High-purity iron rolling method based on sulfur content

Abstract

The invention discloses a high-purity iron rolling method based on the sulfur content, and belongs to the technical field of ferrous metallurgy. The method comprises the following steps: S1, smelting to form a blank; s2, iron blank pretreatment; s3, heating the iron blank; s4, rolling the iron blank; s5, cooling after rolling; the total mass of impurity elements except the Fe element in the high-purity iron billet is 300 ppm or below, S is smaller than or equal to 45 ppm, S + Mn is smaller than or equal to 270 ppm, and other impurities do not exceed 30 ppm; when S is less than or equal to 5ppm, Mn / S is not limited; when S is more than 5ppm and less than or equal to 15ppm, Mn / S is more than or equal to 2; when S is more than 15ppm and less than or equal to 25ppm, Mn / S is more than or equal to 3; when S is more than 25ppm and less than or equal to 35ppm, Mn / S is more than or equal to 4; when S is more than 35 ppm and less than or equal to 45 ppm, Mn / S is more than or equal to 5; mn / S is the mass ratio of manganese to sulfur in the high-purity iron. According to the method, cracks are effectively prevented from being generated in the high-purity iron rolling process, the quality and production efficiency of pure iron products are improved, and the strict quality requirement of the high-end field for high-purity iron materials is met.

Description

TECHNICAL FIELD

[0001] The application relates to a high-purity iron rolling method based on sulfur content and belongs to the technical field of steel metallurgy. BACKGROUND

[0002] High-purity iron materials are widely used in many fields such as high-end electromagnetic fields and semiconductor industries due to their excellent physical and chemical properties. For example, they are used to manufacture pole shoes of traveling wave tubes, magnetrons, klystrons, electromagnetic lenses, magnetic cores of high-sensitivity relays and magnetic sensors, and key components in cavities of etching machines and chemical vapor deposition (CVD) equipment for producing chips, such as substrates of electrostatic chuck (ESC) for fixing wafers. However, high-purity iron has poor processability and is prone to surface, edge or shallow surface cracks during rolling, which greatly reduces the mechanical properties and service life of high-purity iron products.

[0003] It is found through research on steel that sulfur in steel is one of the key factors leading to rolling cracks, and Mn in steel can inhibit the occurrence of cracking of steel during rolling. The reasons are as follows: compared with other metal elements, sulfur has a faster diffusion rate in iron, so it is prone to move and segregate during heating. During high-temperature process, sulfur atoms quickly segregate to the grain boundary to form FeS. FeS has a low melting point and will melt at the grain boundary to form a liquid metal film, weakening the bonding force of the grain boundary. Under the action of rolling stress, the decreased bonding force of the grain boundary leads to its inability to deform together with the grain, thereby causing the grain boundary to crack. However, the Mn element in steel can form MnS compounds with S elements to control the movement of S atoms and prevent them from diffusing freely. In this way, the grain boundary of Fe grain cannot enrich enough S elements, so that the grain boundary cannot produce FeS, and thus the grain boundary will not soften. Therefore, the Mn element in steel can prevent the grain boundary from cracking or "red brittleness". Usually, the reference amount of Mn / S ratio is introduced to control the composition and adjust the reasonable Mn / S ratio, so that the steel does not crack during rolling.

[0004] However, for high-purity iron with a total amount of impurity elements below 300 ppm, so far, there has been no systematic study on the influence of sulfur content on the rolling cracking of high-purity iron, and no reasonable Mn / S ratio for preventing rolling cracks for different sulfur contents. SUMMARY

[0005] The application proposes a high-purity iron rolling method based on sulfur content to effectively prevent the occurrence of cracks during the rolling process of high-purity iron, improve the quality and production efficiency of high-purity iron products, and meet the strict quality requirements of high-purity iron materials in high-end fields by accurately controlling the sulfur content in high-purity iron and determining a reasonable manganese-sulfur ratio for different sulfur contents.

[0006] The application provides a high-purity iron rolling method based on sulfur content, comprising the following steps:

[0007] Step S1, smelting into a billet: adopting a three-step method of converter primary smelting-LF furnace refining-RH refining, and then casting into a billet, to obtain a high-purity iron billet;

[0008] Step S2, iron billet pretreatment: adopting a method of mechanical polishing and chemical cleaning to clean the surface of the iron billet;

[0009] Step S3, iron billet heating: placing the pretreated iron billet into a heating furnace, and adopting a three-stage heating system to heat;

[0010] Step S4, iron billet rolling: adopting a high-precision rolling mill to roll, in the rough rolling stage, the total reduction rate is controlled to be 40-50%, and in the finish rolling stage, the total reduction rate is controlled to be 20-30%;

[0011] Step S5, post-rolling cooling: adopting a staged and gradual cooling method to cool the high-purity iron product to room temperature after rolling;

[0012] In step S1, the total mass of impurity elements in the high-purity iron billet except Fe element is below 300 ppm, wherein S≤45 ppm, S+Mn≤270 ppm, and other impurities are not more than 30 ppm; when S≤5 ppm, Mn / S is not limited; when 5 ppm

[0013] Optionally, step S2 comprises the following sub-steps:

[0014] Step S2.1, soaking the high-purity iron billet into a hydrochloric acid pool, soaking for 15-30 minutes, and taking out the iron billet to clean;

[0015] Step S2.2, using a high-precision grinding machine to clean the insoluble substances on the surface of the iron billet, and then using 800# to 2000# sandpaper to polish the surface of the iron billet to be bright, so that the surface roughness of the iron billet reaches the Ra0.8 μm level;

[0016] Step S2.3, soaking the iron billet in a weak alkaline cleaning liquid at 50-60 ℃ for 15-20 minutes, and after cleaning the iron billet, washing the iron billet with deionized water and drying.

[0017] Optionally, in step S2.1, the mass concentration of hydrochloric acid in the hydrochloric acid pool is 5-20%.

[0018] Optionally, in step S2.3, the pH value of the weakly alkaline cleaning solution is 8 to 10.

[0019] Optionally, step S3 includes the following sub-steps:

[0020] Step S3.1: Increase the temperature from room temperature to 800-900℃ at a heating rate of 15-20℃ / min and hold the temperature to make the internal temperature of the iron billet initially uniform.

[0021] Step S3.2: Increase the temperature to 1100-1200℃ at a heating rate of 10-15℃ / min and hold at that temperature to further promote the temperature uniformity of the iron billet;

[0022] Step S3.3: Increase the temperature to the rolling temperature of 1250-1350℃ at a heating rate of 5-8℃ / min, and maintain the temperature at this temperature to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0023] Optionally, in step S3.1, the holding time is calculated based on a thickness of 1 to 2 minutes per millimeter of the billet; in step S3.2, the holding time is calculated based on a thickness of 0.5 to 1 minute per millimeter of the billet; and in step S3.3, the holding time is calculated based on a thickness of 0.2 to 0.5 minutes per millimeter of the billet.

[0024] Optionally, in step S4, during the rough rolling stage, the reduction rate for each pass is controlled at 10-15%, and the rolling speed is controlled at 1-2 m / s.

[0025] Optionally, in step S4, during the finishing rolling stage, the reduction rate for each pass is controlled at 3-5%, and the rolling speed is controlled at 0.5-1 m / s.

[0026] Optionally, step S5 includes the following sub-steps:

[0027] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 800-900℃.

[0028] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 300-400℃;

[0029] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0030] Optionally, in step S5.1, the cooling air velocity is controlled at 4 m / s to reduce the product temperature to 850°C.

[0031] Compared with the prior art, the advantages of the present invention are as follows:

[0032] (1) For high-purity iron with a purity of 99.97% or higher, the present invention limits the S content to no more than 45 ppm, and makes different limits on Mn / S according to the specific sulfur content, which effectively changes the morphology and distribution of sulfides, improves the strength of grain boundaries, fundamentally reduces the root cause of cracks, and greatly reduces the occurrence rate of rolling cracks in pure iron.

[0033] (2) The comprehensive billet pretreatment and optimized heating process of this invention ensure the surface quality and internal structure uniformity of the billet, improve the plasticity of the billet, provide a good foundation for the subsequent rolling process, and further reduce the risk of inclusions, holes or cracks.

[0034] (3) The high-precision rolling process control and progressive cooling method of this invention make the pure iron products uniformly stressed and have stable temperature changes during rolling and cooling, effectively avoiding cracks caused by stress concentration and uneven temperature, and improving product quality and production efficiency.

[0035] (4) The method of determining Mn / S based on sulfur content in the rolling of high-purity iron in this invention not only finds a suitable range of Mn / S, but also provides a good example for accurately controlling the rolling cracks of high-purity iron. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a scanning electron microscope backscattered image of the surface morphology of the rolled product in Example 1;

[0038] Figure 2 The image shows a secondary electron image of the surface morphology of the rolled product from Example 2 using scanning electron microscopy.

[0039] Figure 3 This is a scanning electron microscope backscattered image of the surface morphology of the rolled product in Example 3;

[0040] Figure 4 This is a secondary electron image of the surface morphology of the rolled product in Example 4, obtained by scanning electron microscopy.

[0041] Figure 5 Metallographic micrographs of the surface morphology of the longitudinal section of the rolled product in Example 5;

[0042] Figure 6 The image shows a secondary electron image of the cross-sectional surface morphology of the rolled product in Comparative Example 1.

[0043] Figure 7 The image shows a secondary electron image of the cross-sectional surface morphology of the rolled product in Comparative Example 2.

[0044] Figure 8 The image shows a scanning electron microscope backscattered image of the cross-sectional surface morphology of the rolled product in Comparative Example 3.

[0045] Figure 9 The image shows a secondary electron image of the cross-sectional surface morphology of the rolled product in Comparative Example 4.

[0046] Figure 10 The image shows a scanning electron microscope backscattered image of the longitudinal section surface morphology of the rolled product in Comparative Example 5.

[0047] Figure 11 Metallographic micrographs of the longitudinal section surface morphology of the rolled product for Comparative Example 6. Detailed Implementation

[0048] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be described in detail below with reference to specific embodiments. It should be understood that the embodiments described in this specification are merely illustrative and not intended to limit the scope of the invention.

[0049] This invention provides a method for rolling high-purity iron based on sulfur content, comprising the following steps:

[0050] Step S1, Smelting into billets: The smelting process is carried out using a three-step method of primary smelting in a converter, refining in an LF furnace, and refining in an RH furnace. The billets are then cast into billets to obtain high-purity iron billets.

[0051] Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning.

[0052] Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system;

[0053] Step S4, billet rolling: Rolling is carried out using a high-precision rolling mill. In the roughing stage, the total reduction rate is controlled at 40-50%, and in the finishing stage, the total reduction rate is controlled at 20-30%.

[0054] Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method.

[0055] Specifically, in step S1, a three-step smelting process—converter primary refining, LF furnace refining, and RH refining—is adopted. This process can precisely and effectively control the content of each element in high-purity iron, ensuring that the total mass of impurity elements other than Fe is below 300 ppm, with S ≤ 45 ppm, S + Mn ≤ 270 ppm, and other impurities not exceeding 30 ppm, such as C ≤ 10 ppm, Si ≤ 8 ppm, and P ≤ 10 ppm. Furthermore, by finely adjusting the process parameters, high-purity iron materials with precisely controlled manganese-sulfur ratios can be obtained. For example, when S ≤ 5 ppm, Mn / S is not limited; when 5 ppm < S ≤ 15 ppm, Mn / S ≥ 2; when 15 ppm < S ≤ 25 ppm, Mn / S ≥ 3; when 25 ppm < S ≤ 35 ppm, Mn / S ≥ 4; and when 35 ppm < S ≤ 45 ppm, Mn / S ≥ 5. Mn / S is the mass ratio of manganese to sulfur in high-purity iron.

[0056] It should be noted that sulfur (S) has very low solubility in iron and readily combines with Fe to form FeS. This FeS forms a thin film distributed along the austenite grain boundaries. Since FeS has a melting point of only 1190℃, during hot working of steel, such as continuous casting, forging, and rolling, this liquid film on the grain boundaries significantly weakens the grain boundary bonding force, leading to a sharp decrease in material plasticity and cracking. This phenomenon is called "hot brittleness" or "red brittleness." Because manganese has a much stronger affinity for sulfur than iron, it preferentially reacts with sulfur to form manganese sulfide (MnS). MnS has a melting point as high as 1610℃, far exceeding FeS's 1190℃ and also significantly higher than the hot working temperature of steel, which is typically below 1350℃. Therefore, MnS remains solid during hot working, dispersing in granular or rod-like forms rather than forming a continuous liquid film. Theoretically, based on atomic weight calculations, the minimum Mn / S mass ratio required to completely fix S is Mn / S = 55 / 32 ≈ 1.7. However, due to kinetic factors, segregation effects, interference from other elements, etc., in actual steel production, this Mn / S ratio of 1.7 is far from sufficient. Based on extensive practical experience and theoretical research, the minimum requirement for Mn / S in steel is above 15, and the higher the better, when conditions permit.

[0057] However, for the high-purity iron of the present invention with a purity of 99.97% or higher, due to the strict limitation on the impurity content, the total amount of S and Mn is required to be below 270ppm. Therefore, in order to eliminate the tendency of sulfides to become hot brittle, one fundamental solution is to limit the absolute content of S in high-purity iron. Secondly, considering the current production technology and production cost, the absolute content of S cannot always be within the ideal range. Therefore, under the premise that S+Mn≤270ppm, different minimum Mn / S ratios need to be determined for relatively high and different S contents. After numerous experiments, this invention concludes that the sulfur (S) content in high-purity iron with a purity of 99.97% or higher should not exceed 45 ppm. Specifically, when S ≤ 5 ppm, since the absolute sulfur content is already low enough, there is no need to use manganese (Mn) to eliminate the harmful effects of S, meaning there is no need to limit the Mn / S ratio. When 5 ppm < S ≤ 15 ppm, although the S content is low, it is still necessary to use manganese (Mn) to eliminate some of the harmful effects of S, meaning the Mn / S ratio needs to be limited to ≥ 2. Similarly, when 15 ppm < S ≤ 25 ppm, the Mn / S ratio needs to be limited to ≥ 3; when 25 ppm < S ≤ 35 ppm, the Mn / S ratio needs to be limited to ≥ 4; and when 35 ppm < S ≤ 45 ppm, the Mn / S ratio needs to be limited to ≥ 5. In other words, as the S content increases, the lower limit of the Mn / S ratio needs to be increased accordingly. However, it is clear that the Mn / S ratio of this invention is far lower than the minimum requirement for Mn / S in steel.

[0058] Specifically, in step S2, a combination of mechanical grinding and chemical cleaning is used to meticulously remove oxide scale, impurities, and oil stains from the surface of the iron billet, substances that can easily alter the composition of high-purity iron. Step S2 includes the following sub-steps:

[0059] Step S2.1: Immerse the high-purity iron billet in a hydrochloric acid bath for 15-30 minutes, then remove the billet and rinse it clean.

[0060] Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level.

[0061] Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 50-60℃ for 15-20 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

[0062] In step S2.1, a 5%–20% hydrochloric acid aqueous solution is used, which quickly dissolves all the loose oxide scale on the surface of the iron billet after acid leaching. The purpose of step S2.2 is to eliminate any microcracks and defects that may exist on the surface of the iron billet. In step S2.3, the weakly alkaline cleaning solution is a metal cleaning solution with a pH of 8–10.

[0063] Specifically, step S3 includes the following sub-steps:

[0064] Step S3.1: Increase the temperature from room temperature to 800-900℃ at a heating rate of 15-20℃ / min and hold the temperature to make the internal temperature of the iron billet initially uniform.

[0065] Step S3.2: Increase the temperature to 1100-1200℃ at a heating rate of 10-15℃ / min and hold at that temperature to further promote the temperature uniformity of the iron billet;

[0066] Step S3.3: Increase the temperature to the rolling temperature of 1250-1350℃ at a heating rate of 5-8℃ / min, and maintain the temperature at this temperature to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0067] In step S3.1, the holding time of the billet is generally calculated based on 1 to 2 minutes per millimeter of billet thickness; in step S3.2, the holding time of the billet is generally calculated based on 0.5 to 1 minute per millimeter of billet thickness; and in step S3.3, the holding time of the billet is generally calculated based on 0.2 to 0.5 minutes per millimeter of billet thickness. Uniforming the temperature of the billet can improve its plasticity and reduce internal stress and cracking caused by uneven temperature.

[0068] Specifically, in step S4, a high-precision rolling mill is used for rolling. In the roughing stage, the total reduction rate is controlled at 40-50%, the reduction rate per pass is controlled at 10-15%, and the rolling speed is controlled at 1-2 m / s. In the finishing stage, the total reduction rate is controlled at 20-30%, the reduction rate per pass is controlled at 3-5%, and the rolling speed is controlled at 0.5-1 m / s.

[0069] Specifically, step S5 includes the following sub-steps:

[0070] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 800-900℃.

[0071] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 300-400℃;

[0072] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0073] During the cooling process, multiple temperature sensors installed on the cooling equipment monitor the product's temperature changes in real time. Based on the temperature changes, the flow rate and velocity of the cooling medium are automatically adjusted to ensure a uniform and stable cooling process, avoiding thermal stress concentration and cracking caused by excessively fast or uneven cooling.

[0074] In summary, firstly, by precisely controlling the sulfur content and manganese-sulfur ratio, this invention effectively alters the morphology and distribution of sulfides, improves grain boundary strength, fundamentally reduces the root causes of crack formation, and significantly lowers the incidence of rolling cracks in pure iron. Secondly, the comprehensive billet pretreatment and optimized heating process of this invention ensure the surface quality and internal structural uniformity of the billet, improves its plasticity, provides a good foundation for subsequent rolling processes, and further reduces the risk of inclusions, voids, or cracks. Thirdly, the high-precision rolling process control and progressive cooling method of this invention ensure that pure iron products are subjected to uniform stress and stable temperature changes during rolling and cooling, effectively avoiding crack formation caused by stress concentration and uneven temperature, and improving product quality and production efficiency.

[0075] Example 1

[0076] A method for rolling high-purity iron based on sulfur content includes the following steps:

[0077] Step S1, Smelting and billet formation: The smelting process adopts a three-step method of converter primary smelting - LF furnace refining - RH refining, and then the billet is cast into a billet with a size of 200mm×200mm×600mm.

[0078] The total mass of impurity elements other than Fe in high-purity iron is less than 300 ppm, of which S is 4 ppm and Mn is 50 ppm.

[0079] Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning.

[0080] Step S2.1: Immerse the high-purity iron billet in a 5% hydrochloric acid solution for 15 minutes, then remove the billet and rinse it clean.

[0081] Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level.

[0082] Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 50°C for 15 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

[0083] Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system;

[0084] Step S3.1: Increase the temperature from room temperature to 800℃ at a heating rate of 15℃ / min and hold for 200 minutes to make the internal temperature of the iron billet initially uniform.

[0085] Step S3.2: Increase the temperature to 1100℃ at a heating rate of 10℃ / min and hold for 100 minutes to further promote the temperature uniformity of the iron billet;

[0086] Step S3.3: Increase the temperature to 1250℃ at a heating rate of 5℃ / min, and hold at this temperature for 40 minutes to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0087] Step S4, Billet Rolling: Rolling is performed using a rolling mill. In the roughing stage, the total reduction rate is controlled at 40%, with a reduction rate of 10-15% per pass, and the rolling speed is controlled at 1-2 m / s. In the finishing stage, the total reduction rate is controlled at 20%, with a reduction rate of 3-5% per pass, and the rolling speed is controlled at 0.5-1 m / s.

[0088] Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method.

[0089] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 800℃.

[0090] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 300℃;

[0091] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0092] Detailed magnification of the rolled steel plate surface revealed no cracks. X-ray flaw detector analysis of the high-purity iron plate also revealed no internal defects. Twenty 10mm x 10mm x 10mm samples were cut from different parts of the iron plate using wire cutting. After grinding and polishing, the microstructure was examined under a scanning electron microscope for microcracks and inclusions. Figure 1 This is a scanning electron microscope backscattered image of the surface morphology of the rolled product in Example 1, from... Figure 1 It is evident that the ferrite grains in the high-purity iron plate are uniformly distributed, with no intergranular cracks or inclusions found.

[0093] Example 2

[0094] A method for rolling high-purity iron based on sulfur content includes the following steps:

[0095] Step S1, Smelting and billet formation: The smelting process adopts a three-step method of converter primary smelting - LF furnace refining - RH refining, and then the billet is cast into a billet with a size of 200mm×200mm×600mm.

[0096] The total mass of impurity elements other than Fe in high-purity iron is below 300 ppm, of which S is 15 ppm, Mn is 30 ppm, and Mn / S = 2.

[0097] Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning.

[0098] Step S2.1: Immerse the high-purity iron billet in a 10% hydrochloric acid solution for 30 minutes, then remove the billet and rinse it clean.

[0099] Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level.

[0100] Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 60°C for 20 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

[0101] Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system;

[0102] Step S3.1: Increase the temperature from room temperature to 900℃ at a heating rate of 20℃ / min and hold for 400 minutes to make the internal temperature of the iron billet initially uniform.

[0103] Step S3.2: Increase the temperature to 1200℃ at a heating rate of 15℃ / min and hold for 200 minutes to further promote the temperature uniformity of the iron billet;

[0104] Step S3.3: Increase the temperature to 1350℃ at a heating rate of 8℃ / min, and hold at this temperature for 100 minutes to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0105] Step S4, Billet Rolling: Rolling is performed using a rolling mill. In the roughing stage, the total reduction rate is controlled at 50%, with a reduction rate of 10-15% per pass, and the rolling speed is controlled at 1-2 m / s. In the finishing stage, the total reduction rate is controlled at 30%, with a reduction rate of 3-5% per pass, and the rolling speed is controlled at 0.5-1 m / s.

[0106] Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method.

[0107] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 900℃;

[0108] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 400℃;

[0109] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0110] Detailed magnification of the rolled steel plate surface revealed no cracks. X-ray flaw detector analysis of the high-purity iron plate also revealed no internal defects. Twenty 10mm x 10mm x 10mm samples were cut from different parts of the iron plate using wire cutting. After grinding and polishing, the microstructure was examined under a scanning electron microscope for microcracks and inclusions. Figure 2 These are secondary electron images of the surface morphology of the rolled product from Example 2, obtained using scanning electron microscopy. Figure 2 It is evident that the ferrite grains in the high-purity iron plate are uniformly distributed, with no intergranular cracks or inclusions found.

[0111] Example 3

[0112] A method for rolling high-purity iron based on sulfur content includes the following steps:

[0113] Step S1, Smelting and billet formation: The smelting process adopts a three-step method of converter primary smelting - LF furnace refining - RH refining, and then the billet is cast into a billet with a size of 200mm×200mm×600mm.

[0114] The total mass of impurity elements other than Fe in high-purity iron is below 300 ppm, of which S is 25 ppm, Mn is 75 ppm, and Mn / S = 3.

[0115] Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning.

[0116] Step S2.1: Immerse the high-purity iron billet in a hydrochloric acid pool with a mass concentration of 15% for 23 minutes, then remove the billet and rinse it clean.

[0117] Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level.

[0118] Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 55°C for 18 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

[0119] Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system;

[0120] Step S3.1: Increase the temperature from room temperature to 850℃ at a heating rate of 18℃ / min and hold for 300 minutes to make the internal temperature of the iron billet initially uniform.

[0121] Step S3.2: Increase the temperature to 1100℃ at a heating rate of 13℃ / min and hold for 150 minutes to further promote the temperature uniformity of the iron billet;

[0122] Step S3.3: Increase the temperature to 1300℃ at a heating rate of 7℃ / min, and hold at this temperature for 70 minutes to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0123] Step S4, Billet Rolling: Rolling is performed using a rolling mill. In the roughing stage, the total reduction rate is controlled at 45%, with a reduction rate of 10-15% per pass, and the rolling speed is controlled at 1-2 m / s. In the finishing stage, the total reduction rate is controlled at 25%, with a reduction rate of 3-5% per pass, and the rolling speed is controlled at 0.5-1 m / s.

[0124] Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method.

[0125] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 850℃;

[0126] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 350℃;

[0127] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0128] Detailed magnification of the rolled steel plate surface revealed no cracks. X-ray flaw detector analysis of the high-purity iron plate also revealed no internal defects. Twenty 10mm x 10mm x 10mm samples were cut from different parts of the iron plate using wire cutting. After grinding and polishing, the microstructure was examined under a scanning electron microscope for microcracks and inclusions. Figure 3 This is a scanning electron microscope backscattered image of the surface morphology of the rolled product in Example 3, from... Figure 3 It is evident that the ferrite grains in the high-purity iron plate are uniformly distributed, with no intergranular cracks or inclusions found.

[0129] Example 4

[0130] A method for rolling high-purity iron based on sulfur content includes the following steps:

[0131] Step S1, Smelting and billet formation: The smelting process adopts a three-step method of converter primary smelting - LF furnace refining - RH refining, and then the billet is cast into a billet with a size of 200mm×200mm×600mm.

[0132] The total mass of impurity elements other than Fe in high-purity iron is below 300 ppm, of which S is 35 ppm, Mn is 140 ppm, and Mn / S = 4.

[0133] Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning.

[0134] Step S2.1: Immerse the high-purity iron billet in a hydrochloric acid pool with a mass concentration of 20% for 20 minutes, then remove the billet and rinse it clean.

[0135] Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level.

[0136] Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 58°C for 16 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

[0137] Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system;

[0138] Step S3.1: Increase the temperature from room temperature to 820℃ at a heating rate of 16℃ / min and hold for 280 minutes to make the internal temperature of the iron billet initially uniform.

[0139] Step S3.2: Increase the temperature to 1180℃ at a heating rate of 12℃ / min and hold for 110 minutes to further promote the temperature uniformity of the iron billet;

[0140] Step S3.3: Increase the temperature to 1280℃ at a heating rate of 6℃ / min, and hold at this temperature for 80 minutes to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0141] Step S4, Billet Rolling: Rolling is performed using a rolling mill. In the roughing stage, the total reduction rate is controlled at 42%, with a reduction rate of 10-15% per pass, and the rolling speed is controlled at 1-2 m / s. In the finishing stage, the total reduction rate is controlled at 22%, with a reduction rate of 3-5% per pass, and the rolling speed is controlled at 0.5-1 m / s.

[0142] Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method.

[0143] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 820℃;

[0144] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 320℃;

[0145] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0146] Detailed magnification of the rolled steel plate surface revealed no cracks. X-ray flaw detector analysis of the high-purity iron plate also revealed no internal defects. Twenty 10mm x 10mm x 10mm samples were cut from different parts of the iron plate using wire cutting. After grinding and polishing, the microstructure was examined under a scanning electron microscope for microcracks and inclusions. Figure 4 The image shows a secondary electron image of the surface morphology of the rolled product from Example 4, obtained via scanning electron microscopy. Figure 4 It is evident that the ferrite grains in the high-purity iron plate are uniformly distributed, with no intergranular cracks or inclusions found.

[0147] Example 5

[0148] A method for rolling high-purity iron based on sulfur content includes the following steps:

[0149] Step S1, Smelting and billet formation: The smelting process adopts a three-step method of converter primary smelting - LF furnace refining - RH refining, and then the billet is cast into a billet with a size of 200mm×200mm×600mm.

[0150] The total mass of impurity elements other than Fe in high-purity iron is below 300 ppm, of which S is 45 ppm, Mn is 225 ppm, and Mn / S = 5.

[0151] Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning.

[0152] Step S2.1: Immerse the high-purity iron billet in a hydrochloric acid pool with a mass concentration of 15% for 20 minutes, then remove the billet and rinse it clean.

[0153] Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level.

[0154] Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 52°C for 18 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

[0155] Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system;

[0156] Step S3.1: Increase the temperature from room temperature to 880℃ at a heating rate of 19℃ / min and hold for 380 minutes to make the internal temperature of the iron billet initially uniform.

[0157] Step S3.2: Increase the temperature to 1120℃ at a heating rate of 14℃ / min and hold for 190 minutes to further promote the temperature uniformity of the iron billet;

[0158] Step S3.3: Increase the temperature to 1330℃ at a heating rate of 7℃ / min, and hold at this temperature for 50 minutes to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

[0159] Step S4, Billet Rolling: Rolling is performed using a rolling mill. In the roughing stage, the total reduction rate is controlled at 48%, with a reduction rate of 10-15% per pass, and the rolling speed is controlled at 1-2 m / s. In the finishing stage, the total reduction rate is controlled at 28%, with a reduction rate of 3-5% per pass, and the rolling speed is controlled at 0.5-1 m / s.

[0160] Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method.

[0161] Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 880℃;

[0162] Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 380℃;

[0163] Step S5.3: Allow the product to cool naturally in the air to room temperature.

[0164] Detailed magnification of the rolled steel plate surface revealed no cracks. X-ray flaw detector testing of the high-purity iron plate also revealed no internal defects. Twenty 10mm × 10mm × 10mm samples were cut from different parts of the iron plate using wire cutting. After grinding and polishing, the microstructure was examined under a metallographic microscope for microcracks and inclusions. Figure 5 Metallographic micrographs of the longitudinal section surface morphology of the rolled product in Example 5 are shown below. Figure 5 It is evident that the high-purity iron plate has a uniform structure, no cracks, and no inclusions were found.

[0165] Comparative Example 1

[0166] This comparative example is identical to Example 1 except that the S content in the high-purity iron is 10 ppm, the Mn content is 19 ppm, and the Mn / S ratio is 1.9.

[0167] Upon close magnification of the rolled steel plate surface, obvious cracks were found. Using wire cutting, a cracked piece of iron was cut off and transversely along the rolled plate. After fine grinding and polishing of the cut surface, the crack was observed again. Figure 6 To obtain the secondary electron images of the cross-sectional surface morphology of the rolled product in Comparative Example 1, from... Figure 6 It is evident that the crack initiation point was at the triangular grain boundary, and the crack extended along the grain boundary into the interior of the rolled plate, remaining only on the shallow surface of the plate. Therefore, when the S content in high-purity iron is 10 ppm and the Mn / S ratio is less than 2, cracks will appear during the rolling of high-purity iron.

[0168] Comparative Example 2

[0169] This comparative example is identical to Example 1 except that the S content in the high-purity iron is 20 ppm, the Mn content is 55 ppm, and the Mn / S ratio is 2.8.

[0170] Upon close magnification of the rolled steel plate surface, occasional cracks were observed. Using wire cutting, a cracked piece of iron was cut off, transversely along the rolled plate, and the cross-section was finely ground and polished before further observation of the crack. Figure 7 To obtain the secondary electron images of the cross-sectional surface morphology of the rolled product in Comparative Example 2, from... Figure 7 As can be seen, fine cracks were found to extend through the transversely cut cross-section. Therefore, when the S content in high-purity iron is 20 ppm and the Mn / S ratio is less than 3, cracks will appear in the rolled high-purity iron.

[0171] Comparative Example 3

[0172] This comparative example is identical to Example 1 except that the S content in the high-purity iron is 30 ppm, the Mn content is 115 ppm, and the Mn / S ratio is 3.8.

[0173] Upon close magnification of the rolled steel plate surface, obvious cracks were found. Using wire cutting, a cracked piece of iron was cut off and transversely along the rolled plate. After fine grinding and polishing of the cut surface, the crack was observed again. Figure 8 To show the scanning electron microscope backscattered image of the cross-sectional surface morphology of the rolled product in Comparative Example 3, from... Figure 8 As can be seen, the cracks extend from the corners into the interior of the rolled plate, but do not penetrate through, resulting in numerous voids. Therefore, when the S content in high-purity iron is 30 ppm and the Mn / S ratio is less than 4, cracks will appear during the rolling of high-purity iron.

[0174] Comparative Example 4

[0175] This comparative example is identical to Example 1 except that the S content in the high-purity iron is 40 ppm, the Mn content is 187 ppm, and the Mn / S ratio is 4.7.

[0176] Upon close magnification of the rolled steel plate surface, obvious cracks were found. Using wire cutting, a cracked piece of iron was cut off and transversely along the rolled plate. After fine grinding and polishing of the cut surface, the crack was observed again. Figure 9 To obtain the secondary electron images of the cross-sectional surface morphology of the rolled product in Comparative Example 4, from... Figure 9 As can be seen, the crack extends along the corner into the interior of the rolled plate, but does not penetrate through. Therefore, when the S content in high-purity iron is 40 ppm and the Mn / S ratio is less than 5, cracks will appear when rolling high-purity iron.

[0177] Comparative Example 5

[0178] In this comparative example, step S2 only roughly removes the oxide scale, impurities, and oil stains from the surface of the iron billet, and the surface roughness of the iron billet does not reach the Ra0.8μm level. Step S3 adopts a constant-rate uninterrupted heating system, raising the temperature from room temperature to 1300℃ at a heating rate of 20℃ / min and holding it at that temperature for 200 minutes. Otherwise, the remaining steps are exactly the same as in Example 1.

[0179] Upon close magnification and examination of the rolled steel plate surface, no obvious cracks were found. Using wire cutting, a cracked piece of iron was cut off, and different cross-sections were cut along the rolling direction (longitudinal direction) of the plate. The cross-sections were then finely ground and polished before observation. Figure 10 For comparative example 5, the longitudinal section surface morphology of the rolled product is obtained from scanning electron microscopy backscattered images. Figure 10 As can be seen, the billet is densely packed with inclusions and pores, including both circular pores and irregularly shaped inclusions. Therefore, when the billet pretreatment and three-stage heating process of this invention are not used, the internal structure of the rolled product is poor.

[0180] Comparative Example 6

[0181] In this comparative example, step S4 is performed using a conventional rolling mill, and the reduction rate and rolling speed at each rolling stage cannot be precisely controlled. Step S5 involves water cooling to room temperature. Otherwise, the remaining steps are exactly the same as in Example 1.

[0182] Upon close magnification of the rolled steel plate surface, occasional inconspicuous cracks were observed. Using wire cutting, a cracked piece of iron was cut off, and different cross-sections were cut along the rolling direction (longitudinal direction) of the plate. After fine grinding and polishing of the cross-sections, the cracks were observed. Figure 11 Metallographic micrographs of the longitudinal section surface morphology of the rolled product in Comparative Example 6 were obtained from... Figure 11 As can be seen, the microcracks extend in a dendritic pattern until they disappear. Therefore, microcracks will appear when rolling high-purity iron without employing the high-precision rolling process control and progressive cooling method of this invention.

[0183] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this invention, and these modifications or substitutions should all be covered within the scope of protection of this invention. Therefore, the scope of protection of this invention should be determined by the scope defined in the claims.

Claims

1. A method for rolling high-purity iron based on sulfur content, characterized in that, The method includes the following steps: Step S1, Smelting into billets: The smelting process is carried out using a three-step method of primary smelting in a converter, refining in an LF furnace, and refining in an RH furnace. The billets are then cast into billets to obtain high-purity iron billets. Step S2, Iron billet pretreatment: The iron billet is cleaned by a combination of mechanical grinding and chemical cleaning. Step S3, Iron billet heating: The pretreated iron billet is placed in the heating furnace and heated using a three-stage heating system; Step S4, billet rolling: Rolling is carried out using a high-precision rolling mill. In the roughing stage, the total reduction rate is controlled at 40-50%, and in the finishing stage, the total reduction rate is controlled at 20-30%. Step S5, Cooling after rolling: The high-purity iron product after rolling is cooled to room temperature in stages using a progressive cooling method. In step S1, the total mass of impurity elements other than Fe in the high-purity iron billet is below 300 ppm, wherein S ≤ 45 ppm, S + Mn ≤ 270 ppm, and other impurities do not exceed 30 ppm; when S ≤ 5 ppm, Mn / S is not limited; when 5 ppm < S ≤ 15 ppm, Mn / S ≥ 2; when 15 ppm < S ≤ 25 ppm, Mn / S ≥ 3; when 25 ppm < S ≤ 35 ppm, Mn / S ≥ 4; when 35 ppm < S ≤ 45 ppm, Mn / S ≥ 5; Mn / S is the mass ratio of manganese and sulfur in high-purity iron.

2. The method according to claim 1, characterized in that, Step S2 includes the following sub-steps: Step S2.1: Immerse the high-purity iron billet in a hydrochloric acid bath for 15-30 minutes, then remove the billet and rinse it clean. Step S2.2: Use a high-precision grinding wheel to clean the insoluble substances from the surface of the iron billet, and then use 800# to 2000# sandpaper to polish the surface of the iron billet to a smooth finish, so that the surface roughness of the iron billet reaches Ra0.8μm level. Step S2.3: Immerse the iron billet in a weakly alkaline cleaning solution at 50-60℃ for 15-20 minutes. After cleaning, rinse the iron billet with deionized water and dry it.

3. The method according to claim 2, characterized in that, In step S2.1, the mass concentration of hydrochloric acid in the hydrochloric acid pool is 5-20%.

4. The method according to claim 2, characterized in that, In step S2.3, the pH value of the weakly alkaline cleaning solution is 8 to 10.

5. The method according to claim 1, characterized in that, Step S3 includes the following sub-steps: Step S3.1: Increase the temperature from room temperature to 800-900℃ at a heating rate of 15-20℃ / min and hold the temperature to make the internal temperature of the iron billet initially uniform. Step S3.2: Increase the temperature to 1100-1200℃ at a heating rate of 10-15℃ / min and hold at that temperature to further promote the temperature uniformity of the iron billet; Step S3.3: Increase the temperature to the rolling temperature of 1250-1350℃ at a heating rate of 5-8℃ / min, and maintain the temperature at this temperature to ensure that the overall temperature of the billet is uniform and reaches the optimal rolling temperature.

6. The method according to claim 5, characterized in that, In step S3.1, the holding time is calculated based on the thickness of the iron billet at 1 to 2 minutes per millimeter; in step S3.2, the holding time is calculated based on the thickness of the iron billet at 0.5 to 1 minute per millimeter; in step S3.3, the holding time is calculated based on the thickness of the iron billet at 0.2 to 0.5 minutes per millimeter.

7. The method according to claim 1, characterized in that, In step S4, during the rough rolling stage, the reduction rate for each pass is controlled at 10-15%, and the rolling speed is controlled at 1-2 meters per second.

8. The method according to claim 1, characterized in that, In step S4, during the finishing rolling stage, the reduction rate for each pass is controlled at 3-5%, and the rolling speed is controlled at 0.5-1 m / s.

9. The method according to claim 1, characterized in that, Step S5 includes the following sub-steps: Step S5.1: Use high-temperature hot steam for cooling, with the cooling wind speed controlled at 3-5 m / s, to rapidly reduce the product temperature to 800-900℃. Step S5.2: Use water cooling, with the water temperature controlled at 20-30℃, to further reduce the product temperature to 300-400℃; Step S5.3: Allow the product to cool naturally in the air to room temperature.

10. The method according to claim 9, characterized in that, In step S5.1, the cooling air velocity is controlled at 4 m / s to reduce the product temperature to 850℃.

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