Low-nickel austenite hot steel material for casting and preparation method and application thereof

Abstract

The invention belongs to the technical field of austenitic steel, and particularly relates to a low-nickel austenitic hot steel material for casting and a preparation method and application thereof, and the low-nickel austenitic hot steel material comprises the following elements in percentage by weight: 0.3-0.5% of C, 2.5-3.5% of Si, 10-16% of Mn, 17-19% of Cr, 1.5-3% of Ni, less than or equal to 0.04% of P, less than or equal to 0.15% of S, 0.2-0.35% of N, and the balance of iron and other inevitable impurity elements. According to the low-nickel austenite hot steel material for casting and application of the low-nickel austenite hot steel material, Si is used for replacing part of Cr, the nickel equivalent needed for stabilizing austenite is reduced on the premise that oxidation resistance at the temperature of 950 DEG C is guaranteed, Mn and N are used for replacing 8% of Ni, the raw material cost of heat-resistant steel is reduced, limitation on S is relaxed, the free-cutting effect of MnS is utilized, and the casting machining cost is not obviously improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of austenitic steel materials, and particularly relates to a low-nickel austenitic hot steel material for casting as well as a preparation method and application thereof. BACKGROUND

[0002] The austenitic heat-resistant steel refers to a heat-resistant steel with an austenitic structure at room temperature. The steel contains Cr about 20%-25%, Ni about 9%-40%, C about 0.3%-0.5%, and Nb about 0.5%-2%.

[0003] In the austenitic heat-resistant steel, nickel is a main austenitizing element, and its main role is to stabilize austenite, that is, to obtain an austenite matrix at room temperature and avoid the precipitation of too much brittle TCP phase in the austenite during high-temperature service, so that the heat-resistant steel has good high-temperature mechanical properties and oxidation resistance and corrosion resistance. Nickel is a valuable metal element, and the production of the austenitic heat-resistant steel needs to use a large amount of nickel element, resulting in high product raw material cost. Developing a low-nickel austenitic heat-resistant steel which is fully or partially replaced by other inexpensive alloy elements instead of nickel is conducive to reducing the cost and improving the market competitiveness of the heat-resistant steel product.

[0004] However, as one of the main materials of the austenitic steel, the decrease of the content of nickel needs to increase other components to maintain the stability of the steel performance. How to design a low-nickel austenitic steel which is applicable to casting and meets the requirements of high-temperature oxidation resistance and cold-hot impact resistance is a technical problem to be solved in the field.

[0005] It should be noted that the above information disclosed in the background section is only used to understand the background of the present application, and therefore, the above description is not considered to constitute the information of the prior art. SUMMARY

[0006] The present application provides at least a low-nickel austenitic hot steel material for casting as well as a preparation method and application thereof.

[0007] In a first aspect, the present application provides a low-nickel austenitic hot steel material for casting. The element composition of the material includes, in terms of weight percentage, C: 0.3-0.5%, Si: 2.5-3.5%, Mn: 10-16%, Cr: 17-19%, Ni: 1.5-3%, P≤0.04%, S≤0.15%, N: 0.2-0.35%, and the balance being iron and other unavoidable impurity elements.

[0008] In an optional embodiment, the element composition of the material includes C: 0.46%, Si: 2.6%, Mn: 11.5%, Cr: 18.9%, Ni: 1.8%, P: 0.015%, S: 0.08%, N: 0.21%, and Nb: 0.007%.

[0009] In an alternative embodiment, the elemental composition of the material comprises: C: 0.33%, Si: 3.1%, Mn: 12.1%, Cr: 17.5%, Ni: 1.6%, P: 0.018%, S: 0.13%, N: 0.27%, Nb: 0.005%.

[0010] In an alternative embodiment, the elemental composition of the material comprises: C: 0.43%, Si: 2.9%, Mn: 14.9%, Cr: 17.9%, Ni: 1.7%, P: 0.016%, S: 0.09%, N: 0.32%, Nb: 0.005%.

[0011] In an alternative embodiment, the elemental composition of the material comprises: C: 0.49%, Si: 3.4%, Mn: 15.6%, Cr: 18.9%, Ni: 2.9%, P: 0.019%, S: 0.08%, N: 0.23%, Nb: 0.006%.

[0012] In an alternative embodiment, the single cast test bar of the low-nickel austenitic heat-resistant steel material for casting has a tensile strength of >100 MPa, a yield strength of >90 MPa, and an elongation after fracture of >40% at 950°C.

[0013] In a second aspect, the embodiments of the present disclosure further provide a preparation method of the low-nickel austenitic heat-resistant steel material for casting as described above, comprising the following steps: S1, melting raw materials by using a medium-frequency electric furnace; S2, adding elements to a preset proportion after determining the composition of the molten steel by using a direct-reading spectrometer; S3, deoxidizing the molten steel by adding a deoxidizer; and S4, pouring the molten steel into a pouring ladle added with a modifier after heating the molten steel.

[0014] In an alternative embodiment, the frequency of the medium-frequency electric furnace is 500 Hz.

[0015] In an alternative embodiment, the temperature range of the molten steel in the S4 is 1600-1700°C.

[0016] In a third aspect, the embodiments of the present disclosure further provide an application of the low-nickel austenitic heat-resistant steel material for casting in the field of automobile engine exhaust pipes and turbocharger housings.

[0017] The low-nickel austenitic heat-resistant steel material for casting, the preparation method and the application thereof have the following beneficial effects: the Si replaces part of the Cr, the nickel equivalent required for stabilizing the austenite is reduced under the premise of ensuring the 950°C oxidation resistance, 8% of the Ni is replaced by the Mn and the N, the raw material cost of the heat-resistant steel is reduced, the limitation on the S is relaxed, the easy cutting effect of the MnS is utilized, and the casting processing cost does not increase obviously.

[0018] Other features and advantages of the present application will be set forth in the description that follows, and in part will be apparent from the description, or can be learned by practice of the application. The purposes and other advantages of the application will be realized and attained by the structure particularly pointed out in the description and claims hereof as well as the appended drawings.

[0019] To make the above objectives, features and advantages of the present application more obvious and comprehensible, the following preferred embodiments are specifically described below, and the accompanying drawings are referred to for detailed description. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or prior art description. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0021] Figure 1 A mass production 1.4826 as-cast-50X metallographic map provided by the embodiment of the present disclosure; Figure 2 An example 6 as-cast-100X metallographic map provided by the embodiment of the present disclosure; Figure 3 A TCP phase diagram precipitated at 850 DEG C for 500h of example 3 provided by the embodiment of the present disclosure; Figure 4 A TCP phase diagram precipitated at 850 DEG C for 500h of example 4 provided by the embodiment of the present disclosure; Figure 5 A TCP phase diagram precipitated at 850 DEG C for 500h of example 5 provided by the embodiment of the present disclosure; Figure 6 A TCP phase diagram precipitated at 850 DEG C for 500h of example 7 provided by the embodiment of the present disclosure. DETAILED DESCRIPTION

[0022] In order to make the objectives, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions of the present application will be described clearly and completely in combination with the drawings below. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0023] As used herein, the phrases "in one embodiment," "according to one embodiment," "in some embodiments," and the like generally mean the particular feature, structure, or characteristic following the phrase can be included in at least one embodiment of the present disclosure. Thus, appearances of such phrases in various places throughout this specification do not necessarily all refer to the same embodiment. As used herein, the terms "example," "exemplary," and the like are utilized merely for the purpose of facilitating this description. Any embodiment, aspect, or design described herein as an "example" or "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments, aspects, or designs. Rather, the terms "example," "exemplary," and the like are used as illustrative only and are not intended to convey a preference or indication that any embodiment, aspect, or design described herein is preferred, advantageous, or superior to other embodiments, aspects, or designs.

[0024] The terminology used herein is for the purpose of describing particular example configurations only and is not intended to be limiting. As used herein, the singular articles "a," "an," and "the" can be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having," are inclusive and therefore specify the presence of stated features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order

[0025] It is to be noted that like-numbered components have similar structures and functions throughout the figures and, thus, once one component is defined in a figure, it is not necessary to further describe and explain it in subsequent figures.

[0026] Some embodiments of the present application will now be described in detail in connection with the accompanying drawings. The embodiments described below and features in the embodiments can be combined with each other, if not in conflict.

[0027] The disclosed embodiments provide a low-nickel austenitic heat-resistant steel material for casting. The material has an elemental composition, in terms of weight percentage, including: C: 0.3-0.5%, Si: 2.5-3.5%, Mn: 10-16%, Cr: 17-19%, Ni: 1.5-3%, P≤0.04%, S≤0.15%, N: 0.2-0.35%, and the balance being iron and other inevitable impurity elements.

[0028] Specifically, C: in the austenitic heat-resistant steel described in the present application, C has two states, i.e. interstitial solid solution and formation of intergranular carbide. Among them, 0.15%-0.2% of C in the solid solution state also has the effect of stabilizing austenite and reducing the addition amount of Ni to reduce the cost of raw materials. The remaining about 0.2% of C forms a carbide with Cr element at the grain boundary, which has high thermal stability, increases the grain boundary strength at high temperature service environment, and reduces the grain boundary creep rate, thereby improving the service life of the part. At the same time, in the present application, when the C content is less than 0.3%, the content of carbide precipitated is relatively small, and the austenite grain boundary strength is significantly lower than that in the grain interior, resulting in intergranular cracking of the part during use, reducing the service life. When the C content exceeds 0.5%, the carbide is connected in a network and thickened, although the high-temperature strength and creep resistance are significantly improved, the brittleness is also significantly increased, the thermal fatigue life is also significantly shortened under the usual service conditions of automobile engine exhaust manifold and turbocharger shell, and the machining performance is also seriously deteriorated, and the processing tool cost increases by 100%-900%. Based on this, the mass percentage of C in the austenitic heat-resistant steel described in the present application is controlled to be 0.3-0.5%.

[0029] Specifically, Si: in the austenitic heat-resistant steel described in the present application, the main role of Si is to replace part of Cr element to provide high-temperature oxidation resistance, thereby reducing the nickel equivalent element required for stabilizing austenite. Si can assist in deoxidation during smelting to improve the fluidity of molten steel, and the deoxidation product of Si can reduce the alkalinity of Mn slag, which helps to reduce the reaction between steel slag and mold material to improve the surface roughness of the casting. However, Si reduces the solubility of N in the molten steel, and the addition of Si increases the brittleness of austenite. When the addition amount of Si is greater than 4%, the casting will crack when air-cooled from red-hot state. When the addition amount is 3.6%-4%, the red-hot casting will also crack when water-cooled. Based on this, the mass percentage of Si in the austenitic heat-resistant steel described in the present application is controlled to be 2.5-3.5%.

[0030] Specifically, manganese is a strong austenite forming element. When used as a substitution type solid solution element, the same mass percentage of manganese has the effect of stabilizing austenite 0.5 times that of nickel. Since the single value of manganese is about one tenth of that of nickel, the use of manganese to replace nickel can greatly reduce the use amount of nickel in the austenitic heat-resistant steel, thereby reducing the cost of raw materials. However, in actual production, the addition of manganese often causes slag hole defects in the casting, and seriously affects the machining and cutting performance of the material.

[0031] Specifically, Mn: In the austenitic heat-resistant steel described in the present application, Mn is the main austenite forming element, and the solid-solved Mn can replace about 0.5 times of Ni. The Mn in the molten steel can significantly increase the solubility of N, avoid N in a supersaturated state, and thus reduce the proportion of nitrogen porosity defects. At the same time, the MnS formed by the combination of Mn and S is the main free-cutting phase. However, when the Mn addition amount > 16%, the material appears significant brittleness, and the machining cutting force also significantly increases. Based on this, in the austenitic heat-resistant steel described in the present application, the mass percentage of Mn is controlled to be 10-16%.

[0032] Specifically, chromium is the most important antioxidant element in heat-resistant steel, and is also the main element for reducing the high-temperature stability of austenite. When it is a substitution type solid-solved element, if the heat-resistant steel part needs to be used in the engine exhaust gas for a long time within 900℃, the mass percentage of chromium needs to be ≥18%, and if the use temperature is increased to 950℃, the mass percentage of chromium needs to be ≥20%. The antioxidant property provided by the same mass percentage of silicon element is close to 3 times of that of chromium, but its influence on the high-temperature stability of austenite is only 1.5 times of that of chromium, so appropriate substitution of chromium with silicon can reduce the demand for nickel and nickel equivalent elements on the premise of maintaining the high-temperature antioxidant property of the heat-resistant steel, thereby reducing the raw material cost of the heat-resistant steel.

[0033] Specifically, Cr: In the austenitic heat-resistant steel described in the present application, the main role of Cr is to provide 950℃ antioxidant property and corrosion resistance, and to increase the solubility of N in the molten steel, thereby increasing the N absorption rate when N-containing alloy is added, reducing the boiling phenomenon of the molten steel at high temperature, and reducing the casting porosity defects. However, Cr is also a ferrite forming element, and a high content will cause ferrite phase to appear in the as-cast state, and reduce the high-temperature stability of the austenite matrix, thereby reducing the service life of the part. When the mass percentage of Cr is less than 17%, the solubility of N in the molten steel is less than 0.25% (about 1500℃), which leads to a decrease in the N element that can actually stabilize the austenite, a decrease in the austenite stability, a tendency to retain high-temperature ferrite phase in the as-cast structure, and a decrease in the 950℃ antioxidant property. When the mass percentage of Cr is greater than 19%, ferrite phase will also appear in the as-cast state when the content of the remaining alloy elements meets the requirements. Based on this, in the technical solution described in the present application, the mass percentage of Cr is controlled to be 17-19%.

[0034] Specifically, Ni: In the austenitic heat-resistant steel described in the present application, Ni is the main austenite forming element, and is usually the alloy element with the highest proportion in the raw material cost of the heat-resistant steel. Ni will reduce the solubility of N in the molten steel, and increase the casting porosity defects. When the content of Ni < 1.3%, the TCP phase precipitated after the heat-resistant steel is kept at high temperature for a long time will significantly increase in size, and the cold and hot impact life will significantly decrease. Based on this, in the technical solution described in the present application, the mass percentage of Ni is controlled to be 1.5-3%.

[0035] Specifically, nitrogen is a strong austenite forming element, and when acting as a gap type solid solution element, the same mass percentage of nitrogen can stabilize austenite 20-30 times that of nickel, which can greatly reduce the use amount of nickel in the austenitic heat-resistant steel, thereby reducing the raw material cost. However, in actual production, the addition of nitrogen often causes nitrogen porosity defects in the castings, and seriously affects the material processing and cutting performance.

[0036] Specifically, in the austenitic heat-resistant steel described in the present application, N is the main austenite forming element, and the solid solution N can replace 20-30 times of Ni. When the content of the remaining alloy elements meets the requirements, the heat-resistant steel is subjected to melting and pouring at a temperature of about 1350-1750℃ during solidification, and the maximum N solubility in the molten steel at this temperature is about 0.35%. When the addition amount exceeds this value, N escapes, increasing the cost of nitrogen-containing additives, and the precipitated nitrogen porosity defects of the castings are significantly increased. Based on this, in the technical solution described in the present application, the mass percentage of N is controlled to be 0.2-0.35%.

[0037] Specifically, the single-cast test bar of the low-nickel austenitic heat-resistant steel material for casting has a tensile strength of >100 MPa at 950℃, a yield strength of >90 MPa, and an elongation of >40% after fracture.

[0038] Specifically, the heat-resistant steel described in the present application replaces 5% of chromium elements with 2% of silicon elements, reduces the nickel equivalent (1%-2%) required for stabilizing austenite on the basis of ensuring the 950℃ oxidation resistance of the heat-resistant steel, replaces a large amount of nickel elements with nitrogen and manganese to stabilize austenite, reduces the raw material cost of the heat-resistant steel, optimizes the addition range of nitrogen and manganese elements, and adds appropriate MnS easy cutting phase, so that the processing cost of the castings is lower than that of ordinary chromium-nickel austenitic heat-resistant steel; the oxidation resistance and cold thermal shock resistance at about 950℃ are similar to those of 1.4826 grade heat-resistant steel.

[0039] The present application also provides an application of the low-nickel austenitic heat-resistant steel material for casting as described above in the field of automobile engine exhaust pipes and turbocharger housings. Embodiment

[0040] A preparation method of a low-nickel austenitic heat-resistant steel material for casting, comprising the following steps: S1, melting raw materials by using a 500Hz intermediate frequency electric furnace; S2, adding elements to a preset ratio after determining the composition of the molten steel by using a direct-reading spectrometer; S3, deoxidizing the molten steel by adding a deoxidizer; and S4, pouring the molten steel into a pouring ladle added with a modifier after heating the molten steel to 1600-1700℃.

[0041] Among them, various iron alloys of raw materials such as electrolytic manganese, chromium iron, silicon iron, niobium iron, nickel plate, iron sulfide, rare earth silicon iron and silicon nitride need to be baked before melting.

[0042] The chemical compositions of Examples 1-7 and the comparative examples (mass-produced 1.4826 stainless steel) are shown in Table 1, and the performance parameters are shown in Table 2.

[0043] Table 1

[0044] Table 2

[0045] Please see Figure 1 and Figure 2 ,like Figure 1 and Figure 2 As shown, the austenitic heat-resistant steel of the present invention has a fully austenitic matrix due to its high nickel equivalent, while the mass-produced 1.4826 heat-resistant steel contains a small amount of ferrite.

[0046] In Example 1, the C content was 0.28%, which is lower than the threshold of 0.3%. When the C content is less than 0.3%, the amount of carbides precipitated is small, and the austenite grain boundary strength is significantly lower than that inside the grains, which leads to intergranular cracking of the parts during use and reduces their service life.

[0047] In Example 2, the Cr content was 16.8%, which is lower than the threshold of 17%. When the mass percentage of Cr is less than 17%, the solubility of N in molten steel is less than 0.25% (around 1500℃), which leads to a reduction in the amount of N that can actually stabilize austenite. This results in a decrease in austenite stability, making it easier for high-temperature ferrite phases to remain in the as-cast structure. Furthermore, the oxidation resistance at 950℃ is reduced at this time.

[0048] Please see Figures 3-6 ,like Figures 3-6 As shown, after high-temperature heat preservation, the amount of brittle TCP phase precipitated in the austenite of Example 3 is significantly greater than that in other examples. The main difference in composition lies in the Ni content, which is also related to the sharp reduction in thermal shock life.

[0049] In summary, this low-nickel austenitic hot-rolled steel material for casting, its preparation method, and its application replace some of the Cr with Si, reducing the nickel equivalent required to stabilize austenite while ensuring oxidation resistance at 950℃. By replacing 8% of Ni with Mn and N, the raw material cost of heat-resistant steel is reduced, the restrictions on S are relaxed, and the easy-machining effect of MnS is utilized, so that the processing cost of castings is not significantly increased.

[0050] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A low nickel austenitic heat-resistant steel material for casting, characterized by, The element composition of the material comprises, in percentage by weight: C: 0.3-0.5%, Si: 2.5-3.5%, Mn: 10-16%, Cr: 17-19%, Ni: 1.5-3%, P≤0.04%, S≤0.15%, N: 0.2-0.35%, and the balance being iron and other inevitable impurity elements.

2. The low nickel austenitic heat steel material for casting as claimed in claim 1, wherein, The element composition of the material comprises: C: 0.46%, Si: 2.6%, Mn: 11.5%, Cr: 18.9%, Ni: 1.8%, P: 0.015%, S: 0.08%, N: 0.21%, and Nb: 0.007%.

3. The low nickel austenitic heat steel material for casting as claimed in claim 1, wherein, The element composition of the material comprises: C: 0.33%, Si: 3.1%, Mn: 12.1%, Cr: 17.5%, Ni: 1.6%, P: 0.018%, S: 0.13%, N: 0.27%, and Nb: 0.005%.

4. The low nickel austenitic heat steel material for casting as claimed in claim 1, wherein, The element composition of the material comprises: C: 0.43%, Si: 2.9%, Mn: 14.9%, Cr: 17.9%, Ni: 1.7%, P: 0.016%, S: 0.09%, N: 0.32%, and Nb: 0.005%.

5. The low nickel austenitic heat steel material for casting as claimed in claim 1, wherein, The element composition of the material comprises: C: 0.49%, Si: 3.4%, Mn: 15.6%, Cr: 18.9%, Ni: 2.9%, P: 0.019%, S: 0.08%, N: 0.23%, and Nb: 0.006%. ​ 6. The low nickel austenitic heat steel material for casting as claimed in claim 1, wherein, The single-cast test bar of the low-nickel austenitic heat-resistant steel material for casting has a tensile strength of >100 MPa, a yield strength of >90 MPa, and an elongation after fracture of >40% at 950℃.

7. A method of producing a low nickel austenitic heat-resistant steel material for casting as claimed in any one of claims 1 to 6, characterized in that, The method comprises the following steps: S1, melting raw materials by using a medium-frequency electric furnace; S2, adding elements to a preset proportion after determining the composition of the molten steel by using a direct-reading spectrometer; S3, deoxidizing the molten steel by adding a deoxidizer; and S4, pouring the molten steel into a ladle with a modifier after heating the molten steel.

8. The production method according to claim 7, wherein The frequency of the medium-frequency electric furnace is 500 Hz.

9. The production method according to claim 7, wherein The temperature range of the molten steel in S4 is 1600-1700℃.

10. Use of the low-nickel austenitic heat-resistant steel material for casting according to any one of claims 1-6 in the field of exhaust pipes and turbocharger housings of automobile engines.

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