Austenitic stainless steel and treatment method thereof

Abstract

The invention provides a treatment method of austenitic stainless steel, which relates to a preparation and application technology of austenitic stainless steel, and comprises the following steps: preheating an austenitic stainless steel ingot so as to decompose a part of primary phase in the austenitic stainless steel ingot into M23C6 carbide and enable the residual primary phase to be in a discontinuous distribution state; the austenitic stainless steel cast ingot obtained after preheating treatment is subjected to homogenization treatment, so that M23C6 carbide is redissolved into a matrix; and carrying out thermal deformation treatment on the homogenized austenitic stainless steel cast ingot. According to the invention, through preheating treatment, a coarse primary phase is decomposed into M23C6 carbide; through homogenization treatment, on one hand, dendritic segregation is eliminated, and on the other hand, M23C6 carbides are re-dissolved into a matrix, so that refining of a primary phase is achieved; the dynamic recrystallization driving force difference caused by dendritic segregation and a coarse primary phase is avoided, and complete dynamic recrystallization is ensured to be realized after thermal deformation treatment, so that a dual-grain structure formed by mixing coarse grains and fine grains is avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation and application of austenitic stainless steel, and in particular relates to austenitic stainless steel and a processing method thereof. Background Art

[0002] Austenitic stainless steel has excellent comprehensive properties such as mechanics, medium and high temperature stability, hot workability, weldability and corrosion resistance. It is an important structural material in the fields of military, chemical industry, thermal power and nuclear power. Compared with conventional austenitic stainless steel, Nb / Ti composite stabilized N-containing austenitic stainless steel has better high temperature strength, corrosion resistance, creep endurance and fatigue life. The most representative ones are supercritical boiler superheater material NF709, and fourth-generation integrated long-life sodium-cooled fast reactor structural candidate material Alloy709 and other austenitic stainless steels. First, nitrogen is an effective solid solution strengthening element. The presence of nitrogen can significantly improve the strength of stainless steel, prolong the incubation time of carbide nucleation, alleviate the tendency of intergranular carbide precipitation phase, and thus improve corrosion resistance. Secondly, Nb / Ti stabilization can form highly stable MX-type carbides, effectively reduce the precipitation tendency of M23C6-type carbides, and thus delay the precipitation of M23C6-type carbides. 23 The formation of brittle intermetallic phases such as sigma phase, G phase and Laves phase induced by C6 carbide can improve the creep rupture performance and fatigue life of stainless steel at high temperature.

[0003] For Nb / Ti composite stabilized N-containing austenitic stainless steel, dendritic segregation of Nb / Ti and other elements and the formation of coarse primary Nb(C,N), Ti(C,N), and (Nb,Ti)(C,N) will inevitably occur during the solidification process, and the distribution of Nb / Ti primary phase is uneven, mainly distributed between dendrites or at the junction between dendrite trunks and dendrites. In the subsequent hot working process, on the one hand, dendritic segregation and unevenly distributed coarse primary phases make the dynamic recrystallization driving force between dendrites and dendrite trunks different; on the other hand, due to the high alloying degree of Nb / Ti composite stabilized N-containing austenitic stainless steel, secondary (Nb,Ti)(C,N) is easily precipitated during hot deformation, making the N-containing austenitic stainless steel recrystallized unevenly, that is, dynamic recrystallization is completed in some areas and not in some areas, so that a dual grain structure of coarse and fine grains appears in the N-containing austenitic stainless steel. The dual grain structure (grain inhomogeneity) will significantly reduce the high temperature fatigue performance and endurance performance, greatly shortening the service life of stainless steel. Summary of the invention

[0004] Therefore, the present invention provides an austenitic stainless steel and a processing method thereof to solve the problem of dual grain structure in austenitic stainless steel in the prior art.

[0005] In order to solve the above problems, the present invention provides a method for treating austenitic stainless steel, comprising the following steps:

[0006] Step 1) preheating the austenitic stainless steel ingot to decompose part of the primary phase in the austenitic stainless steel ingot into M 23 C6 carbide, so that the remaining primary phase is in a discontinuous distribution state;

[0007] Step 2) homogenizing the austenitic stainless steel ingot after the preheating treatment to make M 23 C6 carbides dissolve back into the matrix;

[0008] Step 3) performing a thermal deformation treatment on the homogenized austenitic stainless steel ingot to allow the homogenized austenitic stainless steel ingot to undergo dynamic recrystallization.

[0009] Further, in the step 1), the austenitic stainless steel ingot is a N-containing austenitic stainless steel ingot; preferably, the austenitic stainless steel ingot is a Nb / Ti composite stabilized N-containing austenitic stainless steel ingot; further preferably, the chemical composition of the austenitic stainless steel ingot is, by weight percentage, C≤0.10%, Mn≤1.50%, Si≤1.00%, Cr: 19.5-23.0%, Ni: 23.0-26.0%, Mo: 1.0-4.0%, N: 0.10-0.25%, Nb: 0.10-0.40%, Ti: 0.10-0.50%, P≤0.030%, S≤0.030%, Fe and unavoidable residual elements are the remainder; and / or

[0010] The primary phase in the austenitic stainless steel ingot includes at least one of Nb(C,N), Ti(C,N), and (Nb,Ti)(C,N).

[0011] Furthermore, in the step 1), the preheating treatment includes placing the austenitic stainless steel ingot in a heating furnace, heating it to 850-950° C., and keeping it warm for 3-6 hours;

[0012] Preferably, the temperature is heated to 880-920°C.

[0013] Furthermore, in the step 2), the homogenization treatment includes: heating the austenitic stainless steel ingot after the preheat treatment to 1180-1250°C and performing a heat preservation treatment; preferably, heating to 1200-1220°C.

[0014] Furthermore, the insulation treatment time t≥30h+m×k; wherein m is the thickness of the austenitic stainless steel ingot, in mm; and k=10-15min / mm.

[0015] Furthermore, in the step 3), the temperature of the thermal deformation treatment is 1200-1220°C.

[0016] Furthermore, in the step 3), the thermal deformation treatment is performed by multiple thermal deformation processes; wherein the total deformation amount of the multiple thermal deformation processes is ≥ 40%; and the deformation amount of each process is 15-25%;

[0017] Preferably, the total deformation of the multiple heat deformation treatments is ≥ 60%;

[0018] Preferably, the deformation amount of the last pass is 15-20%.

[0019] Furthermore, in the step 3), after the step of thermal deformation, the step further includes: cooling the stainless steel ingot after the thermal deformation;

[0020] Preferably, the cooling treatment is carried out by water cooling.

[0021] On the other hand, the present invention provides an austenitic stainless steel, in which a dual grain structure of a mixture of coarse grains and fine grains is not observed; in which a secondary precipitation phase and a primary phase with a size >10 μm are not observed.

[0022] Furthermore, the austenitic stainless steel is obtained by any of the above-mentioned processing methods.

[0023] The present invention provides an austenitic stainless steel and a processing method thereof, which have the following beneficial effects:

[0024] 1. On the one hand, the present invention provides a method for treating austenitic stainless steel, comprising: sequentially performing preheating treatment, homogenization treatment, and thermal deformation treatment on an austenitic stainless steel ingot; based on the above method, the present invention decomposes the coarse primary phase in the austenitic stainless steel ingot into M by preheating treatment. 23 C6 carbide, so that the remaining primary phase is in a discontinuous distribution state; through homogenization treatment, on the one hand, the dendrite segregation is eliminated, and on the other hand, M 23 C6 carbide dissolves back into the matrix, thereby achieving the refinement of the primary phase; through thermal deformation treatment, the austenitic stainless steel ingot after homogenization treatment undergoes dynamic recrystallization; after the dendrite segregation is eliminated and the primary phase is refined, the difference in the driving force for dynamic recrystallization can be avoided, ensuring that complete dynamic recrystallization is achieved after thermal deformation treatment, thereby avoiding the appearance of a dual grain structure of coarse and fine grains in the stainless steel, that is, obtaining a more uniform dynamically recrystallized grain.

[0025] 2. Furthermore, the present invention sets the temperature of the thermal deformation treatment to 1200-1220°C to avoid the precipitation of secondary phases (Nb, Ti) (C, N) during the thermal deformation process, thereby hindering dynamic recrystallization; in addition, the present invention limits the deformation amount of the last thermal deformation treatment to 15-20% to avoid the situation where a smaller deformation amount fails to achieve the purpose of crushing the precipitated phase in the stainless steel and forming complete dynamic recrystallization; while a larger deformation amount is likely to promote the formation of secondary precipitation phases, thereby affecting the dynamic recrystallization process, increasing the internal stress of the stainless steel and causing a decrease in the service performance of the subsequent material; therefore, the present invention limits the deformation amount of the last thermal deformation treatment, which is conducive to achieving complete dynamic recrystallization while ensuring the improvement of the subsequent performance of the material.

[0026] 3. On the other hand, the present invention provides an austenitic stainless steel obtained by the above-mentioned treatment method; in the structure of the austenitic stainless steel, no dual grain structure of coarse crystals and fine crystals is observed, and no secondary precipitation phase and primary phase with a size >10 μm are observed; therefore, the austenitic stainless steel of the present invention has higher high temperature fatigue performance and endurance performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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.

[0028] Figure 1 is the metallographic diagram of the austenitic stainless steel ingot in Example 1;

[0029] Figure 2 is a metallographic image of the austenitic stainless steel ingot after preheat treatment in Example 1;

[0030] Figure 3 The metallographic images of the austenitic stainless steel ingot after homogenization treatment in Example 1; wherein: (a) is an OM image at low magnification, and (b) is an OM image at high magnification;

[0031] Figure 4 The metallographic images of the dynamic recrystallization of the austenitic stainless steel obtained in Example 1; wherein: (a) is an OM image at low magnification, and (b) is a SEM image at high magnification;

[0032] Figure 5 is the metallographic diagram of the austenitic stainless steel ingot in Example 2;

[0033] Figure 6 is a metallographic image of the austenitic stainless steel ingot after preheat treatment in Example 2;

[0034] Figure 7 The metallographic images of the austenitic stainless steel ingot after homogenization treatment in Example 2; wherein: (a) is an OM image at low magnification, and (b) is an OM image at high magnification;

[0035] Figure 8 The metallographic images of the dynamic recrystallization of austenitic stainless steel obtained in Example 2; wherein: (a) is an OM image at low magnification, and (b) is a SEM image at high magnification;

[0036] Fig. 9 The metallographic diagram of the dynamic recrystallization of austenitic stainless steel obtained in Comparative Example 1;

[0037] Fig.10 The metallographic images of the dynamic recrystallization of austenitic stainless steel obtained in Comparative Example 2 are as follows; wherein: (a) is an OM image at low magnification, and (b) is an SEM image at high magnification;

[0038] Fig.11 The metallographic images of the dynamic recrystallization of austenitic stainless steel obtained in Comparative Example 3 are as follows; wherein: (a) is an OM image at low magnification, and (b) is an SEM image at high magnification;

[0039] Fig.12 The metallographic diagram of the dynamic recrystallization of austenitic stainless steel obtained in Comparative Example 4;

[0040] Fig.13 : (a) is the metallographic image of the austenitic stainless steel ingot after homogenization treatment in Comparative Example 5; (b) is the metallographic image of the dynamic recrystallization of the austenitic stainless steel obtained in Example 5. DETAILED DESCRIPTION

[0041] 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.

[0042] For Nb / Ti composite stabilized N-containing austenitic stainless steel, dendrite segregation of elements such as Nb / Ti and formation of coarse primary phases Nb(C,N), Ti(C,N), and (Nb,Ti)(C,N) are more likely to occur; therefore, the present invention provides a method for treating austenitic stainless steel, comprising the following steps:

[0043] Step 1) placing the austenitic stainless steel ingot into a heating furnace for preheating, so that part of the primary phase in the austenitic stainless steel ingot is decomposed into M 23C6 carbide, so that the remaining primary phase is in a discontinuous distribution state; wherein the charging temperature is less than 700°C to avoid the ingot cracking caused by the charging temperature being too high; after charging, the furnace is heated to 850-950°C and kept warm for 3-6 hours; preferably, the furnace is heated to 880-920°C;

[0044] Wherein, the austenitic stainless steel ingot (before treatment) is a Nb / Ti composite stabilized N-containing austenitic stainless steel ingot; preferably, the chemical composition of the austenitic stainless steel ingot is, by weight percentage, C≤0.10%, Mn≤1.50%, Si≤1.00%, Cr: 19.5-23.0%, Ni: 23.0-26.0%, Mo: 1.0-4.0%, N: 0.10-0.25%, Nb: 0.10-0.40%, Ti: 0.10-0.50%, P≤0.030%, S≤0.030%, Fe and unavoidable residual elements are the remainder;

[0045] Step 2) homogenizing the austenitic stainless steel ingot after preheating to make M 23 C6 carbide is dissolved back into the matrix; wherein, the homogenization treatment is to heat the austenitic stainless steel ingot after preheating treatment to 1180-1250°C and perform heat preservation treatment; preferably, it is heated to 1200-1220°C.

[0046] The heat preservation time t≥30h+m×k; wherein m is the thickness of the austenitic stainless steel ingot, in mm; k=10-15min / mm; the heat preservation time is limited according to the thickness of the ingot, which can ensure that the dendrite segregation in the ingot is completely eliminated, and the decomposition product M of the primary phase in step 1) is reduced to 23 C6 carbide dissolves back;

[0047] Step 3) performing multiple heat deformation treatments on the homogenized austenitic stainless steel ingot to allow the homogenized austenitic stainless steel ingot to undergo dynamic recrystallization; and then performing rapid water cooling on the ingot after the heat deformation treatment;

[0048] Among them, the thermal deformation treatment can be carried out by rolling, forging and the like; the temperature of the thermal deformation treatment is 1200-1220°C; the total deformation amount of the thermal deformation treatment is ≥40%; the deformation amount of each pass is 15-25%; the deformation amount of the last pass is 15-20%; preferably, the total deformation amount of the thermal deformation treatment is ≥60%.

[0049] The ingot after the preheating treatment may be directly heated to the homogenization temperature, or the ingot after the preheating treatment may be cooled and then heated to the temperature; the ingot after the homogenization treatment may be directly thermally deformed.

[0050] Based on the above method, the present invention decomposes part of the coarse primary phase in the austenitic stainless steel ingot into M by preheating. 23 C6 carbide; through homogenization treatment, on the one hand, the dendrite segregation caused by high Cr / Mo is eliminated, and on the other hand, M 23 C6 carbides are dissolved back into the matrix, thereby achieving the refinement of the primary phase (e.g., the continuous primary phase is partially decomposed to present a discontinuous distribution state, so it is finer); the austenitic stainless steel ingot after homogenization treatment undergoes dynamic recrystallization through thermal deformation treatment; dendrite segregation is eliminated, and after the primary phase is refined, the difference in dynamic recrystallization driving force can be avoided, ensuring that complete dynamic recrystallization is achieved after thermal deformation treatment, thereby avoiding the appearance of a dual grain structure of coarse and fine grains in the stainless steel, that is, obtaining a more uniform dynamically recrystallized grain.

[0051] Among them, the primary phases in the austenitic stainless steel ingot include Nb(C,N), Ti(C,N), (Nb,Ti)(C,N); M 23 C6 carbides include Nb 23 C6, Ti 23 C6; wherein, Nb(C,N) refers to the carbide formed by Nb, C, and N in stainless steel; Ti(C,N) is the carbide formed by Ti, C, and N in stainless steel; (Nb,Ti)(C,N) is a composite carbide formed by Nb(C,N) and Ti(C,N).

[0052] In some embodiments, the temperature of the hot deformation treatment is set to 1200-1220°C. If the temperature is too low, secondary phases (Nb, Ti) (C, N) are easily precipitated, which is not conducive to dynamic recrystallization; if the temperature is too high, high-temperature oxidation is likely to occur and high-temperature ferrite is generated; in addition, the deformation amount of the last hot deformation treatment is limited to 15-20% to avoid the situation where a smaller deformation amount fails to achieve the purpose of crushing the precipitated phase in the stainless steel and forming complete dynamic recrystallization; a larger deformation amount is likely to promote the formation of secondary precipitated phases, thereby affecting the dynamic recrystallization process, increasing the internal stress of the stainless steel and causing a decrease in the service performance of the subsequent material; rapid water cooling can prevent the formation of (Nb, Ti) (C, N) secondary phases during the cooling process of the ingot after the hot deformation treatment, thereby avoiding the influence of the secondary phase on the dynamic recrystallization.

[0053] On the other hand, the present invention provides an austenitic stainless steel, in which a dual grain structure of a mixture of coarse crystals and fine crystals is not observed; and in which a secondary precipitated phase and a primary phase with a size > 10 μm are not observed; the austenitic stainless steel is obtained by any of the above-mentioned treatment methods.

[0054] The present invention is further described below with reference to specific embodiments and comparative examples.

[0055] In the embodiments and comparative examples, materials are prepared according to the chemical composition of austenitic stainless steel, and then smelted by refining equipment, and then poured and cooled to obtain an austenitic stainless steel ingot before treatment; the refining equipment can ensure that the chemical composition of the molten steel meets the composition control requirements; after cooling, the size of the ingot is φ120×150mm 3 Perform corresponding processing.

[0056] Example 1

[0057] This embodiment provides a method for treating austenitic stainless steel, comprising the following steps:

[0058] Step 1) placing an austenitic stainless steel ingot into a heating furnace for preheating; wherein the furnace temperature is 600° C., and then heating to 900° C. and keeping the temperature for 4 hours;

[0059] Wherein, the chemical composition of the austenitic stainless steel ingot (before treatment) is shown in Table 1 in terms of weight percentage; Step 2) heating the austenitic stainless steel ingot after preheat treatment to 1200° C. and keeping the temperature for 40 hours to achieve homogenization treatment;

[0060] Step 3) performing three heat deformation treatments on the homogenized austenitic stainless steel ingot at 1200° C.; followed by water cooling to obtain treated austenitic stainless steel;

[0061] Among them, the total deformation amount of thermal deformation treatment is 60%; the deformation amount of each process is 20%.

[0062] In this embodiment, the metallographic structure of the austenitic stainless steel ingot (before treatment) is as follows: Figure 1 As shown in the figure, it can be seen that the matrix structure of the ingot before treatment is composed of coarse dendrites, and coarse (size>10μm) primary phase Nb(C,N) is formed between the dendrites, and the primary phase is continuous, skeletal and short rod-shaped; the metallographic structure of the austenitic stainless steel ingot after preheating treatment is as follows Figure 2 As shown in the figure, it can be seen that the coarse primary phase Nb(C,N) between the dendrites decomposes, and at the same time, fine secondary precipitates precipitate between the dendrites; the metallographic structure of the austenitic stainless steel ingot after homogenization treatment is as follows Figure 3 As shown, it can be seen that the dendrite segregation has been eliminated (see Figure 3 a), the primary phase Nb(C,N) is spherical or fine needle-shaped, that is, it is significantly refined (see Figure 3 b); The metallographic structure of the austenitic stainless steel after treatment in this embodiment is as follows Figure 4 As shown in Figure 2, it can be seen that complete dynamic recrystallization occurs after thermal deformation treatment, the grain size is relatively uniform, and there is no dual grain structure of coarse and fine grains (see Figure 4a), and the dynamic recrystallization grains of the matrix are clean inside, without secondary precipitation phases and primary phases with a size > 10 μm (see Figure 4 b).

[0063] Example 2

[0064] This embodiment provides a method for treating austenitic stainless steel, comprising the following steps:

[0065] Step 1) placing an austenitic stainless steel ingot into a heating furnace for preheating; wherein the furnace temperature is 600° C., and then heating to 920° C. and keeping the temperature for 4 hours;

[0066] Wherein, the chemical composition of the austenitic stainless steel ingot (before treatment) is shown in Table 1 in terms of weight percentage; Step 2) heating the austenitic stainless steel ingot after preheat treatment to 1220° C. and keeping the temperature for 40 hours to achieve homogenization treatment;

[0067] Step 3) subjecting the homogenized austenitic stainless steel ingot to three heat deformation treatments at 1220° C.; followed by water cooling to obtain treated austenitic stainless steel;

[0068] Among them, the total deformation amount of thermal deformation treatment is 60%; the deformation amount of each process is 20%.

[0069] In this embodiment, the metallographic structure of the austenitic stainless steel ingot (before treatment) is as follows: Figure 5 As shown in the figure, it can be seen that the matrix structure of the ingot before treatment is composed of coarse dendrites, and coarse primary phase Nb(C,N) is formed between the dendrites, and the primary phase is continuous, skeletal and short rod-shaped; the metallographic structure of the austenitic stainless steel ingot after preheating treatment is as follows Figure 6 As shown in Figure 2, it can be seen that the coarse primary phase Nb(C,N) between the dendrites decomposes, and at the same time, fine secondary precipitates precipitate between the dendrites; the metallographic structure of the austenitic stainless steel ingot after homogenization treatment is shown in Figure 2. Figure 7 As shown, it can be seen that the dendrite segregation has been eliminated (see Figure 7 a), the primary phase Nb(C,N) is spherical or fine needle-shaped, that is, it is significantly refined (see Figure 7 b); The metallographic structure of the austenitic stainless steel after treatment in this embodiment is as follows Figure 8 As shown in Figure 2, it can be seen that complete dynamic recrystallization occurs after thermal deformation treatment, the grain size is relatively uniform, and there is no dual grain structure of coarse and fine grains (see Figure 8 a), and the dynamic recrystallization grains of the matrix are clean inside and there is no secondary precipitation phase (see Figure 8 b).

[0070] Table 1 Chemical composition of austenitic stainless steel ingots in Examples 1 and 2 (wt.%)

[0071]

[0072] Comparative Example 1

[0073] This comparative example provides a method for treating austenitic stainless steel, comprising the following steps:

[0074] The austenitic stainless steel ingot is subjected to three heat deformation treatments at 1200°C, followed by water cooling to obtain the treated austenitic stainless steel;

[0075] Wherein, by weight percentage, the chemical composition of the austenitic stainless steel ingot (before treatment) is the same as that of Example 1; the total deformation amount of the thermal deformation treatment is 60%; and the deformation amount of each pass is 20%.

[0076] The metallographic structure of the austenitic stainless steel after treatment in this comparative example is as follows: Fig. 9 As shown, it can be seen that dendrite structure still exists in some areas, dynamic recrystallization is incomplete, dendrite structure affects dynamic recrystallization, and dynamic recrystallization does not occur completely between dendrites, resulting in uneven recrystallized grains; at the same time, a large number of secondary precipitation phases are precipitated at the dendrite positions where dynamic recrystallization does not occur. In this comparative example, since no preheating treatment and homogenization treatment were performed, the coarse (Nb, Ti) (C, N) primary phase and solidification segregation were not eliminated, which significantly affected the recrystallization driving force. Dynamic recrystallization has been completed in some areas, while dynamic recrystallization has not occurred in some areas, resulting in uneven recrystallized grains.

[0077] Comparative Example 2

[0078] This comparative example provides a method for treating austenitic stainless steel, comprising the following steps:

[0079] Step 1) placing an austenitic stainless steel ingot into a heating furnace for preheating; wherein the furnace temperature is 600° C., and then heating to 900° C. and keeping the temperature for 4 hours;

[0080] Wherein, by weight percentage, the chemical composition of the austenitic stainless steel ingot (before treatment) is the same as that of Example 1;

[0081] Step 2) heating the preheated austenitic stainless steel ingot to 1200° C. and keeping the temperature for 40 hours to achieve homogenization;

[0082] Step 3) performing three heat deformation treatments on the homogenized austenitic stainless steel ingot at 1100° C.; followed by water cooling to obtain treated austenitic stainless steel;

[0083] Among them, the total deformation amount of thermal deformation treatment is 60%; the deformation amount of each process is 20%.

[0084] The metallographic structure of the austenitic stainless steel after treatment in this comparative example is as follows: Fig.10 As shown in Figure 2, it can be seen that the recrystallized grains are of different sizes (see Fig.10 a), a large amount of secondary precipitation phases were precipitated in some areas (see Fig.10 b). This is because the temperature of the heat deformation treatment in this comparative example is lower than 1200°C, which is easy to promote the precipitation of secondary phases. The secondary phases will hinder the thermal diffusion movement, resulting in a slowdown in the rate of dynamic recrystallization, which inhibits the dynamic recrystallization process and fails to form complete dynamic recrystallization.

[0085] Comparative Example 3

[0086] This comparative example provides a method for treating austenitic stainless steel, comprising the following steps:

[0087] Step 1) placing an austenitic stainless steel ingot into a heating furnace for preheating; wherein the furnace temperature is 600° C., and then heating to 900° C. and keeping the temperature for 4 hours;

[0088] Wherein, by weight percentage, the chemical composition of the austenitic stainless steel ingot (before treatment) is the same as that of Example 1;

[0089] Step 2) heating the preheated austenitic stainless steel ingot to 1200° C. and keeping the temperature for 40 hours to achieve homogenization;

[0090] Step 3) performing three heat deformation treatments on the homogenized austenitic stainless steel ingot at 1200° C.; followed by water cooling to obtain treated austenitic stainless steel;

[0091] Among them, the total deformation amount of thermal deformation treatment is 60%; the deformation amount of the last pass is 25%.

[0092] The metallographic structure of the austenitic stainless steel after treatment in this comparative example is as follows: Fig.11 As shown in Figure 2, it can be seen that the recrystallized grains are of different sizes (see Fig.11 a), a large amount of secondary precipitation phases were precipitated in some areas (see Fig.11 b). This is because the deformation of the last hot deformation in this comparative example is 25%. The large deformation leads to the formation of a large number of secondary precipitation phases during the dynamic recrystallization process. The secondary precipitation phases have a pinning effect on the grain boundaries, inhibiting the growth of grains, and on the other hand, reduce the driving force of dynamic recrystallization, which is not conducive to the nucleation and growth of dynamic recrystallization, so that the dynamic recrystallization process is inhibited, and finally cause the inhomogeneity of grains.

[0093] Comparative Example 4

[0094] This comparative example provides a method for treating austenitic stainless steel, comprising the following steps:

[0095] Step 1) placing an austenitic stainless steel ingot into a heating furnace for preheating; wherein the furnace temperature is 600° C., and then heating to 900° C. and keeping the temperature for 4 hours;

[0096] Wherein, by weight percentage, the chemical composition of the austenitic stainless steel ingot (before treatment) is the same as that of Example 1;

[0097] Step 2) heating the preheated austenitic stainless steel ingot to 1200° C. and keeping the temperature for 40 hours to achieve homogenization;

[0098] Step 3) performing three heat deformation treatments on the homogenized austenitic stainless steel ingot at 1200° C.; followed by water cooling to obtain treated austenitic stainless steel;

[0099] Among them, the total deformation amount of thermal deformation treatment is 60%; the deformation amount of the last pass is 10%.

[0100] The metallographic structure of the austenitic stainless steel after treatment in this comparative example is as follows: Fig.12 As shown, it can be seen that the recrystallized grain size is uneven. This is because the deformation amount of the last hot deformation in this comparative example is 10%. The smaller deformation amount reduces the efficiency of dynamic softening of austenitic stainless steel, which inhibits the dynamic recrystallization process and cannot produce complete dynamic recrystallization.

[0101] Comparative Example 5

[0102] This comparative example provides a method for treating austenitic stainless steel, comprising the following steps:

[0103] Step 1) heating the austenitic stainless steel ingot to 1200° C. and keeping the temperature for 40 hours to achieve homogenization treatment; wherein, the chemical composition of the austenitic stainless steel ingot (before treatment) is the same as that of Example 1 in terms of weight percentage;

[0104] Step 2) performing three heat deformation treatments on the homogenized austenitic stainless steel ingot at 1200° C.; followed by water cooling to obtain treated austenitic stainless steel;

[0105] Among them, the total deformation amount of thermal deformation treatment is 60%; the deformation amount of each pass is 20%.

[0106] The metallographic structure of the austenitic stainless steel after treatment in this comparative example is as follows: Fig.13 As shown in Figure 2, it can be seen that the primary phase Nb(C,N) in the austenite ingot after homogenization is slowly dissolved and still presents a skeleton-like continuous morphology (see Fig.13a); The grain size of the treated austenitic stainless steel is uneven. This is because the comparative example did not undergo preheating treatment, and the large amount of coarse Nb, Ti (C, N) primary phases affected the driving force of recrystallization, and dynamic recrystallization did not occur in some areas, resulting in uneven recrystallization.

[0107] 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.

[0108] 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 treating austenitic stainless steel, characterized in that: The following steps are involved: Step 1) preheating the austenitic stainless steel ingot to decompose part of the primary phase in the austenitic stainless steel ingot into M 23 C6 carbide, so that the remaining primary phase is in a discontinuous distribution state; Step 2) homogenizing the austenitic stainless steel ingot after the preheating treatment to make M 23 C6 carbides dissolve back into the matrix; Step 3) performing a thermal deformation treatment on the homogenized austenitic stainless steel ingot to allow the homogenized austenitic stainless steel ingot to undergo dynamic recrystallization.

2. The method for treating austenitic stainless steel according to claim 1, characterized in that: In the step 1), the austenitic stainless steel ingot is a N-containing austenitic stainless steel ingot; preferably, the austenitic stainless steel ingot is a Nb / Ti composite stabilized N-containing austenitic stainless steel ingot; further preferably, the chemical composition of the austenitic stainless steel ingot is, by weight percentage, C≤0.10%, Mn≤1.50%, Si≤1.00%, Cr: 19.5-23.0%, Ni: 23.0-26.0%, Mo: 1.0-4.0%, N: 0.10-0.25%, Nb: 0.10-0.40%, Ti: 0.10-0.50%, P≤0.030%, S≤0.030%, Fe and unavoidable residual elements are the remainder; and / or The primary phase in the austenitic stainless steel ingot includes at least one of Nb(C,N), Ti(C,N), and (Nb,Ti)(C,N).

3. The method for treating austenitic stainless steel according to claim 1, characterized in that: In the step 1), the preheating treatment comprises: placing the austenitic stainless steel ingot in a heating furnace, heating it to 850-950° C., and keeping it warm for 3-6 hours; Preferably, the temperature is heated to 880-920°C.

4. The method for treating austenitic stainless steel according to claim 1, characterized in that: In the step 2), the homogenization treatment includes: heating the austenitic stainless steel ingot after the preheating treatment to 1180-1250° C. and performing a heat preservation treatment; Preferably, the temperature is heated to 1200-1220°C.

5. The method for treating austenitic stainless steel according to claim 4, characterized in that: The time of the heat preservation treatment is t≥30h+m×k; wherein m is the thickness of the austenitic stainless steel ingot, in mm; and k=10-15min / mm.

6. The method for treating austenitic stainless steel according to claim 1, characterized in that: In the step 3), the temperature of the thermal deformation treatment is 1200-1220°C.

7. The method for treating austenitic stainless steel according to claim 1 or 6, characterized in that: In the step 3), the thermal deformation treatment is performed by multiple thermal deformation processes; wherein the total deformation amount of the multiple thermal deformation processes is ≥ 40%; and the deformation amount of each process is 15-25%; Preferably, the total deformation of the multiple heat deformation treatments is ≥ 60%; Preferably, the deformation amount of the last pass is 15-20%.

8. The method for treating austenitic stainless steel according to claim 1, characterized in that: In the step 3), after the step of thermal deformation, the method further comprises: cooling the austenitic stainless steel ingot after the thermal deformation; Preferably, the cooling treatment is carried out by water cooling.

9. An austenitic stainless steel, characterized in that: No dual grain structure of coarse grains and fine grains is observed in the austenitic stainless steel; no secondary precipitation phase and primary phase with a size of >10 μm are observed in the austenitic stainless steel.

10. The austenitic stainless steel according to claim 9, characterized in that: The austenitic stainless steel is obtained by the treatment method described in any one of claims 1 to 8.

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