JP2007277628A - Austenitic stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel exhibiting excellent oxidation resistance even in use under the conditions in an oxidizing atmosphere under a high temperature environment heated to ≥1,000°C. <P>SOLUTION: The austenitic stainless steel has a composition comprising 0.04 to 0.15% C, 1.5 to 3.0% Si, ≤2.0% Mn, ≤0.04% P, ≤0.03% S, 15 to 30% Cr, 8 to 15% Ni, 0.05 to 0.20% Al, 0.001 to 0.010% B, 0.15 to 0.30% N and 0.01 to 0.10% Ca and/or REM, and the balance Fe with impurities, and in which Ni(bal) prescribed by a formula, Ni(bal)=30×(C+N)+Ni+0.5×Mn-1.1×(Cr+1.5×Si)+8.2, is -1.0 to +2.0, and Al and Ni satisfy an inequality of log(Al×N)≥-1.845. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an austenitic stainless steel, for example, an austenitic stainless steel exhibiting excellent oxidation resistance even when used in an oxidizing atmosphere under a high temperature environment of 1000 ° C. or higher.

In particular to reach the recently, NOx in the perspective various emissions from a gas to protect the global environment, SOx further reducing the concentration of harmful gases such as CO ₂ have been strongly desired. On the other hand, the necessity of efficient energy use has been emphasized from the viewpoint of effective utilization of fossil energy resources. In order to satisfy both of these demands, there is an increasing need to operate at a higher temperature exceeding 1000 ° C in each industrial field such as thermal power generation, chemical industry, and steel manufacturing. The material is required to have better oxidation resistance.

  Conventionally, austenitic stainless steel is frequently used for such high temperature applications. For example, there are high Cr-high Ni steels such as 18Cr-8Ni type represented by SUS304, 25Cr-20Ni type represented by SUS310S, and 20Cr-32Ni steel known as Alloy 800. Moreover, AlSI302B, JISXM15J1, AISI314 steel, etc. are known as stainless steels whose high temperature characteristics are improved by increasing the Si content.

  In general, the 18Cr-8Ni system is excellent in weldability and economy, but is inferior in high temperature characteristics such as oxidation resistance and high temperature strength. High Cr-high Ni steel can ensure high temperature strength, but its oxidation resistance in an environment exceeding 1000 ° C. is poor. Further, from the viewpoint of cost, a high Ni content is a problem.

  In the past, in order to improve the high-temperature characteristics of materials for high-temperature devices, for example, the inventions disclosed in the publications of Patent Documents 1 to 22 are known. In the inventions disclosed in Patent Documents 1 to 22, there are many things that can improve the high-temperature characteristics by increasing the Si content, and by addition of alloy elements such as Mo, Cu, N, Ti, and Nb. Some say it can be improved.

Furthermore, Patent Document 23 discloses an invention for improving oxidation resistance in a high temperature environment of 800 ° C. or higher.
JP-A-53-90167 Japanese Patent Publication No.53-43370 Japanese Patent Publication No.54-12890 Japanese Patent Publication No.54-33207 Japanese Patent Publication No. 56-17424 Japanese Patent Publication No.56-25507 Japanese Patent Publication No.57-16187 Japanese Patent Publication No.57-42701 Japanese Patent Publication No.57-54543 Japanese Patent Publication No.57-59299 Japanese Patent Publication No.58-2268 Japanese Patent Publication No. 58-42264 JP 59-185663 A JP-A-60-92454 JP-A-63-69949 Japanese Patent Laid-Open No. 63-213643 JP 63-69950 A JP-A-63-69951 JP-A 63-157840 Japanese Patent Laid-Open No. 63-213643 Japanese Patent Publication No. 1-8695 JP-A-1-159351 JP-A-8-319541

  As disclosed in Patent Documents 1 to 22, it is possible to significantly improve the oxidation resistance by increasing the Si content. However, in the use in an environment exceeding 1000 ° C., the oxidation resistance Sex is not enough. Further, even the invention disclosed in Patent Document 23 has a problem in oxidation resistance in an oxidizing atmosphere exceeding 1000 ° C. and has not been put into practical use.

  As a result of intensive studies in order to solve the above problems, the inventor of the present invention disclosed in Patent Document 23 defines the relationship between the Ni balance and the (Al × N) product, thereby increasing the temperature to 1000 ° C. or higher. The inventors have found that excellent oxidation resistance can be obtained even under use in an oxidizing atmosphere under the environment, and have further studied and completed the present invention.

  In the present invention, C: 0.04 to 0.15% (in the present specification, “%” in terms of composition means “mass%” unless otherwise specified), Si: 1.5 to 3.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 15-30%, Ni: 8-15%, Al: 0.05-0.20%, B : 0.001 to 0.010%, N: 0.15 to 0.3%, Ca and / or REM: 0.01 to 0.10%, balance Fe and impurities, defined by the formula (1) The austenitic stainless steel is characterized in that Ni (bal) is -1.0 to +2.0 and Al and N satisfy the formula (2). In addition, the element symbol in (1) and (2) type | formula shows content (mass%) of the element, respectively.

Ni (bal) = 30 × (C + N) + Ni + 0.5 × Mn−1.1 × (Cr + 1.5 × Si) +8.2 (1)
log (Al × N) ≧ -1.845 (2)
The austenitic stainless steel according to the present invention preferably further contains Cu: 2.0% or less and / or Mo: 1.0% or less.

  According to the present invention, it is possible to provide an austenitic stainless steel that exhibits excellent oxidation resistance even when used in an oxidizing atmosphere under a high temperature environment of 1000 ° C. or higher.

Hereinafter, the best mode for carrying out the austenitic stainless steel according to the present invention will be described.
First, the composition of the austenitic stainless steel of the present embodiment, the Ni balance value, the relationship between Al and N, and the initial crystal grain size will be described.
C: 0.04% or more and 0.15% or less C is an austenite-forming element, and is an element effective for promoting the stabilization of the austenite structure and increasing the high-temperature strength and creep rupture strength of the parent phase. Since the reduction of C causes a reduction in steel plate strength, the lower limit of the C content is 0.04% particularly in order to ensure the strength in a high temperature environment exceeding 1000 ° C. On the other hand, when the C content exceeds 0.15%, hot workability is impaired. Therefore, in the present invention, the C content is limited to 0.04% or more and 0.15% or less. A preferable range is 0.06% or more and 0.12% or less.
Si: 1.5% or more and 3.0% or less Si is added as a deoxidizer during the melting of steel, but is a ferrite-forming element and an effective element for precipitation of δ-ferrite in the austenite phase. . Moreover, since the Si type internal oxide is formed in the interface of the oxide film produced | generated and an alloy and the adhesiveness of an oxide film is improved and corrosion resistance is improved, the one where the Si content is higher is desirable. In particular, the lower limit of the Si content is set to 1.5% in order to improve crystal grain growth suppression and oxidation resistance due to δ-ferrite precipitation in a high temperature environment exceeding 1000 ° C. However, when the Si content exceeds 3.0%, the weldability and hot workability deteriorate. Therefore, in the present invention, the Si content is limited to 1.5% or more and 3.0% or less. A preferable range is 1.8% or more and 2.5% or less.
Mn: 2.0% or less Mn is an austenite forming element and is contained as an alternative to Ni together with stabilization of the austenite structure. However, if the Mn content exceeds 2.0%, the amount of Mn contained in the oxide film at the initial stage of heating increases and the steam oxidation resistance is deteriorated. Therefore, the Mn content is limited to 2.0% or less. A preferable range is 1.2% or less.
P: 0.04% or less P is one of elements that segregate in steel and impair weldability and hot workability, so the P content is limited to 0.04% or less. Preferably it is 0.02% or less.
S: 0.03% or less S, like P, segregates in steel and inhibits weldability and hot workability. Therefore, the upper limit of the S content is 0.03%. Preferably it is 0.01% or less.
Cr: 15% or more and 30% or less Cr is an element effective for precipitation of δ-ferrite in an austenite phase and an element effective for improving oxidation resistance, high temperature wear resistance and creep strength. is there. If the Cr content is less than 15%, such an effect cannot be obtained. On the other hand, if it exceeds 30%, the hot workability is deteriorated. Therefore, in the present invention, the Cr content is limited to 15% or more and 30% or less. In particular, the Cr content is preferably 24% or more and 26% or less in order to suppress the growth of crystal grains due to δ-ferrite precipitation and improve the oxidation resistance in a high temperature environment exceeding 1000 ° C.
Ni: 8% or more and 15% or less Ni is an austenite forming element, and is an important element for stabilizing the austenite structure and improving oxidation resistance and creep strength. If the Ni content is less than 8%, such an effect cannot be obtained. On the other hand, if the Ni content exceeds 15%, weldability is impaired and coarse Cr ₂ N precipitation is promoted during use at high temperatures. , Creep rupture strength decreases. Therefore, the Ni content is limited to 8% or more and 15% or less. Preferably they are 11% or more and 15% or less.
Al: 0.05% or more and 0.20% or less Al is an element that improves the oxidation resistance of the alloy in the same manner as Cr and Si, and is also contained as a deoxidizer during the melting of steel as in Si. . Al is also finely dispersed in the steel as AlN fine particles formed by combining with N in the steel, so that the crystal grains are refined, especially for the coarsening of the grains that occurs in a high temperature environment exceeding 1000 ° C. Therefore, it is an effective element for nailing the suppression of crystal grain growth. There are Ti and Nb as similar elements, but the compounds Ti (C, N) and Nb (C, N) with C and N in steel are effective in an environment of 900 ° C. or lower. It is not an effective element that causes abnormal oxidation due to decomposition of the compound.

In the coexistence of N of 0.30% or less, if the Al content is less than 0.05%, the amount of AlN produced is insufficient for staking the growth of crystal grains, while the content exceeding 0.20% is a surface defect. Increase. Therefore, in the present invention, the Al content is limited to 0.05% or more and 0.20% or less. Preferably they are 0.05% or more and 0.10% or less.
B: 0.001% or more and 0.010% or less B is contained in 0.001% or more, so that when hot working is performed in a state where the creep strength and the ferrite and austenite phases coexist, B becomes a grain boundary. Since it precipitates preferentially, it is an element effective in improving hot workability by increasing the bonding strength between different phases. However, when the B content exceeds 0.010%, hot workability is inhibited by forming an intermetallic compound. Therefore, the B content is limited to 0.001% or more and 0.010% or less. A preferable range is 0.001% or more and 0.007% or less.
N: 0.15% or more and 0.3% or less N is an austenite forming element, and is an element effective for stabilizing the austenite structure and improving the creep strength. In addition, Al is finely dispersed as AlN in the alloy matrix due to the inclusion of Al, and has a nailed action of suppressing the growth of crystal grains against the coarsening of crystal grains particularly occurring in a high temperature environment exceeding 1000 ° C. In the coexistence of Si of 1.5% or more and C of 0.15% or less, if it is less than 0.15%, it does not contribute to the improvement of creep strength, and is insufficient for the finely dispersed precipitation of AlN. If the content exceeds 0.3%, not only a significant improvement in creep strength is observed, but also the effect of suppressing the growth of crystal grains due to the precipitation of δ-ferrite cannot be expected, and hot workability is further inhibited. Therefore, the N content is limited to 0.15% or more and 0.3% or less.
Ca and / or REM: 0.01% or more and 0.10% or less Cr ₂ O ₃ , SiO ₂ and Al ₂ O ₃ are dense but brittle and have poor adhesion. Inclusion of rare earth elements (REM) such as Ca, Y, La, and Ce is effective for improving the mechanical properties (plastic deformability, etc.), crack resistance and adhesion of these oxides. These elements contain one or more of these elements. If the content of these elements is less than 0.01% in total, the effect cannot be exhibited. On the other hand, the content exceeding 0.10% inhibits hot workability and weldability. Therefore, Ca and / or REM is contained in a total of 0.01% or more and 0.10% or less.
Cu: 2.0% or less, and / or Mo: 1.0% or less In the present invention, Cu and Mo are both optional added elements and are contained as necessary.

  Cu is an austenite-forming element and is an element effective for stabilizing the austenite structure and improving the creep strength. In the present invention, Cu is contained as needed in the form of replacing part of Ni. When the content exceeds 2.0%, hot workability and weldability are significantly impaired. Therefore, when Cu is contained, the content is desirably limited to 2.0% or less. On the other hand, if the Cu content is 0.05% or more, the above-described effects can be obtained with certainty, so it is more preferably 0.05% or more and 2.0% or less, and even more preferably 0.10% or more and 1%. .80% or less.

On the other hand, Mo is a ferrite forming element and is an effective element for precipitation of δ-ferrite into the austenite phase. Mo is effective for improving the corrosion resistance, and becomes carbonitride during high temperature holding and is finely dispersed to increase the high temperature strength. If the content exceeds 1.0%, there is a danger that a low-melting and highly volatile oxide MoO ₃ is produced at a high temperature of 1000 ° C. or higher, and a hard and brittle σ phase in the temperature range of 800 to 900 ° C. Increases the precipitation sensitivity. Therefore, when Mo is contained, the content is desirably limited to 1.0% or less. On the other hand, if the Mo content is 0.01% or more, the above-described effects can be obtained with certainty, and more preferably 0.01% or more and 1.0% or less.

The balance other than these is Fe and impurities.
Ni (bal) defined by the formula (1): -1.0 to +2.0
Ni (bal) = 30 × (C + N) + Ni + 0.5 × Mn−1.1 × (Cr + 1.5 × Si) +8.2 (1)
The element symbol in the formula (1) indicates the content (% by mass) of each element.

The Ni balance value defined by the formula (1) indicates the stability of the austenite phase in the solidified structure metallurgically, but means that the δ-ferrite phase is formed in the austenite phase when the Ni balance value decreases. . The δ-ferrite phase finely precipitated in the austenite phase acts as a nail that suppresses the refinement of crystal grains and the coarsening of austenite grains in the temperature range exceeding 1000 ° C, and Cr passes through the grain boundaries. By promoting the diffusion to the direction, Cr ₂ O ₃ is generated at an early stage, and there is an effect of improving the oxidation resistance. Further, moderate precipitation is effective in improving weldability and hot workability because P and S are absorbed by δ-ferrite. However, excessive precipitation reduces high temperature strength and creep strength. On the other hand, when the Ni balance value is increased, an austenite single phase structure is formed, and P and S are segregated at the austenite grain boundaries, so that weldability and hot workability are deteriorated. Therefore, in the present invention, considering the high temperature characteristics, weldability and hot workability, the range of the Ni balance value is set to -1.0 or more and +2.0 or less.
Al and N satisfy the formula (2)
log (Al × N) ≧ -1.845 (2)
Each element symbol in the formula (2) indicates the content (% by mass) of the element.

The left side of equation (2) represents the solubility product of AlN, and a large value indicates that AlN is stable at high temperatures. That is, the effect of suppressing the growth of austenite crystal grains is increased by the effect of nailed AlN precipitates. If this value is −1.845 or more, AlN is stably present even at a high temperature exceeding 1000 ° C., and the coarsening of austenite crystal grains in a temperature range exceeding 1000 ° C. is suppressed similarly to the δ ferrite phase. It acts as a nail, promotes the diffusion of Cr outward through the grain boundaries, and has the effect of improving the oxidation resistance by early generation of Cr ₂ O ₃ . Preferably it is -0.173 or more.
Initial crystal grain size: 40 μm or more and 200 μm or less From the viewpoint of high temperature strength and creep strength, the crystal grain size should be large to some extent. On the other hand, a fine grain structure is advantageous for high temperature corrosion resistance and high temperature oxidation resistance. Considering high-temperature strength and creep strength, from the viewpoint of ensuring both high-temperature corrosion resistance and high-temperature oxidation resistance, the solution heat treatment after hot rolling is performed so that the average grain size inside the alloy is 40 μm or more and 200 μm or less. It is desirable to use a steel plate adjusted to.

The composition of the austenitic stainless steel of the present embodiment, the Ni balance value, the relationship between Al and N, and the initial crystal grain size are as described above.
Next, the reason why the austenitic stainless steel of the present embodiment can provide excellent oxidation resistance even when used in an oxidizing atmosphere under a high temperature environment of 1000 ° C. or higher will be described.

The cause of deterioration of oxidation resistance in the temperature range exceeding 1000 ° C is that the formation of an oxide film formed on the substrate interface and the coarsening of the crystal grains proceed at the same time. It is thought that it is because a crack or peeling phenomenon is induced and a stable protective film is not obtained. That is, since the crystal grains are coarsened in a high temperature environment exceeding 1000 ° C., the delay of Cr ₂ O ₃ generation due to the delay of the outward diffusion through the Cr grain boundary due to this, the coarseness of the crystal grains Oxidation resistance cannot be obtained because of the occurrence of cracks and peeling in the oxide film produced by the conversion.

  Therefore, if the growth of crystal grains at the base interface can be suppressed while the Cr-rich oxide film is formed and stably adhered to the base interface, thermal expansion due to the coarsening of the oxide film and crystal grains generated at the base interface is possible. Due to the difference in stress caused by the difference, it is possible to prevent rapid abnormal oxidation from progressing due to the occurrence of cracking or peeling of the oxide film, and to maintain and improve oxidation resistance. At the same time, the refinement of the crystal grains promotes the outward diffusion of Cr through the crystal grain boundaries, and is thus effective for early generation of a Cr-rich oxide film on the surface layer portion. For this reason, in a temperature range exceeding 1000 ° C., it is possible to improve oxidation resistance and obtain excellent high-temperature oxidation characteristics by maintaining the refinement of the crystal grains and suppressing the coarsening. In order to maintain the refinement of the crystal grains and suppress the growth, it is effective to finely disperse the second phase that is stable and does not decompose even in a high temperature region in the metal crystal.

Since the austenitic stainless steel of the present embodiment has the above-described composition, Ni balance value, relationship between Al and N, and the initial crystal grain size,
(A) δ-ferrite can be precipitated in the austenite grain boundary as a second phase, thereby making it possible to prevent grain refinement and coarsening, and (b) metal with AlN as the second phase. It can be finely dispersed and precipitated in the structure, thereby preventing crystal grains from becoming finer and preventing coarsening, thereby improving oxidation resistance even under conditions of an oxidizing atmosphere in a high temperature environment exceeding 1000 ° C. Improved high-temperature oxidation characteristics, specifically, the dimensions (plate thickness, width, length) and weight of three test pieces are measured in advance before heating, as will be described in the examples described later. In addition, the corrosion weight loss obtained as an average value obtained by converting the difference from the weight measurement value after performing the continuous heating at 1200 ° C. for 200 hours in the air atmosphere and descaling the test piece after the continuous heating is converted per unit surface area. But, An excellent oxidation resistance at a high temperature, which has never been seen before, is in the range of 50 mg / cm ² or less.

Furthermore, the present invention will be described more specifically with reference to examples.
(1) Production of test material A 17Kg steel ingot melted by a high-frequency electric furnace (vacuum melting) was hot-forged to a plate thickness of 20 mm, then cold-rolled to a plate thickness of 14 mm, water-cooled after heat treatment at 1100 ° C for 30 minutes By doing this, the test piece for evaluation which consists of this invention steel (or comparative steel) which has the chemical composition (mass%, remainder Fe) shown in Table 1 was manufactured.

(2) Oxidation resistance test method and results These test pieces were tested by the following test methods, and the following results were obtained.
(A) Test method Continuous heating at 1200 ° C. for 200 hours in an air atmosphere was performed to measure the weight loss of the corrosion and evaluate the oxidation resistance. Corrosion weight loss is determined by measuring the dimensions (plate thickness, width, length) and weight of three test pieces in advance before heating, and measuring the weight after descaling the test piece after completion of continuous heating. Was obtained as an average value converted per unit surface area. The results are summarized in Table 2.

(B) Test result:
(I) Regarding the effect of suppressing the coarsening of crystal grains by the addition of Al. 7 and Sample No. in which Al was not actively added. 8 and 9 were examined for the metallographic structure of the specimens obtained by performing a heat treatment of holding at 1200 ° C. for 30 minutes and then water cooling. As a result, sample no. Sample No. 7 in which the crystal grain size hardly changed, whereas Al was not positively added. In both 8 and 9, crystal grains were extremely coarse.

From the above results, it can be seen that the coarsening of crystal grains in the growth temperature range of crystal grains can be effectively suppressed by the fine dispersion and precipitation of AlN produced by the addition of Al in the metal structure.
(Ii) Oxidation resistance test results Table 3 shows the measurement results of the average corrosion weight loss in the 200-hour atmospheric oxidation test at 1200 and 1250 ° C continuous heating.

  From the results shown in Table 3, sample No. 1 to which Al was positively added was obtained. No. 7 shows the effect of suppressing the coarsening of crystal grains due to AlN fine precipitation, so that the average corrosion weight loss is the sample No. 7 in which Al was not actively added. It is about 1/2 compared to 8 and 9, and there is almost no change in weight at 1200 and 1250 ° C., indicating that good oxidation resistance can be secured.

  Further, it can be seen that a better result can be obtained when the heat treatment is performed on the higher temperature side in relation to the heat treatment temperature and the average corrosion weight loss. This is because the rate of grain growth of the crystal grains being heated can be kept lower by heat treatment at a higher temperature.

  From the above results, the finely dispersed precipitation of AlN in the metal structure due to the addition of Al effectively works to maintain fine grains and suppress coarsening of the grains directly under the oxide film in an oxidizing atmosphere in the grain growth temperature range. It can be seen that it acts effectively on the early generation of the oxide film and stabilization of adhesion.

Claims (2)

In mass%, C: 0.04 to 0.15%, Si: 1.5 to 3.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 15-30%, Ni: 8-15%, Al: 0.05-0.20%, B: 0.001-0.010%, N: 0.15-0.3%, Ca and / Or REM: 0.01 to 0.10%, balance Fe and impurities, Ni (bal) defined by the formula (1) is -1.0 to +2.0, and Al and N are (2 Austenitic stainless steel characterized by satisfying the formula
Ni (bal) = 30 × (C + N) + Ni + 0.5 × Mn−1.1 × (Cr + 1.5 × Si) +8.2 (1)
log (Al × N) ≧ -1.845 (2)
The element symbols in the expressions (1) and (2) indicate the content (% by mass) of the element. The austenitic stainless steel according to claim 1, further comprising, by mass%, Cu: 2.0% or less and / or Mo: 1.0% or less.