Low temperature resistant hot rolled H-section steel with 420MPa grade yield strength and its manufacturing method

Abstract

The present invention belongs to the field of metallurgy technology, specifically to a 420 MPa grade low temperature resistant hot rolled H-shaped steel and its manufacturing method, the chemical composition of the H-shaped steel is, in mass percent, C: 0.08%-0.10%, Si≦0.2%, Mn: 1.25%-1.45%, V: 0.03%-0.045%, Ti: 0.015%-0.025%, Cr: 0.15%-0.30%, Als: 0.02%-0.04%, N: 0.007%-0.01%, P≦0.008%, S≦0.005%, O≦0.004%, and the remainder is Fe and unavoidable impurities. The present invention combines the thin flange characteristics of small and medium-sized H-shaped steel produced by rolling rectangular slabs, uses a design of components that combines low C content and V micro-alloying suitable for normalizing rolling, and adds an appropriate amount of Cr element to control the cooling rate to avoid the formation of abnormal structures such as Widmanstätten that reduce the low-temperature impact toughness of the steel, thereby obtaining a high-strength and toughness hot-rolled H-shaped steel of 420 MPa grade or higher that is stably controlled by the rolling mill that hot-rolls the H-shaped steel.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims priority from Chinese Patent Application No. 202110818559.5, filed on July 20, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention belongs to the field of metallurgy, and particularly relates to a 420 MPa grade low temperature resistant hot rolled H-section steel and its manufacturing method. [Background technology]

[0003] As energy mining expands to more complex regions, such as those with extreme cold, hot-rolled H-section steel with higher overall performance is required to replace the current low-grade H-section steel, reducing weight while ensuring high reliability. Furthermore, to adapt to the complex application environments of extreme cold and other regions, there are higher requirements for the low-temperature impact toughness, weldability, and resistance to delamination cracking of steel materials, and the demand for higher-strength hot-rolled H-section steel is gradually increasing.

[0004] At present, Chinese hot-rolled H-beam manufacturers are successively developing different grades of H-beam with a yield strength of 345 MPa or more, which are generally manufactured by combining complex micro-alloying and hot rolling methods. Different companies produce products of different grades and with different overall performance based on their equipment levels.

[0005] Chinese patent application CN201510498771.2 discloses a 420 MPa grade hot-rolled H-section steel with excellent low-temperature toughness and a manufacturing method thereof, whose composition is C: 0.06-0.12%, Si: 0.20-0.40%, Mn: 1.20-1.60%, P≦0.015%, S≦0.010%, V: 0.050-0.070%, Ni: 0.10-0.20%, N: 0.0050-0.0100%, and the remainder is Fe and unavoidable impurities. Compared with conventional technologies, this patent utilizes a rational V, Ni, and N composition design and adaptive controlled rolling and controlled cooling processes to develop a 420 MPa hot-rolled H-section steel with excellent overall performance. The yield strength ReH is 440-520 MPa, the tensile strength Rm is 550-650 MPa, the elongation A is A≥22%, and the low-temperature impact toughness at -40°C is KV2≥100J. This patented controlled cooling method uses a two-stage process: the first stage is rapid cooling and the second stage is air cooling. This requires special cooling equipment to meet these manufacturing requirements. The resulting 420 MPa-grade hot-rolled H-section steel with excellent low-temperature toughness has a through-thickness performance Z of 40-65%. This process design is intended for large-sized deformed slabs and is not suitable for the production of small and medium-sized thin flange H-section steel products.

[0006] Chinese patent application CN201510788520.8 discloses a 420 MPa grade, high-strength, low-yield-ratio H-beam steel and its manufacturing method. The chemical composition of the H-beam steel is, in mass percent, 0.11-0.15% C, 0.20-0.35% Si, 1.35-1.50% Mn, P≦0.035%, S≦0.025%, Cu: 0.25-0.30%, Cr: 0.40-0.45%, Ni: 0.20-0.30%, Nb: 0.20-0.30%, and the remainder is iron and trace impurities. By optimizing the composition, this patent achieves the production of 420 MPa grade, low-yield-ratio H-beam steel, with a yield strength of 427 MPa or higher, a tensile strength of 641 MPa or higher, and a yield ratio of 0.64-0.67. In this patent, elements such as Ni, Nb, and Cu are added and used in a specific highly corrosion-resistant environment. The Cu element affects the manufacturing process, increasing the probability of cracks occurring in the legs of the H-shaped steel at the high-pressure rolling end temperature, and significantly increasing manufacturing costs.

[0007] Patent application CN201510498771.2 discloses a 420 MPa grade high performance earthquake-resistant H-section steel and its manufacturing method, containing 0.15-0.18 wt% C, 0.30-0.45 wt% Si, 1.35-1.55 wt% Mn, 0.070-0.090 wt% V, ≦0.015 wt%, ≦0.020 wt% S, with the remainder being Fe and unavoidable impurities. The manufacturing method includes smelting molten steel, deoxidation alloying, LF furnace refining of the molten steel, casting of the molten steel, and post-treatment steps. The smelting process in this patent uses a nitrogen-rich vanadium micro-alloying process to effectively refine the austenite grains, and the increased nitrogen content during smelting contributes to the formation of vanadium carbonitrides and their dispersion at the grain boundaries, promoting the nucleation of austenite grains and further refining the austenite grains. After rolling, controlled cooling, rapid cooling and other processes, the H-section steel is guaranteed to have excellent shape and surface quality and improve performance. In this patent, the strength index reaches 420 MPa, but the low-temperature toughness is not significantly improved.

[0008] The three 420MPa grade H-beams and manufacturing technologies mentioned above all combine micro-alloying and different cooling methods. The products involved are large-scale, have high strength, and maintain a certain level of impact toughness. This places high demands on cooling equipment, while the products are rolled using conventional equipment, which limits the low-temperature toughness requirements. Therefore, small and medium-sized H-beam products must be redesigned to improve strength and impact toughness under high-temperature normalizing rolling conditions. Furthermore, unlike plate rolling, the complex cross-section of H-beams places more stringent process requirements.

[0009] Currently, the dephosphorization process in steelmaking is generally completed using a converter. However, the dephosphorization rate of a single converter is low, making it difficult to consistently control the phosphorus content in molten steel below 0.01%. To improve the dephosphorization rate, currently disclosed prior art often uses two converters to perform advanced dephosphorization using a dual process, which can consistently control the phosphorus content in molten steel below 0.01%. However, the dual process requires separate dephosphorization and decarburization processes in a dephosphorization converter and a decarburization converter, necessitating the construction of a new dephosphorization furnace. This process requires a smelting time 40 minutes or more longer than that of a single converter, resulting in significant equipment investment and low production efficiency. Some companies use a converter double slag method for advanced dephosphorization, but the primary dephosphorization slag from the converter cannot be fully discharged, making it difficult to consistently control the phosphorus content in molten steel below 0.01%.

[0010] Some companies use three removal processes (desiliconization, desulfurization, and dephosphorization) in hot metal pretreatment to remove phosphorus. However, the dephosphorization reaction occurs at the slag-steel interface, as shown in Equation (1). The unstable reaction product, P2O5, formed by the reaction of phosphorus and oxygen, P2O5, can only be removed by adding P2O5 to the slag and reacting it with CaO to form CaO·P2O5. Dephosphorization only occurs in the molten steel that comes into contact with the steel slag (the molten steel at the bottom of the ladle rarely undergoes dephosphorization). Therefore, the only way to achieve the required P content in the molten steel (i.e., P content of 0.03% or less) is through prolonged stirring to promote contact between the steel slag and the molten steel. This results in low hot metal pretreatment efficiency, requiring the entire three removal processes to take 150-200 minutes. This results in low production efficiency and a large temperature drop in the molten metal, which has a serious negative impact on the profits of steel companies. Therefore, fewer and fewer Chinese steel companies are using this three-removal process for hot metal pretreatment. Therefore, it is necessary to improve the dephosphorization process, so as to achieve the purpose of reducing the phosphorus content and achieve the effect of improving low-temperature toughness.

[0011] 2[P]+5[O]+4(CaO)=(4CaO P2O5)(1) Summary of the Invention [Problem to be solved by the invention]

[0012] To meet the demand for high-strength, high-temperature toughness steel sections in complex environments such as the cold polar regions of the Arctic and Antarctic, we provide low-temperature resistant hot-rolled H-section steel with a yield strength of up to 420 MPa, as well as a manufacturing method for the same. This H-section steel not only meets the demand for steel for building structures in extremely low temperature conditions, such as in the construction of onshore and offshore oil platforms, but also has the advantage of low manufacturing costs, allowing for lightweight building structures and providing convenience in the construction of steel structures. [Means for solving the problem]

[0013] The technical solutions of the present invention are as follows:

[0014] The present invention provides a low-temperature resistant H-beam steel having a yield strength of 420 MPa grade, and its chemical composition, in mass percent, is as follows: C: 0.08%-0.10%, Si≦0.2%, Mn: 1.25%-1.45%, V: 0.03%-0.045%, Ti: 0.015%-0.025%, Cr: 0.15%-0.30%, Als: 0.02%-0.04%, N: 0.007%-0.01%, P≦0.008%, S≦0.005%, O≦0.004%, and the remainder being Fe and unavoidable impurities.

[0015] Preferably, in the present invention, the P and S elements satisfy P + S ≦ 0.01%. H-section steel manufactured based on this requirement can better meet the impact toughness requirements under extreme temperatures. The present invention is particularly suitable for manufacturing small and medium-sized H-section steel products with flange thicknesses of 15 mm or less, but is not limited to products of the above specifications.

[0016] In the high strength hot rolled H-section steel of the present invention, the design principles of each chemical element are as follows:

[0017] Carbon: To achieve a 420 MPa strength grade for hot-rolled H-section steel and meet the low-temperature resistance requirements, the steel matrix structure must be ultrafine sheet pearlite and flat ferrite, and at the same time, a certain amount of nano-level vanadium-containing carbides must be obtained. Given the large compression ratio of small-specification H-section steel, the difficulty of controlling the equipment capacity and rolling end temperature, and other factors, the carbon content must not be high and should be controlled at 0.08% to 0.10%.

[0018] Manganese: In hot-rolled steel, Mn stabilizes the austenite structure, improves the hardenability of the steel, and can improve the strength of the steel. Furthermore, Mn is an element that is prone to segregation, distributing unevenly in different parts of the steel structure, resulting in large differences in performance. To ensure strength, the Mn content is preferably set to 1.25% or more. Furthermore, in order to control crack sensitivity, excessive addition of Mn impairs mechanical property indicators such as low-temperature toughness and plasticity, so it is best not to add too much. Therefore, taking all factors into consideration, the Mn content in the steel of the present invention is controlled to 1.25% to 1.45%.

[0019] Silicon: Si is a deoxidizing element and contributes to improving strength, but if there is too much Si, it will form a large amount of Si-containing Fe2SiO4Si on the steel surface, which will increase the viscosity of the steel, make it difficult to remove mill scale, and affect the surface quality. Therefore, the lower limit of the Si content is set to 0.20% or less.

[0020] Phosphorus: Phosphorus is a harmful impurity in steel. It can significantly expand the two-phase region between the liquid and solid phases and easily segregate at grain boundaries, causing abnormalities in the local structure of the steel, resulting in "embrittlement" of the steel, significantly reducing the low-temperature impact toughness and temper embrittlement of the steel, and causing uneven mechanical performance. Phosphorus also causes corrosion fatigue and weld cracks. Therefore, in converter smelting, properly controlling the P content plays a significant role in improving the low-temperature toughness of H-section steel. Considering the capacity of the equipment, in the present invention, P≦0.008%.

[0021] Sulfur: As one of the five unavoidable elements in steel, solidification segregation can cause weld cracks and reduced toughness. Intermittent inclusions due to the manufacturing process have a serious impact on the low-temperature toughness of steel, so its content should be reduced as much as possible. Sulfur easily forms MnS inclusions, which act as crack initiation sites and reduce workability, so the S content is preferably limited to 0.005% or less. The lower limits for P and S are determined by equipment capacity and cost control; both should be above 0%, with P + S ≦ 0.010%.

[0022] Aluminum: Aluminum is used as a powerful deoxidizing element in the manufacturing process of the H-beam steel in this invention. It reduces the probability of forming spherical impurities by minimizing the oxygen content in the steel. Some aluminum can also combine with nitrogen in the steel to form AlN precipitates, thereby increasing the strength of the steel. Therefore, the aluminum content in this invention is controlled to 0.02% to 0.04%.

[0023] Titanium: Ti is a strong carbide-forming element. Trace amounts of Ti contribute to fixing N in steel. The fine TiN formed at the same time inhibits excessive austenite grain growth during slab heating, thereby refining the original austenite grains. Ti can also form compounds such as TiC, TiS, and Ti4C2S2 in steel, which can inhibit grain growth in the heat-affected zone during welding and improve the weldability of the final H-beam steel. Therefore, in this invention, 0.015% to 0.025% Ti is added.

[0024] Vanadium: As a strong carbonitride-forming element, V carbonitrides form nano-level carbonitrides during the cooling stage of the later rolling process, playing a significant role in precipitation strengthening. VN alloys can act as nucleating particles for ferrite and pearlite structures, contributing to the refinement of the structure. VC also plays a role in precipitation strengthening, and the rolling deformation resistance of vanadium-containing steels is low, which helps reduce the rolling load. For a yield strength of 420 MPa grade, the V content should be controlled between 0.03% and 0.045%.

[0025] Chromium: Adding a certain amount of Cr to steel can improve its strength, hardness, and wear resistance. Adding chromium to steel can significantly improve its hardenability. Experiments have shown that Cr has slightly weaker hardenability than Mo, and it does not form a large amount of bainite at low cooling rates, reducing the toughness of the steel. Too much or too little Cr can adversely affect the hardenability and fracture retardation of the steel, making it more susceptible to defects. To avoid the formation of abnormal structures, Cr is controlled within the range of 0.15% to 0.30%.

[0026] Nitrogen: The N element in steel forms TiN with Ti and VN with V, which strengthens the VC alloy by precipitation, improving its strength. Too much N content can lead to poor quality on the slab surface, causing transverse cracks. Therefore, the nitrogen content in this invention is set to 0.007% to 0.010%.

[0027] Oxygen: By appropriately reducing the oxygen content in the steel, it is possible to avoid the formation of strong oxidizing elements and large particles of oxide inclusions, thereby ensuring the improvement of the toughness and plasticity of the steel. In the present invention, the nitrogen content is set to ≦0.004%.

[0028] The H-shaped steel has a yield strength of 420 MPa or more, a tensile strength of 520 MPa or more, an elongation of 20% or more, and an impact energy at -50°C of 100 J or more.

[0029] The present invention further provides a method for manufacturing the above-mentioned H-section steel having a yield strength of 420 MPa grade, which mainly includes the steps of hot metal pretreatment + deep dephosphorization → converter smelting → argon injection into the ladle → RH / LF refining → casting of rectangular continuous cast slabs → slow cooling of the continuous cast slabs in a slow cooling pit or hot feeding / hot charging → semi-continuous rolling of the steel wire → dense slow cooling on a cooling bed, etc. Specifically, the method includes the following steps:

[0030] (1) Hot metal pretreatment and multiple dephosphorization.

[0031] During the tapping process of the blast furnace, the molten pig iron in the blast furnace flows into the first ladle. As the molten pig iron is continuously poured into the blast furnace, a dephosphorization agent is sprayed into the ladle to dephosphorize it. The liquid level in the ladle rises continuously. When the liquid level of the molten pig iron exceeds the upper edge of the pig iron discharge port by 20-30 cm, the pig iron discharge port is opened, and the molten pig iron flows into the second ladle through the pig iron discharge port. The dephosphorization operation by spraying is then carried out again in the second ladle. The second ladle also has a pig iron discharge port on its side, 20 to 30 cm below the steel-slag interface. The dephosphorized molten iron in the upper part of the second ladle flows downward through the pig iron discharge port into the third ladle, and then repeatedly flows downward through the pig iron discharge port into the Nth ladle. N dephosphorization operations are performed on the molten iron, and the phosphorus content of the molten iron is reduced to 0.02% or less, with 4≦N≦6.

[0032] Every 30 to 40 minutes of dephosphorization, the first ladle to the N-1 ladle undergo one slag exchange operation.

[0033] (2) Converter smelting: The phosphorus content at the end point is controlled to 0.007% or less.

[0034] (3) The processes of argon injection into the ladle, RH / LF refining, casting of rectangular continuous slabs, slow cooling of the continuously cast slabs in a cooling pit or hot feeding / hot charging, semi-continuous rolling of steel wire, and dense slow cooling on a cooling bed are carried out. In the rolling process, the soaking temperature of the heating furnace is 1210~1250℃, the slab casting time is 140~180min, the start temperature of finish rolling is 1000~1050℃, and the finish temperature of finish rolling is 890~930℃.

[0035] Preferably, in step (1), the nominal volumes of the first to (N-1) ladles are 30 to 50 tons of molten pig iron, and the depths are 1.5 to 2 m. The pig iron discharge ports are provided on the side walls of the ladles 50 to 60 cm away from the top of the ladles and extend outward by 20 to 30 cm. A slide plate is used to control the opening and closing of the pig iron discharge ports. The volume of the Nth ladle is matched to the tonnage of the converter.

[0036] Preferably, the dephosphorization agent spraying points include the pouring point of blast furnace hot metal into the first ladle, and the pouring point of the previous ladle into the next ladle, i.e., the pouring point of the first ladle into the second ladle, the pouring point of the second ladle into the third ladle, and similarly the pouring point of the N-2 ladle into the N-1 ladle, where the hot metal is stirred vigorously and the dephosphorization kinetic conditions are good.

[0037] Preferably, the dephosphorization agent is a mixture of sludge balls and lime, the mass ratio of the sludge balls to the lime being 1:1, the components of the sludge balls being, in mass percent, CaO: 2-8%, Fe2O3: 85-92%, SiO2: 1-5%, the components of the lime being, in mass percent, CaO: 85-90%, Fe2O3: 0-3%, SiO2: 0-10%, MgO: 0-10%, and the amount of dephosphorization agent added to the ladle is 2-3 kg / ton of iron / ladle.

[0038] Preferably, the carrier gas used when spraying the dephosphorization agent is oxygen, the spray pressure is 0.2 to 0.3 MPa, and the spray flow rate is 2 to 4 m 3 / min.

[0039] Preferably, the next ladle in the ladle is 60 to 70 cm lower than the previous ladle.

[0040] Preferably, the slag exchange operation continues by removing the slag from the first ladle, placing the slag from the top of the second ladle into the first ladle, placing the slag from the top of the third ladle into the second ladle, and so on until the slag from the top of the N-1 ladle is placed into the N-2 ladle.

[0041] Preferably, the first to (N-1)th ladles are each provided with a slag discharge port at the top, the slag discharge port extending outward by 20 to 30 mm, and the positional difference between the slag discharge port and the pig iron discharge port on the ladle wall is one-fourth of the circumference.

[0042] The ladle cars of ladles 1 through N-1 are all equipped with hydraulic lifting and rotating devices, allowing the ladles to be raised or lowered 1-2 meters and rotated 90 degrees around their center axis. The rolling process is divided into rough rolling and finish rolling. Rough rolling uses reciprocating rolling, typically with seven passes. Finish rolling is performed in five consecutive passes, with water cooling between the finish rolling frames fully activated. To ensure control of the rolling finish temperature, the reduction ratio of the last two passes of finish rolling is 6%-12%. Sufficient cooling after finish rolling promotes the precipitation of V carbonitrides, which plays a role in precipitation strengthening.

[0043] The present invention uses a low-carbon, low-phosphorus vanadium micro-alloying process design, combined with groove rolling of section steel, to realize the industrial production of small and medium-sized high-toughness H-section steel products of 420 MPa grade.

[0044] [About the dephosphorization process]

[0045] In the tapping process of a blast furnace, the hot metal is dephosphorized by multi-stage spraying. The hot metal in the ladle is dephosphorized by spraying. Taking advantage of the fact that the kinetic conditions for dephosphorization of the hot metal at the steel-slag interface are good and dephosphorization is sufficient, a hot metal discharge port is installed on the side of the ladle 20-30 cm below the steel-slag interface. The hot metal that has been sufficiently dephosphorized at the top of the ladle flows into the next ladle through the hot metal discharge port, and is sprayed in the next ladle. The dephosphorization process using mist is repeated, and a pig iron discharge port is provided on the side of the next ladle 20 to 30 cm below the steel-slag interface. The molten pig iron that has been sufficiently dephosphorized in the upper part of the next ladle can then flow downward into the next ladle through the pig iron discharge port. This process is repeated 4 to 6 times. The dephosphorization process for molten pig iron will be described below using an example in which molten pig iron is dephosphorized four times. Since the slag in the previous ladle cannot flow into the next ladle, only the molten pig iron flows into the next ladle. The phosphorus in the molten pig iron in the next ladle again enters a dephosphorization equilibrium reaction with the slag, further removing the phosphorus from the molten pig iron. After multiple dephosphorization processes, the phosphorus in the molten pig iron is reduced to an extremely low level.

[0046] The specific implementation process of this dephosphorization method is as follows:

[0047] In step (1), in the tapping process of the blast furnace, the molten iron in the blast furnace flows into the first ladle, the nominal volume of the first ladle is 30 to 50 tons of molten iron, and the depth is 1.5 to 2 m. A pig iron discharge port is provided on the side wall of the ladle, 50 to 60 cm away from the top of the ladle, and the pig iron discharge port extends outward by 20 to 30 cm. A slide plate is used to control the opening and closing of the pig iron discharge port. With the continuous pouring of molten pig iron into the blast furnace, when the volume of the molten pig iron in the first ladle reaches 50% or more, the dephosphorization agent is sprayed into the ladle to dephosphorize it. At this time, the pig iron discharge port is closed, and the liquid level in the ladle rises continuously. When the liquid level of the molten pig iron exceeds the upper edge of the pig iron discharge port by 20-30 cm, the pig iron discharge port is opened, and the molten pig iron flows into the second ladle through the pig iron discharge port, and the molten pig iron is removed from the first ladle. The flow rate at which the blast furnace hot metal flows into the second ladle from the first ladle is the same as the flow rate at which the blast furnace hot metal flows into the first ladle, and the hot metal level is in a stable state, neither rising nor falling. A dephosphorization reaction occurs continuously at the steel-slag interface in the first ladle, continuously removing the phosphorus contained in the hot metal near the steel-slag interface, and the hot metal that has been sufficiently dephosphorized at the top of the first ladle flows into the second ladle through the hot metal discharge port.

[0048] The second ladle is located 60 to 70 cm lower than the position of the first ladle, and the molten iron flows from the first ladle through the iron discharge port into the second ladle by the action of gravity.

[0049] In step (2), the ladle size parameters and dephosphorization process parameters of the second, third, and fourth ladles are exactly the same as those of the first ladle, and the dephosphorization operation is carried out by spraying into the second, third, and fourth ladles.

[0050] The hot metal that has been sufficiently dephosphorized near the steel-slag interface in the second ladle flows by gravity through the hot metal discharge port into the third ladle, the hot metal that has been sufficiently dephosphorized near the steel-slag interface in the third ladle flows by gravity through the hot metal discharge port into the fourth ladle, and the hot metal that has been sufficiently dephosphorized near the steel-slag interface in the fourth ladle flows by gravity through the hot metal discharge port into the fifth ladle.

[0051] The third, fourth and fifth ladles are each 60 to 70 cm lower than the second, third and fourth ladles.

[0052] The fifth ladle is a large-volume ladle, and its volume matches the tonnage of the converter. The molten iron in the fifth ladle is sent to the KR station for desulfurization. After desulfurization is complete, it is sent to the converter for normal smelting, and the phosphorus content at the end of converter smelting is controlled to be below 0.007%.

[0053] Preferably, every 30-40 minutes of normal dephosphorization, the first to fourth ladles undergo one slag exchange operation. The slag exchange process involves removing the slag from the first ladle, transferring the slag from the top of the second ladle to the first ladle, transferring the slag from the top of the third ladle to the second ladle, and transferring the slag from the top of the fourth ladle to the third ladle. This multi-stage dephosphorization process ensures that the phosphorus content of each subsequent ladle is lower than that of the previous ladle. Under sufficient dephosphorization conditions, the phosphorus distribution ratio in the slag remains essentially unchanged, i.e., the phosphorus content of the slag in the subsequent ladle is lower than that of the previous ladle. Therefore, when the upper slag of the subsequent ladle is transferred to the previous ladle, the slag can still achieve good dephosphorization.

[0054] Preferably, the first to fourth ladles are each provided with a slag outlet at the top, the slag outlet extending outward by 20 to 30 mm, the slag in the ladle can flow out through the slag outlet, and the difference in position between the slag outlet and the pig iron outlet on the ladle wall is one-fourth of the circumference.

[0055] Preferably, the ladle cars of the first to fourth ladles are all equipped with hydraulic lifting and rotating devices, which can lift and lower the ladles by 1 to 2 m and rotate them 90 degrees around the central axis.

[0056] Preferably, when performing the slag exchange operation, the pig iron discharge port is first closed, and then the ladle cart's rotation device is used to rotate the preceding ladle and the next ladle by 90 degrees around the central axis to avoid contact with the pig iron discharge port when the ladles are raised or lowered. The preceding ladle is then lowered and the next ladle is raised using the hydraulic lifting device. When the next ladle is higher than the preceding ladle and the slag in the next ladle can flow into the preceding ladle through the slag discharge port, the slag discharge port is opened, and the slag flows from the following ladle into the preceding ladle. After the slag has completely flowed in, the slag discharge port is closed.

[0057] Any steps not mentioned in this invention can use conventional techniques. [Effects of the Invention]

[0058] The advantages of the technical solution of the present invention are as follows:

[0059] 1. This invention combines the thin flange characteristics of small and medium-sized H-section steel produced by rolling rectangular slabs, and uses a design of components that combines a low C content suitable for normalizing rolling with V micro-alloying. By adding an appropriate amount of Cr and controlling the cooling rate, it is possible to avoid the formation of abnormal structures such as Widmanstätten, which would reduce the low-temperature impact toughness of the steel. This allows the rolling mill used to hot-roll H-section steel to produce stably and controlled high-toughness hot-rolled H-section steel of 420 MPa grade or higher.

[0060] 2. The present invention uses a low-phosphorus smelting process to control the P content to 0.008% or less and the P+S content to 0.011% or less, contributing to improving the low-temperature toughness of the steel and maintaining a high level even under conditions of -50°C.

[0061] 3. The matrix structure of the H-shaped steel of the present invention is refined pearlite + proeutectoid ferrite, and the second phase particles are mainly V(C,N), which has better structural stability and is easier to obtain.

[0062] 4. The present invention achieves fine grain strengthening and precipitation strengthening by refining the matrix structure, and achieves the favorable effects of the H-shaped steel attaining 420 MPa and low-temperature impact toughness of greater than 100 J at -50°C.

[0063] 5. In the present invention, during the tapping process of a blast furnace, the dephosphorization operation is carried out by spraying the molten pig iron 4 to 6 times using 4 to 6 small ladles. This dephosphorization operation is a continuous operation with high production efficiency. The total dephosphorization time, which is the time it takes for the molten pig iron to flow through these 4 to 6 small ladles, is only 20 to 30 minutes, which is much shorter than the processing time of the three removal processes, and the dephosphorization efficiency is high.

[0064] 6. In the present invention, the pig iron discharge port of the small ladle is located near the steel-slag interface of the ladle. At this position, the dephosphorization kinetics conditions are favorable and the dephosphorization reaction at the slag-steel interface is sufficient. Therefore, the fully dephosphorized molten pig iron flows into the next ladle, while the molten pig iron at the bottom of the ladle where the dephosphorization reaction has not yet occurred rises to the vicinity of the slag-steel interface to continue the dephosphorization reaction. This dephosphorization method has a high dephosphorization efficiency.

[0065] 7. In the present invention, the amount of phosphorus concentration in the slag in the next ladle is lower than that in the previous ladle. Therefore, the hot metal flowing into the next ladle can continue to undergo dephosphorization with the slag, which is equivalent to performing a slag exchange dephosphorization operation. After four to six slag exchange dephosphorization operations, the phosphorus content in the hot metal can be reduced to 0.02% or less, which is a high dephosphorization rate of the hot metal and is advantageous for the stable production of low-phosphorus steel.

[0066] 8. In the present invention, the phosphorus concentration of the slag in the next ladle is lower than that of the slag in the previous ladle. Therefore, after the slag in the next ladle flows into the previous ladle, the dephosphorization effect can still be achieved. This is equivalent to recycling the dephosphorization slag, which saves the consumption of dephosphorization materials and reduces the dephosphorization cost. [Brief explanation of the drawings]

[0067] [Figure 1] 1 is a metallographic diagram (×200) of a high-toughness, low-temperature-resistant H-shaped steel having a yield strength of 420 MPa grade manufactured in Example 2 according to the present invention. [Figure 2] FIG. 1 is a schematic diagram of the multi-stage dephosphorization process described herein. [Figure 3] FIG. 1 is a schematic diagram of the structure of a small ladle. [Figure 4] FIG. 2 is a side view of the structure of a small ladle. DETAILED DESCRIPTION OF THE INVENTION

[0068] The present invention will be described below with reference to specific examples, which are merely for the purpose of further illustrating the present invention and are not intended to limit the scope of protection of the present invention, and any non-essential modifications or adjustments made by those skilled in the art based on the present invention are also within the scope of protection of the present invention.

[0069] The continuous cast slabs in the following examples were produced according to a process that uses blast furnace hot metal as the raw material, dephosphorization during the blast furnace tapping process, KR dephosphorization, smelting and refining using a converter, adjusting the contents of C, Si, Mn, S, P, etc., micro-alloying, and, after the components reach the target values, continuous casting and direct heating or soaking of the slab. The production steps for Examples 1 to 4 are as follows.

[0070] The steel undergoes the following steps: dephosphorization during the blast furnace tapping process, KR desulfurization as pretreatment for the molten iron, smelting in a converter, argon injection into the ladle, refining, continuous casting, wire rolling, online cooling, and slow cooling in a cooling bed. Wire rolling involves two stages: rough rolling and finish rolling. The hot rolling process is mainly based on temperature control, with the end temperature detected on the outside of the flange, and the rolled material naturally cooling in a cooling bed after rolling. The chemical compositions and specific processes for Examples 1 to 4 are shown in Table 1 below. [Table 1]

[0071] The specific dephosphorization process during the blast furnace tapping process is as follows:

[0072] In step 1, during the tapping process of the blast furnace, the molten pig iron in the blast furnace flows into the first ladle. The nominal volume of the first ladle is 30 tons of molten pig iron, and the depth is 1.5 m. A pig iron discharge port is installed on the side wall of the ladle 50 cm away from the top of the ladle. The pig iron discharge port extends 20 cm outward, and a slide plate is used to control the opening and closing of the pig iron discharge port. As the molten pig iron is continuously poured into the blast furnace, when the volume of the molten pig iron in the first ladle reaches 50% or more, a dephosphorizing agent is sprayed into the ladle to dephosphorize it. At this time, the pig iron discharge port is closed, and the liquid level in the ladle constantly rises. When the liquid level of the molten pig iron exceeds the upper edge of the pig iron discharge port by 20 cm, the pig iron discharge port is opened, and the molten pig iron flows into the second ladle through the pig iron discharge port. The molten pig iron flows from the first ladle to the second ladle. The flow rate of the blast furnace hot metal into the first ladle is the same as the flow rate of the blast furnace hot metal into the first ladle, and the hot metal level is in a stable state, neither rising nor falling. A dephosphorization reaction occurs continuously at the steel-slag interface in the first ladle, continuously removing phosphorus contained in the hot metal near the steel-slag interface. The hot metal that has been sufficiently dephosphorized in the upper part of the first ladle flows into the second ladle through the hot metal discharge port, as shown in Figures 2 to 4.

[0073] The spray points of the dephosphorization agent include the point where blast furnace hot metal is poured into the first ladle and the point where hot metal from the ladle immediately preceding the next ladle is poured into the next ladle. At these locations, the hot metal is stirred vigorously, providing favorable kinetic conditions for dephosphorization.

[0074] The dephosphorization agent is a mixture of sludge balls and lime, and the amount of dephosphorization agent added to the first ladle is 2 kg / 1 ton of iron / ladle. The mass ratio of sludge balls to lime is 1:1. The components of the sludge balls are, in mass percent, CaO: 5%, Fe2O3: 88%, SiO2: 3%, and the remainder are impurities. The components of the lime are, in mass percent, CaO: 90%, Fe2O3: 1%, SiO2: 3%, MgO: 5%, and the remainder are impurities.

[0075] The carrier gas used when spraying the dephosphorization agent was oxygen, the spray pressure was 0.2 MPa, and the spray flow rate was 2 m3 / min.

[0076] The second ladle is positioned 60 cm lower than the first ladle, and the molten iron flows from the first ladle through the iron discharge port into the second ladle by gravity.

[0077] In step 2, the ladle size parameters and dephosphorization process parameters of the second, third, and fourth ladles are exactly the same as those of the first ladle, and the dephosphorization operation is performed by spraying in all of the second, third, and fourth ladles.

[0078] The hot metal that has been sufficiently dephosphorized near the steel-slag interface in the second ladle flows by gravity through the hot metal discharge port into the third ladle, the hot metal that has been sufficiently dephosphorized near the steel-slag interface in the third ladle flows by gravity through the hot metal discharge port into the fourth ladle, and the hot metal that has been sufficiently dephosphorized near the steel-slag interface in the fourth ladle flows by gravity through the hot metal discharge port into the fifth ladle.

[0079] The third, fourth and fifth ladles are 60 cm lower than the second, third and fourth ladles, respectively.

[0080] The composition and temperature of the hot metal were detected, and the dephosphorization rate and the hot metal temperature in the four small ladles are shown in the table below. [Table 2]

[0081] As can be seen from the table above, after dephosphorization in the four small ladles, the phosphorus content of the hot metal was reduced from 0.163% to 0.017%, the dephosphorization rate of the hot metal was 89.6%, and the dephosphorization rate of the hot metal was high. The temperature of the hot metal was reduced from 1520°C to 1426°C, and the temperature decrease of the hot metal was 94°C, which was small. The temperature of the hot metal after dephosphorization was much higher than the required temperature (higher than 1250°C) before entering the converter.

[0082] The fifth ladle is a large-volume ladle, and its volume matches the tonnage of the converter. The molten iron in the fifth ladle is sent to the KR station for desulfurization. After desulfurization is complete, it is sent to the converter for normal smelting, and the phosphorus content at the end of smelting in the converter is controlled to be 0.007% or less.

[0083] During each normal 30-40 minute dephosphorization, each of the first to fourth ladles undergoes a slag exchange operation. The slag exchange process involves removing the slag from the first ladle, transferring the slag from the top of the second ladle to the first ladle, transferring the slag from the top of the third ladle to the second ladle, and transferring the slag from the top of the fourth ladle to the third ladle. This multi-stage dephosphorization process ensures that the phosphorus content of each subsequent ladle is lower than that of the previous ladle. Under sufficient dephosphorization conditions, the phosphorus distribution ratio in the slag remains essentially unchanged, i.e., the phosphorus content of the slag in each subsequent ladle is lower than that of the previous ladle. The slag in each ladle was subjected to chemical composition testing, and the chemical compositions of the slag in the four small ladles are shown in the table below. [Table 3]

[0084] As can be seen from the above table, the phosphorus content of the slag in the next ladle is lower than that of the previous ladle, and it has higher alkalinity and stronger oxidizing property (T.Fe can be converted to FeO). Therefore, if the slag in the upper part of the next ladle is placed in the previous ladle, the slag can still achieve good dephosphorization effect.

[0085] The first to fourth ladles each have a slag outlet at the top, which extends 20 mm outward, allowing the slag in the ladle to flow out through the slag outlet. The difference in position between the slag outlet and the pig iron outlet on the ladle wall is one-fourth of the circumference.

[0086] The ladle cars for the first to fourth ladles are all equipped with hydraulic lifting and rotating devices, which can lift and lower the ladles by 1.5 m and rotate them 90 degrees around the central axis.

[0087] When performing the slag exchange operation, first close the pig iron discharge port, then use the ladle cart's rotation device to rotate the previous and next ladles 90 degrees around the center axis to avoid contact with the pig iron discharge port when the ladles are raised or lowered, then use the hydraulic lifting device to lower the previous ladle and raise the next ladle, and when the next ladle is higher than the previous one and the slag in the next ladle can flow into the previous ladle through the slag discharge port, the slag discharge port is opened and the slag flows from the next ladle into the previous ladle, and once the slag has completely flowed in, the slag discharge port is closed.

[0088] The hot rolling process conditions for Examples 1 to 4 are shown in Table 2. The reference standard is BS EN ISO 377-1997 "Sampling location and manufacture of mechanical property test specimens", the test methods for yield strength, tensile strength and elongation refer to standard ISO6892-1-2009 "Room temperature tensile test method for metallic materials", the test method for impact energy refer to standard ISO 148-1 "Charpy pendulum impact test for metallic materials", and the results are shown in Table 3. [Table 4] [Table 5]

[0089] As can be seen from the table, the H-section steels in Examples 1 to 4 of the present invention maintain a yield strength of 440 MPa, have good elongation performance, and have high impact energy at -50°C. These steels can meet the extremely low temperature operating conditions of marine engineering components and can be used to manufacture supporting structural parts that require high low temperature toughness, such as for offshore oil platforms and ocean transport vessels.

[0090] As can be seen from Figure 1, the structure of the H-section steel in this application is a granular bainite + ferrite structure. Reducing the P content not only satisfies the strength requirement for 420 MPa grade H-section steel, but also improves toughness, particularly low-temperature toughness. For details not specifically described in this invention, ordinary technical knowledge in this field can be used.

[0091] Finally, the above embodiments are only for illustrating the technical solutions of the present invention, and are not intended to limit the same. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art can understand that, without departing from the spirit and scope of the technical solutions of the present invention, any modifications or equivalent replacements made to the technical solutions of the present invention should be included in the scope of the claims of the present invention. [Explanation of symbols]

[0092] 1...blast furnace, 2...first ladle, 3...second ladle, 4...third ladle, 5...fourth ladle, 6...fifth ladle, 7...powder spray desulfurization device, 8...iron discharge port, 9...slag discharge port

Claims

1. A low-temperature resistant H-shaped steel having a yield strength of 420 MPa or more, The chemical composition of the H-shaped steel is, in mass percent, C: 0.08% to 0.10%, Si≦0.2%, Mn: 1.25% to 1.45%, V: 0.03% to 0.045%, Ti: 0.015% to 0.025%, Cr: 0.15% to 0.30%, Als (acid-soluble Al): 0.02% to 0.04%, N: 0.007% to 0.01%, P≦0.008%, S≦0.005%, O≦0.004%, and the remainder being Fe and unavoidable impurities; An H-shaped steel having a Charpy absorbed energy of 100 J or more at -50°C.

2. 2. The H-beam steel according to claim 1, wherein the chemical composition of the H-beam steel is, in mass percent, P+S≦0.01%.

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