JPS63134649A - Stainless steel excellent in corrosion resistance and hot workability - Google Patents

Abstract

PURPOSE:To improve hot workability and corrosion resistance by specifying respective quantities of C, Si, Mn, P, S, Al, Ni, Cr, Cu, Mo, W, N, B, Ti, Nb, and Fe. CONSTITUTION:This stainless steel has a composition consisting of <=0.03% C, <=1% Si, <=2% Mn, <=0.04% P, <=0.005% S, 0.01-0.3% Sol.Al, 27-40% Ni, 18-25% Cr, 0.3-3% Cu, 3-7% Mo, <=5% W, <=0.05% N, 0.0005-0.008% B, <=1% Ti and/or <=1% Nb, and the balance Fe. Moreover, in the above composition, (P%+8XB%) and (N%+C%) are controlled to <0.064% and <0.07%, respectively, and further, in case of (N%+C%)>0.03%, 8(N%+C%-0.03%) is controlled to <Ti%+Nb%.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is applicable to transportation of oil and natural gas in a corrosive environment containing humid hydrogen sulfide, as well as corrosive components such as carbon dioxide gas and chlorine ions. When manufactured and used as line pipes, it has excellent hot workability and excellent corrosion resistance, especially excellent pitting corrosion resistance, crevice corrosion resistance, and SC resistance.
This relates to stainless steel exhibiting C properties.

(Prior Art) Traditionally, 5US316 for line pipes exposed to highly corrosive environments such as fizs C (h CQ-
．． Austenitic stainless steels such as 904 and Inconel 825 are used. However, these stainless steels do not necessarily have sufficient hot workability, so Ca,
Improvement plans such as adding Mg, 'REM, and B are being considered. For example, in Special Publication No. 56-31345, B:
O,0O05~0.20%, Zr:O,OO1~6゜0
%, Ce:O, OO 1 to 0.5% is disclosed. This method aims to improve hot workability through the combined addition of BsZrsCe, and the impurity elements O and N are replaced with Zr and Ce.
The aim is to fix the Zr by e and make effective use of B. At the same time, the aim is to promote the formation of Zr carbide, thereby further improving the hot workability. Also, JP-A-61
In No. 84348, when B is added to improve hot workability, oxygen is added to 0.0060% in order to suppress the oxygen content to a low level in order to significantly increase the effect of B.
% or less, and a method has been proposed in which B is added in an amount of 0.001 to 0.010%.

By the way, the biggest problem in processing these stainless steels is the blooming process of ingots or CC slabs having a solidification structure or solidification segregation. However, B
is an element that significantly lowers the melt embrittlement temperature, and its careless addition could actually cause problems in the blooming process.

In addition, SCC is the most important type of local corrosion in high-alloy line pipes used in H2S -COz CQ- environments, and pitting and crevice corrosion are caused by SCC.
It has been found that this causes swelling. In particular, 11°S
In a highly corrosive environment with a high concentration of , CO□, even Inconel 825 type materials may suffer from crevice corrosion, and improvements are desired.

(Problems to be Solved by the Invention) The objects of the present invention are to provide excellent corrosion resistance and hot workability, particularly to withstand production and use as a line pipe, to have excellent hot workability, corrosion resistance, and particularly excellent pitting corrosion resistance; Crevice corrosion resistance, SCC resistance
Our objective is to provide stainless steel that exhibits excellent properties.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present inventors first proposed B.
After repeating various experiments to see the effects of addition, B
It was found that although the addition of 5Ju improves workability in the temperature range where hot rolling is performed, excessive twisting of 5Ju actually impairs workability at temperatures above 1200°C.

However, even when the B addition level was relatively low, blooming rolling cracks could occur due to the addition of B, so when we investigated the effects of various trace elements under the addition of an appropriate amount of B, we found the following. I gained knowledge.

(1) When B was added to the 20Cr-3ONi system,
Hot workability (blushing, hot rolling) at a high temperature of 1200° C. does not necessarily improve. Therefore, B, P, N, C,
When we investigated the effects of Ti and Nb contents 1, it was found that hot workability was improved by regulating the amounts of B and P and regulating the amounts of N and Ci.

If the contents of stainless steel, B, and Pi are high, the melting point of this stainless steel will be lowered, and workability will become a problem, especially during blooming rolling.

(2) Furthermore, if the N and Cl contents in the steel are high, nitridation forms boron or carbide, which naturally leads to
Since it is effective for workability at 00°C and reduces jl, fixing N and C with Ti and Nb can prevent excessive addition of B, and even a small amount of B addition can increase its effectiveness. As a result, hot workability could be improved.

(3) When P%+8xB%<0.064%, N%+C%<0.07%, N%+C% is 0.03% or more, 8(
(C%+N%-0.03%)<Ti%+Nb%, not only at 900 to 1000℃ but also at 1200℃
It was found that even the above conditions had good hot workability.

That is, from these findings, (i) P reduces workability at temperatures above 1200°C in the same way as B in stainless steel containing 18-25Cr-27-4ONi, and (ii) Therefore, workability at temperatures above 1200°C is reduced. To maintain workability in the lump process, P<0.040%, P%+8XB%<
It is necessary to limit PSBii to a relationship of 0.064%; (iii) B also segregates at grain boundaries and
It is thought that it improves the workability at ℃, but N or C
The presence of Nb or T, which is a strong carbonitride-forming element, forms a compound with B in steel and impairs the effectiveness of B.
By adding i, B is segregated at grain boundaries (1v) In order to further improve corrosion resistance, 011% M
The present invention was completed based on the knowledge that the coexistence of O%W of each element is effective in achieving the object of the present invention.

Here, the gist of the present invention is that C: 0.03%
Below, Si: 1.0% or less, Mn: 2.0% or less,
I': 0.04% or less, S: 0.005% or less, sol. Al: 0.01~0.3%, Ni: 27~
40%, Cr: 18-25%, Cu: 0.3-
3.0%, Mo: 3 to 7%, W: 5% or less,
N: 0.05% or less, B: O, 0O05 to 0.0
08%, furthermore Ti11% or less and Nb:1
% or less, if desired, REM: 0.001-0.1%, Y: 0.0
01 to 0.05%, Mg: 0.001 to 0.05%, and Ca: 0.001 to 0.05%. .064%, N%+Cχ
<0.07%, 8(NZ+C$0.03%)<TiX+NbX (however, when N%+C% is 0.03% or more) Corrosion resistance characterized by having a composition consisting of the balance Fe and accompanying impurities It is a stainless steel with excellent hot workability.

Thus, one feature of the present invention is that 900-1000
In order to improve not only the hot workability at ゛C but also the hot workability at temperatures above 1200℃, we added a small amount of B, and in order to make effective use of that small amount of B, we added B, P, N, C,
This is because the component ranges of Nb and Ti are limited to the aforementioned components.

On the other hand, in the present invention, in order to prevent crevice corrosion and pitting corrosion, these corrosions are prevented by the upper limit of Moi and the addition of Nb, and this effect prevents SCC.

(Function) Next, the reason why the alloy composition components are limited as described above in the present invention will be explained in detail below. In this specification, "%" means "% by weight" unless otherwise specified.

C: If the content of the C component exceeds 0.03%, carbides will precipitate at the grain boundaries, making stress corrosion cracking more likely to occur at the grain boundaries, so the content was set at 0.03% or less.

Si: The Si component is an essential component as a deoxidizing component, but if its content exceeds 1.0%, hot workability will deteriorate, so the content should be kept at 1.0% or less. Established.

Mn: Like the Si component, the Mn component has a deoxidizing effect, and even if it is contained up to 2.0%, it will not have an adverse effect on the tonality, so its content up to 2.0% is allowed.

P: The P component is an important element in the present invention, and has a content of 0°04
If it exceeds 0.04%, hot workability will be impaired, so the content was set at 0.04% or less. Preferably 0.020%
It is as follows.

The SO3 component has the effect of deteriorating hot workability, and this effect becomes noticeable when it exceeds o, oos%, so its content was determined to be 0.005% or less.

sol. Al: AQ acid component Like the Si and Mn components, it has a deoxidizing effect and is a necessary component for that purpose, but if its content is less than 0.01% of sol, 8Q, the deoxidizing effect is small; If it exceeds 3%, toughness will be impaired, so its content was set at 0.01% or more and 0.3% or less in terms of sol and AQ.

Ni: The Ni component has the effect of improving corrosion resistance, especially stress corrosion cracking resistance, but if its content is less than 27%, desired excellent stress corrosion cracking resistance cannot be secured; Even if the content exceeds 20%, no further improvement effect on stress corrosion cracking resistance appears. Therefore, in consideration of economic efficiency, the content was set at 27% to 40%. Preferably it is 27-35%.

Cr: The Cr component has the effect of significantly improving corrosion resistance when coexisting with N1 and the IO and W components (described later), but if its content is less than 18%, the desired excellent corrosion resistance cannot be achieved. Therefore, it is necessary to contain 18% or more, but even if the content exceeds 25%, no further improvement effect will be obtained. %.

! Ilo and W: As mentioned above, these components contain N
Coexistence with i and Cr has the effect of improving corrosion resistance. Mo is an important component in the present invention, and if its content is less than 3%, crevice corrosion and pitting corrosion cannot be prevented in an 11□S-co2-CQ- environment.On the other hand, if Mo content exceeds 7% , since adding more than 5% gold to W poses a problem from an economic point of view, the Mo content should be 3% or more.
The content of W was set at 7% or less, and the W content was set at 5% or less.

Cu: Cu component has the effect of improving corrosion resistance,
If the content is less than 0.3%, the desired corrosion resistance cannot be ensured, while if the content exceeds 3%, hot workability will deteriorate, so the content should be reduced to 0.3%.
It was set at 3-3%.

Ti and Nb: These components form carbonitrides,
Prevents component B from turning into nitrogen boride and carbon oxides,
It has the effect of indirectly improving hot workability. Also Nb
It is desirable to add Ni because it forms a solid solution in the steel and has the effect of improving corrosion resistance. Even if the content exceeds Ti: 1% and Nb: 1%, a further improvement effect does not appear, so in consideration of economic efficiency, the content of Ti
Nb: 1% or less, Nb: 1% or less.

N: The N component combines with the B component to form nitrogen boride, which indirectly has the effect of deteriorating hot workability. If the content exceeds 0.05%, the adverse effects of the N component will appear even if Ti and Nb components are added, so the content should be reduced to 0.05%.
It was set at 0.05% or less.

If N%+C%<0.07% N%+C%>0.03%, 8(N%+C%-0.
03) <Ti%+Nb% As described later in the Examples section, the B component is 16 to 24 ppm.
The Nb-Cr-Mo steel containing Nb-Cr-Mo is melted and bloomed,
Rolling was carried out, and the degree of edge cracking during the rolling process was observed. As shown in Figure 2, which will be described later, when Ti% + Nb% was plotted on the vertical axis and N% + C% was plotted on the horizontal axis, it was found that N% +
C%<0.07.8×(N%+C%-0.03)<T
It showed the desired excellent hot workability within the composition range of less than i%+Nb%. Based on these results, N,
The content of C, Ti, and Nb components is N%+C%<0.07
%, 8×(N%+C%-0.03%) <Ti%+Nb
%.

In addition, in the case of N%+C%≦0.03%, the amount of (NiC) is sufficiently small, so (Ti←Nb)
There is no need to limit I.

B: Component B has the effect of reducing edge cracking during hot rolling. If the content is less than 0.0005%, the effect will not be exhibited, and if it exceeds 0.008%, the processability will deteriorate at temperatures above 1200°C.
It was set at 0.0005% or more and 0.008% or less.

Preferably it is 0.001 to 0.005%.

P%+8xB%<0.064: N with P and B components changed as described later in the Examples section
b-Cr-Mo steel was melted and bloomed, and the degree of cracking in the blooming process and edge cracking in the rolling process was observed. When P% and B% were replotted on the horizontal axis, the desired excellent hot workability was exhibited within the component range satisfying P%+8XB%<0.064.

Based on these results, stars containing P and B components are set to P%.
+8XB%<0.064.

Next, the present invention will be described in more detail with reference to examples.

Example Steels shown in Table 1 were melted in a 50Kg vacuum induction furnace.

For steel numbers 1 to 15, the influence of P and B components was investigated, and for steel numbers 16 to 27, the influence of C, N, Ti, and Nb components was investigated with the P and B components being appropriate.
Steel No. 28 and later were investigated by changing various components in order to confirm the knowledge obtained so far.

The obtained ingot was heated to 1250°C, forged and bloomed into a 60t x 100w x 150 II (kasu 11).
A block was prepared, this block was externally machined, and hot workability was evaluated based on the depth of scratches. Next, the block from which scratches were removed was heated to 1200°C and heated to 6t (nIl) in 9 baths.
1) was rolled and edge cracking was investigated.

Corrosion resistance is 2tXI from 5t (n++a) plate.
QwX75 j! (mm) test piece was cut out, two pieces were stacked to create a double U-hand test piece, and 5% NaCQ
+0.5% CIl: +C00Il, 10 atm H2S
A test was conducted for 720 hours in an autoclave at 10 atm cot and 100°C, and the crevice corrosion was measured with and without SCC.

In addition, the test pieces used were the as-cut material and the heat-treated material that had been subjected to sensitization treatment at 650° C. for 1 hour, and were used for the test.

Evaluations of hot workability and corrosion resistance are summarized in Table 2.

In addition, each characteristic was evaluated based on the following criteria.

Blossom crack evaluation 2 〇 Maximum crack depth 1mm or less ×
“ 11 Edge cracking in ultra-rolling process 20 Maximum edge side deviation (less than Hn+m on one side This is a graph that summarizes U workability in relation to P% and B%, and from this, P≦0.0
4%, B: 0.0005-0.008%, and P%
It can be seen that when +8×B%<O, 064%, hot workability is improved in both blooming and hot rolling.

Figure 2 also shows B:O,Q016~0.0024
The hot workability for cases in the range of (Ti + N
b) It is a graph collectively showing the relationship between % and (N+C)%. From this, (N+C)%<0.07%, 8
It can be seen that when (N%+Cχ-0°03%)<Ti%+Nb%, the hot workability during hot rolling is impaired.

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are graphs collectively showing data of an example of the present invention.

Claims (2)

[Claims] (1) C: 0.03% or less, Si: 1.0% or less, Mn
: 2.0% or less, P: 0.04% or less, S: 0.005
% or less, sol. Al: 0.01-0.3%, Ni: 2
7-40%, Cr: 18-25%, Cu: 0.3-3.
0%, Mo: 3-7%, W: 5% or less, N: 0.05%
Hereinafter, R: 0.0005 to 0.008% is contained, and Ti
Contains one or two of: 1% or less and Nb: 1% or less, P%+8×B%<0.064%, N%+C%<0.07%, 8(N%+C %-0.03%)<Ti%+Nb%, (however, if N%+C% is 0.03% or more) Corrosion resistance, hot processing characterized by having a composition consisting of the balance Fe and incidental impurities Stainless steel with excellent durability. (2) C: 0.03% or less, Si: 1.0% or less, Mn
: 2.0% or less, P: 0.04% or less, S: 0.005
% or less, sol. Al: 0.01-0.3%, Ni: 2
7-40%, Cr: 18-25%, Cu: 0.3-3.
0%, Mo: 3-7%, W: 5% or less, N: 0.05%
Below, B: 0.0005 to 0.008% is contained, and Ti
: 1% or less and Nb: 1% or less, REM: 0.001-0.1%, Y: 0.001-0.
05%, Mg: 0.001-0.05%, Ca: 0.0
Contains one or more types selected from the group consisting of 01 to 0.05%, and P% + 8 × B% < 0.064%, N% + C% < 0.07%, 8 (N% + C %-0.03%)<Ti%+Nb% (however, when N%+C% is 0.03% or more) Corrosion resistance and hot workability characterized by having a composition consisting of the balance Fe and incidental impurities Made of superior stainless steel.