450MPa-grade thick-specification pipeline steel suitable for hot delivery and hot charging and manufacturing method of 450MPa-grade thick-specification pipeline steel

Abstract

The invention discloses 450MPa-grade thick-specification pipeline steel suitable for hot delivery and hot charging and a manufacturing method of the 450MPa-grade thick-specification pipeline steel. The 450MPa-grade thick-specification pipeline steel comprises the following components in percentage by mass: 0.030 to 0.055 percent of C, 0.10 to 0.30 percent of Si, 1.45 to 1.64 percent of Mn, 0.16 to 0.30 percent of Cr, 0.05 to 0.12 percent of Mo, 0.10 to 0.20 percent of Ni, 0.040 to 0.049 percent of Nb, 0.008 to 0.020 percent of Ti, 0.01 to 0.04 percent of Al, 0.001 to 0.003 percent of Ca, 0.0010 to 0.0045 percent of N, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S and the balance of Fe. C + Mn / 6 + (Cr + Mo) / 5 is more than or equal to 0.34% and less than or equal to 0.38%; 0.90% or less of 1.5 Mo + Cr + Ni + 6Nb; ti / N > = 3.5. The thickness of the pipeline steel is larger than or equal to 28 mm, the yield strength Rt < 0.5 > of the pipeline steel is 450-570 MPa, the tensile strength Rm of the pipeline steel is 535-655 MPa, and the yield ratio Rt < 0.5 > / Rm is smaller than or equal to 0.90; charpy impact energy AKV at-20 DEG C is greater than or equal to 250J; and the-10 DEG C full-thickness DWTT fracture shear area rate SA% is greater than or equal to 85%.

Description

Technical Field

[0001] The present invention relates to the field of pipeline steel, and in particular to a 450MPa grade thick-gauge pipeline steel suitable for hot delivery and hot loading and a manufacturing method thereof. Background Art

[0002] After years of development, the manufacturing technology of pipeline steel has been continuously improved. Nowadays, the demand for natural gas energy is still increasing, and there is still a strong demand for natural gas pipeline construction at home and abroad. Moreover, with the development of technology, the technical requirements for pipeline steel in new pipeline projects are also constantly improving, such as thick walls, low temperatures, large strains, acid resistance, etc. Among them, since large-capacity trunk pipelines and submarine pipelines are usually designed with larger wall thicknesses, the low-temperature crack arrest performance of thick-specification pipeline steel has attracted much attention, namely the drop weight tear test performance (DWTT), which is also an important indicator to measure the manufacturing level of pipeline steel of steel companies.

[0003] In the drop weight tear test, the thinner the specimen is, the closer it is to the plane stress state, and it is only subjected to plane shear force. The thicker the specimen is, the closer it is to the plane strain state, σ z The normal stress is not zero, and it is in a triaxial stress state, which increases the brittle tendency. In addition, during the manufacturing process of thick-gauge pipeline steel, there are organizational differences in the thickness direction, especially the weaker degree of organizational refinement in the thickness center. Therefore, it is very difficult to perform full-thickness DWTT on thick-gauge pipeline steel. Usually, only thinned specimens can be selected according to the API RP5L3 standard. However, with the increasing requirements of the pipeline industry, since the full-thickness DWTT can better represent the crack arrest performance of the solid pipe, it may become a rigid requirement for some high-demand pipeline projects. However, in the existing public patent literature, there are few reports on technical solutions for thick-gauge pipeline steel to achieve full-thickness DWTT performance.

[0004] On the other hand, with the development of carbon emission reduction and extreme energy efficiency in my country's steel industry, steel companies are actively reducing carbon emissions in various processes of steel manufacturing. Among them, hot delivery and hot charging technology can effectively utilize the sensible heat of continuous casting billets, reduce the energy consumption of slabs in heating furnaces, and shorten process time. It is one of the important technical paths for energy conservation and efficiency improvement in the steel industry. Relevant companies are constantly expanding product varieties suitable for hot delivery and hot charging, and strive to further increase the hot charging temperature and hot charging ratio.

[0005] However, when the hot charging temperature of microalloyed steel reaches the two-phase region (Ar1~Ar3), some quality problems are prone to occur. It is mainly manifested in two aspects. First, in this temperature range, the surface of the ingot is prone to form a network of pro-eutectoid ferrite along the austenite grain boundary, and carbonitride second phase particles are precipitated to weaken the austenite grain boundary. When the slab is heated again in the furnace, the tensile stress is formed due to the α→γ phase transformation and contraction, which may cause surface cracks. On the other hand, when the two-phase region is charged into the furnace, the original cast austenite grains are coarse, and the grains of some ferrite that has undergone phase transformation and austenitized again are relatively small, resulting in the austenite grains being in a mixed crystal state. If it cannot be improved by the rolling process, the finished steel plate will show lower toughness. As a typical microalloyed high-strength steel, high-grade pipeline steel has higher quality requirements on the strength and toughness of the steel plate, and it is more necessary to adopt effective technical solutions to solve the quality risks of hot delivery and hot charging processes.

[0006] Chinese patent publication number CN109202029B discloses a "production method for preventing straightening and hot delivery cracks of micro-alloyed steel continuous casting billets". The method reduces the surface temperature of the continuous casting billets to 450°C or above by rapidly cooling the continuous casting billets before straightening the slabs; controls the surface temperature of the billets to be ≤600°C during the straightening stage; and controls the temperature of the continuous casting billets to return to normal after straightening and before cutting. The surface temperature of the continuous casting billets after temperature return is less than the Ar3 temperature of the steel grade. This method can improve the surface quality of the continuous casting billets, but the heat loss of the slabs is large, and the effect of saving energy consumption is relatively limited.

[0007] Chinese patent publication number CN109128074B discloses "a method for producing microalloyed steel capable of hot transport and hot charging". It improves the stability of supercooled austenite, inhibits the nucleation of proeutectoid ferrite on grain boundaries, and inhibits the precipitation of carbonitrides on the interface between austenite and ferrite by adding 0.001-0.004% B element, thereby avoiding the formation of grain boundary ferrite and the resulting star cracking during the austenitization heating process after hot transport and hot charging of the microalloyed steel. However, the B element will reduce the low-temperature toughness of the pipeline steel, and the deliberate addition of the B element is not allowed in the pipeline steel specification.

[0008] Chinese patent publication number CN115341129A discloses "a method for hot delivery and hot loading production of microalloyed low-alloy structural steel", which achieves hot loading and rolling by matching the content of Ti and Nb elements and stipulating the soaking time of the heating furnace. However, the public text does not mention a technical solution to the potential quality problems of high-temperature hot loading.

[0009] Chinese patent publication number CN105861937A discloses "a low-temperature pipeline steel for LNG gas transmission trunk lines and its preparation method", the composition of which is designed to contain 0.21-0.35%wt Ni and 0.05-0.08%wt Nb. The alloy cost is relatively high, and the disclosure does not mention the applicable steel plate thickness and full-thickness DWTT performance.

[0010] Chinese patent publication number CN103952638A discloses “Pipeline steel with excellent low temperature toughness and its manufacturing process”, and its composition design adopts a design with low Mn content (0.55-0.72%) and high Ni content (0.55-0.80%). Summary of the invention

[0011] The object of the present invention is to provide a 450MPa grade thick specification pipeline steel suitable for hot delivery and hot charging and a manufacturing method thereof. By hot charging of slabs, the precipitation of microalloying elements during the cooling process of the slabs is reduced, and the grain refining effect of Nb is more efficiently exerted. At the same time, the thickness center temperature of the hot charged slab is higher than the surface layer. The slab has better temperature uniformity in the thickness direction in the heating furnace, and can ensure that the microalloying elements in the core of the slab are effectively dissolved during the heating process, and that the subsequent rough rolling is fully recrystallized and refined, thereby obtaining good full-thickness direction organizational uniformity, achieving good full-thickness DWTT performance, and solving potential quality problems caused by high-temperature hot charging through manufacturing process control.

[0012] To achieve the above object, the technical solution of the present invention is:

[0013] A 450MPa grade thick specification pipeline steel suitable for hot delivery and hot loading, wherein the composition mass percentage is: C: 0.030-0.055%, Si: 0.10-0.30%, Mn: 1.45-1.64%, Cr: 0.16-0.30%, Mo: 0.05-0.12%, Ni: 0.10-0.20%, Nb: 0.040-0.049%, Ti: 0.008-0.020%, Al: 0.01-0.04%, Ca: 0.001-0.003%, N: 0.0010-0.0045%, P≤0.012%, S≤0.002%; the balance includes Fe and other unavoidable impurities, and needs to meet the following requirements at the same time:

[0014] 0.34%≤C+Mn / 6+(Cr+Mo) / 5≤0.38%;

[0015] 1.5Mo+Cr+Ni+6Nb≤0.90%;

[0016] Ti / N ≥ 3.5.

[0017] Furthermore, the balance is Fe and other inevitable impurities.

[0018] The microstructure of the pipeline steel of the present invention is fine polygonal ferrite + acicular ferrite, with an average grain size of ≤7 μm. Meanwhile, the difference in average grain size between 1 / 4 and 1 / 2 thickness of the steel plate is ≤1 μm.

[0019] The thickness of the pipeline steel of the present invention is ≥28mm, and its yield strength Rt 0.5 The tensile strength Rm is 450-570MPa, the yield strength ratio Rt is 535-655MPa, 0.5 / Rm≤0.90; -20℃ Charpy impact energy AKV≥250J; -10℃ full thickness DWTT fracture shear area rate SA%≥85%.

[0020] In the composition design of the pipeline steel of the present invention:

[0021] C: C is the most basic strengthening element, which has the effects of solid solution strengthening and carbide precipitation strengthening, but too high C content is not conducive to the full thickness DWTT performance of the steel of the present invention. The present invention controls the C content to be 0.030-0.055%.

[0022] Si: Si is a deoxidizing element in steel and also has a solid solution strengthening effect, but too high Si content will affect the welding performance of pipeline steel. In the present invention, the Si content is controlled to be 0.10-0.30%.

[0023] Mn: Mn is an important solid solution strengthening element and can reduce the phase transition point and refine the phase transition structure. However, excessive Mn content increases strength while also reducing low temperature toughness and increasing the difficulty of controlling center segregation, which is not conducive to full thickness DWTT performance. The present invention controls the Mn content to 1.45-1.64%.

[0024] Cr: Cr can effectively improve the hardenability of steel, improve the uniformity of the structure and performance of the steel plate in the thickness direction, and improve the strength of the steel. However, it should be noted that if the Cr content in the steel is too high, the steel plate is prone to form a hard phase structure during the rapid cooling process, which is not conducive to low-temperature toughness. The present invention controls the Cr content to 0.16-0.30%.

[0025] Mo: Mo element has the function of expanding the γ phase area, which can reduce the γ→α phase transformation temperature of steel. Adding a small amount of Mo helps to inhibit the formation of reticular proeutectoid ferrite on the surface of the slab during the slab cooling process. In the present invention, the Mo content is controlled to be 0.05-0.12%.

[0026] Ni: Ni increases stacking fault energy, promotes cross slip of screw dislocations at low temperatures, increases the work consumed by crack propagation, improves local crack arrest capability, and promotes crack toughness expansion, thereby improving the DWTT performance of pipeline steel in extremely low temperature environments. However, Ni is expensive. The present invention controls the Ni content to 0.10-0.20%.

[0027] Nb: Nb is the most important microalloying element for grain refinement. During the rough rolling process, the solid solution Nb can pin the deformed austenite grain boundaries through the solute drag effect, hindering the growth of recrystallized austenite grains. In addition, Nb also has the effect of increasing the recrystallization temperature, which can increase the strain accumulation in the non-recrystallized zone during the finishing rolling process, and the Nb carbonitride precipitates on the deformation band of the deformed austenite, which can play a role in refining the phase transformation structure during the γ→α phase transformation process, and it also has a precipitation strengthening effect. However, it should be noted that when the Nb content in the steel is too high, it will be limited by the solubility product of C and Nb. The present invention controls the Nb content to 0.040-0.049%.

[0028] Ti: Ti has a strong binding force with N and is a strong carbonitride-forming element. TiN formed by Ti element has high thermal stability and can prevent the growth of austenite grains during slab heating and rough rolling recrystallization. In addition, Ti can effectively fix the N element and inhibit the precipitation of AlN at the austenite / ferrite grain boundary on the slab surface, thereby reducing the surface cracking tendency of the hot-charged slab. However, if the Ti content is too high, larger and more numerous Ti carbonitrides will be formed, which is not conducive to the toughness of the steel. The present invention controls the Ti content to be between 0.008% and 0.020%.

[0029] Al: In the economical low yield ratio high strength steel described in the present invention, Al is a deoxidizing element, but if the Al content is too high, inclusions and AlN precipitates are easily formed. The present invention controls the Al content to be 0.01-0.04%.

[0030] Ca: In the present invention, the morphology of sulfides can be controlled by micro-Ca treatment, thereby inhibiting the formation of MnS inclusions, but excessive Ca content is likely to form inclusions. The present invention controls the Ca content to 0.001-0.003%.

[0031] N: N element can form TiN particles with high melting point with Ti, thus inhibiting the coarsening of austenite grains during reheating. However, the N element content must be controlled at a low level to reduce the precipitation of AlN. In the present invention, the N content is controlled at 0.0010-0.0045%.

[0032] Furthermore, the present invention controls C+Mn / 6+(Cr+Mo) / 5 to be between 0.34% and 0.38%.

[0033] The combination of C+Mn / 6+(Cr+Mo) / 5 has a great influence on strength and toughness. Especially for thick specifications, if C+Mn / 6+(Cr+Mo) / 5 is too low, it is difficult to achieve the target strength. C+Mn / 6+(Cr+Mo) / 5 should not be too high either, otherwise, a hard phase structure is easily formed under the low cooling rate and low cooling stop process path adopted by the present invention.

[0034] The present invention needs to control the upper limit of the alloy element combination 1.5Mo+Cr+Ni+6Nb, because when the hot charging temperature is in the two-phase region, a small amount of ferrite precipitates along the cast austenite grain boundary on the slab surface. When the slab is reheated in the heating furnace, the ferrite transforms into austenite, which will shrink and produce tensile stress. In order to ensure that the invention steel has good high-temperature thermoplasticity and avoid surface cracks, the alloy element combination 1.5Mo+Cr+Ni+6Nb needs to be controlled to be ≤0.90%.

[0035] In addition, the present invention also needs to control Ti / N≥3.5, so that Ti can fix N, inhibit the precipitation of AlN on the austenite / ferrite grain boundary on the slab surface, and reduce the surface cracking tendency of the hot-charged slab.

[0036] In the above formulas, C, Mn, Mo, Cr, Ni, Nb, N, and Ti all represent the mass percentages of the corresponding elements.

[0037] Among other inevitable impurities, S is easy to form MnS inclusions, which are in the form of long strips after rolling. P is an element that is easy to segregate. If the content of P and S impurity elements in steel is too high, the performance of the steel will be reduced, which is not conducive to the full thickness DWTT performance of the steel of the present invention. The present invention controls P≤0.012% and S≤0.002%.

[0038] The present invention also provides a method for manufacturing the 450MPa grade thick specification pipeline steel suitable for hot delivery and hot loading, which comprises the following steps:

[0039] 1) Smelting and casting

[0040] Smelting and casting according to the above ingredients;

[0041] 2) Reheating

[0042] The charging temperature is 600~730℃, and the heating rate of the slab below 800℃ is controlled to be ≤1.5℃

[0043] / min, then heat the slab to the target reheating temperature of 1090-1150°C, with a total furnace time T≥0.7H1, where T is in min and H1 is the slab thickness in mm;

[0044] 3) Rolling

[0045] Rough rolling, the total reduction rate of rough rolling is ≥68%, the rolling temperature of the last two passes of rough rolling is controlled at 950-990°C, and the reduction rate of a single pass is ≥20%, and the thickness of the intermediate billet is controlled at 3.4-3.8H2, where H2 is the thickness of the finished steel plate;

[0046] Finish rolling, the final rolling temperature is 760~810℃;

[0047] 4) Cooling

[0048] Control the cooling rate to 10-17°C / s and the cooling stop temperature to 380-450°C.

[0049] Preferably, step 1) is performed by continuous casting, and in a horizontal section of continuous casting, the instantaneous temperature of the slab surface at the outlet of the horizontal section of continuous casting is controlled to be ≤680°C, and then the slab is naturally heated up.

[0050] In the manufacturing method of the present invention:

[0051] In step 1), continuous casting is preferred. In the horizontal section of continuous casting, the surface of the slab is quickly cooled by spraying water to achieve rapid cooling of the surface of the slab, thereby inhibiting the formation of a network-like proeutectoid ferrite along the austenite grain boundaries due to slow cooling on the surface of the slab, while reducing the precipitation of carbonitrides on the surface of the slab along the austenite grain boundaries, and controlling the instantaneous temperature of the slab surface at the outlet of the horizontal section of continuous casting to be ≤680°C, and then the slab is naturally heated up.

[0052] In step 2), after the continuous casting billet is cut, it is sent to the heating furnace as soon as possible for reheating of the billet, and the furnace temperature is 600-730°C. During the heating process, the heating rate of the billet needs to be controlled before the billet reaches the complete austenitizing temperature. The present invention controls the heating rate of the billet below 800°C to ≤1.5°C / min, and then heats the billet to the target reheating temperature of 1090-1150°C.

[0053] The present invention can reduce the precipitation of microalloying elements during the cooling process of the slab by adopting a higher hot charging temperature, so that more microalloying elements are in a solid solution state, which is conducive to more efficient use of the microalloying elements, especially Nb elements. In the subsequent rough rolling process, Nb in a solid solution state inhibits the growth of austenite recrystallization grains through solute drag, and can give full play to the role of grain refinement. In addition, during high-temperature hot charging, the temperature of the core of the slab is higher than that of the surface layer. In the heating furnace, the temperature uniformity of the slab in the thickness direction is better than that of the cold billet charging furnace, which can ensure that the microalloying elements in the core of the slab are effectively dissolved in the heating process, and is conducive to achieving temperature uniformity and uniformity of organizational properties in the thickness direction. However, there are some problems with high-temperature hot charging. When the charging temperature is in the two-phase region, a mixed structure of ferrite and austenite exists on the surface of the slab. During the heating process, ferrite transforms into austenite, and tensile stress is generated locally, which can easily lead to surface defects. Therefore, in order to avoid surface defects, the heating rate of the slab must be controlled before it is completely austenitized. In the present invention, the heating rate of the slab is controlled to be ≤1.5℃ / min below 800℃.

[0054] In addition, the composition of the present invention controls the alloy element combination 1.5Mo+Cr+Ni+6Nb≤0.90% to ensure that the invention steel has good thermoplasticity when the temperature is increased in the two-phase region to avoid surface cracks; at the same time, Ti / N is controlled to be ≥3.5, and Ti is used to fix N to inhibit the precipitation of AlN on the austenite / ferrite grain boundary on the surface of the slab, thereby reducing the surface cracking tendency of the hot-charged slab.

[0055] The present invention controls the reheating temperature to be 1090-1150°C. Grain refinement is an important means to obtain good full-thickness DWTT performance. In order to prevent the austenite grain size of the slab from being too large, the present invention adopts a lower reheating temperature as much as possible. In order to ensure sufficient solid solution of Nb, the reheating temperature should not be too low. At the same time, the total furnace time T is controlled to be ≥0.7H1, where T is in min and H1 is the slab thickness in mm, to ensure that the slab is fully heated.

[0056] In step 3), in the rough rolling stage, the total rough rolling reduction rate is controlled to be ≥68%, the rolling temperature of the last two rough rolling passes is controlled in the range of 950-990°C, and the single pass reduction rate is ≥20%, and the intermediate billet thickness is further controlled to be 3.4-3.8H2 (H2 is the thickness of the finished steel plate), and the finishing rolling temperature is 760-810°C.

[0057] Due to the presence of a mixed structure of ferrite + austenite locally in the slab during high-temperature hot loading, there are mixed crystals in the austenite grains after the slab is reheated and austenitized. It is necessary to use a larger reduction rate during the rough rolling process to achieve austenite grain refinement through sufficient recrystallization. Therefore, the total reduction rate of rough rolling is controlled to be ≥68%, and the lower the recrystallization temperature, the smaller the austenite grain size after recrystallization, but it cannot be lower than the critical recrystallization temperature. In addition, in order to ensure that the rolling deformation penetrates into the core of the slab, a higher single-pass reduction rate needs to be controlled. Therefore, the present invention controls the rolling temperature of the last two passes of rough rolling to be in the range of 950-990°C, and the single-pass reduction rate is ≥20%.

[0058] Furthermore, in step 3), the finishing rolling is carried out in the non-recrystallized zone. The greater the finishing rolling reduction rate, the more deformation bands of the deformed austenite, the greater the strain energy storage, and the more conducive to the subsequent phase deformation nucleus, thereby refining the grains. In order to take into account the rough rolling reduction rate and the finishing rolling reduction rate in an appropriate range, the present invention controls the intermediate billet thickness to 3.4-3.8H2 (H2 is the thickness of the finished steel plate). In addition, during the finishing rolling process, the deformed austenite will recover at high temperature and release the strain energy storage, so the lower the rolling temperature, the less recovery, the more phase deformation nucleus points, and it is also conducive to the deformation-induced precipitation of ferrite. The present invention controls the finishing rolling temperature to 760-810°C.

[0059] In step 4), the present invention controls the cooling rate to be within the range of 10-17°C / s, and controls the cooling stop temperature to be within the range of 380-450°C.

[0060] In order to solve the technical difficulties of full-thickness DWTT performance of thick-gauge pipeline steel, the present invention needs to fully refine the phase change structure of the core of the thick-gauge steel plate and improve the uniformity of the structure in the full-thickness direction. To this end, a low cooling rate process scheme is adopted to reduce the actual cooling rate difference between the surface and the core of the steel plate, extend the total water cooling time, and achieve effective cooling of the core of the steel plate. The present invention controls the cooling rate at 10-17°C / s; at the same time, due to the thick specifications, it is difficult to avoid a certain temperature difference between the core and the surface of the steel plate after stopping water cooling. The present invention adopts a lower stop cooling temperature to ensure that the core of the steel plate is fully cooled and the structure is refined, and to avoid the formation of pearlite and Mahō Island structures that are unfavorable to the full-thickness DWTT performance. However, the stop cooling temperature should not be too low, otherwise the surface of the steel plate is prone to form a low-temperature hard phase structure. Therefore, the stop cooling temperature is controlled at 380-450°C in the present invention.

[0061] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0062] 1. The 450MPa grade thick specification pipeline steel suitable for hot delivery and hot charging of the present invention adopts a lower C content, strengthens by Mo / Cr alloying and Nb microalloying fine grains, and adds Ni element to improve low temperature toughness. At the same time, the upper and lower limits of the element content combination of C+Mn / 6+(Cr+Mo) / 5 and 1.5Mo+Cr+Ni+6Nb are controlled to ensure that the steel of the present invention has good low temperature toughness while obtaining the desired strength and avoids the risk of surface defects caused by high temperature hot charging.

[0063] Furthermore, the 450MPa grade thick-gauge pipeline steel suitable for hot delivery and hot loading described in the present invention has a microstructure of fine polygonal ferrite + acicular ferrite, an average grain size of ≤7μm, and at the same time, the difference in average grain size between 1 / 4 and 1 / 2 of the steel plate thickness is ≤1μm.

[0064] 2. The 450MPa grade thick-gauge pipeline steel suitable for hot delivery and hot charging described in the present invention can effectively save energy consumption by adopting a higher hot charging temperature of 600-730°C, and can more efficiently play the role of micro-alloying elements. Moreover, during high-temperature hot charging, the temperature of the core of the slab is higher than that of the surface layer. In the heating furnace, the temperature uniformity in the thickness direction of the slab is better than that of cold billet charging, and it can ensure that the micro-alloying elements in the core of the slab are effectively dissolved during the heating process, which is conducive to achieving temperature uniformity in the thickness direction and uniformity of organizational properties. Conventional cold billet charging requires a longer insulation time to obtain good temperature uniformity in the thickness direction of the slab, and conventional hot charging processes, in order to avoid the risk of surface defects and reduced toughness caused by high-temperature hot charging, the hot charging temperature can usually only be controlled below 600°C, and the energy-saving effect is limited. In order to solve the potential quality problems caused by high-temperature hot charging, especially hot charging in the two-phase zone, the present invention solves the problem of surface defects by rapidly cooling the surface of the slab, controlling the upper limit of the heating rate before the slab is completely austenitized, controlling the upper limit of the alloy element combination 1.5Mo+Cr+Ni+6Nb in terms of composition, and controlling the Ti / N ratio and other technical solutions. In addition, during the rolling process, with the help of a higher solid solution Nb content, combined with the rough rolling temperature and reduction rate process design, the austenite is fully recrystallized and refined, and further through the cooling scheme of low cooling rate and low stop cooling, the core structure of the thick gauge steel plate is fully cooled and infiltrated, and the good full-thickness structure uniformity is achieved, and good full-thickness DWTT performance is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 This is a metallographic structure diagram of the pipeline steel of Example 3 of the present invention at 1 / 4 of the thickness of the steel plate under a 500x microscope;

[0066] Figure 2 This is a metallographic structure diagram of the pipeline steel in Example 3 of the present invention at a position of 1 / 2 of the thickness of the steel plate under a 500x microscope;

[0067] Figure 3 This is the full-thickness DWTT fracture morphology of the pipeline steel in Example 3 of the present invention. DETAILED DESCRIPTION

[0068] The present invention will be further described below in conjunction with embodiments and drawings.

[0069] The composition of the embodiment of the present invention is shown in Table 1, and the remainder is Fe and other inevitable impurities except P and S. Table 2 shows the specific process parameters of the pipeline steel manufacturing method of the embodiment of the present invention. Table 3 shows the mechanical property test results of the pipeline steel of the embodiment of the present invention.

[0070] It can be seen from Table 3 that the yield strength Rt of each embodiment of the present invention is 0.5 In the range of 450~570MPa, the tensile strength Rm is in the range of 535~655MPa, and the yield strength ratio Rt0.5 / Rm≤0.90, Charpy impact energy AKV≥250J at -20℃, full thickness DWTT fracture shear area rate SA%≥85% at -10℃, with excellent strength and toughness.

[0071] Figure 1 , Figure 2 Shown is the metallographic structure diagram of the 450MPa grade thick-gauge pipeline steel suitable for hot transport and hot loading in Example 3 at 1 / 4 and 1 / 2 of the thickness of the steel plate under a 500x microscope. The microstructure is fine polygonal ferrite + acicular ferrite. The average grain size at the 1 / 4 thickness position is 5.8μm as measured by EBSD, and the average grain size at the 1 / 2 thickness position is 6.3μm.

[0072] Figure 3 This is the full-thickness DWTT fracture morphology of the 450MPa grade thick-gauge pipeline steel suitable for hot delivery and hot loading in Example 3.

[0073] It should be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, and all should belong to the protection scope of the present invention.

[0074]

[0075]

[0076]

Claims

1. A 450MPa grade thick specification pipeline steel suitable for hot delivery and hot loading, the composition mass percentage of which is: C: 0.030-0.055%, Si: 0.10-0.30%, Mn: 1.45-1.64%, Cr: 0.16-0.30%, Mo: 0.05-0.12%, Ni: 0.10-0.20%, Nb: 0.040-0.049%, Ti: 0.008-0.020%, Al: 0.01-0.04%, Ca: 0.001-0.003%, N: 0.0010-0.0045%, P≤0.012%, S≤0.002%; the balance includes Fe and other unavoidable impurities, and needs to meet the following requirements at the same time: 0.34%≤C+Mn / 6+(Cr+Mo) / 5≤0.38%; 1.5Mo+Cr+Ni+6Nb≤0.90%; Ti / N ≥ 3.

5.

2. The 450MPa grade thick gauge pipeline steel suitable for hot delivery and hot loading as claimed in claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The 450MPa grade thick gauge pipeline steel suitable for hot delivery and hot loading as claimed in claim 1 or 2, characterized in that: The microstructure of the pipeline steel is fine polygonal ferrite + acicular ferrite, with an average grain size of ≤7 μm. Meanwhile, the difference in average grain size between 1 / 4 and 1 / 2 of the steel plate thickness is ≤1 μm.

4. The 450MPa grade thick gauge pipeline steel suitable for hot delivery and hot loading as claimed in claim 1, 2 or 3, characterized in that: The thickness of the pipeline steel is ≥28mm, and its yield strength Rt 0.5 The tensile strength Rm is 450-570MPa, the yield strength ratio Rt is 535-655MPa, 0.5 / Rm≤0.90; -20℃ Charpy impact energy AKV≥250J; -10℃ full thickness DWTT fracture shear area rate SA%≥85%.

5. The method for manufacturing 450MPa grade thick gauge pipeline steel suitable for hot delivery and hot loading as claimed in claim 1, 2, 3 or 4, characterized in that: The steps include: 1) Smelting and casting Smelting and casting according to the composition of claim 1 or 2; 2) Reheating The charging temperature is 600-730°C, the heating rate of the slab is controlled to be ≤1.5°C / min below 800°C, and then the slab is heated to the target reheating temperature of 1090-1150°C, and the total time in the furnace is T≥0.7H1, where T is in min and H1 is the slab thickness in mm; 3) Rolling Rough rolling, the total reduction rate of rough rolling is ≥68%, the rolling temperature of the last two passes of rough rolling is controlled at 950-990°C, and the reduction rate of a single pass is ≥20%, and the thickness of the intermediate billet is controlled at 3.4-3.8H2, where H2 is the thickness of the finished steel plate, in mm; Finish rolling, the final rolling temperature is 760~810℃; 4) Cooling Control the cooling rate to 10-17°C / s and the cooling stop temperature to 380-450°C.

6. The method for manufacturing 450MPa grade thick gauge pipeline steel suitable for hot delivery and hot loading as claimed in claim 5, characterized in that: Step 1) The casting is carried out by continuous casting. In the horizontal section of continuous casting, the instantaneous temperature of the slab surface at the outlet of the horizontal section of continuous casting is controlled to be ≤680°C, and then the slab is naturally heated up.

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