Hydrogen-induced delayed fracture-resistant steel for deep well oil casing and preparation method of hydrogen-induced delayed fracture-resistant steel

Abstract

The invention discloses steel for a hydrogen-induced delayed fracture resistant deep well oil casing and a preparation method of the steel. The steel plate comprises the following chemical components in percentage by weight: 0.05 to 0.10 percent of C, 1.50 to 2.50 percent of Si, 1.00 to 2.50 percent of Mn, 0.8 to 1.2 percent of Cr, 0.38 to 1.20 percent of Cu and Ni, 0.10 to 0.50 percent of Mo, 0.05 to 0.28 percent of Nb and V, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S and the balance of Fe and inevitable impurity elements. The production method comprises the steps of steel billet heating, forging, normalizing, quenching and inverse phase change annealing heat treatment. According to the hydrogen-induced delayed fracture resistant steel for the deep well oil casing, after quenching and inverse phase transformation annealing heat treatment, a martensite + ferrite + retained austenite multi-phase structure is obtained, obdurability matching is good, and good hydrogen-induced delayed fracture resistance is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of forging and heat treatment, and particularly relates to a steel for deep well oil casing resistant to hydrogen-induced delayed fracture and a preparation method thereof. Background Art

[0002] As strategic reserve resources, oil and gas have always attracted the attention of countries around the world. In China, the efforts in exploring, exploiting and developing oil and gas have been increasing year by year. The number of wells drilled each year remains at a relatively high level, accounting for about 20% of the world's total. China has become one of the three countries with the largest number of wells drilled in the world. During the process of exploiting oil and gas, oil casing mainly plays the role of well cementing. Its consumption accounts for 70% - 80% of oil well pipes. It is a product with high technological content and high added value, and at the same time, it is also a high-risk product. Once quality problems occur during use, it will cause huge economic losses, and in severe cases, it will lead to well destruction and casualties. With the continuous increase in the demand for oil and gas, the proportion of deep well exploitation has gradually increased, and the geological conditions are more complex, resulting in more severe service conditions for the casing. Therefore, higher requirements are correspondingly put forward for the performance of the steel for deep well oil casing, requiring it to have excellent mechanical properties and good strength-toughness matching.

[0003] It should be noted that the service environment of oil well pipes is humid, which is prone to electrochemical corrosion. Hydrogen is inevitably adsorbed into the steel; and during the transportation of oil and gas, hydrogen in it will also penetrate into the steel when it comes into contact with the pipeline. Hydrogen continuously diffuses and accumulates towards the stress concentration site. When the critical hydrogen concentration is reached, the steel will suddenly become unstable and cause hydrogen-induced delayed fracture, without plastic deformation before fracture. Some studies have shown that with the increase in the strength of high-strength steel, the risk of hydrogen-induced delayed fracture is higher. Therefore, an oil well casing steel with both high strength and impact toughness, as well as good hydrogen-induced delayed fracture resistance, is an important link to ensure the sustainable development of deep-layer oil and gas resources. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a steel for deep well oil casing resistant to hydrogen-induced delayed fracture, with uniform steel plate structure, excellent mechanical properties, good strength-toughness matching, and good hydrogen-induced delayed fracture resistance.

[0005] To solve the above technical problem, the technical solution adopted by the present invention is: A steel for deep well oil casing resistant to hydrogen-induced delayed fracture, its chemical composition and weight percentage are as follows: C: 0.05 - 0.10%, Si: 1.50 - 2.50%, Mn: 1.00 - 2.50%, Cr: 0.8 - 1.2%, Cu + Ni: 0.38 - 1.20%, Mo: 0.10 - 0.50%, Nb + V: 0.05 - 0.28%, P ≤ 0.025%, S ≤ 0.015%, and the balance is Fe and inevitable impurity elements.

[0006] The setting basis for each component and its content in the steel for deep well oil casing of the present invention is as follows: C: Carbon is an effective element to ensure high tensile strength of the steel after quenching + reverse phase transformation annealing heat treatment. It is the basic element for forming martensite and austenite, playing a role in solid solution strengthening. However, too high carbon content will seriously affect the plasticity, impact toughness, corrosion resistance, and hydrogen-induced delayed fracture resistance of the steel. Therefore, the design range of carbon content is 0.05 - 0.10%.

[0007] Si: Silicon also plays a role in solid solution strengthening. Silicon dissolves in ferrite, which can improve the strength and toughness of the steel. Some studies have shown that silicon can inhibit the precipitation of cementite and improve the steel properties. However, too high silicon content will promote the segregation of impurity elements at grain boundaries, reduce the grain boundary energy, and is unfavorable for the delayed fracture resistance of the steel. Therefore, the design range of silicon content is 1.50 - 2.50%.

[0008] Mn: Manganese can dissolve in ferrite, improving the strength of the steel. And it is beneficial to the solubility of vanadium at high temperature, thus enhancing the precipitation strengthening of vanadium. Manganese is an effective element for forming austenite and improving its stability. However, too high manganese content will segregate to grain boundaries, increasing temper brittleness. Therefore, the design range of manganese content is 1.00 - 2.50%.

[0009] Cr: Chromium can improve the hardenability and temper resistance of the steel, ensuring high strength after heat treatment. It forms alloy carbides of chromium, which are finely and dispersedly distributed, refining the microstructure and improving the strength and plasticity of the steel. Therefore, the design range of chromium content is 0.80 - 1.20%.

[0010] Cu + Ni: Copper can improve the corrosion resistance of the steel, thus inhibiting the entry of hydrogen into the steel during service. Nickel can increase the self-corrosion potential of the steel in the positive direction and improve the corrosion resistance of the steel. Both are beneficial to improving the delayed fracture resistance of the steel. Therefore, the design range of Cu + Ni is 0.38 - 1.20%.

[0011] Mo: Molybdenum inhibits the formation of proeutectoid ferrite. In quenched and tempered steels, it can enhance the temper resistance, i.e., temper stability, of the steel, and can strengthen grain boundaries, which is beneficial to improving the delayed fracture resistance of the steel. Therefore, the molybdenum content is designed to be 0.10 - 0.50%.

[0012] Nb + V: Both vanadium and niobium are elements for precipitation strengthening and grain refinement strengthening of steel, which can improve the strength and toughness of steel at the same time. Moreover, the vanadium carbonitride precipitates formed by vanadium can act as strong hydrogen traps, adsorb hydrogen inside the crystal grains, and then inhibit the diffusion and grain boundary aggregation of hydrogen, thereby improving the hydrogen embrittlement resistance of steel. However, the effect becomes saturated when the vanadium content exceeds 0.20%. The carbide of niobium is a very strong hydrogen trap, which further improves the hydrogen embrittlement resistance of steel, but the effect becomes saturated when the niobium content exceeds 0.08%. When vanadium and niobium are added in combination, the effect of resisting hydrogen embrittlement is more significant. Therefore, the vanadium + niobium content is designed to be 0.05 - 0.28%.

[0013] P, S: Both phosphorus and sulfur are harmful elements, which damage the plasticity and grain boundary energy of steel. The lower the content, the better. Based on production costs, therefore, the phosphorus content is designed to be ≤0.025%, and the sulfur content is designed to be ≤0.015%.

[0014] Furthermore, the microstructure of the steel for hydrogen-induced delayed fracture-resistant deep well oil casing described in the present invention after quenching + reverse transformation annealing heat treatment is a multi-phase structure of martensite + ferrite + retained austenite.

[0015] Furthermore, for the steel for hydrogen-induced delayed fracture-resistant deep well oil casing described in the present invention, after quenching + reverse transformation annealing heat treatment, the tensile strength ≥1145 Mpa, the yield strength ≥930 MPa, and the elongation after fracture ≥17%. The mechanical properties are excellent and the strength and toughness are well matched.

[0016] Furthermore, the hydrogen embrittlement sensitivity index HEI of the steel for hydrogen-induced delayed fracture-resistant deep well oil casing described in the present invention C ≤35%.

[0017] The present invention also provides a production method of steel for hydrogen-induced delayed fracture-resistant deep well oil casing, including steel billet heating, forging, normalizing, and quenching + reverse transformation annealing heat treatment processes; in the forging process, the steel plate is forged into a thick slab with a thickness of 14 - 18 mm, and the final forging temperature ≥850°C. In order to maintain the plate shape, it is cooled to room temperature in the air to form a duplex structure of ferrite + pearlite.

[0018] Furthermore, in the steel billet heating process of the present invention, the steel billet is stacked and cooled in an environment below 300°C before being heated in the furnace, and the stacking and cooling time is 35 - 40 h. Some studies have shown that the generation of hydrogen-induced delayed fracture in steel is mainly dominated by diffusible hydrogen, and diffusible hydrogen in steel can be released at temperatures below 300°C, which is beneficial to improving the hydrogen-induced delayed fracture resistance of steel.

[0019] Furthermore, in the billet heating process of the present invention, the billet is heated in a resistance furnace, the heating temperature is 1200 - 1250 °C, and the heating time is 180 - 240 min. The heating process before forging can make the alloy elements in the billet fully diffuse and dissolve, make the steel structure uniform, have excellent mechanical properties, and reduce the internal stress of the steel for deep well oil casing during forging and cooling.

[0020] Furthermore, in the normalizing process of the present invention, the Ac3 temperature of the steel is estimated using Andrews' empirical formula, Ac3 = 910 - 203[C] 1 / 2 + 44.7[Si] - 15.2[Ni] + 31.5[Mo] + 104[V] + 13.1[W]. The normalizing heating temperature of the steel plate is Ac3 + 10 °C, holding for 50 min, and air-cooling to room temperature, which can make the alloy carbides finer and more dispersed, adsorb as strong hydrogen traps, and thus improve the hydrogen embrittlement resistance of the material; Furthermore, in the quenching + reverse transformation annealing heat treatment process of the present invention, the quenching heating temperature of the steel plate is Ac3 °C, holding for 60 - 80 min, and forced air-cooling to room temperature to obtain a quenched lath martensite structure; the reverse transformation annealing process of the steel plate uses electromagnetic induction heating, the heating rate is 40 - 50 °C / s, the Ac1 temperature is estimated using Andrews' empirical formula, Ac1 = 723 - 10.7[Mn] + 29.1[Si] + 16.9[Cr] - 16.9[Ni] + 290[As] + 6.38[W], the specimen is heated to Ac1 + 50 °C, holding for 100 - 120 min, forced air-cooling to 200 - 300 °C, then rapidly heating to 400 - 450 °C at a heating rate of 40 - 50 °C / s, holding for 60 - 70 min, and then air-cooling to room temperature in the air to obtain a multi-phase structure of martensite + ferrite + retained austenite. Research shows that using electromagnetic induction for heat treatment can further make the carbide precipitation finer and more dispersed, refine the grains, provide guarantee for excellent mechanical properties and good strength-toughness matching, and the carbides finely and dispersedly distributed in the matrix can adsorb hydrogen as irreversible hydrogen traps, reduce the diffusible hydrogen content in the steel, reduce the hydrogen segregation at the grain boundaries, improve the grain boundary strength, and thus be beneficial to improving the hydrogen-induced delayed fracture resistance of the steel; the reverse transformation annealing process can obtain a retained austenite structure, further improving the strength and plasticity of the steel.

[0021] The beneficial effects of adopting the above technical solutions are as follows: 1. The steel for deep well oil casing provided by the present invention mainly has C, Si, Mn, Cr, Mo as the main chemical components. By adding trace alloy elements, excellent mechanical properties are formed, and the strength-toughness matching is good.

[0022] 2. The steel for deep well oil casing provided by the present invention forms a ferrite + pearlite duplex structure after being cooled to room temperature in air after forging, and obtains a tempered martensite + ferrite + retained austenite multiphase structure after quenching + reverse transformation annealing. The structure is uniform and has good resistance to hydrogen-induced delayed fracture performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the metallographic structure of the steel for oil casing in Example 1. SPECIFIC EMBODIMENTS

[0024] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Examples 1 - 6

[0025] The chemical compositions and Ac1 and Ac3 temperatures of the steel for deep well oil casing in each example are shown in Table 1.

[0026] Table 1 Chemical compositions (wt.%) and Ac1 and Ac3 temperatures (°C) of the steel for oil casing in each example

[0027] The preparation methods of the steel for deep well oil casing in each example are as follows: 500 kg steel ingots are smelted by a vacuum induction furnace, and then processed into corresponding steel through heating, forging, normalizing, quenching + reverse transformation annealing heat treatment; specifically as follows: (1) Steel billet heating: The steel billet is stacked and cooled in an environment below 300 °C before being put into the furnace for heating, and the stacking and cooling time is 35 - 40 h. The steel billet is heated in a resistance furnace, the heating temperature is 1200 - 1250 °C, and the holding time is 180 - 240 min.

[0028] (2) Forging: The steel plate is forged into a slab with a thickness of 14 - 18 mm, the final forging temperature is ≥900 °C, and then it is cooled to room temperature in air to obtain a ferrite + pearlite duplex structure.

[0029] (3) Normalizing: The heating temperature of the steel plate is Ac3 + 10 °C, it is held for 50 min, and then air-cooled to room temperature.

[0030] The process parameters of steel billet heating, forging and normalizing in each example are shown in Table 2.

[0031] Table 2 Process control parameters of steel billet heating, forging and normalizing in each example

[0032] (4) Quenching + inverse transformation annealing heat treatment: The quenching heating temperature of the steel plate is Ac3 °C, hold for 60 - 80 min, and then forced air-cooled to room temperature to obtain quenched lath martensite structure; The inverse transformation annealing process of the steel plate uses electromagnetic induction heating, with a heating rate of 40 - 50 °C / s. Heat the specimen to Ac1 + 50 °C, hold for 100 - 120 min, then forced air-cooled to 200 - 300 °C, and then quickly heat up to 400 - 450 °C at a heating rate of 40 - 50 °C / s, hold for 60 - 70 min, and then air-cooled to room temperature to obtain tempered martensite + ferrite + retained austenite multiphase structure.

[0033] The control parameters of the quenching + inverse transformation annealing heat treatment process for the steel billets of each embodiment are shown in Table 3.

[0034] Table 3 Control parameters of the quenching and inverse transformation annealing processes for each embodiment

[0035] The metallographic structure of the steel for oil casing in Example 1 is as Figure 1 shown. As can be seen from Figure 1 it, its microstructure is a multiphase structure of martensite + ferrite + retained austenite. The metallographic structures of the steel for oil casing in the remaining examples are the same as those in Example 1 and will not be shown one by one.

[0036] The mechanical properties and hydrogen embrittlement sensitivity index HEI C of the steel for oil casing in each embodiment were detected, and the results are shown in Table 4. Among them, the hydrogen embrittlement sensitivity index HEI C was measured by the constant load notch tensile test (Constant Load Tensile, CLT). The specific measurement method is as follows: Prepare a constant load notch tensile specimen (diameter d = 5 mm, d at the notch N = 3 mm, notch 60° ± 2° / 0.15R ± 0.025); Judge the critical fracture stress σ NC of the specimen under the erosion of Walpole slow-release solution (16.4 g C2H3NaO2 + 15.4 ml HCL + 1000 ml deionized water) with pH = 3.5 ± 0.5. The fracture time is t c = 100 h; Record the minimum stress that fractures within the specified fracture cut-off time t c as σ f , and record the maximum stress that does not fracture within the fracture cut-off time t c as σ n , then the critical stress of the notch specimen is: σ NC = (σ f + σ n ) / 2; The index of the hydrogen embrittlement sensitivity degree of the steel is: HEI C (%) = (σ NCO-σ NC ) / σ NCO ´100%, σ NCO is the tensile strength of the notched specimen; the smaller the value of the index, the better the hydrogen-induced delayed fracture resistance of the material.

[0037] Table 4 Mechanical properties and hydrogen embrittlement sensitivity index of the steel for oil casing in each embodiment

[0038] As can be seen from Table 4, the steel for deep well oil casing provided by the present invention has excellent mechanical properties, good strength and toughness matching, and good hydrogen-induced delayed fracture resistance.

Claims

1. A steel for deep well petroleum casing resistant to hydrogen-induced delayed fracture, characterized in that: The chemical composition of the steel plate and its weight percentage are: C: 0.05-0.10%, Si: 1.50-2.50%, Mn: 1.00-2.50%, Cr: 0.8-1.2%, Cu+Ni: 0.38-1.20%, Mo: 0.10-0.50%, Nb+V: 0.05-0.28%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurity elements.

2. The steel for deep well petroleum casing resistant to hydrogen-induced delayed fracture according to claim 1, characterized in that: The microstructure of the steel plate after quenching+reverse phase transformation annealing heat treatment is a multiphase structure of martensite+ferrite+retained austenite.

3. The steel for deep well petroleum casing resistant to hydrogen-induced delayed fracture according to claim 1, characterized in that: The steel plate has a tensile strength of ≥1145 MPa, a yield strength of ≥930 MPa, and an elongation after fracture of ≥17%.

4. The steel for deep well petroleum casing resistant to hydrogen-induced delayed fracture according to claim 1, characterized in that: The steel plate hydrogen embrittlement sensitivity index HEI C ≤35%.

5. A method for preparing hydrogen-induced delayed fracture resistant deep well petroleum casing steel according to any one of claims 1 to 4, characterized in that: The method comprises the steps of heating, forging, normalizing, quenching and reverse phase transformation annealing of steel billets. In the forging step, the steel plate is forged into a 14-18 mm thick slab with a final forging temperature of ≥900°C, and then cooled to room temperature in air to obtain a ferrite + pearlite dual-phase structure.

6. The method for preparing hydrogen-induced delayed fracture resistant deep well petroleum casing steel according to claim 5, characterized in that: In the steel billet heating process, the steel billet is pile-cooled in an environment below 300° C. before being put into the furnace for heating, and the pile-cooling time is 35 to 40 hours.

7. The method for preparing hydrogen-induced delayed fracture resistant deep well petroleum casing steel according to claim 5, characterized in that: In the billet heating process, the billet is heated in a resistance furnace at a heating temperature of 1200-1250° C. and a holding time of 180-240 minutes.

8. The method for preparing hydrogen-induced delayed fracture resistant deep well petroleum casing steel according to claim 5, characterized in that: In the normalizing process, the steel plate is heated to a temperature of Ac3+10°C, kept at that temperature for 50 minutes, and then air-cooled to room temperature.

9. The method for preparing hydrogen-induced delayed fracture resistant steel for deep well petroleum casing according to claim 5, characterized in that: In the quenching + reverse phase transformation annealing heat treatment process, the steel plate quenching heating temperature is Ac3°C, kept warm for 60 to 80 minutes, and forced air cooling is performed to room temperature to obtain a quenched lath martensite structure; the steel plate reverse phase transformation annealing process adopts electromagnetic induction heating with a heating rate of 40 to 50°C / s, heating the sample to Ac1+50°C, keeping warm for 100 to 120 minutes, forced air cooling to 200 to 300°C, then rapidly heating to 400 to 450°C at a heating rate of 40 to 50°C / s, keeping warm for 60 to 70 minutes, and then cooling to room temperature in air to obtain a martensite + ferrite + retained austenite multiphase structure.

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