JP2010156039A - Ferritic stainless steel superior in heat resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel which has superior thermal fatigue characteristics and repeated oxidation characteristics. <P>SOLUTION: The ferritic stainless steel includes, by mass%, 0.02% or less C, 0.3% or less Si, 1% or less Mn, 0.05% or less P, 0.01% or less S, 0.5% or less Al, 0.02% or less N, 20-40% Cr, 1% or less Ni, 0.2-1% Nb, 0.1-3% Mo, 1-3% Cu and 0.0003-0.01% B so that Si and Mn satisfy the following expression of 2×Si+Mn<1 mass%, wherein symbols of elements represent the contents (mass%) of the elements, respectively. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is in an environment subject to repeated thermal cycles, such as exhaust manifolds and exhaust pipes of automobiles and motorcycles, exhaust system members such as converter cases, exhaust ducts of thermal power plants, heat exchangers and fuel cell-related members. The present invention relates to a ferritic stainless steel used for members used in the above.

  Exhaust system members typified by exhaust manifolds and exhaust pipe converter cases of automobiles and motorcycles are repeatedly subjected to a thermal cycle of temperature rise to a high temperature range and temperature fall to room temperature as the engine starts and stops. For this reason, these members are known to cause fatigue due to thermal stress because they are restrained due to the surroundings of the members and their own shapes, and free expansion and contraction are hindered. For this reason, when a material inferior in thermal fatigue characteristics is used for the above-mentioned member, it breaks during use or leaks exhaust gas.

  In general, the thermal fatigue experienced by the exhaust system member during use becomes more severe as the maximum temperature of the thermal cycle is higher. However, with the trend toward lower emissions, the combustion temperature of automobile engines tends to increase year by year, and materials used for exhaust system members are required to have better thermal fatigue characteristics than ever before. ing.

  As a material for an exhaust system member that can cope with the increase in the exhaust temperature, conventionally, ferritic stainless steel to which Nb or Mo is added, for example, SUS444 (19Cr-1.8Mo-0.2Nb) defined in JIS G4305 is used. Steel, 19Cr-2Mo-0.5Nb steel, etc.) are known, but several other materials for exhaust system members have been developed. For example, in Patent Document 1, after reducing Si to a certain amount or less and maintaining the workability at room temperature, the oxidation resistance at a high temperature exceeding 900 ° C. is enhanced by adding Mo and W in combination. Ferritic stainless steel is disclosed.

  Further, in Patent Document 2, Cu precipitates are finely precipitated at an intermediate temperature range of 600 to 750 ° C., and further, at a high temperature range of 800 ° C. or higher, the finely precipitated Cu precipitates are dissolved again. Ferritic stainless steel with the strength, heat fatigue resistance and oxidation resistance required for exhaust system parts used in exhaust gas environments in a high temperature range of 800 ° C or higher is disclosed by strengthening the solid solution. Patent Reference 3 describes a ferritic stainless steel that has improved thermal fatigue characteristics by keeping the presence of Cu precipitates (ε-Cu phase) in a certain state before being used in an exhaust gas member. Is disclosed.

  Further, in Patent Document 4, in the Cr-containing steel containing Nb, the content of Si and Cr is reduced, and by adding an appropriate amount of Mo in relation to the Si content, precipitation of the Laves phase is suppressed, Disclosed is a steel in which high-temperature strength is increased without increasing the normal-temperature strength and the occurrence of abnormal oxidation is suppressed by making Mo a solid solution-Mo main form. Further, Patent Document 5 discloses Nb, Mo. A heat resistant ferritic stainless steel is disclosed in which the content of is optimized to improve the in-plane anisotropy of room temperature elongation, and high-temperature strength is ensured by solute Cu.

JP 2004-018921 A JP 2000-297355 A JP 2006-117985 A Japanese Patent Laid-Open No. 2002-212585 JP 2001-303204 A

  However, in the above-described prior art ferritic stainless steel, it has become difficult to sufficiently cope with the recent increase in exhaust gas temperature, and development of a material having more excellent thermal fatigue characteristics and repeated oxidation characteristics is desired. Yes.

  Accordingly, an object of the present invention is to provide a ferritic stainless steel that has superior thermal fatigue characteristics and repetitive oxidation characteristics as compared with conventional materials by advantageously solving the above-mentioned problems of conventional materials for exhaust system members. It is to provide.

  The inventors conducted extensive research focusing on the effects of various additive elements on the thermal fatigue properties of ferritic stainless steel. As a result, in the ferritic stainless steel containing Cr exceeding 20 mass%, in addition to the composite addition of Nb, Mo, Cu and B, the thermal fatigue characteristics can be further improved by optimizing the contents of Si and Mn. The inventors have found that repeated oxidation characteristics can be dramatically improved, and have completed the present invention.

That is, the present invention is C: 0.02 mass% or less, Si: 0.3 mass% or less, Mn: 1 mass% or less, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.5 mass% Hereinafter, N: 0.02 mass% or less, Cr: 20 to 40 mass%, Ni: 1 mass% or less, Nb: 0.2 to 1 mass%, Mo: 0.1 to 3 mass%, Cu: 1 to 3 mass%, B: 0.0003-0.01mass% is contained, Si and Mn are following formula;
2 × Si + Mn <1 mass%
However, each element symbol indicates the content (mass%) of each element.
It is a ferritic stainless steel excellent in thermal fatigue characteristics and repeated oxidation characteristics characterized by containing.

  In addition to the above component composition, the present invention further includes V: 0.5 mass% or less, W: 1-5 mass%, Ti: 0.02-0.5 mass%, Zr: 0.02-0.5 mass%, Co : 0.05 to 3 mass% and Ta: 1 mass% or less selected from 2 mass% or less.

  According to the present invention, it is possible to provide a ferritic stainless steel that has excellent thermal fatigue characteristics and repeated oxidation characteristics as compared with conventional materials. Therefore, the ferritic stainless steel of the present invention can be suitably used as a material for exhaust system members of automobiles and motorcycles whose emission is being reduced.

It is a figure which shows the test piece shape used for the thermal fatigue test. It is a figure explaining the heating cycle of a thermal fatigue test condition, and a distortion cycle.

First, the reason why the composition of the ferritic stainless steel of the present invention is limited to the above range will be described.
C: 0.02 mass% or less Since C is an element that lowers the formability and corrosion resistance of steel, it is limited to 0.02 mass% or less. Preferably, it is 0.01 mass% or less.

Si: 0.3 mass% or less Si is an element that improves the oxidation characteristics of steel, and is one of the important elements in the present invention. However, excessive Si addition promotes the precipitation of the Laves phase and lowers the thermal fatigue characteristics, so it is limited to 0.3 mass% or less. Preferably it is 0.2 mass% or less.

Mn: 1 mass% or less Mn is an element that improves scale adhesion and repeatedly improves oxidation resistance. However, according to the studies by the inventors, it has been newly found that Mn is an element that adversely affects thermal fatigue properties. Although the cause is not yet clear, it is considered that the thermal expansion behavior is changed by the addition of Mn. Therefore, Mn is set to 1 mass% or less. Preferably it is the range of 0.1-0.6 mass%.

P: 0.05 mass% or less P is a harmful element that lowers the corrosion resistance and toughness of steel, and is limited to 0.05 mass% or less. Preferably it is 0.03 mass% or less.

S: 0.01 mass% or less S is a harmful element that becomes a starting point of steel cracking and pitting corrosion and lowers corrosion resistance, and is limited to 0.01 mass% or less. Preferably it is 0.002 mass% or less.

Al: 0.5 mass% or less Al is generally an element added as a deoxidizer in the steelmaking stage, and is an element inevitably added when Al deoxidation is performed. Moreover, since there exists an effect which improves oxidation resistance, you may add actively as needed. However, excessive addition of Al reduces toughness, so the upper limit is made 0.5 mass%. Preferably, it is 0.3 mass% or less.

N: 0.02 mass% or less Since N is a harmful element that lowers the formability and corrosion resistance of steel, it is limited to 0.02 mass% or less. Preferably, it is 0.01 mass% or less.

Cr: 20-40 mass%
Cr is a basic component necessary for ensuring the corrosion resistance of ferritic stainless steel. In addition, Cr has the effect of increasing strength and improving thermal fatigue characteristics and the effect of improving oxidation resistance in the entire temperature range of the thermal fatigue test performed in the present invention. However, if Cr is less than 20 mass%, a sufficient effect of improving thermal fatigue properties cannot be obtained, while if it exceeds 40 mass%, workability is impaired. Therefore, Cr is set to a range of 20 to 40 mass%. Preferably, it is the range of 21-35 mass%.

Ni: 1 mass% or less Ni is an element having an effect of improving the toughness of steel. However, since Ni is expensive, it is set to 1 mass% or less.

Nb: 0.2-1 mass%
Nb is an element that increases the high-temperature strength of the steel and improves the thermal fatigue life, and is one of the important elements in the present invention. However, if the amount of Nb added is less than 0.2 mass%, sufficient high-temperature strength cannot be obtained. On the other hand, if it exceeds 1 mass%, a large amount of Laves phase is precipitated, resulting in a decrease in thermal fatigue characteristics. Therefore, Nb is set to a range of 0.2 to 1 mass%. Preferably, it is the range of 0.3-0.6 mass%.

Mo: 0.1-3 mass%
Mo is an important element that increases the high temperature strength of the steel and improves the thermal fatigue life. If the Mo content is less than 0.1 mass%, sufficient high-temperature strength cannot be obtained. On the other hand, excessive addition exceeding 3 mass% precipitates a large amount of Laves phase and σ phase, leading to deterioration of thermal fatigue characteristics. . Therefore, Mo is set to a range of 0.1 to 3 mass%. Preferably, it is the range of 1-2 mass%.

Cu: 1-3 mass%
Cu is an element having the effect of improving the thermal fatigue properties by maintaining high strength in the low temperature range for a long time by repeating fine ε-Cu precipitation and solid solution during repeated thermal cycles. It is one of the essential elements in the present invention. The above effect is manifested by adding more than 1 mass% of Cu, but excessive addition exceeding 3 mass% reduces the toughness of the steel. Therefore, Cu is added in the range of 1 to 3 mass%. Preferably it is the range of 1.2-2 mass%.

B: 0.0003 to 0.01 mass%
B is one of the important elements in the present invention that has the effect of suppressing the precipitation of the Laves phase in the vicinity of the grain boundary, increasing the high temperature strength, and improving the thermal fatigue characteristics. To obtain the above effect, 0.0003 mass% or more must be added. However, excessive addition exceeding 0.01 mass% lowers the melting point of the steel and causes hot cracking during welding. Therefore, B is in the range of 0.0003 to 0.01 mass%. Preferably, it is in the range of 0.0005 to 0.003 mass%.

(2 × Si + Mn): less than 1 mass% The ferritic stainless steel of the present invention, in order to achieve both thermal fatigue characteristics and repeated oxidation characteristics, in addition to Si and Mn satisfying the above composition range, formula;
(2 × Si + Mn) <1 mass%
However, each element symbol indicates the content (mass%) of each element.
It is necessary to satisfy and contain.
In order to obtain sufficient thermal fatigue characteristics, Si and Mn are preferably as low as possible. On the other hand, in order to improve oxidation characteristics, there is a conflicting requirement that Si and Mn are preferable as high as possible. However, in the ferritic stainless steel of the present invention, the deterioration of oxidation characteristics due to low Si and low Mn can be compensated by the addition of Cr, so Si and Mn are limited to a range satisfying the above relational expression. However, it has become possible to achieve both excellent thermal fatigue characteristics and repeated oxidation characteristics. Preferably, (2 × Si + Mn) is 0.7 mass% or less.

The ferritic stainless steel of the present invention further contains one or more selected from V, W, Ti, Zr, Co, and Ta in the following range in addition to the essential components described above. Can do.
V: 0.5 mass% or less V is an element effective for improving the formability of steel. However, excessive addition exceeding 0.5 mass% causes coarse V (C, N) to precipitate and deteriorates the surface properties. Therefore, when adding V, it is preferable to set it as 0.5 mass% or less.

W: 1 to 5 mass%
W is an element that has the effect of increasing the high-temperature strength and oxidation resistance by being dissolved in steel. These effects are recognized by addition of 1 mass% or more. However, excessive addition exceeding 5 mass% leads to an increase in raw material cost. Therefore, when adding W, it is preferable to add in the range of 1-5 mass%. More preferably, it is the range of 2-3.5 mass%.

Ti: 0.02-0.5 mass%
Ti is an element that improves the formability of steel, and has an effect of increasing the solid solution amount of effective Nb by preferentially bonding to them because the affinity for C and N is stronger than Nb. Such an effect is recognized at 0.02 mass% or more. However, when it is added in an amount exceeding 0.5 mass%, coarse Ti (C, N) is precipitated and the surface properties are deteriorated. Therefore, Ti is preferably added in the range of 0.02 to 0.5 mass%. More preferably, it is the range of 0.02-0.4 mass%.

Zr: 0.02-0.5 mass%
Zr, like Ti, is an element that improves formability, has an affinity for C and N that is stronger than that of Nb, and preferentially binds to them, so that it has the effect of increasing the solid solution amount of effective Nb. Such an effect is recognized by addition of 0.02 mass% or more, but addition exceeding 0.5 mass% causes Zr intermetallic compounds to precipitate and embrittles the steel. Therefore, it is preferable to add Zr in the range of 0.02 to 0.5 mass%. More preferably, it is the range of 0.02-0.4 mass%.

Co: 0.05-3 mass%
Co is an element effective for increasing the high-temperature strength of steel, and can be added as necessary. This effect is recognized when 0.05 mass% or more is added. However, excessive addition causes an increase in cost, so the upper limit is preferably set to 3 mass%. More preferably, it is the range of 0.1-1 mass%.

Ta: 2 mass% or less Ta is an element effective for increasing the high-temperature strength of steel, and can be added as necessary. However, since a large amount of addition exceeding 2 mass% deteriorates thermal fatigue properties due to precipitation of the Laves phase, when it is added, it is preferably made 2 mass% or less. More preferably, it is 1.5 mass% or less.

Next, the preferable manufacturing method of the ferritic stainless steel of this invention is demonstrated.
The production conditions of the steel of the present invention are not particularly limited, and any suitable method for producing Cr-containing steel can be used. For example, after the molten steel adjusted to the above-mentioned proper component composition is melted through a secondary refining process such as a refining furnace such as a converter or an electric furnace, a steel refining process, a vacuum refining process, etc.・ Production method to make a slab by the lump method, and then make the slab a cold-rolled annealed plate through each step of hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling Can be adopted. In addition, the cold rolling in the said process is good also as cold rolling of 2 times or more on both sides of intermediate annealing, and each process of cold rolling, finish annealing, and pickling may be performed repeatedly. Furthermore, the hot-rolled sheet annealing step may be omitted.

  No. having the component composition shown in Table 1. 1 to 29 steel was melted in a vacuum melting furnace to form a 100 kg steel ingot, and then the steel ingot was heated to 1200 ° C. and hot-rolled into a sheet bar having a thickness of 30 mm. It was subjected to repeated oxidation tests.

<Thermal fatigue test>
A part of the sheet bar is forged into a square bar having a thickness of 30 mm and a width of 30 mm, and the square bar is subjected to a finish heat treatment of holding at 1000 to 1150 ° C. for 1 to 5 minutes and then cooling at 20 ° C./s or more. After the application, a thermal fatigue test piece (the most detailed diameter of 8 mmφ) having the shape shown in FIG. 1 was collected from the square bar and subjected to a thermal fatigue test. In the thermal fatigue test, as shown in FIG. 2, after maintaining at a minimum temperature of 200 ° C. for 90 s, the temperature was increased to 6 ° C./s up to a maximum temperature of 880 ° C. It was performed under the condition of repeatedly applying a heat treatment in which the heat cycle for lowering the temperature to 6 ° C. at 6 ° C./s was one cycle. The apparent strain in the thermal fatigue test was controlled by an electromechanical system to be 60% of the free thermal expansion and contraction strain (constraint rate 0.40). The thermal fatigue life is the number of cycles when the maximum load is reduced to 90% with respect to the maximum load in the fifth cycle in which the load-strain hysteresis loop is stabilized. It was evaluated that the properties were excellent (◯).

<Repetitive oxidation test>
The sheet bar having a thickness of 30 mm obtained by hot rolling is heated to 1000 to 1200 ° C. and further hot rolled to obtain a hot rolled plate having a thickness of 5 mm. The hot rolled plate has a temperature of 1000 to 1200 ° C. After the hot-rolled annealed plate is applied, pickling, cold rolling at a reduction rate of 60%, finish annealing at a temperature of 1000 to 1200 ° C., pickling, and a cold-rolled annealed plate having a thickness of 2 mm is obtained. .
Two test pieces each having a thickness of 2 mm, a width of 20 mm, and a length of 30 mm were sampled from the cold-rolled annealed plate, and the test pieces were heated in the atmosphere at 1000 ° C. for 8 minutes and then cooled to 200 ° C. for 1 minute. An oxidation test was repeated 1000 times to repeat the heat cycle to be held. In addition, the evaluation of the repeated oxidation characteristics was determined by visually observing the surface of the test piece after the test to determine that no scale peeling occurred (good).

  The results of the thermal fatigue test and the repeated oxidation test are summarized in Table 2. From Table 2, all the ferritic stainless steels satisfying the component composition of the present invention have a thermal fatigue life exceeding 1200 cycles, have good thermal fatigue properties, and are also exfoliated in repeated oxidation tests. No abnormal oxidation was observed. On the other hand, the steel of the comparative example not satisfying the component composition of the present invention has a life of 1200 cycles in the thermal fatigue test and / or scale peeling in the repeated oxidation test. It is not a combined steel.

  The steel sheet of the present invention undergoes repeated thermal cycles such as exhaust manifolds and exhaust pipes of automobiles and motorcycles, exhaust system members such as converter cases, exhaust ducts of thermal power plants, heat exchangers and fuel cell-related members. It can use suitably for the member used in an environment.

Claims (2)

C: 0.02 mass% or less,
Si: 0.3 mass% or less,
Mn: 1 mass% or less,
P: 0.05 mass% or less,
S: 0.01 mass% or less,
Al: 0.5 mass% or less,
N: 0.02 mass% or less,
Cr: 20 to 40 mass%,
Ni: 1 mass% or less,
Nb: 0.2-1 mass%,
Mo: 0.1-3 mass%,
Cu: 1-3 mass%,
B: 0.0003-0.01mass% is contained,
A ferritic stainless steel excellent in thermal fatigue characteristics and repeated oxidation characteristics characterized by containing Si and Mn satisfying the following formula.
2 × Si + Mn <1 mass%
However, each element symbol indicates the content (mass%) of each element. In addition to the above component composition,
V: 0.5 mass% or less,
W: 1 to 5 mass%,
Ti: 0.02-0.5 mass%,
Zr: 0.02 to 0.5 mass%,
The ferritic stainless steel according to claim 1, comprising one or more selected from Co: 0.05 to 3 mass% and Ta: 2 mass% or less.

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