9cr1mo stainless steel suitable for oil gas fields and heat treatment method therefor

Abstract

The present invention relates to the technical field of online heat treatments for 9Cr steel, and provides a 9Cr1Mo stainless steel suitable for oil gas fields and a heat treatment method therefor. The heat treatment method comprises the following steps: S1, cooling after forging or rolling; S2, an annealing process; S3, a quenching process; S4, a tempering process; S5, straightening; and S6, stress relief tempering. The 9Cr1Mo stainless steel is obtained by means of the heat treatment method. The 9Cr1Mo stainless steel prepared by means of the heat treatment method has stable room-temperature tensile properties and low-temperature impact properties, and satisfies the use standards in oil gas fields.

Description

9Cr1Mo stainless steel suitable for oil and gas fields and heat treatment method thereof Technical Field

[0001] The present invention relates to the technical field of 9Cr steel online heat treatment, and in particular to 9Cr1Mo stainless steel suitable for the oil and gas field and a heat treatment method thereof. Background Art

[0002] In recent years, raw materials used in domestic oil and gas equipment have rapidly developed, gaining increasing recognition and adoption by international oilfield services. Varieties have also shifted from low-alloy steels to high-alloy steels. Domestically produced oilfield stainless steels such as 9Cr and 13Cr have been recognized by many renowned international oilfield services and are now being exported in large quantities. 9Cr1Mo, due to its excellent corrosion and high-temperature resistance, is widely used in the petrochemical industry and high-pressure boilers. 9Cr1Mo contains 9% ferrite-forming Cr and 1% Mo, offering excellent resistance to high-temperature oxidation and hydrogen sulfide stress corrosion. However, in the oilfield, steel must not only resist H2S / CO2 corrosion but also possess high strength. Therefore, the carbon content of 9Cr1Mo is generally controlled between 0.09% and 0.15%. This allows the structure of 9Cr1Mo to primarily transform into martensite after quenching, allowing it to meet varying mechanical property requirements through tempering. The standards for 9Cr1Mo established by domestic and international oilfield service companies require a yield strength of ≥552 MPa and a longitudinal impact energy of ≥41 J at -10°C after quenching and tempering. However, in the actual production process, it was found that the conventional heat treatment process could not meet the requirements of technical indicators, and the impact energy after heat treatment fluctuated greatly, resulting in unstable quality.

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

[0004] The object of the present invention is to provide a 9Cr1Mo stainless steel suitable for the oil and gas field and a heat treatment method thereof. The 9Cr1Mo stainless steel produced by the heat treatment method has stable comprehensive mechanical properties and meets the use standards in the oil and gas field.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

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

[0007] S1. Cooling after forging or rolling: air-cool the round billet after forging or rolling to a surface temperature of ≤300°C;

[0008] S2. Annealing process: The cooled round billet is annealed at a holding temperature of 650-750°C for a holding time of ≥ (D / 100mm)*6h, where D is the outer diameter of the round billet. The billet is then taken out of the furnace and air-cooled to room temperature.

[0009] S3, quenching process: quenching the round billet annealed in step S2 at a quenching holding temperature of 925-1000°C for a holding time of (2h / 100mm)*D≤t≤(2h / 100mm)*D+2h, where D is the outer diameter of the round billet; then cooling the round billet to a surface temperature below 150°C;

[0010] S4, tempering process: tempering the round billet after quenching in step S3, with a holding temperature of 650-750°C and a holding time of (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h, where D is the outer diameter of the round billet; then air cooling to room temperature;

[0011] S5, straightening;

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

[0013] Furthermore, the air-cooling storage time of the round billet after forging or rolling in step S1 is ≤ 48 hours.

[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°C / h.

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

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

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

[0020] Furthermore, in step S5, the straightening method is rotation 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°C / h.

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

[0023] Furthermore, the specification range of the round billet is φ50-250mm.

[0024] Furthermore, the round billet includes the following chemical components in 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%, and the rest is Fe and unavoidable impurities.

[0025] In a second aspect, the present invention provides a 9Cr1Mo stainless steel obtained by the above 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. The present invention analyzes the formation mechanism of a large amount of carbides at the crystallization site of the material after forging / rolling, and tempers the cooling and annealing processes of the material after forming, thereby improving the microstructure before tempering. It also further optimizes the heat treatment system of quenching, tempering, stress relief and other processes, ensuring that the mechanical properties of the final 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, discovering the presence of a large amount of carbide precipitation at grain boundaries. Based on this, they analyzed the carbide formation mechanism and proposed a technical solution for the 9Cr1Mo material after forming by combining annealing and tempering. Specifically, by optimizing the cooling, tempering, and quenching processes after forming, the mechanical properties of 9Cr1Mo were stabilized.

[0029] 3. The present invention's easy-to-explode martensitic stainless steel undergoes cooling and annealing after forming to improve the microstructure before quenching and tempering, thereby preventing impact energy instability after subsequent heat treatment. Furthermore, the present invention's heat treatment process reduces energy consumption and accelerates production.

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

[0031] In order to make the purpose, technical solutions and advantages of the present invention clearer, 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. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. The process parameters for which specific conditions are not specified in the following examples are generally in accordance with conventional conditions.

[0032] The endpoints of the ranges and any values ​​disclosed herein 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 endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0033] According to a first aspect of the present invention, a 9Cr1Mo stainless steel suitable for the oil and gas field is provided, comprising the following chemical composition in 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%, and the remainder being Fe and unavoidable impurities.

[0034] In the above-mentioned 9Cr1Mo stainless steel suitable for the oil and gas field, as a preferred embodiment, the 9Cr1Mo stainless steel includes the following chemical components in 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%, and the rest is Fe and unavoidable impurities.

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

[0036] S1. Cooling after forging or rolling: air-cool the round billet after forging or rolling to a surface temperature of ≤300°C;

[0037] S2. Annealing process: The cooled round billet is annealed at a holding temperature of 650-750°C for a holding time of ≥ (D / 100mm)*6h, where D is the outer diameter of the round billet. The billet is then taken out of the furnace and air-cooled to room temperature.

[0038] S3, quenching process: quenching the round billet annealed in step S2 at a quenching holding temperature of 925-1000°C for a holding time of (2h / 100mm)*D≤t≤(2h / 100mm)*D+2h, where D is the outer diameter of the round billet; then cooling the round billet to a surface temperature below 150°C;

[0039] S4, tempering process: tempering the round billet after quenching in step S3, with a holding temperature of 650-750°C and a holding time of (4h / 100mm)*D≤t≤(4h / 100mm)*D+5h, where D is the outer diameter of the round billet; then air cooling to room temperature;

[0040] S5, straightening;

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

[0042] During 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 of ≤300°C (for example, air-cooled to 280°C, 260°C, 240°C, 220°C, 200°C, etc.).

[0043] During the annealing process, the holding temperature is 650-750°C (e.g., 660°C, 680°C, 700°C, 720°C, 740°C, etc.). The annealing step prevents the material from cracking, stabilizes the impact energy performance, and improves it. Excessively high annealing temperatures can cause a decrease in impact energy.

[0044] During 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 be incomplete, and if the temperature is too high, the material will easily become coarse-grained or overburned.

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

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

[0047] In the above-mentioned heat treatment method of 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-mentioned heat treatment method of 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°C / h;

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

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

[0051] In the above-mentioned heat treatment method of 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°C / h;

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

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

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

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

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

[0057] The 9Cr1Mo stainless steel used in the following examples is round steel with an outer diameter of 50-250 mm. 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%. The remainder is 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 field, comprising the following steps:

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

[0061] S2, annealing process: annealing the cooled round bars, heating the annealing furnace, heating the annealing furnace to the annealing temperature at a rate of 80 ° C / h, the annealing temperature is 700 ° C, the holding time is 8 hours, and then the round bars are taken out of the furnace and air-cooled to room temperature;

[0062] S3, quenching process: the round billet annealed in step S2 is quenched, the quenching equipment is heated to the quenching and holding temperature at a heating rate of 60°C / h, the quenching and holding temperature is 930°C, and the holding time is 2.4h, and then water-cooled until the surface temperature of the round billet reaches about 130°C;

[0063] S4, tempering process: tempering the round billet after quenching in step S3, heating the tempering furnace to the holding temperature of the tempering treatment at a heating rate of 60°C / h, the holding temperature of the tempering treatment is 700°C, the holding time is 6.8h, and then air-cooling to room temperature;

[0064] S5, straightening;

[0065] S6. Stress relief tempering: The straightened round billet is subjected to stress relief tempering. The tempering furnace is heated to the holding temperature of stress relief tempering at a heating rate of 60°C / h, the holding temperature of stress relief tempering is 660°C, the holding time is 6.8h, and then 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 field, comprising the following steps:

[0068] S1. Cooling after forging: air-cool the forged 120mm round bar to a surface temperature of 120℃. The air-cooling time has reached 21h.

[0069] S2, annealing process: annealing the cooled round bars, heating the annealing furnace, heating the annealing furnace to the annealing holding temperature at a rate of 80 ° C / h, the annealing holding temperature is 710 ° C, the holding time is 8 hours, and then the round bars are taken out of the furnace and air-cooled to room temperature;

[0070] S3, quenching process: the round billet annealed in step S2 is quenched, the quenching equipment is heated to the quenching and holding temperature at a heating rate of 60°C / h, the quenching and holding temperature is 950°C, and the holding time is 2.4h, and then water-cooled until the surface temperature of the round billet reaches about 130°C;

[0071] S4, tempering process: tempering the round billet after quenching in step S3, heating the tempering furnace to the holding temperature of the tempering treatment at a heating rate of 60°C / h, the holding temperature of the tempering treatment is 700°C, the holding time is 6.8h, and then air-cooling to room temperature;

[0072] S5, straightening;

[0073] S6. Stress relief tempering: The straightened round billet is subjected to stress relief tempering. The tempering furnace is heated to the holding temperature of stress relief tempering at a heating rate of 60°C / h, the holding temperature of stress relief tempering is 660°C, the holding time is 6.8h, and then 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 field, comprising the following steps:

[0076] S1. Cooling after forging: air-cool the forged 250mm round bar to a surface temperature of 30°C. The air-cooling time has reached 47 hours.

[0077] S2, annealing process: annealing the cooled round bars, heating the annealing furnace, heating the annealing furnace to the annealing holding temperature at a rate of 80 ° C / h, the annealing holding temperature is 710 ° C, the holding time is 17 hours, and then the round bars are taken out of the furnace and air-cooled to room temperature;

[0078] S3, quenching process: the round billet annealed in step S2 is quenched, the quenching equipment is heated to the quenching and holding temperature at a heating rate of 60°C / h, the quenching and holding temperature is 950°C, and the holding time is 6 hours, and then water-cooled until the surface temperature of the round billet reaches about 130°C;

[0079] S4, tempering process: tempering the round billet after quenching in step S3, heating the tempering furnace to the holding temperature of the tempering treatment at a heating rate of 60°C / h, the holding temperature of the tempering treatment is 700°C, the holding time is 12h, and then air-cooling to room temperature;

[0080] S5, straightening;

[0081] S6. Stress relief tempering: The straightened round billet is subjected to stress relief tempering. The tempering furnace is heated to the holding temperature of stress relief tempering at a heating rate of 60°C / h, the holding temperature of stress relief tempering is 660°C, the holding time is 12h, and then 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 field, comprising the following steps:

[0084] S1, send the rolled 120mm round bar to the annealing furnace;

[0085] S2, annealing process: the annealing furnace is kept at 400 ° C. After all the materials are rolled and put into the annealing furnace, the annealing furnace is heated. The heating rate of the annealing furnace is 80 ° C / h to the annealing holding temperature. The holding temperature of the annealing treatment is 830 ° C, the holding time is 8 hours, and then the furnace is cooled to room temperature;

[0086] S3, quenching process: the round billet annealed in step S2 is quenched, the quenching equipment is heated to the quenching and holding temperature at a heating rate of 60°C / h, the quenching and holding temperature is 930°C, and the holding time is 2.4h, and then water-cooled until the surface temperature of the round billet reaches about 130°C;

[0087] S4, tempering process: tempering the round billet after quenching in step S3, heating the tempering furnace to the holding temperature of the tempering treatment at a heating rate of 60°C / h, the holding temperature of the tempering treatment is 700°C, the holding time is 6.8h, and then air-cooling to room temperature;

[0088] S5, straightening;

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

[0090] Comparative Example 2

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

[0092] Before annealing, some materials cracked, with fine straight cracks on the surface that could not be salvaged.

[0093] Comparative Example 3

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

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

[0096] Comparative Example 4

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

[0098] Since the core temperature of the material actually exceeds 400°C, there is a large difference between the surface temperature and the core temperature, which results in the internal temperature of the material being continuously kept at 400-600°C. Carbides precipitate along the grain boundaries, and the impact resistance decreases. For specific mechanical properties and impact energy data, please see Table 1.

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

[0100] Table 1

[0101] From the data in Table 1, it can be seen that compared with Example 1, Comparative Example 1 was not cooled after rolling but annealed in a hot annealing furnace, and the annealing temperature was relatively high, resulting in a decrease in the low-temperature impact energy of the material.

[0102] Compared with Example 3, Comparative Example 3 has a relatively high annealing temperature, which results in unstable low-temperature impact energy performance of the material, resulting in the presence of materials with low impact energy performance.

[0103] Compared with Example 1, in Comparative Example 4, the surface temperature of the material entering the annealing furnace is relatively high, resulting in the precipitation of carbides along the grain boundaries, and the low-temperature impact energy performance of the material is unstable, and the low-temperature impact energy performance is relatively low.

[0104] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A heat treatment method for 9Cr1Mo stainless steel, characterized in that, It includes the following steps: S1. Cooling after forging or rolling: Air-cool the forged or rolled round billet until the surface temperature ≤ 300°C; S2. Annealing process: Anneal the cooled round billet. The holding temperature for annealing is 650 - 750°C, and the holding time ≥ (D / 100mm) * 6h, where D is the outer diameter of the round billet; then take it out of the furnace and air-cool to room temperature; S3. Quenching process: Quench the round billet annealed in step S2. The quenching holding temperature is 925 - 1000°C, and the holding time is (2h / 100mm) * D ≤ t ≤ (2h / 100mm) * D + 2h, where D is the outer diameter of the round billet; then cool it to below 150°C of the surface temperature of the round billet; S4. Tempering process: Temper the round billet quenched in step S3. The holding temperature for tempering is 650 - 750°C, and the holding time is (4h / 100mm) * D ≤ t ≤ (4h / 100mm) * D + 5h, where D is the outer diameter of the round billet; then air-cool to room temperature; S5. Straightening; S6. Stress-relieving tempering: Conduct stress-relieving tempering on the straightened round billet. The holding time at the holding temperature for stress-relieving tempering is (4h / 100mm) * D ≤ t ≤ (4h / 100mm) * D + 5h, where the holding temperature for stress-relieving tempering is 30 - 50°C lower than the holding temperature for the tempering process in step S4, and then air-cool to room temperature.

2. The heat treatment method according to claim 1, characterized in that, The air-cooling parking time of the forged or rolled round billet ≤ 48h.

3. The heat treatment method according to claim 1, wherein In the annealing process of step S2, the heating rate of the annealing furnace to the holding temperature for annealing is ≤ 100°C / h.

4. 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°C / h; and / or, the cooling rate is ≥ 300°C / h; and / or, the cooling method includes any one of water cooling, air cooling, oil cooling or polymer cooling.

5. 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 for tempering is ≤ 80°C / h; and / or, the cooling rate is ≥ 300°C / h.

6. The heat treatment method according to claim 1, wherein In step S5, the straightening method is rotary or pressure straightening.

7. The heat treatment method according to claim 1, wherein In step S6, the heating rate of the tempering furnace to the holding temperature for stress-relieving tempering is ≤ 80°C / h; and / or, the cooling rate is ≥ 300°C / h.

8. The heat treatment method according to claim 1, characterized in that, The specification range of the round billet is φ50 - 250mm.

9. The heat treatment method according to claim 1, characterized in that, The round billet includes the following chemical components 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%, and the rest is Fe and inevitable impurities.

10. 9Cr1Mo stainless steel obtained by using the heat treatment method described in any one of claims 1 - 9.

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