Grade of ductile iron with reinforced ferritic matrix

Abstract

The invention relates to a grade of ductile iron having a tensile strength (Rm) equal to or greater than 700 MPa, an elongation at break equal to or greater than 11%, a yield strength (Rp 0.2) equal to or greater than 532 MPa, and a minimum ratio of yield strength (Rp 0.2) to tensile strength (Rm) of 76% in a tensile test performed on a specimen taken from a tensile bar in accordance with standard EN 1563 and having a diameter of 25 mm and a length of 200 mm.

Description

DOMAIN OF THE INVENTION

[0001] The field of the invention is that of ductile irons or spheroidal graphite (SG) irons with a reinforced ferritic matrix (mostly ferritic). TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Ductile irons, or ductile irons with spheroidal graphite, standardized according to the European standard EN 1563 and referred to as having a reinforced ferritic matrix, are characterized by the following mechanical properties: tensile strength (Rm) in MPa, elongation (A) in %, and yield strength (Rp 0.2, i.e., the stress value at which 0.2% of the plastic deformation remains) in MPa. The ratio between yield strength and tensile strength is also an indicator of the quality of ductile irons. The table below lists the various standardized (EN 1563) ductile irons with a reinforced ferritic matrix, along with their properties and structures. Designation R m (MPa) Rp 0.2 (MPa) A% Matrix structure EN-GJS-450-18 450 350 18 Reinforced Ferritic EN-GJS-500-14 500 400 14 Reinforced Ferritic EN-GJS-600-10 600 470 10 Reinforced Ferritic

[0003] These cast irons have very interesting properties in terms of elongation at break but exhibit tensile strengths always below 700MPa.

[0004] It is also known that traditional ductile irons, also standardized (EN1563), have excellent tensile strength up to 700MPa (see table below - grade EN-GJS-700-2), the elongation at break is then limited to 2%. Designation R m (MPa) Rp 0.2 (MPa) A% Matrix structure EN-GJS-700-2 700 420 2 Perlitic EN-GJS-600-3 600 370 3 Perlitic EN-GJS-500-7 500 320 7 Perlito-ferritic EN-GJS-400-15 400 250 15 Ferritic EN-GJS-350-22 350 220 22 Ferritic

[0005] We see that the currently existing ductile grades do not offer both tensile strength values ​​Rm greater than or equal to 700MPa and elongation at break greater than or equal to 8%, 10%, or even 11%, and this with a significant ratio between the yield strength and the tensile strength, typically greater than 75%.

[0006] Document EP3243920A1 proposes a high-strength cast iron alloy with an elongation at break between 2% and 10%. Document WO2018093894 discloses a ductile cast iron alloy with an elongation at break greater than or equal to 2.5%. SUBJECT OF THE INVENTION

[0007] The invention aims to overcome the aforementioned drawbacks by providing a ferritic-matrix reinforced ductile iron with a tensile strength at least comparable to pearlitic-structured ductile irons (typically greater than or equal to 700 MPa), with an elongation at break greater than or equal to 11%, and furthermore, an Rp 0.2 / Rm ratio greater than 76%. The invention also relates to a method for manufacturing such a ductile iron. BRIEF DESCRIPTION OF THE INVENTION

[0008] The invention relates to a ductile iron with a reinforced ferritic matrix according to the claims. It is remarkable in that the structure with a reinforced ferritic matrix comprises more than 70% ferrite and in that the ductile iron exhibits a tensile strength (Rm) greater than or equal to 700 MPa, an elongation at break greater than or equal to 11%, a yield strength (Rp 0.2) greater than or equal to 532 MPa and a yield strength (Rp 0.2) to tensile strength (Rm) ratio greater than or equal to 76%, during a tensile test carried out on a specimen taken from a tensile bar in accordance with EN 1563 with a diameter of 25 mm and a length of 200 mm.

[0009] The ductile iron according to the invention has the following composition, the content of the different elements being given as mass percentages: a carbon content of between 2.8 and 4.5%; a silicon content of between 3 and 5%; a manganese content of between 0.1 and 0.8%; a sulfur content of less than 0.01%; a phosphorus content of between 0.001 and 0.05%; a copper content of between 0.1 and 1%, advantageously between 0.1% and 0.8%, or even preferentially between 0.1% and 0.7%; a magnesium content of less than 0.1%, a nickel content of between 0% and 1%, a chromium content of between 0% and 1%, impurities of less than 0.2%, these impurities coming in particular from the materials and equipment used in the manufacturing process of the cast iron, the balance to reach 100% being made by the iron content.

[0010] According to the invention, this grade of cast iron first allows for a significant increase in elongation at break, while maintaining excellent tensile strength, compared to prior art ductile cast irons. It also allows for a lesser reduction in mechanical properties resulting from an increase in the thickness of the manufactured part, i.e., low thickness sensitivity. The ratio between yield strength and tensile strength, which exceeds 76% for this grade, is also a significant advantage.

[0011] The invention also extends to a method for preparing a ferritic matrix ductile iron for the manufacture of a casting, comprising the following steps: the preparation of a raw material which contains carbon, silicon, manganese, sulfur, phosphorus, copper, magnesium and iron, and potentially nickel and chromium; the melting of the raw material; the inoculation of the raw material; the application of spheroidization; the casting of the inoculated material.

[0012] The ductile iron obtained exhibits a tensile strength (Rm) greater than or equal to 700 MPa, a yield strength greater than or equal to 532 MPa and a yield strength ratio (Rp 0.2) to tensile strength (Rm) of at least 76%, in a tensile test carried out on a specimen taken from a tensile bar in accordance with standard EN 1563 with a diameter of 25 mm and a length of 200 mm; the ductile iron also exhibits an elongation at break greater than or equal to 11%.

[0013] The inoculated material consists (by mass percentage) of 2.8% to 4.5% carbon, 3% to 5% silicon, 0.1% to 0.8% manganese, less than 0.01% sulfur, 0.001% to 0.05% phosphorus, 0.1% to 1% copper, less than 0.1% magnesium, 0% to 1% nickel, 0% to 1% chromium, iron and unavoidable impurities.

[0014] Preferably, the copper content is between 0.1% and 0.8%, or even advantageously between 0.1% and 0.7%.

[0015] The inoculation is carried out in one, two, three or four stages.

[0016] Spheroidization is advantageously carried out until a proportion of form VI graphite particles is obtained greater than 90%, or even greater than 95%.

[0017] The invention also extends to a method of manufacturing a molded part characterized in that the molded part does not undergo any heat treatment, which gives the ductile iron of the invention a strong economic interest.

[0018] Finally, the invention relates to the use of ductile iron for the manufacture of a high-voltage power line insulator cover or for the manufacture of cast mechanical parts in the fields of transport, mining or energy production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be better understood and other advantages will become apparent upon reading the following description and the accompanying drawings, in which: [ Fig.1 ] there figure 1 is a microstructure image of a ferritic-pearlitic matrix GS cast iron according to the invention before nitric etching; [ Fig. 2 ] there figure 2 is a microstructure image of a GS iron with a ferritic-pearlitic matrix according to the invention after nital etching. DETAILED DESCRIPTION OF THE INVENTION

[0020] The development of ductile spheroidal graphite (SG) cast irons with reinforced ferritic matrix stems from the objective of manufacturers to reduce variations in mechanical properties and the machinability costs of parts.

[0021] Traditional ductile irons with a predominantly pearlitic microstructure matrix are known to exhibit the mechanical properties grouped in Table 1, based on tests carried out with the EN-GJS-600-3 grade. [Table 1] # Rm (MPa) Rp 0.2 (MPa) A (%) Rp 0.2 / Rm (in %) 1 713 508 8,4 71,2% 2 646 452 11,8 70,0% 3 689 495 9,8 71,8% 4 708 500 8,8 70,6% 5 719 512 8,3 71,2% 6 686 485 9 70,7% 7 721 512 8,2 71,0% 8 646 452 11,8 70,0% 9 686 480 9,3 70,0% 10 725 511 8,2 70,5%

[0022] These cast irons have in their composition, in addition to iron, the following mass percentage contents according to Table 2 below. [Table 2] % mass C If Mn P S Cu Mg min 3 2,1 0,1 0,2 0,02 max 4 3 0,5 0,1 0,05 0,7 0,06

[0023] Even though the mechanical properties of these cast irons are already remarkable, they do not allow us to achieve very high values ​​of tensile strength (greater than or equal to 700MPa), yield strength (Rp 0.2 / Rm greater than or equal to 76%) and elongation at break (greater than 8%, or even greater than or equal to 11%).

[0024] Furthermore, the mechanical characteristics of these cast irons with a predominantly pearlitic structure suffer from a high sensitivity to the thickness of the manufactured part.

[0025] The ductile iron according to the invention has a reinforced ferritic matrix. This means that its structure is predominantly ferritic (> 70%) and that it may include, in a minor proportion, pearlite; the structure of the ductile iron according to the invention can therefore also be described as a ferritic-pearlitic matrix.

[0026] The cast iron of the present invention provides, on average, a tensile strength (Rm) greater than or equal to 700 MPa, a yield strength (Rp 0.2) greater than or equal to 532 MPa, a yield strength (Rp 0.2) to tensile strength (Rm) ratio of at least 76%, and an elongation at break always greater than 8% and even greater than or equal to 11%, in tensile tests carried out on a specimen taken from a tensile bar conforming to EN 1563 with a diameter of 25 mm and a length of 200 mm. Four test results (A to D) are noted in Table 3 below. [Table 3] # Rm (MPa) Rp 0.2 (MPa) A (%) Rp 0.2 / Rm (in %) A 725 557 11,5 76,8 B 710 556 11,5 78,3 C 718 559 13 77,8 D 709 539 12 77

[0027] This grade of cast iron has in its composition, in addition to iron (and inevitable impurities), the mass percentage contents according to Table 4. The composition of a grade according to the present invention is also given, by way of example, in Table 4. [Table 4] % mass C If Mn P S Cu Mg Neither Cr min 2,8 3 0,1 0,001 0 0,1 0 0 0 max 4,5 5 0,8 0,05 0,01 1 0,1 1 1 Example 3,17 4,30 0,22 <0,012 0,0035 0,59 0,039 0 0

[0028] In the ductile iron according to the invention, silicon contributes to strengthening the ferritic matrix; the percentage of copper is limited to the range [0.1%-1%], or even to the range [0.1%-0.8%] to control the proportion of pearlite in the matrix. Preferably, the copper content is even limited to 0.7%.

[0029] This type of ductile iron reveals a ferritic-pearlitic (mostly ferritic) microstructure, as seen on the figure 1before metallographic etching with nital (a solution of nitric acid and alcohol commonly used for the chemical etching of ferrous metals) and on the figure 2 after metallographic etching with nital.

[0030] Due to its reinforced ferritic or ferritic-pearlitic structure, ductile iron has mechanical properties that are much less sensitive to the thickness of the manufactured part, which is a strong industrial advantage.

[0031] To prepare ductile iron according to the invention for the manufacture of castings, the process first comprises a step of preparing a raw material containing carbon, silicon, manganese, sulfur, phosphorus, copper, magnesium, iron, and potentially nickel and chromium, within the mass percentage ranges previously stated. The process then comprises steps of melting the raw material, inoculating said material, applying spheroidization, before the step of casting the inoculated material.These steps are carried out so that the ductile iron has a reinforced ferritic structure and the mechanical characteristics mentioned above, namely a tensile strength (Rm) greater than or equal to 700 MPa, a yield strength greater than or equal to 532 MPa, a yield strength ratio (Rp 0.2 ) to tensile strength (Rm) of at least 76% and an elongation at break greater than or equal to 11%.

[0032] Inoculation treatment products are those listed in traditional supplier catalogs, but require particular attention to both their composition and particle size to achieve the mechanical properties Rm, Rp 0.2, and A mentioned above. Furthermore, the choice of inoculant must be adapted to the spheroidization treatment product (a subsequent step). In particular, the choice of the active element in the inoculant (barium, strontium, zirconium) must be made according to the composition of the spheroidization alloy. Inoculation is carried out in one, two, three, or even four stages. It is typically performed with each transfer of molten metal, including in the molten stream that brings the molten metal into the mold and / or when placing an ingot in the pattern plate.

[0033] Products for spheroidization treatment are also those listed in traditional supplier catalogs, but require particular attention to both their composition and particle size to achieve the mechanical properties Rm, Rp 0.2, and A mentioned above. The levels of active elements, such as magnesium, calcium, and rare earth elements, must be rigorously controlled. All traditional spheroidization introduction processes (Tundish Cover, Sandwich, InMold, etc.) are applicable to this process.

[0034] As seen on the figure 1The spheroidization quality is very good when the proportion of graphite particles of form VI is greater than 90%. It is optimal when the proportion of graphite particles of form VI is greater than 95%, with 5% or less of particles of form V, i.e., 100% nodularity. Note that the graphite particle count can be performed manually or by software. For the classification of graphite particles, reference is made to the NF-EN-945 standard.

[0035] The magnesium content must also be sufficient, but less than 0.1%, to ensure a spheroidal character of the graphite particles, throughout the entire thickness range of the parts, without being excessive.

[0036] The mechanical characteristics of the ductile iron according to the invention are obtained in particular with a high density of graphite spheroids per unit polished surface, i.e. greater than 1000 / mm² polished surface.

[0037] It is noted here that the composition of the ductile iron according to the invention falls within the ranges defined by the min and max limits, at the end of the manufacturing process. Therefore, care will be taken during the various stages (supply of raw materials, inoculation, spheroidization) to introduce the compounds in such a way as to respect these ranges in the final iron obtained.

[0038] The process also applies to the manufacture of a casting that does not undergo any heat treatment. Heat treatments are usually applied to cast iron parts to correct microstructural imperfections; obtaining a ductile iron casting that directly exhibits the desired structure and mechanical properties, without requiring heat treatment, offers a significant economic advantage.

[0039] The ductile iron according to the invention finds its application for the manufacture of cast mechanical parts, such as a high-voltage power line insulator cover or for the manufacture of cast mechanical parts in the fields of transport, mining or energy production.

[0040] It goes without saying that the present invention cannot be limited to the embodiment described above, which may be modified without departing from the scope of the invention.

Claims

1. Ductile iron having a reinforced ferritic matrix structure, characterised in that the reinforced ferritic matrix structure comprises more than 70% ferrite, in that the ductile iron has a tensile strength (Rm) higher than or equal to 700 MPa, an elongation at break higher than or equal to 11%, a yield strength (Rp0.2) higher than or equal to 532 MPa and a ratio of yield strength (Rp0.2) to tensile strength (Rm) of at least 76%, in a tensile test carried out on a test piece taken from a tensile bar in accordance with EN 1563 with a diameter of 25 mm and a length of 200 mm, and in that it is formed by the following compounds, as a percentage by mass: - carbon, between 2.8% and 4.5%, - silicon, between 3 and 5%, - manganese, between 0.1 and 0.8%, - sulphur, less than 0.01%, - phosphorus, between 0.001 and 0.05%, - copper, between 0.1 and 1%, - magnesium, less than 0.1%, - nickel, between 0% and 1%, - chromium, between 0% and 1%, - impurities, less than 0.2%, and - iron, to achieve a 100% balance.

2. Ductile iron according to claim 1, wherein the mass percentage of copper ranges from 0.1% to 0.8%, or preferentially ranges from 0.1% to 0.7%.

3. A method for preparing a ductile iron according to claim 1, comprising the following steps: - a raw material containing carbon, silicon, manganese, sulphur, phosphorus, copper, magnesium and iron, and potentially nickel and chromium; - melting the raw material; - inoculating the raw material; - applying spheroidisation; - pouring the inoculated material.

4. The preparation method according to the preceding claim, wherein the inoculation step is performed twice, three times or four times.

5. The preparation method according to one of claims 3 and 4, wherein spheroidisation is carried out until a proportion of form VI graphite particles exceeding 90%, or even 95%, is achieved.

6. The preparation method according to one of claims 3 to 6, resulting in the manufacture of a moulded part not subjected to any heat treatment.

7. Application of ductile iron according to one of claims 1 and 2 for the manufacture of a high-voltage electrical line insulator cover or for the manufacture of cast mechanical parts in the fields of transport, mining or energy production.

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