JPH057458B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Fields] Steel used in automobile exhaust systems has traditionally been required to have excellent high-temperature corrosion resistance, but recently chlorides have also been sprayed on roads to melt snow. Therefore, it has become important to ensure high-temperature corrosion resistance in the presence of chlorides. The present invention relates to an austenitic stainless steel that exhibits excellent high-temperature corrosion resistance in such a dry corrosion environment in which chlorides coexist. [Prior Art] When considering high temperature corrosion resistance of metal materials, balance with cost is important. Particularly, when it comes to materials such as automobile exhaust systems, which require cost reduction through mass production, cost requirements are strict. Austenitic stainless steel containing Cr and Ni is well known as a metal material that achieves both cost and high-temperature corrosion resistance. Materials based on austenitic stainless steel are often used for applications that require high-temperature corrosion resistance, such as automobile exhaust systems. For example, Tokuko Sho 57-16187 and Tokuko Sho No.
The austenitic stainless steel described in Japanese Patent No. 58-42264 is given excellent high-temperature corrosion resistance by regulating the amount of S. [Problems to be solved by the invention] However, although such conventional steels do have excellent high-temperature corrosion resistance, they have poor high-temperature corrosion resistance in dry corrosion environments where chlorides coexist, such as when chlorides are attached. Gender is not taken into account. With regard to automobile exhaust systems, it has become important to consider high temperature corrosion resistance in a dry corrosive environment where chlorides coexist, as large amounts of snow melting agents are recently being sprayed on roads. In addition to automobile exhaust systems, we also provide materials for waste incinerators, waste heat utilization equipment, and kitchens.
It is important to ensure high-temperature corrosion resistance in a dry corrosion environment where chlorides coexist. The present invention provides an austenitic stainless steel in which the amount of alloying components added is minimized and the high-temperature corrosion resistance in a dry corrosion environment in which chlorides coexist is improved without deteriorating economic efficiency. [Means to solve the problem] High-temperature corrosion in a dry corrosion environment with chlorides
Although the rate of corrosion thinning is high, it is characterized by extremely high grain boundary erosion. Therefore, simply reducing the rate of corrosion thinning will not improve high-temperature corrosion resistance in a dry corrosion environment where chlorides coexist, and it will not be possible to comprehensively suppress corrosion in conjunction with grain boundary corrosion. becomes important. In other words, corrosion thinning is a general corrosion phenomenon,
Grain boundary erosion is a phenomenon in which only grain boundaries are eroded from the alloy surface toward the inside. In a dry corrosion environment where chlorides coexist, chlorides react with oxides on the alloy surface to form double oxides, and at this time, the liberated Cl penetrates into the alloy through grain boundaries and reacts with the metal, causing the metal to react. Oxidized. The metal chloride formed at this time generally evaporates easily, and once it evaporates, an oxidation reaction occurs again, whereby grain boundaries are selectively eroded. For the above reasons, grain boundary corrosion accounts for a large proportion in a dry corrosion environment where chlorides coexist. From this point of view, the present inventors conducted a detailed study on the high-temperature corrosion of stainless steel in a dry corrosion environment in which chlorides coexist, and as a result, the following findings were obtained. First, although Cr has traditionally been considered to be an element that is universally effective against high-temperature corrosion, the results of investigating its effectiveness in a dry corrosion environment in which chlorides coexist have shown that, contrary to expectations, Cr causes corrosion thinning, grain It was found that it is a harmful element that is harmful to both types of corrosion and significantly promotes high-temperature corrosion as a whole. The reason for this is that the previously mentioned free Cl combines with Cr in the steel and evaporates as Cr chloride, causing grain boundary erosion.The surface layer with reduced Cr has lower oxidation resistance and oxidation progresses. This is thought to be for the purpose of Figure 1 shows the corrosion status of various stainless steels containing 25 to 30% Ni and varying amounts of Cr when immersed in a saturated NaCl solution and heated for 2 hours at -730°C for 10 cycles in relation to the amount of Cr. This is what is shown.
In addition, in order to eliminate the effects of the amount of C, grain size, and amount of Si and Mo, which will be described later, C0.02 to 0.03%, grain size number 6.5, Si 0.5 to 0.7%, and Mo 0.1 to 0.2%. As seen in the figure, as the amount of Cr increases, corrosion thinning becomes more prominent, and intergranular corrosion also becomes more significant, so the total amount of corrosion increases rapidly. Second, when we investigated the effect of C, we found that although C is effective against corrosion thinning, it is extremely harmful to grain boundary erosion, and overall it is an element that increases high-temperature corrosion. It has been found. The reason is that Cr carbide precipitates at grain boundaries when heated to high temperatures. This is thought to be because the Cr carbide decomposes due to the reaction between the carbide and Cl, and grain boundary erosion progresses significantly. Figure 2 shows 18Cr-10Ni stainless steel with varying C content, immersed in saturated NaCl solution -650
C shows the corrosion state after 10 cycles of heating at ℃.
It is shown in relation to quantity. The grain size number is
6.5, Si was 0.4 to 0.6%, and Mo was 0.1 to 0.3%. As seen in the figure, as the amount of C increases, corrosion thinning decreases, but the total amount of corrosion increases. This is simply because an increase in the amount of C significantly increases grain boundary erosion. Thirdly, when the crystal grains are refined, corrosion loss increases, but grain boundary erosion is largely suppressed, and high-temperature corrosion is suppressed as a whole. The reason for this is thought to be that by making the grains finer, the number of grain boundary paths becomes larger and longer, thereby suppressing the progress into the interior. Figure 3 shows the corrosion status of 18Cr-10Ni stainless steel containing 0.02 to 0.03% C and varying the grain size after 10 cycles of immersion in saturated NaCl solution and heating at -650°C. This is shown in the relationship. The crystal grain size is a grain size number defined by JIS G0551, and all crystal grain size numbers in this specification refer to this value. Si 0.5-0.7%, Mo
was set at 0.1 to 0.3%. As seen in the figure, as the grain size becomes smaller, that is, the grain size number becomes larger, corrosion thinning becomes more pronounced depending on the degree, but grain boundary corrosion is largely suppressed, and as a result, the total The amount of erosion will be suppressed. Fourth, Si and Mo are typical constituent elements of stainless steel, and both are effective in suppressing both corrosion thinning and grain boundary erosion. The reason for this is that an oxide scale (SiO ₂ ) mainly composed of Si is formed below the surface scale, and this suppresses the corrosion of Cl into the interior more than the oxide scale of Fe or Cr. Significantly lowers the oxygen partial pressure on the surface,
This is thought to be because it prevents Fe oxidation and is also effective against corrosion and thinning. Figure 4 shows an investigation of the effects of Si and Mo on high-temperature corrosion in a dry corrosion environment in which chlorides coexist. The steel that was investigated was 17Cr-25Ni stainless steel with a grain size number of 6.5 with a C content of 0.02 to 0.03% for the Si influence investigation, and a C0.02 to 0.03% steel for the Mo influence investigation. It is 20Cr-30Ni stainless steel and the grain size number is 7.0. The test consisted of 10 cycles of immersion in a saturated NaCl solution and heating at -750°C.
As is clear from the figure, as the Si content and Mo content increase, corrosion thinning and grain boundary erosion are suppressed, effectively reducing the total amount of corrosion. however,
Since Mo is more expensive than Si, it is desirable to rely on Si as much as possible. Fifth, it was also found that Mn is harmful to high-temperature corrosion resistance in a dry corrosion environment in which chlorides coexist, and N is an effective element. The stainless steel of the present invention was developed based on this knowledge, and has a weight percentage of less than 0.03% C and 10% Cr.
~20%, Ni10~30%, Mn2% or less, Si over2%6%
Contains the following, with the balance consisting of Fe and unavoidable impurities, and the basic structural requirements are that the crystal grain size is fine with grain size number 6 or more, and the total erosion amount shown by formula (1) is 500 or less. It is. 24.4Cr + 2.8Ni + 6.7Mn -48.8Si (-56.9Mo - 148.0Nb) ...(1) Furthermore, if necessary, one or more of the following three component systems (1) to (3) may be used. Added selectively. (1) Any one or two of Mo0.5-5% and N0.02-0.4%. (2) A total of 0.1 to 1% of any one or more of Ti, Nb, Zr, and Ta. (3) A total of 0.001 to 0.1% of one or more of Ca, Mg, Al, Y, and rare earth elements. The reasons for limiting the composition, grain size, and total erosion index of the stainless steel of the present invention will be described below. Γ component composition C: As shown in FIG. 2, in a dry corrosion environment in which chloride coexists, grain boundary corrosion increases significantly and high temperature corrosion resistance decreases, so it should be less than 0.03%. Since the lower the amount of C, the higher the high temperature corrosion resistance is, no lower limit is specified. Si: As shown in Figure 4, it has a large high-temperature corrosion inhibiting effect in a dry corrosion environment with chlorides, and has a 2%
It is assumed that the content of However, from the viewpoint of ensuring workability and weldability, the upper limit is set at 6%. Mn: Mn decreases high temperature corrosion resistance in a dry corrosion environment where chlorides coexist, so the upper limit is set at 2%.
However, on the one hand, this element is essential for steelmaking and ensuring workability, and at a minimum
It is customary to contain about 0.2%. Cr: As shown in Fig. 1, it is limited to 20% or less because it significantly reduces high-temperature corrosivity in a dry corrosion environment in which chlorides coexist. However, this element is an important element in ensuring high-temperature oxidation resistance in normal exhaust gas that does not coexist with chlorides, and requires addition of 10% or more. Ni: Since it does not have a large effect on high temperature corrosion resistance in a dry corrosion environment where chlorides coexist, Ni should be added at a content of 10% or more to ensure a fully austenitic structure. However, since increasing the amount added leads to increased costs, it should be kept at 30% or less. Mo, N: Both are elements effective for high temperature corrosion resistance in a dry corrosion environment where chlorides coexist, and are added as necessary. The addition amount of Mo is required to be 0.5% or more in order to obtain a reliable addition effect, but since it is an expensive element, 5% or more is required.
The upper limit is %. As for N, it is contained at about 0.02% even if it is not specifically added, so when it is added, the content is 0.02% or more. however,
Processability is ensured by setting the upper limit of solid solubility at 0.4%. Ti, Nb, Zr, Ta: All are effective elements for increasing high-temperature corrosion resistance in a dry corrosion environment where chlorides coexist, and a clear addition effect can be obtained when the total amount added is 0.1% or more. . However, as the amount added decreases processability,
Keep the total amount below 1%. Ca, Mg, Al, Y, rare earth elements: These are effective in increasing the cleanliness of steel. Increasing the cleanliness of steel promotes the diffusion of harmful elements such as Cr as well as beneficial elements such as Si in the steel. Although corrosion thinning becomes slightly more pronounced due to the diffusion of harmful elements, the front surface corrosion tendency increases, grain boundary corrosion is easily suppressed by Si, etc., and overall it acts as a beneficial element. When adding, the addition effect can be obtained when the total amount is 0.001% or more, but if it exceeds 0.1%, workability and weldability deteriorate, so 0.001 to 0.1%
The range shall be . Unavoidable impurities: For example, it is preferable to have less P and S in terms of weldability, etc., but each is 0.03%
If it is below, it can be used for normal purposes, so it is not particularly stipulated. Γ Grain size As shown in Figure 3, as the grain size becomes smaller, corrosion thinning becomes more pronounced, but grain boundary erosion, which is a characteristic form of corrosion in a dry corrosion environment with chlorides, is greatly reduced. This will ultimately reduce total erosion. Total erosion is effectively suppressed in the fine grain region where the grain size is 6 or more. Since the smaller the grain size is, the more the total erosion is suppressed, there is no upper limit on the grain size number. However, on the other hand, the grain refinement tends to increase the strength at room temperature,
Since it becomes difficult to process, it is practically desirable to limit the particle size number to 10 or less from the viewpoint of workability. Means for adjusting the grain size to grain size number 6 or higher include adjusting the degree of cold working, solution temperature, and time, and by this adjustment, a predetermined grain size can be easily obtained. In addition, in conventional stainless steels in which only high-temperature corrosion resistance is considered, the grain size is adjusted to about 4 to 5, mainly from the viewpoint of high-temperature ductility and workability. ΓTotal corrosion amount Regarding high-temperature corrosion resistance in a dry corrosion environment in which chlorides coexist, as mentioned above, the total corrosion amount is expressed as the sum of the corrosion thickness reduction amount and the grain boundary erosion amount, and the corrosion reduction Various factors are intricately related to meat and grain boundary erosion. Therefore, it is difficult to obtain a satisfactory total amount of erosion only by reviewing individual factors such as the amount of C and the amount of Cr. As a result of multiple regression analysis of the influence of various components on the total amount of erosion, the present inventors found that the total amount of erosion was
We found that it can be expressed by equation (1). Equation (1) represents the total amount of corrosion when steel with a C content of 0.03% or less is immersed in a saturated NaCl solution and heated at -750°C for 10 cycles, and the target high-temperature corrosion resistance is 1.5 times that of SUS304 steel. In this case, the total erosion amount expressed by equation (1) needs to be 500 or less. In addition, in formula (1), Nb is Ti, Nb, Zr,
Ta is represented by Nb, and it represents the total content of this element group. [Example] Each 17 kg of test steel whose composition is shown in Table 1 was melted by vacuum melting, and then hot rolled and cold rolled to a thickness of 4.9 kg.
After forming steel plates with a thickness of 1.5 mm, they were subjected to solution treatment, and then test specimens with a width of 20 mm, a length of 30 mm, and a thickness of 3 mm were taken from each steel plate. In order to evaluate high-temperature corrosivity in the presence of chlorides, each test piece was immersed in a saturated NaCl solution for 5 minutes, held at 650 to 750°C for 2 hours, and cooled in air for 5 minutes.
Ten cycles were repeated, after which the total amount of corrosion was investigated. The total corrosion amount was determined by investigating the amount of corrosion thinning, measuring the maximum depth of grain boundary corrosion from cross-sectional microscopic observation, and adding the two. The results are shown in Table 1. Note that the crystal grain size was adjusted by the heat treatment temperature and time. In Table 1, test steels No. 1 to 20 are steels of the present invention,
Nos. 21 to 24 are conventional steel. It can be seen that the steel of the present invention has significantly superior high-temperature corrosion resistance in a dry corrosion environment in which chlorides coexist as compared to conventional steel.

〔Effect of the invention〕

As is clear from the above description, the stainless steel of the present invention has excellent high-temperature corrosion resistance in a dry corrosion environment where chlorides coexist, so it can withstand high temperatures even when snow-melting agents adhere to the exhaust system of an automobile. Furthermore, it can be used in waste incinerators, waste heat utilization equipment, kitchens, etc., and exhibits a high degree of durability. In addition, the stainless steel of the present invention has improved high-temperature corrosion resistance by increasing the amount of Si, and on the contrary, it has reduced Cr content, so it is extremely economical.

[Brief explanation of the drawing]

FIGS. 1 to 4 are graphs showing the degree of influence of various factors that affect high temperature corrosion resistance in the presence of chlorides.

Claims (1)

[Claims] 1. Less than 0.03% C by weight, 10-20% Cr, 10-10% Ni
Contains 30%, Mn 2% or less, Si more than 2% and 6% or less,
The balance consists of Fe and unavoidable impurities, and the grain size is fine with grain size number (JIS) 6 or more, and the following formula is 500 or less. Excellent high-temperature corrosion resistance in the coexistence of chlorides. stainless steel. 24.4Cr+2.8Ni+6.7Mn−48.8Si 2 Weight% less than C0.03%, Cr10~20%, Ni10~
30%, Mn2% or less, Si over 2% and 6% or less, and
Contains one or two of Mo0.5-5% and N0.02-0.4%, the balance is Fe and unavoidable impurities, and the crystal grain size is determined by the grain size number (JIS).
A stainless steel with excellent high-temperature corrosion resistance in the coexistence of chlorides, characterized by having fine grains of 6 or more and the following formula of 500 or less. 24.4Cr+2.8Ni+6.7Mn−48.8Si−56.9Mo 3 Weight% less than C0.03%, Cr10~20%, Ni10~
30%, Mn2% or less, Si over 2% and 6% or less, and
Any one or two of Ti, Nb, Zr and Ta
A chloride containing a total of 0.1 to 1% of seeds or more, the remainder consisting of Fe and unavoidable impurities, and further having a fine crystal grain size of grain size number (JIS) 6 or more and the following formula of 500 or less Stainless steel with excellent high-temperature corrosion resistance in the presence of substances. 24.4Cr+2.8Ni+6.7Mn−48.8Si −56.9Mo−148.0Nb 4 Weight% less than C0.03%, Cr10~20%, Ni10~
30%, Mn2% or less, Si over 2% and 6% or less, and
Contains a total of 0.1 to 1.0% of any one or two of Mo0.5 to 5% and N0.02 to 0.4%, and one or two or more of Ti, Nb, Zr, and Ta,
The balance consists of Fe and unavoidable impurities, and the grain size is fine with grain size number (JIS) 6 or more, and the following formula is 500 or less. Excellent high-temperature corrosion resistance in the coexistence of chlorides. stainless steel. 24.4Cr+2.8Ni+6.7Mn−48.8Si −56.9Mo−148.0Nb 5 Weight% C less than 0.03%, Cr10~20%, Ni10~
30%, Mn2% or less, Si over 2% and 6% or less, and
Any one of Ca, Mg, Al, Y, and rare earth elements
Contains a total of 0.001 to 0.1% of a species or two or more species,
The balance consists of Fe and unavoidable impurities, and the grain size is fine with grain size number (JIS) 6 or more, and the following formula is 500 or less. Excellent high-temperature corrosion resistance in the coexistence of chlorides. stainless steel. 24.4Cr+2.8Ni+6.7Mn−48.8Si 6 Weight% C less than 0.03%, Cr10~20%, Ni10~
30%, Mn2% or less, Si over 2% and 6% or less, and
Any one of Mo0.5%~5% and N0.02~0.4%
A total of 0.001 or more of one or more of Ca, Mg, Al, Y, and rare earth elements
0.1%, the balance consists of Fe and unavoidable impurities, and the crystal grain size is fine with grain size number (JIS) 6 or more, and the following formula is 500 or less. Stainless steel with excellent high temperature corrosion resistance. 24.4Cr+2.8Ni+6.7Mn−48.8Si−56.9Mo 7 Weight% less than C0.03%, Cr10~20%, Ni10~
30%, Mn2% or less, Si over 2% and 6% or less, and
Any one of Mo0.5%~5% and N0.02~0.4%
species or two species, any one or more of Ti, Nb, Zr and Ta, total 0.1 to 1%, Ca,
Contains a total of 0.001 to 0.1% of one or more of Mg, Al, Y, and rare earth elements, with the remainder
It consists of Fe and unavoidable impurities, and has a fine crystal grain size of grain size number (JIS) 6 or more and the following formula:
Stainless steel with excellent high-temperature corrosion resistance in the coexistence of chlorides, characterized by a corrosion resistance of 500 or less. 24.4Cr＋2.8Ni＋6.7Mn−48.8Si −56.9Mo−148.0Nb