JPH0382739A - Duplex stainless steel excellent in hot workability and corrosion resistance - Google Patents

Abstract

PURPOSE:To improve corrosion resistance and hot workability by specifying respective contents of C, Si, Mn, P, S, Cr, Ni, Mo, Cu, V, etc., and ferrite. CONSTITUTION:The duplex stainless steel has a composition consisting of, by weight, <=0.03% C, 0.1-2% Si, 0.1-2% Mn, <=0.05% P, <=0.002% S, 17-30% Cr, 1-11% Ni, 1-6% Mo, 0.1.2% Cu, 0.01-0.5% V, 0.01-0.04% Al, 0.1-0.4% N, <=0.005% O, one or more kinds among 0.0005-0.01% Ca, 0.0005-0.01% Mg, and 0.0005-0.01% REM, and the balance Fe. Further, S(%)+O(%), Al(%)+N(%), and ferrite content are regulated to <=0.005%, <=0.007%, and 30-70vol.%, respectively. The above steel has superior corrosion resistance and hot workability.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to stainless steel which is protected from a two-phase structure of austenite and ferrite, and is particularly applicable to stainless steels that require seawater resistance, such as heat exchangers that use seawater. It has good hot workability and corrosion resistance (stress corrosion cracking resistance, pitting corrosion resistance), making it suitable for chemical equipment and structures, piping for various chemical plants, line pipes, oil country tubular goods, etc.

This relates to duplex stainless steel with excellent acid resistance, etc.

<Prior art and its challenges> In recent years, demand for duplex stainless steel has increased as a material with excellent corrosion resistance and weldability, but this duplex stainless steel is particularly superior to ferritic stainless steel in terms of seawater corrosion resistance and strength. Or, because it has better properties than austenitic stainless steel, it is also rated 5US329 in JIS.
It has been standardized as J1!1m, and the fields of its application are continuing to expand.

Furthermore, with the expansion of the field of application of duplex stainless steel, trace elements are added to duplex stainless steel to improve its corrosion resistance in order to exhibit good performance in environments where even the 5US329J1 steel does not have sufficient corrosion resistance. Attempts have also been made to further improve it. For example, there is a proposal to add V or W to improve the seawater corrosion resistance of duplex stainless steel (Japanese Patent Publication No. 51-43807),
Proposals regarding the addition of Cu aimed at improving the acid resistance of duplex stainless steel (Japanese Patent Publication No. 59-5662) have also been made.

However, on the other hand, the above-mentioned duplex stainless steel has a two-phase structure of a ferrite phase and an austenite phase, so its hot workability is poor, and defects such as cracks and flaws occur during blooming and hot rolling. However, the manufacturing yield was very low.

Moreover, V is applied to 5US329J1 steel to improve corrosion resistance.
, W, Cu, etc., the decrease in hot workability was even more remarkable.

Therefore, S is an element that inhibits hot workability of duplex stainless steel.
In addition to reducing the content of solid solution S to C as much as possible,
A proposal was made to improve the hot workability of duplex stainless steel by fixing it as a sulfide and suppressing its segregation to grain boundaries by adding a.
issue). However, although the above-mentioned "means of adding Ca" does indeed greatly improve hot workability and improve manufacturing yield, it comes at the cost of improving the "excellent properties" originally possessed by duplex stainless steel. It has been pointed out that this leads to a decrease in "corrosion resistance" and is therefore undesirable from a practical standpoint, limiting its use.

Separately, JP-A-63157838 discloses a technique to improve crevice corrosion by making inclusions spherical by adding Ca, but Ca-based inclusions were highly water-soluble and spherical. However, they are inherently prone to becoming the starting point for pitting corrosion, and therefore there is a natural limit to technology for controlling the shape of inclusions aimed at improving corrosion resistance.

Furthermore, we added N to duplex stainless steel to ensure high corrosion resistance.
Attempts have also been made to add N, but it has not been possible to eliminate the problem that the desired corrosion resistance cannot be obtained due to the precipitation of nitrides when N is added.

Therefore, an object of the present invention is to provide a duplex stainless steel that maintains its original excellent corrosion resistance and also exhibits sufficiently satisfactory hot workability.

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have specifically found that ``corrosion resistance does not deteriorate even if sulfide-forming elements such as Ca, which are considered to be effective in improving hot workability, are added.'' In addition, with the aim of realizing a two-phase stainless steel with a composition that can fully demonstrate the corrosion resistance improvement effect of the addition of Cu and V, and that does not cause a decrease in corrosion resistance due to nitride precipitation even if N is added, we first developed a two-phase stainless steel. We investigated and examined the cause of the deterioration of the corrosion resistance of stainless steel when Ca is added, which has a significant effect on improving the hot workability of stainless steel. It also has a strong tendency to combine with O to form oxides. Therefore, when Ca is added to steel in which O is present at a higher level, many oxide-based inclusions are formed, which causes a significant decrease in corrosion resistance. Furthermore, this tendency is not limited to Ca, but also Mg and REV (which are presumed to be effective in improving the hot workability of duplex stainless steel). It was also found that the same effect was obtained when other sulfide-forming elements such as rare earth elements were added.

Furthermore, the present inventors used r 10 ZFeCfz・6Hto + 1/2O as a method for evaluating the corrosion resistance of stainless steel.
By introducing a method to determine the temperature (C, P, T,) at which bifting occurs when immersed in a solution of N-H (J) for 24 hours, we developed a method to determine the temperature (C, P, T,) at which bifting occurs when immersed in a solution of In addition to evaluating the corrosion resistance of duplex stainless steel with addition of metal-forming elements, we also investigated
is calculated using a formula based on the content ratio (wt%) of N and
P.1. The corrosion resistance of similar materials was evaluated by applying the same P, It was confirmed that the temperature at which pitting corrosion occurs is significantly lower in the steel with the sulfide-forming elements Mg and REM added than in the steel without the addition of Ca, Mg and REM.

Based on these results, we comprehensively and carefully examined the effects of various elements on the corrosion resistance of duplex stainless steel, and developed a duplex stainless steel with a composition that achieves both corrosion resistance and hot workability. As a result of repeated research on the existence or non-existence of That is, (al) Addition of Cat Mg or REM to normally produced duplex stainless steel is effective in improving hot workability, and in particular reduces the ○ content and S content to specific levels. and S ($) + OCX) ≦0.0050, sulfide-based inclusions that significantly reduce corrosion resistance even if Cat Mg and REM are added,
The formation of oxide inclusions is no longer observed.

(b) In addition, the presence of nitride-based inclusions also causes a decrease in corrosion resistance, and in duplex stainless steel [111(X
) X N (X)) (7) When the value particularly exceeds 0.007, precipitation of AIN becomes a starting point for pitting corrosion, resulting in a decrease in pitting corrosion resistance.

(C) Furthermore, in duplex stainless steel, when the ferrite content ratio in the steel is adjusted to a specific range, the corrosion resistance improvement effect becomes even more stable.

(d) Therefore, the addition of Cat Mg or REM;
If measures such as regulating the O content and S content, regulating the M content and N content, and adjusting the amount of ferrite are used together, duplex stainless steel with extremely excellent corrosion resistance and hot workability can be produced. Realization of steel is possible.

The present invention has been made based on the above-mentioned findings, etc., and is based on the following: ``C: 0.03% or less (hereinafter, % representing the component ratio is % by weight).

Si: 0.0050%.

Mn: 0.0050%, P: 0.05%
below.

S: 0.002% or less, Cr: 17.0
~30.0%.

Ni: 1.0-11.0%, Mo: 1.
0-6.0%.

Cu: 0.0050%, V: 0.01~
0.50%.

A1: o, oi~0.04%, N: 0.1
0-0.40%.

0: 0.0050% or less So, and Ca: o, ooos~o, oto%.

Mg: 0.0005-0.010%.

Contains one or more of R, EM: 0.0005 to 0.010%, or further W: 0.0050%
, Ti: 0.01-0.50%.

Nb: 0101 to 0.50%, and the balance is free from Fe and inevitable impurities, and S(χ)+O(χ)≦0.0050゜AI(X) N (X) ≦0.0070゜, Mg
:0.0005 to 0.010% By configuring it to satisfy the following conditions, it is characterized by having both excellent hot workability and corrosion resistance.

Here, the reason why the content ratio of each component and the ferrite content ratio of the duplex stainless steel are limited as described above is as follows.

<Function> A) Component content ratio C is an element that is unavoidably contained in steel, but if its content exceeds 0.03%, carbides will precipitate, especially in the weld heat affected zone, and corrosion resistance will deteriorate. Since this would lead to a decrease in carbon content, the C content was set at 0.03% or less.

Ni In order to ensure sufficient corrosion resistance, it is essential to reduce the O content, and therefore it is essential to add St for the purpose of deoxidation. In this case, if the Si content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, while if the Si content exceeds 2.0%, it will cause embrittlement. ．．
It was set as 0050%.

Mn is a component added to deoxidize and desulfurize steel, but if its content is less than 0.10%, the deoxidizing and desulfurizing effect will be small; on the other hand, if its content exceeds 2.0%, If the Mn content is 0, it will have a negative effect on corrosion resistance.
．． It was set as 0050%.

Since P is an impurity element that is unavoidably contained in steel and deteriorates hot workability and corrosion resistance, it is preferable to keep its content as low as possible, but considering the cost of dephosphorization,
The content was set at 0.05% or less.

S.

S is also an impurity that is inevitably contained in steel, and is the element that has the greatest effect on the hot workability of duplex stainless steel, so the lower the content, the better. In order to ensure sufficiently satisfactory hot workability, it is necessary to reduce S to a level of 0.002% or less, so the upper limit of the S content was set at 0.002%.

Cr Cr is one of the basic components of duplex stainless steel and is an important component governing corrosion resistance. In order to exhibit an austenite-ferrite two-phase structure, 17.0% is required.
A Cr content of at least 30% is required.
If the Cr content exceeds 0%, the σ phase tends to precipitate, degrading corrosion resistance and toughness, so the Cr content should be 17. O~
It was set at 30.0%.

Ni Ni is a component added in consideration of the Cr content, Mo content, and N content in order to obtain a two-phase structure, but if the Ni content is less than 1.0%, the ferrite phase becomes the main component. Therefore, a two-phase structure cannot be obtained. On the other hand, if Ni is contained in an amount exceeding 11.0%, not only will the phase become mainly austenite, making it impossible to obtain a two-phase structure, but it will also cause economic disadvantages since it is an expensive element. It also becomes. Therefore, the Ni content was determined to be 1.0 to 11.0%.

The Mo component has the effect of improving the corrosion resistance of steel, but if the Mo content is less than 1.0%, the desired effect cannot be obtained by the above effect, while on the other hand, if the Mo content is more than 6.0%, σ The Mo content is 1.0 because it significantly promotes phase precipitation.
It was set at ~6.0%.

Cu Cu component has the effect of improving the acid resistance of steel, but if the content is less than 0.10%, the desired effect due to the above effect cannot be obtained, while on the other hand, if the content exceeds 2.0%, Cu″
The content was determined to be 0.0050%.

The component (■) has the effect of improving local corrosion resistance when added in appropriate amounts along with Cr, Mo, and Cu, but if its content is less than 0.01%, the desired effect due to the above effect cannot be obtained, On the other hand, if the content exceeds 0.50%, hot workability deteriorates, so the content should be 0.01 to 0.
．． It was set at 50%.

Al is also an essential component as a deoxidizing agent, and in order to ensure sufficient corrosion resistance, it is essential to reduce the amount of O by also utilizing the deoxidizing action of Al. However, if the content is less than 0.01%, the desired deoxidizing effect cannot be obtained, while if the content exceeds 0.04%, AfN will precipitate, leading to a decrease in pitting corrosion resistance. Therefore, the N content is 0.01 to 0.0
It was set at 4%. Furthermore, in order to ensure stable corrosion resistance, as described later, tU (X) x N (X) ≦
It is necessary to adjust the N content so as to satisfy the condition of 0.0070.

N is an important component in forming a two-phase structure and is also effective in improving corrosion resistance, but if the N content is less than 0.10%, the above effects are poor, while if it exceeds 0.40%, The N content is 0.10 to 0.4 because it reduces hot workability and tends to cause blowholes during casting.
It was limited to 0%. In addition, the upper limit of the N content is also regulated by corrosion resistance, and the N content is adjusted so as to satisfy the condition Af(χ)×N(χ)≦0.0070 so that AIN is not generated as described later. There is a need.

Ca, M, and REM (earth-element) Ca and Mg,
Alternatively, since REMs such as La and Ce all have the effect of generating sulfides in steel, fixing S, and improving the hot workability of steel, one or more of these can be used. However, if the content is less than 0.0005%, the desired effect cannot be obtained;
．． Even if the content exceeds 0.010%, the above effect will be saturated, so Ca.

The content of Mg or REM is 0.0005 to 0.00, respectively.
It was set as 0.10%.

Since all of these ingredients have the effect of improving the corrosion resistance of the steel, one or more of these ingredients may be added as necessary. The reason for limiting the content range will be explained.

a) W W has the effect of improving crevice corrosion resistance, but if its content is less than 0.01%, the desired effect of the above effect cannot be ensured, and on the other hand, if it is contained more than 1.50%. Since this leads to a decrease in hot workability, the W content is set at 0.0050%.

b) Ti and Nb These elements generate stable carbides in steel and contribute to improving corrosion resistance, but if any of them is less than 0.01%, a sufficient effect cannot be obtained; Since the above effects would be saturated even if the content exceeds 50%, the Ti content and Nb content were determined to be 0.01 to 0.50%.

O is an undesirable impurity element that easily forms compounds with Ca, REM, etc. and easily becomes oxide-based inclusions that reduce corrosion resistance. It is necessary to reduce it to 0.050% or less.

In order to improve corrosion resistance, the lower the O content, the better, and it is preferable to reduce it to 0.0030% or less if possible.

Ca for improving the hot workability of Σ(X)shiΣSakami Kabutsa duplex stainless steel
If the addition is carried out under the condition that S (Ri + O(X) > 0.0050), oxide inclusions and sulfides will be present in the steel even if the respective contents of 8.0 are within the specified ranges. Since a large amount of system inclusions precipitate and greatly reduce corrosion resistance, it is specified that the content of S and 0 should be limited to a range that satisfies the condition of s (z) + O(X) s 0.0050. Ta.

Figure 1 shows (S (X) + O(
It is a graph showing the relationship between the value of X)) and corrosion resistance.

The results shown in FIG. 1 were obtained by evaluating the corrosion resistance of the three types of steel shown in the figure and varying the S and O contents. Here, all of the 3w and 4 types are p,
Since r is not the same, the corrosion resistance test was conducted at temperatures near C, P, and T of each steel. The test environment was 10% Fe (J3, 6Hz○+1/2ON-HC
A Z solution was used, and the test piece was immersed in this solution for 24 hours, and then the corrosion loss was measured to evaluate the corrosion resistance.

From Figure 1, (S (%) + O(X)) is 0.
．． It is clear that when the corrosion weight exceeds 0.0050, the corrosion loss increases significantly regardless of the steel type, and it is clear that Luniha (S(X) + 0(X)) must be 0.0050 or less to ensure good corrosion resistance. .

As mentioned previously, in order to improve the corrosion resistance of steel, it is necessary to sufficiently deoxidize it, so there is a tendency to increase the amount of M added, but duplex stainless steel In steels that normally have a high N content, such as steels, when excessive amounts of AI are added, a large amount of AfN-based inclusions will precipitate, resulting in a significant decrease in corrosion resistance. If the content of AI and N exceeds the value of [AI (χ) × N (χ)] of 0.0070, the corrosion resistance of steel will deteriorate significantly, so the content of M and N It was determined that the amount should be limited within a range that satisfies the condition Af(ri x N (z)≦0.0070).

Figure 2 shows (111(ri)
2 is a graph showing the relationship between the value of and corrosion resistance.

The results shown in FIG. 2 were obtained by evaluating the corrosion resistance of the three types of steel shown in the figure and varying the amount of A1 added. Here, all four types of 3fi above are P,
Since I is not the same, the corrosion resistance test was conducted at temperatures near C, P, and T of each steel.

Corrosion resistance test environment is 10! re(Js・6Hzo +1/
A 2ON-HCf solution was used, and the test piece was immersed in this solution for 24 hours, and then the corrosion loss was measured to evaluate the corrosion resistance.

From this Figure 2, it can be seen that [pJl(z)×
It is clear that when N (X) ) exceeds 0.0070, the corrosion loss becomes significantly large (Ai! (X) X N (%) ) is 0.
．． It can be seen that the value needs to be 0070 or less.

B) Ferrite content The amount of ferrite is 3Qvo1. % or 70vol.

%, it is impossible to secure the desired corrosion resistance, so the ferrite content in the duplex stainless steel was limited to 30 to 0.010%.

Note that the above ratio of the amount of ferrite 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 high frequency induction vacuum melting furnace and cast into a 50 kg steel ingot.

Next, these steel ingots were hot-rolled to a thickness of 12 tm, and then subjected to solution treatment under the following conditions: heating and holding at r1070°C for 30 minutes, followed by water cooling. In addition to investigating the amount, 21
Corrosion test piece of m thickness x 20n width x 5Qn + length and IonφX
High-temperature, high-speed tensile test pieces with a length of 130 m were taken and used for each test.

In addition, the corrosion test was carried out using the above corrosion test pieces at various temperatures as shown in Table 2 at 10% PeC! 5-6HzO+2ON-H (Immerse in J aqueous solution for 24 hours to find the lowest temperature at which pitting occurs on the surface of the test piece, and define this temperature as the "limit pinting temperature (C, P, T,)". However, it is well known that the degree of corrosion resistance of stainless steel is mostly controlled by the content of each element of Cr + N and No, and as mentioned above, The level of corrosion resistance is P, L = Cr(Z) + 3 Mo(f) + 1
6N (χ), so the above P
, 1. Taking into consideration the fact that has been used as an index of corrosion resistance, the evaluation of corrosion resistance in this example was P, 1. C with
Hereinafter, Si, T.

Based on the height of

In addition, the high-temperature high-speed tensile test was conducted to evaluate hot workability, and the strain rate was 90 at a strain rate of 1.Os.
Items 1 that exhibit a reduction of area of 70% or more when subjected to tensile fracture at each temperature of 0°C, 1ooo°C, and 1200°C exhibit sufficient workability and good surface quality even in actual hot rolling. Therefore, those whose aperture value was 70% or more at all temperatures were evaluated as good (◯), and those whose aperture value was less than 70% at any temperature were evaluated as poor (×).

These results are shown in Table 2.

As is clear from the results shown in Table 2, the duplex stainless steel according to the present invention not only has good hot workability because it contains sulfide-forming elements such as Ca, but also has S (χ)+O(χ)≦0.0050゜Al (X) X N (X)≦0.0070 is satisfied, so the corrosion resistance test results are
As can be seen in Figure 3, which is a graph showing the relationship between C, P, and T, P, 1. Even so, it shows much higher C, P, and T than this, and it can be seen that it has even better corrosion resistance.

On the other hand, conventional steels 16 to 21 also have good hot workability due to the addition of sulfide-forming elements such as Ca and Mg, but conventional steels 16, 17, and 18 have low amounts of S and 0. Since the reduction is not sufficient and a large amount of sulfide-based and oxide-based inclusions are precipitated, C and P are lower than p and r compared to the steel of the present invention.
,? , is at a low level, and conventional steel 18-21
Because the value of (Aj (X) Since it is out of the range, p, r
C, P, and T are low in comparison to , and it is hard to say that any of them are excellent in both hot workability and corrosion resistance.

Furthermore, Comparative Steels 22 and 23 had inferior corrosion resistance because their ferrite fractions were outside the range defined by the present invention.

Comparative steel 24.25.26 has an excessive amount of P and no addition of Cu or V, so its corrosion resistance is insufficient, and comparative steel 27.28 has (S(χ) 10(ri) or the value of (Af (X)

Furthermore, Comparative Steel 29 has poor hot workability and corrosion resistance due to excessive S content, and Comparative Steel 3° has poor corrosion resistance because it does not contain any sulfide-forming elements such as Ca, Mg, or REM. Although the results are good, the hot workability is insufficient.

<Summary of Effects> As explained above, according to the present invention, it is possible to realize a duplex stainless steel that is excellent in both corrosion resistance and hot workability, and it is possible to achieve “corrosion resistance for line pipes for submarine flow lines, etc.” The conventional problems associated with duplex stainless steels, which were forced to sacrifice hot workability and significantly reduce production yields in applications where seawater resistance (e.g., seawater resistance) were important, could be solved at once. This brings about unimaginable effects.

[Brief explanation of drawings]

Figure 1 shows (S(X) 10(
2)] is a graph showing the relationship between the value and the corrosion weight loss. Figure 2 shows (Affi(χ)) in duplex stainless steel.
It is a graph showing the relationship between the value of ×N(X)) and corrosion loss. Figure 3 shows the corrosion resistance test results for p, r, C, P, T,
This is a graph expressed in relation to Figure (Sα) + ○α)] Value of A2 Figure 3 [A1(χ) x N (X)] Figure 3 P, 1

Claims (2)

[Claims] (1) C: 0.03% or less, Si: 0.10-2.0%, Mn in weight percentage
: 0.10-2.0%, P: 0.05% or less, S: 0.
002% or less, Cr: 17.0-30.0%, Ni: 1
．． 0-11.0%, Mo: 1.0-6.0%, Cu: 0
．． 10-2.0%, V: 0.01-0.50%, Al:
0.01-0.04%, N: 0.10-0.40%, O
: 0.0050% or less, and also contains one or more of the following: Ca: 0.0005-0.010%, Mg: 0.0005-0.010%, REM: 0.0005-0.010% At the same time, the remainder consists of Fe and unavoidable impurities, and S (%) + O (%) ≦0.0050, Al (%) × N (%) ≦0.0070, ferrite content: 30 to 70 vol. % duplex stainless steel with excellent hot workability and corrosion resistance. (2) C: 0.03% or less, Si: 0.10-2.0%, Mn in weight percentage
: 0.10-2.0%, P: 0.05% or less, S: 0.
002% or less, Cr: 17.0-30.0%, Ni: 1
．． 0-11.0%, Mo: 1.0-6.0%, Cu: 0
．． 10-2.0%, V: 0.01-0.50%, Al:
0.01-0.04%, N: 0.10-0.40%, O
: 0.0050% or less, and one or more of Ca: 0.0005 to 0.010%, Mg: 0.0005 to 0.010%, REM: 0.0005 to 0.010%, and W :0.01~1.50%, Ti:0.01~0.50
%, Nb: 0.01 to 0.50%, the remainder consists of Fe and unavoidable impurities, and S (%) + O (%) ≦ 0.0050, Al ( %)×N(%)≦0.0070, ferrite content: 30 to 70 vol. % duplex stainless steel with excellent hot workability and corrosion resistance.