Steel for 900MPa grade pressure steel pipe, manufacturing, forming and post-heat treatment method

Abstract

The present invention provides a 900MPa grade steel for pressure-bearing steel pipe, manufacturing, forming and subsequent heat treatment methods. The steel has the following components by weight percentage: C 0.14%-0.18%, Si 0.15%-0.35%, Mn 0.70%-1.00%, P≤0.010%, S≤0.003%, Cr 0.40%-0.65%, Mo 0.15%-0.25%, V 0.02%-0.05%, B 0.0005%-0.005%, Alt 0.020%-0.045%, and the balance is Fe and unavoidable inclusions. The manufacturing method includes smelting, continuous casting, heating, rolling and tempering. After forming and subsequent heat treatment, the tensile strength of the steel plate of the present invention is 900MPa, which meets the requirements for manufacturing high-strength pressure-bearing steel pipes.

Description

Technical Field

[0001] The invention belongs to the field of metal materials, and in particular relates to a 900MPa grade steel for pressure-bearing steel pipe, and a manufacturing, forming and subsequent heat treatment method thereof. Background Art

[0002] Pressure steel pipes are used as a link for the transmission of industrial gases, liquids and other media. They transport media from one device to another under the action of internal pressure and are widely used in industrial energy fields such as chemical industry and hydropower. In recent years, with the upgrading of traditional energy industries and the rapid development of new energy, in order to improve the reaction or transmission efficiency of media, design institutes have put forward high-efficiency, high-parameter and high-reliability design requirements for pressure steel pipes, requiring pressure steel pipes to have higher pressure-bearing capacity and impact resistance. The above invention and the strength of existing pressure steel pipes and the strength after forming can no longer meet the development needs of large-scale, high-parameter pressure steel pipes such as chemical industry and hydropower. Therefore, it is urgent to develop 900MPa-level (10-40)mm thin-gauge high-strength key materials for pressure equipment with super strength at room temperature, super strength at high temperature and good plastic toughness after forming, to support the development needs of new, high-parameter pressure equipment steel pipes in my country's energy field.

[0003] Patent document "A steel for nuclear power pressure equipment and its manufacturing method" (publication number CN 103160732 A) discloses that the steel composition is 0.12% to 0.18% C; 0.15% to 0.35% Si; 1.20% to 1.65% Mn; ≤0.015% P; ≤0.010% S; 0.50% to 0.85% Ni; ≤0.15% Cr; 0.020% to 0.050% Al; ≤0.02% V; ≤0.02% Ti, and the rest is Fe; the process adopts smelting, continuous casting, rolling and heat treatment. Judging from the disclosed composition, production method and beneficial effects, this patent is for steel for pressure equipment used in the nuclear power field, and the tensile strength of the steel plate is 560 to 625N / mm 2 Within the range, the tensile strength after simulated post-weld heat treatment is reduced to 510-600N / mm 2 When facing a higher pressure service environment, the product cannot provide the higher strength support equipment requirements, especially there is no technical solution and data after molding to support its sufficient strength to meet the high pressure service requirements.

[0004] Patent document "A hot-rolled weldable steel plate for pressure-bearing equipment and its production method" (publication number CN110983175A), from the disclosed composition, production method and beneficial effects, the tensile strength of the steel plate involved in the patent is 450-610MPa in the supplied state and the simulated post-weld heat treatment state, and its microstructure is composed of relatively low-strength ferrite and pearlite. When faced with a higher pressure service environment, the product cannot provide the higher strength support equipment requirements, especially there is no technical solution and data support after forming to show that it has sufficient strength to meet the high-pressure service requirements.

[0005] The patent document "A hot-rolled dual-phase high corrosion-resistant steel plate with a tensile strength of 900 MPa and its manufacturing method" (publication number CN115094339A) discloses a dual-phase high corrosion-resistant steel plate including the following chemical components by mass percentage: C: 0.015% to 0.040%, Si: 0.40% to 0.70%, Mn: 0.30% to 0.60%, P: 0.015% to 0.030%, S: ≤0.002%, Cr: 3.00% to 5.00%, Ni: ≤0.15%, Cu: 0.15% to 0.30%, Als: 0.020% to 0.050%, Ti: 0.0 10%~0.025%, H≤1.5ppm, the rest are Fe and unavoidable impurity elements. The high corrosion-resistant steel plate has a dual-phase structure of quasi-polygonal ferrite and granular bainite, has excellent atmospheric corrosion resistance, high strength and low yield ratio, and is easy to form; the process is continuous casting→heating→rolling→laminar cooling→coiling; from the disclosed composition, production method and beneficial effects, the patented product is used for the manufacture of atmospheric corrosion-resistant environment equipment, and is a hot-rolled coil product with a relatively thin thickness. Although the strength is at the 900MPa level, the forming heat treatment and its performance required for pressure-bearing equipment are not reflected, and the manufacturing requirements of high-pressure equipment cannot be met.

[0006] The strength of the steel for pressure-bearing equipment of the above invention and the strength after forming can no longer meet the development needs of large-scale, high-parameter pressure-bearing equipment such as coal-fired power, petrochemical, nuclear energy, and hydrogen energy. Therefore, it is urgently necessary to develop 900MPa-level (10-40)mm thin-gauge high-strength key materials for pressure-bearing equipment with super strength at room temperature, super strength at high temperature, and good plasticity and toughness after forming to support the development needs of new, high-parameter pressure-bearing equipment or equipment in my country's energy field. Summary of the invention

[0007] The purpose of the present invention is to overcome the problem that the above-mentioned inventions are insufficient in strength or cannot provide a technical solution to improve the strength of high-strength pressure-bearing steel pipes after forming, and to provide a steel plate with a thickness specification of (10 to 40) mm and a steel product for pressure-bearing steel pipes with a tensile strength of 900 MPa after forming, as well as a manufacturing, forming, and heat treatment process to meet the manufacturing needs of high-strength pressure-bearing steel pipes.

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

[0009] A 900MPa grade steel for pressure-bearing steel pipe, the steel plate has the following components by weight percentage: C 0.14%-0.18%, Si 0.15%-0.35%, Mn 0.70%-1.00%, P≤0.010%, S≤0.003%, Cr 0.40%-0.65%, Mo 0.15%-0.25%, V 0.02%-0.05%, B 0.0005%-0.005%, Alt 0.020%-0.045%, and the balance is Fe and unavoidable inclusions.

[0010] The thickness of the steel used for the pressure-bearing steel pipe is 10 to 40 mm.

[0011] The mechanical properties of the steel used for the pressure steel pipe are 720MPa≤R p0.2 ≤890MPa、900MPa≤R m ≤980MPa, impact energy KV2≥100J 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 can be significantly improved by adding C. In addition, the C in 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 a uniform tempered bainite or martensite structure 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 0.14% to 0.18%.

[0014] Si is used as a reducing agent and deoxidizer in the steelmaking process. Si has a certain influence on the martensite transformation during the tempering process. When the silicon content is higher than 0.5%, it will hinder the activity of C in martensite during the tempering process, so that ε carbide is generated in martensitic steel instead of M3C type 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 0.15% to 0.35%.

[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 0.70% to 1.00%.

[0016] S and P are harmful elements in steel and must be strictly controlled to ensure comprehensive indicators such as steel purity and mechanical properties. Therefore, the present invention limits P to ≤ 0.010% and S to ≤ 0.003%.

[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. It forms stable carbides with C in steel, which improves the pressure-bearing capacity of pressure-bearing equipment. When the Cr content is lower than 0.3%, the hardenability is improved very little, which is not conducive to the formation of lath martensite structure and cannot meet the high-strength performance requirements of steel. Therefore, the present invention limits the Cr content range to 0.40% to 0.65%.

[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 limits the Mo content range to 0.15% to 0.25%.

[0019] V: Adding an appropriate amount of V can form highly stable fine carbides (VC) with C, which are dispersed in the crystal and at the grain boundary, and have a significant inhibitory effect on the slip and climb of dislocations, thereby enhancing the matrix strength and hardness of the steel. However, too high a content will increase the yield strength ratio of the steel, which is not conducive to forming and manufacturing. Therefore, the present invention limits the V content to 0.02% to 0.05%.

[0020] B increases the hardenability of steel and utilizes the high-hardness boride formed by boron to improve the strength of steel. However, a boron content exceeding 0.007% will cause hot brittleness of the steel and affect the forming and processing performance of the pressure-bearing equipment. Therefore, the present invention limits the B content range to 0.0005% to 0.005%.

[0021] 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 to 0.020% to 0.045%.

[0022] The second technical solution of the present invention is to provide a method for manufacturing a steel plate for a 900MPa grade pressure-bearing steel pipe, comprising heating, rolling, and tempering;

[0023] (1) Heating: The heating temperature is controlled at 1180-1230° C., and the total heating time is 4.0-6.0 hours.

[0024] (2) Rolling: A two-stage controlled rolling method is used. The end rolling temperature of the recrystallization zone is ≥1000°C. In order to fully break up the core structure of the steel billet, the total deformation rate is ≥60%. The starting rolling temperature of the non-recrystallization zone is 890-930°C, the final rolling temperature is 860-890°C, 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 (10-40) mm.

[0025] (3) 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 and martensite complex structure with high strength and hardness under air cooling conditions after rolling. However, 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 3.0-5.0 min / mm.

[0026] The third technical solution of the present invention is to provide a steel plate forming process for 900MPa grade pressure steel pipe;

[0027] Hot forming is a necessary process for steel plate manufacturing pressure-bearing equipment. The forming process not only enables it to obtain the shape required by the equipment, but is also an important means to improve the final strength of the pressure-bearing equipment. In order to obtain a post-forming strength of 900MPa, the forming heat treatment parameters and reasons of the steel plate of the present invention are set as follows:

[0028] The heating temperature range of the hot forming process is 900-930°C, the net holding time is 1.0-2.0min / mm, and after being taken out of the furnace, it enters a cold water bath at 0-15°C for rapid cooling and transforms into a fine lath bainite and lath martensite dual-phase structure. The preferred rapid cooling rate is 15-30°C / s; then it undergoes a short-time tempering heat treatment at 600-630°C with a net holding time of 1.0-1.5h; at this time, the microstructure of the steel is tempered bainite and tempered martensite.

[0029] The heating temperature range of the hot forming process is 930-960℃, the net holding time is 1.0-2.0min / mm, and after being taken out of the furnace, it enters the cold water bath to cool to room temperature. In this process, chemical elements C, Mn, Cr, Mo and alloy elements such as B enter the austenite zone, and after a period of heat preservation, they are fully dissolved in the austenite matrix, and then enter the cold water bath at 0-15℃ and rapidly cool at a cooling rate of 15-30℃ / s to transform into a fine lath bainite and lath martensite complex phase structure. At this time, the steel has high strength under the solid solution strengthening of the alloy elements. However, the plasticity and toughness of the steel are poor and the organizational stress is high. It is necessary to eliminate the quenching stress in time, soften the organization, improve the plasticity and toughness of the steel, and complete the steel forming process through a short-term tempering heat treatment at 600-630℃ and a net holding time of 1.0-1.5h to soften the organization, improve the plasticity and toughness of the steel, and complete the steel forming process, so as to avoid poor forming effect or local cracking due to excessive internal stress. At this time, the microstructure of the steel is tempered bainite and tempered martensite.

[0030] The fourth technical solution of the present invention is to provide a post-forming heat treatment process for 900MPa grade pressure steel pipes; the post-forming heat treatment is a secondary hardening heat treatment, the heating temperature is 630-660°C, and the heat preservation is 2.0-4.0h. At this time, a large amount of stable (10-30)nm (Fe, Mn, Cr, Mo) is dispersed and precipitated in the steel. 23 C6, Mo2C and VC second phase particles.

[0031] In order to ensure the ultimate high strength of the pressure-bearing equipment, it is necessary to further adjust the size and type of the second phase particles in the steel, that is, the secondary hardening heat treatment, the process is 630 ~ 660 ℃, the net holding time is 2.0 ~ 4.0h, at this time a large number of 10 ~ 30nm level (Fe, Mn, Cr, Mo) are dispersed in the steel 23 The volume percentage of C6, Mo2C and VC second phase particles accounts for 10% to 15% of the organizational proportion, and the rest is tempered bainite and tempered martensite. The second phase particles interact with the high-density dislocations in tempered bainite and tempered martensite to improve the strength and plasticity of the matrix and meet the use requirements of high-strength pressure-bearing steel pipes. After heat treatment after forming, the mechanical properties of the pressure-bearing steel pipe are 770MPa≤R at room temperature. p0.2 ≤950MPa、900MPa≤R m ≤1030MPa、450MPa≤R p0.2 (400℃)≤560MPa, (-40℃)KV2≥120J.

[0032] The beneficial effects of the present invention are:

[0033] (1) By adding Cr, Mo, V, and B alloy elements to the strengthening elements C, Si, and Mn, and strictly controlling the content of harmful elements P and S, the steel plate of the present invention obtains a tempered bainite and martensite dual phase structure in combination with the manufacturing process.

[0034] (2) The mechanical properties of the steel plate for pressure pipes obtained through a unique production process are 720MPa≤R at room temperature. p0.2 ≤890MPa、900MPa≤R m ≤980MPa, elongation A≥22%; impact energy KV2≥100J at -40℃.

[0035] (3) After forming and secondary hardening heat treatment, the steel plate of the present invention obtains a high-density tempered bainite and tempered martensite complex structure, and a large amount of (10-30) nm-level (Fe, Mn, Cr, Mo) is dispersed and precipitated in the steel. 23 The second phase particles of C6, Mo2C and VC account for 10-15% of the structure, ensuring that the steel plate still has 900MPa strength and good plasticity and toughness after forming.

[0036] (4) The mechanical properties of the steel plate for pressure pipes obtained through a unique production process are 770MPa≤R at room temperature. p0.2 ≤950MPa、900MPa≤R m ≤1030MPa, elongation A≥22%; 450MPa≤R at 400℃ p0.2 ≤560MPa, impact energy KV2≥120J at -40℃.

[0037] (5) The present invention obtains a 900MPa grade high-strength steel plate for pressure-bearing steel pipe with a thickness specification of (10 to 40) mm. DETAILED DESCRIPTION

[0038] The present invention will be further described below by way of examples.

[0039] The embodiment of the present invention performs heating, rolling, heat treatment, forming and heat treatment after forming according to the component ratio of the technical solution.

[0040] Manufacturing method of 900MPa grade pressure steel pipe steel:

[0041] (1) Heating: The heating temperature is controlled at 1180-1230° C., and the total heating time is 4.0-6.0 hours;

[0042] (2) Rolling: adopt two-stage controlled rolling method, the end temperature of recrystallization zone rolling is ≥1000℃, the total deformation rate is ≥60%; the starting rolling temperature of non-recrystallization zone is 890~930℃, the final rolling temperature is 860~890℃, and the total deformation rate is ≥60%;

[0043] (3) Tempering: The tempering temperature is 650-700°C, and the insulation time is 3.0-5.0 min / mm.

[0044] Further; smelting: molten steel smelting is carried out in a converter, using high-quality scrap steel and molten iron as raw materials, the molten iron content is controlled at 75-85%, dephosphorization and decarburization are smelted separately in a converter, wherein the dephosphorization oxygen blowing is controlled at 7-10 minutes, and the decarburization oxygen blowing is controlled at 8-12 minutes, and finally the phosphorus mass fraction is reduced to less than 0.006%; deep desulfurization treatment is carried out in an LF refining furnace, and the sulfur content is controlled below 0.002%; degassing is completed in a VD furnace, the net cycle time is 10-15 minutes, and the calming time before pouring is 3-5 minutes.

[0045] Further; continuous casting: superheat is 20-30℃, the casting rate is 1.0-1.4m / min; the billet is stacked and slowly cooled after it comes off the line, the stacking slow cooling time is 24-36h, and the billet is unstacked below 400℃;

[0046] A forming process for a 900MPa grade pressure-bearing steel pipe. In the forming process, the heating temperature is 900-930°C for insulation, and the insulation time is 1.0-2.0min / mm. After being taken out of the furnace, it enters a cold water bath at 0-15°C and is rapidly cooled to room temperature at a cooling rate of 15-30°C / S to transform into a lath bainite and lath martensite dual-phase structure, and then undergoes a short-time tempering heat treatment at 600-630°C for insulation of 1.0-1.5h.

[0047] A post-forming heat treatment process for a 900MPa grade pressure-bearing steel pipe comprises a secondary hardening heat treatment process, a heating temperature of 630-660°C, and a heat preservation time of 2.0-4.0h.

[0048] Further; after the secondary hardening heat treatment, the steel contains (Fe, Mn, Cr, Mo) with a size of (10 to 30) nm 23 The volume percentage of the second phase particles of C6, Mo2C and VC accounts for 10% to 15% of the microstructure, and the rest is tempered bainite and tempered martensite.

[0049] Further, after heat treatment after forming, the mechanical properties of the pressure steel pipe are 770MPa≤R at room temperature. p0.2 ≤950MPa、900MPa≤R m ≤1030MPa; at 400℃, 450MPa≤R p0.2 ≤560MPa; -40℃ impact energy KV2≥120J.

[0050] The composition of the steel of the embodiment of the present invention is shown in Table 1. The main process parameters of the steel of the embodiment of the present invention are shown in Table 2. The mechanical properties and microstructure of the steel plate of the embodiment of the present invention are shown in Table 3. The mechanical properties and microstructure of the steel plate of the embodiment of the present invention after forming and heat treatment are shown in Table 4.

[0051] Table 1 Composition of steel according to the present invention (wt%)

[0052] Example C Si Mn P S Cr Mo V B Alt 1 0.16 0.17 0.78 0.006 0.001 0.49 0.24 0.02 0.0006 0.039 2 0.15 0.28 0.84 0.006 0.002 0.45 0.22 0.03 0.0039 0.022 3 0.14 0.20 0.70 0.008 0.003 0.58 0.25 0.05 0.001 0.033 4 0.15 0.18 0.92 0.010 0.001 0.40 0.17 0.03 0.004 0.028 5 0.17 0.35 1.00 0.005 0.002 0.55 0.16 0.05 0.0018 0.045 6 0.14 0.20 0.80 0.010 0.001 0.65 0.15 0.03 0.0034 0.031 7 0.18 0.33 0.90 0.006 0.001 0.52 0.19 0.04 0.0042 0.028 8 0.15 0.24 0.86 0.007 0.002 0.53 0.17 0.04 0.0009 0.020 9 0.14 0.15 0.97 0.006 0.001 0.57 0.18 0.03 0.0007 0.023 10 0.17 0.20 0.85 0.008 0.003 0.43 0.15 0.05 0.0028 0.040

[0053] Table 2 Main process parameters of the steel in the embodiment of the present invention

[0054]

[0055] Table 3 Mechanical properties and microstructure of steel plates according to the embodiments of the present invention

[0056] Example Sampling location <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / ％ <![CDATA[(-40℃)KV2 / J]]> Microstructure 1 T / 4 727 950 25.0 149 Tempered bainite + tempered martensite 2 T / 4 733 968 25.5 143 Tempered bainite + tempered martensite 3 T / 4 796 966 26.0 146 Tempered bainite + tempered martensite 4 T / 4 850 970 22.5 135 Tempered bainite + tempered martensite 5 T / 4 865 975 23.5 141 Tempered bainite + tempered martensite 6 T / 4 890 980 25.0 130 Tempered bainite + tempered martensite 7 T / 4 800 971 23.0 148 Tempered bainite + tempered martensite 8 T / 4 780 950 23.0 166 Tempered bainite + tempered martensite 9 T / 4 793 944 24.0 155 Tempered bainite + tempered martensite 10 T / 4 732 945 22.0 160 Tempered bainite + tempered martensite

[0057] Table 4 Mechanical properties and microstructure of steel plates after forming and heat treatment according to the present invention

[0058]

[0059] According to the above results, it can be concluded that the mechanical properties of the steel plate for pressure-bearing steel pipe of the present invention are 720MPa≤R p0.2 ≤890MPa、900MPa≤R m ≤980MPa, elongation A≥22%, -40℃ impact energy KV2≥100J. 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 770MPa≤R p0.2 ≤950MPa、900MPa≤R m ≤1030MPa; at 400℃, 450MPa≤R p0.2 ≤560MPa, elongation A≥22%; -40℃ impact energy KV2≥120J.

[0060] 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 900MPa grade steel for pressure-bearing steel pipe, characterized in that: The composition of the steel is as follows by weight percentage: C 0.14%~0.18%, Si 0.15%~0.35%, Mn 0.70%~0.97%, P ≤0.010%, S ≤0.003%, Cr 0.52%~0.65%, Mo 0.15%~0.19%, V 0.02%~0.05%, B 0.0028%~0.005%, Alt 0.020%~0.045%, the balance being Fe and unavoidable inclusions; The manufacturing method of the 900MPa grade steel for pressure-bearing steel pipe comprises smelting, continuous casting, heating, rolling and tempering; (1) Heating: The heating temperature is controlled at 1180~1230℃, and the total heating time is 4.0~6.0h; (2) Rolling: A two-stage controlled rolling method is used, with the end rolling temperature of the recrystallization zone being ≥1000°C and the total deformation rate being ≥60%; the start rolling temperature of the non-recrystallization zone being 890~930°C, the final rolling temperature being 860~890°C, and the total deformation rate being ≥60%; (3) Tempering: The tempering temperature is 650~700℃, and the net holding time is 3.0~5.0min / mm; The forming process of the 900MPa grade pressure-bearing steel pipe is as follows: in the forming process, the heating temperature is 900-930°C for insulation, the net insulation time is 1.0-2.0min / mm, and after being taken out of the furnace, it enters a cold water bath and is rapidly cooled to room temperature at a cooling rate of 15-30°C / s, and the phase is transformed into a two-phase structure of lath bainite and lath martensite, and then a short-time tempering heat treatment is performed, the tempering temperature is 600-630°C, and the net insulation time is 1.0-1.5h; The post-forming heat treatment process of the 900MPa grade pressure-bearing steel pipe is a secondary hardening heat treatment, and the specific process is: heating temperature 630~660℃, net holding time 2.0~4.0h; after the secondary hardening heat treatment, the steel contains (Fe, Mn, Cr, Mo) with a size of (10~30)nm 23 The volume percentage of the second phase particles of C6, Mo2C and VC is 10%~15% of the tissue proportion.

2. The 900MPa grade steel for pressure-bearing steel pipe according to claim 1, characterized in that: The steel thickness for pressure-bearing steel pipes is 10~40mm.

3. The 900MPa grade steel for pressure-bearing steel pipe according to claim 1, characterized in that: The mechanical properties of steel for pressure pipes are 720MPa≤ R p0.2 ≤890MPa、900MPa≤ R m ≤980MPa; impact energy at -40℃ KV 2≥100J.

4. The 900MPa grade steel for pressure-bearing steel pipe according to claim 1, characterized in that: Smelting: The smelting of molten steel is carried out in a converter, using high-quality scrap steel and molten iron as raw materials. The molten iron content is controlled at 75%~85%. Dephosphorization and decarburization are smelted separately in a converter, with oxygen blowing for dephosphorization controlled at 7~10min and oxygen blowing for decarburization controlled at 8~12min, ultimately reducing the phosphorus mass fraction to less than 0.006%. Deep desulfurization is carried out in the LF refining furnace, and the sulfur content is controlled below 0.002%. Degassing is completed in a VD furnace, with a net cycle time of 10~15min and a calming time of 3~5min before pouring.

5. The 900MPa grade steel for pressure-bearing steel pipe according to claim 1, characterized in that: Continuous casting: superheat is 20~30℃, and the casting rate during casting is 1.0~1.4m / min. The billets are stacked and slowly cooled after coming off the line. The stacking slow cooling time is 24~36h, and the billets are unstacking below 400℃.

6. The 900MPa grade steel for pressure-bearing steel pipe according to claim 1, characterized in that: After heat treatment after forming, the mechanical properties of the pressure steel pipe are 770MPa≤ R p0.2 ≤950MPa、900MPa≤ R m ≤1030MPa; 450MPa≤ at 400℃ R p0.2 ≤560MPa; -40℃ impact energy KV 2≥120J.

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