9Cr1Mo stainless steel and heat treatment method thereof suitable for oil and gas field

Abstract

This invention provides a 9Cr1Mo stainless steel suitable for the oil and gas industry and its heat treatment method, relating to the field of online heat treatment technology for 9Cr steel. The heat treatment method includes the following steps: S1, cooling after forging or rolling; S2, annealing; S3, quenching; S4, tempering; S5, straightening; S6, stress-relief tempering. The 9Cr1Mo stainless steel is obtained by the above heat treatment method. The 9Cr1Mo stainless steel obtained by this heat treatment method exhibits stable room temperature tensile properties and low-temperature impact properties, meeting the standards for use in the oil and gas industry.

Description

Technical Field

[0001] This invention relates to the field of online heat treatment technology for 9Cr steel, and particularly to a 9Cr1Mo stainless steel suitable for the oil and gas industry and its heat treatment method. Background Technology

[0002] 9Cr1Mo is widely used in the petrochemical and high-pressure boiler industries due to its excellent corrosion resistance and high-temperature resistance. 9Cr1Mo has a ferrite-forming element content of 9% (Cr) and a molybdenum content of 1%, giving it good resistance to high-temperature oxidation and hydrogen sulfide stress corrosion. However, in the petroleum industry, steel not only needs resistance to H2S / CO2 corrosion but also high strength. Therefore, the carbon content of 9Cr1Mo is generally controlled between 0.09-0.15%, ensuring that most of the microstructure transforms into martensite after quenching, and that tempering meets different mechanical property requirements. Domestic and international oilfield service companies' standards for 9Cr1Mo require a yield strength ≥552MPa and a longitudinal impact energy ≥41J at -10℃ after tempering. However, in actual production, it has been found that conventional heat treatment processes cannot meet these technical requirements, and large fluctuations in impact energy after heat treatment lead to unstable quality.

[0003] In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] The purpose of this invention is to provide a 9Cr1Mo stainless steel suitable for the oil and gas industry and its heat treatment method. The 9Cr1Mo stainless steel obtained by this heat treatment method has stable comprehensive mechanical properties and meets the standards for use in the oil and gas industry.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides a heat treatment method for 9Cr1Mo stainless steel, comprising the following steps:

[0007] S1. Cooling after forging or rolling: Air cool the forged or rolled round billet until the surface temperature is ≤300℃;

[0008] S2. Annealing process: The cooled round billet is annealed at a holding temperature of 650-750℃ for a holding time of ≥(D / 100mm)*6h, where D is the outer diameter of the round billet; then it is air-cooled to room temperature after being removed from the furnace.

[0009] S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching temperature is 925-1000℃ and the holding time is (2h / 100mm)*D≤t≤(2h / 100mm)*D+2h, where D is the outer diameter of the round billet; then it is cooled to below 150℃.

[0010] S4. Tempering process: The round billet quenched in step S3 is tempered. The holding temperature for tempering is 650-750℃, and the holding time is (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h, where D is the outer diameter of the round billet; then it is air-cooled to room temperature.

[0011] S5, Straightening;

[0012] S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The holding time at the stress-relief tempering temperature is (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h. The holding temperature of stress-relief tempering is 30-50℃ lower than the holding temperature of tempering in step S4. After that, it is air-cooled to room temperature.

[0013] Furthermore, in step S1, the air-cooling and resting time of the forged or rolled round billet is ≤48h.

[0014] Furthermore, in the annealing process of step S2, the heating rate of the annealing furnace to the holding temperature of the annealing treatment is ≤100℃ / h.

[0015] Furthermore, in the quenching process of step S3, the heating rate of the quenching equipment to the quenching holding temperature is ≤80℃ / h.

[0016] Furthermore, in the quenching process of step S3, the cooling rate is ≥300℃ / h.

[0017] Furthermore, in the quenching process of step S3, the cooling method includes any one of water cooling, air cooling, oil cooling, or polymer cooling.

[0018] Furthermore, in the tempering process of step S4, the heating rate of the tempering furnace to the holding temperature of the tempering treatment is ≤80℃ / h.

[0019] Furthermore, in the tempering process of step S4, the cooling rate is ≥300℃ / h.

[0020] Furthermore, in step S5, the straightening method is rotational or pressure straightening.

[0021] Furthermore, in step S6, the heating rate of the tempering furnace to the holding temperature for stress-relief tempering is ≤80℃ / h.

[0022] Furthermore, in step S6, the cooling rate is ≥300℃ / h.

[0023] Furthermore, the specifications of the round blank range from φ50 to 250 mm.

[0024] Furthermore, the round blank comprises the following chemical composition by mass percentage: C: ≤0.15%, Si: ≤1.0%, Mn: 0.30-1.0%, P: ≤0.20%, S: ≤0.010%, Cr: 8.00-10.00%, Ni: ≤0.50%, Mo: 0.90-1.10%, with the remainder being Fe and unavoidable impurities.

[0025] Secondly, the present invention provides a 9Cr1Mo stainless steel obtained by the above-mentioned heat treatment method.

[0026] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0027] 1. This invention analyzes the formation mechanism of a large number of carbides at the crystallization site of the material after forging / rolling, improves the microstructure before tempering by tempering the cooling and annealing process after the material is formed, and further optimizes the heat treatment system of quenching, tempering, stress relief and other processes, so as to ensure that the mechanical properties of the finally obtained 9Cr1Mo stainless steel remain stable and meet the API6A standard requirements in the oil and gas field.

[0028] 2. The inventors studied and observed the microstructure of the materials obtained after forging and / or rolling, and found that a large number of carbide precipitates were present at the grain boundaries. Based on this, the carbide formation mechanism was analyzed, and a technical solution of annealing + tempering heat treatment process was provided for 9Cr1Mo material after forming. Specifically, the cooling, tempering, and quenching processes after material forming were optimized, ultimately stabilizing the mechanical properties of 9Cr1Mo.

[0029] 3. The cooling and annealing process after forming of the easily crackable martensitic stainless steel of this invention improves the microstructure before tempering and avoids instability in impact energy after subsequent heat treatment. At the same time, the heat treatment process of this invention reduces energy consumption and accelerates the production pace.

[0030] 4. The method of the present invention improves the low-temperature impact energy of the material. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0032] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0033] According to a first aspect of the invention, a 9Cr1Mo stainless steel suitable for the oil and gas industry is provided, comprising the following chemical composition by mass percentage: C: ≤0.15%, Si: ≤1.0%, Mn: 0.30-1.0%, P: ≤0.20%, S: ≤0.010%, Cr: 8.00-10.00%, Ni: ≤0.50%, Mo: 0.90-1.10%, with the remainder being Fe and unavoidable impurities.

[0034] In the aforementioned 9Cr1Mo stainless steel suitable for the oil and gas field, as a preferred embodiment, the 9Cr1Mo stainless steel comprises the following chemical composition by mass percentage: C: 0.09-0.15%, Si: ≤1.00%, Mn: 0.30-0.60%, P: ≤0.015%, S: ≤0.004%, Cr: 8.50-9.50%, Ni: ≤0.45%, Mo: 0.90-1.05%, with the remainder being Fe and unavoidable impurities.

[0035] According to a second aspect of the present invention, a heat treatment method for the above-mentioned 9Cr1Mo stainless steel is provided, comprising the following steps:

[0036] S1. Cooling after forging or rolling: Air cool the forged or rolled round billet until the surface temperature is ≤300℃;

[0037] S2. Annealing process: The cooled round billet is annealed at a holding temperature of 650-750℃ for a holding time of ≥(D / 100mm)*6h, where D is the outer diameter of the round billet; then it is air-cooled to room temperature after being removed from the furnace.

[0038] S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching temperature is 925-1000℃ and the holding time is (2h / 100mm)*D≤t≤(2h / 100mm)*D+2h, where D is the outer diameter of the round billet; then it is cooled to below 150℃.

[0039] S4. Tempering process: The round billet quenched in step S3 is tempered. The holding temperature for tempering is 650-750℃, and the holding time is (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h, where D is the outer diameter of the round billet; then it is air-cooled to room temperature.

[0040] S5, Straightening;

[0041] S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The holding time at the stress-relief tempering temperature is (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h. The holding temperature of stress-relief tempering is 30-50℃ lower than the holding temperature of tempering in step S4. After that, it is air-cooled to room temperature.

[0042] In the cooling step after forging or rolling, in order to prevent the core temperature from being too high and the microstructure transformation from being incomplete, the round billet is air-cooled to a surface temperature ≤300℃ (e.g., air-cooled to 280℃, 260℃, 240℃, 220℃, 200℃, etc.).

[0043] In the annealing process, the holding temperature is 650-750℃ (e.g., 660℃, 680℃, 700℃, 720℃, 740℃, etc.). The annealing step ensures that the material does not crack and that its impact performance is stable and improved. Excessively high annealing temperatures will cause a decrease in impact energy.

[0044] In the quenching process, the holding temperature is set to 925-1000℃ (such as 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, etc.). If the temperature is too low, the austenite transformation of the material will not be complete, and if the temperature is too high, the material is prone to coarse grains or overheating.

[0045] During the tempering process, the holding temperature is controlled at 650-750℃ (such as 660℃, 680℃, 700℃, 720℃, 740℃, etc.) to ensure that the mechanical properties of the steel meet the requirements.

[0046] In the above heat treatment method for 9Cr1Mo stainless steel, as a preferred embodiment, the air-cooling and standing time of the forged or rolled round billet is ≤48h, and the longer the standing time, the higher the risk of cracking.

[0047] In the above heat treatment method for 9Cr1Mo stainless steel, as a preferred embodiment, in the annealing process of step S2, the heating rate of the annealing furnace to the holding temperature of the annealing treatment is ≤100℃ / h.

[0048] In the above heat treatment method for 9Cr1Mo stainless steel, as a preferred embodiment, in the quenching process of step S3, the heating rate of the quenching equipment to the quenching holding temperature is ≤80℃ / h.

[0049] Optionally, the cooling rate is ≥300℃ / h;

[0050] Optionally, the cooling method may include any one of water cooling, air cooling, oil cooling, or polymer cooling.

[0051] In the above heat treatment method for 9Cr1Mo stainless steel, as a preferred embodiment, in the tempering process of step S4, the heating rate of the tempering furnace to the holding temperature of the tempering treatment is ≤80℃ / h.

[0052] Optionally, the cooling rate is ≥300℃ / h.

[0053] In the above heat treatment method for 9Cr1Mo stainless steel, as a preferred embodiment, in step S6, the heating rate of the tempering furnace to the holding temperature for stress-relief tempering is ≤80℃ / h.

[0054] Optionally, the cooling rate is ≥300℃ / h.

[0055] In the above-mentioned heat treatment method for 9Cr1Mo stainless steel, as a preferred embodiment, the specifications of the round billet range from φ50-250mm.

[0056] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0057] The 9Cr1Mo stainless steel specifications in the following examples are: round bars with an outer diameter of 50-250mm, and the main chemical composition and content (wt%) of the steel are: C: 0.10%, Si: 0.34%, Mn: 0.41%, P: 0.013%, S: 0.002%, Cr: 8.66%, Ni: 0.05%, Mo: 0.99%, with the remainder being Fe and unavoidable impurities.

[0058] Example 1

[0059] This embodiment provides a heat treatment method for 9Cr1Mo stainless steel applicable to the oil and gas industry, including the following steps:

[0060] S1. Cooling after rolling: The 120mm rolled round bar is air-cooled to a surface temperature of 280℃. At this time, the air-cooling time has reached 15 hours.

[0061] S2. Annealing process: The cooled round bars are annealed. The annealing furnace is heated at a rate of 80℃ / h to the holding temperature of the annealing treatment. The holding temperature of the annealing treatment is 700℃ and the holding time is 8h. After that, the bars are taken out of the furnace and air-cooled to room temperature.

[0062] S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching equipment is heated to the quenching holding temperature at a rate of 60℃ / h, the quenching holding temperature is 930℃, and the holding time is 2.4h. Then, it is water cooled until the surface temperature of the round billet reaches about 130℃.

[0063] S4. Tempering process: The round billet quenched in step S3 is tempered. The heating rate of the tempering furnace to the holding temperature of the tempering treatment is 60℃ / h, the holding temperature of the tempering treatment is 700℃, the holding time is 6.8h, and then it is air-cooled to room temperature.

[0064] S5, Straightening;

[0065] S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The heating rate of the tempering furnace to the stress-relief tempering holding temperature is 60℃ / h, the holding temperature of the stress-relief tempering is 660℃, the holding time is 6.8h, and then it is air-cooled to room temperature.

[0066] Example 2

[0067] This embodiment provides a heat treatment method for 9Cr1Mo stainless steel applicable to the oil and gas industry, including the following steps:

[0068] S1. Cooling after forging: The 120mm forged round bar is air-cooled to a surface temperature of 120℃. At this time, the air-cooling time has reached 21 hours.

[0069] S2. Annealing process: The cooled round bars are annealed. The annealing furnace is heated at a rate of 80℃ / h to the annealing holding temperature. The holding temperature for annealing is 710℃ and the holding time is 8h. After that, the bars are taken out of the furnace and air-cooled to room temperature.

[0070] S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching equipment is heated to the quenching holding temperature at a rate of 60℃ / h, the quenching holding temperature is 950℃, and the holding time is 2.4h. Then, it is water cooled until the surface temperature of the round billet reaches about 130℃.

[0071] S4. Tempering process: The round billet quenched in step S3 is tempered. The heating rate of the tempering furnace to the holding temperature of the tempering treatment is 60℃ / h, the holding temperature of the tempering treatment is 700℃, the holding time is 6.8h, and then it is air-cooled to room temperature.

[0072] S5, Straightening;

[0073] S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The heating rate of the tempering furnace to the stress-relief tempering holding temperature is 60℃ / h, the holding temperature of the stress-relief tempering is 660℃, the holding time is 6.8h, and then it is air-cooled to room temperature.

[0074] Example 3

[0075] This embodiment provides a heat treatment method for 9Cr1Mo stainless steel applicable to the oil and gas industry, including the following steps:

[0076] S1. Cooling after forging: The 250mm forged round bar is air-cooled to a surface temperature of 30℃. At this time, the air-cooling time has reached 47 hours.

[0077] S2. Annealing process: The cooled round bars are annealed. The annealing furnace is heated at a rate of 80℃ / h to the annealing holding temperature. The holding temperature for annealing is 710℃ and the holding time is 17h. After that, the bars are taken out of the furnace and air-cooled to room temperature.

[0078] S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching equipment is heated to the quenching holding temperature at a rate of 60℃ / h, the quenching holding temperature is 950℃, and the holding time is 6h. Then, it is water-cooled until the surface temperature of the round billet reaches about 130℃.

[0079] S4. Tempering process: The round billet quenched in step S3 is tempered. The heating rate of the tempering furnace to the holding temperature of the tempering treatment is 60℃ / h, the holding temperature of the tempering treatment is 700℃, the holding time is 12h, and then it is air-cooled to room temperature.

[0080] S5, Straightening;

[0081] S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The heating rate of the tempering furnace to the stress-relief tempering holding temperature is 60℃ / h, the holding temperature of stress-relief tempering is 660℃, the holding time is 12h, and then it is air-cooled to room temperature.

[0082] Comparative Example 1

[0083] This comparative example provides a heat treatment method for 9Cr1Mo stainless steel suitable for the oil and gas industry, including the following steps:

[0084] S1. The 120mm rolled round bar is sent to the annealing furnace.

[0085] S2. Annealing process: The annealing furnace is heated to 400℃. After all the material is rolled into the annealing furnace, the annealing furnace is heated. The heating rate of the annealing furnace to the annealing holding temperature is 80℃ / h. The holding temperature of the annealing treatment is 830℃ and the holding time is 8h. After that, the furnace is cooled to room temperature.

[0086] S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching equipment is heated to the quenching holding temperature at a rate of 60℃ / h, the quenching holding temperature is 930℃, and the holding time is 2.4h. Then, it is water cooled until the surface temperature of the round billet reaches about 130℃.

[0087] S4. Tempering process: The round billet quenched in step S3 is tempered. The heating rate of the tempering furnace to the holding temperature of the tempering treatment is 60℃ / h, the holding temperature of the tempering treatment is 700℃, the holding time is 6.8h, and then it is air-cooled to room temperature.

[0088] S5, Straightening;

[0089] S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The heating rate of the tempering furnace to the stress-relief tempering holding temperature is 60℃ / h, the holding temperature of the stress-relief tempering is 660℃, the holding time is 6.8h, and then it is air-cooled to room temperature.

[0090] Comparative Example 2

[0091] The remaining steps and parameters of this comparative example are the same as those of Example 3, except that the air-cooling and resting time of the 250mm forged round billet is up to 83 hours.

[0092] Before annealing, some of the material cracked, and the surface had fine, straight cracks that could not be salvaged.

[0093] Comparative Example 3

[0094] The parameters of the remaining steps in this comparative example are the same as those in Example 3, except that the annealing temperature in step S2 is 800°C.

[0095] When annealed at this temperature, the low-temperature impact performance of the material obtained after the final heat treatment is unstable.

[0096] Comparative Example 4

[0097] The remaining steps and parameters of this comparative example are the same as those of Example 1, except that in step S1, the surface temperature is air-cooled to 350°C.

[0098] Since the actual core temperature of the material exceeds 400℃, and the temperature difference between the surface and the core is large, the inside of the material is equivalent to being continuously kept at 400-600℃. Carbides precipitate along the grain boundaries, resulting in a decrease in impact. For specific mechanical properties and impact energy data, please refer to Table 1.

[0099] The 9Cr1Mo stainless steel obtained after heat treatment according to Examples 1-3 and Comparative Examples 1-4 was subjected to room temperature tensile properties and low temperature impact properties tests, referring to standards ASTM A370 and ASTM E23. The actual data comparison is shown in Table 1. Two sets of samples were tested for each example or comparative example.

[0100] Table 1

[0101]

[0102] As can be seen from the data in Table 1, compared with Example 1, Comparative Example 1 was annealed in a hot annealing furnace instead of being cooled after rolling, and the annealing temperature was too high, which resulted in a decrease in the low-temperature impact energy of the material.

[0103] Compared with Example 3, Comparative Example 3 has a higher annealing temperature, which leads to unstable low-temperature impact performance of the material, resulting in materials with low impact performance.

[0104] Compared with Example 1, Comparative Example 4 shows that the surface temperature of the material entering the annealing furnace is higher, which leads to the precipitation of carbides along the grain boundaries. As a result, the low-temperature impact performance of the material is unstable and has a low low-temperature impact performance.

[0105] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heat treatment method for 9Cr1Mo stainless steel, characterized in that, Includes the following steps: S1. Cooling after forging or rolling: Air cool the forged or rolled round billet until the surface temperature is ≤300℃; S2. Annealing process: The cooled round billet is annealed at a holding temperature of 650-750℃ for a holding time of ≥(D / 100mm)*6h, where D is the outer diameter of the round billet; then it is air-cooled to room temperature after being removed from the furnace. S3. Quenching process: The round billet after annealing in step S2 is quenched. The quenching temperature is 925-1000℃ and the holding time is (2h / 100mm)*D≤t≤(2h / 100mm)*D+2h, where D is the outer diameter of the round billet; then it is cooled to below 150℃. S4. Tempering process: The round billet quenched in step S3 is tempered. The holding temperature for tempering is 660-750℃, and the holding time is (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h, where D is the outer diameter of the round billet; then it is air-cooled to room temperature. S5, Straightening; S6. Stress-relief tempering: The straightened round billet is subjected to stress-relief tempering. The holding time at the stress-relief tempering temperature is (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h. The holding temperature of stress-relief tempering is 30-50℃ lower than the holding temperature of tempering in step S4. After that, it is air-cooled to room temperature. The air-cooling and standing time of the forged or rolled round billet is ≤48h; The 9Cr1Mo stainless steel is composed of the following chemical composition by mass percentage: C: 0.09-0.15%, Si: ≤1.00%, Mn: 0.30-0.60%, P: ≤0.015%, S: ≤0.004%, Cr: 8.50-9.50%, Ni: ≤0.45%, Mo: 0.90-1.05%, with the remainder being Fe and unavoidable impurities.

2. The heat treatment method according to claim 1, characterized in that, In the annealing process of step S2, the heating rate of the annealing furnace to the holding temperature of the annealing treatment is ≤100℃ / h.

3. The heat treatment method according to claim 1, characterized in that, In the quenching process of step S3, the heating rate of the quenching equipment to the quenching holding temperature is ≤80℃ / h. And / or, the cooling rate is ≥300℃ / h; And / or, the cooling method includes any one of water cooling, air cooling, oil cooling or polymer cooling.

4. The heat treatment method according to claim 1, characterized in that, In the tempering process of step S4, the heating rate of the tempering furnace to the holding temperature of the tempering treatment is ≤80℃ / h. And / or, the cooling rate is ≥300℃ / h.

5. The heat treatment method according to claim 1, characterized in that, In step S5, the straightening method is rotational or pressure straightening.

6. The heat treatment method according to claim 1, characterized in that, In step S6, the heating rate of the tempering furnace to the holding temperature for stress-relief tempering is ≤80℃ / h; And / or, the cooling rate is ≥300℃ / h.

7. The heat treatment method according to claim 1, characterized in that, The specifications of the round blank range from φ50 to 250 mm.

8. 9Cr1Mo stainless steel obtained by the heat treatment method according to any one of claims 1-7.

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