A heat treatment method for martensitic heat-resistant steel

Abstract

The invention belongs to the field of casting technology, and mainly relates to a heat treatment method for martensitic heat-resistant steel, which includes primary normalizing, secondary normalizing and tempering in sequence; primary normalizing, the normalizing temperature is 1130℃~1180℃, the holding time is equal to the heat penetration time of the casting + 5h~8h, and the cooling method is air cooling; secondary normalizing, the normalizing temperature is 1080℃~1120℃, the holding time is equal to the heat penetration time of the casting, and the cooling method is air cooling; tempering, the tempering temperature is 720℃~750℃, the holding time is equal to the heat penetration time of the casting + 3h~5h, and the cooling method is furnace cooling. The heat treatment method for martensitic heat-resistant steel disclosed in the invention is designed to match the composition and performance index of the new martensitic heat-resistant steel with the heat treatment process; effectively prevent the material from producing defects such as component segregation, uneven organization, and large residual stress; adjust the microstructure of the material, and improve the room temperature and high temperature mechanical properties, corrosion resistance and material stability of the material.

Description

Technical Field

[0001] The invention belongs to the technical field of casting, and mainly relates to a heat treatment method for martensitic heat-resistant steel. Background Art

[0002] As an efficient and clean power generation technology, the selection and application of key heat-resistant materials for ultra-supercritical units are the key to technological development. Ultra-supercritical units increase the pressure and temperature of steam to further improve the thermal efficiency and economy of the unit. However, with the continuous improvement of steam parameters, more stringent requirements are placed on the heat resistance, corrosion resistance and mechanical properties of unit materials. Therefore, the selection and application of key heat-resistant materials have become one of the core issues in the development of ultra-supercritical unit technology.

[0003] Ultra-supercritical units mainly use key heat-resistant materials such as martensitic heat-resistant steel, austenitic heat-resistant steel and nickel-based alloys. Among them, austenitic heat-resistant steel and nickel-based alloys are expensive, so their application is limited. The commonly used materials for steam turbine castings are martensitic heat-resistant steel, such as C91, COSTE, CB2, etc.

[0004] At present, the casting material process for ultra-supercritical units with higher temperature service conditions than steam turbine castings is still imperfect. Therefore, the research and development of new martensitic heat-resistant materials and heat treatment methods with low cost and better performance than C91, COSTE, CB2 and other materials has become a top priority. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a heat treatment method for martensitic heat-resistant steel.

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

[0007] A heat treatment method for martensitic heat-resistant steel comprises primary normalizing, secondary normalizing and tempering in sequence; in primary normalizing, the normalizing temperature is 1130°C to 1180°C, the holding time is equal to the heat-through time of the casting + 5h to 8h, and the cooling method is air cooling; in secondary normalizing, the normalizing temperature is 1080°C to 1120°C, the holding time is equal to the heat-through time of the casting, and the cooling method is air cooling; in tempering, the tempering temperature is 720°C to 750°C, the holding time is equal to the heat-through time of the casting + 3h to 5h, and the cooling method is furnace cooling.

[0008] Furthermore, the heat penetration time calculation formula of the casting is:

[0009]

[0010] In the formula,

[0011] t is the heat penetration time of the casting, unit: h;

[0012] h is the maximum wall thickness of the casting, in mm;

[0013] b is a constant, b = 25.4 mm / in;

[0014] a is a constant, a=1in / h.

[0015] Furthermore, in the primary normalizing step, the heating rate is ≤60°C / h.

[0016] Furthermore, in the secondary normalizing step, the heating rate is ≤60°C / h, and the cooling rate is ≥50°C / h.

[0017] Furthermore, in the tempering step, the heating rate is ≤60°C / h, and the cooling rate is ≤50°C / h.

[0018] Furthermore, the chemical components of the martensitic heat-resistant steel include:

[0019] 0.11%~0.14% C, 0.12%~0.24% Si, 0.42%~0.58% Mn, 9.2%~10.0% Cr, 0.55%~0.75% Mo, 1.65%~1.85% W, 1.7%~2.2% Co, 0.016%~0.024% N, 0.12%~0.28% Ni, 0.17%~0.23% V, 0.03%~0.07% Nb, 0.005%~0.009% B.

[0020] Furthermore, the Creq of the martensitic heat-resistant steel is 6% to 7%;

[0021] Creq=Cr+6Si+4Mo+11V+1.5W+5Nb-40C-2Mn-4Ni-2Co-30N.

[0022] Preferably, the mass percentage content of the chemical components of the martensitic heat-resistant steel includes:

[0023] 0.12% C, 0.15% Si, 0.50% Mn, 9.5% Cr, 0.65% Mo, 1.75% W, 2.0% Co, 0.018% N, 0.20% Ni, 0.20% V, 0.05% Nb, 0.006% B.

[0024] Furthermore, the room temperature mechanical properties of the martensitic heat-resistant steel include:

[0025] The tensile strength is 720Mpa~850Mpa, the yield strength is 500Mpa~600Mpa, the cross-sectional shrinkage is 50%~5%, the elongation after fracture is 19%~31%; the impact absorption energy is 30J~60J.

[0026] Furthermore, the martensitic heat-resistant steel has a high-temperature tensile strength of 300 MPa to 400 MPa at 650°C, and can maintain a loading time of more than 200 hours under high-temperature durability conditions of 630°C and 198 MPa.

[0027] It can be seen from the above technical solution that the advantages and positive effects of the present invention are:

[0028] The heat treatment method of martensitic heat-resistant steel disclosed in the present invention designs a heat treatment process that matches the composition and performance indicators of the new martensitic heat-resistant steel; effectively prevents the material from having defects such as component segregation, uneven structure, and large residual stress; adjusts the material's microstructure, and improves the material's room temperature and high temperature mechanical properties, corrosion resistance, and material stability. DETAILED DESCRIPTION

[0029] The following specific examples further illustrate the present invention so that those skilled in the art can better understand the present invention and implement it, but the examples are not intended to limit the present invention. According to the common technical knowledge and customary means in this field, without departing from the above-mentioned basic technical concept of the present invention, other various forms of modification, replacement or change made to the above-mentioned structure of the present invention should all fall within the protection scope of the present invention.

[0030] The percentages, % and fractions in the examples are calculated by mass unless otherwise specified.

[0031] The embodiments of the present invention are the results of producing the same model of products using raw materials from different batches. Due to the slight differences between the raw materials of each batch, there will be slight fluctuations in the values ​​in the embodiments. However, the effects can be achieved as long as they are within the listed ranges.

[0032] The embodiment of the present invention discloses a martensitic heat-resistant steel, the composition design of which ensures that the steel can still maintain sufficient strength and stability in a high temperature environment, which is achieved by adding alloy elements that can improve the strength of the solid solution and form stable compounds. The corrosion resistance of the steel is taken into consideration during the design for the specific service environment of the material. By adding elements that can form a dense oxide film or improve the passivation ability, the corrosion resistance of the steel is effectively improved. Good plasticity and toughness help the material resist fracture and deformation during processing and service. When designing the composition of the present invention, the content and proportion of alloy elements are reasonably controlled to optimize the plasticity and toughness of the steel. In addition, under the premise of ensuring performance, the present invention reduces the content of alloy elements as much as possible to reduce production costs and improve performance-price ratio.

[0033] In the embodiments disclosed in the present invention, compared with traditional martensitic heat-resistant steels such as CB2, the composition design concept of the martensitic heat-resistant steel is to appropriately reduce the Mo content, add W and form a W-Mo composite solid solution strengthening mainly composed of W. When the Mo equivalent (Mo equivalent = Mo + W / 2) is the same, the higher the W content, the better the high-temperature performance; increase the Co content, improve the proportion of austenite forming elements in the composition, avoid the appearance of high-temperature δ-ferrite in the metallographic structure, and improve the mechanical properties of the material.

[0034] The functions and composition designs of the elements of the martensitic heat-resistant steel disclosed in the present invention are as follows:

[0035] Cr element. Adding enough Cr element to martensitic heat-resistant steel material can obtain sufficient high-temperature strength and corrosion resistance, but too high Cr will cause the casting to generate δ-ferrite due to excessive ferrite-forming elements during solidification, forming soft spots in the structure, reducing the mechanical properties of the material; and promote the formation of harmful phase Z phase during high-temperature service. Therefore, the Cr content needs to be within a suitable range. The Cr content in the martensitic heat-resistant steel of the present invention is controlled in the range of 9.2% to 10.0%, and the optimal is 9.5%.

[0036] Mo element, Mo element plays an important role in solid solution strengthening and precipitation strengthening in the material. Under high temperature service conditions, Mo is an important component element for forming Laves phase. The dispersed precipitated Laves phase improves the high temperature performance of the material, but the Laves phase is easy to coarsen and transform from a performance strengthening phase to a harmful phase. The content of Mo element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.55% to 0.75%, and the optimal content is 0.65%.

[0037] W element, W element is a heavy metal element, and its solid solution strengthening effect is obvious. Compared with materials such as CB2, the present invention adds W element. Due to the addition of W element, when serving at high temperature, it will promote the formation of Laves phase and inhibit the coarsening of Laves phase. W element is the main element to improve the creep strength of materials, but if the W content is too high, it will promote the precipitation of δ-ferrite and form soft spots of the material. The content control range of W element in the martensitic heat-resistant steel of the present invention is 1.65% to 1.85%, and the optimal is 1.75%.

[0038] Co element, adding W and / or Mo elements to martensitic heat-resistant steel will promote the formation of δ-ferrite. The austenite-forming element Co is added to the martensitic heat-resistant steel of the present invention to inhibit the formation of δ-ferrite. While Co strengthens the solid solution, it can increase the atomic activity of Cr and Al and improve the oxidation resistance. However, if the Co content is too high, it may reduce the solid solubility limit of Mo and W, resulting in the premature precipitation of the Laves phase. The content of the Co element in the martensitic heat-resistant steel of the present invention is controlled in the range of 1.7% to 2.2%, and the optimal content is 2.0%.

[0039] C element, in martensitic heat-resistant steel, C element is an important forming element of M23C6 and MX, and plays a role in solid solution strengthening in the form of interstitial solid solution, but excessive C will make the steel brittle; and in order to obtain good material welding performance, the C content must be strictly controlled. The content control range of C element in the martensitic heat-resistant steel of the present invention is 0.11% to 0.14%, and the optimal is 0.12%.

[0040] N element is also an important element of MX strengthening phase, but N element will also promote the transformation of MX to Z phase which is easy to coarsen. Therefore, too high N content can improve strength in early stage or at lower temperature, but too high N content will cause serious creep rupture strength loss in long term or high temperature environment. The content of N element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.016% to 0.024%, and the optimum is 0.018%.

[0041] Ni element can improve the toughness and low temperature performance of steel. In martensitic heat-resistant steel, an appropriate amount of Ni can significantly improve the plasticity and toughness of the steel. The content of Ni element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.12% to 0.28%, and the optimal content is 0.20%.

[0042] Mn element, as a deoxidizer and desulfurizer, can improve the internal quality of steel. At the same time, Mn is also one of the elements that form austenite, which has a certain influence on the organizational structure of steel. The content of Mn element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.42% to 0.58%, and the optimal content is 0.50%.

[0043] V element is a strong carbide and nitride forming element, which is distributed in the martensite matrix as fine dispersed carbide and nitride particles, playing a role of precipitation strengthening. The content of V element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.17% to 0.23%, and the optimal content is 0.20%.

[0044] Nb element, similar to V element, is also a strong carbide forming element, which helps to improve the high temperature strength and creep resistance of steel. The content of Nb element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.03% to 0.07%, and the optimal content is 0.05%.

[0045] B element, adding a trace amount of B element to the material can form grain boundary strengthening, significantly improve the creep performance of the material, improve the grain boundary bonding force, hinder the coarsening of grain boundary carbides, and effectively improve the alloy welding performance. The content of B element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.005% to 0.009%, and the optimal content is 0.006%.

[0046] Si element can improve the strength of ferrite, improve the corrosion resistance and casting performance of the material, but Si element will reduce toughness. The content of Si element in the martensitic heat-resistant steel of the present invention is controlled in the range of 0.12% to 0.24%, and the optimal content is 0.15%.

[0047] Creq, Creq is often used to calculate the ferrite content in materials such as stainless steel.

[0048] In this embodiment, Creq=Cr+6Si+4Mo+11V+1.5W+5Nb-40C-2Mn-4Ni-2Co-30N.

[0049] By adjusting the content of each alloy element in the material, the value of Creq can be controlled, thereby achieving accurate control of the ferrite content. The control range of Creq in the martensitic heat-resistant steel of the present invention is 6% to 7%, and the optimal value is 6.8%.

[0050] In the embodiments disclosed in the present invention, in order to eliminate the casting defects of the martensitic heat-resistant steel casting and improve the comprehensive performance of the martensitic heat-resistant steel casting, a heat treatment process is designed according to the specific material composition ratio of the martensitic heat-resistant steel in the above embodiments. The heat treatment process flow is primary normalizing → secondary normalizing → tempering. Specifically, the process design of each heat treatment step is as follows:

[0051] One normalizing:

[0052] The purpose of primary normalizing is to eliminate the unfavorable structure of the cast state and make the material composition fully uniform. The process parameters of primary normalizing are to slowly heat up to 1130℃~1180℃ at a heating rate of ≤60℃ / h, keep warm for a period of time, and then air cool to below 150℃. The holding time of primary normalizing can be extended by 5h~8h on the basis of the heat penetration time of the casting, that is, the holding time of primary normalizing = the heat penetration time of the casting + 5h~8h.

[0053] Secondary normalizing:

[0054] After the first normalizing, due to the high austenitizing temperature and long holding time, the original austenite grains are coarse and the martensite laths are thicker. Therefore, a secondary low-temperature normalizing is required to refine the grains and further improve the strength and plasticity.

[0055] The process parameters of secondary normalizing are to slowly heat up to 1080℃~1120℃ at a heating rate of ≤60℃ / h, keep warm for a period of time, and then cool to below 80℃ at a cooling rate of ≥50℃ / h. The holding time of secondary normalizing is sufficient to ensure that the casting is heated through, that is, the holding time of secondary normalizing = the heat-through time of the casting.

[0056] Tempering:

[0057] The structure of the material after two normalizing is lath martensite. Tempering can decompose the lath martensite structure with higher stress to balance the strength, hardness and plasticity of the material.

[0058] The process parameters of tempering are set to slowly increase the temperature to 720℃~750℃ at a heating rate of ≤60℃ / h, and then cool the furnace to below 200℃ at a cooling rate of ≤50℃ / h after a period of heat preservation. The holding time of tempering is 3h~5h more than the heat penetration time of the casting to ensure that the decomposition of the core martensite structure is complete; that is, the tempering holding time = the heat penetration time of the casting + 3h~5h.

[0059] In the embodiments disclosed in the present invention, the heat penetration time of the casting can be calculated by the wall thickness of the casting. Specifically, the heat penetration time calculation formula of the casting is:

[0060]

[0061] In the formula,

[0062] t is the heat penetration time of the casting, unit: h;

[0063] h is the maximum wall thickness of the casting, in mm;

[0064] b is a constant, b = 25.4 mm / in;

[0065] a is a constant, a=1in / h.

[0066] During the casting process, martensitic heat-resistant steel castings are prone to internal structural defects due to factors such as cooling rate and metal shrinkage, such as component segregation, uneven structure, and large residual stress. The present invention develops a new martensitic heat-resistant steel material through reasonable component design, and designs the above-mentioned heat treatment process for the material. In this embodiment, the martensitic heat-resistant steel castings after casting and heat treatment are tested, and the room temperature mechanical properties of the castings are as follows:

[0067] The tensile strength is 720Mpa~850Mpa, the yield strength is 500Mpa~600Mpa, the cross-sectional shrinkage is 50%~5%, the elongation after fracture is 19%~31%; the impact absorption energy is 30J~60J.

[0068] The high temperature mechanical properties of castings are:

[0069] The tensile strength at 650℃ is 300Mpa~400Mpa; the high temperature endurance test can maintain the loading time for more than 200h under the conditions of 630℃ and 198Mpa stress.

[0070] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A heat treatment method for martensitic heat-resistant steel, characterized in that: It includes primary normalizing, secondary normalizing and tempering in sequence; One normalizing, the normalizing temperature is 1130℃~1180℃, the holding time = the heat penetration time of the casting + 5h~8h, and the cooling method is air cooling; Secondary normalizing, normalizing temperature is 1080℃~1120℃, holding time = heat penetration time of casting, cooling method is air cooling; Tempering: the tempering temperature is 720℃~750℃, the holding time = the heat penetration time of the casting + 3h~5h, and the cooling method is furnace cooling.

2. The heat treatment method of martensitic heat-resistant steel according to claim 1, characterized in that: The heat penetration time calculation formula of the casting is: In the formula, t is the heat penetration time of the casting, unit: h; h is the maximum wall thickness of the casting, in mm; b is a constant, b = 25.4 mm / in; a is a constant, a=1in / h.

3. The heat treatment method of martensitic heat-resistant steel according to claim 1, characterized in that: In the primary normalizing step, the heating rate is ≤60°C / h.

4. The heat treatment method of martensitic heat-resistant steel according to claim 1, characterized in that: In the secondary normalizing step, the heating rate is ≤60°C / h, and the cooling rate is ≥50°C / h.

5. The heat treatment method of martensitic heat-resistant steel according to claim 1, characterized in that: In the tempering step, the heating rate is ≤60°C / h, and the cooling rate is ≤50°C / h.

6. The heat treatment method of martensitic heat-resistant steel according to any one of claims 1 to 5, characterized in that: The mass percentage content of the chemical components of the martensitic heat-resistant steel includes: 0.11%~0.14% C, 0.12%~0.24% Si, 0.42%~0.58% Mn, 9.2%~10.0% Cr, 0.55%~0.75% Mo, 1.65%~1.85% W, 1.7%~2.2% Co, 0.016%~0.024% N, 0.12%~0.28% Ni, 0.17%~0.23% V, 0.03%~0.07% Nb, 0.005%~0.009% B.

7. The heat treatment method of martensitic heat-resistant steel according to claim 6, characterized in that: The Creq of the martensitic heat-resistant steel is 6% to 7%; Creq=Cr+6Si+4Mo+11V+1.5W+5Nb-40C-2Mn-4Ni-2Co-30N.

8. The heat treatment method of martensitic heat-resistant steel according to claim 6, characterized in that: The mass percentage content of the chemical components of the martensitic heat-resistant steel includes: 0.12% C, 0.15% Si, 0.50% Mn, 9.5% Cr, 0.65% Mo, 1.75% W, 2.0% Co, 0.018% N, 0.20% Ni, 0.20% V, 0.05% Nb, 0.006% B.

9. The heat treatment method of martensitic heat-resistant steel according to claim 6, characterized in that: The room temperature mechanical properties of the martensitic heat-resistant steel include: The tensile strength is 720Mpa~850Mpa, the yield strength is 500Mpa~600Mpa, the cross-sectional shrinkage is 50%~5%, the elongation after fracture is 19%~31%; the impact absorption energy is 30J~60J.

10. The heat treatment method of martensitic heat-resistant steel according to claim 9, characterized in that: The martensitic heat-resistant steel has a high-temperature tensile strength of 300Mpa to 400Mpa at 650°C; the high-temperature endurance is maintained at a stress of 630°C and 198Mpa for more than 200 hours.

Images