JPS59107019A - Production of two phase stainless cast steel product of high cr and low ni having excellent sea water resistance - Google Patents

Abstract

PURPOSE:To suppress efficiently the deposition of a sigmaphase and to produce stably a cast steel product having excellent sea water resistance by charging a molten high Cr low Ni stainless steel having a specific component compsn. into a casting mold, knocking out the mold at a specific temp. after solidification and cooling quickly the steel. CONSTITUTION:A molten high Cr low Ni stainless steel contg., by wt%, <=0.05% C, <=2.0% Si, <=2.0% Mn, 22.0-30.0% Cr, 4.0-11.0% Ni, 0.5-5.0% Mo, 0.10- 0.60% Cu, <=0.04% P, <=0.03% S, <= N, and further contg. 1 kind of 0.05- 1.50% W or 0.05-1.50% V, and consisting of the balance Fe with inevitable impurities is charged into a casting mold and is solidified. The mold is knocked out while the molten steel is in the high temp. state before cooling down to a temp. region of <1,000 deg.C and the steel is immediately and quickly cooled. The two phase stainless cast steel product having excellent sea water resistance is thus mass-produced easily at a low cost.

Description

[Detailed Description of the Invention] This invention provides stable and highly corrosion-resistant duplex stainless steel castings that have extremely good pitting corrosion resistance and crevice corrosion resistance, especially against seawater.
It concerns a method for reliable mass production.

The rapid development of industry in recent years has been remarkable, and along with this, the development of various plants and equipment aimed at the effective use of seawater has also rapidly increased. Even in various types of conventional equipment that have been forced to come into contact with seawater, there is an increasing demand for metal materials that have more stable corrosion resistance against seawater.

Conventionally, to meet these requirements, high Cr, low N1 duplex stainless steel, which has relatively excellent seawater resistance, has been useful, and has been used for pumps and valves in seawater plants, marine power plants, etc. is increasing.

However, 5C8II (25Cr-5Ni -2
Duplex stainless steels typified by Mo) have had the problem of insufficient pitting corrosion resistance and crevice corrosion resistance against relatively high temperature seawater.

Therefore, in order to deal with these problems, C: 0.05%
Below, St: 2.0% or less, Seed: 2.0% or less.

Cr* more than 22% to 35% s Nl: 5.
0-9-0%.

Mo: 0.5-5.0%, Cu: 0.10-0.6
0%, P: 0.04% or less, S: 0.03% or less, N:
Contains 0.30% or more of 1 and further contains W? 0.05-1.5
% or V: High Cr low N1 duplex stainless steel as a material for heat exchangers and offshore structures, containing any one of 0.05 to 1.5%, with the remainder essentially consisting of PE. It was proposed earlier ('Special Publication No. 53-43372).

By the way, materials with excellent seawater resistance are required not only for structural materials, but also for equipment parts manufactured by casting such as pumps and pulp, as mentioned above. To manufacture such a member using duplex stainless steel, 1. Usually, after pouring molten steel into a sand mold and letting it solidify, the mold is left as it is at room temperature, waiting for it to cool down, and then the mold is removed. Previously, the method of taking out the product was adopted, but when trying to manufacture a cast product using duplex stainless steel, which does not easily precipitate the σ phase, part of the ferrite phase is converted into the σ + γ phase during the cooling process to room temperature after solidification. However, it is not possible to avoid the disadvantage that the toughness and ductility are significantly reduced at room temperature and σ-phase embrittlement cracking occurs, making it difficult to stably mass-produce cast steel products. This was no exception to the duplex stainless steel proposed in the above-mentioned Japanese Patent Publication No. 53-43372, which showed good corrosion resistance even in relatively high temperature seawater. The experiments and research conducted by the present inventors in an attempt to manufacture cast steel products with good seawater resistance under the 1. standard casting method have led to the conclusion that the above-mentioned problems cannot be avoided. I had no choice but to make a decision.

From the above-mentioned viewpoint, the present inventors have developed a cast steel product that exhibits good corrosion resistance even against seawater in a relatively high temperature range, without requiring particularly complicated equipment.

In order to achieve stable mass production with high yield, especially the above-mentioned
Structural products Cr low Ni described in 43372 publication
Focusing on the excellent seawater resistance of duplex stainless steel, we have conducted various studies with the aim of solving the above problems that occur when using this duplex stainless steel with cast steel. Basically, a molten high Cr, low Ni duplex stainless steel having a composition similar to that described in the above patent publication is poured into a sand mold or a metal mold, and then solidified into a cast steel product. By rapidly cooling the mold while maintaining the temperature at no lower than 1000℃, the precipitation of the σ phase can be effectively suppressed, resulting in excellent seawater resistance without causing σ phase embrittlement cracking. This led to the discovery that cast steel products could be produced stably.

This invention was made based on the above knowledge, and C: 0.05% or less (hereinafter, % indicates the amount of component, % by weight)
).

Si: 2.0% or less, Mn: 2.0% or less.

Cr: 22.0~30.0%, Ni: 4.0~
11.0%.

Mo: 0.5-5.0%z Cu: 0.10-0.
60%.

P: 0.04% or less, S: 0.03% or less.

N: 0.25% or less.

Contains all of the following: W: 0.05-1.50%, V: 0.05-1
．． 50%.

A molten high Cr, low Ni stainless steel containing any one of the following, with the remainder consisting of Fe and unavoidable impurities is poured into a mold and allowed to solidify. By completely demolishing the mold while it is still in a high temperature state before the temperature drops to the temperature range, and then rapidly cooling it immediately to suppress the precipitation of the σ phase, #f can be formed without causing σ phase embrittlement cracking. It is characterized by being able to stably produce duplex stainless steel castings with excellent seawater resistance.

At the same time, the reason why the compositional content of the steel and the temperature at the time of demolding are limited as described above in the method for manufacturing a duplex stainless steel cast product of the present invention will be explained.

A) Compositional Component Amount ■ C Since C has a negative effect on the crevice corrosion resistance of duplex stainless steel, it is desirable to keep it as low as possible, but in consideration of the manufacturing cost of the steel, its content should be reduced to 0. It was set at 0.05% or less.

■ 5i Si is an effective element as a deoxidizing agent and has the effect of improving castability, especially the fluidity of molten steel, but if it is contained in excess of 2.0%, the susceptibility to weld cracking increases. Its content was set at 2.0% or less.

■ Mn MnFi5t is an element useful as a deoxidizing agent and desulfurizing agent, but 2.0%? If the content exceeds 2.2, the content may deteriorate pitting corrosion and crevice corrosion.
It was set as 0% or less.

■　Cr Cr component is an important element for stainless steel.

In particular, it has the effect of significantly improving pitting corrosion resistance, 11Tt crevice corrosion resistance, and general corrosion resistance, but its content is 22.0
If the amount is less than 30%, the desired effect cannot be obtained.
．． If the content exceeds 0%, the castability and toughness will deteriorate, so the content should be set at 22.0 to 30.0%.
%.

■Nl The Ni component has the effect of significantly improving the mechanical properties and general corrosion resistance of steel, but if its content is less than 4.0%, the desired effect cannot be obtained, and
The desired ferrite-austenite two-phase structure cannot be obtained. On the other hand, Ni is an expensive element, 11
．． If the Ni content exceeds 0%, the steel will be expensive compared to its effectiveness, so the Ni component content should be set at vi-4.0~
It was set at 11.0%.

M.

The Mo component KVi has the effect of significantly improving the seawater resistance of steel, but if its content is less than 0.5%, the desired effect cannot be obtained; However, the content was set at 5 to 5.0% because it would not result in any significant improvement or would lead to an increase in steel prices.

(7Cu Cu component is an important component of the duplex stainless steel in the present invention, and has the effect of significantly improving the pitting corrosion resistance, crevice corrosion resistance, and general corrosion resistance of the steel.
If the content is less than 0.10%, the desired effect cannot be obtained;
If the content exceeds 10%, the corrosion resistance will deteriorate, so the content was set at 10% to 0.60%.

■ P If the P content exceeds 0.04%, the weldability of the steel deteriorates, so the P content in the steel was set at 0.04% or less.

■ SS is an element that should not have a negative effect on the pitting corrosion and crevice corrosion properties of steel, and it is better to keep its content as low as possible.

However, taking into account the total manufacturing cost of steel, the S content was determined to be 0.3% or less.

■ N When coexisting with NI-J and tMo, it has a remarkable effect on improving corrosion resistance, but when it is added in excess of 0.25 inch, it causes blowholes in the steel and deteriorates the quality of the steel. Therefore, the N content 'ff was determined to be 0.25% or less.

Q W and V W components and V components have the same effect of further improving the pitting corrosion resistance and i1 crevice corrosion resistance of steel against seawater by coexisting with Mo and Cr, etc., but when their content is 0. If the content is less than .05%, the desired effect cannot be obtained, and the effect becomes noticeable from a content of 0.05% and gradually increases up to 1.50%.

However, 1.50%? Even if added in excess of this amount, no further improvement in corrosion resistance will be obtained, and the weldability of the steel will deteriorate. .50%.

B) High Cr and high MO duplex stainless steel castings are shown in Figure 1. As is clear from the 950
The transformation of the ferrite phase to the σ0r phase occurs at temperatures below ℃, and as the temperature decreases, the transformation start time becomes short.■ 111, with the nose at 850℃, and the σ phase precipitates in a very short time. do. For this reason, in cast steel products, the transformation of the ferrite phase into the σ+γ phase easily progresses during slow cooling during the cooling process after casting and solidification, resulting in σ phase embrittlement cracking. For this reason, the mold release temperature in the high temperature state is determined by the σ + γ of the ferrite phase.
The temperature was limited to 1000°C, at which no phase transformation occurs.

Note that the rapid cooling in this invention may be any method such as water cooling, water spray cooling, etc. as long as it can obtain a cooling rate sufficient to suppress precipitation of the σ phase.

Further, it is preferable that the duplex stainless steel cast product obtained by the method of the present invention has a ferrite content adjusted to 40 to 60%. because.

The amount of ferrite greatly affects its mechanical properties.

If the amount is less than 40%, the yield strength will be low, while if it exceeds 60%, the strength and toughness will tend to deteriorate.

Next, the present invention will be specifically explained using Examples and in comparison with Comparative Examples.

EXAMPLE First, cast copper A-N having the chemical composition shown in Table 1 was melted by a conventional method. Note that the red stamps in Table 1 indicate that the chemical composition at that point is from the steel subject to the present invention. This indicates that it is off.

Next, the molten metal of each cast steel A-N was cast into a sand mold, and the
A stepped material of ~100 IDIX40~100 pieces x 400 knots (casting weight: 140 kp) was obtained. During this casting, the mold was removed 26 minutes and 27 seconds after pouring into the molten pot, and the mold was immediately cooled with water at a surface temperature of 1000°C. The ferrite N' of the obtained stepped cast steel material was measured, and the values are also shown in Table 1.

Next, each of the obtained cast steel materials was heated in a water-cooled solution housing f! ! After carrying out step 8, a test piece was cut out from each of the cast steel strips and the corrosion rate in a boiling 1% HCt solution and crevice corrosion resistance in seawater were examined.

Figure 2 shows steel castings A-N in boiling 1% HC1 solution.
However, the cast steel products obtained by the method of the present invention have significantly improved corrosion resistance compared to conventional austenitic stainless steel (steel type I), and the -Cr level Duplex stainless steel can ゛1: Compared to products J to N, the corrosion rate is about 1/10th, which is a low value. You can see what it shows.

FIG. 3 shows the results of a laboratory test for crevice corrosion resistance in seawater. It as a test method:
, 3 mm x 20 ram x 30 mI+
+, 3 ws x 12Tnm x 30 II+m
Two test pieces were stacked on top of each other and tightened with Teflon bolts and nuts to form a gap between the test pieces.

The corrosive environment is 3% NaCt+/20 M Na2.
80°C artificial seawater consisting of 5O* was used. The corrosion test period was 30 days.

From the results shown in FIG. 3, it is clear that fr
It is clear that the steel products A-H have very little corrosion and exhibit extremely good seawater resistance even in actual high-temperature seawater.

Next, all of the molten steel having the composition of steel type A shown in Table 1 was cast into two sand molds similar to the above, one of which was the same as above.
After 26 minutes and 27 seconds after pouring, perform mold d and make 1 table m.
The sample was cooled with water at a temperature of 1000°C (method of the present invention).

The other one is 1-! cast in a sand mold! Leave it to room temperature (
conventional method).

When the two thus obtained duplex stainless steel castings were subjected to radiation tests, die check tests, and microscopic tests, the results shown in Table 2 were obtained, and the results shown in Figure 4 were as follows. Microscopic structures as shown were observed.

From the results shown in Table 2, it is clear that the method of the present invention produces a healthy ingot with no cracks observed, whereas the conventional method produces embrittlement cracks due to σ phase precipitation. It is clear that this occurs.

Also, from Fig. 4, an ingot obtained by the method of the present invention (Fig. 4) is shown.
In Figure (a)), a two-phase structure of [ferrite deaustenite] was observed with no precipitation of the σ phase at all, whereas in the ingot obtained by the conventional method (No. 411 (b) )
), a large amount of σ phase precipitation was observed.

As is clear from the above examples, by carrying out the method of the present invention, it is possible to stably obtain a cast steel product that is sound and has excellent seawater resistance without causing embrittlement cracking due to σ phase precipitation. .

As mentioned above, this invention makes it possible to stably mass-produce duplex stainless steel castings that are extremely resistant to seawater at a low cost without requiring complicated equipment or labor. This will bring about industrially useful effects, such as the ability to further expand the fields of application of Zorant and other equipment.

[Brief explanation of drawings]

Figure 1 shows the TTT curve of high Cr, high MO duplex stainless steel, and Figure 2 compares the corrosion rates of each yuzu cast iron product in a boiling 1% HC1 solution. Figure 3 is a graph showing the results of comparing the crevice corrosion resistance of various cast steel products in seawater, Figure 4 is a micrograph of duplex stainless steel cast steel, and Figure 4 (a) is a graph showing the results of comparing the crevice corrosion resistance of various cast steel products in seawater. Fig. 4 shows the structure of the cast steel product obtained by the conventional method. 1 person Figure 2 network 1 Article 3 Steel Figure 4

Claims (1)

[Claims] C: 0.05% or less. St: 2.0% or less. Mne 2-0% or less. Cr: 22.0-30.0%. N164.0-11.0%. Mo: 0.5-5.0%. Cu: 0-10 to 0-60%. P: 0.04% or less. S: 0.03% or less. N: 0.25% or less. In addition to containing. W: 0.05-1.50%. v: o, o s ~1.50%. Contains any one of the following: Fe and unavoidable impurities: Remains. After pouring high Cr, low Ni stainless steel molten metal having a composition (more than 100% by weight) into a mold and solidifying it, while it is in a high temperature state before the temperature drops to a temperature range of less than 1000 ° C. Demold and immediately cool down to σ
A patented method for manufacturing duplex stainless steel castings with excellent seawater resistance that suppresses phase precipitation.