JPH0813034A - Production of superplastic duplex stainless steel sheet - Google Patents

Abstract

PURPOSE:To greatly improve the superplastic properties in a low temp. region and greatly improve, in particular, the superplastic properties of a duplex stain less steel designed for superplastic working at low temp. CONSTITUTION:A cast slab of a duplex stainless steel, having a composition producing superplastic properties, is hot-rolled. The resultant hot rolled steel plate is heat-treated at a temp. in the range between a temp. above the upper limit temp. at which sigma-phase is allowed to occur and 1000 deg.C. Then, the steel plate is cold-rolled or cold-rolled and annealed. By this method, the duplex stainless steel, capable of superplastic working at low temp. as compared with the conventional one, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a superplastic duplex stainless steel sheet, and more particularly, to a method for forming a superplastic duplex stainless steel sheet at a lower temperature (around 900 ° C.) than a conventional one. That is, a method for producing a superplastic duplex stainless steel sheet exhibiting good elongation characteristics with small deformation resistance is proposed.

[0002]

2. Description of the Related Art Conventionally, as a stainless steel exhibiting superplasticity, a pitting resistant duplex stainless steel represented by SUS 329 J4L or the like is well known. This stainless steel is a material originally designed with the aim of improving corrosion resistance and oxidation resistance under a use environment such as seawater.

By the way, superplastic materials include refined superplastic materials and transformed superplastic materials, and it is well known that the former is predominant in majority at present. Such a refined superplastic material maintains the fine recrystallized grains during high-temperature deformation because the matrix phase and the second phase exert an effect of suppressing grain growth on each other, which results in good superplastic properties. Is considered to express. And the above SUS 329 J4L
Consists of a two-phase structure of austenite and ferrite, and superplasticity is relatively easily developed by the action of these two phases.

This SUS 329 J4L type duplex stainless steel is
In applications requiring superplasticity, for example, when applied to an integrally molded product having a complicated shape such as a sink, a sink, or a golf club head, it is necessary to mold at a temperature of 1000 ° C. or more. This is because, if the molding is performed at a lower temperature, a σ phase, which is a hard intermetallic compound, precipitates upon deformation. When this σ phase is generated, each grain boundary of σ / α and σ / γ becomes α / γ, α / α, γ /
Since it is less slippery than the grain boundary of γ, the flow stress required for deformation is increased. Therefore, in order to use the duplex stainless steel as a superplastic material, forming in a temperature range where the σ phase does not precipitate is indispensable.

As described above, the conventional superplastic duplex stainless steel must be processed at a high temperature of about 1000 ° C., and therefore, in a long-time superplastic processing, the oxidation resistance of the material itself is required. . Because, in the case of duplex stainless steel, if oxidation resistance is poor, for example, due to material deformation,
This is because the oxidation easily proceeds to the inside of the material, which causes voids and breaks of the material. For this reason, for duplex stainless steels, the lower temperature range (for example, the temperature range that is common for Ti alloy superplastic materials, 900
Even at (° C.), A material that exhibits superplasticity has been required. If superplasticity can be expressed even at low temperature, molding conditions can be relaxed, oxidation can be suppressed and more stable molding can be realized, and equipment design can be made easier and cheaper to reduce processing cost and molding. This is because it helps improve the cycle.

In order to exhibit excellent superplasticity even in a low temperature range of about 900 ° C., the stress required for deformation, that is, the flow stress, is low in the low temperature range, and the strain rate sensitivity index, That is, it is desired that the material has a high m value and does not precipitate the σ phase. Note that the m value refers to a numerical value of m having a relationship represented by the following equation. Inδ = m × lnε + C (δ; stress, ε; strain rate, C: constant)

On the other hand, the conventional duplex stainless steel has sufficient workability even at a temperature of 1000 ° C. or less,
Various proposals have already been made. For example, Japanese Patent Publication No. 59-14
Japanese Patent No. 099 discloses a duplex stainless steel having improved local corrosion resistance in addition to hot workability.

However, 2 shown in each of the above-mentioned prior arts.
Duplex stainless steel or 2 specified in SUS 329J4L
Duplex stainless steel mainly focuses on corrosion resistance and is not designed for the purpose of developing and improving superplasticity.

[0009] Therefore, the inventors of the present invention have a main purpose of improving superplasticity characteristics, and while maintaining the corrosion resistance which is the original characteristic of stainless steel, have excellent superplasticity (deformation resistance In order to develop a superplastic duplex stainless steel capable of exhibiting a small size and excellent elongation properties, various studies have been conducted on various alloy component compositions. As a result, the inventors have previously developed a practical superplastic duplex stainless steel that has no σ phase precipitation, has superplastic properties even at a low temperature (about 900 ° C.), and has corrosion resistance. Developed (see Japanese Patent Application No. 5-185667).

[0010]

However, the superplastic duplex stainless steel developed by the present inventors has superior superplastic properties at around 900 ° C. as compared with the previous materials exhibiting superplastic properties. However, practically, a material having a higher m value and a lower flow stress is required, and it is still necessary to improve the superplasticity.

An object of the present invention is to further improve superplastic properties in a low temperature range of about 900 ° C., and particularly to further improve the superplastic properties of a duplex stainless steel designed for superplastic working at low temperatures. To improve it.

[0012]

Means for Solving the Problems The present inventors have paid attention to factors other than the alloy component composition that govern the superplastic properties, and have further studied diligently to realize the above object. As a result, the inventors have found that it is effective to adopt the means having the following content as a gist configuration to solve the above-mentioned problem, and have completed the method of the present invention. It is.

That is, the present invention provides: A hot-rolled cast slab of a duplex stainless steel having a component composition exhibiting superplastic properties is then hot-rolled, and the resulting hot-rolled steel sheet is subjected to a temperature exceeding the upper limit temperature at which the σ phase occurs to 1000 ° C, preferably 900 to 1000 ° C. , Followed by cold rolling. ． Hot-rolled cast slab of duplex stainless steel consisting of component composition that exhibits superplasticity, and then the obtained hot-rolled steel sheet exceeds the upper limit temperature at which σ phase is generated up to 1000 ℃, preferably 900 to 1000 ℃ Is a heat treatment, followed by cold rolling and annealing, which is a method for producing a superplastic duplex stainless steel sheet. ． As a duplex stainless steel that exhibits superplasticity,
C: 0.05 wt% or less, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, Cr: 17.0 to 26.0 wt%, Ni: 3.0 to 10.0 wt%, preferably 4.0 to 8.0 wt%, Cu: 0.1 to 2.0 wt%, preferably
1.0 to 2.0 wt%, N: 0.08 to 0.20 wt% and S: 0.002 wt%
% Or less, the balance consisting of Fe and unavoidable impurities 2
A method for producing a superplastic duplex stainless steel sheet according to the above or above, which is characterized by using duplex stainless steel. ． As a duplex stainless steel exhibiting superplasticity characteristics, in addition to the component composition in the invention described above,
The superplastic duplex stainless steel sheet according to the above item 1 or 2, characterized by using a plastic duplex stainless steel containing one or two of Mo: 0.1 to 2.0 wt% and REM: 0.005 to 0.05 wt%. Is a manufacturing method. ． The contents of C, Si, Mn, Cr, Ni, Mo, Cu, and N are defined as Cr _eq defined by the following formula (1) and Ni defined by the following formula (2).
_The method for producing a superplastic duplex stainless steel sheet described above or characterized in that the difference (Cr _eq −Ni _eq ) with _eq is restricted to a range satisfying 12.0 to 17.0, which is favorable. Note Cr _eq = Cr + Mo + 1.5Si (1) Ni _eq = Ni + 30C + 0.5Mn + 0.5Cu + 20N ... (2)

[0014]

The present inventors have paid attention to heat treatment conditions as factors other than alloying components that control superplastic properties, and particularly examined the temperature of heat treatment performed after hot rolling. as a result,
It has been found that heat treatment after hot rolling is performed, and that lowering the temperature is effective for improving superplastic properties. However, when the temperature of the heat treatment is lower than the upper limit of the σ phase precipitation temperature, there is a problem that a σ phase is generated and the test piece is damaged during processing. That is, heat treatment in a temperature range in which a σ phase is generated has a practical problem that the superplasticity performance is improved but the toughness of the material is deteriorated. Thus, in the present invention, the lower limit of the heat treatment temperature after hot rolling is set to the upper limit temperature at which the σ phase is generated, while if it exceeds 1000 ° C, the strain rate sensitivity index (m
Value), flow stress and elongation tend to decrease, so the upper limit was set to 1000 ° C. (see Examples).

By the way, the inventors of the present invention initially thought that a material having a smaller crystal grain size had better superplastic properties, and thus considered that the effect of improving the superplastic properties by limiting the heat treatment temperature was due to the refinement of the crystal grains. I was However, when the microstructure of the duplex stainless steel was observed, almost no change was observed in the sizes of the ferrite grains and the austenite grains of the superplastic material. Therefore, a further microscopic structure was observed. As a result, it was found that when the heat treatment was performed after hot rolling and the temperature was lowered, the growth of the α phase was suppressed, and the γ phase surrounded by the α phase remained after the heat treatment. That is, in the cold-rolled material after the heat treatment, or the material after cold-rolling and annealing, there is almost no change in the size of the α phase and the γ phase itself, but the α / α grain boundary in the α phase is We have found that it is increasing significantly. From this, it was concluded that lowering the temperature of the heat treatment performed after hot rolling increases the α / α grain boundaries in the α phase and greatly improves the superplastic properties.

In the present invention, it is desirable to perform annealing after cold rolling, if necessary. Thereby, the material is softened, and the gas shielding property when performing the gas blow molding is facilitated. This annealing is preferably performed under the condition of the upper limit temperature at which the σ phase is generated to 1000 ° C. × 5 min.

As described above, in the method of the present invention, 90
In order to further improve the superplastic properties in the low temperature range of about 0 ° C., it is important to lower the heat treatment temperature after hot rolling, but it is also necessary to control the composition of the duplex stainless steel. Considering that there was, the component composition was as described below. C: 0.05 wt% or less When the content of C exceeds 0.05 wt%, the intergranular corrosion susceptibility increases, the pitting corrosion resistance deteriorates, and the hot workability decreases due to the precipitation of carbides. In addition, since hardening occurs during cold rolling, subsequent processing and the like are difficult. Therefore, the upper limit of C is 0.05 wt%.

Si: 1.5 wt% or less Si is a constituent element of the σ phase which is an intermetallic compound.
As the amount of Si increases, the speed at which the σ phase precipitates increases, and the precipitation temperature increases. Therefore, in order to prevent the σ phase from being precipitated even at around 900 ° C. by the action of this Si, this Si needs to be 1.5 wt% or less.

Mn: 3.0 wt% or less Mn acts as a deoxidizing element during melting and refining,
It is an element effective for combining with S to purify a sulfurized product and prevent the occurrence of hot embrittlement. However, if it exceeds 3.0 wt%, the oxidation resistance deteriorates. Therefore, this Mn needs to be 3.0 wt% or less.

Cr: 17.0 to 26.0 wt% Cr is a ferrite-forming element and a sigma-phase constituent element. If the Cr content exceeds 26.0 wt%, the precipitation of the σ phase becomes remarkable, and the precipitation of the σ phase occurs at around 900 ° C. even if the amount of elements such as Si that promotes the precipitation of the σ phase is reduced.
Hot workability and superplasticity in the α phase formation temperature range are deteriorated. On the other hand, if the Cr content is less than 17.0 wt%, as in the case of Ni described below, the amount of austenite increases, so that the effect of suppressing the growth of γ grains by the α phase disappears, and the superplastic performance is deteriorated, and the steel Oxidation resistance is reduced, and the material is oxidized remarkably by holding at a high temperature for a long time in superplastic forming, so that good elongation cannot be obtained. Therefore, this Cr is 17.0 ~ 26.0wt
%.

Ni: 3.0 to 10.0 wt% Ni is an austenite forming element, and if it is less than 3.0 wt%, the ratio of γ (austenite) phase is 30 wt% even if adjusted by other ferrite forming elements or austenite forming elements. % Or less, the effect of suppressing the grain growth of the α (ferrite) phase during superplastic deformation is reduced, and the superplastic properties are deteriorated. On the other hand, if it exceeds 10% by weight, the ratio of the γ phase is increased, and the grain growth rate of the γ phase is increased, causing an increase in the flow stress of the material at high temperatures. Therefore, this Ni is 3.
0 to 10.0 wt% Preferably, it is in the range of 4.0 to 8.0 wt%.

Mo: 0.1-2.0 wt% Mo is an element that contributes to improvement in corrosion resistance such as pitting corrosion resistance and crevice corrosion resistance after processing, and is also a solid solution strengthening element. Therefore, when the Mo content exceeds 2.0 wt%, the deformation resistance in superplastic deformation increases significantly. On the other hand, when the Mo content is less than 0.1 wt%, the oxidation of the material during long-time superplastic processing becomes remarkable, The material is likely to break. Therefore, in applications where a large superplastic elongation is required, the Mo content is 0.1-2.0 wt%.
Preferably, it is in the range of 1.0 to 2.0 wt%.

Cu: 0.1 to 2.0 wt% Cu is an element that contributes to the improvement of corrosion resistance such as pitting corrosion resistance and crevice corrosion resistance after processing, but if too much is added, hot workability is deteriorated. Therefore, the upper limit is 2.0 wt% and the lower limit is 0.1 wt% at which the effect of improving corrosion resistance begins to appear. Preferably it is in the range of 1.0 to 2.0 wt%.

N: 0.08 to 0.20 wt% N, like C, is an austenite forming element, so the N content must be determined from the structural balance in consideration of other ferrite forming elements. This N also has the effect of improving pitting corrosion resistance. Therefore, the lower limit of this N is 0.08 wt%. On the other hand, the N content
If it exceeds 0.20 wt%, the hot workability becomes extremely poor,
It is limited to the range of 0.08 to 0.20 wt%.

S: not more than 0.002 wt% S is known to segregate at grain boundaries and significantly degrade the hot workability of the duplex stainless steel. For this reason, it is practically preferable to keep the workability at 0.002 wt% or less to ensure hot workability.

REM: 0.005 to 0.05 wt% REM (Any one selected from rare earth elements or a mixture of two or more such as misch metal,
Particularly, La, Ce and Y are preferred. ) Is an element that contributes to the improvement of oxidation resistance in superplastic working.
If it is added in excess of wt%, it may cause surface flaws or non-metallic inclusions which remain in the steel and cause deterioration of corrosion resistance. Therefore, the upper limit is 0.05 wt% and the lower limit is 0.005 wt% at which the effect of improving the oxidation resistance begins to appear.

Next, the ratio between the α and γ phases will be described. The inventors are considered to play the most important role in superplastic deformation of duplex stainless steel, α / γ
To investigate the effect of grain boundaries, a wide range of α / γ ratios of 2
Experiments were performed on duplex stainless steel. As a result, it has been found that the α / γ ratio has a strong correlation with the strain rate sensitivity index (m value) which is one of the superplastic properties. The α / γ ratio can be represented by [Cr _eq -Ni _eq ], and it has been found that a high m value can be obtained when this value is in a suitable range. By the way, "Progress in Materials Scienc
According to “e” (Vol.33 (1989) p.169), the feature of the micronized superplastic material is that the two-phase Zener effect suppresses grain growth during superplastic deformation. Another reason is that the maintenance of fine recrystallized grains activates grain boundary sliding without reducing the grain boundary area. In order to suppress the grain growth of different phases, the ratio of the two-phase structure should be 5
It is also mentioned that it is desirable to set the time to 0:50, but it is premised that the intensity levels of the two phases are equal to or close to each other. However, between the α and γ phases under superplastic deformation, since the strength of the γ phase is high, it is more advantageous to use a soft matrix than a hard matrix when considering reduction in deformation resistance. . Therefore, it can be said that the ratio of the α and γ phases needs to be higher than 1: 1 for the soft α phase.

Then, the above α / γ ratio, that is, [Cr _eq −N
In order to make i _eq ] appropriate, Cr _eq defined by the above-described equations (1) and (2), which are indicators of the ratio of α layer / γ phase,
The difference from Ni _eq [Cr _eq -Ni _eq ] was restricted to a range satisfying 12.0 to 17.0. The reason for this limitation is [Cr _eq −
By setting Ni _eq ) to 12.0 or more, the matrix can be softened.On the other hand, if (Cr _eq -Ni _eq ) is 17.0 or less, the effect of inhibiting the growth of heterophase grains is not impaired. It is.

[0029]

[Example] 10 kg of duplex stainless steel having the composition shown in Table 1 was induction-melted in a high frequency furnace in an air atmosphere, cast into a 10 kg mold, and then in the temperature range of 1150 to 1200 ° C. Was hot-rolled into a hot-rolled sheet having a thickness of 10 mm. afterwards,
This hot-rolled sheet is subjected to a heat treatment in a temperature range of 850 to 1050 ° C., further subjected to descaling treatment, then subjected to cold rolling at a rolling reduction of 84%, and if necessary, further subjected to annealing at 1000 ° C. × 5 min.
A 1.6 mm thick test piece (cold rolled sheet) was prepared. The shape of this test piece is such that the parallel part length is 10 mm and the width is 5 mm.

The test piece thus obtained was heated to a superplastic forming temperature of 900 ° C. and then heated to about 70 ° C.
After holding for 10 minutes, it was subjected to a tensile test to evaluate superplastic properties. For this tensile test, the step strain rate method, which is one of the high temperature strength test methods, was adopted instead of the normal uniaxial tensile test method with a constant crosshead speed. Initially, tension was started at an extremely low crosshead speed (0.005 mm / min), and after reaching the stress peak, the crosshead speed was gradually increased.
Find the stress peak at each crosshead speed, and perform this operation.
It is a test method that can obtain deformation resistance (flow stress) and strain rate sensitivity index (m value) by continuing to 20 mm / min.

Although there is no clear definition of superplasticity, so far, the elongation is 200% or more and the above m
When the value is 0.3 or more, it is considered that superplasticity can be judged. Therefore, superplasticity was evaluated using three indices of flow stress, m value and elongation. The results are shown in Table 2. As is clear from the results shown in Table 2, the superplastic duplex stainless steel manufactured according to the present invention has a lower flow stress and a lower m-value than the superplastic duplex stainless steel manufactured by the conventional manufacturing method. And high superplasticity with excellent elongation. In particular, by setting the heat treatment temperature after hot working to 925 ° C, the superplastic properties (m
The values, flow stress, and elongation) could be improved by more than 10% compared to the conventional method (when the heat treatment temperature was higher than 1000 ° C).
In addition, the specimens annealed after cold rolling had m-values and flow stresses lower than those of the materials after cold rolling without annealing.
Although it was reduced by about 10%, a new effect of good gas shielding properties was exhibited.

[0032]

[Table 1]

[0033]

[Table 2]

Next, for the duplex stainless steel produced by the production method of the present invention, the effect of [Cr _eq -N
i _eq ] was also tested. The result is shown in FIG. That is, FIG. 1 shows the strain rate sensitivity index (m value) when the strain rate is 2.0 × 10 ⁻⁴ s ⁻¹ and the [Cr _eq −Ni _eq ] of the steel according to the method of the present invention and the steel according to the conventional method. It can be seen that the m value increases when [Cr _eq -Ni _eq ] is in the range of 12.0 to 17.0, and good superplastic properties can be obtained in that range. It should be noted that the mark ● indicates the steel according to the method of the present invention, and the mark ○ indicates the steel according to the conventional method.

[0035]

As described above, according to the method of the present invention, a very simple operation of lowering the heat treatment temperature after hot working can be used to design a superplastic working at a low temperature.
The superplasticity characteristics of the duplex stainless steel can be further improved. In addition, even when forming at a lower temperature (around 900 ° C) than conventional ones, superplastic duplex stainless steel sheets with low deformation resistance and good elongation characteristics can be efficiently manufactured with existing mass production equipment. can do. Therefore, the present invention contributes to further expanding the application range of the iron-based superplastic material, and makes it possible to supply an inexpensive two-phase stainless superplastic material.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a strain rate sensitivity index (m value) and [Cr _eq −Ni _eq ].

Front page continuation (72) Inventor Shinya Koide 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nihon Metallurgical Industry Co., Ltd. Technical Research Center

Claims (5)

[Claims] 1. A cast slab of duplex stainless steel having a composition that exhibits superplasticity is hot-rolled, and then,
The obtained hot rolled steel sheet exceeds the upper limit temperature of σ phase to 1000 ℃
A method for manufacturing a superplastic duplex stainless steel sheet, which comprises heat-treating at 1, then cold rolling. 2. A cast slab of duplex stainless steel having a composition that exhibits superplasticity is hot-rolled, and then,
The obtained hot rolled steel sheet exceeds the upper limit temperature of σ phase to 1000 ℃
A method for producing a superplastic duplex stainless steel sheet, which comprises heat-treating at 1, then cold rolling and annealing. 3. As a duplex stainless steel exhibiting superplasticity characteristics, C: 0.05 wt% or less, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, Cr: 17.0-26.0 wt%, Ni: 3.0- 10.0 wt%, Cu: 0.1 to 2.0 wt%, N: 0.08 to 0.20 wt% and S: 0.002 wt% or less,
The method for producing a superplastic duplex stainless steel sheet according to claim 1 or 2, characterized in that a duplex stainless steel consisting of Fe and inevitable impurities is used as the balance. 4. A duplex stainless steel exhibiting superplastic properties, which is C: 0.05 wt% or less, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, Cr: 17.0-26.0 wt%, Ni: 3.0- 10.0 wt%, Cu: 0.1 to 2.0 wt%, N: 0.08 to 0.20 wt% and S: 0.002 wt% or less,
Furthermore, Mo: 0.1-2.0wt% and REM: 0.005-0.05wt%
3. A method for producing a superplastic duplex stainless steel sheet according to claim 1 or 2, characterized in that a duplex stainless steel containing 1 or 2 of the above and the balance Fe and inevitable impurities is used. 5. The contents of C, Si, Mn, Cr, Ni, Mo, Cu, N are defined as Cr _eq defined by the following formula (1) and Ni _eq defined by the following formula (2). The difference (Cr _eq −Ni _eq ) is regulated to a range satisfying 12.0 to 17.0, The method for producing a superplastic duplex stainless steel sheet according to claim 3 or 4, characterized in that Note Cr _eq = Cr + Mo + 1.5Si (1) Ni _eq = Ni + 30C + 0.5Mn + 0.5Cu + 20N ... (2)