Steel plate for 900MPa grade thick-walled pressure-bearing steel pipe, manufacturing, forming and subsequent heat treatment method

Abstract

The present invention provides a steel plate for 900 MPa grade thick-walled pressure-bearing steel pipes, a manufacturing method, a forming method and a post-heat treatment method. The chemical composition of the steel is as follows by weight percentage: C 0.15% - 0.19%, Si 0.15% - 0.35%, Mn 0.95% - 1.30%, P ≤ 0.010%, S ≤ 0.003%, Cr 0.40% - 0.65%, Mo 0.20% - 0.30%, Ni 0.30% - 0.70%, V 0.02% - 0.05%, Ti 0.01% - 0.10%, Alt 0.020% - 0.045%; the balance is Fe and unavoidable inclusions. The manufacturing method includes smelting, continuous casting, heating, rolling, controlled cooling, and tempering; after forming and heat treatment, the mechanical properties of the steel are as follows: at room temperature, 750 MPa ≤ R p0.2 ≤ 930 MPa, 900 MPa ≤ R m ≤ 1010 MPa; at 400 °C, 430 MPa ≤ R p0.2 ≤ 550 MPa; the impact energy KV2 ≥ 110 J at -40 °C; meeting the use requirements of high-strength pressure-bearing equipment.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and particularly relates to a steel plate for a 900 MPa thick-walled pressure-bearing steel pipe, its manufacturing, forming and heat treatment methods. Background Art

[0002] As a link for transporting media such as industrial gases and liquids, pressure-bearing steel pipes transport the media from one device to another under the action of internal pressure, and are widely used in industrial energy fields such as chemical engineering and hydropower. In recent years, with the upgrading of traditional energy industries and the rapid development of new energy, in order to improve the medium reaction or transmission efficiency, design institutes have put forward design requirements of high efficiency, high parameters and high reliability for pressure-bearing steel pipes, and pressure-bearing steel pipes are required to have high pressure-bearing capacity and impact resistance. The strength of the above-mentioned invention and existing pressure-bearing steel pipes and the strength after forming can no longer meet the development needs of large-scale and high-parameter pressure-bearing steel pipes in chemical engineering, hydropower, etc. Therefore, there is an urgent need to develop a high-strength key material for 900 MPa (40-80) mm thin-specification pressure-bearing equipment with room-temperature ultra-high strength, high-temperature ultra-high strength and good plasticity and toughness after forming to support the development needs of new and high-parameter pressure-bearing equipment steel pipes in the energy field of our country.

[0003] The patent document "A Steel for Nuclear Power Pressure-bearing Equipment and Its Manufacturing Method" (publication number CN 103160732 A) discloses that the composition of the steel is 0.12% - 0.18% C; 0.15% - 0.35% Si; 1.20% - 1.65% Mn; ≤0.015% P; ≤0.010% S; 0.50% - 0.85% Ni; ≤0.15% Cr; 0.020% - 0.050% Al; ≤0.02% V; ≤0.02% Ti, and the remaining content is Fe; in terms of process, smelting, continuous casting, rolling and heat treatment are adopted; judging from the disclosed composition, production method and beneficial effects, this patent is for a steel for pressure-bearing equipment applied to the nuclear power field, and the tensile strength of its steel plate is in the range of 560 - 625 N / mm 2 and the tensile strength after simulated post-weld heat treatment state drops to 510 - 600 N / mm 2 . When facing a service environment with higher pressure, this product cannot provide higher strength to support the 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.

[0004] The chemical composition and its mass percentage content of the steel plate disclosed in the patent document "A Steel Plate for Hot-Rolled Weldable Pressure-Bearing Equipment and Its Production Method" (Publication No. CN110983175A) are as follows: C: 0.06% - 0.12%, Si: 0.20% - 0.50%, Mn: 1.40% - 1.60%, P ≤ 0.020%, S ≤ 0.015%, Ni: 0.25% - 0.50%, Nb: 0.020% - 0.050%, Alt ≥ 0.020%, and the balance is Fe and unavoidable impurities. The carbon equivalent CEV is 0.35 - 0.37%; the production method includes smelting and billet rolling; the obtained steel plate has good weldability and can meet the room-temperature tensile strength and -50°C low-temperature impact performance in both the delivery state and the simulated post-weld heat treatment state; judging from the disclosed composition, production method and beneficial effects, the tensile strength of the steel plate involved in this patent in the supplied state and the simulated post-weld heat treatment state is 450 - 610 MPa, and its microstructure consists of ferrite and pearlite with relatively low strength. When facing a service environment with higher pressure, this product cannot provide sufficient strength to support the 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 duplex high-corrosion-resistant steel plate disclosed in the patent document "A Hot-Rolled Duplex High-Corrosion-Resistant Steel Plate with a Tensile Strength of 900 MPa Grade and Its Manufacturing Method" (Publication No. CN115094339A) includes the following chemical components by mass percentage: C: 0.015% - 0.040%, Si: 0.40% - 0.70%, Mn: 0.30% - 0.60%, P: 0.015% - 0.030%, S: ≤ 0.002%, Cr: 3.00% - 5.00%, Ni: ≤ 0.15%, Cu: 0.15% - 0.30%, Als: 0.020% - 0.050%, Ti: 0.010% - 0.025%, H ≤ 1.5 ppm, and the rest is Fe and unavoidable impurity elements. This high-corrosion-resistant steel plate has a duplex 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 flow cooling → coiling; judging from the disclosed composition, production method and beneficial effects, this steel plate is used for the manufacture of equipment in an atmospheric corrosion environment and is a hot-rolled coil product with a relatively thin thickness. Although the strength is at the 9 MPa grade, the forming heat treatment and its performance required for pressure-bearing equipment are not reflected, and it cannot meet the manufacturing requirements of high-pressure equipment.

[0006] The strength of the above-mentioned invention and existing steel for pressure-bearing equipment before and after forming can no longer meet the development needs of large-scale and high-parameter pressure-bearing equipment such as coal power, petrochemical, nuclear energy, and hydrogen energy. Therefore, there is an urgent need to develop high-strength key materials for pressure-bearing equipment with a thickness specification of (40-80) mm that have ultra-high strength at room temperature, ultra-high strength at high temperature, and good plasticity and toughness after forming, so as to support the development needs of new and high-parameter pressure-bearing equipment or installations in the energy field of our country. Summary of the Invention

[0007] The purpose of the present invention is to overcome the problem that the strength of the above-mentioned invention is insufficient or unable to provide a technical solution for improving the strength of high-strength pressure-bearing equipment after forming, and to provide a steel plate with a thickness specification of (40-80) mm and a steel for pressure-bearing steel pipes with a tensile strength of 900 MPa grade after forming, as well as manufacturing, forming, and heat treatment processes, to meet the manufacturing of high-strength pressure-bearing equipment.

[0008] The purpose of the present invention is achieved as follows:

[0009] A steel for thick-walled pressure-bearing steel pipes with a grade of 900 MPa. The composition of the steel is as follows by weight percentage: C 0.15% - 0.19%, Si 0.15% - 0.35%, Mn 0.95% - 1.30%, P ≤ 0.010%, S ≤ 0.003%, Cr 0.40% - 0.65%, Mo 0.20% - 0.30%, Ni 0.30% - 0.70%, V 0.02% - 0.05%, Ti 0.01% - 0.10%, Alt 0.020% - 0.045%; the balance is Fe and inevitable inclusions.

[0010] The thickness of the steel for pressure-bearing steel pipes is 40 - 80 mm.

[0011] The mechanical properties of the steel for pressure-bearing steel pipes are 700 MPa ≤ R p0.2 ≤ 880 MPa, 900 MPa ≤ R m ≤ 980 MPa, and the impact energy KV2 at -40 °C ≥ 100 J.

[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. By adding C element, the hardenability of steel is significantly improved. In addition, C in the present invention combines with strong carbide alloy elements in the steel to play a precipitation strengthening role and obtain a secondary hardening effect to ensure the high-strength requirements of the steel. When the carbon content is lower than 0.13%, the hardenability is relatively low, and it is difficult to obtain a uniform tempered bainite or martensite structure during 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. Therefore, the carbon content range in the present invention is limited to 0.15% - 0.19%.

[0014] Si acts as a reducing agent and deoxidizer during the steelmaking process. Si has a certain influence on the martensite transformation during tempering. When the silicon content is higher than 0.5%, it will hinder the activity of C in martensite during tempering, causing ε-carbide to form in martensitic steel instead of M3C-type carbide, reducing the hardness and toughness of the steel and increasing the sensitivity to temper brittleness. Therefore, the Si content range in this invention is limited to 0.15% - 0.35%.

[0015] Mn is an element that strongly stabilizes austenite. It can effectively reduce the decomposition rate of austenite, improve the hardenability of steel, and strongly increase the strength and hardness of steel. However, a high Mn content will enhance the temper brittleness of steel. Therefore, the Mn content range in this invention is limited to 0.95% - 1.30%.

[0016] As harmful elements in steel, S and P must be strictly controlled to ensure the purity, plasticity, and toughness of the steel. Therefore, in this invention, it is limited that S ≤ 0.003% and P ≤ 0.010%.

[0017] Adding a certain amount of Cr to 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 the steel, playing a role in improving the pressure-bearing capacity of pressure-bearing equipment. When the Cr content is lower than 0.3%, the improvement of hardenability is very small, which is not conducive to the formation of lath martensite structure and cannot meet the high-strength performance requirements of the steel. Therefore, the Cr content range in this invention is limited to 0.40% - 0.65%.

[0018] Mo can improve the hardenability of steel, effectively refine austenite grains, and also strengthen the solid-solution strengthening effect of ferrite. Adding a certain amount of Mo can improve the temper resistance of steel and play a role in inhibiting temper 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 Mo content range in this invention is limited to 0.20% - 0.30%.

[0019] Ni is an alloying element that expands the austenite region. It has the effect of refining ferrite grains. While ensuring strength, it does not reduce the plasticity and toughness of the steel, especially the low-temperature toughness. However, due to Ni belonging to precious metal elements, considering the cost performance, the Ni content range in this invention is limited to 0.30% - 0.70%.

[0020] Adding an appropriate amount of V can form fine carbides (VC) with relatively high stability with C, which are dispersed in the intragranular and grain boundary regions, playing an obvious role in hindering 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 ratio of the steel, which is not conducive to forming manufacturing. Therefore, the V content range in this invention is limited to 0.02% - 0.05%.

[0021] Ti has a solution strengthening effect. When dissolved in austenite, it improves the hardenability of the steel and the tempering stability after quenching. During the tempering process, stable second-phase (Ti, Mo)2C particles precipitate, which have a strong secondary hardening effect, improve the strength and hot strength of the steel, and also improve the weldability of the steel. Therefore, the Ti content range in this invention is limited to 0.01% - 0.10%.

[0022] Alt is a commonly used deoxidizer in steel. Adding a small amount of aluminum can refine the grain size, improve the strength and impact toughness of the steel. However, if the amount is too high, it will affect the hot working performance, welding performance and cutting performance of the steel. In this invention, the Alt content range is limited to 0.020% - 0.045%.

[0023] The second technical solution of this invention is to provide a manufacturing method for the steel used in 900MPa grade thick-walled pressure-bearing steel pipes, including smelting, continuous casting, heating, rolling, controlled cooling, and tempering;

[0024] Smelting: The steelmaking is carried out in a converter. High-quality scrap steel and hot metal are used as raw materials, and the hot metal content is controlled at 75 - 85%. At the same time, to effectively reduce the content of harmful element P, dephosphorization and decarburization are carried out separately in the converter. Among them, the oxygen blowing for dephosphorization is controlled for 7 - 10 minutes, and the oxygen blowing for decarburization is controlled for 8 - 12 minutes. Finally, the phosphorus mass fraction is reduced to less than 0.006%. Deep desulfurization treatment is carried out in the LF refining furnace, and the sulfur content is controlled below 0.002%. Degassing is completed in the VD furnace, with a net circulation time of 10 - 15 minutes and a calming time of 3 - 5 minutes before pouring;

[0025] Continuous casting: After breaking the vacuum, slab continuous casting machine is used for casting. The superheat is set at 20 - 30°C, and the casting speed is 1.0 - 1.4 m / min during casting. The cast slab is taken offline for stacking slow cooling, with a stacking slow cooling time of 24 - 36 hours and unstacking below 400°C to prevent defects such as cracks in the cast slab caused by rapid cooling;

[0026] Heating: The heating temperature is controlled at 1180 - 1230°C, and the total heating duration is 4.0 - 6.0 hours;

[0027] Rolling: The two-stage controlled rolling method is adopted. The finishing temperature in the recrystallization zone is ≥1000°C. In order to fully crush the core structure of the steel billet, the total deformation rate is ≥50%. The starting rolling temperature in the non-recrystallization zone is 850 - 900°C, the finishing rolling temperature is 820 - 860°C, and the total deformation rate is ≥%55. After rolling, the steel plate obtains a bainite and martensite duplex structure with high strength and hardness under air cooling conditions. At this time, the austenite grains are further flattened and elongated. With the increase of the grain boundary area, the nucleation rate of ferrite increases during the subsequent phase transformation process, and the grains are fully refined. The target thickness of the rolled piece is (40 - 80) mm;

[0028] Controlled cooling: To further refine the internal structure of thick-specification steel plates, the controlled cooling process is adopted during the rolling of the steel plates. The starting cooling temperature is 780 - 820°C, the cooling rate is 20 - 30°C / s, and the recrystallization temperature is 540 - 580°C.

[0029] Tempering: Since a relatively large amount of solid solution strengthening elements such as C, Mn, Cr, Mo, and Ni are added to the steel, the steel plate after rolling obtains a bainite and martensite duplex structure with relatively high strength and hardness under air-cooling conditions. However, at this time, the grain size of the steel plate is coarse, and internal stress is concentrated, making it prone to delayed cracking during flame cutting. Therefore, tempering heat treatment should be carried out in a timely manner for softening and stress relief. In the present invention, while ensuring that the strength of the pressure-bearing steel plate is not lost, the tempering heat treatment makes the steel plate have appropriate plasticity and toughness, which is beneficial to the forming process of the steel plate. Therefore, the tempering temperature of the steel is 650 - 700°C, and the net holding time is 1.0 - 3.0 min / mm.

[0030] The third technical solution of the present invention is to provide a forming process method for 900 MPa grade thick-walled pressure-bearing steel pipes.

[0031] Hot forming is an essential process for manufacturing pressure-bearing equipment from steel plates. The forming process not only gives it the shape required for the equipment but also is an important means to improve the final strength of the pressure-bearing equipment. The forming heat treatment process is crucial. The present invention has formulated the optimal forming heat treatment process for the steel. The set parameters and reasons are as follows:

[0032] The heating temperature range of the forming process is 930 - 960°C, the net holding time is 1.0 - 2.0 min / mm. After being taken out of the furnace, it enters a cold water bath at 0 - 10°C and is rapidly cooled to room temperature at a rate of 25 - 40°C / s to obtain a fine lath bainite and lath martensite duplex structure. Then, it undergoes a short-time tempering heat treatment with a heating temperature of 570 - 600°C and a holding time of 0.5 - 1.0 h. At this time, the microstructure of the steel is tempered bainite and tempered martensite.

[0033] The heating temperature range of the forming process is 930 - 960 °C, the net holding time is 1.0 - 2.0 min / mm, and after being taken out of the furnace, it is cooled to room temperature in a cold water bath. During this process, chemical elements such as C, Mn, Cr, Mo, Ni and micro-alloying elements such as V, Ti enter the austenite region. After a period of holding, they are fully dissolved in the austenite matrix. Subsequently, it enters a cold water bath at 0 - 10 °C and is rapidly cooled at a cooling rate of 25 - 40 °C / s to obtain a duplex structure of fine lath bainite and lath martensite. At this time, the steel has ultra-high strength and meets the requirements of steel for pressure-bearing steel pipes. At this time, the steel has very high strength under the solid solution strengthening of alloying elements. However, the plasticity and toughness of the steel are poor and the tissue stress is relatively high. It is necessary to eliminate the quenching stress in time, soften the tissue and improve the plasticity and toughness of the steel through short-time tempering heat treatment at 570 - 600 °C with a net holding time of 0.5 - 1.0 h, and complete the forming process of the steel 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.

[0034] The fourth technical solution of the present invention is a heat treatment method for a 900 MPa grade thick-walled pressure-bearing steel pipe after forming. To ensure that the pressure-bearing equipment has very high strength and excellent plasticity and toughness, it is also necessary to obtain it by further adjusting the size and type of the second-phase particles in the steel, that is, secondary hardening heat treatment. The specific process is a heating temperature of 600 - 630 °C and a holding time of 2.0 - 4.0 h. At this time, a large number of stable second-phase particles with a size of (10 - 30) nm, such as (Fe, Mn, Cr, Mo) 23 C6, Mo2C, (Ti, Mo)2C and VC, etc., are precipitated in the steel. The volume percentage content thereof accounts for 10% - 15% of the tissue proportion, and the rest is tempered bainite and tempered martensite. The second-phase particles interact with the high-density dislocations in the tempered bainite and tempered martensite to improve the strength and plasticity of the matrix. After forming and heat treatment, the mechanical properties of the steel are shown as 750 MPa ≤ R p0.2 ≤ 930 MPa at room temperature, 900 MPa ≤ R m ≤ 1010 MPa; 430 MPa ≤ R p0.2 ≤ 550 MPa at 400 °C; the impact energy KV2 ≥ 110 J at -40 °C; it meets the use requirements of high-strength pressure-bearing equipment.

[0035] The beneficial effects of the present invention are as follows:

[0036] (1) Based on strengthening elements such as C, Si, Mn, Cr, Mo, Ni, V, Ti are compounded and added, and at the same time, the contents of harmful elements P and S are strictly controlled. Combined with the manufacturing process, the steel plate of the present invention obtains a duplex structure of tempered bainite and martensite.

[0037] (2) The steel for thick-walled pressure-bearing steel pipes obtained by the unique production process has mechanical properties shown as 700 MPa ≤ R at room temperaturep0.2 ≤880 MPa, 900 MPa ≤ R m ≤980 MPa, impact toughness KV2 ≥ 100 J at -40 °C.

[0038] (3) After forming and secondary hardening heat treatment, the steel plate of the present invention obtains a duplex structure of high-density tempered bainite and tempered martensite, and at the same time, a large amount of 10-30 nm grade (Fe, Mn, Cr, Mo) 23 C6, Mo2C, (Ti, Mo)2C and VC and other stable second-phase particles precipitate dispersively in the steel, and the proportion of the structure is 10% - 15%, ensuring that the steel plate still has a strength of 900 MPa grade and good plasticity and toughness after forming.

[0039] (4) The mechanical properties of the steel after forming and heat treatment obtained by the unique production process are as follows: at room temperature, 750 MPa ≤ R p0.2 ≤930 MPa, 900 MPa ≤ R m ≤1010 MPa; at 400 °C, 430 MPa ≤ R p0.2 ≤550 MPa; impact toughness KV2 ≥ 110 J at -40 °C.

[0040] (5) The present invention obtains a steel for 900 MPa grade thick-walled pressure-bearing steel pipes with a thickness specification of (40 - 80) mm. Detailed implementation mode

[0041] The present invention will be further described below through examples.

[0042] In the examples of the present invention, heating, rolling, controlled cooling, tempering, forming and post-forming heat treatment are carried out according to the component ratios of the technical solutions.

[0043] Manufacturing method of the steel for 900 MPa grade thick-walled pressure-bearing steel pipes:

[0044] Heating: The heating temperature is controlled at 1180 - 1230 °C, and the total heating duration is 4.0 - 6.0 hours;

[0045] Rolling: Two-stage controlled rolling method is adopted. The finishing temperature in the recrystallization zone is ≥1000 °C, and the total deformation rate is ≥50%; the starting rolling temperature in the non-recrystallization zone is 850 - 900 °C, the finishing rolling temperature is 820 - 860 °C, and the total deformation rate is ≥55%;

[0046] Controlled cooling: The starting cooling temperature is 780 - 820 °C, the cooling rate is 20 - 30 °C / s, and the return red temperature is 540 - 580 °C;

[0047] Tempering: The tempering temperature is 650 - 700 °C, and the net holding time is 1.0 - 3.0 min / mm.

[0048] Further; smelting: The molten steel smelting is carried out in a converter, using high-quality scrap steel and hot metal as raw materials. The content of hot metal is controlled at 75-85%. Dephosphorization and decarburization are carried out separately in the converter. Among them, the oxygen blowing for dephosphorization is controlled at 7-10 min, and the oxygen blowing for decarburization is controlled at 8-12 min. Finally, the mass fraction of phosphorus is reduced to less than 0.006%; Deep desulfurization treatment is carried out in the LF refining furnace, and the sulfur content is controlled below 0.002%; Degassing is completed in the VD furnace, with a net circulation time of 10-15 min and a calming time of 3-5 min before casting.

[0049] Further; continuous casting: After breaking the vacuum, slab continuous casting machine is used for casting, with the superheat set at 20-30 °C, and the drawing speed during casting is 1.0-1.4 m / min: The cast slab is taken offline for stacking and slow cooling, with a stacking and slow cooling time of 24-36 h, and unstacking is carried out below 400 °C.

[0050] A forming process for 900 MPa grade thick-walled pressure-bearing steel pipes, with a heating temperature of 930-960 °C, a holding time of 1.0-2.0 min / mm, and after being taken out of the furnace, it enters a cold water bath for cooling to room temperature; Subsequently, it enters a cold water bath at 0-10 °C and rapidly cools at a cooling rate of 25-40 °C / s to transform into a duplex structure of lath bainite and lath martensite; Then, through a short-time tempering heat treatment with a heating temperature of 570-600 °C and a net holding time of 1.0-1.5 h, the forming process of the steel is completed. Further, the microstructure of the steel after the forming process is tempered bainite and tempered martensite.

[0051] The heat treatment method after the forming of the 900 MPa grade thick-walled pressure-bearing steel pipe described above. The heat treatment after forming is secondary hardening heat treatment, specifically with a heating temperature of 600-630 °C and a holding time of 2.0-4.0 h. Further; After the secondary hardening heat treatment, (10-30) nm (Fe, Mn, Cr, Mo) 23 C6, Mo2C, (Ti, Mo)2C and VC second-phase particles precipitate in the steel, and their volume percentage content accounts for 10%-15% of the tissue proportion, and the rest is tempered bainite and tempered martensite.

[0052] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1. The main process parameters for the smelting of the steel in the embodiments of the present invention are shown in Table 2. The main process parameters for the rolling of the steel in the embodiments of the present invention are shown in Table 3. The main process parameters for the forming and subsequent heat treatment of the steel in the embodiments of the present invention are shown in Table 4. The properties and microstructure of the steel in the embodiments of the present invention are shown in Table 5. The properties and microstructure of the steel after forming and heat treatment in the embodiments of the present invention are shown in Table 6.

[0053] Table 1 Chemical composition of the steel in the embodiments of the present invention (wt%)

[0054]

[0055] Table 2 Main process parameters for the smelting of the steel in the embodiments of the present invention

[0056]

[0057] Table 3 Main process parameters of steel rolling in the embodiments of the present invention

[0058]

[0059] Table 4 Main process parameters of steel forming and subsequent heat treatment in the embodiments of the present invention

[0060]

[0061] Table 5 Properties and microstructures of the steel in the embodiments of the present invention

[0062] Example Sampling location <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / ％ <![CDATA[(-40℃)KV2 / J]]> Microstructure 1 T / 4 717 912 23.0 168 Tempered bainite + tempered martensite 2 T / 4 750 921 23.0 160 Tempered bainite + tempered martensite 3 T / 4 780 930 22.5 150 Tempered bainite + tempered martensite 4 T / 4 863 980 23.0 152 Tempered bainite + tempered martensite 5 T / 4 852 968 23.5 140 Tempered bainite + tempered martensite 6 T / 4 786 923 25.0 149 Tempered bainite + tempered martensite 7 T / 4 834 967 21.0 136 Tempered bainite + tempered martensite 8 T / 4 825 945 23.5 155 Tempered bainite + tempered martensite 9 T / 4 770 910 25.5 152 Tempered bainite + tempered martensite 10 T / 4 735 927 25.0 161 Tempered bainite + tempered martensite

[0063] Table 6 Properties and microstructures of the formed and heat-treated steel in the embodiments of the present invention

[0064]

[0065] As can be seen from the above, the mechanical properties of the steel for 900MPa grade thick-walled pressure-bearing steel pipes of the present invention show that at room temperature, 700MPa ≤ R p0.2 ≤ 880MPa, 900MPa ≤ R m ≤ 980MPa, and the impact energy KV2 at -40°C ≥ 100J. The mechanical properties of the pressure-bearing steel after forming and heat treatment are 750MPa ≤ R p0.2 ≤ 930MPa, 900MPa ≤ R m ≤ 1010MPa; 430MPa ≤ R p0.2 (400°C) ≤ 550MPa; (-40°C) KV2 ≥ 110J.

[0066] In order to describe the present invention, the present invention has been appropriately and fully described by way of examples above. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill 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 replacements, improvements, etc. should be included within the protection scope of the present invention. The patent protection scope of the present invention shall be defined by the claims.

Claims

1. A steel for 900MPa grade thick-walled pressure-bearing steel pipe, characterized in that, The composition of this steel is as follows by weight percentage: C 0.15% - 0.19%, Si 0.15% - 0.28%, Mn 0.95% - 1.18%, P ≤0.010%, S ≤0.003%, Cr 0.40% - 0.65%, Mo 0.20% - 0.30%, Ni 0.30% - 0.70%, V 0.02% - 0.05%, Ti 0.03% - 0.10%, Alt 0.020% - 0.045%; the balance is Fe and inevitable inclusions.

2. The steel for 900 MPa grade thick-walled pressure-bearing steel pipes according to claim 1, characterized in that, The thickness of the steel for pressure-bearing steel pipes is 40 - 80 mm.

3. The steel for 900MPa grade thick-walled pressure-bearing steel pipes according to claim 1, characterized in that, The mechanical properties of the steel for pressure-bearing steel pipes are 700 MPa ≤ R p0.2 ≤ 880 MPa, 900 MPa ≤ R m ≤ 980 MPa, the impact energy KV 2 ≥ 100 J at -40 °C.

4. A manufacturing method for the steel for 900 MPa grade thick-walled pressure-bearing steel pipes according to any one of claims 1 - 3, including smelting, continuous casting, heating, rolling, controlled cooling, and tempering; characterized in that: Heating: The heating temperature is controlled at 1180 - 1230 °C, and the total heating duration is 4.0 - 6.0 h; Rolling: The two-stage controlled rolling method is adopted. The finishing temperature in the recrystallization zone is ≥1000 °C, and the total deformation rate is ≥50%; the starting rolling temperature in the non-recrystallization zone is 850 - 900 °C, the finishing rolling temperature is 820 - 860 °C, and the total deformation rate is ≥55%; Controlled cooling: The starting cooling temperature is 780 - 820 °C, the cooling rate is 21 - 30 °C / s, and the recalescence temperature is 540 - 580 °C; Tempering: The tempering temperature is 650 - 700 °C, and the net holding time is 1.5 - 3.0 min / mm.

5. The manufacturing method for the steel for 900 MPa grade thick-walled pressure-bearing steel pipes according to claim 4, characterized in that: Smelting: The molten steel smelting is carried out in a converter, using high-quality scrap steel and hot metal as raw materials. The content of hot metal is controlled at 75% - 85%. Phosphorus removal and carbon removal are smelted separately in the converter. Among them, the oxygen blowing for phosphorus removal is controlled at 7 - 10 min, and the oxygen blowing for carbon removal is controlled at 8 - 12 min. Finally, the phosphorus mass fraction is reduced to less than 0.006%; deep desulfurization treatment is carried out in the LF refining furnace, and the sulfur content is controlled below 0.002%; degassing is completed in the VD furnace, the net circulation time is 10 - 15 min, and the pre-pouring calming time is 3 - 5 min.

6. The manufacturing method for the steel for 900 MPa grade thick-walled pressure-bearing steel pipes according to claim 4, characterized in that: Continuous casting: After breaking the vacuum, slab continuous casting machine is used for casting. The superheat is set at 20 - 30 °C, and the casting speed during pouring is 1.0 - 1.4 m / min: The cast slab is taken off the production line and stacked for slow cooling. The stacking slow cooling time is 24 - 36 h, and the unstacking is carried out below 400 °C.

7. A forming process for a 900 MPa grade thick-walled pressure-bearing steel pipe, characterized in that: The said pressure-bearing steel pipe is made of the steel for pressure-bearing steel pipes according to any one of claims 1 - 3 or the steel for pressure-bearing steel pipes obtained by the manufacturing method according to any one of claims 4 - 6; In the forming process, the heating temperature is 930 - 960 °C, the net holding time is 1.0 - 2.0 min / mm, and after being taken out of the furnace and entering the cold water bath, it is rapidly cooled to room temperature at a cooling rate of 25 - 40 °C / s, and the phase transformation is a duplex structure of lath bainite and lath martensite; then short-time tempering heat treatment is carried out, the tempering temperature is 570 - 600 °C, and the net holding time is 0.5 - 1.0 h.

8. The forming process of the 900 MPa grade thick-walled pressure-bearing steel pipe according to claim 7, characterized in that: The microstructure of the steel after the forming process is tempered bainite and tempered martensite.

9. A heat treatment method for a 900MPa grade thick-walled pressure-bearing steel pipe after forming, characterized in that: The pressure-bearing steel pipe is made of the steel for pressure-bearing steel pipes described in any one of claims 1-3 or the steel for pressure-bearing steel pipes obtained by using the manufacturing method described in any one of claims 4-6; The heat treatment after forming is secondary hardening heat treatment, and the specific process is a heating temperature of 600-630 °C and a net holding time of 2.0-4.0 h.

10. The heat treatment method for the formed 900 MPa grade thick-walled pressure-bearing steel pipe according to claim 9, characterized in that: After secondary hardening heat treatment, precipitates of 10-30 nm sized (Fe, Mn, Cr, Mo) 23 C6, Mo2C, (Ti, Mo)2C and VC second-phase particles with a volume percentage of 10%-15% are formed, and the rest are tempered bainite and tempered martensite.

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