JPS6115146B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an austenitic stainless steel that is relatively inexpensive and has excellent oxidation resistance. Currently, it is used as a material for automobile exhaust gas purification equipment.
Si-based austenitic stainless steel is used. High Si austenitic stainless steel
Shows excellent heat resistance due to the oxidation resistance improvement effect of Si and Cr. Furthermore, it has been reported that oxidation resistance is significantly improved by adding rare earth elements and Ca [Iizumi et al., Tetsu to Hagane, 63 (1977), 715]. However, high
When Si-based austenitic stainless steel is used at high temperatures, pit-shaped local oxidation occurs in a dispersed manner.
It has been found that although it appears to have sufficient oxidation resistance based on the apparent oxidation loss, this can be a serious defect in use. When the cross section of this defect is observed, as shown in FIG. 1, it is a cluster of half-moon-shaped scales.
Such defects significantly deteriorated the oxidation resistance of the product. Dry this half-moon-shaped scale mass.
When descaled by honing, the diameter is 100 to 200.
μ, it becomes a depression about 20 to 70 μ deep. Because of this form, this defect is called a pit flaw. The purpose of the present invention is to develop a steel that does not produce pit flaws during use at high temperatures. As a result of detailed research and study on the causes of pit flaws, the present inventor found that pit flaws can be reduced by lowering the S content and at the same time containing an amount of Ca commensurate with the S content, as shown in Figure 2. We have discovered that it is possible to suppress the occurrence of That is, it has been found that there is a certain relationship between the S content and the Ca content, and that pitting defects no longer occur when Ca/S is 0.8 or more. That is, in the present invention, C: 0.03 to 0.08%, Si: 3.00 to
4.00%, Mn: 1.00% or less, S: 0.005% or less,
Ni: 13.00~14.50%, Cr: 18.00~20.00%, Cu:
Summary of an austenitic stainless steel with excellent oxidation resistance, containing 0.49% or less, N: 0.03 to 0.10%, Ca: 0.8×%S or more and 0.010% or less, with the balance consisting of Fe and inevitable impurities. That is. Next, the reason for limiting the content of each alloying element in the present invention will be explained. C: C is an austenite forming element and at the same time is an important element for obtaining high temperature strength. In order to improve high temperature strength, if it is less than 0.03%, it will have little effect, and if it is added in excess of 0.08%, coarse chromium carbide will precipitate at the grain boundaries during high temperature use, causing material embrittlement, so it is not appropriate. range 0.03
Limited to ~0.08%. Si: The higher the Si content, the better the oxidation resistance, but if the Si content exceeds 4%, no further effect can be expected and the price will increase, so the Si content should be 4% or less. On the other hand, if it is too low, the oxidation resistance will deteriorate, so the content should be 3% or more. Mn: Mn is an austenite-forming element, and Ni
However, if too much, the oxidation resistance deteriorates and the price increases, so the amount contained in ordinary stainless steel should be 1% or less. S: S is an important element in order to obtain the excellent oxidation resistance of the present invention. If the S content is high, pitting defects will occur and the oxidation resistance will deteriorate, so it is desirable to keep it as low as possible. At the same time as suppressing the occurrence of pit defects, in relation to the Ca content, it should be 0.005% or less. Ni: Ni is one of the basic elements of austenitic stainless steel. In order to maintain the austenitic structure, Ni should be at least 13%; on the other hand, if it is too high, weldability will deteriorate and the price will also increase.
14.50% or less. Cr: Cr is the most basic element to maintain oxidation resistance at high temperatures. If it is less than 18%, sufficient oxidation resistance cannot be obtained and weldability deteriorates. On the other hand, when it exceeds 20%, a large amount of δ-ferrite is generated, and a large amount of Ni is required to maintain the balance.
Becomes expensive. Therefore, Cr is limited to 18.00 to 20.00%. Cu: Cu is an element that does not affect oxidation resistance. However, since Ni has the effect of stabilizing the austenite structure and is mixed in as an impurity from the Ni raw material, it is set at 0.49% or less. N: Since N is an austenite-forming element,
It is an effective element for saving Ni and improving high temperature strength. Too much will deteriorate cold workability.
0.1% or less. On the other hand, if it is too low, the stability and high temperature strength of the austenite structure will deteriorate.
Must be 0.03% or more. Ca: The most important element in order to obtain the excellent oxidation resistance of the present invention. When Ca is added to the austenitic stainless steel of the present invention, the protective properties of the oxide film composed of Cr, Si, etc. are significantly improved.
Thereby, the occurrence of pit flaws can be suppressed and oxidation resistance can be significantly improved. To obtain this effect, the Ca content must be 0.8 times or more the S content. If the Ca content is too high, inclusions will increase and costs will increase, so the Ca content should be 0.010% or less. Next, the present invention will be specifically explained with reference to Examples. Table 1 shows the chemical compositions of the invention steel and comparative steel used in this test.

[Table] These steels were melted using known methods, hot-rolled, cold-rolled, and solution annealed. Test pieces of 10 mm x 30 mm x plate thickness were prepared and subjected to a continuous heating test at 1200°C for 20 hours. . Table 2 shows the occurrence of pit flaws after heating at 1200°C for 20 hours. Pit flaws are removed by dry honing after continuous heating, and then
Judgments were made by visual observation and according to the criteria in Table 3.

[Table] As shown in Table 2, it can be seen that the steels of the present invention do not develop pitting defects and have superior properties compared to the comparative steels. The results of Tables 1 and 2
Figure 3 shows the results organized by Ca/S. As shown in FIG. 3, when Ca/S is set to 0.8 or more, no pit flaws occur regardless of the S content.
When Ca/S is less than 0.8, pit flaws occur, but when the S content is low, the pit flaw rank shifts to the better side even if the Ca/S ratio is the same. As shown in the above examples, low S and
By containing Ca, it is possible to supply an austenitic stainless steel with excellent oxidation resistance and no pitting defects.

[Table] The applicable temperature range of the present invention is 850 to 1200°C, preferably 900 to 1150°C. When the temperature exceeds 1200°C, the oxidation loss increases rapidly, and at 700 to 850°C, there is a risk of σ embrittlement. As mentioned above, the steel of the present invention can be heated to 850 to 1200℃.
It is a relatively inexpensive austenitic stainless steel that has excellent oxidation resistance in high-temperature oxidizing atmospheres, and is especially excellent in that pit flaws do not occur during oxidation. The steel of the present invention has excellent properties as a material for automobile exhaust gas purification devices, but it can also be used in devices exposed to high-temperature combustion atmospheres, such as garbage incinerators.

[Brief explanation of the drawing]

Figure 1 is a metallographic micrograph of a cross section of a pit flaw (500x magnification), Figure 2 is a diagram showing the influence of S and Ca on the occurrence of pit flaws, and Figure 3 is similar to Figure 2.
It is a diagram organized by Ca/S.

Claims (1)

[Claims] 1 C: 0.03-0.08%, Si: 3.00-4.00%, Mn:
1.00% or less, S: 0.005% or less, Ni: 13.00-14.50
%, Cr: 18.00-20.00%, Cu: 0.49% or less, N:
An austenitic stainless steel with excellent oxidation resistance, characterized by containing 0.03 to 0.10%, Ca: 0.8x% or more and 0.010% or less, and the balance consisting of Fe and inevitable impurities.