A heating method for improving high-temperature oxidation resistance and crack resistance of 9Ni steel billets

Abstract

The present invention discloses a heating method for improving the high-temperature oxidation resistance and crack resistance of a 9Ni steel billet. By controlling the temperature of a preheating section and the O2 content in a mixed gas, and adjusting the O2 content, heating rate, and holding temperature in the mixed gas of the first, second, and third soaking sections, the rapid generation of dense Fe3O4 on the surface of the 9Ni steel billet during the preheating process is promoted, and the formation of pores and thermal cracks in the outer oxide layer is effectively controlled, thereby significantly inhibiting the diffusion of oxygen from the surface to the interior of the 9Ni steel billet during the preheating and high-temperature soaking processes, and achieving stable control of weak oxidation and shallow intergranular cracks on the surface of the 9Ni steel billet during the high-temperature heating process. The method can effectively improve the problem of low metal yield and high production cost in the hot rolling process of the 9Ni steel billet under high nickel conditions, and can fully ensure the surface quality of the rolled plate, while achieving environmentally friendly and cost-reducing production of the 9Ni steel billet by heating the 9Ni steel billet without an oxidation coating.

Description

Technical Field

[0001] The invention belongs to the technical field of steel plate surface quality control, and in particular relates to a heating method for improving the high-temperature oxidation resistance and crack resistance of a 9Ni steel billet. Background Art

[0002] By the end of 2023, China had built 28 liquefied natural gas (LNG) receiving stations with an annual receiving capacity exceeding 116 million tons, ranking among the top in the world. Currently, the main materials capable of safely storing LNG below -162°C include 9Ni steel, aluminum alloys, austenitic stainless steel, and nickel alloys. Compared to other materials, 9Ni steel has become the preferred material for LNG storage and transportation due to its excellent low-temperature toughness, cold workability, weldability, and crack growth resistance, as well as its low ductile-brittle transition temperature and low production cost, making it widely used.

[0003] The production process of 9Ni steel plate mainly includes smelting-continuous casting-ingot heating-controlled rolling and controlled cooling-heat treatment, and its final structure is low-carbon martensite + a small amount of reversed austenite. Before hot continuous rolling production, 9Ni steel billets must be heated to 1150-1200℃ in a heating furnace and kept warm for a certain period of time. The fuel of the heating furnace is mainly coal gas (blast furnace gas / converter gas / coke oven gas) + combustion air. Due to the lack of antioxidant elements such as Cr and Al in the steel and the long-term heating in a high-temperature oxidizing environment, the 9Ni steel billets are severely burned after leaving the heating furnace and obvious intergranular cracks are prone to appear on their surface, which will not only cause a significant reduction in the metal yield in the subsequent rolling process, but also easily lead to obvious cracking on the surface of the steel plate during subsequent thermal deformation and quenching, seriously affecting the surface quality and toughness of the product.

[0004] In order to effectively inhibit the high-temperature oxidation of steel billets, domestic and foreign steel companies mainly use the two methods of "adding antioxidant elements" and "spraying antioxidant coatings".

[0005] For example, the Chinese patents with publication numbers CN 114540712A and CN116219294A published on May 27, 2022 and June 6, 2023, respectively, disclose "a kind of uncoated reinforced high-temperature oxidation-resistant hot stamping steel with added Ce element" and "a kind of uncoated high-temperature oxidation-resistant hot stamping steel with added Y element". The technical scheme mainly adds Ce and Y elements to form Ce and Y oxides on the surface of the steel plate, thereby reducing the growth rate of the oxide scale and increasing its adhesion to the matrix, thereby improving the high-temperature oxidation resistance of the steel. However, the high content of Ce and Y not only significantly increases the production cost, but also easily leads to nodules in the continuous casting nozzle, which is not conducive to the continuous casting of 9Ni steel.

[0006] For example, the Chinese patents with publication numbers CN114438393B, CN 115047000A and CN 110229949A, which were published on March 21, 2023, September 13, 2022 and September 13, 2019, respectively, disclosed "a method for preventing intergranular oxidation cracks on the surface of high-nickel steel plates", "a method for evaluating the protective effect of high-temperature anti-oxidation coatings for nickel-based steels" and "a high-temperature anti-oxidation descaling isolation agent based on nickel-containing steel and a preparation method thereof". The technical solutions disclosed therein mainly suppress the growth of iron oxide scale on the steel surface and the formation of intergranular cracks by grinding the ingots and spraying MgO-based or SiC-Al2O3-based high-temperature anti-oxidation coatings. However, grinding the ingots and spraying anti-oxidation coatings will not only significantly increase production costs, but may also cause significant environmental pollution.

[0007] The above-mentioned existing technology mainly improves the high-temperature oxidation resistance and crack resistance of 9Ni steel by adding antioxidant Ce and Y elements, grinding the cast billets and spraying antioxidant coatings. However, it has disadvantages such as being unfavorable for continuous casting, high production costs and being unfriendly to the environment. It is not suitable for economical and environmentally friendly industrial production of 9Ni steel. Summary of the Invention

[0008] In order to solve the above technical problems, the present invention provides a heating method for improving the high-temperature oxidation resistance and crack resistance of 9Ni steel billets. By controlling the heating atmosphere, heating rate and holding temperature of the four-stage heating process, the reduction and harmless control of iron oxide scale and intergranular cracks on the surface of 9Ni steel billets at high temperatures are achieved. It can effectively improve the problem of low metal yield and high production cost in the hot rolling process of 9Ni steel billets under high nickel conditions, and fully guarantee the surface quality of the rolled plates, while realizing environmentally friendly and cost-reducing production of 9Ni steel billets without oxidation coating heating.

[0009] The specific technical solutions of the present invention are as follows:

[0010] The present invention provides a heating method for improving the high-temperature oxidation resistance and crack resistance of a 9Ni steel billet, the heating method comprising: heating the 9Ni steel billet in a mixed atmosphere of CO2, O2, and N2 through a preheating section, a soaking section, a soaking section, and a soaking section in a heating furnace;

[0011] Preheating section: Control the volume fraction of O2 in the mixed atmosphere to 0.5-1.0%, and the preheating section temperature to 880-920°C;

[0012] Soaking stage: Control the volume fraction of O2 in the mixed atmosphere to be 0.5-1.5%, the temperature of the soaking stage to be 1030-1060°C; the heating rate from the preheating stage temperature to the soaking stage temperature to be 1.4-1.6°C / min;

[0013] Soaking stage 2: Control the volume fraction of O2 in the mixed atmosphere to be 0.5-1.0%, the temperature of the soaking stage 2 to be 1120-1150°C; the heating rate from the temperature of the soaking stage 1 to the temperature of the soaking stage 2 to be 1.8-2.1°C / min;

[0014] Soaking stage three: control the volume fraction of O2 in the mixed atmosphere to be 0.5-1.0%, the temperature of the soaking stage three to be 1190-1220°C; the heating rate from the temperature of the soaking stage two to the temperature of the soaking stage three is 2.2-2.5°C / min.

[0015] Furthermore, the holding time of the preheating section is 20 to 25 minutes.

[0016] The soaking and heat preservation time of the soaking section is 3 to 5 minutes.

[0017] The soaking and heat preservation time of the second soaking stage is 3 to 5 minutes.

[0018] The soaking and heat preservation time of the three soaking stages is 2 to 4 minutes.

[0019] In the mixed atmosphere, the volume fractions of CO2 and N2 are 15% to 35% and 67% to 85% respectively.

[0020] The residence time of the 9Ni steel billet in the heating furnace is 180 to 240 minutes.

[0021] After leaving the heating furnace, the 9Ni steel billet is air-cooled to room temperature.

[0022] After the 9Ni steel billet is heated in a heating furnace, the thickness of its inner oxide layer is ≤520μm; the thickness of its outer oxide layer is ≤620μm; the total oxide layer thickness is ≤1150μm; no intergranular cracks appear in the inner oxide layer; and no cracks occur in the outer oxide layer.

[0023] Because 9Ni steel lacks antioxidant elements such as Cr and Al and requires prolonged heating in a high-temperature, oxidizing environment, it suffers from severe burnout and numerous intergranular cracks on its surface after exiting the heating furnace. This can easily lead to reduced metal yield in the subsequent rolling process and significant cracking on the steel plate surface, seriously affecting the surface quality and toughness of the product. If an anti-oxidation coating is sprayed before entering the heating furnace, the production cost of 9Ni steel will be significantly increased and the environmental pollution during the production process will be aggravated.

[0024] The present invention is based on the heating system and flue gas composition of the hot rolling mill heating furnace. By optimizing and controlling the oxidizing atmosphere, heating rate and preheating (soaking) temperature in the preheating section, the soaking section one, the soaking section two and the soaking section three, the rapid generation of dense Fe3O4 on the surface of the 9Ni steel billet during the high-temperature heating process is promoted, the formation of holes in the outer oxide layer is reduced, and the thickness of the inner and outer oxide layers of the 9Ni steel is simultaneously thinned and the surface intergranular cracks are alleviated without the protection of an anti-oxidation coating. This relaxes the requirement for applying an anti-oxidation coating to the 9Ni steel billet before entering the heating furnace, and can achieve cost-saving and environmentally friendly production of 9Ni steel.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention promotes the rapid formation of dense Fe3O4 on the surface of 9Ni steel billet during preheating by optimizing the temperature of the preheating section and the O2 content in the heating atmosphere, avoids the formation of loose Fe2O3, and thus significantly hinders the inward diffusion of oxygen, thereby improving the problem of thick outer iron oxide scale formed during long-term heating in the heating furnace, which leads to low hot-rolled metal yield.

[0027] (2) The present invention further reduces the diffusion rate of oxygen atoms in the outer oxide layer of 9Ni steel and their reaction with iron atoms during high-temperature soaking by controlling the O2 content in the mixed gas in the first, second and third soaking stages. At the same time, it effectively inhibits the formation of pores in the outer oxide layer, thereby blocking the inward diffusion channel of oxygen atoms and reducing the depth of intergranular cracks on the surface of the 9Ni steel billet.

[0028] (3) The present invention effectively avoids the concentration of thermal stress generated by the temperature difference between the inner and outer oxide layers on the outer oxide layer, causing thermal cracks in the outer oxide layer, by controlling the heating rate and the absorptive temperature of the first, second and third absorptive stages, thereby reducing the risk of sudden oxidation on the surface of the 9Ni steel billet and further achieving stable control of weak oxidation and shallow intergranular cracks on the surface of the 9Ni steel billet during the high-temperature heating process. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the SEM morphology of the oxide layer cross section of Example 1;

[0030] Figure 2 This is the SEM morphology of the oxide layer cross section of Comparative Example 1;

[0031] Figure 3 The energy spectrum analysis results of the surface phase (Fe3O4) of the oxide layer in Example 1;

[0032] Figure 4 This is the energy spectrum analysis result of the surface phase (Fe2O3) of the oxide layer of Comparative Example 1. DETAILED DESCRIPTION

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0034] If no specific techniques or conditions are specified in the examples, they can be carried out according to the techniques or conditions described in the literature in the field or according to the product instructions.

[0035] The chemical composition and weight percentage of the 9Ni steel billet in each embodiment and comparative example are: 0.043% C, 0.254% Si, 0.652% Mn, 0.0041% P, 0.0012% S, 9.12% Ni, 0.032% Al, and the rest are Fe and unavoidable impurities.

[0036] Example 1

[0037] A heating method for improving the high-temperature oxidation resistance and crack resistance of a 9Ni steel billet comprises the following steps:

[0038] 1) Wire-cut 9Ni steel billets were machined into several 10 mm × 10 mm × 10 mm cube specimens. The specimens were ground with sandpaper to 2000 grit, then polished, cleaned with alcohol, and dried for later use.

[0039] 2) The horizontal molybdenum wire furnace was heated to a preheating temperature of 880°C and kept warm. Subsequently, a mixed gas of CO2, O2 and N2 with volume fractions of 15%, 1% and 84% respectively was introduced into the furnace. The sum of the flow rates of CO2, O2 and N2 was fixed at 1 L / min. After the mixed gas was introduced for 15 minutes, the 9Ni steel billet sample was pushed into the molybdenum wire furnace and heated to 880°C with the furnace and kept warm for 25 minutes.

[0040] 3) The molybdenum wire furnace is heated at a rate of 1.5°C / min to a soaking temperature of 1043°C and kept at this temperature for 4 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 15%, 1.5%, and 83.5%, respectively.

[0041] 4) The heating furnace is heated at a rate of 1.85°C / min to a second soaking temperature of 1124°C and maintained at this temperature for 4 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 15%, 0.9%, and 84.1%, respectively.

[0042] 5) Continue to heat the heating furnace at 2.3°C / min to the third-stage absorptive temperature of 1195°C and keep it at that temperature for 3 minutes. Do not change the volume fraction of the mixed gas CO2, O2 and N2 during the heating and absorptive process. After the sample has been kept at that temperature for 3 minutes, take it out and air cool it to room temperature.

[0043] Example 2

[0044] A high-temperature oxidation-resistant and crack-resistant heating atmosphere and heating method for a 9Ni steel billet (whose chemical composition is the same as that of Example 1) comprises the following steps:

[0045] 1) Sample processing: The sample processing method is the same as that in Example 1.

[0046] 2) The horizontal molybdenum wire furnace was heated to a preheating temperature of 891°C and kept warm. Subsequently, a mixed gas of CO2, O2 and N2 with volume fractions of 23%, 0.8% and 76.2% respectively was introduced into the furnace (the sum of the flow rates of CO2, O2 and N2 was fixed at 1 L / min). After the mixed gas was introduced for 15 minutes, the 9Ni steel billet sample was pushed into the molybdenum wire furnace and heated to 891°C with the furnace and kept warm for 23 minutes.

[0047] 3) The molybdenum wire furnace was heated at a rate of 1.4°C / min to a soaking temperature of 1030°C and kept at this temperature for 4.5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 were adjusted to 23%, 1.3%, and 75.7%, respectively.

[0048] 4) The heating furnace was heated at a rate of 1.93°C / min to a second soaking temperature of 1132°C and maintained at this temperature for 3.5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 were adjusted to 23%, 0.8%, and 76.2%, respectively.

[0049] 5) Continue to heat the heating furnace at 2.2°C / min to the third-stage absorptive temperature of 1201°C and keep it at that temperature for 3.5 minutes. The volume fractions of the mixed gas CO2, O2 and N2 do not change during the heating process. The volume fractions of the mixed gas CO2, O2 and N2 do not change during the heating and absorptive process. After the sample has been kept at that temperature for 3.5 minutes, take it out and air cool it to room temperature.

[0050] Example 3

[0051] A high-temperature oxidation-resistant and crack-resistant heating atmosphere and heating method for a 9Ni steel billet (whose chemical composition is the same as that of Example 1) comprises the following steps:

[0052] 1) Sample processing: The sample processing method is the same as that in Example 1.

[0053] 2) The horizontal molybdenum wire furnace was heated to a preheating temperature of 905°C and kept warm. Subsequently, a mixed gas of CO2, O2 and N2 with volume fractions of 29%, 0.6% and 70.4% respectively was introduced into the furnace (the sum of the flow rates of CO2, O2 and N2 was fixed at 1 L / min). After the mixed gas was introduced for 15 minutes, the 9Ni steel billet sample was pushed into the molybdenum wire furnace and heated to 905°C with the furnace and kept warm for 21 minutes.

[0054] 3) The molybdenum wire furnace is heated at a rate of 1.45°C / min to a soaking temperature of 1050°C and kept at this temperature for 5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 29%, 0.95%, and 70.05%, respectively.

[0055] 4) The heating furnace was heated at a rate of 1.99°C / min to a second soaking temperature of 1140°C and maintained at this temperature for 4.5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 were adjusted to 29%, 0.7%, and 70.3%, respectively.

[0056] 5) Continue to heat the heating furnace at 2.4℃ / min to the third-stage absorptive temperature of 1212℃ and keep it at this temperature for 2.5 minutes. The volume fractions of the mixed gas CO2, O2 and N2 do not change during the heating process. The volume fractions of the mixed gas CO2, O2 and N2 do not change during the heating and absorptive processes. After the sample has been kept at this temperature for 2.5 minutes, take it out and air cool it to room temperature.

[0057] Example 4

[0058] A high-temperature oxidation-resistant and crack-resistant heating atmosphere and heating method for a 9Ni steel billet (whose chemical composition is the same as that of Example 1) comprises the following steps:

[0059] 1) Sample processing: The sample processing method is the same as that in Example 1.

[0060] 2) The horizontal molybdenum wire furnace was heated to a preheating temperature of 918°C and kept warm. Subsequently, a mixed gas of CO2, O2 and N2 with volume fractions of 34%, 0.5% and 65.5% respectively was introduced into the furnace (the sum of the flow rates of CO2, O2 and N2 was fixed at 1 L / min). After the mixed gas was introduced for 15 minutes, the 9Ni steel billet sample was pushed into the molybdenum wire furnace and heated to 918°C with the furnace and kept warm for 22 minutes.

[0061] 3) The molybdenum wire furnace was heated at a rate of 1.55°C / min to a soaking temperature of 1058°C and kept at this temperature for 3.5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 were adjusted to 34%, 0.55%, and 65.45%, respectively.

[0062] 4) The heating furnace is heated at a rate of 2.08°C / min to a second soaking temperature of 1148°C and maintained at this temperature for 3 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 34%, 0.55%, and 65.45%, respectively.

[0063] 5) Continue to heat the heating furnace at 2.35°C / min to the third-stage absorptive temperature of 1219°C and keep it at this temperature for 2 minutes. Do not change the volume fractions of the mixed gas CO2, O2 and N2 during the heating and absorptive process. After the sample has been kept at this temperature for 2 minutes, take it out and air cool it to room temperature.

[0064] Comparative Example 1

[0065] A method for heating a 9Ni steel billet comprises the following steps:

[0066] 1) Sample processing: The sample processing method is the same as that in Example 1.

[0067] 2) The horizontal molybdenum wire furnace was heated to a preheating temperature of 840°C and kept warm. Subsequently, a mixed gas of CO2, O2 and N2 with volume fractions of 18%, 1.5% and 80.5% respectively was introduced into the furnace (the sum of the flow rates of CO2, O2 and N2 was fixed at 1 L / min). After the mixed gas was introduced for 15 minutes, the 9Ni steel billet sample was pushed into the molybdenum wire furnace and heated to 840°C with the furnace and kept warm for 24 minutes.

[0068] 3) The molybdenum wire furnace is heated at a rate of 1.6°C / min to a soaking temperature of 1036°C and kept at this temperature for 5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 18%, 1.5%, and 80.5%, respectively.

[0069] 4) The heating furnace was heated at a rate of 1.88°C / min to a second soaking temperature of 1126°C and maintained at this temperature for 4 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 were adjusted to 18%, 0.5%, and 81.5%, respectively.

[0070] 5) The heating furnace is heated at a rate of 2.3°C / min to a third-stage temperature of 1194°C and maintained at this temperature for 3 minutes. The volume fractions of the mixed gas of CO2, O2, and N2 are not changed during the heating process.

[0071] 6) After keeping the sample warm for 3 minutes, take it out and air cool it to room temperature.

[0072] Comparative Example 2

[0073] A method for heating a 9Ni steel billet comprises the following steps:

[0074] 1) Sample processing: The sample processing method is the same as that in Example 1.

[0075] 2) The horizontal molybdenum wire furnace was heated to a preheating temperature of 895°C and kept warm. Subsequently, a mixed gas of CO2, O2 and N2 with volume fractions of 27%, 1% and 72% respectively was introduced into the furnace (the sum of the flow rates of CO2, O2 and N2 was fixed at 1 L / min). After the mixed gas was introduced for 15 minutes, the 9Ni steel billet sample was pushed into the molybdenum wire furnace and heated to 895°C with the furnace and kept warm for 23 minutes.

[0076] 3) The molybdenum wire furnace is heated at a rate of 1.75°C / min to a soaking temperature of 1051°C and kept at this temperature for 4.5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 27%, 2%, and 71%, respectively.

[0077] 4) The heating furnace is heated at a rate of 2.2°C / min to a second soaking temperature of 1130°C and maintained at this temperature for 5 minutes. During the heating process, the volume fractions of the mixed gases CO2, O2, and N2 are adjusted to 27%, 1.5%, and 71.5%, respectively.

[0078] 5) The heating furnace is heated at a rate of 2.6°C / min to a third-stage soaking temperature of 1200°C and maintained at this temperature for 4 minutes. The volume fractions of the mixed gas CO2, O2, and N2 do not change during the heating process.

[0079] 6) After keeping the sample warm for 4 minutes, take it out and air cool it to room temperature.

[0080] The cross-sectional morphology of the oxidized samples of Examples 1-4, Comparative Example 1, and Comparative Example 2 was observed and photographed using a scanning electron microscope (SEM). The presence of pores and cracks in the outer oxide layer was observed. The diameter of the pores in the outer oxide layer, the thickness of the inner and outer oxide layers, and the depth of intergranular cracks in the inner oxide layer were measured for each sample. The results are shown in Table 1.

[0081] Table 1 Oxide layer thickness and pores and cracks of various embodiments of the present invention and comparative examples

[0082]

[0083] It can be seen from the above table that the total oxide layer thickness of the samples of Examples 1 to 4, which adopt high preheating temperature, low mixed gas O2 content and low heating rate, after high-temperature oxidation is 947 to 1134 μm, the outer oxide layer thickness is 526 to 617 μm, and the inner oxide layer thickness is 421 to 517 μm. There are no obvious holes and cracks in the outer oxide layer. The outer oxide layer phase is mainly Fe3O4, and there is basically no intergranular crack in the inner oxide layer.

[0084] Compared with Examples 1 to 4, the preheating temperature of Comparative Example 1 is lower, the volume fraction of O2 in the mixed gas is higher, the thickness of the inner and outer oxide layers increase to 725 and 1031 μm, respectively, and obvious large holes appear in the outer oxide layer, such as Figure 2As shown, and its outermost phase is Fe2O3, as shown Figure 4 As shown in the figure, there are obvious intergranular cracks in the inner oxide layer, and the depth reaches 755μm.

[0085] Compared with Examples 1 to 4, the heating rates of the first, second and third soaking stages of Comparative Example 2 are faster and the volume fraction of O2 in the mixed gas is higher. The thicknesses of the inner and outer oxide layers increase to 834 and 1109 μm, respectively. Obvious large holes and cracks appear in the outer oxide layer, and the intergranular cracks in the inner oxide layer further deepen, with a depth of 823 μm.

[0086] The above-mentioned detailed description of a heating method for improving the high-temperature oxidation resistance and crack resistance of 9Ni steel billets with reference to the embodiment is illustrative rather than restrictive. Several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A heating method for improving the high-temperature oxidation resistance and crack resistance of 9Ni steel billets, characterized in that: The heating method comprises: heating a 9Ni steel billet in a mixed atmosphere of CO2, O2 and N2 through a preheating section, a soaking section, a soaking section, and a soaking section in a heating furnace; Preheating section: Control the volume fraction of O2 in the mixed atmosphere to 0.5~1.0%, and the preheating section temperature to 880~920℃; Soaking stage: Control the volume fraction of O2 in the mixed atmosphere to be 0.5-1.5%, the temperature of the soaking stage to be 1030-1060°C; the heating rate from the preheating stage temperature to the soaking stage temperature to be 1.4-1.6°C / min; Soaking stage 2: Control the volume fraction of O2 in the mixed atmosphere to be 0.5-1.0%, the temperature of the soaking stage 2 to be 1120-1150°C; the heating rate from the temperature of the soaking stage 1 to the temperature of the soaking stage 2 to be 1.8-2.1°C / min; Soaking stage three: control the volume fraction of O2 in the mixed atmosphere to be 0.5~1.0%, the temperature of the soaking stage three to be 1190~1220℃; the heating rate from the temperature of the soaking stage two to the temperature of the soaking stage three is 2.2~2.5℃ / min.

2. The heating method according to claim 1, characterized in that The holding time of the preheating section is 20 to 25 minutes.

3. The heating method according to claim 1, wherein The soaking and heat preservation time of the soaking section is 3 to 5 minutes.

4. The heating method according to claim 1, characterized in that The soaking and holding time of the second soaking stage is 3 to 5 minutes.

5. The heating method according to claim 1, wherein The soaking and heat preservation time of the three soaking stages is 2 to 4 minutes.

6. The heating method according to any one of claims 1 to 5, characterized in that: In the mixed atmosphere, the volume fractions of CO2 and N2 are 15% to 35% and 67% to 85% respectively, and the total volume fraction of the mixed atmosphere is 100%.

7. The heating method according to any one of claims 1 to 5, characterized in that: The residence time of 9Ni steel billet in the heating furnace is 180~240min.

8. The heating method according to any one of claims 1 to 5, characterized in that: After leaving the heating furnace, the 9Ni steel billet is air-cooled to room temperature.

9. The heating method according to any one of claims 1 to 5, characterized in that: After the 9Ni steel billet is heated in a heating furnace, the thickness of its inner oxide layer is ≤520μm; the thickness of its outer oxide layer is ≤620μm; the total oxide layer thickness is ≤1150μm; no intergranular cracks appear in the inner oxide layer; and no cracks occur in the outer oxide layer.

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