Smelting method of low-sulfur low-nitrogen molten steel with large scrap ratio

Abstract

The invention belongs to the technical field of steelmaking, and discloses a smelting method of high-scrap-ratio low-sulfur low-nitrogen molten steel, which comprises the following steps of: controlling the amount of molten iron in an iron ladle according to an expected scrap ratio, heating by using a direct-current iron ladle furnace, continuously adding scrap steel in the heating process, slagging, blowing argon and stirring, melting the scrap steel, desulfurizing and recarburizing, generating low-sulfur molten iron, slagging off, and loading into a converter for steelmaking; the method specifically comprises the stages of direct-current ladle furnace preparation, quantity-controlled iron injection, waste steel melting, desulfurization and recarburization, slagging-off and iron loading for steelmaking. According to the method, the waste steel can be smelted into low-sulfur molten iron for converter steelmaking, large-waste-steel-ratio, low-sulfur, low-nitrogen and low-pollution smelting is achieved, and low-carbon-emission and high-quality production is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of steelmaking, and in particular relates to a method for smelting molten steel with a large scrap steel ratio and low sulfur and low nitrogen content. Background Art

[0002] Scrap steel is an energy-carrying, energy-saving green resource that can be recycled infinitely. It is one of the two indispensable iron resources for steel production. Compared with using molten iron, using 1 ton of scrap steel can reduce the consumption of 1 ton of raw coal and 1.7 tons of concentrate powder. At the same time, it can reduce the emission of a large amount of wastewater, waste gas, solid waste and 1.6 tons of carbon dioxide.

[0003] In order to cope with the severe challenges of climate change to human survival and development, the country has formulated the "dual carbon" strategic goal, which has become a severe challenge faced by the high-quality development of the steel industry and a major opportunity to achieve leapfrog progress. With the continuous increase in steel reserves, social scrap steel resources will increase, and crude steel production will gradually decline after reaching its peak. The proportion of scrap steel used by steel companies will become higher and higher. This is the general trend of the development of the steel and metallurgical industry and an inevitable need for the green development of the steel industry. In the actual situation where the blast furnace-converter long process technology occupies an absolute dominant position, increasing the capacity of the electric furnace short process and improving the scrap steel ratio of the long process are the main measures for energy conservation and carbon reduction.

[0004] Electric furnaces can achieve full scrap steel smelting, but there are three problems. First, the nitrogen content of molten steel is high, usually above 40ppm; second, modern electric furnaces no longer have the desulfurization function, and low-sulfur steel cannot be produced; third, modern electric furnaces basically have the scrap preheating function, which requires special treatment of dioxin issues.

[0005] The upper limit of scrap steel digestion in converter is 20-25%. To increase the scrap steel ratio further, it is necessary to preheat the scrap steel or add a heat supplement. Preheating in the iron ladle only accounts for about 10% of the scrap steel. Preheating in the heating furnace and other places will cause dioxin pollution, and the heat loss will be large when transferring to the converter. Adding a heat supplement in the furnace will cause problems such as low utilization rate of the heat supplement and increase of harmful impurities such as sulfur. After the converter uses these methods to increase the scrap steel ratio, similar problems will occur as in the electric furnace, with high nitrogen and sulfur content in the molten steel.

[0006] To sum up, the research and development of steelmaking technology with high scrap steel ratio, low nitrogen, low sulfur and low pollution is necessary for green and high-quality development. Summary of the invention

[0007] The object of the present invention is to provide a smelting method of low-sulfur and low-nitrogen molten steel with a large scrap ratio, and the technical scheme is as follows: A method for smelting molten steel with a large scrap steel ratio and low sulfur and low nitrogen, specifically comprising: controlling the amount of molten iron in an iron ladle according to an expected scrap steel ratio, heating it in a direct current iron ladle furnace, continuously adding scrap steel, making slag, blowing argon and stirring during the heating process, melting the scrap steel, desulfurizing and carbonizing, generating low sulfur molten iron, and loading the molten iron into a converter for steelmaking after slagging.

[0008] Furthermore, the smelting method of the high scrap ratio low sulfur and low nitrogen molten steel of the present invention includes the stages of preparing a DC ladle furnace, controlling iron injection, melting scrap steel, desulfurizing and carbonizing, slag removal, and iron charging and steelmaking.

[0009] Furthermore, in the preparation stage of the DC iron ladle furnace described in the present invention: a DC iron ladle furnace is installed between the molten iron pretreatment and the blast furnace, and the DC iron ladle furnace has the functions of electrode heating, bunker charging, continuous scrap steel charging, bottom blowing argon stirring, etc.; the DC iron ladle furnace adopts a top single electrode plus iron ladle bottom electrode mode, and the electrode lifting and lowering long stroke design; an ultra-high power power supply system is selected to quickly melt a large proportion of scrap steel, which can match the converter smelting rhythm; the scrap steel adding position is located on the same side of the slag removal direction, and the bottom blowing argon air hole position corresponds to it. If there is no molten iron pretreatment equipment, add molten iron slag removal equipment.

[0010] Furthermore, the controlled iron injection stage of the present invention comprises injecting a planned amount of molten iron into the ladle.

[0011] Furthermore, in the scrap steel melting stage described in the present invention: after the ladle filled with molten iron is transported to the DC ladle furnace, argon gas is connected to the bottom blowing and argon stirring throughout the whole process to heat the molten iron, and scrap steel is continuously added, and the scrap steel adding time is 4-8 minutes shorter than the heating time to ensure that the scrap steel is completely melted and the temperature is appropriately raised; lime, fluorite, wollastonite, light-burned dolomite and other slag materials are added to increase the slag layer thickness to more than 80 mm, and a foaming agent is added to make foamed slag for submerged arc heating.

[0012] Furthermore, in the scrap steel melting stage of the present invention: weak stirring is performed during the heating, and the argon flow rate is controlled at 40-80Nl / min; strong stirring is performed after the heating is stopped, and the argon flow rate is controlled at 600-1000Nl / min.

[0013] Furthermore, in the scrap steel melting stage of the present invention: the heating time is shorter than the converter smelting cycle. When the ladle furnace and the converter are produced one-to-one, the heating time is more than 5-8 minutes shorter than the converter smelting cycle; when the ladle furnace and the converter are produced two-to-one, the heating time is more than 8 minutes shorter than twice the converter smelting cycle.

[0014] Furthermore, in the stage of melting scrap steel of the present invention, the scrap steel is mainly small and light, with the thickness of plate-type scrap steel being less than 10 mm, and the length and width being less than 200 mm; the diameter of strip-type scrap steel being less than 20 mm, and the length being less than 200 mm.

[0015] Furthermore, in the desulfurization and carbon increase stage of the present invention: when adding slag, the ratio is adjusted according to the desulfurization requirements to produce desulfurized slag, and the slag basicity is controlled at 1.8-2.5; after the scrap steel is fully melted and the temperature is appropriate, a large amount of argon is used for strong stirring for desulfurization, and a carbon increase agent is added at 3.5-4% of the scrap steel amount for carbon increase.

[0016] Furthermore, in the slagging stage described in the present invention, after desulfurization, the iron ladle is moved to the slagging position, and 85-90% of the desulfurized molten iron is removed from the iron ladle by using bottom argon blowing and slag driving technology.

[0017] Furthermore, the iron charging and steelmaking of the present invention is as follows: scrap steel is added to the converter according to normal procedures, desulfurized molten iron is charged, and smelting is carried out according to conventional smelting methods.

[0018] The method of the present invention can achieve an overall scrap steel ratio of 25% to 60%, and can control the sulfur content of molten steel to below 0.01%, and the nitrogen content to below 0.0025%.

[0019] The beneficial effects of the present invention are: 1) It can realize steelmaking with a large scrap ratio in the converter process. The DC single-electrode iron-clad furnace has a large distance between the electrode and the cladding wall, which can reduce arc radiation erosion of the cladding wall and can be heated at ultra-high power; the large distance between the electrode and the cladding wall provides a large drop-in space, and scrap steel can be added at a high rate. The overall scrap ratio can be as high as 60%.

[0020] 2) Efficient desulfurization and slag removal. Slag making according to the desulfurization requirements can achieve efficient desulfurization; bottom argon blowing is set on the opposite side of the slag removal direction to blow away the slag and achieve efficient slag removal. The lowest sulfur content in molten iron can reach 0.005%; combined with molten iron pretreatment, it can reach 0.001%.

[0021] 3) Efficient denitrification. After the scrap steel is melted and heated, the recarburizer is added to convert the scrap steel into molten iron, providing heat source and denitrification capacity for the converter.

[0022] 4) Avoid dioxin pollution. Scrap steel is directly added to molten iron, and the temperature instantly rises to over 1000℃, thus avoiding the generation of dioxins. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is further described in detail below in conjunction with embodiments. Example

[0024] Taking the production line of "molten iron pretreatment-150t converter-LF-continuous casting" as an example, five furnaces are smelted according to the method of the present invention, and the furnace numbers are A, B, C, D, and E respectively, as follows: (1) Preparation of DC ladle furnace: A DC ladle furnace is installed between the molten iron pretreatment and the blast furnace. The DC ladle furnace adopts a top single electrode plus a ladle bottom electrode mode, and a long electrode lifting stroke design. 47% of the molten iron in the ladle can be heated; an ultra-high power power supply system is selected, and the transformer capacity is 50MVA; the scrap steel adding position is located on the same side as the slag removal direction, and the bottom argon blowing hole position corresponds to it.

[0025] (2) Controlled iron injection: Pour the planned amount of molten iron into the iron ladle. The loading amount of 150t converter steel material is controlled at about 163t, the converter scrap ratio is controlled at about 15%, and the total scrap ratio is controlled at 25-60%. The planned molten iron volumes are 122.25, 114.1, 97.8, 65.2, and 65.2t respectively. Other relevant information is shown in Table 1 below.

[0026] Table 1 Planned addition of hot metal and scrap steel for 5 heats

[0027] Note: The amount of iron-clad steel includes the amount of molten iron in the iron-clad and the amount of scrap steel in the iron-clad.

[0028] (3) Melting scrap steel: After the ladle filled with molten iron is transported to the ladle furnace, argon gas is connected to the bottom for stirring, the molten iron is heated, and scrap steel is continuously added; lime, fluorite, wollastonite and other slag materials are added to increase the thickness of the slag layer, and foaming agent is added to make foamed slag for submerged arc heating. The three furnaces A, B, and C are produced in accordance with the one-to-one matching of ladle furnaces and converters, and the two furnaces D and E are produced in accordance with the two-to-one matching of ladle furnaces and converters. The weak stirring flow rate, scrap steel adding time, heating time, slag layer thickness, slag addition amount, foaming agent addition amount and converter smelting cycle during the five heating cycles are shown in Table 2.

[0029] Table 2 Melting scrap steel in ladle furnace for 5 batches

[0030] (4) Desulfurization and carbon increase: When adding slag, the proportion is adjusted according to the desulfurization requirements to produce desulfurized slag, and the slag basicity is controlled at 1.8-2.5; after the scrap steel is fully melted and the temperature is appropriate, a large amount of argon is used for strong stirring desulfurization and carbon increase by adding a carbon increaser. The 3 furnaces A, B, and C are subjected to a one-time strong stirring desulfurization and carbon increase by adding a carbon increaser, while the 2 furnaces D and E are subjected to a two-time strong stirring desulfurization and carbon increase by adding a carbon increaser, which is carried out once when heating, half of the scrap steel is added, and once when all of the scrap steel is added. The strong stirring time, strong stirring flow, slag addition amount, basicity, strong stirring flow, sulfur content, carbon increase ratio, etc. after heating for 5 furnaces are shown in Table 3.

[0031] Table 3 Desulfurization and carbon addition of 5-heat iron ladle furnace

[0032] (5) Slag removal: After desulfurization, the iron ladle is moved to the slag removal position, and the slag is removed from the iron ladle. The bottom argon blowing slag removal technology is used for efficient slag removal, and 85-90% of the desulfurization slag is removed.

[0033] (6) Iron charging and steelmaking: The converter is charged with scrap steel and desulfurized molten iron for smelting according to normal procedures. The steel materials are generally charged according to the plan. Within the control deviation range, the sulfur content of the molten steel is controlled below 0.010%, and the nitrogen content of the molten steel is at the same level as that of normal scrap steel smelting. The specific situation is shown in Table 4.

[0034] Table 4 Actual loading of steel materials and sulfur and nitrogen in molten steel for 5 batches

[0035] It can be seen from the above 5 furnaces that the method of the present invention can achieve a large scrap steel ratio, low sulfur and low nitrogen production; at the same time, directly adding scrap steel into molten iron can avoid the generation of dioxins; the scrap steel is melted in the iron ladle, and the heat loss during the transfer process to the furnace is small.

Claims

1. A method for smelting molten steel with a high scrap ratio and low sulfur and low nitrogen content, characterized in that: The amount of molten iron in the ladle is controlled according to the expected scrap steel ratio, and the ladle is heated in a DC ladle furnace. During the heating process, scrap steel is continuously added, slag is made, argon is blown and stirred, the scrap steel is melted, desulfurized and carbonized, and low-sulfur molten iron is generated. After deslagging, the molten iron is loaded into the converter for steelmaking.

2. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 1, characterized in that: Specifically, it includes the preparation of DC ladle furnace, controlled iron injection, melting of scrap steel, desulfurization and carbon addition, slag removal, and iron loading and steelmaking stages.

3. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 2, characterized in that: The preparation stage of the DC iron-clad furnace: a DC iron-clad furnace is equipped between the molten iron pretreatment and the blast furnace. The DC iron-clad furnace adopts a top single electrode plus an iron-clad bottom electrode mode, and a long electrode lifting stroke design; the scrap steel adding position is located on the same side of the slag removal direction, and the bottom argon blowing hole position corresponds to it.

4. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 2, characterized in that: The controlled iron injection stage: injecting a planned amount of molten iron into the ladle.

5. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 2, characterized in that: In the scrap steel melting stage, after the ladle filled with molten iron is transported to the DC ladle furnace, argon gas is connected for bottom blowing and stirring to heat the molten iron, and scrap steel is continuously added, wherein the scrap steel adding time is 4-8 minutes shorter than the heating time; slag materials such as lime, fluorite, wollastonite and light-burned dolomite are added to increase the slag layer thickness to more than 80 mm, a foaming agent is added to make foamed slag, and submerged arc heating is performed.

6. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 5, characterized in that: The scrap steel melting stage: weak stirring is performed during the heating, and the argon flow rate is controlled at 40-80Nl / min. Strong stirring is performed after the heating is stopped, and the argon flow rate is controlled at 600-1000Nl / min.

7. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 5, characterized in that: The scrap steel melting stage: the heating time is shorter than the converter smelting cycle. When the ladle furnace and the converter are produced one-to-one, the heating time is more than 5-8 minutes shorter than the converter smelting cycle; when the ladle furnace and the converter are produced two-to-one, the heating time is more than 8 minutes shorter than twice the converter smelting cycle.

8. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 5, characterized in that: In the desulfurization and carburization stage: when adding slag, the proportion is adjusted according to the desulfurization requirements to produce desulfurization slag, and the slag basicity is controlled at 1.8-2.5; after the scrap steel is fully melted and the temperature is appropriate, a large amount of argon gas is used for strong stirring for desulfurization, and a carburizer is added at 3.5-4% of the scrap steel amount for carburization.

9. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 2, characterized in that: The slag removal stage: after desulfurization, the iron ladle reaches the slag removal position, and the bottom argon blowing slag removal technology is used to remove 85-90% of the desulfurized molten iron from the iron ladle.

10. The method for smelting molten steel with high scrap ratio and low sulfur and nitrogen content according to claim 2, characterized in that: The iron charging and steelmaking are as follows: scrap steel is added to the converter according to normal procedures, desulfurized molten iron is charged, and smelting is carried out according to conventional smelting methods.

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