A martensitic heat-resistant steel and a method for manufacturing the same

Abstract

The application discloses a kind of martensitic heat-resistant steel and its manufacturing method, and its component weight percentage is as follows: C 0.07-0.15%, Si 0.11-0.50%, Mn 0.5-1.0%, Cr 8.0-12.5%, W 0.6-1.4%, Mo 2.1-3.5%, Co 4.1-6.0%, Nb 0.11-0.50%, V 0.36-0.80%, Cu 0.5-1.2%, N 0.003-0.020%, B 0.005-0.020%, S≤0.01%, P≤0.02%, Ni≤0.12%, O≤0.0015%, the balance includes Fe and inevitable impurities, while satisfying: 0.5%≤V+Nb≤1.3%. The application uses low W, high Mo, high Co, V / Nb control to realize the good matching of steel toughness, and the high-temperature strength and creep resistance reach or exceed the level of traditional high-tungsten heat-resistant steel.

Description

Technical Field

[0001] This invention belongs to the field of heat-resistant steel technology, specifically relating to a martensitic heat-resistant steel and its manufacturing method. Background Technology

[0002] The key to improving the thermal efficiency of thermal power units lies in overcoming the technical bottlenecks in the unit's steam parameters (temperature and pressure). Increasing steam temperature and pressure requires materials with higher strength, better oxidation resistance, and corrosion resistance; therefore, material iteration is a crucial foundation.

[0003] The first generation of martensitic heat-resistant steel (such as 12Cr1MoV) uses Cr-Mo as the core alloy system, with Cr content of about 1%~3% and Mo content of about 0.25%-1%, supplemented with a small amount of V, Si, Mn, and a relatively high C content (0.1%-0.15%). It has limited high-temperature strength and creep performance, and is prone to microstructure aging during long-term service.

[0004] Second-generation martensitic heat-resistant steels (such as P91 and P92) reduce the C content to 0.08%-0.12% and increase the Cr content to 8%~9% based on Cr-Mo. The Mo content of P91 is increased to about 1%, while P92 appropriately reduces the Mo content and adds 1.5-2.0% W to further improve high-temperature creep strength and resistance to steam oxidation.

[0005] Third-generation martensitic heat-resistant steels are represented by SAVE12AD, G115, and MARBN steels. SAVE12AD steel has a W content of approximately 3%.

[0006] A new type of martensitic heat-resistant steel, 9Cr-3W-3CoVNbCuBN (G115), with a W content of 2.5-3.5%.

[0007] 9Cr-3W-3CoVNbBN steel (MARBN steel) has a W content of approximately 3%.

[0008] Currently, martensitic heat-resistant steels with mature applications in 620-650℃ ultra-supercritical power plant engineering generally employ 3% tungsten (W) as the main strengthening element, combined with trace elements such as molybdenum (≤1.2%), vanadium (V), and niobium (Nb) to synergistically regulate microstructure and properties. However, high tungsten content easily leads to tungsten segregation, promoting the precipitation of harmful Laves phases, which significantly impairs the strength-toughness balance of the material. Furthermore, as a scarce strategic resource, tungsten has long faced a predicament of tight supply and high prices, which not only increases material manufacturing costs but also brings significant supply chain risks. Therefore, tungsten conservation and consumption reduction have become a new direction for technological research and development in this field. Summary of the Invention

[0009] The purpose of this invention is to provide a martensitic heat-resistant steel and its manufacturing method, which achieves a good balance of strength, toughness, and high-temperature performance of the martensitic heat-resistant steel under low W conditions. Compared with high W heat-resistant steel of the same grade, the tungsten content is reduced by more than 50%, significantly reducing material costs. The room temperature properties of the martensitic heat-resistant steel are: yield strength Rp0.2≥550MPa, tensile strength Rm≥730MPa, elongation A≥15%, and impact KV2≥120J; the high-temperature tensile properties at 650℃ are: yield strength≥300MPa, tensile strength Rm≥350MPa, and elongation A≥20%.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: A martensitic heat-resistant steel comprises the following elements by weight percentage: C: 0.07-0.15%, Si: 0.11-0.50%, Mn: 0.5-1.0%, Cr: 8.0-12.5%, W: 0.6-1.4%, Mo: 2.1-3.5%, Co: 4.1-6.0%, Nb: 0.11-0.50%, V: 0.36-0.80%, Cu: 0.5-1.2%, N: 0.003-0.020%, B: 0.005-0.020%, S≤0.01%, P≤0.02%, Ni≤0.12%, O≤0.0015%, with the balance including Fe and unavoidable impurity elements, and also requiring the following condition to be met simultaneously: 0.5%≤V+Nb≤1.3%.

[0011] Furthermore, the balance consists of Fe and unavoidable impurity elements.

[0012] The microstructure of the martensitic heat-resistant steel of the present invention is tempered martensite + precipitated phase, wherein the precipitated phase is Mo2C, (WMo)6C, VC and / or VN, NbC and / or NbN, wherein the size of Mo2C and (WMo)6C is ≤3μm.

[0013] The room temperature properties of the martensitic heat-resistant steel described in this invention are: yield strength Rp0.2≥550MPa, tensile strength Rm≥730MPa, elongation A≥15%, and impact KV2≥120J; the high temperature tensile properties at 650℃ are: yield strength≥300MPa, tensile strength Rm≥350MPa, and elongation A≥20%.

[0014] In the composition design of the martensitic heat-resistant steel of this invention: Carbon: As the most commonly used strengthening element in steel, C can form carbides such as Mo2C, (WMo)6C, and (VNb)C with alloying elements Mo, W, V, and Nb, effectively reinforcing the lath boundaries and maintaining the material's properties for a long time. However, excessively high C content is detrimental to weldability. Therefore, the C content of the steel in this invention is controlled within the range of 0.07-0.15%.

[0015] Silicon: At high temperatures, Si forms a dense SiO2 protective film, improving the material's oxidation resistance. Additionally, Si is a strong deoxidizer, enhancing the material's purity. However, excessively high Si content can deteriorate the material's strength and toughness. The Si content of the steel in this invention is controlled within the range of 0.11-0.5%.

[0016] Manganese (Mn) can improve hardenability and increase the strength of steel, but Mn also increases the coefficient of thermal expansion and reduces thermal conductivity. The Mn content of the steel in this invention is controlled in the range of 0.5-1.0%.

[0017] Chromium: From the perspective of improving oxidation resistance, a higher Cr content is generally desirable. However, excessively high Cr content can lead to the formation of δ-ferrite in the material, reducing its creep strength and toughness. Furthermore, relevant experimental studies have shown that the steel exhibits the highest creep strength when the Cr content is 9%. Therefore, the Cr content of the steel in this invention is controlled within the range of 8.0-12.5%.

[0018] Tungsten: W exists in steel in two forms: one is as a solid solution in the matrix, providing solid solution strengthening; the other is as a precipitate, providing precipitation strengthening. Tungsten is a scarce strategic resource, with a long-term tight supply and high price. The steel of this invention adopts a W-saving design, controlling the W content within the range of 0.6-1.4%.

[0019] Molybdenum (Mo), similar to wheat (W), plays a role in steel not only in solid solution strengthening but also in precipitation strengthening. Mo and carbon (C) can form fine, dispersed Mo₂C, improving the material's creep and fatigue resistance. This invention compensates for the strengthening effect of wheat by increasing the amount of Mo added, maintaining the high-temperature creep resistance of the matrix. However, excessively high Mo content can lead to the precipitation of the harmful Fe₃Mo₂ phase, thereby reducing the alloy's hot workability and mechanical properties. Therefore, the Mo content of the steel in this invention is controlled at 2.1-3.5%.

[0020] Cobalt (Co) acts as a solid solution strengthener in the matrix, synergistically enhancing the high-temperature strength and creep resistance of the matrix with elements such as Mo and W. Simultaneously, Co improves the material's oxidation and corrosion resistance. Furthermore, Co reduces the diffusion rate of Fe atoms, delaying the nucleation and growth of the harmful Fe3Mo2 phase and promoting the uniform precipitation of Mo as fine, dispersed Mo2C nanoparticles (rather than large particles). Therefore, to further improve the matrix stability and high-temperature strength, the Co content was appropriately increased; the Co content in the steel of this invention is controlled at 4.1-6.0%.

[0021] Vanadium and niobium (V and Nb) primarily form the MC phase with C and N elements, which are important strengthening phases in ferritic heat-resistant steels. This invention has found that the combined addition of V and Nb can significantly improve the creep strength of steel. Insufficient V and Nb content results in an inadequate MC phase quantity and unsatisfactory high-temperature creep strength. V and Nb have large atomic radii, and excessive content can easily lead to segregation and inhomogeneity. Furthermore, excessive V and Nb consume the matrix C, preventing W and Mo from forming sufficient strengthening phases and weakening high-temperature performance. In this invention, the V content is controlled at 0.36-0.80% to ensure sufficient fine VC / VN, providing strong precipitation strengthening while avoiding excessive segregation. The Nb content is controlled at 0.11-0.50% to provide highly stable NbC / NbN, inhibiting coarsening. Controlling the total amount of strengthening phase (0.5% ≤ V + Nb ≤ 1.3%) ensures that the total amount of strengthening phase meets the high-temperature creep strength requirements, while simultaneously suppressing composite segregation at its source.

[0022] Copper: Cu can act as both solid solution strengthening and precipitation strengthening in the form of a nano-copper-rich phase. When the Cu content is low, it mainly exists in the solid solution form, and the strengthening effect is relatively weak. When the Cu content is high, it will seriously reduce the high-temperature plasticity. Therefore, the Cu content of the steel in this invention is controlled at 0.5-1.2%.

[0023] Boron and nitrogen: Boron (B) is a strong grain boundary segregating element, which can effectively enhance grain boundary bonding, thereby significantly improving the creep strength and ductility of the alloy of this invention. Low B content does not achieve the effect of strengthening grain boundaries, while excessively high B content easily leads to the formation of a large number of low-melting-point precipitates, which is detrimental to hot working and processability. Simultaneously, as the B content increases, the nitrogen (N) content must be controlled to prevent the formation of boron (BN). However, excessively low N content reduces the precipitation of the strengthening phase MX, thus reducing the alloy strength; furthermore, from a production economics perspective, excessively low N content results in extremely high production costs. Therefore, the B content of the steel of this invention is controlled at 0.005-0.020%, and the N content is controlled at 0.003-0.020%.

[0024] S, P, Ni, and O are residual elements in steel. In this invention, S is controlled to be ≤0.01%, P to be ≤0.02%, Ni to be ≤0.12%, and O to be ≤0.0015%.

[0025] This invention employs a W-saving design in its composition. Mo and W both belong to Group VIB elements and can dissolve in a martensitic matrix to exert solid solution strengthening effects, thereby enhancing the high-temperature strength of steel. Replacing W with Mo refines the size of the precipitated phase, further improving microstructure stability and ensuring performance stability. The strengthening effect of W is compensated by increasing the amount of Mo added, maintaining the high-temperature creep resistance of the matrix. However, research has found that when "replacing W with Mo," excessively high Mo content can induce the precipitation of the "harmful Fe3Mo2 phase," which impairs the hot working and mechanical properties of the material. Therefore, the addition of Mo content is limited.

[0026] This invention reduces the diffusion rate of Fe atoms by adding Co, thus delaying the nucleation and growth of the harmful Fe3Mo2 phase. This promotes the uniform precipitation of Mo as fine, dispersed Mo2C nanoparticles (rather than large particles), avoiding the precipitation of the harmful Fe3Mo2 phase that may be caused by high Mo content. Consequently, the amount of Mo added can be further increased while the amount of W added can be reduced. Simultaneously, the addition of V and Nb, adjusted to 0.5% ≤ V + Nb ≤ 1.3%, forms stable, fine MC-type carbides (Mo2C and (WMo)6C carbide sizes ≤ 3 μm), which pin dislocations and hinder grain boundary migration, ensuring that the steel achieves a good balance of strength and toughness, as well as high-temperature strength.

[0027] Furthermore, W is a scarce strategic resource, and the market price of metallic W is 2-3 times that of metallic Mo. The tungsten-saving design not only significantly reduces material costs but also aligns with the policy of protective mining of tungsten resources.

[0028] The method for manufacturing martensitic heat-resistant steel according to the present invention includes the following steps: 1) Smelting Smelting is carried out according to the above-mentioned components; 2) Forging The steel ingot is heated to 1100-1250℃ and held for ≥3h, then forged into a forging bar or forging. 3) Heat treatment Perform normalizing and tempering heat treatment.

[0029] Preferably, in step 3), the normalizing temperature is 1060~1100℃ and the normalizing time is ≥0.5h.

[0030] Preferably, in step 3), the tempering temperature is 760~790℃ and the tempering time is ≥2.0h.

[0031] In the method for manufacturing martensitic heat-resistant steel according to the present invention: The heating temperature of the steel ingot is controlled at 1100-1250℃, and the holding time is ≥3h. Within this temperature range, the high-melting-point carbides Mo2C, (WMo)6C, VC / VN, and NbC / NbN are fully dissolved, while alloying elements such as Cr, Mo, W, and Co are fully diffused in the austenite, reducing segregation and preparing for the subsequent precipitation of fine, dispersed secondary precipitates in the matrix. At the same time, excessively high temperatures are avoided to prevent overheating and burning, and to prevent hot brittleness and surface defects.

[0032] The normalizing temperature is controlled at 1060~1100℃ and the normalizing time is ≥0.5h, so that the precipitated phases Mo2C and (WMo)6C undergo partial re-dissolution and lath martensite is formed during the cooling process.

[0033] The tempering temperature is controlled at 760~790℃ and the tempering time is ≥2.0h. Through tempering, the martensite decomposes and a large amount of Mo2C, (WMo)6C and other precipitates are formed in the grain boundaries and laths, forming tempered martensite structure.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves a good balance of strength and toughness in steel, as well as high-temperature strength, by using a lower W content (W: 0.6-1.4%) and a higher Mo content (Mo: 2.1-3.5%), along with the synergistic addition of high levels of Co, V, and Nb elements, and adjusting the ratio of 0.5% ≤ V + Nb ≤ 1.3%. This achieves the goal of saving W and significantly reduces material costs.

[0035] While existing technologies utilize Mo to replace W to improve strength and thus achieve W-saving designs, the addition of excessive Mo can lead to the precipitation of the harmful Fe3Mo2 phase, thus limiting the addition of Mo. The amount of Mo added is generally controlled at ≤1.2%. Therefore, to achieve higher strength, it is necessary to add about 3% W as the main strengthening element.

[0036] This invention utilizes the addition of high Co content to reduce the diffusion rate of Fe atoms, delay the nucleation and growth of the harmful Fe3Mo2 phase, and promote the uniform precipitation of Mo as fine, dispersed Mo2C nanoparticles (rather than large particles). This avoids the precipitation of the harmful Fe3Mo2 phase that may be caused by high Mo content. Thus, by increasing the addition of Mo content, the addition of W can be significantly reduced.

[0037] The room temperature properties of the martensitic heat-resistant steel described in this invention are: yield strength Rp0.2≥550MPa, tensile strength Rm≥730MPa, elongation A≥15%, and impact KV2≥120J; the high temperature tensile properties at 650℃ are: yield strength≥300MPa, tensile strength Rm≥350MPa, and elongation A≥20%.

[0038] Compared with existing 9Cr-3W-3Co series heat-resistant steel, the martensitic heat-resistant steel described in this invention has comparable strength, but can reduce tungsten usage by more than 50%, significantly reducing material costs and adapting to the policy of protective mining of tungsten resources. Attached Figure Description

[0039] Figure 1 The image shows the metallographic structure of steel obtained in Example 1 of this invention. Detailed Implementation

[0040] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0041] The composition of the steel in the embodiments and comparative examples of the present invention is shown in Table 1, the process control of the steel in the embodiments and comparative examples of the present invention is shown in Table 2, and the performance of the steel in the embodiments and comparative examples of the present invention is shown in Table 3.

[0042] Figure 1 The metallographic image of the martensitic heat-resistant steel obtained in Example 1 of the present invention shows that fine and dispersed Mo2C and (WMo)6C precipitates are distributed on the matrix, with the size of Mo2C and (WMo)6C ≤3μm.

[0043] In Comparative Example 1, the W and Mo contents were within the range of this invention, but the Co content was 2.61%, which was significantly lower than the 4.1-6.0% limit of this invention, and the total V+Nb content was less than 0.5%, which did not meet the requirements of this invention. As a result, the steel obtained had low room temperature and high temperature strength.

[0044] In Comparative Example 2, the Co content was within the range of this invention, no Mo was added, and the Nb content was low. The resulting steel exhibited a low room temperature impact KV2.

[0045] In Comparative Example 3, the Co content was within the range of this invention, the Mo content was low, and the V content was also low. The resulting steel exhibited both low strength and low room temperature impact KV2.

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Claims

1. A martensitic heat-resistant steel, characterized in that, Its composition by weight percentage includes: C: 0.07-0.15%, Si: 0.11-0.50%, Mn: 0.5-1.0%, Cr: 8.0-12.5%, W: 0.6-1.4%, Mo: 2.1-3.5%, Co: 4.1-6.0%, Nb: 0.11-0.50%, V: 0.36-0.80%, Cu: 0.5-1.2%, N: 0.003-0.020%, B: 0.005-0.020%, S≤0.01%, P≤0.02%, Ni≤0.12%, O≤0.0015%, with the balance including Fe and unavoidable impurity elements, and must also meet the following requirement: 0.5%≤V+Nb≤1.3%.

2. The martensitic heat-resistant steel as described in claim 1, characterized in that, The balance consists of Fe and unavoidable impurity elements.

3. The martensitic heat-resistant steel as described in claim 1 or 2, characterized in that, The microstructure of the martensitic heat-resistant steel is tempered martensite plus precipitates, wherein the precipitates are Mo2C, (WMo)6C, VC and / or VN, NbC and / or NbN, wherein the size of Mo2C and (WMo)6C is ≤3µm.

4. The martensitic heat-resistant steel as described in claim 1, 2, or 3, characterized in that, The room temperature properties of the martensitic heat-resistant steel are: yield strength Rp0.2≥550MPa, tensile strength Rm≥730MPa, elongation A≥15%, and impact KV2≥120J; the high temperature tensile properties at 650℃ are: yield strength≥300MPa, tensile strength Rm≥350MPa, and elongation A≥20%.

5. The method for manufacturing martensitic heat-resistant steel according to any one of claims 1 to 4, characterized in that, Includes the following steps: 1) Smelting Steel ingots are obtained by smelting according to the composition described in claim 1 or 2; 2) Forging The steel ingot is heated to 1100-1250℃ and held for ≥3 hours, then forged into a forging bar or forging. 3) Heat treatment Normalizing treatment, with a normalizing temperature of 1060~1100℃ and a normalizing time of ≥0.5h; Tempering treatment, tempering temperature is 760~790℃, tempering time is ≥2.0h.

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