JPH06136488A - Ferritic stainless steel with excellent workability, high temperature salt damage resistance and high temperature strength - Google Patents

Abstract

(57) [Abstract] [Purpose] The present invention, as an automobile exhaust system material, is capable of responding to improvements in high fuel efficiency, high output, and exhaust gas purification performance.
Provided is a ferritic stainless steel which is relatively inexpensive and has excellent workability, high temperature salt damage resistance and high temperature strength. [Structure] Solid solution strengthening of Mo and W is used for high temperature strengthening, and Mo and W are also used for improving high temperature salt damage resistance. In order to achieve both workability and high temperature strength, C + N is lowered, and Ti is further added to reduce the amount of solid solution C + N. As a result, solid solution strengthening at high temperature is sufficiently effectively performed, and stable high temperature strength for a long time and high temperature salt damage resistance are secured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a relatively inexpensive ferritic stainless steel used as a high temperature member such as an automobile exhaust pipe or a catalyst outer cylinder material.

[0002]

2. Description of the Related Art In recent years, the exhaust gas temperature has risen to a maximum of about 900 ° C. from the viewpoint of improving fuel efficiency and output of automobiles. Further, exhaust gas purification is also strongly required due to the stricter regulations. From such a background, weight reduction, high temperature high strength, and low heat capacity are desired for the exhaust system material. As a result, conventional castings such as SUS430LX and AISI409 are used for the exhaust manifold and front pipe.
Steel plates and steel pipes of such materials are currently used.
It is conceivable that the exhaust gas temperature will rise to around 900 ° C in the near future.

From the viewpoint of high temperature strength, SUS430LX is used as a high temperature member in which Nb is added from Ti. The addition of Nb improves the high temperature strength but raises the recrystallization temperature. Further, Nb has a strong affinity with C and N and easily forms carbonitrides, and the characteristics such as workability, toughness and high temperature strength are greatly influenced by the manufacturing process conditions, and it is necessary to strictly control many process conditions. Also, Cr
The amount is relatively large and is as high as 16 or more, which is often economically disadvantageous. AISI409 has a low Cr content and is economically inexpensive, but has much lower oxidation resistance, high temperature salt damage resistance, high temperature strength, and the like than SUS430LX. As described above, currently used materials have problems in terms of economy or characteristics.

Further, related publicly known examples can be broadly classified into those for automobile exhaust system materials and petroleum combustor materials and those for high temperature members of energy plants. The contents are as follows. First, regarding materials for automobile exhaust systems and materials for petroleum combustors, Japanese Patent Laid-Open No. 64-8254 and Japanese Patent Laid-Open No.
No. 175843 and JP-A-3-274245 are mentioned. In these chemical composition ranges, the high temperature strength is excellent and the workability is also considered in part, but the Cr content is 15% or more in weight% (hereinafter referred to as%), and Nb is 0.2 to 1. The upper limit is increased to 0%, and Mo is added from the viewpoint of corrosion resistance, which is economically disadvantageous. Also,
No particular consideration is given to high temperature salt damage resistance, which is one of the performances of the exhaust manifold and the front pipe.

Also, for high temperature parts of energy plants,
For example, Japanese Patent Application Laid-Open No. 3-59135 and Japanese Patent Application Laid-Open No. 4-5744 disclose materials for borer tubes and heat exchangers.
These are added with Mo, Nb, W and V to increase C or N, and mainly improve the high temperature strength by precipitation strengthening by carbonitrides and intermetallic compounds, and processing at the time of manufacturing exhaust manifolds and front pipes. Properties and resistance to high temperature salt damage are not fully considered. Further, the operating temperature is 700 ° C. or lower, and the component design differs from the exhaust manifold and the front pipe in the operating environment, that is, the operating temperature (maximum 800 to 900 ° C.) and the use in exhaust gas. It can be said that it is difficult to stably maintain the precipitation morphology that supports high temperature strength for a long time when this type of material is used for an automobile exhaust pipe for a long time.

[0006]

As described above, the presently used materials and known examples have problems in performance or economics in use environment as high temperature members such as automobile exhaust pipes and catalyst outer cylinder materials. . Therefore, the present inventors have conducted various researches and developments in order to achieve both good properties and economical efficiency, and have excellent manufacturability, workability, high-temperature salt damage resistance, and high-temperature strength that are superior to those of conventional SUS430LX. A heat-resistant ferritic stainless steel was completed.

That is, the Cr content is 11 to 17% and low Cr
It is intended to compensate for the deterioration of the corrosion resistance and the high temperature salt damage resistance by the combined addition of Mo and W and simultaneously improve the high temperature strengthening. In addition, Nb, which remarkably raises the recrystallization temperature, has a strong affinity with other elements and easily forms a precipitation phase, is not added, and addition of Ti adds C and N.
To improve the corrosion resistance by fixing the carbon and further suppress the precipitation reaction of carbonitrides of Mo and W to effectively utilize the effects of the corrosion resistance of Mo and W, high temperature salt damage resistance and high temperature strengthening for a long time. It is intended.

[0008]

[Means for Solving the Problems] Solid solution strengthening of Mo or W is used for high temperature strengthening, and Mo or W is also used for improving high temperature salt damage resistance. In addition, in order to achieve both workability and high temperature strength, C + N is lowered, and an appropriate amount of Ti is added to fix them to reduce the amount of solid solution C + N. At the same time, the solid solution strengthening of Mo and W at high temperature is effectively worked, and stable high temperature strength and high temperature salt damage resistance are secured for a long time. Further, addition of Ti also prevents coarsening of the grain size of the welded portion and the weld-affected zone.

On the other hand, Mo and W easily form an intermetallic compound with Fe. Due to this precipitation, toughness, high temperature strength, etc. during use decrease. Although Mo and W improve the high temperature salt damage resistance, as shown in Table 1, Si, Mo and W have the effect of increasing the high temperature salt damage resistance. However, Si deteriorates workability, and excessive addition of Mo and W (4% or more)
Is the opposite effect. Further, these high temperature strengthening or high temperature salt damage improving elements increase the recrystallization temperature, which is disadvantageous in view of manufacturability. In consideration of the above characteristics, the present invention particularly defines the optimum ranges of C, N, Ti, Mo and W.

[0010]

[Action]

C: This steel mainly supports high temperature strength by solid solution strengthening, and it is desired to keep it as low as possible from the viewpoint of improving workability and toughness of hot-rolled sheet. However, since extremely low temperature is not economical, it is 0.03 as a level that does not reduce high temperature strength.
%, And combined with N: C + N ≦ 0.04%. Si: Since it is a deoxidizing element, at least 0.1% is necessary.
Further, although it improves the oxidation resistance and the high temperature salt damage resistance, it reduces the workability, so the content was made 1% or less. Mn: Since it is a deoxidizing element, at least 0.1% is necessary.
Further, it is an austenite forming element and the upper limit was made 1% or less in order to prevent martensitic transformation.

P: It is useful for high temperature high strength (solid solution strengthening), but since it deteriorates weldability, it is set to 0.01 to 0.1%. S: MnS-forming element, which reduces the corrosion resistance to 0.
It was set to 01% or less. Cr: Effective in improving oxidation resistance and high temperature salt damage resistance,
The minimum resistance to oxidation and high temperature salt damage is AISI409
Therefore, the minimum value is set to 11%. Moreover, considering the maximum temperature around 900 ° C as the usage environment of this steel, it is 17%.
The above additions are not very effective, so the upper limit was made this.

Ti: C + N is fixed and is added to improve workability, weldability, and long-term stability of high-temperature strength. Ti is more C, N than W and Mo
It has a strong affinity with and is an important additive element for maintaining the solid solution strengthening effect of W and Mo at high temperature for a long time even during use. In order to fix C and N that do not form a solid solution in the mother phase, the minimum addition amount was set to 0.01%. Further, addition of Ti exceeding 0.5% lowers the high temperature strength, so the upper limit was made 0.5%. Mo: An additive element that enhances high temperature strength and high temperature salt damage resistance, and if it exceeds 3%, the high temperature salt damage resistance deteriorates, so 3
% Or less, and since the present steel has a reduced amount of Cr, it is a necessary additive element from the viewpoint of ensuring the corrosion resistance which is a basic characteristic of stainless steel. Further, from the viewpoint of improving the high temperature strength, 0.5% ≦ (Mo + W) ≦ 4%.

Similar to W: Mo, it is an additive element that enhances the high temperature strength and the high temperature salt damage resistance. If it exceeds 3%, the high temperature salt damage resistance deteriorates. It was set to 5% ≦ (Mo + W) ≦ 4%. N: This steel mainly supports high temperature strength by solid solution strengthening, and it is desired to keep it as low as possible from the viewpoint of improving workability and toughness of hot-rolled sheet. However, since extremely low temperature is not economical, it is 0.03 as a level that does not reduce high temperature strength.
%, And combined with C: C + N ≦ 0.04%.

[0014]

[Examples] Test steels having the chemical composition shown in Table 1 were melted by vacuum melting in an amount of 8 kg each, and then hot-rolled-cold-rolled-annealed pickling was performed to produce a 2 mm thin plate. Using this thin plate, a tensile test, a cycle aging, a high temperature salt damage test and a high temperature tensile test were carried out, and various properties thereof are shown in Table 2.

[0015]

[Table 1]

[0016]

[Table 2]

The addition of Mo and W is effective in improving the high temperature strength. Considering the securing of the high temperature strength after cycle heating,
It can be seen from the results of NUS1 and 5 that Mo and W are particularly effective. However, the comparative steel NUS24,2
As can be seen from 6 and 27, the high temperature salt damage resistance starts to deteriorate when 3% or more is added alone and 4% or more is added in the composite. Nb
It is also known from Material and Process 4 (1991) 6, 1796 that the high temperature strengthening effect of is higher than Mo and W. On the other hand, the addition of Nb alone is not very good, as can be seen from the example of the comparative steel SUS430LX, in consideration of ensuring the high temperature strength after cycle heating.

The amount of Ti added is NUS2 of the comparative steel.
As can be seen from FIG. 9, if it exceeds 0.5%, the initial high temperature strength is lowered, so the upper limit of the Ti addition amount is 0.1.
It was set to 5%. As described above, it is understood that the steel of the present invention is good in each item of high temperature strength, high temperature strength after cycle aging, high temperature salt damage and ductility at room temperature. In addition, the recrystallization temperature is 50 from that of SUS430LX in the comparative steel.
It has also been confirmed to be low near ℃.

[0019]

INDUSTRIAL APPLICABILITY The present invention is to find a component with well-balanced necessary characteristics in consideration of the use environment, especially as an automobile exhaust system material.
It is possible to provide relatively inexpensive ferritic stainless steel that can cope with higher output and improved exhaust gas purification performance.

Claims (1)

[Claims] 1. C: 0.03% or less by weight% Si: 0.1 to 1% Mn: 0.1 to 1% P: 0.01 to 0.1% S: 0.01% or less Cr: 11 to 17% Ti: 0.01 to 0.5% Mo: 0.01 to 3% or less W: 3% or less N: 0.03% or less 0.5% ≦ (Mo + W) ≦ 4% , (C +
N) A ferritic stainless steel which is excellent in workability, high temperature salt damage resistance and high temperature strength, characterized by satisfying N) ≤ 0.04%, and the balance consisting of iron and impurities that can be inevitably mixed in during manufacturing.