JP7599781B2 - Ferritic heat-resistant steel with both high-temperature strength and oxidation resistance - Google Patents

Description

The present invention relates to a ferritic heat-resistant steel with excellent high-temperature strength and oxidation resistance for use in high-temperature and corrosive combustion gas environments, such as recuperator heat exchangers and other heat balance-enhancing heat exchangers and high-temperature exhaust systems.

In conventional heat exchangers for improving the heat balance of recuperators and exhaust systems for various high-temperature gases, the maximum metal temperature in the operating environment was approximately 750°C. Therefore, DIN standard steel type _{X10CrAl24 ,} known as ferritic heat-resistant steel (Cr-Si-Al steel), and the like are used as steel materials that can withstand this temperature range.

To further improve thermal efficiency, it is necessary to raise the operating temperature, for example to 800°C or higher. However, because oxidation and corrosion in high-temperature environments cause significant thinning of steel, the steel is required to have higher resistance to high-temperature oxidation, as well as excellent high-temperature strength that satisfies the design strength in high-temperature environments and suppresses deformation.

Austenitic stainless steels and Ni-based alloys are considered to be useful as steel materials with high temperature resistance, but they are not economically superior because they contain large amounts of alloying elements such as Ni.

On the other hand, ferritic stainless steel is economical because it contains a small amount of alloying elements such as Ni. Therefore, a method has been proposed to improve high-temperature strength by adding rare metals such as Hf, Zr, Ta, and Os to ferritic stainless steel (see, for example, Patent Document 1).

However, this proposal only evaluates creep strength at 650°C, and its applicability in higher temperature environments is unclear. In addition, the carbonitrides that are said to contribute to the high-temperature strength described here generally precipitate quickly, so their strengthening effect at higher temperatures is small, and it is not expected that they will be able to raise the environmental temperature at which current recuperator heat exchangers are used. Furthermore, rare metals are highly valuable, which not only worsens economic efficiency, but also brings concerns about procurement, raising issues regarding the stable supply of raw materials.

Japanese Patent Application Publication No. 11-61342

In ferritic stainless steels and ferritic heat-resistant steels, alloy elements such as Mo, W, Nb, and Ti are sometimes added to utilize the material strengthening mechanisms of solid solution strengthening and precipitation strengthening by intermetallic compounds in order to improve high-temperature strength.
Alternatively, the grain boundary segregation of P, S, and other elements reduces oxidation resistance and secondary workability, but in order to suppress this, boron (B), which segregates preferentially at grain boundaries and is effective in increasing strength by strengthening the grain boundaries, is sometimes added.

In alloy design, the combination of these approaches may result in steels with excellent oxidation resistance and high-temperature strength, but there is a problem in that the precipitation of intermetallic compounds competes with the precipitation of boron compounds, which are not expected to improve high-temperature strength or oxidation resistance.

Therefore, if it were possible to segregate boron to grain boundaries while suppressing the precipitation of boron compounds, which competes with the precipitation of intermetallic compounds, it is expected that the precipitation strengthening due to the precipitation of intermetallic compounds and the grain boundary strengthening due to the grain boundary segregation of boron will maximize the effect of improving high-temperature strength, and excellent oxidation resistance will be achieved through the precipitation of intermetallic compounds that does not reduce oxidation resistance. However, a specific method for achieving this has not yet been found.

The problem that this invention aims to solve is to provide a ferritic heat-resistant steel that is excellent in high-temperature strength, oxidation resistance, and economy.

The first means for solving the problems of the present invention is
This ferritic heat resistant steel contains, in mass%, C: 0.001-0.040%, Si: 0.4-1.2%, Mn: 0.01-1.00%, P: 0.040% or less, S: 0.030% or less, Cr: 21.5-26.0% or less, Mo: 0.01-1.50%, W: 0.01-2.00%, Al: 0.6-1.4%, Ti: 0.01-0.90%, Nb: 0.01-0.90%, N: 0.05% or less, B: 0.0001-0.0150%, and the balance being Fe and unavoidable impurities.

The second aspect of the present invention is a ferritic heat-resistant steel having the chemical components as described in the first aspect, further characterized in that the relationship between formulas (1) and (2) is satisfied.
125×[B]≦[Ti]...Formula (1)
[Ti]≦(1/1174)×(7735-([Si]+289.1×[Cr]+48.5×[Mo]+375.5×[Al]+12.2×[W]+9.1×[Nb]+5000×[B]))...Formula (2)
In addition, the mass percentage of each alloying element is substituted for the [element symbol] in the formula.

According to the present invention, the crystallization or precipitation of _B2Ti , which does not provide the effect of improving high-temperature strength and oxidation resistance, is suppressed, and instead, an intermetallic compound that improves high-temperature strength without decreasing oxidation resistance, such as the precipitation of the Laves phase expressed by (Fe, Cr, Si) ₂ (Nb, Ti, Mo, W), is formed at the grain boundaries and within the grains, thereby achieving both grain boundary segregation of boron and precipitation of intermetallic compounds, thereby realizing grain boundary strengthening and precipitation strengthening by the precipitation of intermetallic compounds, and thus obtaining excellent high-temperature strength, while excellent oxidation resistance can be obtained by the precipitation of the intermetallic compound that does not decrease oxidation resistance, such as the precipitation of the Laves phase. Thus, a ferritic heat-resistant steel excellent in high-temperature strength, oxidation resistance, and economy can be provided.

First, the reasons for specifying each chemical composition of the heat-resistant steel of the present invention and the reasons for specifying formulas (1) and (2) are explained below.

C: 0.001-0.040%
C is an element necessary for ensuring strength. However, if C exceeds 0.040%, workability decreases. Therefore, C is set to 0.001 to 0.040%.

Si: 0.4-1.2%
Silicon is an element that improves oxidation resistance. To achieve this, the silicon content must be 0.4% or more. On the other hand, if the silicon content exceeds 1.2%, the workability decreases. , Si is 0.4 to 1.2%.

Mn: 0.01-1.00%
Mn is an element that improves oxidation resistance and scale spalling resistance. However, if it exceeds 1.00%, the γ phase, which is the starting point of abnormal oxidation, is formed. In addition, the thermal expansion coefficient of the γ phase is α Since the Mn content is large compared to the Fe phase, the dimensional change is large. Therefore, the Mn content is set to 0.01 to 1.00%.

P: ≦0.040%
P is an element that is mixed into steel as an inevitable impurity, but if the P content exceeds 0.040%, hot workability decreases. Therefore, the P content is set to 0.040% or less.

S: ≦0.030%
S is an element that is mixed into steel as an inevitable impurity, but if the S content exceeds 0.030%, hot workability decreases. Therefore, the S content is set to 0.030% or less.

Cr:21.5-26.0%
Cr is an element that improves oxidation resistance, and in the present invention, Cr is added in an amount of 21.5% or more. However, if Cr exceeds 26.0%, the workability decreases. .5 to 26.0%.

Mo: 0.01~1.50%
Mo is an element that improves high-temperature strength. However, if Mo exceeds 1.50%, oxidation resistance decreases. Therefore, Mo is set to 0.01 to 1.50%.

W: 0.01~2.00%
W is an element that improves high-temperature strength and oxidation resistance. However, if W exceeds 2.00%, it reduces oxidation resistance and workability. Therefore, W is limited to 0.01 to 2.00%. do.

Al: 0.6-1.4%
Al is an element that improves oxidation resistance, and 0.6% or more is necessary. However, if it exceeds 1.4%, workability decreases. Therefore, Al is set to 0.6 to 1.4%. .

Ti: 0.01~0.90%
Ti is an element that improves high-temperature strength through solid solution strengthening and precipitation strengthening using intermetallic compounds (e.g., Laves phase), and is also effective in improving oxidation resistance. However, Ti is a strong carbonitride forming element. If the content in the steel exceeds 0.90%, carbonitrides are likely to crystallize or precipitate, which become the starting point for abnormal oxidation, leading to a decrease in oxidation resistance, and at the same time, the formation of intermetallic compounds (e.g., Laves' This leads to an excessive increase in the amount of precipitates of the titanium phase (Ti phase) and coarsening, and inhibits the grain boundary segregation of boron, preventing the steel from exhibiting excellent high-temperature strength. Therefore, the Ti content is set to 0.01 to 0.90%.

Nb: 0.01-0.90%
Nb is an element necessary for improving high-temperature strength by solid solution strengthening and precipitation strengthening using intermetallic compounds (e.g., Laves phase), and 0.01% or more is necessary. However, Nb is a strong carbonitride forming element. If the content in the steel exceeds 0.90%, carbonitrides are likely to crystallize or precipitate, which become the starting point for abnormal oxidation, leading to deterioration of oxidation resistance and a decrease in high-temperature strength. is set to 0.01 to 0.90%.

N: ≦0.05%
N is an element that is mixed in as an impurity. If the N content exceeds 0.05%, the amount of Ti and Nb compounds precipitated increases, and the high-temperature strength decreases. Therefore, the N content is set to 0.05% or less.

B: 0.0001-0.0150%
B is an element that strengthens grain boundaries and improves high-temperature strength, and must be 0.0001% or more. However, if B exceeds 0.0150%, it causes crystallization or precipitation of B ₂ Ti, which reduces the excellent properties. Therefore, the B content is set to 0.0001 to 0.0150%.

Formula (1): 125×[B]≦[Ti]
Formula (1) is an index of improved oxidation resistance. When formula (1) is satisfied, the oxygen derived from the environment in which the steel is used diffuses into the vicinity of the oxide scale formed on the surface of the steel used at high temperatures. The large amount of dissolved Ti and the concentration of Ti in the vicinity of the oxide scale provide excellent oxidation resistance.
On the other hand, when formula (1) is not satisfied and the value of the right side is smaller than the value of the left side, the amount of Ti dissolved in the steel material is reduced due to the crystallization or precipitation of _B2Ti , and the above-mentioned The effect of inhibiting the growth of oxide scale by solute Ti is insufficient, and the effect of improving oxidation resistance cannot be obtained.

Formula (2): [Ti]≦(1/1174)×(7735−([Si]+289.1×[Cr]+48.5×[Mo]+375.5×[Al]+12.2×[W] +9.1×[Nb]+5000×[B]))
Formula (2) is an index of the effect of grain boundary strengthening, i.e., the improvement of high-temperature strength. If the value on the right side of formula (2) is smaller than the value of Ti, the grain boundaries of intermetallic compounds, such as the Laves phase, are strengthened. Since there is a lot of precipitation and the precipitates become coarse, it becomes difficult for the grain boundary segregation of B to coexist. In other words, the effect of grain boundary strengthening due to the coexistence of intermetallic compounds and grain boundary segregation of B, in other words, the effect of improving high-temperature strength, cannot be obtained. This will be the case.

(Example)
For the ferritic heat-resistant steels of Examples No. 1 to 17 and the steels of Comparative Examples No. 18 to 26, each having the chemical composition shown in Table 1, a steel ingot was melted by 100 kg VIM (vacuum induction melting method), heated to 1100°C, forged into a prism shape with a square of 15 mm, held at 1100°C for 15 minutes, and then water-cooled to prepare a specified test piece, and the high-temperature strength and oxidation resistance were evaluated. In Table 1, the cases where each steel of the Examples and Comparative Examples satisfied the formulas (1) and (2) are indicated by ◯, and the cases where each steel did not satisfy the formulas are indicated by ×.

The evaluation of high-temperature strength was carried out in the following manner based on the high-temperature tensile test method specified in JIS G 0567. For each of the examples and comparative examples, high-temperature tensile test pieces with a parallel section diameter of 6 mm were prepared, and after heating the test pieces to 850°C, tensile stress was applied to the test pieces until they broke, and the maximum stress at this time was determined as the high-temperature tensile strength. In Table 1, those with a high-temperature tensile strength of 50 MPa or more are indicated by a circle as having excellent high-temperature strength, and those with a high-temperature tensile strength below 50 MPa are indicated by an X as having poor high-temperature strength.

The oxidation resistance was evaluated by a 100-hour continuous oxidation test described below. For each of the steels of the examples and comparative examples, test pieces with a diameter of 21 mm and a length of 12 mm were prepared, and the surface properties were adjusted with abrasive paper of No. 320 to No. 600, and the weight of the test pieces was measured. Next, a continuous oxidation test was performed in which the test pieces were held in an electric furnace in an air atmosphere heated to 1200°C for 100 hours and then cooled to room temperature. After the test, the surface scale of the test pieces was removed by shot blasting, and the weight of the test pieces was measured. The reduction in the weight of the test pieces before and after the test was used as an evaluation index for oxidation resistance.
In Table 1, specimens with a weight loss of 50 mg or less per unit area were deemed to have excellent oxidation resistance and indicated with a circle, while specimens with a weight loss of 50 mg or more were deemed to have poor oxidation resistance and indicated with a cross.

In addition, in Table 1, as an overall judgment, materials that are inferior in either high-temperature strength or oxidation resistance (marked with an x) are marked with an x as they do not achieve both.

Nos. 1 to 17, which are examples of the present invention, produced steels with excellent high-temperature strength and oxidation resistance.

On the other hand, in Comparative Example No. 18, Ti and N were excessive, and the value of formula (2) was also outside the range specified by the present invention, so that the high-temperature strength was reduced.
In Comparative Example No. 19, the values of formulas (1) and (2) were out of the range, and the high-temperature strength and oxidation resistance were reduced.
In Comparative Example 20, the B content was excessive, the Cr content was too small, and the value of formula (1) was also outside the specified range, so that the oxidation resistance was reduced.
In Comparative Example 21, the Si content was too small and the Mn content was too large, so the oxidation resistance was reduced. Also, since B was not included, the high-temperature strength was reduced.
In Comparative Example 22, the value of formula (2) was outside the specified range, and the high-temperature strength was reduced.
In Comparative Example 23, Nb was not contained, and the value of formula (2) was outside the specified range, and the high-temperature strength was reduced.
In Comparative Example 24, the content of Mo was excessive, and the values of formulas (1) and (2) were outside the prescribed ranges, so that the high-temperature strength and oxidation resistance were reduced.
In Comparative Example 25, the Cr content was too small, the Mo content was too large, the Al content was too small, Ti was not contained, and the value of formula (1) was outside the specified range, so that the high-temperature strength and oxidation resistance were reduced.
In Comparative Example 26, the value of formula (1) was also outside the specified range, and therefore the oxidation resistance was reduced.

Claims (1)

In mass%, C: 0.001 to 0.040%, Si: 0.4 to 1.2%, Mn: 0.01 to 1.00%, P: 0.040% or less, S: 0.030% or less, Cr: 21.5 to 26.0% or less, Mo: 0.01 to 1.50%, W: 0.01 to 2.00%, Al: 0.6 to 1.4%, Ti: 0.01 to 0.90%, Nb: 0.01 to 0.90%, N: 0.05% or less, B: 0.0001 to 0.0150%, and the balance being Fe and unavoidable impurities;
Furthermore, the relationship of formula (1) and formula (2) is satisfied.
Ferritic heat-resistant steel characterized by:
125×[B]≦[Ti]...Formula (1)
[Ti]≦(1/1174)×(7735-([Si]+289.1×[Cr]+48.5×[Mo]+375.5×[Al]+12.2×[W]+9.1×[Nb]+5000×[B]))...Formula (2)
In addition, the mass percentage of each alloying element is substituted for the [element symbol] in the formula.