JPH02258956A - Highly corrosion resistant duplex phase stainless steel excellent in hot workability - Google Patents

Abstract

PURPOSE:To produce a highly corrosion resistant duplex phase stainless steel excellent in hot workability by incorporating specific percentages of C, Si, Mn, P, S, Ni, Cr, Mo, V, solAl, Ca, B, N, and O to Fe and also specifying the proportion of gamma-phase in a cast state. CONSTITUTION:A duplex phase stainless steel having a composition which consists of, by weight ratio, <=0.03% C, 0.1-1.0% Si, 0.3-2.0% Mn, <=0.035% P, <=0.0020% S, 4.0-9.0% Ni, 20-28% Cr, 0.5-5.0% Mo, 0.1-1.0% V, 0.005-0.05% solAl, 0.0010-0.0100% Ca, 0.08-0.3% N, <=0.006% O, and the balance essentially Fe and also having a structure in which the proportion of gamma-phase in the cast state is regulated to 30-70% is prepared. By this method, the highly corrosion resistant duplex phase stainless steel suitable as highly corrosion resistant stock for line pipe for oil well, etc., and excellent in hot workability can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is suitable as a highly corrosion-resistant material for oil well line pipes and chemical industry piping, and is suitable for use during hot processing (for example, when manufacturing steel plates and steel pipes). The present invention relates to a highly corrosion-resistant duplex stainless steel with excellent hot workability and extremely low occurrence of defects.

<Prior art and its problems> Duplex stainless steel containing a large amount of Cr has strong resistance to pitting corrosion, crevice corrosion, stress corrosion, etc. caused by chlorides, so it cannot be used in heat exchangers using seawater. Although it boasts a wide range of uses including chemical manufacturing equipment and oil well piping materials, it has been pointed out that duplex stainless steel also has the drawback of generally being inferior in high-temperature deformability. In other words, duplex stainless steel has low hot workability, and is a material that easily induces surface cracking of steel pieces due to embrittlement mechanisms called "sigma embrittlement" and "475°C embrittlement" during the cooling process after hot working. Met.

Therefore, the following measures have been proposed to improve the high-temperature deformability of this duplex stainless steel, and it has been reported that a corresponding effect can be obtained.

^) In addition to controlling the ratio of γ phase to 15 to 55% and designing the composition to ensure a sufficient amount of α phase, we also balanced the steel ingot components to increase the strength of γ phase and α phase at high temperatures. A method of strengthening grain boundaries by increasing the grain boundaries and adding B (Japanese Patent Publication No. 59-14099).

B) A method of reducing S and O in steel and, if necessary, adding rare earth elements to enhance O and S removal during steel melting (Japanese Patent Publication No. 15660/1983).

C) A method of changing the form of inclusions by adding Aβ and Ca in combination (Japanese Patent Publication No. 54-24364).

By the way, in duplex stainless steel, Mo and N are often added to improve corrosion resistance, but V, W, Cu, etc. are added in addition to the above elements for the purpose of further increasing crevice corrosion resistance. Sometimes. However, although these elements are very effective for improving corrosion resistance, they also significantly impede hot workability on the - side, and it is known that - in particular, significantly deteriorates toughness.

Therefore, as mentioned above, when duplex stainless steel, which has low hot workability and is prone to cracking the surface of steel pieces, contains a large amount of V, etc., although it has a significant effect on crevice corrosion resistance. The embrittlement will be further promoted, and even if the above-mentioned methods for improving high-temperature deformability are applied as they are, it will be difficult to prevent surface cracking of the steel billet, and the steel billet yield will be significantly reduced. As a result, he was forced to do so.

Therefore, the current situation is that there is still no practical duplex stainless steel in which the crevice corrosion resistance is further improved by containing a large amount of (2) or the like.

For this reason, the purpose of the present invention is to add corrosion resistance improving ingredients such as (1) to impart better corrosion resistance performance,
The aim was to provide a duplex stainless steel that also had excellent hot workability.

Means for Solving the Problems> Therefore, in order to achieve the above object, the present inventors first conducted the following steps: a) It is generally known that the hot workability of duplex stainless steel improves as the amount of the γ phase decreases. However, if the γ phase is reduced, the toughness will deteriorate.

b) On the other hand, the precipitation of the sigma phase is promoted when the γ phase increases.

C) Many of the surface cracks of steel slabs caused by sigma embrittlement etc. originate from hot working cracks and are promoted by stress generated during cooling or maintenance of the steel slab.

Focusing on the fact that the In duplex stainless steel containing elements that improve corrosion resistance, the amounts of S and O in the steel are reduced to extremely low levels, combined addition of Ca and B, and the low temperature range of hot working (1000 to 800°C) is achieved. The ratio of γ phase is 30 to 70
If the composition is set to be at a medium level of %, the hot workability will be significantly improved and the processing yield of the steel billet can be improved to over 95%. Therefore, according to the above method, ■
They came to the conclusion that it is possible to create a duplex stainless steel that has improved corrosion resistance and excellent hot workability by adding other ingredients.

This invention was made based on the above knowledge, and it is based on the following: ``C: 0.03% or less of duplex stainless steel.

Si: 0.1-1.0%, Mn: 0.3
~2.0%.

P: 0.035% or less, S: 0.002
Less than 0%.

Ni: 4.0~9.0%, Cr: 20~
28%.

Gate o: 0.5-5.0%, V: 0.1-1.
0%.

Sol, Aj! : 0.005-0.05%.

Ca: 0.0010-0.0100%.

B: 0.0005-0.0100%, N:
0.08-0.2%.

0: Contains 0.006% or less, or in addition, C1l: 0
．． 1-1.5%.

W: 0.1-1.5%.

Contains at least one type of Nb: 0.05 to 0.5%, the remainder is substantially composed of Fe, and the composition has a composition in which the T phase ratio in the cast state is 30 to 70%. This makes it possible to maintain excellent corrosion resistance while also providing excellent hot workability.

Here, the reason why the content ratio of the components of the duplex stainless steel and the ratio of the γ phase in the cast state are limited to the above-mentioned values is as follows.

(Al C If the C content exceeds 0.03%, carbides will precipitate at grain boundaries, and corrosion resistance and toughness will tend to deteriorate. Therefore, the C content was set at 0.03% or less. .

(bl 5i Si is an essential component for deoxidizing steel, and to ensure a sufficient deoxidizing effect, it must be contained at 0.1% or more, but if it is contained in excess of 1.0%, The Si content was set at 0.1 to 1.0% since this would lead to deterioration of toughness.

(e) Mn Although the Mn component also has a deoxidizing effect like Si, if its content is less than 0.3%, the desired effect due to this effect cannot be obtained. On the other hand, since Mn content up to 2.0% does not have any particular adverse effect on steel,? The In content was determined to be 3 to 2.0%.

(d) P P is an impurity element that deteriorates the hot workability and corrosion resistance of steel, but since it is allowed up to 0.035%,
The P content was set at 0.035% or less.

(el S and O 8 and O, if present in excess, segregate at grain boundaries and deteriorate hot workability, and also combine with Ca, AJ!, etc. to form coarse inclusions and deteriorate corrosion resistance. Therefore, it is an impurity element that should be reduced as much as possible.However, if the S content is reduced to 0.0020%
In addition, since the desired high hot workability and corrosion resistance can be obtained by suppressing the C content to 0.006% or less, these contents were each limited as described above.

ffl Ni Ni The Ni component is essential for improving the mechanical properties, workability, and general corrosion resistance of steel, and for making the steel structure into an austenite-ferrite two-phase Ni woven structure. If the Ni content is less than 0%, the above effects cannot be obtained, and on the other hand, if the Ni content exceeds 9.0%, the property improvement effect that is commensurate with the cost cannot be obtained.
．． It was set as 0%.

(gl Cr Cr is an important component that improves the general corrosion resistance of steel,
In order to ensure the desired corrosion resistance required for duplex stainless steel, it is necessary to contain 20% or more. On the other hand, if Cr content exceeds 28%, workability will be adversely affected, so the Cr content should be 20 to 28%.
It was determined that

(h! M.

Since the Mo component has the effect of significantly improving the local corrosion resistance of steel, it is added together with Cr in corrosion-resistant alloys to further improve the corrosion resistance, but if the Mo content is less than 0.5%, the above effect will occur. On the other hand, 5
．． If the Mo content exceeds 0%, not only will it not be possible to ensure an effect commensurate with the cost increase, but there is also concern that it will have a negative impact on workability, so the Mo content should be 0.5 to 5%.
．． It was set as 0%.

(1) Component V (2) has the effect of improving the corrosion resistance and crevice corrosion resistance of steel against chloride solutions etc. by coexisting with Cr, Mo, etc. in appropriate amounts, but if its content is less than 0.1%, the above effect will be reduced. On the other hand, if the content exceeds 1.0%, the hot workability and toughness will be significantly deteriorated. Established.

(Jl sol and AR Af are indispensable as deoxidizing components, and in order to ensure sufficient deoxidizing effects, it is necessary to contain them at 0.005% or more in sot and AI; If it is contained in sol, it will precipitate as AIN, which will deteriorate the corrosion resistance and also cause a decrease in toughness.
, set at 0.005 to 0.05% by AI.

(kl Ca The Ca component has the effect of fixing S in steel as sulfide and improving hot workability, but when its content is 0.0010
If the content is less than 0.0100%, the desired effect cannot be obtained by the above action, while if the content exceeds 0.0100%, it not only impairs hot workability but also deteriorates corrosion resistance. The amount is o, ooio ~ o, oto.

%.

(1) B The B component has the effect of improving the hot workability of steel, but the mechanism is different from that in the case of Ca addition, so by coexisting with Ca, hot workability in a wide temperature range is achieved. be able to. However, if the B content (■) is less than 0.0005%, the effect of improving hot workability is insufficient;
B content exceeding 100% promotes melt embrittlement and impairs hot workability;
,ooos~0.0100%.

(mlN The N component has the effect of improving the pitting corrosion resistance and crevice corrosion resistance not only of the base metal but also of the welded part, but when its content is 0.0
If it is less than 8%, the desired effect cannot be obtained in the above action;
If the N content exceeds 0.30%, nitrides tend to precipitate and have an adverse effect on corrosion resistance and toughness, so the N content was set at 0.08 to 0.30%.

(o) Cu, W, and Nb Each of these components has the effect of further improving the corrosion resistance of duplex stainless steel, so one or more of these components may be included if necessary, but the content of each component is It was established for the following reasons.

b) Cu Cu component has the effect of improving the pitting corrosion resistance, crevice corrosion resistance, and general corrosion resistance of duplex stainless steel, but if its content is less than 0.1%, the desired effects due to the above effects cannot be obtained. On the other hand, if Cu is contained in an amount exceeding 1.5%, it not only impairs high-temperature deformability but also precipitates ε-phase and deteriorates corrosion resistance. was set at 0.1 to 1.5%.

Mouth) W The W component has the effect of improving the local corrosion resistance of duplex stainless steel, but if its content is less than 0.1%, the desired effect due to the above effect cannot be obtained; Since it is difficult to obtain a further corrosion resistance improvement effect even if the content exceeds W, the content is set at 0.1 to 1.5% when W is included.

C) Nb The Nb component has the effect of improving the local corrosion resistance of duplex stainless steel as in (3), but its content is 0.05%.
If it is less than that, the desired effect due to the above action cannot be obtained; on the other hand,
Even if the Nb content exceeds 0.5%, the gas cannot have an effect commensurate with the cost increase, so the Nb content is 0.05%.
It was set at ~0.5%.

(p) Ratio of γ phase in the cast state In normal duplex stainless steel, the ratio of γ phase is 110.
It decreases significantly above 0℃, but in the low temperature range of hot working (10
00 to 800°C), there is no large variation, and it can be represented by the ratio of γ phase in the cast state. and,
If the ratio of the γ phase in the cast state is less than 30%, the toughness will deteriorate and heat crank will easily occur when cleaning the billet after hot working (see Examples below). ), the processing yield of steel billets decreases. On the other hand, if the ratio of the γ phase in the cast state exceeds 70%, hot workability is significantly reduced, and cracks due to the sigma phase are likely to occur, resulting in a decrease in the processing yield of the steel billet.

Therefore, in the duplex stainless steel according to the present invention, the proportion of the γ phase in the cast state is limited to 30 to 70%.

Note that the above ratio of the γ phase can be achieved by adjusting the content ratio of each component within the specified range of the present invention.

Next, the present invention will be described in more detail using Examples and in comparison with Comparative Examples.

<Example> First, each duplex stainless steel having the composition shown in Table 1 was melted in a vacuum melting furnace and cast into a 25 kg round steel ingot.

Next, the black scale existing on the surface of these round steel ingots is removed by grinding, and the bottom surface of the steel ingot is further polished and finished, and then the ferrite phase ratio (
α%) was measured. Then, the γ phase ratio (1%) was determined by “γ%=100−α%”.

Next, the ground steel ingot was heated in a furnace at 1250°C for 2 hours and then taken out of the furnace to start forging.
6011 thickness x 100 baking soda width x 6011 thickness x 100 baking soda width x
A steel piece with a length of 40 ora was made, and the weight was measured after cooling in the air. Then, the weight after the scratches generated on the surface of the steel billet due to the forging were removed by grinding was also measured, and the ratio of this "weight after scratch removal" to the "weight of the billet without maintenance after forging" was calculated. "Steel yield rate".

The above “calculation result of steel single-handed processing yield” is calculated as “γ phase ratio (1%)”
The measurement results are also shown in Table 1.

As is clear from the results shown in Table 1, the duplex stainless steel according to the present invention shows a high value of 95% or more in steel single-hand processing yield, and has excellent hot workability compared to conventional steel. I can see that

In addition, it was also confirmed by a corrosion test that the duplex stainless steel according to the present invention has sufficiently excellent corrosion resistance.

Summary of Effects> As explained above, according to the present invention, it is possible to provide a duplex stainless steel that exhibits significantly high corrosion resistance by adding V and has excellent hot workability.
It is expected to greatly contribute to improving the performance and reducing costs of chemical industry piping, oil well line pipes, etc., and will bring extremely useful effects industrially.

Claims (2)

[Claims] (1) C: 0.03% or less, Si: 0.1-1.0%, Mn:
0.3-2.0%, P: 0.035% or less, S: 0.0
020% or less, Ni: 4.0 to 9.0%, Cr: 20 to
28%, Mo: 0.5-5.0%. V: 0.1-1.0%, sol. Al: 0.005~0
．． 05%, Ca: 0.0010-0.0100%, B: 0.0005-0.0100%, N: 0.08-0
．． 3%, O: 0.006% or less, the balance is substantially made of Fe, and the γ phase ratio in the cast state is 30 to 70%. Highly corrosion-resistant duplex stainless steel with excellent hot workability. steel. (2) C: 0.03% or less, Si: 0.1 to 1.0%, Mn:
0.3-2.0%, P: 0.035% or less, S: 0.0
020% or less, Ni: 4.0 to 9.0%, Cr: 20 to
28%, Mo: 0.5-5.0%, V: 0.1-1.0
%, sol. Al: 0.005-0.05%, Ca: 0
．． 0010-0.0100%, B: 0.0005-0.0100%, N: 0.08-0
．． 3%, O: 0.006% or less, and further contains Cu: 0.1 to 1.5%, W: 0.1 to 1.5%, and Nb: 0.05 to 0.5%. A highly corrosion-resistant duplex stainless steel having excellent hot workability, containing at least one type of stainless steel, the remainder being substantially composed of Fe, and having a γ phase ratio of 30 to 70% in a cast state.