A clean bridge steel plate and a method of manufacturing the same

Abstract

The present application relates to a kind of high purity bridge steel plate and its manufacturing method, steel plate is by the following mass percentage component smelting: C:0.04~0.12%, Si:0.05%~0.55%, Mn:0.90%~1.70%, P≤0.015%, S≤0.001%, Alt:0.015%~0.050, Ca:0.0008%~0.0030%, O:≤0.0012%, H:≤0.00015%, N:0.0010%~0.0050%, the balance is Fe, other small (micro) alloying element and inevitable impurity.The steel plate of the present application is by specific molten steel preliminary refining, refining process, Ca treatment technology, controls molten steel P, S, O, H, N and other harmful element residual content, effectively reduces the residual content of inclusions in molten steel, controls residual inclusion form, reaches pure steel smelting.Continues to reduce molten steel inclusions by slab continuous casting plays the effect of tundish metallurgy, crystallizer metallurgy, effectively controls the inclusions segregation, controls the secondary precipitation behavior of inclusions in solidification process, realizes pure, uniform continuous casting billet casting, satisfies the technical requirements of bridge steel high purity, low inclusions content.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a pure bridge steel plate and its manufacturing method. Background Technology

[0002] As modern bridges gradually develop towards longer spans and heavier loads, fatigue and brittleness are important factors affecting the lifespan and safety of bridges. Certain inclusions in steel are important causes of fatigue cracks or brittleness. Therefore, improving the purity of steel and reducing the content of inclusions in steel are important means to improve the safety and achieve a longer lifespan for large and extra-large bridges, and have gradually gained high attention in bridge design.

[0003] Authorization announcement number CN115074630B discloses a high-ductility FH36 grade marine engineering steel and its manufacturing method. The claims specify that inclusions of types A, B, C, and D in the steel must satisfy the following values: A≤0.5, B≤0.5, C≤0.5, and D≤0.5. The aim is to ensure performance indicators such as tensile elongation after fracture ≥30%, maximum force elongation ≥26%, Charpy impact energy at -60℃ ≥200J, and zero plastic transformation temperature less than -60℃. Inclusion control is not the focus of this application, and the inclusion control measures are not as specific and detailed as those in this application. Furthermore, requiring excessively low inclusion levels would inevitably increase steelmaking costs.

[0004] Application publication number CN116121654A discloses an ultra-high strength steel for key equipment in large hydropower stations and its manufacturing method. It requires that the smelting process ensures that the non-metallic inclusions in the steel are ≤0.5 grade. The purpose is to obtain mechanical and technological properties such as room temperature tensile strength ≥1000MPa, yield strength ≥900MPa, elongation after fracture ≥15%, impact absorption energy at -40℃ ≥150J, 180° cold bending d=2a qualified, and strain aging impact absorption energy at -40℃ ≥100J. Inclusion control is not the focus of this application, and the inclusion control measures are not as specific and detailed as those in this application. At the same time, its requirements for inclusions are relatively general. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-carbon steel plate for bridge steel structures with high purity and low inclusion content, which is in contrast to the above-mentioned prior art. The manufacturing method provided effectively reduces the residual inclusion content in molten steel, controls the morphology of residual inclusions, and achieves pure steel smelting.

[0006] The technical solution adopted by the present invention to solve the above problems is as follows: a pure bridge steel plate, the chemical composition of which, by mass percentage, is C: 0.04-0.12%, Si: 0.05%-0.55%, Mn: 0.90%-1.70%, P≤0.015%, S≤0.001%, Alt: 0.015%-0.050%, Ca: 0.0008%-0.0030%, O: ≤0.0012%, H: ≤0.00015%, N: 0.0010%-0.0050%, with the balance being Fe, other trace (micro) alloying elements and unavoidable impurities.

[0007] The steel plate has high purity, and the non-metallic inclusions meet the requirements specified in GB / T 10561.

[0008]

[0009] This invention utilizes specific primary and refining processes for molten steel, along with Ca treatment technology, to control the residual content of harmful elements such as P, S, O, H, and N in the molten steel. This effectively reduces the residual content and morphology of inclusions, achieving pure steel smelting. Continuous casting of the slab further reduces inclusions by leveraging the effects of tundish and crystallizer metallurgy, effectively controlling inclusion agglomeration and secondary precipitation during solidification. This results in pure, uniform continuous casting, meeting the technical requirements for high purity, low inclusion content, and suitable residual inclusion morphology in bridge steel. This ensures the excellent fatigue resistance and resistance to brittle fracture of this low-carbon bridge steel plate.

[0010] The roles of the components and their contents in the steel plate in this invention are as follows:

[0011] (1) Carbon (C)

[0012] It forms carbides with microalloying elements such as Nb, Ti, and V, inhibiting austenite grain growth and controlling austenite recrystallization behavior, thus exhibiting a certain precipitation strengthening effect and controlling the microstructure and properties of the steel plate. However, excessively high content will severely deteriorate the low-temperature toughness and weldability of the steel; conversely, excessively low content will lead to severe over-oxidation of the molten steel during converter smelting, increasing the deoxidation burden during subsequent refining and also affecting the content and morphology of inclusions in the steel plate. This invention is applicable to steel plates with a carbon (C) content of 0.04–0.12%.

[0013] (2) Silicon (Si)

[0014] It is a relatively economical deoxidizer in the steelmaking process, used for precipitation deoxidation and removal of oxygen from slag; the main problem is that if it remains in the steel as silicate inclusions, it will cause the level of Class C inclusions in the steel to exceed the standard. This invention is applicable to steel plates with a Si content of 0.05% to 0.55%.

[0015] (3) Manganese (Mn)

[0016] Mn is an important strengthening element in steel, increasing the strength of steel plates; it affects the phase transformation point of steel, thus influencing the plasticity and toughness of steel plates; it improves the hot brittleness of steel; improper handling can easily lead to excessive levels of Class A inclusions in the steel, or cause excessive point-like inclusions. This invention is applicable to steel plates with a Mn content of 0.90% to 1.70%.

[0017] (4) Phosphorus (P)

[0018] Phosphorus has extremely low solubility in solid steel. During solidification, it segregates between grains, forming a high-phosphorus brittle layer; or it forms Fe3P inclusions, which are hard and brittle, making the steel prone to brittle cracking and extremely detrimental to low-temperature toughness. This invention primarily treats phosphorus (P) as a harmful element, reducing its content to ≤0.015%.

[0019] (5) Sulfur (S)

[0020] During the refining process or the solidification process of continuously cast billets, segregation occurs, and Mn reacts with Mn to form MnS. This MnS extends along the rolling rheological direction, forming banded inclusions that are large Class A inclusions in the steel. These inclusions significantly reduce the steel's plasticity, toughness, and fatigue properties, and may also cause hydrogen-induced cracking. This invention reduces its content to ≤0.001% and modifies its residual compounds into spherical or dot-like forms.

[0021] 6. Aluminum (AL)

[0022] Strong deoxidizing elements, used for final deoxidation of molten steel, are important for reducing the oxygen content and inclusions in steel. Residual oxides in steel exist as Al2O3, or as composite inclusions of Al2O3 / CaO / MgO / CaS in different proportions. After rolling deformation, these form chain-like Class B inclusions or Class C and D inclusions, which must be strictly controlled. This invention controls the total aluminum (ALt) content in the steel to be between 0.015% and 0.050%.

[0023] (7) Calcium (Ca)

[0024] It has strong deoxidizing ability and can deeply deoxidize molten steel; by changing the composition and morphology of inclusions, it can promote the reduction of the amount of aluminate inclusions in steel; it can transform inclusions from long strips into small, dispersed spherical dots, reducing their harm to toughness and improving the inclusion rating of steel plates. To ensure the Ca treatment effect, this invention controls the Ca content in the steel to be 0.0008% to 0.0030%.

[0025] (8) Oxygen (O)

[0026] Oxygen has extremely low solubility in steel and a severe tendency to segregate. During solidification and cooling, almost all the oxygen in the steel will precipitate out, and most of it will react with Fe, S, etc. to form oxides and oxygen sulfides, which accumulate at the grain boundaries, causing grain boundary embrittlement. Therefore, oxygen is the main source of inclusions in steel and can form various types of inclusions, reducing the ductility, toughness, and fatigue properties of steel. It is necessary to reduce the oxygen content in steel and modify the morphology of inclusions remaining in the steel to avoid or reduce the impact of oxide inclusions on the properties of steel plates. This invention controls the O content in steel to ≤0.0012%.

[0027] (9) Hydrogen (H)

[0028] Even trace amounts can cause hydrogen embrittlement, reducing the toughness of steel; at certain levels, it can cause microscopic defects such as white spots and microcracks within the steel, and even render the steel unusable. Therefore, it is considered a harmful element in steel, and its content should be minimized. This invention controls hydrogen (H) content to ≤0.00015%.

[0029] (10) Nitrogen (N)

[0030] When nitrogen (N) combines with elements such as Nb, V, and Ti, it can partially replace carbon to exert the effects of Nb, V, and Ti; however, it easily forms coarse TiN particles, causing excessive inclusions in the steel. This invention controls the nitrogen (N) content to be between 0.0010% and 0.0050%.

[0031] The present invention also aims to provide a method for manufacturing high-purity, low-impurity steel plates with the above-mentioned chemical composition, suitable for the fabrication of steel structures such as large and extra-large bridges. The specific steps are as follows:

[0032] The first step, initial training

[0033] KR pretreatment reduces the [S] content in molten iron to ≤0.005%. In the BOF converter primary refining process, the [C] content is controlled to be no less than 0.02%, and [P] ≤0.005% before tapping. Slag is blocked during tapping to strictly prevent primary refining slag from entering the ladle with the steel stream. During tapping, at least 1.5 kg / t of silicon manganese and at least 2.0 kg / t of aluminum ingots are added with the steel stream for pre-deoxidation. Sulfur is the most important element forming Class A inclusions, so its residual content is reduced from the beginning of molten iron production. The [C] content is affected by the blowing, which impacts the oxygen content in the molten steel. High oxygen content increases the refining deoxidation burden and affects the quantity and morphology of inclusions in the final steel. P is a harmful element and can cause inclusions in steel; controlling the [P] content in the molten steel before tapping aims to ensure that the final steel [P] ≤0.015%. Poor slag blocking allows primary slag to enter the ladle, increasing the deoxidation burden during refining and affecting the quantity and morphology of inclusions in the final steel, thus increasing the [P] content in the final molten steel. Adding silicon-manganese and aluminum ingots during tapping leverages the advantage of sufficient alloy contact between the molten steel and the alloys to pre-deoxidize the steel.

[0034] The second step is LF refining.

[0035] LF refining uses specialized refining slag to create white slag. Silicon can be used as a deoxidizer in the early stages, while aluminum is used exclusively in the middle and later stages. During LF refining, the [AL] content in the molten steel is maintained above 0.009%. FeSi powder or SiC powder is added during refining to remove free oxygen from the slag, reducing the (FeO) content and preventing oxygen transfer from the steel-slag interface to the molten steel, thus improving the slag's ability to adsorb and stabilize inclusions. The slag content is controlled at approximately 20-36% Al₂O₃, 35-55% CaO, 3.0-4.5 basicity, and no more than 10% MgO, maximizing the deoxidation effect at the steel-slag interface and ensuring sufficient adsorption and stabilization of inclusions by the refined slag. The total LF refining time is no less than 45 minutes, with the white slag refining time ≥38 minutes, ensuring sufficient deoxidation and desulfurization depth and complete removal of chemical reaction products. In the final stage of LF refining, a calcium feed line is used to control the T[Ca] / T[O] ratio in the molten steel, so that calcium aluminate in the molten steel is a low-melting-point liquid inclusion that floats to the slag with Ar blowing and stirring.

[0036] The third step, RH vacuum refining.

[0037] Before vacuum treatment, new slag is formed on the surface of the molten steel. The basicity of the new slag is not less than 3.5, and the fluidity of the molten steel is suitable to ensure that the slag adsorbs and stabilizes inclusions, thus purifying the molten steel. Before vacuum treatment, a calcium feed line is used for calcium treatment to ensure that the Al2O3 inclusions still present in the molten steel are transformed into a CaO·Al2O3 liquid state or a composite inclusion with a core of calcium aluminate, magnesium spinel, etc., and an outer layer of sulfides (Ca, Mn)S. The RH vacuum treatment (<66Pa) is carried out for 18-30 minutes at a high vacuum level, with appropriate booster gas flow rate and a vacuum circulation flow rate of 5-20 t / min. This minimizes the [H] and [N] gas content in the molten steel, further enhancing the reaction capacity of carbon and chemical elements with oxygen under vacuum, promoting the depth of deoxidation and desulfurization reactions, and allowing inclusions to penetrate the steel-slag interface to the maximum extent possible. After RH vacuum is broken, the molten steel is treated with Ar by soft blowing and slag removal. During Ar blowing, inclusions larger than 10μm collide and aggregate, then float to the slag. During the slag removal process, even smaller micron-sized inclusions, such as high-melting-point CaO or CaS shells encasing low-melting-point CaO-Al2O3 cores, are further removed from the molten steel. The final molten steel is guaranteed to have [O] content ≤0.0010%, [N] content ≤0.0050%, [H] ≤0.00015%, [S] content ≤0.001%, and [Ca] content reaching 0.0008%~0.0030%, with a [Ca] / [S] ratio of 1.0~3.2. This ensures that the total inclusion content in the molten steel is reduced to an extremely low level, and MnS inclusions are modified into fine spherical (Ca, Mn)S.

[0038] Step 4: Continuous steel casting

[0039] Molten steel is cast into continuous casting slabs on a straight-arc continuous casting machine. The slab thickness is 150mm to 370mm, the tundish capacity is 60t, and the net weight of the molten steel in the tundish during casting is ≥45t. Slag retaining walls and dams are installed. The amount of covering agent used in the tundish is 6.0 to 10.0 kg / t steel. The flow field of the molten steel in the tundish is controlled to prevent slag entrapment and promote the collision and aggregation of inclusions, allowing them to enter the slag and remain stably there. The superheat of the molten steel in the tundish is controlled within 16℃ ± 10℃ during casting. The flow field of the molten steel in the crystallizer is controlled, and the fluctuation of the liquid level in the crystallizer is kept within ± 4mm to prevent slag entrapment. The amount of protective slag used in the crystallizer is not less than 0.4kg / t steel. A black slag surface is maintained during casting. The Ar gas flow rate for the casting system is not less than 21m³. 3 / h, to prevent oxidation of molten steel and to adsorb inclusions. The casting speed is controlled by A / (slab thickness × slab width), where A is a coefficient that matches the superheat level. The coefficient A is controlled between 0.5 and 4.2. The secondary cooling water volume for continuous casting is 1.03 L / kg steel to 1.28 L / kg steel to promote the flotation and removal of inclusions in the liquid phase cavity. When the solidification fraction of the continuously cast slab is 0.55 to 0.80, a light reduction of 10 mm to 16 mm is applied to the slab, distributed in 3 to 4 sector segments. The ladle pouring is stopped when the molten steel depth in the ladle is not less than 200 mm, and the slag in the ladle is strictly monitored. The tundish pouring is completed when the molten steel depth in the tundish is not less than 100 mm, and protective slag is strictly prevented from entering the continuously cast slab with the eddy current. After the continuously cast slab is pulled out of the casting machine, the hot slabs are stacked. When the slab temperature is ≤450℃, it can be disassembled and proceed to the next process. The ladle, tundish, and crystallizer strictly prevent inclusions from entering the continuously cast billet and promote the purification of molten steel. The superheat, casting speed, and secondary cooling intensity are matched to further promote the flotation of inclusions in the liquid phase cavity. Combined with light reduction technology, the center segregation of the low-magnification microstructure of the continuously cast billet is ≤C1.5 grade. This avoids the generation of new and larger Al2O3-MnS-CaS composite inclusions due to chemical equilibrium shift caused by severe segregation during the solidification and cooling process of the continuously cast billet. MnS is transformed into CaS or CaS·MnS composite inclusions, avoiding the precipitation of large TiN particles. This ensures that the inclusion rating of the steel plate is qualified. Scanning electron microscopy analysis shows that the proportion of inclusions with an average particle size >5μm in the final steel plate is ≤8%.

[0040] Step 5, rolling

[0041] The continuously cast slab is heated in a continuous heating furnace, with the heating section temperature controlled at 900℃~1280℃ and the soaking section temperature not exceeding 1260℃. The total heating time in the furnace at a temperature ≥1180℃ is ≥4.0 min / cm slab thickness, further reducing compositional segregation in the continuously cast slab. After exiting the heating furnace, the continuously cast slab is de-oxidized with high-pressure water and then rolled into finished steel plates on a four-roll reversible thick plate mill. The rolling compression ratio (finished steel plate thickness / continuous cast slab thickness) is 3.3~19.0. It is rolled in the austenite fully recrystallized zone for initial rolling and in the austenite non-recrystallized zone for finish rolling. The initial and final rolling temperatures are ≥980℃, and there are 3~6 rolling passes with a single-pass reduction rate of 8%~40%. The intermediate billet is held at a temperature of 2.0–4.5 h, where h is the thickness of the finished steel plate. The initial rolling temperature for finishing rolling is 990–780℃, with no limit on the single-pass reduction rate. The final rolling temperature is ≥ the Ar3 temperature, where Ar3 temperature is the starting temperature for the transformation of austenite to ferrite, calculated based on the actual composition of the steel plate and verified by laboratory testing. After rolling, the steel plate is held for 10–40 seconds before entering the ACC accelerated cooling system for accelerated cooling. The final cooling temperature is 500–670℃, and the cooling rate is 5.0–30.0℃ / second. Rolling must ensure that the steel plate meets the required performance while avoiding excessive elongation of residual plastic inclusions and excessive fragmentation and expansion of brittle inclusions.

[0042] Compared with the prior art, the advantages of the present invention are as follows:

[0043] This invention relates to a steel plate that, through specific primary and refining processes and Ca treatment technology, controls the residual content of harmful elements such as P, S, O, H, and N in the molten steel, effectively reducing the residual content and morphology of inclusions, thus achieving pure steel smelting. Continuous casting of the slab further reduces inclusions by leveraging the effects of tundish and crystallizer metallurgy, effectively controlling inclusion agglomeration and secondary precipitation during solidification. This results in pure, uniform continuous casting, meeting the technical requirements of high purity, low inclusion content, and suitable residual inclusion morphology for bridge steel. It also ensures excellent fatigue resistance and resistance to brittle fracture in this low-carbon bridge steel plate, making it widely applicable in the construction of large and extra-large bridge steel structures. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the optical microscopy test results of typical inclusions in the pure bridge steel plate of this invention.

[0045] Figure 2 This is a schematic diagram of the surface scan results of typical inclusions in a clean bridge steel plate according to the present invention. Detailed Implementation

[0046] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0047] Example 1

[0048] The steel plate in this embodiment is 8mm thick and of steel grade Q370qE. The steel plate is smelted from the following components by mass percentage: C: 0.08%, Si: 0.15%, Mn: 1.42%, P: 0.012%, S≤0.0008%, Alt: 0.026%, Ca: 0.0018%, O: 0.0010%, H: 0.00011%, N: 0.0023%, with the balance being Fe, other trace (micro) alloying elements, and unavoidable impurities.

[0049] The steps of the pure bridge steel plate manufacturing method in this embodiment are as follows:

[0050] The first step, initial training

[0051] KR pretreatment reduces the [S] content in molten iron to 0.004%. BOF converter primary refining controls the blowing to stop [C] at 0.03% and [P] at ≤0.005% before tapping. Slag is blocked during tapping. During the tapping process, 1.6 kg / t silicon manganese and 2.0 kg / t aluminum ingots are added with the steel stream for pre-deoxidation.

[0052] The second step is LF refining.

[0053] LF refining uses specialized refining slag. Silicon is used as the deoxidizer in the early stages, while aluminum is used entirely in the middle and later stages. The [AL] content in the molten steel during LF refining is between 0.009% and 0.046%. FeSi powder is added during the refining process to remove oxygen from the slag, controlling the Al2O3 content to approximately 20-36%, the CaO content to approximately 35-55%, the basicity to 3.0-4.5, and the MgO content to no more than 10%. The total LF refining time is 55 minutes, with 45 minutes dedicated to white slag refining. Calcium wire is fed at the end of the LF refining process to maintain a T[Ca] / T[O] ratio of approximately 1.0 in the molten steel. This ensures that calcium aluminate in the molten steel is a low-melting-point liquid inclusion, which floats to the surface and enters the slag during Ar blowing and stirring.

[0054] The third step, RH vacuum refining.

[0055] Before vacuum treatment, new slag is formed on the surface of the molten steel, with a basicity of 3.6. A calcium feed line is used to treat the molten steel with calcium before vacuum treatment. The high vacuum treatment (RH < 66 Pa) lasts for 18 minutes, with controlled flow of the lifting gas and a vacuum circulation flow rate of 7 t / min. After the RH vacuum is broken, Ar is soft-blown and the molten steel is settled for 23 minutes. The final molten steel has the following contents: [O] content 0.0010%, [N] content 0.0023%, [H] content 0.00011%, [S] content ≤ 0.0008%, [Ca] content reaching 0.0018%, and [Ca] / [S] ratio reaching 2.25.

[0056] Step 4: Continuous steel casting

[0057] Molten steel is cast into continuously cast slabs on a straight-arc continuous casting machine. The slab thickness is 150 mm. The tundish capacity is 60 t, the net weight of the molten steel during casting is 45 t, and the amount of covering agent used in the tundish is 6.8 kg / t of steel. The superheat of the molten steel in the tundish is controlled between 19 and 24 °C during the casting process. The liquid level fluctuation in the crystallizer is maintained within ±4 mm. The amount of protective slag used in the crystallizer is not less than 0.43 kg / t of steel, and the Ar gas flow rate for the casting system is 21.8 m³ / t. 3 / h. The casting speed is controlled according to A / (slab thickness × slab width), where the coefficient of A is 3.81, and the secondary cooling water volume for continuous casting is 1.09 L / kg steel. The final pour is made when the molten steel depth in the ladle reaches 210 mm, and slag discharge from the ladle is strictly inspected. The final pour is made in the tundish when the molten steel depth in the tundish reaches 120 mm. After the continuously cast slab is pulled from the casting machine, the hot slabs are stacked, and after the slab temperature reaches 450℃, they are disassembled and proceed to the next process. The final continuous cast slab has a low-magnification microstructure with center segregation of C1.5 grade, and scanning electron microscopy analysis shows that the proportion of inclusions with an average particle size >5μm in the final steel plate is 6%.

[0058] Step 5, rolling

[0059] The continuously cast slab is heated in a continuous heating furnace, with the heating section temperature controlled at 900℃~1280℃ and the soaking section temperature not exceeding 1260℃. The total heating time at a furnace temperature ≥1180℃ is 4.3 min / cm slab thickness. The steel plate rolling compression ratio is 18.8. It is initially rolled in the fully recrystallized austenite zone and then finished rolled in the non-recrystallized austenite zone. The initial rolling temperature is 1030℃, and there are 6 rolling passes with a single-pass reduction rate of 8%~40%. The intermediate slab is allowed to heat for 4.5 hours. The finishing rolling starts at 990℃ and ends at a temperature ≥Ar3. After rolling, the steel plate is held for 25 seconds before entering the ACC accelerated cooling system for accelerated cooling. The final cooling temperature is 640℃, and the cooling rate is 28℃ / second.

[0060] The final rating of inclusions in the steel plate is shown in the table below.

[0061]

[0062] Example 2

[0063] The steel plate in this embodiment is 16mm thick and of steel grade Q420qE. The steel plate is smelted from the following components by mass percentage: C: 0.08%, Si: 0.24%, Mn: 1.53%, P: 0.013%, S≤0.001%, Alt: 0.035%, Ca: 0.0025%, O: 0.0011%, H: 0.00010%, N: 0.0028%, with the balance being Fe, other trace (micro) alloying elements, and unavoidable impurities.

[0064] The steps of the pure bridge steel plate manufacturing method in this embodiment are as follows:

[0065] The first step, initial training

[0066] KR pretreatment reduces the [S] content in molten iron to 0.005%. In the BOF converter primary refining, the blowing is controlled to stop [C] at 0.04% and [P] ≤ 0.004% before tapping. Slag is blocked during tapping. During the tapping process, 1.7 kg / t of silicon manganese and 2.1 kg / t of aluminum ingots are added with the steel stream for pre-deoxidation.

[0067] The second step is LF refining.

[0068] LF refining uses special refining slag materials. Silicon is used as the deoxidizer in the early stage and aluminum is used in the middle and later stages. FeSi powder is added during the refining process to remove oxygen from the slag. The content of Al2O3 in the slag is controlled at about 20-36%, the content of CaO is about 35-55%, the basicity is 3.0-4.5, and the MgO content does not exceed 10%. The total refining time of LF is 57 minutes, of which the white slag refining time is 45 minutes. Calcium wire is fed at the end of LF refining to make the [Ca] content in the molten steel about 0.0022% and the T[Ca] / T[O] about 1.1. This makes the calcium aluminate in the molten steel a low-melting-point liquid inclusion, which floats to the slag with Ar blowing and stirring.

[0069] The third step, RH vacuum refining.

[0070] Before vacuum treatment, new slag is formed on the surface of the molten steel, with a basicity of 3.6. A calcium feed line is used to treat the molten steel with calcium before vacuum treatment. The high vacuum treatment (RH < 66 Pa) lasts for 19 minutes, with controlled flow of the booster gas and a vacuum circulation flow rate of 8 t / min. After the RH vacuum is broken, Ar is soft-blown and the steel is settled for 23 minutes. The final molten steel has the following contents: [O] 0.0011%, [N] 0.0028%, [H] 0.00010%, [S] ≤ 0.0010%, [Ca] 0.0025%, and [Ca] / [S] ratio of 2.5.

[0071] Step 4: Continuous steel casting

[0072] Molten steel was cast into continuous casting slabs on a straight-arc continuous casting machine. The slab thickness was 150 mm. The tundish capacity was 60 t, the net weight of the molten steel during casting was 48 t, and the amount of covering agent used in the tundish was 7.2 kg / t of steel. The superheat of the molten steel in the tundish was controlled at 12–16 °C during the casting process. The liquid level fluctuation in the crystallizer was maintained within ±4 mm. The amount of protective slag used in the crystallizer was 0.45 kg / t of steel, and the Ar gas flow rate for the casting system was 22 m³ / t. 3 / h. The casting speed is controlled by A / (slab thickness × slab width), where the coefficient of A is 2.62, and the secondary cooling water volume for continuous casting is 1.10 L / kg steel. The ladle is poured when the molten steel depth in the ladle reaches 210 mm, and slag discharge from the ladle is strictly monitored. The tundish is poured when the molten steel depth in the tundish reaches 110 mm. After being pulled from the casting machine, the hot billets are stacked, and after reaching a billet temperature of 440℃, they are disassembled and proceed to the next process. The final continuous casting billet has a low-magnification microstructure with center segregation of C 0.5 grade. Scanning electron microscopy analysis shows that the proportion of inclusions with an average particle size >5μm in the final steel plate is 8%.

[0073] Step 5, rolling

[0074] The continuously cast slab is heated in a continuous heating furnace, with the heating section temperature controlled at 900℃~1280℃ and the soaking section temperature not exceeding 1260℃. The total heating time at a furnace temperature ≥1180℃ is 4.5 min / cm slab thickness. The steel plate rolling compression ratio is 9.3. It is initially rolled in the fully recrystallized austenite zone and then finished rolled in the non-recrystallized austenite zone. The initial rolling temperature is 1050℃, and there are 5 rolling passes with a single-pass reduction rate of 8%~40%. The intermediate slab is allowed to heat for 4.0 h. The finishing rolling starts at 920℃ and ends at a temperature ≥Ar3. After rolling, the steel plate is held for 20 seconds before entering the ACC accelerated cooling system for accelerated cooling. The final cooling temperature is 600℃, and the cooling rate is 23℃ / second.

[0075] The final rating of inclusions in the steel plate is shown in the table below.

[0076]

[0077] Example 3

[0078] The steel plate in this embodiment is 44mm thick and of steel grade Q420qE. The steel plate is smelted from the following components by mass percentage: C: 0.07%, Si: 0.23%, Mn: 1.54%, P: 0.011%, S≤0.0009%, Alt: 0.025%, Ca: 0.0022%, O: 0.0009%, H: 0.00009%, N: 0.0037%, with the balance being Fe, other trace (micro) alloying elements, and unavoidable impurities.

[0079] The steps of the pure bridge steel plate manufacturing method in this embodiment are as follows:

[0080] The first step, initial training

[0081] KR pretreatment reduces the [S] content in molten iron to 0.004%. BOF converter primary refining controls the blowing to stop [C] at 0.02% and [P] at ≤0.005% before tapping. Slag is blocked during tapping. During the tapping process, 1.5 kg / t silicon manganese and 2.2 kg / t aluminum ingots are added with the steel stream for pre-deoxidation.

[0082] The second step is LF refining.

[0083] LF refining uses special refining slag materials. Silicon can be used as a precipitating deoxidizer in the early stage, while aluminum is used exclusively in the middle and later stages. FeSi powder is added during the refining process to remove oxygen from the slag. The Al2O3 content in the slag is controlled at about 20-36%, the CaO content at about 35-55%, the basicity at 3.0-4.5, and the MgO content at no more than 10%. The total refining time of LF is 50 minutes, of which the white slag refining time is 40 minutes. Calcium wire is fed at the end of LF refining to make the [Ca] content in the molten steel about 0.0033% and the T[Ca] / T[O] about 1.0. This makes the calcium aluminate in the molten steel a low-melting-point liquid inclusion, which floats to the slag with Ar blowing and stirring.

[0084] The third step, RH vacuum refining.

[0085] Before vacuum treatment, new slag is formed on the surface of the molten steel, with a basicity of 3.7. A calcium feed line is used to treat the molten steel with calcium before vacuum treatment. The RH vacuum treatment (<66 Pa) lasts for 20 minutes, with controlled flow of the lifting gas and a vacuum circulation flow rate of 7 t / min. After the RH vacuum is broken, Ar is soft-blown and the steel is settled for 28 minutes. The final molten steel has the following contents: [O] content 0.0009%, [N] content 0.0037%, [H] content 0.00009%, [S] content ≤0.0009%, [Ca] content reaching 0.0022%, and [Ca] / [S] ratio reaching 2.44.

[0086] Step 4: Continuous steel casting

[0087] Molten steel was cast into continuously cast slabs on a straight-arc continuous casting machine. The slab thickness was 370 mm. The tundish capacity was 60 t, the net weight of the molten steel during casting was 50 t, and the amount of covering agent used in the tundish was 8.8 kg / t of steel. The superheat of the molten steel in the tundish was controlled between 11 and 17 °C during the casting process. The liquid level fluctuation in the crystallizer was maintained within ±4 mm. The amount of protective slag used in the crystallizer was 0.5 kg / t of steel, and the Ar gas flow rate for sealing the casting system was 22.1 m³ / t. 3 / h. The casting speed is controlled by A / (slab thickness × slab width), where the coefficient of A is 0.65, and the secondary cooling water volume for continuous casting is 1.12 L / kg steel. When the solidification fraction of the continuously cast slab is 0.66, a light reduction is applied to the slab, with a dynamic soft reduction of 16 mm at the end of solidification, distributed among three sector sections. The final pouring is completed when the molten steel depth in the ladle reaches 210 mm, and slag discharge from the ladle is strictly inspected. The final pouring is completed when the molten steel depth in the tundish reaches 120 mm. After the continuously cast slab is pulled from the casting machine, the hot slabs are stacked, and after the slab temperature reaches 400℃, they are disassembled and proceed to the next process. The final continuously cast slab has a low-magnification microstructure with center segregation of grade C1.0, and scanning electron microscopy analysis shows that the proportion of inclusions with an average particle size >5 μm in the final steel plate is 7%.

[0088] Step 5, rolling

[0089] The continuously cast slab is heated in a continuous heating furnace, with the heating section temperature controlled at 900℃~1280℃ and the soaking section temperature not exceeding 1260℃. The total heating time at a furnace temperature ≥1180℃ is 5.2 min / cm slab thickness. The steel plate rolling compression ratio is 8.3. It is initially rolled in the fully recrystallized austenite zone and then finished rolled in the non-recrystallized austenite zone. The initial rolling temperature is 1010℃, and there are 4 rolling passes with a single-pass reduction rate of 8%~40%. The intermediate slab is allowed to heat for 2.3 hours. The finishing rolling starts at 850℃ and ends at a temperature ≥Ar3. After rolling, the steel plate is held for 19 seconds before entering the ACC accelerated cooling system for accelerated cooling. The final cooling temperature is 580℃, and the cooling rate is 12℃ / second.

[0090] The final rating of inclusions in the steel plate is shown in the table below.

[0091]

[0092] Example 4

[0093] The steel plate in this embodiment is 60mm thick and of steel grade Q370qE. The steel plate is smelted from the following components by mass percentage: C: 0.09%, Si: 0.24%, Mn: 1.56%, P: 0.009%, S≤0.0010%, Alt: 0.032%, Ca: 0.0021%, O: 0.0010%, H: 0.00008%, N: 0.0035%, with the balance being Fe, other trace (micro) alloying elements, and unavoidable impurities.

[0094] The steps of the pure bridge steel plate manufacturing method in this embodiment are as follows:

[0095] The first step, initial training

[0096] KR pretreatment reduces the [S] content in molten iron to 0.003%. In the BOF converter primary refining, the blowing is controlled to stop [C] at 0.04% and [P] ≤ 0.004% before tapping. Slag is blocked during tapping. During the tapping process, 1.6 kg / t of silicon manganese and 2.1 kg / t of aluminum ingots are added with the steel stream for pre-deoxidation.

[0097] The second step is LF refining.

[0098] LF refining uses special refining slag materials. Silicon can be used as a precipitating deoxidizer in the early stage, while aluminum is used exclusively in the middle and later stages. FeSi powder is added during the refining process to remove oxygen from the slag. The Al2O3 content in the slag is controlled at about 20-36%, the CaO content at about 35-55%, the basicity at 3.0-4.5, and the MgO content at no more than 10%. The total refining time of LF is 62 minutes, of which the white slag refining time is 49 minutes. Calcium wire is fed at the end of LF refining to make the [Ca] content in the molten steel about 0.0027% and the T[Ca] / T[O] about 1.1. This makes the calcium aluminate in the molten steel a low-melting-point liquid inclusion, which floats to the slag with Ar blowing and stirring.

[0099] The third step, RH vacuum refining.

[0100] Before vacuum treatment, new slag is formed on the surface of the molten steel, with a basicity of 3.7. A calcium feed line is used to treat the molten steel with calcium before vacuum treatment. The RH vacuum treatment (<66 Pa) is performed for 20 minutes at a high vacuum level, with controlled flow of the lifting gas and a vacuum circulation flow rate of 8 t / min. After the RH vacuum is broken, Ar is soft-blown and the steel is settled for 26 minutes. The final molten steel has the following contents: [O] content 0.0010%, [N] content 0.0035%, [H] content 0.00008%, [S] content ≤0.0010%, [Ca] content reaching 0.0021%, and [Ca] / [S] ratio reaching 2.1.

[0101] Step 4: Continuous steel casting

[0102] Molten steel was cast into continuously cast slabs on a straight-arc continuous casting machine. The slab thickness was 370 mm. The tundish capacity was 60 t, the net weight of the molten steel during casting was 51 t, and the amount of covering agent used in the tundish was 9.4 kg / t of steel. The superheat of the molten steel in the tundish was controlled within 10–18 °C during the casting process. The liquid level fluctuation in the crystallizer was maintained within ±4 mm. The amount of protective slag used in the crystallizer was 0.49 kg / t of steel, and the Ar gas flow rate for the casting system was 22.4 m³ / t. 3 / h. The casting speed is controlled by A / (slab thickness × slab width), where the coefficient of A is 0.81, and the secondary cooling water volume for continuous casting is 1.19 L / kg steel. When the solidification fraction of the continuously cast slab is 0.69, a light reduction is applied to the slab, with a dynamic soft reduction of 16 mm at the end of solidification, distributed among 4 sector segments. The final pouring is done when the molten steel depth in the ladle reaches 220 mm, and slag discharge from the ladle is strictly inspected. The final pouring is done when the molten steel depth in the tundish reaches 110 mm. After the continuously cast slab is pulled from the casting machine, the hot slabs are stacked, and the stacking is removed after the slab temperature reaches 430℃ to proceed to the next process. The final continuously cast slab has a low-magnification microstructure with central segregation of C grade 0.5, and scanning electron microscopy analysis shows that the proportion of inclusions with an average particle size >5 μm in the final steel plate is 6%.

[0103] Step 5, rolling

[0104] The continuously cast slab is heated in a continuous heating furnace, with the heating section temperature controlled at 900℃~1280℃ and the soaking section temperature not exceeding 1260℃. The total heating time at a furnace temperature ≥1180℃ is 6.1 min / cm slab thickness. The steel plate rolling compression ratio is 6.1. It is initially rolled in the fully recrystallized austenite zone and then finished rolled in the non-recrystallized austenite zone. The initial rolling temperature is 1030℃, and there are 5 rolling passes with a single-pass reduction rate of 8%~40%. The intermediate slab is allowed to heat for 2.2 hours. The finishing rolling starts at 820℃ and ends at a temperature ≥Ar3. After rolling, the steel plate is held for 20 seconds before entering the ACC accelerated cooling system for accelerated cooling. The final cooling temperature is 540℃, and the cooling rate is 8℃ / second.

[0105] The final rating of inclusions in the steel plate is shown in the table below.

[0106]

[0107] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing a pure bridge steel plate, characterized in that, The steel plate is smelted from the following components by mass percentage: C: 0.04–0.12%, Si: 0.05%–0.55%, Mn: 0.90%–1.70%, P≤0.015%, S≤0.001%, Alt: 0.015%–0.050%, Ca: 0.0008%–0.0030%, O: ≤0.0012%, H: ≤0.00015%, N: 0.0010%–0.0050%, with the balance being Fe and unavoidable impurities; the steel plate has high purity and conforms to GB / T 10561 Inspection of non-metallic inclusions shall meet the following requirements: Class A: coarse inclusions ≤ 0.5, fine inclusions ≤ 1.0; Class B: coarse inclusions ≤ 0.5, fine inclusions ≤ 1.0; Class C: coarse inclusions ≤ 0.5, fine inclusions ≤ 0.5; Class D: coarse inclusions ≤ 0.5, fine inclusions ≤ 1.0; Class Ds: ≤ 1.5; Scanning electron microscopy analysis shall show that the proportion of inclusions with an average particle size > 5 μm in the final steel plate is ≤ 8%; The method includes the following steps: The first step, initial training KR pretreatment reduces the [S] content in molten iron to ≤0.005%. In the BOF converter primary refining, the blowing stops [C] and the [P] content is controlled to be not less than 0.02% and ≤0.005% before tapping. During the tapping process, silicon manganese and aluminum ingots are added with the steel stream for pre-deoxidation. The second step is LF refining. Refined slag is used to make white slag. Silicon is used before the precipitation deoxidizer, and aluminum is used for deoxidation in the middle and later stages. The [AL] content of the molten steel is above 0.009% during the LF refining process. Free oxygen in the slag is removed during the refining process. The content of Al2O3, CaO, MgO and basicity in the slag are controlled. Calcium wire is fed at the end of the LF refining process to control the T[Ca] / T[O] ratio in the molten steel. This makes the calcium aluminate in the molten steel a low-melting-point liquid inclusion, which floats into the slag with Ar blowing and stirring. The third step, RH vacuum refining. Before vacuum treatment, new slag is formed on the surface of the molten steel. Calcium is fed through a calcium wire before vacuum treatment. After RH vacuum is broken, Ar is soft-blown to treat the molten steel, allowing inclusions larger than 10μm and even smaller micron-sized inclusions to be removed from the molten steel. This ensures that the final molten steel has [O] content ≤0.0010%, [N] content ≤0.0050%, [H] ≤0.00015%, [S] content ≤0.001%, and [Ca] content reaches 0.0008%~0.0030%, while ensuring that [Ca] / [S] reaches 1.0~3.

2. MnS inclusions are modified into fine spherical (Ca, Mn)S. RH vacuum treatment at <66Pa high vacuum, treatment time 18min~30min, and vacuum circulation flow rate 5~20t / min, minimizes the [H] and [N] gas content in the molten steel, further promotes the deoxidation and desulfurization reaction depth, and allows inclusions to penetrate the steel-slag interface to the maximum extent and enter the slag. Step 4: Continuous steel casting Molten steel is cast into continuous casting slabs on a straight-arc continuous casting machine. The slab thickness is 150mm to 370mm, the tundish capacity is 60t, the net weight of molten steel in the tundish is ≥45t during casting, the amount of covering agent used in the tundish is 6.0 to 10.0kg / t steel, the superheat of the molten steel in the tundish is controlled at 16℃±10℃ during casting, the flow field of the molten steel in the crystallizer is controlled, the fluctuation of the liquid level in the crystallizer is kept within ±4mm, the amount of protective slag used in the crystallizer is not less than 0.4kg / t steel, the continuous casting slab is stacked and slowly cooled to the slab temperature ≤450℃ before being disassembled, achieving a low-magnification microstructure with center segregation ≤C1.5 grade, and scanning electron microscopy analysis shows that the proportion of inclusions with an average particle size >5μm in the final steel plate is ≤8%; a black slag surface is maintained during casting, and the Ar gas flow rate of the casting system is not less than 21m³. 3 / h, the casting speed is controlled according to A / (slab thickness × slab width), where the coefficient of A is controlled between 0.5 and 4.2, the secondary cooling water volume for continuous casting is 1.03L / kg steel to 1.28L / kg steel, and a light pressure of 10mm to 16mm is applied to the continuous casting slab when the solidification fraction of the continuous casting slab is 0.55 to 0.

80. Step 5, rolling The continuously cast slab is heated in a continuous heating furnace, with the temperature of the heating section controlled at 900℃~1280℃ and the temperature of the soaking section not exceeding 1260℃. The total heating time in the furnace with a temperature ≥1180℃ is ≥4.0 min / cm slab thickness. After the continuously cast slab exits the heating furnace, the surface iron oxide scale is removed by high-pressure water, and then it is rolled into finished steel plates on a four-roll reversible thick plate mill. After rolling, the steel plates are held for 10~40 seconds before entering the ACC accelerated cooling system for accelerated cooling. The final cooling temperature is 500~670℃, and the cooling rate is 5.0~30.0℃ / second.

2. The method for manufacturing a pure bridge steel plate according to claim 1, characterized in that: In step two, the total refining time of LF shall not be less than 45 minutes, of which the refining time of white residue shall be ≥38 minutes.

3. The method for manufacturing a pure bridge steel plate according to claim 1, characterized in that: In step five, the steel plate rolling compression ratio is 3.3 to 19.0, with initial rolling in the fully recrystallized austenite region and finish rolling in the non-recrystallized austenite region. The initial rolling temperature is ≥980℃, with 3 to 6 rolling passes and a single-pass reduction rate of 8% to 40%. The intermediate billet thickness is 2.0 to 4.5h, where h is the thickness of the finished steel plate. The finish rolling temperature is 990 to 780℃, with no limit on the single-pass reduction rate. The final rolling temperature is ≥Ar3 temperature, where Ar3 temperature is the starting temperature of the transformation from austenite to ferrite.

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