Method for eliminating delta ferrite in austenitic stainless steel and austenitic stainless steel

Abstract

The invention provides a method for eliminating delta ferrite in austenitic stainless steel and the austenitic stainless steel, and belongs to the technical field of metal materials. The method comprises the following steps that an austenitic stainless steel ingot containing the delta ferrite is subjected to heat preservation for first set time at the first set temperature for first heat treatment; heating the austenitic stainless steel ingot subjected to the first heat treatment to a second set temperature, and carrying out heat preservation for a second set time at the second set temperature for second heat treatment; and cooling the austenitic stainless steel ingot subjected to the second heat treatment, or sequentially and repeatedly carrying out the first heat treatment and the second heat treatment on the austenitic stainless steel ingot subjected to the second heat treatment for at least one time, and then cooling to obtain the austenitic stainless steel subjected to delta ferrite elimination. The first heat treatment and the second heat treatment are performed, and the second set time is limited, so that the austenitic stainless steel is prevented from being in a high-temperature environment for a long time, and austenite grains are prevented from obviously growing.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials, and in particular relates to a method for eliminating delta ferrite in austenitic stainless steel and austenitic stainless steel. Background Art

[0002] Fe-Cr-Ni austenitic stainless steel is widely used in nuclear reactor structural materials due to its good high temperature strength and corrosion resistance. Since the main equipment such as the reactor internal components have been in service for a long time under high irradiation and high temperature environment, high requirements are put forward for the grain size, organizational stability, mechanical properties and corrosion resistance of the materials. These indicators are closely related to the organizational uniformity and second phase precipitation of the materials. Therefore, improving the performance of the materials by regulating the high temperature organization of austenitic stainless steel is a key issue that needs to be solved urgently.

[0003] According to the Fe-Cr-Ni alloy phase diagram, austenitic stainless steel should be a completely austenitic structure under equilibrium conditions. However, during the solidification process of austenitic stainless steel ingots, the segregation of alloy elements and the influence of cooling rate lead to local segregation of elements, which triggers the precipitation of δ ferrite. Especially in ultra-thick plate ingots, the δ ferrite content is particularly significantly affected by the thickness direction. The uneven distribution of phases in the organization will lead to differences in the deformation ability of the alloy, resulting in local stress concentration during the later deformation process, and ultimately causing the cracking of components. When the material is in high-temperature service in a nuclear reactor, δ ferrite is very likely to undergo high-temperature phase transformation, triggering the precipitation of brittle phases such as intermetallic compounds such as χ, σ, and η, which accelerate the cracking of the material as a crack source during the deformation process, greatly reducing the mechanical properties of the material. In addition, the carbides formed by the decomposition of δ ferrite during long-term aging will also cause the depletion of surrounding elements, aggravate the intergranular corrosion of austenite, and reduce the corrosion resistance of the material.

[0004] At present, the related art discloses a method for eliminating ferrite in austenitic stainless steel, which mainly eliminates ferrite through a heat treatment process, wherein the heat treatment is to keep the specimen at a temperature above 1200°C for 30 to 200 hours. However, this heat treatment method will cause serious coarsening of austenite grains, and the specimen will suffer from severe oxidation and burning at high temperature for a long time, which will reduce the surface quality of the product, reduce the product yield, and increase the production cost. Summary of the invention

[0005] Therefore, the present invention provides a method for eliminating delta ferrite in austenitic stainless steel and austenitic stainless steel, which can solve the technical problem of coarsening of austenite grains when eliminating delta ferrite in the prior art.

[0006] In order to solve the above problems, the present invention provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0007] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature and for a first set time to decompose the delta ferrite; after the heat preservation, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0008] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition products of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1 to 2 hours;

[0009] Step 3): cooling the austenitic stainless steel ingot after the second heat treatment to obtain austenitic stainless steel after eliminating delta ferrite;

[0010] Alternatively, the austenitic stainless steel ingot after the second heat treatment is subjected to the first heat treatment and the second heat treatment at least once in sequence, and then subjected to cooling treatment to obtain the austenitic stainless steel after the delta ferrite is eliminated.

[0011] Furthermore, in the step 1): the first set temperature is 950±20° C.; and / or

[0012] The first set time is 2 to 5 hours.

[0013] Furthermore, before the step 1), the method further comprises: heating the austenitic stainless steel ingot containing delta ferrite to a first set temperature;

[0014] Preferably, the time t used to heat the austenitic stainless steel ingot containing delta ferrite to the first set temperature is t=v×h; wherein v is 1 to 2 min / mm; and h is the thickness of the austenitic stainless steel ingot containing delta ferrite, in mm.

[0015] Furthermore, in the step 2), the second set temperature is 1150±20°C.

[0016] Furthermore, in the step 3), the cooling treatment is water cooling or air cooling; and / or

[0017] Before the step of sequentially repeating the first heat treatment and the second heat treatment at least once on the austenitic stainless steel ingot after the second heat treatment, the method further comprises: cooling the austenitic stainless steel ingot after the second heat treatment to a first set temperature;

[0018] Preferably, the furnace is cooled to the first set temperature;

[0019] Preferably, the first heat treatment and the second heat treatment are repeated four times in sequence.

[0020] Furthermore, the first heat treatment promotes the decomposition of delta ferrite into sigma phase and γ2 phase.

[0021] Furthermore, the second heat treatment promotes the dissolution of the decomposition products of ferrite and transforms the σ phase into the γ phase.

[0022] Furthermore, the composition of the austenitic stainless steel ingot containing delta ferrite satisfies the following formula:

[0023]

[0024] Wherein, each element symbol represents the weight percentage of the corresponding element in austenitic stainless steel; and / or

[0025] The area fraction of delta ferrite in the delta ferrite-containing austenitic stainless steel ingot is less than 20%.

[0026] On the other hand, the present invention provides an austenitic stainless steel, wherein the austenitic stainless steel is an austenitic stainless steel after eliminating delta ferrite;

[0027] Preferably, the area fraction of delta ferrite in the austenitic stainless steel is 0;

[0028] Preferably, the grain size of the austenitic stainless steel is 0.5 to 1.5 mm.

[0029] Furthermore, the austenitic stainless steel is treated by eliminating delta ferrite using any of the methods described above.

[0030] Compared with the prior art, the present invention has at least the following beneficial effects:

[0031] 1. The present invention provides a method for eliminating delta ferrite in austenitic stainless steel, wherein the delta ferrite is decomposed by a first heat treatment, and the decomposition product of the delta ferrite is dissolved back by a second heat treatment, and then the austenitic stainless steel ingot after the second heat treatment is cooled to obtain the austenitic stainless steel after the delta ferrite is eliminated; or the austenitic stainless steel ingot after the second heat treatment is subjected to the first heat treatment and the second heat treatment at least once in sequence, and then cooled to obtain the austenitic stainless steel after the delta ferrite is eliminated; wherein the first heat treatment is to heat the austenitic stainless steel ingot containing delta ferrite at a first set temperature for a first set time; the second heat treatment is to heat the austenitic stainless steel ingot after the first heat treatment to a second set temperature, and keep it at the second set temperature for a second set time; wherein the second set time is 1 to 2 hours; based on the above method, the present invention performs multi-stage heat treatment, that is, the second heat treatment is performed after the first heat treatment, and the second set time is set to 1 to 2 hours to avoid the austenitic stainless steel being in a high temperature environment for a long time, thereby ensuring that the austenite grains do not grow significantly; at the same time, it can avoid serious oxidation and burning of the ingot at high temperature, reduce the surface quality of the product, reduce the product yield and increase the production cost.

[0032] 2. Furthermore, the present invention performs a first heat treatment on the austenitic stainless steel ingot containing δ ferrite at the peak temperature of δ ferrite decomposition (950±20°C) to promote the nucleation of σ phase in δ ferrite and accelerate the phase transformation of δ→σ+γ2 in the structure; then a short-time heat treatment is performed at the dissolution temperature of the decomposition product of δ ferrite (1150±20°C) to increase the activity of alloy element diffusion, promote element diffusion, and transform the σ phase into the γ phase, that is, by cyclically performing the first heat treatment and the second heat treatment, the δ→σ→γ phase transformation is promoted, and the δ ferrite in the austenitic stainless steel ingot containing δ ferrite is completely eliminated.

[0033] 3. On the other hand, the present invention provides an austenitic stainless steel, which is an austenitic stainless steel after eliminating δ ferrite; the austenitic stainless steel adopts the above method to eliminate δ ferrite, and the area fraction of δ ferrite is less than 1%; the austenitic stainless steel after eliminating δ ferrite by the present invention can effectively avoid the precipitation of brittle phases (intermetallic compounds such as χ, σ, η, etc.) and decomposition of δ ferrite in the alloy during long-term aging, ensure the organizational stability of the austenitic stainless steel during high-temperature service, avoid the uncertainty of plastic changes caused by the decomposition reaction of δ ferrite, thereby further improving the service life of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. The drawings described below are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0035] Figure 1 This is a process diagram of the method for eliminating delta ferrite in austenitic stainless steel of the present invention;

[0036] Figure 2 is the organization diagram of the austenitic stainless steel ingot containing delta ferrite in Example 1;

[0037] Figure 3 The influence of the number of cycles on the ferrite content in the core structure of the ingot is shown in Figure 2.

[0038] Figure 4 The metallographic structure diagram of the austenitic stainless steel after eliminating delta ferrite obtained in Example 4;

[0039] Figure 5 (a) and (b) are the metallographic structures of the austenitic stainless steel obtained in Comparative Examples 2 and 3, respectively;

[0040] Figure 6 This is a metallographic photograph of the morphology of delta ferrite and precipitate phase in austenitic stainless steel obtained in Comparative Example 3;

[0041] Figure 7 (a) and 7(b) are the metallographic structures of austenitic stainless steel obtained in Comparative Examples 4 and 5, respectively;

[0042] Figure 8 The metallographic structure diagram of the austenitic stainless steel obtained in Comparative Example 6;

[0043] Fig. 9 This is the metallographic structure diagram of the austenitic stainless steel after eliminating delta ferrite obtained in Example 5;

[0044] Fig.10 This is the metallographic structure diagram of the austenitic stainless steel after eliminating delta ferrite obtained in Example 6. DETAILED DESCRIPTION

[0045] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. The drawings described below are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0046] The present invention provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0047] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation, an austenitic stainless steel ingot after the first heat treatment is obtained; wherein the first set temperature T1 is 950±20°C; and the first set time is 2 to 5 hours;

[0048] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set temperature T2 is 1150±20°C; and the second set time is 1 to 2h;

[0049] Step 3): when the area fraction of δ ferrite in the austenitic stainless steel ingot containing δ ferrite is less than 2%, cooling the austenitic stainless steel ingot after the second heat treatment to obtain austenitic stainless steel after eliminating δ ferrite;

[0050] When the area fraction of δ ferrite in the austenitic stainless steel ingot containing δ ferrite is greater than 2%, the austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated at least once in sequence, and then water cooling or air cooling is performed to obtain austenitic stainless steel after eliminating δ ferrite; preferably, when the area fraction of δ ferrite in the austenitic stainless steel ingot containing δ ferrite is greater than 8%, the first heat treatment and the second heat treatment are repeated four times, that is, the ingot is subjected to the first heat treatment and the second heat treatment five times in sequence, so that the δ ferrite in the ingot can be completely eliminated.

[0051] The “elimination” in the present invention includes reducing the content of delta ferrite in austenitic stainless steel, as well as completely eliminating delta ferrite in austenitic stainless steel.

[0052] Based on the above method, the present invention sets the second set time to 1 to 2 hours, and cycles the first heat treatment and the second heat treatment to avoid austenitic stainless steel being in a high temperature environment for a long time, thereby ensuring that the austenite grains do not grow significantly; at the same time, it can avoid severe oxidation and burning of the ingot at high temperature, reduce the surface quality of the product, reduce the product yield, and increase the production cost.

[0053] Based on the low atomic density in the ferrite lattice, the fast diffusion rate of elements in ferrite, and the characteristics of providing favorable nucleation positions and nucleation elements for the precipitation of σ phase, the present invention performs a first heat treatment on the austenitic stainless steel ingot containing δ ferrite at the peak temperature of δ ferrite decomposition (950±20℃), promotes the nucleation of σ phase in δ ferrite, accelerates the decomposition of δ ferrite in the organization, and the phase transformation of δ→σ+γ2 occurs; then a short-time heat treatment is performed at the dissolution temperature of the decomposition product of δ ferrite (1150±20℃), the activity of alloy element diffusion is increased, the element diffusion is promoted, and the σ phase is dissolved into γ (i.e., the σ phase is dissolved back), that is, the σ phase is transformed into the γ phase. Finally, by cyclically performing the first heat treatment and the second heat treatment, the δ→σ→γ phase transformation is promoted, and the complete elimination of δ ferrite in the austenitic stainless steel ingot containing δ ferrite is achieved.

[0054] Water cooling or rapid air cooling after cyclic heat treatment can avoid precipitation of precipitated phases caused by uneven diffusion of elements during the stay of the core structure of the ingot in the high temperature zone, and avoid grain coarsening.

[0055] In some embodiments, before step 1), the method further includes: heating the austenitic stainless steel ingot containing δ ferrite to a first set temperature; preferably, the time t used to heat the austenitic stainless steel ingot containing δ ferrite to the first set temperature is t = v × h; wherein v is 1 to 2 min / mm; and h is the thickness of the austenitic stainless steel ingot containing δ ferrite, in mm. When the size of the ingot is large, the time t is limited by the thickness of the ingot, so that the surface and the core of the ingot are both heated to the first set temperature, thereby ensuring that during the first heat treatment, the core of the ingot is fully kept warm at the first set temperature and the δ ferrite is fully decomposed.

[0056] In some embodiments, the composition of the delta ferrite-containing austenitic stainless steel ingot satisfies the following formula:

[0057]

[0058] Among them, each element symbol represents the weight percentage of the corresponding element in austenitic stainless steel;

[0059] The area fraction of delta ferrite in the austenitic stainless steel ingot containing delta ferrite is less than 20%.

[0060] On the other hand, the present invention provides an austenitic stainless steel, which is an austenitic stainless steel after eliminating δ ferrite; the area fraction of δ ferrite in the austenitic stainless steel is 0; the grain size of the austenitic stainless steel is 0.5-1.5 mm; preferably, the austenitic stainless steel adopts any of the above methods to eliminate δ ferrite.

[0061] The present invention is further described below with reference to specific embodiments and comparative examples.

[0062] In the embodiments and comparative examples of the present invention, the size of the austenitic stainless steel ingot containing δ ferrite is 50 mm×200 mm×200 mm; before the austenitic stainless steel ingot containing δ ferrite is kept at the first set temperature (or set temperature) for the first set time (or set time), the austenitic stainless steel ingot containing δ ferrite is heated to the first set temperature (or set temperature); wherein the time t used to heat the austenitic stainless steel ingot containing δ ferrite to the first set temperature (or set temperature) is t=v×h=200 min; wherein v is 1 min / mm; h is 200 mm.

[0063] Example 1

[0064] Reference Figure 1 This embodiment provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0065] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0066] Wherein, the first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; wherein, (wherein the weight percentage of Nb in the austenitic stainless steel is 0); the size of the austenitic stainless steel ingot containing delta ferrite is 50 mm×200 mm×200 mm;

[0067] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0068] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated once in sequence, and then water cooling treatment is performed to obtain austenitic stainless steel after eliminating delta ferrite.

[0069] The content of delta ferrite in the ingot was counted with reference to GB / T 13305-2008 "Metallurgical Test Method for Area Content of α-Phase in Stainless Steel", among which the delta ferrite content in the core of the ingot was the highest; the delta ferrite area fraction in the core of the austenitic stainless steel ingot containing delta ferrite in this embodiment was measured to be 2.73%, and the delta ferrite had a dendritic morphology, and its typical metallographic structure was as follows Figure 2 As shown; after eliminating the delta ferrite by the method of this embodiment (i.e., cyclically performing the first heat treatment and the second heat treatment twice), the area fraction of the delta ferrite in the core of the austenitic stainless steel obtained is 0.

[0070] Example 2

[0071] This embodiment provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0072] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0073] Wherein, the first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; wherein, The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0074] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and heat treating at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0075] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated twice in sequence, and then water cooling is performed to obtain austenitic stainless steel after eliminating delta ferrite.

[0076] In this embodiment, the area fraction of δ ferrite in the core of the austenitic stainless steel ingot containing δ ferrite is 3.44%. After eliminating δ ferrite by the method of this embodiment (i.e., cyclically treating the first heat treatment and the second heat treatment three times), the area fraction of δ ferrite in the core of the austenitic stainless steel obtained is 0.

[0077] Example 3

[0078] This embodiment provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0079] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0080] Wherein, the first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; wherein, The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0081] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0082] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated three times in sequence, and then water cooling is performed to obtain austenitic stainless steel after eliminating delta ferrite.

[0083] In this embodiment, the area fraction of δ ferrite in the core of the austenitic stainless steel ingot containing δ ferrite is 4.85%. After eliminating δ ferrite by the method of this embodiment (i.e., cyclically performing the first heat treatment and the second heat treatment four times), the area fraction of δ ferrite in the core of the austenitic stainless steel obtained is 0.

[0084] Example 4

[0085] This embodiment provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0086] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0087] Wherein, the first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; wherein, The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0088] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0089] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated four times in sequence, and then water-cooled to obtain austenitic stainless steel after eliminating delta ferrite.

[0090] In this embodiment, the area fraction of δ ferrite in the core of the austenitic stainless steel ingot containing δ ferrite is 8%. After eliminating δ ferrite by the method of this embodiment (i.e., cyclically performing the first heat treatment and the second heat treatment five times), the area fraction of δ ferrite in the core of the austenitic stainless steel obtained is 0. The typical metallographic structure of the austenitic stainless steel after eliminating δ ferrite obtained in this embodiment is as follows: Figure 4 As shown, it can be seen that there is no delta ferrite and the average grain size is about 1 mm.

[0091] Example 5

[0092] This embodiment provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0093] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0094] The first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.05%, S: 0.3%, Cr: 18.0%, Ni: 14%, Mo: 2.0%, Nb: 1.5%, Mn: 1.5%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance, The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0095] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0096] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated four times in sequence, and then water-cooled to obtain austenitic stainless steel after eliminating delta ferrite.

[0097] In this embodiment, the area fraction of δ ferrite in the core of the austenitic stainless steel ingot containing δ ferrite is 18%. After eliminating δ ferrite by the method of this embodiment (i.e., cyclically performing the first heat treatment and the second heat treatment five times), the area fraction of δ ferrite in the core of the austenitic stainless steel obtained is 0. The typical metallographic structure of the austenitic stainless steel after eliminating δ ferrite obtained in this embodiment is as follows: Fig. 9 As shown, it can be seen that the structure after cyclic heat treatment does not contain delta ferrite and the grains are relatively uniform.

[0098] Example 6

[0099] This embodiment provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0100] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0101] The first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.05%, Si: 0.3%, Cr: 18.0%, Ni: 14%, Mo: 2.0%, Mn: 1.5%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance, The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0102] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0103] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to the first set temperature, the first heat treatment and the second heat treatment are repeated four times in sequence, and then water-cooled to obtain austenitic stainless steel after eliminating delta ferrite.

[0104] In this embodiment, the area fraction of δ ferrite in the core of the austenitic stainless steel ingot containing δ ferrite is 3%. After eliminating δ ferrite by the method of this embodiment (i.e., cyclically performing the first heat treatment and the second heat treatment three times), the area fraction of δ ferrite in the core of the austenitic stainless steel obtained is 0. The typical metallographic structure of the austenitic stainless steel after eliminating δ ferrite obtained in this embodiment is as follows: Fig.10 As shown, it can be seen that there is no delta ferrite in the cyclic heat treatment thickness structure.

[0105] Comparative Example 1

[0106] This comparative example provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0107] Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature T1 for a first set time to decompose the delta ferrite; after the heat preservation is completed, an austenitic stainless steel ingot after the first heat treatment is obtained;

[0108] Wherein, the first set temperature T1 is 950°C; the first set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is, by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; wherein, The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0109] Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature T2, and performing a second heat treatment at the second set temperature for a second set time to dissolve the decomposition product of delta ferrite; after the heat preservation, obtaining the austenitic stainless steel ingot after the second heat treatment; wherein the second set time is 1h; the second set temperature T2 is 1150°C;

[0110] Step 3): The austenitic stainless steel ingot after the second heat treatment is furnace cooled to a first set temperature, and then water-cooled to obtain austenitic stainless steel after eliminating delta ferrite.

[0111] In this comparative example, the area fraction of delta ferrite in the core of the austenitic stainless steel ingot containing delta ferrite is 8%; see Figure 3 After eliminating δ ferrite by the method of this comparative example (i.e., cyclically performing the first heat treatment and the second heat treatment once), the area fraction of δ ferrite in the core of the austenitic stainless steel obtained is 4.85%. Compared with Example 4, since this comparative example only performs one heat treatment in the temperature range of T1 and T2, the δ ferrite is not fully decomposed, and the decomposition products of the δ ferrite are not fully dissolved back, so the area fraction of δ ferrite in the treated structure is higher.

[0112] Comparative Example 2

[0113] This comparative example provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0114] The austenitic stainless steel ingot containing delta ferrite is kept at a set temperature T1 for a set time, and then water-cooled after the end of the keeping time to obtain the austenitic stainless steel after the delta ferrite is eliminated;

[0115] The set temperature T1 is 950°C; the set time is 2h; the composition of the austenitic stainless steel ingot containing delta ferrite is as follows by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0116] The area fraction of delta ferrite in the core of the austenitic stainless steel ingot containing delta ferrite in this comparative example is 8%. The typical metallographic structure of the austenitic stainless steel after eliminating delta ferrite in this comparative example is as follows: Figure 5 As shown in (a), it can be seen that there is part of δ ferrite. In this comparative example, the ingot is heat treated at 950℃ for 2h, and only part of the δ ferrite is decomposed into σ and γ2, so the austenitic stainless steel obtained has residual δ ferrite.

[0117] Comparative Example 3

[0118] This comparative example provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0119] The austenitic stainless steel ingot containing delta ferrite is kept at a set temperature T1 for a set time, and then water-cooled after the end of the holding period to obtain an austenitic stainless steel with delta ferrite eliminated;

[0120] The set temperature T1 is 950°C; the set time is 5h; the composition of the austenitic stainless steel ingot containing delta ferrite is as follows by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0121] The area fraction of delta ferrite in the core of the austenitic stainless steel ingot containing delta ferrite in this comparative example is 8%. The typical metallographic structure of the austenitic stainless steel after eliminating delta ferrite in this comparative example is as follows: Figure 5 As shown in (b), it can be seen that there is some δ ferrite; the morphology of δ ferrite and precipitate phase in this comparative example is as follows Figure 6 As shown in the figure, it can be seen that black σ phase and white γ2 phase are precipitated inside the δ ferrite, and its outer contour still maintains the original δ phase morphology. It can be seen that when the ingot is kept at 950℃, a δ→σ+γ2 phase transformation occurs.

[0122] Comparative Example 4

[0123] This comparative example provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0124] The austenitic stainless steel ingot containing delta ferrite is kept at a set temperature T2 for a set time, and then water-cooled after the end of the keeping time to obtain an austenitic stainless steel with delta ferrite eliminated;

[0125] The set temperature T2 is 1150°C; the set time is 1h; the composition of the austenitic stainless steel ingot containing delta ferrite is as follows by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0126] The area fraction of delta ferrite in the core of the austenitic stainless steel ingot containing delta ferrite in this comparative example is 8%. The typical metallographic structure of the austenitic stainless steel after eliminating delta ferrite in this comparative example is as follows: Figure 7As shown in (a), it can be seen that there is some δ ferrite, and the area fraction of δ ferrite in the core of the ingot is 5%. After heat treatment at 1150℃ for 1h, the δ ferrite is not fully decomposed, so the austenitic stainless steel obtained has residual δ ferrite.

[0127] Based on the methods in Comparative Examples 1 to 4 above, after the ingot is subjected to the above heat treatment, the kinetics and thermodynamics reach a certain balance, the mutual diffusion of elements between the δ ferrite and the surrounding austenite slows down, resulting in a decrease in the decomposition rate of the δ ferrite in the later stage (that is, the δ ferrite is not fully decomposed), and the σ phase no longer precipitates, so the residual δ ferrite is obtained in the austenitic stainless steel.

[0128] Comparative Example 5

[0129] This comparative example provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0130] The austenitic stainless steel ingot containing delta ferrite is kept at a set temperature T2 for a set time, and then water-cooled after the end of the keeping time to obtain an austenitic stainless steel with delta ferrite eliminated;

[0131] The set temperature T2 is 1150°C; the set time is 10h; the composition of the austenitic stainless steel ingot containing delta ferrite is as follows by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0132] The area fraction of delta ferrite in the core of the austenitic stainless steel ingot containing delta ferrite in this comparative example is 8%. The typical metallographic structure of the austenitic stainless steel after eliminating delta ferrite in this comparative example is as follows: Figure 7 As shown in (b), it can be seen that there is some δ ferrite. After heat treatment at 1150℃ for 10h, the solid solution rate of ferrite gradually decreases during the insulation process. Finally, due to insufficient insulation time, the austenitization of δ ferrite is not sufficient, so the austenitic stainless steel obtained has residual δ ferrite.

[0133] Comparative Example 6

[0134] This comparative example provides a method for eliminating delta ferrite in austenitic stainless steel, comprising the following steps:

[0135] The austenitic stainless steel ingot containing delta ferrite is kept at a set temperature T for a set time, and then water-cooled after the end of the keeping time to obtain an austenitic stainless steel with delta ferrite eliminated;

[0136] The set temperature T is 1200°C; the set time is 30h; the composition of the austenitic stainless steel ingot containing delta ferrite is as follows by weight percentage: C: 0.047%, Si: 0.42%, Cr: 17.0%, Ni: 12.26%, Mo: 2.50%, Mn: 1.5%, N: 0.065%, Cu≤0.10%, S≤0.003%, P≤0.02%, Fe balance; The size of the austenitic stainless steel ingot containing delta ferrite is 50 mm × 200 mm × 200 mm;

[0137] The area fraction of delta ferrite in the core of the austenitic stainless steel ingot containing delta ferrite in this comparative example is 8%. The typical metallographic structure of the austenitic stainless steel after eliminating delta ferrite in this comparative example is as follows: Figure 8 As shown, it can be seen that there is part of delta ferrite. At the same time, the austenite grains in this comparative example are significantly coarsened, and the average grain size is about 2.5 mm. Compared with the embodiment, the austenite grains in this comparative example are subjected to long-term high-temperature heat treatment, and the grain boundary free energy increases, and grain boundary migration occurs, which eventually leads to grain growth and reduced interface energy. In addition, during the long-term high-temperature heat treatment of the ingot, the surface is severely oxidized and burned, the alloy surface is loose, and the surface quality is seriously reduced.

[0138] It is easy for those skilled in the art to understand that, without conflict, the advantageous technical features of the above-mentioned methods can be freely combined and superimposed.

[0139] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present invention. These improvements and variations should also be regarded as the protection scope of the present invention.

Claims

1. A method for eliminating delta ferrite in austenitic stainless steel, characterized in that: The following steps are involved: Step 1): performing a first heat treatment on an austenitic stainless steel ingot containing delta ferrite at a first set temperature and for a first set time to decompose the delta ferrite; after the heat preservation, an austenitic stainless steel ingot after the first heat treatment is obtained; Step 2): heating the austenitic stainless steel ingot after the first heat treatment to a second set temperature, and maintaining the temperature at the second set temperature for a second set time to perform a second heat treatment, so as to dissolve the decomposition product of delta ferrite; After the heat preservation is completed, an austenitic stainless steel ingot after the second heat treatment is obtained; wherein the second setting time is 1 to 2 hours; Step 3): cooling the austenitic stainless steel ingot after the second heat treatment to obtain austenitic stainless steel after eliminating delta ferrite; Alternatively, the austenitic stainless steel ingot after the second heat treatment is subjected to the first heat treatment and the second heat treatment at least once in sequence, and then subjected to cooling treatment to obtain the austenitic stainless steel after the delta ferrite is eliminated.

2. The method for eliminating delta ferrite in austenitic stainless steel according to claim 1, characterized in that: In the step 1): the first set temperature is 950±20° C.; and / or The first set time is 2 to 5 hours.

3. The method for eliminating delta ferrite in austenitic stainless steel according to claim 1 or 2, characterized in that: Before the step 1), the method further comprises: heating the austenitic stainless steel ingot containing delta ferrite to a first set temperature; Preferably, the time t used to heat the austenitic stainless steel ingot containing delta ferrite to the first set temperature is t=v×h; wherein v is 1 to 2 min / mm; and h is the thickness of the austenitic stainless steel ingot containing delta ferrite, in mm.

4. The method for eliminating delta ferrite in austenitic stainless steel according to claim 1, characterized in that: In the step 2): the second set temperature is 1150±20°C.

5. The method for eliminating delta ferrite in austenitic stainless steel according to claim 1, characterized in that: In the step 3): the cooling treatment is water cooling or cold treatment; and / or Before the step of sequentially repeating the first heat treatment and the second heat treatment at least once on the austenitic stainless steel ingot after the second heat treatment, the method further comprises: cooling the austenitic stainless steel ingot after the second heat treatment to a first set temperature; Preferably, the furnace is cooled to the first set temperature; Preferably, the first heat treatment and the second heat treatment are repeated four times in sequence.

6. The method for eliminating delta ferrite in austenitic stainless steel according to claim 1, characterized in that: The first heat treatment promotes the decomposition of delta ferrite into sigma phase and gamma 2 phase.

7. The method for eliminating delta ferrite in austenitic stainless steel according to claim 6, characterized in that: The second heat treatment promotes the dissolution of the decomposition products of ferrite, transforming the σ phase into the γ phase.

8. The method for eliminating delta ferrite in austenitic stainless steel according to any one of claims 1 to 7, characterized in that: The composition of the austenitic stainless steel ingot containing delta ferrite satisfies the following formula: Wherein, each element symbol represents the weight percentage of the corresponding element in austenitic stainless steel; and / or The area fraction of delta ferrite in the delta ferrite-containing austenitic stainless steel ingot is less than 20%.

9. An austenitic stainless steel, characterized in that: The austenitic stainless steel is austenitic stainless steel after eliminating delta ferrite; Preferably, the area fraction of delta ferrite in the austenitic stainless steel is 0; Preferably, the grain size of the austenitic stainless steel is 0.5 to 1.5 mm.

10. The austenitic stainless steel according to claim 9, characterized in that: The austenitic stainless steel is subjected to the method described in any one of claims 1 to 8 to eliminate delta ferrite.

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