Refining method for increasing nitrogen in vacuum process of VD vacuum furnace

Abstract

The invention provides a refining method for increasing nitrogen in the vacuum process of a VD vacuum furnace, and relates to the technical field of ferrous metallurgy, the refining method comprises the following steps: S1, refining molten steel in an LF steel ladle by adopting nitrogen as bottom blowing gas; and S2, the molten steel obtained after LF ladle refining is placed in a VD vacuum furnace, nitrogen serves as bottom blowing gas, the nitrogen flow and the vacuum degree are adjusted in a multi-stage mode, and the nitrogen increasing operation is completed. A multi-stage adjustment mode is adopted, and the nitrogen content in the molten steel is controlled by controlling the nitrogen flow, the vacuum degree and the retention time.

Description

Technical Field

[0001] The invention relates to the technical field of iron and steel metallurgy, and in particular to a refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace. Background Art

[0002] In the iron and steel metallurgical industry, some steel grades need nitrogen addition to improve their chemical and mechanical properties, such as P91, P92, 0Cr17Ni4Cu4Nb, DIN 1.4104, 14Cr17Ni2, etc. During the smelting process, nitrogen needs to be added to 300-900ppm. Currently, there are two mainstream smelting processes:

[0003] 1) Adding nitriding alloy: For example, adding manganese nitride, chromium nitride and other alloys, adding nitriding alloy and then vacuuming, but the nitrogen loss rate in the vacuum process is 40-50%. The loss rate of this process fluctuates widely and is difficult to control. In addition, nitriding alloy is relatively expensive and has high cost. The cost of adding nitriding alloy increases by no less than 77 yuan / ton. If the nitrogen is unqualified after vacuuming, it is difficult to adjust and there is a high risk of producing unqualified products;

[0004] 2) LF refining nitrogen blowing: This method has low cost, but after LF refining nitrogen blowing, it is necessary to reserve a margin for nitriding alloy adjustment, and after VD vacuum weak nitrogen blowing, it is necessary to add nitriding alloy. In both methods, impurities and gases (H and O) entrained in the alloy have low temperatures after VD vacuum, and it is difficult to increase nitrogen. Limited by the production rhythm, it is necessary to control different nitrogen blowing intensities according to different weak nitrogen blowing times, which easily leads to difficulty in removing secondary oxidation of molten steel; and it is also necessary to control a higher temperature to ensure sufficient nitrogen blowing time. High temperature will increase the degree of erosion of the ladle, further increasing the risk of inclusions; in addition, since multiple sampling is required to confirm the nitrogen content in order to ensure that the required nitrogen content is met, the smelting rhythm is seriously affected, and the time and composition analysis cost are wasted, which is not conducive to production organization.

[0005] 3) VD vacuum weak nitrogen blowing to increase nitrogen: After VD vacuum weak nitrogen blowing, nitriding alloy needs to be added. It is difficult to remove impurities and gases (H and O) entrained in the alloy. Moreover, the temperature after VD vacuum is low, and it is difficult to increase nitrogen. Limited by the production rhythm, different nitrogen blowing intensities need to be controlled according to different weak nitrogen blowing times, which easily leads to secondary oxidation of the molten steel.

[0006] Therefore, there is an urgent need for a low-cost, highly stable nitrogen-enriched refining method. Summary of the invention

[0007] The object of the present invention is to provide a refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace. The present invention adopts a multi-stage adjustment method to control the nitrogen content in molten steel by controlling the nitrogen flow rate, vacuum degree and holding time.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace, comprising the following steps:

[0010] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle;

[0011] S2: The molten steel refined by the LF ladle is placed in a VD vacuum furnace, nitrogen is used as the bottom blowing gas, and the nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation.

[0012] Further, based on the above technical solution, in step S1, the nitrogen flow rate is 80-600 NL / min, and the holding time is 40-80 min.

[0013] Further, on the basis of the above technical solution, after the LF ladle refining is completed, the nitrogen content in the molten steel is 200-350 ppm by mass.

[0014] Further, based on the above technical solution, in step S2, the multi-level adjustment includes:

[0015] Level A adjustment: nitrogen flow rate is 30-120NL / min, vacuuming starts until the vacuum degree is ≤67Pa, and the vacuuming time is 6-10min;

[0016] Level B adjustment: nitrogen flow rate is 30-120NL / min, vacuum degree ≤67Pa, maintained for 8-10min;

[0017] C-level adjustment: nitrogen flow rate is 200-700NL / min, vacuum degree is 800-1500Pa, and maintained for 5-10min;

[0018] Level D adjustment: Nitrogen flow rate is 30-120NL / min, vacuum is broken, and sampling is performed when vacuum ends.

[0019] Further, on the basis of the above technical solution, when the nitrogen content in the sampled molten steel is 500-800 ppm by mass percentage, the nitrogen addition operation is completed;

[0020] When the nitrogen content in the sampled molten steel is ≥800ppm, the molten steel in the VD vacuum furnace needs to be adjusted once for level A, level B and level D, where the level B holding time is 1 to 3 minutes, and the nitrogen content is tested after completion;

[0021] When the nitrogen content in the sampled molten steel is 350-500ppm, weak nitrogen blowing operation is performed on the molten steel in the VD vacuum furnace until the tested nitrogen content meets the requirements;

[0022] Among them, the nitrogen flow rate in the weak nitrogen blowing operation is 30~100NL / min;

[0023] When the nitrogen content in the sampled molten steel is less than 350ppm and does not meet the nitrogen content requirement of the steel grade, it is necessary to increase nitrogen again according to steps S1 and S2.

[0024] Further, on the basis of the above technical solution, the H content in the molten steel after the nitrogen addition operation is 0.8-1.2 ppm, and the O content is 5-13 ppm, by mass percentage;

[0025] And / or, the nitrogen loss rate in the molten steel after the nitrogen addition operation is 15 to 35%.

[0026] Furthermore, based on the above technical solution, the steel type of the molten steel includes one of P91, 1Cr17Ni2, P92, 0Cr17Ni4Cu4Nb, DIN 1.4104 or 14Cr17Ni2.

[0027] Furthermore, on the basis of the above technical solution, the molten steel that has completed the nitrogen addition operation is cast.

[0028] Further, on the basis of the above technical solution, the casting includes:

[0029] At the beginning of casting, the molten steel flow rate is 750-1100kg / min, and the casting time is 5-7min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1500-2000kg / min, and the casting time is 10-25min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 400-800kg / min, and the casting time is 6-10min.

[0030] The present invention provides a refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace, and the beneficial effects are as follows:

[0031] 1. The present invention adopts a multi-stage adjustment method to control the nitrogen content in the molten steel by controlling the nitrogen flow rate, vacuum degree and holding time.

[0032] 2. The VD of the present invention does not need to be nitrogenated by means of nitriding alloys after vacuum breaking, nor does it need to increase the nitrogen flow rate and stirring intensity after vacuum breaking. This not only avoids secondary oxidation of the molten steel after nitrogen addition, but also saves costs. There is no need to take samples multiple times to confirm the nitrogen content, which saves time and is beneficial to production.

[0033] 3. In the LF ladle refining of the present invention, it is not necessary to increase the affinity of nitrogen by increasing the temperature, which not only reduces the corrosion of the ladle, but also reduces the inclusions and gases in the steel. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0035] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0036] According to a first aspect of the present invention, a refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace is provided, comprising the following steps:

[0037] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle;

[0038] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation.

[0039] Specifically, the process flow of a refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace provided by the present invention is: EAF steel tapping→LF ladle refining→VD vacuum furnace vacuum refining→casting.

[0040] Specifically, the VD vacuum furnace uses nitrogen as the bottom blowing gas, mainly to ensure the degassing of H and O in the molten steel.

[0041] Furthermore, the LF ladle refining process in step S1 of the present invention and the VD vacuum furnace refining in step S2 both use nitrogen as the bottom blowing gas, the purpose of which is to increase the nitrogen content in the alloy. Compared with the method of first using nitrogen and then using argon to tap steel in the prior art, it can save costs, and the process is stable, and there is basically no pollution to the molten steel; and it also avoids the use of nitriding alloys and other means to increase nitrogen, saving costs, and the process is stable, and there is basically no pollution to the molten steel.

[0042] As an optional embodiment of the present invention, in step S1, the nitrogen flow rate is 80-600NL / min (for example, 100NL / min, 200NL / min, 300NL / min, 400NL / min, 500NL / min, etc.), and the holding time is 40-80min (for example, 50min, 60min, 70min, etc.).

[0043] Specifically, LF ladle refining includes the following process: molten steel (ladle) enters the LF furnace station → connects the nitrogen pipeline and starts nitrogen blowing → supplies power → adds slag → adds deoxidation material → adds alloy to adjust composition → the composition meets the technical requirements, the temperature meets the vacuum requirements, and the LF furnace ends.

[0044] As an optional embodiment of the present invention, after LF ladle refining is completed, the nitrogen content in the molten steel is 200-350 ppm (e.g., 230 ppm, 250 ppm, 270 ppm, 300 ppm, 330 ppm, etc.) by mass percentage.

[0045] As an optional implementation manner of the present invention, in step S2, the multi-level adjustment includes:

[0046] Level A adjustment: nitrogen flow rate is 30-120NL / min (such as 50NL / min, 70NL / min, 90NL / min, 110NL / min, etc.), start vacuuming to vacuum degree ≤67Pa (such as 40Pa, 50Pa, 55Pa, 60Pa, etc.), vacuuming time is 6-10min (such as 7min, 8min, 9min, etc.);

[0047] Specifically, the process of starting to evacuate the air to a vacuum degree of ≤67Pa takes about 6-10 minutes.

[0048] Level B adjustment: nitrogen flow rate is 30-120NL / min (such as 50NL / min, 70NL / min, 90NL / min, 110NL / min, etc.), vacuum degree ≤67Pa (such as 40Pa, 50Pa, 55Pa, 60Pa, etc.), maintained for 8-10min (such as 8.5min, 9min, 9.5min, etc.);

[0049] C-level adjustment: nitrogen flow rate is 200-700NL / min (such as 250NL / min, 300NL / min, 350NL / min, 400NL / min, 450NL / min, 500NL / min, 550NL / min, 600NL / min, 650NL / min, etc.), vacuum degree is 800-1500Pa (such as 900Pa, 1000Pa, 1100Pa, 1200Pa, 1300Pa, 1400Pa, etc.), maintained for 5-10min (such as 6min, 7min, 8min, 9min, etc.);

[0050] Specifically, the reason for increasing the nitrogen flow rate and vacuum degree at the same time in the C-level adjustment is that the nitrogen flow rate is increased to fully contact the nitrogen and the molten steel and increase the contact area;

[0051] In a non-ultimate vacuum (800-1500Pa) state, the nitrogen flow rate is increased at the same time, that is, the contact area between the molten steel and the nitrogen is increased, and the vacuum compressed nitrogen under the condition of 800-1500Pa is used to dissolve into the molten steel, thereby enhancing the solubility of nitrogen; the present invention does not use high temperature to increase the affinity of nitrogen, reduces the corrosion of the ladle, and reduces the inclusions in the steel;

[0052] If the nitrogen flow rate in the C-level adjustment is less than 200NL / min or the vacuum degree is less than 800Pa, the nitrogen content in the molten steel will not meet the requirements, that is, less than 500ppm.

[0053] Furthermore, the nitrogen content in the final molten steel can be controlled by controlling the time of C-level adjustment:

[0054] Specifically, in the C-level adjustment stage, as the nitrogen blowing time increases, the carbon content in the molten steel gradually decreases, resulting in a decrease in the carbon-oxygen (CO) reaction rate. After the carbon-oxygen reaction weakens, the amount of CO gas generated decreases, and its denitrification capacity also decreases accordingly. When the C-level adjustment time is greater than 10 minutes, the nitrogen content in the final molten steel will be ≥800ppm. Therefore, by controlling the C-level adjustment time to 5-10 minutes, the nitrogen content in the final molten steel can be controlled to 500-800ppm.

[0055] Level D adjustment: The nitrogen flow rate is 30-120NL / min (such as 50NL / min, 70NL / min, 90NL / min, 110NL / min, etc.), the vacuum is broken, and sampling is performed when the vacuum ends.

[0056] Specifically, the process of degassing H and O in the molten steel needs to be carried out under ultra-low vacuum (≤67pa), but nitrogen is difficult to increase under ultra-low vacuum, and even negative growth occurs. Therefore, it is impossible to increase nitrogen under ultra-low vacuum. The present invention adopts a multi-stage adjustment method to control the nitrogen content in the molten steel by controlling the nitrogen flow rate, vacuum degree and holding time.

[0057] Specifically, the present invention does not need to increase nitrogen by means of nitriding alloys during VD vacuum breaking, nor does it need to increase the nitrogen flow rate or stirring intensity by breaking the vacuum to increase nitrogen. This not only avoids secondary oxidation of the molten steel after nitrogen addition, but also saves costs, and does not require multiple sampling to confirm the nitrogen content, thus saving time and facilitating production.

[0058] As an optional embodiment of the present invention, when the nitrogen content in the sampled molten steel is 500-800 ppm (e.g., 550 ppm, 600 ppm, 650 ppm, 700 ppm, 750 ppm, etc.) by mass percentage, the nitrogen addition operation is completed;

[0059] Specifically, when the nitrogen content is within the range of 500-800ppm, it can meet the nitrogen content requirements of most steel grades, and can also make the alloy element content of each part of the steel the same, without obvious composition differences, meeting the element content consistency requirements; sampling is performed at most twice within the nitrogen content range of 500-800ppm, and the second time is for reconfirming the results.

[0060] When the nitrogen content in the sampled molten steel is ≥800ppm, the molten steel in the VD vacuum furnace needs to be adjusted again at levels A, B and D, with the B level holding time being 1 to 3 minutes, and the nitrogen content is tested after completion;

[0061] When the nitrogen content in the sampled molten steel is 350-500ppm, weak nitrogen blowing operation is performed on the molten steel in the VD vacuum furnace until the tested nitrogen content meets the requirements;

[0062] Among them, the nitrogen flow rate in the weak nitrogen blowing operation is 30 to 100NL / min (such as 50NL / min, 70NL / min, 90NL / min, etc.).

[0063] When the nitrogen content in the sampled molten steel is less than 350ppm and does not meet the nitrogen content requirement of the steel grade, it is necessary to increase nitrogen again according to steps S1 and S2.

[0064] Specifically, if the nitrogen content in the sampled molten steel is high (≥800ppm), an ultra-low vacuum degassing operation (B-level adjustment) can be performed again. The appropriate ultra-low vacuum time (1 to 3 minutes) needs to be controlled, which can reduce the nitrogen content by about 100ppm.

[0065] As an optional embodiment of the present invention, the H content in the molten steel after the nitrogen addition operation is 0.6-1.2 ppm, and the O content is 5-13 ppm, by mass percentage;

[0066] And / or, the nitrogen loss rate in the molten steel after the nitrogen addition operation is 15 to 35%.

[0067] As an optional embodiment of the present invention, the steel type of the molten steel includes one of P91, 1Cr17Ni2, P92, 0Cr17Ni4Cu4Nb, DIN 1.4104 or 14Cr17Ni2.

[0068] As an optional implementation of the present invention, the molten steel that has completed the nitrogen addition operation is cast.

[0069] As an optional embodiment of the present invention, the casting includes:

[0070] At the beginning of casting, the molten steel flow rate is 750-1100kg / min, and the casting time is 5-7min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1500-2000kg / min, and the casting time is 10-25min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 400-800kg / min, and the casting time is 6-10min.

[0071] The present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0072] The embodiments provided by the present invention are performed on steels selected from P91, P92, 0Cr17Ni4Cu4Nb, DIN 1.4104, and 14Cr17Ni2 for nitrogen addition.

[0073] Example 1

[0074] The steel grade used in this embodiment is P91, and the steel is tapped by EAF to obtain a ladle;

[0075] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle, with a nitrogen flow rate of 300NL / min and a holding time of 60min;

[0076] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation;

[0077] The multi-level adjustment includes:

[0078] Level A adjustment: nitrogen flow rate is 100NL / min, vacuum is drawn, and the vacuum degree is reduced from atmospheric pressure to 50Pa, which takes 9 minutes;

[0079] Level B adjustment: nitrogen flow rate is 80NL / min, vacuum degree is 50Pa, and maintained for 10min;

[0080] C-level adjustment: nitrogen flow rate is 400NL / min, vacuum degree is adjusted from 50Pa to 1200Pa, and maintained at 1200Pa for 8min;

[0081] Level D adjustment: nitrogen flow rate is 100NL / min, vacuum is broken, and sampling is performed after vacuum ends;

[0082] S3: Casting:

[0083] At the beginning of casting, the molten steel flow rate is 900kg / min, and the casting time is 6min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1800kg / min, and the casting time is 15min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 600kg / min, and the casting time is 8min.

[0084] Example 2

[0085] The steel grade used in this embodiment is P92, and the steel is tapped by EAF to obtain a ladle;

[0086] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle, with a nitrogen flow rate of 80 NL / min and a holding time of 80 min;

[0087] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation;

[0088] The multi-level adjustment includes:

[0089] Level A adjustment: nitrogen flow rate is 90NL / min, vacuum is drawn, and the vacuum degree is reduced from atmospheric pressure to 60Pa, which takes 8 minutes;

[0090] Level B adjustment: nitrogen flow rate is 120NL / min, vacuum degree is 60Pa, and maintained for 10min;

[0091] C-level adjustment: nitrogen flow rate is 500NL / min, vacuum degree is adjusted from 60Pa to 1500Pa, and maintained at 1500Pa for 6min;

[0092] Level D adjustment: nitrogen flow rate is 90NL / min, vacuum is broken, and sampling is performed after vacuum is ended;

[0093] S3: Casting:

[0094] At the beginning of casting, the molten steel flow rate is 900kg / min, and the casting time is 7min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1800kg / min, and the casting time is 15min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 500kg / min, and the casting time is 10min.

[0095] Example 3

[0096] The steel used in this embodiment is 14Cr17Ni2, which is tapped by EAF to obtain a ladle;

[0097] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle, with a nitrogen flow rate of 600NL / min and a holding time of 40min;

[0098] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation;

[0099] The multi-level adjustment includes:

[0100] Level A adjustment: nitrogen flow rate is 70NL / min, vacuum is drawn, and the vacuum degree is reduced from atmospheric pressure to 55Pa, which takes 9 minutes;

[0101] Level B adjustment: nitrogen flow rate is 100NL / min, vacuum degree is 55Pa, and it is maintained for 10min;

[0102] C-level adjustment: nitrogen flow rate is 500NL / min, vacuum degree is adjusted from 55Pa to 1000Pa, and maintained at 1000Pa for 10min;

[0103] Level D adjustment: nitrogen flow rate is 80NL / min, vacuum is broken, and sampling is performed after vacuum is ended;

[0104] S3: Casting:

[0105] At the beginning of casting, the molten steel flow rate is 1000kg / min, and the casting time is 6min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1500kg / min, and the casting time is 25min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 400kg / min, and the casting time is 10min.

[0106] Example 4

[0107] The steel grade used in this embodiment is P91, and the steel is tapped by EAF to obtain a ladle;

[0108] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle, with a nitrogen flow rate of 300NL / min and a holding time of 60min;

[0109] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation;

[0110] The multi-level adjustment includes:

[0111] Level A adjustment: nitrogen flow rate is 100NL / min, vacuum is drawn, and the vacuum degree is reduced from atmospheric pressure to 50Pa, which takes 9 minutes;

[0112] Level B adjustment: nitrogen flow rate is 80NL / min, vacuum degree is 50Pa, and maintained for 10min;

[0113] C-level adjustment: nitrogen flow rate is 400NL / min, vacuum degree is adjusted from 50Pa to 1200Pa, and maintained for 12min;

[0114] Level D adjustment: Nitrogen flow rate is 100NL / min, vacuum is broken, and sampling is performed when vacuum ends.

[0115] As shown in Table 1, since the adjustment time of level C is greater than 10 minutes, the nitrogen content in the molten steel after VD refining in Example 4 is 800-1000ppm, and the nitrogen content in the molten steel after sampling is ≥800ppm. The adjustment operations of levels A, B and D are repeated once, wherein level B is maintained for 2 minutes, and the tested nitrogen content is 550-650ppm, which meets the requirements, and then the casting is performed as in step S3 in Example 1.

[0116] Example 5

[0117] The steel grade used in this embodiment is P91-2, and the steel is tapped by EAF to obtain a ladle;

[0118] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle, with a nitrogen flow rate of 300NL / min and a holding time of 60min;

[0119] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation;

[0120] The multi-level adjustment includes:

[0121] Level A adjustment: nitrogen flow rate is 100NL / min, vacuum is drawn, and the vacuum degree is reduced from atmospheric pressure to 50Pa, which takes 9 minutes;

[0122] Level B adjustment: nitrogen flow rate is 80NL / min, vacuum degree is 50Pa, and maintained for 10min;

[0123] Level C adjustment: nitrogen flow rate is 150NL / min, adjusted from 50Pa to 1200Pa, and maintained at 1200Pa for 10min;

[0124] Level D adjustment: nitrogen flow rate is 100NL / min, vacuum is broken, and sampling is performed after vacuum ends;

[0125] As shown in Table 1, since the C-grade nitrogen flow rate is 150NL / min, which is less than 200-700NL / min specified in the present invention, the nitrogen content in the molten steel after VD refining in Example 5 is 350-400ppm, and the molten steel in the VD vacuum furnace is subjected to weak nitrogen blowing operation, the nitrogen flow rate is 70NL / min, and the tested nitrogen content is 550-650ppm, which meets the requirements, and then the casting is performed as in step S3 in Example 1.

[0126] Example 6

[0127] The steel grade used in this embodiment is P91, and the steel is tapped by EAF to obtain a ladle;

[0128] S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle, with a nitrogen flow rate of 300NL / min and a holding time of 60min;

[0129] S2: The molten steel refined in the LF ladle is placed in a VD vacuum furnace and nitrogen is used as the bottom blowing gas. The nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation;

[0130] The multi-level adjustment includes:

[0131] Level A adjustment: nitrogen flow rate is 100NL / min, vacuum is drawn, and the vacuum degree is reduced from atmospheric pressure to 50Pa, which takes 9 minutes;

[0132] Level B adjustment: nitrogen flow rate is 80NL / min, vacuum degree is 50Pa, and maintained for 6min;

[0133] Level C adjustment: nitrogen flow rate is 400NL / min, adjusted from 50Pa to 600Pa, maintained for 8min;

[0134] Level D adjustment: nitrogen flow rate is 100NL / min, vacuum is broken, and sampling is performed after vacuum ends;

[0135] As shown in Table 1, since the vacuum degree in the C-level adjustment is 600Pa, which is less than 800-1500Pa specified in the present invention, the nitrogen content in the molten steel after VD refining in Example 6 is less than 350ppm, the above steps S1 and S2 are repeated to increase nitrogen, and sampling is performed again after the vacuum is ended. The nitrogen content is tested to be 550-650ppm, which meets the requirements, and then casting is performed as in step S3 in Example 1.

[0136] Comparative Example 1

[0137] The difference between this comparative example and Example 1 is that nitrogen is added by adding nitriding alloy, specifically:

[0138] The steel used in this comparative example is P91, which is tapped by EAF to obtain a ladle;

[0139] S1: Nitrogen is used as bottom blowing gas to refine the LF ladle, and the nitrogen flow rate is 300NL / min. Nitriding alloy needs to be added during the process to make the nitrogen content in the molten steel reach more than 900ppm. The holding time is 60min to obtain molten steel;

[0140] S2: placing the molten steel in a VD vacuum furnace for refining, using nitrogen as bottom blowing gas for degassing, and maintaining the vacuum degree for 8 to 10 minutes after the vacuum degree reaches ≤67Pa, and then breaking the vacuum;

[0141] S3: After the VD vacuum is broken, the nitrogen content in the molten steel is sampled and confirmed. The nitrogen content is 200-400ppm. It is necessary to add appropriate nitriding alloy to the molten steel according to the requirements of the steel grade. After the nitriding alloy is melted, the nitrogen content is sampled and confirmed. If it cannot meet the requirements of the steel grade, it needs to be adjusted again. At least two adjustments and three samplings are required to confirm the nitrogen content to obtain the molten steel to be cast;

[0142] S3: Casting:

[0143] At the beginning of casting, the molten steel flow rate is 900kg / min, and the casting time is 6min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1800kg / min, and the casting time is 15min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 600kg / min, and the casting time is 8min.

[0144] Comparative Example 2

[0145] The difference between this comparative example and Example 1 is that the nitrogen is not increased by adjusting the nitrogen flow rate and vacuum degree in multiple stages. Specifically:

[0146] The steel used in this comparative example is P91, which is tapped by EAF to obtain a ladle;

[0147] S1: Using nitrogen as bottom blowing gas to refine the LF ladle, the nitrogen flow rate is 300NL / min, the holding time is 60min, and molten steel is obtained;

[0148] S2: placing the molten steel in a VD vacuum furnace and using nitrogen as bottom blowing gas, with a nitrogen flow rate of 400NL / min and a vacuum degree of 1200Pa, maintaining for 8 minutes to complete the nitrogen addition operation; breaking the vacuum to obtain the molten steel to be cast.

[0149] Since the nitrogen content in the molten steel finally obtained in this comparative example is unqualified, casting cannot be carried out.

[0150] Comparative Example 3

[0151] The difference between this comparative example and Example 1 is that the time for the C-level adjustment is 15 minutes, specifically:

[0152] Level C adjustment: nitrogen flow rate is 400NL / min, vacuum degree is 1200Pa, and it is maintained for 15min; the remaining steps and technical parameters are the same as those in Example 1. Since the nitrogen content in the molten steel obtained in this comparative example is unqualified, casting cannot be carried out.

[0153] Performance Testing

[0154] The test method is as follows:

[0155] The nitrogen content, H and O content and impurity content in the molten steel are quantified and analyzed by EDS spectrometer.

[0156] When alloy is used to adjust nitrogen content, the formula for nitrogen loss rate is:

[0157]

[0158] When nitrogen is used to adjust the nitrogen content, the formula for nitrogen loss rate is:

[0159]

[0160] The nitrogen content test table in Examples 1-6 and Comparative Examples 1-3 is shown in Table 1:

[0161] Table 1 Nitrogen content test table of different steel grades

[0162]

[0163] Note: Nitrogen loss rate should refer to the theoretical loss rate, which cannot be measured in practice. In actual applications, the price of nitrogen is much lower than that of nitrided alloys.

[0164] As shown in Table 1, since Comparative Example 1 uses a nitriding alloy to increase nitrogen, the nitrogen content in the molten steel after VD refining does not meet the standard, and additional nitriding alloy needs to be added for nitrogen increase, which not only increases the cost but may also cause secondary oxidation of the molten steel after nitrogen increase.

[0165] As shown in Table 1, since Comparative Example 2 does not increase nitrogen by adjusting nitrogen flow and vacuum degree in multiple stages, the nitrogen content in the molten steel after VD refining exceeds the specified range.

[0166] As shown in Table 1, since the time for controlling the C-level adjustment in Comparative Example 3 is 15 minutes, which exceeds the range of the adjustment time of 5-10 minutes specified in the present invention, the nitrogen content in the molten steel after VD refining exceeds the specified range, and Comparative Example 3 does not handle the situation when the nitrogen content is ≥800ppm according to the solution proposed in the present invention, the O content in the molten steel in Comparative Example 3 is significantly increased, and the length of the macroscopic water immersion flaw detection defect image is greater than 5mm, which does not meet the quality requirements of the steel grade.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace, characterized in that: The steps include: S1: Nitrogen is used as bottom blowing gas to refine the molten steel in the LF ladle; S2: The molten steel refined by the LF ladle is placed in a VD vacuum furnace, nitrogen is used as the bottom blowing gas, and the nitrogen flow rate and vacuum degree are adjusted in multiple stages to complete the nitrogen addition operation.

2. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 1, characterized in that: In step S1, the nitrogen flow rate is 80-600 NL / min, and the holding time is 40-80 min.

3. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 1, characterized in that: After LF ladle refining, the nitrogen content in the molten steel is 200-350ppm by mass.

4. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 3, characterized in that: In step S2, the multi-level adjustment includes: Level A adjustment: nitrogen flow rate is 30-120NL / min, vacuuming starts until the vacuum degree is ≤67Pa, and the vacuuming time is 6-10min; Level B adjustment: nitrogen flow rate is 30-120NL / min, vacuum degree ≤67Pa, maintained for 8-10min; C-level adjustment: nitrogen flow rate is 200-700NL / min, vacuum degree is 800-1500Pa, and maintained for 5-10min; Level D adjustment: Nitrogen flow rate is 30-120NL / min, vacuum is broken, and sampling is performed when vacuum ends.

5. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 4, characterized in that: When the nitrogen content in the sampled molten steel is 500-800 ppm by mass, the nitrogen enrichment operation is completed.

6. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 5, characterized in that: According to the mass percentage, when the nitrogen content in the sampled molten steel is ≥800ppm, the molten steel in the VD vacuum furnace needs to repeat the A, B and D level adjustment operations, where the B level time is 1 to 3 minutes, and the nitrogen content is tested after the end; When the nitrogen content in the sampled molten steel is 350-500ppm, a weak nitrogen blowing operation is performed on the molten steel in the VD vacuum furnace until the tested nitrogen content meets the requirements; wherein the nitrogen flow rate in the weak nitrogen blowing operation is 30-100NL / min; When the nitrogen content in the sampled molten steel is less than 350ppm and does not meet the nitrogen content requirement of the steel grade, it is necessary to increase nitrogen again according to steps S1 and S2.

7. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 1, characterized in that: According to mass percentage, the H content in the molten steel after the nitrogen addition operation is 0.6-1.2ppm, and the O content is 5-13ppm; And / or, the nitrogen loss rate in the molten steel after the nitrogen addition operation is 15 to 35%.

8. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 1, characterized in that: The steel type of the molten steel includes one of P91, 1Cr17Ni2, P92, 0Cr17Ni4Cu4Nb, DIN 1.4104 or 14Cr17Ni2.

9. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 1, characterized in that: The molten steel that has completed the nitrogen addition operation is cast.

10. The refining method for increasing nitrogen in a vacuum process of a VD vacuum furnace according to claim 9, characterized in that: The casting comprises: At the beginning of casting, the molten steel flow rate is 750-1100kg / min, and the casting time is 5-7min; after the second adjustment of the molten steel flow rate, the molten steel flow rate is 1500-2000kg / min, and the casting time is 10-25min; after the third adjustment of the molten steel flow rate, the molten steel flow rate is 400-800kg / min, and the casting time is 6-10min.

Images