JPH0382740A - Duplex stainless steel excellent in hot workability and corrosion resistance - Google Patents

Abstract

PURPOSE:To maintain superior corrosion resistance and to provide sufficiently satisfactory hot workability by adding specific amounts of Ca, Mg, and REM to a duplex stainless steel with a prescribed composition and also specifying ferrite content. CONSTITUTION:A duplex stainless steel has a composition consisting of, by weight, <=0.03% C, 0.10-2.0% Si, 0.10-2.0% Mn, <=0.05% P, <=0.002% S, 17.0-30.0% Cr, 1.0-11.0% Ni, 0.10-6.0% Mo, <0.10% Cu, 0.01-0.50% V, 0.01-0.10% Al, 0.01-0.40% N, <=0.0050% O, one or more kinds among 0.0005-0.010% Ca, 0.0005-0.010% Mg, and 0.0005-0.010% REM, and the balance Fe with inevitable impurities. Ferrite content is regulated to 30-70vol.%. Further, small amounts of one or more elements among W, Ti, and Nb are incorporated, if necessary. By this method, superior hot workability and excellent corrosion resistance can be obtained by the reduction in O content and the addition of Ca, Mg, and REM.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to stainless steel having 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. The present invention relates to duplex stainless steel that has excellent hot workability and corrosion resistance and is suitable for chemical equipment and structures, piping for various chemical plants, line pipes, oil country tubular goods, etc.

<Prior art and its challenges> In general, "two-phase stainless steel" not only has excellent corrosion resistance and weldability, but also exceeds ferritic stainless steel and austenitic stainless steel in terms of strength. The demand for steel as industrial steel is showing significant growth.

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.

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,
JP-A-60-262946 proposed that the hot workability of duplex stainless steel be improved by fixing it as a sulfide and suppressing its segregation to grain boundaries by adding a.
issue).

However, although it is true that the above-mentioned "means of adding Ca" significantly improves hot workability and improves manufacturing yield, it comes at the cost of improving the "excellent properties" originally possessed by duplex stainless steel. This leads to a decrease in corrosion resistance.
Therefore, it has been pointed out that its use is limited and that it is not practical.

Therefore, an object of the present invention was 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 object, the present inventors first discovered that when Ca is added, which is significantly effective in improving the hot workability of duplex stainless steel, the corrosion resistance of the steel deteriorates. After investigating and examining the cause, it was found that rCa has a strong tendency to combine with solid solution S to form sulfides, and at the same time to combine with ○ in steel to form oxides. niCa
When adding , many oxide-based inclusions are formed,
The fact that "this causes a significant decrease in corrosion resistance" has become clear, and this tendency is not limited to Ca, but is presumed to be effective in improving the hot workability of duplex stainless steel. Mg, REM (rare earth elements)
It was also found that the same effect was obtained when other sulfide-forming elements were added.

Furthermore, the present inventors have developed a method for evaluating the corrosion resistance of stainless steel by determining the temperature at which pitting occurs when immersed in a solution of 10% Fe (J, 6H, O for 24 hours). In addition to evaluating the corrosion resistance of duplex stainless steel without the addition of sulfide-forming elements and duplex stainless steel with the addition of sulfide-forming elements, the generally known rCr,
The corrosion resistance of the same material was also evaluated by applying the method of evaluating by "P, 1." calculated by the formula based on the content ratio (wt%) of Mo and N, and from the results, "the same P, 1." Even if it is a two-phase stainless steel type with ``sulfide-forming elements such as Ca, Mg, and REM added to improve hot workability'', ``Ca, Mg, and RE''
It was confirmed that the pitting corrosion onset temperature was significantly lower than that of Sample 2, which did not contain M.

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 of Can
The addition of Mg or REM is both effective in improving hot workability, and in particular, when the O content is reduced to a certain level, even if Ca, Mg, and REM are added, oxides that significantly reduce corrosion resistance. If the formation of system inclusions is no longer observed and the ferrite content ratio in the steel is adjusted to a specific range, the corrosion resistance improvement effect will become even more stable, but if these measures are used together, We were able to reach new knowledge that it is possible to create duplex stainless steel that has both extremely excellent corrosion resistance and hot workability.

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

Si: 0.03%.

Mn: 0.03%, P: 0.05% or less.

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

Ni: 1.0-11.0%, gate o: 0.1
0-6.0%.

Cu: less than 0.10%, V: 0.01
~0.50%.

Ill: 0.01-0.10%, N: 0.
01-0.40%.

0: 0.0050% or less So, and Ca: 0.0005-0.010%.

Gate g: 0.0005-0.010%.

REM: 1 out of 0.0005-0.010%
W: 0.01-1.5
0%, Ti: 0.01-0.50%.

Nb: 0.01 to 0.50%, and the balance is Fe and unavoidable impurities, and the ferrite content is 30 to 70%. %, it combines 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.

A) $, the metal content 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. Therefore, the C content was set at 0.03% or less.

t In order to ensure sufficient corrosion resistance, it is essential to reduce the ○ content, and therefore it is essential to add Si 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 at 0.3%.

Mn Mn is a component added for deoxidizing and desulfurizing 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 at 0.3%.

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 unavoidably contained in steel, and is the element that has the greatest effect on the hot workability of duplex stainless steel, so the lower its content is, 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%
A Cr content of at least 30% is required.
If it exceeds 0%, the σ phase tends to precipitate, deteriorating corrosion resistance and toughness, so the Cr content should be 17.0~
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%, the phase will be mainly composed of austenite, and not only will a two-phase Mi weave not be obtained.
Since it is an expensive element, it also causes economic disadvantage. Therefore, the Ni content was determined to be 1.0 to 11.0%.

M.

The Mo component has the effect of improving the corrosion resistance of steel, but if the Mo content is less than 0.1%, the desired effect due to the above effect cannot be obtained, while if the Mo content exceeds 6.0%, the σ phase The Mo content is 0.1 because it significantly promotes the precipitation of
It was set at ~6.0%.

Cu Cu is an element that inhibits hot workability, so it is necessary to reduce its amount as much as possible, but if the content is less than 0.10%, the negative effect on hot workability is small, so The content was limited to less than 0.10%.

■The component has the effect of improving pitting corrosion resistance by coexisting with appropriate amounts of Cr+Mo, etc., but the content is 0.0
If the content is less than 1%, the desired effect due to the above action cannot be obtained, and on the other hand, if the content exceeds 0.50%, hot workability deteriorates. Established.

IM is also an essential component as a deoxidizing agent, and the deoxidizing action of M was also utilized to ensure sufficient corrosion resistance. It is essential to reduce l. However, if the content is less than 0.01%, the desired deoxidizing effect cannot be obtained, while if the content exceeds 0.10%, AIN will precipitate, leading to a decrease in corrosion resistance. A1 content is 0.01~o,io%
It was determined that

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 0.01%, the above effect is poor, while if it exceeds 0.40%. N content is 0.01 to 0.4 because hot workability decreases and blowholes are more likely to form during casting.
It was limited to 0%.

Ca and REM () Ca, Mg, La, Ce, and other REMs all have the effect of generating sulfides in steel, fixing S, and improving the hot workability of steel. One or two of these
It is essential to add seeds or more, but the content of each is 0.00
If it is less than 0.05%, the desired effect due to the above action cannot be obtained,
On the other hand, since the above effect is saturated even if Ca is contained in an amount exceeding 0.010%.

The content of ceramic or REM is o, ooos ~ o, o, respectively.
t.

%.

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

a) When added in an appropriate amount together with W Cr and Mo, the effect of improving local corrosion properties can be obtained, but if the content is less than 0.01%, the desired effect cannot be ensured, while on the other hand, if the content exceeds 1.50% If it is included, it will lead to a decrease in hot workability, so
The W content was determined to be 0.01 to 1.50%.

b) Tt 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 effect is 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, LEM, etc., and easily becomes oxide inclusions that reduce corrosion resistance. It is necessary to reduce it to 0.050% or less.

From the viewpoint of corrosion resistance, the lower the 0 content, the better, and it is desirable to reduce it to 0, ooao% or less if possible.

B) Ferrite content The amount of ferrite is 30vol. % or less than 70 sills.

%, the desired corrosion resistance cannot be secured, so the ferrite content in the duplex stainless steel is set to 30 to 70 vol. %.

Note that the above ratio of ferrite I 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 121 m, and then subjected to solution treatment under the following conditions: heating and holding at r1070°C for 30 minutes, followed by water cooling, and the ferrite (α) of each plate thus obtained was In addition to investigating the amount, 2f from the board material
Corrosion test piece with l thickness x 2Qm width x 50tm length and 10mφ
High-temperature, high-speed tensile test pieces with a length of 130 ts were taken and used for each test.

In addition, in the corrosion test, the above corrosion test piece was heated to 10% of various temperatures.
The lowest temperature at which binning occurs on the surface of the test piece after immersion in an aqueous solution of F e Cj s 6H, O for 24 hours was investigated, and this temperature was determined as the "limit binning temperature (C, P, T,)".
According to the method, those with higher corrosion resistance are evaluated as having high corrosion resistance. However, the degree of corrosion resistance of stainless steel is almost Cr,
It is well known that the level of corrosion resistance is controlled by the content of each element of N and Mo, and as already mentioned, the level of corrosion resistance is determined by the content of each element, P, 1. =Cr(X)+3Mo(Ri+16N(Z)) Considering that P, 1. above has been used as an index of corrosion resistance, the evaluation of corrosion resistance in this example is P, , C, P, ?.

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.
Those that exhibit a reduction of area of 70% or more when subjected to tensile fracture at 0°C, 1000°C, 1100°C, and 1200°C exhibit sufficient workability even in actual hot rolling and have a good surface. In terms of properties, 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 (x).

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, but also has corrosion resistance test results of P, 1, C, P, T, As can be seen in Figure 1, which is a graph showing the relationship between P and 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. Note that the numbers attached to the symbols in FIG. 1 indicate the steel type numbers in Table 1.

On the other hand, conventional steels 16 to 21 also contain Ca, Mg,
Although it has good hot workability due to the addition of sulfide-forming elements such as REM, the reduction in the amount of 0 is not sufficient, so compared to the steel of the present invention, C and P are reduced in terms of p and r. , T are at low levels, and it is difficult to say that both hot workability and corrosion resistance are excellent.

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 has insufficient corrosion resistance due to an excessive amount of P, comparative steel 25 has poor hot workability due to an excessive amount of S, and furthermore, comparative steel 26 has Ca, which is a sulfide-forming element,
Since no Mg or REM was added, the corrosion resistance was good, but the hot workability was insufficient.

Summary of Effects> As explained above, according to the present invention, it is possible to realize a duplex stainless steel that has excellent corrosion resistance and heat workability, and improves the corrosion resistance of line pipes for submarine flow lines, etc. Dual-phase stainless steel has been forced to sacrifice hot workability and significantly reduce manufacturing yield in applications where compatibility (seawater, etc.) is important.

It brings about unprecedented industrial effects, such as being able to solve all the conventional problems associated with non-resistance steel.

[Brief explanation of drawings]

Figure 1 shows the corrosion resistance test results for p, r, C, P, T,
This is a graph expressed in relation to

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: 0.10-6.0%, Cu:
Less than 0.10%, V: 0.01-0.50%, Al: 0
．． 01-0.10%, N: 0.01-0.40%, O:
0.0050% or less, and also contains one or more of Ca: 0.0005 to 0.010%, Mg: 0.0005 to 0.010%, and REM: 0.0005 to 0.010%. In addition, the remainder consists of Fe and unavoidable impurities, and the ferrite content is 30
~70vol. % 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: 0.10-6.0%, Cu:
Less than 0.10%, V: 0.01-0.50%, Al: 0
．． 01-0.10%, N: 0.01-0.40%, O:
0.0050% or less, and one or more of Ca: 0.0005-0.010%, Mg: 0.0005-0.010%, REM: 0.0005-0.010%, and W: 0.01-1.50%, Ti: 0.01-0.50
%, Nb: 0.01 to 0.50%, the balance consists of Fe and unavoidable impurities, and the ferrite content is 30%.
~70vol. % duplex stainless steel with excellent hot workability and corrosion resistance.