A method for producing an ultra-low nitrogen steel

Abstract

The application provides a production method of ultra-low-nitrogen steel and relates to the technical field of metallurgy.The production method provided by the application comprises pretreatment desulfurization, converter smelting, RH treatment, continuous casting and the like, and through improvement and optimization of the process, preparation of high-quality ultra-low-nitrogen (less than 10 ppm) steel is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, in particular to a production method of ultra-low nitrogen steel. BACKGROUND

[0002] The nitrogen content in steel will affect the organization, performance and corrosion resistance of the steel. Generally speaking, with the increase of nitrogen content, the plasticity and toughness of the steel will be significantly reduced, the weldability and cold bending will be poor, and the cold brittleness and hot brittleness will be intensified. Too high nitrogen content will also cause the appearance of harmful intermetallic phases in the steel, such as martensite, ferrite, nitride, etc., reducing the corrosion resistance and stability of the steel.

[0003] Generally speaking, low carbon steel, low alloy steel, carbon structural steel and other ordinary steel grades need to reduce the nitrogen content to ensure the plasticity and toughness of the steel and the weldability. Therefore, reducing the nitrogen content in the steel is one of the important ways to improve the quality of the steel. For example, the patent with the application number CN103667581B provides a low-nitrogen SWRH82B steel smelting method, which can reduce the nitrogen content in the SWRH82B wire rod steel to below 40ppm. The patent with the application number CN202110373701.X provides a low-nitrogen steel smelting method, which obtains low-nitrogen steel with a nitrogen content of 20-60ppm. The patent with the application number CN201210164848.9 provides a method for controlling the nitrogen content of ultra-low carbon steel, and finally obtains a billet with a nitrogen content of not more than 35ppm.

[0004] As can be seen, most of the current processes can control the nitrogen content in the steel to a relatively low level, but generally only to about 20-60ppm, and it is difficult to achieve lower nitrogen content steel. However, the nitrogen content of about 20-60ppm cannot meet the control requirements of nitrogen content in steel in some special fields. Therefore, how to further reduce the nitrogen content in the steel is one of the technical problems to be overcome at present. SUMMARY

[0005] To solve the above problems, the present application provides a production method of ultra-low nitrogen steel, which successfully controls the nitrogen content in the steel to less than 10ppm by adjusting and optimizing the process.

[0006] The production method of ultra-low nitrogen steel according to the present application comprises the following steps:

[0007] S1, pretreatment desulfurization: the molten iron in the blast furnace is sent to the desulfurization station, and the passivated magnesium and lime are sprayed as carriers to desulfurize to a sulfur content of <0.001%, and the bright surface of the molten iron is >95%;

[0008] S2, converter smelting: transported to the combined blown converter, clean scrap steel is added, oxygen supply operation is carried out, slagging material is added in the converter, combined blowing process is adopted in the smelting process of the converter, and the molten steel is discharged after smelting is completed;

[0009] S3, RH treatment: special grade lime is added to cover the liquid surface of the ladle, the molten steel is not deoxidized, and the ladle is directly sent to the RH furnace for circulating refining;

[0010] S4, continuous casting: after the molten steel is refined, aluminum is added for deoxidation, then alloy elements are added, and finally ladle covering agent is added, the ladle is lifted to the continuous casting ladle rack, protective slag is added to the tundish, the whole process is protected from pouring, and the molten steel is poured into the slab, the tundish temperature and slab casting speed are controlled, and the slab is cooled.

[0011] Further, the blowing amount of the passivated magnesium is 1.2-2.5 kg / ton of iron, the blowing amount of the lime is 1.5-3.5 kg / ton of iron, the compressed air pressure is 0.3-0.65 MPa, the instantaneous flow is 2350 Nm 3 / h, intermittent blowing three times, each time 5-7 min, and slagging three times.

[0012] Further, the clean scrap steel adopts a low-carbon cold-rolled material intermediate slab flying shear head to ensure that it is clean and free of impurities, and the addition amount of the clean scrap steel is ≤15 t / furnace.

[0013] Further, the slag-making material is one or more of lime, dolomite, limestone, and sintered ore.

[0014] Further, the lime is special grade lime with an activity of >320 ml, an effective CaO content of >90%, an S content of <0.020%, and an addition amount of 35-45 kg / ton of steel; the dolomite is light-burned dolomite with an S content of <0.020%, and an addition amount of 25-35 kg / ton of steel; the limestone has an addition amount of 0-15 kg / ton of steel; and the sintered ore has an addition amount of 0-25 kg / ton of steel.

[0015] Further, the converter smelting further includes a slag adjusting step, specifically using iron oxide scale for slag adjusting, and the addition amount of the iron oxide scale is ≤25 kg / ton of steel.

[0016] Further, in the combined blowing process, the oxygen supply intensity of the top lance is 2.8-4.5 Nm 3 / ton of steel·min, the lance position control adopts a high-low-high-low four-stage method, argon is blown throughout the smelting process, and the corresponding bottom argon blowing intensity is 0.5-1.50 Nm 3 / ton of steel·min, and the bottom blowing mode is a high-low-high-strong four-stage method.

[0017] Further, in the four-stage method, the top blowing instantaneous flow is high (28000-34000 Nm 3 / h and not 28000 Nm 3 / h)-low (instantaneous flow 25000-28000 Nm3 / h) - high (instantaneous flow 32000 ~ 36000 Nm 3 / h and not 32000 Nm 3 / h) - low (instantaneous flow 30000 ~ 32000 Nm 3 / h.

[0018] Further, in the four-stage method, the bottom blowing instantaneous flow is high (800 ~ 900 Nm 3 / h) - low (550 ~ 650 Nm 3 / h) - high (900 ~ 1000 Nm 3 / h) - strong (1200 ~ 1500 Nm 3 / h.

[0019] Further, the carbon content in the molten iron at the end of smelting is 0.035wt% ~ 0.12wt%, and the oxygen content is 400 ~ 850ppm.

[0020] Further, the tapping temperature is 1620℃ ~ 1680℃.

[0021] Further, the addition amount of special grade lime in the ladle is 1.35 ~ 3.5kg / t steel.

[0022] Further, the RH treatment uses a new vacuum tank with tank age < 55 tanks, and before vacuum treatment, it is confirmed that all nitrogen gas pipelines are completely cut off.

[0023] Further, the vacuum degree of the RH treatment is 67 ~ 100Pa, the instantaneous flow of argon in the riser is 550 ~ 1200 Nm 3 / h, the vacuum decarburization time is 6 ~ 15min, and in the pre-evacuation stage, the circulation gas flow is adjusted to 40m 3 / h.

[0024] Preferably, the instantaneous flow of argon in the riser is 650 ~ 1100 Nm 3 / h

[0025] Further, the alloying element is one or more of ferrosilicon, electrolytic manganese, ferrotitanium, and silicon nitride.

[0026] Further, the addition amount of ferrosilicon is 0 ~ 35kg / t steel, the addition amount of electrolytic manganese is 0 ~ 4.5kg / t steel, the addition amount of ferrotitanium is 0 ~ 15kg / t steel, and the addition amount of silicon nitride is 0 ~ 1.5kg / t steel.

[0027] Further, in the aluminum deoxidation, the addition amount of aluminum is 2.5 ~ 4.5kg / t steel.

[0028] Further, the protective slag is ultra-low-carbon and low-nitrogen protective slag, and the adding amount is 0.35-0.5 kg / ton of steel.

[0029] Further, the protective slag is composed of the following raw materials in percentage by weight: SiO2 36.85 wt%, CaO 29.93 wt%, MgO 2.41 wt%, Fe2O3 0.72 wt%, Al2O3 2.13 wt%, N < 80 ppm, C < 2.0 wt%, H2O < 0.35 wt%, and the balance is ash content.

[0030] Further, in the continuous casting process, a new ladle is used for casting the protective sleeve, and the protective sleeve adopts double-ring sealing joint.

[0031] Further, in the continuous casting process, the argon pressure is 0.15 MPa, and the flow rate is 0.4 Nm 3 / h; the tundish is fully sealed, and the flow rate is 0.2 Nm 3 / h; the stopper is protected by argon, the argon pressure is 0.15 MPa, and the flow rate is 0.2 Nm 3 / h; the slide plate surface is sealed by argon, the pressure is 0.15 MPa, and the flow rate is 0.4 Nm 3 / h, and the liquid level fluctuation of the mold is less than ±3 mm.

[0032] Further, the tundish temperature is 1525-1555℃, and the slab casting speed is 0.95-1.35 m / min.

[0033] In each process (link) of the application, a gas sampler is used for sampling, and the gas is processed into a standard shape for gas analysis, and an American leco oxygen-nitrogen-hydrogen gas analyzer is used to analyze the gas content of each link, and the nitrogen content in the final steel product is ≤10 ppm.

[0034] In the application, the pretreated molten iron is charged into a top and bottom combined blowing converter, and clean scrap steel is added in a proper proportion, and oxygen supply operation is carried out, and a proper amount of slag making material is added in the blowing process to ensure stable reaction and good slag melting in the converter. Meanwhile, the intensity of top and bottom combined blowing is strengthened to ensure good degassing in the converter. Once-lap-carbon tapping is carried out at the end of the converter to eliminate supplementary blowing. The molten steel with qualified blowing temperature and composition is tapped from the converter into a ladle, and no deoxidation and alloying operation is carried out in the tapping process. A proper amount of lime is added in the tapping process to cover the molten steel surface, isolate air and prevent the molten steel from increasing N. In the RH treatment link, a new vacuum tank is used to prevent air leakage and increase N. All nitrogen valves are completely closed to eliminate the possibility of nitrogen infiltration. In the pre-vacuum stage, the circulation gas flow is reduced, and in the vacuum stage, the circulation flow control is strengthened to limit the possibility of molten steel turbulence and naked contact with air to increase N. In the continuous casting production process, the whole process protection casting is strengthened. The ladle sleeve, argon seal, tundish seal and slide plate surface seal are systematically optimized to control the continuous casting N increase within 0-2 ppm.

[0035] Compared with the prior art, the present application has the beneficial technical effects:

[0036] The present application realizes the preparation of high-quality ultra-low nitrogen (<10ppm) fine steel through the improvement and optimization of the process of adding raw and auxiliary materials to the converter, the converter bottom blowing process, the process in the argon station treatment process, the RH treatment process and the continuous casting production process. DETAILED DESCRIPTION

[0037] The technical solutions provided by the present application are further described below in combination with examples.

[0038] The production method of ultra-low nitrogen steel is as follows:

[0039] The molten iron from the blast furnace is added with passivated magnesium and lime with compressed air as the carrier, the passivated magnesium and lime are added with compressed air as the carrier, and intermittent blowing is performed three times, each time for 5 minutes, and slagging is performed three times, and the desulfurization is to <0.001%, and the bright surface of the molten iron is >95%. The molten iron is transported to the combined blown converter, the scrap steel is all used in the low-carbon cold-rolled material intermediate billet flying shear head, and is clean and free of impurity pollution. The desulfurized molten iron with a sulfur content of <0.001% is used, and lime, dolomite, limestone and sintered ore are added as the slag material in the converter, the lime used for the lime white ash of the slag material is a special grade lime with an activity of >320mL and an effective CaO content of >90% and a S content of <0.020%, the light-burned dolomite has a S content of <0.020%, and some products in the production process are adjusted with iron oxide scale. The combined blowing process is used in the converter smelting process. Argon blowing is performed throughout the smelting process, the bottom blowing mode is high (instantaneous flow 650Nm 3 / h)-low (instantaneous flow 550Nm 3 / h)-high (instantaneous flow 850Nm 3 / h)-high (instantaneous flow 1000Nm 3 / h)-strong (after stirring instantaneous flow 1500Nm 3 / h). The converter endpoint once pulls carbon, the molten steel is not deoxidized, no alloying, and special grade lime is added to cover the molten steel liquid level of the ladle. The molten steel is not deoxidized and directly sent to the RH furnace for cyclic refining. The original process is followed: RH vacuum treatment, addition of aluminum deoxidation, addition of subsequent ferrosilicon, metal manganese and other alloys, and finally addition of ladle covering agent. The ladle is lifted to the continuous casting ladle rack, the tundish is added with protective slag, the whole process is protected from pouring, the tundish temperature and slab casting speed are controlled, and the molten steel is cooled, and the ultra-low nitrogen steel is obtained. In the continuous casting production process, a new ladle is used for each furnace to cast a protective sleeve, and the protective sleeve must use a double-ring sealed joint.

[0040] In this way, the N content in the steel is strictly controlled to be within 10ppm.

[0041] Table 1

[0042]

[0043]

[0044] The N content (ppm) of each stage of the products of Examples 1-5 was detected, and the results are shown in Table 2:

[0045] Table 2

[0046] No. Furnace No. Steel Grade Converter End Point RH Inlet RH Outlet Casting Tundish Casting Mold Finished Product Example 1 2310204683 PYDC04 6 6 6 7 8 8 Example 2 2310204684 PYDC04 7 7 7 8 9 9 Example 3 2312204792 PYCGO01 5 6 6 6 8 8 Example 4 2312206603 PYW270 6 7 7 8 9 8 Example 5 2312206604 PYW270 8 8 8 9 9 10

[0047] Comparative Example

[0048] The P≤0.12wt%, S≤0.001wt%, Si≤0.45wt% in the molten iron, S≤0.020wt% in the scrap steel, O2≥99.5%, N2≤0.001% in the oxygen, Fe≥53wt% in the sintered ore, CaO≥92wt% in the lime, CaO≥35wt%, MgO≥28wt% in the light-burned dolomite, Fe2O3≥65% in the iron scale, Ar≥99.9% in the argon, and the rest are trace amounts, Al≥99.5wt% in the aluminum particles, the manganese iron alloy is composed of the following raw materials: Mn 65wt%, C≤6.5wt%, S≤0.30wt%, P≤0.15wt%, Al≤0.15wt%, and the rest is Fe; the metal manganese alloy is composed of the following raw materials: Mn 99.5wt%, C≤0.02wt%, S≤0.020wt%, P≤0.010wt%, Al≤0.02wt%, and the rest is Fe; the silicon nitride alloy is composed of the following raw materials: Si 25wt%, N≤28wt%, C≤0.05wt%, S≤0.020wt%, P≤0.15wt%, Al≤0.15wt%, and the rest is Fe; and the silicon iron alloy is composed of the following raw materials: Si 72wt%, C≤0.02wt%, S≤0.020wt%, P≤0.15wt%, Al≤0.02wt%, and the rest is Fe.

[0049] Table 3

[0050]

[0051]

[0052] The N content (ppm) of each stage of the products of Examples 1-5 was detected, and the results are shown in Table 2:

[0053] Table 4

[0054]

[0055]

[0056] Comparative Example 6

[0057] The same as Example 1 except that the bottom blowing gas was replaced by Ar.

[0058] Comparative Example 7

[0059] The same as Example 1 except that the life of the vacuum tank in production was 85 times.

[0060] Comparative Example 8

[0061] The same as Example 1 except that the flow rate of the circulating gas during the pre-vacuum stage was 50 m 3 / h.

[0062] Comparative Example 9

[0063] The same as Example 1 except that the large ladle bushing was not used and instead a new bushing was used every two ladles and a new process was used for the ladle nozzle every ladle.

[0064] Test Example 1

[0065] The N content (ppm) of each stage of the products of Comparative Examples 6-9 was detected and the results are shown in Table 5:

[0066] Table 5

[0067] No. Furnace No. Steel Grade Converter End Point RH Inlet RH Outlet Casting Tundish Casting Mold Finished Product Example 6 2310203557 Q355qD 25 27 27 30 32 32 Example 7 2312203895 SPHC 35 35 34 36 37 38 Example 8 2312205885 Q195L 27 28 27 30 33 35 Example 9 2310226321 Q390qD 31 35 33 35 37 36

[0068] The principles and implementation modes of the present application are described herein using specific examples, and the above examples are only used to help understand the method of the present application and its core idea; at the same time, for those skilled in the art, according to the idea of the present application, there will be changes in the specific implementation mode and application range. In summary, the content of the specification should not be understood as a limitation of the present application.

Claims

1. A method for producing an ultra-low nitrogen steel, characterized by, The method comprises the following steps: S1, pretreatment desulfurization: the blast furnace molten iron is sent to a desulfurization station, and passivated magnesium and lime are sprayed as carriers by compressed air to desulfurize the blast furnace molten iron to a sulfur content of <0.001%, and a bright surface of the molten iron >95%; the spraying amount of the passivated magnesium is 1.2~2.5 kg / ton of iron, the spraying amount of the lime is 1.5~3.5 kg / ton of iron, the compressed air pressure is 0.3~0.65 MPa, the instantaneous flow is 2350 Nm 3 / h, intermittent spraying three times, each time 5~7 min, and slagging three times. S2, converter smelting: transported to the combined blown converter, clean scrap is added at the same time, oxygen supply operation is carried out, slagging material is added in the converter, combined blowing process is used in the converter smelting process, and tapping is carried out after smelting is completed; the argon bottom blowing intensity in the combined blowing process is 0.5-1.50 Nm 3 / t steel·min, the bottom blowing mode is high-low-high-strong four-stage method; the bottom blowing instantaneous flow rate is high 800-900 Nm 3 / h-low 550-650 Nm 3 / h-high 900-1000 Nm 3 / h-strong 1200-1500 Nm 3 / h; S3, RH treatment: adding special grade lime to cover the liquid surface of ladle, the molten steel is not deoxidized and directly sent to the RH furnace for circulating refining; the vacuum degree of the RH treatment is 67-1000 Pa, the instantaneous flow of argon in the riser is 550-1200 Nm 3 / h, the vacuum decarburization time is 6-15 min, in the pre-evacuation stage, the circulation gas flow is adjusted to 40 m 3 / h; the oxygen supply intensity of the top lance in the complex blowing process is 2.8-4.5 Nm 3 / ton of steel·min, the lance position control adopts high-low-high-low four-stage method, argon blowing is used throughout the smelting process, and the corresponding bottom argon blowing intensity is 0.5-1.50 Nm 3 / ton of steel·min, the bottom blowing mode is high-low-high-strong four-stage method; S4, continuous casting: adding aluminum to the molten steel after circulating refining to deoxidize, then adding alloying elements, and finally adding ladle covering agent, hoisting the ladle to the continuous casting ladle frame, adding protective slag in the tundish, full-process protective pouring, continuous casting of the steel billet, controlling the tundish temperature and the slab casting speed, cooling, and obtaining the product; the argon pressure in the continuous casting process is 0.15 MPa, and the flow rate is 0.4 Nm 3 / h; the tundish is fully sealed, and the flow rate is 0.2 Nm 3 / h; the argon protection of the stopper is performed, the argon pressure is 0.15 MPa, and the flow rate is 0.2 Nm 3 / h; the argon protection of the slide plate surface is performed, the pressure is 0.15 MPa, and the flow rate is 0.4 Nm 3 / h, and the fluctuation of the crystallizer liquid level is less than ±3 mm. The RH treatment adopts a new vacuum tank with tank age < 55 furnaces, and before the vacuum treatment, it is confirmed that all nitrogen pipelines are completely cut; In the continuous casting process, a new ladle casting protective sleeve is used for each furnace, and the protective sleeve adopts double-ring sealing joints.

2. The production method according to claim 1, characterized by, In the converter smelting, a slag adjusting step is further included, specifically adopting iron oxide skin for slag adjusting, and the addition amount of the iron oxide skin is 0-25 kg / ton of steel.

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