JPS5852463A - Stainless steel with superior corrosion resistance and mechanical property - Google Patents

Abstract

PURPOSE:To obtain a stainless steel with superior corrosion resistance and mechanical properties by adding specified percentages of C, Si, Mn, Ni, Cr, Mo, Ti, Cu and W to Fe. CONSTITUTION:A stainless steel consisting of, by weight, <=0.1% C, <=2.0% Si, 6-35% Ni, 18-30% Cr, 0.05-10% Mo, 0.01-2% Ti, 0.5-5% Cu and/or 0.05- 3% W and the balance essentially Fe or further contg. 0.01-1% Nb is prepared. At this time, the composition of the steel is regulated so as to make the K value represented by equation III 0.85-1.85. The K value is calculated using F represented by equationIand A represented by equation II. The resulting stainless steel has a 2-phase structure consisting basically of ferrite and austenite. It has favorable mechanical properties, undergoes slight deterioration in an atmosphere contg. H2S, and shows high fatigue strength and superior corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to stainless steel with excellent corrosion resistance and mechanical properties.

Conventionally, JIS SC3I (Cr l 3%) is used as a material used in corrosive environments and requires strength.
Martensitic stainless steels, such as typified by Martensitic stainless steels, are widely used. However, this material does not necessarily have sufficient corrosion resistance and is often unsuitable as a structural material in a corrosive environment, especially in response to the recent diversification and harshness of corrosive environmental conditions. There are many cases where accidents occur. As a countermeasure to this problem, the use of austenitic stainless steel has been considered in some cases, but it has a drawback in terms of strength and cannot be used in actual equipment.

Therefore, the present inventors conducted a detailed investigation into the causes of corrosion accidents in SC5I martensitic stainless steel, and found that in a corrosive environment containing sulfurized IJium (Na2S), hydrogen sulfide (Na2S) is present in the steel. A phenomenon in which H2S (H2S) penetrates occurs, and the passivation film on the steel surface is destroyed by the H2S that has penetrated into the steel, leading to the occurrence of cracks due to a significant decrease in mechanical properties, especially toughness, due to the occurrence of severe pitting corrosion. We obtained the knowledge that It was also found that H2S had no adverse effect on fatigue strength. Based on this knowledge, the present invention was completed as a result of extensive research into the chemical composition of stainless steel in order to exhibit stable corrosion resistance in the corrosive environment and provide the necessary mechanical properties as a structural material. It is something.

That is, the present invention provides CO, 1% or less (% represents weight %. The same applies hereinafter), Si 2% or less, Mn 2% or less,
Ni6-35%, Cr18-30%, Mo0.05-I
O, Ti0.01-2, and CuO, 5-5%,
One type selected from W 0.05~'t% also has L<2
seeds and, if necessary, NbO, OI to 1 H, the remainder substantially consisting of Fe, and F-Cr (%) +1
．． 5 Mo (%) + 2.55i (%) + 0.5
Nb (%) 137i (%) + 0.15 W (%)
... CI] A = Ni (%) + 0.51 Mn (
%) + 29.4 C (%) + 0.4 Cu (%)・
-... [When it is old, the value of K calculated by the following formula is 0.85 to 1
．． A stainless steel having a component composition satisfying 85 is provided.

The stainless steel according to the present invention basically has a two-phase structure consisting of a ferrite phase and an austenite phase, and has good mechanical properties and shows only slight deterioration in yield even when used for a long time in an atmosphere containing H2S. It also has high fatigue strength and exhibits excellent durability in corrosive environments.

The reasons for limiting the components of the stainless steel of the present invention will be explained below. dC: 0.1% or less It is a strong austenite-forming element, and it is an interstitial solid solution that improves the strength as the amount added increases. On the other hand, this leads to a decrease in toughness and a rapid increase in intergranular corrosion and pitting corrosion susceptibility due to the precipitation of Cr carbides. However, the above deterioration in corrosion resistance due to C can be prevented by adding TI or Nb (these elements fix C and suppress the precipitation of Cr carbide).
The upper limit of the amount is set to 0.1 inch, preferably within a range that satisfies the following formula.

Si: 2% or less Used as an effective deoxidizer during steel melting. In addition to improving corrosion resistance and pitting corrosion resistance, since it is a ferrite-forming element, it is effective in controlling the amount of ferrite and improving strength. However, since addition of a large amount leads to a decrease in toughness, the upper limit is set at 20%.

Mn: Has a deoxidizing and desulfurizing effect on steel with a diameter of 2 mm or less, and contributes to improving mechanical properties. It is also an effective element for stabilizing austenite. However, if added in a large amount, corrosion resistance will deteriorate, so the content should be 2% or less.

Ni: 6-35% Has a remarkable effect on stabilizing austenite and improves toughness, corrosion resistance, etc. Its amount should be determined in relation to the amount of other elements, especially MO and W, which are ferrite-forming elements and are strength-imparting elements, from the viewpoint of ensuring balance with ferrite amount, toughness, and strength. It is. In the present invention, from these points, at least 6% is required to stabilize austenite and ensure toughness. As the amount of Ni increases, the toughness improves, but if it increases too much, the structure loses its balance and the strength also decreases, so the upper limit is set at 35%.

Cr: 18-30% This is the main ferrite-forming element in contrast to Ni, and contributes to improving strength, as well as having the effect of passivating steel and improving corrosion resistance. The amount of 41 blades must be determined in consideration of the amount of austenite-forming elements such as Ni in order to stabilize (balance) the structure, and to ensure strength, toughness, etc. In the steel of the present invention, if the amount of 41!4 is less than 18%, the structure cannot be stabilized and the strength will be insufficient, while if it exceeds 30%, the toughness will drop sharply. Therefore, it is set at 18 to 30%.

Mo: 0.05~] 0% A ferrite-forming element, which not only increases strength but also has a remarkable effect on improving corrosion resistance. In particular, combined addition with w1□C* provides excellent sulfide corrosion resistance.

For this reason, 0.05% or more is required.

However, if added in a large amount, the toughness deteriorates and the precipitation of the σ phase is promoted, so the upper limit is set at 10%.

Ti: 0.01 to 2 Ti acts as a strong carbide-forming element and ferrite-forming element. It is particularly effective in improving corrosion resistance, and for this purpose at least 0.01% is required, but it is preferable that the above [ff3 formula is satisfied in relation to C and Ti.

In addition to the above elements, the stainless steel of the present invention has Cu and W.
Contains one or two elements selected from.

Cu: 0.5 to 5% An austenite-forming element, which improves corrosion resistance and is dissolved in the matrix to increase strength. In particular, the combined addition of MO and the like improves the corrosion resistance against sulfides. However, if the S reign is less than 0.5%, the effect will not be sufficient, while if it exceeds 5%, the toughness will be impaired due to partial precipitation. Therefore, it is set to 0.5 to 5 inches.

W: 0.05 to 3 I is an element effective in improving corrosion resistance, and has the effect of increasing sulfide corrosion resistance when coexisting with Mo.

If the amount is less than 0.05%, the effect is insufficient,
On the other hand, if it exceeds 3 inches, the toughness will decrease. Yo-)te005
~3chi.

In addition to the above-mentioned elements, the stainless steel of the present invention may contain Nb for the purpose of improving the material quality.

Nb acts as a strong carbide-forming element and ferrite-forming element, and is particularly effective in improving corrosion resistance.

It also has the effect of suppressing grain boundary precipitation reactions caused by N. The amount added for this purpose is determined by the relationship with C and Ti, but if it is less than 0.01%, the effect will be insufficient, so preferably the amount satisfies the above formula [IV].

Note that the upper limit is preferably 1%.

P, S, and other impurities may be present within the range normally allowed for this type of steel.

The stainless steel of the present invention is further characterized in that the value of K calculated by formula [n11] is within the range of 0.85 to 1.85 using the values of F and A shown in the above [I] and [old formula. It takes.

When the stainless steel according to the present invention satisfies all of the above conditions, it exhibits excellent corrosion resistance and guarantees various mechanical properties necessary as a structural material.

Note that the steel of the present invention can also be suitably used as a cast steel product. Solution treatment and water cooling after melting and casting may be carried out by conventional methods.

Next, 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 to prepare various test steels. The corrosion resistance and mechanical properties of each were compared. Test steel Al~
No. 12 is the steel of the present invention, and Al0I◆ to 107 are comparative steels (among them, AlO3 to 107 are martensitic conventional steels). Figure 1 shows the corrosion resistance of each sample steel, passivation potential (m
V vs.

S, C, E, ) [Ce-20 ppm solution with H2S30
ppm added solution (pH 3,5)].

The horizontal axis of the figure is the value according to the above formula [[[1]], and the numbers in the figure indicate the test steel layers. In addition, as shown in Table 2 and Mechanical properties of each test steel, the passivation potential of each test stainless steel changes depending on the K value, and
~1.85) has a low potential and exhibits good corrosion resistance in a corrosive environment containing H2S, which is particularly noticeable in composite additives of W and Cu.

Still, as shown in Table 2, the steel of the present invention has good strength and is found to be sufficiently durable as a structural material.

As mentioned above, the stainless steel according to the present invention has excellent corrosion resistance, particularly withstands H2S-containing corrosive environments, and also has good mechanical properties, so it has excellent corrosion resistance in corrosive environments compared to conventional materials such as SC5I. Guaranteed durability.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between values and passivation potential of stainless steel. Patent applicant Kubota Iron Works Co., Ltd. Agent Patent attorney Shinhachibe Miyazaki

Claims (1)

[Claims] +11 CO, 1% or less, Si 2.0% or less, Mn
2.0 inches or less, Ni6-35%, Cr I8-30e1
6, Mo 0.05-IO%, Ti 0.01-2
%, and Cu 0.5-5%, Wo, 05-3% 1
1. A stainless steel having excellent corrosion resistance and mechanical properties, comprising one or two elements, the remainder being substantially Fe, and having a value of 0.85 to 1.85 as calculated by the following formula. A+2.77 However, F=Cr(%)+1.5Mo(%)+2.5Si(
%) + 8Ti (%) 10, 15Wf section) A = Ni (%) + 0.5 l Mn (%) + 29.
4 C (%) + 〇, 4 Cu1%) + 21 CO,
1% or less, Si2.0% or less, Mn2.0% or less, Ni
6-35%, Cr l 8-'30%, Mo 0.
05~IO%, Ti 0.01~2%, NbO
, 0'1~1% and CuO, 5~5%, Wo, 05~
consisting of one type of 3rd element or 2y element, the remainder being substantially KFe, and having a value of 0.85 to l as calculated by the following formula,
Stainless steel with excellent corrosion resistance and mechanical properties characterized by a rating of 85. A+2.77 However, F=Cr(%)+1.5 Mo(%)+2.55i
(%) + 〇, 5 Nb (%) + 37i (%) 0.15
W (%) A = Ni (%) + 0.51 Mn (%) + 2
9.4 C (%) + 〇, 4 Cu (%)