A 550mpa grade cryogenic steel with low brittle transition temperature and a method of manufacturing

Abstract

This invention belongs to the field of metallic materials technology, and provides a low-temperature steel with a brittle-to-ductile transition temperature of 550MPa and its manufacturing method. The composition design employs a low-carbon, low-manganese chemical system to reduce the segregation tendency of continuously cast billets in the thickness direction. Microalloying elements such as Nb, V-N, and Ti are added to refine the grain size, while alloying elements such as Ni, Cr, and Mo improve the hardenability of the steel and enhance the uniformity of properties in the thickness direction. Elements such as La, Ca, Mg, and Zr are used to improve the purity of the molten steel and the quality of the continuously cast billets, further enhancing the uniformity of properties in the thickness direction of the steel plate. The microstructure of the steel plate is polygonal ferrite + tempered sorbite. The yield strength of the low-temperature steel is 600~650MPa, the tensile strength is 700~750MPa, the elongation after fracture is ≥23.5%, and the impact energy at -85℃ is ≥175J; the full-thickness brittle-to-ductile transition temperature T0 is [not specified]. kb ≤-65℃; the steel plate has excellent mechanical properties and a low brittle transition temperature.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a 550MPa grade low-temperature steel with a low brittle-to-ductile transition temperature and its manufacturing method. Background Technology

[0002] Marine equipment must withstand harsh environments such as wind, waves, rain, snow, and low temperatures during its service life. In particular, marine equipment serving in polar waters will also be subject to continuous impacts such as sea ice. This requires the construction materials to have good low-temperature performance. As the most important material for building marine equipment, it is required to have key characteristics such as high strength and toughness and low brittle transition temperature.

[0003] Currently, the commonly used methods for testing brittle transition temperature are mainly FATT and NDTT. FATT is an impact test specimen with a size of 10×10×55mm, while NDTT is a drop hammer test specimen with a maximum thickness of only 25mm. Due to the limitation of specimen size, neither of these two methods can fully reflect the brittle transition temperature of thick steel plates. This invention is aimed at the production technology of thick steel plates and evaluates the brittle transition temperature of the steel plates across their entire thickness. The steel plates exhibit a low brittle transition temperature.

[0004] Patent document “A Production Method of 440MPa Grade Normalized Steel Plate for Ship Hulls” (application number: 202510119758.5) discloses a 440MPa grade normalized high-strength structural steel with the following chemical composition: C: 0.06%~0.10%, Si≤0.40%, Mn: 0.90%~1.20%, P≤0.015%, S≤0.005%, Ni: 0.90%~1.30%, Cr: 0.50%~0.65%, V: 0.030%~0.050%, Cu: 0.15%~0.35%, N≤0.0040%, with the remainder being Fe and unavoidable impurities. The production process involves smelting, ingot casting, rolling, rapid cooling, finishing, normalizing, and tempering. The produced steel plates have a thickness of 8-20mm, a yield strength ≥440MPa, tensile strength ≥600MPa, yield-to-tensile ratio ≤0.75, impact energy at -60℃ ≥200J, and a ductile-brittle transition temperature not exceeding -80℃. However, the maximum plate thickness is only 20mm, and the yield strength is only 440MPa, which does not meet the application requirements. Furthermore, the ductile-brittle transition temperature is FATT, which is only the surface transition temperature.

[0005] Patent document "A High-Strength Thick Steel Plate with Low Brittle-Temperature Transition Temperature for Storage Tanks and Its Preparation Method" (Application No.: 202510635514.2) discloses a high-strength thick steel plate with low brittle-temperature transition temperature for storage tanks. Its chemical composition is: C: 0.22%~0.26%, Si: 0.13%~0.144%, Mn: 0.73%~0.92%, P≤0.015%, S≤0.01%, Cr: 0.03%~0.049%, V: 0.01%~0.019%, Nd: 0.00016%~0.00047%, with the balance being Fe and impurities. The process includes smelting, four-stage slab heating, and three-stage controlled rolling. The steel plate has a yield strength ≥506MPa, tensile strength ≥675MPa, elongation after fracture ≥28.0%, a brittle-temperature transition temperature ≤-80℃, and a non-plastic transition temperature ≤-85℃. Its ductile-brittle transition temperature and non-plastic transition temperature are both determined by small surface samples, which cannot meet the requirements of polar marine equipment for the low-temperature toughness of steel plates.

[0006] Patent document "A 460MPa Grade Marine Engineering Steel with Excellent Fracture Resistance and Its Preparation Method" (Application No.: 202411023359.0) discloses a 460MPa grade marine engineering steel with excellent fracture resistance. Its chemical composition is: C: 0.05%~0.10%, Si: 0.15%~0.40%, Mn: 1.40%~2.00%, P≤0.008%, S≤0.002%, Cu: 0.15%~0.45%, Ni: 0.50%~1.00%, Cr: 0.15%~0.4%. 0%, Nb: 0.015%~0.05%, Ti: 0.007%~0.02%, Alt: 0.015%~0.055%, O≤0.001%, N≤0.005%, with the balance being Fe and unavoidable impurities. Its process includes smelting, continuous casting, primary rolling, primary cooling, secondary rolling, secondary cooling, and slow cooling. The process is complex and involves two heating cycles and two rolling cycles, resulting in a long production cycle and high cost. Furthermore, it does not evaluate the brittle transition temperature of the steel plate across its entire thickness, which cannot meet the requirements for use in polar and marine equipment.

[0007] Patent document "A High-Strength, High-Toughness Low-Temperature Marine Engineering Steel Plate and its Manufacturing Method" (application number: 202211610358.7) discloses a high-strength, high-toughness low-temperature marine engineering steel plate and its manufacturing method. Its chemical composition is: C: 0.085%~0.13%, Si: 0.16%~0.4%, Mn: 0.8%~1.4%, P≤0.02%, S≤0.01%, Als: 0.02%~0.04%, Ni: 3.2%~4.5%, Cr: 0.3%~0.7%, Mo: 0.3%~0.06%, Cu: 0.4%~0.7%, Nb: 0.01%~0.04%, V: 0.1%~0.25%, N: 0.003%~0.006%, with the remainder being Fe and unavoidable impurities. The steel plate production method includes smelting, casting, low-temperature heating, controlled rolling, controlled cooling, and low-temperature tempering. The steel plate has excellent strength and toughness, but its process includes a casting process, which will inevitably increase the production cycle of the steel plate. This patent can only produce steel plates with a maximum thickness of 60mm, which cannot meet the needs of large marine equipment. Furthermore, it does not evaluate the full-thickness brittle transition temperature of the steel plate, which cannot meet the construction requirements of polar equipment.

[0008] In summary, the following problems exist in the production of high-strength ship plates with low brittle transition temperatures.

[0009] 1) The steel plates are thin and have low strength, which cannot meet the construction requirements of high-end polar equipment. 2) Steel plate production process is complex, with long production cycle and high cost; 3) The brittle transition temperature of the steel plate is high throughout its thickness, which cannot meet the requirements of polar equipment. Summary of the Invention

[0010] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a 550MPa grade low-temperature steel with reasonable composition design, simple process, yield strength of 600~650MPa, tensile strength of 700~750MPa, elongation after fracture ≥23.5%, impact energy at -85℃ ≥175J, and full-thickness brittle transition temperature Tkb≤-65℃, as well as a manufacturing method thereof.

[0011] The objective of this invention is achieved as follows: A low-temperature steel with a low brittle-to-ductile transition temperature and a pressure of 550 MPa, wherein the elemental composition of the steel by mass percentage is as follows: C: 0.050%~0.150%, Si: 0.030%~0.500%, Mn: 0.60%~1.60%, P≤0.008%, S≤0.002%, Nb: 0.040%~0.100%, V: 0.050%~0.240%, N: 0.0250%~0.0380%, Cu: 0.50%~1.00%, Cr: 0.20 %~0.50%, Ni: 0.95%~1.95%, Mo: 0.20%~0.70%, Ti: 0.020%~0.050%, Als: 0.020%~0.050%, W: 0.010%~0.070%, La: 0.0100%~0.0700%, Ca: 0.0050%~0.0100%, Mg: 0.0030%~0.0055%, Zr: 0.0040%~0.0090%, balance Fe and unavoidable impurities.

[0012] Furthermore, the low-temperature steel has Ca / Mg ratio of 1.5 to 3.0 and Ni / (Cu+Cr) ratio of 1.2 to 1.9.

[0013] Furthermore, the microstructure of the low-temperature steel includes polygonal ferrite and tempered sorbite, wherein the volume percentage of each microstructure is: ferrite 15.0%~30.0%, tempered sorbite 70.0%~85.0%, and the average size of the precipitated phase is 30~50nm.

[0014] Furthermore, the low-temperature steel has a yield strength of 600~650MPa, a tensile strength of 700~750MPa, an elongation after fracture ≥23.5%, and an impact energy at -85℃ ≥175J; the full-thickness brittle transition temperature T0 kb ≤-65℃.

[0015] The rationale behind the ingredient design is as follows: C: An important strengthening element in steel, it forms interstitial solid solutions, effectively improving the strength of steel. It also forms carbide precipitates with microalloying elements, hindering grain growth. Low C content reduces carbide formation, affecting grain refinement during rolling. High C content increases cementite content, significantly raising the brittle transition temperature and negatively impacting the low-temperature toughness, plasticity, and weldability of the steel plate. Therefore, considering cost and performance factors, this invention controls the C content to be between 0.050% and 0.150%.

[0016] Si (Silicon): In the steelmaking process, silicon plays a deoxidizing role. Dissolving in the ferrite matrix, it effectively improves the strength of steel, inhibits temper brittleness, and does not reduce weldability. However, excessive silicon content can cause an increase in the ductile-brittle transition temperature, reduce the low-temperature toughness of the steel plate, and affect its surface quality. Considering factors such as cost and performance, this invention controls the Si content within the range of 0.030% to 0.500%.

[0017] Mn: An element that expands the austenite phase region, it can lower the temperature of the austenite-ferrite transformation, allowing steel to obtain more fine ferrite under the same cooling conditions, thereby improving the strength and low-temperature toughness of the steel. It forms a substitutional solid solution in steel, which can dissolve extensively in the Fe matrix, improving the strength of the steel through solid solution strengthening; it can delay the transformation of ferrite and pearlite in steel, significantly increasing the hardenability of the steel, lowering the ductile-brittle transition temperature, improving impact toughness, and eliminating the influence of S in the steel, thus improving the hot working properties of the steel. However, excessive Mn content easily leads to the formation of segregation and banded structures in the steel, which adversely affects both the plasticity and toughness of the steel, and easily causes temper brittleness, especially affecting the properties in the thickness direction of the steel plate. Considering all factors, this invention controls the Mn content to be in the range of 0.60%~1.60%.

[0018] Nitrogen (Nb): A grain-refining element. When heated, undissolved Nb carbon and nitride particles are distributed along the austenite grain boundaries, hindering austenite grain growth during heating. Upon cooling, it effectively delays the recrystallization of deformed austenite, preventing austenite grain growth and refining ferrite grains, thus improving the strength and toughness of the steel. However, if the Nb content is too high, Nb (CN) will precipitate in large, micron-sized forms or as a thin film at grain boundaries, becoming the origin of microcracks or a pathway for crack propagation along grain boundaries. Considering all factors, this invention controls the Nb content to be in the range of 0.040% to 0.100%.

[0019] Vanadium (V): A key strengthening and toughening element in this invention, it has a strong affinity for nitrogen and carbon, forming extremely stable carbonitrides. Its key characteristics include high solid solubility in austenite and a strong tendency to precipitate in ferrite, enhancing the strength of steel through grain refinement, precipitation strengthening, and solid solution strengthening. As the V content increases, the ductile-brittle transition temperature of the steel decreases. However, excessively high V content can actually increase the ductile-brittle transition temperature. Vanadium carbonitrides can precipitate at lower temperatures, hindering dislocation movement and thus strengthening and toughening the steel, significantly improving the overall mechanical properties of the steel plate. In the production of thick steel plates, by utilizing the cross-sectional temperature gradient, the differentiated precipitation of V phases across the cross-section is controlled, ensuring a low brittle-brittle transition temperature across the entire thickness of the steel plate. Considering all factors, this invention controls the V content within the range of 0.050% to 0.240%.

[0020] Nitrogen (N): Another important strengthening and toughening element in this invention. The addition of nitrogen promotes the formation of a large amount of v (CN) in the steel, thereby refining its grains and improving its plasticity and toughness. Nitrogen-containing steel not only eliminates the cost increase caused by degassing and refining denitrification during steelmaking, but also allows the addition of nitrogen to fully utilize the role of microalloying elements, saving the amount of alloying elements used, thus greatly reducing production costs. It can optimize the precipitation behavior of v in the steel cross-section, making its properties more uniform. In addition, the addition of nitrogen can replace the role of carbon, forming interstitial solid solutions in the steel to improve its strength. It also reduces the carbon equivalent of the steel without increasing the brittle transition temperature of the steel plate, promoting uniform property distribution. Considering both performance and cost, this invention controls the range of nitrogen to be 0.0250%~0.0380%.

[0021] Cu can improve the stability of austenite in steel, increase the hardenability of steel, and improve the strength, plasticity, and low-temperature toughness of steel when added in appropriate amounts. When dissolved in steel, it will increase the ductile-brittle transition temperature of steel. During low-temperature treatment, it can produce ε-Cu precipitation, thereby improving the strength of steel. However, if the content is too high, the hot brittleness of steel will deteriorate, and hot cracking will easily occur. The range of Cu content controlled in this invention is Cu: 0.50%~1.00%.

[0022] Cr: a solid solution element, which can improve the strength of steel without significantly reducing its toughness; it lowers the transformation temperature of austenite to ferrite and promotes the formation of finer ferrite structure during rolling, thus playing a positive role in reducing the brittle transition temperature of steel plates. However, if the content is too high, it will increase the temper brittleness of steel, thereby reducing the low-temperature toughness of steel. Taking all factors into consideration, the Cr content in this invention is controlled within the range of 0.20% to 0.50%.

[0023] Ni has no adverse effects on the hardening and toughness of the weld heat-affected zone of steel, and can improve the plasticity and low-temperature toughness of steel, significantly reducing the brittle transition temperature of steel. The addition of Ni can increase the solid solubility of Cu in steel, preventing Cu segregation at grain boundaries. In addition, the addition of Ni can reduce the tendency for hot cracking when the Cu content is high. Considering factors such as cost and performance, the range of Ni content controlled in this invention is 0.95%~1.95%.

[0024] Mo improves the hardenability of steel and reduces the differences in microstructure and properties caused by the different cooling rates between the surface and core of the steel plate. This is beneficial for improving the uniformity of properties across the entire thickness of the steel plate. Mo accumulates at austenite grain boundaries, lowering grain boundary energy and hindering the growth of austenite grains during hot working, thereby refining the grains and improving the strength and toughness of the steel. Taking all factors into consideration, the Mo content in this invention is controlled within the range of 0.20% to 0.70%.

[0025] Ti exhibits strong precipitation strengthening, increasing the strength of steel and preventing austenite recrystallization. Simultaneously, it refines grain size, improving the yield strength of the steel. The high formation temperature of Ti's C and N compounds, resulting in TiN and Ti(CN) during billet solidification, effectively inhibits austenite grain growth. Its high remelting temperature during heating further inhibits austenite grain growth, thus refining the grain size of the steel plate, improving its strength and toughness, lowering the brittle transition temperature, and promoting the overall mechanical properties of the steel plate. Considering all factors, this invention controls the Ti content within the range of 0.020% to 0.050%.

[0026] Al: A strong deoxidizer in steel. Adding a small amount can generate highly fine, ultra-microscopic oxides, which has a beneficial effect on improving the purity of steel and can also improve the uniformity of performance in the thickness direction of the steel plate. Taking all factors into consideration, the Al content in this invention is controlled within the range of 0.020% to 0.050%.

[0027] W forms refractory carbides in steel, reducing its thermal sensitivity, increasing hardenability and hardness, and improving its tempering stability, red hardness, and hot strength. W is a strong carbide-forming element in steel, delaying the transformation of austenite to ferrite and pearlite. Under appropriate cooling rates, it promotes the formation of acicular ferrite or lower bainite, thereby improving the uniformity of properties across the entire thickness of the steel plate. Taking all factors into consideration, the range of W controlled in this invention is 0.040%~0.080%.

[0028] La has excellent deoxidizing and desulfurizing effects, improves the fluidity of steel, reduces non-metallic inclusions, makes the steel structure denser and purer, improves the mechanical properties in the thickness direction of thick steel plates, especially low-temperature toughness, and has a beneficial effect on reducing the brittle transition temperature of steel. Taking all factors into consideration, the range of La controlled in this invention is 0.0100%~0.0700%.

[0029] Ca and Mg: Ca reacts with Al2O3 inclusions produced by deoxidation to form calcium aluminate compounds, which in turn react with MnS to form (Ca,Mn)S composite inclusions, transforming them into spherical inclusions. This improves the overall properties of the steel, especially the mechanical properties of thick steel plates. However, excessively high Ca content can easily lead to large, non-deformable inclusions, which become crack initiation sites and reduce the mechanical properties of the steel plate. Considering all factors, this invention controls the Ca content to be in the range of 0.0050% to 0.0100%. Mg has a strong affinity for oxygen and sulfur, forming fine precipitates. These precipitates are uniformly dispersed in the steel, avoiding stress concentration caused by large oxide particles remaining in the steel, thus improving the plasticity and toughness of the steel. Considering all factors, this invention controls the Mg content to be in the range of 0.0030% to 0.0055%. To better utilize the effects of Ca and Mg, the Ca / Mg ratio is controlled to be in the range of 1.5 to 3.0.

[0030] Zr: A strong carbide-forming element, its addition in small amounts has degassing, purifying, and grain-refining effects. It combines with N to form fine, dispersed ZrN particles, which can refine the grain size in the as-cast and hot-worked states, thus improving the low-temperature properties of steel and lowering the brittle transition temperature. However, excessive content can lead to the formation of large inclusions, which become crack initiations in the steel. Considering all factors, this invention controls the Zr content to be in the range of 0.0040% to 0.0090%.

[0031] The second technical solution of this invention provides a method for manufacturing 550MPa grade low-temperature steel with a low brittle-to-ductile transition temperature, including smelting, continuous casting, heating, rolling, cooling, and tempering. Smelting: including converter smelting, ladle refining, and RH treatment; Converter smelting: Adjust the content of elements such as C, Si, Mn, P, and S to bring them within the range of this invention, and control the basicity of the converter steel R = 3.00~4.00; RH treatment: The refined molten steel is subjected to RH treatment for 40-60 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 650-670 Pa to ensure that the final N content of the steel is within the range of this invention. The [H] content in the steel is controlled to be ≤1.5 ppm and [O] content to be ≤10 ppm. Before the end of smelting, Ca and Mg treatment is performed, and La and Zr elements are added.

[0032] Continuous casting: The tundish uses a high superheat, 40~65℃, with full-process protective casting. The casting speed is controlled at 1.00~1.70m / min, and the secondary cooling water ratio is 1.10~1.70m³. 3 / t, to ensure the proportion of columnar crystals formed in the continuously cast billet, the proportion of columnar crystals is ≥98.5% to reduce segregation in the core. Electromagnetic stirring is used in the later stage of continuous casting process, with a stirring current of 550~650A. At the end of continuous casting, a light reduction process is used, with a reduction of 10.0~20.0mm.

[0033] Preferably, in order to control the grain size of the continuously cast billet, a strong cooling process is adopted for the billet after continuous casting, with a cooling start temperature of 1000~1150℃ and a final cooling temperature of 450~600℃. Then, it is put into a slow cooling pit for slow cooling, with a slow cooling start temperature of 400~500℃ and a cooling rate of 10~20℃ / h, until the cooling ends at room temperature.

[0034] heating: The billet is heated to 1150℃~1250℃ in two stages. In the first stage, the heating temperature is below 600℃, and rapid heating is adopted to reduce the time spent in the furnace. The heating rate is 8.0~15.0℃ / min. In the second stage, the heating temperature is above 600℃, and slow heating is adopted to ensure the uniformity of billet heating. The heating rate is 6.0~12.0℃ / min, and the total time spent in the furnace is 8.0~13.0min / cm.

[0035] Rolling: The process includes three stages of rolling. The first stage, to fully break down the columnar crystals of the continuously cast billet and prepare for subsequent grain refinement, employs a high-temperature, high-speed rolling method with a large reduction. The billet is rolled directly after descaling, with a rolling speed of 2.0~3.0 m / s. The average reduction per pass in the first stage is ≥45 mm, and the final rolling temperature is 1050~1140℃. The thickness of the billet I awaiting heating is 3.5~4.0 times the finished product thickness. Preferably, to suppress grain growth in the intermediate billet and increase the temperature gradient between the billet surface and core, water spray cooling is used on the billet awaiting heating, with a cooling rate of 5.0~ The first stage involves a rolling temperature of 15.0℃ / s and a cooling time of 10.0~20.0s, ensuring a temperature difference of >100℃ between the surface and core of the billet. The second stage begins with a rolling temperature of 900~950℃, ensuring a reduction rate of over 15.5% per pass to achieve complete recrystallization of the steel plate. The final rolling temperature is 820~870℃, and the thickness of the billet (II) is 1.50~2.00 times the thickness of the finished product. The third stage employs a low-temperature, high-reduction process to fully deform the grains and further reduce the grain size. The initial rolling temperature is 750~800℃, the reduction rate is 16.0~20.0%, and the final rolling temperature is 700~740℃.

[0036] cool down: To maintain fine grains after rolling and prevent grain growth, the rolled steel plate is cooled using UFC at a rate of 7.0~14.0℃ / s until it reaches below 200℃.

[0037] Tempering: In order to release the internal stress formed during the rolling and cooling process of steel plates and to further form fine precipitates, the cooled steel plates are tempered at a temperature of 600~700℃ and a holding time of 3.00~5.00min / mm.

[0038] The technical advantages of this invention are as follows: (1) The present invention adopts a low carbon and low manganese chemical composition system to reduce the segregation tendency of the continuous casting billet in the thickness direction, and adds micro-alloying elements such as Nb, VN, and Ti to refine the grains, and alloying elements such as Ni, Cr, and Mo to improve the hardenability of the steel and improve the uniformity of the performance in the thickness direction. Furthermore, the purity of the molten steel and the quality of the continuous casting billet are improved by elements such as La, Ca, Mg, and Zr, which further enhances the uniformity of the performance in the thickness direction of the steel plate.

[0039] (2) In the smelting process, a high-yield addition process is adopted for elements such as La, Ca, Mg and Zr. In the continuous casting process, a process of high superheat + strong secondary cooling water + electromagnetic stirring + light pressure is adopted to control the uniformity of the billet and obtain fine initial austenite grains. The continuous casting billet adopts a process of strong cooling + slow cooling pit slow cooling to control grain growth and regulate the precipitated phase of the billet.

[0040] (3) The rolling process adopts the TMCP process based on three-stage controlled rolling. In the first stage, the intermediate billet is heated by an accelerated cooling process to reduce the heating time, prevent grain growth, and form a larger temperature gradient on the billet cross section, so that the rolling deformation can better penetrate into the core of the steel plate. In the second stage, a deep recrystallization controlled rolling process with a large reduction rate is adopted to promote grain refinement. In the third stage, a low-temperature rolling process is adopted to further refine the grain size.

[0041] (4) The rolled steel plate is subjected to UFC cooling and tempering treatment. The thickness of the steel plate is ≥60mm. The microstructure of the steel plate includes polygonal ferrite and tempered sorbite. The volume percentage of each microstructure is: ferrite 15.0%~30.0%, tempered sorbite 70.0%~85.0%, precipitate size 30~50nm, and 1 / 4t precipitate quantity 10~20μm. -2 The number of precipitated phases in 1 / 2t is 30~40μm. -2 The yield strength of low-temperature steel is 600~650MPa, tensile strength is 700~750MPa, elongation after fracture is ≥23.5%, and impact energy at -85℃ is ≥175J; the brittle-brittle transition temperature at full thickness is T0. kb ≤-65℃; the steel plate has excellent mechanical properties and a low brittle transition temperature. Detailed Implementation

[0042] The present invention will be further illustrated below through examples.

[0043] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, heating, rolling, cooling and tempering.

[0044] heating: The billet is heated in two stages to 1150℃~1250℃. In the first stage, when the heating temperature is below 600℃, a rapid heating method is used with a heating rate of 8.0~15.0℃ / min. In the second stage, when the heating temperature is above 600℃, a slow heating method is used with a heating rate of 6.0~12.0℃ / min. The total time in the furnace is 8.0~13.0min / cm. Rolling: The process includes three stages of rolling. The first stage uses a high-temperature, high-speed rolling process with large reduction. After descaling, the cast billet is rolled directly at a speed of 2.0~3.0 m / s, with an average reduction of ≥45 mm per pass and a final rolling temperature of 1050~1140℃. The thickness of the billet I waiting to be heated is 3.5~4.0 times the thickness of the finished product. The second stage starts rolling at 900~950℃, with a reduction rate of ≥15.5% per pass and a final rolling temperature of 820~870℃. The thickness of the billet II waiting to be heated is 1.50~2.00 times the thickness of the finished product. The third stage uses a low-temperature, high-reduction process, starting at 750~800℃, with a reduction rate of 16.0~20.0% per pass and a final rolling temperature of 700~740℃. cool down: The rolled steel plate is cooled using UFC at a rate of 7.0~14.0℃ / s until it reaches below 200℃. Tempering: The cooled steel plate is then tempered at a temperature of 600-700℃ for a holding time of 3.00-5.00 min / mm.

[0045] Furthermore, after the first stage of rolling, the billet to be heated is cooled by water spraying at a rate of 5.0~15.0℃ / s and a cooling time of 10.0~20.0s, with a temperature difference of >100℃ between the surface and core of the continuously cast billet.

[0046] Further; smelting: including converter smelting, ladle refining, and RH treatment; Converter smelting: Basicity of molten steel in converter R = 3.00~4.00; RH treatment: The refined molten steel is subjected to RH treatment for 40~60 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 650~670Pa. The [H] in the steel is controlled to be ≤1.5ppm and [O] to be ≤10ppm. Before the end of smelting, Ca and Mg are treated and La and Zr elements are added.

[0047] Further; continuous casting: The tundish uses a high superheat, 40~65℃, with full-process protective casting. The casting speed is 1.00~1.70m / min, and the secondary cooling water volume is 1.10~1.70m³. 3 / t, to ensure the proportion of columnar crystals formed in the continuously cast billet, the proportion of columnar crystals is ≥98.5% to reduce segregation in the core. Electromagnetic stirring is used in the later stage of continuous casting process, with a stirring current of 550~650A. At the end of continuous casting, a light reduction process is used, with a reduction of 10.0~20.0mm.

[0048] Furthermore, after continuous casting, the billet is subjected to a strong cooling process with a starting cooling temperature of 1000~1150℃ and a final cooling temperature of 450~600℃. It is then placed in a slow cooling pit for slow cooling with a starting temperature of 400~500℃ and a cooling rate of 10~20℃ / h until it reaches room temperature.

[0049] The composition of the steel in this embodiment is shown in Table 1. The smelting and continuous casting process of the steel in this embodiment is shown in Table 2. The main process parameters for cooling the steel billet in this embodiment are shown in Table 3. The main process parameters for heating the steel billet in this embodiment are shown in Table 4. The main process parameters for the first stage rolling of the steel in this embodiment are shown in Table 5. The main process parameters for the second stage rolling of the steel in this embodiment are shown in Table 6. The main process parameters for the third stage rolling and tempering of the steel in this embodiment are shown in Table 7. The microstructure of the steel in this embodiment is shown in Table 8. The mechanical properties of the steel in this embodiment are shown in Table 9. The brittle-brittle transition temperature of the steel at full thickness is shown in Table 10.

[0050] Table 1. Composition (wt%) of steel in embodiments of the present invention C 0.058 0.092 0.138 0.063 0.107 0.143 0.077 0.114 0.086 0.126 0.053 Si 0.492 0.368 0.046 0.433 0.205 0.034 0.103 0.136 0.327 0.094 0.057 Mn 1.56 1.11 0.77 1.44 1.07 0.63 1.38 0.98 1.23 0.84 1.32 P 0.005 0.007 0.004 0.008 0.006 0.005 0.007 0.006 0.005 0.006 0.007 S 0.002 0.001 0.002 0.001 0.001 0.002 0.001 0.002 0.001 0.002 0.001 Nb 0.042 0.073 0.099 0.048 0.081 0.054 0.088 0.061 0.092 0.067 0.058 V 0.236 0.161 0.104 0.217 0.142 0.084 0.198 0.123 0.063 0.179 0.221 N 0.0377 0.0316 0.0266 0.0368 0.0273 0.0307 0.0354 0.0292 0.0332 0.0258 0.0283 Cu 0.67 0.82 0.56 0.87 0.52 0.96 0.62 0.91 0.73 0.79 0.54 Cr 0.32 0.44 0.36 0.29 0.23 0.47 0.39 0.26 0.41 0.49 0.31 Ni 1.33 1.64 1.14 1.76 0.98 1.93 1.28 1.84 1.42 1.56 1.09 Mo 0.41 0.64 0.24 0.47 0.67 0.28 0.52 0.33 0.58 0.36 0.26 Ti 0.024 0.037 0.047 0.027 0.041 0.021 0.031 0.043 0.034 0.033 0.036 Als 0.032 0.043 0.036 0.046 0.022 0.049 0.026 0.038 0.029 0.039 0.037 W 0.011 0.038 0.066 0.017 0.044 0.023 0.051 0.029 0.063 0.032 0.047 La 0.0337 0.0556 0.0123 0.0379 0.0174 0.0628 0.0428 0.0248 0.0473 0.0286 0.0168 Ca 0.0073 0.0092 0.0059 0.0052 0.0066 0.0061 0.0078 0.0083 0.0087 0.0097 0.0094 Mg 0.0043 0.0052 0.0036 0.0031 0.0041 0.0034 0.0049 0.0041 0.0038 0.0044 0.0037 Zr 0.0062 0.0044 0.0068 0.0082 0.0048 0.0073 0.0088 0.0051 0.0077 0.0056 0.0054 Ca / Mg 1.70 1.77 1.64 1.68 1.61 1.79 1.59 2.02 2.23 2.20 2.54 Ni / (Cu+Cr) 1.34 1.30 1.24 1.52 1.31 1.35 1.27 1.57 1.25 1.22 1.28 Table 2 Steel smelting and continuous casting processes in embodiments of the present invention 1 3.06 57 666 41 1.38 1.42 98.6 572 14.6 2 3.46 46 669 64 1.02 1.13 98.5 617 18.7 3 3.83 59 656 56 1.23 1.48 98.8 554 19.3 4 3.14 44 664 43 1.44 1.56 98.9 608 10.3 5 3.52 48 667 62 1.09 1.19 98.6 628 12.7 6 3.96 42 651 54 1.54 1.63 98.7 593 13.8 7 3.27 43 657 58 1.16 1.24 98.6 643 15.3 8 3.64 56 662 47 1.62 1.67 98.7 587 11.8 9 3.33 53 653 49 1.68 1.31 98.9 631 17.3 10 3.72 49 658 51 1.31 1.36 99.0 564 16.4 11 3.18 47 659 44 1.56 1.27 98.8 558 10.8 Table 3 Main process parameters for cooling steel billets in embodiments of the present invention. 1 1013 459 404 14 2 1146 597 452 16 3 1088 476 418 19 4 1039 539 496 17 5 1104 563 467 13 6 1024 487 429 18 7 1077 553 478 12 8 1048 506 437 11 9 1124 586 486 16 10 1069 524 444 14 11 1056 518 447 12 Table 4 Main process parameters for heating steel billets in embodiments of the present invention 1 1154 8.3 11.6 9.6 2 1178 10.3 9.4 10.6 3 1194 14.2 6.7 8.3 4 1159 13.8 7.3 11.3 5 1182 8.6 11.1 12.7 6 1196 14.6 6.2 8.8 7 1163 13.3 7.6 11.6 8 1169 9.2 10.8 9.1 9 1172 12.6 8.8 12.2 10 1188 9.7 10.4 10.4 11 1166 10.6 9.1 10.7 Table 5. Main process parameters for the first stage rolling of steel in the embodiments of the present invention. 1 65 48 2.14 1058 3.8t 13.4 11.2 2 70 51 2.63 1106 3.6t 12.2 12.3 3 85 46 2.27 1092 3.8t 7.2 17.4 4 60 47 2.74 1064 3.9t 14.1 10.3 5 90 49 2.36 1118 3.6t 5.3 19.2 6 85 53 2.82 1139 3.6t 6.1 18.3 7 80 46 2.09 1074 3.7t 8.3 16.8 8 75 52 2.42 1124 3.8t 11.4 13.2 9 80 52 2.94 1089 3.7t 9.2 15.3 10 75 51 2.58 1133 3.7t 10.1 14.6 11 85 48 2.26 1079 3.6t 8.2 16.7 Note: t: thickness of the final product Table 6 Main process parameters for the second-stage rolling of steel in the embodiments of the present invention. 1 901 15.8 821 1.73t 2 921 17.2 844 1.52t 3 928 16.2 848 1.77t 4 908 17.4 828 1.91t 5 942 17.1 861 1.58t 6 948 15.9 866 1.84t 7 913 16.7 832 1.63t 8 934 16.8 853 1.88t 9 916 15.9 837 1.66t 10 937 17.3 859 1.96t 11 936 16.4 824 1.64t Table 7 Main process parameters for the third stage rolling and tempering of steel in the embodiments of the present invention. 1 754 18.7 703 10.1 651 3.82 2 771 16.4 723 12.8 607 4.89 3 776 19.2 728 7.2 666 3.63 4 759 16.8 706 13.6 626 4.47 5 792 19.7 726 7.9 673 3.47 6 798 17.2 737 10.8 613 4.66 7 764 16.6 712 8.6 684 3.26 8 784 17.6 731 11.4 648 4.03 9 768 17.8 717 9.3 696 3.03 10 788 18.1 739 12.1 636 4.28 11 756 18.3 704 7.4 658 3.19 The microstructure of the steel in the embodiments of the present invention was examined, and the results are shown in Table 8. The mechanical properties of the steel in the embodiments of the present invention were examined, and the results are shown in Table 9.

[0051] Table 8 Microstructure of steel in embodiments of the present invention 1 22.4 77.6 36 12 30 2 23.3 76.7 31 14 32 3 18.8 81.2 42 11 33 4 26.4 73.6 47 13 31 5 19.3 80.7 34 16 31 6 27.7 72.3 41 12 32 7 24.3 75.7 43 11 34 8 21.6 78.4 48 10 33 9 29.8 70.2 38 13 31 10 25.6 74.4 45 12 34 11 29.7 70.3 48 12 30 Table 9 Mechanical properties of steel in embodiments of the present invention 1-1 / 4t-Transverse 628 732 27 213 198 1-1 / 4t-Longitudinal 618 720 27 227 210 1-1 / 2t-Horizontal 609 713 26.5 201 181 1-1 / 2t-Longitudinal 603 710 27 207 188 2-1 / 4t-Transverse 628 728 26 224 204 2-1 / 4t-Longitudinal 624 721 26.5 235 213 2-1 / 2t-Horizontal 617 708 24 207 191 2-1 / 2t-Longitudinal 605 702 25 212 186 3-1 / 4t-Transverse 644 741 24 225 200 3-1 / 4t-Longitudinal 637 733 25 230 211 3-1 / 2t-Horizontal 625 732 23.5 209 191 3-1 / 2t-Longitudinal 611 721 23.5 202 198 4-1 / 4t-Transverse 635 736 26.5 219 185 4-1 / 4t-Longitudinal 631 727 27 224 200 4-1 / 2t-Horizontal 629 725 27 207 186 4-1 / 2t-Longitudinal 612 726 27 219 198 5-1 / 4t-Transverse 625 738 27.5 209 183 5-1 / 4t-Longitudinal 612 722 27.5 214 198 5-1 / 2t-Horizontal 619 713 26.5 200 181 5-1 / 2t-Longitudinal 610 700 27 209 186 6-1 / 4t-Transverse 626 729 29 219 201 6-1 / 4t-Longitudinal 604 725 29 232 215 6-1 / 2t-Transverse 610 728 27.5 210 190 6-1 / 2t-Longitudinal 602 724 28 225 201 7-1 / 4t-Transverse 629 730 27 225 206 7-1 / 4t-Longitudinal 612 711 27.5 240 221 7-1 / 2t-Horizontal 615 726 27 217 201 7-1 / 2t-Longitudinal 607 707 27.5 220 208 8-1 / 4t-Transverse 638 741 29 215 191 8-1 / 4t-Longitudinal 626 733 29.5 227 196 8-1 / 2t-Transverse 617 728 27.5 203 181 8-1 / 2t-Longitudinal 610 718 28 210 190 9-1 / 4t-Transverse 620 729 26 217 198 9-1 / 4t-Longitudinal 613 727 27.5 220 210 9-1 / 2t-Horizontal 609 719 26.5 209 201 9-1 / 2t-Longitudinal 604 700 27 213 210 10-1 / 4t-Horizontal 630 740 27 210 189 10-1 / 4t-Longitudinal 619 733 27.5 218 201 10-1 / 2t-Horizontal 612 729 26.5 202 178 10-1 / 2t-Longitudinal 608 724 27.5 209 190 11-1 / 4t-Transverse 601 712 28 215 184 11-1 / 4t-Longitudinal 589 704 28.5 216 199 11-1 / 2t-Horizontal 582 702 27.5 204 178 11-1 / 2t-Longitudinal 578 700 28 210 193 The full-thickness brittle transition temperature (Tkb) of the steel of the present invention was determined by means of bending the full-thickness sample at different temperatures, measuring the proportion of fibrous fracture surface of the bent sample after fracture, and the temperature corresponding to 70% fibrous fracture surface is the full-thickness brittle transition temperature of the steel plate.

[0052] Table 10 Brittle-to-brittle transition temperatures of the steel of the present invention (full thickness) 1 -65 2 -70 3 -65 4 -65 5 -70 6 -65 7 -65 8 -65 9 -65 10 -70 11 -65 The low-temperature steel produced using this invention has a microstructure of polygonal ferrite + tempered sorbite, wherein the volume percentages of each microstructure are: ferrite 15.0%~30.0%, tempered sorbite 70.0%~85.0%, and the average size of the precipitated phases is 30~50 nm. It exhibits a yield strength of 600~650 MPa, a tensile strength of 700~750 MPa, an elongation after fracture ≥23.5%, and an impact energy at -85℃ ≥175 J; the brittle-to-ductile transition temperature Tkb ≤ -65℃, demonstrating excellent mechanical properties and a low brittle-to-ductile transition temperature.

[0053] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A low-brittle transition temperature 550MPa grade low-temperature steel, characterized in that, The elemental composition of the steel, by mass percentage, is as follows: C: 0.05%~0.15%, Si: 0.03%~0.50%, Mn: 0.60%~1.60%, P≤0.008%, S≤0.002%, Nb: 0.04%~0.10%, V: 0.05%~0.24%, N: 0.025%~0.038%, Cu: 0.50%~1.00%, Cr: 0.20%~0.50%, Ni: 0.95%~1.95%, Mo... 0.20%~0.70%, Ti: 0.02%~0.05%, Als: 0.02%~0.05%, W: 0.01%~0.07%, La: 0.01%~0.07%, Ca: 0.005%~0.010%, Mg: 0.0030%~0.0055%, Zr: 0.004%~0.009%, Ca / Mg = 1.5~3.0, Ni / (Cu+Cr) = 1.2~1.9; the balance is Fe and unavoidable impurities. The manufacturing method of a 550MPa grade low-temperature steel with a low brittle transition temperature includes smelting, continuous casting, heating, rolling, cooling, and tempering. Continuous casting: The tundish uses a high superheat, 40~65℃, with full-process protective casting. The casting speed is 1.0~1.7m / min, and the secondary cooling water volume is 1.1~1.7m³ / min. 3 / t, columnar crystal ratio ≥98.5%, electromagnetic stirring is used in the later stage of continuous casting process, stirring current 550~650A, and a light reduction process is used at the end of continuous casting, with a reduction amount of 10~20mm. After continuous casting, the billet is subjected to a strong cooling process with an initial cooling temperature of 1000~1150℃ and a final cooling temperature of 450~600℃. Then it is placed in a slow cooling pit for slow cooling with an initial cooling temperature of 400~500℃ and a cooling rate of 10~20℃ / h until it reaches room temperature. heating: The billet is heated in two stages to 1150℃~1250℃. In the first stage, when the heating temperature is below 600℃, a rapid heating method is used with a heating rate of 8.0~15.0℃ / min. In the second stage, when the heating temperature is above 600℃, a slow heating method is used with a heating rate of 6.0~12.0℃ / min. The total time in the furnace is 8.0~13.0min / cm. Rolling: The process includes three stages of rolling. The first stage uses a high-temperature, high-speed rolling method with large reduction. After descaling, the cast billet is rolled directly at a speed of 2.0-3.0 m / s, with an average reduction of ≥45 mm per pass and a final rolling temperature of 1050-1140℃. The thickness of the billet I waiting to be heated is 3.5-4.0 times the finished product thickness. After the first stage rolling, the billet is cooled by water spray at a rate of 5-15℃ / s for 10-20 seconds, with a temperature difference of >100℃ between the surface and core of the continuously cast billet. The second stage starts rolling at 900-950℃ with a reduction of ≥15.5% per pass and a final rolling temperature of 820-870℃. The thickness of the billet II waiting to be heated is 1.5-2.0 times the finished product thickness. The third stage uses a low-temperature, large-reduction process, starting at 750-800℃ with a reduction of 16%-20% per pass and a final rolling temperature of 700-740℃. cool down: The rolled steel plate is cooled by UFC at a rate of 7~14℃ / s until it reaches below 200℃. Tempering: The cooled steel plate is then tempered at a temperature of 600-700℃ for 3-5 minutes per mm.

2. The 550MPa grade low-temperature steel with a low brittle-to-ductile transition temperature according to claim 1, characterized in that, The microstructure of low-temperature steel includes polygonal ferrite and tempered sorbite. The volume percentages of each microstructure are: ferrite 15%~30%, tempered sorbite 70%~85%, and the average size of the precipitated phases is 30~50nm.

3. The 550MPa grade low-temperature steel with a low brittle-to-ductile transition temperature according to claim 1, characterized in that, The low-temperature steel has a tensile strength of 700~750MPa, an elongation after fracture ≥23.5%, and an impact energy of -85℃ ≥175J; the full-thickness brittle transition temperature T0 kb ≤-65℃.

4. The method for manufacturing a 550MPa grade low-temperature steel with a low brittle-to-ductile transition temperature according to claim 1, characterized in that, Smelting: including converter smelting, ladle refining, and RH treatment; Converter smelting: Basicity of molten steel in converter R = 3.00~4.00; RH treatment: The refined molten steel is subjected to RH treatment for 40~60 minutes. Nitrogen is blown throughout the RH treatment process at a pressure of 650~670Pa. The [H] in the steel is controlled to be ≤1.5ppm and [O] to be ≤10ppm. Before the end of smelting, Ca and Mg are treated and La and Zr elements are added.