Matrix is low carbon martensite / ferrite duplex spheroidal graphite cast iron and its preparation method

Abstract

The disclosed base body is low-carbon martensite / ferrite dual-phase nodular cast iron, which is composed of the following raw material components in percentage by mass: C 3.3% to 3.6%, Si 2.4% to 2.7%, Mn 0.1% to 0.3%, P ≤0.1%, S ≤0.015%, Ni 6.1% to 6.5%, Mg 0.08% to 0.11%, and the balance of Fe, and the structure is 10% spherical graphite, 60% to 70% low-carbon martensite and 20% to 30% ferrite. The preparation method comprises the following steps: weighing scrap steel, ferrosilicon, nickel plate, manganese iron and carbon additive according to the component composition of nodular cast iron, putting the raw materials into a medium-frequency induction furnace to heat and smelt into molten iron; adding spheroidizing agent and inoculant into the molten iron; continuously casting according to the horizontal continuous casting process specification to obtain a casting; putting the casting into a vacuum tube furnace to heat and keep warm, cooling and secondary keeping warm, taking out after furnace cooling and air cooling; then putting into a muffle furnace to keep warm, taking out and quenching to obtain the low-carbon martensite / ferrite dual-phase nodular cast iron with good plasticity and toughness.

Description

Technical Field

[0001] This invention belongs to the field of metal materials technology, specifically relating to low-carbon martensitic / ferritic duplex ductile iron with a matrix of low carbon. This invention also relates to a method for preparing low-carbon martensitic / ferritic duplex ductile iron with a matrix of low carbon. Background Technology

[0002] To meet the requirements of lightweight manufacturing, aluminum alloys, due to their low density, corrosion resistance, and high strength, are widely used in aerospace, automotive engineering, and construction engineering industries. After 20 years of development, my country's annual aluminum production has exceeded half of the global total and is still in a phase of rapid growth. The booming aluminum industry inevitably drives demand in its related industrial chains, such as the aluminum extrusion die industry. Extrusion is the main method of aluminum forming. Currently, most aluminum extrusion dies are made of H13 steel, but it is prone to adhesion problems with molten aluminum. Taking 5Cr series die steel in H13 steel as an example, it was an early die material developed to meet the service conditions of steel processing. Studies have shown that aluminum-iron adhesion can cause various failure problems in dies, such as thermal fatigue cracking, surface fragmentation, and corrosion.

[0003] Based on the physicochemical properties of aluminum melt not wetting graphite at temperatures not exceeding 800℃, the nearly 10 vol.% spherical graphite in ductile iron can not only reduce stress concentration at irregular spherical interfaces but also lower the coefficient of friction and reduce aluminum-iron adhesion strength, making it a promising material for aluminum alloy extrusion dies. However, testing after traditional heat treatment of ductile iron revealed drawbacks in material replacement. While isothermal quenching can achieve strength similar to H13 steel, its toughness is lower, often resulting in thermal fatigue cracking and brittle fracture. Furthermore, the isothermal quenching temperature should not exceed 500℃. At 500℃, the austenitic structure of isothermal quenched ductile iron (ADI) decomposes, leading to high-temperature softening, accelerated wear, and reduced die life. Additionally, the high carbon content in the austenitic structure of the ADI matrix, being a brittle phase, further contributes to poor plasticity and toughness. Therefore, if the current problems of insufficient room-temperature toughness and short service life of ductile iron can be solved, ductile iron could replace H13 steel in aluminum alloy extrusion dies. Summary of the Invention

[0004] The purpose of this invention is to provide a low-carbon martensitic / ferritic duplex ductile iron matrix, which solves the problems of cracking and short service life caused by insufficient plasticity and toughness of existing ductile iron when used as aluminum alloy extrusion die material.

[0005] Another object of the present invention is to provide a method for preparing low-carbon martensitic / ferritic duplex ductile iron.

[0006] The technical solution adopted in this invention is that the matrix is ​​low-carbon martensitic / ferritic duplex ductile iron, which is composed of the following raw material components by mass percentage: C: 3.3%~3.6%, Si: 2.4%~2.7%, Mn: 0.1%~0.3%, P≤0.1%, S: ≤0.015%, Ni: 6.1%~6.5%, Mg: 0.08%~0.11%, with the balance being Fe.

[0007] The invention is further characterized in that,

[0008] By volume fraction, ductile iron has a microstructure of 10% spheroidal graphite, 60%–70% low-carbon martensite, and 20%–30% ferrite.

[0009] Another technical solution adopted in this invention is a method for preparing low-carbon martensitic / ferritic duplex ductile iron, which is specifically implemented according to the following steps:

[0010] Step 1: Weigh out the following materials according to the composition of ductile iron: scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser. Place the weighed raw materials into a medium-frequency induction furnace and heat them to melt into molten iron.

[0011] Step 2: Add spheroidizing agent and inoculant to the molten iron smelted in Step 1;

[0012] Step 3: Pour the molten iron that has been spheroidized and inoculated in Step 2 into the preheated horizontal continuous casting furnace, and continuously cast the castings according to the horizontal continuous casting process specifications.

[0013] Step 4: Place the casting obtained in Step 3 into a vacuum tube furnace for heating and holding, then cool it down and hold it for a second time before cooling it with the furnace, and finally take it out and air cool it.

[0014] Step 5: Place the air-cooled casting from Step 4 into a muffle furnace for heat preservation, then remove and quench it to obtain a low-carbon martensitic / ferritic duplex ductile iron matrix.

[0015] Another feature of the technical solution of the present invention is that,

[0016] In step 1, the total carbon equivalent in the raw materials is 4.3 wt%; the heating rate of the medium-frequency induction furnace is 70℃ / min~80℃ / min, the melting time is 10min~40min, and the melting temperature is 1450℃~1550℃.

[0017] In step 2, the mass of the spheroidizing agent accounts for 1.1% to 1.4% of the total mass of the molten iron, and the mass of the inoculant accounts for 1.2% to 1.5% of the total mass of the molten iron.

[0018] In step 2, the inoculant is a silicon-magnesium-calcium-barium inoculant, and the spheroidizing agent is silicon carbide.

[0019] In step 3, the drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the resulting casting is not less than 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 .

[0020] In step 4, the vacuum degree in the vacuum tube furnace is -0.1MPa, the heating temperature is 900℃~950℃, and the holding time is 4h~6h; the cooling is specifically to cool down to 500℃ at a rate of 0.2℃ / min, and the secondary holding time is 4h~6h; and then the furnace is cooled to below 300℃.

[0021] In step 5, the heat preservation temperature is 750℃~800℃, and the heat preservation time is 1h~2h;

[0022] The quenching medium is water, the quenching temperature is 750℃-800℃, and the interval between the casting being taken out of the muffle furnace and entering the water for quenching is no more than 6 seconds.

[0023] The beneficial effects of this invention are:

[0024] 1) The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, which is obtained by horizontal continuous casting, and the ductile iron density is not less than 500 particles / mm². 2 This provides more in-situ carbon points for carbon diffusion and re-dissolution during subsequent heat treatment;

[0025] 2) When Ni alloying element is added to ductile iron, it can be held at 750℃-800℃ during heat treatment. The temperature range is in the α / γ dual-phase region, where carbon diffusion is slow and the carbon content dissolved into the γ phase is low. After quenching, the carbon content in the martensite is low, which is low-carbon martensite. This greatly improves the plasticity and toughness of the material. Attached Figure Description

[0026] Figure 1 This is a SEM image of the low-carbon martensitic / ferritic duplex ductile iron prepared in Example 1 of the present invention.

[0027] Figure 2 This is a comparison diagram of stress-strain curves of Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0028] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings.

[0029] The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, which is composed of the following raw material components by mass percentage: C: 3.3%~3.6%, Si: 2.4%~2.7%, Mn: 0.1%~0.3%, P≤0.1%, S: ≤0.015%, Ni: 6.1%~6.5%, Mg: 0.08%~0.11%, with the balance being Fe.

[0030] By volume fraction, the microstructure of low-carbon martensitic / ferritic duplex ductile iron consists of 10% spheroidal graphite, 60%–70% low-carbon martensite, and 20%–30% ferrite.

[0031] The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to the present invention is implemented according to the following steps:

[0032] Step 1: Weigh the raw materials according to the composition of ductile iron, including: scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser. The total carbon equivalent in the raw materials is 4.3 wt%. Place the weighed raw materials into a medium-frequency induction furnace and heat them to 1450℃~1550℃ at a heating rate of 70℃ / min~80℃ / min. Melt for 10min~40min to form molten iron.

[0033] Step 2: Add spheroidizing agent and inoculant to the molten iron smelted in Step 1;

[0034] The inoculant is a silicon-magnesium-calcium-barium inoculant, and the spheroidizing agent is silicon carbide;

[0035] The mass of the spheroidizing agent accounts for 1.1% to 1.4% of the total mass of the molten iron, and the mass of the inoculant accounts for 1.2% to 1.5% of the total mass of the molten iron.

[0036] Step 3: Pour the molten iron that has been spheroidized and inoculated in Step 2 into the furnace of a preheated horizontal continuous casting furnace, and obtain the casting by horizontal continuous casting method; the casting is as-cast material containing Ni ductile iron; the subsequent steps require heat treatment to control the lath-shaped low-carbon martensite.

[0037] The drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the resulting casting is 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 The microstructure of the casting consists of high-carbon martensite, ferrite, and bainite.

[0038] Step 4: Place the casting obtained in Step 3 in a vacuum tube furnace at 900℃~950℃ and hold for 4h~6h to obtain an austenitic structure and fully austenitize it. Then, cool the vacuum tube furnace to 500℃ at a cooling rate of 0.2℃ / min and hold it at 500℃ for 4h~6h to allow as much carbon in the austenite as possible to dissolve back into the graphite. After that, cool it with the furnace to below 300℃ and then take it out and air cool it. The cooling process yields a material with a full ferritic matrix. The purpose is to reduce the carbon content in the matrix structure to a minimum before quenching, which is more conducive to obtaining low-carbon martensite during quenching.

[0039] The vacuum level of the vacuum tube furnace is -0.1 MPa;

[0040] The purpose of selecting a vacuum tube furnace is to avoid decarburization of the material surface during the austenitization process.

[0041] Step 4 is to anneal to obtain a fully ferrite matrix. Because the carbon content dissolved in ferrite is relatively low, and the addition of the alloying element Ni to the material reduces the diffusion rate of carbon and expands the austenite phase region, subsequent heat treatment can be carried out at a lower temperature, resulting in a lower amount of dissolved carbon in the matrix.

[0042] Step 5: Place the air-cooled casting from Step 4 into a muffle furnace at 750℃~800℃ and hold for 1h~2h to allow less carbon to dissolve in the γ phase. Then, take it out and quench it in water at a temperature of 750℃-800℃ to obtain a low-carbon martensitic / ferritic duplex ductile iron matrix.

[0043] The interval between removing the casting from the muffle furnace and quenching it in water should not exceed 6 seconds.

[0044] In step 5, quenching after holding at a lower temperature slows down the diffusion rate of carbon. This allows for the formation of lath-shaped low-carbon martensite after holding at a lower temperature for a longer period of time, eliminating the need for short-term high-temperature holding and enabling time control.

[0045] The present invention relates to a method for preparing low-carbon martensitic / ferritic duplex ductile iron. The principle is as follows: Low-carbon martensite, due to its good strength and toughness, can be obtained by short-time holding followed by quenching. However, this method is difficult to apply to large parts because carbon diffuses rapidly, and a slightly longer holding time results in a high carbon content in the quenched martensite, failing to achieve the desired microstructure. This invention draws on the alloying principle of steel by adding the alloying element Ni to the ductile iron. Ni has the same crystal structure as γ-Fe, thus it can be infinitely dissolved in γ-Fe, thereby lowering the austenite transformation temperature, hindering the diffusion rate of carbon, and expanding the temperature range of the austenite single-phase region. This allows for quenching at a lower temperature, reducing the carbon content dissolved in the γ-phase. The resulting martensite also has a lower carbon content, which is more conducive to obtaining a low-carbon martensite microstructure. Simultaneously, strict control over the proportions of each component in the raw materials and each step of the casting process ensures the quality and quantity of graphite spheres in the material, resulting in a graphite density of not less than 500 spheres / mm². 2 The castings were prepared to fully utilize the self-lubricating properties of ductile iron. Then, a suitable heat treatment process was used, namely, quenching in the dual-phase region to obtain a low-carbon martensitic matrix structure. By extending the holding time before quenching, the process window was expanded. Finally, while ensuring the strength of the material, its plasticity and toughness were improved, and a low-carbon martensitic / ferritic dual-phase ductile iron matrix was prepared, thus extending its service life.

[0046] The present invention relates to a method for preparing low-carbon martensitic / ferritic duplex ductile iron, wherein the ductile iron castings obtained by horizontal continuous casting have a ductile iron density number ≥ 500 ductile iron particles / mm². 2 By using a suitable heat treatment process, the casting is first annealed to obtain full ferrite, and then quenched after holding at 750℃-800℃ in the two-phase region to obtain a low-carbon martensitic / ferrite duplex ductile iron matrix.

[0047] Example 1

[0048] The matrix prepared in Example 1 of this invention is a low-carbon martensitic / ferritic duplex ductile iron, composed of the following raw material components by mass percentage: C: 3.3%, Si: 2.4%, Mn: 0.1%, P≤0.1%, S: ≤0.015%, Ni: 6.1%, Mg: 0.08%, with the balance being Fe. By volume fraction, the microstructure of the low-carbon martensitic / ferritic duplex ductile iron matrix consists of 10% spheroidal graphite, 65% lath-like low-carbon martensite, and 25% ferrite.

[0049] The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to the present invention is implemented according to the following steps:

[0050] Step 1: Weigh out scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser according to the composition of ductile iron, and put the weighed raw materials into a medium frequency induction furnace. Heat them to 1500℃ at a heating rate of 75℃ / min and melt them for 25 minutes to form molten iron.

[0051] Step 2: Add a spheroidizing agent and an inoculant to the molten iron prepared in Step 1; the spheroidizing agent is a silicon-magnesium-calcium-barium inoculant, accounting for 1.2% of the total mass of the molten iron; the inoculant is silicon carbide, accounting for 1.2% of the total mass of the molten iron.

[0052] Step 3: Pour the molten iron containing the spheroidizing agent and inoculant from Step 2 into the preheated horizontal continuous casting furnace, and obtain the casting using the horizontal continuous casting method; the drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the obtained casting is 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 .

[0053] Step 4: Place the casting obtained in Step 3 in a vacuum tube furnace at 930℃ and hold for 5 hours to fully austenitize it. Then, cool the vacuum tube furnace to 500℃ at a cooling rate of 0.2℃ / min. Hold the material at 500℃ for 5 hours and then cool it with the furnace to below 300℃. Finally, remove it and air cool it. The vacuum degree of the vacuum tube furnace is -0.1MPa.

[0054] Step 5: Place the product from Step 4 in a muffle furnace at 750℃ and hold for 2 hours. Then remove it and quench it at 780℃ to obtain a low-carbon martensitic / ferritic duplex ductile iron.

[0055] The matrix prepared in this invention is low-carbon martensitic / ferritic duplex ductile iron, and the SEM image of the microstructure is shown below. Figure 1 As shown, the black spheres are spherical graphite, the lath-like structure is low-carbon martensite, and the dark gray blocks are ferrite.

[0056] Example 2

[0057] The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, composed of the following raw material components by mass percentage: C: 3.6%, Si: 2.7%, Mn: 0.3%, P≤0.1%, S: ≤0.015%, Ni: 6.5%, Mg: 0.11%, with the balance being Fe. By volume fraction, the microstructure of the ductile iron is 10% spheroidal graphite, 80% low-carbon martensite, and 10% ferrite. The microstructure of the low-carbon martensitic / ferritic duplex ductile iron matrix is ​​10% spheroidal graphite by volume, 60% lath-like low-carbon martensite by volume, and 30% ferrite by volume.

[0058] The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to the present invention is implemented according to the following steps:

[0059] Step 1: Weigh out scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser according to the composition of ductile iron. Heat the weighed raw materials in a medium-frequency induction furnace to 1450°C at a heating rate of 80°C / min and melt for 40 minutes to form molten iron.

[0060] Step 2: Add a spheroidizing agent and an inoculant to the molten iron prepared in Step 1; the spheroidizing agent is a silicon-magnesium-calcium-barium inoculant, accounting for 1.4% of the total mass of the molten iron; the inoculant is silicon carbide, accounting for 1.5% of the total mass of the molten iron.

[0061] Step 3: Pour the molten iron containing the spheroidizing agent and inoculant from Step 2 into the preheated horizontal continuous casting furnace, and obtain the casting using the horizontal continuous casting method; the drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the obtained casting is 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 .

[0062] Step 4: Place the casting obtained in Step 3 in a vacuum tube furnace at 900℃ and hold for 6 hours to fully austenitize it. Then, cool the vacuum tube furnace to 500℃ at a cooling rate of 0.2℃ / min. Hold the material at 500℃ for 6 hours and then cool it with the furnace to below 300℃. Finally, remove it and air cool it. The vacuum degree of the vacuum tube furnace is -0.1MPa.

[0063] Step 5: Place the material from Step 4 in a muffle furnace at 800℃ and hold for 1 hour. Then remove it and quench it at 750℃ to obtain a low-carbon martensitic / ferritic duplex ductile iron matrix.

[0064] Example 3

[0065] The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, composed of the following raw material components by mass percentage: C: 3.5%, Si: 2.6%, Mn: 0.2%, P≤0.1%, S: ≤0.015%, Ni: 6.3%, Mg: 0.09%, with the balance being Fe. The microstructure of the low-carbon martensitic / ferritic duplex ductile iron matrix consists of 10% by volume spheroidal graphite, 70% by volume lath-like low-carbon martensite, and 20% by volume ferrite.

[0066] The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to the present invention is implemented according to the following steps:

[0067] Step 1: Weigh out scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser according to the composition of ductile iron. Heat the weighed raw materials in a medium-frequency induction furnace at a heating rate of 70℃ / min to 1550℃ and melt for 10 minutes to form molten iron.

[0068] Step 2: Add a spheroidizing agent and an inoculant to the molten iron prepared in Step 1; the spheroidizing agent is a silicon-magnesium-calcium-barium inoculant, accounting for 1.1% of the total mass of the molten iron; the inoculant is silicon carbide, accounting for 1.3% of the total mass of the molten iron.

[0069] Step 3: Pour the molten iron containing the spheroidizing agent and inoculant from Step 2 into the preheated horizontal continuous casting furnace, and obtain the casting using the horizontal continuous casting method; the drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the obtained casting is 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 .

[0070] Step 4: Place the casting obtained in Step 3 in a vacuum tube furnace at 950℃ and hold for 4 hours to fully austenitize it. Then, cool the vacuum tube furnace to 500℃ at a cooling rate of 0.2℃ / min. Hold the material at 500℃ for 4 hours and then cool it with the furnace to below 300℃. Finally, remove it and air cool it. The vacuum degree of the vacuum tube furnace is -0.1MPa.

[0071] Step 5: Place the material from Step 4 in a muffle furnace at 780℃ and hold for 2 hours. Then remove it and quench it at 800℃ to obtain a low-carbon martensitic / ferrite duplex ductile iron with a matrix of 10% spheroidal graphite, 85% lath low-carbon martensite, and 5% ferrite.

[0072] Example 4

[0073] The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, composed of the following raw material components by mass percentage: C: 3.5%, Si: 2.6%, Mn: 0.1%, P≤0.1%, S: ≤0.015%, Ni: 6.4%, Mg: 0.1%, with the balance being Fe. The microstructure of the low-carbon martensitic / ferritic duplex ductile iron matrix consists of 10% by volume spheroidal graphite, 68% by volume lath-like low-carbon martensite, and 22% by volume ferrite.

[0074] The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to the present invention is implemented according to the following steps:

[0075] Step 1: Weigh out scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser according to the composition of ductile iron. Heat the weighed raw materials in a medium frequency induction furnace at a heating rate of 75℃ / min to 1500℃ and melt for 25 minutes to form molten iron.

[0076] Step 2: Add spheroidizing agent and inoculant to the molten iron obtained in Step 1; the spheroidizing agent is 1.2% of the mass of the molten iron obtained in Step 1, and the inoculant is 1.3% of the mass of the molten iron obtained in Step 1;

[0077] Step 3: Pour the molten iron containing the spheroidizing agent and inoculant from Step 2 into the preheated horizontal continuous casting furnace, and obtain the casting using the horizontal continuous casting method; the drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the obtained casting is 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 .

[0078] Step 4: Place the casting obtained in Step 3 in a vacuum tube furnace at 900℃ and hold for 5 hours to fully austenitize it. Then, cool the vacuum tube furnace to 500℃ at a cooling rate of 0.2℃ / min and hold the material at 500℃ for 6 hours. After holding at 500℃, cool the material with the furnace to below 300℃ and then remove it for air cooling. The vacuum degree of the vacuum tube furnace is -0.1MPa.

[0079] Step 5: Place the material from Step 4 in a muffle furnace at 750℃ and hold for 2 hours. Then remove it and quench it at 780℃ to obtain a low-carbon martensitic / ferrite duplex ductile iron with a matrix of 10% spheroidal graphite, 85% lath low-carbon martensite, and 5% ferrite.

[0080] Example 5

[0081] The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, which is composed of the following raw material components by mass percentage: C: 3.5%, Si: 2.5%, Mn: 0.1%, P≤0.1%, S: ≤0.015%, Ni: 6.5%, Mg: 0.11%, with the balance being Fe.

[0082] By volume fraction, ductile iron has a microstructure of 10% spheroidal graphite, 62% low-carbon martensite, and 28% ferrite.

[0083] The matrix of this invention is low-carbon martensitic / ferritic duplex ductile iron, and it is implemented according to the following steps:

[0084] Step 1: Weigh out scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser according to the composition of ductile iron. Heat the weighed raw materials in a medium-frequency induction furnace to 1500℃ at a heating rate of 75℃ / min and melt for 25 minutes to form molten iron.

[0085] Step 2: Add a spheroidizing agent and an inoculant to the molten iron obtained in Step 1; the spheroidizing agent is a silicon-magnesium-calcium-barium inoculant, and the mass of the spheroidizing agent is 1.2% of the mass of the molten iron obtained in Step 1; the inoculant is silicon carbide, and the mass of the inoculant is 1.3% of the mass of the molten iron obtained in Step 1.

[0086] Step 3: Pour the molten iron containing the spheroidizing agent and inoculant added in Step 2 into the preheated horizontal continuous casting furnace and obtain the casting by horizontal continuous casting.

[0087] Step 4: Place the casting obtained in Step 3 in a vacuum tube furnace at 900℃ and hold for 6 hours to fully austenitize it. Then, cool the vacuum tube furnace to 500℃ at a cooling rate of 0.2℃ / min. Hold the material at 500℃ for 6 hours and then cool it with the furnace to below 300℃. Finally, remove it and air cool it. The vacuum degree of the vacuum tube furnace is -0.1MPa.

[0088] Step 5: Place the material from Step 4 in a muffle furnace at 800℃ and hold for 2 hours. After the holding time is up, immediately remove it and quench it at 750℃ to obtain a low-carbon martensitic / ferrite duplex ductile iron with a matrix of 10% spheroidal graphite, 83% lath low-carbon martensite, and 7% ferrite.

[0089] Comparative Example 1

[0090] The specific steps for preparing ordinary ductile iron are as follows:

[0091] Step 1: Weigh out scrap steel, ferrosilicon, ferromanganese, and carbon raiser according to the composition of ductile iron, and put the weighed raw materials into a medium frequency induction furnace. Heat them to 1500℃ at a heating rate of 75℃ / min and melt them for 25 minutes to form molten iron.

[0092] Step 2: Add a spheroidizing agent and an inoculant to the molten iron prepared in Step 1; the spheroidizing agent is a silicon-magnesium-calcium-barium inoculant, accounting for 1.2% of the total mass of the molten iron; the inoculant is silicon carbide, accounting for 1.2% of the total mass of the molten iron.

[0093] Step 3: Pour the molten iron containing the spheroidizing agent and inoculant added in Step 2 into the preheated horizontal continuous casting furnace and obtain the casting by horizontal continuous casting.

[0094] Step 4: Place the casting from Step 3 in a muffle furnace at 850℃ and hold for 2 hours, then remove and quench to obtain quenched ductile iron.

[0095] Figure 2 The stress-strain curves of the low-carbon martensitic / ferritic duplex ductile iron prepared in Example 1 of the present invention and the ordinary quenched ductile iron prepared in Comparative Example 1 are compared. It can be seen that the strength and toughness of Example 1 of the present invention are significantly higher than those of Comparative Example 1.

Claims

1. A method for preparing low-carbon martensitic / ferritic duplex ductile iron, characterized in that, The specific steps are as follows: Step 1: Weigh out the following materials according to the composition of ductile iron: scrap steel, ferrosilicon, nickel plate, ferromanganese, and carbon raiser. Place the weighed raw materials into a medium-frequency induction furnace and heat them to melt into molten iron. Step 2: Add spheroidizing agent and inoculant to the molten iron smelted in Step 1; Step 3: Pour the molten iron that has been spheroidized and inoculated in Step 2 into the preheated horizontal continuous casting furnace, and continuously cast it into castings according to the horizontal continuous casting process specifications. Step 4: Place the casting obtained in Step 3 into a vacuum tube furnace for heating and holding, then cool it down and hold it for a second time before cooling it with the furnace, and finally take it out and air cool it. Step 5: Place the air-cooled casting from Step 4 into a muffle furnace for heat preservation, then remove it and quench it to obtain a low-carbon martensitic / ferritic duplex ductile iron. The ductile iron is composed of the following raw material components by mass percentage: C: 3.3%~3.6%, Si: 2.4%~2.7%, Mn: 0.1%~0.3%, P≤0.1%, S: ≤0.015%, Ni: 6.1%~6.5%, Mg: 0.08%~0.11%, with the balance being Fe; The microstructure of the ductile iron, by volume fraction percentage, is 10% spheroidal graphite, 60%~70% low-carbon martensite, and 20%~30% ferrite. In step 4, the vacuum degree in the vacuum tube furnace is -0.1MPa, the heating temperature is 900℃~950℃, and the holding time is 4h~6h; the cooling is specifically to cool down to 500℃ at a rate of 0.2℃ / min, and the second holding time is 4h~6h; and then cool down to below 300℃ with the furnace. In step 5, the heat preservation temperature is 750℃~800℃, and the heat preservation time is 1h~2h; The quenching medium is water, the quenching temperature is 750℃-800℃, and the interval between the casting being taken out of the muffle furnace and entering the water for quenching is no more than 6 seconds.

2. The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to claim 1, characterized in that, In step 1, the total carbon equivalent in the raw materials is 4.3 wt%; the heating rate of the medium-frequency induction furnace is 70℃ / min~80℃ / min, the melting time is 10min~40min, and the melting temperature is 1450℃~1550℃.

3. The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to claim 1, characterized in that, In step 2, the mass of the spheroidizing agent accounts for 1.1% to 1.4% of the total mass of the molten iron, and the mass of the inoculant accounts for 1.2% to 1.5% of the total mass of the molten iron.

4. The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to claim 1 or 3, characterized in that, In step 2, the inoculant is a silicon-magnesium-calcium-barium inoculant, and the spheroidizing agent is silicon carbide.

5. The method for preparing low-carbon martensitic / ferritic duplex ductile iron according to claim 1, characterized in that, In step 3, the drawing speed during horizontal continuous casting is 1 m / min; the volume fraction of spheroidal graphite in the resulting casting is 10%, and the spheroidal graphite density number is ≥500 spheroids / mm². 2 .

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