A martensitic heat-resistant steel and a smelting method thereof

Abstract

The invention belongs to the technical field of casting and mainly relates to a martensitic heat-resistant steel. The mass percentage content of chemical components comprises: 0.11%-0.14% of C, 0.12%-0.24% of Si, 0.42%-0.58% of Mn, 9.2%-10.0% of Cr, 0.55%-0.75% of Mo, 1.65%-1.85% of W, 1.7%-2.2% of Co, 0.016%-0.024% of N, 0.12%-0.28% of Ni, 0.17%-0.23% of V, 0.03%-0.07% of Nb and 0.005%-0.009% of B. The martensitic heat-resistant steel disclosed in the present invention is a new type of material. In terms of composition design, the Mo content is appropriately reduced, W is added to form a W-Mo composite solid solution strengthening mainly composed of W; the Co content is increased, the proportion of austenite forming elements in the composition is increased, and high-temperature δ-ferrite is avoided in the metallographic structure. Compared with traditional martensitic heat-resistant materials, the martensitic heat-resistant steel disclosed in the present invention has better high-temperature mechanical properties and corrosion resistance, and lower production cost.

Description

Technical Field

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

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

[0007] A martensitic heat-resistant steel, the chemical components by weight percentage comprising:

[0008] 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.

[0009] Further, Creq is 6% to 7%;

[0010] Among them, Creq=Cr+6Si+4Mo+11V+1.5W+5Nb-40C-2Mn-4Ni-2Co-30N.

[0011] Furthermore, the mass percentage content of chemical components includes:

[0012] 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.

[0013] The above-mentioned method for smelting the martensitic heat-resistant steel comprises, in sequence, an EAF furnace smelting step, an LF1 furnace smelting step, a VOD furnace smelting step and a LF2 furnace smelting step.

[0014] Furthermore, in the EAF furnace smelting step, the charge includes scrap steel and carbon steel return material, and 30kg to 60kg of lime is added per ton of steel; oxygen blowing decarburization is performed, and steel is tapped when the C content is 0.06% to 0.15% and the P content is ≤0.005%, and a silicon-calcium-manganese-rare-earth-barium composite deoxidizer and / or a synthetic refining agent is added during the tapping process; the addition amount of the silicon-calcium-manganese-rare-earth-barium composite deoxidizer is 2kg to 4kg per ton of molten steel, and the addition amount of the synthetic refining agent is 3kg to 5kg per ton of molten steel.

[0015] Furthermore, in the LF1 furnace smelting step, low carbon ferrochrome, electrolytic cobalt and ferromolybdenum are added in batches, Cr, Co and Mo are adjusted to be close to the lower limit, the C content is controlled between 0.3% and 0.6%, and the S content is controlled to be ≤0.007%.

[0016] Furthermore, in the VOD furnace smelting step, including the oxygen blowing decarburization stage and the vacuum carbon deoxidation stage, the C content is controlled to be ≤0.06%.

[0017] Furthermore, the LF2 furnace smelting step sequentially includes a reduction stage, a deep deoxidation stage and an alloy composition precise adjustment stage.

[0018] Furthermore, in the reduction stage, a reduction process using aluminum, silicon-calcium particles, or silicon-calcium-manganese-rare-earth-barium composite deoxidizer is adopted.

[0019] Furthermore, in the deep deoxidation stage, calcium silicon wire and aluminum particles are fed, the feeding amount of the calcium silicon wire is 1kg to 3kg per ton of molten steel, and the feeding amount of the aluminum particles is 0.5kg per ton of molten steel.

[0020] It can be seen from the above technical solution that the advantages and positive effects of the present invention are:

[0021] The martensitic heat-resistant steel disclosed in the present invention, in terms of composition design, appropriately reduces the Mo content, adds W to form a W-Mo composite solid solution strengthening with W as the main component; increases the Co content, improves the proportion of austenite forming elements in the composition, and avoids the appearance of high-temperature δ-ferrite in the metallographic structure. The martensitic heat-resistant steel disclosed in the present invention is a new type of heat-resistant steel material. Compared with traditional martensitic heat-resistant steel materials, such as CB2, the martensitic heat-resistant steel of the present invention has better high-temperature mechanical properties and corrosion resistance, and has lower production costs.

[0022] The smelting method of martensitic heat-resistant steel disclosed in the present invention sequentially adopts EAF furnace smelting step, LF1 furnace smelting step, VOD furnace smelting step and LF2 furnace smelting step, wherein the deoxidation process of the EAF furnace tapping process and the LF deoxidation reduction process can make the oxygen content in the molten steel less than or equal to 30ppm.

[0023] The martensitic heat-resistant steel disclosed in the present invention has a tensile strength of 720Mpa-850Mpa, a yield strength of 500Mpa-600Mpa, a cross-sectional shrinkage of 50%-65%, and an elongation of 19%-31% at room temperature after a heat treatment of primary normalizing at 1130-1180°C + secondary normalizing at 1080-1120°C + tempering at 720-750°C; the tensile strength at 650°C high temperature reaches 300Mpa-400Mpa; the impact energy (AKv) at room temperature reaches 30J-60J; and the high-temperature endurance can maintain a loading time of more than 200h before breaking under the stress conditions of 630°C and 198Mpa. DETAILED DESCRIPTION

[0024] 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.

[0025] The percentages, % and fractions in the examples are calculated by mass unless otherwise specified.

[0026] 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.

[0027] The embodiment of the present invention discloses a martensitic heat-resistant steel, the composition design 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.

[0028] Compared with traditional martensitic heat-resistant steels such as CB2, the martensitic heat-resistant steel disclosed in the present invention has a composition design concept of appropriately reducing the Mo content, adding W and forming a W-Mo composite solid solution strengthening mainly composed of W. Under the condition of the same Mo equivalent (Mo equivalent = Mo + W / 2), the higher the W content, the better the high-temperature performance; increasing the Co content, improving the proportion of austenite forming elements in the composition, avoiding the appearance of high-temperature δ-ferrite in the metallographic structure, and improving the mechanical properties of the material.

[0029] The functions and composition designs of the elements of the martensitic heat-resistant steel disclosed in the present invention are as follows:

[0030] 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%.

[0031] 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%.

[0032] 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%.

[0033] 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%.

[0034] 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%.

[0035] 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%.

[0036] 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%.

[0037] 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%.

[0038] 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%.

[0039] 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%.

[0040] 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%.

[0041] 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%.

[0042] Creq, Creq is often used to calculate the ferrite content in materials such as stainless steel.

[0043] In this embodiment, Creq=Cr+6Si+4Mo+11V+1.5W+5Nb-40C-2Mn-4Ni-2Co-30N.

[0044] 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%.

[0045] In the embodiments disclosed in the present invention, according to the above material composition ratio, considering that the martensitic heat-resistant steel material in the embodiments of the present invention has many elements that need to be controlled, and the Si and Al contents in the steel affect the T[O] content of the molten steel, which is one of the key points for controlling the quality of the molten steel. Combined with the foundry smelting equipment and process methods, after comprehensive consideration, the smelting process method of EAF furnace smelting → LF1 furnace smelting → VOD furnace smelting → LF2 furnace smelting → steel pouring is adopted.

[0046] The smelting process is as follows:

[0047] The oxidation smelting of EAF furnace is mainly to provide crude molten steel for the subsequent LF1 furnace, that is, to melt low C and low P molten steel. According to the residual element requirements, Ni≤0.28%, P≤0.019%, the furnace charge can mainly be scrap steel and carbon steel return material, the addition ratio of the two can be 7:3; the amount of lime added is 30-60kg / t steel. The electric furnace controls the decarburization amount to 0.4%-0.6%. At the end of oxidation, the stirring power of the molten steel in the electric furnace can be increased by inserting double-tube deep oxygen blowing into the molten steel, promoting the carbon-oxygen reaction and generating a large number of CO bubbles, so as to achieve the purification effect of degassing and removing inclusions. Oxygen blowing and decarburization can be started when the molten steel temperature is ≥1620℃, and the decarburization rate is 0.1-0.15% / min. During the decarburization process, the furnace door is tilted to perform slag flow operation. After the oxygen blowing is completed, samples are taken to analyze the C / P content. The end point C content is 0.06%-0.15%) and the P content is ≤0.005%, which meets the requirements and can be tapped from the EAF furnace. During the tapping process of the EAF furnace, 2-4kg / t of silicon-calcium-manganese-rare-earth-barium composite deoxidizer and 3-5kg / t of synthetic refining agent are added to remove most of the free oxygen and other inclusions in the crude steel liquid.

[0048] After the molten steel arrives at the LF1 furnace station, it is mainly subjected to preliminary reduction and alloying operations. Lime and fluorite are added to make slag, and Al is added for preliminary reduction; low-carbon ferrochrome, electrolytic cobalt, and ferromolybdenum are added in small batches, and Cr, Co, and Mo are adjusted to near the lower limit, and the C content is controlled between 0.3% and 0.6%, and the S content is ≤0.007%. When the LF1 furnace is mixed with alloys such as W and Co, due to the large amount of alloy added and W not only has a high melting point, but also a very large density, which is about 2.5 times the density of Fe, the problem of slow dissolution of W and Co should be solved by increasing the alloy addition temperature, i.e. ≥1620℃), increasing the argon gas flow rate, i.e. ≥350NL / min, and adding alloys in small batches. When all components meet the requirements, they can be transferred to the VOD furnace.

[0049] VOD refining furnace smelting can remove carbon from molten steel to a very low level under vacuum conditions, and control the C content to ≤0.06% after two stages of oxygen decarburization and vacuum carbon deoxidation. Before entering the VOD furnace, the C content of the molten steel is controlled at 0.3-0.6%, the Al content is controlled at 0.02-0.04%, and the lower the sulfur content, the better. The temperature of the molten steel is 1600℃-1640℃, and the VOD furnace maintains a high vacuum degree of ≤67Pa, and the carbon deoxidation time is ≥15min. During this period, the argon flow rate is increased to promote carbon-oxygen reaction and further degassing and inclusion removal; after breaking the vacuum, slag-making materials such as CaO, fluorite, Al particles, ferrotitanium, and calcium silicon can be added, and the reduction time is maintained at ≥10min.

[0050] After the molten steel is transported from the VOD furnace to the LF2 furnace, the reduction process of Al, silicon calcium particles, silicon calcium manganese rare earth barium composite deoxidizer is adopted in the pre-reduction period. After the initial reduction, 1-3kg / t of silicon calcium wire and 0.5kg / t of Al particles are fed for deep deoxidation, and the Al content is controlled to be ≤0.01%, and the Ti content is ≤0.020%. After the slag changes from black to yellow and then to white and is maintained for a certain period of time, ferroboron and ferroniobium can be added to adjust the content of Nb and B. The refining time is extended to more than 60 minutes under low argon supply intensity and small flow rate. Under the condition of argon soft blowing for a certain period of time, most of the inclusions in the steel are ensured to float up. After the alloy components are accurately adjusted to their control range and the temperature reaches the requirements, it can be transferred to the casting process.

[0051] The invention discloses a smelting method of martensitic heat-resistant steel. Through the above control measures, the total oxygen content in the molten steel can be finally reduced to below 30ppm, the inclusion area content can be below 0.1%, and the quality of the molten steel meets the casting requirements of casting materials under high temperature service conditions of ultra-supercritical units.

[0052] The martensitic heat-resistant steel casting formed after pouring, after heat treatment at 1130℃~1180℃ primary normalizing + 1080℃~1120℃ secondary normalizing + 720℃~750℃ tempering, has a tensile strength of 720Mpa~850Mpa, a yield strength of 500Mpa~600Mpa, a cross-sectional shrinkage of 50%~65%, and an elongation of 19%~31% at room temperature; the high-temperature tensile strength at 650℃ reaches 300Mpa~400Mpa; the room-temperature impact energy (AKv) reaches 30J~60J; under the high-temperature endurance condition of 630℃ and 198Mpa stress, the loading time can be maintained for more than 200h before breaking.

[0053] 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 martensitic heat-resistant steel, characterized in that: The mass percentage of chemical components, including: 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.

2. The martensitic heat-resistant steel according to claim 1, characterized in that: Creq is 6% to 7%; Creq=Cr+6Si+4Mo+11V+1.5W+5Nb-40C-2Mn-4Ni-2Co-30N.

3. The martensitic heat-resistant steel according to claim 1, characterized in that: The mass percentage of chemical components, including: 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.

4. A method for smelting martensitic heat-resistant steel according to any one of claims 1 to 3, characterized in that: The process includes an EAF furnace melting step, an LF1 furnace melting step, a VOD furnace melting step and a LF2 furnace melting step in sequence.

5. The method for smelting martensitic heat-resistant steel according to claim 4, characterized in that: In the EAF furnace smelting step, the furnace charge includes scrap steel and carbon steel return material, and 30kg to 60kg of lime is added per ton of steel; Oxygen blowing decarburization, steel is tapped when the C content is 0.06% to 0.15% and the P content is ≤0.005%, and silicon-calcium-manganese-rare-earth-barium composite deoxidizer and / or synthetic refining agent is added during the tapping process; The addition amount of the silicon-calcium-manganese-rare-earth-barium composite deoxidizer is 2kg-4kg per ton of molten steel, and the addition amount of the synthetic refining agent is 3kg-5kg per ton of molten steel.

6. The method for smelting martensitic heat-resistant steel according to claim 4, characterized in that: In the LF1 furnace smelting step, low carbon ferrochrome, electrolytic cobalt and ferromolybdenum are added in batches, Cr, Co and Mo are adjusted to be close to the lower limit, the C content is controlled between 0.3% and 0.6%, and the S content is controlled to be ≤0.007%.

7. The method for smelting martensitic heat-resistant steel according to claim 4, characterized in that: In the VOD furnace smelting step, including the oxygen blowing decarburization stage and the vacuum carbon deoxidation stage, the C content is controlled to be ≤0.06%.

8. The method for smelting martensitic heat-resistant steel according to claim 4, characterized in that: The LF2 furnace smelting steps include a reduction stage, a deep deoxidation stage and an alloy composition precise adjustment stage in sequence.

9. The method for smelting martensitic heat-resistant steel according to claim 8, characterized in that: In the reduction stage, a reduction process using aluminum, silicon calcium particles, and silicon calcium manganese rare earth barium composite deoxidizer is adopted.

10. The method for smelting martensitic heat-resistant steel according to claim 8, characterized in that: In the deep deoxidation stage, calcium silicon wire and aluminum particles are fed, the feeding amount of the calcium silicon wire is 1kg to 3kg per ton of molten steel, and the feeding amount of the aluminum particles is 0.5kg per ton of molten steel.