CN111607678A

CN111607678A - Production method for improving graphite form of nodular cast iron

Info

Publication number: CN111607678A
Application number: CN202010625706.2A
Authority: CN
Inventors: 崔炜; 李胜柱; 李伟柱; 吴宝成; 郭孝江; 陶前昭
Original assignee: Guangxi Yuchai Machine Parts Manufacturing Co Ltd
Current assignee: Guangxi Yuchai Foundry Co ltd
Priority date: 2020-07-01
Filing date: 2020-07-01
Publication date: 2020-09-01
Anticipated expiration: 2040-07-01
Also published as: CN111607678B

Abstract

The invention provides a production method for improving the graphite form of nodular cast iron, which selects low-sulfur low-titanium pig iron, carbon steel and recycled iron as main furnace charges, and adds 75 ferrosilicon, 65 high-carbon ferromanganese, electrolytic copper and graphite carburant; smelting at a specific frequency in an intermediate frequency furnace, adding a nodulizer, an inoculant and a SiCaMn deoxidizer, tapping and nodulizing, wherein the nodulizing process requires specific tapping position, speed and tapping steps, and the specific inoculant flow rate and total amount are selected in the pouring process, so that the spheroidization of the pearlite matrix and the nodular cast iron casting graphite of the mixed matrix can reach 2-grade or above, the nodulizing rate reaches 93-99 percent, and the spheroidization diameter is 7-grade round graphite. The production method has the advantages of simple and convenient treatment process, high reliability and low production cost.

Description

Production method for improving graphite form of nodular cast iron

Technical Field

The invention belongs to the field of casting technology, and relates to a production method for improving the graphite form of nodular cast iron.

Background

The nodular cast iron is a widely used high-quality engineering material, has mechanical properties similar to those of common cast steel materials, is superior to steel castings in yield strength and density, has the manufacturing cost 30-50% lower than that of the steel castings, and has good manufacturability.

However, graphite plays a role as a "defect" in the nodular cast iron material, and the existence of graphite breaks the continuity of the metal matrix and has a remarkable dividing effect on the matrix. In failed cast iron parts, cracks all start from graphite. The graphite of the nodular cast iron is round, and the graphite of the gray cast iron is flaky. The edge of the flake graphite is sharp and is easily developed into a crack source under the influence of stress concentration, and the graphite in the nodular cast iron is round, so that the stress concentration phenomenon of a graphite edge matrix is improved to a certain extent. The relationship between the graphite morphology and the mechanical properties of gray cast iron and ductile cast iron proves that the influence of the graphite morphology on the mechanical properties of cast iron is very critical.

For nodular cast iron, how to enable graphite to become more round and finer is a key control element in the production process of the nodular cast iron.

1. In the production of nodular cast iron, the common level is that the graphite spheroidization grade is 2-4 grade, and the spheroidization rate is 70-94 percent; the spheroidization can reach grade 1-2, the spheroidization rate is more than 90 percent, particularly, the center of the casting can only reach grade 3, and the spheroidization rate is 70-89 percent, and because the graphite roundness is not high, the graphite is ubiquitous, the national standard considers the actual manufacturing level of most enterprises, and the spheroidization grade is not taken as a necessary concern.

2. The diameter of graphite is generally leveled to 5-6 grade, the diameter of graphite is 0.03-0.12mm, and the graphite can be evaluated to be very small as the diameter of 7 grade graphite, and particularly, large-wall-thickness castings like crankshafts can reach the diameter of 6 grade and the diameter is 0.03-0.06mm, which is a very good level.

3. The spheroidization process is generally adopted in China by adopting a rushing-in method, although the graphite grade can be improved by wire feeding spheroidization, the rushing-in method still occupies a dominant position at present no matter the technology, equipment reliability, core-spun yarn quality and the like do not reach the well-popularized ground step.

4. The fluctuation of the process of the flushing method is large, the flushing angle of the molten iron, the flushing speed, the positions and the granularity of a nodulizer and an inoculant can influence the absorption rate of magnesium, and further influence the graphite roundness. The addition of the nodulizer is generally increased to 1.3-1.5% in the industry, but the excessive addition of the nodulizer increases slag inclusion in molten iron, the reaction in the spheroidization process is severe, the molten iron is wasted greatly, and the residual rare earth content in the molten iron is increased, so that the white cast of a casting and the shrinkage tendency of the casting are increased, and the improvement of the quality of the casting is not facilitated.

5. The good graphite form is obtained by adopting a flushing method process, and besides reasonable iron liquid components, the method for effectively improving the magnesium absorption rate, a scientific and reasonable inoculation method and various detailed control are required to ensure the stability so as to obtain the nodularity of more than 2 grades and the graphite diameter of more than 7 grades.

The production method of high-strength high-elongation nodular cast iron which refers to Chinese patent document with the patent number of 2017108487584 comprises the following steps: (1) selecting Q10 low-sulfur low-titanium pig iron, common carbon steel and self-produced recycled iron as main furnace charges, and adding 75 ferrosilicon accounting for 0.65-0.8% of the total mass of the main furnace charges, 65 high-carbon ferromanganese accounting for 0.15-0.19%, electrolytic copper accounting for 0.15-0.20% and graphite type carburant accounting for 2.3-2.5%; smelting and melting by an intermediate frequency furnace to obtain molten iron; (2) preparing 2-6 and 3-7 of low rare earth nodulizer according to a mass ratio of 4:1, adding a special nodulizer prepared by mixing the components in the proportion into a nodulizing ladle to realize that the residual RE of the final molten iron is 0.02-0.025 percent and the residual Mg0.03-0.04 percent, and controlling the residual RE/Mg ratio to 2/3; (3) adding a BS-1A inoculant and 75 ferrosilicon as a primary inoculant before spheroidizing the molten iron; (4) tapping liquid iron from the intermediate frequency furnace to a spheroidizing bag, and tapping for 2 times by adopting a flushing method; (5) during pouring, 75 silicon is added with the flow as a secondary inoculant. In the production method, stable residual Mg and residual RE are obtained through reasonable components and nodulizer proportion and a stable nodulizing process, and a reasonable inoculation process is adopted, so that the nodulizing grade of graphite can reach more than grade 2, the diameter grade of graphite is 5-8, round graphite nodules but the nodulizing rate is only 90-92%, and the diameter of graphite is 0.015-0.06 mm.

Disclosure of Invention

The invention relates to a production method which is characterized in that a general punching method is adopted as a basis through production practice, graphite with the graphite grade of 1-2, the spheroidization rate of 93-99% and the graphite diameter of more than 7 grades is stably obtained in the production of nodular cast iron, and the mechanical properties of the nodular cast iron, such as graphite roundness, graphite quantity and graphite diameter of more than 7 grades, are stably improved.

The invention solves the technical problems through the following technical scheme:

a process for improving the shape of graphite cast iron includes such steps as adding nodulizer and primary inoculant including BS-1A inoculant, SiCaMn deoxidant with granularity of 3-6mm, BS-1SB and 75 Si-Fe in nodulizing ladle.

Further, in the step of spheroidizing the molten iron, the packaging sequence is as follows: firstly, 1/3 parts of 75 ferrosilicon are placed on one side of a spheroidizing dam, then a spheroidizing agent is poured, the spheroidizing agent is paved, then a BS-1A, BS-1SB and SiCaMn mixture is placed on the spheroidizing agent, the rest 75 ferrosilicon of 2/3 parts is added after paving, finally 1 kg of pearl sand is scattered in the middle of a ladle material, two corners are exposed, electrolytic copper is placed on the pearl sand, and the ladle is finished.

Further, the molten iron is pearlite matrix molten iron or mixed matrix molten iron.

Further, the pearlite matrix iron liquid is mainly prepared by adding 75 ferrosilicon which is 0.18 percent of the total mass of the main furnace charge, 0.35 percent of 65 high-carbon ferromanganese, 0.4 percent of electrolytic copper and 2.45 percent of graphite type carburant into main furnace charge which comprises 20 percent of low-sulfur low-titanium pig iron, 55 percent of carbon steel and 25 percent of pearlite type recycled iron by mass ratio, and smelting by an intermediate frequency furnace to obtain the following components: c: 3.8-3.9%, Si: 0.7-0.9%, Mn: 0.4-0.5%, Cu: 0.4-0.5%, S is less than or equal to 0.03%, and Ti is less than or equal to 0.035%.

Further, the mixed matrix iron liquid mainly comprises raw materials, 10% of low-sulfur low-titanium pig iron, 50% of carbon steel and 30% of mixed matrix type return iron in a mass ratio of Q10 as main furnace materials, and is added with 75 ferrosilicon accounting for 0.5% of the total mass of the main furnace materials, 0.05% of 65 high-carbon ferromanganese, 0.12% of electrolytic copper and 2.6% of graphite type carburant, and the mixed matrix iron liquid is smelted by an intermediate frequency furnace to obtain the following components: c: 3.8-3.9%, Si: 1.3-1.5%, Mn: 0.18-0.28%, Cu: 0.2-0.3%, S is less than or equal to 0.03%, and Ti is less than or equal to 0.035%.

Further, in the pouring step, a sulfur-oxygen inoculant is added along with the flow to serve as a secondary inoculant, and the adding point is the position with the height of 10cm at the outlet of the casting ladle.

Further, in the tapping step, 85-90% of tapping liquid is tapped at the 1 st time, the tapping speed is 500 kg/S, the tapping liquid is flushed into the ladle wall below 50% of the height, and the tapping liquid time is less than or equal to 10S; and 2, after 8-10S of the start of the spheroidization reaction, adding the rest 10-15% of iron liquid.

Further, the molten iron is poured into the casting by adopting a gating system of an open cross runner.

Further, in the step of smelting the molten iron, the stirring frequency of the intermediate frequency furnace is 350-420Hz, and the heat preservation time of the molten iron is less than or equal to 40 minutes.

Further, the method for improving the graphite roundness of the nodular cast iron comprises the following process steps:

(1) smelting main furnace charge, 75 ferrosilicon, 65 high-carbon ferromanganese, electrolytic copper and graphite type carburant into pearlite matrix iron liquid or mixed matrix iron liquid through specific stirring frequency of an intermediate frequency furnace, and preserving heat of the pearlite matrix iron liquid or mixed matrix iron liquid;

(2) spheroidizing the molten iron: adding a low rare earth 1-6 nodulizer accounting for 0.95-1% of the mass of the iron liquid into a nodulizing ladle, uniformly mixing a BS-1A inoculant accounting for 0.28-0.6% of the mass of the iron liquid and a SiCaMn deoxidizer accounting for 0.08-0.1% of the mass of the iron liquid and having the granularity of 3-6mm, taking 0.2-0.25% of BS-1SB and 0.4% of 75 ferrosilicon as primary inoculants, and completing ladle filling according to the ladle filling sequence;

(3) tapping molten iron from the intermediate frequency furnace to a spheroidizing bag by adopting a flushing method, wherein the tapping and spheroidizing temperature is 1530 and 1550 ℃, and tapping the molten iron for 2 times;

(4) during pouring, a sulfur-oxygen inoculant with the granularity of 0.3-1mm is used as a secondary inoculant and added along with the flow, the addition point is the height position of 10cm at the outlet of a pouring ladle, the secondary inoculant amount is 0.15-0.2 percent of the mass of the poured iron liquid, and the secondary inoculant is added by a funnel with the flow rate of 21 g/s;

(5) molten iron is injected into a casting through a pouring system containing an open cross gate of 20-25cm, and the flow-resisting section ratio of the pouring system is sigma straight, sigma horizontal, sigma total in sigma 1:1.05-1.08: 0.94-0.98.

THE ADVANTAGES OF THE PRESENT INVENTION

The invention adopts the most common rushing-in process and the raw materials which can be purchased in the market, selects proper raw materials, reasonable furnace burden proportion and component design, improves the graphite roundness by innovating the nodulizing process of the nodular cast iron in front of the furnace, and has the following beneficial effects:

(1) in the existing spheroidizing process, a rare earth magnesium silicon iron spheroidizing agent is added, in the spheroidizing process, a part of free sulfur and oxygen in molten iron can be removed by utilizing high reaction activity of rare earth and magnesium, and the spheroidizing dose of 1.3-1.5% is usually added to achieve the graphite rounding effect. Most nodulizers react with sulfur in molten iron, and have little effect on removing oxygen. The addition of the nodulizer is increased, so that magnesium sulfide is mixed, primary slag and secondary slag inclusion are caused to form a casting, and the formation probability of cementite in a matrix is increased due to the excessively high magnesium residue; meanwhile, excessive nodulizer is added, so that the reaction in the nodulizing process is severe, the waste of molten iron is high, the smoke is high, and the magnesium absorption in the nodulizing process is unstable. In the spheroidizing process, 0.08-0.1% of SiCaMn deoxidizer is added, which plays an important role in removing free sulfur and oxygen in molten iron, and is used as a diffusion deoxidizer in the smelting of stainless steel castings, and when the temperature of molten steel in a furnace is raised to about 1600 ℃, the molten steel is added on the surface of the molten steel and is rapidly covered, and the furnace is stopped and stands for 5 minutes, so that the aim of diffusion deoxidation is fulfilled. The SiCaMn deoxidizer added in the invention can reduce the amount of free oxygen in molten iron in the smelting process of nodular cast iron, has an important effect on improving the smelting quality, particularly the graphite roundness, can reduce the addition of a nodulizer, can compensate the deoxidation effect on the basis of nodulizer desulfurization, ensures that the molten iron is deoxidized fully in the spheroidizing process, and the residual free oxygen does not cause excessive interference on the graphite growth. The highest cast iron can only reach about 1500-.

(2) Sulfides and oxides are generated in the spheroidizing process and float upwards to form slag, so that oxide cores such as magnesium oxide and magnesium sulfide are lacked in molten iron, the number of graphite cores is reduced, and graphite crystallization is influenced. The iron liquid is desulfurized and deoxidized fully by the SiCaMn deoxidizer, a good environment is created for the growth roundness of graphite, 0.15-0.2% of sulfur-oxygen inoculant is added during iron liquid pouring, the sulfur-oxygen inoculant is in a combined state, free oxygen and sulfur in the iron liquid cannot be increased, the sulfur-oxygen inoculant is sulfide and oxide, the sulfide and oxide are cores of efficient graphite nucleation, a large amount of carbon radicals can be promoted to attach to the sulfide and oxide, and the graphite nucleation is promoted, and the number of graphite is increased. The increase in the amount of graphite will significantly reduce the diameter of the graphite.

(3) The common spheroidizing process by the pouring method only can ensure that the spheroidization grade of the spherical graphite can only reach 2-3 grades, the spheroidization rate reaches 80-90 percent, the diameter of the graphite is 5-6 grades, and the quantity of the graphite is 100 plus one 250 pieces/mm²The core of the casting reaches the spheroidization grade of 3-4, and the spheroidization rate is 70-85%. The treatment process for improving the graphite roundness of the nodular cast iron is simple and convenient, has high reliability, can stably improve the spheroidization grade of the nodular graphite to reach 1-2 grade and above, the spheroidization rate reaches 93-99 percent, the spheroidization grade of the core of the casting reaches 2 grade, the spheroidization rate reaches 90-95 percent, the diameter of the graphite reaches 7 grade, and the number of the graphite can reach 300 and 400/mm²。

(4) The method is suitable for producing the nodular cast iron with a mixed matrix and a single matrix.

Drawings

FIG. 1: schematic diagram of the sequence of the spheroidized packaging bag.

FIG. 2: the amount of the first tapping iron is shown schematically.

FIG. 3: schematic drawing of tapping and punching positions of the intermediate frequency furnace.

FIG. 4: the iron liquid amount from the second tapping to the end is shown schematically.

FIG. 5: the pouring process is schematically illustrated as secondary inoculation.

FIG. 6: the structure of the crankshaft gating system used in embodiments 1 to 3 is schematically illustrated.

In the figure: 1. covering with pearl sand; 2. 75 silicon iron layer; 3. a mixed agent layer of Bs-1A, BS-1Sb and SiCaMn; 4. 1-6 low rare earth nodulizer layer; 5. 75 silicon iron layer; 6. a tapping furnace mouth of the intermediate frequency furnace; 7. pouring the tapping iron liquid into the position; 8. packaging materials in the reaction; 9. the height of the liquid level of the first tapping; 10. the height of the secondary tapping; 11. a secondary inoculant funnel; 12. a secondary inoculant; 13. the distance between a hopper feed opening and molten iron; 14. liquid iron; 15. casting a crankshaft; 16. pouring a ladle; 17. a crankshaft gating system; 18. a cross gate; 19. and (5) casting.

Detailed Description

The present invention is further explained below with reference to the accompanying drawings and specific embodiments, it should be noted that the present embodiments are not intended to limit the scope of the present invention. The invention adopts the following technical scheme to obtain the nodular iron casting with the nodularity of more than 85 percent and the graphite diameter of 6-7 grade, wherein the nodularity is grade 2.

(1) The main furnace material is composed of 20% of Q10 low-sulfur low-titanium pig iron, 55% of carbon steel and 25% of pearlite type return iron or 25% of mixed matrix return iron, and then 75 ferrosilicon accounting for 0.18-0.5% of the total mass of the main furnace material, 0.05-0.35% of 65 high-carbon ferromanganese, 0.12-0.5% of electrolytic copper (low lead, purity 99.99%) and 2.45-2.5% of graphite type carburant are added, and the medium-frequency stirring frequency is controlled to be 350-420Hz through medium-frequency furnace smelting, and the smelting component is: c: 3.8-3.9%, Si: 0.7-1.5%, Mn: 0.18-0.5%, Cu: 0.2 to 0.5 percent of crude iron liquid, less than or equal to 0.03 percent of S and less than or equal to 0.035 to 0.05 percent of Ti; the medium-low frequency stirring frequency can promote carbon atoms to be uniformly dispersed and exist in a proper atom group size form, and the heat preservation time of the molten iron is less than or equal to 40 minutes;

(2) spheroidizing the molten iron: adding a low rare earth 1-6 nodulizer accounting for 0.95-1% of the mass of the molten iron into a nodulizing ladle 5 before the molten iron nodulizing treatment, so as to realize 0.005-0.008% of residual RE and 0.03-0.04% of residual Mg in the final molten iron, and controlling the ratio of the residual RE to the Mg to be 1/5.5; respectively adding a BS-1A inoculant accounting for 0.28-0.32% of the mass of the iron liquid, 0.2-0.25% of BS-1SB, 0.08-0.1% of SiCaMn deoxidizer and 0.4% of 75 silicon iron serving as primary inoculants into the spheroidizing ladle 5, wherein the SiCaMn deoxidizer has the particle size of 3-6mm and needs to be uniformly mixed with BS-1A in advance; during packaging, the materials are packaged according to the sequence of figure 1, firstly 75 ferrosilicon with 1/3 accounting for 0.4% of the weight of the tapping liquid is placed on one side of a spheroidizing dam, then low rare earth 1-6 nodulizing agents 4 are poured, the low rare earth 1-6 nodulizing agents 4 are paved, then BS-1A, BS-1SB and SiCaMn mixture 3 are placed on the low rare earth 1-6 nodulizing agents 4, the remaining 75 ferrosilicon of 2/3 is added after paving, finally 1 kg of pearl sand is scattered in the middle of the packaging materials, two corners are leaked, electrolytic copper is placed on the pearl sand, and packaging is completed. The SiCaMn deoxidizer is used for reducing free oxygen in molten iron and preventing the free oxygen from occupying a graphite growth path when a carbon atom stacking structure is formed, so that the graphite growth in a certain direction is restricted and the spherical graphite morphology is deformed.

(3) Tapping liquid from the intermediate frequency furnace to a spheroidizing ladle, adopting an injection method, wherein the tapping and spheroidizing temperature is 1530-1550 ℃, the tapping step is divided into 2 times, 85-90% of liquid iron is discharged for the 1 st time (as shown in figure 2), the tapping speed is 500 kg/S, the height of the tapping liquid iron injected into the ladle wall is below 50% (as shown in figure 3, the spheroidizing agent cannot be directly injected), the tapping time for one time is less than or equal to 10 seconds, and after 8-10 seconds of the spheroidizing reaction, the remaining 10-15% of liquid iron is added (as shown in figure 4).

(4) During pouring, a sulfur-oxygen inoculant with the particle size of 0.3-1mm is used as a secondary inoculant and is added along with the flow, the addition point is the height position of 10cm at a ladle outlet, the inoculation amount is 0.15-0.2 percent of the mass of the poured iron liquid, and the secondary inoculant is added by a funnel with the flow speed of 21g/s (shown in figure 5).

(5) The casting pouring system comprises an open 20-25cm transverse pouring channel, the flow resisting section ratio is sigma straight, sigma is divided into horizontal sections, sigma is 1:1.05-1.08:0.94-0.98, and the purpose is to enable the secondary inoculant to enter the pouring system along with the flow, and enter the casting after being fully dissolved with molten iron in the pouring system (as shown in figure 6).

The following specific examples are made of a single pearlite matrix nodular cast iron crankshaft and a mixed matrix nodular cast iron bracket:

example 1: production of single pearlite matrix nodular cast iron crankshaft using prior art methods

The part weight was 48Kg, the basic wall thickness 25mm, and the external dimensions 600 x 240 x 245 mm.

(1) Using 20% of Q10 low-sulfur low-titanium pig iron, 55% of carbon steel and 25% of pearlite type return iron; then 75 ferrosilicon accounting for 0.18 percent of the total mass of the main furnace charge, 0.35 percent of 65 high-carbon ferromanganese, 0.4 percent of electrolytic copper (low lead, purity 99.99 percent) and 2.45 percent of graphite type carburant are added. Smelting to obtain base iron with the following components: c: 3.85%, Si: 0.78%, Mn: 0.42%, Cu: 0.44%, S: 0.012%, Ti: 0.031%.

(2) Spheroidizing: 1-6 nodulizer of low rare earth accounting for 0.95 percent of the mass of the molten iron, 0.32 percent of BS-1A inoculant, 0.21 percent of BS-1SB and 0.4 percent of 75 ferrosilicon are added into the nodulizing ladle to be used as primary inoculant.

(3) And (2) tapping molten iron in steps 2, tapping 85-90% of molten iron in step 1, wherein the tapping speed reaches 500 kg/S, the tapping molten iron is flushed into the ladle wall below 50% of the height, the tapping time for one time is 10 seconds, and after the spheroidization reaction starts to 8-10S h, the remaining 10-15% of molten iron is added.

(4) When pouring, BS-1 with the granularity of 0.3-1mm is taken as a secondary inoculant and added with the stream, the addition point is the height position of 10cm at a pouring ladle outlet, the inoculation agent is 0.15 percent of the mass of the poured molten iron, and the inoculation agent is added by a funnel with the flow speed of 21 g/s. The residual magnesium of the final molten iron is 0.036%, and the content of rare earth: 0.006 percent.

The body is dissected, the spheroidization rate of the dangerous section is 90 percent, the graphite diameter is 0.05-0.06mm, the graphite diameter grade is 6, and the number of graphite is 230/mm²。

Example 2: crankshaft for producing single pearlite matrix nodular cast iron

(2) Spheroidizing: 1-6 nodulizer of low rare earth accounting for 0.95 percent of the mass of the molten iron, 0.32 percent of BS-1A inoculant and 0.21 percent of BS-1SB are added into the nodulizing ladle, 0.1 percent of SiCaMn deoxidizer and 0.4 percent of 75 ferrosilicon are added as primary inoculants.

(4) When pouring, BS-1 with the granularity of 0.3-1mm is taken as a secondary inoculant and added with the stream, the addition point is the height position of 10cm at a pouring ladle outlet, the inoculation agent is 0.15 percent of the mass of the poured molten iron, and the inoculation agent is added by a funnel with the flow speed of 21 g/s. The residual magnesium of the final molten iron is 0.036%, and the content of rare earth: 0.006%, dissection of the body, grade 2 of dangerous section spheroidization rate (spheroidization rate 94%), 0.05-0.06mm of graphite diameter, grade 6 of graphite diameter, and 250 graphite/mm²。

Example 3: crankshaft for producing single pearlite matrix nodular cast iron

The part weight was 70Kg, the basic wall thickness 25mm, and the external dimensions 800 x 250 mm.

(1) Using 20% of Q10 low-sulfur low-titanium pig iron, 55% of carbon steel and 25% of pearlite type return iron; then adding 75 ferrosilicon accounting for 0.18 percent of the total mass of the main furnace charge, 0.35 percent of 65 high-carbon ferromanganese, 0.4 percent of electrolytic copper (low lead, purity 99.99 percent) and 2.45 percent of graphite type carburant, and smelting to obtain the original iron liquid with the following components: c: 3.8-3.9%, Si: 0.7-0.9%, Mn: 0.4-0.5%, Cu: 0.4-0.5%, S is less than or equal to 0.03%, and Ti is less than or equal to 0.035%.

(2) Spheroidizing: 1-6 nodulizer of low rare earth accounting for 0.95 percent of the mass of the tapping liquid, 0.32 percent of BS-1A inoculant, 0.22 percent of BS-1SB, 0.1 percent of SiCaMn deoxidizer and 0.4 percent of 75 ferrosilicon are added into the nodulizing ladle to be used as a primary inoculant.

(3) Tapping molten iron in 2 steps, tapping 85-90% of molten iron in the 1 st time, tapping at a tapping speed of 500 kg/S, pouring the molten iron into the ladle wall at a height of below 50%, and tapping for 8 seconds for one time; and 2, after 8-10S h of the start of the spheroidization reaction, adding the rest 10-15% of iron liquid.

(4) During pouring, a sulfur-oxygen inoculant with the particle size of 0.3-1mm is used as a secondary inoculant and added with the stream, the addition point is the height position of 10cm at a ladle outlet, the inoculation amount is 0.15 percent of the mass of the poured iron liquid, and the secondary inoculant is added with a funnel with the flow speed of 21 g/s. The residual magnesium of the final iron liquid is 0.037%, rare earth: 0.007%.

Through the dissection of the body, the spheroidization grade of the dangerous section reaches 1 grade, the spheroidization rate reaches 98 percent, the diameter of the graphite is 0.02-0.028mm, the diameter of the graphite is 7 grades, and the number of the graphite is 334/mm²。

Single pearlite base ductile iron crankshafts produced by the methods of examples 1-3 were compared as shown in table 1:

table 1:

in table 1, the common flushing process is adopted in the examples 1, 2 and 3, but the SiCaMn deoxidizer and the secondary inoculant of the sulfur-oxygen inoculant with the particle size of 0.3-1mm are added into the primary inoculant in the example 3, so that the graphite spheroidization grade is improved to 1 grade, the spheroidization rate is obviously improved to 98 percent, the diameter of the graphite is obviously refined, and the quantity of the graphite is obviously improved. Therefore, the SiCaMn deoxidizer is applied to the field of nodular cast iron, the sulfur-oxygen inoculant with the granularity of 0.3-1mm is added along with the flow to serve as the secondary inoculant, and the secondary inoculant is complementary in the spheroidization process, and reasonable process and material proportion are adopted, so that the improvement on the graphite roundness and the improvement on the graphite form have great significance.

Example 4: method for producing mixed matrix nodular cast iron bracket by using prior art

The part weight was 53Kg, the basic wall thickness was 17mm, and the external dimension was 680 x 400 x 440 mm.

(1) Q10 low-sulfur low-titanium pig iron 10%, carbon steel 50%, mixed matrix type return iron 30%; then 75 ferrosilicon accounting for 0.5 percent of the total mass of the main furnace charge, 0.05 percent of 65 high-carbon ferromanganese, 0.12 percent of electrolytic copper (low lead, purity 99.99 percent) and 2.6 percent of graphite type carburant are added. The raw iron obtained by smelting comprises the following components: c: 3.87%, Si: 1.42%, Mn: 0.26%, Cu: 0.25%, S: 0.013%, Ti: 0.031%.

(2) Spheroidizing: 2-6 and 3-7 of low rare earth nodulizer are selected and mixed according to the mass ratio of 4:1 to prepare the special nodulizer. Adding a special nodulizer accounting for 1.1 percent of the mass of the molten iron into the nodulizing ladle, adding a BS-1A inoculant accounting for 0.45 percent of the mass of the molten iron, and taking 0.4 percent of 75 ferrosilicon as a primary inoculant.

(3) And (2) tapping molten iron in steps 2, tapping 85-90% of molten iron in step 1, wherein the tapping speed reaches 500 kg/S, the tapping molten iron is flushed into the ladle wall below 50% of the height, tapping is completed within 10 seconds, and after the spheroidization reaction starts to be 8-10S h, the remaining 10-15% of molten iron is added.

(4) During pouring, 75 ferrosilicon with the granularity of 0.3-1mm is taken as a secondary inoculant and added with the stream, the addition point is the height position of 10cm at a pouring ladle outlet, the inoculation agent is 0.17 percent of the mass of the poured molten iron, and the secondary inoculant is added by a funnel with the flow speed of 12 g/s. The final molten iron realizes that the residual RE of the final molten iron is 0.022 percent, the residual Mg is 0.038 percent, the ratio of the residual RE to the Mg is controlled to 2/3, the residual Mg is 0.036 percent, and the ratio of rare earth: 0.008 percent.

The body is dissected, the nodulizing rate of the section is 90 percent, the graphite diameter is 0.04-0.05mm, the graphite diameter grade is 6, and the number of graphite is 250/mm²。

Example 5:

(2) Spheroidizing: 2-6 and 3-7 of low rare earth nodulizer are selected and mixed according to the mass ratio of 4:1 to prepare the special nodulizer. Adding a special nodulizer accounting for 1.1 percent of the mass of the molten iron into the nodulizing ladle, adding a BS-1A inoculant accounting for 0.45 percent of the mass of the molten iron, adding a SiCaMn deoxidizer of 0.09 percent, and taking 0.4 percent of 75 ferrosilicon as a primary inoculant.

The body is dissected, the section spheroidization grade is 2 grade, the spheroidization rate is 93 percent, the graphite diameter is 0.04-0.05mm, the graphite diameter grade is 6 grade, and the graphite quantity is 290/mm²。

Example 6: bracket for producing mixed matrix nodular cast iron

The part weight was 48Kg, the basic wall thickness was 15mm, and the external dimensions were 680 x 400 x 380 mm.

(1) Q10 low-sulfur low-titanium pig iron 10%, carbon steel 50%, mixed matrix type return iron 30%; then 75 ferrosilicon accounting for 0.5 percent of the total mass of the main furnace charge, 0.05 percent of 65 high-carbon ferromanganese, 0.12 percent of electrolytic copper (low lead, purity 99.99 percent) and 2.6 percent of graphite type carburant are added. The raw iron obtained by smelting comprises the following components: c: 3.85%, Si: 1.41%, Mn: 0.26%, Cu: 0.27%, S: 0.015%, Ti: 0.028 percent.

(2) Spheroidizing: 1-6 percent of nodulizer accounting for 1 percent of the mass of the molten iron, 0.6 percent of BS-1A inoculant, 0.09 percent of SiCaMn deoxidizer and 0.4 percent of 75 ferrosilicon serving as a primary inoculant are added into the nodulizing ladle.

(4) During pouring, a sulfur-oxygen inoculant with the granularity of 0.3-1mm is used as a secondary inoculant and added along with the flow, the addition point is the height position of 10cm at a pouring ladle outlet, the inoculation agent is 0.17 percent of the mass of the poured molten iron, and the secondary inoculant is added by a funnel with the flow speed of 21 g/s. The residual magnesium of the final molten iron is 0.036%, and the content of rare earth: 0.008 percent.

The body is dissected, the section spheroidization grade is 1 grade, the spheroidization rate is 99 percent, the graphite grade is 7 grade, and the number of graphite is 380 per mm². After the method is used, the spheroidization grade of the graphite is obviously improved, the diameter of the graphite is reduced, and the quantity of the graphite is increased.

Mixed matrix ductile iron brackets produced by the methods of examples 6-6 were compared as shown in table 2:

TABLE 2

In table 2, the common pour-in process is adopted in the examples 4, 5 and 6, but the SiCaMn deoxidizer and the secondary inoculant of the sulfur-oxygen inoculant with the particle size of 0.3-1mm are added into the primary inoculant in the example 6, so that the graphite spheroidization grade is improved to 1 grade, the spheroidization rate is obviously improved to 99 percent, the diameter of the graphite is obviously refined, and the quantity of the graphite is obviously improved. Therefore, the SiCaMn deoxidizer is applied to the field of nodular cast iron, the sulfur-oxygen inoculant with the granularity of 0.3-1mm is added along with the flow to serve as the secondary inoculant, and the secondary inoculant is complementary in the spheroidization process, and reasonable process and material proportion are adopted, so that the improvement on the graphite roundness and the improvement on the graphite form have great significance.

Claims

1. A method for improving graphite form of nodular cast iron is characterized in that in the step of spheroidizing molten iron, a nodulizer and a primary inoculant are added into a spheroidizing ladle, wherein the primary inoculant comprises a BS-1A inoculant, a SiCaMn deoxidizer with the granularity of 3-6mm, BS-1SB and 75 silicon iron.

2. The method for producing nodular cast iron having improved graphite morphology according to claim 1, wherein in the step of spheroidizing the molten iron, the packing sequence is as follows: firstly, 1/3 parts of 75 ferrosilicon are placed on one side of a spheroidizing dam, then a spheroidizing agent is poured, the spheroidizing agent is paved, then a BS-1A, BS-1SB and SiCaMn mixture is placed on the spheroidizing agent, the rest 75 ferrosilicon of 2/3 parts is added after paving, finally 1 kg of pearl sand is scattered in the middle of a ladle material, two corners are exposed, electrolytic copper is placed on the pearl sand, and the ladle is finished.

3. The method for producing ductile iron with improved graphite morphology according to claim 2, wherein the molten iron is pearlite matrix molten iron or mixed matrix molten iron.

4. The method for producing nodular cast iron with improved graphite morphology according to claim 3, wherein the pearlite matrix iron liquid is prepared by melting a main furnace charge, 20% of low-sulfur low-titanium pig iron, 55% of carbon steel and 25% of pearlite type recycled iron according to the mass ratio of Q10, and then adding 75 ferrosilicon accounting for 0.18% of the total mass of the main furnace charge, 0.35% of 65 high-carbon ferromanganese, 0.4% of electrolytic copper and 2.45% of graphite type carburant through an intermediate frequency furnace, wherein the medium frequency furnace comprises the following components: c: 3.8-3.9%, Si: 0.7-0.9%, Mn: 0.4-0.5%, Cu: 0.4-0.5%, S is less than or equal to 0.03%, and Ti is less than or equal to 0.035%.

5. The method for producing nodular cast iron with improved graphite morphology according to claim 3, wherein the mixed matrix iron liquid is prepared by using raw materials and 10% of low-sulfur low-titanium pig iron, 50% of carbon steel and 30% of mixed matrix type recycled iron as main charging materials, and further adding 0.5% of 75 ferrosilicon, 0.05% of 65 high-carbon ferromanganese, 0.12% of electrolytic copper and 2.6% of graphite type carburant by mass of the main charging materials, and obtaining the following components by intermediate frequency furnace smelting: c: 3.8-3.9%, Si: 1.3-1.5%, Mn: 0.18-0.28%, Cu: 0.2-0.3%, S is less than or equal to 0.03%, and Ti is less than or equal to 0.035%.

6. The method as claimed in claim 1, wherein the pouring step is carried out by adding a sulfur-oxygen inoculant as a secondary inoculant into the stream at a height of 10cm from the outlet of the ladle.

7. The method for producing ductile iron with improved graphite morphology according to claim 1, wherein in the tapping step, 85-90% of the tapping iron liquid is tapped at the 1 st time, the tapping speed is 500 kg/S, the tapping iron liquid is flushed into the ladle wall at a height of less than 50%, and the tapping time is less than or equal to 10S; and 2, after 8-10S of the start of the spheroidization reaction, adding the rest 10-15% of iron liquid.

8. The method for producing ductile iron with improved graphite morphology according to claim 1, wherein said molten iron is poured into the casting using a gating system of open cross runners.

9. The method as claimed in claim 1, wherein the stirring frequency of the intermediate frequency furnace is 350-420Hz, and the holding time for the molten iron is less than or equal to 40 minutes.

10. A method for producing spheroidal graphite cast iron with improved graphite morphology according to any of claims 1-9, characterized by comprising the following process steps:

(2) spheroidizing the molten iron: adding a low rare earth 1-6 nodulizer accounting for 0.95-1% of the mass of the iron liquid into the nodulizing ladle to realize that the residual RE of the final iron liquid is 0.005-0.008% and the residual Mg is 0.03-0.04%, and controlling the ratio of the residual RE to the Mg to be 1/5.3-1/5.5; firstly, uniformly mixing a BS-1A inoculant accounting for 0.28-0.6 percent of the mass of the discharged iron liquid and a SiCaMn deoxidizer accounting for 0.08-0.1 percent of the particle size of 3-6mm, then taking 0.2-0.25 percent of BS-1SB and 0.4 percent of 75 ferrosilicon as primary inoculants, and completing the packaging according to the packaging sequence;