A raw material pure iron steel coil for powder metallurgy and a method for producing the same

Abstract

The application discloses a raw material pure iron steel coil for powder metallurgy and a production method thereof. The raw material pure iron steel coil comprises the following chemical components in percentage by weight: C<=0.0015%; Si<=0.005%; Mn<=0.020%; P<=0.008%; Als<=0.015%; S<=0.005%; Cr<=0.010%; Ni<=0.010%; Cu<=0.008%; Ti<=0.005%; [O]<=0.004%; N<=0.005%; and the rest is Fe and inevitable inclusions. The raw material pure iron steel coil is produced by adopting the production method of steelmaking+continuous casting+hot continuous rolling+cross cutting, has very low strength, high elongation and high purity, and can be used in powder metallurgy. The production method has high production efficiency, high material yield, short delivery cycle, can greatly reduce the production process cost and improve the material yield of users.

Description

Technical Field

[0001] This invention belongs to the field of steel technology, specifically relating to a pure iron steel coil used in powder metallurgy and its production method. Background Technology

[0002] Pure iron is a ferroalloy with a very low carbon content. Due to its excellent properties such as low coercivity, good thermal and electromagnetic conductivity, softness, and high toughness, it is widely used as an iron-based raw material in the smelting of high-temperature alloys, heat-resistant alloys, precision alloys, and maraging steels. Based on its application, it is mainly divided into three categories: raw material pure iron, electromagnetic pure iron, and military pure iron. Currently, the pure iron produced by steel companies is mainly raw material pure iron.

[0003] Raw pure iron is mainly used in industries such as powder metallurgy, permanent magnet materials, lithium batteries, amorphous ribbons, and soft magnets. Currently, steel companies primarily use two methods for producing pure iron: one is to use converter smelting and continuous casting to produce slabs, which are then cut into small billets or further processed into bars; the other is to use medium-frequency furnaces or electric furnaces to directly produce small billets or wire rods, which are then rolled into bars. Due to production process costs and material yield considerations, a small number of pure iron plants also use single-sheet rolling of medium plates for production.

[0004] Regarding existing technologies for producing pure iron slabs using converter smelting and continuous casting, Chinese patent CN105986053A discloses a method for producing industrial pure iron. This method uses all iron in the converter smelting process, without adding scrap steel, resulting in high costs. Furthermore, it does not specify the control parameters for the three-stage converter smelting process or the ladle refining method for obtaining ultra-low [Mn] content. LF dephosphorization and slag removal are required, making the process complex and costly. Chinese patent CN113774277A discloses an ultra-low carbon and ultra-low manganese industrial pure iron and its preparation method. Through KR desulfurization pretreatment of molten iron, low-sulfur molten iron is obtained and then added to the converter. Converter smelting, through a series of technical measures such as single slag removal and control of flux addition, molten steel temperature, oxygen lance position, and free oxygen content in the molten steel, can produce high-purity molten steel with a carbon content ≤0.002% and a manganese content ≤0.035%. However, it cannot obtain molten steel with extremely low residual Cr, Ni, and Cu content.

[0005] Existing technologies for producing raw material pure iron slabs using medium-frequency furnaces or electric furnaces include, for example, Chinese patent CN102352421A, which discloses a "process for smelting industrial pure iron using converter waste slag granules." However, this method has limitations because the raw material is converter waste slag granules and the equipment used is a medium-frequency induction furnace or a vacuum induction furnace. Another example is Chinese patent CN105603312A, which discloses a "method for manufacturing ultra-pure industrial pure iron," which uses an EBT electric furnace for smelting and adds lanthanum-cerium alloy for deoxidation and desulfurization, resulting in higher costs.

[0006] Regarding existing technologies for producing raw material pure iron using single-sheet rolling of medium plates, such as Chinese patent CN114959488A which discloses "An Industrial Pure Iron Medium-Thick Plate and Its Production Method", the production process route is: molten iron pretreatment - converter smelting + RH vacuum refining - continuous casting - cutting medium plate billet - billet heating - high-pressure water descaling - rolling - air cooling finishing - inspection and delivery, which has low production efficiency. Summary of the Invention

[0007] To reduce production costs and improve production efficiency and material yield, this invention provides a pure iron steel coil for powder metallurgy and its production method. The pure iron steel coil is produced using a steelmaking + continuous casting + hot rolling + cross-cutting process. This steel coil has extremely low strength, high elongation, and high purity, making it suitable for powder metallurgy. This production method offers high production efficiency, high material yield, and short delivery cycle, significantly reducing production process costs and improving material yield for users.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] The present invention provides a pure iron steel coil for powder metallurgy, comprising the following chemical composition by weight percentage: C≤0.0015%; Si≤0.005%; Mn≤0.020%; P≤0.008%; Als≤0.015%; S≤0.005%; Cr≤0.010%; Ni≤0.010%; Cu≤0.008%; Ti≤0.005%; [O]≤0.004%; N≤0.005%; the remainder being Fe and unavoidable inclusions; the metallographic structure of the pure iron steel coil for powder metallurgy is ferrite.

[0010] The raw material pure iron steel coil for powder metallurgy has a yield strength ≤180MPa, tensile strength ≤280MPa, elongation A50 ≥70%, yield strength ratio ≤0.60, cold bending performance 180°, and no cracks at D=0a.

[0011] The production method of pure iron steel coils for powder metallurgy provided by the present invention includes the following steps: hot metal pretreatment → converter smelting → alloy fine-tuning station → RH refining → continuous casting → hot rolling → cross cutting.

[0012] During the pretreatment of molten iron, scrap steel with Cr, Ni, and Cu contents of ≤80ppm should be used, and the use of slag steel, small pieces of iron, and casting scraps is prohibited.

[0013] During converter smelting, the double-slag method is adopted. Before slag removal, the converter basicity is 1.5-2.0, and after slag removal, the converter basicity is 3.5-4.0. The free oxygen content in the molten steel at the end of the converter is controlled to be above 0.0800%.

[0014] During continuous casting, ultra-low carbon steel protective slag with a carbon content of ≤4.5% is used for protective casting, and the billet head is cut off by ≥4.6m and the tail is cut off by ≥4.6m.

[0015] During hot rolling, both the inlet and outlet nozzles for primary descaling are fully open; the longitudinal temperature difference of R2DT is ≤40℃; and the finishing rolling temperature is ≥900℃.

[0016] Furthermore, in the hot metal pretreatment step, a group of ultra-low carbon steel grades are arranged to clean the furnace and washing tank before production to determine whether manganese alloy was weighed in the previous heat of the converter. If manganese alloy was weighed, the empty vibrating hopper or the empty belt is run for 1-2 minutes. After hot metal desulfurization, the target [S] is ≤0.0010%, and the total time for adding scrap steel and mixing iron is ≤16min.

[0017] In the converter smelting process, the auxiliary blowing and post-stirring in the converter adopt a weak stirring mode ≤0.05Nm. 3 / t.min; endpoint temperature control 1650±5℃, endpoint oxygen control ≥800ppm.

[0018] In the RH refining step, the vacuum degree is ≤2.6mbar, the decarbonization time is ≥10min, and the CO concentration in the exhaust gas is <1.0%.

[0019] During the RH refining process, it is strictly forbidden to add cold materials to lower the temperature. When the temperature is high, carbon powder should be added in the early or middle stages of decarburization or the treatment time should be extended appropriately to lower the temperature.

[0020] During the continuous casting process, the argon blowing flow rate of the stopper rod is ≥6NI / min.

[0021] During the hot rolling process, the slab exit temperature is 1180~1250℃; during the finish rolling process, the crown is controlled at 60±20μm, the wedge is controlled at 0±30μm, and the flatness is controlled at ±150I.

[0022] After hot rolling, a front-end centralized cooling process is adopted, eliminating the head-end non-cooled section. The coiling temperature is controlled at 650±20℃, the coiler tower shape is controlled at ±50mm, the overflow edge is <20mm, and the coil is air-cooled to room temperature after coiling.

[0023] During the cross-cutting process, the cross-cutting production speed is 20-40m / s, the centering error of the steel coil width is controlled within ±20mm, the longitudinal tilt of the straightening roller is controlled within ±2.0mm, and the transverse tilt is controlled within ±1.0mm.

[0024] This invention, in its steelmaking process design, avoids using scrap steel with high nickel, copper, and chromium content, and prohibits the use of slag steel, scrap iron, and casting residue, thereby reducing the content of impurities such as nickel, copper, and chromium in the steel. It employs a double-slag method to minimize the amount of slag in the steel, thus reducing the return of residual elements from the slag to the molten steel in subsequent processes and improving the purity of the molten steel. The converter basicity before slag removal is 1.5-2.0, using a low basicity to facilitate slag removal; the basicity after slag removal is controlled at 3.5-4.0, using a higher basicity to effectively reduce residual elements in the molten steel; and the free oxygen content in the molten steel at the converter endpoint is controlled to be above 0.0800%. A high endpoint oxygen value is also used. Promoting the oxidation of Mn into the steel slag helps reduce the manganese content in the final steel of the converter, achieving a demanganese rate of ≥80% and a dephosphorization rate of ≥90%. Using ultra-low carbon steel protective slag reduces the amount of carbon in the protective slag entering the molten steel and causing carbon increase. Cutting the billet head to ≥4.6m and tail to ≥4.6m reduces the residual element content in the billet head and tail, avoids downgrading and re-judgment of the billets, and improves the yield.

[0025] In the rolling process design of this invention, to maintain the surface quality of the raw pure iron plate, all descaling inlet and outlet nozzles are fully open, and all descaling water pressures are ≥180 Bar; the longitudinal temperature difference of R2DT is ≤40℃ to reduce rolling force instability caused by temperature fluctuations; the finishing rolling temperature is controlled at ≥900℃ to reduce rolling force fluctuations during rolling caused by microstructure changes, ensuring the rolling stability of subsequent finishing mills; crown is controlled at 60±20μm, wedge at 0±30μm, and straightness at ±150I to ensure a good rolled plate shape; in addition, a front-end centralized cooling process is adopted for cooling, and the head non-cooling section is eliminated. Since the raw pure iron has low strength, there is no need to add an auxiliary head non-cooling section for coiling, which can effectively reduce costs and improve efficiency. In the cross-cutting process design, the cross-cutting production speed is selected as 20-40m / s, the steel coil width centering error is controlled within ±20mm, the longitudinal tilt of the straightening rolls is controlled at ±2.0mm, and the transverse tilt is controlled at ±1.0mm, which can effectively improve the plate shape and increase the yield.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] This invention is applicable to the large-scale production of raw pure iron, with low production costs and high efficiency. The hot-rolled raw pure iron steel plate produced using this invention can achieve a purity of over 99.94%, with central porosity, central cracks, corner cracks, triangular cracks, Al2O3 inclusions, and honeycomb bubbles all classified as grade 0, and central segregation ≤ B0.5 grade. The microstructure is a coarse ferrite structure, with a ferrite volume fraction ≥ 99%. The product has a yield strength ≤ 180 MPa, tensile strength ≤ 280 MPa, elongation A50 ≥ 70%, yield strength ratio ≤ 0.60, cold bending performance of 180°, and no cracks at D=0a. It has extremely low strength, high elongation, and high purity. Under extremely low strength conditions, its shearing efficiency is higher than that of high-strength steel coils, and the energy consumption costs of equipment required by customers can be significantly reduced.

[0028] The production method for pure iron steel coils provided by this invention improves production efficiency by more than 3 times compared to the medium plate production method; compared to the continuous casting billet flame cutting method, it increases utilization rate by 5 percentage points, reduces cost by 200 yuan / ton, and doubles delivery cycle. It can significantly reduce production process costs and improve user material yield, achieving a win-win situation. Attached Figure Description

[0029] Figure 1 The image shows the pickled low-magnification microstructure of the hot-rolled steel plate produced in Example 1. It can be seen that the central segregation of the pickled low-magnification microstructure is ≤B0.5 grade, and the central porosity, central cracks, corner cracks, triangular cracks, Al2O3 inclusions, and honeycomb bubbles are all grade 0, with no bright white band. Detailed Implementation

[0030] The present invention provides a pure iron steel coil for powder metallurgy, comprising the following chemical composition by weight percentage: C≤0.0015%; Si≤0.005%; Mn≤0.020%; P≤0.008%; Als≤0.015%; S≤0.005%; Cr≤0.010%; Ni≤0.010%; Cu≤0.008%; Ti≤0.005%; [O]≤0.004%; N≤0.005%; the remainder being Fe and unavoidable inclusions; the metallographic structure of the pure iron steel coil for powder metallurgy is ferrite.

[0031] The production process of the pure iron steel coil used in powder metallurgy includes molten iron pretreatment, converter smelting, alloy fine-tuning station, RH, continuous casting, heating, rolling, cooling, and cross-cutting. Specifically:

[0032] 1) Hot metal pretreatment: Before production, arrange a group of ultra-low carbon steel grade furnace washing tanks to determine whether manganese alloy was weighed in the previous heat of the converter. If necessary, use an empty vibrating hopper or an empty belt for 1-2 minutes. After hot metal desulfurization, the target [S] is ≤0.0010%. Use scrap steel with Cr, Ni and Cu contents ≤80ppm. Slag steel, palm iron, and casting scrap are prohibited. The total time for adding scrap steel and mixing iron is ≤16min.

[0033] 2) Converter smelting: A double-slag method is adopted. Converter basicity before slag removal is 1.5-2.0; after slag removal, basicity is controlled at 3.5-4.0. Auxiliary blowing and post-stirring in the converter adopt a weak stirring mode ≤0.05 Nm. 3 / t.min; endpoint temperature control 1650±5℃, endpoint oxygen control ≥800ppm.

[0034] 3) RH refining: vacuum degree ≤ 2.6 mbar, decarburization time ≥ 10 min and CO concentration in exhaust gas < 1.0%. It is strictly forbidden to add cold material to cool down. When the temperature is high, add carbon powder in the early and middle stages of decarburization or extend the treatment time appropriately to cool down.

[0035] 4) Continuous casting: Argon blowing flow rate of stopper rod ≥6NI / min, ultra-low carbon steel protective slag is used, billet head cut ≥4.6m, tail cut ≥4.6m.

[0036] 5) Hot rolling: Slab exit temperature: 1180-1250℃. To maintain surface quality, all descaling inlet and outlet nozzles are fully open, with all descaling water pressure ≥180 Bar, and longitudinal temperature difference ≤40℃ for R2DT. Finishing rolling temperature ≥900℃, crown control 60±20μm, wedge control 0±30μm, and straightness control ±150I. A front-end centralized cooling process is adopted, eliminating the head-end uncooled section. Coiling temperature control: 650±20℃, coiler tower type control ±50mm, overflow edge <20mm, and air cooling to room temperature after coiling.

[0037] 6) Cross-cutting: Cross-cutting production speed is 20-40m / s, the centering error of the steel coil width is controlled within ±20mm, the longitudinal tilt of the straightening roller is controlled within ±2.0mm, and the transverse tilt is controlled within ±1.0mm.

[0038] The present invention will now be described in detail with reference to the embodiments.

[0039] The chemical composition of the raw material pure iron steel coils in the examples and comparative examples is shown in Table 1, with the balance being iron and unavoidable inclusions. The steelmaking process parameters of the raw material pure iron steel coils in the examples and comparative examples are shown in Table 2. The rolling process and mechanical properties of the raw material pure iron steel coils in the examples and comparative examples are shown in Table 3.

[0040] Table 1. Measured chemical composition (mass percentage, wt%) of the examples.

[0041] serial number C Si Mn P S Als Cr Ni Cu Ti O N Example 1 0.0008 0.0011 0.0193 0.0057 0.0032 0.0081 0.0050 0.0047 0.0067 0.0001 0.0035 0.0025 Example 2 0.0008 0.001 0.0197 0.0062 0.0025 0.0085 0.0051 0.0049 0.0071 0.0001 0.0030 0.0019 Example 3 0.0007 0.0011 0.0145 0.0044 0.0026 0.0065 0.0044 0.0049 0.0078 0.0001 0.0038 0.0015 Example 4 0.0009 0.0015 0.0185 0.0057 0.0022 0.0088 0.0045 0.0046 0.0072 0.0002 0.0027 0.0021 Comparative Example 1 0.0023 0.003 0.0501 0.0185 0.0071 0.0152 0.0223 0.0121 0.0131 0.0010 0.0086 0.0042 Comparative Example 2 0.0022 0.004 0.0562 0.0203 0.0082 0.0162 0.0231 0.0102 0.0125 0.0020 0.0080 0.0045

[0042] Table 2. Main steelmaking parameters for each embodiment and comparative example.

[0043]

[0044] Table 3. Main rolling process parameters and mechanical properties of the embodiments.

[0045]

[0046]

[0047] As can be seen from the above, the raw material pure iron steel coil produced by the production method of powder metallurgy raw material pure iron steel coil provided by the present invention has a central porosity, central crack, corner crack, triangular area crack, Al2O3 inclusion, and honeycomb bubble all of grade 0, and a central segregation ≤ B0.5 grade; its yield strength ≤180MPa, tensile strength ≤280MPa, elongation A50≥70%, yield strength ratio ≤0.60, cold bending performance 180°, D=0a no crack, and has extremely low strength, high elongation and high purity. Its performance is close to the limit of steel, and the shearing processing efficiency is high, which significantly reduces the equipment power energy consumption cost required by customers.

[0048] The above detailed description of a pure iron steel coil used in powder metallurgy and its production method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, variations and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A method for producing pure iron steel coils as raw material for powder metallurgy, characterized in that, The pure iron steel coil used in powder metallurgy comprises the following chemical composition by weight percentage: C≤0.0015%; Si≤0.005%; Mn≤0.020%; P≤0.008%; Als≤0.015%; S≤0.005%; Cr≤0.010%; Ni≤0.010%; Cu≤0.008%; Ti≤0.005%; [O]≤0.004%; N≤0.005%; the remainder is Fe and unavoidable inclusions; the metallographic structure of the pure iron steel coil used in powder metallurgy is ferrite; The process includes the following steps: hot metal pretreatment → converter smelting → alloy fine-tuning station → RH refining → continuous casting → hot rolling → cross cutting; During the pretreatment of molten iron, scrap steel with Cr, Ni, and Cu contents of ≤80ppm should be used, and the use of slag steel, small pieces of iron, and casting scraps is prohibited. In the hot metal pretreatment step, the target [S] after hot metal desulfurization is ≤0.0010%, and the total time for adding scrap steel and mixing iron is ≤16min; During converter smelting, the double-slag method is adopted. Before slag removal, the converter basicity is 1.5-2.0, and after slag removal, the converter basicity is 3.5-4.

0. The free oxygen content in the molten steel at the end of the converter is controlled to be above 0.0800%. In the converter smelting process, the auxiliary blowing and post-stirring in the converter adopt a weak stirring mode ≤0.

05. The endpoint temperature is controlled at 1650±5℃, and the endpoint oxygen content is controlled at ≥800ppm. In the RH refining process, the vacuum level is ≤2.6 mbar, the decarburization time is ≥10 min, and the CO concentration in the exhaust gas is <1.0%. During continuous casting, ultra-low carbon steel protective slag with a carbon content of ≤4.5% is used for protective casting, and the billet head is cut off by ≥4.6m and the tail is cut off by ≥4.6m. During hot rolling, both the inlet and outlet nozzles for primary descaling are fully open; the longitudinal temperature difference in R2DT is ≤40℃; the finishing rolling temperature is ≥900℃. The raw material pure iron steel coil for powder metallurgy has a yield strength ≤180MPa, tensile strength ≤280MPa, and elongation A. 50 ≥70%, yield strength ratio ≤0.60, cold bending performance 180°, no cracks when D=0a.

2. The method for producing pure iron steel coils for powder metallurgy according to claim 1, characterized in that, During the continuous casting process, the argon blowing flow rate of the stopper rod is ≥6NL / min.

3. The method for producing pure iron steel coils for powder metallurgy according to claim 1, characterized in that, During the hot rolling process, the slab exit temperature is 1180~1250℃; during finish rolling, the crown is controlled at 60±20μm, the wedge is controlled at 0±30μm, and the flatness is controlled at ±150μm.

4. The method for producing pure iron steel coils for powder metallurgy according to claim 1, characterized in that, After hot rolling, a front-end centralized cooling process is adopted, eliminating the head-end non-cooled section. The coiling temperature is controlled at 650±20℃, the coiler tower shape is controlled at ±50mm, the overflow edge is <20mm, and the coil is air-cooled to room temperature after coiling.

5. The method for producing pure iron steel coils for powder metallurgy according to claim 1, characterized in that, During the cross-cutting process, the cross-cutting production speed is 20-40m / s, the centering error of the steel coil width is controlled within ±20mm, the longitudinal tilt of the straightening roller is controlled within ±2.0mm, and the transverse tilt is controlled within ±1.0mm.

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