Nitrogen-increasing process and analysis method for gray cast iron

Abstract

This invention relates to a nitrogen-enhancing process and analytical method for gray cast iron, belonging to the field of foundry metallurgy and metallic material analysis technology. The nitrogen-enhancing method involves mixing N-5 type manganese ferronitride with 75SiFe inoculant in a predetermined ratio to form a composite additive with uniform particle size and a mixing homogeneity ≥95%. This additive is added simultaneously with the molten iron as it is poured into the ladle, under controlled conditions. The amount of inoculant added is controlled to ensure a stable nitrogen increase of 25-30 ppm and a final nitrogen content in the molten iron of 80-100 ppm. The analytical method employs a standardized process of secondary sampling, water-cooled rapid quenching, ethanol protection, and pulse inert gas melting detection using an ON-3000 oxygen-nitrogen analyzer to achieve accurate determination and process feedback of nitrogen content. This method enables efficient introduction and stable control of nitrogen, significantly improving the compositional consistency and overall performance of gray cast iron. It is suitable for the industrial production of gray cast iron products requiring high uniformity of microstructure and quality stability.

Description

Technical Field

[0001] This invention relates to the fields of casting metallurgy and metal material analysis technology, specifically to a nitrogen enrichment process and analysis method for gray cast iron. Background Technology

[0002] Gray cast iron is widely used in automobiles, machine tools, and pipe fittings due to its excellent casting properties and comprehensive mechanical properties. Nitrogen in gray cast iron plays a role in refining pearlite, passivating the ends of lamellar graphite, and strengthening the matrix through solid solution, thereby improving strength and fatigue resistance, and reducing casting defects such as white cast iron and shrinkage porosity. However, existing nitrogen enrichment methods still face the following common problems: (1) Absorption rate and fluctuation problem: The method of directly adding manganese nitride iron in the furnace results in a low nitrogen absorption rate and large fluctuation, making it difficult to stably control the nitrogen content within a suitable narrow range, which leads to increased dispersion of microstructure and properties. (2) Mismatch between materials and particle size: The components, impurities and particle size of the inoculant and nitrogen-enhancing agent are not uniform, and the mixing and dissolution dispersion behaviors are inconsistent, resulting in local enrichment or deterioration; (3) Non-standard testing procedures: The lack of an integrated procedure for secondary sampling, rapid cooling, ethanol protection, and pulse inert gas melting method for gaseous elements in gray cast iron results in insufficient repeatability and comparability of tests, making it difficult to form a closed-loop process control. The above problems limit the stable promotion and batch consistency of nitrogen enrichment technology for gray cast iron.

[0003] (4) Some existing nitrogen-enhancing processes often require additional manual or mechanical stirring to promote dispersion after addition, which not only increases the complexity of operation, but also poses risks of uneven stirring, temperature drop or secondary oxidation. Summary of the Invention

[0004] The present invention aims to solve the problems of low nitrogen absorption rate, large fluctuation, poor composition uniformity and lack of closed-loop detection in the existing nitrogen enrichment process for gray cast iron.

[0005] To achieve the above objectives, this invention proposes a method for in-flow nitrogen enrichment of gray cast iron, comprising the following steps: N-5 type manganese ferronitride and 75SiFe inoculant are selected as mixed additives; wherein, the nitrogen content of the N-5 type manganese ferronitride is 5.0%~7.0%, the manganese content is not less than 65%, and the particle size is 1~5mm; the silicon content of the 75SiFe inoculant is 72.0%~80.0%, the aluminum content is not more than 1.0%, the calcium content is not more than 1.0%, and the particle size is 1~5mm; The N-5 type manganese iron nitride and the 75SiFe inoculant are mixed at a mass ratio of 1:(2~5) to obtain a mixed additive with a mixing uniformity of not less than 95%. When the molten iron tapping temperature is 1500℃~1520℃, the mixed additive is continuously fed into the ladle along with the molten iron flow.

[0006] Furthermore, the 75SiFe inoculant contains no more than 0.5% manganese, no more than 0.04% phosphorus, and no more than 0.02% sulfur, and has a bulk density of 2.8~3.2 g / cm³. 3 The melting point is 1200℃~1250℃; and the particle size deviation between the 75SiFe inoculant and the N-5 type manganese nitride ferronitride is no greater than 1mm.

[0007] Furthermore, the particle size distribution of the N-5 type manganese ferronitride is as follows: 60% ± 5% of the particles are 1-3 mm, and 40% ± 5% of the particles are 3-5 mm.

[0008] Furthermore, the mixing step is carried out using a double-helix mixer, and the stirring time is 10-15 minutes; the mixing uniformity is verified by measuring the mass fraction of the main elements by sieving at least 5 random sampling points, and the relative deviation of any sampling point from the batch average value is no more than 3%.

[0009] Furthermore, the molten iron tapping temperature is monitored in real time using an immersion thermocouple, with a measurement error not exceeding ±5℃; the in-flow feeding process utilizes the impact kinetic energy of the molten iron flow to achieve rapid dispersion and melting of additives.

[0010] Furthermore, the continuous feeding process is matched with the flow rate of molten iron, wherein the amount of 75SiFe inoculant added is 0.3% to 0.5% of the mass of molten iron, and the nitrogen increment is controlled at 25 ppm to 30 ppm under the addition conditions, so that the final nitrogen content of the molten iron is controlled at 80 ppm to 100 ppm.

[0011] This invention also proposes a method for analyzing the nitrogen content of gray cast iron, comprising the following steps: One-time sampling: After the molten iron temperature is not lower than 1450℃ and the slag removal is completed, insert the sampling spoon into the middle area 200mm below the surface of the molten iron to take a sample; Secondary sampling: Sampling is performed when the surface of the molten iron in the sampling spoon is covered by a film of three-quarters and the temperature of the molten iron is 1300℃~1350℃. Sample preparation: After sampling, the sample was immediately placed in a 20℃ constant temperature water bath and rapidly cooled to room temperature. Then, the sample was polished to remove the oxide film and cut into 3mm~5mm analytical segments, which were then sealed and stored in ethanol. Nitrogen content determination: The nitrogen content of the sample was determined using an oxygen-nitrogen analyzer.

[0012] Furthermore, during the grinding and polishing of the sample, ethanol was used for cooling to suppress nitrogen loss caused by overheating, and after the sample was sheared, it was immediately immersed in ethanol and sealed for preservation until testing.

[0013] Furthermore, the secondary sampling step is performed using a quartz sampling tube with a diameter of 5 mm, under conditions of an oblique insertion angle of 30° to 60° and an insertion depth of 10 to 15 mm.

[0014] Furthermore, the oxygen-nitrogen analyzer determines the nitrogen content of the sample using a pulse-heated inert gas melting method.

[0015] The present invention has the following beneficial effects: (1) The nitrogen increase process is quantitatively and precisely controlled with high stability. By continuously adding the mixed additives into the ladle molten iron within the tapping temperature window of 1500℃~1520℃, the amount of 75SiFe inoculant added is limited to 0.3%~0.5% of the molten iron mass, so that the nitrogen increase is stably within the range of 25ppm~30ppm; under this constraint, the final nitrogen content of the molten iron is stably maintained within the target window of 80ppm~100ppm. Compared with the experience-based one-time feeding method, this mode replaces the fuzzy feeding with "real-time flow matching + quantitative addition ratio", reducing the fluctuation of nitrogen content in different furnaces and within the same furnace. This invention simultaneously injects the mixed additives into the highly turbulent molten iron flow, and uses the strong shear force and impact force of the fluid to make the additives instantly wrapped, entrained and rapidly dissolved by the molten iron, achieving a mixing uniformity and reaction efficiency far exceeding that of manual stirring. While simplifying the operation, it ensures a high absorption rate and uniform distribution of nitrogen.

[0016] (2) Improved the synergistic efficiency of nitrogen addition and inoculation. By uniformly controlling the particle size range of N-5 type manganese iron nitride and 75SiFe inoculant within the same range of 1mm to 5mm, and using a double-helix mixer with a mixing time of 10min to 15min and a mixing uniformity of not less than 95%, it was ensured that the additive could achieve synchronous and uniform dissolution and diffusion upon entering the molten iron. This avoided the problems of turbulent separation and excessively high local concentration caused by uneven particle size, thereby simultaneously improving the nitrogen absorption efficiency and the effectiveness of the inoculation treatment.

[0017] (3) The adverse effects of impurity elements on the nitrogen enrichment process have been reduced. By strictly limiting the chemical composition of the 75SiFe inoculant, especially controlling the aluminum and calcium content to ≤1.0% and the manganese content to ≤0.5%, the interference of these impurity elements on nitrogen absorption and the potential negative impact on the microstructure of cast iron are minimized while ensuring good inoculation effect. This is conducive to obtaining more stable and consistent comprehensive performance within the preset final nitrogen control window of 80ppm~100ppm.

[0018] (4) A closed-loop system for nitrogen content analysis and detection was established. Through a standardized detection process of "secondary sampling—water cooling—ethanol protection—pulse heating inert gas melting method," especially by controlling the timing and operation of primary and secondary sampling, and combining specific sample treatment with an oxygen and nitrogen analyzer, the detection error introduced by factors such as oxidation and volatilization was greatly reduced. This detection method has good repeatability and comparability, providing accurate and reliable quantitative feedback for in-flow nitrogen enhancement processes, and realizing complete closed-loop quality control from "process setting → process control → result detection → parameter adjustment."

[0019] (5) The comprehensive performance of cast iron and casting defects were synergistically improved. Under the premise of precise control of nitrogen increment and final nitrogen content, the microstructure and mechanical properties of cast iron were simultaneously optimized through comprehensive measures such as continuous quantitative addition, uniform additive particle size and high uniformity mixing. Attached Figure Description

[0020] Figure 1 This is an overview diagram of the method flow of the present invention; Figure 2 This is a schematic diagram of continuous feeding and packaging. Figure 3 This is a schematic diagram showing the sampling points and angles for primary and secondary sampling. Figure 4 This is a comparison chart of final nitrogen content and nitrogen uptake rate; Figure 5 A comparison chart of final nitrogen content RSD; Figure 6 This is a comparison chart of final nitrogen content Cpk. Detailed Implementation

[0021] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. The following embodiments are used to illustrate the present invention and do not constitute a limitation on the scope defined by the claims. Any equivalent substitutions or modifications made without departing from the spirit of the present invention should be considered to fall within the protection scope of the present invention.

[0022] Example 1 This embodiment provides a method for analyzing the nitrogen content of gray cast iron, which is suitable for real-time and stable determination of the nitrogen content of molten iron throughout the entire process from tapping to ladle loading.

[0023] The specific steps are as follows: First sampling: After the molten iron has been melted and the temperature has risen to no less than 1450℃, and the surface slag has been removed, use a preheated and dried standard sampling spoon to take a sample from the middle area, 200mm below the surface of the molten iron. The operation should be continuous to avoid stirring the surface molten iron or introducing oxide slag, so as to ensure the representativeness of the sample taken.

[0024] Secondary sampling: Observe the molten iron in the initial sampling spoon. When the surface film covers more than three-quarters of the spoon and the temperature drops to 1300℃~1350℃, perform a secondary sampling. Use a 5mm diameter quartz sampling tube, inserting it obliquely into the molten iron at an angle of 30°~60° to a depth of 10~15mm, to obtain the liquid sample in the middle of the spoon. After sampling, immediately immerse the entire quartz sampling tube containing the sample in a 20℃ constant temperature water bath. After cooling to 20℃, remove the sample.

[0025] Sample preparation: Polishing: After cooling, remove the iron sample from the quartz sampling tube and gently polish its surface using fine-grit sandpaper or a polishing cloth to thoroughly remove the oxide film. During the polishing process, anhydrous ethanol should be continuously added for cooling and lubrication to suppress nitrogen loss due to overheating caused by friction.

[0026] Shearing: Use pliers to cut the polished sample into analytical segments of 3-5 mm in length. When taking samples, avoid irregular areas at both ends of the sample.

[0027] Storage: Immediately place the cut analytical segment into a sealed container filled with anhydrous ethanol and store until nitrogen content determination.

[0028] Nitrogen content determination: Oxygen and nitrogen were measured using a pulse-heated inert gas melting method. Each sample was measured twice in parallel. If the relative deviation was ≤5% (or the absolute difference was ≤0.0003%), the arithmetic mean was taken; otherwise, a second measurement was performed and the median of the three measurements was taken as the final result.

[0029] Example 2 This embodiment provides a method for in-flow nitrogen addition to gray cast iron, the specific steps of which are as follows: Step 1, Preparation of the mixed additives: Raw material selection and proportioning: Weigh N-5 type manganese iron nitride and 75SiFe inoculant at a mass ratio of 1:3.

[0030] Raw material specifications: The nitrogen content of the N-5 type manganese iron nitride is 6.2%, the manganese content is 67.5%, and the particle size range is 1mm~5mm; its particle size distribution is: 1mm~3mm particles account for 62% of the total mass, and 3mm~5mm particles account for 38%.

[0031] The chemical composition of the 75SiFe inoculant is: 75.8% silicon, 0.5% aluminum, 0.3% calcium, 0.45% manganese, 0.03% phosphorus, and 0.015% sulfur; its physical properties are: bulk density 3.0 g / cm³. 3 Melting point 1220℃, particle size range 1mm~5mm.

[0032] Mixing: Add the two raw materials to a twin-screw mixer and mix for 12 minutes. After mixing, take samples from at least 5 points in the mixture for testing. If the relative deviation of the mass fraction of the main elements (silicon, manganese, and nitrogen) at each point does not exceed ±3%, the mixing uniformity is judged to be above 95%.

[0033] Step 2, in-stream nitrogen addition treatment: The molten iron was melted in a medium-frequency induction furnace, and the tapping temperature was 1515℃ (monitored in real time using an immersion thermocouple, with the measurement error controlled within ±5℃). During the process of the molten iron flowing from the furnace outlet into the ladle, the mixed additive prepared in step 1 was continuously added along the molten iron inflow line, ensuring that the addition process matched the molten iron flow rate in real time. The actual amount of 75SiFe inoculant added was approximately 0.4% of the molten iron mass, and it was added simultaneously with N-5 type manganese nitride ferronitride at a mass ratio of 1:3. As the mixed additive flowed into the ladle with the molten iron, the intense turbulence and impact of the molten iron flow ensured thorough mixing and rapid melting with the molten iron, eliminating the need for additional stirring.

[0034] Nitrogen content determination: Samples of the treated molten iron were taken and the nitrogen content was determined using the same analytical method as in Example 1.

[0035] Example 3 This embodiment provides a method for in-flow nitrogen addition to gray cast iron, the specific steps of which are as follows: Step 1, Preparation of the mixed additives: Raw material selection and proportioning: Weigh N-5 type manganese iron nitride and 75SiFe inoculant at a mass ratio of 1:2.

[0036] Raw material specifications: The N-5 type manganese ferronitride has a nitrogen content of 5.1%, a manganese content of 65.8%, and a particle size range of 1mm to 5mm. Its particle size distribution is: 58% particles are 1mm to 3mm, and 42% are 3mm to 5mm. The chemical composition of the 75SiFe inoculant is: silicon 72.5%, aluminum 0.8%, calcium 0.5%, manganese 0.48%, phosphorus 0.03%, and sulfur 0.015%; its physical properties are: bulk density 2.9 g / cm³. 3 Melting point 1220℃, particle size range 1mm~5mm. The particle size deviation between the two additives is no greater than 1mm.

[0037] Mixing: Add the above raw materials to a twin-screw mixer and mix for 10 minutes. Multiple sampling tests showed that the relative deviation of the mass fraction of the main elements did not exceed ±3%, and the mixing uniformity reached over 95%.

[0038] Step 2, in-stream nitrogen addition treatment: After the molten iron is melted, subsequent operations are performed at a tapping temperature of 1500℃, with a temperature monitoring error not exceeding ±5℃. During the flow of the molten iron into the ladle, the mixed additive obtained in step 1 is continuously added with the flow, ensuring precise matching between the addition and the molten iron flow rate. The amount of the 75SiFe inoculant added is approximately 0.35% of the molten iron mass, and it is added to the molten iron along with N-5 type manganese ferronitride at a mass ratio of 1:2. As the mixed additive flows into the ladle with the molten iron, it achieves thorough mixing and rapid melting due to the intense turbulence and impact of the molten iron flow, eliminating the need for additional stirring. Nitrogen content determination: Samples of the treated molten iron are taken and the nitrogen content is determined using the same analytical method as in Example 1.

[0039] Example 4 This embodiment provides a method for in-flow nitrogen addition to gray cast iron, the specific steps of which are as follows: Step 1, Preparation of the mixed additives: Raw material selection and proportioning: Weigh N-5 type manganese iron nitride and 75SiFe inoculant at a mass ratio of 1:5.

[0040] Raw material specifications: The N-5 type manganese ferronitride has a nitrogen content of 6.8%, a manganese content of 68.2%, and a particle size range of 1mm to 5mm. Its particle size distribution is: 59% particles are 1mm to 3mm, and 41% are 3mm to 5mm. The chemical composition of the 75SiFe inoculant is: silicon 79.5%, aluminum 0.9%, calcium 0.8%, manganese 0.48%, phosphorus 0.035%, and sulfur 0.018%; its physical properties are: bulk density 3.1 g / cm³. 3 Melting point 1240℃, particle size range 1mm~5mm. The particle size deviation between the two additives is no greater than 1mm.

[0041] Mixing: Add the above raw materials to a twin-screw mixer and mix for 15 minutes. Multiple sampling tests showed that the relative deviations in the mass fraction of the main elements did not exceed ±2.7%, and the mixing uniformity was higher than 95%.

[0042] Step 2, Nitrogen Addition Treatment: After the molten iron is melted, subsequent operations are performed at a tapping temperature of 1520℃, with a temperature monitoring error not exceeding ±5℃. At the instant the molten iron enters the ladle, the mixed additives from Step 1 are continuously added synchronously with the molten iron flow; the amount of 75SiFe inoculant added is approximately 0.5% of the molten iron mass, and it is added to the molten iron along with N-5 type manganese ferronitride at a mass ratio of 1:5. During the process of the mixed additives flowing into the ladle with the molten iron, the intense turbulence and impact of the molten iron flow ensure thorough mixing and rapid melting with the molten iron, eliminating the need for additional stirring. Nitrogen Content Determination: Samples of the treated molten iron are taken and the nitrogen content is determined using the same nitrogen content analysis method as in Example 1.

[0043] Comparative Example 1 Except for the feeding method, the operating conditions of this comparative example are the same as those of Example 2.

[0044] The difference lies in the following: This comparative example eliminates the continuous feeding operation. Instead, before the molten iron is tapped from the furnace, the same mass of the mixed additive as in Example 2 is added to the molten iron in the induction furnace all at once. The mixture is then manually stirred in the furnace for 30 seconds using a graphite rod to promote dispersion and melting. The tapping temperature remains set at 1515℃, after which the iron is poured into the ladle according to the standard procedure.

[0045] After the molten iron was processed, first and second sampling were performed according to the method described in Example 1, and the nitrogen content was tested.

[0046] Comparative Example 2 Except for the particle size range and particle size deviation control requirements of the mixed additives, the operating conditions of this comparative example are the same as those of Example 2.

[0047] The difference lies in the following: the N-5 type manganese nitride iron used in this comparative example has a particle size of 2mm~6mm, and the 75SiFe inoculant has a particle size of 0.5mm~2mm. The particle size deviation of the two additives exceeds 1mm, and no particle size control was performed. The raw materials were still prepared at a mass ratio of 1:3 and mixed in a conventional manner. However, due to the large particle size difference, a stratification trend appeared during the stirring process, and the mixing uniformity was difficult to pass the five-point sampling test.

[0048] The feeding process was carried out in the same manner as in Example 2. After completion, the first and second samplings were performed according to the method in Example 1, and the nitrogen content was tested.

[0049] Comparative Example 3 Except for the shortened mixing time in the mixing step, which resulted in substandard mixing uniformity, the operating conditions in this comparative example were consistent with those in Example 2.

[0050] The difference lies in the following: In this comparative example, during the preparation of the mixed additives, when N-5 type manganese iron nitride and 75SiFe inoculant were mixed at a mass ratio of 1:3, only a twin-screw mixer was used for stirring for 6 minutes, while the specifications and weighing ratios of the remaining raw materials remained unchanged. After mixing, the mass fraction of the main elements at 5 points was measured as usual. The results showed that the maximum relative deviation between the sampling points reached 4.6%, and the average deviation exceeded 3%.

[0051] Subsequent feeding operations were performed according to the procedure in Example 2. After feeding was completed, sampling and nitrogen content testing were conducted according to the method in Example 1.

[0052] Comparative Example 4 Except for the impurity element content in the 75SiFe inoculant used in this comparative example exceeding the specified range, all other operating conditions were consistent with those in Example 2.

[0053] The difference lies in the following: the 75SiFe inoculant used in this comparative example contains 1.4% aluminum, 1.2% calcium, 0.06% phosphorus, and 0.035% sulfur, significantly exceeding the set upper limit standards (aluminum and calcium not exceeding 1.0% each, phosphorus not exceeding 0.04%, and sulfur not exceeding 0.02%). Other proportions, particle sizes, and mixing conditions are the same as in Example 2, and the mixing uniformity meets the standards.

[0054] The in-flow nitrogen enrichment process is the same as step 2 of Example 2. After the molten iron is treated, sampling and nitrogen content detection are performed according to the method of Example 1.

[0055] To verify the technical effectiveness of the in-flow nitrogen enrichment method for gray cast iron proposed in this invention, a series of nitrogen enrichment tests and control experiments were designed. Each group of tests adopted a unified raw material system and testing process to ensure that the data are comparable and technically traceable.

[0056] This experiment consisted of 7 experimental groups, including Example 3 (T1-T3) and Comparative Example 4 (C1-C4), with each group undergoing 3 parallel tests. For each heat, one sampling and one secondary sampling were performed, followed by two parallel measurements to evaluate the final nitrogen content (ppm) and nitrogen uptake rate.

[0057] Total number of samples = 7 groups × 3 furnace cycles × 2 sampling times = 42 pieces; Total number of measurements = 42 samples × 2 parallel measurements = 84 sets of data; If the relative deviation of any set of parallel measurements is greater than 5%, then one additional test should be performed and the median of the three tests should be taken as the final result.

[0058] The main equipment used in the experiment included: Medium-frequency induction melting furnace is used for induction heating and melting of gray cast iron molten iron, with a temperature control accuracy within ±10℃; An immersion thermocouple temperature measurement system is used to monitor the temperature of molten iron when it taps out of the furnace in real time, with a measurement error of no more than ±5℃. A twin-helix mixer is used for high-uniformity mixing of N-5 type manganese iron nitride and 75SiFe inoculant, with a stirring time range of 1 to 60 minutes; A standard molten iron sampling spoon, made of graphite-coated carbon steel, with a volume of 50 mL, is used to take a single sample at a position 200 mm below the surface of molten iron. Quartz sampling tube (SiO2), 5mm in diameter and about 200mm in length, is used for secondary sampling. It is inserted obliquely to a depth of 10-15mm below the surface of the molten iron to draw the sample. A constant temperature water bath is used to cool the samples, and the water temperature is controlled at 20℃±0.2℃. Anhydrous ethanol storage bottle, 250mL sealed glass bottle, used to store the metal analysis section after shearing to prevent oxidation and nitrogen loss; The ON-3000 oxygen and nitrogen analyzer is used to determine the nitrogen mass fraction in metal samples by pulse heating inert gas melting method, with a measurement accuracy of ±0.0005%.

[0059] Evaluation indicators and methods: Final nitrogen content: determined using an oxygen-nitrogen analyzer, in ppm; each sample is expressed as the average of two parallel determinations; Nitrogen increment: The difference between the final nitrogen content of the second sampling and the initial nitrogen content of the first sampling, expressed in ppm, is used to measure the effective increase in nitrogen content brought about by in-flow nitrogen addition. Component stability: evaluated using the relative deviation (RSD) of final nitrogen content and the process capability index (Cpk); where Cpk was calculated based on a target control window of 80ppm~100ppm final nitrogen content (LSL=80ppm, USL=100ppm); Mechanical properties: tensile strength (MPa) and Brinell hardness HBW, tested according to standard methods.

[0060] The test results are shown in Tables 1 and 2. Table 1. Results of nitrogen content and uptake rate determination

[0061] Table 2 Evaluation results of mechanical properties and component stability

[0062] Note: The average final nitrogen content in the table is rounded to the nearest 1 ppm. For example, the actual average value of the second sample in group T3 is about 90 ppm, which is in the middle of the target range of 80 ppm to 100 ppm. Therefore, Cpk ≥ 1.7.

[0063] The following analysis is based on the experimental data shown in Table 1-2: In Examples T1, T2, and T3, under the condition of continuous in-flow dosing and limited inoculant addition, the final nitrogen content measured by secondary sampling reached 82 ppm, 86 ppm, and 90 ppm, respectively, with corresponding nitrogen increments of 27 ppm, 28 ppm, and 30 ppm, respectively. The nitrogen absorption rates were all not less than 72%, and the highest reached 78.3%. In contrast, the nitrogen increments of Comparative Examples C1-C4 were only 15-16 ppm, the nitrogen absorption rates were only 41.8%-47.1%, and the final nitrogen content generally remained below or close to 70 ppm, which is inferior to the scheme proposed in this invention.

[0064] By adopting continuous in-flow addition that matches the molten iron flow rate in real time, and using the amount of 75SiFe inoculant added as a quantitative constraint, the actual mass transfer efficiency and retention of nitrogen in the molten iron can be significantly improved. This avoids the problems of local enrichment, nitrogen escape, and uneven nitrogen distribution commonly seen in the traditional "one-time furnace addition" process. The nitrogen increment is stably locked within the range of 25ppm to 30ppm, and the final nitrogen content can be stably controlled within the target window of 80ppm to 100ppm, meeting the precise setting requirements of final nitrogen level in gray cast iron production.

[0065] In the preparation of the mixed additives, this invention employs a twin-helix mixer for 10-15 minutes, strictly controlling the particle size deviation to no more than 1 mm to ensure a mixing uniformity of over 95%. This measure is clearly reflected in the data: the relative deviation (RSD) of nitrogen content in groups T1-T3 is only 2.0%-2.8%, and the process capability index Cpk is higher than 1.7, indicating that the final nitrogen content distribution is concentrated and the process is highly controllable.

[0066] In contrast, although group C2 had the same feeding method and raw material type as group T2, its final nitrogen content RSD was as high as 4.9% and its Cpk was only 1.05, significantly lower than that of group T2, due to the lack of control over particle size and mixing uniformity. This comparison verifies the core role of a uniform particle size window and high mixing uniformity in improving component stability and process consistency.

[0067] While maintaining stable control of the final nitrogen content and keeping the dispersion low, the mechanical properties of groups T1-T3 were significantly better than those of the comparative groups C1-C4, with smaller fluctuation ranges. This indicates that the quantitative and repeatable control of the nitrogen enrichment process not only improves the solid solution strengthening effect of nitrogen in the matrix and the passivation effect of graphite morphology, but also improves the uniformity of microstructure and the consistency of mechanical properties, meeting the manufacturing requirements of high-consistency gray cast iron parts.

[0068] This invention employs a standardized nitrogen analysis method of "secondary sampling - water-cooling rapid cooling - ethanol protection - pulse inert gas melting method", which achieves consistent feedback between online and post-furnace detection results, with no excessive deviation between samples, and demonstrates excellent detection repeatability and comparability overall.

[0069] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for increasing nitrogen content in gray cast iron, characterized in that, Includes the following steps: N-5 type manganese ferronitride and 75SiFe inoculant are selected as mixed additives; wherein, the nitrogen content of the N-5 type manganese ferronitride is 5.0%~7.0%, the manganese content is not less than 65%, and the particle size is 1~5mm; the silicon content of the 75SiFe inoculant is 72.0%~80.0%, the aluminum content is not more than 1.0%, the calcium content is not more than 1.0%, and the particle size is 1~5mm; The N-5 type manganese iron nitride and the 75SiFe inoculant are mixed at a mass ratio of 1:(2~5) to obtain a mixed additive with a mixing uniformity of not less than 95%. The mixing step is carried out using a twin-screw mixer, and the stirring time is 10~15 min. The mixing uniformity is verified by measuring the mass fraction of the main elements at at least 5 random sampling points through sieve sampling. The relative deviation of any sampling point from the batch average value is not greater than 3%. The mixing uniformity is judged to be above 95%. When the molten iron tapping temperature is 1500℃~1520℃, the mixed additive is continuously fed into the ladle along with the molten iron flow.

2. The nitrogen-enhancing method for gray cast iron according to claim 1, characterized in that, The 75SiFe inoculant contains no more than 0.5% manganese, no more than 0.04% phosphorus, and no more than 0.02% sulfur, and has a bulk density of 2.8~3.2 g / cm³. 3 The melting point is 1200℃~1250℃; and the particle size deviation between the 75SiFe inoculant and the N-5 type manganese nitride ferronitride is no greater than 1mm.

3. The nitrogen-enhancing method for gray cast iron according to claim 1, characterized in that, The particle size distribution of the N-5 type manganese ferronitride is as follows: 60% ± 5% of the particles are 1-3 mm, and 40% ± 5% of the particles are 3-5 mm.

4. The nitrogen-enhancing method for gray cast iron according to claim 1, characterized in that, The molten iron tapping temperature is monitored in real time using an immersion thermocouple, with a measurement error not exceeding ±5℃. The in-flow feeding process utilizes the impact kinetic energy of the molten iron flow to achieve rapid dispersion and melting of additives.

5. The nitrogen-enhancing method for gray cast iron according to any one of claims 1 to 4, characterized in that, The continuous feeding process is matched with the flow rate of molten iron, wherein the amount of 75SiFe inoculant added is 0.3% to 0.5% of the mass of molten iron, and the nitrogen increment is controlled at 25ppm to 30ppm under the addition conditions, so that the final nitrogen content of the molten iron is controlled at 80ppm to 100ppm.

Images