Patent Publication Number: US-2004042925-A1

Title: Method for production of ductile iron

Description:
FIELD OF INVENTION  
       [0001] The present invention relates to a method for the manufacture of ductile cast iron with spheroidal or compacted graphite and a ferrosilicon based nodularizer alloy.  
       BACKGROUND ART  
       [0002] Cast iron is typically produced in cupola or induction furnaces, and generally contains between 2 to 4 percent carbon. The carbon is intimately mixed with the iron and the form, which the carbon takes in the solidified cast iron, is very important to the characteristics and properties of the iron castings. If the carbon upon solidification takes the form of iron carbide, then the cast iron is referred to as white or carbidic cast iron and has the physical characteristics of being hard and brittle which in certain applications are undesirable. If the carbon takes the form of flake-like graphite, the cast iron is soft and machinable and is referred to as grey cast iron. If magnesium and rare earth&#39;s are used to treat the liquid grey iron, the carbon will take the form of spheroidal or nodular graphite and is referred to as ductile cast iron.  
       [0003] The form, size and shape the graphite takes as well as the amount of graphite versus iron carbide, can be controlled with certain additives that promote the formation of graphite during solidification of cast iron. These additives are referred to as nodularizers and inoculants and their addition to the cast iron as nodularizing and inoculation. In casting iron products from liquid cast iron, there will always be a risk for the formation of iron carbides in thin sections of castings. The formation of iron carbide is brought about by the rapid cooling of the thin sections as compared to the slower cooling of the thicker sections of the casting. The formation of iron carbide in a cast iron product is referred to as “chill”. The formation of chill is quantified by measuring “chill dept” and the power of a nodularizer or inoculant to prevent chill and reduce chill depth is a convenient way in which to measure and compare the power of nodularizers and inoculants.  
       [0004] In cast iron containing spheroidal graphite the power of nodularizers and inoculants is also commonly measured by the number density per unit area of spheroidal graphite particles in the as-cast microstructure. A higher number density per unit area of graphite spheroids means that the power of nodularizing and inoculation has been improved.  
       [0005] There is a constant need to develop nodularizers and inoculants which reduce chill depth and improve the machinability of ductile cast irons as well as increase the number density of graphite spheroids.  
       [0006] Since the exact chemistry and mechanism of nucleation and why nodularizers and inoculants function as they do is not completely understood, a great deal of research goes into providing the industry with new and improved such alloys.  
       [0007] It is thought that rare earth&#39;s, such as cerium, lanthanum, praseodymium, and neodymium and certain other elements suppress the formation of iron carbide and promote the formation of graphite. A majority of nodularizers contains rare earth&#39;s in the form of a mixture between cerium, lanthanum, praseodymium and neodymium, often knows as misch metal. The addition of these elements is usually facilitated by the addition of a magnesium ferrosilicon alloy and the most widely used alloys contain 40 to 50% silicon, 4 to 6% magnesium and 1 to 2% misch metal.  
       [0008] The suppression of carbide formation is associated by the nucleating properties of the nodularizer and inoculant. By nucleating properties it is understood the number of nuclei formed by an alloy addition. A high number of nuclei formed improves the effectiveness and improves the carbide suppression. Further a high nucleation rate may also give better resistance to fading effects during prolonged holding time of the molten iron after nodularizing and inoculation.  
       [0009] The nodularizer and inoculant alloys also affect ductile iron solidification shrinkage. Some alloys may give good protection against shrinkage while others tend to promote more shrinkage. The use of various rare earth elements may have a pronounced impact on this condition. For nodularizer alloys it is also important that composition of the alloy is such so that a minimum of shrinkage occurs during solidification of the iron.  
       [0010] The nodularizing process is carried out in two basically different ways. In the so-called “ladle treatment method” the nodularizer alloy is placed in the bottom of the ladle whereafter liquid cast iron is filled into the ladle on the top of the nodularizer alloy. Depending on how the nodularizer alloy is placed in the ladle, the ladle treatment method is known as overpour, sandwich, or tundish cover treatment methods. The inoculation is normally carried out after the nodularizing process is done, by adding inoculant to the metal stream during transfer of the cast iron to a pouring vessel or to a mould.  
       [0011] In the so-called “in-the-mould” method the nodularizing treatment is taking place inside the casting mould cavity itself. The in-the-mould nodularizing method is thus significantly different from the ladle treatment nodularizing method.  
       [0012] It is known that the addition of lanthanum rare earth in magnesium ferrosilicon alloy has proven successful for the purpose of minimizing chill and shrinkage in ductile iron when using the in-the-mould nodularizing method. In the in-the-mould treatment method the magnesium ferrosilicon alloy will act both as nodularizer and inoculant simultaneously integrated in the gating system of the casting mould. For magnesium treatment of cast iron in the ladle treatment nodularizing method such integrated or combined nodularizing and inoculation is not known.  
       DISCLOSURE OF INVENTION  
       [0013] It has now been found that the use of pure lanthanum as the only rare earth source in the MgFeSi nodularizer alloy surprisingly further improves the performance of the ductile iron ladle treatment method compared to such nodularizers containing cerium or misch metal. Thus the number of nuclei is substantially increased and the risk for chill and shrinkage formation in the ductile or compacted graphite iron is minimized.  
       [0014] The present invention thus relates to a method for nodularizing treatment of ductile iron using a ladle treatment method for introduction of a magnesium ferrosilicon alloy, which method is characterized in that a magnesium ferrosilicon alloy consisting essentially of 40 to 80% by weight of silicon, 2 to 15% by weight of magnesium, 0.3 to 5% by weight of lanthanum, 0 to 6% by weight of calcium, 0 to 5% by weight of aluminum, the balance being iron, is added to the ladle where after the molten iron is supplied to the ladle.  
       [0015] According to a preferred embodiment the magnesium ferrosilicon alloy added to the ladle comprises 0.5 to 1.5% by weight of lanthanum.  
       [0016] The present invention further relates to the use of a magnesium ferrosilicon alloy comprising 40 to 80% by weight of silicon. 2 to 15% by weight of magnesium 0.3 to 5% by weight of lanthanum, 0 to 6% by weight of calcium 0 to 5% by weight of aluminum, the balance being iron, as nodularizer in the ladle treatment method for the production of ductile cast iron.  
       [0017] It has surprisingly been found that when a magnesium ferrosilicon alloy is used in the ladle treatment method according to the present invention, the number of nuclei formed when the magnesium ferrosilicon alloy is added to cast iron, is increased thus obtaining higher nodule counts and an improved suppression of iron carbide formation using the same amount of alloy as with conventional ladle treatment methods and alloys.  
       [0018] Further, by the present invention it has been found that the shrinkage tendency of the ductile cast iron is greatly reduced or even eliminated when using the method according to the present invention. It has also even been found that the present invention may provide sufficient nucleation power to the ductile cast iron to avoid chill and shrinkage formations only with a very small, or even without, inoculant addition following the nodularizing treatment process. 
     
    
    
     SHORT DESCRIPTION OF THE FIGURES  
     [0019]FIG. 1 is a schematic view of ladle treatment method used in example 1,  
     [0020]FIG. 2  a - h  show microstructures of test castings, and,  
     [0021]FIG. 3  a - d  show shrinkage porosity in test castings.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Example 1  
     [0022] Ductile irons were produced in an induction furnace using a charge based on 50% by weight of steel, 20% by weight of iron returns and 30% pig iron. The target analysis for the castings was 3.7% by weight of C, 2.4% by weight of Si, 0.4% by weight of Mn, 0.010% by weight of S and 0.040% by weight of Mg. Prior to tapping into a nodularizing treatment ladle, 1.5% by weight of magnesium ferrosilicon alloy (MgFeSi) based on the weight of the cast iron was placed into the ladle and covered by 0.5 kg steel punchings, i.e. nodularizing according to the sandwich treatment method. FIG. 1 shows a schematic representation of the treatment ladle used. Two minutes after the iron was tapped into the ladle, the iron was transferred into pouring ladles. Thus no inoculation was carried out after the nodularizating treatment. Coin shaped samples for chemical composition were extracted from the melt, and the ductile iron heats were then cast into sand moulds to produce a 20 mm thick plate and a 5 mm thin plate, a standard chill wedge sample and a cross bar sample for shrinkage evaluation.  
     [0023] Four different tests were performed. In two of the tests magnesium ferrosilicon alloy according to the present invention were used, and for comparison purpose, one test was done with a rare earth free magnesium ferrosilicon alloy and one test was run with a magnesium ferrosilicon alloy containing 1.0% by weight misch metal.  
     [0024] Table 1 shows the chemical composition of magnesium ferrosilicon alloys compared in this test, and Table 2 gives the chemical composition of the produced ductile iron castings.  
               TABLE 1                          Chemical composition of magnesium ferrosilicon nodularizer alloys.                                             Nodularizer   % Si   % Mg   % Ca   % Al   % TRE   % Ce   % La                                                     0.5% La   45.0   5.8   1.0   0.9   0.5   0.0   0.5       Invention       1.0% La   45.5   6.0   1.0   0.9   1.0   0.0   1.0       Invention       RE-free   45.8   6.1   1.0   0.9   0.0   0.0   0.0       Prior art       1.0% Misch   45.0   5.9   1.1   0.8   1.0   0.5   0.25       Prior art                          
 
     [0025]               TABLE 2                          Chemical composition of produced ductile iron castings.                                                 Ductile   %   %   %   %   %   %   %   %       iron   C   Si   Mn   P   S   Mg   Ce   La                                                         0.5% La   3.75   2.28   0.43   0.020   0.008   0.043   &lt;0.004   0.008       1.0% La   3.73   2.25   0.42   0.024   0.010   0.040   &lt;0.004   0.015       RE-free   3.73   2.51   0.46   0.027   0.009   0.046   &lt;0.004   &lt;0.004       1.0%   3.74   2.37   0.45   0.021   0.008   0.047   0.010   0.005       Misch                    
     [0026] Results from metallographic evaluation of graphite structures in cast plates samples are shown in Table 3. Microstructures of the test castings are shown in FIG. 2, while Table 4 gives an evaluation of the chill condition of the microstructures of the test castings. Table 5 and FIG. 3 show results from measurement of shrinkage porosity in the experimental cross-bar castings.  
               TABLE 3                          Characteristic graphite data for cast 5 and 20 mm plates.                                     Nodule       Average   Average       Nodularizer   count   Nodularity   diameter   shape       alloy   (N/mm 2 )   (%)   (μm)   factor                         5 mm plates                                 0.5% La   595   93   13.4   0.88       1.0% La   488   93   13.2   0.88       RE-free   110   81   9.3   0.75       1.0% MM   418   93   14.8   0.86                 20 mm plates                                 0.5% La   224   78   17.5   0.74       1.0% La   188   69   19.1   0.67       RE-free   112   42   20.2   0.50       1.0% MM   178   69   20.8   0.67                  
 
     [0027]               TABLE 4                          Evaluation of chill condition in micrographs of FIG. 2.                                 Nodularizer   5 mm plates   20 mm plates                       0.5% La   (e) no chill   (f) no chill           1.0% La   (g) only trade of chill   (h) no chill           RE-free   (a) massive chill   (b) some chill           1.0% Misch   (c) some chill   (d) no chill                        
     [0028] From Table 3 it can be seen that a substantial increase in the number of nodules was found in the two test runs according to the invention compared to the method using rare earth free magnesium ferrosilicon alloy and to the method using magnesium ferrosilicon alloy containing misch metal. This is true both for the 5 mm plates and the 20 mm plates.  
     [0029] From FIG. 2 and Table 4 it can be seen that the lanthanum containing magnesium ferrosilicon alloys strongly reduces and nearly eliminates the chill in the 5 mm plates and that no chill can be found in the 20 mm plates.  
     [0030] Table 5 and FIG. 3 shows that shrinkage of the ductile iron is eliminated when using the method of the present invention.  
     [0031] From the Example it is clear that ductile iron having practically no chill can be produced by the present invention without carrying out the conventional inoculation after the nodularizing treatment. Further the method according to the invention results in a ductile cast iron having no shrinkage porosity.  
               TABLE 5                          Relative shrinkage porosity area in cross-bar castings.                             Nodularizer   Area % pores                                         0.5% La   0.0           Invention           1.0% La   0.0           Invention           RE-free   8.3           Prior art           1.0% Misch   38.2           Prior art                      
 
     [0032] Because of the low chill and shrinkage porosity formation tendency, especially for the 0.5% lanthanum containing magnesium ferrosilicon alloy, the need for a subsequent addition of post inoculant material is minimized or even eliminated. Thus, the present invention describes a unique new ladle treatment nodularizing method that will be cost effective also in the sense that a minimum requirement for inoculation is needed.