JPS5881956A - Austenitic stainless steel - Google Patents

Abstract

PURPOSE:To provide austenitic stainless steel to which excellent sea water resistance, strength and hot processability are imparted, by a method wherein the use amounts of Ni and Cr in an alloy are limitted to min. necessary amounts and the amount of N is made high to increase a Cr equivalent. CONSTITUTION:This stainless steel consists of, on the wt. basis, 0.08% or less C, 1.0% or less Si, 2.0-7.0% Mn, 6.0-8.8% Ni, 20.0-22.5% Cr, 0.20-0.50% N, 0.5-2.8% Mo and the remainder Fe and inevitable impurities and 0.0050% or less 0 and 0.015% S are further contained therein. In addition, according to necessity, one kind or more of 0.03% or less Al, 0.02% or less Ca, 0.01% or less B, 0.01% or less Mg and 0.05% rare earth elements and one kind or more of Ti, Ta, Zr, Nb in an appropriate amount are contained. By this composition, a Cr equivalent is brought to 31% or more and austenite type stainless steel suitable for a sea water plant is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a seawater-resistant austenitic stainless steel used in various seawater plants such as offshore oil field development, marine resource exploration, wave power generation, and desalination plants.

Up until now, the only stainless steel for seawater resistance was SUS.
316.317 has been used, but with the industrialization and expansion of various seawater plants, the environment in which they are used has become increasingly harsh with high loads and high temperatures, and SUS 816 has been used.

317 developed pitting corrosion and crevice corrosion after several years, and its seawater resistance became insufficient. Furthermore, the tensile strength is about 60 kti/-, which is insufficient in terms of strength.

Stainless steel, SUS B29JI (25Cr -
5Ni-9Nb-0,1ON), xM 19 (22
Cr-18Ni-5Nb-2MO-0+3ON-0,
2V-0, 2Nb), xM17 (20Cr-6Ni-2
,5M.

-3Nb-o, 85N)-21Cr-12Ni-2
, 5Nb-1, 5Nb-o, 2 ON 25Cr-
14Ni-IMo-0.85Nm etc. have been developed and have recently been used in various plants.

However, 808829J1 has a tensile strength of '15k.
Although it has an excellent strength of Q/-, since it has a two-phase structure, the ferrite phase with a high Cr concentration easily generates a sigma phase by thermal operations such as welding, and the formation of a sigma phase makes the material brittle. Moreover, it is sensitive to 475°C brittleness, making it difficult to consistently obtain excellent seawater resistance.

Also, ×M19 and 25Cr-14Ni-IMo-0,8
5Nm has excellent seawater resistance and strength, but since the former contains Nb and N, and the latter contains as much as 25% Cr, it has poor high temperature deformability and is difficult to hot roll. be. Although xM17 has excellent strength.

Since it contains as much as 80% Mn, it is somewhat insufficient in terms of hot workability and seawater resistance.

Furthermore, 2 ICr-12Ni-2, 5Nb-1,5
Although Nb-0,2ONm has excellent seawater resistance and hot workability, it has the drawback of low tensile strength of 66/-.

The object of the present invention is to eliminate such drawbacks of conventional steels, and to obtain an austenitic stainless steel that has excellent seawater resistance, hot workability, and high strength, and is relatively inexpensive. Therefore, as a result of repeated research on the needle pitting corrosion resistance of austenitic stainless steel, the present inventors found that the pitting corrosion resistance is Cr equivalent = Cr-1-2,5Nb + 0.2Ni
It can be roughly evaluated by the relational expression: +15N, and in order to obtain excellent pitting corrosion resistance, the Cr equivalent needs to be 81 or more, and seawater resistance is improved by containing 2.0 to 7.0% Mn. I found out that it can be done.

Using this relational expression, the inventors investigated the chemical composition of austenitic stainless steel that is relatively inexpensive and has excellent seawater resistance, hot workability, and high strength. ~22.5%.

Ni6.0~8.8chi 1MoQ, 5~2.8chi, N
A steel having a chemical composition of O, 20-0.50 and Mn 2.0-7.0 was discovered.

Among these, N provides excellent seawater resistance and high strength, stabilizes the austenite phase, saves Nj content, and improves hot workability (00% without fear of loss).
．． The content of N was 20 to 0.50%.

Cr and Mo are important elements that improve seawater resistance, but if they are contained in large amounts, it is necessary to contain a large amount of Ni to stabilize the austenite phase, which increases hot embrittlement as the alloy content increases. Therefore, in the present invention, in order to minimize the amounts of Cr and Mo necessary to obtain seawater resistance, we have succeeded in using N.

Regarding Ni, the minimum amount of Ni required to stabilize the austenite phase is 6.0 to 8.8%.It is included to prevent N from forming bubbles in the steel ingot, and to improve seawater resistance. It was made to contain 2.0 to 7.0 to improve the content.

That is, the present invention is based on the above-mentioned research results and uses the -Cr equivalent equation to solve the problems associated with island-phase Cr and high-alloy seawater resistant steels. .0
- Excellent seawater resistance with 8.8% Ni, 0.5-2.8% Mo, lower alloy content than conventional steel, and 0.20-0.50% N content. has hot workability, and
Tensile strength 70. We have succeeded in developing an austenitic stainless steel for seawater resistance that has excellent mechanical properties of #/- or higher.

The present invention V will be explained in detail below.

20.0 to 22.5 inches, MoQ, 5 to 2.8 inches, N
The second invention steel contains O of the first invention steel by 0.0050% or less and S by 0.01%.
5% or less, further improving the seawater resistance and hot workability of the first invention steel, and the eighth invention steel is the first invention w41 steel.
Kosara tulo, below 08 inches, ca O, below 02 inches,
BOf) 1% or less Mg 0.01% or less, rare earth elements 0.
The fourth invention steel further improves the hot workability and corrosion resistance of the first invention by containing one to 211 or more of 05% or less, and the fourth invention steel further contains Ti, Ta, Zr, and Nb in addition to the first invention steel. The strength and hot workability of the first invention steel are further improved by containing 0.05 to 1.0 of one or more of these, respectively.

The reasons for limiting the composition of the steel of the present invention will be explained below.

C is a strong austenite phase-forming element, and when dissolved in the matrix, increases strength and improves seawater resistance. On the other hand, C combines with Cr to form carbides and impairs general corrosion resistance by increasing grain boundary # and corrosion sensitivity, so the upper limit was set at 0.081. When used in special applications where carbides are likely to form in the field, it is desirable to keep the content to 0.08% or less.

Cr is a basic element that imparts seawater resistance to the steel of the present invention,
It is also an element that increases the N solid solubility limit.

As shown in Figure 1, in order to obtain excellent seawater resistance, C.
It is necessary to set the r equivalent to 81 or more, and the lower limit of the Cr content to 20. However, Cr is a strong ferrite phase-forming element, and the content of 22.51 or more impairs the austenite-ferrite balance at high temperatures and reduces hot workability, so the upper limit was set at 22.5. .

Ni is a strong austenite phase forming element and has seawater resistance.
It is an element that improves cold and hot workability.

In order to obtain an austenitic structure with the composition balance of the steel of the present invention, it is necessary to contain at least 6.0 inches of Ni, and the lower limit is set at 6.0 inches.

The hot workability of the steel of the present invention is improved due to the improved austenite-ferrite balance at high temperatures as the amount of Ni increases.
It is best between 60 and 8.8 inches, and decreases again above 9 inches. This is because in chemical compositions where the austenite phase is unstable, Ni contributes to improving hot workability by stabilizing it.

However, if the austenite phase is sufficiently stable, the hot embrittlement characteristic of high alloy steel becomes worse as the Ni content increases.

Since Ni is an expensive element, its content should be kept to the minimum necessary, and its upper limit was set at 8.8 inches.

N is the main element of the steel of the present invention and has excellent seawater resistance-9
It is an indispensable element for obtaining strength and austenite phase. In order to fully exhibit these properties, the content must be 0.20% or more.

Since the solid solubility limit of N depends on the amount of CrlMn and the amount of delnu ferrite during solidification, in the steel of the present invention 1, Mail is increased as the N content increases. Furthermore, if the content of NO is 50% or more, the deformation resistance during hot working increases significantly, making hot rolling difficult. Furthermore, since the risk of generating bubbles in the steel ingot during ingot formation increases, the upper limit was set at 0.501.

Mn is a main element in the steel of the present invention, and is an essential element in order to increase the N solid solubility limit and obtain excellent seawater resistance, strength, and austenite phase.

Furthermore, Mn itself is seawater resistant and austenal.

At least 2.0%, which also contributes to the stabilization of the odo phase
The above content is necessary. However, if the content exceeds 7.0%, it will impair seawater resistance and hot workability, so the upper limit should be set at 7.0%.
%.

Mo, together with Cr and N, is an element that imparts seawater resistance to the steel of the present invention, and must be contained in an amount of at least tl=5% or more. The seawater resistance improvement effect of MO is 2.5 times that of Cr;
If Cr-4-M is added, the same seawater resistance can be obtained even with 2.5% less Cr.

The amount decreases by 1.5 inches.

This makes it possible to reduce Ni by 2.0%. That is, the inclusion of Mo1d can reduce Cr + Ni + Mo by 8.5 inches to obtain the same seawater resistance, and can alleviate the hot embrittlement peculiar to high alloy steel. On the other hand, Mo is a very expensive element and its content should be kept to the minimum necessary, 2. If the content exceeds -, the hot brittleness peculiar to high-alloy steel will significantly increase, making hot rolling difficult, so the upper limit for sheep was set at 2.8 inches. Note that it is preferable to suppress the content by 1.5 to 2.2%.

Si is an element necessary for deoxidation during steel manufacturing, but the inclusion of more than necessary is harmful in W4 of the present invention. That is, Si is a ferrite-forming element that is twice as strong as Cr and significantly impairs the ottenite-ferrite balance, and Si reduces the amount of solid solution of N, so the upper limit was set at 1.0. .

In addition, during manufacturing, the amount of Si is 0.680% to 0.50%.
It is desirable to increase the amounts of Cr and N as much as possible.

0 and S are impurity elements that significantly impair seawater resistance and hot workability, but on the other hand, by lowering them from normal levels through special melting, seawater resistance and hot workability can be improved. . The upper limit for these effects is 0.0050%.

Regarding S, it is necessary to set it to 0.015%.

AI, Ca, B, Mg, and rare earth elements are all elements that improve the hot workability of the steel of the present invention. but.

If it is contained in a large amount, it will impair the cleanliness of the steel and even deteriorate the hot workability, so if you have a problem with the upper limit of AI,
o, 08%, Ca O, 02chi, B 0.01*, Mg
o, olm, and rare earth elements were set at 0.05%. Note that rare earth elements include Ce, La, Y, and the like.

Ti, Ta, Zr, and Nb are elements that refine the crystal grains and improve the strength and hot workability of the steel of the present invention.
It is necessary to contain one or more. However, these elements reduce seawater resistance, so their content should be suppressed.
If the content exceeds #C1,011, a large amount of carbonitrides will precipitate, greatly reducing seawater resistance, so the upper limit should be set at 1.01.
It was set as 096.

Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel.

Table 1 shows the chemical composition of these test steels.

Table 1 In Table 1, A1 to A7 steels are conventional steels, and At is 5US.
804, A2 t! SUS+31a A8) f,
5US817. A4 is Danshichi Rumor XM19. A5 is &&&-
XMI7. A6 is 21Cr-12Ni-2,5Mo-0
,2ON,AT is 25cr-14Ni-1Mo-0,
85N, and Bl to B8 steels are the steels of the present invention.

B1 to B4 are the first invention steels, B5 is the second invention steel, B6 Ming is the eighth invention steel, B7 is the fourth invention steel, B8 is the fifth invention steel, and CI and C2 steels are comparison steels. . Table 2 shows the test materials that were melted in the 10* high-frequency melting furnace shown in Table 1 and forged and stretched once to 80φ, and heated at 1050°C for 80 minutes.

After holding, A1 to A were subjected to solid solution heat treatment called W and Q.
7 steel, B1 to B8 steel, CL, C2 steel strength, seawater resistance,
This shows hot workability.

For strength 1, proof tensile strength and elongation were measured using a JIS No. 4 test piece.

Regarding seawater resistance, the corrosion weight loss and pitting corrosion potential when immersed in a 4-FeCl s aqueous solution at 40° C. for 4 hours are remarkable. Hot workability was evaluated by heating and holding at 1,250°C, cooling to 1,000°C, performing a high-temperature torsion test, and determining the torsion value.

As is known from Table 2, Al steel, which is a conventional steel, has excellent hot workability, but has a strength of 1.

All of them are inferior in seawater resistance.

2.2% with increasing Nit for Al m
Regarding A2 steel containing MO, there is a considerable improvement in seawater resistance and good hot workability, but the strength is inferior to A2 steel. A3 with increased Cr tu and MO amount
Compared to A2 steel, the steel has improved seawater resistance and good hot workability, but is inferior in strength. 22Cr-18Ni-5Ni-2Ni
-0,8ON-0,2V-0,2Nb or -8MJ? -0
A6 steel made of 21Cr-12Ni-2,5 has excellent strength, but contains as much as 80% Mn1i, so it is slightly lacking in hot workability and seawater resistance.
A6 steel made of Ni-1,5Ni-0,20Q has excellent seawater resistance and hot workability, but has a tensile strength of 6
It has a low strength of 6kti/-, and 25Cr-14
Composed of Ni-IMo-0.85N; 4 steel has excellent seawater resistance and strength, but because it contains as much as 25 Cr, it has a low twist value of 4.8 turns and is difficult to hot work. It is of inferior quality.

In addition, C1 steel, which is a comparison steel, has a low Mn content of 0.52% (and has a low solid solubility limit of N, so bubbles are generated in the steel ingot and C
On the contrary, steel No. 2 has a high Mn content of 9.0%, so its seawater resistance and hot workability are rather reduced. On the other hand, B1 to B8 steels, which are the steels of the present invention, have Cr, Ni
, N1Mn is appropriately contained and the Cr equivalent is 8
1 or more.Thus, if seawater resistance is compromised, the corrosion resistance against FeCl3 is 1, 0-0, 6.
f/m'Hr, pitting potential against NaCltn is 6
90-740mV vs SCE, 14T Ni, 25
It has excellent seawater resistance equivalent to A7 steel containing Cr, and has a yield strength of 40/- or more, a tensile strength of 7.2 to 9/- or more, and an elongation of 50 cm or more. It has excellent mechanical properties, and also has a torsion value of 5 in terms of hot workability.
．． This is excellent, with 0 or more times.

In the future, the steel of the present invention will not only have seawater resistance but also strength.
It can be seen that the hot workability is also excellent.

As mentioned above, in order to obtain excellent seawater resistance at low cost, the steel of the present invention uses the minimum necessary amount of expensive N1 and Cr, and by increasing the N content, the Cr equivalent value is increased to 81 or more. , and has improved strength, with excellent seawater resistance and 1.
We succeeded in obtaining an austenitic stainless steel with high strength and hot workability, and it has high practicality in offshore oil field development, marine resource exploration, wave power generation, desalination plants, etc.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of Cr equivalent on seawater resistance. Part 2 @ 1 $ 幇 * Ho J Kaoru - 1 - ~ E゛T gate now 5
Gumi attack ξ1 old man, south Ru. Patent applicant Figure 1 Figure 2 Figure 274-

Claims (1)

[Claims] 1. Weight ratio &! Knee L Te C0,081 or less, Si
1.0% or less Mn 2.0-7.0%, Ni 6.
0-8.8%, Cr 20.0-22.5%N O,2
An austenitic stainless steel characterized by containing 0-0.50%, Mo 0 , 5-2, s*, and the balance consisting of Fe and impurity elements. 2. In terms of weight ratio, Co, 0811 or less, Si 1
．． 096 or less 1Mn 2.0-'lO%,
Ni 6.0-8.81Cr 20.0-22.
5%, Mo 0.5-2.8%, NO, 20-0,
Contains 5096° and less than E OO,0050°'
, S 0.015% or less, and the remainder consists of Fe and impurity elements. 8. Weight ratio EL Te C 0.08% or less, Si I, Q% or less Mn 2.0-7.0%, Ni 6.0-8.8%,
Austenite characterized by containing one or more of Cr 20.0-22.5%\lower 1Mg 0.011 or less, rare earth elements 0.05% or less, and the balance consisting of Fe and impurity elements. stainless steel. 4. Less than co, osti, Si l, Q in terms of weight ratio
Below H, Mn 2.0-7.0%, Ni 6.0-8
．． 81, Cr 20.0-22.59b, Mo 0.
Contains 5 to 2.8 H, NO, 20 to 0.50 H, and 0.05 to 1.0 H of one or more of Ti, Ta, Zr, and Nb, and the remainder Austenitic stainless steel characterized by consisting of Fe and impurity elements. 5. Weight ratio: C (1.08 cm or less, Si 1.0 cm or less, Mn 2.0-7.0%, Ni 6.0-s, s
*, cr 20.0-22.5chi, Mo0.5-2.
8chi, containing N0020 to 0.50chi, further OO, 0050chi or less, SO, 015" or less, AIo, oas or less, Ca O, 02% or less,
BO, 0.1% or less. and 0.05% to 1.0% of each of two or more species. Contains O1. An austenitic stainless steel characterized in that the remainder consists of Fe and impurity elements.