JP2017145437A - Ferritic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel with improved high temperature creep strength, without deteriorating room temperature ductility.SOLUTION: A ferritic stainless steel includes 0.05 mass% or less C, 0.1 to 3.0 mass% Si, 0.1 to 2.0 mass% Mn, 13.5 to 16.5 mass% Cr, 0.15 to 1.2 mass% Zr, 1.0 to 4.0 mass% Ni, 0.5 to 2.0 mass% Al, 0.5 to 3.0 mass% Cu, 0.0002 to 0.005 mass% B, 0.05 mass% or less N, 0.005 to 0.05 mass% P, and the balance Fe with inevitable impurities. S included as the inevitable impurities is regulated to be 0.005 mass % or less.SELECTED DRAWING: None

Description

  The present invention relates to a ferritic stainless steel, and particularly relates to a material excellent in both high temperature creep strength and room temperature ductility.

  Steel pipes used for heat exchangers in thermal power plants and chemical plants require not only oxidation resistance and corrosion resistance, but also excellent heat resistance at a limit temperature of 650 ° C, so austenitic stainless steel and ferritic stainless steel Formed with. Austenitic stainless steel has good oxidation resistance, corrosion resistance, and high-temperature strength including creep strength, and has good characteristics as a heat-resistant material, but contains a large amount of Ni (for example, 8 in SUS304). The raw material cost is high. Ferritic stainless steel, on the other hand, has low Ni content and low raw material costs, and has better characteristics as a heat exchanger such as thermal expansion coefficient and thermal conductivity, but it is hotter than austenitic stainless steel. Inferior in strength.

  Therefore, ferritic stainless steel with improved high temperature strength has been developed. For example, 9Cr heat resistant steel containing 9% by mass of Cr has been developed and put into practical use. However, 9Cr heat-resistant steel does not have high-temperature creep strength comparable to that of austenitic stainless steel, and has higher oxidation resistance at higher temperatures than austenitic stainless steel containing 18% by mass of Cr, which improves oxidation resistance. Not enough.

  Therefore, 15Cr ferritic stainless steel containing 15% by mass of Cr has been developed (Non-patent Document 1). The 9Cr heat-resistant steel has a metal structure having tempered martensite, whereas the 15Cr ferritic stainless steel has a ferrite single phase structure by containing a large amount of Cr. Furthermore, Patent Document 1 discloses 15Cr ferritic steel that further improves high-temperature creep strength by adding Mo, W, Nb, Co, and the like to precipitate intermetallic compounds, carbides, and nitrides at high temperatures. It is disclosed.

Japanese Patent No. 3777421

Y. TODA, M. IIJIMA, H. KUSHIMA, K. KIMURA, F. ABE, “Effects of Ni and Heat Treatment on Long-term Creep Strength of Precipitation Strengthened 15Cr Ferritic Heat Resistant Steels”, ISIJ International, Vol.45, No.11, pp.1747-1753, 2005

  The 15Cr ferritic steel described in Non-Patent Document 1 and Patent Document 1 can provide sufficient oxidation resistance with a large amount of Cr. However, in order to further improve the high temperature creep strength, a rare metal such as W or Co is added as in Patent Document 1, and the intermetallic compound, carbide, or nitride is precipitated at a high temperature. The raw material cost increases, and the advantage of low cost of ferritic stainless steel is impaired. In addition, the added metal exists in a solid solution state in the Fe matrix after solution treatment, lowering the ductility at room temperature, and the steel pipe in cold work and high temperature equipment assembly in the steel pipe manufacturing stage. Bending process becomes difficult.

  The present invention has been made in view of the above-mentioned problems, and its problem is that the steel pipe used in high-temperature equipment such as a heat exchanger does not deteriorate heat resistance, oxidation resistance, etc., and is high in cost. Therefore, it is an object of the present invention to provide a ferritic stainless steel that achieves both excellent high temperature creep strength and room temperature ductility that enables cold working.

The present inventors pay attention to a precipitate called G phase, which has been confirmed in the field of duplex stainless steels and the like that require corrosion resistance, and the fine precipitate consisting of this G phase is dislocated due to creep deformation. It was conceived that high creep strength was developed by inhibiting the movement of. As a result of diligent research, the present inventors have found that a 15Cr ferritic stainless steel having a ferrite single phase structure forms a G phase in a high temperature environment without lowering the ductility after solution treatment. Ni, Zr, and Si were found as non-elements. In other words, by adding an appropriate amount of Ni, Zr, Si to a ferritic stainless steel composed of a ferrite single phase structure containing a high concentration of Cr with excellent heat resistance and oxidation resistance, the temperature environment of high temperature equipment can be improved. Under a corresponding high temperature environment of about 450 to 750 ° C., a G phase composed of the intermetallic compound Ni ₁₆ Zr ₆ Si ₇ is formed and finely dispersed in the steel. This G phase is presumed to be semi-consistently precipitated in the ferritic stainless steel due to the relationship between the ferrite phase and Cube on Cube, forming a strain field around the precipitate and efficiently pinning dislocation movement accompanying creep deformation. It is thought that there is. Furthermore, the present inventors do not fix Zr as a carbide or nitride, so that the content of C and N is suppressed, and Ni having an effect of making it difficult to maintain a ferrite single-phase structure is sufficiently added. For this reason, the inventors conceived to add together Al having an effect of stabilizing the ferrite phase.

  That is, the ferritic stainless steel according to the present invention has C: 0.05 mass% or less, Si: 0.1-3.0 mass%, Mn: 0.1-2.0 mass%, Cr: 13.5 To 16.5 mass%, Zr: 0.15 to 1.2 mass%, Ni: 1.0 to 4.0 mass%, Al: 0.5 to 2.0 mass%, Cu: 0.5 to 3 0.0% by mass, B: 0.0002 to 0.005% by mass, N: 0.05% by mass or less, P: 0.005 to 0.05% by mass, with the balance being Fe and inevitable impurities The S contained as the inevitable impurity is limited to 0.005 mass% or less.

The ferritic stainless steel having such a structure suppresses the content of C and N, adds Zr together with Si and Ni, and precipitates a G phase composed of an intermetallic compound Ni ₁₆ Zr ₆ Si ₇ at a high temperature. By finely dispersing in, it has excellent high-temperature creep strength, and ductility after solution treatment does not decrease. In addition, since ferritic stainless steel contains Al, the stability of the ferrite phase is not reduced by Ni, and it has a ferrite single-phase structure. As a result, it has excellent heat resistance and high concentration of Cr. Has sufficient oxidation resistance.

  According to the ferritic stainless steel according to the present invention, because it is excellent in oxidation resistance and heat resistance, it becomes a steel pipe suitably used for high-temperature equipment such as a heat exchanger, and further, the raw material cost does not increase, Since the creep strength at high temperatures is high and the creep deformation resistance is excellent, the thickness of high-temperature equipment can be reduced, manufacturing costs can be reduced, and cold working is possible. In addition, it is possible to reduce the processing cost in bending a steel pipe when manufacturing a high-temperature device.

It is a figure which illustrates typically the minimum strain rate for evaluation of the high temperature creep strength in an Example, (a) is a graph which shows the relation between creep strain and test time, (b) was computed from (a). It is a graph which shows the relationship between a strain rate and test time.

Hereinafter, the form for implement | achieving the ferritic stainless steel which concerns on this invention is demonstrated.
[Ferrite stainless steel]
Ferritic stainless steel according to the present invention is C: 0.05% by mass or less, Si: 0.1-3.0% by mass, Mn: 0.1-2.0% by mass, Cr: 13.5-16 0.5 mass%, Zr: 0.15-1.2 mass%, Ni: 1.0-4.0 mass%, Al: 0.5-2.0 mass%, Cu: 0.5-3.0 % By mass, B: 0.0002 to 0.005% by mass, N: 0.05% by mass or less, P: 0.005 to 0.05% by mass, with the balance being Fe and inevitable impurities, S contained as an inevitable impurity is limited to 0.005 mass% or less. The chemical component composition of the ferritic stainless steel according to the present invention will be described below.

(C: 0.05% by mass or less)
C is an element that has an action of forming carbides in a high temperature environment and improving high temperature strength and high temperature creep strength. However, since C fixes Zr as a carbide, the ferritic stainless steel according to the present invention preferably has a low content in order to sufficiently precipitate Zr as a G phase and improve creep strength. Is not specified. Specifically, the C content is 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less.

(Si: 0.1-3.0% by mass)
Si is an element having a deoxidizing action in molten steel, and effectively acts to improve oxidation resistance even in a trace amount. Further, since Ni, Zr and the intermetallic compound Ni ₁₆ Zr ₆ Si ₇ (G phase) are formed in a high temperature environment and finely dispersed in the steel to improve the creep strength, the ferritic stainless steel according to the present invention It is an essential element for exhibiting high temperature creep strength. In order to exert these effects, the Si content is 0.1% by mass or more, preferably 0.15% by mass or more, and more preferably 0.25% by mass or more. On the other hand, if Si is contained excessively, the hardness of the ferritic stainless steel increases and ductility is lowered, so the Si content is 3.0% by mass or less, preferably 2.5% by mass or less, more preferably Is 1.5 mass% or less.

(Mn: 0.1 to 2.0% by mass)
Mn is an element having a deoxidizing action in molten steel like Si, and in order to exert this effect, the Mn content is 0.1% by mass or more, preferably 0.3% by mass or more, more preferably Is 0.5% by mass or more. On the other hand, if Mn is contained excessively, the stability of the austenite phase becomes high and it becomes difficult to maintain the ferrite single phase structure. Therefore, the Mn content is 2.0% by mass or less, preferably 1.5% by mass or less. More preferably, it is 1.0 mass% or less.

(Cr: 13.5 to 16.5% by mass)
Cr is an essential element in order to develop corrosion resistance (oxidation resistance) as stainless steel. In order to exhibit sufficient oxidation resistance, the Cr content is 13.5% by mass or more, preferably 14.0% by mass or more, more preferably 14.5% by mass or more. On the other hand, if Cr is excessively contained, spinodal decomposition may occur in a high temperature environment and ductility and toughness may be reduced. Therefore, the Cr content is 16.5% by mass or less, preferably 16.0% by mass or less. More preferably, it is 15.5 mass% or less.

(Zr: 0.15 to 1.2% by mass)
Zr does not lower the ductility even if it dissolves in the Fe matrix at room temperature, and also forms Ni, Si and the intermetallic compound Ni ₁₆ Zr ₆ Si ₇ (G phase) in a high temperature environment. Since it is finely dispersed therein to improve the creep strength, it is an essential element for achieving both high temperature creep strength and room temperature ductility of the ferritic stainless steel according to the present invention. In order to sufficiently exhibit this effect, the Zr content is 0.15% by mass or more, preferably 0.3% by mass or more, more preferably 0.5% by mass or more. On the other hand, when the Zr content is excessive, the hot workability in the vicinity of 900 ° C. is lowered, so that it is 1.2% by mass or less, preferably 1.0% by mass or less, more preferably 0.8% by mass or less. is there.

(Ni: 1.0-4.0% by mass)
Ni does not lower the ductility at room temperature, and forms Zr, Si and an intermetallic compound Ni ₁₆ Zr ₆ Si ₇ (G phase) in a high temperature environment to finely disperse in the steel and improve the creep strength. Therefore, it is an essential element in order to achieve both high temperature creep strength and room temperature ductility of the ferritic stainless steel according to the present invention. In order to exert this effect, the Ni content is 1.0% by mass or more, preferably 1.5% by mass or more, more preferably 2.0% by mass or more. On the other hand, since a raw material cost will become high when Ni is contained in large quantities, Ni content shall be 4.0 mass% or less, Preferably it is 3.0 mass% or less, More preferably, it is 2.5 mass% or less.

(Al: 0.5-2.0 mass%)
Al has an effect of stabilizing the ferrite phase, and is an essential element for sufficiently adding Ni in the ferritic stainless steel according to the present invention, and does not inhibit the precipitation of intermetallic compounds, The effect on room temperature ductility is also small. In order to sufficiently obtain the above action, the Al content is 0.5% by mass or more, preferably 0.7% by mass or more, and more preferably 0.9% by mass or more. On the other hand, when Al is contained excessively, the hot workability is lowered, so the Al content is 2.0% by mass or less, preferably 1.5% by mass or less, more preferably 1.2% by mass or less. is there.

(Cu: 0.5-3.0% by mass)
Cu precipitates alone in the ferrite phase, but has less interaction with other precipitated phases, improves strength by precipitation strengthening, and contributes to strengthening in a shorter time than intermetallic compounds, especially in high-temperature environments. In addition, the room temperature ductility is not reduced. In order to sufficiently exhibit this effect, the Cu content is 0.5% by mass or more, preferably 1.0% by mass or more, and more preferably 1.3% by mass or more. On the other hand, if Cu is contained excessively, cracking may occur during hot working during production, so the Cu content is 3.0% by mass or less, preferably 2.5% by mass or less, more preferably 2%. 0.0 mass% or less.

(B: 0.0002 to 0.005 mass%)
B has the effect of improving hot workability by dissolving in steel, and in order to obtain sufficient hot workability, the B content is 0.0002% by mass or more, preferably 0.0005. It is at least mass%, more preferably at least 0.001 mass%. On the other hand, when B is contained excessively, ductility at the time of welding is lowered, so the content is 0.005% by mass or less, preferably 0.003% by mass or less, more preferably 0.0025% by mass or less.

(N: 0.05% by mass or less)
N is an element having an action of forming nitrides in a high temperature environment and improving high temperature strength and high temperature creep strength. However, since N fixes Zr as a nitride, the ferritic stainless steel according to the present invention preferably has a small content in order to sufficiently precipitate Zr as a G phase and improve the creep strength. There is no specific lower limit. Specifically, the N content is 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less.

(P: 0.005 to 0.05 mass%)
P has the effect of forming a phosphide and improving the high temperature strength in a situation where it is exposed to a high temperature environment for a long period of time, so the P content is 0.005% by mass or more, preferably 0.015% by mass or more, More preferably, it is 0.020 mass% or more. On the other hand, if P is contained in excess of 0.05% by mass, weld cracking is likely to occur. Therefore, the P content is 0.05% by mass or less, preferably 0.04% by mass or less, more preferably 0.8%. It suppresses to 035 mass% or less.

(Inevitable impurities, S: 0.005 mass% or less)
The ferritic stainless steel according to the present invention may contain S as an inevitable impurity. However, when the content is increased, the hot workability is deteriorated. Is 0.002% by mass or less, more preferably 0.001% by mass or less.

In addition, low melting point impurities such as Sn, Pb, Sb, As, and Zn derived from scrap raw materials reduce the strength of the grain boundaries during hot working or when used in a high temperature environment. More specifically, each is suppressed to 0.015 mass% or less, and the total amount is set to 0.04 mass% or less. In addition, transition metals such as Mo, W, Nb, Hf, Ta, Co, and V improve the strength by forming an intermetallic compound of Fe ₂ M type (M: transition metal, etc.) at a high temperature, but at room temperature. Then, in order to reduce the ductility by solid solution in the Fe matrix, each is suppressed to 0.3% by mass or less. Ti, when contained in a large amount, lowers toughness at high temperatures, but since the properties are relatively close to the same group 4 element as Zr, it lowers ductility even when dissolved in the Fe matrix at room temperature. Therefore, it may be contained as long as it is 0.2% by mass or less. Furthermore, these components are 1.0% by mass or less in total (excluding S). With such a content, the desired effect of the present invention is not affected.

(Manufacturing method of ferritic stainless steel)
The steel material (ferritic stainless steel material) having the chemical composition of ferritic stainless steel according to the present invention can be easily obtained by appropriately adjusting the ratio of raw materials by melting, and the obtained ingot is obtained as necessary. After adjusting the shape by soaking (homogenization heat treatment) or hot working, an appropriate solution treatment is performed. About soaking, if it is an agglomeration method, solidification segregation will be eliminated by hold | maintaining for about 10 hours, for example in the range of 1250-1300 degreeC, and when it produces by continuous casting, it can also be a shorter time or abbreviate | omitting. Hot working can be carried out in a state of being heated to approximately 1000 ° C. or higher. The steel material obtained in this way can be appropriately controlled by combining cold working and heat treatment as an intermediate process, and the crystal grain size after heat treatment can be refined by adding cold working.・ It may be easy to homogenize. In the production of the ferritic stainless steel material according to the present invention, the final process is a heat treatment for solution treatment regardless of the presence or absence of an intermediate process. The solution treatment is preferably held at a temperature of 1100 ° C. or higher for 60 seconds or more, and then cooled from the holding temperature to 300 ° C. or lower within 30 seconds. As such a cooling method, water cooling is preferred to air cooling more easily than air cooling.

  As mentioned above, although the form for implementing this invention has been described, the Example which confirmed the effect of this invention is demonstrated concretely compared with the comparative example which does not satisfy | fill the requirements of this invention below. It should be noted that the present invention is not limited by this embodiment, and can be implemented with modifications within the scope shown in the claims, all of which are included in the technical scope of the present invention.

[Test material preparation]
A steel material having the chemical composition shown in Table 1 was melted and subjected to a soaking treatment at 1280 ° C. for 8 hours on a 20 kg ingot melted in a vacuum melting furnace (VIF), and then heated to a width of 60 mm and a thickness of 20 mm. Inter-forged. Further, after holding at 1200 ° C. for 5 minutes, a soaking water cooling solution treatment was performed to obtain a base material.

[Evaluation]
(Room temperature ductility)
As an evaluation of room temperature ductility, tensile elongation at break was measured. A 14A round bar tensile test piece described in JIS Z 2241 was prepared from the base material. The test piece had a gauge portion of φ6 mm × 30 mm. A tensile test was performed at room temperature in accordance with JIS Z 2241, and the tensile strength at break (room temperature tensile elongation at break) was measured by matching the fracture specimens. Table 1 shows the room temperature tensile elongation at break. The acceptance criterion is that the room temperature tensile elongation at break is 15% or more.

(High temperature creep strength)
As an evaluation of the high temperature creep strength, the amount of creep deformation (creep strain) was measured. The allowable stress of a heat-resistant material is determined by the creep strain that occurs at a rupture time or a fixed time. ing. In the creep test, the deformation process from loading to fracture includes steady creep deformation and subsequent accelerated creep deformation. Since it is large, there is a tendency for large deformation to proceed locally without breaking. In an actual heat-resistant member, such local deformation causes various problems, and creep strain generated in a certain time may be an important evaluation parameter. Against this background, in this example, creep strain was continuously measured, and the minimum creep strain rate corresponding to the creep strain generated in a certain time was determined and used as an evaluation index.

From the base material, a creeped creep test piece having a gauge portion size of φ6 mm × 30 mm was produced. The creep rate test was conducted at 650 ° C. and 80 MPa, and the elongation between the gauges of the creep test piece was continuously measured with a linear gauge. Then, the creep strain rate is calculated based on the slope of the creep strain with respect to the test time from the relationship between the test time and the creep strain shown in FIG. 1 (a), and the transition of the creep strain rate is obtained as shown in FIG. 1 (b). The minimum value was taken as the minimum creep strain rate. Table 1 shows the minimum creep strain rate. The acceptance criterion is that the minimum creep strain rate is 1.0 × 10 ⁻⁸ sec ⁻¹ or less.

  As shown in Table 1, the steel material No. Nos. 1 to 11 were examples in which the chemical composition was within the range of the present invention, and both room temperature ductility and high temperature creep strength were good.

  On the other hand, the steel material No. Nos. 12 to 15 are comparative examples in which the chemical composition is outside the scope of the present invention, and at least one of room temperature ductility and high temperature creep strength is unacceptable. Steel No. No. 12 has a Zr content of Steel No. No. 13 has a Si content of steel no. No. 14 was insufficient in Ni content, so that the high temperature creep strength was insufficient. Steel No. In No. 15, since the Si content was excessive, the high temperature creep strength was good, but the hardness increased and the room temperature ductility decreased.

Claims (1)

  C: 0.05 mass% or less, Si: 0.1-3.0 mass%, Mn: 0.1-2.0 mass%, Cr: 13.5-16.5 mass%, Zr: 0.15 -1.2 mass%, Ni: 1.0-4.0 mass%, Al: 0.5-2.0 mass%, Cu: 0.5-3.0 mass%, B: 0.0002-0 0.005% by mass, N: 0.05% by mass or less, P: 0.005 to 0.05% by mass, the balance being Fe and inevitable impurities, and S contained as the inevitable impurities being 0.005 Ferritic stainless steel characterized by being limited to mass% or less.

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