However, since Ni is an expensive element, when it exceeds 2.0%, the economic efficiency is lowered, and there is also a problem of deterioration in weldability. Therefore, the content of Ni is preferably limited to the range of 0.01 to 2.0%.

V: 0.005-0.3%

The temperature of V is lower than that of other fine alloys, and it is preferable to add 0.005% or more because it has an effect of preventing the drop in strength by depositing in the weld heat affected zone. However, when exceeding 0.3%, since it can reduce, it is preferable to limit said V to 0.005 to 0.3%.

Co: 0.005 ~ 0.2%

Co is an element advantageous to prevent softening of the matrix structure and to form carbides, but is expensive, so it is preferably added in the range of 0.005 to 0.2%.

W: 0.005-0.2%

W is an effective element that prevents softening of the matrix structure and precipitates carbide, but is expensive, so it is preferably added in the range of 0.005 to 0.2%.

Ca: 0.0005-0.0060%

Ca may be added to form CaS to suppress non-metallic inclusions of MnS, for which 0.0005% or more may be added. However, if the content is more than 0.0060%, and reacts with oxygen contained in the steel to produce CaO, a nonmetallic substitute, the content of Ca is preferably 0.0005 to 0.0060%.

The steel material of the present invention preferably satisfies the above component conditions and at the same time, the carbide index (CI) represented by the following relational formula 1 has a range of 0.04 to 0.15.

[Relationship 1] = 0.146 x Nb + 0.272 x Ti + 0.261 x V + 0.071 x Mo + 0.081 x W + 0.167 x Cr + 0.037 x Co

When the carbide index is less than 0.04, the carbide-forming elements are insufficient, and the number of carbides is insufficient in the base after rolling, so that the nucleation site of reverse transformation austenite cannot be formed at the time of normalizing heat treatment, so that fine austenite cannot be formed. do. When it exceeds 0.15, carbides in the substrate increase after rolling, and in particular, coarse carbides are formed to serve to inhibit toughness.

The steel proposed by the present invention preferably contains 5 × 10 ⁵ to 20 × 10 ⁵ carbides per mm ² . BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows that reverse transformation austenite is formed at the time of normalizing heat treatment from carbide, (a) is an existing material, (b) shows an invention material. As can be seen in Figure 1, the carbide acts as a reverse transformation austenite nucleation site during normalizing heat treatment, unlike the existing material, by producing a large amount of carbide, to form fine austenite grains to fine during air cooling By transforming to ferrite, the strength and low temperature impact properties of the steel can be improved. In order to express the effect, it is necessary to form 5 × 10 ⁵ or more carbides per 1 mm ² inside the steel, but when it exceeds 20 × 10 ⁵ , coarse carbides are also promoted to cause side effects that impair toughness. do.

As described above, steel materials satisfying the component system, component relations, and number of carbides proposed by the present invention have a tensile strength of 500 MPa or more, and Charpy impact energy at -50 ° C also has 150 J or more, which is excellent in strength and low temperature impact. Toughness is secured.

Hereinafter, the production method of the present invention will be described.

Reheating is performed at 1050 to 1250 ° C for the steel that satisfies the above compositional components and component relations. In order to solidify the Ti, Nb carbides and their composite carbides formed during casting, it is preferable to reheat at a temperature of 1050 ° C or higher. However, when reheating excessively high temperature, austenite may coarsen, so the upper limit of the reheating temperature is preferably limited to 1250 ° C.

Thereafter, the reheated steel is rolled at 1250 ° C to Ar3. At this time, rolling can apply rolling of normal conditions. Ar3 temperature is as follows.

Ar3 = 916-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo

Thereafter, the rolled steel is heated at Ac 3 to 1000 ° C., and then maintained for (1.3 × t + 10 to 30) minutes (where t denotes the thickness of the steel (mm)), where Ac 3 is normal. At the time of rising heat treatment, there is a minimum temperature for heating the steel to the austenite region. Heating below the Ac3 temperature results in the coexistence of austenite and ferrite to form coarse ferrite after air cooling. When the temperature exceeds 1000 ° C., austenite enters a coarse growth stage, and coarse ferrite is formed after air cooling, thereby decreasing strength and toughness. Ac3 temperature is as follows.

Ac3 = 910-203 × C ¹ /2-15.2 × Ni + 44.7 × Si + 104 × V + 31.5 × Mo + 13.1 × W

Subsequently, the steel proposed by the present invention can be secured by air-cooling the heated and maintained steel.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are merely examples for describing the present invention in more detail, and do not limit the scope of the present invention.

(Example)

The steel slabs having the component systems shown in Tables 1 and 2 below were reheated and then rolled to prepare steel materials. Then, after the heat treatment and maintenance normalized under the conditions of Table 3, it was air-cooled. At this time, the reheating temperature was 1150 ° C and the rolling temperature was 950 ° C. Carbide density and mechanical properties of the prepared steels were measured, and the results are shown in Table 3 below.

division Chemical composition (% by weight) C Si Mn P S Al Ni Cu Cr Inventory 1 0.04 0.2 1.55 0.013 0.002 0.015 0.25 0.1 0.3 Inventive Example 2 0.08 0.4 1.7 0.013 0.005 0.032 0.1 0 0.2 Inventory 3 0.087 0.2 1.55 0.012 0.002 0.013 0 0 0.2 Honorable 4 0.1 0.3 1.4 0.013 0.002 0.013 0.21 0.2 0.05 Inventory 5 0.072 0.2 1.5 0.013 0.002 0.013 0 0.22 0.3 Inventory 6 0.086 0.3 1.4 0.013 0.003 0.013 0 0 0.24 Honorable 7 0.09 0.4 1.5 0.013 0.002 0.013 0 0 0.18 Comparative Example 1 0.005 0.2 1.5 0.014 0.003 0.034 0 0 0.11 Comparative Example 2 0.18 0.3 0.8 0.013 0.001 0.038 0 0 0 Comparative Example 3 0.08 0.4 1.2 0.013 0.005 0.024 0 0 0.15 Comparative Example 4 0.06 0.2 1.4 0.015 0.009 0.043 0 0 0.1

division Chemical composition (% by weight) Carbide
Indices
Mo Ti Nb V Co W N
(ppm) Ca Inventory 1 0.05 0.015 0.04 0.03 0.02 0.05 35 - 0.076 Inventive Example 2 0.1 0.014 0.02 0 0 0 55 0.0015 0.047 Inventory 3 0.25 0.023 0.06 0 0.04 0.06 42 - 0.072 Honorable 4 0.3 0.013 0.03 0.05 0 0.03 42 0.0035 0.053 Inventory 5 0.04 0.02 0.03 0 0.02 0.05 42 - 0.068 Inventory 6 0.2 0.022 0.07 0.04 0.02 0.07 42 - 0.087 Honorable 7 0.11 0.03 0.04 0 0.03 0.04 42 - 0.056 Comparative Example 1 0 0.012 0.03 0 0 0.03 32 - 0.028 Comparative Example 2 0.04 0.013 0.04 0 0 0.04 28 - 0.022 Comparative Example 3 0 0.027 0.02 0.05 0.1 0.05 26 - 0.036 Comparative Example 4 0.03 0.019 0.04 0 0 0.04 43 - 0.025

division Normalizing Temperature (℃) Thickness
(mm) Retention time
(minute) Carbide density
(× 10 ⁵ pieces / mm ² ) The tensile strength
(MPa) vE _{-50 ℃}
(J) Inventory 1 907 75 113 11.62 656 251 Inventive Example 2 902 30 54 9.15 598 211 Inventory 3 898 55 87 11.48 652 248 Honorable 4 896 60 93 8.87 591 206 Inventory 5 896 70 109 11.61 656 251 Inventory 6 901 30 54 11.2 646 244 Honorable 7 901 50 80 8.15 574 194 Comparative Example 1 935 20 41 1.35 414 84 Comparative Example 2 869 25 49 0.4 391 69 Comparative Example 3 901 50 80 2.26 435 99 Comparative Example 4 901 60 93 3.33 460 116

As can be seen from Tables 1 to 3, Inventive Examples 1 to 7 having compositional components and ranges according to the present invention were confirmed to have excellent tensile strength and low temperature impact toughness values as a large amount of carbides were formed. .

However, Comparative Examples 1 to 4, which do not satisfy the composition and range of the present invention, as it secures a low level of carbide, it can be seen that not only tensile strength, but also low-temperature impact toughness.

2 is a graph showing carbide density according to the carbide index of Inventive Examples 1 to 7. As can be seen in Figure 2, it can be seen that the carbide density also increases linearly as the carbide index increases.

Claims (6)

delete delete delete delete By weight%, C: 0.04-0.20%, Mn: 0.3-2.5%, Si: 0.01-0.6%, Nb: 0.005-0.10%, Mo: 0.01-1.0%, Cr: 0.05-1.0%, Ti: 0.005-- 0.1%, Al: 0.005-0.5%, N: 15-150 ppm, P: 0.02% or less, S: 0.01% or less, Cu: 0.010-1.0%, Ni: 0.01-2.0%, V: 0.005 It comprises at least one selected from the group consisting of ˜0.3%, Co: 0.005 to 0.2%, W: 0.005 to 0.2%, consisting of the balance Fe and other unavoidable impurities,
A reheating step of reheating the steel having a carbide index (CI) of 0.04 to 0.15 represented by the following relation 1 at 1050 to 1250 ° C;
Rolling the reheated steel at 1250 ° C. to Ar 3;
After heating the rolled steel at Ac3 ~ 1000 ℃, maintaining the heating step for (1.3 × t + 10 ~ 30 minutes) (where t means the thickness of the steel (mm)); And
Method of producing a steel having excellent strength and impact toughness comprising the step of air-cooling the heated and maintained steel.
[Relationship 1] = 0.146 x Nb + 0.272 x Ti + 0.261 x V + 0.071 x Mo + 0.081 x W + 0.167 x Cr + 0.037 x Co
The method of claim 5, wherein the steel further comprises Ca: 0.0005 to 0.0060%.

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