JP5958412B2 - Ferritic stainless steel with excellent thermal fatigue properties - Google Patents

Description

  The present invention relates to a ferritic stainless steel having excellent thermal fatigue characteristics.

  Among exhaust system members of automobiles, particularly, an exhaust manifold directly connected to an engine is used in a severe environment where the maximum use temperature reaches 800 ° C. to 900 ° C. Therefore, excellent thermal fatigue characteristics are required for the material, and ferritic stainless steel added with Nb is mainly used. In recent years, in particular, exhaust manifolds have been used at higher temperatures with the aim of improving fuel efficiency and cleaning exhaust gases as environmental countermeasures. Therefore, further improvement of thermal fatigue characteristics is desired for the stainless steel material.

  Nb added to the ferritic stainless steel is dissolved in the steel, thereby increasing the high temperature strength and improving the thermal fatigue characteristics. Furthermore, Mo and W are similarly dissolved in steel, and the thermal fatigue characteristics are improved by increasing the high temperature strength. However, even if the addition amount of these elements is increased, the effect of improving the high temperature strength is saturated at some point. Furthermore, since the strength at room temperature also increases, there is a problem that workability decreases.

  As a means for improving the thermal fatigue characteristics, there is a reduction in thermal expansion coefficient in addition to an increase in high temperature strength. This is because if the thermal expansion coefficient is reduced, the amount of strain caused by restraint from surrounding members is reduced by suppressing thermal expansion during temperature rise and thermal contraction during temperature drop.

  As a method for reducing the thermal expansion coefficient of ferritic stainless steel, Patent Document 1 discloses a steel sheet to which W is added and the amount of precipitated W is limited. Patent Document 2 discloses a steel material having a thermal expansion coefficient reduced by adding Co.

Japanese Patent No. 4604714 JP 2009-221581 A

However, in order to reduce the amount of precipitated W as in Patent Document 1, it is necessary to increase the finish annealing temperature during production, and W may precipitate depending on the subsequent cooling rate. Moreover, since W precipitates as a Laves phase (Fe ₂ W) even when the temperature is raised during use, the amount of precipitated W increases with time, and the thermal expansion coefficient increases accordingly.

  In Patent Document 2, even when the coefficient of thermal expansion is reduced by the addition of Co, excellent thermal fatigue characteristics may not be obtained.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a ferritic stainless steel having excellent thermal fatigue characteristics without restricting manufacturing conditions.

  The inventors conducted research on the thermal expansion coefficient of Nb-added ferritic stainless steel, and that the thermal expansion coefficient is reduced by adding Co. Furthermore, the addition amount of Mn and Ni is also within a specific range. It has been found that excellent thermal fatigue characteristics can be obtained by limiting to the above.

  Thereby, the ferritic stainless steel excellent in thermal fatigue characteristics can be obtained without limiting the production conditions and without increasing the thermal expansion coefficient during use.

  In the present invention, “excellent thermal fatigue characteristics” means that the life is 600 cycles or more in a thermal fatigue test at a maximum temperature of 930 ° C. and a constraint ratio of 0.40.

  The present invention has been made on the basis of the above-described findings, and the gist thereof is as follows.

  [1] By mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 0.2% or less, P: 0.040% or less, S: 0.030% or less, Al: 0 0.01 to 3.0%, Cr: 10.0 to 30.0%, Ni: 0.01 to 0.9%, N: 0.020% or less, Nb: more than 0.2% and 1.0% or less Co: 0.3 to 10%, B: 0.0001 to 0.0050%, and the balance is made of Fe and inevitable impurities, and ferritic stainless steel having excellent thermal fatigue characteristics.

  [2] Further, by mass, Mo: 0.01 to 3.0%, W: 0.3 to 5.0%, Cu: 0.4 to 2.0%, V: 0.1 to 1.%. The ferritic stainless steel having excellent thermal fatigue properties as described in [1] above, containing one or more selected from 0%.

  [3] Furthermore, by mass%, one or more selected from Ti: 0.01 to 0.50%, Zr: 0.005 to 0.50%, REM: 0.001 to 0.1% The ferritic stainless steel having excellent thermal fatigue properties according to the above [1] or [2], characterized by containing.

  [4] The above-mentioned [1], further comprising one or more selected from Ca: 0.0005 to 0.0030% and Mg: 0.0002 to 0.0020% by mass%. Or a ferritic stainless steel excellent in thermal fatigue properties according to any one of [3].

  According to the present invention, a ferritic stainless steel having excellent thermal fatigue characteristics can be obtained. Since the ferritic stainless steel of the present invention has a small coefficient of thermal expansion and is excellent in thermal fatigue characteristics, it can be suitably used for a member in which the temperature rise and fall are repeated, such as an automobile exhaust system member.

It is a figure explaining a thermal fatigue test piece. It is a figure explaining thermal fatigue test conditions.

1. About a component composition The reason which prescribed | regulated the component composition of the steel of this invention below is demonstrated. In addition, the component% shown below means the mass% altogether unless there is particular notice.

C: 0.020% or less C is an element effective for increasing the strength of steel, but if it exceeds 0.020%, toughness and workability are remarkably deteriorated, so 0.020% or less. . From the viewpoint of ensuring workability, 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, it is preferably 0.001% or more, and more preferably 0.003% or more.

Si: 3.0% or less Si is an element effective 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%, workability is lowered. Therefore, the Si amount is 3.0% or less. In addition, Preferably it is 2.0% or less. More preferably, it is 1.0% or less.

Mn: 0.2% or less Mn is an element that increases the strength of steel and also has an action as a deoxidizer. The effect appears with a content of 0.02% or more. However, Mn is a strong γ-phase-forming element and easily forms a γ-phase when the temperature is raised. Since the thermal expansion coefficient increases when the γ phase is formed, the Mn content is 0.2% or less. Preferably it is 0.05 to 0.15% of range. More preferably, it is 0.08 to 0.12% of range.

P: 0.040% or less P is an element that lowers toughness, and is desirably reduced as much as possible. The amount of P is 0.040% or less. Preferably, it is 0.035% or less.

S: 0.030% or less S decreases the moldability and corrosion resistance, so it is desirable to reduce it as much as possible. The amount of S is 0.030% or less. Preferably, it is 0.010% or less. More preferably, it is 0.006% or less.

Al: 0.01 to 3.0%
Al is an element effective for deoxidation. Al is not only used for deoxidation, but also has the effect of improving the oxidation resistance and high temperature fatigue properties of steel. The effect appears with a content of 0.01% or more. On the other hand, if the content exceeds 3.0%, the steel is hardened and the workability is lowered, so the Al content is in the range of 0.01 to 3.0%. Preferably it is 0.2 to 2.0% of range. More preferably, it is 0.3 to 0.5% of range.

Cr: 10.0-30.0%
Cr is an element necessary for improving the oxidation resistance of ferritic stainless steel, and in order to obtain good oxidation resistance, it is necessary to contain 10.0% or more. On the other hand, if the content exceeds 30.0%, the steel becomes hard and the productivity and workability are reduced. Therefore, the Cr content is in the range of 10.0 to 30.0%. Preferably, it is 12.0 to 21.0% of range. More preferably, it is 15.0 to 19.0% of range.

Ni: 0.01-0.9%
Ni is an element that improves the toughness of the steel, and the effect appears with a content of 0.01% or more. If Ni is not added, cracks are likely to occur due to insufficient toughness and ductility of steel, and excellent thermal fatigue characteristics cannot be obtained. In Patent Document 2, it is considered that this is because excellent thermal fatigue characteristics cannot be obtained even when the coefficient of thermal expansion is reduced by adding Co. However, since it is a γ-phase forming element like Mn, the Ni content is in the range of 0.01 to 0.9%. Preferably it is 0.05 to 0.5% of range. More preferably, it is 0.1 to 0.3% of range.

N: 0.020% or less Since N decreases the toughness and formability of steel, it is desirable to reduce it as much as possible, and it is 0.020% or less. Preferably, it is 0.003 to 0.015% of range. More preferably, it is 0.003 to 0.010% of range.

Nb: More than 0.2% and 1.0% or less Nb is an element having an effect of improving the thermal fatigue characteristics by remarkably increasing the high temperature strength by solid solution strengthening. On the other hand, it has an effect of improving intergranular corrosion resistance by being combined with C and N and precipitated as carbonitride. Therefore, the effect of increasing the high temperature strength appears when the content exceeds 0.2%. When Nb is 0.2% or less, even if the thermal expansion coefficient is reduced by adding Co as described later, the high temperature strength of the steel is insufficient, so that excellent thermal fatigue characteristics cannot be obtained. On the other hand, the content exceeding 1.0% not only lowers the toughness of the steel, but also forms a Laves phase (Fe ₂ Nb) at a high temperature to lower the high temperature strength. Therefore, the Nb content is in the range of more than 0.2% and 1.0% or less. Preferably it is 0.30 to 0.60% of range. More preferably, it is 0.35 to 0.55% of range.

Co: 0.3 to 10.0%
Co is an important element in the present invention. By adding Co, the amount of thermal expansion in the vicinity of the magnetic transformation temperature at the time of temperature rise is greatly reduced, and the thermal expansion coefficient can be reduced. In order to reduce the thermal expansion coefficient of steel, it is necessary to contain 0.3% or more. On the other hand, even if the content exceeds 10.0%, the effect of reducing the thermal expansion coefficient is saturated. Therefore, the amount of Co is set in the range of 0.3 to 10.0%. Preferably it is 0.6 to 7.0% of range. More preferably, it is in the range of more than 3.0% and 7.0% or less.

B: 0.0001 to 0.0050%
B is an element effective for improving workability, particularly secondary workability. If it is less than 0.0001%, there is no effect. On the other hand, when it contains exceeding 0.0050%, the workability and toughness of steel will fall. Therefore, the B amount is set in the range of 0.0001 to 0.0050%. Preferably it is 0.0003 to 0.0010% of range. More preferably, it is 0.0005 to 0.0010% of range.

  The present invention is a ferritic stainless steel excellent in thermal fatigue characteristics, characterized in that it contains the essential components and the balance consists of Fe and inevitable impurities, but from the viewpoint of improving thermal fatigue characteristics and high temperature strength One or more selected from Mo, W, Cu, and V can be contained within the following ranges.

Mo: 0.01 to 3.0%
Mo is an element that increases the strength of the steel by solid solution strengthening and improves the thermal fatigue characteristics, and the effect is obtained with a content of 0.01% or more. However, even if the content exceeds 3.0%, the effect of improving thermal fatigue properties is saturated and surface defects are generated. Therefore, when it contains Mo, it is preferable to make Mo amount into the range of 0.01 to 3.0%. More preferably, it is 0.3 to 2.0% of range. More preferably, it is 0.5 to 1.0% of range.

W: 0.3-5.0%
W, like Mo, is an element that increases the strength of the steel by solid solution strengthening, and the effect is obtained by containing 0.3% or more. However, even if the content exceeds 5.0%, the effect of improving thermal fatigue characteristics is saturated and surface defects are generated. In order to obtain good surface properties, the content is made 5.0% or less. Therefore, when it contains W, it is preferable to make W amount into 0.3 to 5.0% of range. More preferably, it is 1.0 to 3.5% of range. More preferably, it is 2.0 to 3.0% of range.

Cu: 0.4 to 2.0%
Cu precipitates as ε-Cu and strengthens the steel to improve thermal fatigue characteristics. In order to acquire the effect, it is necessary to contain 0.4% or more. However, even if the content exceeds 2.0%, the effect of improving thermal fatigue properties is not only saturated, but the steel becomes hard and workability is lowered. Therefore, when it contains Cu, it is preferable to make Cu amount into the range of 0.4 to 2.0%. More preferably, it is 0.75% or more and less than 1.5% of range. More preferably, it is 1.1 to 1.4% of range.

V: 0.01 to 1.0%
V is an element effective for improving the high-temperature strength. The effect is obtained with a content of 0.01% or more. However, if the content exceeds 1.0%, coarse V (C, N) precipitates and the toughness decreases. Therefore, when it contains V, it is preferable to make V amount into the range of 0.01 to 1.0%. More preferably, it is 0.05 to 0.5% of range. More preferably, it is 0.1 to 0.3% of range.

  Furthermore, you may contain 1 or more types chosen from Ti, Zr, and REM from the viewpoint of corrosion resistance or oxidation resistance in the following range as a selection element.

Ti: 0.01 to 0.50%
Ti fixes C and N, prevents formation of Nb carbonitride, and improves corrosion resistance, formability, and intergranular corrosion of welds. For this purpose, a content of 0.01% or more is necessary. On the other hand, if the content exceeds 0.50%, the toughness of the steel is remarkably lowered and the thermal fatigue properties are deteriorated. Therefore, when Ti is contained, the Ti content should be in the range of 0.01 to 0.50%. Is preferred. More preferably, it is 0.05 to 0.20% of range. More preferably, it is 0.10 to 0.15% of range.

Zr: 0.005 to 0.50%
Zr is an element that improves oxidation resistance. In order to acquire the effect, containing 0.005% or more is preferable. However, if the content exceeds 0.50%, the Zr intermetallic compound precipitates and embrittles the steel. Therefore, when Zr is contained, the Zr content is preferably in the range of 0.005 to 0.50%. . More preferably, it is 0.01 to 0.30% of range. More preferably, it is 0.05 to 0.10% of range.

REM: 0.001 to 0.1%
REM (rare earth element) is an element that improves oxidation resistance. In order to acquire the effect, containing 0.001% or more is preferable. However, since inclusion exceeding 0.1% embrittles the steel, when REM is included, the REM content is preferably in the range of 0.001 to 0.1%. More preferably, it is 0.005 to 0.05% of range. More preferably, it is 0.01 to 0.03% of range.

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

Ca: 0.0005 to 0.0030%
Ca is an effective component for preventing nozzle clogging due to precipitation of Ti-based inclusions that are likely to occur during continuous casting. If it is less than 0.0005%, the effect is not obtained. In order to obtain good surface properties without generating surface defects, the content must be 0.0030% or less. Therefore, when it contains Ca, it is preferable to make Ca amount into the range of 0.0005 to 0.0030%. More preferably, it is 0.0005 to 0.0020% of range. More preferably, it is 0.0005 to 0.0015% of range.

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 Nb and Ti are added as in the present invention also has an effect of suppressing the coarsening of Nb and Ti carbonitrides. The effect appears with a content of 0.0002% or more. When the Ti carbonitride becomes coarse, it becomes a starting point for brittle cracking, so that the toughness is greatly reduced. When Nb carbonitrides become coarse, the amount of Nb solid solution in steel decreases, leading to a decrease in thermal fatigue characteristics. On the other hand, when the Mg content exceeds 0.0020%, the surface properties of the steel are deteriorated. Therefore, when it contains Mg, it is preferable to make Mg amount into the range of 0.0002 to 0.0020%. More preferably, it is 0.0002 to 0.0015% of range. More preferably, it is 0.0004 to 0.0010% of range.

2. About a manufacturing method Next, the manufacturing method of the ferritic stainless steel excellent in the thermal fatigue characteristic of this invention is demonstrated.

  For the ferritic stainless steel excellent in thermal fatigue characteristics of the present invention, an ordinary method for producing stainless steel can be used. Steel having the above composition is melted in a melting furnace such as a converter or electric furnace, and further subjected to secondary refining such as ladle refining, vacuum refining, etc., and then steel slab by continuous casting or ingot-bundling rolling (Slab), hot-rolled, hot-rolled sheet annealed and pickled to give a hot-rolled annealed pickled sheet. Furthermore, a method of forming a cold-rolled annealed plate through each process such as cold rolling, finish annealing, pickling and the like is recommended. An example is as follows.

  It is melted in a converter or an electric furnace, secondary refining is performed by AOD method or VOD method to melt molten steel having the above composition, and a slab is formed by continuous casting method. This slab is heated to 1000 to 1250 ° C., and hot rolled into a desired thickness by hot rolling. The hot-rolled sheet is subjected to continuous annealing at a temperature of 900 ° C. to 1100 ° C., and then descaled by shot blasting and pickling to obtain a hot-rolled annealed pickled sheet.

  It is possible to use this hot-rolled annealed pickled plate as it is for the purpose of the present invention, such as an exhaust manifold, but further, cold-rolled and annealed / pickled to obtain a cold-rolled annealed pickled plate. You can also. In this cold rolling process, two or more cold rolling processes including intermediate annealing may be performed as necessary. The total rolling reduction in the cold rolling process comprising one or more cold rollings is 60% or more, preferably 70% or more. Cold-rolled sheet annealing temperature is 900-1150 degreeC, Preferably it is 950-1100 degreeC.

  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. Moreover, it can also be set to BA finishing which annealed in the reducing atmosphere containing hydrogen and abbreviated the pickling.

  Using hot-rolled sheet products or cold-rolled annealed sheet products obtained as described above, bending processing is performed according to each application, and exhaust pipes for automobiles and motorcycles, catalyst outer cylinders, and thermal power plant plants. It is formed into an exhaust duct or a fuel cell related member. The welding method for welding these members is not particularly limited, and various arc welding methods such as TIG, MIG, and MAG, resistance welding methods such as spot welding and seam welding, and electric resistance welding methods, etc. High frequency resistance welding and high frequency induction welding can be applied.

  No. having the component composition shown in Table 1. 1-30 steel was melted and cast in a vacuum melting furnace to form 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 was divided into two. One of them was used to form a square bar having a cross section of 30 mm × 30 mm by forging, and after annealing within a range of 950 to 1050 ° C., it was machined to produce a thermal fatigue test piece shown in FIG. Using this test piece, the measurement of the thermal expansion coefficient and the thermal fatigue test were performed as described later. About annealing temperature, it set for every component, confirming a structure within the temperature range of 950-1050 degreeC.

<Measurement of thermal expansion coefficient and thermal fatigue test>
Using the test piece for thermal fatigue test, a thermal fatigue test was performed with a maximum temperature of 930 ° C., a minimum temperature of 200 ° C., and a restraint ratio of 0.35. FIG. 2 shows a thermal fatigue test method. First, the heat fatigue test piece was heated at a heating rate of 10 ° C./s and a cooling rate of 10 ° C./s between 200 ° C. and 930 ° C., and heating and cooling were repeated for 3 cycles without stress. The amount of thermal expansion was measured from the displacement meter reading at the third cycle when the behavior of thermal expansion was stable, and the coefficient of thermal expansion was determined. Subsequently, based on the obtained amount of thermal expansion, a strain amount such that the restraint rate was 0.40 was obtained, and a thermal fatigue test was performed. In the thermal fatigue test, heating and cooling are repeated at a heating rate of 10 ° C./s and a cooling rate of 10 ° C./s between 200 ° C. and 930 ° C., and at the same time, strain is repeatedly applied so that the constraint rate is 0.40. The fatigue life was measured. The holding times at 200 ° C. and 930 ° C. were both 2 min. In addition, about the said thermal fatigue life, according to the Japan Society of Materials Standard high temperature low cycle test method standard, the load detected at 200 degreeC is divided by the cross-sectional area of the test piece soaking | uniform-heating parallel part, and stress is obtained. The number of cycles when the stress value decreased to 75% with respect to the stress value at the fifth cycle was defined as the thermal fatigue life. For comparison, the same test was performed on Nb-Si composite added steel (15% Cr-0.9% Si-0.4% Nb).

In Table 1, the determination criteria are as follows.

Thermal expansion coefficient: 13.2 × ^{10 -6} / ℃ following are the ○ (pass), was × (fail) those exceeding 13.2 × ^{10 -6} / ℃.

  Thermal fatigue life: ◯ (pass) for 600 cycles or more and x (fail) for less than 600 cycles.

  From Table 1, No. which is an example of the present invention. 1 to 23 all had a thermal fatigue life of 600 cycles or more. Moreover, the surface properties of all the hot-rolled annealed pickling plates of the examples of the present invention were good with no surface defects.

  On the other hand, Comparative Example No. Co having a low Co content of 0.2% outside the scope of the present invention was used. In No. 24, since the thermal expansion coefficient was large, the thermal fatigue characteristics were rejected. Comparative Example No. Mn is 0.28%, which is high outside the scope of the present invention. Comparative Example No. 25 and Ni are 1.02%, which is high outside the scope of the present invention. In No. 26, the thermal expansion coefficient was large, and the thermal fatigue characteristics were unacceptable. Comparative Examples Nos. Nb of 0.17% and 0.18% are low outside the scope of the present invention. 27 and no. In No. 28, the thermal fatigue characteristics were rejected due to insufficient strength at high temperatures. Comparative Example No. 29 is a component that satisfies Patent Document 2, and its thermal expansion coefficient was reduced by the addition of Co, but because Ni was not added, an excellent thermal fatigue life could not be obtained and it was rejected. Conventional Example No. Reference numeral 30 denotes a conventional SUS444 steel.

  Therefore, it is clear that the steels within the scope of the present invention are excellent in thermal fatigue characteristics.

  Since the ferritic stainless steel of the present invention has a small coefficient of thermal expansion and excellent thermal fatigue characteristics, it is suitable for exhaust system members such as exhaust manifolds, various exhaust pipes, converter cases and mufflers. Furthermore, it is also suitable as an exhaust system member or a fuel cell member of a thermal power generation system.

Claims (4)

  In mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 0.2% or less, P: 0.040% or less, S: 0.030% or less, Al: 0.01 to 3.0%, Cr: 10.0 to 30.0%, Ni: 0.01 to 0.9%, N: 0.020% or less, Nb: more than 0.2% and 1.0% or less, Co: A ferritic stainless steel having excellent thermal fatigue characteristics characterized by containing 0.3 to 10%, B: 0.0001 to 0.0050%, and the balance being Fe and inevitable impurities. % By mass, Mn: 0.19% or less , Mo : 0.01-3.0%, W: 0.3-5.0%, Cu: 0.4-2.0%, The ferritic stainless steel having excellent thermal fatigue characteristics according to claim 1, comprising at least one selected from V: 0.1 to 1.0%.   Furthermore, it contains at least one selected from Ti: 0.01 to 0.50%, Zr: 0.005 to 0.50%, and REM: 0.001 to 0.1% by mass%. The ferritic stainless steel excellent in thermal fatigue characteristics according to claim 1 or 2.   Furthermore, by mass%, it contains 1 or more types chosen from Ca: 0.0005-0.0030% and Mg: 0.0002-0.0020%, Any of Claim 1 thru | or 3 characterized by the above-mentioned. Ferritic stainless steel with excellent thermal fatigue properties.

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