A production method of centrifugal ductile iron pipe

Abstract

The present invention relates to the field of centrifugal casting technology, and specifically discloses a method for producing a centrifugal ductile iron pipe. The method can directly produce a ferrite-based ductile iron pipe in the as-cast state, eliminating the need for a heat treatment process in conventional processes. This effectively reduces energy consumption and production costs, improves production efficiency, and avoids issues such as inconsistent product size and quality often associated with conventional heat treatment processes. More importantly, while eliminating the need for conventional heat treatment processes, the performance of the ductile iron pipe is significantly improved. The resulting ductile iron pipe has a tensile strength of 600 MPa to 650 MPa, a yield strength of 440 MPa to 525 MPa, and an elongation greater than 12%, significantly exceeding the requirements of the GB / T 13295 standard.

Description

Technical Field

[0001] The invention relates to the technical field of centrifugal casting, in particular to a production method of a centrifugal ductile iron pipe. Background Art

[0002] Ductile iron pipes are mainly produced by centrifugal casting using metal models. In actual production, in order to speed up production efficiency and protect the service life of the metal model, a circulating water cooling system is generally installed on the outer wall of the metal model to speed up cooling, protect the metal model, and improve production efficiency. However, the problem caused by this is that carbide structures such as cementite and pearlite often appear in the matrix structure of the ductile iron pipe obtained in the cast state. If a subsequent heat treatment step to eliminate cementite or pearlite is not carried out, the plasticity and toughness of the ductile iron pipe will not meet the product's use requirements. Therefore, the current existing centrifugal ductile iron pipe production process must have a high-temperature heat treatment step to eliminate cementite or pearlite.

[0003] The adverse effects of heat treatment include: (1) energy waste; (2) during the heat treatment process, due to the decomposition of carbides, the free expansion of graphitization of ductile iron pipes will cause large changes in the size of the ductile iron pipes; (3) the heat treatment process will produce tracks or wheel marks on the surface of the ductile iron pipes, affecting the consistency of the appearance quality; (4) during the high-temperature annealing stage to eliminate cementite, the ductile iron pipes may also become oval due to excessive temperature and be scrapped; (5) the high-temperature elimination of cementite takes a long time, resulting in low production efficiency. Therefore, it is necessary to provide a new process with high production efficiency, low energy consumption, and the ability to further improve the performance of ductile iron pipes. Summary of the Invention

[0004] Aiming at the problem that the existing production process of centrifugal ductile iron pipes requires heat treatment, which leads to high production cost, high energy consumption and affects the performance and product qualification rate of ductile iron pipes, the present invention provides a production method of centrifugal ductile iron pipes.

[0005] To solve the above technical problems, the technical solutions provided by the embodiments of the present invention are:

[0006] A method for producing a centrifugal ductile iron pipe comprises the following steps:

[0007] S1, obtaining molten iron with the required composition through electric furnace smelting;

[0008] S2, according to the pearlescent factor P x Determine the carbon equivalent Ceq of the molten iron after carbon reduction, then determine the carbon content of the molten iron after carbon reduction according to Ceq = [C] + 1 / 3 (n + [P]) + m, and add carbon steel to the molten iron according to the carbon content requirement to obtain the carbon-reduced molten iron; m = 0.15 to 0.25;

[0009] Among them, P x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb];

[0010] If the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, n = 3.4 ~ 3.6, a = 3; if the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, n = 2.6 ~ 2.8, a = 2;

[0011] If the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting:

[0012] If P x ≤2.0, the carbon equivalent Ceq in the molten iron after carbon reduction is controlled to be 4.5% to 4.65%; if 2.0<P x When ≤3.0, the Ceq in the hot metal after carbon reduction is controlled to be 4.7% to 4.9%;

[0013] If the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting:

[0014] If P x ≤2.0, the Ceq in the molten iron after carbon reduction is controlled to be 4.1% to 4.25%; if 2.0<P x When ≤2.5, the Ceq in the molten iron after carbon reduction is controlled to be 4.3% to 4.4%;

[0015] S3, determining the magnesium content in the molten iron after spheroidization according to the anti-spheroidization factor K1, and then adding a magnesium spheroidizing agent to the reduced carbon molten iron for spheroidization according to the magnesium content requirement in the molten iron;

[0016] If K1≤1.0, the magnesium content in the molten iron after spheroidization is controlled to be 0.05%~0.06%; if 1.0<K1≤2.0, the magnesium content in the molten iron after spheroidization is controlled to be 0.035%~0.045%;

[0017] S4, adding a silicon-containing inoculant to the spheroidized molten iron for a primary inoculation treatment;

[0018] S5, adding a silicon-containing inoculant to the fan-shaped ladle, pouring the molten iron after the primary inoculation treatment into the fan-shaped ladle for secondary inoculation treatment;

[0019] S6, pouring the molten iron after the secondary inoculation treatment into the launder, adding a silicon-containing inoculant during the pouring process, and performing a third inoculation treatment;

[0020] S7, coating the inner wall of the pipe mold with a silicon-containing inoculant, then feeding the molten iron flowing into the launder into the pipe mold for four inoculation treatments, and then rotating and centrifuging, cooling and forming to obtain a centrifugal ductile iron cast pipe.

[0021] With respect to the prior art, the present invention provides a method for producing centrifugal ductile iron pipes, in which the primary inoculation treatment is placed after the spheroidizing treatment, which serves to intensify the fluctuation of the composition of the molten iron, so that silicon-rich areas exist in the molten iron, increase the activity of the carbon element, and promote the formation of graphite; a secondary inoculation is performed in a fan-shaped bag, which can form carbon atom groups and larger carbon molecular chains Cn in the molten iron, further increase the fluctuation of the composition of the molten iron, reduce the nucleation supercooling of the molten iron, and thus inhibit the formation of carbides; in the process of pouring molten iron, with-stream inoculation is adopted to increase the external core of graphite nucleation and improve the morphology and quantity of graphite organization; in-mold inoculation is performed in the mold tube, which can reduce the sensitivity to a large cooling rate during the solidification process. Avoid the formation of carbides on the outer surface of the ductile iron pipe; further, the magnesium content in the molten iron after spheroidization treatment is accurately controlled by the anti-spheroidization factor, and the roundness of the graphite nodules in the ductile iron is improved. At the same time, the C content in the molten iron is accurately controlled by the pearlite factor, ferrite formation is promoted, and the formation of pearlite and carbides is inhibited, thereby obtaining ferrite + graphite structure, which significantly improves the strength and plasticity of the ductile iron. Therefore, high-strength and high-plasticity ductile iron can be prepared without the need for traditional heat treatment steps. The process is simple, the energy consumption is low, the production efficiency of ductile iron can be effectively improved, and the production cost can be reduced, thereby significantly improving the market competitiveness of the enterprise, and it has high promotion and application value.

[0022] As a specific embodiment of the present invention, in S1, the chemical composition of the molten iron is: C 4.2% to 4.6%, Si 0.6% to 1.2%, Mn 0.2% to 0.4%, P ≤ 0.08%, S ≤ 0.03%, and the balance is Fe and unavoidable impurities.

[0023] Carbon is the element with the strongest graphitization ability and can strongly promote the formation of graphite. However, excessive carbon can easily cause defects such as graphite floating and slag inclusions. Si is also an element with strong graphitization ability. At the same time, increasing the amount of silicon can expand the temperature interval between the two phases. Too low Si content can easily lead to the formation of a large amount of carbides.

[0024] By controlling the C and Si contents within the above ranges, the formation of carbides can be reduced as much as possible, which is beneficial for obtaining a ferrite matrix and provides conditions for omitting the subsequent heat treatment process.

[0025] It should be noted that the thermal insulation coating mentioned above in the present invention is a conventional thermal insulation coating in the art, such as diatomaceous earth.

[0026] Furthermore, in S1, the content requirements of other impurity elements in the molten iron are: Cr≤0.035%, Ni≤0.035%, Mo≤0.001%, Cu≤0.03%, Al≤0.03%, Ti≤0.06%, V≤0.008%, B≤0.001%, Sb≤0.005%, As≤0.007%, Sn≤0.007%, Pb≤0.001%, Ce≤0.003%, Bi≤0.002%, and Te≤0.001%.

[0027] By controlling the content of the above-mentioned impurity elements, the roundness of the graphite nodules in the ductile iron can be ensured and the formation of carbides can be inhibited, thereby improving the strength and plasticity of the ductile iron.

[0028] As a specific embodiment of the present invention, in S3, the temperature of the molten iron is controlled to be 1480°C to 1500°C when the magnesium spheroidizing agent is added.

[0029] The optimal molten iron temperature can improve the spheroidization effect, promote uniform mixing of molten iron components, and reduce the content of impurity elements.

[0030] As an embodiment of the present invention, in S3, if the anti-spheroidization factor is 1.0<K1≤2.0, a cerium alloy is added to the carbon-reduced molten iron during the spheroidization treatment to control the cerium content in the carbon-reduced molten iron to be 0.001% to 0.003%.

[0031] It should be noted that the calculation formula of the anti-spheroidization factor in the present invention is as follows:

[0032] Anti-spheroidization factor K1 = 4.4×[Ti]+2.0×[As]+2.3×[Sn]+5.0×[Sb]+290×[Pb]+370×[Bi]+1.6×[Al].

[0033] Preferably, in S4, if the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, the silicon content after one inoculation treatment is controlled to be 3.4% to 3.6%; if the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, the silicon content after one inoculation treatment is controlled to be 2.6% to 2.8%.

[0034] By controlling the above silicon content, the formation of pearlite can be effectively inhibited and the structure and quantity of ferrite can be improved.

[0035] Preferably, in S5, the silicon-containing inoculant is silicon carbide or ferrosilicon inoculant with a particle size of 2 mm to 3 mm, and the amount added is 0.3% to 0.5% of the mass of the molten iron after one inoculation treatment.

[0036] Preferably, in S5, the temperature of the molten iron after the primary inoculation treatment is controlled to be 1350°C to 1380°C.

[0037] Preferably, in S6, the particle size of the silicon-containing inoculant is 1 mm to 3 mm. If the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, the amount of the silicon-containing inoculant added is 0.2% to 0.4% of the mass of the molten iron after the secondary inoculation treatment; if the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, the amount of the silicon-containing inoculant added is 0.5% to 0.75% of the mass of the molten iron after the secondary inoculation treatment.

[0038] Preferably, in S6, the temperature of the molten iron after the secondary inoculation treatment is controlled to be 1310°C to 1350°C.

[0039] Preferably, in S7, before the molten iron enters the tube mold, the rotation speed of the tube mold is controlled to be 700 r / min to 1200 r / min.

[0040] Preferably, in S7, the amount of the silicon-containing inoculant added is 0.08% to 0.12% of the mass of the molten iron.

[0041] It should be noted that if the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, in S7, before the molten iron enters the tube mold, the rotation speed of the tube mold is controlled to be 1000r / min~1200r / min; if the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, in S7, before the molten iron enters the tube mold, the rotation speed of the tube mold is controlled to be 700r / min~900r / min.

[0042] Preferably, in S7, after cooling and molding, the tube mold continues to rotate for 1 minute to 5 minutes, and is cooled to 600° C. to 700° C. for demoulding.

[0043] Preferably, in S7, the particle size of the silicon-containing inoculant is 0.3 mm to 1 mm.

[0044] Preferably, in S7, the temperature of the molten iron after the three inoculation treatments is controlled to be 1250°C to 1320°C.

[0045] Through specific four-time inoculation treatment, specific carbon reduction treatment, and control of magnesium content in the spheroidizing process, the graphitization of ductile iron can be effectively promoted, and the formation of carbides in ductile iron can be inhibited. At the same time, the number of eutectic groups in ductile iron can be increased, the grains can be refined, the graphite morphology can be improved, and the sensitivity to large cooling rates during the solidification of molten iron can be further reduced, and the formation of carbides on the surface of ductile iron pipes can be further reduced, thereby significantly improving the mechanical properties of ductile iron. There is no need for the high-temperature process of eliminating cementite or pearlite in traditional processes, which significantly improves the product quality and performance of ductile iron pipes and has high practical value.

[0046] Preferably, if the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, the chemical composition of the obtained centrifugal ductile iron pipe is: C 3.3% to 3.8%, Si 3.8% to 4.4%, Mn 0.2% to 0.4%, Mg 0.035% to 0.060%, P≤0.08%, S≤0.03%, Cr≤0.035%, Ni≤0.035%, Mo≤0.001%, Cu≤0.03%, Al≤0.03%, Ti≤0.06%, V≤0.008%, B≤0.001%, Sb≤0.005%, As≤0.007%, Sn≤0.007%, Pb≤0.001%, Ce≤0.003%, Bi≤0.002%, Te≤0.001%; the balance is Fe and unavoidable impurities.

[0047] If the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, the chemical composition of the resulting centrifugal ductile iron pipe is: C 3.1% to 3.4%, Si 3.2% to 3.8%, Mn 0.2% to 0.4%, Mg 0.035% to 0.060%, P≤0.08%, S≤0.03%, Cr≤0.035%, Ni≤0.035%, Mo≤0.001%, Cu≤0.03%, Al≤0.03%, Ti≤0.06%, V≤0.008%, B≤0.001%, Sb≤0.005%, As≤0.007%, Sn≤0.007%, Pb≤0.001%, Ce≤0.003%, Bi≤0.002%, Te≤0.001%; the balance is Fe and unavoidable impurities.

[0048] The centrifugal ductile iron pipe production method provided by the present invention can directly obtain ferrite-based ductile iron pipes in the cast state, without the need for the heat treatment process in traditional processes, thereby effectively reducing energy consumption and production costs, improving production efficiency, and avoiding problems such as inconsistent product size and quality of ductile iron pipes that are easily caused by traditional heat treatment processes. More importantly, while eliminating the traditional heat treatment process, the performance of the ductile iron pipe is significantly improved. The prepared ductile iron pipe has a tensile strength of 600MPa to 650MPa, a yield strength of 440MPa to 525MPa, and an elongation of greater than 12%, which are significantly higher than the requirements of the GB / T 13295 standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is the metallographic structure diagram (×100) of the ductile iron prepared in Example 1 of the present invention;

[0050] Figure 2 This is the metallographic structure diagram (×100) of the ductile iron prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0052] In order to better illustrate the present invention, further examples are given below.

[0053] Example 1

[0054] The embodiment of the present invention provides a method for producing a centrifugal ductile iron pipe DN800 (the interior of the centrifuge metal mold is not sprayed with a thermal insulation coating), comprising the following steps:

[0055] S1, smelting pig iron in a medium frequency electric furnace to obtain molten iron with a composition of C 4.2%, Si 0.6%, Mn 0.2%, P 0.02%, S 0.02%, Cr 0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti 0.04%, V 0.007%, B 0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.001%, and Te 0.0008%;

[0056] S2, calculate P according to the composition of molten iron in medium frequency electric furnace x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb], n=3.4, a=3, calculate the pearlescence influence factor P x =1.209, that is, P x ≤2.0, therefore, Ceq is taken as 4.5%. According to Ceq=[C]+1 / 3([n]+[P])+0.2, the C content after carbon reduction is 3.53%. According to the carbon content requirement, carbon steel is added to the molten iron to obtain the carbon-reduced molten iron in the medium frequency furnace;

[0057] The composition of molten iron after carbon reduction in the medium frequency furnace is: C 3.53%, Si 0.6%, Mn 0.2%, P 0.02%, S0.02%, Cr0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti0.04%, V 0.007%, B0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.001%, Te 0.0008%, and the balance is iron and unavoidable impurity elements;

[0058] S3, heating the molten iron in the medium frequency electric furnace to 1480° C. and pouring it into a spheroidizing bag, transporting it to the intermediate spheroidizing position, calculating the anti-spheroidizing factor K1 = 0.897 according to K1 = 4.4×[Ti] + 2.0×[As] + 2.3×[Sn] + 5.0×[Sb] + 290×[Pb] + 370×[Bi] + 1.6×[Al], determining that the magnesium content in the molten iron after spheroidization is 0.05%, and then adding a magnesium spheroidizing agent to the molten iron according to the above magnesium content requirement to carry out spheroidization treatment;

[0059] S4, according to the above Si content requirements, add 75SiFe inoculant to the spheroidized molten iron, perform an inoculation treatment, and control the Si content after the inoculation treatment to 3.4%;

[0060] The composition of the molten iron after the first inoculation is: C: 3.33%, Si: 3.4%, Mn: 0.2%, Mg 0.05%, P 0.02%, S 0.02%, Cr 0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti 0.04%, V 0.007%, B 0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.001%, Te 0.0008%, and the rest are Fe and impurity elements;

[0061] S5, adding 0.3% 75SiFe inoculant by weight of the molten iron to the bottom of the fan-shaped ladle, heating the molten iron after the primary inoculation treatment to 1350°C and pouring it into the centrifuge fan-shaped ladle for secondary inoculation treatment; the particle size of the 75SiFe inoculant is 2-3mm;

[0062] S6, heating the molten iron after the secondary inoculation treatment to 1310°C and pouring it into the launder, adding 0.2% of the molten iron mass of 75SiFe inoculant during the pouring process, and performing a third inoculation treatment; the particle size of the 75SiFe inoculant is 1-3mm;

[0063] S7, coating the inner wall of the tube mold with 0.1% 75SiFe inoculant by weight of the molten iron, and then feeding the molten iron flowing into the launder into the tube mold for four inoculation treatments; the particle size of the 75SiFe inoculant is 0.3-1 mm;

[0064] The composition of the molten iron after four inoculations is: C: 3.33%, Si: 3.85%, Mn: 0.2%, Mg: 0.050%, P0.02%, S 0.02%, Cr 0.021%, Ni 0.018%, Mo 0.0008%, Cu0.025%, Al 0.021%, Ti0.04%, V 0.007%, B 0.0007%, Sb 0.002%, As0.004%, Sn 0.004%, Pb 0.001%, Bi0.001%, Te 0.0008%, and the rest are Fe and impurity elements;

[0065] S8, before the molten iron enters the mold tube, the mold tube rotation speed is controlled to 1000r / min. After the molten iron is poured into the mold tube, it is cooled by circulating water on the outer wall. After cooling and forming, the mold tube continues to rotate for 3 minutes. The casting tube temperature drops to 600℃, and the mold is demolded to obtain a centrifugal ductile iron casting tube.

[0066] Example 2

[0067] The embodiment of the present invention provides a method for producing a centrifugal ductile iron pipe DN800 (the interior of the centrifuge metal mold is not sprayed with a thermal insulation coating), comprising the following steps:

[0068] S1, smelting the pig iron in a medium frequency electric furnace to obtain molten iron with a composition of C 4.4%, Si 1.0%, Mn 0.3%, P 0.05%, S 0.01%, Cr 0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V 0.005%, B 0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, and Te 0.001%;

[0069] S2, calculate P according to the composition of molten iron in medium frequency electric furnace x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb], n=3.5, a=3, calculate the pearlescence influence factor P x =1.679, that is, P x ≤2.0, therefore, Ceq is taken as 4.6%. According to Ceq=[C]+1 / 3([n]+[P])+0.2, the carbon content after carbon reduction is calculated to be 3.62%. According to the carbon content requirement, carbon steel is added to the molten iron to obtain the carbon-reduced molten iron;

[0070] The composition of molten iron after carbon reduction in the medium frequency furnace is: C 3.62%, Si 1.0%, Mn 0.3%, P 0.05%, S0.01%, Cr0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V 0.005%, B0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, Te 0.001%, and the balance is iron and unavoidable impurity elements;

[0071] S3, heating the molten iron in the medium frequency electric furnace to 1490° C. and pouring it into a spheroidizing bag, transporting it to the intermediate spheroidizing position, calculating the anti-spheroidizing factor K1=0.992 according to K1=4.4×[Ti]+2.0×[As]+2.3×[Sn]+5.0×[Sb]+290×[Pb]+370×[Bi]+1.6×[Al], determining that the magnesium content in the molten iron after spheroidization is 0.06%, and then adding a magnesium spheroidizing agent to the molten iron according to the above magnesium content requirement to carry out spheroidization treatment;

[0072] S4, adding 75SiFe inoculant to the spheroidized molten iron for a first inoculation treatment, controlling the Si content in the molten iron to 3.5% after the first inoculation;

[0073] The composition of the molten iron after the first inoculation is: C: 3.42%, Si: 3.5%, Mn: 0.3%, Mg 0.06%, P 0.05%, S 0.01%, Cr 0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V0.005%, B 0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, Te 0.001%, and the rest are Fe and impurity elements;

[0074] S5, adding 0.4% 75SiFe inoculant by weight of the molten iron to the bottom of the fan-shaped ladle, heating the molten iron after the primary inoculation treatment to 1360°C and pouring it into the centrifuge fan-shaped ladle for secondary inoculation treatment; the particle size of the 75SiFe inoculant is 2-3mm;

[0075] S6, heating the molten iron after the secondary inoculation treatment to 1330°C and pouring it into the launder, adding 0.3% of the molten iron mass of 75SiFe inoculant during the pouring process, and performing a third inoculation treatment; the particle size of the 75SiFe inoculant is 1-3mm;

[0076] S7, coating the inner wall of the tube mold with 0.1% 75SiFe inoculant by weight of the molten iron, and then feeding the molten iron flowing into the launder into the tube mold for four inoculation treatments; the particle size of the 75SiFe inoculant is 0.3-1 mm;

[0077] The composition of the molten iron after four inoculations is: C: 3.42%, Si: 4.1%, Mn: 0.3%, Mg: 0.060%, P0.05%, S 0.01%, Cr 0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V 0.005%, B 0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, Te 0.001%, and the rest are Fe and impurity elements;

[0078] S8, before the molten iron enters the mold tube, the mold tube rotation speed is controlled to 1100r / min. After the molten iron is poured into the mold tube, it is cooled by circulating water on the outer wall. After cooling and forming, the mold tube continues to rotate for 2 minutes. The casting tube temperature drops to 650℃, and the mold is demolded to obtain a centrifugal ductile iron casting tube.

[0079] Example 3

[0080] The embodiment of the present invention provides a method for producing a centrifugal ductile iron pipe DN800 (the interior of the centrifuge metal mold is not sprayed with a thermal insulation coating), comprising the following steps:

[0081] S1, smelting pig iron in a medium frequency electric furnace to obtain molten iron with a composition of C 4.6%, Si 1.2%, Mn 0.4%, P 0.08%, S 0.03%, Cr 0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti 0.06%, V 0.004%, B 0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Ce 0.001%, Bi 0.002%, and Te 0.0007%;

[0082] S2, calculate P according to the composition of molten iron in medium frequency electric furnace x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb], n=3.6, a=3, calculate the pearlescence influence factor P x =2.252, or 2.0 <P x≤3.0, therefore, Ceq is taken as 4.8%. According to Ceq=[C]+1 / 3([n]+[P])+0.2, the carbon content after carbon reduction is calculated to be 3.81%. According to the carbon content requirement, carbon steel is added to the molten iron to obtain the carbon-reduced molten iron;

[0083] The composition of molten iron after carbon reduction in the medium frequency furnace is: C 3.81%, Si 1.2%, Mn 0.4%, P 0.08%, S0.03%, Cr0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti 0.06%, V 0.004%, B0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Bi 0.002%, Te 0.0007%, and the balance is iron and unavoidable impurity elements;

[0084] S3, heating the molten iron in the medium frequency electric furnace to 1500° C. and pouring it into a spheroidizing ladle, transporting it to the intermediate spheroidizing station, calculating the anti-spheroidizing factor K1 = 1.385 based on K1 = 4.4×[Ti] + 2.0×[As] + 2.3×[Sn] + 5.0×[Sb] + 290×[Pb] + 370×[Bi] + 1.6×[Al], determining that the magnesium content in the molten iron after spheroidization is 0.04%, and then adding a magnesium spheroidizing agent and a cerium alloy to the molten iron according to the above magnesium content requirements to perform spheroidizing treatment, and controlling the Ce content in the molten iron to 0.001%;

[0085] S4, adding 75SiFe inoculant to the spheroidized molten iron for a first inoculation treatment, controlling the Si content after the first inoculation treatment to 3.6%;

[0086] The composition of the molten iron after the first inoculation is: C: 3.61%, Si: 3.6%, Mn: 0.4%, Mg: 0.040%, P0.08%, S 0.03%, Cr 0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti0.06%, V 0.004%, B 0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Ce0.001%, Bi 0.002%, Te 0.0007%, and the rest are Fe and impurity elements;

[0087] S5, adding 0.5% 75SiFe inoculant by weight of the molten iron to the bottom of the fan-shaped ladle, heating the molten iron after the primary inoculation treatment to 1380°C and pouring it into the centrifuge fan-shaped ladle for secondary inoculation treatment; the particle size of the 75SiFe inoculant is 2-3mm;

[0088] S6, heating the molten iron after the secondary inoculation treatment to 1350°C and pouring it into the launder, adding 0.4% 75SiFe inoculant by weight of the molten iron during the pouring process, and performing a third inoculation treatment; the particle size of the 75SiFe inoculant is 1-3mm;

[0089] S7, coating the inner wall of the tube mold with 0.1% 75SiFe inoculant by weight of the molten iron, and then feeding the molten iron flowing into the launder into the tube mold for four inoculation treatments; the particle size of the 75SiFe inoculant is 0.3-1 mm;

[0090] The composition of the molten iron after four inoculations is: C: 3.61%, Si: 4.35%, Mn: 0.4%, Mg: 0.04%, P0.08%, S 0.03%, Cr 0.031%, Ni 0.028%, Mo 0.0009%, Cu0.03%, Al 0.025%, Ti0.06%, V 0.004%, B 0.001%, Sb 0.005%, As0.005%, Sn 0.007%, Pb 0.001%, Bi0.0018%, Te 0.0007%, and the rest are Fe and impurity elements;

[0091] S8, before the molten iron enters the mold tube, the mold tube rotation speed is controlled to 1200r / min. After the molten iron is poured into the mold tube, it is cooled by circulating water on the outer wall. After cooling and forming, the mold tube continues to rotate for 5 minutes. The casting tube temperature drops to 700℃, and the mold is demolded to obtain a centrifugal ductile iron casting tube.

[0092] Example 4

[0093] The embodiment of the present invention provides a method for producing a centrifugal ductile iron pipe DN1400 (spraying a thermal insulation coating inside a centrifuge metal mold), comprising the following steps:

[0094] S1, smelting pig iron in a medium frequency electric furnace to obtain molten iron with a composition of C 4.2%, Si 0.6%, Mn 0.2%, P 0.02%, S 0.02%, Cr 0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti 0.04%, V 0.007%, B 0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.001%, and Te 0.0008%;

[0095] S2, calculate P according to the composition of molten iron in medium frequency electric furnace x=3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb], n=2.7, a=2, calculate the pearlescence influence factor P x =0.414, that is, P x ≤2.0, therefore, Ceq is taken as 4.2%. According to Ceq=[C]+1 / 3([n]+[P])+0.2, the carbon content after carbon reduction is calculated to be 3.49%. According to the carbon content requirement, carbon steel is added to the molten iron to obtain the carbon-reduced molten iron;

[0096] The composition of molten iron after carbon reduction in the medium frequency furnace is: C 3.49%, Si 0.6%, Mn 0.2%, P 0.02%, S0.02%, Cr0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti0.04%, V 0.007%, B0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.001%, Te 0.0008%, and the balance is iron and unavoidable impurity elements;

[0097] S3, heating the molten iron in the medium frequency electric furnace to 1480° C. and pouring it into a spheroidizing bag, transporting it to the intermediate spheroidizing position, calculating the anti-spheroidizing factor K1 = 0.897 according to K1 = 4.4×[Ti] + 2.0×[As] + 2.3×[Sn] + 5.0×[Sb] + 290×[Pb] + 370×[Bi] + 1.6×[Al], determining that the magnesium content in the molten iron after spheroidization is 0.05%, and then adding a magnesium spheroidizing agent to the molten iron according to the above magnesium content requirement to carry out spheroidization treatment;

[0098] S4, adding 75SiFe inoculant to the spheroidized molten iron for a first inoculation treatment, controlling the Si content in the molten iron to 2.7% after the inoculation treatment;

[0099] The composition of the molten iron after the first inoculation is: C: 3.29%, Si: 2.7%, Mn 0.2%, Mg: 0.05%, P 0.02%, S 0.02%, Cr 0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti 0.04%, V 0.007%, B 0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.001%, Te 0.0008%, and the rest are Fe and impurity elements;

[0100] S5, adding 0.4% 75SiFe inoculant by weight of the molten iron to the bottom of the fan-shaped ladle, heating the molten iron after the primary inoculation treatment to 1350°C and pouring it into the centrifuge fan-shaped ladle for secondary inoculation treatment; the particle size of the 75SiFe inoculant is 2-3mm;

[0101] S6, heating the molten iron after the secondary inoculation treatment to 1310°C and pouring it into the launder, adding 0.6% of the molten iron mass of 75SiFe inoculant during the pouring process, and performing a third inoculation treatment; the particle size of the 75SiFe inoculant is 1-3mm;

[0102] S7, coating the inner wall of the tube mold with 0.1% 75SiFe inoculant by weight of the molten iron, and then feeding the molten iron flowing into the launder into the tube mold for four inoculation treatments; the particle size of the 75SiFe inoculant is 0.3-1 mm;

[0103] The composition of the molten iron after four inoculations is: C 3.3%, Si 3.5%, Mn 0.2%, Mg 0.050%, P 0.02%, S 0.02%, Cr 0.021%, Ni 0.018%, Mo 0.0008%, Cu 0.025%, Al 0.021%, Ti 0.04%, V 0.007%, B 0.0007%, Sb 0.002%, As 0.004%, Sn 0.004%, Pb 0.001%, Bi 0.002%, Te 0.001%; the balance is Fe and unavoidable impurities;

[0104] S8, before the molten iron enters the mold tube, the mold tube rotation speed is controlled to 700r / min. After the molten iron is poured into the mold tube, it is cooled by circulating water on the outer wall. After cooling and forming, the mold tube continues to rotate for 5 minutes. The casting tube temperature drops to 600℃, and the mold is demolded to obtain a centrifugal ductile iron casting tube.

[0105] Example 5

[0106] The embodiment of the present invention provides a method for producing a centrifugal ductile iron pipe DN1400 (spraying a thermal insulation coating inside a centrifuge metal mold), comprising the following steps:

[0107] S1, smelting pig iron in a medium frequency electric furnace to obtain molten iron with a composition of C 4.4%, Si 1.0%, Mn 0.3%, P 0.05%, S 0.01%, Cr 0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V 0.005%, B 0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, and Te 0.001%;

[0108] S2, calculate P according to the composition of molten iron in medium frequency electric furnace x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb], n=2.8, a=2, calculate the pearlescence influence factor P x =0.885, that is, P x ≤2.0, therefore, Ceq is taken as 4.1%. According to Ceq=[C]+1 / 3([n]+[P])+0.2, the carbon content after carbon reduction is calculated to be 3.39%. According to the carbon content requirement, carbon steel is added to the molten iron to obtain the carbon-reduced molten iron;

[0109] The composition of molten iron after carbon reduction in the medium frequency furnace is: C 3.39%, Si 1.0%, Mn 0.3%, P 0.05%, S0.01%, Cr0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V 0.005%, B0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, Te 0.001%, and the balance is iron and unavoidable impurity elements;

[0110] S3, heating the molten iron in the medium frequency electric furnace to 1490° C. and pouring it into a spheroidizing bag, transporting it to the intermediate spheroidizing position, calculating the anti-spheroidizing factor K1 = 0.99 according to K1 = 4.4×[Ti] + 2.0×[As] + 2.3×[Sn] + 5.0×[Sb] + 290×[Pb] + 370×[Bi] + 1.6×[Al], determining the magnesium content in the molten iron after spheroidization to be 0.06%, and then adding a magnesium spheroidizing agent to the molten iron according to the above magnesium content requirement to carry out spheroidization treatment;

[0111] S4, adding 75SiFe inoculant to the spheroidized molten iron for a first inoculation treatment, controlling the Si content in the molten iron to be 2.8% after the first inoculation treatment;

[0112] The composition of the molten iron after the first inoculation is: C: 3.19%, Si: 2.8%, Mn: 0.3%, Mg: 0.060%, P0.05%, S 0.01%, Cr 0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti0.05%, V 0.005%, B 0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi0.0012%, Te 0.001%, and the rest are Fe and impurity elements;

[0113] S5, adding 0.5% 75SiFe inoculant by weight of the molten iron to the bottom of the fan-shaped ladle, heating the molten iron after the primary inoculation treatment to 1360°C and pouring it into the centrifuge fan-shaped ladle for secondary inoculation treatment; the particle size of the 75SiFe inoculant is 2-3mm;

[0114] S6, heating the molten iron after the secondary inoculation treatment to 1330°C and pouring it into the launder, adding 0.75% of the molten iron mass of 75SiFe inoculant during the pouring process, and performing a third inoculation treatment; the particle size of the 75SiFe inoculant is 1-3mm;

[0115] S7, coating the inner wall of the tube mold with 0.1% 75SiFe inoculant by weight of the molten iron, and then feeding the molten iron flowing into the launder into the tube mold for four inoculation treatments; the particle size of the 75SiFe inoculant is 0.3-1 mm;

[0116] The composition of the molten iron after four inoculations is: C 3.2%, Si 3.8%, Mn 0.3%, Mg 0.060%, P 0.05%, S 0.01%, Cr 0.035%, Ni 0.035%, Mo 0.001%, Cu 0.015%, Al 0.03%, Ti 0.05%, V 0.005%, B 0.0005%, Sb 0.004%, As 0.007%, Sn 0.006%, Pb 0.0008%, Bi 0.0012%, Te 0.001%; the balance is Fe and unavoidable impurities;

[0117] S8, before the molten iron enters the mold tube, the mold tube rotation speed is controlled to 800r / min. After the molten iron is poured into the mold tube, it is cooled by circulating water on the outer wall. After cooling and forming, the mold tube continues to rotate for 4 minutes. The casting tube temperature drops to 680℃, and the mold is demolded to obtain a centrifugal ductile iron casting tube.

[0118] Example 6

[0119] The embodiment of the present invention provides a method for producing a centrifugal ductile iron pipe DN1400 (spraying a thermal insulation coating inside a centrifuge metal mold), comprising the following steps:

[0120] S1, smelting pig iron in a medium frequency electric furnace to obtain molten iron with a composition of C 4.6%, Si 1.2%, Mn 0.4%, P 0.08%, S 0.03%, Cr 0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti 0.06%, V 0.004%, B 0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Ce 0.001%, Bi 0.002%, and Te 0.0007%;

[0121] S2, calculate P according to the composition of molten iron in medium frequency electric furnace x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb], n=2.6, a=2, calculate the pearlescence influence factor P x =2.25, or 2.0 <P x ≤2.5, therefore, Ceq is taken as 4.3%. According to Ceq=[C]+1 / 3([n]+[P])+0.2, the carbon content after carbon reduction is calculated to be 3.57%. According to the carbon content requirement, carbon steel is added to the molten iron to obtain carbon-reduced molten iron;

[0122] The composition of molten iron after carbon reduction in the medium frequency furnace is: C 3.57%, Si 1.2%, Mn 0.4%, P 0.08%, S0.03%, Cr0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti 0.06%, V 0.004%, B0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Ce 0.001%, Bi 0.002%, Te0.0007%, and the balance is iron and unavoidable impurity elements;

[0123] S3, heating the molten iron in the medium frequency electric furnace to 1500° C. and pouring it into a spheroidizing ladle, transporting it to the intermediate spheroidizing station, calculating the anti-spheroidizing factor K1 = 1.38 based on K1 = 4.4×[Ti] + 2.0×[As] + 2.3×[Sn] + 5.0×[Sb] + 290×[Pb] + 370×[Bi] + 1.6×[Al], determining the magnesium content in the molten iron after spheroidization to be 0.04%, then adding a magnesium spheroidizing agent and a cerium alloy to the molten iron according to the above magnesium content requirements to perform spheroidizing treatment, and controlling the Ce content in the molten iron to be 0.001%;

[0124] S4, adding 75SiFe inoculant to the spheroidized molten iron for a first inoculation treatment, controlling the Si content in the molten iron to be 2.6% after the first inoculation treatment;

[0125] The composition of the molten iron after the first inoculation is: C: 3.37%, Si: 2.6%, Mn: Mn 0.4%, Mg: 0.040%, P0.08%, S 0.03%, Cr 0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti0.06%, V 0.004%, B 0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Ce0.001%, Bi 0.002%, Te 0.0007%, and the rest are Fe and impurity elements;

[0126] S5, adding 0.3% 75SiFe inoculant by weight of the molten iron to the bottom of the fan-shaped ladle, heating the molten iron after the primary inoculation treatment to 1380°C and pouring it into the centrifuge fan-shaped ladle for secondary inoculation treatment; the particle size of the 75SiFe inoculant is 2-3mm;

[0127] S6, heating the molten iron after the secondary inoculation treatment to 1350°C and pouring it into the launder, adding 0.5% of the molten iron mass of 75SiFe inoculant during the pouring process, and performing a third inoculation treatment; the particle size of the 75SiFe inoculant is 1-3mm;

[0128] S7, coating the inner wall of the tube mold with 0.1% 75SiFe inoculant by weight of the molten iron, and then feeding the molten iron flowing into the launder into the tube mold for four inoculation treatments; the particle size of the 75SiFe inoculant is 0.3-1 mm;

[0129] The composition of the molten iron after four inoculations is: C 3.37%, Si 3.3%, Mn 0.4%, Mg 0.040%, P 0.08%, S 0.03%, Cr 0.031%, Ni 0.028%, Mo 0.0009%, Cu 0.03%, Al 0.025%, Ti 0.06%, V 0.004%, B 0.001%, Sb 0.005%, As 0.005%, Sn 0.007%, Pb 0.001%, Ce 0.001%, Bi 0.002%, Te 0.0007%; the balance is Fe and unavoidable impurities;

[0130] S8, before the molten iron enters the mold tube, the mold tube rotation speed is controlled to 900r / min. After the molten iron is poured into the mold tube, it is cooled by circulating water on the outer wall. After cooling and forming, the mold tube continues to rotate for 1 minute. The casting tube temperature drops to 600℃, and the mold is demolded to obtain a centrifugal ductile iron casting tube.

[0131] Comparative Examples 1-3

[0132] This comparative example provides a method for preparing a ductile iron pipe, and its preparation process is exactly the same as that of the corresponding embodiments 1-3, except that: (1) the order of step S2 and step S3 is reversed, that is, a primary inoculation treatment is performed before spheroidization treatment, and the silicon content in the molten iron after the primary inoculation is 1.8%-2.3% (silicon content 1: 1.8% in comparative example 1, 2.0% in comparative example 2, and 2.3% in comparative example 3); (2) inoculation in the fan-shaped package is not performed, that is, no silicon-containing inoculant is added to the fan-shaped package and no secondary inoculation treatment is performed; (3) the demoulded ductile iron pipe is annealed; and the remaining operations are exactly the same.

[0133] The specific annealing steps are as follows:

[0134] The ductile iron pipe after demoulding is heated to 960℃ and kept at this temperature for 18 minutes to undergo high-temperature heat treatment to eliminate carburization. It is then cooled to below 650℃ and air-cooled before being taken out of the furnace.

[0135] Comparative Examples 4-6

[0136] This comparative example provides a method for preparing a ductile iron pipe, and its preparation process is exactly the same as that of the corresponding embodiments 4-6, except that: (1) the order of step S2 and step S3 is reversed, that is, a primary inoculation treatment is performed before spheroidization treatment, and the silicon content in the molten iron after the primary inoculation is 1.8%-2.3% (silicon content 1 is 1.8% in comparative example 1, 2.0% in comparative example 2, and 2.3% in comparative example 3); (2) inoculation in the fan-shaped package is not performed, that is, no silicon-containing inoculant is added to the fan-shaped package and no secondary inoculation treatment is performed; (3) the demoulded ductile iron pipe is annealed; and the remaining operations are exactly the same.

[0137] The ductile iron pipe after demoulding is heated to 760℃ and kept at this temperature for 8 minutes to carry out heat treatment to eliminate pearlite, and then cooled to below 650℃ in the furnace and air-cooled.

[0138] The ductile iron pipes prepared in the above Examples 1-6 and Comparative Examples 1-6 were processed into standard test bars according to the national standard for ductile iron pipes, and tensile tests were carried out on a universal testing machine to measure performance indicators. The reference standard was GB / T228.1-2021, and the results are shown in Table 1. According to the ductile iron metallographic detection standard, the as-cast centrifugal ductile iron pipes were metallographically detected. The reference standard was GB / T 9441-2021. The metallographic structures of the ductile iron pipes prepared in Example 1 and Comparative Example 1 are shown in Table 1. Figure 1 and Figure 2 shown.

[0139] Table 1

[0140]

[0141] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for producing a centrifugal ductile iron pipe, characterized in that: The steps include: S1, obtaining molten iron with the required composition through electric furnace smelting; S2, according to the pearlescent factor P x Determine the carbon equivalent Ceq of the molten iron after carbon reduction, then determine the carbon content of the molten iron after carbon reduction according to Ceq = [C] + 1 / 3 (n + [P]) + m, and add carbon steel to the molten iron according to the carbon content requirement to obtain the carbon-reduced molten iron; m = 0.15 to 0.25; Among them, P x =3.0×[Mn]-2.65×(na)+7.75×[Cu]+90×[Sn]+357×[Pb]+333×[Bi]+20.1×[As]+9.6×[Cr]+71.7×[Sb]; If the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, n = 3.4 ~ 3.6, a = 3; if the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, n = 2.6 ~ 2.8, a = 2; If the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting: If P x ≤2.0, the carbon equivalent Ceq in the molten iron after carbon reduction is controlled to be 4.5% to 4.65%; if 2.0<P x When ≤3.0, the Ceq in the molten iron after carbon reduction is controlled to be 4.7% to 4.9%; If the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting: If P x ≤2.0, the Ceq in the molten iron after carbon reduction is controlled to be 4.1% to 4.25%; if 2.0<P x When ≤2.5, the Ceq in the molten iron after carbon reduction is controlled to be 4.3% to 4.4%; S3, determining the magnesium content in the molten iron after spheroidization according to the anti-spheroidization factor K1, and then adding a magnesium spheroidizing agent to the reduced carbon molten iron for spheroidization according to the magnesium content requirement in the molten iron; If K1≤1.0, the magnesium content in the molten iron after spheroidization is controlled to be 0.05%~0.06%; if 1.0<K1≤2.0, the magnesium content in the molten iron after spheroidization is controlled to be 0.035%~0.045%; S4, adding a silicon-containing inoculant to the spheroidized molten iron for a primary inoculation treatment; S5, adding a silicon-containing inoculant to the fan-shaped ladle, pouring the molten iron after the primary inoculation treatment into the fan-shaped ladle for secondary inoculation treatment; S6, pouring the molten iron after the secondary inoculation treatment into the launder, adding a silicon-containing inoculant during the pouring process, and performing a third inoculation treatment; S7, coating the inner wall of the pipe mold with a silicon-containing inoculant, then feeding the molten iron flowing into the launder into the pipe mold for four inoculation treatments, and then rotating and centrifuging, cooling and forming to obtain a centrifugal ductile iron cast pipe.

2. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S1, the chemical composition of the molten iron is: C 4.2% to 4.6%, Si 0.6% to 1.2%, Mn 0.2% to 0.4%, P≤0.08%, S≤0.03%, and the balance is Fe and unavoidable impurities.

3. The method for producing a centrifugal ductile iron pipe according to claim 2, wherein: In S1, the content requirements of other impurity elements in the molten iron are: Cr≤0.035%, Ni≤0.035%, Mo≤0.001%, Cu≤0.03%, Al≤0.03%, Ti≤0.06%, V≤0.008%, B≤0.001%, Sb≤0.005%, As≤0.007%, Sn≤0.007%, Pb≤0.001%, Ce≤0.003%, Bi≤0.002%, and Te≤0.001%.

4. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S3, when adding magnesium nodulizer, the temperature of the carbon-reduced molten iron is controlled to be 1480° C. to 1500° C.; and / or In S3, if the anti-spheroidization factor is 1.0<K1≤2.0, a cerium alloy is added to the carbon-reduced molten iron during the spheroidization treatment to control the cerium content in the carbon-reduced molten iron to be 0.001% to 0.003%.

5. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S4, if the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, the silicon content after the first inoculation treatment is controlled to be 3.4% to 3.6%; if the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, the silicon content after the first inoculation treatment is controlled to be 2.6% to 2.8%; and / or In S5, the particle size of the silicon-containing inoculant is 2 mm to 3 mm, and the amount of the silicon-containing inoculant added is 0.3% to 0.5% of the mass of the molten iron after one inoculation treatment.

6. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S5, the temperature of the molten iron after the primary inoculation treatment is controlled to be 1350°C to 1380°C.

7. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S6, the particle size of the silicon-containing inoculant is 1 mm to 3 mm; if the metal inner wall of the centrifuge is not sprayed with a thermal insulation coating before casting, the amount of the silicon-containing inoculant added is 0.2% to 0.4% of the mass of the molten iron after the secondary inoculation treatment; if the metal inner wall of the centrifuge is sprayed with a thermal insulation coating before casting, the amount of the silicon-containing inoculant added is 0.5% to 0.75% of the mass of the molten iron after the secondary inoculation treatment; and / or In S6, the temperature of the molten iron after the secondary inoculation treatment is controlled to be 1310°C to 1350°C.

8. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S7, the amount of the silicon-containing inoculant added is 0.08% to 0.12% of the mass of the molten iron; and / or In S7, before the molten iron enters the tube mold, the rotation speed of the tube mold is controlled to be 700 rpm to 1200 rpm; and / or In S7, after cooling and molding, the tube mold continues to rotate for 1 minute to 5 minutes, and the temperature is lowered to 600° C. to 700° C. for demoulding.

9. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: In S7, the particle size of the silicon-containing inoculant is 0.3 mm to 1 mm; and / or In S7, the temperature of the molten iron after the three inoculation treatments is controlled to be 1250°C to 1320°C.

10. The method for producing a centrifugal ductile iron pipe according to claim 1, wherein: If the metal inner wall of the centrifuge is not sprayed with thermal insulation coating before casting, the chemical composition of the obtained centrifugal ductile iron pipe is: C 3.3%-3.8%, Si 3.8%-4.4%, Mn 0.2%-0.4%, Mg 0.035%-0.060%, P≤0.08%, S≤0.03%, Cr≤0.035%, Ni≤0.035%, Mo≤0.001%, Cu≤0.03%, Al≤0.03%, Ti≤0.06%, V≤0.008%, B≤0.001%, Sb≤0.005%, As≤0.007%, Sn≤0.007%, Pb≤0.001%, Ce≤0.003%, Bi≤0.002%, Te≤0.001%; the balance is Fe and unavoidable impurities; If the metal inner wall of the centrifuge is sprayed with thermal insulation coating before casting, the chemical composition of the resulting centrifugal ductile iron pipe is: C 3.1% to 3.4%, Si 3.2% to 3.8%, Mn 0.2% to 0.4%, Mg 0.035% to 0.060%, P≤0.08%, S≤0.03%, Cr≤0.035%, Ni≤0.035%, Mo≤0.001%, Cu≤0.03%, Al≤0.03%, Ti≤0.06%, V≤0.008%, B≤0.001%, Sb≤0.005%, As≤0.007%, Sn≤0.007%, Pb≤0.001%, Ce≤0.003%, Bi≤0.002%, Te≤0.001%; the balance is Fe and unavoidable impurities.

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