JP2004018921A - Ferritic stainless steel which is soft at room temperature and excellent in high temperature oxidation resistance - Google Patents

Abstract

A ferritic stainless steel with improved high-temperature oxidation resistance without impairing workability at room temperature.
SOLUTION: In mass%, C: 0.02% or less, Si: 0.1% or less, Mn: 2.0% or less, Cr: 12.0 to 16.0%, Mo: 1 0.0 to 5.0%, W: more than 2.0%, 5.0% or less, Nb: 5 (C + N) to 1.0% and N: 0.02% or less, with the balance being Fe and unavoidable Is adjusted to the composition range of the target impurity.
[Selection diagram] Fig. 1

Description

【００３４】
【表２】

【００３５】
表２から明らかなように、この発明に従う鋼板はいずれも、加工性はいうまでもなく、優れた耐高温酸化性が得られている。
【００３６】
【発明の効果】
かくして、この発明によれば、室温での加工性を損なうことなしに、耐高温酸化性に優れるフェライト系ステンレス鋼を得ることができる。
従って、この発明によれば、エンジン性能の向上により、排ガス温度が　９００℃を超えるような使途においても、それに耐え得る排気系部材を安定して供給することができる。また、燃料電池関連部材のような高い加工性と耐酸化性が要求される用途にも有利に適合する。
【図面の簡単な説明】
【図１】１４％Ｃｒ−０．０５％Ｓｉ　−０．５％Ｎｂ−１．８％Ｍｏ鋼をベースに、Ｗを種々の割合で添加した時の耐高温酸化性について調べた結果を示したグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is suitable for use in a member used in a high temperature environment such as an exhaust pipe of a car or motorcycle, an outer casing of a catalyst, an exhaust duct of a thermal power plant, or a member related to a fuel cell. It relates to a ferritic stainless steel having excellent properties.
[0002]
[Prior art]
Exhaust system members used in an exhaust system environment of a vehicle, such as an exhaust manifold, an exhaust pipe, a converter case, and a muffler, are required to have excellent moldability and heat resistance. At present, for such applications, Cr-containing steels which are soft at room temperature, have excellent moldability, and have relatively high high-temperature proof strength, such as Nb and Si-added Cr, for example, Type 429 (14Cr-0.9Si-0.4Nb system) Steel is heavily used.
However, this Type 429 steel has a problem that the high-temperature proof stress becomes insufficient when the exhaust gas temperature rises to about 900 ° C., which is higher than the current temperature, due to the improvement in engine performance.
[0003]
To solve the above problem, a Cr-containing steel in which Nb and Mo are added to improve the high-temperature proof stress, and a SUS444 (19Cr-0.2Nb-1.8Mo) steel specified in JIS G4305 have been developed. However, this SUS 444 steel has a problem that workability is low due to low elongation.
[0004]
In consideration of the workability, a Cr-containing steel excellent in high-temperature strength, workability and surface properties as a material applicable over a wide range from a high-temperature portion to a low-temperature portion of an exhaust system member is disclosed in JP-A-2000-73147. Is disclosed. This material is a Cr-containing steel containing C: 0.02 mass% or less, Si: 0.10 mass% or less, Cr: 3.0 to 20 mass%, and Nb: 0.2 to 1.0 mass%. Is reduced to 0.10 mass% or less, thereby suppressing precipitation of Fe ₂ Nb Laves phase, suppressing increase in room temperature yield strength, and imparting excellent high-temperature strength, workability, and good surface properties. [0005]
[Problems to be solved by the invention]
By the way, recently, further improvement in engine performance has been pursued, and the exhaust gas temperature has been getting higher accordingly. However, in the case of conventional materials, oxidation resistance at high temperatures such as 900 ° C. to 1000 ° C. Had left the problem.
In other words, in order to further improve engine performance, a further increase in exhaust gas temperature is inevitable, but when the exhaust gas temperature rises from 900 ° C to a high temperature such as 1000 ° C, any of the current materials are abnormal. Oxidation occurred, causing a problem that it could not withstand actual use.
Here, abnormal oxidation means that when a material is exposed to a high-temperature exhaust gas, an Fe oxide is generated. Since the oxidation speed of the Fe oxide is abnormally high, the oxidation proceeds rapidly, and It refers to the phenomenon of becoming ragged.
[0006]
The present invention advantageously solves the above-mentioned problems, and maintains high ductility at room temperature, while maintaining good workability, and has an oxidation resistance at a high temperature exceeding 900 ° C., that is, a high-temperature oxidation resistance. It is an object of the present invention to propose a ferritic stainless steel having a significantly improved steel.
[0007]
[Hands to solve the problem]
By the way, the present inventors have conducted intensive studies to achieve the above-mentioned object. As a result, in order to maintain the softness at room temperature, the amount of Si is limited to a certain amount or less, and improvement of high-temperature oxidation resistance is required. In order to achieve this, it has been found that the addition of W, particularly the combined addition of Mo and W, is effective.
The present invention is based on the above findings.
[0008]
That is, the gist configuration of the present invention is as follows.
1. In mass%,
C: 0.02% or less,
Si: 0.1% or less,
Mn: 2.0% or less,
Cr: 12.0 to 16.0%,
Mo: 1.0 to 5.0%,
W: more than 2.0%, 5.0% or less,
Nb: 5 (C + N) to 1.0% and N: 0.02% or less, the balance being Fe and unavoidable impurities, characterized by being soft at room temperature and excellent in high-temperature oxidation resistance. Ferritic stainless steel.
[0009]
2. In the above item 1, the total amount of Mo and W is (Mo + W) ≧ 4.3% by mass%.
A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, characterized by satisfying the following conditions.
[0010]
3. In the above 1 or 2, the steel further contains Ti: 0.5% or less by mass%;
A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, having a composition containing at least one selected from Zr: 0.5% or less and V: 0.5% or less.
[0011]
4. In the above 1, 2 or 3, the steel further contains Ni: 2.0% or less by mass%;
Cu: 1.0% or less,
A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, having a composition containing at least one selected from Co: 1.0% or less and Ca: 0.01% or less.
[0012]
5. The ferritic stainless steel according to any one of the above items 1 to 4, wherein the steel further has a composition containing 0.5% or less by mass of Al by mass%, which is soft at room temperature and excellent in high-temperature oxidation resistance. .
[0013]
6. In any one of the above 1 to 5, the steel further contains B: 0.01% or less by mass%;
Mg: A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, characterized by having a composition containing at least one selected from among 0.01% or less.
[0014]
7. The ferritic stainless steel according to any one of the above items 1 to 6, characterized in that the steel further has a composition containing 0.1% or less by weight of REM: soft at room temperature and excellent in high-temperature oxidation resistance. .
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the reason for limiting the component composition to the above range in the present invention will be described. In addition, "%" display about a component shall mean the mass% unless there is particular notice.
C: not more than 0.02% C deteriorates toughness and workability, so its inclusion is preferably reduced as much as possible. From this viewpoint, in the present invention, the C content is limited to 0.02% or less. More preferably, it is 0.008% or less.
[0016]
Si: 0.1% or less Si is effective for improving the oxidation resistance. However, in the present invention, since the addition of W is intended to improve the high-temperature oxidation resistance, it is not particularly necessary in this aspect. Rather, if the amount of Si increases, the strength increases and the workability decreases, so in the present invention, the content is set to 0.1% or less.
[0017]
Mn: 2.0% or less Mn effectively contributes as a deoxidizing agent, but excessive addition forms MnS and lowers the corrosion resistance, so it was limited to 2.0% or less. More preferably, it is 1.0% or less. From the viewpoint of scale peeling resistance, the higher the amount of Mn, the better. Therefore, from this viewpoint, it is preferable to contain 0.3% or more.
[0018]
Cr: 12.0 to 16.0%
Cr is a basic element that improves corrosion resistance and oxidation resistance, but has the disadvantage of increasing strength at room temperature and reducing workability. In the present invention, since the high-temperature oxidation resistance is improved by adding W, the Cr content is set to 16.0% or less from the viewpoint of workability. On the other hand, if the Cr content is less than 12.0%, the corrosion resistance is significantly reduced even if W is added, so the lower limit was set to 12.0%. More preferably, it is in the range of 14.0 to 16.0%.
[0019]
Mo: 1.0 to 5.0%
Mo effectively contributes to improvement of not only high temperature strength but also oxidation resistance and corrosion resistance. Therefore, in the present invention, Mo is contained in an amount of 1.0% or more. However, if the content is too large, the strength at room temperature increases and the workability decreases, so the upper limit was set to 5.0%. More preferably, it is in the range of 1.8 to 2.5%.
[0020]
W: more than 2.0%, 5.0% or less W is an element that is particularly important in the present invention. That is, by adding W in the above-mentioned ferritic stainless steel to which Mo is added, remarkable improvement in high-temperature oxidation resistance can be achieved. Also, it effectively contributes to improvement of high-temperature strength. However, when the W content is 2.0% or less, the effect of the addition is poor. On the other hand, when the W content is more than 5.0%, the cost is increased. Therefore, W is more than 2.0% and 5.0% or less. In the range described above. More preferably, it is in the range of 3.0 to 3.5%.
[0021]
FIG. 1 shows the results of examining high-temperature oxidation resistance when W is added at various ratios based on 14% Cr-0.05% Si-0.5% Nb-1.8% Mo steel. .
In the high-temperature oxidation resistance test, the test piece was kept in an air atmosphere at 950 ° C. for 100 hours, and the weight of the test piece after this test was evaluated. If the weight change after the test is 10 mg / cm ² or less, it can be said that the high-temperature oxidation resistance is excellent.
As shown in the figure, when W is contained more than 2.0%, the high-temperature oxidation resistance is remarkably improved.
[0022]
(Mo + W) ≧ 4.3%
As described above, the high-temperature oxidation resistance can be significantly improved by adding Mo and W in combination. For that purpose, the total amount of these elements is preferably set to 4.3% or more. It is more preferably at least 4.7%.
[0023]
Nb: 5 (C + N) to 1.0%
Nb is an element effective for improving the high-temperature strength, and in order to exhibit this effect, it is necessary to contain 5 (C + N) or more in consideration of the amounts of C and N. However, too much addition increases the strength at room temperature and lowers workability, so the upper limit was 1.0%. More preferably, it is in the range of 0.4 to 0.7%.
[0024]
N: 0.02% or less N also deteriorates the toughness and workability similarly to C, so it is preferable to minimize the incorporation of N. From this viewpoint, in the present invention, the N content is limited to 0.02% or less. More preferably, it is 0.008% or less.
[0025]
As described above, the basic components have been described. However, in the present invention, other elements described below can be appropriately contained.
At least one of Ti, Zr and V selected from Ti: 0.5% or less, Zr: 0.5% or less and V: 0.5% or less: From this viewpoint, it is preferable to contain each of them in an amount of 0.02% or more. However, if the content exceeds 0.5%, the steel material will be embrittled. Therefore, each content is set to 0.5% or less.
Since these elements are also effective in improving the high-temperature strength, it is preferable that the total (W + Ti + Zr + V + Cu) amount of the above-mentioned W and Cu described later is more than 3%.
[0026]
Ni: 2.0% or less, Cu: 1.0% or less, Co: 1.0% or less, and Ca: 0.01% or less Ni, Cu, Co and Ca are all toughness. Are effective elements for improving Ni, and are contained at Ni: 2.0% or less, Cu: 1.0% or less, Co: 1.0% or less, and Ca: 0.01% or less, respectively. In particular, Ca, when Ti is contained, effectively contributes to prevention of nozzle clogging during continuous casting. In order to sufficiently exhibit the effects of these elements, Ni: 0.5% or more, Cu: 0.05% or more, Co: 0.03% or more, and Ca: 0.0005% or more. It is preferable to include them.
[0027]
Al: 0.5% or less Al is useful as a deoxidizing agent, and for that purpose, it is preferable to contain 0.01% or more. Further, Al forms a fine scale on the surface of the welded portion, prevents absorption of oxygen and nitrogen during welding, and effectively contributes to improvement in toughness of the welded portion. For this purpose, the content is preferably 0.02% or more. However, if the content exceeds 0.5%, the effect reaches saturation, so in the present invention, the content is set to 0.5% or less.
[0028]
At least one of B and Mg selected from B: 0.01% or less and Mg: 0.01% or less, both of which effectively contribute to improvement of the brittleness in secondary processing, but the content exceeds 0.01%. In addition, since the strength at room temperature is increased and the ductility is reduced, the content of each is set to 0.01% or less. More preferably, B: 0.0003% or more and Mg: 0.0003% or more.
[0029]
REM: 0.1% or less REM effectively contributes to the improvement of oxidation resistance. More preferably, it is 0.002% or more. In the present invention, REM means a lanthanoid element and Y.
[0030]
Next, a preferred method for producing the steel of the present invention will be described. The production conditions of the steel according to the present invention are not particularly limited, and a general production method of Cr-containing steel can be suitably used.
For example, after smelting molten steel adjusted to the above appropriate composition range using smelting furnaces such as converters and electric furnaces, and further using smelting such as ladle refining and vacuum refining, continuous casting or ingot casting -It is preferable that after the slab is formed by the lumping method, each of the steps of hot rolling, hot rolling annealing, pickling, cold rolling, finish annealing, and pickling is sequentially performed to obtain a cold rolled annealed plate. The cold rolling may be performed once or two or more times including intermediate annealing. The steps of cold rolling, finish annealing, and pickling may be repeated. In some cases, the hot-rolled sheet annealing may be omitted. Further, when glossiness is required, it is advantageous to apply a skin pass or the like.
[0031]
【Example】
50 kg steel ingots having the component compositions shown in Table 1 were produced, and these ingots were heated to 1100 ° C., and then hot-rolled into hot-rolled sheets having a thickness of 5 mm. Then, for these hot rolled sheets, hot rolled sheet annealing (annealing temperature: 1000 ° C)-pickling-cold rolling (cold rolling reduction: 60%)-finish annealing (annealing temperature: 1000 ° C)-pickling In order to obtain a cold-rolled annealed plate having a thickness of 2 mm.
Table 2 shows the results of examining the workability and the high-temperature oxidation resistance of the thus obtained cold rolled annealed sheet.
[0032]
In addition, each characteristic was evaluated as follows.
(1) Workability From each cold-rolled annealed plate, two JIS No. 13B tensile test specimens each having a rolling direction as a tensile direction were sampled, and a tensile test at room temperature was performed in accordance with JIS G 0567. Then, the average of the elongation values of the two test pieces was obtained. When the elongation value is 34% or more, it can be said that the workability is excellent.
(2) High-temperature oxidation resistance Two test pieces (2 mm thick x 20 mm width x 30 mm length) were sampled from each cold-rolled annealed sheet, and these test pieces were placed in an air atmosphere at 950 ° C for 100 hours. Held. The weight of each test piece before and after the test was measured, the change in weight before and after the test was calculated, and the average value of the two pieces was obtained. The smaller the change in weight, the better the resistance to high-temperature oxidation. And if the weight change is 10 mg / cm ² or less, it can be said that high temperature oxidation resistance is excellent.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
As is clear from Table 2, all the steel sheets according to the present invention have excellent high-temperature oxidation resistance as well as workability.
[0036]
【The invention's effect】
Thus, according to the present invention, a ferritic stainless steel excellent in high-temperature oxidation resistance can be obtained without impairing workability at room temperature.
Therefore, according to the present invention, by improving the engine performance, it is possible to stably supply an exhaust system member that can withstand the use even when the exhaust gas temperature exceeds 900 ° C. It is also advantageously applicable to applications requiring high workability and oxidation resistance, such as fuel cell-related members.
[Brief description of the drawings]
FIG. 1 shows the results of examining high-temperature oxidation resistance when W is added at various ratios based on 14% Cr-0.05% Si-0.5% Nb-1.8% Mo steel. FIG.

Claims (7)

In mass%,
C: 0.02% or less,
Si: 0.1% or less,
Mn: 2.0% or less,
Cr: 12.0 to 16.0%,
Mo: 1.0 to 5.0%,
W: more than 2.0%, 5.0% or less,
Nb: 5 (C + N) to 1.0% and N: 0.02% or less, the balance being Fe and unavoidable impurities, characterized by being soft at room temperature and excellent in high-temperature oxidation resistance. Ferritic stainless steel. 2. The composition according to claim 1, wherein the total amount of Mo and W is (Mo + W) ≧ 4.3% by mass%.
A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, characterized by satisfying the following conditions. 3. The steel according to claim 1, wherein the steel further comprises Ti: 0.5% or less by mass%.
A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, having a composition containing at least one selected from Zr: 0.5% or less and V: 0.5% or less. The steel according to claim 1, 2, or 3, further comprising: Ni: 2.0% or less by mass%.
Cu: 1.0% or less,
A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, having a composition containing at least one selected from Co: 1.0% or less and Ca: 0.01% or less. The ferritic stainless steel according to any one of claims 1 to 4, wherein the steel further has a composition containing 0.5% by mass or less of Al by mass%, which is soft at room temperature and excellent in high-temperature oxidation resistance. steel. The steel according to any one of claims 1 to 5, wherein the steel further contains B: 0.01% or less by mass%.
Mg: A ferritic stainless steel which is soft at room temperature and excellent in high-temperature oxidation resistance, characterized by having a composition containing at least one selected from among 0.01% or less. The ferritic stainless steel according to any one of claims 1 to 6, wherein the steel further has a composition containing 0.1% by mass or less of REM by mass%. steel.

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