CN114807769A

CN114807769A - Double-phase heat-resistant steel with controllable TCP phase distribution and application thereof

Info

Publication number: CN114807769A
Application number: CN202210717477.6A
Authority: CN
Inventors: 王寅杰; 宫高全; 陈小华
Original assignee: Kehua Holdings Co ltd
Current assignee: Kehua Holdings Co ltd
Priority date: 2022-06-23
Filing date: 2022-06-23
Publication date: 2022-07-29
Anticipated expiration: 2042-06-23
Also published as: CN114807769B

Abstract

The invention belongs to the technical field of stainless steel-heat-resistant steel material metallurgy, and particularly relates to a dual-phase heat-resistant steel with controllable TCP phase distribution and application thereof, wherein the dual-phase heat-resistant steel comprises the following components in percentage by mass: c: 0.2 to 0.6 percent; si: 0.5 to 1.0 percent; mn is less than 2.0 percent; cr: 20 to 24 percent; ni: 1.5% -4%; p is less than or equal to 0.04 percent; s is less than or equal to 0.3 percent; w + Mo: 0.5 to 2 percent; n: 0.1 to 0.25 percent; the balance of iron and other inevitable impurity elements; the prepared dual-phase heat-resistant steel with controllable TCP phase distribution controls the distribution of TCP phases during long-time use at high temperature through the ferrite phase in the cast state, the dual-phase heat-resistant steel has 20-50% of island-shaped distributed ferrite phase in the cast state, and the TCP phases are separated out at the ferrite-austenite phase interface when the dual-phase heat-resistant steel is used at high temperature, are distributed into an isolated spherical surface, and have little influence on the creep resistance and fatigue resistance of materials.

Description

Double-phase heat-resistant steel with controllable TCP phase distribution and application thereof

Technical Field

The invention belongs to the technical field of stainless steel-heat-resistant steel material metallurgy, and particularly relates to a dual-phase heat-resistant steel with controllable TCP phase distribution and application thereof.

Background

In duplex stainless steel, the ferrite phase and the austenite phase account for about half of each other in the solid solution structure, and the content of the minor phase is generally required to be 30%. Under the condition of low C content, the Cr content is 18-28%, and the Ni content is 3-10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, N, etc. The steel has the characteristics of both austenitic stainless steel and ferritic stainless steel, and compared with ferrite, the steel has higher plasticity and toughness, no room temperature brittleness, obviously improved intergranular corrosion resistance and welding performance, and simultaneously keeps the characteristics of high 475 ℃ brittleness and heat conductivity coefficient, superplasticity and the like of the ferritic stainless steel. Compared with austenitic stainless steel, the strength is high, and the intergranular corrosion resistance and the chloride stress corrosion resistance are obviously improved. The duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

However, duplex stainless steels still have various brittleness tendencies of high chromium ferritic stainless steels and are not suitable for use in working conditions above 300 ℃.

Disclosure of Invention

The invention provides a dual-phase heat-resistant steel with controllable TCP phase distribution and application thereof, aiming at solving the technical problem that dual-phase stainless steel is not suitable for working in a high-temperature environment.

In order to solve the technical problems, the invention provides a dual-phase heat-resistant steel with controllable TCP phase distribution, which comprises the following components in percentage by mass: c: 0.2 to 0.6 percent; si: 0.5 to 1.0 percent; mn is less than 2.0 percent; cr: 20 to 24 percent; ni: 1.5% -4%; p is less than or equal to 0.04 percent; s is less than or equal to 0.3 percent; w + Mo: 0.5 to 2 percent; n: 0.1 to 0.25 percent; the balance being iron and other unavoidable impurity elements.

In still another aspect, the invention also provides application of the dual-phase heat-resistant steel with the controllable TCP phase distribution in an exhaust manifold and a turbocharger housing of an automobile engine.

The dual-phase heat-resistant steel with the controllable TCP phase distribution has the beneficial effects that the distribution of the TCP phase is controlled by the ferrite phase in the casting state during long-time use at high temperature, the island-shaped distribution of the ferrite phase with the volume percentage of 20-50% exists in the dual-phase heat-resistant steel in the casting state, and the TCP phase is separated out at the ferrite-austenite phase interface during high-temperature use, is an isolated spherical surface in distribution and has little influence on the creep resistance and fatigue resistance of the material.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

FIG. 1 is an as-cast condition of example 1 of a dual phase heat resistant steel of the present invention in which the phase distribution of TCP is controlled; FIG. 2 is a spherical distribution map of example 1 of the dual phase heat resistant steel of the present invention in which the phase distribution of TCP is controlled; FIG. 3 is an as-cast condition of comparative example 2 of the dual phase heat resistant steel of the present invention in which the phase distribution of TCP is controlled; fig. 4 is a net distribution analysis chart of comparative example 2 of the dual phase heat resistant steel of the present invention in which the TCP phase distribution is controlled.

Detailed Description

To make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings, and it is apparent that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Duplex stainless steels are stainless steels in which the ferrite and austenite phases are about half each. The steel has the advantages of both ferritic stainless steel and austenitic stainless steel, and has better welding performance, normal temperature strength and chloride stress corrosion resistance. The continuous use temperature range of the existing duplex stainless steel is-50-250 ℃, the highest use temperature is in a methanol synthesis reactor, and the temperature is less than 300 ℃, which is caused by low-temperature brittle transition and 475 ℃ brittleness of a ferrite phase.

In the iron-based austenitic stainless steel/heat-resistant steel, nickel is a main austenitizing element, and the main function of the nickel is to form and stabilize austenite, so that the heat-resistant steel obtains good high-temperature mechanical property, oxidation resistance and corrosion resistance, and not only is the phase change caused by cooling to room temperature avoided, but also the TCP phase is prevented from being separated out when the heat-resistant steel is used for a long time at high temperature. Nickel is a precious metal element and belongs to strategic resources, and the mass percentage of nickel in the austenitic heat-resistant steel is usually more than 9 percent, so that the product price is high. The nickel content in the duplex stainless steel is usually less than 7%, and the addition of nickel can be further reduced by adding a small amount of nitrogen and carbon elements, so that the aim of reducing the cost of raw materials is fulfilled.

In the iron-based austenitic heat-resistant steel/high-temperature alloy, the electron vacancy number of nickel is 0.66, and the nickel is the most important element for reducing the average electron vacancy number of a matrix and plays a key role in inhibiting the precipitation of a TCP phase at high temperature. The pure reduction of nickel in the iron-based austenitic heat-resistant steel/high-temperature alloy and the supplement of nickel equivalent by elements such as manganese, nitrogen and the like can form a complete austenite matrix in an as-cast state, but during high-temperature use, a TCP phase can be separated out in a net shape, and the creep property and the fatigue property of the material are seriously reduced.

Nitrogen and carbon act as strong austenite forming elements, and when they act as interstitial solid solution elements, they act as austenite stabilizing elements about 30 times as much as nickel, so that the amount of nickel used in the austenitic heat-resistant steel can be greatly reduced, and the material cost can be reduced. However, in actual production, the addition of nitrogen often causes the defects of nitrogen pores on castings, and seriously affects the machining and cutting performance of materials, and carbon can also form carbide when being excessive, so that the cost of machining tools is increased, and the comprehensive production cost is increased and cannot be paid back. The carbon content of the existing mass-produced duplex stainless steel is usually less than 0.03 percent, and because the service condition is low-temperature corrosion resistance, the precipitation of carbide can reduce the normal-temperature corrosion resistance of the material.

As shown in fig. 1 and fig. 2, specifically, the dual-phase heat-resistant steel of the present invention controls the distribution of TCP phase during long-time use at high temperature through the ferrite phase in the as-cast state, and the dual-phase heat-resistant steel has 20% to 50% volume percentage of island-shaped distribution of ferrite phase in the as-cast state, and during high-temperature use, the TCP phase is precipitated at the ferrite-austenite phase interface, and the distribution is an isolated spherical surface, which has little influence on the creep resistance and fatigue resistance of the material, and avoids the net precipitation of TCP phase in austenite, thereby improving the stability of austenite high-temperature stability, i.e. high-temperature mechanical property, of the heat-resistant material with high average electronic air number (low nickel and low cost), and eliminating the precipitation type nitrogen pore defect; the adding range of nitrogen and carbon elements is optimized, and a proper amount of free-cutting elements are added, so that the processing cost of the casting is lower than that of common chromium-nickel austenitic heat-resistant steel; the tensile strength at 900 ℃ is 2 times that of D5S austenitic heat-resistant ductile iron, the creep rupture time at 30MPa is 20 times that of D5S austenitic heat-resistant ductile iron, and the cost of raw materials is greatly reduced.

In this embodiment, specifically, in the dual-phase heat-resistant steel of the present invention, C may form a carbide with high thermal stability with Cr and W elements at the grain boundary, so as to perform a precipitation strengthening effect on the grain boundary, reduce the creep speed of the grain boundary, and thereby improve the service life of the component. Meanwhile, in the invention, C and N replace noble metal Ni together, which plays the roles of stabilizing austenite matrix and reducing raw material cost. At compositions defined in the present invention, C below 0.2%, the volume fraction of ferrite phase in the as-cast state exceeds 50%, destroying the network distribution of austenite and thus seriously degrading the high-temperature mechanical properties of the material. As shown in fig. 3 and 4, when the C content exceeds 0.6%, the volume fraction of the ferrite phase in the as-cast state is less than 5%, and the TCP phase is precipitated in the austenite phase in a network form rather than in the ferrite-austenite interface when used for a long time at 900 ℃ or more, which seriously deteriorates the creep resistance and fatigue resistance of the material. Based on this, the mass percent of C in the austenitic heat-resistant steel of the present invention is controlled to be 0.2 to 0.6%.

In this example, in the dual phase heat resistant steel of the present invention, in particular, Si functions as auxiliary deoxidation during melting to improve the fluidity of molten steel, reduce the defects of cast slag holes, and slightly improve the high temperature oxidation resistance and corrosion resistance. Since Mn is considered as a harmful element and is limited, Si is the only strong deoxidizing element in molten steel during smelting, thereby reducing the burning loss of other precious alloys and controlling the acidity and alkalinity of slag. When the content is more than 1.0% or less than 0.5%, the fluidity of molten steel is greatly reduced, and the defects of casting slag holes are increased rapidly. Si is a main ferrite forming element, the electron vacancy number is as high as 6.66, and the high Si content can increase the ferrite phase content in an as-cast state, reduce the strength of the material above 900 ℃, reduce the high-temperature stability of an austenite matrix and carbide, promote the precipitation of harmful TCP phases and reduce the service life of parts. In the austenitic heat-resistant steel of the present invention, the mass percentage of Si is controlled to 0.5 to 1.0%.

In this example, specifically, in the dual phase heat resistant steel of the present invention, Mn is a harmful element, and the content thereof needs to be controlled to < 2.0%. Generally, Mn has the effect of reducing the cost of raw materials by replacing Ni, and can also react with a harmful element S to form spherical MnS, thereby reducing the hot brittleness of grain boundary FeS. However, when the mass percent of the slag is more than 2.0%, the precipitated air hole defects are obviously increased, the alkalinity of the steel slag is enhanced, and the corrosion of a furnace lining is accelerated. According to Pauling's theory, the electron vacancy number of Mn is 3.66, which is higher than 2.66 of the main component Fe element, and is the highest among austenite forming elements, and in the dual-phase heat-resistant steel with low nickel equivalent, the formation of TCP phase is promoted, the stability of austenite matrix is reduced, and the creep speed is increased. Mn is brought in by raw materials, and high-manganese scrap steel is forbidden.

In this embodiment, specifically, in the dual-phase heat-resistant steel of the present invention, Cr mainly functions to provide 900-. However, Cr is a ferrite-forming element, and if the content of Cr is too high, a ferrite phase appears in an as-cast state, and the ferrite is less than one tenth of austenite in tensile strength at the temperature of over 900 ℃, so that the high-temperature mechanical properties of the part are seriously reduced. Cr is also a TCP phase forming element, the electron vacancy number reaches 4.66, and when the mass percent of Cr is less than 20%, the solubility of N in molten steel is less than 0.15%, so that a TCP phase appears in an as-cast structure, and the defect of casting pores is increased. When the mass percent of Cr is more than 24 percent, and the content of the rest alloy elements meets the requirement, the volume fraction of a ferrite phase in an as-cast state exceeds 50 percent, and the net distribution of austenite is destroyed, so that the high-temperature mechanical property of the material is seriously reduced. Based on this, in the technical scheme of the invention, the mass percent of Cr is controlled to be 20-24%.

In this example, specifically, in the dual-phase heat-resistant steel of the present invention, Ni is a main austenite forming element, the number of electron vacancies is 0.66, and it is the strongest element that suppresses the TCP phase among the main alloy elements of the iron-based heat-resistant steel. Ni is the alloy element that accounts for the highest proportion of raw material costs. Ni also decreases the solubility of N, and at higher levels of both elements, increases the cast porosity defects. When the content of other alloying elements meets the requirement and the mass percent of Ni is less than 1.5%, the volume fraction of ferrite phase in an as-cast state exceeds 50%, and the net distribution of austenite is destroyed, so that the high-temperature mechanical property of the material is seriously reduced. When the Ni content exceeds 4%, the volume fraction of ferrite phase in cast state is less than 20%, and TCP phase can be separated out in network form in austenite phase instead of ferrite/austenite interface when it is used for a long time at 900 deg.C, so that it can seriously damage creep resistance and fatigue property of material. Based on this, in the technical scheme of the invention, the mass percent of Ni is controlled to be 1.5-4%.

In this example, in the dual phase heat resistant steel of the present invention, in particular, W and Mo mainly function to form carbide with C to perform precipitation strengthening, and W solid-dissolved in the matrix may perform solid-solution strengthening to improve creep rupture time and suppress nitrogen pore defects. W and Mo may also enhance the pitting corrosion resistance of the material against Cl-ions at high temperatures. When the addition amount of W and Mo exceeds 2%, the creep rupture time does not increase along with the addition amount, and the equivalent weight of chromium and the average electron vacancy number of the material increase, so that the mass percent of W and Mo is controlled to be 0.5-2% in the technical scheme of the invention.

In this example, specifically, in the dual phase heat resistant steel of the present invention, N is a main austenite forming element, and solid-dissolved N can replace about 30 times Ni to reduce the raw material cost. N may also enhance the resistance of the material to Cl at high temperatures ^- Pitting corrosion performance of the ions. The solid solution content of N in the ferrite phase is very small, and the solid solubility in the austenite phase is less than 0.5%, so when the ferrite phase and the austenite phase respectively account for about 50%, the mass percent of N in the dual-phase heat-resistant steel is less than 0.25%, so as to avoid the precipitation type nitrogen pore defect. Based on this, in the technical scheme of the invention, the mass percent of N is controlled to be 0.1-0.25%.

Wherein the tensile strength of the single-cast test bar of the dual-phase heat-resistant steel at 900 ℃ is not lower than 110MPa, the yield strength is not lower than 80MPa, and the elongation after fracture is not lower than 25%.

The creep rupture time of the dual-phase heat-resistant steel at 900 ℃ and 30MPa is not less than 60 h.

The TCP phase of the dual-phase heat-resistant steel is separated out along an as-cast ferrite-austenite interface in a spherical distribution.

Specifically, the upper limit of the working temperature of the dual-phase heat-resistant steel is 950 ℃.

The dual phase heat resistant steel was prepared with reference to the components and proportions in table 1.

TABLE 1 elemental composition and proportions of the two-phase heat-resistant steels in examples and comparative examples

After testing the relevant properties in the examples and comparative examples in table 1, the data are summarized in table 2.

Of these, comparative example 5 is D5S heat-resistant ductile iron.

As can be seen from the data in fig. 1 and 2 and table 2, the dual phase heat resistant steel of the present invention controls the distribution of TCP phase by ferrite phase in an as-cast state for a long time use at high temperature, so that TCP phase is precipitated in a spherical distribution along an as-cast ferrite/austenite interface, thereby improving high temperature stability and eliminating the precipitation type nitrogen pore defect; meanwhile, the mechanical property of the dual-phase heat-resistant steel is greatly improved due to the optimized addition range of nitrogen and carbon elements and the addition of easy-cutting elements.

From comparative example 1, it is understood that when the content of C element is less than 0.2%, the volume fraction of ferrite phase in the as-cast state exceeds 50%, and the net distribution of austenite is broken to seriously lower the high temperature mechanical properties of the material, so that the properties are remarkably lower than those of the examples.

From comparative example 2 and fig. 3 and 4, it is understood that when the C content exceeds 0.6%, the volume fraction of ferrite phase in the as-cast state is less than 5%, TCP phase is precipitated in the austenite phase in the form of network rather than in the ferrite/austenite interface when used for a long period of time at 900 ℃ or more, and creep resistance and fatigue property of the material are seriously deteriorated, and although tensile strength and yield strength are both clearly indicated when the C content is excessive, elongation and creep rupture time are both affected.

As is clear from comparative example 3, when the Ni content exceeds 4%, the volume fraction of the ferrite phase in the as-cast state is less than 20%, and the TCP phase is precipitated in the austenite phase in a network form, not in the ferrite-austenite interface, upon long-term use at 900 ℃ or more, which seriously deteriorates the creep resistance and fatigue resistance of the material.

As is clear from comparative example 4, when Si is more than 1.0%, the fluidity of molten steel is greatly reduced and the defects of casting slag holes are rapidly increased. Si is a main ferrite forming element, the electron vacancy number is as high as 6.66, the content of the Si is too high, the content of a ferrite phase in an as-cast state is increased, the strength of the material at the temperature of over 900 ℃ is reduced, the high-temperature stability of an austenite matrix and carbide is reduced, the precipitation of harmful TCP phases is promoted, and the service life of parts is shortened; when the mass percent of Cr is more than 24 percent, and the content of the rest alloy elements meets the requirement, the volume fraction of a ferrite phase in an as-cast state exceeds 50 percent, and the net distribution of austenite is destroyed, so that the high-temperature mechanical property of the material is seriously reduced.

In conclusion, the prepared dual-phase heat-resistant steel with controllable TCP phase distribution controls the distribution of the TCP phase during long-time use at high temperature through the ferrite phase in the cast state, the dual-phase heat-resistant steel has 20-50% of island-shaped distributed ferrite phase in the cast state, and the TCP phase is separated out at the ferrite-austenite phase interface during high-temperature use, the distribution is an isolated spherical surface, the creep resistance and fatigue resistance of the material are slightly influenced, and the net separation of the TCP phase in austenite is avoided, so that the high-temperature stability of the austenite, namely the stability of high-temperature mechanical properties of the heat-resistant material with high average electronic air number (low nickel and low cost) is improved, and the defect of nitrogen gas separation holes is eliminated; the adding range of nitrogen and carbon elements is optimized, and a proper amount of free-cutting elements are added, so that the processing cost of the casting is lower than that of common chromium-nickel austenitic heat-resistant steel; the tensile strength at 900 ℃ is 2 times that of D5S austenitic heat-resistant ductile iron, and the creep rupture time at 30MPa is 20 times that of D5S austenitic heat-resistant ductile iron.

In light of the foregoing description of the preferred embodiment of the present invention, many modifications and variations will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. The dual-phase heat-resistant steel with the controllable TCP phase distribution is characterized by comprising the following components in percentage by mass: c: 0.2 to 0.6 percent; si: 0.5 to 1.0 percent; mn is less than 2.0 percent; cr: 20 to 24 percent; ni: 1.5% -4%; p is less than or equal to 0.04 percent; s is less than or equal to 0.3 percent; w + Mo: 0.5 to 2 percent; n: 0.1 to 0.25 percent; the balance being iron and other unavoidable impurity elements.

2. A dual phase heat resistant steel as set forth in claim 1, wherein said dual phase heat resistant steel has a tensile strength of not less than 110MPa, a yield strength of not less than 80MPa, and a post fracture elongation of not less than 25% at 900 ℃.

3. A dual phase heat resistant steel according to claim 1, characterized in that it has a creep rupture time of not less than 60h at 900 ℃ and 30 MPa.

4. A dual phase heat resistant steel as claimed in claim 1, wherein TCP phase of said dual phase heat resistant steel is precipitated in a spherical distribution along an as-cast ferrite-austenite interface.

5. Use of the dual phase heat resistant steel of claim 1 in exhaust manifolds and turbocharger housings of automotive engines.

6. Use of a dual phase heat resistant steel according to claim 5 in automotive engine exhaust manifolds and turbocharger housings, wherein the dual phase heat resistant steel has an upper operating temperature limit of 950 ℃.