Low-temperature-resistant hot-rolled h-shaped steel and preparation method thereof

Abstract

The application relates to a low-temperature-resistant hot-rolled H-shaped steel and a preparation method thereof. The chemical component composition of the low-temperature-resistant hot-rolled H-shaped steel comprises the following components in percentage by weight: C: 0.10-0.13; Si: <=0.25; Mn: 0.8-1.0; V: 0.07-0.11; P <=0.01; S <=0.005; Nb: 0.005-0.015; Al: 0.008-0.015; N: 0.08-0.012; O <=0.003; RE: 0.008-0.015; wherein As+Sn+Zn+Ca+Mg+S <=0.03, and the rest is iron Fe and inevitable impurities. Through online structure regulation and micro-alloying process design, combined with the reciprocating rolling characteristics of the H-shaped steel, the industrial production of large-size high-strength and high-toughness H-shaped steel products is realized, and the requirement of transverse impact toughness is met.

Description

Technical Field

[0001] This invention relates to the field of building structural steel smelting and rolling technology, specifically to a low-temperature resistant hot-rolled H-beam and its preparation method. Background Technology

[0002] China is rich in oil and natural gas resources, with immense development potential. With the establishment of the Arctic Silk Road in polar regions, the oil reserves in polar ocean areas are increasingly valued by various countries. Marine equipment, represented by floating vessels, and marine engineering projects, represented by offshore oil platforms, are gradually expanding into more complex deep-sea and polar regions, placing more stringent demands on the construction quality and requirements of these equipment. Therefore, the demand for hot-rolled H-beams used in marine equipment and engineering is not only increasing, but also requiring higher comprehensive performance in low-temperature environments. Currently, the impact toughness requirements for hot-rolled H-beams used worldwide are mainly along the rolling direction. Due to the complex shape and microstructure of H-beams, with significant differences in microstructure along the transverse direction of the flanges, specific requirements for transverse impact toughness are generally not specified. However, with the development of engineering quality and quantity under complex environmental conditions, platform structures are becoming more complex, and while ensuring strength and toughness, higher requirements are gradually being placed on the transverse impact toughness index. Profiles with good transverse impact toughness are beginning to be promoted and used in some standards and projects. Due to the complexity of the H-beam cross-section, controlling the metal flow and temperature during the rolling process to simultaneously improve the transverse and longitudinal impact toughness adds greater difficulty to the production and preparation of H-beams.

[0003] For medium to large-sized H-beams, shaped billets are often rolled into their final form. Furthermore, the application of microalloying in H-beam manufacturing presents greater challenges in ensuring microstructure. Additionally, the presence of manganese sulfide significantly affects transverse impact toughness. Therefore, the combined effect of numerous factors ultimately influences transverse impact toughness. Especially for industrial mass production, maintaining stable and uniform transverse impact toughness at -40℃ is a significant challenge. Published patents provide corresponding technical approaches for different manufacturing conditions to ensure the stability of the steel's transverse impact toughness.

[0004] Patent application CN1421286A discloses a rolling method for niobium-containing H-beams. This method utilizes the principles of physical metallurgy to adjust and optimize conventional process conditions. It employs controlled rolling in both the recrystallization and non-recrystallization zones, controlling the deformation amount per pass to allow ferrite to nucleate in the deformation zone, resulting in fine ferrite grains and a uniformly refined microstructure. This yields H-beams with high strength, high toughness, and good weldability. The obtained H-beams have a tensile strength of 490–610 MPa and a transverse impact energy of 34–98 J at -20℃. However, this invention places stringent demands on the control of deformation temperature and deformation amount, which also increases the load on the rolling mill. Real-time adjustment of the reduction amount is extremely difficult, significantly impacting product performance. The resulting fluctuations are difficult to control, leading to a significant decrease in the product yield.

[0005] Patent CN 103556055B discloses a hot-rolled H-beam for offshore natural gas extraction platform structures and its production method. This invention provides hot-rolled H-beams for offshore natural gas extraction platform structures, with the following composition by weight percentage: C: 0.10–0.17, Si: 0.10–0.40, Mn: 1.00–1.60, P≤0.025, S≤0.015, Nb: 0.02–0.05, Ti≤0.025, with the remainder being iron and unavoidable impurities. The mechanical properties, transverse and longitudinal impact energy at -20℃, and surface quality of the hot-rolled H-beams for offshore natural gas extraction platform structures disclosed in this invention fully meet the technical requirements for H-beams used in offshore natural gas extraction platform structures. This invention primarily employs an Nb,Ti composite microalloying composition design. Due to the influence of the Nb,Ti composite microalloying mechanism, the rolling force is relatively large during the actual rolling process, placing high demands on the rolling equipment.

[0006] Patent application CN110938778A discloses a low-temperature resistant hot-rolled H-beam for marine engineering and its preparation method. The chemical composition of the H-beam, by weight percentage, includes: C: 0.040–0.070; Si: 0.015–0.30; Mn: 1.20–1.50; P ≤ 0.015; S ≤ 0.010; Nb: 0.02–0.040; Ti: 0.008–0.025; Ni: 0.10–0.50; Al: 0.015–0.050; B: 0.0001–0.0008; As+Sn+Cu+Zn ≤ 0.04; N ≤ 0.0040; T.[O] ≤ 0.0015. The remainder is Fe and unavoidable impurities. This invention employs composite microalloying for controlled rolling and cooling production, enabling batch production of products with different billet shapes and specifications. It meets the production needs of multi-specification, small-batch steel sections for marine engineering projects in low-temperature environments. However, the project utilizes an ultra-low carbon content design, resulting in higher production costs. Therefore, its economic viability for industrial-scale mass production is limited.

[0007] In summary, special designs are needed for the smelting and rolling processes and heating equipment of H-beams rolled from irregularly shaped billets. These designs aim to not only meet smelting requirements and reduce rolling load, but also ensure that the microstructure after rolling has high transverse impact toughness under certain low-temperature conditions, while improving uniformity. Summary of the Invention

[0008] To meet the needs of marine equipment and marine engineering construction in different regions and harsh and complex environments, this invention provides a method for preparing large-size low-temperature resistant hot-rolled H-steel based on the rolling of irregularly shaped billets, which exhibits good comprehensive impact toughness. The technical solution of this invention is as follows:

[0009] This invention provides a large-size, low-temperature resistant hot-rolled H-beam with good transverse impact toughness and its preparation method. Its chemical composition by weight percentage (%) is: C: 0.10–0.13; Si: ≤0.25; Mn: 0.8–1.0; V: 0.07–0.11; P ≤0.01; S ≤0.005; Nb: 0.005–0.015; Al: 0.008–0.015; N: 0.08–0.012; O ≤0.003; RE: 0.008–0.015; wherein As+Sn+Zn+Ca+Mg+S ≤0.03, and the remainder is iron (Fe) and unavoidable impurities. The production process mainly includes converter smelting, LF refining, continuous casting and hot rolling, online homogenization heat treatment with a dedicated electromagnetic induction heating device, and online controlled cooling.

[0010] The high-strength, high-toughness, and low-temperature resistant hot-rolled H-beam has a yield strength ≥420MPa, tensile strength ≥500MPa, elongation ≥19%, transverse impact energy at -50℃ ≥120J, and Z-direction properties: reduction of area ≥50%.

[0011] Preferably, the RE comprises lanthanum and / or cerium.

[0012] The design principle of the main chemical elements in the high-strength hot-rolled H-beams described in this invention is as follows:

[0013] Carbon: To meet the mechanical property requirements of large-size, low-temperature resistant hot-rolled H-beams with good transverse impact toughness, an ultrafine ferrite and lamellar pearlite microstructure is obtained, with a grain size of grade 8 or higher. Considering the low compression ratio of thick H-beams, excessive carbon content can easily lead to abnormal microstructures such as Widmanstätten and modified pearlite. To improve the peritectic reaction during smelting, avoid the peritectic zone, improve the surface quality of the cast billet, and ensure a yield strength of over 420 MPa, the carbon content is controlled within the range of 0.10–0.13% based on the comprehensive strength and toughness requirements.

[0014] Manganese: In steel, it increases hardenability, stabilizes austenite structure, exerts solid solution strengthening, and significantly improves yield strength. In large-diameter H-beams, different parts exhibit varying deformation and microstructure transformation. Excessive Mn content can easily lead to severe segregation, causing significant performance differences. To ensure the strength of marine steel while reducing segregation and improving the uniformity of the microstructure between the center and edges, a comprehensive consideration of Mn content control at 0.8–1.0% is recommended.

[0015] Phosphorus: Although high phosphorus content can improve corrosion resistance, excessive phosphorus can lead to embrittlement of grain boundaries and deterioration of low-temperature performance. Therefore, the lower the phosphorus content, the better the effect and the better the low-temperature toughness. P should be controlled below 0.01%.

[0016] Sulfur: Sulfur reacts with Mn to form MnS inclusions. Excessive sulfur content can easily lead to the formation of more MnS and other sulfides, resulting in a large number of long strip-shaped MnS inclusions in different parts of complex cross-section steels. This reduces transverse low-temperature toughness, reduces the reduction of area, and indirectly reduces the Z-axis properties of the steel. Therefore, it is essential to strictly control S ≤ 0.005%.

[0017] Aluminum: It can improve the cleanliness of steel and is used as a strong deoxidizing element; it forms a large amount of AlN with nitrogen, which plays a role in refining grains during the rolling process. On the one hand, this ensures that the oxygen content in the steel is reduced, thus reducing the probability of large oxide inclusions; on the other hand, aluminum and nitrogen in the steel form AlN precipitates, which can refine the grains and thus improve the strength of the steel. However, excessively high Al content can easily increase casting difficulty, causing nodules or pinholes at the nozzle. Therefore, in this invention, the aluminum content is controlled at 0.008-0.015%.

[0018] Rare earth elements (RE) primarily purify steel, altering inclusions and reducing pitting and intergranular corrosion. To control the shape of MnS and avoid the presence of numerous elongated MnS inclusions that affect the uniformity of the transverse structure and thus reduce impact toughness, a certain amount of RE is added to improve the shape of MnS. RE is a composite additive element; considering economic efficiency and cost-effectiveness, this invention mainly uses lanthanum and / or cerium series elements to spheroidize inclusions. Since excessive RE can negatively impact other steel properties, it needs to be added in appropriate amounts; therefore, the selected range is RE: 0.008–0.015%.

[0019] Vanadium (V(C,N)) is a strong carbonitride forming element, playing a precipitation strengthening role. Furthermore, V(C,N) nanoparticles can act as nucleation sites for ferrite and pearlite structures, contributing to grain refinement and austenite refinement. Due to the low C and high N content in this invention, the amount of VC precipitated is relatively small. Therefore, it mainly exists in the form of VN during rolling and post-rolling cooling, thus increasing precipitation strengthening. VN in steel is mainly added in the form of alloys. To meet the requirements of 420 MPa yield strength for low-temperature resistant steel, the V content is controlled between 0.07% and 0.11%.

[0020] Nitrogen: A certain concentration of nitrogen (N) in steel combines with v (V) to form VN second-phase particles, resulting in precipitation strengthening and significantly improving strength. However, excessively high N content can easily induce surface defects in the billet, such as transverse cracks and corner cracks. Conversely, insufficient nitrogen content results in fewer VN particles, leading to poor precipitation strengthening and insufficient strength. This invention requires a nitrogen content of 0.08–0.012%.

[0021] Oxygen: Oxygen easily forms large-particle oxide inclusions with elements such as Al, Mg, and Ca. At the same time, controlling the oxygen content to a low level will lead to a significant increase in cost. Therefore, in order to ensure that the strength is improved while the transverse impact toughness and plasticity of the steel are enhanced, this invention takes into account the oxygen content and controls it to ≤0.003%.

[0022] This invention provides a method for preparing low-temperature resistant hot-rolled H-beams, comprising the following steps:

[0023] 1) Smelting;

[0024] 2) Rolling: Electromagnetic heating devices are installed at the entrance and exit of the finishing mill; a cooling system is installed after the electromagnetic heating device at the exit of the finishing mill.

[0025] Preferably, step 1) includes:

[0026] Hot metal pretreatment: The sulfur content of the raw hot metal entering the furnace is ≤0.006%, and the arsenic content of the hot metal is less than 60ppm;

[0027] Converter smelting: The basicity of the final slag is controlled within the range of 2.8-3.8. The slag material is added 3 minutes before the end point, and the final lance pressing time is ≥1 minute until the injection ends.

[0028] The steel tapping process uses aluminum manganese ferrooxidant for deoxidation, with an addition amount of 1.2-2.3 kg / t of steel. Alloying is carried out using metallic manganese, low-phosphorus and low-carbon silicon manganese, vanadium nitrogen, and niobium ferrooxidant, controlled according to the target composition value. After tapping, 3-6 kg / t of lime is added, and the tapping time is not less than 3 minutes.

[0029] LF Refining: The refining process involves bottom blowing argon stirring throughout, with soft argon blowing for more than 15 minutes; after entering the station, feed in ≥50 meters of aluminum wire, and then blow argon into the LF furnace for 7-12 minutes for steel ladle smelting; before soft argon blowing for refining, feed in 80-100m / furnace of high-calcium wire for calcification treatment.

[0030] Casting of irregular continuous casting billets: The continuous casting process adopts a casting stopper rod ladle, and the sprue immersion depth is 60-75mm; the casting superheat is controlled to be less than 25℃, and the continuous casting speed is 0.85~1.0m / min;

[0031] The continuously cast billet is cooled slowly.

[0032] Preferably, step 2) includes: the heating furnace temperature during the rolling process is 1220-1260℃, the billet time in the furnace is 240-360 min; the roughing rolling speed is ≥2.5 m / s; the reduction rate of the vertical roll in the last pass of the finishing rolling stage is controlled to be no less than 12%, and the reduction rate of the remaining passes is no less than 18%; the starting temperature of the finishing rolling is 930-950℃, and the finishing rolling temperature is 780-850℃; the compression ratio of the last pass of the finishing rolling is 8%-12%, and the mill speed is controlled below 1.6 m / s for the speed reduction rolling; the yield strength difference between the upper and lower flanges is controlled to be 10-15 MPa.

[0033] Preferably, the electromagnetic heating device includes a plurality of heating units arranged in sequence, wherein each heating unit includes sub-heating units symmetrically arranged on the outer side of the flange of the H-beam, and the sub-heating units cover the upper and lower flanges and at least part of the web for temperature compensation heating.

[0034] Preferably, when the workpiece enters the mill and when the number of rolling passes is even, the electromagnetic heating device at the entrance of the finishing mill is operated to reduce the temperature difference between the upper and lower flanges caused by the roughing process.

[0035] When exiting the final pass of the finishing mill, the electromagnetic heating device at the mill exit is activated.

[0036] Preferably, when exiting the final pass of the finishing mill, or when the number of rolling passes is odd, a cooling system at the exit of the finishing mill is used for cooling to obtain the required inter-pass temperature or phase transformation temperature.

[0037] Preferably, the finishing rolling process has 7 or fewer rolling passes.

[0038] Specifically, this invention provides a method for preparing hot-rolled H-beams with good transverse impact toughness. The method includes the following steps: hot metal pretreatment (KR desulfurization) → 120-ton converter smelting → ladle argon blowing → LF refining → casting of shaped continuous casting billets → slow cooling in a slow cooling pit for the continuous casting billets → heating in a heating furnace → BD rough rolling → TM reversible finishing rolling → inter-stand cooling → electromagnetic induction online rapid heat treatment → intensive slow cooling on a cooling bed → straightening by a straightener → finishing treatment of the finished product. The above process mainly achieves the stability and improvement of transverse impact toughness by controlling the MnS shape and improving the microstructure through subsequent online heat treatment.

[0039] (1) Smelting process: High-quality, low-sulfur molten iron and scrap steel are fed into the converter for smelting. The raw materials fed into the furnace meet the following requirements: sulfur content of molten iron ≤ 0.006%, arsenic content of molten iron ≤ 60 ppm, and high-quality internally adjusted scrap steel is used. The basicity of the final slag is controlled within the range of 2.8-3.8. The slag material is added 3 minutes before the end point, and the end point lance pressing time is ≥ 1 minute. Ar gas is used for bottom blowing throughout the process. During the steel tapping process, aluminum manganese ferrooxidizer is used, with an addition amount of 1.2-2.3 kg / t of steel. Any shortfall is supplemented during refining. Metallic manganese, low-phosphorus, low-carbon silicon manganese (P content ≤ 0.06%, S content ≤ 0.02%), vanadium nitrogen, and niobium ferroalloys are used for alloying, controlled according to the target composition values. 3-6 kg / t of lime is added after steel tapping. The tapping time is not less than 3 minutes. The refining process involves bottom-blowing argon stirring throughout. Before leaving the station, soft blowing at 0.2-0.25 MPa pressure is used to ensure inclusions float to the surface, and the soft blowing time for refining is guaranteed to be greater than 15 minutes. Upon entering the station, ≥50 meters of aluminum wire is fed in to control the total aluminum content and minimize subsequent aluminum blending. The ladle enters the LF furnace for argon blowing for 7-12 minutes. Before the soft blowing of argon for refining, 80-100 m / furnace of high-calcium wire is fed for calcification treatment. A special stopper rod ladle is used in the continuous casting process, with the nozzle immersion depth at 60-75 mm. Weak cooling is used for the secondary cooling (this is a standard technique in the field); the continuous casting speed is 0.85-1.0 m / min. A special protective slag for peritectic steel is used, and the thickness of the liquid slag layer is measured during the process to ensure it is greater than the crystallizer amplitude. Carbonized rice husks are used for protection in the tundish, ensuring good liquid surface coverage. The billet is slowly cooled in the slow cooling pit, and the surface is inspected; any surface cracks are cleaned. Among them, the protective slag for peritectic steel is a conventional and commonly used protective slag by those skilled in the art.

[0040] (2) Rolling process: The billet first enters a walking beam furnace for heating to obtain austenitic grains of suitable size. After roughing and finishing rolling, it is rolled into finished products. The rolling process is controlled by controlling rolling and cooling. The heating furnace temperature during the rolling process is 1220-1260℃, and the billet time in the furnace is 240-360 min; the roughing rolling speed (BD) is ≥2.5 m / s. During the finishing rolling stage, the rolling load of the vertical rolls is controlled. The reduction rate of the vertical rolls in the last pass shall not be less than 12%, and the reduction rate of the remaining passes shall not be less than 18%. The compression ratio is ensured during the roughing rolling stage, and the intermediate billet undergoes sufficient recrystallization to increase the austenitic nucleation rate and refine the austenitic structure. The starting temperature of the finishing rolling is 930-950℃, and the cooling water between the finishing mill stands is fully turned on. The finishing rolling temperature is 780-850℃. To ensure controlled finishing temperature, the compression ratio of the final pass is 8%–12%, while the mill speed is controlled below 1.6 m / s for reduced rolling. A cooling system at the finishing mill exit is used to achieve the required inter-pass temperature, facilitating the precipitation strengthening effect of subsequently precipitated vanadium-containing carbonitrides. It should be noted that due to the large flange thickness of thick-gauge steel sections and the significant temperature difference between the upper and lower flanges, the final microstructure transformation is severely affected by external factors. Therefore, a special online heat treatment system is designed after the final finishing pass (see [link to relevant documentation]). Figure 1 The heat treatment apparatus (see diagram) performs normalizing treatment to ensure microstructure homogeneity, resulting in uniform phase transformation along the transverse direction of the upper and lower flanges. This leads to a more uniform and consistent microstructure, thereby improving transverse impact toughness and enhancing stability. Simultaneously, it guarantees the consistency of flange Z-direction performance under homogeneous microstructure conditions, ensuring structural stability and system safety when used in special environments such as marine equipment steel structures, offshore oil platforms in polar environments, and steel structures in extremely cold regions.

[0041] This invention achieves the industrial production of large-size, high-strength and high-toughness H-beams by combining online microstructure control and microalloying process design with the reciprocating rolling characteristics of H-beams, thus meeting the requirements for transverse impact toughness.

[0042] Compared with other inventions, the advantages of the technical solution of this invention are as follows:

[0043] 1. This invention adopts low-cost VN alloying + fine rolling process to refine the microstructure, achieving a matrix microstructure grain size refinement to level 9 or above; while the composition design does not contain Ni element, it meets the requirements of low-cost composition design and ensures a significant improvement in the overall low-temperature resistance in both the transverse and longitudinal directions.

[0044] 2. By controlling the sulfur content during the smelting process and strictly controlling the Mn / S ratio, this invention achieves a lower number and smaller size of strip-shaped inclusions, refines the grain size of the transverse structure, and significantly improves the transverse impact toughness under -50℃ conditions.

[0045] 3. This invention combines the characteristics of rolling large-size H-beams with different compression ratios of 16-50mm, and adopts a VN micro-alloying composition design suitable for temperature-controlled rolling. By controlling the N content during the LF refining process, the amount and size of VN precipitation are increased. At the same time, Nb recrystallization is controlled to meet the -50℃ toughness requirements of different specifications of products, thereby obtaining stable low-temperature resistant hot-rolled H-beams on a general hot-rolled H-beam mill.

[0046] 4. In the rolling process of this invention, specially designed online homogenizing and cooling equipment are used before and after the finishing mill to ensure stable and uniform rolling. Temperature compensation heating is performed in a timely manner according to the size and temperature changes of the rolled piece, which significantly reduces the temperature difference between the upper and lower flanges. The temperature difference is controlled below 10°C, which ensures that the difference in grain size between the upper and lower flanges is below 0.5 grade, achieving uniformity of the microstructure between the upper and lower flanges. Ultimately, this results in a significant improvement in transverse impact toughness and achieves the effect of refining the microstructure.

[0047] 5. The inter-stand and post-rolling cooling devices of this invention, based on real-time rolling parameter models, use cooling water to cool the entire flange area, reducing the temperature difference to below 10℃ and ensuring the final rolling temperature is controlled between 780 and 850℃. The yield strength difference between the upper and lower flanges is controlled within the range of 10 to 15 MPa, and the transverse impact energy is stably controlled above 120 J, meeting the requirement of uniform performance. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the online heating and cooling equipment before and after the finishing mill in the present invention;

[0049] Figure 2 A schematic diagram of the overall online electromagnetic heating device of the present invention;

[0050] Figure 3 Front view of the schematic diagram of the online electromagnetic heating device of the present invention;

[0051] Figure 4 Microstructure of the hot-rolled low-temperature resistant H-beam involved in Example 2;

[0052] Figure label:

[0053] 1. H-beam rolled piece, 2. Upper flange, 3. Lower flange, 4. Induction heating coil, 5. Roller conveyor for rolled piece, 6. Cooling water pipe on the outer side of the flange. Detailed Implementation

[0054] The present invention will be further described below with reference to specific embodiments.

[0055] To achieve uniform microstructure in the upper and lower flanges and improve lateral impact toughness, this invention designs an online heating and cooling device. The process and equipment layout are as follows: Figure 1As shown. An online electromagnetic heating device is installed at both the inlet and outlet of the finishing mill. The electromagnetic heating device comprises several heating units arranged sequentially. Each heating unit includes sub-heating units symmetrically arranged on the outer side of the flanges of the H-beam. These sub-heating units cover the upper and lower flanges and at least part of the web, and are used for temperature compensation heating. Each sub-heating unit consists of an induction heating coil 4, which performs timely temperature compensation heating based on changes in the rolled piece temperature. This heating device has an instant heating function, based on feedback from the PLC regarding the flange location of the rolled piece and 2 and 2. Figure 3 When the temperature difference between the upper flange 2 and the lower flange 3 exceeds 10℃, the electromagnetic induction device is activated to reduce the temperature difference. In the pre-mill heating zone I, the electromagnetic induction device is activated at the entry of the workpiece 1 and in even-numbered passes according to the temperature conditions, gradually reducing the large temperature difference between the upper and lower flanges caused by the roughing process. In the post-mill heating zone II, the electromagnetic induction device works in conjunction with the cooling system, which consists of the workpiece transport roller table 5 and cooling water pipes on the outer side of the flanges. This system controls the phase transformation process to ensure uniformity and refinement of the microstructure, ensuring that the grain size difference between the upper and lower flanges is below 0.5 grade, ultimately achieving a significant improvement in transverse impact toughness. The above operations are adjusted according to the specifications and dimensions of the H-beam workpiece 1 to meet the temperature control requirements. Especially in the last pass of the rolling process, precise cooling control of the billet temperature is performed to stabilize the temperature difference changes in different parts during the rolling process, ensuring uniform temperature throughout the entire workpiece in the transverse direction. Through the operation of the above devices, the temperature difference between the upper and lower flange sections is kept below 10℃, while simultaneously reducing the temperature difference between the flanges and the web, thus improving the overall performance stability and uniformity of the H-beam. The above-mentioned equipment, combined with microalloying design, can achieve the requirements for improved transverse impact toughness of the final hot-rolled H-beam; at the same time, it can ensure that the transverse impact energy at -50℃ is ≥120J, with a low ductile-brittle transition temperature; and the Z-direction performance is stably controlled at a reduction of area of ​​≥50%.

[0056] This invention achieves the industrial production of large-size, high-strength and high-toughness H-beams by combining online microstructure control and microalloying process design with the reciprocating rolling characteristics of H-beams, thus meeting the requirements for transverse impact toughness.

[0057] The present invention will now be described in detail:

[0058] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0059] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0060] Table 3 is a list of performance tests for each embodiment and comparative example of the present invention.

[0061] Each embodiment of the present invention is manufactured according to the following steps:

[0062] 1) Selected low-phosphorus, low-sulfur, and low-residual-element molten iron is smelted in the converter before entering the LF furnace for composition and inclusion control. The raw materials are strictly controlled to ensure the molten iron sulfur content is ≤0.006% and the arsenic content is less than 60ppm. The final slag basicity is controlled within the range of 2.8-3.8, with slag added 3 minutes before the end point, and the final lance pressing time ≥1 minute. Ar gas is used for bottom blowing throughout the process. During the steel tapping process, 1.2-2.3 kg / t of ferroaluminum manganese is added for deoxidation, with the remainder added during refining. 3-6 kg / t of lime is added after tapping. The tapping time is 3-5 minutes. Argon stirring is used throughout the refining process, with low-pressure soft blowing before leaving the station to allow various inclusions to float. Soft argon blowing during refining lasts 15-20 minutes. After entering the station, 50-100 meters of aluminum wire is fed to meet the Al content requirements in the steel. The ladle is then fed 80-100 meters / furnace of high-calcium wire for calcification treatment in the LF furnace. The continuous casting process uses a special stopper rod ladle, with the nozzle immersed to a depth of 60-75mm. Secondary cooling is performed using a weak cooling mode; the casting speed is 0.85–1.0 m / min. A special protective slag for peritectic steel is used. The tundish is protected with well-coated carbonized rice husks covering the liquid surface. The billet is slowly cooled in a slow cooling pit, and its surface is inspected and defects are cleaned.

[0063] The final product is continuously cast into irregularly shaped billets, which are divided into three types according to the different flange thicknesses. The residual elements in the molten iron are strictly controlled within the range of As+Sn+Zn+Pb+Ca+Mg≤0.03; the casting superheat is controlled to be less than 25℃ during the casting process; one of the values ​​within the range of 0.85~1.0m / min is selected as the constant casting speed; and the straightening temperature is not lower than 860℃.

[0064] 2) Rolling process: The billet is reheated in a furnace at a temperature controlled at 1220–1260℃ for 240–360 minutes, and then rolled. Rolling and cooling processes are controlled. The roughing (BD) process has fewer than 9 passes; the BD rolling speed is ≥2.5 m / s. The finishing (TM) process has fewer than 7 passes. During the finishing stage, the rolling load on the vertical rolls is controlled; the reduction rate of the vertical rolls in the last pass must not be less than 12%, and for the remaining passes, not less than 18%. The starting temperature for finishing is 930–950℃, and the final temperature is controlled at 780–850℃. The compression ratio in the final finishing pass is 8%–12%, and the mill speed is controlled below 1.6 m / s for speed reduction. In the pre-mill heating zone I, the electromagnetic induction device is activated at the workpiece inlet 1 and in even-numbered passes according to the temperature, gradually reducing the large temperature difference between the upper and lower flanges caused by the roughing process. A special online heat treatment system was designed after the final pass of the finishing mill (see [link]). Figure 1The product undergoes heat treatment (see diagram), followed by cooling in the final or odd-numbered passes to achieve the desired inter-pass temperature or phase transition temperature. This ensures consistent microstructure uniformity, enhances transverse impact toughness, and improves stability. Simultaneously, it guarantees consistent flange Z-axis performance under uniform microstructure conditions, ensuring structural stability and system safety when using the product in special environments. The cooling bed maintains a temperature above 300°C, and the product undergoes concentrated, slow cooling for at least 15 minutes. Once the product temperature drops below 200°C, it is then straightened in a straightening machine.

[0065] 3) Finishing process: After the product comes off the production line, it undergoes surface and dimensional finishing treatment; during the finishing process, samples are taken and the product performance is analyzed.

[0066] Table 1. Chemical composition (wt.%) of various embodiments and comparative examples of the present invention.

[0067]

[0068] The main refining process parameters are shown in Table 2.

[0069] Table 2 Refining Process Parameters

[0070]

[0071] The specific process parameters for continuous casting are shown in Table 3.

[0072] Table 3 Process parameters for continuous casting

[0073]

[0074] Table 3 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0075] Table 4 Rolling process parameters

[0076]

[0077] The performance tests of the various embodiments and comparative examples of the present invention are shown in Table 5.

[0078] Table 5: Record of Mechanical Properties of Rolled Products

[0079]

[0080]

[0081] Performance testing was conducted on samples of the products from the embodiments involved in the patent. For mechanical properties, the sample locations were one-third of the way from the edge to the center of the H-beam flange, following EN ISO 377-1 "Sampling location and preparation of test specimens for mechanical properties test". The test methods for yield strength, tensile strength, elongation, and other indicators followed EN ISO 6892-1 "Metallic materials, tensile testing at room temperature"; the impact energy test method followed ISO 148-1 "Metallic materials, Charpy impact test". Comparison with the mechanical properties in the embodiments revealed that the H-beam flanges produced using the method of this patent exhibit superior performance compared to currently disclosed patented products, and also possess excellent Z-direction properties.

[0082] Furthermore, any content not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing low-temperature resistant hot-rolled H-beams, characterized in that, The chemical composition of the low-temperature resistant hot-rolled H-beam, by weight percentage, includes: C: 0.10~0.13; Si: ≤0.25; Mn: 0.8~1.0; V: 0.07~0.11; P≤0.01; S≤0.005; Nb: 0.005~0.015; Al: 0.008~0.015; N: 0.08~0.012; O≤0.003; RE: 0.008~0.015; wherein As+Sn+Zn+Ca+Mg+S≤0.03, and the remainder is iron (Fe) and unavoidable impurities; The preparation method includes the following steps: 1) Smelting; 2) Rolling: Electromagnetic heating devices are installed at the entrance and exit of the finishing mill; a cooling system is installed after the electromagnetic heating device at the exit of the finishing mill to keep the temperature difference between the upper and lower flanges below 10℃. The electromagnetic heating device includes several heating units arranged in sequence. Each heating unit includes sub-heating units symmetrically arranged on the outside of the flanges of the H-beam. The sub-heating units cover the upper and lower flanges and at least part of the web, and are used for temperature compensation heating. When the workpiece enters the mill, and when the number of rolling passes is even, the electromagnetic heating device at the entrance of the finishing mill is activated to reduce the temperature difference between the upper and lower flanges caused by the roughing process. When exiting the final pass of the finishing mill, the electromagnetic heating device at the mill exit is activated; When exiting the final pass of the finishing mill, or when the number of rolling passes is odd, a cooling system at the exit of the finishing mill is used for cooling to obtain the required inter-pass temperature or phase transformation temperature.

2. The preparation method according to claim 1, characterized in that, The low-temperature resistant hot-rolled H-beam has a yield strength ≥420MPa, tensile strength ≥500MPa, elongation ≥19%, transverse impact energy at -50℃ ≥120J, and Z-direction properties: reduction of area ≥50%.

3. The preparation method according to claim 1, characterized in that, The RE includes lanthanum and / or cerium.

4. The preparation method according to claim 1, characterized in that, Step 1) includes: Hot metal pretreatment: The sulfur content of the raw hot metal entering the furnace is ≤0.006%, and the arsenic content of the hot metal is less than 60ppm; Converter smelting: The basicity of the final slag is controlled within the range of 2.8-3.

8. The slag material is added 3 minutes before the end point, and the final lance pressing time is ≥1 minute until the injection ends. The steel tapping process uses aluminum manganese ferrooxidant for deoxidation, with an addition amount of 1.2-2.3 kg / t of steel. Alloying is carried out using metallic manganese, low-phosphorus and low-carbon silicon manganese, vanadium nitrogen, and niobium ferrooxidant, controlled according to the target composition value. After tapping, 3-6 kg / t of lime is added, and the tapping time is not less than 3 minutes. LF Refining: The refining process involves bottom blowing argon stirring throughout, with soft argon blowing for more than 15 minutes; after entering the station, feed in ≥50 meters of aluminum wire, and then blow argon into the LF furnace for 7-12 minutes for steel ladle smelting; before soft argon blowing for refining, feed in 80-100m / furnace of high-calcium wire for calcification treatment. Casting of irregular continuous casting billets: The continuous casting process adopts a casting stopper rod ladle, and the sprue immersion depth is 60-75mm; the casting superheat is controlled to be less than 25℃, and the continuous casting speed is 0.85~1.0m / min; The continuously cast billet is cooled slowly.

5. The preparation method according to claim 1, characterized in that, Step 2) includes: the heating furnace temperature during the rolling process is 1220-1260℃, and the billet time in the furnace is 240-360 min; the roughing rolling speed is ≥2.5 m / s; the reduction rate of the vertical roll in the last pass of the finishing rolling stage is controlled to be no less than 12%, and the reduction rate of the remaining passes is no less than 18%; the starting temperature of the finishing rolling is 930-950℃, and the finishing rolling temperature is 780-850℃; the compression ratio of the last pass of the finishing rolling is 8%-12%, and the mill speed is controlled below 1.6 m / s for reduced rolling speed; the yield strength difference between the upper and lower flanges is controlled to be 10-15 MPa.

6. The preparation method according to claim 1, characterized in that, The finishing rolling process has 7 or fewer rolling passes.

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