Steel for coffee machine blades and preparation method thereof

Abstract

The present invention relates to a steel for coffee machine blades, comprising the following components by weight: C: 0.30-0.35%; Si ≤ 0.40%; Mn ≤ 0.80%; P ≤ 0.030%; S ≤ 0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.15%, with the remainder being Fe and unavoidable impurities. The manufacturing method comprises the following steps in sequence: molten iron pretreatment + K-OBM-S smelting + VOD vacuum degassing + LF refining → casting steel ingots → rolling and blanking → rolling into finished products → water cooling → annealing. The material of the present invention has the characteristics of high heat treatment hardness, good wear resistance, and excellent pitting corrosion resistance.

Description

Technical Field

[0001] The present invention relates to the composition design, smelting, thermal processing and heat treatment of steel, belonging to the field of thermal processing of metal materials, and more particularly to a steel for coffee machine blades and a preparation method thereof. Background Art

[0002] Coffee machine blades are required to have good wear resistance and corrosion resistance. The commonly used material has always been 30Cr13. The hardness of 30Cr13 after heat treatment is 50-53HRC, and the wear resistance is insufficient. It is necessary to develop higher hardness, better wear resistance and corrosion resistance and long-life blade steel.

[0003] The present invention aims to develop a steel for coffee machine blades with enhanced hardness, wear resistance, and corrosion resistance, thereby extending blade life. By precisely and rationally combining the solid solution strengthening elements C+N, high levels of Cr+Mo+W (elements that enhance corrosion resistance and temper stability), and small amounts of grain-refining elements V and Nb, the present invention has designed a steel for coffee machine blades with superior hardness, wear resistance, and corrosion resistance. Both corrosion resistance and hardness surpass those of the commonly used blade material 30Cr13.

[0004] This invention utilizes a combination of high-temperature diffusion of the steel ingot, solution heating of the billet, rapid cooling with hot water using residual heat after rolling, and annealing at 760-780°C. This resolves the severe structural segregation issues of the steel ingot and refines the grain size of the network carbide segregation, achieving uniform microstructural refinement for the blade steel. The blades utilize a heat treatment process of high-temperature quenching followed by low-temperature tempering, achieving a hardness of 55-58 HRC. Salt spray corrosion (5% NaCl solution, 35°C, 24 hours) meets ISO 10289-1999 Grade 8 or higher. Summary of the Invention

[0005] The purpose of the present invention is to address the above problems and provide a steel for coffee machine blades and a preparation method.

[0006] The object of the present invention is achieved as follows: a steel for coffee machine blades, the chemical composition of the steel for coffee machine blades is as follows: C: 0.30-0.35%; Si≤0.40%; Mn≤0.80%; P≤0.030%; S≤0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.12%, and the rest is Fe and unavoidable impurities; the hardness is 55-57HRC, and the grain size is ≥10 level.

[0007] A method for preparing steel for coffee machine blades comprises the following steps: Step 1: Smelting: Pretreated molten iron is subjected to converter smelting, VOD vacuum degassing, and LF refining, so that the weight percentages of the molten steel reach C: 0.30-0.35%; Si≤0.40%; Mn≤0.80%; P≤0.030%; S≤0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.15%, The rest is Fe and inevitable impurities, and then the steel is cast into small steel ingots of φ420~380×2850~2950mm; Step 2: Ingot heating and rolling: After the steel ingot is demoulded, it is sent to the hot rolling process, heated to 600~800℃ at 100℃~150℃ / h and kept warm for 2~4 hours, then heated to 1280~1300℃ at a heating rate of 120℃~180℃ / h, kept warm for 6~8h for high temperature diffusion homogenization; then the temperature is lowered to 1180~1200℃ and kept warm for 1~2 hours, and the ingot after being kept warm is rolled into 150~220×150~ 220×2800~3200mm specification billet, final rolling temperature 950~1000℃; Step 3: Rolling into product: the rolled billet is transferred to the rolling mill heating furnace, heated at a rate of 150℃~200℃ / h to 1080~1100℃ and kept warm for 30~60 minutes. While meeting the rolling temperature, the chain and network carbides precipitated in the billet during the hot-rolling process due to the temperature drop are all dissolved into the steel matrix; the billet after temperature equalization is rolled into φ30-80mm round steel by the rolling mill, and the final rolling temperature is controlled at 850-900℃; Step 4: Controlled cooling and annealing of the round steel: water cooling is used after rolling Cooling, the cooling water temperature is controlled at 53 ~ 85 ℃, the round steel is cooled to 300-450 ℃, the round steel after water cooling is heated to 350 ~ 450 ℃ at a heating rate of ≤ 50 ℃ / h and kept warm for 3 hours, then heated to 760 ~ 780 ℃ at 80 ℃ ~ 120 ℃ / h and kept warm. The holding time is calculated as "(8 + 0.6 × furnace load) h", the furnace load is calculated in tons, and h is hours; Step 5: Blade heat treatment: After the round steel is processed into a blade, it is quenched at 1050-1080 ℃. The blade quenching cooling method is oil cooling. After quenching, the workpiece is subjected to 180-200 ℃ low-temperature tempering heat treatment.

[0008] The beneficial effects of the present invention are as follows: (1) Through the precise and rational combination of C+N solid solution strengthening elements, high levels of Cr+Mo+W elements that improve corrosion resistance and tempering stability, and small amounts of V and Nb, the present invention designs a steel for coffee machine blades with high hardness, wear resistance, and corrosion resistance. Both corrosion resistance and hardness surpass those of the commonly used blade material 30Cr13. (2) The present invention employs the technical measures of "high-temperature diffusion of the steel ingot followed by solid solution heating of the billet followed by rapid cooling in warm water using residual heat after rolling followed by annealing at 760-780°C," addressing the serious problems of ingot microstructure segregation, network carbide segregation, and grain refinement, achieving uniform microstructure refinement in the blade steel. (3) The blades utilize a "high-temperature quenching followed by low-temperature tempering" heat treatment, achieving a hardness of 55-57 HRC, a grain size ≥ Grade 10, and salt spray corrosion (5% NaCl solution, 35°C, 24 hours) results that meet ISO 10289-1999 Grade 8 or higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will be further described below in conjunction with the accompanying drawings.

[0010] Figure 1 It is an annealing organization diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0011] 1. Chemical composition design concept: Based on the composition of 30Cr13, without increasing the C content, N is added. The C+N solid solution strengthening element improves the heat treatment hardness, wear resistance and corrosion resistance of the material; a higher Mo+W content is added to improve the tempering stability of the material, ensuring that the hardness of the workpiece after quenching decreases little or not during the tempering process, and achieving high hardness and high wear resistance of the material; V+Nb composite grain refinement ensures that the grains do not grow at a higher quenching temperature; Cr+Mo+W+N synergistically improves corrosion resistance. Its pitting resistance equivalent PREN=Cr+3.3(Mo+0.5W)+30N=19.475, which is 7.375 higher than that of 30Cr13 (13.5).

[0012] C: 0.30-0.35%; Si≤0.40%; Mn≤0.80%; P≤0.030%; S≤0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.12%, the remainder being Fe and unavoidable impurities. The round steel used for the blade has a diameter of 30-80 mm and a length of 1000-6000 mm. The annealing hardness ranges from 210-260 HB and the quenching and tempering hardness ranges from 55-57 HRC.

[0013] The material prepared by the invention can reach a hardness of 55-57HRC and a grain size of more than 10 grades after high-temperature quenching at 1050-1080 DEG C and low-temperature tempering at 160-200 DEG C.

[0014] The manufacturing method for coffee machine blade steel includes the following steps: Step 1: Smelting: Pretreated molten iron undergoes K-OBM-S smelting, VOD vacuum degassing, and LF refining until the molten steel meets the following compositional weight percentage requirements. To achieve low segregation and high density, the steel is cast into small ingots measuring φ420 / 380×2850mm. The following compositional elements are present: C: 0.30-0.35%; Si≤0.40%; Mn≤0.80%; P≤0.030%; S≤0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.15%, with the remainder being Fe and unavoidable impurities. To achieve low-segregation, high-density steel, it is cast into small ingots of φ420 / 380×2850mm. K-OBM-S smelting, VOD vacuum degassing, and LF refining are specifically performed as follows: (1) Pre-treatment and dephosphorization of molten iron from blast furnaces (P≤0.015), with the temperature of the treated molten iron reaching 1230-1300°C. (2) Converter smelting: During converter smelting, decarburization, dephosphorization, and alloying are performed, resulting in a molten steel composition by weight of 0.30-0.33% C; 0.25% Si; 0.75% Mn; 0.025% P; 0.010% S; 13.50-14.00% Cr; 0.40-0.55% Mo; 0.40-0.55% W; 0.10-0.15% V; 0.02-0.08% Nb; the remainder being Fe and unavoidable impurities. After exiting the furnace, the steel temperature is 1620-1700°C. (3) VOD vacuum degassing: N2 is supplied throughout the VOD process, with a vacuum of ≤3 mbar and strong agitation for dehydrogenation. The treatment time is 20-25 minutes. At the end of the treatment, [H] is ≤2.5 ppm. The strong agitation gas flow rate is 620-650 NL / min. (4) LF refining: During LF refining, the slag basicity is controlled to R(CaO / SiO2) = 4.0-6.0, and the Al2O3 content in the refined slag is 20%-27%. The reduction slag is held for 25-35 minutes after formation. During the LF ladle refining process, bottom-blown nitrogen is used for nitrogen alloying. Other ferroalloys are then added to adjust the chemical composition by weight to: C: 0.30-0.35%; Si ≤ 0.40%; Mn ≤ 0.80%; P ≤ 0.030%; S ≤ 0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.15%. The remainder is Fe and unavoidable impurities. The molten steel temperature is 1557-1567°C before ingot casting.

[0015] Step 2: Ingot Heating and Rolling: After the ingot is demoulded, it is sent to the hot rolling process. After heating to 600-800℃ at 100℃ / h and holding for 3 hours, it is heated to 1280-1300℃ at a heating rate of 120℃ / h and held for 6-8 hours for high-temperature diffusion homogenization. The temperature is then lowered to 1180-1200℃ and held for 2 hours to prevent the billet from further heating during the rolling process, which may cause overheating. The ingot is rolled into a 220×220×3000mm specification billet through a 1000mm blooming mill, and the final rolling temperature is 950-1000℃.

[0016] Step 3: Rolling: The rolled billet is transferred to the φ650 mill heating furnace, where the temperature is raised to 1080-1100°C at a rate of 180°C / h and held for 30-60 minutes. While meeting the rolling temperature, the chain and network carbides that precipitate during the hot-rolling process due to the temperature drop are dissolved into the steel matrix. The billet, after being heated, is rolled into φ30-80mm round steel in the φ650 mill. To avoid coarse grains in the rolled product or the precipitation of carbides at grain boundaries to form network and chain carbides, the final rolling temperature is controlled at 850-900°C.

[0017] Step 4: Controlled Cooling and Annealing of the Round Steel: To prevent the steel from cooling too slowly, leading to the precipitation of chain and network carbides and grain growth during the cooling process, water cooling is used after rolling. To prevent cracking caused by excessive cooling, the cooling water temperature is controlled between 53°C and 85°C. The round steel cools to 300-450°C, at which point the carbides are almost completely dissolved in the matrix, achieving a solid solution effect. The water-cooled round steel is heated to 400°C at a rate of ≤50°C / h and held for 3 hours to minimize internal and external temperature differences and thermal stress. The temperature is then raised to 760-780°C at 80°C / h and held for (8 + 0.6 × furnace charge) hours. During this holding period, carbides uniformly and dispersedly precipitate from the matrix and further grow, reducing the hardness of the round steel. After annealing, the hardness of the round steel is 210-260HB. The round steel treated by this process has a grain size of ≥10 and a uniformly distributed sorbite microstructure, eliminating network and chain carbides. The "post-rolling water-cooling solution treatment followed by 760-780°C annealing" method achieves the homogenized sorbite structure required for quenching, replacing the pre-quenching conditioning process.

[0018] 3. Blade Heat Treatment: After being processed into blades from round steel, they undergo high-temperature quenching at 1050-1080°C. This dissolves most of the workpiece's alloy carbides into the steel matrix, improving quench hardness and corrosion resistance while retaining a small amount of VC and NbC carbides to inhibit grain growth. The blades are cooled during quenching using oil. After quenching, they undergo low-temperature tempering at 180-200°C. This dissolves most of the alloy into the workpiece matrix, ensuring the steel's hardness and corrosion resistance. After this heat treatment, the blades achieve a hardness of ≥55 HRC. Salt spray corrosion resistance testing (5% NaCl solution, 35°C, 24 hours) according to ASTM B117-2016 shows results meeting ISO 10289-1999 Grade 8 or higher. Their service life is 1.5 times that of 30Cr13 blades. Example 1

[0019] Step 1: Smelting: The pretreated molten iron is subjected to K-OBM-S smelting, VOD vacuum degassing, and LF refining. The steel composition weight percentage reaches the following requirements and is tapped and cast into a φ420 / 380×2850mm small ingot.

[0020]

[0021] Step 2: Ingot heating and rolling: After the ingot is demoulded, it is sent to the hot rolling process. After the 001 heat is heated to 750℃ at 100℃ / h and kept warm for 3 hours, it is heated to 1283℃ at a heating rate of 120℃ / h and kept warm for 8 hours for high-temperature diffusion homogenization; then the temperature is lowered to 1181℃ and kept warm for 2 hours. The steel ingot after soaking is rolled into 200×200×3000mm square billet by 1000mm reversible blooming mill, and the final rolling temperature is 960℃. The rolled billet is hot transferred to the φ650 soaking furnace of the rolling mill.

[0022] Heat 002 was heated at 800℃ at a rate of 150℃ / h and kept warm for 3 hours, then heated to 1298℃ at a rate of 150℃ / h and kept warm for 6 hours for high-temperature diffusion homogenization; then the temperature was lowered to 1200℃ and kept warm for 1 hour. The steel ingots after equalization were rolled into 180×180×3000mm square billets through a 1000mm reversible primary rolling mill, with a final rolling temperature of 985℃. The billets were then red-transferred to the φ650 mill equalization furnace.

[0023]

[0024] Step 3: Rolling: After the billets are heated in a soaking furnace, they are rolled into φ30mm and φ80mm round steel bars, respectively. The soaking temperature, time, and final rolling temperature are shown in Table 3.

[0025]

[0026] Step 4: Cooling: After rolling, the round steel is directly dropped into the warm water tank for cooling. The water cooling temperature and cooling time of the round steel are shown in Table 4.

[0027]

[0028] Annealing: Load the cooled round steel into a trolley furnace, heat it to 350℃ at a heating rate of 50℃ / h and keep it at that temperature for 3 hours, then heat it to 760℃ (001 heat) and 780℃ (002 heat) at a heating rate of 80℃ / h and keep it at that temperature for (8+0.6×60)=34 hours, then cool it to 400℃ and air-cool it to room temperature.

[0029]

[0030] Implementation Results: The product obtained in Example 1 and 30Cr13 material were heat treated, and their hardness was tested. Salt spray corrosion testing was also conducted according to ASTM B117-2016 (5% NaCl solution, 35°C, 24 hours). The results met the ISO 10289-1999 standard for rating. Table 6 shows the sample hardness and salt spray corrosion rating.

[0031]

[0032] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by any technician in this field within the scope of the present invention are included in the patent scope of the present invention.

Claims

1. A method for preparing steel for coffee machine blades, characterized by: The following steps are involved: Step 1: Smelting: The pretreated molten iron is subjected to converter smelting, VOD vacuum degassing, and LF refining, and the composition weight percentage of the molten steel reaches C: 0.30-0.35%; Si≤0.40%; Mn≤0.80%; P≤0.030%; S≤0.010%; Cr: 13.50-14.50%; Mo: 0.40-0.60%; W: 0.40-0.60%; V: 0.10-0.20%; Nb: 0.02-0.10%; N: 0.080-0.15%, and the rest is Fe and unavoidable impurities, and then the steel is tapped and cast into small steel ingots of φ420-380×2850-2950mm; Step 2: Ingot heating and rolling: After the ingot is demoulded, it is sent to the hot rolling process, heated to 600-800℃ at 100℃-150℃ / h and kept warm for 2-4 hours, then heated to 1280-1300℃ at a heating rate of 120℃-180℃ / h and kept warm for 6-8 hours for high-temperature diffusion homogenization; then the temperature is lowered to 1180-1200℃ and kept warm for 1-2 hours. The ingot after holding is rolled into 150-220×150-220×2800-3200mm specification billet through the blooming mill, and the final rolling temperature is 950-1000℃; Step 3: Rolling into products: The rolled billet is transferred to the rolling mill heating furnace and heated at a rate of 150℃~200℃ / h to 1080~1100℃ and kept at this temperature for 30~60 minutes. While meeting the rolling temperature, the chain and network carbides precipitated in the billet during the hot-rolling process due to the temperature drop are completely dissolved into the steel matrix. The billet after temperature equalization is rolled into φ30-80mm round steel by the rolling mill, and the final rolling temperature is controlled at 850-900℃. Step 4: Controlled cooling and annealing of round steel: After rolling, the round steel is cooled by water cooling, and the cooling water temperature is controlled at 53-85℃. The round steel is cooled to 300-450℃. The water-cooled round steel is heated to 350-450℃ at a heating rate of ≤50℃ / h and kept at this temperature for 3 hours. Then, the round steel is heated to 760-780℃ at a heating rate of 80-120℃ / h and kept at this temperature. The holding time is calculated as "(8+0.6×charge)h". The charge is calculated in tons, and h is hours. Step 5: Blade heat treatment: After the round steel is processed into a blade, it is quenched at a high temperature of 1050-1080℃. The blade quenching cooling method is oil cooling. After quenching, the workpiece is subjected to a low temperature tempering heat treatment at 180-200℃.

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