X80 grade thick-wall hot bending pipe with low-temperature impact energy of not less than 90 J at -45 DEG C and preparation method thereof

Abstract

This invention discloses an X80 grade thick-walled hot-bent pipe with a low-temperature impact energy of not less than 90J at -45℃, relating to the field of pipeline steel production technology. Its chemical composition and mass percentages are: C: 0.06~0.09, Si: 0.10~0.40, Mn: 1.60~1.90, P: ≤0.02, S: ≤0.005, Cr: 0.10~0.20, Ni: 0.50~0.80, Cu: 0.10~0.35, Nb: 0.03~0.08, Ti: 0.010~0.040, Mo: 0.15~0.50, V: 0.030~0.050, Zn: 0.06~0.09. r: 0.001~0.010, O: 0.0001~0.0030, B+N: 0.002~0.004, with the remainder being Fe and unavoidable impurities; wherein, the above chemical composition and mass percentage also satisfy: 612≤Za≤639, 2.66≤Cp≤4.10, 0.81≤Ip≤2.43, where Za=814-273C-74Mn-56Ni-16Cr-9Mo-5Cu, Cp=C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, Ip=Nb / C+O / Zr. The microstructure of the bent pipe obtained by this invention is composed of granular bainite, acicular ferrite, and M / A components, which is suitable for the development needs of high-pressure, thick-walled pipeline steel.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel production technology, and in particular to an X80 grade thick-walled hot-bent pipe with an impact energy of not less than 90J at -45℃ and its preparation method. Background Technology

[0002] With the continuous increase in steel grade, pipe diameter, wall thickness, and conveying pressure, hot-bent pipes have now developed to specifications with X80 steel grade, an outer diameter of 1422mm, and a wall thickness of over 30mm. The China-Russia East Route urgently needs two types of X80 large-diameter, thick-walled hot-bent pipes: low-temperature station bends with a D1422×(≥)30mm specification (applicable temperature -45℃). The trial production technology is quite challenging, especially for thick-walled steel pipes where the medium-frequency induction heating temperature is unevenly distributed along the wall thickness. This induction heating method results in a significant temperature difference (100-200℃) between the inner and outer walls of the bend. Furthermore, localized quenching cooling has limitations, causing significant differences in cooling rates between the outer surface, the center of the wall thickness, and the inner surface. These process characteristics greatly complicate the processing and ensure the strength and toughness of X80 thick-walled hot-bent pipes, and also place higher demands on the steel used for hot-bent pipes.

[0003] Currently, X80 hot-bent pipes mainly employ a production process of medium-frequency local heating, quenching, and overall tempering. X80 steel primarily uses controlled rolling and cooling low-carbon microalloyed steel. Due to the characteristics of the hot-bent pipe process, the strengthening and toughening effect generated by controlled rolling and cooling is weakened. To ensure that the final strength and low-temperature toughness of the bent pipe meet design requirements, it is necessary to improve the hardenability of the steel by increasing the content of microalloying elements such as Mo, Ni, and Nb on the basis of mainline steel. On the other hand, because the process window for thick-walled hot-bent pipes is relatively small, higher requirements are placed on the chemical composition design and microstructure control of X80 thick-walled hot-bent pipes.

[0004] The invention patent CN10326315B, entitled "A 900MPa Grade Ultra-High Strength and High Toughness Pipeline Steel Plate and Its Manufacturing Method," effectively improves the hardenability of steel by controlling appropriate C, Ni, Mo, and Nb contents (0.035~0.065%, 0.10~0.50%, 0.001~0.50%, 0.030~0.055%) and rapid cooling processes. It has certain solid solution strengthening and fine grain strengthening effects, effectively improving the strength of the steel and obtaining good low-temperature toughness. However, the Mn content (1.90~2.50%) is too high, and excessive Mn is prone to segregation, resulting in uneven steel composition and structure. The Cr content (0.35~0.60%) is too high, which can easily lead to a significant increase in the ductile-brittle transition temperature of the steel, which is not conducive to service in harsh low-temperature environments. Moreover, this invention only provides an impact value of -20℃, and it is uncertain whether it can reach the impact performance standard of -45℃. It is unknown whether it can be applied to the needs of new engineering construction.

[0005] The invention patent CN107099745B, entitled "High Carbon Equivalent Low Temperature High Toughness X80 Bend Pipeline Steel Plate and its Manufacturing Method," is based on the design principle of high carbon equivalent. It adds appropriate amounts of C and Mn, trace amounts of Nb, V, Ti, and other trace alloying elements, and small amounts of Mo, Cu, Ni, etc., combined with a specific TMCP process (two-stage rolling + intermediate billet cooling), resulting in steel plates with excellent low-temperature toughness, meeting the requirement of -45℃ KV2≥230J. The segmented gradient cooling process requires no additional equipment investment and can be implemented under existing on-site conditions, resulting in high productivity and a simple process. However, its yield strength ratio is ≤0.93. A high yield strength ratio implies a low work hardening index and low uniform elongation, weak plastic deformation capacity, reduced allowable deformation before fracture, and brittle fracture, which is detrimental to the safety of pipeline construction. Furthermore, the -45℃ impact energy in the invention is ≥230J, while in this invention it is ≥90J, indicating that the low-temperature impact toughness of this invention is superior to that of the invention.

[0006] The invention patent with publication number CN109402500A, entitled "X80 wide and thick steel plate for hot bending of pipe with good low temperature toughness and its production method", is based on low C (0.055~0.080%) and low Mn (1.55~1.75%), reducing the proportion of precious metal elements used. It utilizes the precipitation strengthening and grain refinement strengthening effects of elements such as V and Nb in the deformation and heat treatment process, which refines the structure and makes it more uniform, thereby improving the strength and toughness of the steel and obtaining a steel plate for hot bending of ultra-wide and thick walls at low temperature. However, the addition of a heat treatment process in the production of the steel plate significantly increases the production time and cost, resulting in financial and material losses and increased material costs, which is not conducive to the conservation and efficient use of resources.

[0007] The invention patent with publication number CN107988548B, entitled "An X80 pipeline steel plate adapted to low-temperature exposed environment and its production method," features alloying elements characterized by low C (0.03~0.06%), low Mn (0.45~0.75%), high Ni (1.50~2.0%), and high Cu (1.10~1.40%). Combined with its quenching and tempering process, the large amount of ε-Cu precipitation during aging compensates for the strength deficiency caused by low C and low Mn, ensuring the strength and toughness of the steel plate and giving it suitable weldability. However, Cu is prone to hot brittleness and reduced plasticity during hot working, and the excessively high Ni content, coupled with the increased quenching and tempering process, prolongs the manufacturing process of the steel plate, increases the production cost of steel, and is not conducive to the economic benefits of the product and the maximization of resource utilization.

[0008] Building upon the existing low-C, high-Mn, Cr(Mo), Ni, Cu, and Nb / Ti / V alloy system for X80 hot-bent pipes, it is necessary to increase the content of key alloys to ensure good hardenability during induction heating and quenching to guarantee pipe strength. Furthermore, appropriate addition of microalloying elements is required to control grain growth and achieve good low-temperature toughness. A lower carbon equivalent must also be controlled to ensure good field circumferential welding performance. Further optimization of the composition of the experimental steel and research on key process parameters for controlled rolling and cooling are needed to obtain a uniform and fine original microstructure, reducing the strength loss caused by the secondary hot-bending process and improving low-temperature toughness. Therefore, by optimizing the steel plate alloy composition and rolling process, the overall performance of X80 large-diameter thick-walled hot-bent pipes can be improved to better meet the needs of engineering construction. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an X80 grade thick-walled hot-bent pipe with an impact energy of not less than 90J at -45℃ and its preparation method.

[0010] To solve the above technical problems, the technical solution of the present invention is as follows: A type of thick-walled hot-bent pipe of grade X80 with a low-temperature impact energy of not less than 90J at -45℃ has the following chemical composition and mass percentage: C: 0.06~0.09, Si: 0.10~0.40, Mn: 1.60~1.90, P: ≤0.02, S: ≤0.005, Cr: 0.10~0.20, Ni: 0.50~0.80, Cu: 0.10~0.35, Nb: 0.03~0.08, Ti: 0.010~0.040, Mo: 0.15~0.50, V: 0.030~0.050, Zr: 0.001~0.010, O: 0.0001~0.0030, B+N: 0.002~0.004, with the remainder being Fe and unavoidable impurities; The chemical composition and mass percentages mentioned above also satisfy the following: 612≤Za≤639, 2.66≤Cp≤4.10, 0.81≤Ip≤2.43, where Za=814-273C-74Mn-56Ni-16Cr-9Mo-5Cu, Cp=C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, and Ip=Nb / C+O / Zr.

[0011] As a preferred embodiment of the X80 grade thick-walled hot-bent pipe with an impact energy of not less than 90J at -45℃ as described in this invention, the microstructure of the X80 grade thick-walled hot-bent pipe is a multiphase structure of granular bainite + acicular ferrite + M / A, the wall thickness of the pipe is 30~45mm, the yield strength is 540~660MPa, the tensile strength is 620~820MPa, the yield strength ratio is ≤0.92, the average transverse impact energy at -45℃ is ≥90J, and the impact energy of the weld heat-affected zone at -45℃ is ≥70J.

[0012] This invention also provides a method for preparing an X80 grade thick-walled hot-bent tube with an impact energy of not less than 90J at -45℃, comprising: Steel billet raw material preparation; Smelting: Steel billets are loaded into the furnace, vacuumed, and heated to melt. The steel is refined under high temperature and high vacuum conditions for 10 minutes, protective gas is added, alloying components are added, the temperature is adjusted, and the billets are poured into steel ingots. The steel ingots are then heated to the complete austenitization temperature of 1150~1250℃ and held for more than 3 hours. Rolling: Rough rolling and finish rolling are performed sequentially, using 3-5 passes of large reduction in rough rolling and 5-7 passes of small reduction in finish rolling, with the total reduction rate of rough rolling not less than 60%; Cooling: Control the cooling temperature to 700~780℃ and the red temperature to 400~500℃; Hot bending: The entire pipeline steel pipe is placed on the platform, the rear end of the main pipe is fixed, and the front end of the main pipe is passed through the induction coil and connected to the bending arm that moves along the predetermined bending radius trajectory. When the main pipe is induction heated and bent, the end of the main pipe is pushed forward at a predetermined speed by the pushing device. Then, the steel pipe is heated by the medium frequency induction coil, so that the heated main pipe is bent and deformed under the action of the bending arm. Water cooling is then performed to fix the pipe shape. Finally, the bent pipe is tempered as a whole in a bogie furnace.

[0013] As a preferred embodiment of the X80 grade thick-walled hot-bent pipe with an impact energy of not less than 90J at -45℃ as described in this invention, wherein: in the smelting process, the heating rate of the steel ingot is 8-12℃ / s.

[0014] As a preferred embodiment of the X80 grade thick-walled hot-bent tube with an impact energy of not less than 90J at -45℃ as described in this invention, in the rolling process, the rough rolling temperature is controlled at 1000~1100℃; the finishing rolling stage involves multiple passes, with the initial rolling temperature controlled at 800~960℃, and the finishing rolling stage controls the final rolling temperature at 760~800℃.

[0015] As a preferred embodiment of the X80 grade thick-walled hot-bent pipe with an impact energy of not less than 90J at -45℃ as described in this invention, the cooling process controls the cooling rate to be 5~30℃ / s.

[0016] As a preferred embodiment of the X80 grade thick-walled hot-bent tube with an impact energy of not less than 90J at -45℃ as described in this invention, in the hot-bending process, the quenching temperature of induction heating is controlled to be 900~1100℃, and the advancing speed is 0.35mm / s.

[0017] As a preferred embodiment of the X80 grade thick-walled hot-bent pipe with an impact energy of not less than 90J at -45℃ as described in this invention, in the hot-bent process, the cooling rate of water cooling is controlled to be 5-30℃ / s; the tempering temperature is 500~650℃.

[0018] The beneficial effects of this invention are: (1) This invention controls the austenite-ferrite initiation temperature Ar3 at 612~639℃ by adjusting the relationship 611≤Za≤639. The austenite-ferrite initiation temperature Ar3 is determined by the six main components C, Mn, Ni, Cr, Mo and Cu according to “Za=814-273C-74Mn-56Ni-16Cr-9Mo-5Cu”. By adjusting the Ni and Mo elements, the austenite-ferrite transformation temperature is reduced, which can reduce the transformation temperature of the intermediate temperature transformation structure bainite, promote the formation of AF, and facilitate the refinement of M / A, thereby improving its low temperature toughness. If Za is too high, the driving force will increase, the bainite structure will coarsen, which is not conducive to obtaining good low temperature toughness.

[0019] (2) This invention controls the addition amounts of the key alloying components C, Mn, Si, Ni, Cr, Mo, Cu, and N in low-carbon bainitic steel billets by adjusting the relationship Cp=C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, ensuring that 2.66≤Cp≤4.10. Among these, C, Mn, Ni, Cu, and N elements play a stabilizing role in austenite, significantly improving the stability of retained austenite and thus increasing the size and content of the MA component. Cr and Mo are both medium-strong carbide-forming elements, inhibiting carbon diffusion in austenite and significantly improving the stability of retained austenite, allowing more supercooled austenite to transform into the MA component, thereby increasing the size and content of the MA component. Although silicon (Si) is not a carbide-forming element, in the medium- and low-temperature phase transformation range, Si can enhance the bonding force of iron atoms, increase the diffusion activation energy of carbon atoms, and correspondingly reduce the diffusion coefficient of carbon in austenite. This effectively increases the content of retained austenite and its mechanical and thermal stability in steel, causing more supercooled austenite to transform into austenite (MA) components, thereby increasing the size and content of MA. By controlling the scientific and reasonable proportion of these elements added to the steel, 1.40 ≥ Cp ≥ 0.70 can be achieved. If the Cp value is too high, the size of the MA components in the steel will increase, and the content will also increase; if the Cp value is too low, a higher heating temperature is required to achieve complete austenitization, and the austenite grains will easily grow and coarsen, thus causing the size and coarsening of the MA components in the steel.

[0020] (3) The control relationship of this invention is 0.81≤Ip≤2.43, which requires that the sum of the Nb:C ratio and Zr:O ratio determined by the three key alloying elements such as Nb, Zr and C according to “Ip=Nb / C+ O / Zr” be controlled between 0.81 and 2.43. By controlling the content of Nb and Zr in the steel plate, it is beneficial to the precipitation of NbC, ZrN and ZrO2. The precipitation of NbC pins dislocations and prevents the migration of subgrain boundaries during recrystallization, thereby increasing the recrystallization temperature of austenite, hindering the growth of austenite grains, and refining the grain size. During hot deformation, Nb strain-induced precipitation occurs at austenite grain boundaries, which greatly delays austenite recrystallization, facilitates controlled rolling in the non-recrystallized zone, ensures the accumulation of distortion in the austenite matrix, promotes an increase in the number of bainite transformation nuclei, refines the grain size, and improves overall performance. Zr can form micron-sized inclusions or micron-sized composite inclusions during solidification, which is beneficial for improving the low-temperature toughness of steel. When Ip is too high, more NbC precipitates appear, including large-sized precipitates. It also easily forms a large number of coarse-grained ZrO2 particles in the steel. These inclusions cannot effectively promote the nucleation of acicular ferrite and easily form a large amount of grain boundary ferrite and lamellar ferrite in the heat-affected zone. The fracture turns into brittle fracture, which destroys the continuity of the matrix and is detrimental to the material properties. When Ip is too low, there are too few precipitates, the austenite grains grow, the grain refinement effect is not obvious, and the low-temperature toughness is reduced.

[0021] (4) In this invention, the roughing rolling adopts 3 to 5 passes with large reduction and the finishing rolling adopts 5 to 7 passes with small reduction. The roughing rolling adopts a large reduction, which is ≥60%. The small number of passes with large reduction is conducive to ensuring the full recrystallization of austenite, refining the austenite grains, and preventing mixed crystals. The finishing rolling adopts multiple passes with small reduction, which is conducive to accumulating a large amount of deformation, obtaining high-density deformation twins and deformation bands. At the same time, strain-induced carbonitride precipitation increases the deformation nucleation sites of the ferrite phase, giving the steel good low-temperature toughness. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the microstructure at the 1 / 4 thickness position of the X80 grade thick-walled hot-bent tube with a low-temperature impact energy of not less than 90J at -45℃ prepared in Example 3. Figure 2 This is a schematic diagram of the microstructure of the hot-bent tube prepared in Comparative Example 2 at the 1 / 4 thickness position. Detailed Implementation

[0024] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0025] This application provides an X80 grade thick-walled hot-bent pipe with a low-temperature impact energy of not less than 90J at -45℃. Its chemical composition and mass percentage are as follows: C: 0.06~0.09, Si: 0.10~0.40, Mn: 1.60~1.90, P: ≤0.02, S: ≤0.005, Cr: 0.10~0.20, Ni: 0.50~0.80, Cu: 0.10~0.35, Nb: 0.03~0.08, Ti: 0.010~0.040, Mo: 0.15~0.50, V: 0.030~0.050, Zr: 0.001~0.010, O: 0.0001~0.0030, B+N: 0.002~0.004, with the remainder being Fe and unavoidable impurities.

[0026] It should be noted that the above chemical composition and mass percentage must also meet the following conditions: 612≤Za≤639, 2.66≤Cp≤4.10, 0.81≤Ip≤2.43. Wherein, Za = 814-273C-74Mn-56Ni-16Cr-9Mo-5Cu, Cp = C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, and Ip = Nb / C+O / Zr.

[0027] It is understandable that the symbols for each element in the above formula represent the absolute value of the mass percentage of each element.

[0028] The functions and mechanisms of the above-mentioned main elements are as follows: C: Carbon, the most economical and basic strengthening element, can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. When the carbon content is <0.01%, interstitial carbon atoms gradually decrease, and carbon atoms after thermal cycling precipitate with alloying elements, weakening the grain boundaries. Increasing the carbon content can improve the strength of steel, but the increase in yield strength is less than that in tensile strength, so the yield strength ratio decreases. Excessive carbon content will increase the carbon equivalent, which is detrimental to toughness and weldability. Therefore, in this application, the C content is controlled at 0.06~0.09%.

[0029] Si: Silicon increases the strength of steel and increases the amount of retained austenite or martensite-austenite components in bainitic steel. However, excessively high Si content will reduce the low-temperature toughness of the steel plate and the weld heat-affected zone. Therefore, the Si content in this application is controlled at 0.10~0.40%.

[0030] Mn: Manganese lowers the transformation temperature of ferrite in steel, and during cooling, it easily forms a medium-temperature transformation structure, thereby refining the size of the ferrite matrix in the steel and enhancing the grain refinement strengthening effect. It dissolves in ferrite, increasing its contribution to solid solution strengthening and improving yield and tensile strength. However, excessively high Mn content can cause center segregation, increasing carbon equivalent and crack susceptibility index, and worsening the weldability of the steel. Therefore, in this application, the Mn content is controlled at 1.60~1.90%.

[0031] Ni: Nickel can strengthen ferrite, refine grains, and improve low-temperature impact toughness. Therefore, the Ni content in this application is controlled at 0.50~0.80%.

[0032] Niobium (Nb) inhibits austenite grain growth during heating, increasing the coarsening temperature. During steel plate rolling, Nb (C, N) exhibits grain refinement and precipitation hardening effects, resulting in high strength and toughness. During hot deformation, it inhibits grain growth after recrystallization, refining the grains. It also suppresses recrystallization, increases the recrystallization temperature, and expands the non-recrystallized austenite region. Furthermore, it improves the hardenability of the steel, enabling the acquisition of a medium-temperature transformation microstructure over a wide range of cooling rates. Therefore, the Nb content in this application is controlled at 0.03~0.08%.

[0033] Mo: Molybdenum can lower the transformation temperature of bainite in intermediate-temperature transformation structures, inhibit the formation of polygonal ferrite, and promote the formation of ferrite (AF) and bainite. Molybdenum in ferrite has a solid solution strengthening effect, making carbides less prone to decomposition and improving strength; it also improves corrosion resistance when combined with copper and nickel; however, excessive molybdenum affects the carbon equivalent, which is detrimental to welding. Therefore, the Mo content in this application is controlled at 0.15~0.50%.

[0034] Ti: Titanium can combine with N at high temperatures to form stable TiN particles, which can control the size of austenite grains at relatively high homogenization temperatures. This is beneficial for grain control in the heat-affected zone during welding. Moreover, titanium preferentially combines with N, increasing the solid solution content of Nb in austenite, enhancing the precipitation strengthening effect of Nb(C,N) in ferrite, improving performance inhomogeneity, and increasing impact toughness. Therefore, the Ti content in this application is controlled at 0.010~0.040%.

[0035] Cu: When copper is added in appropriate amounts to pipeline steel, it can improve the steel's resistance to HIC (hydrogen-induced cracking). Among alloying elements, Cu has the most significant effect on hydrogen-induced cracking. Therefore, the Cu content in this application is controlled at 0.10~0.35%.

[0036] V: Vanadium precipitates during the ferrite transformation process, thereby increasing the contribution of precipitation strengthening, refining grains, and improving welding performance. When the V content is too high, the precipitation strengthening effect is greater than required, resulting in a mismatch between strength and toughness, which is not conducive to obtaining good low-temperature toughness. Therefore, the V content in this application is controlled at 0.030~0.050%.

[0037] Zr and O: Zirconium can form micron-sized inclusions or micron-sized composite inclusions during solidification, which have little impact on strengthening and grain refinement, but are beneficial to improving the low-temperature toughness of steel. When the zirconium content is too high, the area percentage and number of inclusion particles per unit area in the steel increase significantly, and large single-particle coarse ZrO2 is easily formed in the steel. This type of inclusion cannot effectively promote the nucleation of acicular ferrite, and easily forms a large amount of grain boundary ferrite and side lath ferrite in the heat-affected zone, turning the fracture into brittle fracture, which is not conducive to improving low-temperature toughness. When the O content is too high, oxides are easily formed, which is harmful to the toughness of the steel plate. Therefore, in this application, the Zr content is controlled at 0.001~0.010%, and the O content is controlled at 0.0001~0.0030%.

[0038] B and N: Boron lowers the transformation temperature of bainite, promotes bainite transformation, refines the transformed bainite structure, and improves the strength and toughness of steel; it expands the bainite phase region, allowing for the formation of bainite structure over a wider cooling rate range; it is a grain boundary segregating element, inhibiting ferrite nucleation at austenite grain boundaries, but excessive B content can lead to hot brittleness; N helps form TiN and (Ti, Nb) (C, N), but excessive amounts will reduce the toughness of pipeline steel. In this application, the B and N content is controlled at B+N = 0.002~0.004%.

[0039] This application, by adjusting the relationship 611≤Za≤639, requires that the austenite-ferrite initiation temperature Ar3, determined by the six main elemental compositions of C, Mn, Ni, Cr, Mo, and Cu according to "Za=814-273C-74Mn-56Ni-16Cr-9Mo-5Cu", be controlled within 612~639℃. By adjusting the Ni and Mo elements, the transformation temperature of austenite-ferrite is reduced, which can lower the transformation temperature of the intermediate-temperature transformation structure bainite, promote the formation of AF, and facilitate the refinement of M / A, thereby improving its low-temperature toughness. If Za is too high, the driving force increases, the bainite structure coarsens, which is not conducive to obtaining good low-temperature toughness.

[0040] This invention controls the addition amounts of key alloying components C, Mn, Si, Ni, Cr, Mo, Cu, and N in low-carbon bainitic steel billets by adjusting the relationship Cp=C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, ensuring that 2.66≤Cp≤4.10. C, Mn, Ni, Cu, and N elements stabilize austenite in steel, significantly improving the stability of retained austenite and thus increasing the size and content of the austenite-forming alloy (MA) component. Cr and Mo are both medium-strong carbide-forming elements that inhibit carbon diffusion in austenite, significantly improving the stability of retained austenite and allowing more supercooled austenite to transform into MA components, thereby increasing the size and content of MA components. Although silicon (Si) is not a carbide-forming element, in the medium- and low-temperature phase transformation range, Si can enhance the bonding force of iron atoms, increase the diffusion activation energy of carbon atoms, and correspondingly reduce the diffusion coefficient of carbon in austenite. This effectively increases the content of retained austenite and its mechanical and thermal stability in steel, causing more supercooled austenite to transform into austenite (MA) components, thereby increasing the size and content of MA. By controlling the scientific and reasonable proportion of these elements added to the steel, 1.40 ≥ Cp ≥ 0.70 can be achieved. If the Cp value is too high, the size of the MA components in the steel will increase, and the content will also increase; if the Cp value is too low, a higher heating temperature is required to achieve complete austenitization, and the austenite grains will easily grow and coarsen, thus causing the size and coarsening of the MA components in the steel.

[0041] This application, by controlling the relationship 0.81≤Ip≤2.43, requires that the sum of the Nb:C ratio and Zr:O ratio, determined by the composition of three key alloying elements (Nb, Zr, and C) according to "Ip=Nb / C+ O / Zr", be controlled between 0.81 and 2.43. By controlling the Nb and Zr content in the steel plate, the precipitation of NbC, ZrN, and ZrO2 is facilitated. The precipitation of NbC pins dislocations and prevents the migration of subgrain boundaries during recrystallization, thereby increasing the recrystallization temperature of austenite, hindering the growth of austenite grains, and refining the grains. During hot deformation, Nb strain-induced precipitation occurs at austenite grain boundaries, significantly delaying austenite recrystallization, facilitating controlled rolling in the non-recrystallized zone, ensuring the accumulation of distortion in the austenite matrix, promoting an increase in the number of bainite transformation nuclei, refining the grains, and improving overall performance. Zr can form micron-sized inclusions or micron-sized composite inclusions during solidification, which is beneficial for improving the low-temperature toughness of the steel. When Ip is too high, more NbC precipitates appear, including large-sized precipitates. It also easily forms a large number of coarse-grained ZrO2 particles in the steel. These inclusions cannot effectively promote the nucleation of acicular ferrite and easily form a large amount of grain boundary ferrite and lamellar ferrite in the heat-affected zone. The fracture turns into brittle fracture, which destroys the continuity of the matrix and is detrimental to the material properties. When Ip is too low, there are too few precipitates, the austenite grains grow, the grain refinement effect is not obvious, and the low-temperature toughness is reduced.

[0042] In addition, this application also provides a method for preparing an X80 grade thick-walled hot-bent tube with a low-temperature impact energy of not less than 90J at -45℃, the method specifically including the following steps: Step S101: Preparation of steel billet raw materials.

[0043] Specifically, the billets for furnace steel are prepared according to the following composition percentages: C: 0.06~0.09, Si: 0.10~0.40, Mn: 1.60~1.90, P: ≤0.02, S: ≤0.005, Cr: 0.10~0.20, Ni: 0.50~0.80, Cu: 0.10~0.35, Nb: 0.03~0.08, Ti: 0.010~0.040, Mo: 0.15~0.50, V: 0.030~0.050, Zr: 0.001~0.010, O: 0.0001~0.0030, B+N: 0.0001~0.0006.

[0044] Step S102: Smelting: Steel billets are loaded into the furnace, vacuumed to 1 Pa, and then heated to melt. The molten steel is refined for 10 minutes under high temperature and high vacuum conditions. Protective gas is added, alloy components are added, the temperature is adjusted, and the billets are cast into steel ingots. The steel ingots are then heated at a rate of 8-12℃ / s to the complete austenitization temperature of 1150~1250℃ and held for more than 3 hours.

[0045] Step S103: Rolling: Rough rolling and finish rolling are performed sequentially. Rough rolling is carried out with 3-5 passes of large reduction and finish rolling with 5-7 passes of small reduction. The total reduction rate of rough rolling is not less than 60%. Among them, the finishing temperature of rough rolling is controlled at 1000-1100℃ to ensure sufficient recrystallization of austenite during the rough rolling stage. In the finish rolling stage, multiple passes are rolled, and the initial rolling temperature is controlled at 800-960℃. The finishing temperature is controlled at 760-800℃ to promote the formation of deformation bands and the accumulation of distortion.

[0046] Step S104: Cooling: Control the cooling temperature to 700~780℃, the cooling rate to 5~30℃ / s, and the red temperature to 400~500℃.

[0047] Step S105: Hot bending: Place the entire pipeline steel pipe on the platform, fix the rear end of the main pipe, and connect the front end of the main pipe through the induction coil to the bending arm that moves along the predetermined bending radius trajectory. When the main pipe is induction heated and bent, the end of the main pipe is pushed forward at a predetermined speed by the pushing device. Then, the steel pipe is heated by the medium frequency induction coil, so that the heated main pipe is bent and deformed under the action of the bending arm. Water cooling is then performed to fix the pipe shape. Finally, the bent pipe is tempered as a whole by a bogie furnace.

[0048] The induction heating quenching temperature is controlled at 900~1100℃, the feed rate is 0.35mm / s, the water cooling rate is controlled at 5-30℃ / s, and the tempering temperature is controlled at 500~650℃.

[0049] The above technical solution will be further explained below through specific embodiments.

[0050] Examples 1-5 all provide an X80 grade thick-walled hot-bent pipe with a low-temperature impact energy of not less than 90J at -45℃. The main components of the steel used for this hot-bent pipe include: C, Si, Mn, P, S, Cr, Ni, Cu, Nb, Ti, Mo, V, Zr, O, B, N, with the remainder being Fe and unavoidable impurities. Furthermore, the content of alloying elements, by mass percentage, must meet the following requirements: 612≤Za≤639, 2.66≤Cp≤4.10, 0.81≤Ip≤2.43, where: Za=814-273C-74Mn-56Ni-16Cr-9Mo-5Cu, Cp=C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, Ip=Nb / C+O / Zr. The compositions of the examples and comparative examples are shown in Table 1.

[0051] Table 1

[0052] Eight heats of experimental steel were prepared in a 75kg vacuum furnace according to the compositions of Examples 1-5 and Comparative Examples 1-3, using the methods described above. The parameters for the rolling and cooling processes are shown in Table 2.

[0053] Table 2

[0054] The performance of the hot-bent pipes was tested, and the results are shown in Table 3.

[0055] Table 3

[0056] As can be seen from Table 3, the X80 grade thick-walled hot-bent pipes with a low-temperature impact energy of not less than 90J at -45℃ prepared in Examples 1 to 5 have a yield strength of 540~660MPa, a tensile strength of 640~820MPa, a yield strength ratio of ≤0.93, and an average transverse impact energy of ≥90J for the pipes at -45℃.

[0057] After grinding, polishing, and etching, the metallographic structure of the samples from Example 3 and Comparative Example 2 was observed, and the results are shown in the appendix. Figure 1 and 2As shown, the metallographic structures of Example 3 and Comparative Example 2 are both granular bainite + acicular ferrite + M / A components. However, Example 3 has smaller grains, a finer microstructure, and better low-temperature toughness.

[0058] Therefore, the technical solution of this application produces an X80 grade thick-walled hot-bent pipe with a low-temperature impact energy of not less than 90J at -45℃, a yield strength of 540~660MPa, a tensile strength of 620~820MPa, a yield-to-tensile ratio of ≤0.93, an average transverse impact energy of ≥90J at -45℃, and an average impact energy of ≥70J in the weld heat-affected zone at -45℃, exhibiting good strength, toughness, and weldability.

[0059] In addition to the above embodiments, the present invention may have other implementation methods; all technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A thick-walled hot-bent pipe of grade X80 with an impact energy of not less than 90J at -45℃, characterized in that: Its chemical composition and mass percentage are as follows: C: 0.06~0.09, Si: 0.10~0.40, Mn: 1.60~1.90, P: ≤0.02, S: ≤0.005, Cr: 0.10~0.20, Ni: 0.50~0.80, Cu: 0.10~0.35, Nb: 0.03~0.08, Ti: 0.010~0.040, Mo: 0.15~0.50, V: 0.030~0.050, Zr: 0.001~0.010, O: 0.0001~0.0030, B+N: 0.002~0.004, with the remainder being Fe and unavoidable impurities; The chemical composition and mass percentages mentioned above also satisfy the following: 612≤Za≤639, 2.66≤Cp≤4.10, 0.81≤Ip≤2.43, where Za=814-273C-74Mn-56Ni-16Cr-9Mo-5Cu, Cp=C+Mn / 6+Si / 5+Ni / 15+Cr / 2+Mo / 4+Cu / 13+N / 0.001, and Ip=Nb / C+O / Zr.

2. The X80 grade thick-walled hot-bent pipe with a low-temperature impact energy of not less than 90J at -45℃ as described in claim 1, characterized in that: The microstructure of the X80 grade thick-walled hot-bent pipe is a multiphase structure of granular bainite + acicular ferrite + M / A. The wall thickness of the pipe is 30~45mm, the yield strength is 540~660MPa, the tensile strength is 620~820MPa, the yield strength ratio is ≤0.92, the average transverse impact energy at -45℃ is ≥90J, and the impact energy of the weld heat-affected zone at -45℃ is ≥70J.

3. A method for preparing an X80 grade thick-walled hot-bent tube with a low-temperature impact energy of not less than 90J at -45℃ as described in claim 1 or 2, characterized in that: include: Steel billet raw material preparation; Smelting: Steel billets are loaded into the furnace, vacuumed, and heated to melt. The steel is refined under high temperature and high vacuum conditions for 10 minutes, protective gas is added, alloying components are added, the temperature is adjusted, and the billets are poured into steel ingots. The steel ingots are then heated to the complete austenitization temperature of 1150~1250℃ and held for more than 3 hours. Rolling: Rough rolling and finish rolling are performed sequentially, using 3-5 passes of large reduction in rough rolling and 5-7 passes of small reduction in finish rolling, with the total reduction rate of rough rolling not less than 60%; Cooling: Control the cooling temperature to 700~780℃ and the red temperature to 400~500℃; Hot bending: The entire pipeline steel pipe is placed on the platform, the rear end of the main pipe is fixed, and the front end of the main pipe is passed through the induction coil and connected to the bending arm that moves along the predetermined bending radius trajectory. When the main pipe is induction heated and bent, the end of the main pipe is pushed forward at a predetermined speed by the pushing device. Then, the steel pipe is heated by the medium frequency induction coil, so that the heated main pipe is bent and deformed under the action of the bending arm. Water cooling is then performed to fix the pipe shape. Finally, the bent pipe is tempered as a whole in a bogie furnace.

4. The method for preparing X80 grade thick-walled hot-bent tubes with an impact energy of not less than 90J at -45℃ as described in claim 3, characterized in that: In the smelting process, the heating rate of the steel ingot is 8-12℃ / s.

5. The method for preparing X80 grade thick-walled hot-bent tubes with an impact energy of not less than 90J at -45℃ according to claim 3, characterized in that: In the rolling process, the roughing rolling temperature is controlled at 1000~1100℃; the finishing rolling stage involves multiple passes, with the initial rolling temperature controlled at 800~960℃, and the finishing rolling stage controls the final rolling temperature at 760~800℃.

6. The method for preparing X80 grade thick-walled hot-bent tubes with an impact energy of not less than 90J at -45℃ according to claim 3, characterized in that: In the cooling process, the cooling rate is controlled to be 5~30℃ / s.

7. The method for preparing X80 grade thick-walled hot-bent tubes with an impact energy of not less than 90J at -45℃ according to claim 3, characterized in that: In the hot bending process, the quenching temperature of induction heating is controlled at 900~1100℃, and the advancing speed is 0.35mm / s.

8. The method for preparing X80 grade thick-walled hot-bent pipe with a low-temperature impact energy of not less than 90J at -45℃ according to claim 3, characterized in that: In the hot bending process, the cooling rate of the water cooling is controlled at 5-30℃ / s; the tempering temperature is 500~650℃.

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