JP5268105B2 - Duplex stainless steel and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a duplex phase stainless steel in which a fine crystal grain structure can be readily produced with addition of as small amounts of rare elements as possible. <P>SOLUTION: The duplex phase stainless steel containing an austenitic phase and a ferritic phase is manufactured by: performing cold rolling with rolling reduction rate of &ge;80%; and subsequently performing annealing. In the obtained duplex phase stainless steel, the ferritic phase accounts for 30-60% by volume, an aspect ratio in the ferritic phase is &lt;0.40, and grain size in the austenitic phase is &le;5 &mu;m. A fine grain size in the austenitic phase can be achieved without any variation, thereby imparting excellent mechanical characteristics with high strength and high toughness. The duplex phase stainless steel preferably comprises, by mass: &le;0.10% C; &le;2.0% Si; &le;2.0% Mn; 5.0-15.0% Ni; 16.0-28.0% Cr; &le;5% Mo; if desired, &le;0.20% N; and the balance being Fe and unavoidable impurities. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention includes, for example, a duplex stainless steel used for a material requiring strength, ductility, corrosion resistance, such as a member for a nuclear device, a clad steel plate, and a member for piping, and also in an annealing treatment after cold rolling, The present invention relates to a method for producing a duplex stainless steel capable of preventing the coarsening of steel.

  Since the duplex stainless steel is a material in which an austenite phase and a ferrite phase are mixed, it is a material excellent in strength and ductility balance. In recent years, further improvement in strength and ductility has been demanded, and many inventions using a grain refinement process have been reported. For example, by aggressively imparting precipitate generating elements such as Ti, Zr, Nb, V, W, and Cu to the material, fine precipitates such as carbides and nitrides can be used to improve strength and ductility. (See Patent Document 1). Moreover, the prior art has also proposed a heat treatment method that suppresses the coarsening of the ferrite phase by selectively changing the annealing temperature (see Patent Document 2). Furthermore, a manufacturing method for obtaining fine-grained steel by carrying out a heat treatment process in which the cooling rate in the annealing treatment process after cold rolling is controlled to 20 ° C./min or more has been proposed (see Patent Document 3). Furthermore, a method of obtaining a refined material by performing secondary annealing again after setting the temperature of the primary annealing treatment after cold rolling to a ferrite + austenite two-phase structure or a ferrite single-phase region has also been proposed. . (See Patent Document 4).

JP-A-2005-48203 JP-B-60-45251 JP-B 61-59382 Japanese Patent Laid-Open No. 3-191025

By the way, in recent years, the manufacturing cost has increased in general with the rapid increase of the raw material cost. Therefore, in developing new materials in the future, it is necessary to suppress the use of precipitate-generating elements such as rare metal elements Ti, Zr, Nb, V, and W as much as possible. Moreover, the annealing conditions after cold rolling of conventional duplex stainless steel are set to 1050 ° C. to 1200 ° C. with the intention of promoting recrystallization in the ferrite phase. As a result, the austenite phase is extremely coarse at present. Therefore, methods for increasing the cooling rate in the annealing process and applying a double annealing process have also been proposed as methods for refining the austenite crystal grains. There are problems such as an increase in cost and difficulty in application to large members.
From such a viewpoint, it is desired to develop a duplex stainless steel capable of reducing the rare elements as much as possible and easily producing a fine grain structure.

  This invention is made | formed in view of said actual condition, and the annealing process after cold rolling is controlled by controlling the volume ratio of a ferrite phase to 30 to 60% and the aspect-ratio of the structure | tissue form to less than 0.40. An object of the present invention is to provide a duplex stainless steel capable of easily producing a fine grain structure formed by the above process and a method for producing the same.

That is, among the duplex stainless steels of the present invention, the first present invention is mass%, C: 0.10% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 7 0.0-15.0%, Cr: 16.0-28.0%, Mo: 2.0-5.0%, with the balance being composed of Fe and unavoidable impurities , includes a ferrite phase, the volume ratio of the ferrite phase is a 30% to 60%, the aspect ratio of the ferrite phase is less than 0.40, the crystal grain size of the austenite phase, characterized in that it is 5μm or less.

The duplex stainless steel according to the second aspect of the present invention is characterized in that, in the first aspect of the present invention, the composition contains mass% and further contains N of 0.20% or less.

The duplex stainless steel according to the third aspect of the present invention is characterized in that, in the first or second aspect of the present invention, the wall thickness is 20 to 150 mm.

The method for producing a duplex stainless steel according to the fourth aspect of the present invention comprises subjecting the stainless steel having the composition according to any one of the first to third aspects of the present invention to cold rolling at a rolling reduction of 80% or more, A temperature of 900 to 1050 ° C. with an aspect ratio of less than 0.40, followed by heating at a heating rate of 0.3 ° C./second or more to cause recrystallization of the austenite phase and no crystal growth of the phase Annealing is performed to reduce the crystal grain size of the austenite phase to 5 μm or less.

That is, according to the duplex stainless steel of the present invention, it has fine crystal grains in which coarsening of crystal grains is surely prevented by the pinning effect of the ferrite phase, and has high strength and high toughness. Show properties.
Here, when the aspect ratio of the ferrite phase is 0.4 or more, the pinning effect of the ferrite phase is reduced, and the effect of suppressing the coarsening of the austenite phase is not sufficiently obtained. The variation in particle size increases. For this reason, in the duplex stainless steel of the present invention, it is necessary that the aspect ratio of the ferrite phase satisfies less than 0.4.

  Moreover, in the duplex stainless steel of this invention containing an austenite phase and a ferrite phase, the volume ratio of the said ferrite phase is controlled to 30 to 60%. By appropriately determining the volume fraction of ferrite, it is possible to ensure a good balance of ductility, strength, and corrosion resistance.

In the duplex stainless steel of the present invention, as a suitable composition, C: 0.10% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 7 . 0 to 15.0%, Cr: 18.0 to 28.0%, Mo: 2.0 to 5.0 % or less, and optionally containing N in an amount of 0.20% by mass or less. Those composed of Fe and inevitable impurities can be shown. Below, the reason for limitation of each component in this suitable composition is demonstrated. In addition, all the contents in the following mean the mass%.

C: 0.10% or less C combines with Cr and Mo to form M ₆ C, M ₇ C ₃ and M ₂₃ C ₆ type carbides, thereby contributing to the pinning effect at the grain boundaries. Moreover, it has the effect | action which improves a solid solution strengthening ability with respect to an austenite phase. For these actions, C is added as much as possible. However, when carbon is contained excessively, more Cr carbide precipitates at the grain boundary during the stabilization treatment, weakening the grain boundary and reducing ductility. Therefore, the addition amount of C is limited to 0.10% or less. In order to obtain the above effect sufficiently, it is desirable to set the lower limit of C to 0.01%, and it is desirable to set the upper limit of C to 0.05% for the same reason as described above.

Si: 2.0% or less Si is an essential element for enhancing oxidation resistance, corrosion resistance, and strength, and at the same time contributes to refinement of crystal grains by precipitation of intermetallic compounds. However, excessive addition deteriorates manufacturability and causes embrittlement due to coarsening of intermetallic compounds. Therefore, the content is limited to a range of 2.0% or less. In order to obtain the above effect sufficiently, the lower limit of Si is desirably 0.4%, and the upper limit of Si is desirably 1.0% for the same reason as described above.

Mn: 2.0% or less Mn is added as a deoxidizer and contributes to refinement of crystal grains by generating precipitates such as MnS. However, excessive inclusion deteriorates workability and corrosion resistance. Therefore, the content is 2.0% or less. In order to obtain the above effect sufficiently, the lower limit of Mn is desirably 0.5%, and the upper limit of Mn is desirably 1.5% for the same reason as described above.

Ni: 7 . 0 to 15.0%
Ni is an austenite phase stabilizing element, but excessive addition causes an increase in raw material cost. Therefore, the content is 7 . 0 to 15.0%. Incidentally, for the 13.0% upper limit of Ni for the same reason as described above it is desirable.

Cr: 16.0 to 28.0%
Cr needs to be added in an amount of 16.0% or more in order to improve corrosion resistance. Moreover, it contributes to the suppression of the austenite phase coarsening by promoting the formation of ferrite. However, excessive addition also significantly reduces hot workability. Therefore, the content is made 16.0 to 28.0%. Desirably, it is 18.0 to 26.0%.

Mo: 2.0 to 5.0 %
Mo is an element that contributes to solid solution strengthening in the matrix, and by generating precipitates such as Mo ₂ C, it contributes to refinement of crystal grains. It is an unsuitable element. Therefore, the content is set to 2.0 to 5.0% . Desirably, it is 2.0 to 4.0% of range.

N: 0.20% or less N is an element that improves corrosion resistance, and is also an element useful for controlling the ferrite volume fraction, and is added as desired. However, if it is contained in a large amount, it causes blow holes, so the content is 0.2% or less. Desirably, it is 0.16% or less.

Ti, Zr, Nb, V, W, and Cu combine with C and N to generate carbides and nitrides, thereby contributing to grain refinement. In the alloy of the present invention, these products are Even without use, crystal grain refinement can be achieved. Therefore, it is not necessary to add Ti, Zr, V, W, or Cu, but it is possible to add one or more if desired. For example, Ti, Zr, and V can each contain 0.5% or less, W3.0% or less, and Cu 2.0% or less.

Cold rolling reduction ratio: 80% or more When producing duplex stainless steel, the rolling reduction ratio in cold rolling is set to 80% or more. This reduces the aspect ratio of the ferrite phase before annealing, prevents the austenite phase from coarsening due to the pinning effect of the ferrite phase during annealing, and achieves grain refinement that makes the austenite phase substantially 5 μm or less. The If the rolling reduction in cold rolling is less than 80%, the crystal grains are not sufficiently refined, and it becomes difficult to make the austenite phase substantially 5 μm or less. For the same reason, the cold rolling reduction is desirably 90% or more. The term “substantially 5 μm or less” means that the crystal grain size is 5 μm or less except for those rarely exceeding 5 μm, and more preferably 5 μm or less for all crystal grain sizes.

Aspect ratio: less than 0.40 The above rolling reduction ratio is set to 80% or more, and the ferrite volume ratio is controlled to 30% or more, so that the aspect ratio of the ferrite phase is set to less than 0.40%, and the pinning effect is ensured. Can be a thing. When the aspect ratio is less than 0.40%, the effect of suppressing the coarsening of crystal grains in the austenite phase becomes small.

Annealing temperature After cold rolling, annealing heat treatment is performed for the purpose of improving toughness. The volume ratio of austenite phase: ferrite phase can be adjusted by setting the annealing temperature. The annealing conditions may be any conditions as long as recrystallization of the austenite phase occurs and crystal grain coarsening does not occur, but the temperature can be set to, for example, 900 to 1050 ° C.

Temperature raising rate during annealing During annealing, crystal grain growth can be suppressed even if the annealing rate is slowed down to 0.3 ° C./second in the temperature raising process. Thereby, even a large member having a wall thickness of 20 mm to 150 mm can be produced with a fine material with little variation in crystal grain size.

As described above, according to the duplex stainless steel of the present invention, the austenite phase and the ferrite phase are included, and in mass%, C: 0.10% or less, Si: 2.0% or less, Mn: 2.0 %: Ni: 7.0 to 15.0%, Cr: 16.0 to 28.0%, Mo: 2.0 to 5.0%, with the balance being Fe and inevitable impurities. The ferrite phase has a volume fraction of 30 to 60%, the ferrite phase has an aspect ratio of less than 0.40 , and the crystal grain size of the austenite phase is 5 μm or less. It is obtained without variation, and mechanical properties with high strength and high toughness can be obtained.

  In addition, according to the method for producing a duplex stainless steel of the present invention, cold rolling is performed at a reduction rate of 80% or more, and then annealing is performed without using a rare metal or the like, and an excessive heat treatment step. The austenite phase can be refined only by controlling the ferrite phase structure without burden. Therefore, it is possible to produce a duplex stainless steel having high strength and high toughness at low cost and efficiently.

FIG. 1 is a diagram showing the EBSD observation results of 1050 ° C. and 1300 ° C. annealed materials after 90% cold rolling in Fe-19Cr-12Ni. FIG. 2 is a diagram showing an EBSD structure observation result of 1050 ° C. and 1300 ° C. annealed materials after 90% cold rolling in Fe-22Cr-12Ni. FIG. 3 is a diagram showing an EBSD structure observation result of a 1050 ° C. annealed material after 90% cold rolling in the duplex stainless steel Fe-25Cr-7Ni. FIG. 4 is a view showing an EBSD structure observation result of 1050 ° C. and 1300 ° C. annealed materials after 70% cold rolling in Fe-22Cr-12Ni. FIG. 5 is a diagram showing the austenite phase grain size accompanying the change in the ferrite phase volume fraction of the materials shown in FIGS. FIG. 6 is a diagram showing the austenite phase grain size accompanying the change in the ferrite phase aspect ratio of the materials shown in FIGS. FIG. 7 is a diagram showing the austenite phase grain size accompanying the increase in the heating rate during annealing at 950 ° C. in the comparative steel SUS316L and the Fe-22Cr-12Ni material. FIG. 8 is a diagram showing an EBSD structure observation result of the comparative steel SUS316L and the Fe-22Cr-12Ni material at a temperature rising rate of 3.1 ° C./second.

Hereinafter, an embodiment of the present invention will be described.
Preferably, in mass%, C: 0.10% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 5.0-15.0%, Cr: 18.0-28 0.0%, Mo: 5% or less, and further, if desired, N: 0.20% or less, and the balance is adjusted to a composition comprising Fe and inevitable impurities, and the steel is melted. However, the present invention is not limited to the above composition as long as the structure of the duplex stainless steel can be obtained.
Moreover, the melting method of the said steel can be performed by a conventional method, and the manufacturing method is not specifically limited.

The duplex stainless steel is first hot forged. For example, forging can be performed by heating to 1150 ° C. to 1250 ° C., and the conditions in the forging process are not particularly limited to the present invention.
After hot forging, solution treatment can be performed. Examples of the heat treatment conditions include a heating temperature of 1050 to 1200 ° C. and a holding time of 1 to 3 hours.
After the solution treatment, cold rolling with a compression rate of 80% or more is performed. A predetermined shape is obtained by rolling in one pass or multiple passes. The number of passes at this time is not particularly limited in the present invention. In the duplex stainless steel after cold rolling, the aspect ratio of the ferrite phase is preferably less than 0.40.

After cold rolling, an annealing process is performed. For example, a heating temperature of 950 ° C. to 1050 ° C. and holding for 1 minute to 1 hour are exemplified. In the annealing, the austenite phase is prevented from being coarsened by the pinning effect of the ferritic steel, and preferably an austenite phase having a particle size of substantially 5 μm or less is obtained.
The duplex stainless steel of the present invention is suitable for materials that require strength, ductility, corrosion resistance, etc., such as members for nuclear equipment, clad steel plates, and piping members, and particularly in the annealing treatment after cold rolling. It is suitable as a material that can prevent coarsening.

Hereinafter, embodiments of the present invention will be described in detail.
Four alloys having the composition shown in Table 1 (remainder: impurities) were melted in a vacuum induction melting furnace to obtain a 25 kg steel ingot. The obtained steel ingot was made into a plate material having a thickness of 30 mm × 120 mm × L by hot forging (heating temperature 1250 ° C.). Each sample material was subjected to a solution treatment of 1050 ° C. × 3 h and cut into a material of 20 mm × 20 mm × L. This was cold-rolled at a reduction ratio of 70% and 90% to obtain a cold-rolled material. Furthermore, these were subjected to annealing treatment at 950 ° C. to 1300 ° C., and alloys with various volume fractions of ferrite phases were produced.

  FIGS. 1 and 2 show EBSD (Electron Back Scattering) of each test material held at 1050 ° C. to 1300 ° C. for 1 hour after cold rolling at a reduction rate of 90% with respect to Fe-19Cr-12Ni and Fe-22Cr-12Ni materials. diffraction pattern) Observation results are shown. As the annealing temperature increased, the volume fraction of the ferrite phase changed from 7.7 to 64.4%, and the crystal grain size of the austenite phase at that time changed from 3.86 to 16.4 μm. These crystal grain sizes were all derived by removing twins and performing image analysis to extract only large-angle grain boundaries with an orientation difference angle of 15 degrees or more. As a result of observation, in particular, in Fe-22Cr-12Ni having an annealing temperature of 1050 ° C., the ferrite phase structure was a rod-shaped structure with an aspect ratio of 0.38.

  In FIG. 3, the EBSD observation result after 1050 degreeC annealing of Fe-25Cr-7Ni duplex stainless steel is shown. The duplex stainless steel has a ferrite phase volume fraction of 59.4%. At this time, the aspect ratio of the ferrite phase was 0.39, which was a rod-like structure. At that time, the crystal grain size of the austenite phase was 4.47 μm.

  In FIG. 4, each of the Fe-22Cr-12Ni materials whose refinement of the austenite phase was confirmed as described above was cold-rolled by changing the reduction ratio to 70%, and held at 1050 ° C. to 1300 ° C. for 1 hour. The EBSD observation result of a test material is shown. As the annealing temperature increased, the volume fraction of the ferrite phase changed from 26.2 to 52.6%, and the crystal grain size of the austenite phase at that time changed from 6.22 to 20.6 μm. In particular, in the material having an annealing temperature of 1050 ° C. in which the crystal grain size of the austenite phase is 6.22 μm, the ferrite phase structure has an aspect ratio of 0.40.

  FIG. 5 shows a summary of the effect of the ferrite phase volume ratio on the austenite phase grain size of the material shown in FIGS. 1 to 4 (referred to as CW + AN in this drawing). For example, a material having an ferrite phase aspect ratio of less than 0.40 has an austenite crystal grain size of 5 μm or less.

  FIG. 6 shows a summary of the aspect ratio of the ferrite phase on the austenite phase grain size of the material shown in FIG. Obviously, the austenite phase grain size becomes coarser as the aspect ratio of the ferrite phase increases. In particular, the material with an austenite phase crystal grain size of 5 μm or less had an aspect ratio of less than 0.40, which was lower than other materials.

FIG. 7 shows the change in the crystal grain size of the austenite phase accompanying the change in the heating rate up to the annealing temperature of 950 ° C. in the comparative steel SUS316L and Fe-22Cr-12Ni, and FIG. The EBSD structure | tissue observation result when doing is shown. Each material is cold-rolled at a reduction rate of 90%.
In addition, after heating up to 950 degreeC, in order to prevent an overshoot, it hold | maintained for 1 minute. In SUS316L steel, the crystal grain size increased rapidly when the heating rate was 9.3 ° C./sec or more. On the other hand, in the Fe-22Cr-12Ni material, a rod-like structure as observed in FIG. 2B was observed even at a temperature rising rate of 0.3 ° C./second. The crystal grain size of the austenite phase at that time was found to be very fine, about 1 μm, regardless of the rate of temperature increase. In other words, it became clear that the ferrite phase in a rod-like structure is effective in maintaining the long-term structural stability of the austenite phase.

Claims (4)

In mass%, C: 0.10% or less, Si: 2.0% or less, Mn: 2.0% or less, Ni: 7.0-15.0%, Cr: 16.0-28.0%, Mo: contains 2.0 to 5.0%, the balance is composed of Fe and inevitable impurities , includes an austenite phase and a ferrite phase, the volume fraction of the ferrite phase is 30 to 60%, , the aspect ratio of the ferrite phase is less than 0.40, two-phase stainless steel, wherein the crystal grain size of the austenite phase is 5μm or less. The composition,% by mass, still two-phase stainless steel according to claim 1, characterized in that it comprises 0.20% hereinafter containing a N. The duplex stainless steel according to claim 1 or 2, wherein the thickness is 20 to 150 mm . The stainless steel having the composition according to any one of claims 1 to 3 is cold-rolled at a reduction rate of 80% or more so that the aspect ratio of the ferrite phase is less than 0.40, and then 0.3 ° C / second. Austenite phase is recrystallized by heating at the above temperature rising rate, and annealing is performed at a temperature of 900 to 1050 ° C. at which the crystal grain growth of the phase does not occur to reduce the crystal grain size of the austenite phase to 5 μm or less. A process for producing a duplex stainless steel characterized by