JPH07118805A - Duplex stainless steel excellent in workability and working method therefor - Google Patents

Abstract

PURPOSE:To obtain a duplex stainless steel constituted of a ferritic phase and a martensitic phase excellent in workability including superplastic working. CONSTITUTION:This duplex stainless steel has a compsn. contg., as essential components, 12 to 20% Cr, <=8% Ni, <=2.5% Si, <=4% Mn, and the balance Fe with inevitable impurities and shows two phases of a ferritic phase and a martensitic phase at an ordinary temp. At the time of subjecting the duplex stainless steel to warm or cold working of >=50%, it shows superplastic deformation in the temp. range of 700 to 1150 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless steel having excellent workability such as hot workability and superplastic working, and further excellent corrosion resistance, and a working method including the superplastic working method.

[0002]

2. Description of the Related Art Duplex stainless steel composed of a ferrite (α) phase and an austenite (γ) phase is well known, and superplasticity can be obtained by combining a proper heat treatment and working to obtain a fine two-phase structure. Various methods for improving the deformability have been proposed.

For example, Japanese Patent No. 1521683 or Japanese Patent Laid-Open No. 3-183
In 215625, a two-phase stainless steel composed of α phase and γ phase is heated to a high temperature (1200 to 1360 ° C.) close to that of α single phase, and then rapidly cooled, followed by cold or warm working, and then superplasticity. A method has been proposed in which the superplastic deformability is increased by heating to the processing temperature to finely precipitate the γ phase (hereinafter referred to as prior art 1).

Further, in Japanese Patent Laid-Open No. 3-215625, a two-phase stainless steel consisting of α phase and γ phase is melted and solidified, and then α
A method has been proposed in which the material is rapidly cooled from a high temperature that becomes a single phase, and then cold working or warm working is performed, followed by superplastic working (hereinafter referred to as prior art 2).

[0005]

In the above-mentioned prior art 1,
As is clear from the example, 1200 to 1360 are used for the purpose of bringing a two-phase stainless steel consisting of α phase and γ phase close to α single phase.
Since it needs to be heated to a high temperature of ℃, the production yield decreases due to the high temperature oxidation of the material, and it cannot be manufactured in the normal solution heat treatment process of stainless steel (heating at 1000 to 1200 ℃). The manufacturing cost increases because a special step of heating is required.

Further, in the conventional technique 2, since special equipment for quenching the solidified steel ingot or slab is required, the manufacturing cost also increases. As described above, in the conventional technique, there is a problem that the production is not easy and the production cost is high because a special process or equipment is required to obtain the duplex stainless steel having the enhanced superplastic deformability. .

The present invention does not require a special process such as conventional high temperature heating or rapid cooling of a steel ingot or a slab, and after heating to a temperature range similar to the solution heat treatment of ordinary stainless steel, it is cooled. Then, the structure near room temperature is made into a two-phase structure of a ferrite phase and a martensite phase, and then by appropriate warm working or cold working, the workability including superplastic deformability is excellent at low cost. The following is the finding of the duplex stainless steel and the finding of the superplastic working method thereof.

(1) The invention of claim 1 is a duplex stainless steel (having a composition of wt%) which has the following characteristics and is excellent in workability. (A) The main components are Cr: 12 to 20%, Ni: 8% or less, Si: 2.5% or less, Mn: 4% or less, the balance is Fe, and (b) the metal structure at room temperature is ferrite. It is composed of two phases, a phase and a martensite phase.

(2) The invention of claim 2 is the duplex stainless steel excellent in workability according to claim 1, wherein the volume ratio of the martensite is 20 to 80%.

(3) A third aspect of the present invention is a superplastic working method for duplex stainless steel, which comprises the following steps. (A) As the main component composition, Cr: 12-20%, Ni: 8
% Or less, Si: 2.5% or less, Mn: 4% or less,
A duplex stainless steel having the balance of Fe and having a metallic structure at room temperature composed of two phases of a ferrite phase and a martensite phase is prepared, and (b) the stainless steel having the dual phase structure is warmed. 50 by processing or cold working
% Or more of the plastic working, and (c) next, heating to 700 to 1150 ° C. to perform superplastic working.

(4) According to the invention of claim 4, the volume ratio of the martensite phase of the duplex stainless steel at room temperature is 20.
The superplastic working method for duplex stainless steel according to claim 3, wherein the superplastic working ratio is -80%.

Regarding the chemical composition of the steel, in order to more stably obtain a two-phase structure composed of a ferrite (α) phase and a martensite phase at around room temperature, the following ranges of main components are necessary. Cr: 12 to 20%. If the Cr content is 12% or less, the corrosion resistance as stainless steel is poor, while the other content is 20%.
This is because the γ phase is generated when the value exceeds.

Ni: 8% or less. 8% Ni content
This is because if it exceeds, the γ phase will be partially generated instead of the α phase. Si: 2.5% or less. It is desirable that the Si content be at least about 0.05% for deoxidizing the steel. On the other hand, if the content exceeds 2.5%, the superplastic deformability is impaired, so the content is set to the above range.

Mn: 4% or less. Mn content is 4%
If it exceeds, the transformation of the γ phase to the martensite phase is inhibited, so the above range is set.

The above are the reasons for limiting the main components.
Other components, such as C, deteriorate the corrosion resistance of stainless steel, so it is desirable that the content be as low as possible.
Even if contained up to 0.05%, it is acceptable from the viewpoint of corrosion resistance.

It is desirable that Sol.Al be contained to some extent when the stainless steel is melted and solidified from the viewpoint of suppressing bubble generation, but if it exceeds 0.05%, the effect is saturated and inclusions are present. Since it increases, it is desirable to set it to 0.05% or less. Although a small amount of N contributes to the improvement of strength, if it exceeds 0.2%, the recovery and recrystallization behavior during hot workability is significantly delayed and the hot workability is impaired. 0.2% or less.

Cu is contained in order to further improve the corrosion resistance, but if it exceeds 3.0%, the effect is saturated, so
3.0% or less.

Ca, Zr and rare earth elements are contained in order to improve the hot workability of steel, but Ca is 0.01%, Z
If the r content exceeds 0.1% and the rare earth element content exceeds 0.1%, the toughness deteriorates.

B improves the hot workability of steel, so if it is added, its effect is saturated if the content exceeds 0.01%, and the toughness deteriorates. Further, it is desirable that the amount of unavoidable impurities such as P and S is as small as possible, but 0.05% is allowable as long as the characteristics are not impaired.

Regarding the Microstructure of Steel In order for this duplex stainless steel to have superplastic deformability, it is necessary that the metallic structure at room temperature be composed of two phases, a ferrite phase and a martensite phase, as described above. . The reason is that warm working or cold working is performed in order to enhance the superplastic deformability of this stainless steel. At this time, since the strain energy accumulated in the martensitic structure is large, the transformation to the γ phase occurs at the same time. This is because miniaturization due to recrystallization is promoted.

There is a certain degree of superplastic deformability even if the above-mentioned warm working or cold working is not performed. Therefore,
The concept of being composed of two phases, a ferrite phase and a martensite phase, includes both a state before performing the warm working or cold working and a state after performing the warm working or cold working. It means.

When the above-mentioned warm working or cold working is carried out, the energy of working strain during this working is accumulated in this martensite phase, so that when it is heated to the superplastic working temperature, it transforms to the γ phase. At the same time, miniaturization due to recrystallization is promoted. Therefore, in order to increase the superplastic deformability, the volume ratio of the martensite phase is at least 20%.
It is necessary to be at least 80%.

This means that the volume ratio of the α phase on the other surface is 20 to 80.
It means%. The above are the reasons for limiting the component composition and metal structure of the duplex stainless steel described in claims 1 and 2.

The duplex stainless steel having excellent workability according to claim 1 is also excellent in hot workability as described later in Examples. As a empirical formula, the following AF values were introduced and investigated. When the AF value is 16 or less, ear cracking, biting flaws, and lap flaws do not occur during hot working, and these occur when the AF value exceeds 16. .

Here, AF value = 1.2 (Cr + Si) -0.9Mn-0.6Ni-
8 (C-N). That is, by controlling the AF value determined from the component composition within the range defined in claim 1 as described above, excellent hot rolling property is exhibited in hot seamless rolling or plate rolling.

Further, the duplex stainless steel generally has good corrosion resistance, but the duplex stainless steel having excellent workability according to claim 1 has extremely excellent corrosion resistance. Within the component composition according to claim 1, MsA ≧ 0 defined by the following formula.
When it is, corrosion resistance and strength are excellent.

Here, MsA = 1950-2400 (CN) -42Si-50Mn-63C
r-93Ni. That is, MsA is C, Si, or austenite phase at the cooling start temperature after hot working or solution treatment.
It is a value determined by N, Si, Mn, Cr and Ni. This value is a value obtained by calculating the distribution ratio of each alloying element to the austenite phase and the ferrite phase at the cooling start temperature after hot working or solution treatment and calculating from the average composition.

Superplastic Working Method for Duplex Stainless Steel In order to superplastically work the above-mentioned superplastic working duplex stainless steel, it is necessary to first prepare a duplex stainless steel having the above-mentioned composition. Becomes In this case, specifically, for example, a steel ingot or a slab is subjected to plastic working by a normal rolling process to be worked into a thin plate, shaped steel or steel bar. In this state, the metal structure needs to have a two-phase structure of a ferrite phase and a martensite phase.

Next, at least 50% of plastic working is carried out by warm working or cold working in a state of once having a two-phase structure of a ferrite phase and a martensite phase at room temperature. In this case, the processing method may be normal rolling or processing such as forging. The reason for this is that in order to obtain excellent superplastic deformability, it is important to refine the structure before the superplastic deformation.

At this time, 2 consisting of α phase and martensite
In martensitic stainless steel, there is a martensite phase with a higher transition density than in the case where the martensite phase is the γ phase, and since it undergoes cold work or warm work, this martensite has a larger strain. Energy is stored. In order to increase the dislocation density of the martensite phase before cold or warm working, it is effective to perform a quenching treatment before the working. For example, when the thin steel sheet is in a state before superplastic working, quenching treatment is performed after hot rolling, and then cold rolling is performed for at least 50%.
It is advisable to carry out plastic working.

When this martensite is subsequently heated in the superplastic temperature range (700 to 1150 ° C.), it transforms to the γ phase, but since the strain energy accumulated by the cold working and the like is large, it is transformed into the γ phase. It recrystallizes at the same time as the transformation to generate fine crystals, which increases the superplastic deformability.
The above-mentioned warm working degree is a working degree of 700 ° C. or less, and cold working means working in the vicinity of normal temperature.

The reason why the workability in warm working or cold working is 50% or more is that sufficient strain energy is accumulated in the martensite phase, so that fine crystals are formed when heated later in the superplastic temperature range. This is because it is necessary to generate it. In addition, in order to obtain excellent superplastic deformability, in addition to the refinement of the structure in the stage before superplastic deformation as described above, each phase is uniformly dispersed during superplastic deformation and the equiaxed structure is obtained. It is important to be able to form easily, but in stainless steel consisting of α phase and γ phase at room temperature, the coarse γ phase generated by processing at a temperature of 1150 ° C. or less is easily formed during deformation. Do not divide / equalize.

On the other hand, in the case of a stainless steel consisting of α phase and martensite phase at around room temperature, the corrosion resistance of the stainless steel is ensured and the two phase structure of α phase and martensite at room temperature is obtained. , Cr content is reduced to the required minimum, and Si and Mn content are also reduced.

Further, Cr, Si and Mn are all γ
Since it is an element that suppresses the high temperature deformation of the phase, the deformability of γ phase crystal grains during superplastic deformation is improved by reducing the content of these elements, and easy crystal grain deformation and fragmentation due to grain boundary sliding etc. Axialization progresses and superplastic deformability improves.

Next, regarding a specific method of superplastic working,
As described above, it may be heated to 700 to 1150 ° C. and processed into a desired final product by a conventionally known processing method such as rolling, forging or pressing within the temperature range.

[0036]

【Example】

Example 1 No. 1 shown in Table 1 Two-phase stainless steel having a chemical composition of 1 to 6 was melted in a high-frequency vacuum melting furnace at 10 kg each,
After casting a steel ingot and hot rolling to a thickness of 10 mm, 1050 ° C
For 30 minutes, and then water-cooled heat treatment was performed to further increase the dislocation density of the martensite phase. No. 1 to
No. 6 is a steel of the present invention, and each has a two-phase structure of a ferrite phase and a martensite phase at room temperature.

On the other hand, in No. 6 to 9 are comparative steels,
Both have a two-phase structure of a ferrite phase and an austenite phase at room temperature. 80% cold rolling was applied to these duplex stainless steels, and a tensile test piece was taken from the obtained cold rolled plate, and subjected to a tensile test at a temperature of 900 ° C. and a strain rate of 1 × 10 ⁻³ / s. I did. The steel of the present invention having a two-phase structure of a ferrite phase and a martensite phase at room temperature exhibits an elongation of 450 to 700%, and among them, Cr is 20% or less, Ni is 8% or less, Si is 2.5% or less, No. with Mn of 4% or less.
The invention steels Nos. 3 to 5 showed a large elongation of 650% or more.

On the other hand, in the comparative steels having the two-phase structure of the ferritic phase and the austenitic phase at room temperature, that is, the conventional two-phase stainless steels, the elongation is low at 300% or less, and the elongation is 300% or less. The difference was clear. 1 is the same as No. 1 in Table 1. For the invention steels of No. 3,
This shows the result of a tensile test conducted at a strain rate of 1 × 10 ⁻³ / s at a temperature of 1 ° C. to 1150 ° C., and an elongation of 300% or more is obtained.

[0039]

[Table 1]

Example 2 Stainless steels having the composition No. 1 to No. 30 shown in Table 2 were melted in a high frequency melting furnace to produce a steel ingot of 50 kg.
No 1 to 22 are the duplex stainless steels of the present invention, and No 23 to
27 is a comparative duplex stainless steel. Also, No 28,
Numerals 29 and 30 are SUS 329J1L, SUS 329J3L, SUS 329J4L, which are two-phase stainless steel specified in JIS. A round bar test piece with a parallel part of the side surface of these steel ingots of 6 mmφ was collected,
A high temperature tensile test was performed, and the hot workability was evaluated by the reduction value.

When the drawing value is less than 65%, unacceptable cracking occurs in actual manufacturing, so this value was used as a criterion for determining the hot workability. FIG. 2 shows the minimum drawing value when the above test piece was heated to 800 to 1100 ° C. and subjected to a high temperature tensile test. As is clear from this figure, when the AF value is less than 16.0, the minimum aperture value is 65% or more, indicating good hot workability.

[0042]

[Table 2]

[0043]

[Table 3]

FIG. 3 shows the relationship between the hot working temperature, the hot working finish temperature and the working flaw of the steel of the present invention. In the steel of the present invention, workability flaws do not occur in the hot working temperature range of 1100 to 1250 ° C, and workability flaws occur outside this range. Also, when the hot working finish temperature is 800 ℃ or higher, no work flaws occur and
If the temperature is lower than ℃, the defects will be significantly generated.

Example 3 Stainless steel having the composition shown in Table 3 was melted in a high frequency melting furnace to produce a steel ingot of 50 kg. This steel ingot
Hot rolled 0 mm thick steel plate and air cooled to room temperature,
And re-heated to 1000 ° C. and cooled with water were prepared. In order to make the cooling conditions uniform, the room temperature during air cooling and the water temperature during water cooling were both adjusted to 25 ° C.

A round bar tensile test piece having a parallel portion of 6 mm and a plate-like corrosion test piece having a size of 5 × 30 × 50 mm were sampled from the obtained steel sheet, and the tensile strength was evaluated and the corrosion resistance was evaluated by an atmospheric exposure test for one month. I did. The atmospheric exposure test was evaluated by the presence or absence of rust. The results are also shown in Table 3.

Since the steels 1 to 14 of the present invention have MsA ≧ 0, each of them has a 0.2% proof stress of 60 kgf / mm ² or more and is excellent in corrosion resistance. In contrast, comparative steels 15-17,
Since 19 has MsA <0, it is a ferritic / austenitic duplex stainless steel with a maximum of 43 kg.
Only a yield strength of f / mm ² was obtained.

Comparative steel 18 has MsA ≧ 0,
Since the Cr content is lower than 16% described in the claims, sufficient corrosion resistance is not obtained. Further, the comparative steel 20 has Ms
A ≧ 0, but the Ni content is 0.
Since it is lower than 5%, sufficient corrosion resistance is not obtained.

[0049]

[Table 4]

[0050]

As described above, this duplex stainless steel is excellent in normal hot workability in the range of 1100 to 1250 ° C, and there are few rolling defects in seamless rolling and plate rolling. Further, this stainless steel has almost the same degree of corrosion resistance as conventional two-phase stainless steel of a ferrite phase and an austenite phase, and can be used for various purposes.

Further, the duplex stainless steel according to the present invention is then subjected to appropriate warm working or cold working to
It exhibits excellent superplastic deformability when heated to 700 ℃ ~ 1150 ℃. It has an elongation of at least about 300% at around 700 ° C, and an elongation near 700% in the range of 700 ° C to 1000 ° C.

[Brief description of drawings]

FIG. 1 is a diagram showing superplastic deformability of a duplex stainless steel of the present invention.

FIG. 2 shows the AF value and 800 of the duplex stainless steel of the present invention.
It is a figure which shows the relationship with the minimum aperture value in 1100 degreeC.

FIG. 3 is a diagram showing the hot workability of the duplex stainless steel of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Inazumi 1-2, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Steel Pipe Co., Ltd. (72) Katsumi Masamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo No. Nippon Steel Tube Co., Ltd.

Claims (4)

[Claims] 1. A machine having excellent workability having the following characteristics:
Duplex stainless steel (component composition is wt%). (A) Cr: 12 to 20%, Ni: 8% or less, Si: 2.5% or less, Mn: 4% or less as the main component, the balance is Fe, and (b) the metal structure near room temperature is It is composed of two phases, a ferrite phase and a martensite phase. 2. The volume ratio of the martensite phase is 20 to 80.
%, The duplex stainless steel having excellent workability according to claim 1. 3. A superplastic working method for duplex stainless steel, comprising the following steps. (A) Cr: 12 to 20%, Ni: 8% or less, Si: 2.5% or less, Mn: 4% or less as the main component, the balance is Fe, and the metal structure near room temperature is ferrite. A duplex stainless steel composed of two phases, a phase and a martensite phase, is prepared, and (b) 50% or more of plastic working is performed on the stainless steel having the two-phase structure by warm working or cold working. (C) Next, superplastic working is performed by heating at 700 to 1150 ° C. 4. The superplastic working method for duplex stainless steel according to claim 3, wherein the volume ratio of the martensite phase of the duplex stainless at room temperature is 20 to 80%.