JP2810606B2 - Superplastic duplex stainless steel with low deformation resistance at low temperature and excellent elongation properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superplastic duplex stainless steel, and more particularly to a superplastic duplex stainless steel having a small deformation resistance and excellent elongation characteristics even in forming at a lower temperature range than in the past.

[0002]

2. Description of the Related Art Conventionally, as a stainless steel exhibiting superplasticity, a corrosion-resistant duplex stainless steel represented by SUS329 J2L or the like has been known. This steel is a material originally designed with the aim of improving corrosion resistance under a use environment such as seawater.

[0003] By the way, superplastic materials include micronized superplastic materials and transformed superplastic materials, and it is well known that the former predominates at present. In this refined superplasticity, the fact that the parent phase and the second phase exert an action of suppressing grain growth with each other and the fine recrystallized grains are maintained during high-temperature deformation contributes to the development of good superplasticity. It is believed that. SUS329 J2L is composed of a two-phase structure of austenite and ferrite, and although the corrosion resistance is improved by the action of these two phases, it is also a structure in which superplasticity is relatively easily developed.

[0004] When this SUS329 J2L is used for applications requiring superplasticity, for example, as an integral molding process having a complicated shape, such as a sink for a jet airliner, the molding temperature is usually set to 1000 ° C or higher. is there. This is because, when molding at a lower temperature, the volume fraction of the sigma phase, which is a hard intermetallic compound, which precipitates due to stress upon deformation, increases, so that the two-phase structure of austenite and ferrite, which is convenient for superplastic deformation. Because it collapses. The σ phase makes the material flow non-uniform at the time of molding, and may cause breakage during molding due to uneven thickness. On the other hand, if superplasticity develops at lower temperatures, high-temperature forming conditions are relaxed, and more stable forming can be realized, and the facility design can be easily and inexpensively implemented. There is a strong demand for the development of duplex stainless steel.

A duplex stainless steel having sufficient workability even at a temperature of 1000 ° C. or less is disclosed in
JP 14099 discloses that in addition to hot workability, the local corrosion resistance is improved.

[0006]

However, the conventional duplex stainless steels represented by the duplex stainless steel or SUS329 J2L disclosed in the above publication mainly focus on corrosion resistance in many cases. None was designed to improve plasticity. Therefore, the conventional duplex stainless steel is disadvantageous in the following points from the viewpoint of improving superplasticity, particularly, the development of superplasticity in a lower temperature range than before.

[0007] Among the alloy components, Mo, W and the like contribute to the improvement of corrosion resistance. However, since these components are originally solid solution strengthening elements, they hinder the movement of intragranular dislocations, so that superplasticity is developed. It is difficult to expect. Because, in order to activate superplastic deformation, intragranular dislocations move quickly and deposit on the grain boundaries, which are displaced upward by a mechanism such as the Wartman creep model, which suppresses the increase in dislocation density. It is desirable to do so.

The inclusion of Mo facilitates the precipitation of a σ phase containing Cr-Mo as a main component. The precipitation of the σ phase promotes the transition of the α-γ-σ three-phase coexistence state, which destroys the two-phase structure composed of α and γ, which is convenient for superplasticity, and causes large variation in strength. At this time, if the deformation is continued, there is a possibility that the material flow at the interface of σ / α and σ / γ becomes uneven and a cavity is formed.

In a temperature range where the σ phase is easily precipitated, that is, below the σ precipitation temperature, the temperature is lower than the recrystallization temperature and no superplasticity is exhibited. Therefore, superplastic forming cannot be performed. Molding must be performed.

[0010] In SUS329 J2L, the N content is about 0.15%. Although there is a report that this solid solution N improves superplastic performance, since N is a typical solid solution strengthening element,
Raises the tensile properties of the material and causes the material to strengthen. This is considered to increase the deformation resistance during superplastic deformation. From the above points, the conventional duplex stainless steel is hard to say that it is excellent in superplasticity especially at low temperatures.

Therefore, an object of the present invention is to improve these points and to develop a superplastic duplex stainless steel having a wide range of use conditions.

[0012]

In order to develop superplasticity, it is known that dislocation motion is facilitated, intragranular and grain boundary diffusion is active, and grain deformation is easy. . Regarding the superplastic mechanism of duplex stainless steel,
In “Iron and Steel,” Vol. 73, pp. 1722 of 1987, it was stated that, in part, the continuous occurrence of dynamic recrystallization and recovery occurred during superplastic deformation due to the huge elongation of over 1000%. Although there is an idea that there is, it is somewhat difficult to explain 1000% deformation.

The present inventors have conducted intensive studies based on the hypothesis that grain boundary slip triggered by a specific slip surface governs the generation of superplasticity. To develop a duplex duplex stainless steel, the following (a) ~
It has been found that, by making the alloy component satisfying (d), the appearance of superplasticity having excellent elongation and low deformation resistance at low temperature can be expected as compared with the conventional case.

(A) Either a grain boundary which is easy to slip or a factor which makes it hard to slip is removed as much as possible. (b) It does not hinder intragranular dislocation motion, since local deformation of grains is relatively easy. However, in this case, fine grains are necessary in order to increase the grain boundary area, and when the grain size is small, the movable dislocation distance is short, dislocations are easily deposited on the grain boundaries, and the dislocations are increased by the ascending motion of the deposited dislocations. It is assumed that extinction is likely to occur. (c) Softening the matrix to facilitate deformation of the matrix.

That is, according to the present invention, C: not more than 0.010 wt% (hereinafter simply referred to as%), Si: not more than 2.0%, Mn: not more than 2.0%
Below, Cr: 15.0-30.0%, Ni: 5.0-10.0%, N: 0.01
This is a superplastic duplex stainless steel (first invention) containing 5% or less, substantially consisting of iron and having low deformation resistance at low temperatures and excellent elongation characteristics.

Further, the present invention relates to a duplex stainless steel containing 0.5% or less in total of any one or more of Mo, W, V, Ti, Nb and Cu in addition to the above component composition. Second invention).

Here, the contents of Cr and Ni are expressed by the following formula:
(1) it is advantageous that the ratio Cr _eq / Ni _eq and Ni _eq defined in the Cr _eq and the following formula is defined (2) is restricted to a range satisfying 3.0 to 3.3 at. Cr _eq = Cr + Mo + 1.5Si + Nb + 10Ti --- (1) Ni _eq = Ni + 30C + 30N + 0.5Mn ---- (2)

Next, the reasons for limiting the content of each alloy component in the duplex stainless steel according to the present invention will be described. C: 0.010% or less When the C content is more than 0.010%, precipitation of grain boundary carbides becomes easy, deteriorates the integrity of grain boundaries, inhibits annihilation of grain boundary dislocations, and also reduces the cold rolled material. In order to harden,
The upper limit is 0.010%. On the other hand, the lower limit is preferably set to 0.005% or more that enables production in a vacuum melting furnace.

Si: 2.0% or less Since Si is a constituent element of the σ phase which is an intermetallic compound,
It must be 2.0% or less, more preferably 1.0% or less. On the other hand, the lower limit is 0.2 to obtain a sufficient deoxidizing effect.
% Is preferable.

Mn: 2.0% or less When the Mn content exceeds 2.0%, the tensile strength in the cold state is improved, and work hardening is facilitated. Therefore, the Mn content needs to be 2.0% or less, more preferably 1.5% or less. is there. On the other hand, the lower limit is required as a deoxidizing component like Si, and if it is 0.5% or less, the deoxidizing effect is insufficient, so it is preferable to set the lower limit to 0.5% or more.

Cr: 15.0-30.0% Cr is a ferrite-forming element and a sigma-phase constituent element. And, as in the case of Ni, if it is less than 15.0, the amount of austenite increases, so that the effect of suppressing the growth of γ grains by the α phase disappears, leading to deterioration of superplastic performance. on the other hand,
If it exceeds 30.0%, precipitation of the σ phase becomes remarkable, so that hot workability and superplasticity in the α-phase formation temperature range are deteriorated. Therefore, the upper limit is 30.0%.

Ni: 5.0 to 10.0% Ni is an austenite-forming element. When the Ni content is less than 5.0%, the ratio of the γ phase becomes 30% or less, and the effect of suppressing the grain growth of the α phase during superplastic deformation. Disappears, leading to deterioration of superplasticity. On the other hand, if the content exceeds 10.0%, the ratio of the γ phase becomes high, and the grain growth rate of the γ phase increases to deteriorate superplasticity. Therefore, the upper limit is set to 10.0%.

N: 0.015% or less N has been reported to be effective against superplastic phenomena, but is a typical solid solution strengthening element in stainless steel. When the N content exceeds 0.015%, the material is strengthened by this solid solution strengthening, dislocation motion is inhibited, dislocations are fixed, and the load stress required for deformation increases, and the deformation resistance in superplastic deformation is reduced. Will rise.

In addition, conventional superplastic duplex stainless steels are required to have corrosion resistance and cold strength at the same time. Therefore, in addition to the above basic components, Cu, M
It usually contains o, Ti, Nb, W and V. In the present invention, any one of these or two
More than one species may be contained, but in this case, one or more of them is limited to a total range of 0.5% or less.

That is, although Mo has been an essential component in conventional superplastic duplex stainless steels, Mo itself has W or V
Similarly to the above, it is an element for improving corrosion resistance and also a σ phase forming element. The σ phase is a phase that is strengthened compared to the α and γ phases,
The precipitation does not improve the superplastic performance, and the material flow at the interface between the σ phase and the matrix becomes uneven during the deformation at a high temperature, which leads to the formation of a cavity and causes a fracture.

Ti and Nb precipitate fine carbides, hinder grain boundary movement during recrystallization, cause grain boundary strengthening by coherent precipitation, and hinder grain boundary sliding during deformation. Cu deteriorates hot workability. For the reasons described above, these third components are, in total,
It is important that the content be 0.5% or less.

Further, regarding the ratio of α and γ phases, “Pr
ogress in Materials Science '' (Vol.33 (1989) p.16
According to 9), as a feature of miniaturized superplasticity, the two phases constituting each other exert a Zener effect, suppress the grain growth during superplastic deformation, and maintain fine recrystallized grains. In other words, the grain boundary slip increases without decreasing the grain boundary area. Further, it is also mentioned that it is desirable to set the structure ratio of the two phases to 50:50 in order to suppress the grain growth of the different phases, but this is because the strength levels of the two phases are equal to or close to each other. It is assumed that However, since the strength of the γ phase is high between the α and γ phases under superplastic deformation, it is advantageous to use a soft matrix rather than a hard matrix in consideration of reduction in deformation resistance. Therefore, it is necessary to increase the ratio of the α and γ phases in the soft α phase.

Therefore, an index of the ratio of the α and γ phases,
Cr _eq and Ni _eq defined by the above equations (1) and (2)
And the ratio Cr _eq / Ni _eq to be in a range satisfying 3.0 to 3.3. In other words, by making the Cr _eq / Ni _eq 3.0 or more, it is possible to soften the matrix phase, whereas Cr _eq
When / Ni _eq is 3.3 or less, the effect of suppressing the growth of heterophase grains is not impaired.

[0029]

As described above, in the present invention, the component composition is regulated based on the fact that the grain boundary slip triggered by a specific slip surface governs the generation of superplasticity. That is,
Generating grain boundary slip on a specific slip surface is based on widening the grain boundary area, that is, refining the crystal grains, and forming smooth grain boundaries that are easy to slip.

In order to satisfy these conditions, first, precipitates such as carbides which are coherently precipitated at the grain boundaries are eliminated, and the movement of the grain boundaries at the time of recrystallization is not hindered. A very small amount or no addition, and then a very small amount or no addition of the solid solution strengthening elements Mo, W and N which inhibit the dislocation motion in the grains; In order to increase the number of the parent phases, it is advantageous to increase the α phase having an α / γ phase ratio after setting the C low.

In order to lower the temperature at which superplasticity develops, it is essential to suppress the formation of a σ phase having material properties different from those of the α and γ phases. It is advantageous to regulate or not to add.

The deformation resistance can be reduced by reducing the stress that contributes to grain boundary sliding. Therefore, it is only necessary to suppress or remove various strengthening elements and realize a soft matrix, and to improve superplasticity. It contributes to reduction of deformation resistance.

[0033]

EXAMPLE 10 kg of a duplex stainless steel having the composition shown in Table 1 was induction-melted in a high-frequency furnace in an air atmosphere, then cast into a 10 kg mold, and then in a temperature range of 1150 to 1200 ° C. Hot forging to a thickness of 10 mm, then heat treatment in a temperature range of 1000 to 1200 ° C, and after descaling, cold rolling at a reduction of 75% and adjusting the sheet thickness to 2.5 mm To prepare a test piece.

[0034]

[Table 1]

The test piece thus obtained was heated to 900 ° C., kept at this temperature for about 70 minutes, and then subjected to a tensile test. For the tensile test, a step strain rate method, which is performed as one of the high-temperature strength test methods, was employed instead of the normal uniaxial tensile test method in which the crosshead speed was constant. This is because, at first, tension starts at an extremely low crosshead speed (0.005 mm / min), and after the stress peaks, the crosshead speed is gradually increased and the stress peak at each crosshead speed is increased. This is a test method in which the deformation resistance and strain rate sensitivity index m can be determined by continuing this operation up to 20 mm / min.

Although there is no clear definition of superplasticity, so far, the elongation is 200% or more and the above m
It is said that when the value is 0.3 or more, it may be judged that the steel exhibits superplasticity. Therefore, superplasticity was evaluated using three indices: deformation resistance, strain rate sensitivity index m, and elongation. The present invention steel, deformation resistance 3.5 kgf / mm ² or less, whereas the m value is greater than 0.5, the comparative steels is high and deformation resistance 4.0 kgf / mm ² or more, m values as low as 0.5 or less, the It can be seen that the invention steel shows superior superplasticity.

The results of the steel of the present invention arranged using Cr _eq / Ni _eq as indices are shown in FIGS. That is, FIG.
In which the strain rate is a measurement result of the deformation resistance at the time of 2.0 × 10 ^-4 s ^-1, indicated for Cr _eq / Ni _eq of the present invention steels, Cr _eq
It can be seen that the deformation resistance decreases when / Ni _eq is in the range of 3.0 to 3.5. In addition, the mark ● indicates that a third element such as Mo or W is added.

Next, FIG. 2 shows that the strain rate is also 2.0 × 10 ^-4.
Strain rate sensitivity index m at s ⁻¹ and Cr _eq / Ni of the steel of the present invention
_It shows the relationship with _eq, and the m value is from 3.0 to Cr _eq / Ni _eq .
It turns out that it becomes high in the range of 3.5. Note that ● is the same as in FIG.

[0039]

According to the present invention, the precipitation of the .sigma. Phase can be delayed to improve the superplasticity at a low temperature, and the superplastic working at a low temperature can be realized. Further, since excellent properties can be imparted without adding expensive components such as Mo and W, superplastic duplex stainless steel can be produced at low cost. Therefore, the present invention contributes to further expanding the application range of the iron-based superplastic material, and further suggests the possibility of application of the duplex stainless steel in the operating temperature range of the Ti and Cu-based superplastic material. Things.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between deformation resistance and Cr _eq / Ni _eq .

FIG. 2 is a graph showing a relationship between a strain rate sensitivity index m and Cr _eq / Ni _eq .

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-206430 (JP, A) JP-A-62-227597 (JP, A) JP-A-63-223145 (JP, A)

Claims (2)

(57) [Claims] C: 0.010 wt% or less, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, Cr: 15.0 to 30.0 wt%, Ni: 5.0 to 10.0 wt%, N: 0.015 wt% or less. And the content of Cr and Ni, the following formula
(1) and Cr _eq defined in the Ni _eq is defined by the following formula (2)
The ratio of Cr _eq / Ni _eq is regulated within the range of 3.0 to 3.3.
And which, with the balance being substantially superplastic duplex stainless steel deformation resistance at a low temperature of Fe excellent small and elongation properties. Serial _{Cr eq = Cr + Mo + 1.5Si} + Nb + 10Ti ---- (1) Ni eq = Ni + 30C + 30N + 0.5Mn ---- (2) 2. C: 0.010 wt% or less, Si: 2.0 wt% or less, Mn: 2.0 wt% or less, Cr: 15.0 to 30.0 wt%, Ni: 5.0 to 10.0 wt%, N: 0.015 wt% or less , Mo, W, V, Ti , selected inner shell of Nb and Cu
That any one or two or more kinds containing in total less than 0.5 wt%, and the content of Cr and Ni, the following formula (1)
The ratio of Cr _eq defined by the formula below to Ni _eq defined by the following formula (2)
Cr _eq / Ni _eq is regulated to satisfy 3.0 to 3.3.
Ri, and the balance substantially superplastic duplex stainless steel deformation resistance at a low temperature of Fe excellent small and elongation properties. Serial _{Cr eq = Cr + Mo + 1.5Si} + Nb + 10Ti ---- (1) Ni eq = Ni + 30C + 30N + 0.5Mn ---- (2)