A 460mpa grade cryogenic steel with low brittle transition temperature and a method of manufacturing

Abstract

The application belongs to the technical field of metal materials, and provides a 460MPa-grade low-temperature steel with low brittle transition temperature and a manufacturing method.The low-temperature steel adopts a chemical composition system with low carbon and low manganese, reduces the segregation tendency of the continuous casting billet in the thickness direction, adds micro-alloy elements such as Nb, V-N and Ti to refine the grains, and adds elements such as La, Ca, Mg and Zr to improve the performance uniformity of the steel plate in the thickness direction.The manufacturing method comprises smelting, continuous casting, heating, rolling, cooling and tempering.The microstructure of the steel plate produced by the application comprises polygonal ferrite, tempered sorbite and dispersedly distributed pearlite, and the volume content percentage of each microstructure is as follows: ferrite 45.0%~60.0%, pearlite 5.0%~13.0%, and tempered sorbite 35.0%~41.0%, the size of the precipitated phase is 30~50nm, and the steel plate has excellent mechanical properties and low brittle transition temperature.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a 460MPa 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. Especially for marine equipment operating in polar waters, it is subject to continuous impacts from sea ice, requiring construction materials with excellent low-temperature performance. As the most important material for constructing marine equipment, steel plates are required to possess key characteristics such as high strength and toughness, and a low brittle-to-ductile 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 range in thickness from 8 to 20 mm, with a yield strength ≥440 MPa, tensile strength ≥600 MPa, yield-to-tensile ratio ≤0.75, impact energy at ~60℃ ≥200 J, and a ductile-brittle transition temperature not exceeding ~80℃. However, the maximum plate thickness is only 20 mm, and the yield strength is only 440 MPa, 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] In summary, the following problems exist in the production of high-strength ship plates with low brittle transition temperatures.

[0008] 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

[0009] The purpose of this invention is to overcome the above-mentioned problems and shortcomings and provide a 460MPa grade low-temperature steel with reasonable composition design, simple process, and low brittle transition temperature throughout the thickness, as well as a manufacturing method thereof.

[0010] The objective of this invention is achieved as follows: A 460MPa grade low-temperature steel with a low brittle-to-ductile transition temperature, the elemental composition of which is as follows by mass percentage: C: 0.040%~0.120%, Si: 0.030%~0.350%, Mn: 0.80%~1.40%, P≤0.008%, S≤0.002%, Nb: 0.050%~0.100%, V: 0.040%~0.090%, N: 0.0200%~0.0350%, Cu: 0.40%~0.60%, Cr: 0.10%~0.40%, Ni: 0.50%~1.50%, Ti: 0.010%~0.045%, Als: 0.015%~0.040%, La: 0.0150%~0.0850%, Ca: 0.0030%~0.0070%, Mg: 0.002%~0.005%, Zr: 0.0050%~0.0100%, balance Fe and unavoidable impurities.

[0011] Furthermore, the low-temperature steel has Ca / Mg ratio of 1.1 to 1.5 and Ni / (Cu+Cr) ratio of 1.0 to 1.8.

[0012] Furthermore, the microstructure of the low-temperature steel includes ferrite, pearlite, and tempered sorbite, with the following volume percentages: ferrite 45.0%~60.0%, pearlite 5.0%~13.0%, tempered sorbite 35.0%~41.0%, and the average size of the precipitated phases is 30~50 nm.

[0013] Furthermore; the low-temperature steel has a yield strength of 500~550MPa, a tensile strength of 600~650MPa, an elongation after fracture ≥25.0%, and an impact energy at -85℃ ≥200J; the brittle transition temperature T of the steel plate throughout its thickness is... kb ≤-65℃.

[0014] 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 0.040% and 0.120%.

[0015] Silicon (Si) plays a deoxidizing role in steelmaking. Dissolving in the ferrite matrix, it effectively improves the strength of steel, inhibits temper brittleness, and does not reduce weldability. However, excessive Si content can raise 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 to be between 0.030% and 0.350%.

[0016] 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, excessively high Mn content easily leads to the formation of segregation and banded structures in the steel, adversely affecting both the plasticity and toughness of the steel, and easily causing 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 0.80%~1.40%.

[0017] 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 0.050% to 0.100%.

[0018] 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 its overall mechanical properties. In the production of thick steel plates, by utilizing the cross-sectional temperature gradient, the differentiated precipitation of V phases across the cross-section ensures 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 0.040% to 0.090%.

[0019] 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 0.0200%~0.0350%.

[0020] 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. In this invention, the Cu content is controlled within the range of 0.40% to 0.60%.

[0021] Cr: a solid solution element that 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 structures during rolling, thus playing a positive role in reducing the brittle transition temperature of steel plates. However, excessive content can increase the temper brittleness of steel, thereby reducing its low-temperature toughness. Taking all factors into consideration, this invention controls the Cr content to be in the range of 0.10% to 0.40%.

[0022] 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 and prevent Cu segregation at grain boundaries. Taking into account factors such as cost and performance, the range of Ni controlled in this invention is Ni: 0.50%~1.50%.

[0023] 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 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 0.010% to 0.045%.

[0024] Al: A strong deoxidizer in steel. Adding a small amount can generate highly fine, ultramicroscopic 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 0.015% to 0.040%.

[0025] 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 0.0150%~0.0850%.

[0026] 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 0.0030% to 0.0070%. 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 0.0020% to 0.0050%. To better utilize the effects of Ca and Mg, the Ca / Mg ratio is controlled to be in the range of 1.1 to 1.5.

[0027] 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 initiation sites in the steel. Considering all factors, this invention controls the Zr content to be in the range of 0.0050% to 0.0100%.

[0028] The second technical solution of the present invention is to provide a method for manufacturing 460MPa grade low-temperature steel with a low brittle transition temperature, including smelting, continuous casting, heating, rolling, cooling, and tempering. Smelting: including converter smelting, ladle refining, and RH treatment; a) Converter smelting: Control the basicity of the molten steel in the converter to R=3.00~4.00.

[0029] b) RH treatment: RH treatment time is 40~60min. Nitrogen is blown throughout the RH treatment process at a pressure of 650~670Pa to ensure that the final N content of the steel is within the range of this invention. [H] in the steel is controlled to be ≤1.5ppm and [O] to be ≤10ppm. Ca and Mg treatment is carried out before the end of smelting, and La and Zr elements are added. Continuous casting: The tundish uses a high superheat, 40~65℃, with full-process protective casting. The casting speed is controlled at 0.90~1.60m / min, and the secondary cooling water ratio is 0.90~1.50m³. 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 500~600A. At the end of continuous casting, a light reduction process is used, with a reduction of 10.0~20.0mm. Preferably, in order to control the grain size of the continuously cast billet, a strong cooling process is used on the billet after continuous casting, with a cooling start temperature of 1000~1150℃ and a final cooling temperature of 500~650℃. Then, slow cooling is carried out by stacking, with a stacking start temperature of 440~490℃ and a slow cooling time of ≥24h. heating: The billet is heated in two stages to 1100℃~1250℃. In the first stage, below 600℃, rapid heating is used to reduce the time spent in the furnace, with a heating rate of 10.0~20.0℃ / min. Above 600℃, slow heating is used to ensure the uniformity of billet heating, with a heating rate of 5.0~15.0℃ / min. The total time spent in the furnace is 4.0~6.0h.

[0030] Rolling: A two-stage rolling process is adopted. The first stage rolling is carried out by high-temperature fast rolling + large reduction process in order to fully break the columnar crystals of the continuously cast billet and prepare for subsequent grain refinement. After the billet is descaled after leaving the furnace, it is heated to 1000~1100℃ before rolling. The roll speed is controlled at 2.00~3.00m / s, and the average reduction of the longitudinal rolling pass after the continuously cast billet is widened is controlled at 45~55mm. The final rolling temperature is 900~950℃, and the thickness of the billet at the waiting temperature is (2.0~3.5)t, where t is the final product thickness. During the waiting temperature period, the billet is accelerated to promote the temperature gradient of the billet cross section. The cooling rate is 5.0~10.0℃ / s, and the cooling time is 10.0~20.0s, so that the temperature difference of the billet cross section is ≥100℃. The second stage rolling starts at a rolling temperature of 770~820℃ and ends at a rolling temperature of 690~740℃.

[0031] cool down: To maintain fine grains after rolling and prevent grain growth, the rolled steel plate is cooled using UFC at a rate of 8.0~15.0℃ / s until it reaches below 200℃.

[0032] 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 550~680℃ and a holding time of 1.00~2.00min / mm.

[0033] The technical advantages of this invention are as follows: This invention employs a low-carbon, low-manganese chemical composition system to reduce the segregation tendency of continuously cast billets in the thickness direction, and adds microalloying elements such as Nb, VN, and Ti to refine the grains. By adding elements such as La, Ca, Mg, and Zr, the purity of molten steel and the quality of continuously cast billets are improved, thereby enhancing the performance uniformity of steel plates in the thickness direction.

[0034] The smelting process employs a high-yield addition process for elements such as La, Ca, Mg, and Zr. The continuous casting process uses a combination of high superheat, strong secondary cooling water, electromagnetic stirring, and light pressure to control the uniformity of the billet and obtain fine initial austenite grains.

[0035] The continuous casting billet adopts a process of strong cooling + slow cooling in stacking to control grain growth and regulate the precipitated phases of the billet. The rolling process adopts the TMCP process based on two-stage controlled rolling. During the intermediate billet waiting to be heated, an accelerated cooling process is used to reduce the waiting 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. The rolled steel plate is subjected to UFC cooling and tempering treatment. The final microstructure of the steel plate includes polygonal ferrite, tempered sorbite, and dispersed pearlite. The volume percentage of each microstructure is: ferrite 45.0%~60.0%, pearlite 5.0%~13.0%, tempered sorbite 35.0%~41.0%, and the precipitated phase size is 30~50nm, which gives the steel plate excellent mechanical properties and a low brittle transition temperature.

[0036] The low-temperature steel plate of this invention has a thickness ≥60mm, a yield strength of 500~550MPa, a tensile strength of 600~650MPa, an elongation after fracture ≥25.0%, and an impact energy at -85℃ ≥200J; the brittle-brittle transition temperature T of the steel plate is [missing information]. kb ≤-65℃; 1 / 4t precipitate quantity 10~20μm -2 The number of precipitated phases in 1 / 2t is 30~40μm. -2 . Detailed Implementation

[0037] The present invention will be further illustrated below through examples.

[0038] 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.

[0039] heating: The billet is heated in two stages to 1100℃~1250℃. When the first stage heating temperature is below 600℃, rapid heating is used with a heating rate of 10.0~20.0℃ / min. When the second stage heating temperature is above 600℃, slow heating is used with a heating rate of 5.0~15.0℃ / min. The total furnace time is 4.0~6.0h. Rolling: A two-stage rolling process is adopted. The first stage rolling adopts a high-temperature fast rolling + large reduction process. After the billet is descaled after exiting the furnace, it is heated to 1000~1100℃ before rolling. The roll speed is controlled at 2.00~3.00m / s. After the continuous casting billet is widened, the average reduction of the longitudinal rolling pass is 45~55mm. The final rolling temperature is 900~950℃. The thickness of the billet after heating is (2.0~3.5)t, where t is the final product thickness. The second stage rolling starts at a rolling temperature of 770~820℃ and ends at a rolling temperature of 690~740℃. cool down: The rolled steel plate is cooled using UFC at a rate of 8.0~15.0℃ / s until it reaches below 200℃. Tempering: The cooled steel plate is then tempered at a temperature of 550~680℃ for a holding time of 1.00~2.00 min / mm.

[0040] Furthermore, the billet is accelerated to cool while waiting to reach the desired temperature, thereby promoting the temperature gradient across the billet cross section. The cooling rate is 5.0~10.0℃ / s, and the cooling time is 10.0~20.0s.

[0041] Further; smelting: including converter smelting, ladle refining, and RH treatment; a) Converter smelting: Control the basicity of the molten steel in the converter to R = 3.00~4.00; b) RH treatment: RH treatment time is 40~60min. 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. Ca and Mg treatment is carried out before the end of smelting, and La and Zr elements are added.

[0042] Further; continuous casting: The tundish uses a high superheat, 40~65℃, with full-process protective casting. The casting speed is controlled at 0.90~1.60m / min, and the secondary cooling water ratio is 0.90~1.50m³. 3 / t, electromagnetic stirring is used in the later stage of continuous casting process, with a stirring current of 500~600A, and a light reduction process is used at the end of continuous casting, with a reduction of 10.0~20.0mm; Furthermore, 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 500~650℃. Then, slow cooling is carried out by stacking, with a stacking start temperature of 440~490℃ and a slow cooling time of ≥24h.

[0043] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for smelting and continuous casting of the steel in this embodiment of the invention are shown in Table 2. The main process parameters for cooling the steel billet in this embodiment of the invention are shown in Table 3. The main process parameters for heating the steel billet in this embodiment of the invention are shown in Table 4. The main process parameters for the first stage rolling of the steel in this embodiment of the invention are shown in Table 5. The main process parameters for the second stage rolling, cooling, and tempering of the steel in this embodiment of the invention are shown in Table 6. The microstructure of the steel in this embodiment of the invention is shown in Table 7. The mechanical properties of the steel in this embodiment of the invention are shown in Table 8. The full-thickness brittle transition temperature of the steel in this embodiment of the invention is shown in Table 9. Table 1. Composition (wt%) of steel in embodiments of the present invention C 0.042 0.116 0.047 0.068 0.104 0.098 0.072 0.054 0.058 0.086 0.063 Si 0.146 0.038 0.334 0.238 0.049 0.052 0.078 0.093 0.126 0.067 0.086 Mn 1.33 0.84 1.02 1.18 0.86 0.93 1.06 1.24 1.27 0.97 1.12 P 0.004 0.006 0.007 0.006 0.007 0.004 0.006 0.007 0.004 0.003 0.006 S 0.002 0.001 0.002 0.001 0.001 0.002 0.001 0.002 0.001 0.002 0.001 Nb 0.051 0.096 0.057 0.074 0.091 0.088 0.076 0.063 0.066 0.084 0.064 V 0.088 0.044 0.084 0.069 0.047 0.053 0.062 0.077 0.071 0.057 0.064 N 0.0346 0.0211 0.0337 0.0306 0.0233 0.0264 0.0293 0.0324 0.0311 0.0287 0.0246 Cu 0.44 0.56 0.46 0.52 0.47 0.58 0.49 0.41 0.54 0.41 0.56 Cr 0.39 0.18 0.34 0.37 0.24 0.27 0.16 0.22 0.31 0.12 0.14 Ni 0.92 1.26 1.07 1.13 0.88 1.44 0.76 0.64 1.32 0.59 0.77 Ti 0.013 0.042 0.018 0.021 0.026 0.033 0.036 0.039 0.028 0.016 0.044 Als 0.016 0.038 0.026 0.019 0.024 0.032 0.034 0.036 0.029 0.021 0.018 La 0.0169 0.0208 0.0466 0.0749 0.0652 0.0334 0.0841 0.0709 0.0588 0.0682 0.0766 Ca 0.0068 0.0032 0.0066 0.0051 0.0048 0.0036 0.0044 0.0057 0.0034 0.0063 0.0038 Mg 0.0048 0.0024 0.0046 0.0039 0.0037 0.0031 0.0033 0.0042 0.0028 0.0044 0.0031 Zr 0.0089 0.0064 0.0076 0.0068 0.0084 0.0053 0.0097 0.0058 0.0092 0.0073 0.0081 Ca / Mg 1.41 1.33 1.43 1.31 1.3 1.16 1.33 1.36 1.21 1.43 1.23 Ni / (Cu+Cr) 1.11 1.70 1.34 1.27 1.24 1.69 1.17 1.02 1.55 1.11 1.10 Table 2 Main process parameters for steel smelting and continuous casting in the embodiments of the present invention. 1 3.26 44 662 43 1.03 0.94 523 10.4 2 3.18 48 653 62 1.26 1.02 568 12.6 3 3.44 42 658 54 1.47 1.42 502 13.6 4 3.82 43 666 58 1.52 1.36 559 15.2 5 3.69 56 669 47 1.08 1.29 577 11.9 6 3.52 53 656 49 1.14 0.98 544 17.1 7 3.38 49 664 51 0.93 1.07 592 16.3 8 3.72 57 667 41 1.56 1.16 536 14.7 9 3.06 46 651 64 1.18 1.24 584 18.8 10 3.93 59 657 56 1.33 1.48 511 19.1 11 3.12 41 652 46 0.96 1.13 508 10.7 Table 3 Main process parameters for cooling steel billets in embodiments of the present invention. 1 1008 509 442 29 2 1147 643 488 28 3 1089 524 448 27 4 1032 588 468 31 5 1104 618 476 33 6 1029 539 453 27 7 1077 603 472 28 8 1047 556 459 29 9 1129 631 482 32 10 1066 571 464 31 11 1019 529 443 27 Table 4 Main process parameters for heating steel billets in embodiments of the present invention 1 1108 10.4 14.3 4.4 2 1154 15.6 9.2 5.3 3 1234 18.3 8.4 4.6 4 1169 11.6 13.2 4.8 5 1124 16.8 7.1 4.9 6 1229 19.6 6.6 5.1 7 1183 12.3 12.3 5.2 8 1139 17.1 5.8 5.9 9 1198 13.7 11.7 5.4 10 1214 14.3 10.4 5.7 11 1153 10.8 13.6 4.3 Table 5. Main process parameters for the first stage rolling of steel in the embodiments of the present invention. 1 60 1032 2.18 52 904 3.2t 5.3 19.3 2 70 1064 2.61 49 907 3.3t 7.1 14.3 3 90 1022 2.26 51 917 2.2t 7.8 15.6 4 65 1008 2.73 46 929 2.2t 8.7 12.3 5 80 1088 2.39 47 932 2.9t 5.7 18.2 6 85 1074 2.88 48 924 2.2t 9.1 11.4 7 80 1017 2.08 53 938 2.6t 8.3 13.6 8 75 1092 2.44 54 944 2.7t 6.4 17.4 9 80 1049 2.93 46 948 2.3t 9.6 10.6 10 75 1053 2.57 49 911 2.6t 6.6 16.3 11 90 1034 2.19 51 906 2.2t 7.7 15.8 Note: t is the thickness of the final product. Table 6. Main process parameters for the second stage rolling, cooling, and tempering of steel in the embodiments of the present invention. 1 774 694 14.3 554 1.06 2 778 697 12.3 676 1.92 3 784 702 8.2 563 1.17 4 793 713 13.2 658 1.74 5 813 731 10.4 576 1.28 6 802 722 8.7 639 1.57 7 817 738 9.1 594 1.33 8 786 708 10.6 621 1.44 9 809 729 9.7 661 1.83 10 797 717 11.2 642 1.61 11 776 703 8.6 562 1.48 The microstructure of the steel in the embodiments of the present invention was examined, and the results are shown in Table 7. The mechanical properties of the steel in the embodiments of the present invention were examined, and the results are shown in Table 8.

[0044] Table 7 Microstructure of steel in the embodiments of the present invention 1 52.4 37.4 10.2 32 11 33 2 53.3 40.4 6.3 34 10 32 3 48.8 39.7 11.5 46 11 30 4 56.4 36.6 7.0 31 15 34 5 59.3 35.7 5.0 49 14 31 6 47.7 39.8 12.5 37 12 32 7 54.3 37.8 7.9 33 16 33 8 51.6 38.3 10.1 41 13 31 9 49.8 40.3 9.9 31 14 32 10 55.6 38.7 5.7 43 12 33 11 59.6 35.3 5.1 47 11 31 Table 8 Mechanical properties of steel in the embodiments of the present invention 1-1 / 4t-Transverse 536 633 28.5 262 246 1-1 / 4t-Longitudinal 527 621 28.5 276 258 1-1 / 2t-Horizontal 518 614 28.0 248 229 1-1 / 2t-Longitudinal 503 611 28.5 256 236 2-1 / 4t-Transverse 537 629 27.5 273 252 2-1 / 4t-Longitudinal 533 622 28.0 284 261 2-1 / 2t-Horizontal 526 609 25.5 256 239 2-1 / 2t-Longitudinal 514 603 26.5 261 234 3-1 / 4t-Transverse 553 642 25.5 274 248 3-1 / 4t-Longitudinal 546 634 26.5 279 259 3-1 / 2t-Horizontal 534 633 25.0 258 239 3-1 / 2t-Longitudinal 520 622 25.0 251 246 4-1 / 4t-Transverse 544 637 28.0 268 233 4-1 / 4t-Longitudinal 540 628 28.5 273 248 4-1 / 2t-Transverse 538 626 28.5 256 234 4-1 / 2t-Longitudinal 521 627 28.5 268 246 5-1 / 4t-Transverse 534 639 29.0 258 231 5-1 / 4t-Longitudinal 521 623 29.0 263 246 5-1 / 2t-Horizontal 528 614 28.0 249 229 5-1 / 2t-Longitudinal 519 601 28.5 258 234 6-1 / 4t-Transverse 526 630 30.5 268 249 6-1 / 4t-Longitudinal 513 626 30.5 281 263 6-1 / 2t-Transverse 519 629 29.0 259 238 6-1 / 2t-Longitudinal 511 625 29.5 274 249 7-1 / 4t-Transverse 538 631 28.5 274 254 7-1 / 4t-Longitudinal 522 612 29.0 289 269 7-1 / 2t-Horizontal 524 627 28.5 266 249 7-1 / 2t-Longitudinal 516 608 29.0 269 256 8-1 / 4t-Transverse 547 642 30.5 264 239 8-1 / 4t-Longitudinal 536 634 31.0 276 244 8-1 / 2t-Transverse 526 629 29.0 252 229 8-1 / 2t-Longitudinal 519 619 29.5 259 238 9-1 / 4t-Transverse 529 630 27.5 266 246 9-1 / 4t-Longitudinal 522 628 29.0 269 258 9-1 / 2t-Horizontal 518 620 28.0 258 249 9-1 / 2t-Longitudinal 513 601 28.5 262 258 10-1 / 4t-Horizontal 539 641 28.5 259 237 10-1 / 4t-Longitudinal 528 634 29.0 267 249 10-1 / 2t-Horizontal 521 630 28.0 251 226 10-1 / 2t-Longitudinal 517 625 29.0 258 238 11-1 / 4t-Transverse 517 613 29.5 266 238 11-1 / 4t-Longitudinal 506 601 29.5 273 249 11-1 / 2t-Horizontal 493 606 29.0 259 230 11-1 / 2t-Longitudinal 482 600 28.5 253 246 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.

[0045] Table 9. Brittle-to-ductile transition temperatures of the steel in the embodiments of the present invention. 1 -65 2 -70 3 -65 4 -65 5 -70 6 -65 7 -65 8 -65 9 -65 10 -70 11 -65 The 460MPa grade low-temperature steel with a low brittle-to-ductile transition temperature produced using this invention has a microstructure of ferrite + pearlite + tempered sorbite, with the following volume percentages: ferrite 45.0%~60.0%, pearlite 5.0%~13.0%, tempered sorbite 35.0%~41.0%, and the average size of the precipitated phases is 30~50nm. The low-temperature steel has a yield strength of 500~550MPa, a tensile strength of 600~650MPa, an elongation after fracture ≥25.0%, and an impact energy at -85℃ ≥200J; the brittle-to-ductile transition temperature T of the steel plate is [missing information]. kb ≤-65℃.

[0046] 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 460MPa grade low-temperature steel, characterized in that, The elemental composition of the steel, by mass percentage, is as follows: C: 0.040%~0.120%, Si: 0.030%~0.146%, Mn: 0.80%~1.40%, P≤0.008%, S≤0.002%, Nb: 0.050%~0.100%, V: 0.040%~0.090%, N: 0.0200%~0.0350%, Cu: 0.40%~0.60%, Cr: 0.10%~0.40%, Ni: 1.07%~1.50%, Ti: 0.010%~0.045%, Als: 0.015%~0.040%, La: 0.0150%~0.0850%, Ca: 0.0030%~0.0070%, Mg: 0.002 0%~0.0050%, Zr: 0.0050%~0.0100%, balance being Fe and unavoidable impurities; the low-temperature steel has Ca / Mg = 1.1~1.5, Ni / (Cu+Cr) = 1.0~1.8; the microstructure of the low-temperature steel is ferrite, pearlite, and tempered sorbite, with the following volume percentages: ferrite 45%~60%, pearlite 5%~13%, tempered sorbite 35%~41%, and the average size of the precipitated phases is 30~50nm; the low-temperature steel has a tensile strength of 600~650MPa, elongation after fracture ≥25%, and impact energy at -85℃ ≥200J; the brittle-brittle transition temperature T of the steel plate is... kb ≤-65℃.

2. A method for manufacturing a 460MPa grade low-temperature steel with a low brittle-to-ductile transition temperature as described in claim 1, characterized in that, This includes smelting, continuous casting, heating, rolling, cooling, and tempering; 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 500~650℃. Then, it is subjected to slow cooling by stacking with an initial stacking temperature of 440~490℃ and a slow cooling time of ≥24h. heating: The billet is heated in two stages to 1100℃~1250℃. When the first stage heating temperature is below 600℃, rapid heating is used with a heating rate of 10.0~20.0℃ / min. When the second stage heating temperature is above 600℃, slow heating is used with a heating rate of 5.0~15.0℃ / min. The total furnace time is 4.0~6.0h. Rolling: A two-stage rolling process is adopted. The first stage rolling adopts a high-temperature fast rolling + large reduction process. After the billet is descaled after exiting the furnace, it is heated to 1000~1100℃ before rolling. The roll speed is controlled at 2.00~3.00m / s. After the continuous casting billet is widened, the average reduction of the longitudinal rolling pass is 45~55mm. The final rolling temperature is 900~950℃. The thickness of the billet at the waiting temperature is (2.0~3.5)t, where t is the final product thickness. During the waiting temperature, the billet is accelerated cooling at a cooling rate of 5.0~10.0℃ / s and a cooling time of 10~20s. The second stage rolling starts at a rolling temperature of 770~820℃ and ends at a rolling temperature of 690~740℃. cool down: The rolled steel plate is cooled by UFC at a rate of 8~15℃ / s until it reaches below 200℃. Tempering: The cooled steel plate is then tempered at a temperature of 550~680℃ for a holding time of 1.00~2.00 min / mm.

3. The method for manufacturing a 460MPa grade low-temperature steel with a low brittle-to-ductile transition temperature according to claim 2, characterized in that, Smelting: including converter smelting, ladle refining, and RH treatment; a) Converter smelting: Control the basicity of the molten steel in the converter to R = 3.00~4.00; b) RH treatment: RH treatment time is 40~60min. 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. Ca and Mg treatment is carried out before the end of smelting, and La and Zr elements are added.

4. The method for manufacturing a 460MPa grade low-temperature steel with a low brittle-to-ductile transition temperature according to claim 2, characterized in that, Continuous casting: The tundish uses a high superheat, 40~65℃, with full-process protective casting. The casting speed is controlled at 0.9~1.6m / min, and the secondary cooling water ratio is 0.90~1.50m³. 3 / t, electromagnetic stirring is used in the later stage of continuous casting process, with a stirring current of 500~600A, and a light reduction process is used at the end of continuous casting, with a reduction of 10~20mm.