Method for controlling d-type inclusions in hot-rolled steel, hot-rolled steel

Abstract

The present application relates to the technical field of steelmaking continuous casting process, and particularly relates to a hot-rolled steel D-type inclusion control method and hot-rolled steel. The hot-rolled steel D-type inclusion control method converts traditional calcium-magnesium-aluminum-silicon complex oxides into high-melting-point magnesium-aluminum spinel through four-step cooperation of "aluminum deoxidization-high-alkali slag-vacuum aluminum control-magnesium modification", and makes the magnesium-aluminum spinel sufficiently float before solidification, so that the D-type inclusions are less than or equal to 0.5 level.

Description

Technical Field

[0001] This invention relates to the field of steelmaking continuous casting technology, and in particular to a method for controlling Class D inclusions in hot-rolled steel, and hot-rolled steel. Background Technology

[0002] The increasing demand for lightweight automotive components and ultra-long-life structural parts necessitates that hot-rolled steel simultaneously meet the following requirements: finished product thickness 2.0 mm–6.0 mm, yield strength 580 MPa–650 MPa, tensile strength 780 MPa–850 MPa, and elongation at break (A50) ≥ 18%. Longitudinal impact energy at 40℃ ≥80 J, fatigue life ≥1×10 6 Secondly, D-type and Ds-type complex oxide inclusions of calcium, magnesium, aluminum, and silicon in hot-rolled steel, due to their high hardness and sharp edges, induce microcracks under cyclic loading, becoming the primary source of early fatigue failure. The traditional "converter + LF + RH + calcium treatment + continuous casting" route introduces a large amount of high-melting-point calcium aluminates in the calcium treatment stage, leading to frequent nodule formation in the immersion nozzle. Furthermore, due to the high magnesium vapor pressure and short reaction window in the magnesium alloy ladle feed wire, the magnesium yield is <10%, resulting in large fluctuations in inclusion modification effects. The D-type inclusions in hot-rolled steel are rated as follows: The levels are often higher than 1.5, failing to meet the internal control requirement of ≤0.5; magnesium volatilization leads to dust overflow, deteriorating the operating environment; therefore, developing a core technology to efficiently, stably, and with low dust levels spray passivating granular magnesium powder into the controlled aluminum-steel liquid flow stream during the continuous casting tundish stage, transforming the complex calcium-magnesium-aluminum-silicon oxide D-type inclusions in hot-rolled steel into magnesium-aluminum spinel, and controlling the D-type inclusion rating in hot-rolled steel to ≤0.5 and the Ds-type inclusion rating in hot-rolled steel to ≤0.5 has become a common industry challenge for the cleanliness control of high-end hot-rolled steel. Summary of the Invention

[0003] In view of this, embodiments of this application provide a method for controlling Class D inclusions in hot-rolled steel, and hot-rolled steel, to solve the following technical problem: how to control Class D inclusions in hot-rolled steel to be ≤0.5 level.

[0004] In a first aspect, embodiments of this application provide a method for controlling Class D inclusions in hot-rolled steel, comprising the following steps: An aluminum-based deoxidizer is added to the molten steel tapped from the converter. The amount of aluminum-based deoxidizer added is controlled to reduce the oxygen content of the molten steel at the end point to no more than 0.06 wt%, thereby obtaining deoxidized molten steel. The deoxidized molten steel is subjected to top slag treatment, wherein the basicity of the top slag is ≥5 and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.0 to 1.5. The deoxidized molten steel is then subjected to desulfurization and deoxidation inclusion removal under the top slag cover to obtain impurity-free molten steel. The impurity-removed steel liquid is subjected to vacuum circulation degassing treatment. At the moment of vacuum circulation degassing treatment, the acid-soluble aluminum content of the impurity-removed steel liquid is controlled to be 0.025 wt% to 0.050 wt%, thus obtaining aluminum-controlled steel liquid. During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, passivating magnesium powder particles are sprayed into the stream of the aluminum-controlled steel liquid using inert gas as the carrier gas to obtain cast magnesium steel liquid. The magnesium content of the cast magnesium steel liquid is controlled to be 2 ppm to 6 ppm, so that the complex oxides of calcium, magnesium, aluminum and silicon in the cast magnesium steel liquid are transformed into magnesium aluminum spinel to obtain a continuous casting billet. The D-type inclusion rating of the continuous casting billet is ≤0.5 and the Ds-type inclusion rating of the continuous casting billet is ≤0.5.

[0005] Optionally, the aluminum-based deoxidizer is aluminum-iron, and the amount of aluminum-iron added to every 226 tons of molten steel tapped from the converter is 270 kg to 300 kg.

[0006] Optionally, the top slag treated in the top slag treatment contains, by mass fraction: CaO 44%–50%, Al2O3 27%–29%, MgO 7%–9%, ​​SiO 27%–9%, ​​FeO ≤1%, and MnO ≤0.5%.

[0007] Optionally, the blowing rate VMg of the passivated magnesium powder particles is expressed as: VMg=382×α×WMg×Qst, where α represents the magnesium yield of 15% to 20%, WMg represents the target magnesium mass content of the aluminum-controlled steel liquid of 0.0003 wt% to 0.0005 wt%, and Qst represents the real-time casting mass flow rate of the aluminum-controlled steel liquid.

[0008] Optionally, the inert gas is argon, and the flow rate of the argon is 35 NL / min to 45 NL / min.

[0009] Optionally, the spraying position of the passivating magnesium powder particles is 200 mm to 400 mm below the surface of the molten steel between the tundish impact zone and the tundish dam.

[0010] Optionally, the chemical composition of the continuously cast billet by mass fraction includes: C 0.06%–0.07%, Si 0.08%–0.25%, Mn 1.60%–1.84%, P≤0.012%, S≤0.003%, Ti 0.10%–0.12%, Al 0.015%–0.07%, Mg 0.0003%–0.0005%, and the matrix element Fe.

[0011] Optionally, the continuously cast billet meets the following requirements: finished product thickness of 220 mm to 250 mm, equiaxed crystal ratio ≥35%, and center segregation ≤C1.0 grade.

[0012] Secondly, embodiments of this application provide a hot-rolled steel, which is produced by hot rolling and laminar cooling of a continuously cast billet obtained by the method described in any one of the first aspects at 1100 ℃~1250 ℃. The hot-rolled steel satisfies the following requirements: longitudinal impact energy at -40 ℃ ≥80 J, and fatigue life ≥1×10⁻⁶. 6 Second-rate.

[0013] Optionally, the hot-rolled steel meets the following requirements: finished thickness of 2.0 mm to 6.0 mm, yield strength of 580 MPa to 650 MPa, tensile strength of 780 MPa to 850 MPa, and elongation after fracture A50 ≥ 18%.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for controlling Class D inclusions in hot-rolled steel. This method employs a four-step synergistic process of "aluminum deoxidation—high-alkali slag—vacuum-controlled aluminum—magnesium modification" to transform traditional complex calcium-magnesium-aluminum-silicon oxides into high-melting-point magnesium-aluminum spinel, ensuring its full flotation before solidification, thus achieving Class D inclusions ≤0.5. ① During the converter tapping stage, an aluminum-based deoxidizer is added to the molten steel to reduce the dissolved oxygen mass fraction to ≤0.06%, leaving only acid-soluble aluminum for subsequent magnesium-aluminum spinel formation and inhibiting Al2O3 clusters. ② During the top slag formation stage, top slag is applied to the deoxidized steel liquid. The basicity of the top slag is ≥5, and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.0~1.5. The high activity of CaO fixes the sulfur in the deoxidized steel liquid into the top slag. At the same time, MgO and Al2O3 dilute CaO, preventing the formation of 12CaO·7Al2O3, and encapsulating the already formed Al2O3 into the top slag. ③ During the vacuum circulation degassing stage, the mass fraction of acid-soluble aluminum in the depurified steel liquid is locked at 0.025%~0.050% at the moment of vacuum breaking. This ensures the aluminum source required for subsequent magnesium aluminate spinel formation and prevents secondary precipitation of Al2O3. ④ During the continuous casting stage, inert gas is used as a carrier to spray passivating magnesium powder particles into the controlled aluminum steel molten stream, so that the magnesium mass fraction of the cast magnesium steel molten is 2~6 ppm. Mg reacts with [Al] and [O] in the controlled aluminum steel molten to form MgAl2O4, and at the same time assimilates the remaining complex oxides of calcium, magnesium, aluminum and silicon into magnesium aluminum spinel. High melting point magnesium aluminum spinel is easy to aggregate and float, and the final D-type inclusion rating of the continuous casting billet is ≤0.5 and the Ds-type inclusion rating is ≤0.5. Attached Figure Description

[0015] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0017] Figure 1 A flowchart illustrating a method for controlling Class D inclusions in hot-rolled steel, as provided in this application embodiment. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0020] Partial Glossary: Molten steel tapped from the converter: Liquid steel tapped from the converter and into the ladle. Aluminum-based deoxidizer: A deoxidizing alloy with aluminum as the main element, used to react with dissolved oxygen in the molten steel tapped from the converter to generate Al2O3, thereby reducing the oxygen content of the molten steel. Final oxygen mass content: The mass fraction of dissolved oxygen in the molten steel tapped from the converter at the end of tapping. Top slag: Refining slag covering the surface of the deoxidized molten steel, used for desulfurization and adsorption of deoxidation products. Top slag basicity: The ratio of the mass of CaO to the mass of SiO2 in the top slag. Vacuum circulation degassing treatment: A process of vacuum degassing and inclusion removal of the de-purified molten steel in an RH unit. Breaking the vacuum moment: The moment the RH vacuum chamber returns to normal pressure. Acid-dissolved aluminum mass content: The mass fraction of aluminum in the de-purified molten steel measured by acid dissolution. Passivated particulate magnesium powder: Metallic magnesium particles with passivated surfaces, used to introduce magnesium into the aluminum-controlled molten steel. Calcium-magnesium-aluminum-silicon complex oxides: CaO-MgO-Al2O3-SiO2 series multi-element oxide inclusions present in cast magnesium steel. Magnesium-aluminum spinel: MgAl2O4, a high-melting-point, high-stability inclusion. Class D inclusion rating: Oxide inclusion level assessed according to GB / T 10561 standard. Class Ds inclusion rating: Single-particle large-size oxide inclusion level assessed according to GB / T 10561 standard.

[0021] Figure 1 A flowchart illustrating a method for controlling Class D inclusions in hot-rolled steel, as provided in this application embodiment.

[0022] Please see Figure 1 In a first aspect, embodiments of this application provide a method for controlling Class D inclusions in hot-rolled steel, comprising the following steps: S1. Add an aluminum-based deoxidizer to the molten steel tapped from the converter, wherein the amount of aluminum-based deoxidizer added is controlled to reduce the oxygen content of the molten steel tapped from the converter to no more than 0.06 wt%, thereby obtaining deoxidized molten steel; S2. The deoxidized molten steel is subjected to top slag treatment, wherein the basicity of the top slag is ≥5 and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.0 to 1.5, and the deoxidized molten steel is subjected to desulfurization and deoxidation inclusion removal under the top slag cover to obtain impurity-free molten steel. S3. The impurity-removed steel liquid is subjected to vacuum circulation degassing treatment. At the moment of breaking the vacuum during the vacuum circulation degassing treatment, the acid-soluble aluminum content of the impurity-removed steel liquid is controlled to be 0.025 wt% to 0.050 wt%, thereby obtaining aluminum-controlled steel liquid; S4. During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, passivating magnesium powder particles are sprayed into the stream of the aluminum-controlled steel liquid using inert gas as the carrier gas to obtain cast magnesium steel liquid. S5. Control the magnesium mass content of the cast magnesium steel liquid to be 2 ppm to 6 ppm, so that the complex oxides of calcium, magnesium, aluminum and silicon in the cast magnesium steel liquid are transformed into magnesium aluminum spinel to obtain a continuous casting billet, wherein the D-type inclusion rating of the continuous casting billet is ≤0.5 and the Ds-type inclusion rating of the continuous casting billet is ≤0.5.

[0023] An aluminum-based deoxidizer is added to the dissolved oxygen in the molten steel tapped from the converter, reducing the final oxygen content to ≤0.06 wt%. Simultaneously, the dissolved oxygen in the molten steel is converted into Al2O3 and discharged, reducing the total amount of oxides in the deoxidized steel. Top slag is generated from the deoxidized steel, with a basicity ≥5 and a CaO to (MgO+Al2O3) mass ratio of 1.0–1.5. This allows the top slag to adsorb Al2O3 from the deoxidized steel and remove sulfur, inhibiting the formation of low-melting-point calcium aluminates. The degassed steel undergoes vacuum circulation degassing, controlling the acid-soluble aluminum content at 0.025 wt%–0.050 wt% at the moment of vacuum breaking, preventing excessive Al2O3 precipitation and reserving the aluminum source required for the formation of magnesium aluminum spinel in the aluminum-controlled steel. During continuous casting, inert gas is used as a carrier gas to spray passivating magnesium powder particles into the stream of controlled-flow aluminum-steel, achieving a magnesium content of 2 ppm to 6 ppm in the cast magnesium-steel. The magnesium in the cast magnesium-steel reacts with complex oxides of calcium, magnesium, aluminum, and silicon to form magnesium-aluminum spinel. Magnesium-aluminum spinel has a high melting point and is easy to float, ultimately resulting in a D-class inclusion rating of ≤0.5 and a Ds-class inclusion rating of ≤0.5 in the continuously cast billet.

[0024] Numerical examples include, but are not limited to, the following: endpoint oxygen mass content: 0.06, 0.05, 0.04, 0.03, 0.02, 0.01, 0.005 wt%; top slag basicity: 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0; the ratio of CaO mass to the sum of MgO and Al2O3 mass: 1.0, 1.1, 1.2, 1.3, 1.4, 1.5; acid-soluble aluminum mass content: 0.025, 0.030, 0.035, 0.040, 0.045, 0.050 wt%; magnesium mass content: 2, 3, 4, 5, 6 ppm.

[0025] In some embodiments, the aluminum-based deoxidizer is ferroaluminum, and the amount of ferroaluminum added to every 226 tons of molten steel tapped from the converter is 270 kg to 300 kg.

[0026] Ferroaluminum: A type of aluminum-based deoxidizer, typically containing ≥40 wt% aluminum, used for deoxidation of molten steel tapped from converters. 270 kg to 300 kg of ferroaluminum is added to every 226 tons of molten steel tapped from the converter to ensure the final oxygen content in the tapped steel is ≤0.06 wt%, while simultaneously providing the acid-soluble aluminum required for the subsequent formation of magnesium aluminum spinel. Numerical examples: the amount of ferroaluminum added includes, but is not limited to: 270, 275, 280, 285, 290, 295, and 300 kg.

[0027] In some embodiments, the top slag in the top slag treatment contains, by mass fraction: CaO 44%–50%, Al2O3 27%–29%, MgO 7%–9%, ​​SiO 27%–9%, ​​FeO ≤1%, and MnO ≤0.5%.

[0028] Top slag composition: the mass fraction of each oxide in the top slag. The mass fraction of CaO in the top slag is controlled to be 44%–50%, Al2O3 27%–29%, MgO 7%–9%, ​​SiO2 7%–9%, ​​FeO ≤1%, and MnO ≤0.5%. This allows the top slag to remove sulfur from the deoxidized molten steel through high CaO activity, and inhibits the formation of calcium aluminates in the deoxidized molten steel through the Al2O3 to MgO ratio, while simultaneously reducing secondary oxidation of the deoxidized molten steel. Numerical examples: the mass fraction of CaO includes, but is not limited to: 44, 45, 46, 47, 48, 49, 50%; the mass fraction of Al2O3 includes, but is not limited to: 27, 27.5, 28, 28.5, 29%; the mass fraction of MgO includes, but is not limited to: 7, 7.5, 8, 8.5, 9%; the mass fraction of SiO2 includes, but is not limited to: 7, 7.5, 8, 8.5, 9%; the mass fraction of FeO includes, but is not limited to: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0%; and the mass fraction of MnO includes, but is not limited to: 0.1, 0.2, 0.3, 0.4, 0.5%.

[0029] In some embodiments, the blowing rate VMg of the passivated magnesium powder particles is expressed as: VMg=382×α×WMg×Qst, where α represents the magnesium yield of 15% to 20%, WMg represents the target magnesium mass content of the aluminum-controlled molten steel of 0.0003wt% to 0.0005wt%, and Qst represents the real-time casting mass flow rate of the aluminum-controlled molten steel.

[0030] VMg: Injection rate of passivated magnesium powder particles, in kg / min. α: Magnesium yield, the proportion of magnesium actually entering the aluminum-controlled molten steel to the injected magnesium. WMg: Target magnesium mass content of the aluminum-controlled molten steel. Qst: Real-time casting mass flow rate of the aluminum-controlled molten steel, in kg / min.

[0031] The blowing rate of passivating magnesium powder particles is calculated using the formula VMg = 382 × α × WMg × Qst to stabilize the magnesium mass content in the aluminum-controlled molten steel at 2 ppm to 6 ppm, ensuring that the complex oxides of calcium, magnesium, aluminum, and silicon in the aluminum-controlled molten steel are completely converted into magnesium aluminum spinel in the cast magnesium steel. Example: α: 15, 16, 17, 18, 19, 20%; WMg: 0.0003, 0.00035, 0.0004, 0.00045, 0.0005 wt%; Qst: 6500, 6600, 6700, 6800, 6900, 7000 kg / min, corresponding to VMg calculated as 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 kg / min.

[0032] In some embodiments, the inert gas is argon, and the flow rate of the argon is 35 NL / min to 45 NL / min.

[0033] Argon flow rate: The volumetric flow rate of argon gas used to spray passivating magnesium powder particles, in NL / min. The volumetric flow rate of the carrier argon gas is set to 35 NL / min to 45 NL / min to ensure that the inert argon gas carries the passivating magnesium powder particles and disperses them evenly into the aluminum-controlled steel liquid, preventing oxidation and aggregation of the passivating magnesium powder particles and improving the magnesium recovery rate in the aluminum-controlled steel liquid. Argon flow rates include, but are not limited to: 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, and 45 NL / min.

[0034] In some embodiments, the sprayed passivating magnesium powder particles are sprayed at a position 200 mm to 400 mm below the molten steel surface between the tundish impact zone and the tundish dam.

[0035] The passivating magnesium powder particles are sprayed at a depth of 200mm to 400mm below the molten steel surface between the tundish impact zone and the tundish dam. This avoids contact between the passivating magnesium powder particles and air, prolonging their residence time in the aluminum-controlled molten steel and improving the reaction efficiency between magnesium and the complex oxides of calcium, magnesium, aluminum, and silicon in the aluminum-controlled molten steel. Spraying depths include, but are not limited to: 200, 225, 250, 275, 300, 325, 350, 375, and 400 mm.

[0036] In some embodiments, the chemical composition of the continuously cast billet by mass fraction includes: C 0.06%–0.07%, Si 0.08%–0.25%, Mn 1.60%–1.84%, P ≤0.012%, S ≤0.003%, Ti 0.10%–0.12%, Al 0.015%–0.07%, Mg 0.0003%–0.0005%, and the matrix element Fe.

[0037] The continuous casting billet is designed to contain the following mass fractions: C 0.06%–0.07%, Si 0.08%–0.25%, Mn 1.60%–1.84%, P ≤0.012%, S ≤0.003%, Ti 0.10%–0.12%, Al 0.015%–0.07%, and Mg 0.0003%–0.0005%. This ensures that Mg exists in the form of magnesium aluminum spinel and eliminates Class D inclusions in the billet.

[0038] Numerical examples: the mass fraction of C includes, but is not limited to: 0.06, 0.065, 0.07%; the mass fraction of Si includes, but is not limited to: 0.08, 0.10, 0.12, 0.15, 0.18, 0.20, 0.22, 0.25%; the mass fraction of Mn includes, but is not limited to: 1.60, 1.65, 1.70, 1.75, 1.80, 1.84%; the mass fraction of P includes, but is not limited to: 0.012, 0.011, 0.010, 0.009, 0.008, 0.007, 0.006, 0.005, 0.004, 0.003, 0.002, 0.001 The mass fractions of S include, but are not limited to: 0.003, 0.002, 0.001, 0%; the mass fractions of Ti include, but are not limited to: 0.10, 0.105, 0.11, 0.115, 0.12%; the mass fractions of Al include, but are not limited to: 0.015, 0.020, 0.025, 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060, 0.065, 0.070%; the mass fractions of Mg include, but are not limited to: 0.0003, 0.00035, 0.0004, 0.00045, 0.0005%. In some embodiments, the continuously cast billet meets the following requirements: finished product thickness of 220 mm to 250 mm, equiaxed crystal ratio ≥35%, and center segregation ≤C1.0 grade.

[0039] Continuous casting billet thickness: The cross-sectional thickness of the continuous casting billet, in mm. Equiaxed grain ratio: The percentage of equiaxed grain area in the central region of the continuous casting billet relative to the total area. Center segregation: The positive segregation level of carbon, manganese, and other elements in the center of the continuous casting billet, evaluated according to YB / T 4003. Controlling the finished thickness of the continuous casting billet to 220 mm–250 mm, the equiaxed grain ratio in the central region of the continuous casting billet ≥35%, and the center segregation level of the continuous casting billet ≤C1.0 ensures the internal density and compositional uniformity of the continuous casting billet, avoiding fatigue cracks induced by banded structures in the continuous casting billet. Numerical examples: Finished billet thickness includes, but is not limited to: 220, 225, 230, 235, 240, 245, 250 mm; equiaxed grain ratio includes, but is not limited to: 35, 36, 37, 38, 39, 40%; center segregation includes, but is not limited to: C0.5, C1.0 levels.

[0040] Secondly, embodiments of this application provide a hot-rolled steel, which is produced by hot rolling and laminar cooling of a continuously cast billet obtained by the method described in any one of the first aspects at 1100 ℃~1250 ℃. The hot-rolled steel satisfies the following requirements: longitudinal impact energy at -40 ℃ ≥80 J, and fatigue life ≥1×10⁻⁶. 6 Second-rate.

[0041] Laminar flow cooling: The cooling rate is controlled using laminar flow after hot rolling. -40℃ longitudinal impact energy: The energy absorbed by hot-rolled steel under Charpy V-notch longitudinal impact at -40℃, measured in J. Fatigue life: The number of cycles until fracture under alternating stress in hot-rolled steel. The continuously cast billet is heated to 1100℃~1250℃ and then hot-rolled, followed by laminar flow cooling to ensure that the magnesium-aluminum spinel in the hot-rolled steel remains fine and dispersed, ensuring that the longitudinal impact energy at -40℃ is ≥80 J and the fatigue life is ≥1×10⁻⁶. 6 Second-rate.

[0042] In some embodiments, the hot-rolled steel satisfies the following requirements: finished thickness of 2.0 mm to 6.0 mm, yield strength of 580 MPa to 650 MPa, tensile strength of 780 MPa to 850 MPa, and elongation at break A50 ≥ 18%.

[0043] Hot-rolled steel thickness: The finished thickness of hot-rolled steel, in mm. Yield strength: The stress at which hot-rolled steel undergoes 0.2% plastic deformation, in MPa. Tensile strength: The maximum stress of hot-rolled steel before fracture, in MPa. Elongation at fracture (A50): The percentage elongation of hot-rolled steel after tensile fracture at a gauge length of 50 mm. This design ensures that the finished thickness of hot-rolled steel is 2.0 mm to 6.0 mm, the yield strength is 580 MPa to 650 MPa, the tensile strength is 780 MPa to 850 MPa, and the elongation at fracture (A50) is ≥18%, meeting the dual requirements of strength and formability for advanced automotive structural components. Furthermore, the magnesium aluminum spinel in the hot-rolled steel does not reduce its plasticity.

[0044] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0045] Example 1 Step 1: Add ferroaluminum to the molten steel tapped from the converter, the amount of ferroaluminum added is 270 kg / 226 t of molten steel tapped from the converter, so that the final oxygen mass content of the molten steel tapped from the converter is reduced to 0.0497 wt%, and deoxidized molten steel is obtained; Step 2: The deoxidized molten steel is subjected to top slag treatment. The basicity of the top slag is 6.3, and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.25. Under the cover of the top slag, the deoxidized molten steel is desulfurized and deoxidized to remove inclusions, resulting in impurity-free molten steel. Step 3: Perform vacuum circulation degassing on the impurity-removed steel liquid. At the moment of breaking the vacuum during the vacuum circulation degassing process, control the acid-soluble aluminum content of the impurity-removed steel liquid to 0.037 wt% to obtain aluminum-controlled steel liquid. Step 4: During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, passivating magnesium powder particles are sprayed into the stream of the aluminum-controlled steel liquid using argon gas as the carrier gas to obtain cast magnesium steel liquid. Step 5: Control the magnesium content of the cast magnesium steel liquid to 3 ppm, so that the complex oxides of calcium, magnesium, aluminum and silicon in the cast magnesium steel liquid are transformed into magnesium aluminum spinel, and a continuously cast billet is obtained. Step 6: The continuously cast billet is hot-rolled at 1100 ℃ and then cooled in laminar flow to obtain hot-rolled steel.

[0046] Example 2 Step 1: Add ferroaluminum to the molten steel tapped from the converter, wherein the amount of ferroaluminum added is 290 kg / 226 t of molten steel tapped from the converter, so that the final oxygen mass content of the molten steel tapped from the converter is reduced to 0.0531 wt%, thereby obtaining deoxidized molten steel; Step 2: The deoxidized molten steel is subjected to top slag treatment. The basicity of the top slag is 6.1, and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.30. Under the cover of the top slag, the deoxidized molten steel is desulfurized and deoxidized to remove inclusions, resulting in impurity-free molten steel. Step 3: Perform vacuum circulation degassing on the impurity-removed steel liquid. At the moment of breaking the vacuum during the vacuum circulation degassing process, control the acid-soluble aluminum content of the impurity-removed steel liquid to 0.032 wt% to obtain aluminum-controlled steel liquid. Step 4: During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, passivating magnesium powder particles are sprayed into the stream of the aluminum-controlled steel liquid using argon gas as the carrier gas to obtain cast magnesium steel liquid. Step 5: Control the magnesium content of the cast magnesium steel liquid to 4 ppm, so that the complex oxides of calcium, magnesium, aluminum and silicon in the cast magnesium steel liquid are transformed into magnesium aluminum spinel, and a continuously cast billet is obtained. Step 6: The continuously cast billet is hot-rolled at 1200 ℃ and then cooled in laminar flow to obtain hot-rolled steel.

[0047] Example 3 Step 1: Add ferroaluminum to the molten steel tapped from the converter, wherein the amount of ferroaluminum added is 300 kg / 226 t of molten steel tapped from the converter, so that the final oxygen mass content of the molten steel tapped from the converter is reduced to 0.0450 wt%, thereby obtaining deoxidized molten steel; Step 2: The deoxidized molten steel is subjected to top slag treatment. The basicity of the top slag is 7.0, and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.40. Under the cover of the top slag, the deoxidized molten steel is subjected to desulfurization and deoxidation and removal of inclusions to obtain impurity-free molten steel. Step 3: Perform vacuum circulation degassing on the impurity-removed steel liquid. At the moment of breaking the vacuum during the vacuum circulation degassing process, control the acid-soluble aluminum content of the impurity-removed steel liquid to 0.040 wt% to obtain aluminum-controlled steel liquid. Step 4: During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, passivating magnesium powder particles are sprayed into the stream of the aluminum-controlled steel liquid using argon gas as the carrier gas to obtain cast magnesium steel liquid. Step 5: Control the magnesium content of the cast magnesium steel liquid to 5 ppm, so that the complex oxides of calcium, magnesium, aluminum and silicon in the cast magnesium steel liquid are transformed into magnesium aluminum spinel, and a continuously cast billet is obtained. Step 6: The continuously cast billet is hot-rolled at 1250 ℃ and then cooled in laminar flow to obtain hot-rolled steel.

[0048] Comparative Example 1 (magnesium blowing omitted) Step 1: Add ferroaluminum to the molten steel tapped from the converter, wherein the amount of ferroaluminum added is 290 kg / 226 t of molten steel tapped from the converter, so that the final oxygen mass content of the molten steel tapped from the converter is reduced to 0.0530 wt%, thereby obtaining deoxidized molten steel; Step 2: The deoxidized molten steel is subjected to top slag treatment. The basicity of the top slag is 6.1, and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.30. Under the cover of the top slag, the deoxidized molten steel is desulfurized and deoxidized to remove inclusions, resulting in impurity-free molten steel. Step 3: Perform vacuum circulation degassing on the impurity-removed steel liquid. At the moment of breaking the vacuum during the vacuum circulation degassing process, control the acid-soluble aluminum content of the impurity-removed steel liquid to 0.032 wt% to obtain aluminum-controlled steel liquid. Step 4: During the continuous casting process of injecting the aluminum-controlled molten steel into the tundish, passivating magnesium powder particles are not sprayed, and the continuously cast billet is obtained directly. Step 5: The continuously cast billet is hot-rolled at 1200 ℃ and then cooled in laminar flow to obtain hot-rolled steel.

[0049] Comparative Example 2 (Calcium treatment instead of magnesium blowing) Step 1: Add ferroaluminum to the molten steel tapped from the converter, wherein the amount of ferroaluminum added is 290 kg / 226 t of molten steel tapped from the converter, so that the final oxygen mass content of the molten steel tapped from the converter is reduced to 0.0530 wt%, thereby obtaining deoxidized molten steel; Step 2: The deoxidized molten steel is subjected to top slag treatment. The basicity of the top slag is 6.1, and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.30. Under the cover of the top slag, the deoxidized molten steel is desulfurized and deoxidized to remove inclusions, resulting in impurity-free molten steel. Step 3: Perform vacuum circulation degassing on the impurity-removed steel liquid. At the moment of breaking the vacuum during the vacuum circulation degassing process, control the acid-soluble aluminum content of the impurity-removed steel liquid to 0.032 wt% to obtain aluminum-controlled steel liquid. Step 4: During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, calcium is fed into the stream of the aluminum-controlled steel liquid instead of passivating granular magnesium powder using calcium-iron wire to obtain cast calcium steel liquid. Step 5: Control the calcium content in the cast calcium steel liquid to 25 ppm to obtain a continuously cast billet; Step 6: The continuously cast billet is hot-rolled at 1200 ℃ and then cooled in laminar flow to obtain hot-rolled steel.

[0050] The above embodiments and comparative examples were tested for effectiveness data using the following methods: Class D inclusion rating: GB / T 10561-2023; Class D inclusion rating: GB / T 10561-2023; -40℃ longitudinal impact energy: GB / T 229-2020 (Charpy V-notch); Fatigue life: GB / T 3075-2020 (axial stress control, R=-1, 10) 6 (Second termination).

[0051] Table 1

[0052] As shown in Table 1, Examples 1, 2, and 3 all meet the following requirements: D-type inclusion rating = 0, Ds-type inclusion rating = 0, longitudinal impact energy at -40 ℃ ≥ 92 J, and fatigue life ≥ 2.0 × 10⁻⁶. 6 The entire batch met the technical target of "hot-rolled steel Class D inclusions ≤ 0.5". Compared to Example 1, which did not undergo passivation particle magnesium powder spraying, the D-class inclusion rating increased to 1.5, the Ds-class inclusion rating increased to 1.0, the longitudinal impact energy at -40 ℃ decreased to 65 J, and the fatigue life decreased to 1.1×10⁻⁶. 6 The technical objectives were not met in the first instance. In Comparative Example 2, calcium-iron wire was used instead of passivated particulate magnesium powder for spraying, further improving the D-class inclusion rating to 2.0 and the Ds-class inclusion rating to 1.5. The longitudinal impact energy at -40 ℃ decreased to 58 J, and the fatigue life decreased to 0.9 × 10⁻⁶. 6 The technical objectives were also not met this time.

[0053] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for controlling Class D inclusions in hot-rolled steel, characterized in that, Includes the following steps: An aluminum-based deoxidizer is added to the molten steel tapped from the converter. The amount of aluminum-based deoxidizer added is controlled to reduce the oxygen content of the molten steel at the end point to no more than 0.06 wt%, thereby obtaining deoxidized molten steel. The deoxidized molten steel is subjected to top slag treatment, wherein the basicity of the top slag is ≥5 and the ratio of the mass of CaO to the sum of the masses of MgO and Al2O3 in the top slag is 1.0 to 1.

5. The deoxidized molten steel is then subjected to desulfurization and deoxidation inclusion removal under the top slag cover to obtain impurity-free molten steel. The impurity-removed steel liquid is subjected to vacuum circulation degassing treatment. At the moment of vacuum circulation degassing treatment, the acid-soluble aluminum content of the impurity-removed steel liquid is controlled to be 0.025 wt% to 0.050 wt%, thus obtaining aluminum-controlled steel liquid. During the continuous casting process of injecting the aluminum-controlled steel liquid into the tundish, passivating magnesium powder particles are sprayed into the stream of the aluminum-controlled steel liquid using inert gas as the carrier gas to obtain cast magnesium steel liquid. The magnesium content of the cast magnesium steel liquid is controlled to be 2 ppm to 6 ppm, so that the complex oxides of calcium, magnesium, aluminum and silicon in the cast magnesium steel liquid are transformed into magnesium aluminum spinel to obtain a continuous casting billet. The D-type inclusion rating of the continuous casting billet is ≤0.5 and the Ds-type inclusion rating of the continuous casting billet is ≤0.

5.

2. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The aluminum-based deoxidizer is aluminum-iron, and the amount of aluminum-iron added to every 226 tons of molten steel from the converter is 270 kg to 300 kg.

3. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The top slag treated in the above-mentioned process contains, by mass fraction: CaO 44%–50%, Al2O3 27%–29%, MgO 7%–9%, ​​SiO2 7%–9%, ​​FeO ≤1%, and MnO ≤0.5%.

4. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The expression for the blowing rate VMg of the passivated magnesium powder particles is: VMg=382×α×WMg×Qst, where α represents the magnesium yield of 15% to 20%, WMg represents the target magnesium mass content of the aluminum-controlled steel liquid of 0.0003 wt% to 0.0005 wt%, and Qst represents the real-time casting mass flow rate of the aluminum-controlled steel liquid.

5. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The inert gas is argon, and the flow rate of the argon is 35 NL / min to 45 NL / min.

6. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The spraying location for the passivating magnesium powder particles is 200 mm to 400 mm below the molten steel surface between the tundish impact zone and the tundish retaining dam.

7. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The chemical composition of the continuously cast billet, by mass fraction, includes: C 0.06%–0.07%, Si 0.08%–0.25%, Mn 1.60%–1.84%, P≤0.012%, S≤0.003%, Ti 0.10%–0.12%, Al 0.015%–0.07%, Mg 0.0003%–0.0005%, and the matrix element Fe.

8. The method for controlling Class D inclusions in hot-rolled steel according to claim 1, characterized in that, The continuously cast billet meets the following requirements: finished product thickness is 220 mm to 250 mm, equiaxed crystal ratio is ≥35%, and center segregation is ≤C1.0 grade.

9. A hot-rolled steel, characterized in that, The continuously cast billet obtained by the method according to any one of claims 1 to 8 is hot-rolled at 1100℃ to 1250℃ and then cooled by laminar flow. The hot-rolled steel satisfies the following requirements: longitudinal impact energy at -40℃ ≥ 80 J, and fatigue life ≥ 1×10⁻⁶. 6 Second-rate.

10. The hot-rolled steel according to claim 9, characterized in that, The hot-rolled steel shall meet the following requirements: finished thickness of 2.0 mm to 6.0 mm, yield strength of 580 MPa to 650 MPa, tensile strength of 780 MPa to 850 MPa, and elongation after fracture A50 ≥ 18%.

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