High-purity steel strip and its preparation method

Abstract

This invention discloses a high-purity steel strip and its preparation method. The high-purity steel strip, by weight percentage, comprises 0.25-0.38% C, 0.10-0.20% Si, 0.60-0.80% Mn, 0.25-0.35% Cr, 0.020-0.035% Ti, and 0.0015-0.0045% Ce, with the balance being Fe and unavoidable impurities. This application achieves deep deoxidation and desulfurization by adding rare earth elements, reducing the formation of brittle inclusions at the metallurgical source. Simultaneously, it promotes the spheroidization, refinement, and dispersion of existing inclusions, thereby improving the strength and wear resistance of the steel, effectively eliminating stress concentration points, and significantly inhibiting the initiation of fatigue cracks. Furthermore, by inhibiting the segregation behavior of harmful elements such as sulfur and phosphorus at grain boundaries, it strengthens the grain boundary bonding strength, thereby effectively improving the impact toughness, fatigue strength, and resistance to hydrogen-induced cracking of the steel.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and more specifically, to high-purity steel strip and its preparation method. Background Technology

[0002] Core safety components and critical structural parts of new energy heavy-duty trucks, such as drive axles, high-stress suspension systems, and steering systems, must withstand high instantaneous torque, multi-dimensional impact loads, and harsh wear environments under extreme operating conditions. Therefore, the materials used must possess excellent strength characteristics, fatigue resistance, impact toughness, and wear resistance. While the widely used 20Cr steel generally meets these performance requirements, it still has several technical limitations: its high alloy content results in poor machinability and weldability, requiring cold rolling and annealing during manufacturing and relying on specialized welding equipment. Furthermore, the high alloy content easily introduces impurities, which can form brittle inclusions and become initiation points for fatigue cracks; simultaneously, the segregation of harmful elements at grain boundaries significantly reduces the material's impact toughness. From an economic perspective, the high production cost of 20Cr steel makes it difficult to meet the increasingly demanding requirements for cost control and wear resistance in the load-bearing components of new energy heavy-duty trucks.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to provide a high-purity steel strip and its preparation method, which solves the problem that existing 20Cr steel cannot meet the increasing cost and wear resistance requirements of load-bearing components in new energy heavy-duty trucks.

[0005] This invention is implemented as follows: In a first aspect, the present invention provides a high-purity steel strip comprising, by weight percentage: 0.25-0.38% C, 0.10-0.20% Si, 0.60-0.80% Mn; 0.25-0.35% Cr, 0.020-0.035% Ti, 0.0015-0.0045% Ce, with the balance being Fe and unavoidable impurities.

[0006] In an optional embodiment, by weight percentage, it comprises 0.290-0.311% C, 0.135-0.175% Si, 0.66-0.75% Mn; 0.28-0.33% Cr, 0.025-0.030% Ti, 0.0015-0.0026% Ce, with the balance being Fe and unavoidable impurities.

[0007] Secondly, the present invention provides a method for preparing the high-purity steel strip described in the foregoing embodiments, comprising the following steps: Step S1: Molten iron and scrap steel are added to a converter for smelting to obtain primary molten steel; dephosphorization is carried out during the smelting process, and ferrosilicon and ferroaluminum are added to the ladle for alloying during the tapping process; Step S2: The primary molten steel is transferred to the LF refining furnace for refining to obtain refined molten steel; the composition of the molten steel is adjusted during the refining process, and cerium-iron alloy is added at the end to meet the requirements of the aforementioned implementation method, followed by desulfurization and soft blowing. Step S3: The refined molten steel is continuously cast to obtain a continuously cast slab; Step S4: The slab is heated in a weakly reducing atmosphere, and then the heated slab is subjected to descaling, rough rolling, descaling and finish rolling in sequence to obtain a finish rolled steel strip; Step S5: After cooling the precision-rolled steel strip, it is coiled into a steel coil, and then the steel coil is slowly cooled to obtain the high-cleanliness steel strip.

[0008] In an optional embodiment, the S content in the molten iron is ≤ 0.005 wt%; And / or, the molten iron content in the converter is 85-95%; And / or, the dephosphorization method adopts the dual-slag method; And / or, the converter is a top-and-bottom blown converter.

[0009] In an optional embodiment, the smelting endpoint requires P ≤ 0.010%, C content 0.075-0.085%, tapping temperature 1625-1665℃, and tapping time ≥ 5min; And / or, during the tapping process, add 10-12.0 kg / t of silicon-manganese alloy and 150-400 kg / furnace of aluminum-iron alloy.

[0010] In an optional embodiment, during the refining step, lime and pre-melted slag are added to the molten steel to form slag and desulfurize it, so that S ≤ 0.005%; And / or, in the refining step, the temperature of the molten steel is adjusted to 1540-1570℃, and argon gas is gently blown for 5-10 minutes before hoisting the ladle to obtain the refined molten steel.

[0011] In an optional implementation, the superheat during continuous casting does not exceed 20°C; And / or, during continuous casting, the tundish temperature is 1505-1530℃; And / or, electromagnetic stirring is carried out during continuous casting, with an electromagnetic stirring frequency of 6-8Hz, a current of 280-320A, and a continuous casting billet pulling speed of 0.8-1.2m / min.

[0012] In an optional embodiment, the continuously cast slab is heated in a heating furnace by hot charging. The furnace entry temperature of the continuously cast slab is ≥500℃, and the continuously cast slab is heated to 1200-1250℃. A reducing atmosphere is maintained in the heating furnace.

[0013] In an optional embodiment, the entry temperature of the finishing mill is 1000-1070℃, and the final rolling temperature is 860-910℃.

[0014] In an optional embodiment, the precision-rolled steel strip is cooled by a laminar flow cooling device with edge slow cooling and then wound into a steel coil. And / or, the cooling adopts a first-half cooling mode; And / or, the winding temperature is 610-660℃; And / or, during the winding process, the tail heating control mode is used to control the tail heating length of the finished steel strip to be 19-21m and the heating range to be 19-21℃. And / or, the slow cooling time is greater than 72 hours.

[0015] The present invention has the following beneficial effects: This application, while moderately increasing the carbon content and simultaneously reducing the overall alloying element content, introduces a small amount of cerium and combines it with optimized casting and hot rolling process control to fully utilize the steel's purifying and microstructure-refining effects, thereby comprehensively improving the mechanical properties and wear resistance of the steel. By adding rare earth elements, deep deoxidation and desulfurization are achieved, reducing the formation of brittle inclusions at the metallurgical source; simultaneously, existing inclusions are encouraged to spheroidize, refine, and disperse, thereby improving the steel's strength and wear resistance, effectively eliminating stress concentration points, and significantly inhibiting the initiation of fatigue cracks; furthermore, by suppressing the segregation of harmful elements such as sulfur and phosphorus at grain boundaries, the grain boundary bonding strength is strengthened, thus effectively improving the steel's impact toughness, fatigue strength, and resistance to hydrogen-induced cracking. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a flowchart of Example 1; Figure 2 The image shows the metallographic microstructure of the high-purity steel strip prepared in Example 1 under 1000x magnification. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0019] This invention provides a high-purity steel strip, comprising, by weight percentage: C 0.25-0.38%, Si 0.10-0.20%, Mn 0.60-0.80%; Cr 0.25-0.35%, Ti 0.020-0.035%, Ce 0.0015-0.0045%, with the balance being Fe and unavoidable impurities.

[0020] The high-purity steel strip for new energy trucks provided in this application, while increasing carbon content and reducing the total amount of alloying elements, fully leverages the purification and microstructure refinement effects of rare earth elements through the introduction of a small amount of rare earth elements and optimized casting and hot rolling process control, thereby improving the mechanical properties and wear resistance of the steel. The addition of rare earth elements achieves deep deoxidation and desulfurization, reducing the formation of brittle inclusions at the source; simultaneously, it causes inclusions to spheroidize, refine, and disperse, improving the strength and wear resistance of the steel, eliminating stress concentration points, and inhibiting fatigue crack initiation; furthermore, by inhibiting the segregation of harmful elements such as sulfur and phosphorus at grain boundaries, it effectively strengthens grain boundaries, thereby improving the impact toughness, fatigue strength, and resistance to hydrogen-induced cracking of the steel.

[0021] Cerium is the most abundant rare earth element in the Earth's crust, far exceeding the content of other rare earth elements. In rare earth ores, cerium typically accounts for as much as 49% of the total, calculated as light rare earth elements. However, in the past, high-end applications such as neodymium-iron-boron alloys primarily utilized neodymium and praseodymium, leading to cerium being long left idle or disposed of at low prices as a surplus rare earth element. Cerium exhibits variable valence, commonly being +3 and +4, which gives it excellent reactivity in various chemical reactions: firstly, it possesses strong reducing and deoxidizing / desulfurizing capabilities, effectively removing impurities such as oxygen, sulfur, and hydrogen from alloys during metallurgical processes, refining grains, and improving the toughness and processing performance of materials; secondly, it has high chemical reactivity, a large atomic radius, and readily forms stable intermetallic compounds, significantly enhancing the overall performance of alloys. When the cerium content is less than 0.0015%, it cannot effectively purify and microalloy the steel; while when the cerium content is greater than 0.0045%, it leads to grain coarsening and a decrease in mechanical properties, as well as the deterioration and increase in the morphology and number of inclusions, and also increases production costs and process difficulty. The addition of cerium may exacerbate the risk of cracking, and the microalloying of cerium will lead to poor weldability. High-purity steel for new energy trucks requires welding in subsequent processing, so adding cerium to medium-carbon steel is extremely difficult and requires a comprehensive balance between process and performance. The high-purity steel strip for new energy trucks provided by this invention adopts a medium-carbon content and chromium-manganese-titanium-cerium alloying method. By controlling the cerium content within the range of 0.0015% to 0.0045%, it achieves deep deoxidation and desulfurization of the steel, completes the modification treatment of inclusions, inhibits the segregation of harmful elements at grain boundaries, and thus improves the strength, toughness and low-temperature impact toughness of the steel. Compared to other rare earth elements, cerium is the easiest to separate among rare earth elements and belongs to the light rare earth element category. It has a strong affinity for oxygen and sulfur, thereby reducing the oxygen and sulfur content in steel. It can also alter the morphology of non-metallic inclusions, making them easier to remove. However, cerium is relatively reactive, resulting in a low yield during steelmaking, typically between 30% and 50%. Increasing the cerium content would significantly increase costs; therefore, the cerium content is controlled within the range of 0.0015% to 0.0045%.

[0022] By controlling the chromium content within the range of 0.25% to 0.35%, the decomposition rate of austenite is slowed down, significantly improving the hardenability of the steel. By controlling the manganese content within the range of 0.60% to 0.80%, the martensitic transformation temperature and the rate of phase transformation in the steel can be greatly reduced, improving its hardenability. By controlling the titanium content within the range of 0.020% to 0.035%, titanium forms highly dispersed carbonitride particles with carbon and nitrogen. These carbonitride particles do not dissolve under the high temperatures of heating or welding, significantly improving the steel's resistance to grain coarsening and increasing the grain size of automotive structural steel from approximately grade 9 to grade 11 to 12.

[0023] The high-purity steel strip for new energy trucks provided by this invention has significantly reduced inclusion content and size, and significantly refined grains and intragranular structure. It has high strength, high fatigue resistance and wear resistance, and good impact resistance, which can meet the manufacturing requirements of core safety components and key structural components of new energy trucks.

[0024] In an optional embodiment, by weight percentage, it comprises 0.290-0.311% C, 0.135-0.175% Si, 0.66-0.75% Mn; 0.28-0.33% Cr, 0.025-0.030% Ti, 0.0015-0.0026% Ce, with the balance being Fe and unavoidable impurities.

[0025] This invention also provides a method for preparing the high-cleanliness steel strip described in the foregoing embodiments, comprising the following steps: Step S1: Molten iron and scrap steel are added to a converter for smelting to obtain primary molten steel; dephosphorization is carried out during the smelting process, and ferrosilicon and ferroaluminum are added to the ladle for alloying during the tapping process; Step S2: The primary molten steel is transferred to the LF refining furnace for refining to obtain refined molten steel; the composition of the molten steel is adjusted during the refining process, and cerium-iron alloy is added at the end to meet the requirements of the aforementioned implementation method, followed by desulfurization and soft blowing. Step S3: The refined molten steel is continuously cast to obtain a continuously cast slab; Step S4: The slab is heated in a weakly reducing atmosphere, and then the heated slab is subjected to descaling, rough rolling, descaling and finish rolling in sequence to obtain a finish rolled steel strip; Step S5: After cooling the precision-rolled steel strip, it is coiled into a steel coil, and then the steel coil is slowly cooled to obtain the high-cleanliness steel strip.

[0026] The method for preparing high-purity steel strip in this application has a simple preparation process, reasonable preparation cost, and the hot-rolled steel strip obtained has uniform material and good processing and heat treatment performance, which can meet the manufacturing requirements of core safety components and key moving parts of new energy trucks.

[0027] In an optional embodiment, the S content in the molten iron is ≤ 0.005 wt%. And / or, the molten iron content in the converter is 85-95%; And / or, the dephosphorization method adopts the dual-slag method; And / or, the converter is a top-and-bottom blown converter.

[0028] In an optional embodiment, the smelting endpoint requires P ≤ 0.010%, C content 0.075-0.085%, tapping temperature 1625-1665℃, and tapping time ≥ 5min; And / or, during the tapping process, add 10-12.0 kg / t of silicon-manganese alloy and 150-400 kg / furnace of aluminum-iron alloy.

[0029] In step S1, the sulfur content of molten iron is controlled, the molten iron ratio is increased, and dephosphorization is carried out using top and bottom blowing and double slag method. The final composition and tapping conditions are also adjusted to inhibit the enrichment of harmful elements such as P and S from the source and stabilize the purity and composition of the initial molten steel.

[0030] In an optional embodiment, during the refining step, lime and pre-melted slag are added to the molten steel to form slag and desulfurize it, so that S ≤ 0.005%; And / or, in the refining step, the temperature of the molten steel is adjusted to 1540-1570℃, and argon gas is gently blown for 5-10 minutes before hoisting the ladle to promote the removal of inclusions by floating, thereby obtaining the refined molten steel.

[0031] In step S2, LF refining implements white slag desulfurization and long-term soft blowing, combined with temperature and slag system control, to achieve deep desulfurization, efficient flotation of inclusions and fine-tuning of composition, ensuring the recovery rate and uniformity of microalloying elements such as rare earth.

[0032] In an optional implementation, the superheat during continuous casting does not exceed 20°C; And / or, during continuous casting, the tundish temperature is 1505-1530℃; And / or, electromagnetic stirring is carried out during continuous casting, with an electromagnetic stirring frequency of 6-8Hz, a current of 280-320A, and a continuous casting billet pulling speed of 0.8-1.2m / min.

[0033] In an optional embodiment, the continuously cast slab is heated in a heating furnace by hot charging. The furnace entry temperature of the continuously cast slab is ≥500℃, and the continuously cast slab is heated to 1200-1250℃. A reducing atmosphere is maintained in the heating furnace.

[0034] In step S3, the superheat, tundish temperature, and electromagnetic stirring parameters of continuous casting are limited to effectively suppress macroscopic segregation and central porosity, resulting in high-quality slabs with dense structure and uniform composition.

[0035] In an optional embodiment, the entry temperature of the finishing mill is 1000-1070℃, and the final rolling temperature is 860-910℃.

[0036] In step S4, the weak reducing atmosphere heating inhibits surface oxidation and decarburization, and the hot conveying and charging reduces energy consumption and temperature drop fluctuations. Combined with dual descaling and precise temperature control between roughing and finishing rolling, it ensures that surface quality, thickness accuracy and austenite recrystallization behavior are controllable.

[0037] In an optional embodiment, the precision-rolled steel strip is cooled by a laminar flow cooling device with edge slow cooling and then wound into a steel coil. And / or, the cooling adopts a first-half cooling mode; And / or, the winding temperature is 610-660℃; And / or, during the winding process, the tail heating control mode is used to control the tail heating length of the finished steel strip to be 19-21m and the heating range to be 19-21℃. And / or, the slow cooling time is greater than 72 hours.

[0038] In step S5, the cooling in the first half and the heating at the tail are coordinated to ensure the consistency of the microstructure along the entire length. Slow cooling at the edges alleviates overcooling at the edges. After winding, long-term slow cooling promotes hydrogen diffusion and stress release, ultimately resulting in a stable and uniform microstructure with fine grains, low inclusions, and high strength and toughness.

[0039] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0040] Example 1 This embodiment provides a method for preparing high-purity steel strip, such as... Figure 1 As shown, the specific steps include the following: 1.1 Desulfurization pretreatment was performed on blast furnace hot metal with an S content of 0.0301% to obtain pretreated hot metal with an S content of 0.0043%; 1.2. Smelting is carried out using pretreated molten iron and scrap steel as raw materials. The pretreated molten iron is 200.61t, the scrap steel is 15.620t, the molten iron ratio is 92.78%, and the scrap steel includes 17% heavy mixed scrap steel, 61% other ordinary scrap steel, and 22% self-produced ordinary scrap steel. 1.3. A top-and-bottom blowing converter was used for smelting. The bottom blowing adopted nitrogen-argon switching and the double-slag method was used for smelting. The converter smelting endpoint was 0.0779% C content and 0.0093% P content. The tapping temperature was 1632℃ and the tapping time was 6min. A double slag blocking method of "slag blocking ball + sliding plate" was used during tapping. 2230kg of silicon-manganese alloy was added to the molten steel in the ladle for alloying and 260kg of aluminum-iron alloy was added for deoxidation to obtain primary molten steel. 1.4. The initial molten steel was tested at the argon station and the temperature was 1518℃. A sample was taken and sent for testing. Then it entered the LF refining furnace. During the LF refining process, 1120kg of ferrochrome alloy and 56kg of ferrotitanium alloy were added for alloying. Ferromanganese alloy and ferrosilicon alloy were added for fine-tuning of the composition. Finally, 20kg of ferrocerium alloy was added to adjust the content of C, Si, Cr, Mn, Ti and Ce elements in the molten steel to the target composition. 1076kg of lime and 709kg of pre-melted slag were added to the molten steel for slag formation and desulfurization. The desulfurization was carried out until the S content was 0.0033%. The temperature was heated to 1554℃. The soft argon blowing time was 6 minutes before hoisting the ladle to obtain refined molten steel. 1.5. Refined molten steel is cast into slabs with a cross-section of 230×1250mm using a slab continuous casting machine. The tundish temperature is controlled at 1509-1514℃. High-carbon steel-specific protective slag is used in the crystallizer. Electromagnetic stirring is employed to prevent center segregation. The electromagnetic stirring frequency is 7Hz and the current is 300A. The continuous casting speed is 0.83-1.15m / min, achieving the production of 10 high-quality, defect-free slabs. The slabs are sampled and analyzed using a direct-reading spectrometer. The analysis standard is GB / T 223 "Methods for Chemical Analysis of Iron and Steel and Alloys". The chemical composition by mass fraction is: C: 0.2903%, Si: 0.136%, Mn: 0.692%, Cr: 0.2971%, Ti: 0.0272%, Ce: 0.00173%, P: 0.0103%, S: 0.0028%, with the remainder being Fe and other unavoidable impurities. 1.6. All slabs are hot-charged and fed into the furnace. The slab temperature is controlled at 540-630℃, the total heating time is 247-255 min, the soaking time is 26-28 min, and the tapping temperature is 1205-1217℃. A weak reducing atmosphere is maintained throughout the heating process for descaling. No cracks or defects are found after descaling. The slabs are then rough-rolled in three passes (E1R1 + E2R2) to form intermediate slabs. The transport track is insulated with heat-insulating covers to control the entry temperature of the finishing mill at 1043-1054℃. After descaling and "F1-F7 seven-pass finishing rolling", the finished steel strip is obtained. The final rolling temperature is controlled at 886-896℃. In order to ensure the stability of the finishing rolling, the descaling manifold used before finishing rolling is halved for specifications with a thickness of less than 3.5mm. The specifications of the finished steel strip are (3.00-3.75)mm×(1150-1160)mm. 1.7 The finished steel strip is cooled by a laminar flow cooling device using a "first-half cooling" mode. The first half of the laminar flow cooling participates in the coiling temperature control, while the second half does not, only undergoing air cooling. The coiling temperature measurement point is located in the middle of the laminar flow cooling process, and the coiling temperature is controlled between 628-649℃. Afterward, the strip is coiled by a coiler. Because the tail of the finished steel strip cools faster than the middle section after rolling, differences in microstructure and properties between the tail and middle sections can easily occur during cooling. To reduce this difference, a tail-end heating control mode is adopted, where the tail-end heating length of the finished steel strip is 20m, and the heating range is 20℃. This treatment method ensures that the tensile strength difference along the length direction is controlled within 60MPa. After coiling, the strip is quickly removed from the production line and placed in a slow cooling chamber for slow cooling for more than 72 hours to obtain a high-cleanliness steel strip for new energy trucks.

[0041] Example 2 This embodiment provides a method for preparing a high-purity steel strip, which differs from Embodiment 1 only in its chemical composition. The chemical composition of the steel strip in this embodiment is: C: 0.3109%, Si: 0.173%, Mn: 0.691%, Cr: 0.2897%, Ti: 0.0278%, Ce: 0.00154%, P: 0.0135%, S: 0.0023%, with the remainder being Fe and other unavoidable impurities.

[0042] Example 3 This embodiment provides a method for preparing a high-purity steel strip, which differs from Embodiment 1 only in its chemical composition. The chemical composition of the steel strip in this embodiment is: C: 0.2962%, Si: 0.160%, Mn: 0.690%, Cr: 0.2824%, Ti: 0.0296%, Ce: 0.00259%, P: 0.0123%, S: 0.0030%, with the remainder being Fe and other unavoidable impurities.

[0043] Comparative Example 1 This embodiment provides a method for preparing a high-purity steel strip, which differs from Embodiment 1 only in its chemical composition. The chemical composition of the steel strip in this embodiment is: C: 0.2911%, Si: 0.171%, Mn: 0.701%, Cr: 0.2914%, Ti: 0.0306%, P: 0.0113%, S: 0.0031%, with the remainder being Fe and other unavoidable impurities.

[0044] Comparative Example 2 This embodiment provides a method for preparing a high-purity steel strip, which differs from Embodiment 1 only in its chemical composition. The chemical composition of the steel strip in this embodiment is: C: 0.3021%, Si: 0.165%, Mn: 0.694%, Cr: 0.2719%, Ti: 0.0281%, Ce: 0.0051%, P: 0.0107%, S: 0.0027%, with the remainder being Fe and other unavoidable impurities.

[0045] Comparative Example 3 This embodiment provides a method for preparing a high-purity steel strip, which differs from Embodiment 1 only in its chemical composition. The chemical composition of the steel strip in this embodiment is: C: 0.2021%, Si: 0.177%, Mn: 0.716%, Cr: 0.971%, Ce: 0.0051%, P: 0.0107%, S: 0.0027%, with the remainder being Fe and other unavoidable impurities.

[0046] Comparative Example 4 This embodiment provides a method for preparing a high-purity steel strip. The only difference from Embodiment 1 is that the chemical composition is different and cerium-iron alloy is added after desulfurization and soft blowing in step 1.4. The chemical composition of the steel strip in this embodiment is: C: 0.3109%, Si: 0.191%, Mn: 0.734%, Cr: 0.296%, Ce: 0.0041%, P: 0.0112%, S: 0.0019%, with the remainder being Fe and other unavoidable impurities.

[0047] The products obtained in the above embodiments and comparative examples were sampled and analyzed. The specific methods are as follows, and the results are shown in Tables 1 and 2: Mechanical properties were tested using a universal testing machine, and the testing standard was GB / T 13239 "Metallic materials - low temperature tensile testing".

[0048] The metallographic structure was examined using an optical metallographic microscope, and the evaluation criteria were GB / T 13299 "Methods for evaluating the microstructure of steel", GB / T 34474.1 "Evaluation of banded structures in steel - Part 1: Standard rating chart method", GB / T 10561 "Determination of non-metallic inclusion content in steel - Standard rating chart microscopic examination method" and GB / T 224 "Determination of decarburized layer depth in steel".

[0049] in, Figure 2 The image shows the metallographic microstructure of the product prepared in Example 1 under 1000x magnification. The metallographic structure is "pearlite + ferrite" with good uniformity.

[0050] Table 1 Mechanical Properties of Products

[0051] Table 2 Metallographic Test Data of Products

[0052] According to Tables 1 and 2, comparing Example 1 and Comparative Example 1, it can be seen that the addition of cerium to the steel significantly increases the tensile strength and elongation after fracture, while decreasing the content of non-metallic inclusions and the banded structure level, and refining the grain size. Comparing Example 1 and Comparative Example 2, it can be seen that when the cerium content exceeds 0.0045%, the elongation after fracture decreases, and the content of non-metallic inclusions, the banded structure level, and the grain size level are not significantly improved. Comparing Example 1 and Comparative Example 3, it can be seen that when the cerium content meets the requirements of this product, but C and Cr do not meet the requirements, the yield strength and tensile strength cannot meet the requirements. Comparing Example 1 and Comparative Example 4, it can be seen that when the timing of adding cerium-iron alloy is incorrect, the non-metallic inclusion index deteriorates severely, causing a decrease in both strength and elongation after fracture.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-cleanliness steel strip, characterized in that, By weight percentage, it comprises 0.25-0.38% C, 0.10-0.20% Si, 0.60-0.80% Mn, 0.25-0.35% Cr, 0.020-0.035% Ti, 0.0015-0.0045% Ce, with the balance being Fe and unavoidable impurities.

2. The high-cleanliness steel strip according to claim 1, characterized in that, By weight percentage, it comprises C 0.290-0.311%, Si 0.135-0.175%, Mn 0.66-0.75%; Cr 0.28-0.33%, Ti 0.025-0.030%, Ce 0.0015-0.0026%, with the balance being Fe and unavoidable impurities.

3. A method for preparing a high-cleanliness steel strip as described in claim 1 or 2, characterized in that, Includes the following steps: Step S1: Molten iron and scrap steel are added to a converter for smelting to obtain primary molten steel; dephosphorization is carried out during the smelting process, and ferrosilicon and ferroaluminum are added to the ladle for alloying during the tapping process; Step S2: The primary molten steel is transferred to the LF refining furnace for refining to obtain refined molten steel; the composition of the molten steel is adjusted during the refining process, and ferrocerium alloy is finally added to meet the requirements of claim 1 or 2, followed by desulfurization and soft blowing. Step S3: The refined molten steel is continuously cast to obtain a continuously cast slab; Step S4: The slab is heated in a weakly reducing atmosphere, and then the heated slab is subjected to descaling, rough rolling, descaling and finish rolling in sequence to obtain a finish rolled steel strip; Step S5: After cooling the precision-rolled steel strip, it is coiled into a steel coil, and then the steel coil is slowly cooled to obtain the high-cleanliness steel strip.

4. The method for preparing high-purity steel strip according to claim 3, characterized in that, The S content in the molten iron is ≤0.005 wt%; And / or, the molten iron content in the converter is 85-95%; And / or, the dephosphorization method adopts the dual-slag method; And / or, the converter is a top-and-bottom blown converter.

5. The method for preparing high-purity steel strip according to claim 3, characterized in that, The smelting endpoint requirements are P≤0.010%, C content0.075-0.085%, tapping temperature1625-1665℃, and tapping time≥5min; And / or, during the tapping process, add 10-12.0 kg / t of silicon-manganese alloy and 150-400 kg / furnace of aluminum-iron alloy.

6. The method for preparing high-purity steel strip according to claim 3, characterized in that, In the refining step, lime and pre-melted slag are added to the molten steel to form slag and desulfurize it, so that S ≤ 0.005%; And / or, in the refining step, the temperature of the molten steel is adjusted to 1540-1570℃, and argon gas is gently blown for 5-10 minutes before hoisting the ladle to obtain the refined molten steel.

7. The method for preparing high-purity steel strip according to claim 3, characterized in that, The superheat during continuous casting shall not exceed 20°C; And / or, during continuous casting, the tundish temperature is 1505-1530℃; And / or, electromagnetic stirring is carried out during continuous casting, with an electromagnetic stirring frequency of 6-8Hz, a current of 280-320A, and a continuous casting billet pulling speed of 0.8-1.2m / min.

8. The method for preparing high-purity steel strip according to claim 3, characterized in that, The continuously cast slab is hot-charged into a heating furnace for heating. The furnace entry temperature of the continuously cast slab is ≥500℃. The continuously cast slab is heated to 1200-1250℃, and a reducing atmosphere is maintained in the heating furnace.

9. The method for preparing high-purity steel strip according to claim 3, characterized in that, The entry temperature of the finishing mill is 1000-1070℃, and the final rolling temperature is 860-910℃.

10. The method for preparing high-purity steel strip according to claim 3, characterized in that, The fine-rolled steel strip is cooled by a laminar flow cooling device with edge slow cooling and then wound into a steel coil. And / or, the cooling adopts a first-half cooling mode; And / or, the winding temperature is 610-660℃; And / or, during the winding process, the tail heating control mode is used to control the tail heating length of the finished steel strip to be 19-21m and the heating range to be 19-21℃. And / or, the slow cooling time is greater than 72 hours.

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