Method for controlling inclusions in a hole expanding steel

Abstract

The application provides a method for controlling inclusions of hole expanding steel, comprising the following steps: converter, LF refining, RH vacuum refining, continuous casting; the mass percentage of N in the hole expanding steel is less than or equal to 0.0040%. The method adopts a three-in-one reduction slagging process of tapping-argon station-LF furnace, two-step titanium ferroalloying and auxiliary vacuum pre-calcium treatment, so that the Al2O3 inclusions are more completely denatured, and the number of B-type and DS-type inclusions in the steel is less. The continuous casting of the method adopts high-temperature gradient control of the crystallizer cooling intensity, improves the temperature gradient of the solidification front, and the size of the precipitates obtained is more fine. The method provided by the application significantly reduces the number and size of Al2O3 and Al2O3-MgO type B-type inclusions, Al2O3-CaO type DS-type inclusions with an aluminum-calcium element content ratio of 0.8-1.5, and TiN type inclusions, and solves the hole expanding cracking problem caused by inclusions.

Description

Technical Field

[0001] This invention relates to the field of ladle refining technology, specifically to a method for controlling inclusions in expanded steel. Background Technology

[0002] Expanded steel requires high tensile strength, good formability, and good flange-flanging performance. The expansion ratio, as a formability indicator of expanded steel, reflects the steel plate's ability to resist localized cracking in the direction perpendicular to the hole edge due to excessive local elongation deformation at the hole edge during the expansion process. A higher expansion ratio indicates greater fatigue resistance and better expansion performance.

[0003] One of the factors that significantly affects the hole-expanding performance is the quantity, type, and size of inclusions in steel. Among these, inclusions of types B, D, and DS have the greatest impact on the hole-expanding rate. In the new edition of the "Standard Rating Chart Microscopic Examination Method for Determination of Non-metallic Inclusion Content in Steel" (GB / T10561-2023), TiN inclusions can also be compared with types B, C, D, and DS inclusions based on their morphology, and their chemical characteristics are indicated by subscripts. For example, a high-hole-expanding steel produced by a certain factory using the LF+RH refining process experienced hole-expanding cracking during the user's hole-expanding process. Physical dissection revealed (e.g.) Figure 1 As shown, the steel contains Al2O3 and Al2O3-MgO inclusions larger than 17 μm, Ti2O3-Al2O3-CaS inclusions larger than 27 μm, and Al2O3-CaO inclusions with an aluminum-calcium ratio between 0.8 and 1.5, which are classified as DS-type inclusions with a rating exceeding 1.0. There are also TiN-type inclusions larger than 10 μm, either singly or composed of multiple TiN inclusions. To address the issue of large-sized B-type and DS-type inclusions affecting porosity expansion, targeted control methods are needed.

[0004] To improve porosity, metallurgists have conducted extensive research. For example, Chinese patent application CN108048734A discloses a hot-rolled multiphase steel with a tensile strength of 700 MPa and its production method. This method adds rare earth element RE to the composition design to optimize the shape and size of inclusions in the steel, thereby improving the porosity. Chinese patent application CN117802419A discloses a 1.0 GPa grade hydrogen embrittlement resistant cold-rolled CH steel and its preparation method. By controlling the Mg content between 0.005% and 0.50%, the size and distribution of carbides in DH steel are improved. Chinese invention patent CN114107792B discloses a 780 MPa grade high-surface-area ultra-high porosity steel and its manufacturing method. It points out that in the steelmaking process, Mg deoxidation is used to preferentially form dispersed fine MgO in the molten steel, creating more nucleation points for TiN formation during subsequent continuous casting, effectively refining TiN particles and improving the stability of the porosity.

[0005] However, most existing technologies optimize the shape and size of inclusions in steel by adding rare earth elements or magnesium treatment. There are very few inclusion control methods that can meet the hole expansion rate requirements without the need for additional materials through the production process of expanded steel itself. Summary of the Invention

[0006] To overcome the shortcomings of the above-mentioned technologies, the purpose of this invention is to provide a method for controlling inclusions in expanded steel, reducing the size of inclusions, thereby reducing the impact of inclusions on the expansion rate, and solving the problem of expansion cracking caused by inclusions.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for controlling inclusions in expanded steel includes the following steps: converter, LF refining, RH vacuum refining, and continuous casting to obtain expanded steel billets; controlling the mass percentage of nitrogen in the steel product to be ≤0.0040%.

[0009] Preferably, during the converter process, the carbon content at the end of the main blowing is ≥0.30% (weight percentage) to ensure a good carbon-oxygen reaction during auxiliary blowing and reduce gas intake at the furnace mouth; the bottom blowing argon intensity during auxiliary blowing is increased to 0.12–0.16 Nm. 3 / (tons of liquid·min), forming a localized argon bubble vacuum to promote N2 escape; aluminum granules, lime, and bauxite are added at the tapping of the converter. Here, tons of liquid·min means one ton of liquid per minute, and the liquid refers to all liquids in the furnace, including molten steel and slag in liquid form.

[0010] Preferably, after the converter tapps the steel, the slag is pre-reduced by strong argon blowing through an argon station and maintains good spreadability, so as to achieve early slag formation and rapid formation of white slag; the strong argon blowing intensity is 2.0 to 4.0 standard liters / (minute·ton of steel).

[0011] Preferably, the total number of heating times during the LF refining process is controlled to be 2 to 3 times. The heating time for the first, second, and third heating times is controlled in a step-like manner according to the initial temperature. The first heating time is 10 to 16 minutes, the second heating time is 6 to 8 minutes, and the third heating time is 4 to 6 minutes. Compressed air is circulated through the ladle furnace cover to maintain a slight positive pressure inside the furnace, so as not to produce dense smoke.

[0012] Preferably, during the LF refining process, after the LF forms white slag and is maintained for 15 minutes, temperature and oxygen levels are measured. The molten steel temperature is maintained at 1590–1605℃, and the active oxygen content in the molten steel is 10–30 ppm. The slag surface is blown open, and ferrotitanium alloy is added. After stirring with medium-intensity bottom-blowing argon for 3 minutes, aluminum particles are added for final deoxidation and alloying. The amount of ferrotitanium alloy added is 40–50% of the total weight of the ferrotitanium alloy, and the amount of aluminum particles added is the total amount required for the steel grade. The medium-intensity bottom-blowing argon intensity is 0.20–1.0 BL / (min·ton steel). The purpose is to allow a portion of the ferrotitanium to undergo an oxidation reaction to generate fine titanium oxides, which then polymerize with alumina inclusions in the molten steel to form titanium-aluminum-oxygen composite oxides. This type of titanium-aluminum-oxygen composite oxide has a small wetting angle, making it easy to separate from the molten steel. It can be removed during the subsequent long-term RH vacuum circulation refining process, thereby reducing the number of inclusions in the steel.

[0013] Preferably, in the later stage of LF refining, after all components except Ti are qualified, the steel is gently stirred for 3-5 minutes, and then pure calcium wire is fed in. After feeding the pure calcium wire, the mass percentage of Ca in the molten steel is 20-50 ppm. After feeding the wire, the steel is directly sent to the RH station for vacuum refining. The bottom blowing argon intensity of the gentle stirring is 0.02-0.35 BL / (min·ton steel). After the RH station achieves dehydrogenation at a high vacuum, the remaining ferrotitanium alloy is added, and the contents of elements such as Ti, Als, and Mn are finely adjusted to meet the steel composition requirements. The purpose of this is to achieve more thorough modification of Al2O3 inclusions and reduce the number of B-type and DS-type inclusions in the steel.

[0014] Preferably, in the RH vacuum refining process, after achieving high vacuum to meet the dehydrogenation requirements, the remaining titanium-iron alloy is added. Subsequently, a low vacuum large-circulation gas is used to perform denitrification, driving the gas flow rate to the maximum capacity of the equipment, and finely adjusting the content of elements such as Ti, Als, and Mn to meet the requirements of the steel composition. The vacuum degree of the high vacuum dehydrogenation process is <67 Pa; the vacuum degree of the low vacuum circulation process is 1-5 kPa.

[0015] Preferably, a high temperature gradient is used to control the cooling intensity of the crystallizer during the continuous casting process. The actual cooling effect of the crystallizer is monitored by the hot surface temperature measured by thermocouples on the first row of copper plates and the heat flux density of the narrow copper plates on both sides of the crystallizer. The hot surface temperature measured by thermocouples on the first row of copper plates is controlled between 90 and 125°C, and the difference in heat flux density between the narrow copper plates on both sides of the crystallizer is controlled between 0 and 0.06 MW / mm. 2 The purpose of this is to make the temperature of the billet at the exit of the crystallizer 50-100°C lower than the billet temperature under conventional cooling intensity, thereby increasing the temperature gradient at the solidification front, suppressing the precipitation and growth of titanium nitride inclusions, and resulting in finer precipitates.

[0016] Preferably, in addition to traditional protective casting, the continuous casting step uses a ladle covering agent containing 5-10% (by weight) vermiculite. Because vermiculite has good heat insulation and spreading properties, it can prevent nitrogen increase caused by slag crusting, cracking, and air leakage in the ladle.

[0017] Preferably, the expanded steel is HR600 / 780HE expanded steel, and the mass percentage of N in the steel is ≤0.0040%.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] The present invention provides a method for controlling inclusions in expanded steel, which significantly reduces the quantity and size of B-type inclusions such as Al2O3 and Al2O3-MgO, DS-type inclusions such as Ti2O3-Al2O3-CaS and Al2O3-CaO with an aluminum-calcium ratio between 0.8 and 1.5, as well as TiN-type inclusions, and ultimately reduces the incidence of expanded steel cracking caused by inclusions.

[0020] The present invention employs a ladle covering agent containing 5-10% (by weight) vermiculite to prevent nitrogen increase caused by air leakage due to slag crusting and cracking in the ladle. The method utilizes a three-in-one reduction slag-forming process involving tapping, argon station, and LF furnace, with a two-step titanium-iron alloying method supplemented by pre-vacuum calcium treatment, resulting in more thorough Al2O3 inclusion modification and fewer B- and DS-type inclusions in the steel. The continuous casting process of the present invention uses a high temperature gradient to control the cooling intensity of the crystallizer, increasing the temperature gradient at the solidification front and resulting in finer precipitates. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of inclusions in expanded steel produced by a steel mill. Figure 1 a represents Al2O3-CaO inclusions in steel with an aluminum-calcium ratio of 0.8 to 1.5 and a size of 37.1 μm. Figure 1 b represents Ti2O3-Al2O3-CaS inclusions with a size greater than 27.6 μm in the steel;

[0022] Figure 2 This is a comparison diagram of the types of inclusions in molten steel in Comparative Example 1 and Example 1;

[0023] Figure 3 This is a comparison diagram of the types of inclusions in the steel plates of Comparative Example 1 and Example 1. Detailed Implementation

[0024] To better explain the present invention, the main contents of the present invention are further illustrated below with reference to specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0025] Example 1

[0026] The steel grade being smelted is HR600 / 780HE expanded steel, and the methods and steps for controlling inclusions include converter, LF refining, RH vacuum refining, and continuous casting.

[0027] During the converter process, the carbon content at the end of the main blowing was 0.42% (by weight), and the bottom blowing argon intensity during auxiliary blowing was 0.14 Nm. 3 / (tons of liquid·min). Aluminum granules, lime and bauxite are added at the converter tapping point, and the argon station performs strong argon blowing to pre-reduce the slag.

[0028] In LF refining, the LF furnace is heated twice, with heating times of 16 min and 7 min respectively, maintaining a slight positive pressure inside the furnace. After white slag forms in the LF furnace and is maintained for 15 min, temperature and oxygen levels are measured. The molten steel temperature is 1596℃, and the active oxygen content is 12 ppm. Subsequently, the slag surface is blown open, and a titanium-iron alloy with a titanium content of 70% is added at a rate of 0.71 kg / t of steel. Medium-intensity bottom-blowing argon stirring is performed for 3 min, followed by the addition of aluminum granules for final deoxidation and alloying. In the later stages of LF refining, after all components except Ti are within acceptable limits, weak stirring is performed for 3 min. 420 meters of pure calcium wire is fed into the LF furnace, resulting in a Ca content in the molten steel of approximately 32 ppm. After the wire is fed, the molten steel is directly transferred to the RH station for vacuum refining.

[0029] In RH vacuum refining, after dehydrogenation under high vacuum (<67 Pa), the remaining ferrotitanium alloy is added. Low vacuum cycling is then employed, with fine-tuning of the content of each alloying element to meet the steel composition requirements. The total RH cycle time (start-break-vacuum) is 25 min, and the amount of remaining ferrotitanium alloy (titanium content 70%) added is 0.95 kg / t steel. The vacuum level during the low vacuum cycle is 3 kPa.

[0030] The continuous casting tundish covering agent is a tundish covering agent containing 7% (by weight) vermiculite. The hot surface temperature of the first row of copper plates in the crystallizer, measured by thermocouples, is 120℃. The heat flux density difference between the narrow copper plates on both sides of the crystallizer is 0.02MW / mm². 2 .

[0031] The final product was HR600 / 780HE expanded steel, with a nitrogen content of 0.0032% by mass.

[0032] like Figure 2 As shown in b, the inclusions in the molten steel in the tundish have relatively uniform composition, and the Al2O3 is fully modified, mainly consisting of Al2O3-CaO inclusions.

[0033] like Figure 3 As shown in b, the steel plate samples obtained after hot rolling of the billet have fewer inclusions, more concentrated components, and no independent CaS inclusions or Al2O3-CaS inclusions.

[0034] Inclusions classified as B are rated 0–0.5, while those classified as D and DS are rated 0–0.5. The size of Al₂O₃ and Al₂O₃-MgO inclusions in the steel ranges from 3.25 to 7.34 μm, with an average of 4.42 μm, and they account for 0.87% of the total inclusions. The size of Ti₂O₃-Al₂O₃-CaS inclusions ranges from 3.26 to 10.23 μm, with an average of 4.04 μm, and they account for 11.21% of the total inclusions. Al₂O₃-CaO inclusions with an aluminum-to-calcium ratio between 0.8 and 1.5 range from 4.43 to 5.27 μm, with an average of 4.99 μm, and they account for 0.35% of the total inclusions.

[0035] As shown in Table 1, the number density of oxide inclusions in the above steel plate samples was 4.89 inclusions / mm². 2 The size ranges from 2.68 to 11.01 μm, with an average size of 4.54 μm. There are no oxide inclusions larger than 13 μm. The number density of TiN inclusions is 12.83 inclusions / mm². 2 The size ranges from 0.87 to 9.00 μm, with an average size of 4.09 μm, and the proportion of TiN inclusions larger than 8 μm is 0.78%.

[0036] The final HR600 / 780HE expanded steel plate had an expansion rate of 56-85% and a cracking rate of 0.3%.

[0037] Example 2

[0038] The steel grade being smelted is HR600 / 780HE expanded steel, and the methods and steps for controlling inclusions include converter, LF refining, RH vacuum refining, and continuous casting.

[0039] During the converter process, the carbon content at the end of the main blowing was 0.37% (by weight), and the bottom blowing argon intensity during auxiliary blowing was 0.14 Nm. 3 / (tons of liquid·min). Aluminum granules, lime and bauxite are added at the converter tapping point, and the argon station performs strong argon blowing to pre-reduce the slag.

[0040] In the LF refining process, the LF furnace is heated twice, with heating times of 14 min and 8 min respectively, maintaining a slight positive pressure inside the furnace. After the LF furnace forms white slag and is held for 15 min, the temperature and oxygen content are measured. The molten steel temperature is 1597℃, and the active oxygen content is 18 ppm. Subsequently, the slag surface is blown open, and a titanium-iron alloy with a titanium content of 70% is added at a rate of 0.73 kg / t steel. Medium-intensity bottom-blowing argon stirring is performed for 3 min, followed by the addition of aluminum granules for final deoxidation and alloying. In the later stages of LF refining, after all components except Ti are within acceptable limits, weak stirring is performed for 3 min. 432 meters of pure calcium wire is fed into the LF furnace, resulting in a Ca content in the molten steel of approximately 29 ppm. After the wire is fed, the molten steel is directly transferred to the RH station for vacuum refining.

[0041] In RH vacuum refining, after dehydrogenation under high vacuum (<67 Pa), the remaining ferrotitanium alloy is added. Low vacuum cycling is then employed, with fine-tuning of the content of each alloying element to meet the steel composition requirements. The total RH cycle time (start-break-vacuum) is 24 min, and the amount of remaining ferrotitanium alloy (titanium content 70%) added is 0.93 kg / t steel. The vacuum level during the low vacuum cycle is 2.7 kPa.

[0042] The continuous casting tundish covering agent is a tundish covering agent containing 7% (by weight) vermiculite. The hot surface temperature of the first row of copper plates in the crystallizer, measured by thermocouples, is 118℃. The heat flux density difference between the narrow copper plates on both sides of the crystallizer is 0.015MW / mm². 2 .

[0043] The final product was HR600 / 780HE expanded steel with a nitrogen content of 0.0029% by mass.

[0044] In the steel plate samples obtained after hot rolling of the cast billet, the B-type inclusions were rated 0–0.5, and the D and DS-type inclusions were rated 0–0.5. The size of Al2O3 and Al2O3-MgO inclusions in the steel ranged from 3.25 to 7.39 μm, with an average of 4.21 μm, accounting for 0.16% of the total inclusions. The size of Ti2O3-Al2O3-CaS inclusions ranged from 3.26 to 10.03 μm, with an average of 4.14 μm, accounting for 9.21% of the total inclusions. The size of Al2O3-CaO inclusions with an aluminum-to-calcium ratio between 0.8 and 1.5 ranged from 4.43 to 5.52 μm, with an average of 4.87 μm, accounting for 0.21% of the total inclusions.

[0045] As shown in Table 1, the number density of oxide inclusions in the above steel plate samples was 3.37 inclusions / mm. 2 The size ranges from 1.02 to 12.51 μm, with an average size of 3.65 μm, and there are no oxide inclusions larger than 13 μm; the number density of TiN inclusions is 12.34 inclusions / mm².2 The size ranges from 1.0 to 9.18 μm, with an average size of 4.32 μm, and the proportion of TiN inclusions larger than 8 μm is 1.14%.

[0046] The final HR600 / 780HE expanded steel plate had an expansion rate of 53-81% and no cracking.

[0047] Example 3

[0048] The steel grade being smelted is HR600 / 780HE expanded steel, and the methods and steps for controlling inclusions include converter, LF refining, RH vacuum refining, and continuous casting.

[0049] During the converter process, the carbon content at the end of the main blowing is 0.33% (by weight), and the bottom blowing argon intensity during auxiliary blowing is 0.15 Nm. 3 / (tons of liquid·min). Aluminum granules, lime and bauxite are added at the converter tapping point, and the argon station performs strong argon blowing to pre-reduce the slag.

[0050] In the LF refining process, the LF furnace is heated twice, with heating times of 15 min and 7 min respectively, maintaining a slight positive pressure inside the furnace. After the LF furnace forms white slag and is held for 15 min, the temperature and oxygen content are measured. The molten steel temperature is 1601℃, and the active oxygen content is 21 ppm. Subsequently, the slag surface is blown open, and a titanium-iron alloy with a titanium content of 70% is added at a rate of 0.76 kg / t of steel. Medium-intensity bottom-blowing argon stirring is performed for 3 min, followed by the addition of aluminum granules for final deoxidation and alloying. In the later stages of LF refining, after all components except Ti are within acceptable limits, weak stirring is performed for 3 min. 432 meters of pure calcium wire is fed into the LF furnace, resulting in a Ca content in the molten steel of approximately 29 ppm. After the wire is fed, the molten steel is directly transferred to the RH station for vacuum refining.

[0051] In RH vacuum refining, after dehydrogenation under high vacuum (<67 Pa), the remaining ferrotitanium alloy is added. Low vacuum cycling is then employed, with fine-tuning of the content of each alloying element to meet the steel composition requirements. The total RH cycle time (start-break-vacuum) is 24 min, and the amount of remaining ferrotitanium alloy (titanium content 70%) added is 0.90 kg / t steel. The vacuum level during low vacuum cycling is 3.5 kPa.

[0052] The continuous casting tundish covering agent is a tundish covering agent containing 7% (by weight) vermiculite. The hot surface temperature of the first row of copper plates in the crystallizer, measured by thermocouples, is 108℃. The heat flux density difference between the narrow copper plates on both sides of the crystallizer is 0.018MW / mm². 2 .

[0053] The final product was HR600 / 780HE expanded steel, with a nitrogen content of 0.0031% by mass.

[0054] In the steel plate samples obtained after hot rolling of the cast billet, the B-type inclusions were rated 0–0.5, and the D and DS-type inclusions were rated 0–0.5. The size of Al2O3 and Al2O3-MgO inclusions in the steel ranged from 2.01 to 6.37 μm, with an average of 3.98 μm, accounting for 0.26% of the total inclusions. The size of Ti2O3-Al2O3-CaS inclusions ranged from 3.26 to 9.3 μm, with an average of 4.02 μm, accounting for 6.21% of the total inclusions. The size of Al2O3-CaO inclusions with an aluminum-to-calcium ratio between 0.8 and 1.5 ranged from 3.41 to 9.52 μm, with an average of 4.07 μm, accounting for 0.08% of the total inclusions.

[0055] As shown in Table 1, the number density of oxide inclusions in the above steel plate samples was 2.93 inclusions / mm. 2 The size ranges from 2.01 to 17.1 μm, with an average size of 3.93 μm. The proportion of oxide inclusions larger than 13 μm is 0.08%. The number density of TiN inclusions is 11.03 inclusions / mm². 2 The size ranges from 1.31 to 10.19 μm, with an average size of 3.61 μm, and the proportion of TiN inclusions larger than 8 μm is 0.21%.

[0056] The final HR600 / 780HE expanded steel plate had an expansion rate of 61-87% and no cracking.

[0057] Comparative Example

[0058] The steel grade to be smelted is HR600 / 780HE expanded hole steel. The process steps are converter, LF refining, RH vacuum refining, and continuous casting.

[0059] The carbon content at the end of the main blowing in the converter is 0.12% (by weight), and the bottom blowing argon intensity during auxiliary blowing is increased to 0.08 Nm. 3 / (ton liquid·min).

[0060] The LF was heated a total of 4 times, with heating times of 12 min, 8 min, 5 min, and 3 min for two separate heating cycles. After the LF formed white slag, a total of 1.66 kg / t steel of titanium-iron alloy (containing 70% titanium) was added.

[0061] The total RH circulation time (start-break) is 25 minutes. After breaking the void, 450 meters of pure calcium wire is fed in and then gently stirred for 8 minutes. The Ca content in the steel is between 30 ppm.

[0062] The tundish covering agent used in continuous casting is a common tundish covering agent. The hot surface temperature measured by the thermocouples on the first row of copper plates in the crystallizer is 132℃, and the heat flux density difference between the narrow copper plates on both sides of the crystallizer is 0.079MW / mm².2 .

[0063] The final HR600 / 780HE expanded steel has a nitrogen content of 0.0053% by mass.

[0064] like Figure 2 As shown in Figure a, the components of the inclusions in the molten steel in the ladle are dispersed, including magnesium aluminum spinel, insufficiently modified Al2O3, and Al2O3-CaO-MgO inclusions.

[0065] like Figure 3 As shown in Figure a, the steel plate sample obtained after hot rolling of the billet has a large number of inclusions, the components of the inclusions are relatively dispersed, and it contains independent CaS inclusions.

[0066] Inclusions classified as B are rated 0–1.5, while those classified as D and DS are rated 0–2.0. The size of Al₂O₃ and Al₂O₃-MgO inclusions in the steel ranges from 2.69 to 20.12 μm, with an average of 7.89 μm; they account for 4.7% of the total inclusions. The size of Ti₂O₃-Al₂O₃-CaS inclusions ranges from 5.89 to 27.61 μm, with an average of 10.65 μm; they account for 13.98% of the total inclusions. Al₂O₃-CaO inclusions with an aluminum-to-calcium ratio between 0.8 and 1.5 range from 6.83 to 37.1 μm, with an average of 12.31 μm; they account for 0.96% of the total inclusions.

[0067] As shown in Table 1, the number density of oxide inclusions in the steel plate samples was 6.18 inclusions / mm². 2 The size ranges from 3.01 to 27.01 μm, with an average size of 5.61 μm. Oxide inclusions larger than 13 μm account for 2.72% of the total. The number density of TiN inclusions is 13.72 inclusions / mm². 2 The size ranges from 3.01 to 20.46 μm, with an average size of 4.61 μm. The proportion of TiN inclusions larger than 8 μm is 3.50%.

[0068] The final HR600 / 780HE expanded steel plate had an expansion rate of 35-62%. Customer feedback indicated that cracking occurred, and the cracking rate of the supplied batches was around 5%.

Claims

1. A method for controlling inclusions in expanded steel, characterized in that: Includes the following steps: The expanded steel billet is obtained by converter refining, LF refining, RH vacuum refining and continuous casting; the mass percentage of N in the expanded steel is ≤0.0040%; after the steel is tapped from the converter, the slag is pre-reduced by strong argon blowing through an argon station; The LF refining process involves 2-3 heating cycles, with the heating time controlled in steps based on the initial temperature. The first heating cycle is 10-16 minutes, the second is 6-8 minutes, and the third is 4-6 minutes. After white slag forms in the LF furnace and is maintained for 15 minutes, the temperature and oxygen levels are measured and controlled. The molten steel temperature is maintained at 1590-1605℃, and the active oxygen content is 10-30 ppm. The slag surface is then blown open, and 40-50% of the total mass of ferro-titanium alloy is added. After medium-intensity bottom-blowing argon stirring, aluminum particles are added for final deoxidation and alloying. Once all components except Ti are qualified, the mixture is gently stirred for 3-5 minutes. Then, pure calcium wire is fed in, resulting in a Ca content of 20-50 ppm in the molten steel. After feeding the pure calcium wire, the steel is directly sent to the RH station for RH vacuum refining. The RH vacuum refining process involves high-vacuum dehydrogenation followed by the addition of the remaining ferrotitanium alloy, and then low-vacuum cycling with fine-tuning of the content of each alloying element to meet the steel composition requirements. The vacuum degree of the high-vacuum dehydrogenation process is <67 Pa, and the vacuum degree of the low-vacuum cycling process is 1~5 kPa. The expanded steel is HR600 / 780HE expanded steel.

2. The control method according to claim 1, characterized in that: In the converter step, the carbon content in the molten steel is ≥0.30% by mass at the end of the main blowing, and the bottom blowing argon intensity is 0.12~0.16 Nm during auxiliary blowing. 3 / (tons of liquid per min); aluminum granules, lime and bauxite are added during the converter tapping process.

3. The control method according to claim 1, characterized in that: During the continuous casting process, a high temperature gradient is used to control the cooling intensity of the crystallizer. The hot surface temperature measured by thermocouples on the first row of copper plates in the crystallizer is controlled at 90~125℃, and the heat flux density difference between the narrow copper plates on both sides of the crystallizer is controlled at 0~0.06MW / mm. 2 .

4. The control method according to claim 1, characterized in that: The tundish covering agent used in the continuous casting step contains vermiculite, and the weight percentage of vermiculite in the tundish covering agent is 5-10%.

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