700MPa grade pressure pipe steel, manufacturing, forming and post-heat treatment method

Abstract

The invention provides a 700MPa grade pressure pipe steel, a manufacturing method, a forming method and a subsequent heat treatment method. The steel has the following components by weight percentage: C 0.13%-0.17%, Si 0.40%-0.80%, Mn 0.80%-1.10%, P≤0.010%, S≤0.003%, Cr 0.50%-0.80%, Mo 0.18%-0.28%, Zr 0.05%-0.15%, Alt 0.020%-0.045%, and the balance is Fe and unavoidable inclusions. The manufacturing method comprises heating, rolling and tempering. After the produced steel plate is hot formed and post-heat treated, the mechanical properties of the steel are 520MPa≤Rel≤620MPa and 700MPa≤Rm≤850MPa at room temperature; 380MPa≤Rp0.2≤480MPa at 400°C and impact energy KV2≥200J at -40°C.

Description

Technical Field

[0001] The invention belongs to the field of metal materials, and in particular relates to a 700MPa grade pressure pipe steel, and a manufacturing, forming and subsequent heat treatment method thereof. Background Art

[0002] As a special equipment for material transportation, petrochemical pressure pipeline plays a role as a link of transmission, making the entire petrochemical device an organic whole, and also providing important guarantee for the transportation safety of various fluids. In recent years, the scale of my country's petrochemical industry has continued to develop and grow. The pursuit of safety and efficiency will be an important task for the future development of the petrochemical industry. With the substantial increase in demand for petrochemical products, it is necessary to continuously improve the scale and operation parameters of equipment to meet production capacity needs, which puts forward higher and higher requirements on the pressure resistance and high strength of petrochemical pressure pipelines.

[0003] The invention patent "Composite process and device for corrosion-resistant and pressure-resistant composite pipe (glass fiber reinforced plastic, steel pipe)" (publication number CN 1097852) disclosed before the present invention, from the disclosed process method, the patent combines resin and glass fiber with steel pipe by pressing to form a composite steel pipe. The invention involves finished steel pipes, not steel plates and their production process, and the strength level is not mentioned, which cannot meet the use requirements of high-strength pressure pipes.

[0004] The disclosed invention patent "Heat-resistant, pressure-resistant, and corrosion-resistant marine oil and gas transmission pipe" (publication number CN205447005U), from the disclosed invention effect, the corrosion and pressure resistance of the oil and gas transmission pipe of the invention patent is mainly achieved by adding high tensile fiber, spiral steel wire and synthetic rubber inside and outside the steel pipe. The invention does not involve steel plates and their production process, and the strength level is not mentioned, which cannot meet the use requirements of high-strength pressure-bearing pipelines.

[0005] The chemical composition of the pipeline structural steel disclosed in the invention patent "600MPa grade excellent hot working performance pipeline structural steel and production method" (publication number CN112063925A) is as follows: C: 0.03% ~ 0.05%, Si: 0.20% ~ 0.35%, Mn: 0.55% ~ 0.85%, Nb: 0.02% ~ 0.03%, Ti: 0.011% ~ 0.025%, Ni: 0.7% ~ 1.0%, Cr: 0.60% ~ 1.0%, Cu : 0.80% ~ 1.0%, Mo: 0.20% ~ 0.30%, Als: 0.015% ~ 0.040%, N: 0.00; the balance is iron; from the disclosed composition, production method and beneficial effects, this patented product is mainly used in oil and gas transportation environments. Although the strength of the steel reaches 600MPa by adding alloy elements and combining the rolling process, there is still a certain gap compared to 700MPa, and it has not been adjusted through the heat treatment process, which should be quite different from the petrochemical pressure pipeline.

[0006] The above invention and existing pressure-bearing pipelines can no longer meet the development needs of large-scale petrochemical energy equipment. Therefore, there is an urgent need to develop 700MPa-grade petrochemical high-strength pipeline materials with excellent pressure resistance to support the development needs of my country's large-scale petrochemical energy equipment. Summary of the invention

[0007] The purpose of the present invention is to overcome the problem that the above inventions are insufficient in strength or cannot provide a technical solution to improve the strength of high-strength pressure pipes after forming, and to provide a steel plate with a thickness specification of 8 to 30 mm and a steel product for pressure pipes with a tensile strength of 700 MPa after forming, manufacturing, forming and subsequent heat treatment process to meet the manufacturing needs of high-strength pressure pipes.

[0008] The object of the invention is achieved in this way:

[0009] A 700MPa grade pressure pipe steel, the components of which are as follows by weight percentage: C 0.13%-0.17%, Si 0.40%-0.80%, Mn 0.80%-1.10%, P≤0.010%, S≤0.003%, Cr 0.50%-0.80%, Mo 0.18%-0.28%, Zr 0.05%-0.15%, Alt 0.020%-0.045%, and the balance is Fe and unavoidable inclusions.

[0010] The thickness of steel used for pressure pipes is 8 to 30 mm.

[0011] The mechanical properties of steel for pressure pipes are 500MPa≤R eL ≤590MPa、700MPa≤R m ≤820MPa, elongation A≥25%, impact energy KV2≥160J at -40℃.

[0012] The reasons for the composition design of the present invention are as follows:

[0013] C is the most important element for improving the strength of steel. The hardenability of steel is significantly improved by adding C. In addition, the C of the present invention combines with the strong carbide alloying elements in the steel to play a precipitation strengthening role, obtain a secondary hardening effect, and ensure the high strength requirements of the steel. When the carbon content is lower than 0.13%, the hardenability is low, and it is difficult to obtain uniform tempered bainite in the subsequent tempering treatment, and the strength of the steel cannot meet the use requirements. However, too high a carbon content will affect the machining performance of the steel, so the present invention limits the C content range to 0.13% to 0.17%.

[0014] Si is used as a reducing agent and deoxidizer in the steelmaking process. Si has a certain influence on the transformation of bainite during tempering. When the silicon content is higher than 0.5%, it will hinder the activity of C in bainite during tempering, so that ε carbide is generated in bainitic steel instead of M3C carbide, which reduces the hardness and toughness of the steel and increases the sensitivity of temper brittleness. Therefore, the present invention limits the Si content range to 0.40% to 0.80%.

[0015] Mn is an element that strongly stabilizes austenite, can effectively reduce the decomposition rate of austenite, improve the hardenability of steel, and can strongly increase the strength and hardness of steel. However, a high Mn content will enhance the temper brittleness of steel, so the present invention limits the Mn content range to 0.80% to 1.10%.

[0016] S and P are harmful elements in steel and must be strictly controlled to ensure the purity and plasticity of the steel. Therefore, the present invention limits S to ≤ 0.003% and P to ≤ 0.010%.

[0017] Adding a certain amount of Cr to Cr steel significantly improves the hardenability of the steel and ensures the matrix strength of the steel. In addition, Cr is a strong carbide-forming element, which forms stable carbides with C in the steel, and exerts a precipitation strengthening effect, further improving the compression resistance of the pipeline. Therefore, the present invention limits the Cr content range to 0.50% to 0.80%.

[0018] Mo can improve the hardenability of steel, effectively refine austenite grains, and strengthen the solid solution strengthening effect of ferrite. The addition of a certain amount of Mo can improve the tempering resistance of steel and inhibit tempering embrittlement. At the same time, molybdenum is a strong carbide-forming element, and the Mo2C carbide formed during tempering has a secondary hardening effect. Therefore, the present invention sets the Mo content range to 0.18% to 0.28%.

[0019] Zr can generate highly dispersed strong carbide ZrC, which can strongly hinder the growth of austenite grains and has the effect of refining grains. In addition, zirconium dissolved in austenite can significantly improve the hardenability of steel and improve the strength of steel. However, Zr is a precious metal element, and excessive addition leads to increased costs. Therefore, the present invention limits the Zr content to 0.05% to 0.15%.

[0020] Alt is a commonly used deoxidizer in steel. Adding a small amount of aluminum can refine the grains and improve the strength and impact toughness of steel. Too high an amount will affect the hot working performance, welding performance and cutting performance of steel. Therefore, the present invention limits the Alt content range to 0.020% to 0.045%.

[0021] The second technical solution of the present invention is to provide a method for manufacturing 700MPa grade pressure pipe steel, including heating, rolling and tempering;

[0022] Heating: The heating temperature is controlled at 1180-1230°C, and the total heating time is 4.0-6.0h;

[0023] Rolling: A two-stage controlled rolling method is adopted. The rolling end temperature of the recrystallization zone is ≥1000℃. In order to fully crush the core structure of the steel billet, the total deformation rate is ≥50%; the rolling temperature of the non-recrystallization zone is 830-870℃, the final rolling temperature is 800-850℃, and the total deformation rate is ≥60%; at this time, the austenite grains are further flattened and elongated. With the increase of the grain boundary area, the ferrite nucleation rate increases in the subsequent phase transformation process, and the grains are fully refined. The target thickness of the rolled piece is (8-30) mm.

[0024] Tempering: Since more solid solution strengthening elements such as C, Mn, Cr, and Mo are added to the steel, the steel plate obtains a bainite structure with high strength and hardness under air cooling conditions after rolling. However, at this time, the grain size of the steel plate is coarse, the internal stress is concentrated, and delayed cracks are easily generated during flame cutting. Therefore, tempering heat treatment should be used in time to soften and eliminate stress. The present invention uses tempering heat treatment to ensure that the strength of the pressure-bearing steel plate is not lost, while making the steel plate have suitable plastic toughness, which is conducive to forming processing. Therefore, the tempering heat treatment temperature of the steel is 650-700°C, and the net holding time is 1.0-2.0min / mm.

[0025] The third technical solution of the invention is to provide a forming process method for 700MPa grade pressure pipe steel:

[0026] Hot forming is a necessary process for making high-pressure pressure-bearing pipes. The forming process not only enables the pipe to obtain the required shape, but is also an important means to improve the final strength of the pipe. In order to obtain a post-forming strength of 700MPa, the forming heat treatment parameters and reasons of the steel plate of the present invention are set as follows:

[0027] In the molding process, the heating temperature is 900-930°C, and the net holding time is 1.0-2.0 min / m; after being taken out of the furnace, it is cooled to room temperature at a cooling rate of 10-25°C / s by water mist spraying; then short-term tempering is carried out, the tempering temperature is 700-730°C, and the holding time is 1.0-1.5h.

[0028] Since more solid solution strengthening elements such as C, Mn, Cr, and Mo are added to the steel, a lath bainite structure with high strength and hardness can be obtained after the steel plate is rolled. However, the grain size of the steel plate is coarse, and there is concentration of structural stress and thermal stress, which is prone to delayed cracking during flame cutting. Therefore, heat treatment should be used in time to soften and eliminate stress. The heat treatment process of the present invention with water mist cooling after normalizing has a specific heating temperature of 900-930°C, a net insulation time of 1.0-2.0min / mm, and is cooled to room temperature by water spraying after being taken out of the furnace. In this process, chemical elements C, Mn, Cr, Mo and micro-alloying elements such as Zr enter the austenite region. After a period of insulation, the alloying elements are dissolved in the austenite matrix, and then a water mist spray method is used to cool at a cooling rate of 10-25°C / s to obtain a fine lath bainite structure. At this time, the steel has a higher strength. However, the steel has poor plasticity and toughness and high tissue stress. Through short-time tempering heat treatment at 700-730℃ and net holding time of 1.0-1.5h, the quenching stress can be eliminated in time, the structure can be softened and the plasticity and toughness of the steel can be improved. At this time, the microstructure of the steel is tempered bainite.

[0029] The fourth technical solution of the invention is to provide a post-forming heat treatment process for 700MPa grade pressure pipe steel. In order to ensure the pressure resistance of pressure pipe steel, it is necessary to further adjust the size and type of the second phase particles in the steel to obtain high strength and toughness, that is, secondary hardening heat treatment. The process is a heating temperature of 730-760°C and a net holding time of 2.0-4.0h. At this time, a large amount of (20-50)nm (Fe, Mn, Cr, Mo) is dispersed and precipitated in the steel. 23 The volume percentage of the second phase particles of C6, Mo2C and ZrC is 5% to 10%; the second phase particles interact with the high-density dislocations in the bainite to improve the strength and plasticity of the matrix; the mechanical properties of the steel after forming and heat treatment are 520MPa≤Rel≤620MPa, 700MPa≤Rm≤850MPa at room temperature; 380MPa≤Rp0.2≤480MPa at 400℃, and KV2≥200J at -40℃; the pressure resistance requirements of pressure-bearing pipelines are met.

[0030] The beneficial effects of the present invention are:

[0031] (1) By adding Cr, Mo, and Zr alloy elements in combination with C, Si, and Mn strengthening elements, and strictly controlling the content of harmful elements P and S, the steel plate of the present invention obtains a tempered bainite structure in combination with the manufacturing process.

[0032] (2) The mechanical properties of the steel plate for pressure pipes obtained through a unique production process are 500MPa≤R at room temperature. el ≤590MPa、700MPa≤R m ≤820MPa, (-40℃)KV2≥160J.

[0033] (3) After forming and secondary hardening heat treatment, the steel plate of the present invention obtains a high dislocation density tempered bainite structure, at which a large amount of (20-50) nm (Fe, Mn, Cr, Mo) is dispersed and precipitated in the steel. 23 The second phase particles such as C6, Mo2C and ZrC account for 5-10% of the structure, ensuring that the steel plate still has 700MPa level strength and good plasticity and toughness after forming.

[0034] (4) The mechanical properties of the steel for pressure-bearing equipment of the present invention after forming and heat treatment are 520MPa≤R el ≤620MPa、700MPa≤R m ≤850MPa、380MPa≤R p0.2 (400℃)≤480MPa, (-40℃)KV2≥200J.

[0035] (5) The present invention obtains a 700MPa grade high-strength steel plate for pressure-bearing pipes with a thickness specification of (8 to 30) mm. DETAILED DESCRIPTION

[0036] The present invention will be further described below by way of examples.

[0037] The embodiment of the present invention performs smelting, continuous casting, rolling, tempering, forming and heat treatment according to the component ratio of the technical solution.

[0038] A method for manufacturing 700MPa grade pressure pipe steel: wherein;

[0039] Heating: The heating temperature is controlled at 1180-1230°C, and the total heating time is 4-6 hours;

[0040] Rolling: adopt two-stage controlled rolling method, the end rolling temperature of recrystallization zone is ≥1000℃, the total deformation rate is ≥50%; the starting rolling temperature of non-recrystallization zone is 830~870℃, the final rolling temperature is 800~850℃, and the total deformation rate is ≥60%;

[0041] Tempering: Tempering heat treatment temperature is 650~700℃, and the net holding time is 1.0~2.0min / mm.

[0042] A 700MPa grade pressure pipe forming process, in which the heating temperature is 900-930°C and the net holding time is 1.0-2.0min / m; after being taken out of the furnace, the pipe is quickly cooled to room temperature at a cooling rate of 10-25°C / s by water mist spraying; and then short-time tempering is performed, the short-time tempering temperature is 700-730°C, and the net holding time is 1.0-1.5h.

[0043] Furthermore, after rapid cooling in a cold water bath, the microstructure of the steel changes to lath bainite; after short-time tempering, the microstructure of the steel is tempered bainite.

[0044] A post-forming heat treatment process for a 700MPa-level pressure-bearing pipe, wherein the post-forming heat treatment process is a secondary hardening heat treatment, and the specific process is as follows: a heating temperature of 730 to 760°C and a net insulation time of 2.0 to 4.0 hours.

[0045] Further; after forming and heat treatment, 20 to 50 nm level (Fe, Mn, Cr, Mo) are precipitated in the steel 23 The volume percentage of the second phase particles of C6, Mo2C and ZrC is 5% to 10%.

[0046] The composition of the steel of the present invention is shown in Table 1. The main process parameters of the steel of the present invention are shown in Table 2. The mechanical properties of the steel of the present invention are shown in Table 3. The mechanical properties and microstructure of the steel of the present invention after forming and heat treatment are shown in Table 4.

[0047] Table 1 Composition of steel according to the present invention (wt%)

[0048] Example C Si Mn P S Cr Mo Zr Alt 1 0.14 0.47 0.88 0.006 0.001 0.78 0.23 0.05 0.039 2 0.13 0.58 0.94 0.006 0.002 0.72 0.20 0.09 0.022 3 0.17 0.80 0.83 0.008 0.001 0.50 0.18 0.08 0.033 4 0.13 0.68 1.02 0.007 0.001 0.75 0.19 0.06 0.028 5 0.17 0.55 1.10 0.005 0.002 0.52 0.21 0.07 0.045 6 0.15 0.60 0.90 0.010 0.003 0.66 0.23 0.12 0.031 7 0.14 0.73 0.80 0.006 0.001 0.57 0.26 0.10 0.028 8 0.16 0.44 0.98 0.007 0.002 0.69 0.27 0.13 0.020 9 0.14 0.52 1.07 0.009 0.003 0.80 0.24 0.15 0.023 10 0.13 0.40 1.05 0.008 0.001 0.62 0.20 0.14 0.040

[0049] Table 2 Main process parameters of steel rolling in the embodiment of the present invention

[0050]

[0051] Table 3 Mechanical properties and microstructure of steel in the embodiment of the present invention

[0052] Example Sampling location <![CDATA[R eL / MPa]]> <![CDATA[R m / MPa]]> A / ％ <![CDATA[(-40℃)KV2 / J]]> Microstructure 1 T / 4 589 808 25.0 179 Tempered bainite 2 T / 4 526 749 26.0 173 Tempered bainite 3 T / 4 550 767 26.5 176 Tempered bainite 4 T / 4 580 796 24.5 165 Tempered bainite 5 T / 4 590 820 25.0 161 Tempered bainite 6 T / 4 515 735 28.0 170 Tempered bainite 7 T / 4 500 700 27.0 188 Tempered bainite 8 T / 4 575 798 27.0 176 Tempered bainite 9 T / 4 534 759 28.0 174 Tempered bainite 10 T / 4 510 728 27.5 180 Tempered bainite

[0053] Table 4 Mechanical properties and microstructure of steel formed and heat treated in the present invention

[0054]

[0055] According to the above results, it can be concluded that the mechanical properties of the steel plate for pressure pipes of the present invention are 500MPa≤R el ≤590MPa、700MPa≤R m ≤820MPa, (-40℃)KV2≥160J. The mechanical properties of the steel pipe after forming and heat treatment are as follows: The mechanical properties of the pressure steel pipe at room temperature are 520MPa≤R el ≤620MPa、700MPa≤R m ≤850MPa、380MPa≤R p0.2 (400℃)≤480MPa, (-40℃)KV2≥200J.

[0056] In order to describe the present invention, the present invention is appropriately and fully illustrated by the examples in the above. The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A 700MPa grade pressure pipe steel, characterized in that: The composition of the steel is as follows by weight percentage: C0.13%~ 0.17%, Si 0.40%~0.80%, Mn 0.80%~1.10%, P ≤0.010%, S ≤0.003%, Cr 0.50% ~0.80%, Mo 0.18%~0.28%, Zr 0.05%~0.15%, Alt 0.020%~0.045%, the balance being Fe and unavoidable inclusions; A method for manufacturing 700MPa grade pressure pipe steel, including smelting, continuous casting, heating, rolling and tempering; Heating: The heating temperature is controlled at 1180~1230℃, and the total heating time is 4~6h; Rolling: adopt two-stage controlled rolling method, the end rolling temperature of recrystallization zone is ≥1000℃, the total deformation rate is ≥50%; the starting rolling temperature of non-recrystallization zone is 830~870℃, the final rolling temperature is 800~850℃, and the total deformation rate is ≥60%; Tempering: Tempering heat treatment temperature is 650~700℃, net holding time is 1.0~2.0min / mm; 700MPa grade pressure pipe forming process, in the forming process, the heating temperature is 900~930℃, and the net holding time is 1.0~2.0min / m; after being taken out of the furnace, it is quickly cooled to room temperature by water mist spraying at a cooling rate of 10~25℃ / s; then short-time tempering is carried out, the short-time tempering temperature is 700~730℃, and the net holding time is 1.0~1.5h; The heat treatment process after forming of 700MPa grade pressure pipe is secondary hardening heat treatment. The specific process is: heating temperature 730~760℃, net insulation time 2.0~4.0h.

2. The 700MPa grade pressure pipe steel according to claim 1, characterized in that: The thickness of steel used for pressure pipes is 8~30mm.

3. The 700MPa grade pressure pipe steel according to claim 1, characterized in that: The mechanical properties of steel for pressure pipes are 500MPa≤ R eL ≤590MPa、700MPa≤ R m ≤820MPa, A ≥25%, impact energy at -40℃ KV 2≥160J.

4. The 700MPa grade pressure pipe steel according to claim 1, characterized in that: After heat treatment after forming, 20~50nm level (Fe, Mn, Cr, Mo) are precipitated in the steel 23 The volume percentage of the second phase particles of C6, Mo2C and ZrC is 5%~10%.

5. The 700MPa grade pressure pipe steel according to claim 1, characterized in that: After heat treatment after forming, the mechanical properties of the steel are 520MPa≤ R el ≤620MPa、700MPa≤ R m ≤850MPa; 380MPa≤ at 400℃ R p0.2 ≤480MPa; impact energy at -40℃ KV 2 ≥200J.

6. The 700MPa grade pressure pipe steel according to claim 1, characterized in that: After rapid cooling in a cold water bath, the microstructure of the steel changes to lath bainite; after short-time tempering, the microstructure of the steel is tempered bainite.

Images