A method for producing an industrial pure iron having a Cr content of less than 40 ppm

Abstract

The application discloses a preparation method of industrial pure iron with a Cr content lower than 40 ppm, and the preparation method comprises the following steps: hot metal pretreatment: using hot metal with Mn less than or equal to 0.25% and scrap steel with Cr, Ni and Cu all lower than 0.0080%, and the hot metal quantity / (the hot metal quantity+the scrap steel quantity) is less than or equal to 0.85; converter smelting: adopting a double-slag method, and the end-point oxygen control is greater than or equal to 1000 ppm; LF refining: forming an oxidizing slag, and the oxygen position is greater than or equal to 850 ppm; adopting a strong stirring mode before outlet; RH refining: adding scrap steel with a chromium content less than or equal to 0.0080% to reduce the temperature, the target temperature is controlled in 1580+ / -10 DEG C, and then oxygen is blown; continuous casting: controlling the proportion of chromium-containing drainage sand to be less than or equal to 50%; and the industrial pure iron prepared by the method can be used as low-Cr industrial pure iron for a new energy battery positive electrode material lithium iron phosphate, wherein the center porosity, the center crack, the corner crack, the triangular area crack, the Al2O3 inclusion and the honeycomb-shaped bubble are all 0 level, and the center segregation is less than or equal to B0.5 level.

Description

Technical Field

[0001] This invention belongs to the field of industrial pure iron preparation technology, specifically relating to a method for preparing industrial pure iron with a Cr content of less than 40 ppm. Background Technology

[0002] With the rapid development of the global new energy industry, lithium-ion batteries have been widely used in power batteries, energy storage batteries, and other fields due to their advantages such as high energy density, long cycle life, and environmental friendliness. Among them, lithium iron phosphate (LiFePO4) cathode material has become one of the most promising cathode materials in the current power battery field due to its excellent thermal stability, stable electrochemical performance, controllable cost, and outstanding safety. Its performance directly determines the range, cycle life, and safety reliability of lithium-ion batteries.

[0003] As a core raw material for the preparation of lithium iron phosphate (LFP) cathode materials, the purity and impurity content of industrial pure iron play a decisive role in the final performance of LFP. During the synthesis of LFP, industrial pure iron participates in reactions such as acid dissolution and oxidation, and its impurities are directly introduced into the final product. Among these, Cr, Ni, Cu, and Zn significantly affect the recycling performance and subsequent safety of new energy batteries. Mn and Zn can be controlled relatively easily during production, and mainstream steel mills producing pure iron for new energy batteries can meet the requirements. Cu and Ni can be controlled through ore blending, molten iron selection, and specialized scrap steel management to meet the requirements of different users. However, due to its chemical properties, Cr is prone to fluctuations and chromium reversion during steelmaking, making Cr content control more difficult.

[0004] The industrial pure iron used in lithium iron phosphate cathode materials for new energy batteries generally requires the chromium content to be controlled at an extremely low level, usually ≤100ppm, and ≤50ppm for high-end power-grade batteries. The preparation of such low-Cr industrial pure iron has become one of the key bottlenecks restricting the development of the high-end lithium iron phosphate industry.

[0005] Chinese patent CN 117626099A discloses a method for controlling ultra-low Cr content in pure iron steel for ultra-low carbon new energy batteries, resulting in short-process billets with Cr ≤ 0.0060%. This process requires that the ratio of molten iron content to (molten iron content + scrap steel content) ≥ 0.9. The patent involves a small amount of scrap steel and uses a short electric arc at the LF station for rapid heating of the molten steel, resulting in higher costs. Furthermore, the final billet maintains a Cr level of approximately 40-60 ppm.

[0006] Chinese patent CN 117684085A discloses a method for producing high-purity cathode steel, yielding the following composition: C: 0.001–0.020%, Si ≤ 0.012%, Mn ≤ 0.016%, P ≤ 0.015%, S ≤ 0.010%, Alt ≤ 0.020%, Cu ≤ 0.008%, Cr ≤ 0.006%, Ni ≤ 0.005%, K ≤ 0.010%, Na ≤ 0.010%, Mg ≤ 0.010%, Zn ≤ 0.010%, Ca ≤ 0.015%, Pb ≤ 0.005%, and Nb+V+Ti ≤ 0.010%. The scrap steel loading in this process accounts for 5-10% of the total loading, resulting in higher costs due to the small amount of scrap steel added. The tapping temperature is 1640±20℃ and the converter uses a high temperature (1620-1660℃), which makes it easy for Mn to return during tapping. Furthermore, the Cr element level in the final finished billet is controlled at around 40-60ppm.

[0007] Chinese patent CN 117867389A discloses a method for preparing low-copper industrial pure iron for lithium iron phosphate batteries. This process uses a converter and RH-OB dual-stage process to deeply remove copper, carbon, manganese, potassium, sodium, and other impurities, yielding low-copper industrial pure iron with Cu ≤ 80 ppm. However, the final product's Cr level is controlled at around 116-134 ppm, which is insufficient to meet the requirements for ultra-low Cr pure iron in new energy applications.

[0008] Patent CN 117701806A discloses a method for producing high-purity ultra-low manganese industrial pure iron using a refining duplex process. This method includes hot metal pretreatment, converter dephosphorization, converter decarburization, LF refining, RH refining, and slab continuous casting. It can control the Mn content in the molten steel to below 0.015wt% and stably control the purity of the cast slab to 99.90-99.95wt%. However, the Cr element is difficult to meet the requirements for use in new energy pure iron.

[0009] In summary, current research on pure iron for new energy batteries mainly focuses on the control of Mn, Cu, and Cr elements. While there are relatively good control methods for Mn, control methods for Cu and Cr are relatively limited. The best control level for Cr is only ≤60ppm, which is insufficient to meet the requirements for mass production of lithium iron phosphate cathode materials for new energy batteries. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a method for preparing industrial pure iron with a Cr content of less than 40 ppm. The method employs a production process route of molten iron pretreatment → converter smelting → alloy fine-tuning station → LF refining → RH refining → continuous casting. Through layer-by-layer dechromium removal steelmaking technology, it can not only stably produce pure iron for new energy batteries with ultra-low chromium residual elements (≤40 ppm), but also increase the number of continuous casting furnaces, reduce production process costs, and improve material yield. Simultaneously, it effectively addresses the material yield issue for downstream users, achieving a win-win situation.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] This invention provides a method for preparing industrial pure iron with a Cr content of less than 40 ppm, comprising the following steps:

[0013] Hot metal pretreatment: Use hot metal with Mn≤0.25% and scrap steel with Cr, Ni and Cu all below 0.0080%, and hot metal content / (hot metal content + scrap steel content)≤0.85;

[0014] Converter smelting: using the double slag method, with final oxygen control ≥1000ppm;

[0015] LF refining: produces oxidizing slag with an oxygen content ≥850ppm; strong stirring mode is used before leaving the station;

[0016] RH refining: Add scrap steel with chromium content ≤0.0080% for cooling, with the target temperature controlled at 1580±10℃, and then blow oxygen. Aluminum particles must not be added during the oxygen blowing process.

[0017] Continuous casting: Control the proportion of chromium-containing guide sand to ≤50%.

[0018] Furthermore, in the converter smelting step, the converter tapping temperature is ≤1600℃.

[0019] In the LF refining step, the oxidizing slag has a total mass percentage of FeO and MnO ≥15% and an alkalinity ≤2.0.

[0020] In the LF refining step, the bottom blowing argon time is ≥6min, and the outlet temperature is controlled at 1590±5℃.

[0021] In the RH refining step, the oxygen blowing time is ≥20 min.

[0022] In the RH refining step, aluminum particles are added for deoxidation before leaving the RH station according to the oxygen content of the molten steel, with a target oxygen content of ≤25ppm.

[0023] In the continuous casting step, the casting speed of the slab is ≤1.4m / s.

[0024] In the continuous casting process, quick ladle change technology is used for production.

[0025] In the continuous casting process, the first and last billets of the casting are downgraded and reclassified as scrap steel for the next production run.

[0026] The composition and weight percentage of the industrial pure iron with a Cr content of less than 40 ppm are as follows: C: ≤0.0015%; Si: ≤0.0020%; Mn: ≤0.0130%; P: ≤0.0050%; S: ≤0.0060%; Al: ≤0.0100%; Cr: ≤0.0040%; Ni: ≤0.0055%; Cu: ≤0.0070%; Ti: ≤0.0005%; [O]: ≤0.0050%; N: ≤0.0030%; Fe ≥99.95%.

[0027] The purity of the industrial pure iron with a Cr content of less than 40 ppm is 3N level.

[0028] The industrial pure iron with a Cr content of less than 40 ppm has a central porosity, central crack, corner crack, triangular crack, Al2O3 inclusion, and honeycomb-like bubbles, all of which are grade 0, and central segregation is grade ≤ B0.5.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] In the steelmaking process design, this invention uses scrap steel with low nickel, copper, and chromium content (Cr, Ni, and Cu all below 0.0080%) to reduce the sources of impurities such as nickel, copper, and chromium in the steel; the ratio of molten iron to (molten iron + scrap steel) is ≤0.85 to maximize the amount of scrap steel, increase the iron-to-steel ratio, and reduce costs; the oxygen level at the converter endpoint is controlled at ≥1000ppm, using a high-oxygen environment to promote the oxidation of Cr into the steel slag, which helps reduce the Cr content of the molten steel entering the LF process in the next stage of the converter; the LF process generates oxidizing slag with an oxygen level ≥850ppm, a bottom blowing argon time ≥6min, and the outlet temperature is controlled at 1590±5℃. Before exiting the station, strong stirring is used in conjunction with the oxidizing slag to further reduce residual Cr.

[0031] Low-chromium scrap steel is added to the RH process to lower the temperature and prevent chromium from other sources from increasing the chromium content in the molten steel. The target temperature is controlled at 1580±10℃. Then, oxygen is blown to remove residual chromium for ≥20 minutes. The lower steel temperature and sufficient oxygen blowing environment are conducive to the oxidation of residual chromium in the steel into the slag, further reducing the residual chromium content. Aluminum particles must not be added during the oxygen blowing process to prevent the neutralization of oxygen and aluminum from weakening the chromium removal reaction.

[0032] The continuous casting process employs a quick-change ladle technology. Increasing the number of consecutive castings helps maintain the recycling of pure iron ladles, further improving the purity of the ladles in subsequent smelting processes and reducing residual chromium in the steel. Ladle guide sand is a crucial material used to guide the flow between the ladle and tundish during steelmaking. Its main function is to ensure smooth steel flow and prevent ladle nozzle blockage. The proportion of chromium-containing guide sand is ≤50%, ensuring smooth production while reducing costs. Controlling the continuous casting slab casting speed to ≤1.4m / s improves the quality of pure iron slabs while ensuring precise slab size control. Since the Cr content of the first and last slabs in pure iron steelmaking is difficult to guarantee, downgrading and reclassifying them as scrap for the next production run increases the yield, reducing costs and preventing potential leakage of substandard products.

[0033] The industrial pure iron with a Cr content of less than 40 ppm produced by the preparation method described in this invention can be used as low-Cr industrial pure iron for lithium iron phosphate cathode material in new energy batteries. Its residual element control is as follows: Mn≤130ppm, Cr:≤0.0040%; Ni:≤0.0055%; Cu:≤0.0070%; Fe≥99.95%, and the purity reaches the 3N level. Its low magnification structure is as follows: central porosity, central cracks, corner cracks, triangular cracks, Al2O3 inclusions, and honeycomb bubbles are all grade 0, and central segregation is ≤B0.5 grade. Attached Figure Description

[0034] Figure 1 The pickled low-magnification microstructure of the pure iron continuous casting slab produced by the method in Example 1 has a central segregation of ≤B0.5 grade, and central porosity, central cracks, corner cracks, triangular cracks, Al2O3 inclusions, and honeycomb bubbles are all grade 0, with no bright white band. Detailed Implementation

[0035] The industrial pure iron with a Cr content of less than 40 ppm provided by this invention has the following composition and weight percentage: C: ≤0.0015%; Si: ≤0.0020%; Mn: ≤0.0130%; P: ≤0.0050%; S: ≤0.0060%; Al: ≤0.0100%; Cr: ≤0.0040%; Ni: ≤0.0055%; Cu: ≤0.0070%; Ti: ≤0.0005%; [O]: ≤0.0050%; N: ≤0.0030%; Fe ≥99.95%.

[0036] The production process for industrial pure iron with a Cr content below 40 ppm includes hot metal pretreatment, converter smelting, alloy fine-tuning station, LF (sulfurized iron), RH (reverse oxidizer), and continuous casting. The specific controls for each process are as follows:

[0037] 1) Hot metal pretreatment: Select hot metal with Mn≤0.25%, and use scrap steel with Cr, Ni and Cu content all below 0.0080% and low content of nickel, copper and chromium. Hot metal quantity / (hot metal quantity + scrap steel quantity) ≤0.85.

[0038] 2) Converter smelting: The double slag method is adopted. In order to reduce the content of Cr and Mn elements, the final oxygen content is controlled at ≥1000ppm and the converter tapping temperature is ≤1600℃.

[0039] 3) LF refining: produce oxidizing slag, oxygen level ≥850ppm, bottom blowing argon time ≥6min, and control the outlet temperature at 1590±5℃. Before leaving the station, strong stirring is used in conjunction with the oxidizing slag to further reduce residual Cr.

[0040] 4) RH refining: Scrap steel can be added for cooling. The chromium content of the scrap steel should be ≤0.0080%. The target temperature should be controlled at 1580±10℃. Then, oxygen is blown to remove residual Cr elements. The oxygen blowing time should be ≥20min. Aluminum particles should not be added during the oxygen blowing process. Before leaving the RH station, a very small amount of aluminum particles should be added according to the oxygen content of the molten steel. The oxygen target should be ≤25ppm.

[0041] 5) Continuous casting: Production is carried out using tundish quick change technology, the proportion of chromium-containing guiding sand is ≤50%, the casting speed of continuous casting slab is controlled to be ≤1.4m / s, and the first and last billets of the casting cycle are downgraded and re-judged.

[0042] The present invention will now be described in detail with reference to the embodiments.

[0043] The chemical composition of the examples and comparative examples is shown in Table 1, the process of the examples and comparative examples is shown in Table 3, and the quality of the cast billets of the examples and comparative examples is shown in Table 2.

[0044] Table 1. Measured chemical composition (mass percentage, wt%) of each example and comparative example.

[0045]

[0046] Table 2. Main steelmaking parameters for each embodiment and comparative example.

[0047]

[0048] Table 3. Pickled low-magnification microstructure of pure iron continuous casting slabs in each embodiment and comparative example.

[0049]

[0050] As can be seen from the above, the industrial pure iron with a Cr content of less than 40 ppm produced by the preparation method described in this invention has the following residual element control: Mn ≤ 130 ppm, Cr: ≤ 0.0040%; Ni: ≤ 0.0055%; Cu: ≤ 0.0070%; Fe ≥ 99.95%, and the purity reaches the 3N level. Its low-magnification structure has the following defects: central porosity, central cracks, corner cracks, triangular cracks, Al2O3 inclusions, and honeycomb bubbles are all grade 0, and central segregation is ≤ B0.5 grade.

[0051] The essence of dechromium removal is the reaction between chromium in molten steel and ferrous oxide (FeO) in slag: 2[Cr] + 3(FeO) = (Cr2O3) + 3[Fe]. This reaction allows chromium to enter the slag in the form of Cr2O3, achieving separation from the molten steel. However, in Comparative Examples 1 and 2, the converter tapping temperature is 1628-1638℃, and the active oxygen in the converter is less than 600ppm. The high temperature and low oxygen atmosphere is not conducive to the mass transfer of Cr between steel and slag. Although the bottom blowing argon process in the LF process and the weak stirring mode before exiting the station can promote the uniformity of molten steel temperature and composition, and drive non-metallic inclusions to float and purify the molten steel, it cannot enhance the steel-slag reaction, which is not conducive to the oxidation of chromium and its migration into the slag. In addition, although the use of chromium-based guiding sand in the continuous casting process has the advantages of high melting point, good fluidity, strong resistance to molten steel penetration, and high automatic casting rate, its chromium content is too high. Chromium enters the molten steel through molecular diffusion, which is not conducive to low-chromium production.

[0052] The above detailed description of a method for preparing industrial pure iron with a Cr content of less than 40 ppm, with reference to the embodiments, is illustrative rather than limiting. Several embodiments may be listed within the defined scope. Therefore, variations and modifications that do not depart from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing industrial pure iron with a Cr content of less than 40 ppm, characterized in that, The preparation method includes the following steps: Hot metal pretreatment: Use hot metal with Mn≤0.25% and scrap steel with Cr, Ni and Cu contents all below 0.0080%, hot metal quantity / (hot metal quantity + scrap steel quantity)≤0.85; Converter smelting: using the double slag method, with final oxygen control ≥1000ppm; LF refining: produces oxidizing slag with an oxygen content ≥850ppm; strong stirring mode is used before leaving the station; RH refining: Add scrap steel with chromium content ≤0.0080% for cooling, with the target temperature controlled at 1580±10℃, and then blow oxygen. Aluminum particles must not be added during the oxygen blowing process. Continuous casting: Control the proportion of chromium-containing guide sand to ≤50%.

2. The preparation method according to claim 1, characterized in that, In the converter smelting step, the converter tapping temperature is ≤1600℃.

3. The preparation method according to claim 1, characterized in that, In the LF refining step, the bottom blowing argon time is ≥6min, and the outlet temperature is controlled at 1590±5℃.

4. The preparation method according to claim 1, characterized in that, In the RH refining step, the oxygen blowing time is ≥20 min.

5. The preparation method according to claim 1, characterized in that, In the RH refining step, aluminum particles are added for deoxidation before leaving the RH station according to the oxygen content of the molten steel, with a target oxygen content of ≤25ppm.

6. The preparation method according to claim 1, characterized in that, In the continuous casting step, the casting speed of the slab is ≤1.4m / s.

7. The preparation method according to claim 1, characterized in that, In the continuous casting process, the tundish quick-change technology is used for production; the first and last billets of the casting are downgraded and reclassified as scrap steel for the next production.

8. The preparation method according to claim 1, characterized in that, The composition and weight percentage of the industrial pure iron with a Cr content of less than 40 ppm are as follows: C: ≤0.0015%; Si: ≤0.0020%; Mn: ≤0.0130%; P: ≤0.0050%; S：≤0.0060%； Al: ≤0.0100%; Cr: ≤0.0040%; Ni: ≤0.0055%; Cu: ≤0.0070%; Ti: ≤0.0005%; [O]: ≤0.0050%; N: ≤0.0030%, Fe≥99.95%.

9. The preparation method according to claim 1, characterized in that, The purity of the industrial pure iron with a Cr content of less than 40 ppm is 3N level.

10. The preparation method according to claim 1, characterized in that, The industrial pure iron with a Cr content of less than 40 ppm has a central porosity, central crack, corner crack, triangular crack, Al2O3 inclusion, and honeycomb-like bubbles, all of which are grade 0, and central segregation is grade ≤ B0.5.

Images