CN110643896A

CN110643896A - Ultra-supercritical high-nitrogen martensite cast steel and preparation method thereof

Info

Publication number: CN110643896A
Application number: CN201910933237.8A
Authority: CN
Inventors: 董治中; 孔维雄; 宁保群; 王志奇
Original assignee: Tianjin University of Technology
Current assignee: Tianjin University of Technology
Priority date: 2019-09-29
Filing date: 2019-09-29
Publication date: 2020-01-03

Abstract

An ultra-supercritical high-nitrogen martensite cast steel and a preparation method thereof, wherein the cast steel mainly comprises the following components in percentage by weight: c: 0.005% -0.05%; n: 0.04 to 0.65 percent; cr: 8.0% -12.0%; w: 3.5% -6.5%; co: 3.5% -4.5%; mo: 0.5 to 1.5 percent; v: 0.4% -0.8%; nb: 0.01 to 0.15 percent; mn: 0.03 to 0.80 percent; si: 0.02% -0.10%; ni: 0.005% -0.04%; hf: 0.01 to 0.10 percent; la + Ce: 0.008 percent to 0.10 percent; fe: and (4) the balance. The high-temperature performance and the oxidation resistance of the heat-resistant cast steel are improved by reducing carbon elements, increasing the content of nitrogen elements and strengthening by nitrides; the proper amount of the trace element Hf and the rare earth element La + Ce improve the structure of the steel and ensure that a complete martensite structure is finally obtained. The preparation process combining the pressurized vacuum induction and the pressurized electroslag remelting effectively increases the solid solubility of nitrogen in steel and enables the steel ingot structure to be more uniform and compact.

Description

Ultra-supercritical high-nitrogen martensite cast steel and preparation method thereof

Technical Field

The invention belongs to the technical field of heat-resistant steel, and relates to novel ultra-supercritical high-nitrogen martensite cast steel and a preparation method thereof.

Background

At present, the energy utilization of China is mainly primary energy application, and the thermal power generation capacity of China accounts for more than 75% of the total power generation amount. In order to alleviate the problem of energy shortage, improving the energy utilization efficiency is the primary task of development of all countries in the world. The development of ultra supercritical technology can improve the energy utilization rate, and the steel for ultra supercritical material is the key point for developing the technology. At present, the research on the material in many countries keeps an active attitude, and the America, the Japan and the Europe are the leading technical leaders of the ultra-supercritical material in the world. China starts to research ultra supercritical later and is in the stages of analysis and exploration.

The heat-resistant materials for the ultra-supercritical unit cylinder body at home and abroad are mainly divided into three major types, namely martensite heat-resistant steel, austenite heat-resistant steel and nickel-based alloy. Because of the high cost of nickel-based alloys, the high expansion coefficient and thermal fatigue resistance of austenitic heat-resistant steels are inadequate, while martensitic steels have high strength, high thermal conductivity and low expansion coefficient, martensite is an ideal heat-resistant steel for supercritical coal-fired power plants. The 9% Cr heat-resistant steel is an ideal steel grade for the heat-resistant steel of the ultra-supercritical coal-fired power plant due to the properties of high strength, high thermal conductivity and low expansion coefficient. Since the 50 s of the 20 th century, a four-generation heat-resistant steel system has been formed through alloying development on the basis of 9Cr steel to date. In the COST project of Europe, steel CB2 for a cylinder body is successfully developed, and the service temperature of the steel CB is 620 ℃. In order to further increase the steam temperature and improve the power generation efficiency, it is urgently needed to further develop novel heat-resistant cast steel with higher service temperature.

Researches find that the strength, the ductility, the toughness and the corrosion resistance of the steel can be effectively improved by adding nitrogen into the steel, and the nitrogen is inexhaustible resource in the atmosphere, so the high-nitrogen steel is more and more valued by the steel enterprises in China due to the low cost and excellent comprehensive performance of the high-nitrogen steel. At present, the research on high-nitrogen steel materials replacing nickel with nitrogen is vigorously carried out in various countries in the world, and the research on high-nitrogen steel is carried out in various countries such as Germany, Japan, Switzerland, Bulgaria and the like, and the high-nitrogen steel is applied to industries such as power generation industry, shipbuilding industry, railway industry and the like. However, the solubility of nitrogen in steel is low, and segregation of nitrogen and escape of nitrogen are easily caused during solidification, which severely restricts the development and application of high-nitrogen steel.

Disclosure of Invention

The invention aims to solve the problems that the existing nitrogen-containing heat-resistant cast steel has low solubility of nitrogen and is easy to cause the segregation of nitrogen and the escape of nitrogen in the solidification process, and provides the ultra-supercritical martensite cast steel added with Hf element and rare earth element, which is prepared by a high-pressure vacuum technology and a pressurized electroslag remelting technology, has a complete martensite structure and has good comprehensive mechanical property and high-temperature property.

The technical scheme of the invention is as follows:

the ultra-supercritical high-nitrogen martensitic cast steel is characterized by that it utilizes the addition of high-content nitrogen element to replace expensive nickel element, and uses nitride strengthening mode to raise yield strength, tensile strength, wear resistance and corrosion resistance of steel. The cast steel comprises the following main alloy element components in percentage by mass: n: 0.04-0.65% (preferably 0.15-0.6%, more preferably 0.2-0.5%); cr: 8.0% -12.0%; w: 3.5% -6.5%; co: 3.5% -4.5%; mo: 0.5 to 1.5 percent; v: 0.4% -0.8%; nb: 0.01 to 0.15 percent; mn: 0.03 to 0.80 percent; si: 0.02% -0.10%; ni: 0.005% -0.04%; fe: and (4) the balance. The cast steel further comprises: c: 0.005% -0.05% (preferably 0.008% -0.04%, more preferably 0.01% -0.03%); trace elements and rare earth elements: hf: 0.01 to 0.10 percent (preferably 0.02 to 0.07 percent, and more preferably 0.03 to 0.06 percent); la + Ce: 0.008% -0.10% (preferably 0.01% -0.08%, more preferably 0.02% -0.08%), wherein the weight ratio of lanthanum to cerium is 1: 2.

the preparation method of the ultra-supercritical high-nitrogen martensite cast steel provided by the invention comprises the following steps:

the first step is as follows: the raw materials are proportioned according to the above component proportion and alloy burning loss;

the second step is that: adding alloy raw materials in sequence in a vacuum induction furnace according to the burning loss and volatilization characteristics of alloy elements, and introducing nitrogen protective atmosphere. Wherein the nitrogen element is added in the form of chromium nitride iron. A pressurizing mode is adopted in the process to ensure that nitrogen cannot escape in the form of nitrogen, and the pressure is more than or equal to 10 atmospheric pressures;

the third step: rare earth alloying is carried out on the smelted molten steel; preparing a lanthanum-cerium rare earth alloy, wherein the weight ratio of lanthanum to cerium in the lanthanum-cerium rare earth alloy is 1: 2, using rare earth alloy with the rare earth element accounting for 0.08 wt% in the alloy; adding rare earth alloy into the ladle, directly adding the rare earth alloy into molten steel in a bell jar form, and reacting after passing through the slag surface of the steel slag for 500 mm; after reacting for 5 minutes, pouring is started;

the fourth step: and after the steel ingot is cooled, carrying out pressurized electroslag remelting treatment on the steel ingot. The method comprises the specific steps of slag preparation, slag melting, pressure increasing, smelting state entering, power decreasing, feeding, pressure releasing and demoulding.

The fifth step: cutting small steel ingot samples for spectral analysis after the steel ingots are cooled;

and a sixth step: carrying out heat treatment on the prepared martensite heat-resistant steel ingot: homogenizing 1050-10 h-air cooling, normalizing 1100-2 h-air cooling, and tempering 760-2 h-air cooling.

And in the second step, a pressurized vacuum induction furnace is used for smelting the ultrahigh nitrogen heat-resistant cast steel, the temperature is controlled to be 1500-1650 ℃, and the pressure is greater than or equal to 10 atmospheric pressures.

According to the invention, a proper amount of trace element Hf and rare earth element La + Ce are added to improve the structure of steel, and finally, the obtained cast ingot is subjected to pressurized electroslag remelting, so that on one hand, the nitrogen element dissolved in the steel is prevented from escaping, and on the other hand, a clean steel ingot with uniform and compact structure can be obtained. The tempered cast steel structure is distributed with fine dispersed Laves phases and various nitride strengthening phases, so that dislocation can be effectively pinned, creep strain accumulation is delayed, and excellent high-temperature durable heat strength is ensured.

The principle of the invention is that:

(1) the rare earth element is taken as a surface active substance, is easy to be subjected to segregation at a grain boundary, reduces austenite grain boundary energy and surface tension, so that grains are refined, in addition, the rare earth element can be enriched around a high-melting-point carbide, and the carbide is difficult to nucleate on the grain boundary, so that the carbide is organized to be separated out and grow along the grain boundary, and is changed into granular carbide which is distributed discontinuously; the rare earth elements can also purify molten steel deeply and deteriorate harmful inclusions; in addition, tests show that the rare earth can reduce the austenite fault energy of steel, so that the steel can easily obtain lath martensite structures, and the rare earth can also refine the width of the martensite laths; the addition of the rare earth elements can also improve the creep activation energy of steel, so that the energy barrier which needs to be overcome when the material is subjected to creep is increased, and the creep rate is reduced; the generation and the expansion of the cracks are delayed, and partial cracks can be changed into hollow shapes from wedges, and the nucleation, the expansion and the connection speed of the cracks are slower than those of the cracks, so that the service life is obviously prolonged. Tests also show that the rare earth elements can reduce the austenite fault energy and inhibit proeutectoid ferrite, so that the steel can obtain lath martensite structure more easily.

(2) The proper amount of Hf in the steel can effectively change the redistribution of alloy elements between precipitated phases and gamma solid solutions, so that the strength and the plasticity of the steel are well matched; hf entering the gamma phase is subjected to segregation to the grain boundary, so that the grain boundary can be strengthened, and the plasticity is improved; hf can improve the form of primary carbonitride, so that Chinese character cursive-writing-shaped carbonitride is converted into discontinuous particles, thereby eliminating fracture channels, delaying the initiation and propagation of cracks, and improving the high-temperature creep resistance and crack propagation resistance of the alloy; besides, Hf is beneficial to improving the oxidation resistance of steel and the comprehensive mechanical property of alloy.

(3) The nitrogen can interact with other elements in the steel, and a large amount of VN, NbN and Cr can be separated out from the steel after tempering treatment₂The precipitated phases of nitrides such as N, CrN and the like can be effectively strengthenedThe martensite lath boundary is changed, and dislocation is pinned, so that the yield strength, tensile strength, wear resistance, corrosion resistance and high-temperature resistance of the martensite lath are improved; in addition, nitrogen can replace nickel to be used as an austenite stabilizing element to inhibit the precipitation of phases, so that the performance of the steel is improved, and the production cost is greatly reduced.

(4) The obtained steel ingot has smooth surface, cleanness, uniformity, compactness, uniform metallographic structure and chemical components through the pressurized electroslag remelting process, and the high pressure mode ensures that nitrogen elements in the steel cannot escape in the form of nitrogen in the remelting process.

The invention has the advantages and beneficial effects that:

(1) the content of nitrogen element added into the steel is more than 0.04 percent, so that the nitrogen element and elements such as vanadium, niobium, chromium, iron and the like form a precipitation phase and a dispersion phase such as vanadium nitride, niobium nitride and the like, thereby improving the creep strength and the oxidation resistance of the heat-resistant steel. In addition, nitrogen can replace nickel as an austenite stabilizing element, so that the precipitation of a delta phase can be effectively inhibited, and the production cost can be reduced.

(2) The content of carbon element in the steel is reduced to be less than 0.05 percent, so that the strengthening effect of nitride in the high-nitrogen martensite heat-resistant cast steel plays a leading role and is matched with a certain carbide strengthening effect.

(3) The nitrogen element in the steel is added in the form of chromium nitride iron and high-pressure nitrogen charging. This is because the high nitrogen ferrochrome is composed of Cr, N and small amounts of Si and C, and thus less inclusions are introduced. The high-pressure nitrogen filling mode plays a role in gas protection on one hand, and on the other hand, nitrogen dissolved in steel cannot escape under the high-pressure effect.

(4) The trace Hf element is added into the steel, so that redistribution of alloy elements between a precipitated phase and a gamma solid solution can be effectively changed, the strength and plasticity of the steel can be well matched, and the high-temperature creep resistance and crack propagation resistance of the alloy can be improved; besides, Hf element is beneficial to improving the oxidation resistance of steel and the comprehensive mechanical property of alloy.

(5) A proper amount of rare earth element La + Ce is added to refine original austenite grains, refine the width of a martensite lath, prolong the service life of steel, reduce austenite fault energy, inhibit proeutectoid ferrite and enable the steel to obtain lath martensite structures more easily.

(6) The preparation method of the cast steel comprises the steps of pressurizing vacuum induction and pressurizing electroslag remelting. This is because, in the case of martensitic steels, the nitrogen content of 0.08% reaches its limit nitrogen content, and therefore the production of high-nitrogen or ultra-high-nitrogen martensitic heat-resistant cast steels requires high pressures. The alloying of nitrogen can be realized by the pressurized vacuum induction melting, so that the nitrogen is uniformly distributed, the nitrogen content of the ingot can be accurately controlled, and the ingot is ensured not to volatilize in the form of nitrogen. And in the subsequent pressurizing electroslag remelting process, the contents of impurities and sulfur are effectively reduced, the segregation is improved, and the steel ingot is purified, homogenized and densified.

Drawings

FIG. 1 is a photograph of the as-cast and tempered metallographic structure of the steel of example 1.

FIG. 2 is a photograph of the as-cast and tempered metallographic structure of the steel of example 2.

FIG. 3 is a photograph of the as-cast and tempered metallographic structure of the steel of example 3.

FIG. 4 is a photograph of grain boundary corrosion and statistical data of grain size after normalizing for the steel of example 3.

FIG. 5 is a transmission electron micrograph of the steel of example 3 after tempering showing a fully martensitic lath structure.

FIG. 6 is a transmission electron micrograph of a precipitated phase of the steel of example 3 after tempering treatment.

FIG. 7 shows the results of mechanical properties after tempering of the steels of examples 1 to 4.

The specific implementation mode is as follows:

example 1: preparation of ultra-supercritical high-nitrogen martensite cast steel

The first step is as follows: raw materials are proportioned according to the composition proportion (C: 0.03%, N: 0.1%, Cr: 9.0%, V: 0.5%, Nb: 0.03%, W: 6.5%, Co: 3.9%, Mo: 0.9%, Mn: 0.06%, Si: 0.05%, Ni: 0.015%, Hf: 0.02%, La + Ce: 0.01%, and the balance of Fe) and the burning loss amount of the alloy.

The second step is that: introducing nitrogen protective atmosphere into the vacuum induction furnace, and adding alloy raw materials in sequence according to the burning loss and volatilization characteristics of alloy elements. Wherein the nitrogen element is added in the form of chromium nitride iron. A pressurizing mode is adopted in the process to ensure that nitrogen cannot escape in the form of nitrogen, and the pressure is more than or equal to 10 atmospheric pressures;

the third step: and cutting small steel ingot samples after the steel ingots are cooled for spectral analysis, wherein the measured chemical components are qualified.

The fourth step: carrying out heat treatment on the prepared cast steel: homogenizing (1050 ℃ -10 h-air cooling), normalizing (1100 ℃ -2 h-air cooling), and tempering (760 ℃ -2 h-air cooling).

Example 2:

based on the alloy components of example 1, the nitrogen content is increased to 0.25%, and the rare earth alloy is added, in a specific manner as follows:

the first step is as follows: raw materials are proportioned according to the composition proportion (C: 0.03%, N: 0.25%, Cr: 9.0%, V: 0.5%, Nb: 0.03%, W: 6.5%, Co: 3.9%, Mo: 0.9%, Mn: 0.06%, Si: 0.05%, Ni: 0.015%, Hf: 0.03%, La + Ce: 0.03%, and the balance of Fe) and the burning loss amount of the alloy.

the third step: and carrying out rare earth alloying on the smelted molten steel. A lanthanum cerium rare earth alloy (in the lanthanum cerium rare earth alloy, the weight ratio of lanthanum to cerium is 1: 2) is prepared, and a rare earth alloy in which the rare earth element in the alloy accounts for 0.08 wt% is used. Adding rare earth alloy into the ladle, adopting a bell jar form, directly adding the rare earth alloy into molten steel, penetrating through steel slag, and reacting at a temperature of 300-500mm below the slag surface. After 5 minutes of reaction, casting was started.

The fourth step: and cutting small steel ingot samples after the steel ingots are cooled for spectral analysis, wherein the measured chemical components are qualified.

The fifth step: carrying out heat treatment on the prepared cast steel: homogenizing (1050 ℃ -10 h-air cooling), normalizing (1100 ℃ -2 h-air cooling), and tempering (760 ℃ -2 h-air cooling).

Example 3:

based on the alloy composition of example 2, the nitrogen content was adjusted to 0.3%, and the cast steel was subjected to vacuum pressure electroslag remelting process. The specific mode is as follows:

the first step is as follows: the raw materials are proportioned according to the composition proportion (C: 0.03%, N: 0.3%, Cr: 9.0%, V: 0.5%, Nb: 0.03%, W: 6.5%, Co: 3.9%, Mo: 0.9%, Mn: 0.06%, Si: 0.05%, Ni: 0.015%, Hf: 0.05%, La + Ce: 0.04%, and the balance of Fe) and the burning loss amount of the alloy.

the third step: and carrying out rare earth alloying on the smelted molten steel. A lanthanum cerium rare earth alloy (in the lanthanum cerium rare earth alloy, the weight ratio of lanthanum to cerium is 1: 2) is prepared, and a rare earth alloy in which the rare earth element in the alloy accounts for 0.08 wt% is used. Adding rare earth alloy into the ladle, adopting a bell jar form, directly adding the rare earth alloy into molten steel, penetrating through steel slag, and reacting at a temperature of 300-500mm below the slag surface. After 5 minutes of reaction, casting was started. .

The fifth step: and cutting small steel ingot samples after the steel ingots are cooled for spectral analysis, wherein the measured chemical components are qualified.

And a sixth step: carrying out heat treatment on the prepared cast steel: homogenizing (1050 ℃ -10 h-air cooling), normalizing (1100 ℃ -2 h-air cooling), and tempering (760 ℃ -2 h-air cooling).

Example 4:

the nitrogen content was adjusted to 0.5% based on the alloy composition of example 3. The specific mode is as follows:

the first step is as follows: the raw materials are proportioned according to the composition proportion (C: 0.03%, N: 0.5%, Cr: 9.0%, V: 0.5%, Nb: 0.03%, W: 6.5%, Co: 3.9%, Mo: 0.9%, Mn: 0.06%, Si: 0.05%, Ni: 0.015%, Hf: 0.05%, La + Ce: 0.06%, and the balance of Fe) and the burning loss amount of the alloy.

The as-cast and tempered metallographic structure observations were made on the steels of examples 1, 2 and 3 above and the results are shown in FIGS. 1 to 3, respectively. It was found that in example 1, a part of the ferrite structure was present in the as-cast state, and in example 2, the ferrite structure in the metallurgical phase was reduced as compared with example 1 in both the cast steel and the tempered steel, but nitride inclusions partially rich in chromium and vanadium were present, which were generated during the smelting process and were difficult to eliminate. The cast steel obtained in example 3 had the most uniform and dense microstructure composition, and the ferrite in the cast steel structures of examples 1 and 2 was removed, thereby obtaining a completely martensitic structure. The samples of example 3 after tempering were subjected toThe grain size measurement is carried out, and as shown in figure 4, the average diameter of the grains is 20.32um, and the grain size is eight grades. TEM observation shows that the heat-resistant steel in example 3 has fine laths after tempering and a large amount of fine dispersed Laves phases and VN, CrN and Cr₂N and other nitride strengthening phases are precipitated in the martensite lath boundary and the martensite lath (as shown in figures 5 and 6), and can effectively pin the dislocation and the lath boundary and delay the accumulation of creep strain, thereby improving the creep endurance performance. The four tempered test steels are subjected to conventional mechanical property tests, the test results are shown in figure 7, and the steel of example 3 has the optimal comprehensive mechanical property through comparison.

The novel high-nitrogen martensite heat-resistant cast steel finally obtains a uniform and compact steel ingot with good metallurgical quality by a preparation method of pressurizing vacuum induction and pressurizing electroslag remelting; the high-content nitrogen, a proper amount of rare earth element La + Ce and a trace element Hf in the steel can effectively improve the structure of the steel, and ensure good basic mechanical property and high-temperature heat strength of the steel.

Claims

1. An ultra-supercritical high-nitrogen martensite cast steel is characterized in that: the cast steel comprises the following main alloy components in percentage by mass: n: 0.04 to 0.65 percent; cr: 8.0% -12.0%; w: 3.5% -6.5%; co: 3.5% -4.5%; mo: 0.5 to 1.5 percent; v: 0.4% -0.8%; nb: 0% -0.15%; mn: 0.03 to 0.80 percent; si: 0.02% -0.10%; ni: 0.005% -0.04%; fe: and (4) the balance.

2. The ultra supercritical high nitrogen martensitic cast steel according to claim 1, characterized in that: the cast steel also comprises the following components: c: 0.005-0.05%.

3. The ultra supercritical high nitrogen martensitic cast steel according to claim 1, characterized in that: the cast steel also contains the following microelements and rare earth elements: hf: 0.01 to 0.10 percent; la + Ce: 0.008% -0.10%, wherein the weight ratio of lanthanum to cerium is 1: 2.

4. a method for preparing ultra supercritical high nitrogen martensitic cast steel as claimed in any one of claims 1 to 3, characterized in that: at least comprises the following steps:

the first step is as follows: proportioning raw materials according to the composition proportion and the alloy burning loss amount of any one of claims 1 to 3;

the second step is that: sequentially adding alloy raw materials in a vacuum induction furnace according to the burning loss and volatilization characteristics of alloy elements, and introducing nitrogen protective atmosphere; wherein the nitrogen element is added in a form of chromium nitride iron, a pressurizing mode is adopted in the process to ensure that the nitrogen element cannot escape in a form of nitrogen, and the pressure is more than or equal to 10 atmospheric pressures;

the fourth step: after the steel ingot is cooled, carrying out pressurized electroslag remelting treatment on the steel ingot; the method comprises the specific steps of preparing slag, melting the slag, increasing pressure to enter a melting state, decreasing power feeding, releasing pressure and demoulding;

5. The method for producing an ultra supercritical high nitrogen martensitic cast steel as claimed in claim 4, characterized in that: and secondly, smelting the ultrahigh nitrogen heat-resistant cast steel by using a pressurized vacuum induction furnace, wherein the temperature is controlled to be 1500-1650 ℃, and the pressure is greater than or equal to 10 atmospheric pressures.