KR20130043160A - Method for determining amounts of inoculant to be added to a cast-iron melt - Google Patents

Abstract

A method for determining the amount of an inoculant added to a molten cast iron in a particular casting process, comprising the steps of: providing a first sample holder (1) and a second sample holder (2) each having thermocouples (3, 4) (2) is filled with a predetermined amount of molten iron, and a first cooling curve during solidification of iron in the first sample holder (1) and a second cooling curve during the solidification of iron in the second sample holder (2) A method of determining the amount of an inoculant added to a cast iron melt, comprising the step of recording a second cooling curve during solidification of iron within the furnace, wherein one of the sample holders is subjected to a specific casting process (TE _low ) on the first cooling curve and the lowest melting temperature (TE _low ) on the second cooling curve, and a pre-determined amount of inoculant, Determining the amount of inoculant added to the melt in the process Wherein the amount of the inoculant added to the molten cast iron is determined.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining an amount of an inoculant to be added to a cast iron melt,

The present invention relates to a method for determining the amount of an inoculant added to a cast iron melt according to the preamble of claim 1.

Cast iron is a construction material commonly used for component parts of trucks. Cast iron is used, for example, in engine blocks, cylinder heads, cylinder liner, main-bearing cap and spring mounting.

Since most cast products do not undergo subsequent heat treatment or plastic machining to change the microstructure or remove defects in the product, the properties of the product are predominantly determined primarily at the casting stage. Therefore, the amount and appearance of the solidification structure, namely the primary phase (austenite) and the eutectic structure (austenite and graphite) is extremely important to the properties of the cast product. This amount of phase / tissue can be determined by so-called thermal analysis based on the cooling curve recorded during cooling of the sample from the melt. Thermal analysis is described, for example, in Swedish Patent Publications SE 516136 and SE 515026 and International Patent Publication No. WO 97/355184.

A known method of using thermal analysis to check phases and textures in cast iron is based on the liquidus temperature of the molten iron, i.e., the temperature at which the melt begins to solidify. Depending on how the measured liquidus temperature differs from the predetermined value, the texture of the final cast product can be changed, for example by carbon addition.

However, these known methods do not provide an accurate measure of how internal tissue nucleates or grows, but merely provide a rough estimate of the amount of each of the initial and eutectic phases.

Another known practice is to control the nucleation of the inner tissue of iron by adding an inoculant, but it has been found that it is difficult to optimize the amount of inoculant added by known methods.

Thus, the nucleation of the internal structure of iron significantly affects its properties, particularly the generation and severity of defects. In the final properties of the finished casting product, the process organization is particularly important. However, the fact that already known methods do not provide a valid evaluation of this phase organization can lead to strength problems and scrapping.

Accordingly, it is an object of the present invention to disclose a method for reliably determining the amount of the inoculant added to the cast iron melt and solving or at least minimizing the above-mentioned problems.

SUMMARY OF THE INVENTION [0006]

- providing a first sample holder (1) and a second sample holder (2), each having a thermocouple (3, 4) connected to the analysis equipment (5)

A predetermined amount of molten iron is filled in each of the sample holders 1 and 2 and the first cooling curve during the solidification of iron in the first sample holder 1 and the solidification curve of the iron in the second sample holder 2 A method of determining the amount of an inoculant added to a cast iron melt in a specific casting process, comprising the step of recording a second cooling curve of the casting process,

Before being filled with molten iron, one of the sample holders is provided with an inoculum of a predetermined amount representing a saturation level for the inoculant in a particular casting process, the amount of inoculant to be added is solved by a method characterized in that it is determined based on the difference of the minimum processing temperature in the first cooling curve (TE _low) and the minimum processing temperature in the second cooling curve (TE _low).

The method of the present invention provides precise control of the casting process so that there is only a slight variation in the quality of the finished casting product resulting in reduced scrap and reduced risk of accident during casting, .

According to an alternative example, the predetermined amount of the inoculant for the casting process is an average value based on the saturation level for the inoculant in a plurality of cast iron melts in a specific casting process.

According to an alternative example, the predetermined amount of the inoculant for the casting process is a value selected from the saturation level for the inoculant in a plurality of cast iron melts in a specific casting process.

If the inoculum contains silicon, the effect of silicon on the process temperature is preferably calculated and removed from the inoculated sample based on a predetermined relationship.

The amount of inoculant added in a particular casting process may be adjusted to be over-inoculation or under-inoculation of the melt, if desired.

The method is preferably related to determining the texture of the iron-lamellar graphite form.

Figure 1 is a part of the Fe-C state diagram.
2 shows the cooling form of the molten cast iron.
Figure 3 shows a test set-up for carrying out an experiment according to the method of the present invention.
4 is a diagram showing the difference between the process temperatures of the inoculated sample from the cast iron melt and the un-inoculated sample, respectively.

The theoretical background of the present invention will be explained as an introduction explanation.

)가 증가한다. The solidification of the melt involves the formation of small solid nuclei of aggregated atoms, after which the melt solidifies around the nucleus. The formation of these nuclei, i.e. nucleation, is controlled by the total energy in the melt. Systems such as metal melts tend to have as little energy as possible. Whether a material with a lower energy content is solid or liquid is temperature dependent. The formation of a solid phase in the melt involves the formation of a volume in which the atoms are arranged in an energy-efficient manner. At the same time, a surface is formed between the solid phase and the liquid phase. The atoms in the surface are pressured to a position that normally does not occupy, and the atoms need energy to reach that position. If solidification occurs at the solidification temperature of the molten metal below the solidification temperature (T _M ), the free energy per unit volume (G _v ) decreases and the surface energy

) Increases.

A로 표현될 수 있는데, 여기서 ΔG는 에너지 전체 변화이고, V는 고체의 체적이고, A는 고체의 표면 면적이다. The change in the total amount of energy in the system is given by the equation ΔG = ΔG _v V +

A, where ΔG is the total energy change, V is the volume of the solid and A is the surface area of the solid.

Nucleation occurs only when the total energy amount of the system decreases due to nucleation, that is, when ΔG becomes negative. Since the surface energy counteracts the phase change, the melt retains the liquid even if the temperature passes the solidification point. This is called supercooling of molten metal. As the temperature decreases below the freezing point, the driving force of the phase change further increases. By sufficiently reducing the temperature, nucleation occurs when the decrease in energy per unit volume becomes greater than the amount of energy required to produce the surface. In a homogeneous melt, this supercooling angle may be several hundred degrees, but is generally much smaller. How much of the supercooling is necessary for nucleation is an indicator of how easy nucleation can occur in the melt, which is also known as the nucleation potential of the melt.

철이라고도 불리는 오스테나이트가 정출한다. 공정 온도에서, 카본이 잔류 용탕으로부터 또한 정출하기 시작한다. 카본은 흑연 형태로 정출하며, 흑연은 그 형상에 따라서 재료의 특성에 큰 영향을 미친다. The solidification pattern of the cast iron can be explained by the Fe-C phase diagram in which the longitudinal axis is the temperature and the transverse axis is the weight percentage of carbon content, as in Fig. Figure 1 shows the area associated with cast iron, i.e. up to 5% carbon content. The lines in the figure represent the boundaries of the various phases of iron at various temperatures and carbon contents. The figure is characterized by a line representing the liquidus line (1) and the eutectic temperature (2). These two lines are important in predicting the organization of cast products. At the liquidus temperature,

Austenite, also called iron, is formed. At the process temperature, carbon begins to pour out of the remaining molten metal. Carbon is crystallized in the form of graphite, and graphite has a great influence on the properties of the material depending on its shape.

During cooling from the completely molten state to the solidified state, the melt passes through various phases in the Fe-C phase diagram. Fig. 2 schematically shows the solidification curve for cast iron. The plateau "a" at 1200 DEG C represents the liquidus temperature. At this point, austenite starts to form from the molten metal. When the temperature passes through 1150 占 폚, eutectic solidification starts at "b" which is the lowest process temperature of the melt (TE _low ). Process coagulation is expressed by a slight temperature rise in the melt. When the temperature is again reduced stably (region "c"), the entire melt changes to a solid form.

It has been found that what degree of supercooling is required to initiate process crystallization, i.e. TE _low , is a valid indicator of the nucleation potential of the melt. Small supercooling means good nucleation potential. It is important to achieve high nucleation potential during process solidification, because crystallization of graphite also occurs during solidification. Increasing the volume of graphite during solidification hinders the reduction of the volume of austenite, so the crystallization of graphite is also required for good mechanical properties of the cast product.

The nucleation potential can be controlled by adding a nucleation point in the form of an inoculum. Inoculant addition increases the temperature at which process coagulation takes place. In other words, smaller molten metal and cooling are required to initiate process coagulation.

It is important to optimize the amount of inoculant added to the melt. Adding too little inoculant may result in uneven and insufficient crystallization of the graphite, which results in the formation of carbides and in the worst case so-called white solidification. Adding too much inoculum increases production costs and may result in defects in the tissue of the cast product, for example by graphite expansion.

Thus, as described above, it is desirable to determine the nucleation potential of the melt at point "b" which is the minimum in the process of cooling curve. This is because the supercooling during the process solidification is directly related to the final structure of the casting product.

We have found by measurement that the cast iron melt can be saturated with the inoculant. This means that even with the addition of an additional inoculant during the inoculum addition, the temperature for the process coagulation ceases to increase at a given level. As a result, this "saturation level" represents the highest process temperature, which is the absolute limit for the TE _low of the melt, i.e., the minimum supercooling that can actually be achieved in the melt.

In the process according to the present invention described in more detail below, this relationship is important because it presents a certain level as to how high the process temperature will actually be in the molten cast iron. Thus, the nucleation potential in uninoculated melt can be compared with a stable reference value. Other methods according to the present invention will be described in detail below.

< Step 1 >

As a first step, molten cast iron is produced, which is prepared by dissolving initial materials such as scrap, return, pig iron and machined waste. The composition and temperature of the melt are controlled and necessary adjustments are made.

< Step 2 >

As a second step, a thermal analysis test apparatus is provided. The test apparatus (see Fig. 3) comprises two sample cups 1, 2, for example a sand cup, each cup being provided with thermocouples 3, 4, respectively. The thermocouple is connected to a set of analytical instruments (5) through which thermal analysis software is run. A pre-determined amount of inoculum is placed in one sample cup, indicating the inoculum saturation level of the particular casting process.

The amount of inoculant added to the cast iron melt to vary the inoculum saturation level, i.e. the inoculum is saturated, varies with the process conditions during the melt preparation. These conditions include, for example, the composition of the initial material (scrap) and the setting of process equipment.

However, in the so-called casting process, which is an industrial process for producing cast iron products, efforts have been made to keep the process conditions as constant as possible, that is, within a specific range. In the manufacture of certain components, such as engine blocks, we are always trying to use the same casting equipment to always use scrap with a composition that is maintained within prescribed limits and / or to manufacture certain components. This is to ensure that the results of several casting operations are as consistent as possible.

Thus, the amount of inoculant added to one sample cup to ensure that the injected melt reaches a saturation level can be predetermined as a representative value for a particular casting process. The representative value can then be used each time a new cast iron melt is produced in the casting process.

This representative value may be an average value of the inoculant saturation level determined for a plurality of the molten metals in a specific casting process, for example, a process for casting a predetermined type of engine block. The representative value may be a value selected from a number of different inoculant saturation levels determined for a plurality of molten metals in a particular casting process. For example, the representative value may be a median value or a maximum value or a minimum value. The representative value of the saturation level for a particular casting process can then be stored, for example, in a non-volatile memory in the computer, and retrieved and used therefrom whenever a particular casting process is performed.

It is also possible to determine the inoculum saturation value for a number of different cast iron melts and then to fit these saturation values into a linear relationship.

The method for determining the amount of inoculum needed to saturate the individual cast iron melts is as follows. The process temperature (TE _low ) is simultaneously determined for the two samples taken from the melt. One sample is not inoculated and the other is inoculated. For determination of the saturation level, TE _low is determined for each sample from the cooling curve recorded during the solidification of the sample, after the sample from the melt is simultaneously injected into each of two identical test cups. After the nucleation potential of the inoculated sample from the difference between the TE _low TE _low and non-inoculated sample of the inoculated sample it is determined. This process is repeated several times, and one sample is inoculated with an increasing number of inoculations each time than in the previous batch. By comparing the nucleation potential of the repeated samples it is possible to ascertain the level at which the increase in nucleation potential ceases with increasing inoculum quantity. This level is the inoculum saturation level of each molten metal. As discussed above, the saturation level is determined for a number of molasses, based on whether an average saturation level can be determined for a particular casting process or a representative value can be selected.

Figure 4, which will be discussed in greater detail later, shows how the increase in process temperature (TE _low ) ceases when more than 0.2 wt.% Of the inoculum is added to a given cast iron melt.

The inoculum saturation level is determined by repeated experiments involving two simultaneous measurements in the same two separate test cups under the same conditions. This is because, in the thermal analysis in which the cooling curve is recorded during the solidification of the cast iron in the test cup, the curve is influenced by various factors that conflict with the nucleation potential of the melt. The following are examples of these arguments.

- Test cup material

- Changes in thermocouple location and placement

- Changes in air draft due to walls, ventilation, etc.

- amount of molten metal in the cup

-Chemical composition of the melt

The temperature of the molten metal when it is injected into the cup

The aforementioned distortion factors affect the measurement result, for example by moving the cooling curve up or sideways, thereby placing the process temperature at an inaccurate level.

However, if errors occur in the measurements made in the two test cups under the same conditions, all of the results are affected by the same error. This distortion factor can be eliminated by subtracting the process temperature (TE _low ) of each sample from each other.

Another factor affecting the results is the silicon content of the inoculum. Inoculants generally contain silicon, for example in FeSi form. Silicon changes the chemical composition of the melt in the inoculated test specimen. The temperature at which process coagulation is initiated is based on the chemical composition of the melt, so that when silicon is added, the process temperature in the inoculated sample increases. In order to also exclude this distortion factor, the effect of silicon on the process temperature is subtracted from the inoculated sample.

It is also possible to derive a relationship to the effect of silicon on the process temperature. This can be done by reviewing the state diagram and by linear adaptation. In addition, it is possible to derive the relationship by an appropriate operation program, for example, ThermoCalc. This relationship can be used to calculate and separate the effect of silicon on process temperature.

The inoculant may be alloyed with other materials that are not alloyed with silicon and affect the equilibrium line in the state diagram, ie, other materials that affect the process temperature. It is also possible to obtain a relationship as to how such a substance affects the process temperature and to use this relationship to compensate the process temperature in the inoculated sample.

< Step 3 >

As a third step, the test cup is filled with molten metal and a cooling curve is recorded during solidification of the sample. From the two cooling curves, the lowest process temperature (TE _low ) for each sample is determined. Thereafter, the lowest process temperature of the non-inoculated sample is subtracted from the lowest process temperature of the inoculated sample.

According to the present invention, the influence of the silicone content of the inoculum on the process temperature of the inoculated sample is eliminated before the temperature is subtracted from the process temperature of the unopened sample.

< Step 4 >

As a fourth step, the difference calculated in degrees Celsius is used as a criterion to determine how much inoculum should be added to the melt to ensure that the final cast product has the desired texture. The difference in the obtained unit of ° C, that is, the nucleation potential of the melt serves as a measure of how far the process temperature is actually from the reachable process temperature. The difference in ° C obtained may be related to the final texture of the cast product.

The relationship between the temperature difference, the amount of inoculant added and the final texture of the cast product can be determined empirically. This can be determined for a considerable period of time by preparing various mangles and determining the amount of inoculant added by the process according to the invention and observing the texture of the cast product. For each melt produced, the relationship between the number of degrees of temperature difference and the amount of inoculum added is adjusted until there is a sufficiently good correspondence with the final texture of the cast product.

If the amount of the inoculant added to the molten metal is desired to be over-inoculated or unsaturated inoculated with respect to a predetermined product, it is also possible to manage such an inoculum.

It should be noted that it is not possible to add precisely the amount of inoculant to each melt that corresponds to the saturation level of a particular casting process for each melt. This is because the saturation level of each molten metal spreads with time, centered on the representative value of the saturation level of the casting process. For example, the dispersion can be attributed to the compositional variation of each melt within the prescribed limits. This means that the saturation level of the inoculant for each individual melt differs from the saturation level for the casting process. Adding an inoculant to each melt precisely in an amount expressed by the saturation level for the casting process can result in over-or inadmissive inoculation. As mentioned above, over-inoculation and unsaturated inoculation have several disadvantages with regard to cost and material properties.

< Step 5 >

As a fifth step, after the target amount of the inoculum material is added to the molten metal, the molten metal is injected into the appropriate casting mold.

Of course, there may be cases where only one molten metal is produced and there is no predetermined value for the inoculum saturation level. In this case, the saturation level is determined by repeated experiments with two test cups as described above. After the determined amount of the inoculant is added to the molten metal, the molten metal is used for casting.

&Lt; Description of Embodiments >

The present invention will be described below with reference to specific examples.

In a medium-frequency furnace manufactured by the manufacturer ABB, a 10-tone molten steel of gray iron type containing lamellar graphite was prepared.

As a first step, the amount of inoculum needed to saturate the molten metal with the inoculum was determined. Four test batches numbered from 1 to 4 were prepared for this purpose. The sample collection equipment for each batch (see FIG. 3) included two sample holders 1, 2 in the form of a sand cup provided with thermocouples 3, 4, respectively.

The specifications of the sand cup and thermocouple were as follows.

- Cup material: 65g shell-molded sand cup made of silica sand.

- Sand coated with 3.5% phenolic resin, iron oxide and stearin-based lubricant.

Thermocouple: NiCr-NiAI alloy insulated with high purity quartz glass tube

- Thermocouple position as shown in Fig. The average volume of the measurement cup was 4.86cm ^3. (Spot (spot) based on the weight 350g, density 7.22g / cm ^3).

Each thermocouple was connected to a separate channel of analysis equipment, including a computer with a processor used to run the ATAS evaluation software, a product of Novacast, a manufacturer.

The average weight of the melt contained in the test cup was calculated and an inoculant 6 of Superseed type which is an alloy of FeSi containing strontium was placed in the test cup 2 of Layout 2 to Layout 4. The amount of inoculant was calculated as weight percent of cast iron relative to the amount of molten metal contained in the test cup. The amount of added inoculum is shown in Table 1. No inoculum was placed in all test cups in batch 1.

The test cups were each then filled with the same amount of molten metal and cooling curves were recorded during solidification of the samples and analyzed in the evaluation program.

The lowest process temperature (TE _low ) for each cooling curve was then determined by the evaluation program. This is TE _low Quot; measurement &quot;. Since the inoculum contained silicon, the effect of silicon on process temperature was removed from each of the three inoculated samples. The sample thus corrected will be referred to as " TE _low "calibration. Table 1 shows the process temperature TE _low "measurement" for Sample 1 and Sample 2 in each batch and the process temperature TE _low "calibration" for the inoculated sample after compensation for silicon.

The process temperature TE _low "measurement" of each inoculated sample was subtracted from the process temperature for each un-inoculated sample. The result will be referred to as " ΔTE _low "measurement. After the process temperature of the inoculated sample was calibrated against the silicon content, it was subtracted from the process temperature of the uninoculated sample. The result will be referred to as " DELTA TE _low "calibration. These values are shown in Table 1.

Results from determination of inoculum saturation amount Test
arrangement Inoculant
weight% Uninoculated TE _low Inoculation TE _low
"Measure" Inoculation TE _low
"correction" ΔTE _low
"Measure" ΔTE _low
"correction" One 0 1138.24 1138.04 1138.04 -0.2 -0.2 2 0.1 1135.975 1139.725 1138.975 3.75 3 3 0.2 1132.425 1139.075 1137.575 6.65 5.15 4 0.4 1132.075 1139.95 1136.95 7.875 4.875

Figure 4 is a plot of the &lt; RTI ID = 0.0 &gt; ΔTE _low "Measurement" and ΔTE _low Indicates "calibration &quot;.

It can be seen that when the samples are calibrated against the effect of silicon on the process temperature from the figure, the difference in process temperatures of the inoculated and unopened samples is halted if the inoculum content exceeds 0.2% by weight. Therefore, this content represents the inoculum saturation level of the molten metal. The diagram in Figure 4 also shows that the difference in process temperature continues to increase with increasing inoculant content when samples are not compensated for the effect of silicon. Thus, inoculum saturation levels cannot be determined.

Claims (6)

A method for determining the amount of an inoculant added to a cast iron melt in a particular casting process,
- providing a first sample holder (1) and a second sample holder (2), each having a thermocouple (3, 4) connected to an analytical instrument (5)
A predetermined amount of molten iron is filled in each of the sample holders 1 and 2 and the first cooling curve during the solidification of iron in the first sample holder 1 and the solidification curve of the iron in the second sample holder 2 &Lt; / RTI &gt; comprising the steps of: &lt; Desc / Clms Page number 18 &gt; recording a second cooling curve of the casting iron,
One of the sample holders is provided with a pre-determined amount of inoculum indicating the level of saturation for the inoculant in a particular casting process, prior to filling with molten iron, and the lowest process temperature on the first cooling curve (TE _low ) Characterized in that the amount of the inoculant added to the melt in the particular casting process is determined based on the difference in the lowest process temperature (TE _low ) on the cooling curve. The method of claim 1,
Wherein the predetermined amount of inoculant for the master casting process is an average value based on the inoculum saturation level of a plurality of melts in a particular casting process. The method of claim 1,
Wherein the predetermined amount of inoculant for the master casting process is a value selected from the inoculum saturation levels of a plurality of melts in a particular casting process. 4. The method according to any one of claims 1 to 3,
The inoculant contains silicon,
_Wherein the effect of silicon on the process temperature (TE _low ) is calculated and removed based on a predetermined relationship between silicon and the process temperature. 5. The method according to any one of claims 1 to 4,
A method for determining the amount of an inoculant added to a molten cast iron, characterized in that the amount of the inoculant added to the melt in a particular casting process is adjusted to be an over-or inadmissible inoculation of the melt. The method according to any one of claims 1 to 5,
Wherein the iron is a lamellar graphite type. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;

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