JP2012211379A - FERRITIC STAINLESS STEEL EXCELLENT IN SECONDARY WORKABILITY AND Cr EVAPORATION RESISTANCE - Google Patents

Abstract

【the purpose】
Improve secondary work brittleness and Cr evaporation resistance of ferritic stainless steel
In mass%, C: 0.03% or less, Si: 0.3-2.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 0.2% or less, Cr: 16 to 20%, N: 0.03% or less, Al: 0.8 to less than 2.0%, Nb: 8 (C + N) to 0.6% or less, B: 0.0. Secondary workability containing 0005 to less than 0.01, consisting of the balance Fe and inevitable impurities, having a transition temperature in the drop weight test of −20 ° C. or lower, and satisfying the following formula (1): Ferritic stainless steel with excellent Cr evaporation resistance.
24 ≦ Cr mass% + 1.5 × Si mass% + 6 × Al mass% <30 (1)
[Selection figure] None

Description

  The present invention relates to an environment exposed to 1% or more oxygen and 5% or more water vapor at a high temperature of 600 ° C. or higher, for example, a solid oxide fuel cell that is a system that uses heat as energy, and high-temperature reforming of a fuel cell. Materials used for exhaust gas flow path parts of equipment, micro gas engines, micro gas turbines, exhaust gas power generation and other cogeneration systems, various high-temperature combustion equipment, and automobiles, dramatically reducing the amount of hexavalent chromium that evaporates in the atmosphere In particular, the present invention relates to a ferritic stainless steel excellent in secondary work brittleness and Cr evaporation resistance, which is particularly suitable for a member that requires severe secondary work.

In recent years, due to problems such as the depletion of fossil fuels typified by petroleum and the global warming phenomenon caused by CO ₂ emissions, there has been a demand for practical use of a new system that replaces the conventional power generation system. Various energy-saving systems used at high temperatures are being put into practical use as power sources for new power generation systems, distributed power supplies or automobiles.

In addition, there are a wide range of devices and parts that are exposed to an atmosphere containing oxygen and water vapor at a high temperature of 600 ° C. or higher, such as a combustion tube of a gas burner, a combustion part of an automobile, and a part exposed to these exhaust gas atmospheres at a high temperature. Over.

In general, in the combustion of a gas burner, an engine, etc., it is normal that fuel is burned with oxygen and a large amount of water vapor is generated. Moreover, since the member which hits the exhaust gas flow path of these combustion equipment was produced | generated by combustion but gas passes, it becomes the atmosphere where oxygen and water vapor | steam of 600 degreeC or more are mixed.
Usually, ferritic or austenitic stainless steels or high alloys are used for such members. From the standpoint of equipment and equipment durability, selecting the right stainless steel is not a difficult problem. There wasn't.

  In recent years, as a result of examining these combustion exhaust gases, a slight amount of hexavalent chromium is present in the water vapor present in these exhaust gases, and as a cause thereof, a part of the chromium contained in the alloy components in stainless steel is excessive. It has become clear that this is because it is oxidized and evaporated. As a steel to solve this problem, in Japanese Patent Application Laid-Open No. 2009-167443, Cr: 11-22 mass%, C: 0.03 mass% or less, N: 0.03 mass% or less, Mn: 1.5 mass % Or less, S: 0.008 mass% or less, Si: 2 mass% or less, Al: 1.0 to 6.0 mass% or less and Cr mass% + 1.5 × Si mass% + 6 × Al mass% ≧ 30 An Al-containing ferritic stainless steel is disclosed.

  However, the exhaust gas path members of automobiles, various combustors, and the parts constituting the fuel cell often have very complicated structures and product shapes, and particularly the temperature sensors used in such parts are stretched. It is important not to cause cracking even in overhanging performance, secondary workability, especially processing at low temperatures in winter. In this regard, in manufacturing the above-described site, the steel described in JP-A-2009-167443 is not always satisfactory in terms of secondary workability.

JP 2009-167443 A

  The present invention has been made to improve the secondary work brittleness and Cr evaporation resistance of ferritic stainless steel.

The present invention is an Fe-Cr-Al-Si ferrite system that is excellent in Cr evaporation resistance in an environment exposed to 1% or more oxygen and 5% or more water vapor at a high temperature of 600 ° C or higher and excellent in secondary work brittleness It is characterized by providing stainless steel.
The specific configuration of the present invention is as follows.
In mass%, C: 0.03% or less, Si: 0.3 to 2.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 0.2% or less, Cr: 16 to 20%, N: 0.03% or less, Al: 0.8 to less than 2.0%, Nb: 8 (C + N) to 0.6% or less, B: 0.0. It is a ferritic stainless steel having an excellent secondary workability and Cr evaporation resistance satisfying the following formula (1) that is less than 001 to less than 0.01.
24 ≦ Cr mass% + 1.5 × Si mass% + 6 × Al mass% ≦ 30 (1)
If necessary, this steel is Ti: 0.05 or less, Mo: 0.05-0.5%, Cu: 0.05-0.5%, V: 0.05-0.5%, Zr: 0 Chromium in the oxide layer when it is analyzed by WDS or EDX from the surface layer, if necessary, comprising 0.05 to 0.5% of one or more types, the balance being Fe and unavoidable impurities An oxide layer having a concentration of 5% by mass or less is formed on the surface layer.

  In the ferritic stainless steel of the present invention, hexavalent chromium from steel is added by adding a predetermined amount or more of Cr, Al, and Si, and preferably suppressing the Cr concentration of the oxide formed on the surface layer to 5% by mass or less. Can be remarkably suppressed. In addition to this, in this steel, the secondary workability, which was a problem of the conventional Al-containing steel, has been greatly improved. Therefore, this material can be applied to parts with a broader and more complex shape than conventional parts for high-temperature parts such as various energy supply systems, combustion systems, cogeneration systems, and automobile exhaust systems. It is expected that a clean flue gas environment will be obtained.

Stainless steel containing 11% by mass or more of chromium is a metal material with a slow growth of an oxide film to be produced and less material deterioration at high temperatures. However, in an environment exposed to oxygen and water vapor, such as a combustion exhaust gas atmosphere of 600 ° C. or higher, the chromium in steel is produced by the reaction of chromium oxide generated on the steel surface with oxygen and water vapor, for example, according to the following reaction formula. Is produced in a hexavalent form and evaporates with water vapor.
Cr ₂ O ₃ (solid) + 2H ₂ O + 1.50 ₂ → 2CrO ₂ (OH) ₂ (gas)
In order to suppress this reaction, it is most effective to prevent the chromium-based oxide from being generated on the surface layer as much as possible and to block the atmosphere from chromium or chromium oxide. This mechanism and effect are as described in JP-A-2009-167443.

Al forms a very dense Al ₂ O ₃ film on the surface of the steel material, so by rapidly forming an alumina film, it not only improves high-temperature oxidation resistance but also suppresses Cr evaporation at the initial stage of oxidation. can do. Further, in addition to the effects of suppressing the occurrence of abnormal oxidation and preventing the peeling of the oxide film, it has an effect of improving red heat. However, the addition of excess Al or Si must be limited to an amount that does not cause deterioration from the viewpoints of manufacturability and workability because the toughness of the material is greatly deteriorated.

In the present invention, by adding Nb and B in combination and suppressing the addition of Ti, good Cr evaporation resistance can be obtained even in a state where the addition amount of Al or Si that reduces the secondary workability is reduced. I found that I could secure it. The reason for this has not been clarified yet, but as one of the effects of B, diffusion segregation at the grain boundary preferentially over P or S which inhibits the formation of the Al ₂ O ₃ film makes it possible to improve oxidation resistance and It is presumed that the diffusion of P and S, which are harmful to the next processability, to the grain boundary can be suppressed. For this reason, it is considered that not only the secondary workability but also the oxidation resistance is greatly improved. Furthermore, by adding Nb, which has a slightly higher affinity for oxygen than chromium and a lower affinity than aluminum, Cr oxidation is performed at the initial stage of oxidation while minimizing the amount of Si that provides the same effect. It is possible to suppress the formation of objects on the surface layer. On the other hand, when Ti having an affinity for oxygen close to Al is added, the formation of alumina in this case competes with the formation of oxide of Ti, and the formation of the alumina film is relatively hindered. It was found that some Cr oxide was formed at the initial stage, which deteriorated chromium evaporation resistance. As a result of examining the appropriate range of each element based on this knowledge, by adding Nb and B in combination and limiting the addition amount of Ti, a value smaller than that of JP2009-167443A, that is, 24 ≦ It was found that the chromium evaporation was sufficiently suppressed by regulating Cr mass% + 1.5 × Si mass% + 6 × Al mass% <30, and the present invention was achieved. The reasons for limiting each element are described below.

C: 0.03 mass% or less When the C content is high, abnormal oxidation tends to occur. Moreover, in high Al content ferritic stainless steel, when C content becomes high, the toughness of a slab and a hot coil will deteriorate, and manufacturability will deteriorate. Therefore, the upper limit of the C content is limited to 0.03% by mass or less.

Si: 0.3-2.0 mass%
Si has the effect of suppressing the generation of red scale in ferritic stainless steel. Therefore, the Si content must be added at 0.3% or more. However, excessive addition deteriorates toughness and workability. Therefore, Si addition amount is limited to 0.3 to 2.0 mass% or less.

Mn: Less than 0.5% by mass Mn generates a Mn-based oxide, inhibits formation of a dense Al oxide layer, and adversely affects high-temperature oxidation resistance. Therefore, in order to maintain high temperature oxidation resistance, the Mn content is limited to 0.5% or less. Preferably, it is less than 0.5% by mass.

P: 0.04% by mass or less P has an adverse effect on high-temperature oxidation resistance and toughness of hot-rolled sheet, so its content is limited to 0.04% by mass or less.

S: 0.005 mass% or less S is a component inevitably contained in steel, and remarkably inhibits the formation of an Al ₂ O ₃ film. Therefore, the S content is limited to 0.005% by mass or less.

Cr: 16 to 20% by mass
Cr is a basic and effective element as an element for improving high-temperature oxidation resistance. In order to obtain good high-temperature oxidation resistance, addition of 16% or more is necessary. However, excessive addition degrades the toughness of slabs and hot coils. Therefore, the Cr content is limited to 16 to 20% by mass.

N: 0.03 mass% or less N combines with Al in steel to form AlN, and becomes a starting point for abnormal oxidation. Therefore, the N content is limited to 0.03% by mass or less in order to improve high temperature oxidation resistance.

Al: Less than 2.0% by mass Al, like Cr, is the most important element for suppressing high-temperature oxidation resistance and Cr evaporation, but if excessively contained, slab and hot coil toughness deterioration and product processing In order to raise the secondary work brittle temperature at the time, the upper limit is limited to less than 2.0%. Excellent high-temperature oxidation resistance is obtained by a dense Al oxide formed on the surface of steel when 0.8% or more is added, and is preferably 0.8% by mass or more and less than 2.0%.

Nb: 0.6% by mass or less Nb has an effect of significantly improving toughness by combining with C and N in steel. Addition of Nb increases the high-temperature strength of the steel, promotes the formation of an alumina film for the reasons described above, and relaxes the stress generated during the growth of the oxide film, thereby preventing deformation of the material. In order to obtain these Nb addition effects, it is necessary to add Nb in excess of 8 (C + N) or more. However, if added excessively, the toughness of the steel deteriorates, so the upper limit is limited to 0.6%.

Ti: 0.05 mass% or less It is desirable to suppress the addition amount of Ti as much as possible for the reason described above. However, Ti is also an element that improves the ductility and workability by fixing solute C and N in the steel as carbides. Moreover, the effect which suppresses the grain boundary precipitation of Cr carbide and improves corrosion resistance can also be expected. In order to obtain these effects, an addition amount of Ti of 0.05% by mass or less, preferably less than 0.03% by mass is allowed as necessary.

Rare earth elements: 0.10% by mass or less Rare earth elements: These elements are important elements for improving high-temperature oxidation resistance. Rare earth elements such as La and Ce are thought to improve high temperature oxidation resistance by stabilizing the Al oxide film formed on the surface and improving the adhesion between the matrix and the oxide film. . This effect is effective when a rare earth element is added in an amount of 0.01% by mass or more. However, if added excessively, hot workability and toughness are deteriorated, and inclusions that become the starting point of abnormal oxidation are likely to be generated, and the high-temperature oxidation resistance is lowered. Therefore, the upper limit of addition amount is 0.10% by mass.

Table 1 shows the chemical component values of the test materials. The steel shown in Table 1 was melted in vacuum and subjected to hot rolling, and then annealing and cold rolling were repeated to produce a plate material having a thickness of 0.5 mm. Using the obtained plate material, high temperature oxidation resistance and Cr evaporation resistance were investigated. In addition, Cr evaporation resistance was subjected to an oxidation test of 900 ° C. × 50 h in an atmosphere in which air containing 20% H ₂ O flows at a flow rate of 300 ml / min with the same specimen size as that of high temperature oxidation resistance. According to the method described in 2009-167443, the hexavalent Cr concentration in the condensed water was measured, and the evaporation amount of hexavalent chromium (Cr ⁶⁺ ) after 900 ° C. × 50 h was 0.6 mg / concentration in the condensed water. Those that were 1 or less were evaluated as good (◯), and those that were not so were determined as poor (×). The evaluation sample for secondary processing brittleness was melted and hot-rolled in the same manner as in the preparation of the oxidation test piece, and then a plate material having a thickness of 0.6 mm was prepared by repeating annealing and cold rolling. From the obtained plate material, a blank size was cut out with a diameter of 40 mm and subjected to drawing with a drawing ratio of 2.25. Thereafter, a drop weight test was performed by changing various temperatures using the primary drawn product. In the drop weight test, the weight of the weight was 3 kg, the height of the weight was 100 mm, and the temperature when the crack occurred was defined as the transition temperature.
As seen in the test results of Table 2, all of the steel types of Invention Examples 1 to 13 are excellent in high-temperature oxidation resistance, secondary workability, and Cr evaporation resistance.

  In Comparative Examples Nos. 14 to 27, none of the secondary processing brittleness and / or Cr evaporation resistance is satisfied. Steel types No. 14 to 16 and No. No. 23 satisfies neither the secondary work brittleness nor Cr evaporation resistance. No. Nos. 17-20, 25, 26 and 28 have insufficient secondary processability. 21, 22, 24, and 27 have insufficient Cr evaporation resistance. It should be noted that no. In spite of strict test condition settings such as 14 and 15, such as NG even in the component system considered to be sufficient in terms of Cr evaporation resistance in JP 2009-167443 A, Nb and B are added in combination. In the case where Ti is reduced, no. As shown in Fig. 2, even if Al and Si are reduced to about 1% by mass, the present test conditions are satisfied. In addition, although the film after the test was investigated according to Japanese Patent Application Laid-Open No. 2009-167443, the result completely corresponded to the amount of Cr evaporation, and all the ones with good Cr evaporation resistance were in the oxide film. Those with poor Cr concentration of 5% or less had a Cr concentration in the oxide film exceeding 5%.

  The ferritic stainless steel according to the present invention can remarkably suppress the evaporation of hexavalent chromium. In addition to this, in this steel, the secondary workability, which was a problem of the conventional Al-containing steel, has been greatly improved. For this reason, this material can be applied to parts with a broader and more complex shape than conventional parts for high-temperature parts such as various energy supply systems, combustion systems, cogeneration systems, and automobile exhaust systems. It is expected that a clean flue gas environment will be obtained.

Claims (4)

In mass%, C: 0.03% or less, Si: 0.3-2.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Ni: 0.2% or less, Cr: 16 to 20%, N: 0.03% or less, Al: 0.8 to less than 2.0%, Nb: 8 (C + N) to 0.6% or less, B: 0.0. Secondary workability containing 0005 to less than 0.01, consisting of the balance Fe and inevitable impurities, having a transition temperature in the drop weight test of −20 ° C. or lower, and satisfying the following formula (1): Ferritic stainless steel with excellent Cr evaporation resistance.
24 ≦ Cr mass% + 1.5 × Si mass% + 6 × Al mass% <30 (1)   Ti: 0.05 or less in terms of mass%, Mo: 0.05-0.5%, Cu: 0.05-0.5%, V: 0.05-0.5%, Zr: 0.05-5 The ferritic stainless steel excellent in high-temperature oxidation resistance, secondary workability and Cr evaporation resistance according to claim 1, comprising 0.5% of one kind or two kinds or more and comprising the balance Fe and inevitable impurities.   The secondary workability according to claim 1 or 2, further comprising one or more of REM (rare earth element): 0.001 to 0.05% and Ca: 0.001 to 0.01%. Ferritic stainless steel with excellent Cr evaporation resistance.   The oxide layer is formed in the surface layer, and the chromium concentration in the oxide layer when analyzed by WDS or EDX from the surface layer is 5% by mass or less. Ferritic stainless steel.