JPH02153044A - Duplex stainless steel having high cracking resistance - Google Patents

Abstract

PURPOSE:To obtain a duplex stainless steel excellent in corrosion resistance, corrosion fatigue strength, and cracking resistance by adding and incorporating specific amounts of N to Cr-Ni-Mo stainless steel. CONSTITUTION:As duplex stainless steel used, e.g., for a suction roll, in a paper- making machine, a duplex stainless steel having a composition containing, by weight, <0.05% C, <2.0% Si, <2.0% Mn, 20-25% Cr, 3-8% Ni, 1.5-3.5% Mo, and 0.1-0.3% N is used. Since an austenitic phase can be subjected to solid- solution strengthening by the addition of N and also the precipitation of sigma-phase in a ferritic layer can be prevented in the above steel, threshold of stress intensity factor value can be increased and, as a result, the cracking resistance of a member can be improved and also superior corrosion resistance and corrosion fatigue strength can be provided.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to the improvement of duplex stainless steel used, for example, in suction rolls in paper manufacturing IR machines, and more specifically, the invention relates to improvement of duplex stainless steel that is used for suction rolls, etc. in paper manufacturing IR machines, and more specifically, to improve the property of preventing cracking. Regarding duplex stainless steel with.

(Last technology and its problems) For example, suction rolls in the paper industry are used in white water environments, so the roll materials are required to have excellent corrosion resistance, high corrosion fatigue strength, and the like. Conventionally,
A variety of duplex stainless steels are used for this type of application.

However, in recent years, despite the use of materials with excellent corrosion resistance and high corrosion fatigue strength, early breakage accidents of rolls have occurred one after another. As a result of investigating the accident product,
It was found that when casting defects, inclusions, etc. were present inside the roll, cracks were generated starting from the defects, and the cracks propagated to lead to breakage.

As a result of examining broken rolls, it was found that the smaller the lower limit stress intensity factor value, the earlier the breakage accident occurred5. It provides high duplex stainless steel.

(Technical means and effects) The duplex stainless steel according to the present invention has C:
0.05% or less, Si: 2.0% or less, Mn: 2.0
% or less, Cr: 20-25%, Ni: 3-8%, Mo:
1.5 to 3.5%, N:Q, and 1 to 0.3%, with the balance essentially consisting of Fe.

The duplex stainless steel of the present invention has at least the same corrosion resistance and corrosion fatigue strength as conventional steel, and has excellent cracking resistance.

The reasons for limiting the components of the duplex stainless steel of the present invention are as follows.

C: O, o 5% or less C is an austenite-forming element, and has the ability to strongly damage the matrix dissolved in the austenite phase. However, when the content increases, C "forms carbonizers and C" which is effective for corrosion resistance is consumed, resulting in a decrease in resistance to local corrosion.

Therefore, the upper limit is set to 0.05%.

Si: 2.0% or less Si is an element necessary for deoxidizing molten steel and ensuring good castability. However, the upper limit is set at 2.0% since containing a large amount causes deterioration of material properties such as deoxidization.

Mn: 2.0% or less Mn acts as a deoxidizing agent like the above-mentioned Si, and also has the role of fixing sulfur (S) during melting. However, if a large amount is included, the corrosion resistance will decrease, so the upper limit is set at 280%.

Cr: 20-25% Cr contributes to improving corrosion resistance and stress corrosion cracking resistance.

Further, Cr is a ferrite-forming element, and the strength is increased due to the formation of a ferrite phase in a two-phase structure. Therefore, at least 20% is required. Containing too much will not only lead to a decrease in the toughness of the steel, but also the formation of a brittle sigma phase during casting. The upper limit is 25%.
stipulates.

Ni: 3-8% Ni is an element that stabilizes the austenite phase and contributes to improving the toughness of steel. It is also an important element from the viewpoint of ensuring corrosion resistance. If the content is less than 3%, these effects cannot be fully exhibited, whereas if the content is in a large amount, the corresponding effects cannot be obtained, which is economically disadvantageous. Furthermore, the austenite phase in the two-phase structure becomes excessive,
The quantitative balance of the two-phase structure is lost. Therefore, the upper limit is set at 8%.

Mo: 1.5-3.5% Mo contributes to improving the corrosion resistance of stainless steel.

Therefore, it is necessary to contain at least 1.5%.

However, even if added in a large amount, the effect of improving corrosion resistance not only becomes saturated, but also the sigma phase tends to precipitate at ji7i3''a, resulting in significant embrittlement, so the upper limit is set at 3.5%.

N: 0.1-0.3% N is an important element that characterizes the duplex stainless steel of the present invention. It has the effect of increasing the solid solubility of the austenite phase and suppressing the precipitation of the sigma phase within the ferrite phase, increasing the lower limit stress intensity factor value and increasing the cracking resistance of the member. In order to exhibit these effects, it is necessary to have a content of at least 0.1% or more. Note that although N is inevitably included in normal atmospheric dissolution, its content is less than 0.1%, so it is necessary to actively add it. These effects increase as the amount of N increases, but 0.3
%, nitrides such as CrzN tend to precipitate, reducing corrosion resistance. Therefore, the upper limit is set at 0.3%.

The stainless steel of the present invention contains the above-mentioned component elements, and the remainder consists of unavoidably mixed impurity elements and Fe.

Next, the results of improving the cracking resistance of the duplex stainless steel of the present invention will be specifically explained with reference to Examples.

(Example) The mechanical properties and crack initiation resistance of test steels having the compositions shown in Table 1 were investigated.

Samples W4 No. 1 and No. 12 are duplex stainless steels of the present invention, and sample steel No. 013 is a conventional steel. Each steel sample was kept at 1050° C. for 2 hours per inch of wall thickness, and then cooled in air.

Table 2 shows the mechanical property test results.

Figure 1 shows the relationship between stress intensity factor and crack growth rate. This test was conducted in accordance with ^STM %47, and 1
An inch square test piece was used. In addition, the test was conducted at 50℃, CC
The test was carried out in a corrosive solution of l-1000pp, pH 3.5.

As shown in Table 2, no significant difference in mechanical properties was observed between the N-containing steel of the present invention and conventional N-free steel.

By the way, the lower limit stress intensity factor value is the value at which no crack occurs even if a smaller stress is applied, ΔKLb=σ/i7'−(△Ktb: lower limit stress intensity factor It is expressed as a numerical value, σ: stress, a: allowable defect size). If ΔKtl+ is large, even large values of a are allowed when σ is the same.

In other words, it can be seen that even if a large internal defect exists, no crack occurs, and the resistance to crack occurrence of the member is improved.

According to STM E647, the stress intensity factor value is specified to be measured at a growth rate of 10-' m/cycle (3,94 to 10-1 inch/cycle) or more. Therefore, with reference to FIG. 1, test steel No. 1 to No.
When determining Ktl+ to the lower limit stress intensity factor value of 3, No.
, 1 is about 12ksi, /"'TM, No. 2 is about 10.
5ksir blindness, and No. 3 is determined to be about 9ks. As described above, the steel of the present invention has a higher lower limit stress intensity factor value than the conventional steel.

By the way, the stress that acts on the roll during actual operation is 5 k.
g/an” (3225 kg/1112), the allowable defect size a is determined from the above formula △Ktl+=σ5, and test steel No. 1 is approximately 23 mm.
, sample JI No. 2 is approximately 181, sample jll No. 3
is approximately 13f1.

The larger this value is, the less cracks will occur even in the presence of large defects, which means that the steel of the present invention has better crack initiation resistance than conventional steel. I can say that.

(Effects of the Invention) The duplex stainless steel according to the present invention has corrosion resistance and corrosion fatigue strength at least comparable to conventional steel, and has far superior crack initiation resistance than conventional steel.

Therefore, it is suitable as a roll member of a paper manufacturing machine used under conditions where repeated stress is applied in a corrosive environment.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between stress intensity factor value and crack growth rate. (inch/cycle) Stress intensity factor value Δk (ksi)

Claims (1)

[Claims] (1) In weight%, C: 0.05% or less, Si: 2.0
% or less, Mn: 2.0% or less, Cr: 20-25%, N
i: 3~8%, Mo: 1.5~3.5%, N: 0.1~
Duplex stainless steel with high resistance to cracking, containing 0.3% Fe and the remainder substantially consisting of Fe.