Super martensitic stainless steel with high strength and toughness and CO2 corrosion resistance as well as preparation method and application of super martensitic stainless steel

Abstract

The invention relates to super martensitic stainless steel with high strength and toughness and CO2 corrosion resistance as well as a preparation method and application of the super martensitic stainless steel, and belongs to the technical field of martensitic stainless steel. The super martensitic stainless steel comprises less than or equal to 0.01% of C, less than or equal to 0.50% of Si, less than or equal to 0.80% of Mn, less than or equal to 0.020% of P, less than or equal to 0.005% of S, 15.00%-17.00% of Cr, 6.00%-7.00% of Ni, 2.00%-3.50% of Cu, 1.80%-2.50% of Mo, 0.01%-0.15% of Nb and 0.01%-0.20% of V. Through mutual cooperation of chemical elements, especially strict control of the contents of C, Cr, Cu and other elements, and addition of trace V and Nb elements, the super martensitic stainless steel with highly matched strength, toughness and corrosion resistance is obtained, and the requirement for preparation of the 125 ksi steel grade oil well pipe is met.

Description

Technical Field

[0001] The invention relates to the technical field of martensitic stainless steel, and in particular to a super martensitic stainless steel with high strength, toughness and resistance to CO2 corrosion, and a preparation method and application thereof. Background Art

[0002] In oil fields, the most widely used oil well pipe material is super 13Cr martensitic stainless steel with a steel grade of 95-110ksi. With the growing demand for oil and gas resources, the mining depth of oil and gas fields is developing towards 10,000-meter deep wells. When the bottom hole pressure exceeds 125MPa and the temperature exceeds 170°C, the requirements for the strength-toughness-corrosion resistance matching of martensitic stainless steel are very high. However, the 13Cr series martensitic oil well pipe materials will suffer from severe corrosion during use, which greatly affects the sealing of the oil well pipe. Therefore, materials with higher strength-toughness-corrosion resistance matching or martensitic stainless steel are required.

[0003] At present, there is only one martensitic stainless steel, JFE-UHPTM-15Cr-125, and the oil well pipe made of it has achieved high strength, but the margin of impact energy is not large. In addition, the oil well pipe will face stress corrosion problems during service. Under the premise of not losing strength, further improving the plasticity and toughness of the material can inhibit crack propagation and further improve the material's stress corrosion resistance, which is the direction of improving the performance of oil pipe materials.

[0004] Therefore, there is an urgent need for a material with highly matched strength, toughness and corrosion resistance or a super martensitic stainless steel that can meet the requirements for the preparation of 125ksi steel grade oil well pipes. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a super martensitic stainless steel with high strength, toughness and resistance to CO2 corrosion, and a preparation method and application thereof, so as to obtain a super martensitic stainless steel with highly matched strength, toughness and corrosion resistance, which can meet the preparation of 125ksi steel grade oil well pipes.

[0006] On the one hand, the present invention provides a super martensitic stainless steel with high strength, toughness and resistance to CO2 corrosion, wherein the chemical composition thereof includes, by weight percentage, C≤0.01%, Si≤0.50%, Mn≤0.80%, P≤0.020%, S≤0.005%, Cr: 15.00%-17.00%, Ni: 6.00%-7.00%, Cu: 2.00%-3.50%, Mo: 1.80%-2.50%, Nb: 0.01%-0.15%, V: 0.01%-0.20%, and the balance is Fe and other inevitable impurities.

[0007] Furthermore, the chemical composition of the super martensitic stainless steel includes C≤0.0018%, Si≤0.50%, Mn≤0.80%, P≤0.020%, S≤0.005%, Cr: 16.00%~17.00%, Ni: 6.6%~7.00%, Cu: 2.50%~3.50%, Mo: 2.0%~2.50%, Nb: 0.01~0.15%, V: 0.1%~0.20%, and the balance is Fe and other inevitable impurities.

[0008] On the other hand, the present invention provides a method for preparing super martensitic stainless steel with high strength, toughness and resistance to CO2 corrosion, which is prepared by smelting, forging and heat treatment;

[0009] The heat treatment includes quenching and tempering, the heating temperature of the quenching is 1030°C to 1060°C, and the heat treatment is followed by oil cooling or water cooling; the heating temperature of the tempering is 485°C to 550°C, and the heat treatment is followed by air cooling.

[0010] Furthermore, the heating temperature of the quenching is 1040°C-1060°C, and the holding time is 1-2h.

[0011] Furthermore, the heating temperature of the tempering is 485° C. to 525° C., and the holding time is 2-3 hours.

[0012] Furthermore, the super martensitic stainless steel has a tensile strength of more than 931 MPa, a yield strength of 862-1034 MPa, and a V-type impact energy of ≥200 J.

[0013] Furthermore, at a temperature of 200° C. in a CO 2 environment, the corrosion rate of the super martensitic stainless steel is ≤0.01 mm / a.

[0014] Furthermore, the microstructure of the super martensitic stainless steel is martensite and retained austenite.

[0015] Furthermore, the content of martensite is 75% to 85%.

[0016] Furthermore, the super martensitic stainless steel provided by the present invention can be used to prepare 125ksi steel grade oil well pipes.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0018] 1. In the present invention, by the mutual coordination of chemical elements, especially by strictly controlling the contents of elements such as C, Cr and Cu, and adding trace amounts of V and Nb elements, a super martensitic stainless steel with highly matched strength, toughness and corrosion resistance is obtained, which meets the requirements for the preparation of 125ksi steel grade oil well pipes;

[0019] 2. In the present invention, the preparation method adopts smelting, forging and heat treatment, and the heat treatment is quenching and tempering. The heating temperature of the quenching is 1030℃~1060℃, and the heat preservation is followed by oil cooling; the heating temperature of the tempering is 485℃~550℃, and the heat preservation is followed by air cooling.

[0020] 3. Under the interaction of composition and heat treatment, the obtained super martensitic stainless steel has high strength, impact toughness and corrosion resistance, with a tensile strength of more than 931 MPa, a yield strength of 862-1034 MPa, and a V-type impact energy of ≥200 J; at a temperature of 200°C and a CO2 environment, the corrosion rate of the super martensitic stainless steel is ≤0.01 mm / a.

[0021] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0023] Figure 1 This is the structure of Example 1 after quenching at 1040°C (without tempering);

[0024] Figure 2 This is the structure of comparative example 1 after quenching at 1040°C (without tempering);

[0025] Figure 3 The phase ratio of Example 1 and Comparative Example 1 after quenching at 1040°C (without tempering);

[0026] Figure 4 The organization of the heat treatment system of Example 1;

[0027] Figure 5 The organization of the heat treatment system of Example 2;

[0028] Figure 6 The organization of the heat treatment system of Example 3;

[0029] Figure 7 This is the organization of the heat treatment system of Example 4. DETAILED DESCRIPTION

[0030] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0031] Oil well pipes are important equipment used in the oil industry for drilling, oil production and oil and gas transportation. They need to work in high temperature, high pressure and corrosive environments.

[0032] At present, the most commonly used material for making oil well pipes is 13Cr super martensitic stainless steel, which is a 95-110ksi steel grade material. As the mining depth increases, the bottom hole pressure and temperature will rise accordingly. When the bottom hole pressure exceeds 125MPa and the temperature exceeds 170℃, the oil well pipe made of 13Cr series super martensitic stainless steel will suffer serious corrosion during use, which greatly affects the sealing of the oil well pipe, and the strength cannot meet the requirements.

[0033] In order to improve the strength of oil well pipes, there is a martensitic stainless steel with the brand name JFE-UHPTM-15Cr-125, whose strength can meet the preparation of 125ksi steel grade oil well pipes, but its impact energy margin is not large. In addition, oil well pipes also face stress corrosion problems during service.

[0034] In order to obtain super martensitic stainless steel with very high strength-toughness-corrosion resistance matching, its chemical composition includes C≤0.01%, Si≤0.50%, Mn≤0.80%, P≤0.020%, S≤0.005%, Cr: 15.00%~17.00%, Ni: 6.00%~7.00%, Cu: 2.00%~3.50%, Mo: 1.80%~2.50%, Nb: 0.01%~0.15%, V: 0.01%~0.20%, and the balance is Fe and other inevitable impurities.

[0035] Compared with the prior art, the super martensitic stainless steel provided by the present invention needs to strictly control the contents of elements such as C, Cr and Cu, requiring the C element to be below 0.01%, the Cr element to be 15.00% to 17.00% and the Cu element to be 2.00% to 3.50%; and adding trace amounts of V and Nb elements, and controlling the Nb element to be 0.01% to 0.15%, and the V element to be 0.01% to 0.20%. In the mutual coordination of chemical elements, a super martensitic stainless steel with highly matched strength, toughness and corrosion resistance is obtained, which meets the preparation of 125ksi steel grade oil well pipes.

[0036] It should be noted that in the present invention, reducing the C content to no more than 0.01% can simultaneously improve the toughness and corrosion resistance of super martensitic stainless steel; and appropriately increasing the Cr element can further improve the corrosion resistance. In order to compensate for the strength loss caused by the decrease in C content, the Cu content is increased to 2.0-3.5%, increasing the precipitation strengthening effect of the Cu-rich phase, while changing the phase transition point of the material. Through tempering at the aging temperature, sufficient reverse austenite can be produced, greatly improving the toughness. Adding trace amounts of V and Nb elements forms a highly stable nano-strengthening phase, slowing down the recovery of the matrix structure heat treated at the over-aging temperature, and ensuring that the strength reaches the 125ksi level.

[0037] C element: Reducing the C content from no more than 0.04% to no more than 0.01% can reduce strength and improve toughness and corrosion resistance. In maraging steel, the C element has a great influence on the phase transition point and second phase precipitation of the material. The tempering temperature of 13-15% Cr super martensitic stainless steel is generally in the range of 450-650℃. Reducing the C content can prevent the precipitation of the second phase along the grain boundary during tempering of high Cr stainless steel, which is beneficial to improving toughness and corrosion resistance.

[0038] Cr element: Appropriately increase the Cr content to 15% to 17% to further improve corrosion resistance.

[0039] Cu element: Controlling the Cu content within 2.0-3.5% can produce three beneficial effects: (1) Cu element can increase the Ni equivalent, balance the Ni / Cr equivalent after reducing C and increasing Cr, and prevent the formation of a large amount of delta ferrite; (2) The increase of Cu element can delay the coarsening of Cu-rich phase at the overaging temperature and compensate for the strength loss caused by the reduction of C content; (3) Cu element can change the phase transformation point of stainless steel, and a certain amount of reverse austenite phase will be formed in the subsequent tempering process, which greatly improves the toughness of martensitic stainless steel.

[0040] V and Nb elements: Adding trace amounts of V and Nb elements will form nano-MX phase during tempering, slowing down the recovery of the matrix structure tempered at the over-aging temperature, and ensuring that the strength of 125ksi can be achieved.

[0041] Mn element: Mn is an austenite-forming element that can inhibit the formation of delta ferrite and improve hot working performance.

[0042] Ni element: Ni is the basic element to ensure Ni equivalent. Reasonable Ni equivalent design can avoid the formation of delta ferrite. At the same time, Ni is also the key to ensure the toughness of super martensitic steel.

[0043] Mo element: Mo can produce solid solution strengthening effect in super martensitic stainless steel, improve the material's resistance to tempering, and is also the key to improving the material's corrosion resistance.

[0044] Specifically, the chemical composition of the super martensitic stainless steel includes C≤0.0018%, Si≤0.50%, Mn≤0.80%, P≤0.020%, S≤0.005%, Cr: 16.00%-17.00%, Ni: 6.6%-7.00%, Cu: 2.50%-3.50%, Mo: 2.0%-2.50%, Nb: 0.01-0.15%, V: 0.1%-0.20%, and the balance is Fe and other inevitable impurities.

[0045] The present invention provides a method for preparing super martensitic stainless steel with high strength, toughness and resistance to CO2 corrosion, which is prepared by smelting, forging and heat treatment;

[0046] The heat treatment includes quenching and tempering. The heating temperature of the quenching is 1030°C to 1060°C, followed by oil cooling or water cooling after heat preservation. The heating temperature of the tempering is 485°C to 550°C, followed by air cooling after heat preservation.

[0047] Compared with the existing technology, all super martensitic stainless steels are quenched and tempered. Quenching is used to obtain martensite, and tempering is used to adjust the strength and toughness.

[0048] Specifically, the heating temperature of the quenching is 1040° C.-1060° C., and the holding time is 1-2 hours.

[0049] Specifically, the heating temperature of the tempering is 485° C. to 525° C., and the holding time is 2-3 hours.

[0050] Specifically, the super martensitic stainless steel has a tensile strength of more than 931 MPa, a yield strength of 862-1034 MPa, and a V-type impact energy of ≥200 J.

[0051] Specifically, at a temperature of 200° C. and a CO 2 environment, the corrosion rate of the super martensitic stainless steel is ≤0.01 mm / a.

[0052] Specifically, the structure of the super martensitic stainless steel is martensite+retained austenite, and the content of martensite phase is 75% to 85%.

[0053] In order to more clearly describe the present invention, it is further illustrated by the following examples and comparative examples.

[0054] Example 1

[0055] The preparation of super martensitic stainless steel is as follows:

[0056] Smelting, obtaining a steel ingot by an electric furnace, wherein the steel ingot composition is, by weight percentage, C: 0.0018%, Si: 0.17%, Mn: 0.27%, P: 0.005%, S: 0.0018%, Cr: 16.5%, Ni: 6.64%, Cu: 2.50%, Mo: 2.30%, Nb: 0.10%, V: 0.10%, and the balance is Fe;

[0057] Forging, after forging at 1150℃, a tube billet steel bar with a diameter of φ155mm is prepared;

[0058] Heat treatment: quenching and tempering are used for heat treatment. The heating temperature of quenching is 1040°C, and it is kept warm for 1 hour before oil cooling. The heating temperature of tempering is 485°C, and it is kept warm for 2 hours before air cooling.

[0059] Example 2

[0060] The preparation process of Example 2 is substantially the same as that of Example 1, except that the heating temperature of the quenching in Example 2 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 500°C, which is kept warm for 2 hours and then air-cooled.

[0061] Example 3

[0062] The preparation process of Example 3 is substantially the same as that of Example 1, except that the heating temperature of the quenching in Example 3 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 525°C, which is kept warm for 2 hours and then air-cooled.

[0063] Example 4

[0064] The preparation process of Example 4 is substantially the same as that of Example 1, except that the heating temperature of the quenching in Example 4 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 550°C, which is kept warm for 2 hours and then air-cooled.

[0065] Example 5

[0066] The preparation process of Example 5 is substantially the same as that of Example 1, except that the composition of the steel ingot in Example 5 is C: 0.0016%, Si: 0.17%, Mn: 0.27%, P: 0.005%, S: 0.0018%, Cr: 16.32%, Ni: 6.60%, Cu: 2.05%, Mo: 2.28%, Nb: 0.13%, V: 0.15%, and the balance is Fe.

[0067] Example 6

[0068] The preparation process of Example 6 is substantially the same as that of Example 1, except that the heating temperature of the quenching in Example 6 is 1050°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 510°C, which is kept warm for 2 hours and then air-cooled.

[0069] Example 7

[0070] The preparation process of Example 7 is substantially the same as that of Example 1, except that the heating temperature of the quenching in Example 7 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 525°C, which is kept warm for 2 hours and then air-cooled.

[0071] Example 8

[0072] The preparation process of Example 8 is substantially the same as that of Example 1, except that the heating temperature of the quenching in Example 8 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 550°C, which is kept warm for 2 hours and then air-cooled.

[0073] Comparative Example 1

[0074] The preparation process of Comparative Example 1 is substantially the same as that of Example 1, except that the composition of the steel ingot in Comparative Example 1 is C: 0.018%, Si: 0.16%, Mn: 0.26%, P: 0.005%, S: 0.0027%, Cr: 15.22%, Ni: 6.51%, Cu: 1.02%, Mo: 2.29%, and the balance is Fe.

[0075] Comparative Example 2

[0076] The preparation process of Comparative Example 2 is substantially the same as that of Comparative Example 1, except that the heating temperature of the quenching in Comparative Example 2 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 500°C, which is kept warm for 2 hours and then air-cooled.

[0077] Comparative Example 3

[0078] The preparation process of Comparative Example 3 is substantially the same as that of Comparative Example 1, except that the heating temperature of the quenching in Comparative Example 3 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 525°C, which is kept warm for 2 hours and then air-cooled.

[0079] Comparative Example 4

[0080] The preparation process of Comparative Example 4 is substantially the same as that of Comparative Example 1, except that the heating temperature of the quenching in Comparative Example 4 is 1040°C, which is kept warm for 1 hour and then oil-cooled; the heating temperature of the tempering is 550°C, which is kept warm for 2 hours and then air-cooled.

[0081] Comparative Example 5

[0082] The preparation process of Comparative Example 5 is substantially the same as that of Example 1, except that the heating temperature for tempering in Comparative Example 5 is 575° C., and the mixture is kept at this temperature for 2 hours and then air-cooled.

[0083] Performance Testing

[0084] The above-mentioned Examples 1-8 and Comparative Examples 1-5 were subjected to performance tests, mainly including strength, impact and hardness, and the test results are shown in Table 1; their corrosiveness was tested in a high temperature and high pressure CO2 environment, and the test results are shown in Table 2.

[0085] Table 1 Performance test results

[0086]

[0087]

[0088] Table 2 Corrosion test results

[0089]

[0090] Combining Examples 1-8 and Comparative Examples 1-5 and referring to Table 1, it can be seen that the super martensitic stainless steel prepared by the composition and preparation method provided by the present invention has high performance and corrosion resistance. The tensile strength of the obtained super martensitic stainless steel is above 931MPa, the yield strength is between 862 and 1034MPa, the V-type impact energy is ≥200J, and the cross-sectional shrinkage Z is above 70%. And at a temperature of 200°C and a CO2 environment, the corrosion rate of the super martensitic stainless steel is ≤0.01mm / a.

[0091] Combining Examples 1-4 and Comparative Examples 1-3 and referring to Table 1, it can be seen that the structure obtained after heat treatment of the steel examples of the present invention is 79% martensite + 20% austenite, the martensite phase can guarantee the strength requirement of 125ksi, and the austenite phase can guarantee high toughness. The tempering temperature is increased from 485℃ to 550℃, all of which are tempered martensite. As the temperature increases, the lath recovers, but retains the complete morphology and can still meet the strength requirement of 125ksi. After quenching, the martensite content of the comparative steel is about 92%, and the austenite content is about 8%. The austenite content is small, so the toughness is lower.

[0092] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A super martensitic stainless steel with high strength, toughness and resistance to CO2 corrosion, characterized in that: Calculated in percentage by weight, its chemical composition includes C≤0.01%, Si≤0.50%, Mn≤0.80%, P≤0.020%, S≤0.005%, Cr: 15.00%~17.00%, Ni: 6.00%~7.00%, Cu: 2.00%~3.50%, Mo: 1.80%~2.50%, Nb: 0.01~0.15%, V: 0.01%~0.20%, and the balance is Fe and other inevitable impurities.

2. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 1, characterized in that: The chemical composition of the super martensitic stainless steel includes C≤0.0018%, Si≤0.50%, Mn≤0.80%, P≤0.020%, S≤0.005%, Cr: 16.00%-17.00%, Ni: 6.6%-7.00%, Cu: 2.50%-3.50%, Mo: 2.0%-2.50%, Nb: 0.01-0.15%, V: 0.1%-0.20%, and the balance is Fe and other inevitable impurities.

3. A method for preparing the high-strength, toughness and CO2 corrosion-resistant super martensitic stainless steel as claimed in claim 1 or 2, characterized in that: Prepared by smelting, forging and heat treatment; The heat treatment includes quenching and tempering. The heating temperature of the quenching is 1030°C to 1060°C, and the heat treatment is followed by oil cooling or water cooling. The heating temperature of the tempering is 485°C to 550°C, and the heat treatment is followed by air cooling.

4. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 3, characterized in that: The heating temperature of the quenching is 1040° C.-1060° C., and the holding time is 1-2 hours.

5. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 3, characterized in that: The heating temperature of the tempering is 485° C. to 525° C., and the holding time is 2-3 hours.

6. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 3, characterized in that: The super martensitic stainless steel has a tensile strength of more than 931 MPa, a yield strength of 862-1034 MPa, and a V-type impact energy of ≥200 J.

7. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 3, characterized in that: At a temperature of 200° C. and a CO 2 environment, the corrosion rate of the super martensitic stainless steel is ≤0.01 mm / a.

8. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 3, characterized in that: The microstructure of the super martensitic stainless steel is martensite and retained austenite.

9. The high-strength, toughness, CO2 corrosion-resistant super martensitic stainless steel according to claim 8, characterized in that: The content of martensite is 75% to 85%.

10. Use of the super martensitic stainless steel according to claim 1 or 2 or the super martensitic stainless steel obtained by any one of claims 3 to 9, characterized in that: Used to prepare 125ksi steel grade oil well pipe.

Images