700mpa grade thick walled pressure pipe steel, manufacture, forming and post heat treatment method

Abstract

The application provides a 700MPa thick-wall pressure-bearing pipeline steel, a manufacturing method, a forming method and a post-heat treatment method, the steel has the following chemical elements: C: 0.14% to 0.18%, Si: 0.15% to 0.40%, Mn: 1.10% to 1.50%, P: less than or equal to 0.010%, S: less than or equal to 0.003%, Cr: 0.50% to 0.80%, Mo: 0.25% to 0.38%, Nb: 0.02% to 0.06%, Alt: 0.020% to 0.045%, and the balance is Fe and inevitable inclusions; the manufacturing method comprises smelting, continuous casting, heating, controlled rolling, controlled cooling and heat treatment; after the forming and heat treatment, the mechanical properties of the steel are 9.500MPa <= Rel <= 580MPa, 700MPa <= Rm <= 820MPa, 370MPa <= Rp0.2 <= 460MPa at 400 DEG C, and the impact energy KV2 is greater than or equal to 180J at -40 DEG C.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials, and particularly relates to a 700MPa-grade thick-wall pressure-bearing pipe steel, a manufacturing method, a forming method and a post-heat treatment method. BACKGROUND

[0002] As a special equipment for material transportation, the petrochemical pressure-bearing pipe plays a role of transmission link, makes the whole petrochemical device become an organic whole, and also provides an important guarantee for the safety of various fluid transportation. In recent years, the scale of China's petrochemical industry is continuously developed and expanded, and the pursuit of safety and high efficiency will be an important task for the future development of the petrochemical industry. With a substantial increase in the demand for petrochemical products, it is necessary to continuously improve the scale and operating parameters of the equipment to meet the capacity demand, which puts forward higher and higher requirements for the pressure-bearing property and high strength of the petrochemical pressure-bearing pipe.

[0003] In the prior application patent "Corrosion-resistant and pressure-resistant composite pipe (glass steel and steel pipe) composite process and device" (publication number CN 1097852), according to the disclosed process method, the patent combines resin and glass fiber with the steel pipe through a pressing method to form a composite steel pipe. The invention relates to a finished steel pipe, does not involve a steel plate and its production process, and does not mention the strength grade.

[0004] The disclosed application patent "Temperature-resistant, pressure-resistant and corrosion-resistant marine oil and gas conveying pipe" (publication number CN 205447005U), according to the disclosed invention effect, the corrosion-resistant and pressure-resistant performance of the oil and gas conveying pipe is mainly completed by adding high-tensile fiber, spiral steel wire and synthetic rubber inside and outside the steel pipe. The invention does not involve a steel plate and its production process, and does not mention the strength grade.

[0005] The disclosed application patent "800MPa-grade high-strength steel plate for water power station pressure pipe and production method" (CN 103290339A), according to the disclosed composition, production method and beneficial effect, the patent product is mainly applied to a water conservancy environment. Although it is also a steel grade of 800MPa, the strength of the steel is improved by adding more alloying elements, which is essentially different from the composition design of the present application, and there is also a certain difference in the heat treatment process.

[0006] The above-mentioned inventions and existing pressure-bearing pipes cannot meet the development needs of large-scale petrochemical energy equipment, and therefore it is urgent to develop a 700MPa-grade high-strength thick-wall pipe material for petrochemical use with excellent pressure resistance to support the development needs of large-scale petrochemical energy equipment in China. SUMMARY

[0007] The present application aims to overcome the above-mentioned problems of the prior art and provide a steel plate with a thickness of 30-60 mm and a steel product for a pressure pipeline with a tensile strength of 700 MPa after forming, a manufacturing method, a forming method and a heat treatment process after forming, so as to meet the manufacturing requirements of a high-strength pressure pipeline.

[0008] The present application is achieved as follows:

[0009] A 700 MPa grade thick-walled pressure pipeline steel, which contains the following chemical elements by mass percentage: C: 0.14%-0.18%, Si: 0.15%-0.40%, Mn: 1.10%-1.50%, P≤0.010%, S≤0.003%, Cr: 0.50%-0.80%, Mo: 0.25%-0.38%, Nb: 0.02%-0.06%, Alt: 0.020%-0.045%, and the balance of Fe and unavoidable inclusions.

[0010] The thick-walled pressure pipeline steel has a thickness of 30-60 mm.

[0011] The thick-walled pressure pipeline steel has a mechanical property of 480 MPa≤R eL ≤560 MPa, 700 MPa≤R m ≤800 MPa, A≥25%, and an impact energy KV2 of≥150 J at-40 ℃.

[0012] The present application is achieved as follows:

[0013] C is the most important element for improving the strength of steel, and the addition of C element significantly improves the hardenability of steel. In addition, the combination of C and strong carbide alloying elements in steel plays a precipitation strengthening role, obtaining a secondary hardening effect, and ensuring the high strength requirement of the steel. When the carbon content is lower than 0.13%, the hardenability is lower, and it is difficult to obtain uniform tempered bainite structure after subsequent tempering treatment, and the strength of the steel cannot meet the use requirements. Therefore, the C content range is limited to 0.14%-0.18% in the present application.

[0014] Si is used as a reducing agent and deoxidizer in the steelmaking process. Si has a certain influence on the bainite transformation during tempering. When the silicon content is higher than 0.5%, it will hinder the activity of C in the bainite during tempering, so that ε carbide is generated in the bainite steel instead of M3C type carbide, which reduces the hardness and toughness of the steel and increases the sensitivity of temper brittleness. Therefore, the Si content range is limited to 0.15%-0.40% in the present application.

[0015] Mn is an element that strongly stabilizes austenite, can effectively reduce the decomposition rate of austenite, improve the hardenability of the steel, and can strongly increase the strength and hardness of the steel, but high Mn content can enhance the temper brittleness of the steel, therefore the Mn content range is limited to 1.10% to 1.50%.

[0016] S and P are harmful elements in steel, and must be strictly controlled to ensure the purity and plasticity and toughness of the steel, therefore the S content is limited to ≤0.003%, and the P content is limited to ≤0.010%.

[0017] Adding a certain amount of Cr in the steel can significantly improve the hardenability of the steel and ensure the matrix strength of the steel. In addition, Cr is a strong carbide forming element, which generates stable carbides with C in the steel, and plays a precipitation strengthening effect, further improving the pressure resistance of the pipeline, therefore the Cr content range is limited to 0.50% to 0.80%.

[0018] Mo can improve the hardenability of the steel, effectively refine the austenite grains, and also can strengthen the solid solution strengthening effect of ferrite. A certain amount of Mo can improve the temper resistance of the steel and play a role in inhibiting temper brittleness. 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 is limited to 0.25% to 0.38%.

[0019] Nb can generate highly dispersed strong carbide NbC, has the effect of refining grains, and can still prevent grain growth when heated to 1100-1200℃, thereby improving the strength and toughness of the steel, but too high content will generate ferrite δ phase or other brittle phases, thereby reducing the toughness, therefore the Nb content range is limited to 0.02% to 0.06%.

[0020] Alt is a common deoxidizer in steel, and a small amount of aluminum can refine the grains and improve the strength and impact toughness of the steel. Too high will affect the hot working performance, welding performance and cutting performance of the steel, therefore the Alt content range is limited to 0.020% to 0.045%.

[0021] The second technical scheme of the present application provides a manufacturing method of a 700MPa grade thick-walled pressure-bearing pipeline steel, which comprises heating, controlled rolling, controlled cooling and tempering.

[0022] Heating: the heating temperature is controlled at 1180-1230℃, and the total heating time is 4.0-6.0h.

[0023] Controlled rolling: two-stage controlled rolling method is adopted, recrystallization zone rolling end temperature ≥ 1000℃, in order to fully fragment the core organization of the billet, total deformation rate ≥ 50%; non-recrystallization zone starting rolling temperature 890-930℃, finish rolling temperature 830-870℃, total deformation rate ≥ 55%; at this time, the austenite grains are further flattened and elongated, and 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, and the target thickness of the rolled piece is (30-60) mm.

[0024] Controlled cooling: in order to further refine the internal organization of the thick gauge steel plate, the steel plate rolling opening adopts controlled cooling process, opening cooling temperature 800-840℃, cooling rate 20-30℃ / s, and red temperature is 550-600℃.

[0025] Tempering: because the steel is added with more C, Mn, Cr, Mo and other solid solution strengthening elements, the steel plate after rolling obtains bainite organization with high strength and hardness under the condition of air cooling. However, at this time, the grain size of the steel plate is coarse, the internal stress is concentrated, and the delayed cracks are easy to occur during flame cutting. Therefore, the tempering heat treatment should be adopted in time for softening and stress relief. The tempering heat treatment adopted in the present application ensures that the strength of the pressure-bearing steel plate is not lost, and at the same time, the steel plate has appropriate plasticity and toughness, which is beneficial to the forming processing. Therefore, the tempering heat treatment temperature of the steel is 600-650℃, and the net holding time is 1.0-2.0min / mm.

[0026] The third technical scheme of the present application provides a forming process method of the 700MPa thick wall pressure-bearing pipe steel:

[0027] Hot forming is a necessary process for making high-pressure pressure-bearing pipe. The forming process not only makes it obtain the shape required by the pipe, but also is an important means to improve the final strength of the pipe. In order to obtain the 700MPa level of the strength after forming, the setting parameters and reasons of the forming heat treatment of the steel plate of the present application are as follows:

[0028] In the forming process, the heating temperature is 930-960℃, and the net holding time is 1.0-2.0min / mm; after discharging, water mist spraying method is adopted to rapidly cool to room temperature at a cooling rate of 5-15℃ / s; then short-term tempering is carried out, the tempering temperature is 670-700℃, and the holding time is 0.5-1.0h.

[0029] Because of adding more C, Mn, Cr, Mo and other solid solution strengthening elements in the steel, the steel plate can obtain lath bainite structure with high strength and hardness after rolling. But the grain size of the steel plate is coarse, and there exists stress and thermal stress concentration, so that the steel plate is easy to produce delayed cracks during flame cutting. Therefore, the steel plate should be softened and stress relieved by heat treatment in time. The heat treatment process of the normalizing and water mist cooling according to the present application is that the steel plate is heated to 930-960 DEG C and kept for 1.0-2.0 min / mm, and then the steel plate is cooled to room temperature by water mist spraying. In this process, C, Mn, Cr, Mo and other micro-alloy elements enter the austenite zone, and after a period of keeping, the alloy elements are solid-solved in the austenite matrix, and then the steel plate is cooled by water mist spraying at a cooling speed of 5-15 DEG C / s to obtain fine lath bainite structure. At this time, the steel has high strength. But the plasticity and toughness of the steel are poor, and the stress is high. The quenching stress is eliminated in time by short-term tempering heat treatment at 670-700 DEG C and keeping for 0.5-1.0 h, the structure is softened, the plasticity and toughness of the steel are improved, and the forming process of the steel is completed. At this time, the microstructure of the steel is tempered bainite.

[0030] The fourth technical scheme of the present application provides a heat treatment process after forming of the steel for 700MPa thick-wall pressure-bearing pipe. In order to ensure the pressure resistance of the steel for pressure-bearing pipe, the size and type of the second phase particles in the steel should be further adjusted to obtain high strength, i.e. the secondary hardening heat treatment process is at 700-730 DEG C and keeping for 2.0-4.0 h. At this time, stable (20-50) nm (Fe, Mn, Cr, Mo) 23 C6, Mo2C, NbC and other second phase particles with a volume percentage of 5%-10% interact with the high-density dislocations in the bainite to improve the strength and plasticity of the matrix. After the heat treatment, the mechanical properties of the steel are 500MPa≤R el ≤580MPa, 700MPa≤R m ≤820MPa; 370MPa≤R p0.2 ≤460MPa at 400 DEG C; the impact energy KV2 is greater than or equal to 180J at -40 DEG C; and the high pressure-bearing requirement of the pressure-bearing pipe is met.

[0031] The present application has the following advantages:

[0032] (1) By adding Cr, Mo and Nb alloy elements on the basis of C, Si and Mn strengthening elements, and strictly controlling the contents of harmful elements P and S, and combining with the manufacturing process, the steel plate of the present application obtains tempered bainite structure.

[0033] (2) The steel plate for pressure-bearing steel pipe obtained by the special production process has the mechanical properties of 480MPa≤R eL ≤560MPa, 700MPa≤Rm ≤ 800 MPa, A ≥ 25%, (-40℃) KV2 ≥ 150 J.

[0034] (3) The steel plate of the present application obtains high dislocation density tempered bainite structure after forming and secondary hardening heat treatment, at this time a large number of (20-50) nm (Fe, Mn, Cr, Mo) 23 C6, Mo2C and NbC and other second phase particles, the structure accounts for 5-10%, which ensures that the steel plate still has 700 MPa grade strength and good plasticity and toughness after forming.

[0035] (4) The mechanical properties of the steel for pressure equipment of the present application after forming heat treatment are 500 MPa ≤ R el ≤ 580 MPa, 700 MPa ≤ R m ≤ 820 MPa, 370 MPa ≤ R p0.2 (400℃) ≤ 460 MPa, (-40℃) KV2 ≥ 180 J.

[0036] (5) The present application obtains a 700 MPa grade high-strength steel plate for thick-walled pressure pipeline with a thickness of (30-60) mm. DETAILED DESCRIPTION

[0037] The present application will be further described below by examples.

[0038] The examples of the present application are smelted, continuously cast, controlled rolled, controlled cooled, tempered, formed and heat treated according to the component ratio of the technical scheme.

[0039] A manufacturing method of a 700 MPa grade thick-walled pressure pipeline steel includes smelting, continuous casting, heating, controlled rolling, controlled cooling and tempering.

[0040] Heating: the heating temperature is controlled at 1180-1230℃, and the total heating time is 4-6h;

[0041] Controlled rolling: two-stage controlled rolling method is adopted, the recrystallization zone rolling end temperature is ≥ 1000℃, and the deformation rate is ≥ 50%; the non-recrystallization zone starting rolling temperature is 890-930℃, the final rolling temperature is 830-870℃, and the total deformation rate is ≥ 55%;

[0042] Controlled cooling: the open cooling temperature is 800-840℃, the cooling rate is 20-30℃ / s, and the re-red temperature is 550-600℃;

[0043] Tempering: the tempering heat treatment temperature is 600-650℃, and the net holding time is 1.0-2.0 min / mm.

[0044] A forming method of a 700MPa thick-wall pressure-bearing pipe, in the forming process, the heating temperature is 930-960 DEG C, the net holding time is 1.0-2.0 min / mm; after discharging, the water mist spraying mode is used to rapidly cool to room temperature at a cooling rate of 5-15 DEG C / s; then short-time tempering is carried out, the short-time tempering temperature is 670-700 DEG C, and the net holding time is 0.5-1.0 h. The mechanical properties of the thick-wall pressure-bearing pipe steel are 480MPa <= ReL <= 560MPa, 700MPa <= Rm <= 800MPa, A >= 25%, and the impact energy KV2 at -40 DEG C is >= 150J.

[0045] Further, after the rapid cooling in the cold water bath, the microstructure phase change of the steel is lath bainite; after the short-time tempering, the microstructure of the steel is tempered bainite.

[0046] A post-forming heat treatment method of a 700MPa thick-wall pressure-bearing pipe, the post-forming heat treatment is secondary hardening heat treatment, and the specific process is as follows: the heating temperature is 700-730 DEG C, and the net holding time is 2-4 h.

[0047] Further, after the heat treatment, 20-50nm (Fe, Mn, Cr, Mo) 23 C6, Mo2C, NbC second phase particles with a volume percentage of 5-10%.

[0048] Further, after the heat treatment, the mechanical properties of the steel are 500MPa <= R el <= 580MPa, 700MPa <= R m <= 820MPa; 370MPa <= R p0.2 <= 460MPa at 400 DEG C; and the impact energy KV2 at -40 DEG C is >= 180J.

[0049] The composition of the steel in the embodiment of the application is shown in Table 1. The main process parameters of the steel in the embodiment of the application are shown in Table 2. The mechanical properties and microstructure of the steel in the embodiment of the application are shown in Table 3. The mechanical properties and microstructure of the steel in the embodiment of the application after forming and heat treatment are shown in Table 4.

[0050] Table 1 Composition of the steel in the embodiment of the application (wt%)

[0051] Example C Si Mn P S Cr Mo Nb Alt 1 0.15 0.22 1.48 0.006 0.001 0.80 0.34 0.02 0.039 2 0.16 0.18 1.24 0.008 0.002 0.50 0.36 0.03 0.022 3 0.16 0.20 1.30 0.008 0.001 0.75 0.38 0.05 0.033 4 0.14 0.30 1.15 0.007 0.003 0.64 0.26 0.05 0.028 5 0.17 0.26 1.35 0.010 0.002 0.75 0.30 0.02 0.045 6 0.18 0.15 1.10 0.008 0.001 0.65 0.27 0.03 0.031 7 0.15 0.37 1.40 0.009 0.001 0.69 0.33 0.06 0.028 8 0.16 0.40 1.16 0.007 0.002 0.52 0.25 0.04 0.020 9 0.18 0.15 1.50 0.006 0.003 0.55 0.35 0.03 0.023 10 0.17 0.28 1.45 0.008 0.001 0.58 0.30 0.04 0.040

[0052] Table 2 Main process parameters of the steel in the embodiment of the application

[0053]

[0054] Table 3 Mechanical properties of the steel in the embodiment of the application

[0055] Example Sampling location [R eL / MPa]]> [R m / MPa]]> A / ％ [(-40°C) KV2 / J] Microstructure composition 1 T / 4 490 722 25.0 178 tempered bainite 2 T / 4 491 743 26.0 160 tempered bainite 3 T / 4 536 790 25.5 150 tempered bainite 4 T / 4 503 772 25.0 172 tempered bainite 5 T / 4 511 767 25.5 170 tempered bainite 6 T / 4 560 800 25.0 209 tempered bainite 7 T / 4 526 765 25.0 176 tempered bainite 8 T / 4 497 726 26.5 195 tempered bainite 9 T / 4 480 700 27.5 202 tempered bainite 10 T / 4 485 715 26.0 181 tempered bainite

[0056] Table 4 Mechanical properties and microstructure of the steel plate of the embodiment of the present application after forming and heat treatment

[0057]

[0058] According to the above results, it can be concluded that the mechanical properties of the steel plate for pressure pipelines of the present application are 480 MPa≤R eL ≤560 MPa, 700 MPa≤R m ≤800 MPa, A≥25%, (-40℃)KV2≥150 J. The mechanical properties of the steel pipe after forming and heat treatment are 500 MPa≤R el ≤580 MPa, 700 MPa≤R m ≤820 MPa, 370 MPa≤R p0.2 (400℃)≤460 MPa, (-40℃)KV2≥180 J.

[0059] In order to describe the present application, the above embodiment is appropriately and sufficiently described in the above description, the above embodiment is only used to illustrate the present application, and is not limited to the present application. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art without departing from the spirit and scope of the present application shall be included in the protection scope of the present application, and the patent protection scope of the present application shall be defined by the claims.

Claims

1. A method for forming a 700MPa thick-walled pressure-bearing pipe, characterized in that, The chemical element mass percentages of steel used for 700MPa grade thick-walled pressure-bearing pipelines are as follows: C: 0.14%~0.18%, Si: 0.15%~0.40%, Mn: 1.10%~1.50%, P: ≤0.010%, S: ≤0.003%, Cr: 0.50%~0.80%, Mo: 0.25%~0.38%, Nb: 0.02%~0.06%, Alt: 0.020%~0.045%, with the balance being Fe and unavoidable inclusions; The manufacturing method of steel for 700MPa thick-walled pressure pipelines includes smelting, continuous casting, heating, controlled rolling, controlled cooling, and tempering. Heating: The heating temperature is controlled at 1180~1230℃, and the total heating time is 4~6 hours; Controlled rolling: A two-stage controlled rolling method is adopted. The rolling end temperature in the recrystallization zone is ≥1000℃, and the deformation rate is ≥50%. The rolling start temperature in the non-recrystallization zone is 890~930℃, the final rolling temperature is 830~870℃, and the total deformation rate is ≥55%. Cooling control: Cooling temperature 800~840℃, cooling rate 20~30℃ / s, red temperature 550~600℃; Tempering: The tempering heat treatment temperature is 600~650℃, and the net holding time is 1.0~2.0min / mm; In the molding process, the heating temperature is 930~960℃, and the net holding time is 1.0~2.0min / mm; after exiting the furnace, the furnace is rapidly cooled to room temperature by water mist spraying at a cooling rate of 5~15℃ / s; then a short tempering is performed at a temperature of 670~700℃ and a net holding time of 0.5~1.0h. The heat treatment after molding is a secondary hardening heat treatment, specifically: heating temperature 700~730℃, net holding time 2~4h.

2. The forming method of the 700MPa thick-walled pressure-bearing pipeline according to claim 1, characterized in that, The thickness of steel used for thick-walled pressure pipelines is 30~60mm.

3. The forming method of the 700MPa thick-walled pressure-bearing pipeline according to claim 1, characterized in that, The mechanical properties of steel for thick-walled pressure pipelines are ≤480MPa. R eL ≤560MPa, 700MPa≤ R m ≤800MPa A ≥25% impact energy at -40℃ KV 2≥150J.

4. The forming method of the 700MPa thick-walled pressure-bearing pipeline according to claim 1, characterized in that: After rapid cooling in a cold water bath, the microstructure of the steel is lath bainite; after short-time tempering, the microstructure of the steel is tempered bainite.

5. The forming method of the 700MPa thick-walled pressure-bearing pipeline according to claim 1, characterized in that: After heat treatment, 20-50 nm-sized particles (Fe, Mn, Cr, Mo) precipitate in the steel. 23 The second phase particles consist of C6, Mo2C, and NbC, with a volume percentage of 5% to 10%.

6. The forming method of the 700MPa thick-walled pressure-bearing pipeline according to claim 1, characterized in that: After heat treatment, the mechanical properties of the steel are ≤ 500MPa. R el ≤580MPa, 700MPa≤ R m ≤820MPa; 370MPa≤ at 400℃ R p0.2 ≤460MPa; Impact energy KV2≥180J at -40℃.

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