A low-temperature high-toughness pipe-steel plate and a method for manufacturing the same

Abstract

The application provides a kind of low-temperature high-toughness pipe fitting steel plate, comprising the following mass percentage of chemical elements: C: 0.05~0.07%, Si: 0.15~0.19%, Mn: 1.5~1.7%, Cu: 0.1~0.35%, Ni: 0.2~0.5%, Cr: 0.1~0.35%, Mo: 0.1~0.3%, V: 0.001~0.02%, Nb: 0.02~0.05%, Ti: 0.008~0.015%, Ca: 0.002~0.004%, Al: 0.02~0.04%. The application also provides a preparation method of the steel plate, which comprises heating, rough rolling, finish rolling and cooling of the slab. The steel plate has excellent impact toughness and can be used to manufacture pipe fittings for long-distance oil and gas pipeline projects in high-cold areas, so as to improve the safety of pipe fittings in long-distance oil and gas pipeline projects.

Description

Technical Field

[0001] This invention belongs to the field of steel plate production technology, and particularly relates to a low-temperature high-strength and high-toughness steel plate for pipe fittings and its preparation method. Background Technology

[0002] In long-distance oil and gas pipeline projects, stations, valve chambers, and compressor stations require a large number of pipe fittings to meet the needs of oil and gas transmission pipelines for functions such as distribution, direction changing, diameter changing, pressurization, and metering. Pipe fittings are indispensable and important components of oil and gas pipeline transmission systems. Due to their special structure, the loads they bear are more severe than those of the pipeline itself, making them one of the weakest links in the entire pipeline. Developing high-quality steel for pipe fittings and improving their service safety are of great significance to ensuring the safe and efficient operation of oil and gas pipelines.

[0003] In recent years, long-distance oil and gas pipeline projects have extensively adopted high-grade X70 and X80 steel for construction. Consequently, the strength of the steel used in the supporting pipe fittings has also increased accordingly, with the maximum thickness reaching 50-70mm. Furthermore, as pipeline projects operate in increasingly cold and high-altitude environments, higher requirements have been placed on the low-temperature toughness of the steel used in pipe fittings. For example, the China-Russia East Route pipeline requires pipe fitting steel with an impact energy ≥135J at -45℃. Some international polar pipeline projects are designed for temperatures as low as -60℃, requiring an impact energy ≥70J.

[0004] While thicker steel, higher strength, lower temperature toughness, and ease of welding are current development trends for pipe fitting steel, there are inherent contradictions among these requirements. To ensure lower temperature toughness and ease of welding, a low-carbon content and low-carbon equivalent composition design is necessary. However, this reduces the material's hardenability, especially when the steel plate is thick, making it difficult to guarantee that the strength of the steel after quenching and tempering meets the requirements of the corresponding steel grade. Furthermore, when the steel plate is thick, the compression ratio during rolling is small, making it difficult for deformation to penetrate to the core. Combined with the poor hardenability of the steel plate, quenching and tempering cannot effectively refine the grains in the steel plate's core, resulting in a coarse core structure and deteriorated impact toughness.

[0005] How to simultaneously meet the requirements of thick steel, high strength, high toughness at low temperatures, and easy welding for pipe fittings through reasonable composition and process design is a technical problem that needs to be solved by those skilled in the art.

[0006] In the prior art, Chinese patent document CN101880818A, published on June 4, 2010, discloses a method for preparing X80 bends and pipe fittings steel; Chinese patent document CN102912250A, published on November 10, 2010, discloses an economical low yield strength ratio pipe fitting steel for oil and gas transportation and its production method. Both patents address the performance requirements of pipe fitting steel from the perspectives of composition design, smelting, continuous casting, thermomechanical control process (TMCP), and optimization of quenching and tempering heat treatment processes. However, to ensure the strength of the pipe fitting steel after quenching and tempering, both adopt a high carbon content design, thus making it difficult to guarantee good weldability of the pipe fitting steel. Summary of the Invention

[0007] To address the issues of insufficient strength to meet the required steel grade after tempering heat treatment, and the resulting coarse microstructure and deteriorated impact toughness in the core of existing steel plates used for pipe fittings, a low-temperature, high-strength, high-toughness steel plate is provided. This plate exhibits excellent impact toughness and can be used to manufacture pipe fittings for long-distance oil and gas pipelines operating in cold regions, thereby improving the safety of pipe fittings in such projects.

[0008] This invention provides a steel plate for low-temperature high-strength and high-toughness pipe fittings, comprising the following chemical elements by mass percentage:

[0009] C: 0.05~0.07%, Si: 0.15~0.19%, Mn: 1.5~1.7%, Cu: 0.1~0.35%, Ni: 0.2~0.5%, Cr: 0.1~0.35%, Mo: 0 .1~0.3%, V: 0.001~0.02%, Nb: 0.02~0.05%, Ti: 0.008~0.015%, Ca: 0.002~0.004%, Al: 0.02~0.04%.

[0010] According to another specific embodiment of the present invention, the steel plate for low-temperature high-strength and high-toughness pipe fittings provided by the present invention is composed of the following chemical elements by mass percentage:

[0011] C: 0.05–0.07%, Si: 0.15–0.19%, Mn: 1.5–1.7%, Cu: 0.1–0.35%, Ni: 0.2–0.5%, Cr: 0.1–0.35%, Mo: 0.1–0.3%, V: 0.001–0.02%, Nb: 0.02–0.05%, Ti: 0.008–0.015%, Ca: 0.002–0.004%, Al: 0.02–0.04%; balance Fe and unavoidable impurities.

[0012] Using the above-mentioned scheme, the steel plate provided by the present invention adopts a low-C, low-Mn alloy composition system, which ensures the weldability of the steel for pipe fittings, while improving the central segregation and banded structure, which is beneficial to improving low-temperature toughness; appropriate addition of Cu, Ni, Cr, and Mo improves the hardenability of the steel plate, and can form stable alloy carbides during quenching and tempering, ensuring the strength and toughness matching and performance stability of the pipe fittings; and the addition of trace amounts of V, Nb, and Ti forms micro-alloyed carbonitrides, which refines the grains and plays a role in precipitation strengthening.

[0013] According to another specific embodiment of the present invention, the steel plate for low-temperature high-strength and high-toughness pipe fittings provided by the present invention has P, S and B in unavoidable impurities controlled within the following mass percentages: P≤0.015%, S≤0.0015%, B≤0.0005%.

[0014] By adopting the above scheme, the contents of impurity elements P, S, and B in the steel are strictly controlled at a low level, reducing the number of inclusions in the steel plate, reducing the degree of segregation, and ensuring low-temperature toughness.

[0015] According to another specific embodiment of the present invention, the steel plate for low-temperature high-strength and high-toughness pipe fittings provided by the present invention has a microstructure of ferrite + tempered sorbite + dispersed carbide particles; wherein the volume percentage of ferrite is ≤5% and the ferrite grain size is ≤5μm; the average effective grain size of tempered sorbite is ≤25μm.

[0016] The above scheme enables the steel plate to have a good combination of strength and toughness, especially at -45℃ and -60℃, the center of the wall thickness still has sufficient low-temperature toughness.

[0017] According to another specific embodiment of the present invention, the steel plate for low-temperature high-strength and high-toughness pipe fittings provided by the present invention has an average carbide particle size of ≤1μm.

[0018] This invention also provides a method for preparing steel plates for low-temperature high-strength and high-toughness pipe fittings, the method comprising the following steps:

[0019] S1: Heat the slab at a temperature of 1100~1160℃;

[0020] S2: The slab is rough rolled at an initial rolling temperature of 1000-1100℃ and a final rolling temperature of 950-1000℃.

[0021] S3: The slab is finished rolled at an initial rolling temperature of 790-870℃ and a final rolling temperature of 780-820℃;

[0022] S4: Cooling temperature is 700~760℃, cooling rate is 10~35℃ / s, and cooling is stopped at 300~500℃.

[0023] Using the above scheme, the steel plate preparation method adopts a two-stage controlled rolling process. The rough rolling stage effectively refines the austenite grains and promotes the penetration of rolling deformation into the core. The fine rolling is carried out in the non-recrystallization zone of austenite to improve the uniformity of the microstructure and the core toughness of the steel plate. The cooling rate is controlled within a reasonable range to obtain a refined rolled microstructure.

[0024] According to another specific embodiment of the present invention, the method for preparing low-temperature high-strength and high-toughness pipe fitting steel plate provided by the present invention further includes:

[0025] S5: Perform simulated quenching and tempering heat treatment, with a quenching temperature of 850~930℃, a holding coefficient of 1~2.5min / mm, a quenching medium of water, a tempering temperature of 580~660℃, and a holding coefficient of 1~4min / mm.

[0026] By adopting the above scheme and setting reasonable quenching and tempering heat treatment process parameters, the steel plate obtains a microstructure of tempered sorbite + a small amount of ferrite after quenching and tempering, which gives it a good combination of strength and toughness and a large range of adjustable strength.

[0027] According to another specific embodiment of the present invention, the method for preparing low-temperature high-strength and toughness pipe steel plates provided by the present invention, for steel plates with a thickness greater than or equal to 50 mm, further includes, in step S2, subjecting the steel plate surface to high-pressure water spraying treatment before the start of the last rolling process of rough rolling, so that the surface temperature of the steel plate is reduced to 600°C; and after the surface temperature is warmed back to 800°C, the last rolling process is carried out.

[0028] By adopting the above scheme, differential temperature rolling technology is applied in the last pass of rough rolling to promote the penetration of rolling deformation into the core, thereby refining the core structure and improving the low-temperature toughness of the material.

[0029] According to another specific embodiment of the present invention, in the method for preparing low-temperature high-strength and toughness pipe fitting steel plate provided by the present invention, the compression ratio of the finishing rolling in step S3 is 3 to 6.

[0030] By adopting the above scheme, the finishing rolling compression ratio is guaranteed to be 3 to 6, which can further refine the microstructure and improve the low-temperature toughness of the steel plate.

[0031] According to another specific embodiment of the present invention, the method for preparing steel plates for low-temperature high-strength and tough pipe fittings provided by the present invention has a slab thickness of 250mm, 300mm or 350mm and a steel plate thickness of 20mm to 60mm.

[0032] By adopting the above scheme, a suitable slab thickness is selected based on the thickness of the finished steel plate to ensure sufficient rolling deformation.

[0033] The beneficial effects of this invention are:

[0034] The steel plates for pipe fittings produced according to the technical solution of this invention meet the following performance requirements after quenching and tempering heat treatment: yield strength Rt0.5: 480~680MPa, tensile strength Rm: 550~750MPa, yield ratio Rt0.5 / Rm≤0.93, elongation after fracture A50≥20%; Charpy impact energy at 45℃≥135J, Charpy impact energy at -60℃≥70J.

[0035] Therefore, the steel plate for low-temperature high-strength and high-toughness pipe fittings provided by the present invention has sufficient strength and toughness, especially the excellent impact toughness at the center of the wall thickness at -45℃ and -60℃. It can be used to manufacture pipe fittings for long-distance oil and gas pipeline projects in high-altitude and cold regions. As oil and gas pipeline projects gradually expand to high-altitude and cold regions, it has broad application prospects. Attached Figure Description

[0036] Figure 1 This is a microstructure diagram of the steel plate for low-temperature high-strength and high-toughness pipe fittings according to Embodiment 2 of the present invention;

[0037] Figure 2 This is a microstructure diagram of the steel plate for low-temperature high-strength and high-toughness pipe fittings according to Embodiment 3 of the present invention;

[0038] Figure 3 This is a microstructure diagram of the steel plate for low-temperature high-strength and high-toughness pipe fittings according to Embodiment 8 of the present invention;

[0039] Figure 4 This is a microstructure diagram of the steel plate for low-temperature high-strength and high-toughness pipe fittings according to Embodiment 10 of the present invention. Detailed Implementation

[0040] For the purposes of the detailed description below, it should be understood that, except in any operational instance or otherwise indicated, all figures representing the amounts of ingredients used, for example, in the specification and claims, should be understood to be modified in all cases by the term "about". Therefore, unless otherwise stated, the numerical parameters set forth in the following specification and appended claims are approximations varying according to the desired performance to be obtained in this application. It is not at least not an attempt to limit the application of the doctrine of equivalents to the scope of the claims, and each numerical parameter should be interpreted at least according to the number of significant figures reported and by applying ordinary rounding techniques.

[0041] The terminology used in this application is for the purpose of describing particular embodiments only and is not to be construed as limiting. As used herein, the singular forms “a” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. Expressions such as “at least one of…” modify the entire list of elements when preceding or following it, without modifying any individual elements in the list.

[0042] Furthermore, the terms "comprising" or "including" as used in this application, when used in this specification, indicate the presence of the stated features, areas, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more additional features, areas, integrals, steps, operations, elements, components, and / or combinations thereof.

[0043] As used herein, “about” or “approximately” includes the described value and means, for example, an acceptable range of deviation for a specific value, determined by a person of ordinary skill in the art, taking into account the measurement in question and errors associated with the measurement of the specific quantity (i.e., limitations of the measurement system). Unless otherwise specified, all parameter ranges disclosed include endpoint values ​​and all values ​​in between.

[0044] In the description of this invention, unless otherwise specified, the meanings of the terms are the same as those generally understood by those skilled in the art, but if there are any differences, the definitions of this invention shall prevail; unless otherwise specified, the test methods are all conventional methods; unless otherwise specified, the raw materials and test materials used in this invention are all conventionally available.

[0045] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0046] This invention provides a steel plate for low-temperature high-strength and high-toughness pipe fittings, comprising the following chemical elements by mass percentage:

[0047] C: 0.05~0.07%, Si: 0.15~0.19%, Mn: 1.5~1.7%, Cu: 0.1~0.35%, Ni: 0.2~0.5%, Cr: 0.1~0.35%, Mo: 0 .1~0.3%, V: 0.001~0.02%, Nb: 0.02~0.05%, Ti: 0.008~0.015%, Ca: 0.002~0.004%, Al: 0.02~0.04%.

[0048] According to another specific embodiment of the present invention, the steel plate for low-temperature high-strength and high-toughness pipe fittings is composed of the following chemical elements by mass percentage:

[0049] C: 0.05–0.07%, Si: 0.15–0.19%, Mn: 1.5–1.7%, Cu: 0.1–0.35%, Ni: 0.2–0.5%, Cr: 0.1–0.35%, Mo: 0.1–0.3%, V: 0.001–0.02%, Nb: 0.02–0.05%, Ti: 0.008–0.015%, Ca: 0.002–0.004%, Al: 0.02–0.04%; balance Fe and unavoidable impurities.

[0050] The design principle of the chemical element content of the steel plate for low-temperature high-strength and tough pipe fittings of this invention is as follows:

[0051] Carbon (C): C is the most basic strengthening element in steel, playing a role in solid solution strengthening and precipitation strengthening, and can effectively improve the hardenability of steel. The strength of steel for pipe fittings increases significantly with increasing C content, but excessively high C content will reduce the material's low-temperature toughness and weldability. Therefore, the C content in this invention is controlled at 0.05% to 0.07%.

[0052] Si (silicon): Si is a solid solution strengthening element and also a deoxidizing element in steel. However, excessive Si content will deteriorate the weldability of steel, especially the toughness of the heat-affected zone. It is also not conducive to the removal of iron oxide scale during hot rolling. Therefore, the Si content in this invention is controlled at 0.15 to 0.19%.

[0053] Manganese (Mn): Mn is a solid solution strengthening element and is the most important and economical strengthening element besides carbon. Mn also expands the γ phase region, which can improve the hardenability of steel, lower the γ→α phase transformation temperature of steel, help obtain fine phase transformation products, and improve the toughness of steel. Therefore, the Mn content in this invention is 1.5-1.7%.

[0054] Cu (copper) and Ni (nickel): Cu and Ni are solid solution strengthening elements that can improve the corrosion resistance of steel. Cu can improve the hardenability of steel plates and has a significant effect on counteracting the harmful effects of sulfur in steel. Ni has a significant effect on improving the low-temperature toughness of steel. However, too high a Ni content can easily cause the formation of highly viscous iron oxide scale on the slab surface, which is difficult to remove during subsequent processing and manufacturing, thus affecting the surface quality and fatigue performance of the steel plate. Excessive Ni content is also detrimental to the weldability of thick steel plates. Furthermore, Ni is expensive, and excessive content will increase the cost of the alloy. In addition, the combined use of Cu and Ni has a better effect on improving the corrosion resistance of steel. In this invention, the Cu and Ni contents are controlled at 0.1-0.35% and 0.2-0.5%, respectively.

[0055] Cr (chromium) and Mo (molybdenum): Cr and Mo are solid solution strengthening elements and important elements for improving the hardenability of steel. They can promote the formation of acicular ferrite, and the resulting carbides have good high-temperature stability, which helps to stabilize the properties of steel used in pipe fittings. In this invention, the contents of Cr and Mo are controlled at 0.1-0.35% and 0.1-0.3%, respectively.

[0056] Vanadium (V): V is a typical precipitation strengthening element that can combine with carbon (C) to form vanadium (VC). The precipitation temperature of VC is lower than that of TiC and NbC, and it can precipitate during tempering and quenching. The small particle size of VC is beneficial for improving the strength of the material. However, excessive V content can adversely affect the toughness of the material. Therefore, the V content in this invention is controlled at 0.001–0.02%.

[0057] Niobium (Nb): Nb is one of the important elements in low-carbon microalloyed steel. During hot rolling, the strain-induced precipitation of Nb in solution forms Nb(C,N) particles, which can delay austenite recrystallization, increase the austenite recrystallization temperature of the steel, and is beneficial for refining the microstructure after transformation, thereby improving the strength and toughness of the material. Furthermore, during the cooling process, the solution-treated Nb precipitates as second-phase particles (NbC) dispersed in the matrix, playing a precipitation strengthening role. However, excessively high Nb content cannot achieve complete solution, which not only fails to exert its effect but also increases production costs. It also causes NbC to precipitate prematurely at high temperatures, forming larger NbC particles, which is detrimental to improving the material's strength through precipitation strengthening. Therefore, in this invention, the Nb content is controlled at 0.02–0.05%.

[0058] Ti (Titanium): Ti is a strong carbonitride forming element, which can play a role in nitrogen fixation, thereby improving the toughness of materials. Furthermore, undissolved Ti carbonitrides can inhibit austenite grain growth during steel heating, and TiN and TiC precipitated during rough rolling in the high-temperature austenitic region can effectively suppress austenite grain growth. Additionally, during welding, TiN and TiC particles in the steel can significantly inhibit grain growth in the heat-affected zone, thereby improving the weldability of the steel plate. Therefore, the Ti content in this invention is controlled at 0.008–0.015%.

[0059] Ca (calcium): Ca is mainly used to modify inclusions, making them spherical in shape and uniformly distributed, which is beneficial to improving low-temperature toughness. In this invention, the Ca content is controlled at 0.002-0.004%.

[0060] Al (aluminum): Al is mainly used for deoxidation of steel. Adding an appropriate amount of Al helps refine the grains and improve the strength and toughness of the steel. However, increasing the Al content will cause Al oxides to form in the steel, thereby reducing the toughness of the base metal and the weld heat-affected zone. Therefore, the Al content in this invention is controlled at 0.02-0.04%.

[0061] The low-temperature high-strength and high-toughness pipe fitting steel plate provided by this invention adopts a low-C and low-Mn alloy composition system, which ensures the weldability of the steel for pipe fittings, while improving central segregation and banded structure, which is beneficial to improving low-temperature toughness. Appropriate addition of Cu, Ni, Cr, and Mo improves the hardenability of the steel plate and can form stable alloy carbides during quenching and tempering, ensuring the strength and toughness matching and performance stability of the pipe fittings. In addition, trace amounts of V, Nb, and Ti are added to form micro-alloyed carbonitrides, which refine the grains and play a role in precipitation strengthening.

[0062] Among the unavoidable impurity elements in steel, phosphorus (P) is a prone-segregating element, and sulfur (S) easily forms sulfides, both of which severely reduce the acid resistance of steel. Boron (B) tends to precipitate at grain boundaries, leading to a decrease in the material's plasticity and toughness; excessive content will significantly deteriorate the material's low-temperature toughness. Therefore, according to another specific embodiment of the present invention, the unavoidable impurities P, S, and B in the present invention are controlled at the following mass percentages: P ≤ 0.015%, S ≤ 0.0015%, and B ≤ 0.0005%. Strictly controlling the content of impurity elements P, S, and B in steel at low levels reduces the number of inclusions in the steel plate, reduces the degree of segregation, and ensures low-temperature toughness.

[0063] According to another specific embodiment of the present invention, in order to enable the steel plate for pipe fittings to have a good balance of strength and toughness, especially to maintain sufficient low-temperature toughness at the center of the wall thickness at -45℃ and -60℃, the microstructure of the steel plate for pipe fittings after quenching and tempering heat treatment is designed to be ferrite + tempered sorbite + dispersed carbide particles; wherein, the ferrite is formed due to the low quenching holding temperature or the low quenching cooling rate at the center of the wall thickness of the steel for thick-walled pipe fittings. The volume percentage of ferrite in the present invention is ≤5%, and the ferrite grain size is ≤5μm. Ferrite with fine grain size, as a soft phase, can effectively reduce the yield strength of the material and improve the low-temperature toughness of the material, thus playing a role in controlling the balance of strength and toughness of the steel for pipe fittings.

[0064] Carbide particles are dispersed on the tempered sorbite matrix. The coarsening of the tempered sorbite grain size will cause the deterioration of low-temperature toughness. Therefore, it is necessary to control the grain size of the tempered sorbite matrix. The present invention controls the average effective grain size of the tempered sorbite matrix to be ≤25μm.

[0065] According to another specific embodiment of the present invention, as above, since grain size coarsening will cause deterioration of low-temperature toughness, the present invention controls the carbide particle size to be ≤1μm.

[0066] Furthermore, this invention also provides a method for preparing steel plates for low-temperature high-strength and high-toughness pipe fittings. The process route includes smelting → casting → slab heating → rough rolling → finish rolling → cooling. The specific preparation method includes the following steps:

[0067] S1: The steel plate for low-temperature high-strength and tough pipe fittings according to the present invention is smelted and cast to obtain a slab, wherein the smelting added elements are specifically shown in Table 1.

[0068] Table 1 Chemical composition (wt%)

[0069] element C Si Mn Cu Ni Cr Mo V Nb Ti Ca Al scope 0.05～0.07 0.15-0.19 1.5-1.7 0.1-0.35 0.2-0.5 0.1-0.35 0.1-0.3 0.001-0.02 0.02-0.05 0.008-0.015 0.002-0.004 0.02-0.04

[0070] Preferably, in addition to the above-mentioned components, the balance is Fe and unavoidable impurities; further preferably, in the unavoidable impurities, P, S and B are controlled within the following mass percentages: P≤0.015%, S≤0.0015%, B≤0.0005%.

[0071] The slab is heated, and to avoid coarsening of austenite grains, the heating temperature should not be too high. In this invention, the heating temperature is controlled at 1100–1160℃. Furthermore, a suitable slab thickness is selected based on the thickness of the finished steel plate to ensure sufficient rolling deformation; specifically, if a slab with a thickness of 250mm, 300mm, or 360mm is selected, the resulting steel plate thickness will be 20mm–60mm.

[0072] S2: The slab is rough rolled at an initial rolling temperature of 1000-1100℃ and a final rolling temperature of 950-1000℃.

[0073] This step involves rough rolling in the fully recrystallized austenite region to avoid mixed grain formation, thereby improving the low-temperature toughness of the steel plate. However, for steel plates used in pipe fittings with a finished thickness ≥ 50 mm, the total reduction during rolling is relatively low, making it difficult for deformation to penetrate to the core. This can lead to core grain coarsening and deterioration of low-temperature toughness. To address this issue, a differential temperature rolling process is preferred. Specifically, before the final rough rolling pass, the steel plate surface is subjected to high-pressure water spraying, rapidly reducing the surface temperature to 600°C. This creates a large temperature gradient between the surface and the core, with the core temperature significantly higher than the surface temperature, making the core more susceptible to deformation. The final rolling pass is then performed after the surface temperature returns to 800°C. Rolling at this point allows deformation to penetrate more easily to the core, thus refining the core microstructure and improving the material's low-temperature toughness.

[0074] S3: The slab is finished rolled at an initial rolling temperature of 790-870℃ and a final rolling temperature of 780-820℃.

[0075] This step involves finishing rolling in the non-recrystallization region of austenite with a certain compression ratio. Preferably, the compression ratio of the finishing rolling is controlled to be 3 to 6, which further refines the microstructure and is beneficial to improving the low-temperature toughness of the steel plate, as well as the uniformity of the microstructure and the core toughness of the steel plate.

[0076] S4: The starting cooling temperature is 700~760℃, the cooling rate is 10~35℃ / s, and the stopping cooling temperature is 300~500℃ for controlled cooling. After cooling to room temperature, a refined rolled structure is obtained.

[0077] The present invention discloses a two-stage controlled rolling method for preparing low-temperature high-strength and high-toughness pipe fitting steel plates. The roughing stage effectively refines austenite grains. For extra-thick pipe fitting steel plates, differential temperature rolling technology can be applied in the last pass of roughing to promote the penetration of rolling deformation into the core, while ensuring a sufficient finishing rolling compression ratio to improve the uniformity of the steel plate microstructure and the core toughness of the steel plate. The cooling rate is controlled within a reasonable range to obtain a refined rolled microstructure. Furthermore, reasonable quenching and tempering heat treatment process parameters are set according to the differences in plate thickness, composition, and strength to obtain a good balance of strength and toughness. While ensuring the strength of the pipe fitting, the impact toughness at -45℃ to -60℃ can meet the requirements of engineering projects.

[0078] During the manufacturing process of pipe fittings, the steel plates used for the fittings need to undergo hot pressing forming, including rolling, welding, and multiple localized heating and quenching processes. The final formed pipe fittings also require overall quenching and tempering heat treatment. Since the steel plates used for pipe fittings are generally delivered in a rolled state, in order to assess the suitability of the steel plates for manufacturing pipe fittings, pipe fitting manufacturers generally require suppliers to perform quenching and tempering heat treatment on the steel plates under laboratory conditions to simulate the final state of the finished pipe fittings. The mechanical properties of the steel plates after quenching and tempering heat treatment are used as the acceptance criteria for the steel plates used for pipe fittings.

[0079] Therefore, according to another specific embodiment of the present invention, the method further includes: S5: For the test block used for performance testing, a simulated tempering heat treatment is performed, wherein the quenching temperature is 850-930℃, the holding coefficient is 1-2.5min / mm, the quenching medium is water, the tempering temperature is 580-660℃, the holding coefficient is 1-4min / mm, and after tempering, it is air-cooled to room temperature.

[0080] The microstructure after quenching and tempering heat treatment consists of tempered sorbite + a small amount of ferrite + carbide particles dispersed on the tempered sorbite matrix, exhibiting a good balance of strength and toughness. Combined with the steel plate thickness and appropriate quenching and tempering temperatures, a wide range of adjustable strength is achieved. Water is used as the quenching medium, and the cooling rate during the quenching process is controlled to ensure that the volume percentage of ferrite in the microstructure after quenching and tempering heat treatment is ≤5%, and the ferrite grain size is ≤5μm; the average effective grain size of the tempered sorbite is ≤25μm.

[0081] The preparation method of the steel plate for low-temperature high-strength and tough pipe fittings of the present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0082] Examples 1-12

[0083] The specific mass percentages of each chemical element in the steel plates for low-temperature high-strength and high-toughness pipe fittings in Examples 1-12 are shown in Table 2.

[0084] Table 2 Chemical composition of Examples 1-12 (wt%, balance is Fe and other unavoidable impurities other than P, S and B)

[0085] Example C Si Mn P S Cu Ni Cr Mo V Nb Ti Ca Al B 1 0.050 0.19 1.50 0.0080 0.0015 0.25 0.20 0.20 0.10 0 0.050 0.01 0.0035 0.028 0.0005 2 0.066 0.17 1.70 0.0075 0.0013 0.16 0.24 0.18 0.13 0 0.042 0.012 0.0026 0.038 0.0003 3 0.061 0.18 1.53 0.0119 0.0007 0.16 0.23 0.10 0.26 0.02 0.020 0.011 0.002 0.04 0.0003 4 0.070 0.15 1.55 0.0092 0.0013 0.10 0.22 0.30 0.20 0 0.045 0.008 0.0028 0.035 0.0004 5 0.051 0.16 1.61 0.0121 0.0015 0.35 0.21 0.15 0.23 0.01 0.040 0.015 0.004 0.02 0.0005 6 0.056 0.19 1.51 0.0105 0.0014 0.10 0.29 0.35 0.23 0.02 0.048 0.009 0.0033 0.031 0.0004 7 0.058 0.19 1.58 0.0880 0.0011 0.15 0.33 0.11 0.25 0.02 0.035 0.01 0.0023 0.025 0.0004 8 0.055 0.18 1.53 0.0116 0.0012 0.20 0.35 0.13 0.27 0.01 0.044 0.014 0.0024 0.033 0.0004 9 0.054 0.18 1.56 0.0079 0.0010 0.22 0.35 0.12 0.26 0.02 0.050 0.0011 0.034 0.036 0.0003 10 0.053 0.19 1.54 0.0078 0.0011 0.23 0.42 0.11 0.28 0.02 0.050 0.0013 0.0240 0.022 0.0003 11 0.052 0.16 1.55 0.0088 0.0008 0.19 0.50 0.15 0.27 0.02 0.050 0.001 0.0280 0.033 0.0003 12 0.050 0.19 1.50 0.0078 0.0009 0.20 0.50 0.22 0.30 0.02 0.050 0.0014 0.0028 0.029 0.0003

[0086] Furthermore, the preparation method includes the following steps:

[0087] S1: Smelt and cast according to the above composition to obtain slabs with a thickness of 250mm, 300mm or 360mm, and heat the slabs at a temperature of 1100~1160℃;

[0088] S2: The slab is rough rolled at an initial rolling temperature of 1000-1100℃ and a final rolling temperature of 950-1000℃.

[0089] S3: The slab is finished rolled at an initial rolling temperature of 790-870℃ and a finishing rolling temperature of 780-820℃.

[0090] S4: Cooling temperature is 700~760℃, cooling rate is 10~35℃ / s, and cooling is stopped at 300~500℃.

[0091] The hot rolling process parameters of Examples 1-12 are shown in Table 3. The roughing and final rolling temperature parameters marked as "differential temperature" indicate that the differential temperature rolling process is adopted. That is, before the start of the last pass of roughing, the surface of the steel plate is subjected to high-pressure water spraying treatment to rapidly reduce the surface temperature of the steel plate to 600°C. After the surface temperature returns to 800°C, the last pass of rolling is carried out.

[0092] Table 3 Hot rolling process parameters for Examples 1-12

[0093]

[0094] S5: For the performance test blocks, simulated quenching and tempering heat treatment was performed, with a quenching temperature of 850–930℃, a holding coefficient of 1–2.5 min / mm, water as the quenching medium, a tempering temperature of 580–660℃, a holding coefficient of 1–4 min / mm, and air cooling to room temperature after tempering. The simulated quenching and tempering heat treatment process parameters for Examples 1-12 are shown in Table 4.

[0095] Table 4. Simulated tempering heat treatment process parameters for Examples 1-12

[0096]

[0097]

[0098] Performance testing:

[0099] (1) Tensile properties at room temperature

[0100] At room temperature, tensile test specimens of φ12.5mm round bars taken from the center of the steel plate thickness were used. The steel plate samples of each embodiment were tested according to the national standard GB / T228.1-2010 "Metallic Materials - Tensile Testing at Room Temperature" to measure the yield strength, tensile strength and elongation after fracture of the steel plates of each embodiment.

[0101] (2) Low temperature toughness

[0102] Using a full-size V-notch impact specimen taken from the center of the steel plate thickness, impact tests were conducted on the test steel plate according to the national standard GB / T 229-2007 "Charpy Impact Pendulum Test Method for Metallic Materials". The Charpy impact energy at the center of the steel plate thickness in each embodiment was measured at -45℃ and -60℃.

[0103] The performance test results are shown in Table 5.

[0104] Table 5 Performance test results of Examples 1-12

[0105]

[0106]

[0107] As shown in Table 5, according to the composition and process designed in this invention, steel plates for low-temperature high-strength and tough pipe fittings with a thickness of 20mm to 60mm can be produced. The steel plates can meet the strength requirements of X65 to X80 steel grades, and the room temperature tensile properties can reach: yield strength Rt0.5: 480 to 680 MPa, tensile strength Rm: 550 to 750 MPa, yield ratio Rt0.5 / Rm ≤ 0.93, and elongation after fracture A50 ≥ 20%. Furthermore, the steel plates exhibit excellent low-temperature impact toughness at the center of thickness at -45℃ and -60℃, with Charpy impact energy ≥ 135J at -45℃ and ≥ 110J at -60℃.

[0108] Among them, the steel plate for pipe fittings in Example 3 has the best low-temperature toughness among all examples, with Charpy impact energy of 312J at -45℃ and 292J at -60℃, and meets the strength requirements of X70 steel grade; this is mainly due to the low quenching temperature during the simulated tempering heat treatment process, which results in a large number of fine ferrites in the final microstructure, thus achieving the best low-temperature toughness.

[0109] Examples 9-12 are steel plates with a finished thickness greater than 50 mm. Examples 9 and 12 did not undergo differential temperature rolling during the roughing process, while Examples 10 and 11 did. The results show that although the low-temperature toughness of Examples 9-12 all met the requirements of Charpy impact energy ≥135 J at -45℃ and ≥70 J at -60℃, the low-temperature toughness of Examples 9 and 12 was relatively poor. In particular, Example 9 had the worst low-temperature toughness among all examples, demonstrating that differential temperature rolling can improve the low-temperature toughness of the material.

[0110] (3) Microstructure

[0111] Figures 1-4 Microstructure images of the low-temperature high-strength and high-toughness pipe plates from Examples 2, 3, 8, and 10 are shown respectively. Figures 1-4 As shown, the microstructure of the steel plate for pipe fittings obtained by the present invention is ferrite + tempered sorbite + dispersed carbide particles; analysis shows that the volume percentage of ferrite is ≤5%, and the ferrite grain size is ≤5μm; the average effective grain size of tempered sorbite is ≤25μm; and the average size of carbide particles is ≤1μm.

[0112] Among them, from Figure 1 , Figure 3 and Figure 4 It can be seen that the final microstructures of Examples 2, 8, and 10 exhibit a lamellar morphology. The lamellar morphology is more pronounced in Example 2, while it is less pronounced in Examples 8 and 10. This is mainly due to the different steel plate thicknesses for Examples 2, 8, and 10, which are 22, 44, and 54 mm respectively. Smaller thicknesses result in better hardenability and a more pronounced lamellar morphology in the final microstructure. Figure 2 It can be seen that the microstructure of the steel plate for pipe fittings in Example 3 contains a relatively large amount of fine ferrite and the carbide particles are relatively dense. This is mainly due to the difference in the simulated quenching and tempering process. The quenching temperature of Example 3 is relatively low, only 850°C. At this temperature, the austenitization of the microstructure is not sufficient. Therefore, a large amount of fine ferrite and carbide particles are formed in the subsequent cooling and tempering process.

[0113] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A steel plate for low-temperature high-strength and high-toughness pipe fittings, characterized in that, The steel plate contains the following chemical elements by mass percentage: C: 0.05~0.07%; Si: 0.15~0.19%; Mn: 1.5~1.7%; Cu: 0.1~0.35%; Ni: 0.2~0.5%; Cr: 0.1~0.35%; Mo: 0.1~0.3%; V: 0.001~0.02%; Nb: 0.02~0.05%; Ti: 0.008~0.015%; Ca: 0.002~0.004%; Al: 0.02~0.04%; The microstructure of the steel plate is ferrite + tempered sorbite + dispersed carbide particles; wherein the volume percentage of ferrite is ≤5%, and the ferrite grain size is ≤5μm; the average effective grain size of the tempered sorbite is ≤25μm. The steel plate meets the following performance requirements: yield strength Rt0.5: 480~680MPa, tensile strength Rm: 550~750MPa, yield ratio Rt0.5 / Rm≤0.93, elongation after fracture A50≥20%; Charpy impact energy at 45℃≥135J, Charpy impact energy at -60℃≥70J.

2. The steel plate for low-temperature high-strength and high-toughness pipe fittings as described in claim 1, characterized in that, The steel plate is composed of the following chemical elements by mass percentage: C: 0.05~0.07%; Si: 0.15~0.19%; Mn: 1.5~1.7%; Cu: 0.1~0.35%; Ni: 0.2~0.5%; Cr: 0.1~0.35%; Mo: 0.1~0.3%; V: 0.001~0.02%; Nb: 0.02~0.05%; Ti: 0.008~0.015%; Ca: 0.002~0.004%; Al: 0.02~0.04%; balance Fe and unavoidable impurities.

3. The steel plate for low-temperature high-strength and high-toughness pipe fittings as described in claim 2, characterized in that, Of the unavoidable impurities, P, S, and B are controlled within the following mass percentages: P ≤ 0.015%, S ≤ 0.0015%, and B ≤ 0.0005%.

4. The steel plate for low-temperature high-strength and high-toughness pipe fittings as described in claim 1, characterized in that, The average size of the carbide particles is ≤1μm.

5. A method for preparing a steel plate for low-temperature high-strength and high-toughness pipe fittings as described in any one of claims 1-4, characterized in that, The method includes the following steps: S1: Heating the slab at a temperature of 1100~1160℃; S2: The slab is rough rolled at an initial rolling temperature of 1000~1100℃ and a final rolling temperature of 950~1000℃; S3: The slab is finished rolled at an initial rolling temperature of 790~870℃ and a final rolling temperature of 780~820℃; S4: Cooling temperature is 700~760℃, cooling rate is 10~35℃ / s, and cooling is stopped at 300~500℃. S5: Perform simulated quenching and tempering heat treatment, with a quenching temperature of 850~930℃, a holding coefficient of 1~2.5min / mm, a quenching medium of water, a tempering temperature of 580~660℃, and a holding coefficient of 1~4min / mm.

6. The method for preparing steel plates for low-temperature high-strength and high-toughness pipe fittings as described in claim 5, characterized in that, For steel plates with a thickness of 50 mm or more, step S2 further includes subjecting the steel plate surface to high-pressure water spraying before the start of the last rolling pass of the rough rolling process, so that the surface temperature of the steel plate is reduced to 600°C; and the last rolling pass is performed after the surface temperature is restored to 800°C.

7. The method for preparing steel plates for low-temperature high-strength and high-toughness pipe fittings as described in claim 5, characterized in that, In step S3, the compression ratio of the finishing mill is 3 to 6.

8. The method for preparing low-temperature high-strength and high-toughness pipe fitting steel plate according to any one of claims 5-7, characterized in that, The thickness of the slab is 250mm, 300mm or 360mm, and the thickness of the steel plate is 20mm to 60mm.

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