JPS5852464A - Two-phase stainless steel with high corrosion fatigue strength - Google Patents

Abstract

PURPOSE:To obtain a 2-phase stainless steel with superior corrosion fatigue strength by adding specified amounts of Mo, Cu, W and Co to a Cr-Ni stainless steel having a specified composition and by specifying the weight ratio between austenite and ferrite phases in the steel structure. CONSTITUTION:A 2-phase stainless steel consisting of <=0.1% C, <=2.0% Si, <=2.0% Mn, <=0.04% P, <=0.04% S, 4.0-10.0% Ni, 20,0-28.0% Cr, 1.0-5.0% Mo, 1.0-4.0% Cu, 0.2-2.0% W, 0.1-1.0% Co and the balance essentially Fe is prepared. The steel has a ferrite-austenite 2-phase structure contg. 30-80% ferrite. To provide high toughness the value of 10C%+Si% is adjusted to <=2.0%, and by adding <=0.08% N, the solubility of austenite and the corrosion resistance of the steel are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenite-ferrite duplex stainless steel having excellent corrosion fatigue strength.

Stainless steel is widely used as a material used in corrosive environments, but in order to obtain good durability under usage conditions where corrosion and repeated loads act simultaneously, it is necessary to have not only general corrosion resistance but also corrosion fatigue strength. It must be made of excellent material. An example where high corrosion fatigue strength is required is the suction roll used in paper machines. Conventional materials for this suction roll include martensitic stainless steel such as JIS 5C5I, JIS 5C5I3,
Austenitic stainless steels such as SC5I4 are used, but these do not have sufficient corrosion fatigue strength and fatigue relatively quickly in corrosive environments containing chlorine ions (Ce-), making it difficult to use them stably over long periods of time. The problem is that it is unbearable.

The present invention has been made in view of the above circumstances, and
By adding Mo, Cu1W and CO in combination to the Cr-Ni-Fe system, making the steel structure into two phases of austenite and fecite, and specifying the quantitative ratio of these two phases, it is possible to add ce-ions etc. to the Cr-Ni-Fe system. We succeeded in significantly increasing fatigue strength in corrosive environments and guaranteeing excellent durability.

That is, the present invention provides carbon dioxide of 1% or less ("chi" means "wt%". The same applies hereinafter), Si of 2.0% or less, Mn of 2.0% or less.
0% or less, Po, 04% or less, 50.04 Chi or less, Ni
4.0~IO, 0%, Cr 20.0~28.
0 rice, Mo 1. '0~5.0%, Cu 1.0~
4.0%, WO, 2~2.0 q6, Co O1l~1
．． To provide a duplex stainless steel having excellent corrosion fatigue strength, which has a two-phase structure of austenite and ferrite, and the amount of ferrite in the structure is 30 to 80%.

Next, we will explain the reasons for limiting the composition of the steel of the present invention. OC: 01C or less Strength is increased by solid solution in the matrix, precipitation of carbides, etc., but if it is contained in a large amount, chromium carbide CCr2B C
6 etc.), corrosion resistance and toughness deteriorate.
The upper limit is set to 0.1 inch.

Si: 2.0% or less It is a strong deoxidizing agent and is added to improve castability, but if it exceeds 20%, it will cause the steel to become brittle.
2.0% or less.

Mn: 2.0% or less It is added for the purpose of deoxidation and desulfurization, and has the effect of improving castability, but if it is included in a large amount, corrosion resistance will be impaired.
% or less.

P and S: 0.04 or less each.Both are impurity elements and deteriorate corrosion resistance and mechanical properties, so it is desirable that they be as low as possible, but from the viewpoint of industrial manufacturing methods, each should be 0.011. Allow existence as an upper limit.

Ni: 4.0 to 10.0% Ni is a strong austenite-forming element, indispensable for providing the required amount of austenite to the steel structure, and has a remarkable effect on improving toughness and corrosion resistance. For this purpose, addition of at least 4.0 parts is required. However, if it is included in a large amount, the amount of austenite becomes excessive and the quantitative balance between austenite and ferrite is lost. In order to ensure good corrosion fatigue strength, the steel of the present invention needs to have a two-phase structure with an area ratio of ferrite of 30 to 80 inches, so addition of a large amount must be avoided. Of course, the quantitative ratio of both phases depends on the quantitative ratio with ferrite-forming elements such as Cr. In view of this point, the present invention sets the upper limit of the Ni amount to 1000 chi.

Cr: 20.0-28.0% It is a ferrite-forming element and is an important element for exhibiting the corrosion resistance necessary for stainless steel, and at least 20.0% is required to ensure high strength and high corrosion resistance. . However, if it is contained in a large amount, the toughness will deteriorate, so 28.0
% or less.

Mo: 1.0-5.0% It is a ferrite-forming element and is also effective in improving corrosion resistance, especially pitting corrosion resistance. If it is less than 1.0%, the effect will not be sufficient, while if it exceeds 5.0%, material deterioration will occur due to a decrease in toughness and promotion of precipitation of the σ phase. Therefore, the above range is set.

W: 0.2-20 inches It is a ferrite-forming element and contributes to improving corrosion resistance.

In particular, it has a great effect on improving corrosion fatigue strength, and when it is added in combination with Cu etc., the effect is noticeable, so it is necessary to add at least 0.2%. however,
When the ratio exceeds 2.0, the effect is almost saturated, and the effect against the increase in cost is reduced, which is ultimately disadvantageous.

Cu: 1.0 to 4.0% It is an austenite forming element, and also brings about solid solution strengthening of the austenite phase and improved corrosion resistance against non-oxidizing acids. In order to obtain these effects and the combined effect (improvement of corrosion fatigue strength) of W mentioned above, it is necessary to add 1.0% or more, but if it is included in a large amount, material deterioration such as embrittlement may occur due to the precipitation of intermetallic compounds. Therefore, the upper limit is set at 4.0%.

Co: 0.1 to I', 0% It is an austenite forming element and is noticeable when it is still strong. To obtain these effects, at least 0.1%
However, if l exceeds 0%, the effect is almost saturated, so 1.0% is the practical upper limit.

Since the stainless steel of the present invention has the above-described composition, it forms a two-phase structure of 30 to 80 ferrite, and has excellent corrosion resistance, especially as shown in the examples below. It has excellent fatigue strength as well as good mechanical properties.

The steel of the present invention has excellent corrosion resistance and mechanical properties as described above, but if high toughness is desired for the purpose of use, the sum of 10 times the amount of C and the amount of Si [10C (%) + Si (chi) )] is 20
It is desirable to limit the amounts of C and SI to below %, thereby ensuring a high impact value as described later.

In general, in duplex stainless steel, N, which is a strong austenite-forming element, is actively added to strengthen the austenite as a solid solution and improve corrosion resistance. Not only the occurrence of
Since this is accompanied by deterioration of corrosion resistance and embrittlement of the material, in the steel of the present invention, the content is preferably 0.08% or less, more preferably 0.004% or less. This avoids the occurrence of blowholes, prevents the tendency for corrosion resistance to deteriorate, and has the effect of improving toughness.

The stainless steel of the present invention is suitably used as a cast steel product, and the solution treatment after melting and casting may be carried out according to a conventional method.

Next, the material properties of the steel of the present invention will be specifically explained with reference to Examples.

Examples Stainless steels having various compositions shown in Table 1 were melted and cast, solution treated and water cooled at ++00°C x 2 hours, and the corrosion resistance and mechanical properties of each of the obtained test steels were measured. . The test steels i1-6 are the steels of the present invention, Al01-1
05 is duplex stainless steel for comparison (AIOI is JI
S SC5I I equivalent material).

Table 2 shows the test results. The corrosion fatigue strength in the table is C1
1-I O00ppm, 504-250pH1rr
In a corrosive environment with a pH of 3 containing
The figure shows the breaking strength of a rotating bending fatigue test under the conditions of 08 cycles. Furthermore, FIG. 1 shows the relationship between the C and Si content [10C (%) + Si (%)] and the impact value of each sample steel.

In the figure, "." indicates the steel of the present invention, and roJ indicates the comparative steel.

As is clear from the above test results, the steel of the present invention has a corrosion resistance of about 13 to 2.
Shows 4 times the fatigue strength. By the way, Comparative Steel Sales 105 is W.
Because the content of etc. is insufficient, yet! 104 is W,
Although it contains all the elements such as GulCo, the amount of Ni is too large and the two-phase balance is lost, so the corrosion fatigue strength is extremely low.

It can be seen that the steel of the present invention is equivalent to or better than the comparative steel in terms of mechanical properties such as strength, elongation, and toughness. In particular, as shown in Figure 1, 10C (%) + Si
By restricting the amounts of C and Si so that (%) is 2.0% or less, a high impact value of about 10 to 9''2 or more can be secured.

As described above, the stainless steel according to the present invention has corrosion resistance,
In particular, it has excellent corrosion fatigue strength and good mechanical properties. The stainless steel of the present invention can be used in environments where corrosion and fatigue occur simultaneously, such as in suction rolls for paper manufacturing, and guarantees durability that exceeds that of conventional steels.

[Brief explanation of drawings]

Figure 1 shows the amount of C and Si (10C (%)
) + si (%) 1 is a graph showing the influence of patent applicant Kubota Tekko Co., Ltd. agent Patent attorney Arata Miyazaki Department Figure 1 10C (%) + 5j ('%)

Claims (1)

[Claims] fil C0.1% or less, Si2.0% or less, Mn2
．． 0% or less, Po, 04% or less, 50.04% or less, N
i 4.0~IO, 0%, Cr 20.0~28
．． 0%, Mo 1.0-5.0%, Cut, 0
~4.0%, Wo, 2~2. (1%, Co O, 1~
A high corrosion fatigue strength duplex stainless steel characterized by having a ferrite-austenite duplex structure with a ferrite content of 1.0%, the balance being substantially free of Fe, and a ferrite content of 30 to 80%. (High corrosion fatigue strength duplex stainless steel according to item (1) above, characterized by containing 08% or less of 2+N O. (3) C content (7) Sum of JO@i and Sr content [ The high corrosion fatigue strength duplex stainless steel according to item (1) or item (2) above, characterized in that 10C (% Si [%'')] is 2.0% or less.