JPH0380867B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention is based on Ni-Cr stainless steel SUS304, which improves its corrosion resistance and machinability, and which can be particularly preferably used as a material for food equipment. It is related to. <Prior art> The chemical components of SUS304 specified by JIS are shown in Table 1.

[Table] SUS304 is widely used as a corrosion-resistant material, but its machinability is very poor. When free machinability is required, conventional methods have generally been adopted in which sulfide-based inclusions (MnS) are intentionally generated at the expense of significant corrosion resistance. However, in order to place special emphasis on corrosion resistance and make it compatible with strongly corrosive environments (e.g., chloride environments, acidic beverage environments, etc.), it is necessary to
By lowering the composition ratio Mn/S ratio of MnS, solid solution Cr in MnS
It is said that increasing the amount is effective ("Tetsu to Hagane", 70 (1984), p. 741). <Problems to be Solved by the Invention> Although it is possible to some extent to improve machinability without impairing corrosion resistance by adjusting the above-described production of MnS and reduction of the Mn/S ratio in a well-balanced manner, I still wasn't completely satisfied. Therefore, this invention is based on SUS304,
This was done with the aim of providing a Ni-Cr stainless steel with even better corrosion resistance and machinability. <Means and effects for solving the problems> Corrosion resistance and machinability have been improved by this invention.
Ni-Cr stainless steel is a basic component of SUS304 stainless steel with some modifications, and has the following chemical composition: C0.08% by weight or less,
Si 1.0% by weight or less, Mn 0.7% by weight or less, P 0.04% by weight or less, S 0.005% by weight or less, Ni 8.0 to 12.0% by weight,
Cr17.0~20.0wt%, Mo0.40~0.80wt%,
Cu0.3wt% or less, Bi0.03~0.1wt%, Sn0.03~
0.2% by weight, and the balance is Fe. Bi is an element that improves machinability, and if it is less than 0.03% by weight, it has little effect on improving machinability, while if it is less than 0.1% by weight, it impairs forgeability, and furthermore, it tends to cause pitting corrosion, which has a negative impact on corrosion resistance. affect Therefore, in this invention, Bi is used in a range of 0.03 to 0.1% by weight. Sn not only improves machinability but also general corrosion resistance and crevice corrosion resistance. In particular, the stainless steel of this invention with a combination of Sn and Bi added,
Overall corrosion resistance, pitting corrosion resistance, compared to the addition of Bi only.
Can improve crevice corrosion resistance. In addition, in a dilute sulfuric acid aqueous solution, Sn precipitates on the steel surface, increases hydrogen overvoltage, and improves sulfuric acid resistance (in this case, general corrosion resistance). The corrosion resistance improvement effect of Sn addition as described above is less effective at 0.03% by weight or less;
On the other hand, if it is added in excess of 0.2% by weight, forging cracks will occur, making it impossible to use it as a processed material. Furthermore, although Mo and Cu have the effect of improving corrosion resistance in general, too much Cu may reduce organic acid corrosion resistance. However, as in the present invention, by suppressing Cu to 0.3% by weight or less, organic acid corrosion resistance can be improved. Furthermore, if Mo is less than 0.40% by weight, it may become ineffective in corrosion resistance.
If it is less than 0.80% by weight, the effect on improving corrosion resistance will be small compared to the amount added, and the cost will increase, so 0.40 to 0.80% by weight is optimal in this invention. Regarding S and Mn, as described above, reducing these amounts improves corrosion resistance, but on the other hand, machinability is reduced. In this invention, corrosion resistance is improved by setting S to 0.005% by weight or less and Mn to 0.7% by weight, while machinability decreases.
This can be supplemented by adding Bi and Sn in combination. Ni is the basic element of austenitic (γ) stainless steel and stabilizes the γ phase. In terms of strength, it contributes to improving toughness. With low Ni, the γ phase becomes unstable, which induces martensite during processing, which hardens and reduces toughness. Since Ni is electrochemically more noble than Fe and Cr, it suppresses corrosion in the active state region. It also provides significant resistance to corrosion by neutral chloride solutions and non-oxidizing acids, and enhances passivity. In this invention, since Sn, which is a ferrite-forming element, is added, the γ phase is stabilized by adding more Ni than the SUS304 standard. Cr is a basic component of stainless steel and contributes to passivation of stainless steel in oxidizing environments.
That is, the corrosion resistance of stainless steel is maintained by this passive film, and Cr is an essential element for stainless steel. Regarding the value of C, as specified in JIS, it has an important meaning from the viewpoints of metallographic stability, mechanical properties, corrosion resistance, manufacturing cost, etc. of the material.
That is, C is a strong austenite-forming element, and in order to increase the strength by interstitial solid solution, it is necessary to add a large amount within the solid solubility limit. However, if the amount of C is excessively increased, and when subjected to thermal effects such as heat treatment or welding, Cr ₂₃ C ₆ , Cr ₇ C ₃ , NbC,
Forms carbides such as TiC, leading to decreased corrosion resistance. In particular, when Cr carbide is formed, the amount of Cr in the matrix is reduced and the corrosion resistance is extremely deteriorated, so it is necessary to keep the amount below a certain level. Furthermore, considering the fact that it can be manufactured at a lower cost if the C content is determined according to SUS304, which is the most commonly produced material, the C content is set to 0.08% by weight or less. The value of Si also has important meaning in terms of manufacturability, mechanical properties, manufacturing cost, etc. The addition of Si increases tensile strength and elastic limit, and also improves corrosion resistance.
Forms a SiO ₂ film, improving corrosion resistance. on the other hand,
Si is a strong ferrite-forming element and destabilizes austenite. Si is also used in the rolling process.
If there is too much, problems such as cracking will occur. Taking these things into consideration, it is best to suppress the amount of Si to 1.0% by weight or less as stipulated by JIS in terms of various material properties and manufacturing costs. P is an element that has many harmful effects. As P increases, susceptibility to pitting corrosion increases, and P is also polarized at grain boundaries of austenite, causing intergranular corrosion. In addition, since it impairs stress corrosion cracking properties, it is better to have as little as possible. However, achieving extremely low P in the steelmaking process is technically difficult and extremely costly. Considering these points, the amount of P is
It is best from both performance and cost standpoints to keep the amount below 0.04% by weight as stipulated by JIS. <Example> Sample 2 (steel type with composite addition of Bi and Sn) of the present invention and comparative sample 1 (steel type with only Bi added) having the chemical compositions shown in Table 2 were prepared.

[Table] (1) Improvement in general corrosion resistance
This shows the corrosion rate in boiling water. The corrosion rate of Bi and Sn composite added steel is less than 1/3 of that of Bi only added steel, indicating improved corrosion resistance. Figure 2 shows samples 1 and 2 of dilute sulfuric acid (5% sulfuric acid,
boiling) and [lactic acid + salt] solution (5% lactic acid +
Corrosion rate in 1% salt (boiling) is shown. In both solutions, the corrosion rate of the steel with Bi and Sn composite additions was lower than that of the steel with only Bi added, indicating improved corrosion resistance. (2) Evaluation from the amount of Fe and Cr eluted Figure 3 shows samples 1 and 2 immersed in a [lactic acid + salt] solution (10% lactic acid + 0.3% salt) and heated at 40℃.
The results of measuring the amount of Fe and Cr eluted after being left for 55 days are shown. In terms of Fe elution amount, sample 1 is
The amount of elution is 150 ppm or more, but the amount of elution for steel with Bi and Sn composite addition is less than 1/10 of that of sample 1. Also, in terms of Cr elution amount, sample 2 is 1/1 of sample 1.
It is about 0. In this way, Bi and Sn composite additive steel types have significantly improved corrosion resistance, as evaluated from the amounts of Fe and Cr eluted. (3) Improvement in pitting corrosion resistance Figure 4 shows the pitting corrosion occurrence potential (V'c100) of samples 1 and 2 in a salt solution (3% salt, 30℃).
This is the result of measuring. Generally, a higher value means that pitting corrosion is less likely to occur. Bi,
The V′c100 of Sn composite added steel is the same as that of Bi only added steel.
The average value is 50 mV nobler than V′c100, and Bi,
It can be seen that Sn composite addition improves pitting corrosion resistance. (4) Improvement of crevice corrosion resistance Figure 5 shows the repassivation potential (E _R ) of samples 1 and 2 in a salt solution (3% salt, 30°C).
This is the result of measuring. Generally, the higher this value, the easier it is to stop crevice corrosion. According to FIG. 5, the E _R of the Bi- and Sn-added steel is higher than that of the Bi-only added steel, indicating that the Bi-Sn combined addition improves the crevice corrosion resistance. (5) Improvement in machinability Figure 6 shows the tool life when drilling holes in samples 1 and 2 using high-speed steel (SKH-51 (φ4)). The tool life for steels with Bi and Sn composite additions is greater than that for steels with only Bi additions;
It can be seen that the combined addition of Sn improves machinability compared to the case of adding Bi alone. <Effects of the Invention> As can be seen from the above explanation, the stainless steel of the present invention can significantly improve both the corrosion resistance and machinability of SUS304 stainless steel, and is particularly preferably used as a material for food equipment where corrosion resistance is important. It is possible.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between Samples 1 and 2 and the corrosion rate in dilute hydrochloric acid. FIG. 2 is a diagram showing the relationship between Samples 1 and 2 and the corrosion rate in dilute sulfuric acid and [lactic acid + salt] solutions. Figure 3 shows [lactic acid + salt]
FIG. 2 is a diagram showing the relationship between Samples 1 and 2 and the amounts of Fe and Cr eluted in the solution. FIG. 4 is a diagram showing the relationship between Samples 1 and 2 in a saline solution and the pitting corrosion occurrence potential (V'c100). Figure 5 shows sample 1 in saline solution,
2 and the repassivation potential (E _R ).
FIG. 6 is a diagram showing the relationship between Samples 1 and 2 and tool life when holes are drilled using high-speed steel.

[Claims]

1 Vanes used in liquid or gas compressors have a composition of 0.7 to 3.0% C and 0.1% Si by weight.
~1.5%, Cr2.5~7.0%, W and 2Mo total 10.0~
A sintered body consisting of 20.0%, V0.5~6.0%, Co10.0% or less, the balance Fe and unavoidable impurities, and the sintered body has pores with a maximum diameter of 100μ or less in area% of 1~10.
%, the hardness of the base part is Hv800-1000, and in the base, M ₆ C with an average particle size of 5μ or less and carbide having an MC type crystal structure are uniformly dispersed in the volume% of 9-15%. Vanes with excellent wear resistance and sliding properties. 2 Vanes used in liquid or gas compressors have a composition of 0.7 to 3.0% C and 0.1% Si by weight.
~1.5%, Cr2.5~7.0%, W and 2Mo total 10.0~
20.0%, V0.5~6.0%, Co10.0% or less, Mn0.3~
A sintered body consisting of 2.0% S, 0.2~0.7% S, the balance Fe and unavoidable impurities, with a maximum diameter of 100μ in the base.
The following pores have an area percentage of 1 to 10%, the hardness of the base part is Hv800 to 1000, and the base has M ₆ C with an average grain size of 5μ or less and carbides having an MC type crystal structure by volume. %, and sulfide inclusions are uniformly dispersed in the base by 0.5 to 3.0% in area %.It has excellent wear resistance and sliding properties. Vane.