KR100605678B1 - A heat resistant stainless steel having good heat resistant strength - Google Patents

Abstract

The present invention relates to a heat-resistant stainless steel, by adding an appropriate amount of tungsten, niobium, nitrogen, and cerium when the limit temperature is 1000 ℃, by adjusting the (Cr / Ni) _eq . Ratio of the alloy component system to 1.48 or more, It is an object of the present invention to provide a heat resistant stainless steel having excellent heat resistance.

In the present invention, C: 0.08% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.045% or less, Si: 1.0% or less, Cr: 18-22%, Ni: 8-12% , W: 1.0-2.5%, N: 0.1-0.2%, Nb: 0.2-0.6%, Ti: 0.045% or less, B: 0.005% or less, balance Fe and other unavoidable impurities, (Cr / Ni) _eq . Heat-resistant stainless steel with excellent heat strength with a ratio of 1.48 or more is the technical gist.

Heat resistant steel, tungsten, niobium, creep, high temperature strength

Description

Heat-resistant stainless steel with excellent heat resistance {A HEAT RESISTANT STAINLESS STEEL HAVING GOOD HEAT RESISTANT STRENGTH}

1 is a graph showing the hot workability of the invention steel and conventional steel

Figure 2 is a hot-annealed tissue photograph of the invention steel and conventional steel

3 is a graph showing creep characteristics at 700 ° C.

Figure 4 is a graph showing the weight change by isothermal oxidation for 2000 hours at 800 ℃ atmosphere

Figure 5 is a graph showing the weight change by isothermal oxidation for 500 hours at 1000 ℃ atmosphere

Figure 6 is a graph showing the change in weight by repeated oxidation in the 800 ℃ air atmosphere.

Figure 7 is a graph showing the change in weight by repeated oxidation in an atmosphere of 1000 ℃.

The present invention relates to heat-resistant steels used at temperatures of about 900 ° C. or lower, such as heat exchangers, furnace parts, incinerators, and more particularly tungsten (W), nitrogen (N), Nb (niobium), and optionally Ce (cesium). The present invention relates to a heat-resistant stainless steel that has significantly improved high-temperature mechanical properties and creep strength by adding an appropriate amount).

Representative heat-resistant stainless steels include 309S steel having a basic composition of 22% Cr-12% Ni and 310S steel having a basic composition of 25% Cr-20% Ni. These steels are excellent in oxidation resistance, but the high temperature strength is similar to that of conventional 304 steels. When applied to structures that are stressed at high temperatures, they are not suitable for use as general purpose heat resistant steels because they are easily deformed like creep fracture and are very expensive. not. Therefore, it is common to use conventional 304 or 316 steel for heat exchangers, small incinerators, etc., but there is a disadvantage that frequent replacement due to material deterioration during use is inevitable.

In order to solve this problem, advanced stainless steel manufacturers are focusing on developing heat resistant steel having excellent heat resistance, but excellent oxidation resistance has been obtained in most heat resistant steels, but high progress has not been made in high temperature strength.

Accordingly, the present invention is to provide a heat-resistant stainless steel excellent in heat resistance by adding a suitable amount of tungsten, niobium, nitrogen, and cerium, if necessary, and adjusting the (Cr / Ni) _eq . Ratio of the alloy component system to 1.48 or more. There is a purpose.

In the present invention, C: 0.08% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.045% or less, Si: 1.0% or less, Cr: 18-22%, Ni: 8-12% , W: 1.0-2.5%, N: 0.1-0.2%, Nb: 0.2-0.6%, Ti: 0.045% or less, B: 0.005% or less, balance Fe and other unavoidable impurities, and Ce added as needed It relates to a heat-resistant stainless steel having a composition and excellent in heat resistance with a (Cr / Ni) _eq . Ratio of 1.48 or more.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the composition of the heat-resistant stainless steel of the present invention, the C forms a chromium carbide when excessively added, thereby lowering the corrosion resistance.

The Mn is preferably added at 2.0% or less, which is the usual standard range of austenitic stainless steel, because when added in excess of 2.0%, oxidation resistance is significantly decreased at high temperature and corrosion resistance is reduced by MnS formation. Because.

When P and S are excessively added, not only the hot workability is significantly lowered due to segregation, but especially S forms MnS, which lowers the corrosion resistance. It is preferable.

The Si has an advantage of improving the oxidation resistance when added in a large amount, but can easily form SiO ₂ inclusions in the steelmaking industry can reduce the cleanliness, hot workability and mechanical properties. Therefore, the content is preferably 1.0% or less, which is the normal specification range of austenitic stainless steel.

The Cr is preferably added at least 18% in order to form a protective film by Cr alone at 1000 ℃. However, if the content is too high, (Cr / Ni) _eq. Since excessive Ni must be added for the non-adjustment, the economical efficiency is preferably 22% or less.

Ni serves to stabilize the austenite structure, but because it is expensive (Cr / Ni) _eq. Care should be taken to control the ratio and minimize the addition. In 18% Cr steel and 22% Cr steel, Ni must contain at least 8% and 12%, respectively, in order to retain the austenite structure. However, since the role of Ni which improves oxidation resistance will fall when the content is 12% or more, especially 14 to 19%, it is preferable to make addition amount into 8 to 12%.

W is an element that improves high temperature strength, but when excessively added, W is deteriorated in oxidation resistance due to vaporization of the WO ₃ compound at a high temperature, and promotes the formation of σ phase when maintained at 700 ° C. for a long time, thereby degrading mechanical properties. Therefore, in order to maximize the effect of strengthening employment and at the same time minimize other negative effects, the amount of W added is preferably 1 to 2.5%.

N is an element that improves strength and corrosion resistance at the same time, but if the content is 0.1% or less, it is difficult to expect a great effect. On the other hand, when 0.2% or more is added, a large amount of pinhole defects are generated during the playing process, thereby inhibiting the surface quality. Therefore, the content is preferably 0.1 to 0.2%.

The Nb is a very effective element to form a Nb (C, N) compound to improve the high temperature strength and creep strength, the content is determined by the content of C and N in the steel. In consideration of the N content of 0.1 to 0.2% in the inventive steel, the amount of Nb added is preferably 0.2 to 0.6%.

When Ti is added in an appropriate amount during solidification, Ti has an effect of improving hot workability and controlling SiO ₂ compounds. However, when excessively added, since oxidation resistance is reduced by promotion of TiO ₂ oxide, the content thereof is preferably 0.045% or less.

B is an element that improves hot workability at high temperature, but the solubility of B decreases as the alloy ratio increases. In 304 steel and 310S steel at 1100 ° C, the solid solubility of B is about 100 ppm and about 50 ppm, respectively. In the present invention, the alloy ratio is between 304 and 310S, and the amount of B added is preferably 50 ppm or less.

The Ce is an element that can be added to significantly improve the oxidation resistance, the more the content thereof, the more effective the oxidation resistance, but because it is expensive, it is preferable to add 0.1% or less in consideration of economical efficiency.

Next, in consideration of the economics of steel production, the (Cr / Ni) _eq . Ratio is preferably 1.48 or more, because the (Cr / Ni) _eq . Ratio is too low, due to solidification of primary austenite during playing It causes P and S segregation and entails coagulation cracking during welding. That is, in order to manufacture steel economically, continuous casting and hot rolling are required. Generally, continuous casting of steel is most effective when the solidification mechanism is L → L + δ → L + δ + γ → γ + δ. great. In order to have such a solidification mechanism, the ratio of (Cr / Ni) _eq . Of the alloy component system based on the relations 1 and 2 should be 1.48 or more.

[Relationship 1]

Cr _eq. = (% Cr) +1.37 (% Mo) +1.5 (% Si) +2 (% Nb) +3 (% Ti)

[Relationship 2]

Ni _eq . = (% Ni) + 0.31 (% Mn) + 22 (% C) + 14.2 (% N) + (% Cu)

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1)

The inventive steels of the present invention and conventional steels of 304, 309S and 310S were manufactured in the form of 30 kg ingot in a vacuum induction melting furnace. The prepared ingot was heated in a 1250 ℃ electric furnace for 150 minutes and then hot rolled to a thickness of 14m and 6mm. Table 1 shows the alloy component system in the molten steel, where conventional steel (1) is 304 steel, conventional steel (2) is 309S steel, conventional steel (3) is 310S steel, invention steel (1) and invention steel (2) is a steel that satisfies the conditions of the present invention, the invention steel (1) is also referred to as 2009W steel, and the invention steel (2) is also referred to as 2009WR steel.

division Cr Ni Mn Si C N B Ti Nb W Ce equivalent weight Conventional Steel 1 17.8 7.69 0.84 0.61 0.046 0.04 － － － － － 1.88 Conventional Steel 2 22.56 13.1 1.55 0.34 0.066 0.045 0.0023 0.012 － － － 1.47 Conventional Steel 3 25.27 19.01 1.49 0.54 0.047 0.051 0.0034 0.011 － － － 1.23 Inventive Steel 1 19 8.61 0.97 0.44 0.067 0.170 0.0018 0.014 0.37 1.92 － 1.70 Inventive Steel 2 19.24 8.6 0.98 0.4 0.061 0.170 0.0018 0.012 0.34 1.92 0.022 1.73

As can be seen in Table 1, since the (Cr / Ni) _eq . Ratio of the present invention steel is 1.70, 1.73, respectively, satisfies the scope of the present invention, the continuous castability is evaluated to be very excellent.

Figure 1 shows the hot workability of the invention steel and conventional steel. The hot workability was evaluated by the change in RA.Reduction of Area due to high temperature tensile. As shown in Fig. 1, the invention steel 1 has a hot workability similar to that of the conventional steel, and the invention steel 2 has a significantly better hot workability than the conventional steel 2 and the conventional steel 3. The reason for this is that, as shown in the microstructure of Fig. 2, the 2009WR steel, the inventive steel 2, contains an appropriate amount of δ-ferrite. In other words, the δ / γ interface not only promotes recrystallization nucleation during processing, but also greatly improves workability by suppressing grain growth. As such, when the hot workability is excellent, surface defects or edge cracks can be suppressed during hot rolling, and post-processing such as surface polishing or edge slitting can be omitted, which is very economical.

(Example 2)

For the steels produced in Example 1, the room temperature and high temperature mechanical properties of the steel at 25 to 600 ° C. were measured and the results are shown in Table 2 below.

Steel grade Item 25 ℃ 300 ℃ 400 ℃ 500 ℃ 600 ℃ Conventional Steel 1 YS (kg / mm2) 23.3 13.7 12.8 11.5 9.7 TS (kg / mm2) 65.6 41.5 41.5 39.0 34.9 EI (%) 69.8 49.7 51.3 49.9 50.8 Conventional Steel 2 YS (kg / mm2) 27.0 18.3 17.7 16.2 13.3 TS (kg / mm2) 60.0 48.1 47.9 44.7 38.7 EI (%) 59.6 45.5 47.2 47.4 45.5 Conventional Steel 3 YS (kg / mm2) 25.3 17.6 17.0 14.9 12.3 TS (kg / mm2) 59.4 48.3 47.7 45.4 41.0 EI (%) 61.7 49.6 51.0 50.5 52.7 Inventive Steel 1 YS (kg / mm2) 44.6 28.4 27.4 24.5 22.6 TS (kg / mm2) 76.8 59.1 58.0 54.1 50.3 EI (%) 52.3 43.6 43.7 40.7 40.4 Inventive Steel 2 YS (kg / mm2) 45.3 29.7 27.8 25.9 23.4 TS (kg / mm2) 77.3 59.4 58.0 54.7 50.2 EI (%) 50.0 42.9 46.3 43.3 42.1

As can be seen in Table 2, the conventional steel (1) ~ (3) has a room temperature yield strength of 23 ~ 27kg / mm2, while the invention steel is about 45kg / mm2, about two times, the room temperature tensile strength is It can be seen that the conventional steel is 60 to 65 kg / mm 2, and the invention steel is about 77 kg / mm 2, and the invention steel of the present invention is very excellent. In the case of an elongation, although it is weak compared with the conventional steel, it shows 50 to 52% and is very excellent. In addition, the yield strength of conventional steel at 600 ℃ is about 10kg / mm2, while the yield strength of the invention steel is about 23kg / mm2, it can be seen that the tensile strength is also excellent.

As described above, the steel of the present invention is excellent in high temperature mechanical properties, and is excellent in deformation resistance by load during use, thereby extending the life of the material.

On the other hand, the high temperature stress strain related to the life of the material is usually evaluated by creep characteristics. As can be seen in Figure 3, under the load of 12kg / ㎜ the conventional steel (2) creep rupture occurred in 141 hours, while the invention steel (1) 2,692 hours to creep rupture, in the case of invention steel (2) It took 2,785 hours. That is, the creep life of the inventive steel was about 15 times or more superior to that of the conventional steel (2). This may be due to precipitation strengthening by adding tungsten and niobium and solid solution strengthening by adding nitrogen.

Figure 4 shows the results of an isothermal oxidation experiment for 2000 hours in an atmosphere of 800 ℃, it can be seen that the invention steel is similar to or superior to the conventional steel (2), (3) by weight by oxidation.

In addition, in FIG. 5 showing the results of isothermal oxidation experiments for 500 hours in an atmosphere of 1000 ° C., the weight of invention steel 2 is similar to that of conventional steels 2 and 3, but is similar to that of conventional steels 1. It can be seen that it has a remarkably excellent oxidation resistance. The difference in oxidation resistance between the inventive steel (1) and the inventive steel (2) is due to the rare earth element cerium (Ce) contained in the inventive steel (1). That is, Ce is located between the oxidized scale and the base metal to prevent peeling of the scale, and when a crack occurs in the scale, an oxide is formed at the cracked portion to prevent oxygen from penetrating. 4 and 5, although the invention steel 2 has excellent oxidation resistance even at 1000 degreeC, since the invention steel 1 does not form a safe protective film at 1000 degreeC, the service temperature of invention steel 1 is It is preferable to limit to 900 ° C.

Figure 6 shows the results of 800 repeated oxidation experiments in the atmosphere of 800 ℃. The repeated oxidation experiment was to be repeatedly performed with one cycle of 50 minutes oxidation + 10 minutes cooling. In the case of repeated oxidation experiments, it is usually accompanied by a slight weight increase at the initial stage, followed by weight loss due to scale peeling due to thermal stress as the thickness of the scale becomes thick. However, as shown in Figure 6, it can be seen that the weight increase and decrease of the invention steel is significantly superior to the conventional steel (1) in a similar level to the conventional steel (2), (3).

FIG. 7 shows the results of 450 repeated oxidation experiments in an atmosphere of 1000 ° C., and the inventive steels are accompanied by weight loss due to scale peeling, but are superior to those of the conventional steels 1. In particular, the invention steel 2 is superior to the invention steel 1 due to the addition of Ce as described above.

As described above, the present invention relates to a heat-resistant stainless steel having excellent heat resistance, and by controlling the additive component, there is an effect of obtaining a general-purpose heat resistant steel having excellent high temperature oxidation resistance and mechanical properties through an economical manufacturing process.                     

Claims (2)

By weight%, C: 0.08% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.045% or less, Si: 1.0% or less, Cr: 18-22%, Ni: 8-12%, W: 1.0-2.5%, N: 0.1-0.2%, Nb: 0.2-0.6%, Ti: 0.045% or less, B: 0.005% or less, having a composition consisting of residual Fe and other unavoidable impurities, and having a (Cr / Ni) _eq . Heat-resistant stainless steel with excellent heat resistance of 1.48 or more. (Where Cr _eq. = (% Cr) + 1.37 (% Mo) + 1.5 (% Si) + 2 (% Nb) + 3 (% Ti), Ni _eq . = (% Ni) + 0.31 (% Mn) +22 (% C) +14.2 (% N) + (% Cu).) The heat resistant stainless steel of claim 1, wherein the steel contains 0.1% or less of Ce.

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