Niobium-containing steel casting blank transverse crack control method

Abstract

The invention discloses a transverse crack control method for a niobium-containing steel casting blank, which comprises the following steps of: for a steel type with the niobium mass content of 0.02-0.15%, adopting tundish cover argon blowing and tundish sealing to reduce the nitrogen content in the tundish atmosphere to be below 2%; according to the mass content of steel grade carbon, soft reduction is not adopted or the reduction amount is smaller than 3 mm; the amplitude of the crystallizer is reduced, and the amplitude is controlled within 5 mm; and the amount of cooling water in continuous casting secondary cooling 2-5 zones is reduced, and the specific water amount is not more than 0.8 L / kg. According to the method, through tundish cover argon blowing and tundish sealing, nitrogen in air entering molten steel is reduced, nitrogen increasing of continuous casting molten steel is greatly reduced, and then the separation amount of niobium nitride in the casting blank cooling process is reduced; meanwhile, different soft reduction processes are adopted for steel types with different carbon contents, and under the condition that the requirements for center segregation and center looseness in the casting blank are met, the expansion of cracks on the surface of the casting blank is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a method for controlling transverse cracks in a niobium-containing steel ingot. Background Art

[0002] Adding a small amount of niobium to steel can increase the strength and toughness of the steel, while also being relatively low in cost. It is mainly used in the fields of oil pipeline steel, high-strength plates, and automotive steel. However, during the continuous casting and cooling process, niobium-containing steel will precipitate niobium nitride and niobium carbonitride at the grain boundaries, which in turn will cause cracks on the surface of the ingot.

[0003] In the prior art, patent CN101899613A discloses a method for reducing surface cracks in medium-carbon niobium-containing steel, which reduces the amount of Nb added and partially replaces Nb with Ti. Patent CN104741560A relates to a method for reducing transverse cracks in the corners of niobium-containing steel, which uses a chamfered crystallizer with a narrow surface and a double taper to manufacture niobium-containing steel; controls the N content of the molten steel to be less than or equal to 50ppm, and the Nb content to be less than or equal to 0.12%; controls the water flow rate in the wide surface water tank of the crystallizer to be greater than the water flow rate in the narrow surface water tank; uses a secondary cooling zone to cool the ingot, controls the water volume of the secondary cooling zone to be 0.55-0.80L / t, and controls the ratio of the total water volume of cooling water in zones 2-4 to the total water volume of cooling water in zones 6-8 to be 0.6-1.0. The above patent mainly improves the quality of the ingot by controlling the smelting composition and cooling water, and does not involve the control of continuous casting nitrogen addition and the influence of light pressure reduction on crack propagation. In the actual continuous casting process, the long nozzle, the tundish and the crystallizer nozzle are all in contact with the air. The 78% nitrogen in the air can easily cause nitrogen addition to the molten steel, aggravating the precipitation of niobium nitride during the solidification process of the molten steel. On the other hand, the niobium-containing precipitates at the grain boundaries are prone to microcracks. These microcracks on the surface of the ingot (depth <1mm) will be removed during heating, but the mechanical stress of continuous casting straightening and pressing will cause the cracks to extend into the interior of the ingot, ultimately leading to peeling defects in the rolled product. Summary of the invention

[0004] In order to overcome the above-mentioned defects of the prior art, the present invention reduces the nitrogen in the air entering the molten steel by blowing argon through the tundish cover and sealing the tundish, greatly reduces the nitrogen increase of the continuous casting molten steel, and further reduces the precipitation of niobium nitride during the cooling process of the ingot. At the same time, for steels with different carbon contents, different soft reduction processes are adopted to reduce the expansion of cracks on the surface of the ingot while meeting the requirements of central segregation and central porosity inside the ingot.

[0005] In order to achieve the above-mentioned invention object, the present invention provides a method for controlling transverse cracks in a niobium-containing steel ingot. For a steel having a niobium mass content of 0.02% to 0.15%, the method comprises the following steps:

[0006] ① Use argon blowing on the tundish cover and seal the tundish to reduce the nitrogen content in the tundish atmosphere to below 2%, reduce the nitrogen in the air entering the molten steel, and thus reduce the nitrogen increase during the continuous casting process.

[0007] Furthermore, the argon blowing flow rate of the tundish cover is 1600-4000L / min, wherein, before the slide plate of the first ladle of the tundish is opened, the argon blowing flow rate is 3000-4000L / min; after the liquid level of the tundish rises to the maximum value, the argon blowing flow rate is stabilized at 1600-2000L / min; and the argon blowing flow rate of 3000-4000L / min is also used when changing the ladle.

[0008] Furthermore, asbestos cloth is used to seal the steel overflow hole, impact area and sampling hole of the middle package to prevent nitrogen from entering the middle package to the greatest extent.

[0009] ② According to the carbon content of the steel grade, choose not to use light reduction or choose to use a reduction less than 3mm; when the carbon content of the steel grade is less than 0.15%, do not use light reduction; when the carbon content is 0.15% to 0.2%, use a light reduction of 1 to 3mm; avoid crack expansion in the ingot caused by excessive reduction.

[0010] ③Reduce the amplitude of the crystallizer and control the amplitude within 5mm.

[0011] ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting, and the specific water volume shall not exceed 0.8L / kg.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] The present invention reduces the nitrogen in the air from entering the molten steel by blowing argon through the tundish cover and sealing the tundish, greatly reduces the nitrogen increase in the continuous casting molten steel, and further reduces the precipitation of niobium nitride during the cooling process of the ingot. At the same time, different soft reduction processes are adopted for steels with different carbon contents, and the expansion of cracks on the surface of the ingot is reduced while meeting the requirements of the center segregation and center looseness inside the ingot. The method for producing continuous casting ingots can improve the quality of the ingots and reduce surface defects. DETAILED DESCRIPTION

[0014] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0015] A method for controlling transverse cracks in a niobium-containing steel ingot, for a steel having a niobium mass content of 0.02% to 0.15%, the method comprises the following steps:

[0016] ① Use argon blowing and sealing of the middle bag cover to reduce the nitrogen content in the middle bag atmosphere to less than 2%;

[0017] ② According to the carbon content of the steel grade, light reduction is not adopted or the reduction is less than 3mm;

[0018] ③Reduce the amplitude of the crystallizer and control the amplitude within 5mm;

[0019] ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting, and the specific water volume shall not exceed 0.8L / kg.

[0020] The matters not described in the following embodiments are the same as the description of the above specific implementation methods.

[0021] Example 1

[0022] A method for controlling transverse cracks in a niobium-containing steel ingot, wherein the specific process flow adopts the following technical solution.

[0023] For steels with 0.15wt% niobium and 0.08-0.14wt% carbon content:

[0024] ① Two minutes before the first ladle slide of the tundish was opened, the tundish cover was blown with argon at a rate of 4000L / min; after the tundish liquid level rose to the maximum value, the argon blowing rate was stabilized at 2000L / min; when changing tundishes, the argon blowing rate was also 4000L / min. Secondly, asbestos cloth was used to seal the overflow hole, impact area, and sampling hole of the tundish.

[0025] ② Continuous casting does not use light reduction;

[0026] ③ Crystallizer amplitude 5mm;

[0027] ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting to a specific water volume of 0.8L / kg.

[0028] Example 1 By using the above technical solution, the nitrogen content of the tundish atmosphere is 1.8%, the liquid nitrogen content of the tundish steel is 30ppm, the central segregation of the ingot is C0.5, and the peeling defect rate of the rolled material is 2.6%.

[0029] Example 2

[0030] A method for controlling transverse cracks in a niobium-containing steel ingot, wherein the specific process flow adopts the following technical solution.

[0031] For steels with 0.02% niobium and 0.08-0.14% carbon content:

[0032] ① Two minutes before the first ladle slide of the tundish is opened, the tundish cover adopts an argon blowing flow rate of 3000L / min; after the tundish liquid level rises to the maximum value, the argon blowing flow rate is stabilized at 2000L / min; when changing tundishes, an argon blowing flow rate of 3000L / min is also adopted. Secondly, asbestos cloth is used to seal the overflow hole, impact area and sampling hole of the tundish;

[0033] ② Continuous casting does not use light reduction;

[0034] ③Reduce the amplitude of the crystallizer to 4.5mm;

[0035] ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting, with a specific water volume of 0.77L / kg.

[0036] Example 2: Through the above technical scheme, the nitrogen content of the middle package atmosphere is 2.4%, the liquid nitrogen content of the middle package steel is 38ppm, the central segregation of the ingot is C0.5, and the peeling defect rate of the rolled material is 0.6%.

[0037] Example 3

[0038] A method for controlling transverse cracks in a niobium-containing steel ingot, wherein the specific process flow adopts the following technical solution.

[0039] For steels with 0.15% niobium and 0.15% to 0.2% carbon content:

[0040] ① Two minutes before the first ladle slide of the tundish was opened, the tundish cover was blown with argon at a rate of 4000L / min; after the tundish liquid level rose to the maximum value, the argon blowing rate was stabilized at 2000L / min; when changing tundishes, the argon blowing rate was also 4000L / min. Secondly, asbestos cloth was used to seal the overflow hole, impact area, and sampling hole of the tundish.

[0041] ② Continuous casting adopts a relatively small light pressure reduction, with a pressure reduction of 1mm;

[0042] ③ Crystallizer amplitude 5mm;

[0043] ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting, with a specific water volume of 0.77L / kg.

[0044] Example 3: Through the above technical scheme, the nitrogen content of the middle package atmosphere is 2.2%, the liquid nitrogen content of the middle package steel is 32ppm, the central segregation of the ingot is C1.0, and the peeling defect rate of the rolled material is 1.6%.

[0045] Example 4

[0046] A method for controlling transverse cracks in a niobium-containing steel ingot, wherein the specific process flow adopts the following technical solution.

[0047] For steels with 0.02% niobium and 0.15% to 0.2% carbon content:

[0048] ① Two minutes before the first ladle slide of the tundish was opened, the tundish cover was blown with argon at a rate of 3000L / min; after the tundish liquid level rose to the maximum value, the argon blowing rate was stabilized at 1600L / min; when changing tundishes, the argon blowing rate was also 3000L / min. Secondly, asbestos cloth was used to seal the overflow hole, impact area, and sampling hole of the tundish.

[0049] ② Continuous casting adopts a relatively small light pressure reduction, with a pressure reduction of 3mm;

[0050] ③ Crystallizer amplitude 5mm;

[0051] ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting to a specific water volume of 0.8L / kg.

[0052] Example 4 Through the above technical scheme, the nitrogen content of the middle package atmosphere is 2.8%, the liquid nitrogen content of the middle package steel is 35ppm, the central segregation of the ingot is C1.0, and the peeling defect rate of the rolled material is only 0.9%.

[0053] Comparative Example 1

[0054] A method for controlling transverse cracks in a niobium-containing steel ingot, wherein the specific process flow adopts the following technical solution.

[0055] For steels with 0.15% niobium and 0.08-0.2% carbon content:

[0056] (1) No argon blowing or sealing of the package cover is used.

[0057] (2) Continuous casting lightly press down 5mm.

[0058] (3) Crystallizer amplitude: 5.4 mm;

[0059] (4) Secondary cooling water volume 1.1L / kg;

[0060] In Comparative Example 1, the nitrogen content of the tundish atmosphere is 76%, the liquid nitrogen content of the tundish steel is 65 ppm, the center segregation of the ingot is C0.5-1.0, and the peeling defect rate of the rolled material is 35%.

[0061] Comparative Example 2

[0062] A method for controlling transverse cracks in a niobium-containing steel ingot, wherein the specific process flow adopts the following technical solution.

[0063] For steels with 0.02% niobium and 0.08-0.2% carbon content:

[0064] (1) No argon blowing or sealing of the package cover is used.

[0065] (2) Continuous casting lightly press down 5mm.

[0066] (3) Crystallizer amplitude: 5.4 mm;

[0067] (4) Secondary cooling water volume 1.1L / kg;

[0068] In Comparative Example 2, the nitrogen content of the tundish atmosphere is 78%, the liquid nitrogen content of the tundish steel is 60 ppm, the center segregation of the ingot is C0.5-1.0, and the peeling defect rate of the rolled material is 26%.

[0069] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for controlling transverse cracks in a niobium-containing steel ingot, characterized in that: For a steel grade having a niobium mass content of 0.02% to 0.15%, the method comprises the following steps: ① Use argon blowing and sealing of the middle bag cover to reduce the nitrogen content in the middle bag atmosphere to less than 2%; ② According to the carbon content of the steel grade, light reduction is not adopted or the reduction is less than 3mm; ③Reduce the amplitude of the crystallizer and control the amplitude within 5mm; ④ Reduce the amount of cooling water in the secondary cooling zones 2 to 5 of continuous casting, and the specific water volume shall not exceed 0.8L / kg.

2. The method according to claim 1, characterized in that: In step ①, the argon blowing flow rate of the tundish cover is 1600-4000 L / min; wherein, before the slide plate of the first ladle of the tundish is opened, the argon blowing flow rate is 3000-4000 L / min; after the liquid level of the tundish rises to the maximum value, the argon blowing flow rate is stabilized at 1600-2000 L / min; when changing the ladle, the argon blowing flow rate is 3000-4000 L / min.

3. The method according to claim 1, characterized in that: In step ①, asbestos cloth is used to seal the steel overflow hole, impact area and sampling hole of the middle bag.

4. The method according to claim 1, characterized in that: In step ②, when the carbon content of the steel is less than 0.15%, soft reduction is not used; when the carbon content is 0.15% to 0.2%, the soft reduction is 1 to 3 mm.