JP2007247013A - Ferritic stainless steel with excellent oxidation resistance, workability and high temperature strength - Google Patents

Abstract

Provided is a ferritic stainless steel having excellent high-temperature strength capable of reducing Mn, which has excellent long-term oxidation resistance even when the Si content is low, and which causes a decrease in workability.
SOLUTION: C: 0.02 mass% or less, Si: 0.1 mass% or less, Mn: less than 0.4 mass%, P: 0.04 mass% or less, S: 0.015 mass% or less, N: 0.02 mass% Hereinafter, Cr: 13 to 20 mass%, Nb: 0.3 to 0.7 mass%, Ti: 5 × N to 0.3 mass%, Mo: 0.4 to 3.0 mass%, Ni: 0.5 mass% or less Ferritic stainless steel which is contained and the balance is made of Fe and inevitable impurities, and is excellent in oxidation resistance, workability and high temperature strength.
[Selection figure] None

Description

  The present invention relates to a ferritic stainless steel excellent in oxidation resistance, workability, and high-temperature strength suitable for use in parts that come into contact with high-temperature gas, such as automobile and motorcycle exhaust system parts and power plant exhaust ducts. .

  As materials for parts that come into contact with high-temperature gas such as exhaust parts of automobile exhaust manifolds and front pipes and exhaust ducts of power plants, it is necessary to have excellent high-temperature strength, workability, and oxidation resistance. Therefore, Mo and Nb-containing ferritic stainless steel such as SUS444 may be used for these applications.

  However, although SUS444 steel shows excellent properties in terms of high-temperature strength and oxidation resistance, it still has an insufficient level of workability and has a problem that it may not be processed into a desired shape. Therefore, as a method for improving workability, it is considered to reduce the Si content.

  For example, Patent Document 1 discloses a technique for improving high-temperature strength and workability by reducing the Si content of Cr-containing steel to 0.10 mass% or less. This is to reduce Si to suppress Laves phase precipitation, and to prevent deterioration in high temperature strength and workability due to Laves phase. On the other hand, Patent Document 2 also describes that Si is an element that improves the oxidation resistance of a steel sheet by forming a thin film on the surface of the steel sheet and lowering the diffusion coefficient of oxygen.

  As described above, according to the conventional knowledge, it is desirable to reduce Si in order to improve high temperature strength and workability, and to add Si to improve oxidation resistance. As an element added to steel used for system parts, it has a contradictory property.

In high-temperature exhaust system parts, oxidation resistance is one of the important characteristics.For example, if the oxidation resistance of a material exposed to high temperature for a long time is low, the plate thickness decreases due to scale peeling or the exhaust due to the peeled scale. There is a risk of damage to downstream parts of the tube and clogging of the catalyst. Therefore, Patent Document 3 discloses a technique for improving the scale peel resistance by adding Mn in order to improve the oxidation resistance of the low Si ferritic stainless steel.
Japanese Patent No. 3546714 Japanese Patent No. 3710302 JP 2005-314740 A

  However, there is a concern that the addition of Mn decreases the workability and that the scale generated by oxidation becomes easy to peel off when exposed to a high temperature for a long time.

  Therefore, the object of the present invention is suitable for use in materials for exhaust systems such as automobiles and motorcycles and exhaust ducts of power plants, and is excellent in long-term oxidation resistance even when the Si content is low, and further processed. An object of the present invention is to provide a ferritic stainless steel excellent in high-temperature strength capable of reducing Mn causing a decrease in properties.

  Based on ferritic stainless steel with a reduced amount of Si, the inventors have melted steel with various components added, and the effects of various components on long-term oxidation resistance, high-temperature strength, and workability investigated. As a result, even in steels with reduced Si, long-term oxidation resistance, high-temperature strength, and workability required for exhaust system parts can be obtained by controlling the Mn content to an appropriate range and adding an appropriate amount of Ti. I found out.

Specifically, Mn has an effect of suppressing the scale peeling property at a high temperature oxidation for a short time, but rather promotes the scale peeling at a high temperature oxidation for a long time. However, it became clear that, by reducing Mn and adding an appropriate amount of Ti, scale peeling can be prevented even during long-time high-temperature oxidation, and thus good oxidation resistance can be obtained. . In addition, although the mechanism of the oxidation resistance improvement by Mn reduction | decrease and Ti addition is not fully elucidated yet, factors, such as an elastic modulus improvement of a Mn containing oxide film, can be considered.
The present invention has been completed based on the above findings and further studies.

  That is, the present invention includes C: 0.02 mass% or less, Si: 0.1 mass% or less, Mn: less than 0.4 mass%, P: 0.04 mass% or less, S: 0.015 mass% or less, N: 0.00. 02 mass% or less, Cr: 13 to 20 mass%, Nb: 0.3 to 0.7 mass%, Ti: 5 × N to 0.3 mass%, Mo: 0.4 to 3.0 mass%, Ni: 0.5 mass% It is a ferritic stainless steel containing the following, the balance being excellent in oxidation resistance, workability and high-temperature strength consisting of Fe and inevitable impurities.

The ferritic stainless steel of the present invention is characterized by further containing at least one component of the following groups A to E in addition to the above component composition.
Group A; B: one or two of B: 0.0003 to 0.0050 mass% and Co: 0.01 to 0.25 mass%; Group B; Al: 0.15 mass% or less, Group C; Cu: 0.5 mass % Or less D group; V: 0.2 mass% or less E group; Ca: 0.0005-0.0050 mass% and Mg: 0.0005-0.0050 mass%, 1 type or 2 types

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel excellent in oxidation resistance, workability, and high temperature strength can be provided. Accordingly, the ferritic stainless steel of the present invention can be suitably used as a material for exhaust system parts such as exhaust manifolds, front pipes and converter shells of automobiles and motorcycles, and as a material for exhaust ducts of power plants and the like. .

The reason why the component 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 C is an element that increases the strength of steel and decreases toughness and workability. In particular, when the C content exceeds 0.02 mass%, adverse effects on toughness and workability become significant, so the upper limit is made 0.02 mass%. Preferably it is 0.008 mass% or less.

Si: 0.1 mass% or less Si has an effect of improving the oxidation resistance, but at the same time promotes the precipitation of the Laves phase and lowers the high-temperature strength and workability required for materials used for exhaust system parts. Therefore, in the present invention, the Si content is set to 0.1 mass% or less. Preferably it is 0.05 mass% or less.

Mn: Less than 0.4 mass% Mn is an element having an important meaning in the present invention. In general, Mn is said to improve the oxidation resistance. However, according to the investigation by the inventors, this is for a relatively short-time oxidation resistance, and for improving the long-term oxidation resistance. It became clear that the effect was not obtained so much. The reason for this is not clear yet, but it is thought that excessive Mn may rather promote scale peeling due to long-term oxidation. Mn is also added as a deoxidizer for steel, but in the present invention, the deoxidation effect can be replaced by Ti described later. Furthermore, as described above, Mn is an element that reduces workability. Therefore, in the present invention, the Mn content is less than 0.4 mass%. In addition, Mn content is so preferable that it is low, and it is not necessary to provide a lower limit in particular.

P: 0.04 mass% or less Since P is an element that deteriorates the toughness, corrosion resistance, and weldability of steel, the smaller the amount, the better. Therefore, in the present invention, the content of P is set to 0.04 mass% or less. Preferably it is 0.03 mass% or less, More preferably, it is 0.015 mass% or less.

S: 0.015 mass% or less S is an element that forms MnS, lowers the workability of steel, and lowers corrosion resistance and weldability. Therefore, in this invention, it is set as 0.015 mass% or less. Preferably it is 0.01 mass% or less.

N: 0.02 mass% or less N is an element having an effect similar to that of C, and deteriorates toughness and workability. Therefore, in the present invention, the N content is set to 0.02 mass% or less. Preferably it is 0.01 mass% or less.

Cr: 13-20 mass%
Cr is an element useful for improving oxidation resistance and corrosion resistance. In order to ensure the desired oxidation resistance of the present invention, it is necessary to add 13 mass% or more. However, if the Cr content exceeds 20 mass%, the workability deteriorates, so 20 mass% is made the upper limit. From the viewpoint of ensuring workability, a range of 15 to 17 mass% is preferable.

Nb: 0.3-0.7 mass%
Nb is an element that increases the high-temperature strength, workability of steel, and intergranular corrosion resistance of welds. In order to obtain such an effect, addition of 0.3 mass% or more is necessary. However, if added excessively, it becomes a Laves phase and precipitates to reduce toughness, so the upper limit needs to be 0.7 mass%.

Ti: 5 × N to 0.3 mass%
Ti is the most important element in the present invention. By adding an appropriate amount of Ti, it is possible to compensate for a decrease in oxidation resistance due to Si reduction and a decrease in deoxidation effect due to Mn reduction. In order to obtain sufficient oxidation resistance, Ti needs to be added in an amount of 5 × N (mass%) or more. Furthermore, the reduction of Si and Mn by addition of Ti can also achieve the effect of improving workability. As a result, even if Si is reduced, the oxidation resistance, workability, and high-temperature strength necessary for materials such as exhaust system parts such as automobiles and motorcycles and exhaust ducts of power plants can be ensured. However, if Ti is added excessively, coarse Ti (C, N) precipitates and the surface properties deteriorate, so the upper limit is set to 0.3 mass%. Preferably it is 0.15 mass% or less.

Mo: 0.4-3.0 mass%
Mo has the effect of increasing the high temperature strength by solid solution strengthening of steel. Mo also has an effect of improving corrosion resistance, and the effect is manifested by addition of 0.4 mass% or more. However, addition of a large amount causes an increase in cost, so it is preferable to set the upper limit to 3.0 mass%.

Ni: 0.5 mass% or less Ni improves the toughness of the steel and has an effect. However, since Ni is an austenite stabilizing element, it makes the ferrite structure unstable. Therefore, in this invention, it is 0.5 mass% or less. Preferably it is the range of 0.05-0.3 mass%.

In addition to the above components, the ferritic stainless steel of the present invention further includes at least one selected from B, Co, Al, Cu, V, Ca, and Mg in the following range according to required characteristics. be able to.
B: 0.0003 to 0.0050 mass%
B is an element effective for improving secondary work brittleness resistance, and this effect can be obtained by adding 0.0003 mass% or more. However, if added in a large amount, BN is produced and the workability is lowered, so the upper limit is preferably made 0.0050 mass%. More preferably, it is the range of 0.0004-0.0015 mass%.

Co: 0.01 to 0.25 mass%
Co is an element effective for improving the toughness and high temperature strength of the weld heat affected zone. In order to exhibit the said effect, it is preferable to add 0.01 mass% or more. However, since Co is an expensive component, the upper limit is preferably set to 0.25 mass%.

Al: 0.15 mass% or less Al is usually added as a deoxidizing agent, but in the present invention, deoxidation with Ti is possible, so that a large amount of Al is not required. Rather, excessive addition of Al reduces workability. Therefore, in this invention, it is preferable to set it as 0.15 mass% or less. More preferably, it is 0.05 mass% or less.

Cu: 0.5 mass% or less Cu is effective for improving the corrosion resistance. However, excessive addition causes embrittlement of the steel due to precipitation of ε-Cu, and is preferably 0.5 mass% or less.

V: 0.2 mass% or less V has an effect of improving deep drawability (r value). However, excessive addition of V causes the surface properties to deteriorate due to precipitation of V (C, N), so the upper limit is preferably set to 0.2 mass%. More preferably, it is 0.15 mass%.

Ca: 0.0005 to 0.0050 mass%
Ca has the effect of suppressing nozzle clogging during continuous casting. In order to acquire the effect, it is preferable to add 0.0005 mass% or more. However, excessive addition of Ca facilitates pitting corrosion of steel, so the upper limit is preferably set to 0.0050 mass%.

Mg: 0.0005 to 0.0050 mass%
Mg is an element that improves high temperature fatigue properties. In order to obtain such an effect, 0.0005 mass% or more is preferably added. However, excessive addition of Mg decreases the toughness of the steel, so 0.0050 mass% or less is preferable.

  In the ferritic stainless steel of the present invention, components other than the above are inevitable impurities. However, addition of components other than those described above is not rejected as long as the effects of the present invention are not impaired.

Steel having various component compositions shown in Table 1 was melted to obtain a steel slab, which was heated to 1100 to 1200 ° C. and hot-rolled to obtain a hot-rolled sheet having a thickness of 5 mm. Next, this hot-rolled sheet was hot-rolled sheet annealed, pickled, then cold-rolled and finish-annealed to obtain a cold-rolled annealed sheet having a thickness of 2.0 mm. A test piece was collected from the cold-rolled annealed plate thus obtained and subjected to the following high-temperature strength, workability, and oxidation resistance evaluation tests.
<Oxidation resistance test>
A test piece (2 mm × 20 mm × 30 mm) is taken from each cold-rolled annealed plate, placed in an atmospheric furnace maintained at 950 ° C. and held for 500 hours. From the change in mass of the test piece before and after the test, the oxidation increase (Mg / cm ² ) was determined. However, the thing which caused scale peeling described it.
<High temperature strength test>
A JIS No. 13 B tensile test piece with the rolling direction as the tensile direction was taken from each cold-rolled annealed plate, and subjected to a high-temperature tensile test at a strain rate of 0.3% / min at 900 ° C. in accordance with JIS G0567. The 0.2 mass% yield strength was measured.
<Processability evaluation test>
A JIS No. 13 B tensile test piece with the rolling direction as the tensile direction was taken from each cold-rolled annealed plate, and subjected to a tensile test at room temperature (25 ° C.) in accordance with JIS Z2241, to measure the elongation El.

  The measurement results are shown in Table 2. From Table 2, all the steels A to S that conform to the composition of the present invention are excellent in oxidation resistance, workability and high temperature strength. On the other hand, the steel of T to X deviating from the component composition of the present invention is greatly inferior to the present invention example in any one or more of oxidation resistance, workability and high temperature strength. I understand.

  The technology of the present invention can be used for materials used for automobiles and power plants, as well as for applications exposed to high-temperature oxidizing atmosphere for a long time.

Claims (6)

C: 0.02 mass% or less, Si: 0.1 mass% or less, Mn: less than 0.4 mass%, P: 0.04 mass% or less, S: 0.015 mass% or less, N: 0.02 mass% or less, Cr: Contains 13-20 mass%, Nb: 0.3-0.7 mass%, Ti: 5 × N-0.3 mass%, Mo: 0.4-3.0 mass%, Ni: 0.5 mass% or less, the balance Is a ferritic stainless steel excellent in oxidation resistance, workability and high-temperature strength composed of Fe and inevitable impurities. 2. In addition to the said component composition, 1 or 2 types of B: 0.0003-0.0050mass% and Co: 0.01-0.25mass% are contained, It is characterized by the above-mentioned. Ferritic stainless steel. The ferritic stainless steel according to claim 1 or 2, further comprising Al: 0.15 mass% or less in addition to the above component composition. The ferritic stainless steel according to any one of claims 1 to 3, further comprising Cu: 0.5 mass% or less in addition to the above component composition. The ferritic stainless steel according to any one of claims 1 to 4, further comprising V: 0.2 mass% or less in addition to the above component composition. 6. In addition to the above component composition, it further contains one or two of Ca: 0.0005 to 0.0050 mass% and Mg: 0.0005 to 0.0050 mass%. The ferritic stainless steel according to any one of the above.