KR20160057541A - High temperature structural steel containing titanium and method for manufacturing the same - Google Patents

Abstract

Disclosed are titanium-containing steel for high temperature structures, and a manufacturing method thereof. According to an aspect of the present invention, the titanium-containing steel for high temperature structures comprises: 0.01-0.5 wt% of carbon (C); 5.0-15.0 wt% of titanium (Ti); 1-10 wt% of nickel (Ni); 0.01-5 wt% of aluminum (Al); 0.01-3 wt% of silicon (Si); 0.01-5 wt% of chrome (Cr); 0.1-1.0 wt% of manganese (Mn); and the remaining consisting of iron (Fe) and unavoidable impurities. The steel for high temperature structures has excellent cost-efficiency by minimizing the addition of highly valued alloy elements, and is able to be manufactured by a conventional manufacturing method of steel materials such as casting, dissolution, rolling, etc.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-temperature structural steel containing titanium, and a method of manufacturing the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a titanium-containing high-temperature structural steel and a method for producing the same.

It is known that high-temperature structural materials used as materials for boilers, steam generators, ovens, heat exchangers, and generators should have a high yield strength at a high temperature of 500 to 1000 ° C. The present high temperature structural materials are mainly composed of nickel (Ni) Or titanium (Ti) as a main component. However, these nickel-based or titanium-based heat-resistant alloys have excellent mechanical and chemical properties, but are disadvantageous in that they are expensive because they contain a large amount of nickel (Ni) or titanium (Ti), which are expensive alloying elements.

Accordingly, steels having Fe as a main component as a high-temperature structural material have been steadily developed. Typical examples thereof include Fe-Al alloys, Fe-Si alloys, Fe-Cr alloys, Fe- Alloys and the like are known. However, in the case of Fe-Al alloys and Fe-Si alloys, there is a disadvantage in that a coarse ordered phase is formed and hot rolling is very difficult. In the case of Fe-Cr alloys and Fe-Ni alloys, It is disadvantageous in that it can not be produced by a conventional steel manufacturing process such as casting or melting. In addition, in the case of an Fe-Ti based alloy, a coarse Laves phase is formed, which results in a low hot rolling property and a high brittleness.

Therefore, there is a demand for development of a high-temperature structural steel which can be produced by a conventional steel manufacturing method such as casting, melting, rolling and the like with high economic efficiency by using Fe as a main component and minimizing the addition of expensive alloying elements.

One aspect of the present invention relates to a high-temperature structural steel which can be produced by a conventional method for producing steel, such as casting, melting, and rolling, which is excellent in economy by minimizing the addition of expensive alloying elements, .

The object of the present invention is not limited to the above description. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

The titanium-containing high-temperature structural steel according to one aspect of the present invention comprises 0.005 to 0.5% of carbon (C), 5.0 to 15.0% of titanium (Ti), 1 to 10% of nickel (Ni) 0.01 to 5% silicon (Si), 0.01 to 3% chromium (Cr), 0.1 to 1.0% manganese (Mn), and the balance Fe and unavoidable impurities.

In another aspect of the present invention, there is provided an aluminum alloy comprising 0.005 to 0.5% of carbon (C), 5.0 to 15.0% of titanium (Ti), 1 to 10% of nickel (Ni) Heat treating the steel ingot including 0.01 to 3% silicon (Si), 0.01 to 5% chromium (Cr), 0.1 to 1.0% manganese (Mn), and the balance Fe and unavoidable impurities; Subjecting the heat treated steel ingot to hot rolling to obtain a hot rolled steel sheet; Winding and co-cooling the hot-rolled steel sheet; And a step of post-heat-treating the hot-rolled steel sheet after the winding and air-cooling, thereby producing a steel containing high-temperature structural steel containing titanium.

The high-temperature structural steel according to the present invention is advantageous in economical efficiency by using Fe as a main component and minimizing addition of expensive alloying elements.

The high-temperature structural steel according to the present invention is excellent in high-temperature strength and high-temperature ductility, and can be suitably applied to a material such as a boiler, a steam generator, an oven, a heat exchanger and a generator.

Further, the high-temperature structural steel according to the present invention has an advantage that it can be manufactured by a conventional steel manufacturing method such as casting, melting, and rolling.

Hereinafter, the titanium-containing high-temperature structural steel as one aspect of the present invention will be described in detail. First, the composition and composition range of the steel will be described in detail.

Carbon (C): 0.005 to 0.5 wt%

Carbon is an essential element added to ensure strength of the steel. In order to obtain such an effect in the present invention, the content is preferably 0.005% by weight or more, more preferably 0.07% by weight or more, and even more preferably 0.008% by weight or more. However, when the content is excessive, carbide precipitates are excessively formed to lower the consistency of phases with the precipitates, resulting in deterioration of hot rolling property and room temperature ductility, and also drastically increasing the strength in the mouth to reduce ductility. Therefore, the upper limit of the carbon content is preferably 0.5 wt%, more preferably 0.3 wt%, and even more preferably 0.1 wt%.

Titanium (Ti): 5.0-15.0 wt%

Titanium is an essential element added to ensure the corrosion resistance and high temperature characteristics of steel. In order to obtain such an effect in the present invention, it is preferable that the content is at least 5.0 wt%, more preferably at least 6.0 wt%, and even more preferably at least 8.0 wt%. However, when the content is excessive, a middle phase and a coarse Laves phase are formed to lower the hot rolling property and increase the brittleness. Therefore, the upper limit of the titanium content is preferably 15.0 wt%, more preferably 13.0 wt%, and even more preferably 12.0 wt%.

Nickel (Ni): 1.0 to 10.0 wt%

Nickel is an element added to prevent the deterioration of the hot rolling property of steel caused by the addition of a large amount of titanium. In order to exhibit such an effect in the present invention, the content is preferably 1.0 wt% or more, more preferably 2.0 wt% or more, and even more preferably 3.0 wt% or more. However, when the content is excessive, not only the economical efficiency is lowered but also there is a problem of promoting formation of rules. Therefore, the upper limit of the nickel content is preferably 10.0 wt%, more preferably 8.0 wt%, and even more preferably 7.0 wt%.

Aluminum (Al): 0.01 to 5.0 wt%

Aluminum is an element added to improve the strength of steel. In order to exhibit such an effect in the present invention, the content is preferably 0.01% by weight or more, more preferably 0.1% by weight or more, and even more preferably 0.8% by weight or more. However, when the content is excessive, there is a problem in that it is combined with Fe, Mn and C to form a precipitate of capalcarbide, or solidified in Fe to form a B2 phase or a DO3 phase which is an Fe-Al ordered phase to lower the hot rolling property . Therefore, the upper limit of the aluminum content is preferably 5.0 wt%, more preferably 4.0 wt%, and even more preferably 3.0 wt%.

Silicon (Si): 0.01 to 3.0 wt%

Silicon is an element added for the purpose of improving the strength of a steel and stabilizing a ferrite phase at a high temperature. In order to exhibit such an effect in the present invention, the content is preferably 0.01% by weight or more, more preferably 0.05% by weight or more, and even more preferably 0.1% by weight or more. However, when the content is excessive, there is a problem that the Fe-Si ordered phase B2 or DO3 phase is formed in solid solution in the Fe to decrease the hot rolling property. Therefore, the upper limit of the silicon content is preferably 3.0 wt%, more preferably 2.0 wt%, and even more preferably 1.0 wt%.

Cr (Cr): 0.01 to 5.0 wt%

Chromium is an element added to improve the ductility of the steel by suppressing the formation of a rule. In order to exhibit such an effect in the present invention, the content is preferably 0.01 wt% or more, more preferably 0.03 wt% or more, and even more preferably 0.05 wt% or more. However, when the content is excessive, there is a problem that the hot rolling property is lowered and the ductility at room temperature and the impact toughness are lowered. Accordingly, the upper limit of the chromium content is preferably 5.0 wt% or more, more preferably 3.0 wt%, and even more preferably 1.0 wt%.

Manganese (Mn): 0.1 to 1.0 wt%

Manganese is an element added to improve the high temperature strength of steel. In addition, manganese is an austenite stabilizing element and plays a role in inducing reverse transformation of martensite to austenite during the post heat treatment to be described later. In order to exhibit such an effect in the present invention, the content is preferably 0.1% by weight or more, more preferably 0.11% by weight or more, and even more preferably 0.12% by weight or more. On the other hand, if the content is excessive, there is a problem that the ductility and impact properties of steel are deteriorated due to excessive formation of Mn precipitates. Therefore, the upper limit of the manganese content is preferably 1.0 wt%, more preferably 0.8 wt%, and even more preferably 0.6 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

Of these, phosphorus, sulfur and nitrogen are generally referred to as impurities, and a brief description thereof is as follows.

Phosphorus (P): not more than 0.02% by weight

Phosphorus is an impurity which is inevitably contained and is an element which is segregated in the grain boundaries and is a main cause for lowering the toughness of the steel. Therefore, it is preferable to control the content as low as possible. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is controlled to 0.02 wt%.

Sulfur (S): not more than 0.01% by weight

Sulfur is an inevitably contained impurity, and is an element that causes hot brittleness. Therefore, it is preferable to control the content as low as possible. Theoretically, it is advantageous to limit the content of sulfur to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is controlled to 0.01 wt%.

Nitrogen (N): not more than 0.01% by weight

Nitrogen is an impurity inevitably contained. When the content is excessive, the ductility of the steel is lowered, and there is a problem that the hot rolling property is deteriorated by forming a middle phase. In theory, it is advantageous to limit the content of nitrogen to 0%, but it is inevitably contained in the manufacturing process normally. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the nitrogen content is controlled to 0.01 wt%.

In addition, the high-temperature structural steel of the present invention may include other impurities that may be included in the industrial production process of steel. These impurities can be known to anyone with ordinary knowledge in the art to which the present invention belongs, so that the kind and content of the impurities are not specifically limited in the present invention.

Hereinafter, a preferable microstructure of the steel will be described in detail.

According to an embodiment of the present invention, the microstructure of the steel may include 50 to 80% of ferrite and 20 to 50% of austenite. By securing such microstructure, it is possible to secure a yield strength of 250 MPa or more and an elongation of 30% or more at a high temperature of 700 캜.

Also, according to an embodiment of the present invention, after the post-heat treatment, the average grain size of the ferrite in the structure may be 50 탆 or less. If the average particle diameter of the ferrite exceeds 50 탆, there is a fear that the high-temperature yield strength is lowered. On the other hand, the mean grain size of the austenite formed together is not limited because it is influenced by the average grain size of the ferrite. Theoretically, it is advantageous to decrease the average particle diameter of ferrite, but it is not easy to industrially control it to less than 10 mu m, so the lower limit of the average particle diameter of the ferrite is set to 10 mu m. Here, the average particle diameter means an equivalent circular diameter of particles detected by observing a cross section of a steel sheet.

The high-temperature structural steel of the present invention has an advantage of extremely high yield strength at room temperature. According to an embodiment of the present invention, the flow stress at room temperature outside the stress-strain curve of the steel by 0.2% may be 750 MPa or more. Here, the flow stress measurement reference temperature is 25 占 폚.

The high-temperature structural steel of the present invention is excellent in high-temperature strength and high temperature ductility, and can be suitably applied to a material such as a boiler, a steam generator, an oven, a heat exchanger, and a generator. According to an embodiment of the present invention, the yield strength of the steel at 700 ° C may be 250 MPa or more, and the elongation at 700 ° C may be 30% or more.

The high-temperature structural steel of the present invention described above can be produced by various methods, and the production method thereof is not particularly limited. However, it can be produced by the following method as one embodiment thereof.

Hereinafter, a method for manufacturing a high-temperature structural steel, which is another aspect of the present invention, will be described in detail.

Heat treatment step

The steel ingot satisfying the above composition is heat-treated. This step is a step for homogenizing the microstructure. In the case of a conventional heat resistant alloy containing Fe as a main component, a long heat treatment process of about 500 hours or more is required for microstructure homogenization and stabilization. However, in the high temperature structural steel of the present invention, the Ti and Ni contents are suitably controlled, There is an advantage that the desired effect can be obtained.

At this time, the heat treatment temperature is preferably 1000 to 1100 ° C. If the heat treatment temperature is less than 1000 占 폚, there is a possibility that the structure stabilization is insufficient. If the heat treatment temperature is more than 1100 占 폚, there is a concern that a regulated phase may be formed along with phase separation.

The heat treatment time is preferably 8 hours or more. If the heat treatment time is less than 8 hours, there is a possibility that the structure stabilization is insufficient. The upper limit of the heat treatment time is not particularly limited, but may be limited to, for example, 12 hours from the viewpoint of economical efficiency such as energy consumption and process cost.

Hot rolling step

Thereafter, the heat treated steel ingot is hot-rolled to obtain a hot-rolled steel sheet. This step is a step for securing the material and shape. In the case of the conventional Fe-Ti based alloy, it is difficult to manufacture by the hot rolling due to the formation of coarse Laves phase. However, in the case of the steel of the present invention, the alloy composition is appropriately controlled so that the steel can be manufactured by hot rolling There is an advantage.

At this time, the hot rolling temperature is preferably 1100 to 1250 ° C. If the heat treatment temperature is less than 1100 占 폚, the structure may be reduced to deteriorate the hot rolling property, and if it exceeds 1250 占 폚, defective shape may occur.

Coiling  And Air cooling  step

The hot-rolled steel sheet is wound. At this time, the coiling temperature is preferably 550 to 600 占 폚. If the coiling temperature is less than 550 캜, the ferrite structure may be deformed. If the coiling temperature exceeds 600 캜, the grain size may excessively increase.

The hot rolled steel sheet thus wound is air-cooled. At this time, the air cooling method is not particularly limited, and it may suffice to be carried out under conditions conventionally used in the art.

Post heat treatment  step

The air-cooled hot-rolled steel sheet is post-heat treated. This step is a step for inverting the martensite structure to the austenite structure to secure the ferrite and austenite as the final microstructure.

At this time, the post-heat treatment temperature is preferably 750 to 800 ° C. If the post-heat treatment temperature is less than 750 ° C, it is difficult to induce a tissue change, and if it exceeds 800 ° C, excessive phase separation and a regular phase may be formed.

The post-heat treatment time is preferably 2 hours or more. If the heat treatment time is less than 2 hours, it is difficult for the tissue change to occur. The upper limit of the heat treatment time is not particularly limited, but may be limited to, for example, 4 hours from the viewpoint of economical efficiency such as energy consumption and process cost.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate and specify the present invention and not to limit the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

(Thickness: 8 mm, width: 35 mm, length: 100 mm) having the composition shown in Table 1 was prepared by vacuum induction melting. The steel ingot was heat-treated at a temperature of 1050 占 폚 for 8 hours and hot-rolled at a temperature of 1150 占 폚 to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was rolled up at a temperature of 600 ° C and air-cooled to room temperature (25 ° C). Then, the air-cooled hot-rolled steel sheet was heat-treated at a temperature of 750 ° C for 2 hours.

Then, the room temperature tensile test and the 700 ° C high temperature tensile test were carried out, and the microstructure, phase fraction and average ferrite particle size were obtained by optical microscope photographs.

division Alloy composition (% by weight) C Ti Ni Al Si Cr P S N Mn Inventory 1 0.009 10.94 5.35 0.89 0.65 0.13 0.0089 0.0055 0.0045 0.31 Inventory 2 0.019 10.24 4.87 0.98 0.78 0.45 0.0112 0.0056 0.0047 0.22 Inventory 3 0.015 9.13 5.42 1.34 0.62 0.62 0.0105 0.0045 0.0041 0.37 Honorable 4 0.019 9.54 5.74 1.12 0.57 0.45 0.0091 0.0061 0.0052 0.13 Inventory 5 0.017 10.61 4.74 1.41 0.65 0.08 0.0085 0.0058 0.0068 0.55 Comparative Example 1 0.013 17.45 6.42 1.54 0.48 0.07 0.0094 0.0055 0.0065 0.34 Comparative Example 2 0.011 16.89 4.87 1.78 0.61 0.69 0.0106 0.0043 0.0054 0.29 Comparative Example 3 0.015 10.32 0.05 1.45 0.52 0.78 0.0103 0.059 0.0052 0.26 Comparative Example 4 0.009 10.98 6.12 11.67 0.66 0.41 0.0119 0.0066 0.0053 0.36 Comparative Example 5 0.013 10.34 5.14 1.13 0.54 0.43 0.0102 0.0051 0.0034 0.02 Comparative Example 6 0.011 9.76 4.89 0.78 0.56 0.55 0.0095 0.0043 0.0031 2.26

division Microstructure Properties Tissue fraction
(area%) Ferrite average particle diameter (占 퐉) Hot rolling property Room Temperature Yield Strength (MPa) 700 ° C Yield strength (MPa) 700 ° C Elongation (%) Inventory 1 F65 + A35 16 O 758 260 38 Inventory 2 F68 + A32 19 O 798 275 37 Inventory 3 F62 + A38 21 O 802 284 35 Honorable 4 F69 + A31 24 O 834 259 32 Inventory 5 F71 + A29 22 O 812 264 33 Comparative Example 1 X X X Χ Χ Χ Comparative Example 2 X X X Χ Χ Χ Comparative Example 3 F66 + A34 23 △ 694 91 22 Comparative Example 4 X X X Χ Χ Χ Comparative Example 5 F68 + A32 32 O 620 153 38 Comparative Example 6 F55 + A45 12 △ 892 287 17 In the above structural fraction, F means ferrite and A means austenite.
The room temperature yield strength refers to "flow stress" which is 0.2% off the steel stress-strain curve (the measurement temperature is 25 ° C).

Table 2 shows that Inventive Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, are excellent in both hot rolling resistance and high temperature yield strength and high temperature ductility.

On the other hand, in the case of Comparative Examples 1 and 2, the Ti content was excessively large to form a coarse Lavess phase and regular phase, which made hot rolling impossible. In the case of Comparative Example 3, the Ni content was insufficient and the formation of the coarse Laves phase formed by Ti could not be sufficiently suppressed, so that the high temperature strength and high temperature ductility were deteriorated. In the case of Comparative Example 4, the Al content was excessive and a coarse-grained phase was formed, which made hot rolling impossible. In Comparative Example 5, the Mn content was insufficient and the high temperature strength was deteriorated. In Comparative Example 6, the Mn content was excessive and the high temperature ductility deteriorated.

Claims (13)

(Si): 0.01 to 5%, aluminum (Al): 0.01 to 5%, carbon (C): 0.005 to 0.5%, titanium (Ti): 5.0 to 15.0% To 3% of Cr, 0.01 to 5% of Cr, 0.1 to 1.0% of Mn, and the balance of Fe and unavoidable impurities.
The method according to claim 1,
Wherein the impurity comprises 0.02% or less phosphorus (P), 0.01% or less sulfur (S), and 0.01% or less nitrogen (N) by weight.
The method according to claim 1,
Wherein the microstructure of the steel comprises an area fraction of 50 to 80% of ferrite and 20 to 50% of austenite.
The method of claim 3,
Wherein said ferrite has an average grain size of 10 to 50 占 퐉.
The method according to claim 1,
A high temperature structural steel containing titanium having a flow stress at room temperature exceeding 0.2 MPa from the stress-strain curve of the steel of not less than 750 MPa.
The method according to claim 1,
Wherein the yield strength of the steel at 700 캜 is 250 MPa or more and the elongation at 700 캜 is 30% or more.
(Si): 0.01 to 5%, aluminum (Al): 0.01 to 5%, carbon (C): 0.005 to 0.5%, titanium (Ti): 5.0 to 15.0% Heat treating the steel ingot, the steel ingot including 0.01 to 5% of chromium (Cr), 0.1 to 1.0% of manganese (Mn), the balance of Fe and unavoidable impurities;
Subjecting the heat treated steel ingot to hot rolling to obtain a hot rolled steel sheet;
Winding and co-cooling the hot-rolled steel sheet; And
And after the coiling and air cooling, post-heat-treating the hot-rolled steel sheet.
8. The method of claim 7,
Wherein the heat treatment temperature during the heat treatment is 1000 to 1100 占 폚.
8. The method of claim 7,
Wherein the heat treatment time is 8 to 12 hours at the time of the heat treatment.
8. The method of claim 7,
Wherein the rolling temperature during the hot rolling is 1100 to 1250 ° C.
8. The method of claim 7,
Wherein the coiling temperature is 550 to 600 占 폚 during the winding.
8. The method of claim 7,
Wherein the post-heat treatment temperature during the post-heat treatment is 750 to 800 占 폚.
8. The method of claim 7,
Wherein the post-heat treatment time is 2 to 4 hours in the post-heat treatment.