A method for segmented regulation of delta ferrite in high silicon and high chromium heat-resistant steel

Abstract

The present invention discloses a method for segmented regulation of delta ferrite in high-silicon and high-chromium heat-resistant steel, and belongs to the technical field of nuclear heat-resistant steel production and manufacturing. The method is to regulate delta ferrite in the preparation process of heat-resistant steel with Si greater than 0.8% and Cr greater than 10%. The present invention reduces delta ferrite by coordinated regulation of composition and processing technology, and proposes control standards and control methods for the content and morphology of delta ferrite in the four stages of cast, homogenized, hot-worked and normalized structures, thereby achieving a significant reduction or elimination of delta ferrite in the final structure of the material. The method of the present invention reduces the adverse effects of conventional composition regulation on mechanical properties and corrosion resistance, and obtains high-silicon and high-chromium heat-resistant steel with excellent comprehensive performance.

Description

Technical Field

[0001] The invention relates to the technical field of heat-resistant steel production, and in particular to a method for segmentally regulating delta ferrite in high-silicon and high-chromium heat-resistant steel. Background Art

[0002] Liquid lead-bismuth eutectic (LBE) alloy has the characteristics of low melting point, high boiling point, excellent thermal conductivity and strong neutron shielding ability. It is an ideal cooling medium for the primary loop of a nuclear reactor. The lead-bismuth cooled fast neutron reactor (LFR) is one of the key reactor types developed in the fourth generation of reactors.

[0003] The 12Cr system ferrite / martensitic steel heat-resistant steel has good high-temperature mechanical properties and is one of the candidate materials for manufacturing the fuel cladding with the highest service temperature in fast neutron reactors. However, when LBE alloy is used as a coolant, it has a high operating temperature, a large flow rate, and strong corrosiveness, which will seriously corrode the fuel cladding structure in contact with it, thereby threatening the safety of reactor operation. In order to improve the lead-bismuth corrosion resistance of ferrite / martensite, more than 0.8% Si is usually added to the alloy, and then by controlling the oxygen content in the liquid lead (lead-bismuth), a stable and uniform oxide protective film is produced on the surface of the alloy. However, Si is a delta ferrite-forming element, and its addition will cause an increase in the delta ferrite content, which seriously deteriorates the mechanical properties of the material.

[0004] In the existing public technology, the tendency of δ ferrite precipitation is usually reduced by limiting the alloy CNB value to less than 12 through component optimization, or by dissolving the δ ferrite back through high-temperature long-term solution treatment to obtain a structure with less δ ferrite. The above two methods are feasible for heat-resistant steels with a Cr content of less than 10% and a low Si content, but are not feasible for heat-resistant steels with a Cr content of more than 10% and a Si content of more than 0.8%. On the one hand, high-chromium and high-silicon ferrite martensitic heat-resistant steel has a greater tendency to ferrite precipitation. In order to make its CNB value less than 12, it is necessary to significantly increase the content of austenite stabilizing elements C and N or reduce the content of δ ferrite stabilizing elements V and Nb. The increase in C or N content or the reduction in the weldability of the alloy, and the reduction in V and Nb will reduce the high-temperature mechanical properties of the alloy. On the other hand, it can be seen from the alloy phase diagram that the range of δ ferrite disappearance in high silicon high chromium ferrite / martensite heat-resistant steel is narrow or non-existent, and high temperature solution treatment alone cannot reduce / eliminate δ ferrite. On the contrary, if the process parameters are not set properly, there is a risk of further increase in δ ferrite content. Therefore, solving the problem of stable reduction / elimination of δ ferrite in high silicon high chromium heat-resistant steel has become an urgent problem to be solved. Summary of the invention

[0005] The object of the present invention is to provide a method for segmentally regulating delta ferrite in high-silicon high-chromium ferrite / martensite heat-resistant steel, which solves the problem of high precipitation tendency of delta ferrite and ultimately reduces the delta ferrite content.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] A method for segmented control of delta ferrite in high silicon and high chromium heat-resistant steel, wherein the method is to control delta ferrite during the preparation process of high silicon and high chromium ferrite / martensite heat-resistant steel, wherein the control comprises four stages: composition design control, homogenization control, hot working control and normalizing control, and reasonable delta ferrite content and morphology control targets are set for each stage.

[0008] The preparation process of the high-silicon high-chromium ferrite / martensite heat-resistant steel comprises the following steps:

[0009] (1) Designing the chemical composition of high-silicon and high-chromium ferrite / martensite heat-resistant steel, smelting the ingredients according to the designed chemical composition, and casting the ingot;

[0010] (2) Homogenization treatment: The ingot is homogenized at a temperature of 950-1120°C. When the homogenization temperature is greater than 1080°C, the homogenization time is greater than 3 hours but less than 15 hours. When the homogenization temperature is less than 1080°C, the homogenization time is greater than 5 hours but less than 40 hours.

[0011] (3) Hot deformation treatment: Hot deformation treatment is carried out by forging, rolling, extrusion or pipe threading; the holding temperature before deformation is less than 1120℃, and the holding time after reaching the temperature is less than 5h; the hot deformation process can be returned to the furnace for re-firing, the temperature during re-firing is not higher than 1080℃, and the holding time after reaching the temperature is not more than 2h; the deformation ratio is ≥6.

[0012] (4) Heat treatment (normalizing): The temperature control range during normalizing is 960-1080°C, and the normalizing holding temperature is 10 min to 60 min.

[0013] The high-silicon and high-chromium ferrite / martensite heat-resistant steel has Cr≥10wt.%, Si≥0.8wt.%, and C≤0.2wt.%; the heat-resistant steel also contains elements such as Mn, Ni, Mo, W, Nb, V, N, and B.

[0014] The high-silicon high-chromium ferrite / martensite heat-resistant steel component system should have a temperature zone in which delta ferrite is completely transformed into austenite in a thermodynamically stable state.

[0015] In the chemical composition of the high silicon high chromium ferrite / martensite heat resistant steel, the contents of V and Nb are both greater than 0.10wt.% and less than 0.40wt.%, and 11<CNB value≤13.5, and the CNB value is calculated according to the following formula (1), wherein the symbols of each element in the formula (1) represent the mass percentage of the corresponding element in the high silicon high chromium ferrite / martensite heat resistant steel:

[0016] CNB=Cr+6Si+4Mo+1.5W+5Nb+11V+12Al+9Ti-40C-2Mn-4Ni-1Cu-30N (1).

[0017] The control objectives achieved at each stage of this method are as follows:

[0018] (a) After the chemical composition of the high silicon high chromium ferrite / martensite heat resistant steel is designed, the total content of delta ferrite in the cast ingot should be less than 15%, and the length of a single delta ferrite should be less than 200 μm.

[0019] (b) After homogenization, the delta ferrite morphology should be spheroidized / ellipsoidal, the major axis to minor axis ratio should be less than 3, and the total delta ferrite content should be less than 12%.

[0020] (c) After heat treatment, the delta ferrite content should be less than 7% and the width of a single delta ferrite should be less than 10 μm.

[0021] (d) After heat treatment, the delta ferrite content is less than 1% and the length of a single delta ferrite should be less than 10 μm.

[0022] The design mechanism and beneficial effects of the present invention are as follows:

[0023] 1. The first stage of the present invention mainly reduces the precipitation tendency of δ ferrite in the cast structure by optimizing the composition, and obtains a composition system that is thermodynamically free of δ ferrite. For ferrite / martensitic heat-resistant steel, the composition is the decisive factor in determining the δ ferrite content of the alloy. Within the appropriate composition range, the reduction or elimination of non-equilibrium δ ferrite can be achieved by homogenization, hot working and strict heat treatment. However, if the composition range is not appropriate, that is, a large amount of δ ferrite will exist under thermodynamic steady state, it cannot be completely eliminated by subsequent treatment. The present invention provides a suitable composition range for high-silicon and high-chromium heat-resistant steel, the criteria for composition optimization, and the control target of δ ferrite in this stage. The details are as follows:

[0024] (A) To ensure that the high temperature mechanical properties and lead-bismuth corrosion resistance of the alloy are not reduced, the chemical composition of the high silicon and high chromium ferrite / martensite heat-resistant steel is preferably in the range of C 0.14-0.20%, Si 0.8-2.0%, Mn 0.5-3.0%, Cr 10.0-12.5%, Ni 0.5-1.5%, Mo 0.5-1.6%, W 0.5-1.5%, Nb 0.1-0.4%, V 0.1-0.35%, B≤0.005%, N≤0.05%, and the remainder is iron and other inevitable residual elements.

[0025] (B) In order to achieve the reduction of delta ferrite without affecting other properties, when adjusting the composition during the smelting process, the total CNB value needs to be controlled within the range of 11 to 13.5. The calculation formula of the CNB value is as follows:

[0026] CNB=Cr+6Si+4Mo+1.5W+5Nb+11V+12Al+9Ti-40C-2Mn-4Ni-1Cu-30N

[0027] (C) In this stage, the δ ferrite needs to achieve the following control targets: the total content of δ ferrite in the ingot should be less than 15%, and the length of a single δ ferrite should be less than 200 μm.

[0028] 2. In the second stage of the present invention, the morphology of delta ferrite needs to be regulated by homogenization treatment, so as to promote the dissolution phase transformation of part of delta ferrite produced by non-equilibrium phase transformation during solidification. The morphology of delta ferrite is transformed from irregular shape in cast state to spherical / ellipsoidal shape. The ratio of major axis to minor axis of the ellipsoid is less than 3. The content of delta ferrite is reduced to less than 10%.

[0029] When the homogenization temperature is low and the holding time is short, the non-equilibrium structure in the cast alloy cannot be fully restored, and the element segregation cannot be eliminated. However, when the solution temperature is too high and the holding time is long, the delta ferrite will further increase and become larger, making it difficult to eliminate it in subsequent treatment. In order to achieve the control target of delta ferrite in this step, the homogenization process of high silicon high chromium ferrite / martensite heat-resistant steel needs to be performed according to the following process:

[0030] The preferred homogenization temperature of the present invention is 950-1120° C. Further, when the homogenization temperature is greater than 1080° C., the homogenization time is greater than 3 h but less than 20 h, and when the homogenization temperature is less than 1080° C., the homogenization time is greater than 5 h but less than 40 h.

[0031] 3. The third stage of the present invention needs to achieve further optimization of the δ ferrite morphology and crushing of large-sized δ ferrite through a hot working process. The control target is to reduce the total content of δ ferrite to less than 6%. The δ ferrite of the alloy after forging or rolling will usually be distributed in strips along the forging / rolling direction, and the strip width is required to be less than 10μm. To achieve the above goals, the partial forging or rolling process of high-silicon high-chromium ferrite / martensite heat-resistant steel is defined as follows:

[0032] The holding temperature during hot working is less than 1120℃, and the holding time after the core reaches temperature is less than 5h; the hot working process can be returned to the furnace for re-firing, and the re-firing temperature is not higher than 1080℃, and the holding time after the core reaches temperature does not exceed 2h; the forging ratio / rolling ratio is ≥6.

[0033] 4. The fourth stage of the present invention is normalizing process control. δ ferrite is suppressed and reduced by recrystallization and growth of austenite phase during normalizing. The control target is to reduce δ ferrite to less than 1%. The heat treatment process of high chromium ferrite / martensite heat-resistant steel is usually divided into two steps: normalizing and tempering. The normalizing temperature is usually greater than 900°C, and the tempering temperature is usually lower than 800°C. The regulation of δ ferrite during the normalizing process determines the content of δ ferrite in the final service state organization. Based on this, for the normalizing process, the present invention proposes the following process limitations: the temperature range during normalizing is 960-1080°C, and the normalizing holding temperature is 10min-60min.

[0034] The advantages and beneficial effects of the present invention are that the control target of delta ferrite is divided into different process stages, thereby avoiding the influence of other properties of the alloy due to excessive composition adjustment. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is the delta ferrite in the as-cast structure of the alloy in Example 1;

[0036] Figure 2 It is the delta ferrite in the homogenized structure of the alloy in Example 1;

[0037] Figure 3 is the delta ferrite in the forged structure of the alloy in Comparative Example 1;

[0038] Figure 4 is the delta ferrite in the heat-treated structure of the alloy in Comparative Example 1;

[0039] Figure 5 is the delta ferrite in the as-cast structure of the alloy in Example 2;

[0040] Figure 6 is the delta ferrite in the heat-treated structure of the alloy in Example 2;

[0041] Figure 7 is the delta ferrite in the as-cast structure of the alloy in Comparative Example 1;

[0042] Figure 8 is the delta ferrite in the heat-treated structure of the alloy in Comparative Example 1;

[0043] Fig. 9 It is the delta ferrite in the homogenized structure of the alloy in Comparative Example 2;

[0044] Fig.10 is the delta ferrite in the heat-treated structure of the alloy in Comparative Example 2;

[0045] Fig.11 It is the delta ferrite in the heat-treated structure of the alloy in Comparative Example 3; DETAILED DESCRIPTION

[0046] The present invention is described in detail below through the accompanying drawings and embodiments.

[0047] For high-silicon high-chromium ferrite / martensite heat-resistant steel with Si content greater than 0.8% and Cr content greater than 10%, the precipitation tendency of the harmful phase δ ferrite is relatively high. If the precipitation of δ ferrite is suppressed only by composition regulation, the C content needs to be added to more than 0.21%, which will reduce the weldability of the material. In addition, it is necessary to limit the addition amount of elements such as Cr, Mo, W, V, Nb, etc. that are beneficial to the corrosion resistance and strength of the material but promote the precipitation of δ ferrite, and the final material has poor performance. To this end, the present invention proposes a method for reducing δ ferrite by coordinated regulation of composition and processing technology, and proposes control standards and control methods for the content and morphology of δ ferrite in the four stages of cast, homogenized, hot-worked and normalized, so as to achieve a significant reduction or elimination of δ ferrite in the final state of the material. The method disclosed in the present invention reduces the adverse effects of conventional composition regulation on mechanical properties and corrosion resistance, and obtains high-silicon high-chromium heat-resistant steel with excellent comprehensive performance. The following examples further illustrate the present invention, but the present invention is not limited thereto.

[0048] Embodiment 1:

[0049] This embodiment is a method for segmentally controlling delta ferrite in high silicon and high chromium heat-resistant steel, which is specifically as follows:

[0050] (1) In the composition regulation stage, the chemical composition is (wt.%):

[0051] C: 0.18%, Si: 1.2%, Mn: 0.7%, Cr: 10.8%, Ni: 1%, Mo: 1%, W: 1%, Nb: 0.4%, V: 0.1%, B: 0.003%, N: 0.03%, O: 0.003%; the balance is iron.

[0052] Its CNB=13.1, which is within the range of 11-14.

[0053] The smelting adopts 25kg vacuum induction furnace, and its as-cast structure is as follows Figure 1 As shown, the delta ferrite content is about 10%

[0054] (2) Homogenization stage: homogenization temperature is 1100 degrees Celsius, and the temperature is kept for 10 hours. After homogenization, the tissue is as follows Figure 2 As shown, the delta ferrite is spheroidized, with a total content of about 4%;

[0055] (3) The hot working process is: first, keep the temperature at 1100℃ for 1 hour, the forging ratio is greater than 3, and forge into a 35mm thick slab; then keep the temperature at 1100℃ for 40 minutes, and roll it into a 13mm thick plate in a two-roll mill; the microstructure after rolling is as follows Figure 3 As shown, the total content is about 3%;

[0056] (4) The rolled plate is heat treated as follows: normalizing, holding temperature 1050°C, holding time 45 min; tempering holding temperature 720°C, holding time 2.5 h. Figure 4 It is the microstructure after normalizing + tempering, with a delta ferrite content of less than 1%, occasional delta ferrite, and a total length of less than 10μm.

[0057] above Figures 1 to 4 It is proved that after adopting the segmented control method provided by the present invention, a microstructure with a delta ferrite content of less than 1% can be obtained for a high Si and high Cr alloy.

[0058] Embodiment 2:

[0059] (1) In the composition regulation stage, the chemical composition is (wt.%):

[0060] C: 0.16%, Si: 1.3%, Mn: 1.5%, Cr: 10.8%, Ni: 1.5%, Mo: 1%, W: 1%, Nb: 0.3%, V: 0.3%, B: 0.003%, N: 0.035%, O: 0.003%; the balance is iron.

[0061] Its CNB=12.65, which is within the range of 11-14.

[0062] The smelting adopts 25kg vacuum induction furnace, and its as-cast structure is as follows Figure 5 As shown, the delta ferrite content is about 6%;

[0063] (2) Homogenization stage: the homogenization temperature is 1080 degrees Celsius and the temperature is kept for 5 hours. After homogenization, the delta ferrite in the structure has been spheroidized and its total content is about 3%;

[0064] (3) The hot working process is the same as that in Example 1. The total content of delta ferrite in the microstructure after rolling is about 1%.

[0065] (4) Positive tempering is the same as in Example 1. Figure 6 This is the microstructure after normal tempering, and no delta ferrite is found.

[0066] Comparative Example 1:

[0067] (1) In the composition regulation stage, the chemical composition is (wt.%):

[0068] C: 0.16%, Si: 1.3%, Mn: 0.5%, Cr: 11.5%, Ni: 0.8%, Mo: 1%, W: 1%, Nb: 0.3%, V: 0.3%, B: 0.003%, N: 0.025%, O: 0.003%; the balance is iron.

[0069] Its CNB = 18.25, which is much larger than the range of 11 to 14. Its cast structure is as follows Figure 7 As shown, the delta ferrite content is about 30%;

[0070] (2) Homogenization stage: the homogenization temperature is 1080 degrees Celsius, and the temperature is kept for 5 hours. After homogenization, the delta ferrite in the structure has been spheroidized, and the total content is about 25%;

[0071] (3) The hot working process is the same as that in Example 1. The total content of delta ferrite in the microstructure after rolling is about 25%.

[0072] (4) The tempering process is the same as that in Example 1. Figure 8 It is the microstructure after normalizing + tempering, and its delta ferrite content is about 22%.

[0073] Comparative Example 2:

[0074] (1) In the composition control stage, the chemical composition, CNB value and as-cast structure are the same as those in Example 1.

[0075] (2) Homogenization stage: homogenization temperature is 1200 degrees Celsius, and the temperature is kept for 10 hours. After homogenization, the tissue is as follows Fig. 9 As shown, its delta ferrite increases again, and the total content reaches about 20%;

[0076] (3) The hot working process is the same as that of Example 1, and the delta ferrite content is about 16%

[0077] (4) The heat treatment process is the same as that in Example 1, and the final structure is as follows Fig.10 As shown, the delta ferrite content is about 12%.

[0078] Comparative Example 3:

[0079] (1) In the composition control stage, the chemical composition, CNB value and as-cast structure are the same as those in Example 1.

[0080] (2) homogenization stage, the homogenization process and organization are the same as those in Example 1;

[0081] (3) The thermal processing process is the same as that of Example 1;

[0082] (4) During heat treatment, the normalizing temperature is 1200℃ and the temperature is kept for 45 minutes. The final microstructure is as follows Fig.11 As shown, the delta ferrite content is about 10%, that is, some delta ferrite is precipitated in the alloy.

[0083] In the above embodiment 2 Figure 5 and Figure 6 Another example of the effectiveness of the method of the present invention is provided. Figure 7 and Figure 8 and Figure 1 to Figure 6The comparison proves the importance of the first-stage component control in the staged control method provided by the present invention. Fig. 9 and Fig.10 and Figure 1 to Figure 6 The comparison proves the importance of the second stage homogenization control in the staged control method provided by the present invention. Fig.11 and Figure 1 to Figure 6 The comparison proves the importance of the fourth stage normalizing process control in the staged control method provided by the present invention.

Claims

1. A method for segmentally regulating delta ferrite in high silicon and high chromium heat-resistant steel, characterized in that: The method is to control the delta ferrite of high-silicon high-chromium ferrite / martensite heat-resistant steel during its preparation process, wherein the control includes four stages: composition design control, homogenization control, hot working control and normalizing control, and each stage sets a reasonable delta ferrite content and morphology control target; The preparation process of the high-silicon high-chromium ferrite / martensite heat-resistant steel comprises the following steps: (1) Designing the chemical composition of high-silicon and high-chromium ferrite / martensite heat-resistant steel, smelting the ingredients according to the designed chemical composition, and casting the ingot; (2) Homogenization treatment: The ingot is subjected to homogenization treatment at a homogenization temperature of 950 to 1120°C; when the homogenization temperature is greater than 1080°C, the homogenization time is greater than 3 hours but less than 15 hours; when the homogenization temperature is less than 1080°C, the homogenization time is greater than 5 hours but less than 40 hours; (3) Hot deformation treatment: Forging, rolling, extrusion or pipe threading are used for hot deformation treatment; the holding temperature before deformation is less than 1120℃, and the holding time after reaching the temperature is less than 5h; the hot deformation process is returned to the furnace for re-firing, the temperature during re-firing is not higher than 1080℃, and the holding time after reaching the temperature is not more than 2h; the deformation ratio is ≥6; (4) Heat treatment: The temperature control range during normalizing is 960-1080°C, and the normalizing holding time is 10-60 minutes; The high-silicon high-chromium ferrite / martensite heat-resistant steel contains Cr≥10wt.%, Si≥0.8wt.%, and C≤0.2wt.%; the heat-resistant steel also contains Mn, Ni, Mo, W, Nb, V, N, and B elements; The chemical composition of the high silicon high chromium ferrite / martensite heat resistant steel comprises C 0.14-0.20%, Si 0.8-2.0%, Mn 0.5-3.0%, Cr 10.0-12.5%, Ni 0.5-1.5%, Mo 0.5-1.6%, W 0.5-1.5%, Nb 0.1-0.4%, V 0.1-0.35%, B≤0.005%, N≤0.05%, and the remainder is iron and other unavoidable residual elements, and 11<CNB value≤13.5, and the CNB value is calculated according to the following formula (1), wherein the symbol of each element in the formula (1) represents the mass percentage of the corresponding element in the high silicon high chromium ferrite / martensite heat resistant steel: CNB=Cr+6Si+4Mo+1.5W+5Nb+11V+12Al+9Ti-40C-2Mn-4Ni-1Cu-30N (1).

2. The method for segmentally controlling delta ferrite in high silicon and high chromium heat resistant steel according to claim 1, characterized in that: The high-silicon high-chromium ferrite / martensite heat-resistant steel component system should have a temperature zone in which delta ferrite is completely transformed into austenite in a thermodynamically stable state.

3. The method for segmentally controlling delta ferrite in high silicon and high chromium heat resistant steel according to claim 1, characterized in that: The control objectives achieved at each stage of this method are as follows: (a) after the chemical composition of the high silicon high chromium ferrite / martensite heat resistant steel is designed, the total content of delta ferrite in the cast ingot is less than 15%, and the length of a single delta ferrite is less than 200 μm; (b) After homogenization, the delta ferrite morphology should be spheroidized / ellipsoidal, the major axis to minor axis ratio should be less than 3, and the total delta ferrite content should be less than 12%; (c) After heat treatment, the delta ferrite content is less than 7%, and the width of a single delta ferrite is less than 10 μm; (d) After heat treatment, the delta ferrite content is less than 1%, and the length of a single delta ferrite is less than 10 μm.

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