A microalloying method for promoting refinement and dissolution of NbX phase in austenitic heat-resistant steel

Abstract

The application provides a micro-alloying method for promoting refinement and dissolution of NbX (X=C, N) phases in austenitic heat-resistant steel, and belongs to the technical field of preparation and application of austenitic heat-resistant steel. The application introduces element B which is easy to segregate to the phase interface in the austenitic heat-resistant steel, so as to limit the size of the NbX phase, reduce the dissolution temperature of the NbX phase, and accelerate the dissolution of the NbX phase; the content of the element B is less than or equal to 0.008% according to the weight percentage. The method can inhibit the growth of the NbX phase, limit the size of the NbX phase, promote the dispersion of the NbX phase, and also can reduce the dissolution temperature of the NbX phase and shorten the dissolution time of the NbX phase.

Description

TECHNICAL FIELD

[0001] The present application relates to the preparation and application of austenitic heat-resistant steel, in particular to a micro-alloying method for promoting the refinement and dissolution of NbX (X=C, N) phase in austenitic heat-resistant steel. BACKGROUND

[0002] Electricity is an important energy pillar to ensure the sustainable development of national economy. At present, coal-fired thermal generating units account for more than 75% of the total capacity of generating units, occupying a dominant position. In order to improve the thermal efficiency of coal-fired generating units and reduce emissions and protect the environment, the development of ultra-supercritical power generation technology with high parameters and large capacity has become the only way for China's thermal power generating units. At present, there are hundreds of 600℃ ultra-supercritical units in service, and the key components with the highest temperature in the 600℃ high-parameter steam boiler of the ultra-supercritical boiler-turbine generating unit are the superheater and reheater pipes, which requires the superheater material to have excellent high-temperature mechanical properties. Among them, Nb-containing heat-resistant accounts for a large proportion, because the NbX phase precipitated can inhibit the growth of grains during hot working, and the NbX phase as a strengthening phase is beneficial to the improvement of high-temperature mechanical properties, but after hot working, there are often large-size initial NbX precipitates in the heat-resistant steel, which can easily lead to local cracking damage in the application process, thereby accelerating the failure of the overall part. However, there is little research on how to refine the initial coarse NbX phase and improve the mechanical properties by improving its distribution, therefore, it is of great significance to the development of austenitic heat-resistant steel to invent a method that can eliminate the initial coarse NbX phase and further promote its dispersion and uniform distribution. SUMMARY

[0003] Therefore, the present application aims to provide a micro-alloying method for promoting the refinement and dissolution of NbX phase in austenitic heat-resistant steel, which introduces B element that is easy to segregate to the phase interface into austenitic heat-resistant steel, so that B element is distributed at the phase interface of NbX phase, limiting the growth of NbX phase, controlling the size of initial NbX phase, improving the dispersion degree of NbX phase, and reducing the dissolution temperature of NbX phase, shortening the dissolution time of NbX phase, and accelerating the dissolution of NbX phase.

[0004] In order to achieve the above-mentioned purpose, the present application provides the following technical scheme: a micro-alloying method for promoting the refinement and dissolution of NbX phase in austenitic heat-resistant steel. By adding a small amount of B element in austenitic heat-resistant steel, the growth of NbX phase is limited, the size of NbX phase is controlled, the dissolution temperature of NbX phase is reduced, the dissolution of NbX phase is accelerated, and the solid solution treatment is used as the heat treatment method; the element B is ≤0.008% by weight; the solid solution treatment temperature is 1150℃-1250℃, and the solid solution treatment time is 50min-150min.

[0005] Preferably, the solid solution treatment temperature is 1200℃, and the solid solution treatment time is 60 min.

[0006] Preferably, the chemical composition of the austenitic heat-resistant steel comprises the following components in percentage by weight: C≤0.09%, Si≤0.3%, Mn≤0.8%, P≤0.03%, S≤0.005%, 8.5%≤Ni≤9.5%, 2.5%≤Cu≤3.5%, 0.050%≤N≤0.12%, 17.5%≤Cr≤18.5%, Co≤0.3%, 0.15%≤Mo≤0.25%, 8.50%≤Ni≤9.50%, 0.003%≤Al≤0.006%, 0.3%≤Nb≤0.6%, B≤0.008%, and the balance being Fe and other inevitable impurity elements.

[0007] Preferably, the method for manufacturing the austenitic heat-resistant steel into a billet comprises: casting, cooling, demolding, homogenizing at 1150-1250℃, cooling to room temperature, heating to 1200-1300℃, holding, and manufacturing the austenitic heat-resistant steel into a billet.

[0008] Beneficial technical effects:

[0009] 1. The B-microalloyed austenitic heat-resistant steel containing Nb of the present application has controlled NbX phase size, more fine and dispersed NbX phase, and smaller average grain size, as compared with the austenitic heat-resistant steel without B.

[0010] 2. The B-microalloyed austenitic heat-resistant steel of the present application has reduced NbX phase re-dissolution temperature, significantly accelerated re-dissolution speed, and small grain size, and the re-dissolution and re-distribution of the precipitated phase make the heat-resistant steel have excellent high-temperature performance and good comprehensive mechanical properties, and can be applied to harsh high-temperature environments. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 Figure 2 is a micro-morphology diagram of the rolled sample under high-resolution electron microscope in Example 1, wherein (a) is the NbX phase in the sample containing 75ppm B, and (b) is the energy spectrum diagram of the B element in the NbX phase in the sample containing 75ppm B;

[0012] Figure 2 Figure 3 is a micro-morphology diagram of the rolled sample, wherein (a) is the micro-morphology diagram of the sample without solid solution in Comparative Example 1, and (b) is the micro-morphology diagram of the sample without solid solution containing 75ppm B in Example 1;

[0013] Figure 3Microstructure of solution treated samples in Example 3; where (a) is the microstructure of 0B sample after solution treatment at 1150°C for 60 min, (b) is the microstructure of sample containing 75 ppm B after solution treatment at 1150°C for 60 min;

[0014] Figure 4 Microstructure of solution treated samples in Example 4; where (a) is the microstructure of 0B sample after solution treatment at 1200°C for 120 min, (b) is the microstructure of sample containing 75 ppm B after solution treatment at 1200°C for 60 min. DETAILED DESCRIPTION

[0015] The present application provides a micro-alloying method for promoting the refinement and re-dissolution of NbX phase in austenitic heat-resistant steel. By adding trace amount of B element in the austenitic heat-resistant steel, the growth of NbX phase is limited, the size of NbX phase is controlled, the re-dissolution temperature of NbX phase is reduced, and the re-dissolution of NbX phase is accelerated. The heat treatment mode is solution treatment. The element B is ≤0.008% by weight. The solution treatment temperature is 1150°C-1250°C, and the solution treatment time is 50 min-150 min. X=C, N in the NbX. In the present application, the solution treatment is carried out in an inert atmosphere, which is helpful to observe the accelerated re-dissolution of NbX phase in the B-containing sample. In some specific embodiments, the billet can be cut into small pieces of 15x15x3 mm for solution treatment.

[0016] By adding B micro-alloying, the B element is distributed at the phase interface of NbX phase in the present application, which limits the growth of NbX phase and promotes the re-dissolution of NbX phase. The B element can not only improve the strength and hot working performance of the alloy, but also segregate to the grain boundary, optimize the distribution of secondary phase, and is widely used in low-alloy high-strength steel, stainless steel, heat-resistant steel, and nickel-based alloy materials. For Nb-containing austenitic heat-resistant steel materials, the distribution of B element at the phase interface of NbX phase can control the size of NbX phase, reduce the re-dissolution temperature of NbX phase, shorten the re-dissolution time of NbX phase, and accelerate the re-dissolution of NbX phase.

[0017] In some embodiments, the solution treatment temperature is 1200°C, and the solution treatment time is 60 min.

[0018] In some embodiments, the chemical composition of the austenitic heat-resistant steel comprises the following components in the following weight percentages: C≤0.09%, Si≤0.3%, Mn≤0.8%, P≤0.03%, S≤0.005%, 8.5%≤Ni≤9.5%, 2.5%≤Cu≤3.5%, 0.050%≤N≤0.12%, 17.5%≤Cr≤18.5%, Co≤0.3%, 0.15%≤Mo≤0.25%, 8.50%≤Ni≤9.50%, 0.003%≤Al≤0.006%, 0.3%≤Nb≤0.6%, B≤0.008%, and the balance being Fe and other inevitable impurity elements.

[0019] In some embodiments, the chemical composition of the austenitic heat-resistant steel comprises the following components in the following weight percentages: C is 0.0818%, Si is 0.1358%, Mn is 0.7856%, P is 0.0288%, S is 0.0014%, Ni is 8.1893%, Cu is 2.6915%, N is 0.0585%, Cr is 18.021%, Co is 0.2318%, Mo is 0.1854%, Al is 0.0058%, Nb is 0.3453%, B is 0.0075%, and the balance being Fe and other inevitable impurity elements.

[0020] In some embodiments, the method for producing the austenitic heat-resistant steel into a billet comprises: casting the alloy components in the austenitic heat-resistant steel, cooling, demolding, homogenizing at 1150-1250℃, cooling to room temperature, heating to 1200-1300℃, holding, and producing the billet.

[0021] In the present application, specifically, the method for producing the austenitic heat-resistant steel into a billet comprises: smelting the alloy components in a vacuum induction furnace, air cooling after casting into an ingot under vacuum, demolding when cooled to room temperature, then homogenizing the ingot in a resistance heating furnace at 1150-1250℃ for 12-24 hours, air cooling to room temperature, continuously heating the ingot in the resistance heating furnace to 1200-1300℃, holding, and producing the billet.

[0022] In the present application, the homogenizing temperature is preferably 1200℃, the heating temperature in the resistance heating furnace is preferably 1250℃, and the holding time is preferably 30 minutes.

[0023] In the present application, the billet is hot-rolled into a steel plate, the billet is an ingot with a size of 120×100×500mm, and the steel plate is a medium-thick plate with a thickness of 25mm.

[0024] In some embodiments, the method further comprises solid solution treatment followed by rapid quenching.

[0025] In order to better understand the present application, the content of the present application is further illustrated below in combination with examples, but the content of the present application is not limited to the examples below.

[0026] Example 1

[0027] Preparation of B micro-alloyed austenitic heat-resistant steel

[0028] The chemical composition of the austenitic heat-resistant steel in the embodiment of the present application is as follows in terms of percentage by weight: C is 0.0818%, Si is 0.1358%, Mn is 0.7856%, P is 0.0288%, S is 0.0014%, Ni is 8.1893%, Cu is 2.6915%, N is 0.0585%, Cr is 18.021%, Co is 0.2318%, Mo is 0.1854%, Al is 0.0058%, Nb is 0.3453%, B is 0.0075%, and the balance is Fe and other unavoidable impurity elements.

[0029] The austenitic heat-resistant steel with the above composition is smelted in a vacuum induction furnace, and after being cast into an ingot with a size of 120×100×500 mm under vacuum and air-cooled, the ingot is demolded when cooled to room temperature; then the ingot is placed in a resistance heating furnace at 1200℃ for homogenization treatment for 12-24 h, air-cooled to room temperature, and then placed in the resistance heating furnace and heated to 1250℃, and kept for 30 min, and finally hot-rolled into a medium-thick plate with a thickness of 25 mm.

[0030] Comparative Example 1

[0031] The same as Example 1, except that the austenitic heat-resistant steel does not contain element B.

[0032] Example 2

[0033] Distribution of NbX precipitated phase in rolled B micro-alloyed austenitic heat-resistant steel sample

[0034] A 15 mm×15 mm×3 mm sample is cut from the austenitic heat-resistant steel plate prepared according to Example 1 and Comparative Example 1, and after the prepared sample is ground by sandpaper with different mesh, mechanically polished and etched, the sample is analyzed for microscopic morphology by a scanning electron microscope. As shown in Figs. Figure 2 As shown in Figs. (a) and (b), in the rolled comparative sample, there are relatively large-sized NbX phases and a small number of fine dispersed NbX phases, and the grain size is large. In contrast, in the rolled sample containing 75 ppm B, there are very few large-sized NbX phases, a large number of fine dispersed NbX phases, and a smaller grain size. It is shown that the addition of element B to the austenitic heat-resistant steel has a significant inhibitory effect on the size and precipitation of NbX phase compared with the sample without the addition of element B.

[0035] Example 3

[0036] Distribution of NbX precipitates in microalloyed austenitic heat-resistant steel samples after solution treatment at 1150℃

[0037] Several 15×15×3mm samples were cut from the austenitic heat-resistant steels prepared in Example 1 and Comparative Example 1 and placed in a muffle furnace at 1150℃ for 60 minutes of solution treatment. Afterwards, the samples were removed and quenched in water. The prepared samples were then mechanically polished and subjected to metallographic etching after grinding with sandpaper of different grits. The microstructure of the quenched samples after solution treatment was analyzed using a scanning electron microscope. All solution treatments were performed under an inert atmosphere. Figure 3 As shown in (a) and (b), the dissolution temperature of the NbX phase is generally greater than 1200℃. Under the condition of solution treatment at 1150℃ for 60 min, the comparative sample still contained relatively large NbX phases, and the NbX phase showed almost no dissolution. However, in the sample with added 75 ppm B, no relatively large NbX phases were found, and the number of fine NbX phases was significantly reduced. This indicates that the addition of B element lowers the dissolution temperature of the NbX phase and accelerates its dissolution.

[0038] Example 4

[0039] Distribution of NbX precipitates in microalloyed austenitic heat-resistant steel samples after solution treatment at 1200℃

[0040] Several 15×15×3mm samples were cut from the austenitic heat-resistant steel prepared in Comparative Example 1 and placed in a muffle furnace at 1200℃ for 60 minutes of solution treatment. Then, several more 15×15×3mm samples were cut from the austenitic heat-resistant steel prepared in Example 1 and placed in a muffle furnace at 1200℃ for 120 minutes of solution treatment. Afterward, all samples were removed and quenched in water. The prepared samples were then mechanically polished and subjected to metallographic etching after grinding with sandpaper of different grits. The microstructure of the samples was analyzed using a scanning electron microscope. All solution treatments were performed under an inert atmosphere. Figure 4 As shown in (a) and (b), the sample with 75 ppm B added showed similar results to the comparative sample after solution treatment at 1200℃ for 60 min. This indicates that at the same solution temperature, the addition of B promoted the dissolution of the NbX phase in the austenitic heat-resistant steel and shortened the dissolution time.

[0041] In summary, the NbX phase in the austenitic heat-resistant steel after adding B is more fine, less in quantity and more dispersed, and the grain size is also smaller, which further reduces the re-dissolution temperature of the NbX phase and accelerates the re-dissolution of the NbX phase. The experimental results show that by means of micro-alloying, the inhibitory effect of B element on the growth of the NbX phase at the phase interface can be fully utilized, the size of the NbX phase is limited, and then the distribution of the NbX phase is more fine and dispersed, and the mechanical properties and hot working properties of the material are improved.

[0042] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A microalloying method for promoting the refinement and dissolution of the NbX phase in austenitic heat-resistant steel, characterized in that... The austenitic heat-resistant steel contains element B, and the content of the element B is 0.0075% in terms of percentage by weight; after the austenitic heat-resistant steel is made into a billet, solid solution treatment is carried out, and then rapid quenching is carried out; the solid solution treatment temperature is 1150-1250 DEG C, and the solid solution treatment time is 50-150 min; The chemical components of the austenitic heat-resistant steel are as follows in terms of percentage by weight: C is 0.0818%, Si is 0.1358%, Mn is 0.7856%, P is 0.0288%, S is 0.0014%, Ni is 8.1893%, Cu is 2.6915%, N is 0.0585%, Cr is 18.021%, Co is 0.2318%, Mo is 0.1854%, Al is 0.0058%, Nb is 0.3453%, B is 0.0075%, and the balance is Fe and other inevitable impurity elements; The method for making the austenitic heat-resistant steel into a billet comprises the following steps: pouring the alloy components in the austenitic heat-resistant steel, cooling, demolding, homogenizing at 1150-1250 DEG C, cooling to room temperature, heating to 1200-1300 DEG C, keeping warm, and making the billet.

2. The microalloying method for promoting the refinement and re-dissolution of NbX phase in austenitic heat-resistant steel according to claim 1, characterized in that, The solid solution treatment temperature is 1200 DEG C, and the solid solution treatment time is 60 min.

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