US20240035126A1 - Method for increasing the quality of graphite balls - Google Patents

Method for increasing the quality of graphite balls Download PDF

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Publication number
US20240035126A1
US20240035126A1 US18/125,093 US202318125093A US2024035126A1 US 20240035126 A1 US20240035126 A1 US 20240035126A1 US 202318125093 A US202318125093 A US 202318125093A US 2024035126 A1 US2024035126 A1 US 2024035126A1
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molten iron
rare earth
added
content
nodularization
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Jianchen CONG
Yufang Wang
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Henglilai New Materials Ltd
Shandong University of Technology
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Henglilai New Materials Ltd
Shandong University of Technology
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C1/00—Refining of pig-iron; Cast iron
    • C21C1/10—Making spheroidal graphite cast-iron
    • C21C1/105—Nodularising additive agents
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/08—Making cast-iron alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B3/00—General features in the manufacture of pig-iron
    • C21B3/02—General features in the manufacture of pig-iron by applying additives, e.g. fluxing agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C1/00—Refining of pig-iron; Cast iron
    • C21C1/10—Making spheroidal graphite cast-iron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C22C37/04—Cast-iron alloys containing spheroidal graphite

Definitions

  • the present invention relates to the technical field of metallurgy and cast iron alloys, and particularly to a method for increasing the quality of graphite balls.
  • Nodular cast iron is widely used, and the roundness and size of the graphite balls in nodular cast iron are critical factors affecting its performance. It is well known that sulfur is a main factor causing poor nodularization of graphite, and the molten iron is required to have a sulfur content that is as low as possible to obtain round graphite balls. Where fine graphite balls are desired, ferrosilicon is required for inoculation.
  • the nodularization allows graphite to grow spherically, and the inoculation increases the nucleation rate of graphite, ensuring the number and roundness of graphite balls. Therefore, the traditional nodularization technology includes desulfurization, and then nodularization and inoculation.
  • nodular cast iron with a thick and large cross-section to increase the number of graphite balls and improve the roundness of graphite balls, generally a multi-stage inoculation technology including ladle inoculation, ladle-to-ladle inoculation, in-stream inoculation, and in-mould inoculation is employed.
  • the process is cumbersome, and difficult to control, and the nodularization effect is unstable.
  • the present invention provides a method for increasing the quality of graphite balls.
  • sulfur and rare earth are added in molten iron in advance, to produce a large amount of rare earth sulfide that forms a large number of nuclei of graphite crystallization, and then nodularization is performed, to extend the time of inoculation fade, and allow nodular cast iron with a thick and large cross-section to have good nodularization effect.
  • An object of the present invention is to provide a method for increasing the quality of graphite balls.
  • the method specifically comprises the following steps: increasing the sulfur content in base molten iron in advance, and adding rare earth to a nodularizing unit previously, so that dispersed rare earth sulfide particles are formed during the nodularization process, wherein the rare earth sulfide particles serve as the nuclei of graphite crystallization to increase the number of graphite balls, and improve the roundness of graphite balls.
  • the nodularizing unit is a nodularizing ladle.
  • the rare earth is selected from the group consisting of cerium, a lanthanum-cerium alloy and a cerium-iron alloy and any combination thereof.
  • the rare earth is added to the nodularizing unit previously, and the molten iron is poured into the nodularizing unit and nodularized, wherein the rare earth covers the surface of a nodularizer.
  • the temperature change in the nodularization process is such that the molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle and nodularized.
  • the sulfur content is increased by adding sulfur or ferric sulfide.
  • the rare earth accounts for 0.01-0.08 wt % of the molten iron.
  • the sulfur content in the sulfur-increased molten iron is 0.03-0.07 wt %.
  • the raw material of the base molten iron is selected from steel scrap and recycled scrap; the steel scrap is selected from carbon steel and/or alloy steel.
  • the sources of steel scrap include, but are not limited to, scraps of stamping parts, for example, scraps of automobile stamping parts. The use of steel scraps as a raw material can avoid the hereditary effects of pig iron as a raw material.
  • the steel scrap in the raw material accounts for 50-100 wt %, and the recycled scrap accounts for 0-50 wt %.
  • the method further comprises adding ferrosilicon, and a recarburizer to the base molten iron, to give a carbon content of 3.6-4.0 wt % and a silicon content of 1.8-2.1 wt % in the molten iron.
  • the method further comprises adding ferromanganese to the molten iron after nodularization, wherein the ferromanganese has a particle size of 5 to 15 mm.
  • the manganese content in the molten iron is 0.4-0.6 wt %.
  • the method for increasing number of graphite balls and improving the roundness of graphite balls in nodular cast iron comprises the following operations:
  • Steel scrap and recycled scrap are used as main raw materials and melted to produce nodular cast iron in During melting, an electric furnace.
  • Ferrosilicon, a recarburizer, sulfur powder (or ferric sulfide), and rare earth are added, where the molten iron is controlled to have a carbon content of 3.6-4.0 wt %, a silicon content of 1.8-2.1 wt %, a sulfur content of 0.03-0.07 wt %, and a rare earth content of 0.01 wt %-0.08 wt %.
  • the method comprises specifically the following steps:
  • Sulfur in molten iron can react with rare earth to form rare earth sulfide and also react with manganese to form manganese sulfide; and large manganese sulfide is difficult to be used as the nuclei of graphite nucleation, and reduces the mechanical performances of the material. Therefore, in the present invention, sulfur and rare earth are added before nodularization, to form fine rare earth sulfide acting as the nuclei of graphite balls. After nodularization, ferromanganese is added, to reduce the formation of manganese sulfide inclusions.
  • the obtained thick and large castings of 200 mm ⁇ 200 mm ⁇ 200 mm have a nodularization rate of graphite balls increased by 20% or more at the surface and in the core, a diameter of graphite balls increased by 1 grade, and a number of graphite balls per unit area increased by 50% or more.
  • FIG. 1 schematically shows a sampling position in Comparative Example 3 of the present invention.
  • the materials such as ferrosilicon, the recarburizer, ferrosulfur, the rare earth alloy, and the nodularizer etc. used in the present invention are all commercially available.
  • the present invention provides a method for increasing the quality of graphite balls.
  • the method comprises the following steps.
  • Steel scrap and recycled scrap are used as raw materials and melted to produce nodular cast iron.
  • the sources of steel scrap include, but are not limited to, scraps of stamping parts, for example, scraps of automobile stamping parts.
  • the steel scrap may be carbon steel, alloy steel or a mixture of thereof.
  • the use of steel scraps as a raw material can avoid the hereditary effects of pig iron as a raw material.
  • ferrosilicon and a recarburizer are added to adjust the components in molten iron, and the sulfur content in the base molten iron in the electric furnace is increased to adjust the sulfur content in the base molten iron to 0.03%-0.07%.
  • Rare earth is added before nodularization, to allow the molten iron to have a cerium content reaching 0.01%-0.08%, and a manganese content of 0.4-0.6 wt %.
  • ferromanganese is added to the molten iron, and then a casting is formed by casting according to a conventional procedure.
  • This example provides a method for increasing the quality of graphite balls.
  • the method comprises the following steps:
  • This example provides a method for increasing the quality of graphite balls.
  • the method comprises the following steps:
  • This example provides a method for increasing the quality of graphite balls.
  • the method comprises the following steps:
  • This example provides a method for increasing the quality of graphite balls.
  • the method comprises the following steps:
  • This example provides a method for increasing the quality of graphite balls.
  • the method comprises the following steps:
  • nodular cast iron is produced according to an existing conventional technology. That is, pig iron, steel scrap, and recycled scrap are used as raw materials, and the sulfur content is controlled to be low. Ferrosilicon is added during the melting process for inoculation, followed by nodularization and casting of a nodular cast iron specimen of 200 mm ⁇ 200 mm ⁇ 200 mm. The carbon content, silicon content, and manganese content are controlled according to the ranges in the examples; and the sulfur content is ⁇ 0.03 wt %.
  • the comparative example differs from the examples mainly in that the molten iron is subjected to sulfur-increasing treatment in the electric furnace in the examples, rare earth is added to molten iron before nodularization, and ferromanganese is added to molten iron after nodularization.
  • the sulfur content is controlled to be low, and no rare earth is added before nodularization.
  • pig iron, steel scrap and recycled scrap are used as raw materials specific proportion of the raw materials is 50 wt % pig iron+30 wt % carbon steel scrap+20 wt % recycled scrap.
  • a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed, added to an electric furnace, and melted.
  • Ferrosilicon, ferromanganese, and a recarburizer are added during the melting process of molten iron. After melting, a sample is taken and analyzed, and the components in molten iron are adjusted to have a carbon content of 3.6 wt %, a silicon content of 2.1 wt %, and a manganese content of 0.4 wt %; and the sulfur content in molten iron is detected to be 0.02 wt %.
  • the molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized.
  • the molten iron is casted to form a nodular cast iron specimen of 200 mm ⁇ 200 mm ⁇ 200 mm.
  • nodular cast iron is produced according to an existing conventional technology. Steel scrap, and recycled scrap are used as raw materials, and the sulfur content is controlled to be low. Ferrosilicon is added during the melting process for inoculation, followed by nodularization and casting of a nodular cast iron specimen of 200 mm ⁇ 200 mm ⁇ 200 mm. The carbon content, silicon content, and manganese content are controlled according to the ranges in the examples; and the sulfur content is ⁇ 0.02 wt %.
  • the comparative example differs from the examples mainly in that the molten iron is subjected to sulfur-increasing treatment in the electric furnace in the examples, rare earth is added to molten iron before nodularization, and ferromanganese is added to molten iron after nodularization.
  • the sulfur content is controlled to be low, and no rare earth is added before nodularization.
  • Ferrosilicon, ferromanganese, and a recarburizer are added during the melting process of molten iron. After melting, a sample is taken and analyzed, and the components in molten iron are adjusted to have a carbon content of 4.0 wt %, a silicon content of 1.8 wt %, and a manganese content of 0.6 wt %; and the sulfur content in molten iron is detected to be 0.015 wt %.
  • the molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized.
  • the molten iron is casted to form a nodular cast iron specimen of 200 mm ⁇ 200 mm ⁇ 200 mm.
  • the comparative example differs from the examples mainly in that the molten iron is subjected to sulfur-increasing treatment in the electric furnace in the examples, rare earth is added to molten iron before nodularization, and ferromanganese is added to molten iron after nodularization.
  • the sulfur content is controlled to be extremely low, and no rare earth is added before nodularization.
  • waste steel alone is used as raw material, and specific proportion of the raw material is 100 wt % carbon steel scrap.
  • a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed, added to an electric furnace, and melted.
  • Ferrosilicon, ferromanganese, and a recarburizer are added during the melting process of molten iron. After melting, a sample is taken and analyzed, and the components in molten iron are adjusted to have a carbon content of 3.8 wt %, a silicon content of 2.0 wt %, a manganese content of 0.5 wt %; and the sulfur content in molten iron is detected to be 0.008 wt %.
  • the molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized.
  • the molten iron is casted to form a nodular cast iron specimen of 200 mm ⁇ 200 mm ⁇ 200 mm.
  • test samples are taken from 4 surface positions and a central part of a cubic nodular iron casting of 200 mm ⁇ 200 mm ⁇ 200 mm, respectively.
  • the 4 surface samples are numbered 1, 2, 3, and 4, and the central sample is numbered 5, as shown in Table 1 below.
  • E1 refers to Example 1
  • E2 refers to Example 2
  • E3 refers to Example 3
  • E4 refers to Example 4
  • Average refers to the average of Examples 1-4
  • CE1 refers to Comparative Example 1
  • CE2 refers to Comparative Example 2
  • CE3 refers to Comparative Example 3
  • Average′ refers to the average of Comparative Examples 1-3.
  • the method for increasing the quality of graphite balls has a nodularization effect that is obviously better than the existing common methods. For a thick and large casting, the nodularization rate is increased by 20% or more, the diameter is increased by 1 grade, and the number density of graphite is increased by 50% or more.

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
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  • Mechanical Engineering (AREA)
  • Manufacturing & Machinery (AREA)
  • Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

Abstract

The invention provides a method for increasing the quality of graphite balls. The method comprises melting molten iron in an electric furnace, increasing the sulfur content in the molten iron during the melting process, and adding rare earth in the electric furnace or in a nodularizing ladle; after the molten iron is completely melted, pouring the molten iron into the nodularizing ladle and nodularizing; and after nodularization, adding ferromanganese to a transfer ladle. In the present invention, sulfur is added to molten iron in advance, and rare earth is added to a nodularizing ladle previously, so that a large number of dispersed rare earth sulfide particles are formed in the molten iron during the nodularization process. Rare earth sulfide particles serve as the nuclei of graphite crystallization to increase the number of graphite balls, and improve the roundness of graphite balls.

Description

    FIELD OF THE INVENTION
  • The present invention relates to the technical field of metallurgy and cast iron alloys, and particularly to a method for increasing the quality of graphite balls.
    • DESCRIPTION OF THE RELATED ART
  • Nodular cast iron is widely used, and the roundness and size of the graphite balls in nodular cast iron are critical factors affecting its performance. It is well known that sulfur is a main factor causing poor nodularization of graphite, and the molten iron is required to have a sulfur content that is as low as possible to obtain round graphite balls. Where fine graphite balls are desired, ferrosilicon is required for inoculation. The nodularization allows graphite to grow spherically, and the inoculation increases the nucleation rate of graphite, ensuring the number and roundness of graphite balls. Therefore, the traditional nodularization technology includes desulfurization, and then nodularization and inoculation. For nodular cast iron with a thick and large cross-section, due to the long solidification time of molten iron after inoculation, the inoculation effect is gradually deteriorated, as shown by a decrease in the number of graphite balls per unit area of cast iron, the size of graphite balls is increased, and the degree of nodularization becomes worse, affecting the mechanical performances of castings.
  • Therefore, for nodular cast iron with a thick and large cross-section, to increase the number of graphite balls and improve the roundness of graphite balls, generally a multi-stage inoculation technology including ladle inoculation, ladle-to-ladle inoculation, in-stream inoculation, and in-mould inoculation is employed. The process is cumbersome, and difficult to control, and the nodularization effect is unstable.
  • Therefore, a method for producing nodular cast iron is desired, to solve the above problems.
    • SUMMARY OF THE INVENTION
  • To solve the above technical problems, the present invention provides a method for increasing the quality of graphite balls. In the method of the present invention, sulfur and rare earth are added in molten iron in advance, to produce a large amount of rare earth sulfide that forms a large number of nuclei of graphite crystallization, and then nodularization is performed, to extend the time of inoculation fade, and allow nodular cast iron with a thick and large cross-section to have good nodularization effect.
  • An object of the present invention is to provide a method for increasing the quality of graphite balls. The method specifically comprises the following steps: increasing the sulfur content in base molten iron in advance, and adding rare earth to a nodularizing unit previously, so that dispersed rare earth sulfide particles are formed during the nodularization process, wherein the rare earth sulfide particles serve as the nuclei of graphite crystallization to increase the number of graphite balls, and improve the roundness of graphite balls.
  • In an embodiment of the present invention, the nodularizing unit is a nodularizing ladle.
  • In an embodiment of the present invention, the rare earth is selected from the group consisting of cerium, a lanthanum-cerium alloy and a cerium-iron alloy and any combination thereof.
  • In an embodiment of the present invention, the rare earth is added to the nodularizing unit previously, and the molten iron is poured into the nodularizing unit and nodularized, wherein the rare earth covers the surface of a nodularizer.
  • In an embodiment of the present invention, the temperature change in the nodularization process is such that the molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle and nodularized.
  • In an embodiment of the present invention, the sulfur content is increased by adding sulfur or ferric sulfide.
  • In an embodiment of the present invention, the rare earth accounts for 0.01-0.08 wt % of the molten iron.
  • In an embodiment of the present invention, the sulfur content in the sulfur-increased molten iron is 0.03-0.07 wt %.
  • In an embodiment of the present invention, the raw material of the base molten iron is selected from steel scrap and recycled scrap; the steel scrap is selected from carbon steel and/or alloy steel. The sources of steel scrap include, but are not limited to, scraps of stamping parts, for example, scraps of automobile stamping parts. The use of steel scraps as a raw material can avoid the hereditary effects of pig iron as a raw material.
  • In an embodiment of the present invention, the steel scrap in the raw material accounts for 50-100 wt %, and the recycled scrap accounts for 0-50 wt %.
  • In an embodiment of the present invention, the method further comprises adding ferrosilicon, and a recarburizer to the base molten iron, to give a carbon content of 3.6-4.0 wt % and a silicon content of 1.8-2.1 wt % in the molten iron.
  • In an embodiment of the present invention, the method further comprises adding ferromanganese to the molten iron after nodularization, wherein the ferromanganese has a particle size of 5 to 15 mm.
  • In an embodiment of the present invention, the manganese content in the molten iron is 0.4-0.6 wt %.
  • In an embodiment of the present invention, the method for increasing number of graphite balls and improving the roundness of graphite balls in nodular cast iron comprises the following operations:
  • Steel scrap and recycled scrap are used as main raw materials and melted to produce nodular cast iron in During melting, an electric furnace. Ferrosilicon, a recarburizer, sulfur powder (or ferric sulfide), and rare earth are added, where the molten iron is controlled to have a carbon content of 3.6-4.0 wt %, a silicon content of 1.8-2.1 wt %, a sulfur content of 0.03-0.07 wt %, and a rare earth content of 0.01 wt %-0.08 wt %. The method comprises specifically the following steps:
      • (1) Waste steel tube is used as raw material; a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed.
      • (2) Waste steel tube is added to an electric furnace, then ferrosilicon and a recarburizer are added to the base molten iron, and the sulfur content in the base molten iron in the electric furnace is increased by adding a sulfur increasing material selected from sulfur power (or ferric sulfide). After the molten iron is melted, a sample is taken to analyze the contents of various elements. The contents of various auxiliary materials are adjusted to allow the molten iron to have a carbon silicon, manganese, and sulfur content reaching the above contents.
      • (3) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into a nodularizing ladle previously added with a nodularizer and nodularized, where rare earth is added to the nodularizing ladle previously.
      • (4) After nodularization, FeMn68 ferromanganese is added to a transfer ladle, where the manganese content is controlled to 0.4-0.6%. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm is casted.
  • Compared with the prior art, the technical solution of the present invention has the following advantages:
  • In view of the problems of reduced number of graphite balls and poor roundness of graphite balls, caused by inoculation fade during the nodularization of nodular iron castings with a thick and large cross-section, the traditional ideal of desulfurization and then nodularization and inoculation in the production of nodular cast iron is overturned in the present invention. Instead, sulfur and rare earth are added in molten iron in advance, to produce a large amount of rare earth sulfide that forms a large number of nuclei of graphite crystallization, and then nodularization is performed, to extend the time of inoculation fade, and allow nodular cast iron with a thick and large cross-section to have good nodularization effect.
  • Sulfur in molten iron can react with rare earth to form rare earth sulfide and also react with manganese to form manganese sulfide; and large manganese sulfide is difficult to be used as the nuclei of graphite nucleation, and reduces the mechanical performances of the material. Therefore, in the present invention, sulfur and rare earth are added before nodularization, to form fine rare earth sulfide acting as the nuclei of graphite balls. After nodularization, ferromanganese is added, to reduce the formation of manganese sulfide inclusions.
  • By using the method for increasing the quality of graphite balls according to the present invention, the obtained thick and large castings of 200 mm×200 mm×200 mm have a nodularization rate of graphite balls increased by 20% or more at the surface and in the core, a diameter of graphite balls increased by 1 grade, and a number of graphite balls per unit area increased by 50% or more.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • To make the disclosure of the present invention more comprehensible, the present invention will be further described in detail by way of specific embodiments of the present invention with reference the accompanying drawings, in which:
  • FIG. 1 schematically shows a sampling position in Comparative Example 3 of the present invention.
    •  DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • The present invention will be further described below with reference to the accompanying drawings and specific examples, so that those skilled in the art can better understand and implement the present invention; however, the present invention is not limited thereto.
  • The materials such as ferrosilicon, the recarburizer, ferrosulfur, the rare earth alloy, and the nodularizer etc. used in the present invention are all commercially available.
  • To enhance the effect of nodularization and inoculation of nodular cast iron, the present invention provides a method for increasing the quality of graphite balls. The method comprises the following steps.
  • Steel scrap and recycled scrap are used as raw materials and melted to produce nodular cast iron. The sources of steel scrap include, but are not limited to, scraps of stamping parts, for example, scraps of automobile stamping parts. The steel scrap may be carbon steel, alloy steel or a mixture of thereof. The use of steel scraps as a raw material can avoid the hereditary effects of pig iron as a raw material.
  • After the steel scrap and recycled scrap are melted, ferrosilicon and a recarburizer are added to adjust the components in molten iron, and the sulfur content in the base molten iron in the electric furnace is increased to adjust the sulfur content in the base molten iron to 0.03%-0.07%. Rare earth is added before nodularization, to allow the molten iron to have a cerium content reaching 0.01%-0.08%, and a manganese content of 0.4-0.6 wt %. After nodularization, ferromanganese is added to the molten iron, and then a casting is formed by casting according to a conventional procedure.
    • EXAMPLE 1
  • This example provides a method for increasing the quality of graphite balls. The method comprises the following steps:
      • (1) Waste steel tube and recycled scrap of nodular cast iron are used as main raw materials and melted in an electric furnace, where the waste steel tube and recycled scrap each account for 50 wt %. Ferrosilicon, a recarburizer, and ferrosulphur are added during the melting process, where the molten iron is controlled to have a carbon content of 3.6 wt %, a silicon content of 2.1 wt %, and a sulfur content of 0.03 wt %.
      • (2) In this example, waste steel tube and recycled scrap of nodular cast iron are used as raw materials, a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed.
      • (3) The waste steel tube and recycled scrap of nodular cast iron are added to the electric furnace at a ratio of waste steel tube to recycled scrap of nodular cast iron of 1:1. Then, FeSi75 ferrosilicon, the recarburizer, and FeS40 ferrosulphur are added. After the molten iron is melted, a sample is taken to analyze the contents of various elements. The contents of various auxiliary materials are adjusted to allow the molten iron to have a carbon, silicon, and sulfur content reaching the above contents.
      • (4) A nodularizer is added to a nodularizing ladle, and Ce covers the nodularizer, where the amount of cerium added is calculated according to a content of 0.02 wt % in the molten iron.
      • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle and nodularized.
      • (6) After nodularization, FeMn68 ferromanganese is added to a transfer ladle, where the manganese content is controlled to 0.4 wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm is casted.
    EXAMPLE 2
  • This example provides a method for increasing the quality of graphite balls. The method comprises the following steps:
      • (1) Waste steel tube and recycled scrap of nodular cast iron are used as main raw materials and melted in an electric furnace, where the weight ratio of the waste steel tube and the recycled scrap is 7:3. Ferrosilicon, a recarburizer, and ferrosulphur are added during the melting process, where the molten iron is controlled to have a carbon content of 4.0 wt %, a silicon content of 1.8 wt %, and a sulfur content of 0.07 wt %.
      • (2) In this example, waste steel tube and recycled scrap of nodular cast iron are used as raw materials, a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed.
      • (3) The waste steel tube and recycled scrap of nodular cast iron are added to the electric furnace, where the weight ratio of the waste steel tube and the recycled scrap is 7:3. Then, FeSi75 ferrosilicon, the recarburizer, and FeS40 ferrosulphur are added. After the molten iron is melted, a sample is taken to analyze the contents of various elements. The contents of various auxiliary materials are adjusted to allow the molten iron to have a carbon, silicon, and sulfur content reaching the above contents.
      • (4) A nodularizer is added to a nodularizing ladle, and a lanthanum-cerium alloy covers the nodularizer, where the amount of the lanthanum-cerium alloy added is calculated according to a content of 0.08 wt % in the molten iron.
      • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle and nodularized.
      • (6) After nodularization, FeMn68 ferromanganese is added to a transfer ladle, where the manganese content is controlled to 0.4 wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm is casted.
    EXAMPLE 3
  • This example provides a method for increasing the quality of graphite balls. The method comprises the following steps:
      • (1) Waste steel tube is used as main raw material and melted in an electric furnace, where no recycled scrap is added. Ferrosilicon, a recarburizer, sulfur powder, and a lanthanum-cerium alloy are added during the melting process, where the molten iron is controlled to have a carbon content of 3.8 wt %, a silicon content of 2.0 wt %, a sulfur content of 0.03 wt %, and a cerium content of 0.01 wt %.
      • (2) In this example, waste steel tube is used as raw material; a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed.
      • (3) The waste steel tube is added to the electric furnace. Then, FeSi75 ferrosilicon, the recarburizer, sulfur powder, and the lanthanum-cerium alloy are added. After the molten iron is melted, a sample is taken to analyze the contents of various elements. The contents of various auxiliary materials are adjusted to allow the molten iron to have a carbon, silicon, manganese, sulfur, and cerium content reaching the above contents.
      • (4) A nodularizer is added to a nodularizing ladle, and a lanthanum-cerium alloy covers the nodularizer, where the amount of the lanthanum-cerium alloy added is calculated according to a content of 0.01 wt % in the molten iron.
      • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized,
      • (6) After nodularization, FeMn68 ferromanganese is added to a transfer ladle, where the manganese content is controlled to 0.6 wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm is casted.
    EXAMPLE 4
  • This example provides a method for increasing the quality of graphite balls. The method comprises the following steps:
      • (1) Waste steel tube is used as main raw material and melted in an electric furnace, where no recycled scrap is added. Ferrosilicon, a recarburizer, sulfur powder, and cerium are added during the melting process, where the molten iron is controlled to have a carbon content of 3.9 wt %, a silicon content of 1.9 wt %, a sulfur content of 0.07 wt %, and a cerium content of 0.08 wt %.
      • (2) In this example, waste steel tube is used as raw material; a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed.
      • (3) The waste steel tube is added to the electric furnace. Then, FeSi75 ferrosilicon, the recarburizer, sulfur powder, and a lanthanum-cerium alloy are added. After the molten iron is melted, a sample is taken to analyze the contents of various elements. The contents of various auxiliary materials are adjusted to allow the molten iron to have a carbon, silicon, manganese, sulfur, and cerium content reaching the above contents.
      • (4) A nodularizer is added to a nodularizing ladle, and a lanthanum-cerium alloy covers the nodularizer.
      • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized,
      • (6) After nodularization, FeMn68 ferromanganese is added to a transfer ladle, where the manganese content is controlled to 0.5 wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm is casted.
    EXAMPLE 5
  • This example provides a method for increasing the quality of graphite balls. The method comprises the following steps:
      • (1) Waste steel tube is used as main raw material and melted in an electric furnace, where no recycled scrap is added. Ferrosilicon, a recarburizer, ferrosulfur, and cerium are added during the melting process, where the molten iron is controlled to have a carbon content of 3.8 wt %, a silicon content of 2.1 wt %, a sulfur content of 0.05 wt %, and a cerium content of 0.08 wt %.
      • (2) In this example, waste steel tube is used as raw material; a material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed.
      • (3) The waste steel tube is added to the electric furnace. Then, FeSi75 ferrosilicon, the recarburizer, sulfur powder, and a cerium-iron alloy are added. After the molten iron is melted, a sample is taken to analyze the contents of various elements. The contents of various auxiliary materials are adjusted to allow the molten iron to have a carbon, silicon, manganese, sulfur, and cerium content reaching the above contents.
      • (4) A nodularizer is added to a nodularizing ladle, and a cerium-iron alloy covers the nodularizer.
      • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized,
      • (6) After nodularization, FeMn68 ferromanganese is added to a transfer ladle, where the manganese content is controlled to 0.5 wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm is casted.
    COMPARATIVE EXAMPLE 1
  • (1) In the comparative example, nodular cast iron is produced according to an existing conventional technology. That is, pig iron, steel scrap, and recycled scrap are used as raw materials, and the sulfur content is controlled to be low. Ferrosilicon is added during the melting process for inoculation, followed by nodularization and casting of a nodular cast iron specimen of 200 mm×200 mm×200 mm. The carbon content, silicon content, and manganese content are controlled according to the ranges in the examples; and the sulfur content is ≤0.03 wt %.
  • (2) The comparative example differs from the examples mainly in that the molten iron is subjected to sulfur-increasing treatment in the electric furnace in the examples, rare earth is added to molten iron before nodularization, and ferromanganese is added to molten iron after nodularization. In the comparative example, the sulfur content is controlled to be low, and no rare earth is added before nodularization.
  • (3) In the comparative example, pig iron, steel scrap and recycled scrap are used as raw materials specific proportion of the raw materials is 50 wt % pig iron+30 wt % carbon steel scrap+20 wt % recycled scrap. A material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed, added to an electric furnace, and melted.
  • (4) Ferrosilicon, ferromanganese, and a recarburizer are added during the melting process of molten iron. After melting, a sample is taken and analyzed, and the components in molten iron are adjusted to have a carbon content of 3.6 wt %, a silicon content of 2.1 wt %, and a manganese content of 0.4 wt %; and the sulfur content in molten iron is detected to be 0.02 wt %.
  • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized.
  • (6) After nodularization, the molten iron is casted to form a nodular cast iron specimen of 200 mm×200 mm×200 mm.
    • COMPARATIVE EXAMPLE 2
  • (1) In the comparative example, nodular cast iron is produced according to an existing conventional technology. Steel scrap, and recycled scrap are used as raw materials, and the sulfur content is controlled to be low. Ferrosilicon is added during the melting process for inoculation, followed by nodularization and casting of a nodular cast iron specimen of 200 mm×200 mm×200 mm. The carbon content, silicon content, and manganese content are controlled according to the ranges in the examples; and the sulfur content is ≤0.02 wt %.
  • (2) The comparative example differs from the examples mainly in that the molten iron is subjected to sulfur-increasing treatment in the electric furnace in the examples, rare earth is added to molten iron before nodularization, and ferromanganese is added to molten iron after nodularization. In the comparative example, the sulfur content is controlled to be low, and no rare earth is added before nodularization.
  • (3) In the comparative example, steel scrap and recycled scrap are used as raw materials, and specific proportion of the raw materials is 70 wt % carbon steel scrap+30 wt % recycled scrap. A material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed, added to an electric furnace, and melted.
  • (4) Ferrosilicon, ferromanganese, and a recarburizer are added during the melting process of molten iron. After melting, a sample is taken and analyzed, and the components in molten iron are adjusted to have a carbon content of 4.0 wt %, a silicon content of 1.8 wt %, and a manganese content of 0.6 wt %; and the sulfur content in molten iron is detected to be 0.015 wt %.
  • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized.
  • (6) After nodularization, the molten iron is casted to form a nodular cast iron specimen of 200 mm×200 mm×200 mm.
    • COMPARATIVE EXAMPLE 3
  • (1) In this comparative example, waste steel alone is used as raw material, and the sulfur content is controlled to be extremely low. Ferrosilicon is added during the melting process for inoculation, followed by nodularization and casting of a nodular cast iron specimen of 200 mm×200 mm×200 mm. The carbon content, silicon content, and manganese content are controlled according to the ranges in the examples; and the sulfur content is ≤0.01 wt %.
  • (2) The comparative example differs from the examples mainly in that the molten iron is subjected to sulfur-increasing treatment in the electric furnace in the examples, rare earth is added to molten iron before nodularization, and ferromanganese is added to molten iron after nodularization. In the comparative example, the sulfur content is controlled to be extremely low, and no rare earth is added before nodularization.
  • (3) In the comparative example, waste steel alone is used as raw material, and specific proportion of the raw material is 100 wt % carbon steel scrap. A material list is formulated, and entered into an automatic material weighing system; and the materials are automatically weighed, added to an electric furnace, and melted.
  • (4) Ferrosilicon, ferromanganese, and a recarburizer are added during the melting process of molten iron. After melting, a sample is taken and analyzed, and the components in molten iron are adjusted to have a carbon content of 3.8 wt %, a silicon content of 2.0 wt %, a manganese content of 0.5 wt %; and the sulfur content in molten iron is detected to be 0.008 wt %.
  • (5) The molten iron is heated to 1500° C., held for 4 min, and then cooled to 1450° C. at which the molten iron is poured into the nodularizing ladle previously added with the nodularizer and nodularized.
  • (6) After nodularization, the molten iron is casted to form a nodular cast iron specimen of 200 mm×200 mm×200 mm.
  • The nodularization effects of graphite in each part of the nodular iron castings of 200 mm×200 mm×200 mm obtained in the above examples and comparative examples are shown in Table 1.
  • The test samples are taken from 4 surface positions and a central part of a cubic nodular iron casting of 200 mm×200 mm×200 mm, respectively. The 4 surface samples are numbered 1, 2, 3, and 4, and the central sample is numbered 5, as shown in Table 1 below.
  • According to GB/T9441 “Metallographic Test for Spheroidal Graphite Cast Iron” in connection with image analysis, the nodularization rate, the ball diameter and the number density of graphite in the sample are determined.
  • TABLE 1
    Comparison of nodularization rate and number density of
    graphite of Examples 1-4 and Comparative Examples 1-3
    No. Test item E1 E2 E3 E4 Average CE1 CE2 CE3 Average′
    Sample Nodularization 86.5 87.2 83.1 88.5 86.3 71.3 66.2 69.1 68.9
    1 rate (%)
    Ball diameter 5 5 5 5 5 4 4 4 4
    (grade)
    Number 159 172 152 169 163.0 97 116 98 103.7
    density of
    graphite
    (balls/mm2)
    Sample Nodularization 86.3 85.4 82.7 89 85.9 66.7 67.3 70.1 68.0
    2 rate (%)
    Ball diameter 5 5 5 5 5 4 4 4 4
    (grade)
    Number 200 185 163 190 184.5 88 106 91 95.0
    density of
    graphite
    (balls/mm2)
    Sample Nodularization 84.7 80.9 83.2 85.5 83.6 68.3 65.8 65.6 66.6
    3 rate (%)
    Ball diameter 5 5 5 5 5 4 4 4 4
    (grade)
    Number 175 162 155 176 167.0 103 112 99 104.7
    density of
    graphite
    (balls/mm2)
    Sample Nodularization 80.8 83.5 87.2 84.7 84.1 69.8 72.1 66.7 69.5
    4 rate (%)
    Ball diameter 5 5 5 5 5 4 4 4 4
    (grade)
    Number 147 163 181 177 167.0 117 98 109 108.0
    density of
    graphite
    (balls/mm2)
    Sample Nodularization 84.3 81.3 80.8 84.6 82.8 64.5 62.1 66.6 64.4
    5 rate (%)
    Ball diameter 5 5 5 5 5 4 4 4 4
    (grade)
    Number 181 166 172 178 174.3 131 105 111 115.7
    density of
    graphite
    (balls/mm2)
  • In the above table 1, E1 refers to Example 1, E2 refers to Example 2, E3 refers to Example 3, E4 refers to Example 4, and Average refers to the average of Examples 1-4; CE1 refers to Comparative Example 1, CE2 refers to Comparative Example 2, CE3 refers to Comparative Example 3, and Average′ refers to the average of Comparative Examples 1-3. As can be seen from Table 1, the method for increasing the quality of graphite balls has a nodularization effect that is obviously better than the existing common methods. For a thick and large casting, the nodularization rate is increased by 20% or more, the diameter is increased by 1 grade, and the number density of graphite is increased by 50% or more.
  • Apparently, the above-described embodiments are merely examples provided for clarity of description, and are not intended to limit the implementations of the present invention. Other variations or changes can be made by those skilled in the art based on the above description. The embodiments are not exhaustive herein. Obvious variations or changes derived therefrom also fall within the protection scope of the present invention.

Claims (11)

What is claimed is:
1. A method for increasing the quality of graphite balls, comprising steps of: increasing the sulfur content in base molten iron, and adding rare earth to a nodularizing unit, so that dispersed rare earth sulfide particles are formed during a nodularization process, wherein the rare earth sulfide particles serve as the nuclei of graphite crystallization to increase the number of graphite balls, and improve the roundness of graphite balls.
2. The method according to claim 1, wherein the rare earth is selected from the group consisting of cerium, a lanthanum-cerium alloy and a cerium-iron alloy and any combination thereof.
3. The method according to claim 1, wherein the sulfur content is increased by adding sulfur or ferric sulfide.
4. The method according to claim 1, wherein the rare earth accounts for 0.01%-0.08% by weight of the base molten iron.
5. The method according to claim 1, wherein the sulfur content in the sulfur-increased molten iron is 0.03-0.07 wt %.
6. The method according to claim 1, wherein the rare earth is added to the nodularizing unit previously, and the molten iron is poured into the nodularizing unit and nodularized, wherein the rare earth covers the surface of a nodularizer.
7. The method according to claim 1, wherein a raw material of the base molten iron is selected from steel scrap and recycled scrap; and the steel scrap is selected from carbon steel and/or alloy steel.
8. The method according to claim 7, wherein the steel scrap accounts for 50-100 wt %, and the recycled scrap accounts for 0-50 wt % in the raw material.
9. The method according to claim 1, further comprising adding ferrosilicon, and a recarburizer to the base molten iron, to give a carbon content of 3.6-4.0 wt % and a silicon content of 1.8-2.1 wt % in the molten iron.
10. The method according to claim 1, further comprising adding ferromanganese to the molten iron after nodularization, and the ferromanganese has a particle size of 5 to 15 mm.
11. The method according to claim 10, wherein the manganese content in the molten iron is 0.4-0.6 wt %.
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