A stable precipitation phase strengthened martensitic heat-resistant steel and its preparation method

Abstract

The present invention relates to the technical field of high-temperature metal structural materials. Specifically, a high-stability precipitation-strengthened martensitic heat-resistant steel and a preparation method thereof are proposed. The martensitic heat-resistant steel exhibits high strength and excellent creep resistance. The steel's composition, calculated by weight percentage (wt%), is as follows: C: 0.013-0.016%, Cr: 9.1%, Co: 3.06%, Mn: 1.52%, Si: 0.238%, V: 1.16%, N: 0.048-0.052%, W: 0.101%, with the balance being Fe. To this composition, Cu: 1-2% by weight is added. When the Cu content is 2%, the alloy exhibits optimal high-temperature creep resistance. The preparation method includes the following steps: after weighing the corresponding mass of elemental single substances according to the element ratio, smelting and casting are carried out, and then the alloy ingot is forged and rolled to open the ingot, and then normalizing and tempering are carried out. Compared with martensitic heat-resistant steel without Cu addition and containing only a single MX phase, it exhibits better high-temperature creep resistance at high temperature, which is helpful to promote the engineering practice application of martensitic heat-resistant steel.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-temperature metal structural materials, and in particular relates to a stable precipitation phase strengthened martensitic heat-resistant steel and a preparation method thereof. Background Art

[0002] The power industry is a pioneering industry for the sustainable development of the national economy. In my country, 70% of electricity comes from thermal power generation. It is estimated that every 1% increase in thermal power generation efficiency can save 10% of standard coal per year. 7 t or above. This will not only bring huge economic benefits but also significantly reduce CO2 and other pollutant emissions, achieving green and sustainable development. Thermal power generation heats liquid water to a gaseous state, using high-temperature, high-pressure steam to propel the turbine to perform external work. Improving steam parameters can significantly increase power generation efficiency. Therefore, increasing the service temperature of the steam contact piping of the generator set is key to improving generator set operating efficiency.

[0003] 9~12% Cr martensitic heat-resistant steel is widely used in thermal power generation in key equipment components such as main steam pipes, boilers, and steam turbines in thermal power plants due to its high strength, toughness, creep resistance, and good corrosion resistance. However, it will produce precipitated phase M under long-term service in high temperature and high pressure environment. 23 The coarsening of C6 and the formation of coarse precipitates (Laves and Z phases) weaken the pinning of the precipitates on the martensite lath boundaries, causing structural degradation and ultimately leading to material failure. Therefore, addressing the failure of martensitic heat-resistant steels caused by coarse precipitates during long-term service is an urgent issue. Summary of the Invention

[0004] To address the technical problems encountered in the aforementioned background technology, the present invention provides a stable precipitation-strengthened martensitic heat-resistant steel and a method for preparing the same. This results in a martensitic heat-resistant steel with no coarse precipitations present in its structure when in service at 550-650°C, with only highly stable MX-type and Cu-rich precipitations. The resulting alloy exhibits a low steady-state creep rate and excellent high-temperature service life. The alloy comprises a tempered martensite structure with a high-density dispersion of nano-MX and Cu-rich phases. The synergistic strengthening effect of the highly stable nano-Cu-rich and MX phases effectively hinders the movement of dislocations during creep, reducing the steady-state creep rate of the martensitic steel at high temperatures, thereby improving its high-temperature creep resistance.

[0005] The present invention is specifically implemented by the following technical solutions:

[0006] A first aspect of the present invention provides a stable precipitation phase strengthened martensitic heat-resistant steel, which comprises the following components calculated by weight percentage:

[0007] C: 0.013-0.016%, Cr: 9.1%, Co: 3.06%, Mn: 1.52%, Si: 0.238%, V: 1.16%, N: 0.048-0.052%, W: 0.101%, Cu: 1-2%, and the balance is Fe.

[0008] Preferably, calculated by weight percentage, it includes the following components:

[0009] C: 0.013%, Cr: 9.1%, Co: 3.06%, Mn: 1.52%, Si: 0.238%, V: 1.16%, N: 0.048%, W: 0.101%, Cu: 2%, and the balance is Fe.

[0010] As a further illustration of the present invention, the equilibrium matrix structure of the martensitic heat-resistant steel is tempered martensite, and the precipitated phases are Cu-rich phase and MX phase with high stability, wherein the Cu-rich phase and MX phase are uniformly dispersed in the martensitic lath boundaries and inside the laths.

[0011] As a further illustration of the present invention, the martensitic heat-resistant steel was subjected to a compression creep test at 650°C and 120 MPa, and the steady-state creep rate was .

[0012] A second aspect of the present invention provides a method for preparing any one of the above-mentioned stable precipitation phase strengthened martensitic heat-resistant steels, comprising the following steps:

[0013] S1: According to the original composition ratio of the martensitic heat-resistant steel, weigh the raw materials and set them aside;

[0014] S2: melting and casting the raw materials weighed in S1 to obtain alloy ingots;

[0015] S3: forging and rolling the alloy ingot cast in S2 to obtain an alloy slab;

[0016] S4: performing normalizing heat treatment on the alloy slab obtained in S3 and then air cooling;

[0017] S5: The alloy slab obtained in S4 is subjected to tempering heat treatment and then air-cooled to obtain a martensitic heat-resistant steel having a target microstructure.

[0018] As a further illustration of the present invention, a vacuum induction melting furnace is used for alloy melting and subsequent forging and rolling processes; a tubular heat treatment furnace is used for normalizing heat treatment; and a tubular heat treatment furnace is used for tempering heat treatment.

[0019] As a further illustration of the present invention, in the smelting process in S2, metal elements with similar melting points are first smelted together, and then the alloy ingots are smelted together six times.

[0020] As a further illustration of the present invention, the number of remelting times in S2 is 6, and the vacuum degree in the melting furnace is controlled at 0.07-0.10 MPa.

[0021] As a further illustration of the present invention, the forging and rolling process in S3 includes:

[0022] The alloy ingot cast in S2 was forged at 1100°C into a cross-section of 100 mm. For a 100 mm square billet, the final forging temperature must not be lower than 1000 °C; then it is kept at 1200 °C for 2 h, and then rolled to a thickness of 12 mm to obtain an alloy slab.

[0023] As a further illustration of the present invention, the normalizing heat treatment process in S3 includes:

[0024] The alloy slabs after forging and rolling in S3 were placed in a heat treatment furnace at 1180 °C under inert gas protection for 60 min and then air-cooled.

[0025] As a further illustration of the present invention, the inert gases used in S2 and S3 are both argon.

[0026] As a further illustration of the present invention, the tempering heat treatment process in S5 includes:

[0027] The alloy slab obtained in S4 was kept at 750 °C for 120 min and then air-cooled to obtain a martensitic heat-resistant steel with the target microstructure.

[0028] In the design of the matrix (without Cu), the present invention strictly controls the carbon content and increases the V element (1.16%), so that the MX phase (VC) is precipitated while reducing the Cr 23 The content of C6 phase (Cr 23 C6 tends to coarsen during long-term service. The addition of Cu is intended to produce nano-sized copper-rich precipitates during heat treatment and tempering. Both precipitates pin free dislocations within the grain, increasing precipitation strengthening increments and thus enhancing long-term service stability.

[0029] In terms of process, the present invention adopts vacuum induction melting, strictly limits the contents of impurity elements P and S, and ensures that the C content is within the range of 0.013-0.016% (wt%).

[0030] In summary, compared with the prior art, the present invention has the following beneficial technical effects:

[0031] The present invention provides a novel martensitic heat-resistant steel composition, wherein the matrix structure is tempered lath martensite, and the strengthening phase is a highly stable nano-MX type precipitation phase and a large amount of nano-Cu-rich precipitation phase. The nano-Cu and MX dispersed in the matrix can synergistically strengthen and more effectively hinder the movement of dislocations during creep.

[0032] When the martensitic heat-resistant steel with improved creep properties provided by the present invention is used in high-temperature environments, the dispersed nano-Cu and MX in the alloy jointly pin dislocations, effectively hindering their movement during creep and reducing the creep rate. Simultaneously, the steel exhibits excellent strength and ductility at both room temperature and 650°C. Therefore, the improved martensitic heat-resistant steel of the present invention can be used in high-temperature environments, such as gas turbines and nuclear reactors. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a property diagram of the martensitic heat-resistant steel prepared in Example 1 of the present invention.

[0034] Figure 2 The microstructure of the martensitic heat-resistant steel prepared in Example 1 of the present invention is as follows: Figure 2 (a1) is the microstructure of Example 1 after tempering under EBSD, Figure 2 (a2) to (a4) are the microstructures of Example 1 after tempering under TEM.

[0035] Figure 3 The bar graphs show the original microstructures of the martensitic heat-resistant steels prepared in Comparative Example 1 and Examples 1 and 2 of the present invention, and the hardness changes after aging at 650°C for 1, 10, 100, and 1000 hours.

[0036] Figure 4 Compression creep curves of martensitic heat-resistant steels prepared in Examples 1 and 2 of the present invention and Comparative Example 1 at 650°C and 120 MPa are shown. Figure 4 (a) is the compression creep test curve, Figure 4 (b) is the compression creep rate curve. DETAILED DESCRIPTION

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0038] Example 1

[0039] The raw materials were weighed according to the following mass percentages: C: 0.013%, Cr: 9.1%, Mn: 1.52%, V: 1.16%, Co: 3.06%, Cu: 2%, Si: 0.238%, N: 0.048%, W: 0.101%, and the balance Fe.

[0040] Then the raw materials were placed in a vacuum induction melting furnace and repeatedly melted six times in an inert gas atmosphere.

[0041] Alloy billet; the billet obtained by melting is forged at 1100 ℃ into a cross-section of 100 mm For a 100 mm square billet, the final forging temperature must not be lower than 1000°C. The billet is then held at 1200°C for 2 hours. The resulting alloy ingot, now at 1200°C, is then immediately hot-rolled using a 450 mm diameter roll mill. Ten passes are performed on the ingot, reducing the thickness from 100 mm to 12 mm. The ingot is then air-cooled to room temperature to produce the hot-rolled and cooled alloy plate.

[0042] The slab-like alloy was then kept at 1180 °C for 1 h in a heat treatment furnace for normalizing heat treatment, taken out for air cooling, and then the normalized alloy was kept at 750 °C for 120 min for tempering heat treatment, and taken out for air cooling.

[0043] Φ5×8 mm compression specimens were taken along the rolling direction of the slab using wire-cut electrospark cutting. The surfaces and end faces were ground and polished to 800 mesh before compression creep testing. The remaining specimens were subsequently subjected to creep testing under various stresses. The creep tests were conducted at a creep temperature of 650°C and stresses ranging from 50 to 200 MPa using an RDL creep testing machine.

[0044] Example 2

[0045] The raw materials were weighed according to the following mass percentages: C: 0.013%, Cr: 9.1%, Mn: 1.52%, V: 1.16%, Co: 3.06%, Cu: 1%, Si: 0.238%, N: 0.048%, W: 0.101%, and the balance Fe.

[0046] Then the raw materials were placed in a vacuum induction melting furnace and repeatedly melted six times in an inert gas atmosphere.

[0047] Alloy billet; the billet is forged at 1100 ℃ to a cross section of 100 mm For a 100 mm square billet, the final forging temperature must not be lower than 1000°C. The billet is then held at 1200°C for 2 hours. The resulting alloy ingot, now at 1200°C, is then immediately hot-rolled using a 450 mm diameter roll mill. Ten passes are performed on the ingot, reducing the thickness from 100 mm to 12 mm. The ingot is then air-cooled to room temperature to produce the hot-rolled and cooled alloy plate.

[0048] The slab-shaped alloy was then placed in a heat treatment furnace at 1180°C for 1 hour for normalizing heat treatment, taken out for air cooling, and then the normalized alloy was tempered and taken out for air cooling.

[0049] Φ5×8 mm compression specimens were taken along the rolling direction of the slab using wire-cut electrospark cutting. The surfaces and end faces were ground and polished to 800 mesh before compression creep testing. The remaining samples were subsequently subjected to creep tests under different stresses.

[0050] Example 3

[0051] The raw materials were weighed according to the following mass percentages: C: 0.016%, Cr: 9.1%, Mn: 1.52%, V: 1.16%, Co: 3.06%, Cu: 2%, Si: 0.238%, N: 0.052%, W: 0.101%, and the balance Fe.

[0052] Then the raw materials were placed in a vacuum induction melting furnace and repeatedly melted six times in an inert gas atmosphere.

[0053] Alloy billet; the billet obtained by melting is forged at 1100 ℃ into a cross-section of 100 mm For a 100 mm square billet, the final forging temperature must not be lower than 1000°C. The billet is then held at 1200°C for 2 hours. The resulting alloy ingot, now at 1200°C, is then immediately hot-rolled using a 450 mm diameter roll mill. Ten passes are performed on the ingot, reducing the thickness from 100 mm to 12 mm. The ingot is then air-cooled to room temperature to produce the hot-rolled and cooled alloy plate.

[0054] The slab-like alloy was then kept at 1180 °C for 1 h in a heat treatment furnace for normalizing heat treatment, taken out for air cooling, and then the normalized alloy was kept at 750 °C for 120 min for tempering heat treatment, and taken out for air cooling.

[0055] Comparative Example 1

[0056] The mass percentages are C: 0.013%, Cr: 9.1%, Mn: 1.52%, V: 1.16%, Co: 3.06%, Cu: 0%, Si: 0.238%, N: 0.048%, W: 0.101%, and the balance is Fe. Convert the proportions into mass percentages and weigh the raw materials.

[0057] Then the raw materials were placed in a vacuum induction melting furnace and repeatedly melted six times in an inert gas atmosphere.

[0058] Alloy billet; the billet is forged at 1100 ℃ to a cross section of 100 mm For a 100 mm square billet, the final forging temperature must not be lower than 1000°C. The billet is then held at 1200°C for 2 hours. The alloy ingot, which has reached the end of the 1200°C hold, is then immediately hot-rolled using a 450 mm diameter roll mill. Ten passes are performed on the ingot, reducing the thickness from 100 mm to 12 mm. The ingot is then air-cooled to room temperature to produce the hot-rolled and cooled alloy plate.

[0059] The slab-shaped alloy was then placed in a heat treatment furnace at 1180°C for 1 hour for normalizing heat treatment, taken out for air cooling, and then the normalized alloy was tempered and taken out for air cooling.

[0060] Φ5×8 mm compression specimens were taken along the rolling direction of the slab using wire-cut electrospark cutting. The surfaces and end faces were ground and polished to 800 mesh before compression creep testing. The remaining samples were subsequently subjected to creep tests under different stresses.

[0061] Depend on Figure 1 It can be seen that a large amount of Cu-rich phase and MX phase precipitate when the martensitic heat-resistant steel prepared in Example 1 of the present invention is tempered at 750°C.

[0062] Depend on Figure 2 It can be seen that there is no ferrite precipitation in the martensitic heat-resistant steel prepared in Example 1 of the present invention, and the structure is dense. It can be seen that the addition of Cu can inhibit the precipitation of ferrite; Figure 2 (a2)~(a4) show that there are a large number of rod-shaped Cu-rich precipitates in the martensitic matrix prepared in Example 1, which together with the nanoscale MX phase pin the free dislocations in the grain, thereby improving the high-temperature service stability of the martensitic steel structure.

[0063] Depend on Figure 3 It can be seen that after aging in a heat treatment furnace at 650°C for 1, 10, 100, and 1000 hours, the hardness of the martensitic heat-resistant steel prepared in the embodiment of the present invention decreases the least, and the hardness decreases the most when Cu is not added. This shows that the addition of Cu to martensitic heat-resistant steel can improve the thermal stability of the structure, and the structure with a Cu content of 2% (wt%) is the most stable.

[0064] Depend on Figure 4 It can be seen that the martensitic heat-resistant steel prepared in Example 1 of the present invention has a high MPa compression creep test, the creep rate of the alloy is significantly reduced compared with that of Comparative Example 1. This is mainly because Comparative Example 1 contains only a single MX phase. Although MX has high thermal stability, its number density is low, the pinning effect is weak, and dislocations are easy to climb, resulting in premature recovery and recrystallization of the martensite structure, resulting in a higher creep rate. In Example 1, the addition of Cu causes a large amount of nano-Cu to precipitate in the matrix. The rich Cu is similar to MX and is a nano-scale precipitate (<50 nm). The joint strengthening effect of these two nano-phases compensates for the shortcomings of single MX strengthening, such as weak pinning force and easy dislocation climbing, and more effectively hinders the movement of dislocations during creep, reducing the steady-state creep rate of martensitic steel at high temperature and improving the high-temperature creep resistance of martensitic steel. Moreover, the creep rate of Example 1 is also reduced compared with Example 2, indicating that within a certain range, the increase in copper content can reduce the creep rate of martensite and improve the high-temperature creep resistance of martensitic steel.

[0065] It should be noted that, in this document, terms such as "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements that are inherent to such process, method, article or apparatus.

[0066] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A stable precipitation phase strengthened martensitic heat-resistant steel, characterized in that: Calculated by mass percentage, it includes the following components: C: 0.013-0.016%, Cr: 9.1%, Co: 3.06%, Mn: 1.52%, Si: 0.238%, V: 1.16%, N: 0.048-0.052%, W: 0.101%, Cu: 1-2%, the balance is Fe; The preparation method comprises the following steps: S1: According to the original composition ratio of the martensitic heat-resistant steel, weigh the raw materials and set them aside; S2: melting and casting the raw materials weighed in S1 to obtain alloy ingots; S3: forging and rolling the alloy ingot cast in S2 to obtain an alloy slab; S4: performing normalizing heat treatment on the alloy slab obtained in S3 and then air cooling; S5: subjecting the alloy slab obtained in S4 to tempering heat treatment and then air cooling to obtain a martensitic heat-resistant steel having a target microstructure; The equilibrium matrix structure of the martensitic heat-resistant steel is tempered martensite, and the precipitated phases are Cu-rich phase and MX phase with high stability, wherein the Cu-rich phase and MX phase are uniformly dispersed in the martensitic lath boundaries and inside the laths.

2. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 1, characterized in that: The martensitic heat-resistant steel was subjected to compression creep tests at 650 °C and 120 MPa, and the steady-state creep rate was .

3. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 1, characterized in that: A vacuum induction melting furnace is used for alloy melting and subsequent forging and rolling processes; a tubular heat treatment furnace is used for normalizing heat treatment; and a tubular heat treatment furnace is used for tempering heat treatment.

4. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 1, characterized in that: During the smelting process in S2, metal elements with similar melting points are first smelted together, and then the alloy ingots are smelted together six times.

5. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 4, characterized in that: The vacuum degree in the smelting furnace is controlled at 0.07-0.10 MPa.

6. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 1, characterized in that: The forging and rolling process in S3 includes: The alloy ingot cast in S2 was forged at 1100°C into a cross-section of 100 mm. For a 100 mm square billet, the final forging temperature must not be lower than 1000 °C; then it is kept at 1200 °C for 2 h, and then rolled to a thickness of 12 mm to obtain an alloy slab.

7. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 1, characterized in that: The normalizing heat treatment process in S3 includes: The alloy slabs after forging and rolling in S3 were placed in a heat treatment furnace at 1180 °C under inert gas protection for 60 min and then air-cooled.

8. The stable precipitation phase strengthened martensitic heat-resistant steel according to claim 1, characterized in that: The tempering heat treatment process in S5 includes: The alloy slab obtained in S4 was kept at 750 °C for 120 min and then air-cooled to obtain a martensitic heat-resistant steel with the target microstructure.

Images