JPH1060598A - Seawater resistant precipitation strengthening type duplex stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To develop precipitation hardening type duplex stainless steel excellent in seawater resistance and the balance of strength/toughness, in a Cr-Ni series stainless steel, by specifying the PT value showing the total content of Cr, Mo, W and N contained therein and the G value of a austenite fractional rate. SOLUTION: In Cr-Ni series stainless steel contg., by weight, <0.03% C, <1% Si, <1.5% Mn, <0.04% P, <0.01% S, 20 to 26% Cr, 3 to 7% Ni, <0.03% sol.Al, <0.25% N and 1 to 4% Cu, furthermore contg. 2 to 6% Mo and 4 to 10% W and moreover contg. specified amounts of Ca, Mg, Zr and B according to necessary, the PT value as the index of seawater resistance expressed by Cr%+3.3Mo%+1.7W%+16N% is regulated to >=35, and the G value of the austenite fractional rate expressed by 152C%-6.1Si%-2.8Mn%-5.7Cr%+6.5Ni%-3.8Mo%-1.9W%+0.6Cu%+209N%+130 is limited to 70>=G>=30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a seaside structure,
Bridges, sluices, seawater heat exchangers, seawater desalination equipment, etc.
The present invention relates to a precipitation-strengthened duplex stainless steel for applications requiring both seawater resistance and structural strength.

[0002]

2. Description of the Related Art In recent years, along with the increasing demand for utilization development in waterfront areas and maintenance-free use of structural materials for plumbing or seawater utilization equipment, etc., high-strength materials which have seawater resistance and can be used as structural materials have been developed. It has been demanded. At present, marine structures and other members are mainly used by applying heavy corrosion protection to structural steel.However, the enormous reduction in running costs caused by frequent repainting has become an issue. In some cases, it is unavoidable to use unpainted for reasons of appearance or appearance, and it is expected that the use of duplex stainless steel with high seawater resistance and few problems of stress corrosion cracking will be supported. However, in general, alloy steels having high corrosion resistance to withstand seawater do not have high strength, and ASTM N06625 (21% C
r-9% Mo-4% Nb-64% Ni), N08825
(22% Cr-3% Mo-1.7% Cu-30% Fe-
42% Ni), and all have a tensile rupture strength of at most 50.
It only has about 0 MPa and the proof stress is 250-3.
It is as low as 00 MPa. Conventional duplex stainless steel is also promising as a structural material with a strength of more than 700 MPa, but is an expensive material, and therefore, from the viewpoint of thinning and lightening, improvement in strength is expected in the future.

[0003]

With the above background, attempts have been made to achieve both corrosion resistance and high strength by strengthening duplex stainless steel by aging precipitation.
The duplex stainless steel for oil country tubular goods disclosed in Japanese Patent Application Laid-Open No. 61-157626 is one in which a parent phase having high corrosion resistance is precipitated and strengthened by aging heat treatment. However, in this technique, there is a problem that not only improvement in strength is limited to about 100 MPa but also reduction in toughness. As for high-strength oil country tubular goods, there are steels disclosed in JP-A-61-23713, JP-A-6-271939, and JP-A-7-207337, etc., but both high corrosion resistance and high strength are achieved. However, all of the considerations regarding toughness are insufficient. Further, even in the steel disclosed in Japanese Patent Application Laid-Open No. 63-4045, although the overall corrosion resistance and the strength are both compatible, the austenite rate is 15 to As low as 35%, there still remains a problem in toughness. In the steel disclosed in Japanese Patent Application Laid-Open No. 5-302151, consideration for pitting corrosion resistance is not quantitative, and stable corrosion resistance cannot be expected.

SUMMARY OF THE INVENTION An object of the present invention is to provide a seawater-resistant precipitation-strengthened type having excellent seawater resistance, strength / toughness balance which can be applied to structural materials for seawater resistance, and excellent hot workability during rolling production. It is to provide a duplex stainless steel.

[0005]

In order to solve the above problems and achieve the object, the present invention uses the following means.

(1) The precipitation-strengthened duplex stainless steel for seawater resistance of the present invention has a C content of 0.03% by weight.
Or less, Si: 1% or less, Mn: 1.5% or less,
P: 0.04% or less, S: 0.01% or less, Cr:
20 to 26%, Ni: 3 to 7%, and Sol. Al: 0.03
%, N: 0.25% or less, Cu: 1 to 4%, and one or two of Mo: 2 to 6% and W: 4 to 10%, and Ca: 0 0.005% (including the case without addition), Mg: 0 to 0.05% (including the case without addition), and B: 0 to 0.03% (including the case without addition). , Zr: 0 to 0.3% (including the case of no addition)
And Y, La and Ce as a total content of 0 to 0.03
% (Including the case of no addition) and the following (1),
It is steel satisfying the expression (2).

PT = Cr% + 3.3Mo% + 1.7W%
When +16 N%, PT ≧ 35 (1) G = 152 C% −6.1 Si% −2.8 Mn% −5.7
Cr% + 6.5Ni% -3.8Mo% -1.9W% +
When 0.6 Cu% + 209 N% + 130, 70
≧ G ≧ 30 (2)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The inventor of the present invention has made it possible to precipitate a duplex stainless steel by aging to obtain a seawater-resistant material having an excellent balance between strength and toughness applicable to a structural material for seawater-resistant applications, and to have a heat resistance during rolling production. The authors conducted extensive research on precipitation-strengthened duplex stainless steel for seawater with excellent hot workability.

As a result, first, from the knowledge that sufficient addition of Cr, Mo, W, and N is necessary to ensure sufficient seawater resistance of the mother phase, Cr ≧ 20% and Mo ≧ 2% , W ≧ 4
%, N ≦ 0.25% is required, and when the addition amounts satisfy the expression (1), ie, PT = Cr%
+ 3.3Mo% + 1.7W% + 16N% ≧ 35 (1)
Then, it was confirmed that sufficient seawater resistance could be secured.
In addition, at the same time, by balancing the ferrite stabilizing element and the austenite stabilizing element and setting the austenite fraction to 30% to 70%, the strength and toughness of the material are well balanced, and sufficient reliability as a structural material is obtained. I found something that I could demonstrate. This is because the G value representing the austenite fraction based on the amount of the element added: G = 152 C% −6.1 Si% −
2.8Mn% -5.7Cr% + 6.5Ni% -3.8M
o% -1.9W% + 0.6Cu% + 209N% + 130
When it is calculated, it is necessary that 70 ≧ G ≧ 30 (2) is satisfied.

In the actual steel material manufacturing process, hot workability is important, and Ca, Mg, B, Zr, Y, La, C
It has been found that the addition of e is effective in reducing grain boundary cracking during hot working, but all of these are manufactured by subjecting steel to hot working such as plate rolling and slow cooling. In this case, it is an element that can be selectively added. According to the analysis of the inventors, in the high-strength duplex stainless steel of the precipitation strengthening type, if there is a problem in hot working, some of the precipitated phases precipitate during cooling, and the intracrystalline strength increases. In some cases, there is a case where the strength of each phase of ferrite / austenite is inconsistent, and a crack is generated in the phase interface. In any of these cases, Ca, Mg, B, Zr, Y,
It is considered that the addition of La and Ce was effective in increasing the grain boundary strength and the phase boundary strength.

Based on the above findings, the present inventor has defined the lower limit of the PT value (the total amount of Cr, Mo, W, and N) under specific conditions to ensure seawater resistance, and to maintain the G value ( By controlling the austenitic fraction) to a specific value and balancing the strength and toughness of the material well, seawater with excellent seawater resistance, strength / toughness balance, and excellent hot workability during rolling production. A precipitation-strengthened duplex stainless steel for steel has been found and the present invention has been completed.

That is, the present invention provides a PT
Value (total amount of Cr, Mo, W, N) and G value (austenite fraction) are limited to the following ranges, so that seawater resistance, strength / toughness balance is excellent, and hot workability during rolling production is achieved. A precipitation-strengthened duplex stainless steel for seawater resistance excellent in seawater resistance can be obtained.

The reasons for adding the components of the alloy of the present invention and the reasons for limiting the components will be described below.

(1) Component composition range C is an element which forms a Cr carbide and lowers the corrosion resistance of the alloy.
If the content is less than or equal to the above, the deterioration of the corrosion resistance can be tolerated, so the content is 0.03% or less.

Since Si is an element effective for deoxidation, 1%
The content may be as follows, but if it exceeds 1%, hot workability is impaired, so the content is 1% or less.

Mn is an element effective for phase stability and may contain 1.5% or less. However, if it exceeds 1.5%, it impairs hot workability. 5% or less.

P is an element that segregates at the grain boundaries and impairs the ductility during rolling. The smaller the content, the better. In order to prevent cracking due to a decrease in ductility during rolling, the allowable amount of the content is 0.04% or less.

S, like P, is an element that segregates at grain boundaries and impairs ductility during rolling. The smaller the content, the better. In order to prevent cracking due to a decrease in ductility during rolling, the allowable amount of the content is 0.01% or less.

Cr is important as a basic element that imparts seawater resistance to the alloy. If its content is less than 20%,
Insufficient effect on seawater resistance. On the other hand, if the content exceeds 26%, it becomes difficult to maintain a proper phase balance even by the limiting formula described below.
26% or less.

Ni is a main element that enhances austenite stability. The content of Ni is required to be at least 3% or more from the viewpoint of maintaining the toughness of the steel. In this case, it is difficult to maintain an appropriate phase balance, and the economical efficiency is significantly impaired.

Since Al is an element effective for deoxidation, So
l. Al may contain 0.03% or less, but 0.03% or less.
If the content exceeds 0.2%, problems such as hot workability and ground flaws occur, so the content is 0.03% or less.

N is an element that enhances austenite stability and pitting corrosion resistance when added, but when added in excess of 0.25%, toughness and hot workability are deteriorated. 25% or less.

Cu is an element that precipitates uniformly in the matrix and has a strengthening action. However, excessive addition leads to a decrease in the toughness, ductility, and hot workability of the alloy.
Not less than 4%.

Mo and W are elements that impart seawater resistance to the alloy. If the content is too small, sufficient properties cannot be obtained, and if the content is excessive, the austenite phase becomes unstable, leading to insufficient toughness.
And 1 to 2 of 4 to 10% W. When Ca is added in a small amount, it is effective as an element for improving hot workability due to desulfurization and deoxidation effects. 005% or less.

When a small amount of Mg is added, it is effective as an element for improving hot workability by a deoxidizing effect.
If it exceeds, the hot workability is reduced, so that the content is 0.05% or less. B and Zr are effective elements for strengthening grain boundaries and improving hot workability. However, if added excessively, hot workability and weldability are deteriorated, so that B is 0.03% or less. Z
r is 0.3% or less.

Rare earth elements such as Y, La and Ce are desulfurized,
It is an element that improves hot workability by a deoxidizing effect, and may contain one or more of these elements. If the content exceeds 0.03% in total, the hot workability is adversely affected, so the total content is 0.03% or less.

Further, Ca, Mg, B, Zr, Y, La,
Since Ce can be added as needed, the case without addition is also included. In addition to the above, it is necessary that the seawater resistance satisfies the expression (1) as described above. That is, P
When T = Cr% + 3.3Mo% + 1.7W% + 16N%, PT ≧ 35 (1) In order to ensure the strength characteristics and toughness balance, ferrite is further added within the specified range of the expression (2). It is necessary to control the fraction of / austenite. That is, the austenite fraction is G = 152C% -6.1Si% -2.8.
Mn% -5.7Cr% + 6.5Ni% -3.8Mo%-
It can be expressed as 1.9 W% + 0.6 Cu% + 209 N% + 130, and it is necessary to satisfy the following expression (2).

70 ≧ G ≧ 30 (2) By adjusting to the above-mentioned component composition range, seawater resistance,
It is possible to obtain characteristics excellent in strength / toughness balance and excellent in hot workability during rolling production.

Hereinafter, the effects of the present invention will be proved by giving examples of the present invention.

[0030]

EXAMPLES Tables 1 and 3 show the chemical compositions of the steels studied. Table 1 shows the invention steel grade (No. 1) satisfying the component range of the present invention.
21), and Table 3 shows comparative steel types (Nos. 26 to 47).

Tables 2 and 4 show the PT value defined by the equation (1), the G value defined by the equation (2), the applied solution heat treatment temperature (TS) (time is 15 Water cooling). These steels were melted in a 50 kgw vacuum melting furnace /
After casting, the plate was heated to 1125 ° C and hot rolled. After a plurality of passes, a reduction ratio of 10 was applied to cool the plate to a plate thickness of 12 mm. did. Aging heat treatment (480 ° C x 4)
After time, the test piece was cut out and subjected to a tensile test, an impact test, and a pitting corrosion test.

In the tensile test, a round bar tensile test piece having a parallel part size of φ6 × 13 mmL was used.
% Proof stress (PS) was measured. The impact test uses a V-notch Charpy (JIS No. 4) full-size test piece, and is -165.
The fracture surface transition temperature was determined from the results of 破 50 ° C. The pitting corrosion test was carried out at a temperature of 50 ° C. in accordance with JIS G0578, and the corrosion loss was evaluated.

The test results are also shown in Tables 2 and 4, where the tensile strength,
Rolling resistance (evaluated by 最大: good, Δ: generation of microcracks (<1 mm), ×: generation of cracks (≧ 1 mm)) evaluated from the proof stress (unit MPa or more) and the maximum length of the edge cracks of the rolled sheet.
Fracture transition temperature (in ° C) in Charpy impact test,
The results are collectively shown in the order of the corrosion weight loss (unit: g / m ² .h) in the pitting corrosion test. As can be seen from Table 2, no. The invention steels Nos. 1 to 21 are characterized by good rolling crack resistance and particularly excellent balance between tensile strength and toughness. This is due to the control of the G value described above.
The relationship between S, PS, and fracture surface transition temperature is shown. In addition, the corrosion resistance is good, and the corrosion weight loss in the pitting test is 0.2 g / m2.
² . h or less, the pitting corrosion resistance is also good. This is because a sufficient PT value is satisfied by containing Cr, Mo, W, and N in a certain amount or more. FIG. 2 shows the relationship between the PT value and the corrosion weight loss in the pitting corrosion test. As described above, the inventive steel is excellent from the viewpoints of balance between strength and toughness and compatibility with corrosion resistance.

On the other hand, referring to Tables 3 and 4, comparative steel No. In No. 26, due to an excessive amount of C, sufficient corrosion resistance was not obtained although the PT value was sufficient. Comparative steel No.
27, 28, 29, and 30 represent Si, Mn, P,
The added amount of S was too large, cracks occurred during rolling, and workability was reduced. Comparative steel No. 35, 41, 42, 43,
47 also has a large edge crack during hot rolling due to excessive addition of Al, Ca, Mg, B, and Y + La + Ce (total amount), which are elements that deteriorate hot workability when added in excess. The same is true.

Comparative steel No. In Comparative Steel Nos. 31, 37, and 39, the pitting corrosion resistance was not sufficient because the Cr, Mo, and W amounts were insufficient, and the PT values were not sufficiently large even when the respective addition amounts were within the specified ranges. . At 44, the pitting corrosion resistance is insufficient. On the other hand, if the austenite fraction quantified by the G value is not within the appropriate range as described above, the mechanical properties of the structure will be impaired. Comparative steel No. In Nos. 32 and 46, the G value is insufficient and the toughness is not good. Comparative steel No. On the other hand, in the case of Nos. 34 and 45, the G value was too large, and sufficient strength and proof stress were not obtained. Also, Ni alone has a large effect on toughness, and the comparative steel No. In No. 33, the toughness is deteriorated even when the G value is within the specified range. Comparative steel No. 38, 4
In the case of 0, the addition of Mo and W is excessive, and the toughness is also insufficient.

As is clear from the above Examples and Comparative Examples, according to the composition of the invention steel, high pitting corrosion resistance and strength / toughness balance can be obtained, while being a component of high economical efficiency, and the rolling production An alloy excellent in hot workability at the time can be obtained. In addition, the present invention steel can apply an existing two-phase welding material at the time of welding, and has good weldability.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

According to the present invention, by specifying the alloy composition, the present invention is suitable as a structural material for seawater resistance use, is weldable, and has a high strength of 900 MPa level.
The present invention can provide a Ni-based precipitation strengthened duplex stainless steel at a low cost and with a good yield, and has an industrially beneficial effect.

[Brief description of the drawings]

FIG. 1 shows an austenite fraction (G) according to an embodiment of the present invention.
FIG. 4 is a diagram showing a relationship between the tensile strength (TS), the proof stress (PS), and the transition temperature of the Charpy fracture surface.

FIG. 2 shows a parameter PT of a steel component according to an embodiment of the present invention.
The figure which shows the relationship between a value and corrosion weight loss ((DELTA) W) in a pitting corrosion test.

Claims (1)

[Claims] 1. Substantially, by weight%, C: 0.03% or less, Si: 1% or less, Mn: 1.5% or less, P:
0.04% or less, S: 0.01% or less, Cr: 20
~ 26%, Ni: 3 ~ 7%, Sol.Al:0.03%
And N: 0.25% or less, Cu: 1 to 4%, and one or two of Mo: 2 to 6% and W: 4 to 10%; 0.005% (including the case without addition), Mg: 0 to 0.05% (including the case without addition), B: 0 to 0.03% (including the case without addition), Zr: 0 to 0.3% (including the case of no addition)
And Y, La and Ce as a total content of 0 to 0.03
% (Including the case of no addition) and the following (1),
A precipitation strengthened duplex stainless steel for seawater resistance that satisfies the formula (2). PT = Cr% + 3.3Mo% + 1.7W% + 16N
%, PT ≧ 35 (1) G = 152 C% −6.1 Si% −2.8 Mn% −5.7
Cr% + 6.5Ni% -3.8Mo% -1.9W% +
When 0.6 Cu% + 209 N% + 130, 70
≧ G ≧ 30 (2)