WO2014050016A1

WO2014050016A1 - Ferritic stainless steel

Info

Publication number: WO2014050016A1
Application number: PCT/JP2013/005486
Authority: WO
Inventors: 徹之中村; 太田　裕樹; 尾形　浩行
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-09-25
Filing date: 2013-09-17
Publication date: 2014-04-03
Also published as: EP2902523A1; US20150218683A1; TWI493057B; JPWO2014050016A1; EP2902523A4; KR101673217B1; CN104662188A; MY175890A; CN104662188B; EP2902523B1; ES2693781T3; KR20150055028A; TW201420782A; JP5700175B2

Abstract

Provided is a ferritic stainless steel in which the content of Mo, W and Nb, which are expensive elements, is kept to a minimum and the content of Cu, which reduces oxidation resistance and workability, is kept to a minimum and which exhibits excellent thermal fatigue properties and oxidation resistance. This ferritic stainless steel is characterized in that it contains, in terms of mass %, not more than 0.020% of C, not more than 3.0% of Si, not more than 1.0% of Mn, not more than 0.040% of P, not more than 0.030% of S, at least 10.0% but less than 16.0% of Cr, not more than 0.020% of N, 1.4-4.0% of Al, more than 0.15% but not more than 0.5% of Ti and 0.05-0.5% of Ni, with remainder consisting of Fe and unavoidable impurities, and satisfies formula (1). Al%/Cr% ≥ 0.14… (1) In the formula, Al% and Cr% denote the content (mass %) of Al and Cr respectively.

Description

Ferritic stainless steel

The present invention relates to a ferritic stainless steel suitable for use in an exhaust system member used in a high temperature environment such as an exhaust pipe of an automobile or a motorcycle, a catalyst outer cylinder (also referred to as a converter case), or an exhaust duct of a thermal power plant. .

Exhaust manifolds, exhaust pipes, converter cases, and mufflers used as exhaust system parts in automobiles have thermal fatigue characteristics (oxidation resistance) (hereinafter collectively referred to as oxidation resistance). (Referred to as “heat resistance (property)”).

For applications requiring such heat resistance, steels containing Nb and Si (for example, JFE429EX (15 mass% Cr-0.9 mass% Si-0.4 mass% Nb system) (hereinafter referred to as Nb-) are used. A Cr-containing steel such as Si composite added steel)) is often used. In particular, Nb is known to greatly improve heat resistance. Furthermore, steel (for example, SUS444 (18 mass% Cr-2 mass% Mo-0.5 mass% Nb)) with Mo or W added to improve heat resistance in addition to Nb has been developed. Used for members that require heat resistance.

Further, Patent Document 1 discloses a stainless steel plate with improved heat resistance by adding Ti, Cu, and B in combination. Patent Document 2, Patent Document 3 and Patent Document 4 disclose heat-resistant ferritic stainless steel to which Al is added. Patent Document 5 also discloses a ferritic stainless steel excellent in steam oxidation resistance with Al added.

JP 2010-248620 A JP 2009-68113 A JP 2004-307918 A JP 2001-316773 A JP 2009-167443 A

However, in the technique described in Patent Document 1, since Cu is added, there is a problem in that abnormal oxidation (abnormal oxidation, breakaway) occurs in the continuous oxidation test and the necessary continuous oxidation resistance cannot be obtained.

The techniques described in Patent Document 2 and Patent Document 3 have a problem that Al is added but thermal fatigue characteristics are not considered. Although Al is also added to the technique described in Patent Document 4, there may be cases where necessary oxidation resistance cannot be obtained, for example, abnormal oxidation occurs in the continuous oxidation test, or peeling of the oxide scale occurs in the repeated oxidation test. There is a problem that there is. The technique described in Patent Document 5 is also related to the steam oxidation resistance property with the addition of Al, but there is a problem that peeling of the oxide scale may occur due to repeated oxidation, or excellent repeated oxidation resistance may not be obtained. .

On the other hand, from the viewpoint of alloying elements, Mo and W are expensive elements, and there are problems of reducing hot workability and causing surface defects or reducing workability. Nb is not only an expensive element, but also raises the recrystallization temperature of the steel, so that there is a problem that the annealing temperature needs to be raised and the manufacturing cost becomes high. Cu also has a problem of reducing oxidation resistance and workability.

For this reason, it is desired to develop steel having high heat resistance while suppressing the addition amount of the above alloy elements as much as possible.

The present invention is a ferrite that is excellent in thermal fatigue characteristics and oxidation resistance while minimizing the contents of Mo, W and Nb, which are expensive and deteriorate various properties, and Cu, which reduces oxidation resistance and workability. An object is to provide a stainless steel.

The inventors have conducted intensive research on the effects of Al content and Ti content on thermal fatigue properties, as well as the effects of Cr and Ni content and Al / Cr content ratio on oxidation resistance. The optimum content range of Al, Ti, Cr and Ni was found. The present invention has been made by further studying the above findings, and the gist thereof is as follows.

[1] By mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 10 0.0% or more and less than 16.0%, N: 0.020% or less, Al: 1.4-4.0%, Ti: more than 0.15%, 0.5% or less, Ni: 0.05-0. A ferritic stainless steel containing 5%, the balance being Fe and inevitable impurities and satisfying the following formula (1).
Al% / Cr% ≧ 0.14 (1)
In the formula, Al% and Cr% represent the contents (mass%) of Al and Cr, respectively.

[2] Further, by mass%, it contains one or more selected from Nb: 0.01 to 0.15%, Cu: 0.01% or more and less than 0.4% [1] ] Ferritic stainless steel described in the above.

[3] Further, by mass%, it contains at least one selected from Mo: 0.02 to 0.5% and W: 0.02 to 0.3% [1] or The ferritic stainless steel according to [2].

[4] Further, in terms of mass%, REM: 0.001 to 0.1%, Zr: 0.01 to 0.5%, V: 0.01 to 0.5%, Co: 0.01 to 0. The ferritic stainless steel according to any one of [1] to [3], which contains at least one selected from 5%.

[5] Further, by mass%, one or more selected from B: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0020%, Ca: 0.0005 to 0.0030% The ferritic stainless steel according to any one of [1] to [4], which is contained.

In addition, oxidation resistance means both continuous oxidation resistance and repeated oxidation resistance. Continuous oxidation resistance is evaluated by an increase in oxidation after isothermal holding at high temperature, and repeated oxidation resistance is measured by increasing and decreasing temperature. The evaluation is based on the increase in oxidation after the repetition and the presence or absence of peeling of the oxide scale.

If the continuous oxidation resistance is insufficient, the oxide scale increases during high temperature use and the thickness of the base material decreases, so that excellent thermal fatigue characteristics cannot be obtained. In addition, when the resistance to repeated oxidation is low, the oxide scale peels off during use, and the influence on other members such as a downstream converter becomes a problem.

According to the present invention, a ferritic stainless steel having thermal fatigue characteristics and oxidation resistance equivalent to or better than those of a Nb—Si composite added steel can be obtained by minimizing the contents of Mo, W, Nb and Cu. It is extremely effective for exhaust system members.

It is a figure explaining a thermal fatigue test piece. It is a figure explaining the temperature in a thermal fatigue test, and constraint conditions. It is a figure showing the influence of Al (%) / Cr (%) which has on continuous oxidation resistance (oxidation increase). It is a figure showing the influence of Al (%) / Cr (%) which gives to the repeated oxidation resistance (oxidation increase and the presence or absence of scale peeling).

Hereinafter, the reasons for limitation of each component of the present invention will be described.

1. About a component composition The reason which prescribed | regulated the component composition of the ferritic stainless steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.020% or less C is an element effective for increasing the strength of steel, but if it exceeds 0.020%, the toughness and formability are significantly reduced. Therefore, in the present invention, C is made 0.020% or less. In addition, from the viewpoint of ensuring moldability, C is preferably as low as possible, and is preferably 0.015% or less. More desirably, it is 0.010% or less. On the other hand, in order to ensure the strength as an exhaust system member, C is preferably 0.001% or more, and more preferably 0.003% or more.

Si: 3.0% or less Si is an important element for improving oxidation resistance. The effect is acquired by containing 0.1% or more. When higher oxidation resistance is required, the content is preferably 0.3% or more. However, if the content exceeds 3.0%, not only the workability is lowered, but also the oxide scale is easily peeled off, and the repeated oxidation resistance is lowered. Therefore, the Si amount is 3.0% or less. More preferably, it is in the range of 0.3 to 2.0%. More preferably, it is in the range of 0.5 to 1.0%.

Mn: 1.0% or less Mn is an element that increases the strength of steel and also has an action as a deoxidizer. It also has an effect of suppressing oxide scale peeling when Si is added. In order to acquire the effect, 0.1% or more is preferable. However, excessive inclusion not only significantly increases the oxidation rate, but also tends to form a γ phase at high temperatures, thus reducing heat resistance. Therefore, in the present invention, the amount of Mn is set to 1.0% or less. Preferably, it is in the range of 0.1 to 0.5%. More preferably, it is in the range of 0.15 to 0.4%.

P: 0.040% or less P is a harmful element that lowers toughness, and is desirably reduced as much as possible. Therefore, in the present invention, the P amount is 0.040% or less. Preferably, it is 0.030% or less.

S: 0.030% or less S is reduced as much as possible because it lowers the elongation and r value (Lankford value), adversely affects the formability, and is also a harmful element that lowers the corrosion resistance, which is a basic characteristic of stainless steel. It is desirable to do. Therefore, in the present invention, the S amount is 0.030% or less. Preferably, it is 0.010% or less. More preferably, it is 0.005% or less.

Cr: 10.0% or more and less than 16.0% Cr is an important element effective for improving the corrosion resistance and oxidation resistance, which are the characteristics of stainless steel. If it is less than 10.0%, sufficient oxidation resistance is achieved. Sex cannot be obtained. On the other hand, Cr is an element that solidifies and strengthens steel at room temperature, hardens it, and reduces ductility. In the Al-added steel as in the present invention, when Cr is contained in an amount of 16.0% or more, the above-described adverse effects become remarkable and it becomes difficult to process into a complicated shape, for example, an exhaust manifold. Therefore, the Cr amount is in the range of 10.0% or more and less than 16.0%. More preferably, it is in the range of 11.0 to 15.0%. More preferably, it is in the range of 12.0 to 14.0%.

N: 0.020% or less N is an element that lowers the toughness and formability of steel, and when it exceeds 0.020%, the decrease in formability becomes significant. Therefore, the N amount is 0.020% or less. The N content is preferably reduced as much as possible from the viewpoint of ensuring toughness and formability, and is preferably 0.015% or less. More preferably, it is 0.012% or less.

Al: 1.4 to 4.0%, Al% / Cr% ≧ 0.14
Al is an important element that improves thermal fatigue characteristics. Al acts as a solid solution strengthening element, and greatly improves thermal fatigue characteristics particularly in a thermal fatigue test in which the maximum temperature exceeds 700 ° C. The effect is acquired by containing 1.4% or more.

Further, Al improves the oxidation resistance by making the oxide scale mainly composed of dense and stable Al ₂ O ₃ . When the Al content is less than 1.4%, the oxide scale is mainly composed of Cr oxide, and sufficient Al ₂ O ₃ is not formed. When not less than 1.4% Al is contained and Cr and Al are contained so as to satisfy Al% / Cr% ≧ 0.14, dense and stable Al ₂ O ₃ is generated and excellent oxidation resistance is obtained. can get.
Among the results of Example 1 to be described later, particularly the steel shown in Table 2, the influence on the oxidation resistance of Al% / Cr% was investigated. FIG. 3 shows the influence of Al% / Cr% on the increase in oxidation in the continuous oxidation test held at 1050 ° C. for 400 hours. When Al% / Cr% is less than 0.14, abnormal oxidation (oxidation increase ≧ 50 g / m ² ) occurs despite containing 1.4% or more of Al. On the other hand, when Al% / Cr% is 0.14 or more, abnormal oxidation does not occur.

Further, FIG. 4 shows the influence of Al% / Cr% on the increase in oxidation in the repeated oxidation test of 400 cycles at 1050 ° C. When Al% / Cr% is less than 0.14, abnormal oxidation (oxidation increase ≧ 50 g / m ² ) occurs despite the Al content of 1.4% or more, and scale peeling is observed. It was. On the other hand, when Al% / Cr% is 0.14 or more, neither abnormal oxidation nor scale peeling occurs.

When the value of Al% / Cr% is smaller than 0.14, that is, when the ratio of Cr amount is large with respect to Al amount, Cr oxide is formed and formation of Al ₂ O ₃ oxide film is inhibited. This is considered to be because excellent oxidation resistance cannot be obtained. On the other hand, if Al% / Cr% is 0.14 or more, a dense and stable Al ₂ O ₃ oxide film is formed preferentially over Cr oxide, and thus excellent oxidation resistance can be obtained. Conceivable. Therefore, the Al amount and the Cr amount must satisfy Al% / Cr% ≧ 0.14.

As described above, Al has the effect of improving the thermal fatigue characteristics and oxidation resistance. However, if it exceeds 4.0%, the steel becomes extremely hard and not only the workability and toughness are greatly reduced but also the thermal fatigue characteristics. Also decreases. Therefore, the Al content is in the range of 1.4 to 4.0%. Preferably, it is in the range of 1.5% to 3.5%. More preferably, it is in the range of 2.0 to 3.0%.

Ti: more than 0.15% and less than 0.5% Ti is an important element that has the effect of fixing C and N to improve the corrosion resistance, formability, and intergranular corrosion resistance of the weld. is there. Further, when Al is contained in an amount of 1.4% or more as in the present invention, it is an important element for preventing Al that improves thermal fatigue properties from being precipitated as AlN and not acting as a solid solution strengthening element. In order to prevent the formation of AlN, it is necessary to contain Ti exceeding 0.15%. When the Ti content is less than this, Al is combined with N, and is precipitated as AlN to reduce the solid solution amount of Al, so that excellent thermal fatigue characteristics cannot be obtained.

Furthermore, when Ti is contained in an amount exceeding 0.15%, it not only precipitates as Ti (C, N) but also finely precipitates at the grain boundaries as FeTiP. Ti (C, N) precipitates coarsely and does not contribute to strengthening of the steel, but FeTiP finely precipitated at the grain boundaries strengthens the grain boundaries and improves thermal fatigue characteristics. Therefore, Ti contains more than 0.15%. On the other hand, excessive content lowers the toughness of the steel and the adhesion of the oxide scale (repetitive oxidation resistance), so the upper limit is 0.5%. Accordingly, the Ti content is set to a range of more than 0.15% and 0.5% or less. Preferably it is 0.18 to 0.4% of range. More preferably, it is in the range of 0.20 to 0.3%. Good Ti content is in the range of more than 0.15% and 0.50% or less, and more preferably in the range of 0.18 to 0.40%. More preferably, it is in the range of 0.20 to 0.30%.

Ni: 0.05 to 0.5%
Ni is an important element in the present invention. Ni is an element that not only improves the toughness of the steel but also improves the oxidation resistance, particularly the repeated oxidation resistance, in the Ti-containing steel. In order to acquire the effect, it is necessary to contain 0.05% or more. When Ni is not contained or when the amount of Ni is less than 0.05%, the repeated oxidation resistance is insufficient. If the resistance to repeated oxidation is insufficient, the oxide scale peels off each time the temperature rises or falls, the oxidation progresses and the thickness of the base metal decreases, or the oxide scale peels off and the crack starts. As a result, excellent thermal fatigue characteristics cannot be obtained. On the other hand, Ni is an expensive element and is a strong γ-phase forming element. Therefore, excessive inclusion generates a γ-phase at high temperatures and reduces oxidation resistance. Therefore, the upper limit is 0.5%. Preferably it is 0.05 to 0.50% of range. More preferably, it is in the range of 0.10 to 0.30%. More preferably, it is in the range of 0.15 to 0.25%.

The above are the basic chemical components of the ferritic stainless steel of the present invention, and the balance consists of Fe and inevitable impurities. Further, from the viewpoint of improving heat resistance, one or more selected from Nb and Cu are selected elements as follows. You may contain in the range of.

Nb: 0.01 to 0.15%
Nb forms and fixes carbonitride with C and N, and has the effect of enhancing corrosion resistance, formability, and intergranular corrosion resistance of welds, and also significantly increases high-temperature strength to improve thermal fatigue characteristics and high-temperature fatigue. It is an element having the effect of improving the characteristics. In order to acquire the effect, containing 0.01% or more is preferable. However, if the content exceeds 0.15%, Nb is an expensive element and also raises the recrystallization temperature of the steel. Therefore, it is necessary to increase the annealing temperature, leading to an increase in production cost. Therefore, when Nb is contained, the amount is preferably in the range of 0.01 to 0.15%. More preferably, it is in the range of 0.02 to 0.12%. More preferably, it is in the range of 0.05 to 0.10%.

Cu: 0.01% or more and less than 0.4% Cu is an element effective for improving thermal fatigue characteristics. In order to acquire the effect, containing 0.01% or more is preferable. However, when the content is 0.4% or more, Al ₂ O ₃ production on the oxide scale is inhibited, and the oxidation resistance is lowered. Therefore, when it contains Cu, it is preferable to make the quantity into the range of 0.01% or more and less than 0.4%. More preferably, it is in the range of 0.01 to 0.2%. More preferably, it is in the range of 0.01 to 0.1%. Good Cu content is in the range of 0.01% or more and less than 0.40%, and more preferably in the range of 0.01 to 0.20%. More preferably, it is in the range of 0.01 to 0.10%.

Furthermore, from the viewpoint of improving heat resistance, one or more selected from Mo and W may be contained in the following range as selective elements.

Mo: 0.02 to 0.5%
Mo is an element that improves the heat resistance by increasing the strength of the steel by solid solution strengthening. In order to acquire the effect, containing 0.02% or more is preferable. However, Mo is an expensive element, and a content exceeding 0.5% lowers the oxidation resistance in a steel containing 1.4% or more of Al as in the present invention. Therefore, when Mo is contained, the amount is preferably in the range of 0.02 to 0.5%. More preferably, it is in the range of 0.02 to 0.3%. More preferably, it is in the range of 0.02 to 0.1%. Good Mo content is in the range of 0.02 to 0.50%, and more preferably in the range of 0.02 to 0.30%. More preferably, it is in the range of 0.02 to 0.10%.

W: 0.02-0.3%
W, like Mo, is an element that improves the heat resistance by increasing the strength of the steel by solid solution strengthening. In order to acquire the effect, containing 0.02% or more is preferable. However, in addition to being an expensive element like Mo, the content exceeding 0.3% stabilizes the oxide scale generated during annealing and makes it difficult to descal by pickling after cold rolling annealing. Therefore, when W is contained, the amount is preferably in the range of 0.02 to 0.3%. More preferably, it is in the range of 0.02 to 0.1%. A good W content is in the range of 0.02 to 0.30%, and more preferably in the range of 0.02 to 0.10%.

Furthermore, from the viewpoint of improving heat resistance, one or more selected from REM, Zr, V, and Co may be contained as selected elements in the following range.

REM: 0.001 to 0.10%
REM (rare earth element) is an element that improves oxidation resistance, and is contained as necessary in the present invention. In order to acquire the effect, containing 0.001% or more is preferable. However, if the amount of REM exceeds 0.10%, the steel is embrittled. Therefore, when REM is added, the amount is preferably in the range of 0.001 to 0.10%. More preferably, it is in the range of 0.005 to 0.06%. More preferably, it is in the range of 0.01 to 0.05%. A good REM content is in the range of 0.001 to 0.100%, more preferably in the range of 0.005 to 0.060%. More preferably, it is in the range of 0.010 to 0.050%.

Zr: 0.01 to 0.5%
Zr is an element that improves oxidation resistance, and is contained as necessary in the present invention. In order to acquire the effect, containing 0.01% or more is preferable. However, if the amount of Zr exceeds 0.5%, a Zr intermetallic compound precipitates and embrittles the steel. Therefore, when Zr is contained, the amount is preferably in the range of 0.01 to 0.5%. More preferably, it is in the range of 0.02 to 0.1%. More preferably, it is in the range of 0.01 to 0.10%. A good Zr content is in the range of 0.01 to 0.50%, and more preferably in the range of 0.02 to 0.10%.

V: 0.01 to 0.5%
V is an element effective not only for improving the oxidation resistance but also for improving the high temperature strength. In order to acquire the effect, containing 0.01% or more is preferable. However, if it exceeds 0.5%, coarse V (C, N) is precipitated and the toughness is lowered. Therefore, when V is contained, the amount is preferably in the range of 0.01 to 0.5%. More preferably, it is in the range of 0.05 to 0.4%. More preferably, it is in the range of 0.10 to 0.25%. A favorable V content is in the range of 0.01 to 0.50%, and more preferably in the range of 0.05 to 0.40%.

Co: 0.01 to 0.5%
Co is an element effective for improving toughness and an element for improving high-temperature strength. In order to acquire the effect, containing 0.01% or more is preferable. However, Co is an expensive element, and even if it contains more than 0.5%, the above effect is saturated. Therefore, when Co is contained, the amount is preferably in the range of 0.01 to 0.5%. More preferably, it is in the range of 0.02 to 0.2%. More preferably, it is in the range of 0.02 to 0.1%.
A good Co content is in the range of 0.01 to 0.50%, and better still in the range of 0.02 to 0.20%. More preferably, it is in the range of 0.02 to 0.10%.

Furthermore, from the viewpoint of improving processability and manufacturability, one or more selected from B, Mg and Ca may be contained in the following ranges as selective elements.

B: 0.0002 to 0.0050%
B is an element that improves workability, particularly secondary working embrittlement. In order to obtain the effect, the content is preferably 0.0002% or more. However, the content exceeding 0.0050% lowers the workability and toughness of steel. Therefore, when B is contained, the content is preferably in the range of 0.0002 to 0.0050%. More preferably, it is in the range of 0.0002 to 0.0030%. More preferably, it is in the range of 0.0002 to 0.0010%.

Mg: 0.0002 to 0.0020%
Mg is an element that improves the equiaxed crystal ratio of the slab and is effective in improving workability and toughness. The steel to which Ti is added as in the present invention also has an effect of suppressing the coarsening of Ti carbonitride. In order to obtain the effect, the content is preferably 0.0002% or more. This is because when the Ti carbonitride is coarsened, it becomes a starting point for brittle cracking and the toughness of the steel is greatly reduced. However, if the Mg content exceeds 0.0020%, the surface properties of the steel are deteriorated. Therefore, when it contains Mg, it is preferable to set it as 0.0002 to 0.0020% of range. More preferably, it is in the range of 0.0002 to 0.0015%. More preferably, it is in the range of 0.0004 to 0.0010%.

Ca: 0.0005 to 0.0030%
Ca is an effective component for preventing clogging of the casting nozzle due to precipitation of Ti inclusions that are likely to occur during continuous casting. In order to obtain the effect, the content is preferably 0.0005% or more. However, since surface defects are easily generated, it is necessary to be 0.0030% or less in order to obtain good surface properties. Therefore, when Ca is contained, the Ca content is preferably in the range of 0.0005 to 0.0030%. More preferably, it is in the range of 0.0005% to 0.0020%. More preferably, it is in the range of 0.0005% to 0.0015%.

2. About a manufacturing method Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.

The method for producing stainless steel of the present invention can be suitably used as long as it is an ordinary method for producing ferritic stainless steel, and is not particularly limited. For example, a steel having the above-described component composition of the present invention is obtained by melting a steel in a known melting furnace (such as a converter or an electric furnace), or further through secondary refining such as ladle refining or vacuum refining, It is made into a steel slab by continuous casting or ingot-bundling rolling, and then cold-rolled through various processes such as hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing and pickling. It is preferable to use an annealed plate.

In addition, the said cold rolling may perform cold rolling of 1 time or 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, depending on the case, the hot-rolled sheet annealing may be omitted, and when the gloss of the steel sheet surface is required, skin pass rolling may be performed after cold rolling or after finish annealing.

It is preferable that a more preferable manufacturing method uses specific conditions for a partial condition of the hot rolling process and the cold rolling process. In steelmaking, molten steel containing the above-mentioned essential components and components added as necessary is melted in a converter or an electric furnace, etc. It is preferable to perform secondary refining. The molten steel can be made into a steel material according to a known production method, but from the viewpoint of productivity and quality, it is preferable to use a continuous casting method.

The steel material obtained by continuous casting is heated to 1000 to 1250 ° C., for example, and hot rolled into a desired thickness by hot rolling. Of course, it can be processed as other than the plate material. This hot-rolled sheet is subjected to batch annealing (batch-annealing, box-annealing) at 600 to 900 ° C. or continuous annealing at 850 ° C. to 1050 ° C. as necessary, and then descaled by pickling etc. Become a product. If necessary, the scale may be removed by shot blasting before pickling.

Furthermore, in order to obtain a cold-rolled annealed plate, the hot-rolled annealed plate obtained above is made into a cold-rolled plate through a cold rolling process. In this cold rolling process, two or more cold rollings including intermediate annealing may be performed as necessary for the convenience of production. The total rolling reduction of the cold rolling process comprising one or more cold rollings is set to 60% or more, preferably 70% or more.

The cold-rolled sheet is subjected to continuous annealing (finish annealing) at 850 to 1000 ° C., followed by pickling to form a cold-rolled annealed sheet. Depending on the application, the shape and quality of the steel sheet can be adjusted by adding mild rolling (skin pass rolling or the like) after pickling.

Using the hot-rolled sheet product or cold-rolled annealed sheet product obtained in this way, bending according to each application, etc., exhaust pipes for automobiles and motorcycles, catalyst outer cylinder materials, and thermal power plants It is formed into an exhaust duct or a fuel cell-related member (for example, a separator, an interconnector, a reformer, etc.).

The welding method for welding these members is not particularly limited, and a normal arc welding method such as MIG (Metal Inert Gas), MAG (Metal Active Gas), TIG (Tungsten Inert Gas), or spot Resistance welding methods such as welding and seam welding, and electric resistance welding methods (such as high-frequency resistance welding and high-frequency induction welding are applicable.

No. having the component composition shown in Table 1-1 to Table 1-6. 1 to 80 steel (component% means mass%) was melted and cast in a vacuum melting furnace to obtain a 30 kg steel ingot. After heating to 1170 ° C., hot rolling was performed to obtain a sheet bar having a thickness of 35 mm and a width of 150 mm. This sheet bar is divided into two parts, one of which is made into a square bar with a cross section of 30 mm x 30 mm by hot forging, annealed in the temperature range of 850 to 1000 ° C, and then machined to the thermal fatigue test with the dimensions shown in Fig. 1 A piece was prepared and subjected to a thermal fatigue test. In addition, about annealing temperature, it set for every component within the range described, confirming a structure | tissue.

Using the other sheet bar divided into two, the sheet bar was heated to 1050 ° C. and hot-rolled to obtain a hot-rolled sheet having a thickness of 5 mm. Thereafter, annealing was performed in the temperature range of 850 to 1050 ° C., and the surface scale was removed by pickling or polishing. At this stage, the presence or absence of normal surface of the steel sheet was visually confirmed. This was cold-rolled to a plate thickness of 2 mm, and finish-annealed in a temperature range of 850 to 1000 ° C. to obtain a cold-rolled annealed plate. Test pieces were cut out from this cold-rolled annealed plate with dimensions of 30 mm length × 20 mm width, and all six surfaces were polished with # 320 emery paper, and subjected to the following continuous oxidation test and repeated oxidation test. .

1.1 Thermal fatigue test Fig. 2 shows the thermal fatigue test method. The thermal fatigue test piece was repeatedly heated and cooled between 100 ° C. and 850 ° C. at a heating rate of 10 ° C./s and a cooling rate of 10 ° C./s, and at the same time, strain was repeatedly applied at a restraint ratio of 0.3, The thermal fatigue life was measured. The holding times at 100 ° C. and 850 ° C. were both 2 min.

The thermal fatigue life is 100 ° C. in accordance with the standard test method for high temperature and low-cycle fatigue testing of the standard of the society of materials science, Japan. The stress was calculated by dividing the detected load by the cross-sectional area of the gauged portion of the specimen shown in FIG. 1, and the stress was reduced to 75% with respect to the stress at the fifth cycle. Defined as the number of cycles. For comparison, the same test was performed on Nb—Si composite added steel (15 mass% Cr-0.9 mass% Si-0.4 mass% Nb).
The criterion for the thermal fatigue test was that a thermal fatigue life of Nb—Si composite added steel (940 cycles) or more passed, and less than 940 cycles failed. The determination results are shown in Table 1-2, Table 1-4, and Table 1-6.

1.2 Continuous Oxidation Test The above oxidation test piece is held in an air atmosphere furnace heated to 1050 ° C. for 400 hours, the difference in mass between the test pieces before and after holding is measured, and the oxidation increase per unit area (g / M ² ). Each test was performed twice.
Criteria for continuous oxidation test, if the oxidized amounts after the continuous oxidation test passes the test of less than 50 g / m ^2, the 50 g / m ² or more results were even once failed the test. The determination results are shown in Table 1-2, Table 1-4, and Table 1-6.

1.3 Repeated Oxidation Test Using the above-mentioned oxidation test piece, in the air, 400 cycles of heat treatment that repeats heating and cooling to temperatures of 100 ° C. × 1 min and 1050 ° C. × 20 min are performed, and the mass difference between the test piece before and after the test Was measured, and the increase in oxidation per unit area (g / m ² ) was calculated, and the presence or absence of a scale peeled off from the surface of the test piece was confirmed. The heating rate and cooling rate in the above test were 5 ° C./sec and 1.5 ° C./sec, respectively.
The determination result of the repeated oxidation test is that the surface of the test piece after the repeated oxidation test passed if the oxide scale did not peel off, rejected if the peel was observed, abnormal oxidation (oxidation increase was 50 g / m ² or more) was regarded as reject (abnormal oxidation). The determination results are shown in Table 1-2, Table 1-4, and Table 1-6.

From Table 1-1 to Table 1-6, the No. 1 to 17 and 31 to 75 were all excellent in thermal fatigue characteristics, continuous oxidation resistance and repeated oxidation resistance. In all of the examples of the present invention, the surface of the hot-rolled annealed pickling plate had no defects and had good surface properties.

On the other hand, Comparative Example No. No. 18 had a low Ti content of 0.14%, so its thermal fatigue characteristics were unacceptable. Comparative Example No. No. 19 had a low Ni content of 0.02%, so its resistance to repeated oxidation was not acceptable. Comparative Example No. 20 and no. Since 76 to 80 had a low Al% / Cr% value of less than 0.14, the oxidation resistance (both continuous and repeated) failed. Comparative Example No. No. 21 has a low Al content of 0.89%, so its thermal fatigue property (850 ° C.) is not acceptable, and since the Al% / Cr% value is as low as 0.07, it has oxidation resistance (continuous or repeated). Was also rejected. Comparative Example No. No. 22 had a high Al content of 4.12%, so its thermal fatigue characteristics were unacceptable. Comparative Example No. No. 23 had a low Cr content of 9.4%, so its oxidation resistance (both continuous and repeated) failed. Comparative Example No. No. 24 had a high Cu content of 1.06%, so its oxidation resistance (both continuous and repeated) failed.

Comparative Example No. 25 has low Al content and Ti content, so its thermal fatigue characteristics are unacceptable, and Cu is high at 1.25%, so its oxidation resistance (both continuous and repeated) fails, and Ni is added. As a result, the repeated oxidation characteristics were unacceptable. Comparative Example No. Since No. 26 had low Ti content, the thermal fatigue characteristic was disqualified. Comparative Example No. 27, and no. Since 28 had a small value of Al% / Cr%, the oxidation resistance (both continuous and repeated) failed. Comparative Example No. Since 29 did not contain Ni, its oxidation characteristics repeatedly failed.
Therefore, it is clear that the steels within the scope of the present invention are excellent in thermal fatigue characteristics and oxidation resistance.

The steel of the present invention is not only suitable for exhaust system members such as automobiles, but also suitably used as exhaust system members for thermal power generation systems and solid oxide fuel cell members that require similar characteristics. be able to.

Claims

In mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 10.0% More than less than 16.0%, N: 0.020% or less, Al: 1.4 to 4.0%, Ti: more than 0.15% and 0.5% or less, Ni: 0.05 to 0.5% Ferritic stainless steel which contains, the remainder consists of Fe and an inevitable impurity, and satisfy | fills following formula (1).
Al% / Cr% ≧ 0.14 (1)
In the formula, Al% and Cr% represent the contents (mass%) of Al and Cr, respectively.
2. The composition according to claim 1, further comprising one or more selected from Nb: 0.01 to 0.15% and Cu: 0.01% to less than 0.4% by mass%. Ferritic stainless steel.
3. The composition according to claim 1, further comprising one or more selected from Mo: 0.02 to 0.5% and W: 0.02 to 0.3% by mass%. Ferritic stainless steel described in 1.
Further, in terms of mass%, REM: 0.001 to 0.10%, Zr: 0.01 to 0.5%, V: 0.01 to 0.5%, Co: 0.01 to 0.5% The ferritic stainless steel according to any one of claims 1 to 3, comprising at least one selected from the inside.
Furthermore, it contains at least one selected from B: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0020%, and Ca: 0.0005 to 0.0030% by mass%. The ferritic stainless steel according to any one of claims 1 to 4.