JPH07333180A - Chilling tendency evaluation method for molten metal and device therefor, and manufacture of cast iron using the evaluation method - Google Patents

Abstract

PURPOSE:To provide an evaluation method for the chilling tendency of molten metal, indispensable to the development and manufacture of such molten metal as hardly chilled, and useful for the quality control of molten metal as well CONSTITUTION:A critical cooling rate for chill crystallization is obtained via the analysis of the cooling curve where the cooling rate is variously changed, and a chilling tendency is evaluated on the basis of a difference in the critical cooling rate. In this case, the obtainable critical cooling rate is quantitative and universal with the unit expressed by deg.C/S. Thus, the chilling tendency can be examined via an objective comparison. Also, a measurement section and a tissue observation section can be made to agree to each other on the cooling curve. Accurate evaluation can, therefore, be made with cooling state properly related to chill crystallization. Furthermore, cast iron free from chill can be efficiently produced, using molten metal adjusted to optimum properties, according a cast iron manufacturing method using the chilling tendency evaluation method for the molten metal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chilling tendency evaluation method for molten metal, an apparatus therefor, and a method for producing cast iron using the same.

[0002]

2. Description of the Related Art In the production of castings, it is necessary to prevent crystallization of chill, which impairs mechanical properties such as ductility. The chill generation factors include the molten metal properties, and the factors other than the molten metal properties, such as the molten metal flow and the cooling rate. When factors other than the molten metal properties are excluded and the molten metal properties themselves are considered to be less likely to be chilled and improved, first of all, external factors other than the molten metal properties such as the plan, product shape and type of molding sand are considered. It is necessary to establish a universal and quantitative method for evaluating the net chilling tendency of molten metal, which is not affected by the above, and an apparatus therefor.

Conventionally widely used methods for evaluating the chilling tendency of molten metal include a wedge type test and a plate chilling test. The wedge type test is a method in which the shape of the test piece is wedge-shaped, the cooling rate of the molten metal is continuously changed, and the chilling tendency is judged by the size of the depth of chill appearing on the fracture surface. The plate chill test is a method in which one end of a test piece is forcibly cooled with a water-cooled copper plate or the like, and the chilling tendency is judged by the size of the depth of the chill that also appears on the fracture surface.

[0004]

However, in the conventional method for determining the chilling tendency, the obtained data (chill depth) largely depends on the subjectivity of the measurer, and it is difficult to match it with the behavior in the actual casting. Is. Therefore, it lacks generality, lacks reliability, and has to be limited to qualitative evaluation, and it is difficult to make a quantitative determination. Further, the above conventional chilling tendency determination method is for ordinary cast iron in which chill is clearly observed, and it is difficult to use it for spheroidal graphite cast iron.

On the other hand, there has been proposed a method of using a plate chill test piece to quantify the properties of the molten metal by using the cooling rate of the molten metal as a parameter. However, since the test piece for measuring the cooling rate and the test piece for evaluating the chill are different from each other, it is difficult to correctly correspond the relationship between the cooling state of the molten metal and the chill crystallization. The present invention has been made in view of the above problems in the prior art, is indispensable for developing and manufacturing molten metal that is difficult to chill, and can be used for quality control of molten metal, chilling tendency of molten metal An object of the present invention is to provide an evaluation method, an apparatus therefor, and a method for manufacturing cast iron using the evaluation method.

[0006]

Means for Solving the Problems The inventors of the present invention have made various studies to achieve the above-mentioned objects, examined the cooling state and structure of a cast iron molten metal, and examined the cooling rate and the amount of chill crystallized in the molten metal. The inventors have found that there is a correlation between them and have conceived the present invention. (1) That is, the molten metal chilling tendency evaluation method of the present invention is characterized in that the chilling tendency is evaluated based on the critical cooling rate of chill crystallization during the cooling process of the cast iron molten metal. (2) Then, the critical cooling rate is obtained from the maximum cooling rate after solidification of the cast iron molten metal. (3) Further, the chilling tendency is evaluated based on the slope of the chill crystallization amount-maximum cooling rate function from the critical cooling rate of chill crystallization in the process of cooling the cast iron melt. (4) Further, the same molten metal is solidified at various cooling rates,
The critical cooling rate is determined based on the presence or absence of chilling at that time. (5) In determining the critical cooling rate, the presence or absence of crystallization of chill and the amount thereof are measured as a chill area ratio or a chill volume ratio in the temperature measuring section. (6) The molten metal chilling tendency evaluation device of the present invention is characterized in that the cast iron molten metal is simultaneously injected into a plurality of molds having a temperature measuring means and capable of changing the cooling rate. (7) The method for producing cast iron using the molten metal chilling tendency evaluation method according to the third aspect of the present invention evaluates the chilling tendency based on the critical cooling rate of chill crystallization in the cooling process of the cast iron molten metal,
It is characterized in that the molten metal properties are adjusted based on the evaluation results, and the chill-free cast iron is manufactured using the molten metal.

Means (1) to (7) for solving the above problems will be described in detail below. (1) Evaluation of chilling tendency based on the critical cooling rate of chill crystallization
A method of evaluating the chilling tendency of the molten metal according to the present invention will be described. The cooling curve obtained by thermal analysis should correspond to the solidification structure of the temperature measuring part. By solidifying the molten metal at various cooling rates, it is possible to find a critical cooling rate at which chill occurs (critical cooling rate). That is, "critical cooling rate" means that when the molten metal is cooled and solidified at various rates,
The cooling rate at which chill begins to crystallize, chill does not occur at a cooling rate slower than that, and conversely, at a cooling rate higher than that cooling rate, chill occurs. An excellent molten metal having a small tendency of chilling is a molten metal which hardly chills even when cooled quickly, and has a high critical cooling rate.

(2) The critical cooling rate of the cast iron melt after solidification
Figure 2 obtained from the maximum cooling rate is a diagram for explaining the maximum cooling rate after solidification of cast iron. For example, when the cooling rate is changed by a chiller or the like, even with molten metal having the same properties, the time-temperature diagram (cooling curve) shown in FIG. 2 is obtained. In the present invention, the cooling rate takes the maximum value of the differential value of the time-temperature after solidification of the cast iron melt, that is, the "maximum cooling rate" as a parameter. According to the cooling rate after the solidification of the cast iron, the cast iron is in the temperature range where the phase transformation does not occur, so that it is not affected by the heat input and output due to the phase transformation. FIG. 1 shows the relationship of the area ratio of chill crystallized with respect to various maximum cooling rates in FIG. As shown in FIG. 1, it can be seen that the maximum cooling rate after solidification and the chill area ratio that crystallizes have a correlation. Here, instead of the chill area ratio, the chill volume ratio can be similarly evaluated. Less than,
In the present invention, the chill area ratio will be representatively described. On the other hand, for comparison, the eutectic temperature of the spheroidal graphite cast iron melt is 1 or higher.
FIG. 8 shows the relationship between the average cooling rate from 200 ° C. to 1170 ° C. and the area ratio of crystallized chill. It can be seen that at a cooling rate from the eutectic temperature or higher, a good correspondence between the chill crystallization and the cooling rate is not obtained.

(3) Chill crystallization amount from the critical cooling rate / maximum
Chilling tendency is evaluated based on the slope of the large cooling rate function . As shown in the cooling rate and the chill area ratio in FIG. 1, the chill crystallization amount from the critical cooling rate / the gradient of the maximum cooling rate after solidification (α) is It can be judged that the smaller the molten metal is, the more difficult it is to chill.

(4) The same molten metal is solidified at various cooling rates.
The critical cooling rate based on the presence or absence of chilling at that time.
When the same molten metal to be determined is cooled and solidified at various speeds, there is a cooling rate at which chill begins to crystallize, and at a cooling rate slower than that cooling rate, chill does not occur, and conversely it is faster than that cooling rate. Chilling occurs at the cooling rate. The "critical cooling rate" can be determined by the same molten metal.

(5) The critical cooling rate is whether chill is crystallized or not.
And its amount as the chill area ratio or chill volume ratio of the temperature measuring unit.
What is important in the method for quantitatively evaluating the molten metal chilling tendency of the present invention, which is measured by the above method, is to match the measurement site and the observation site. In other words, the cooling curve represents the behavior of a limited part of the test piece, so when considering the structure corresponding to this, the limited part must be viewed.
Therefore, it is preferable that the tissue observation is performed in the vicinity of the temperature detecting portion of the thermocouple for temperature measurement, and the chilling tendency is obtained by the chill area ratio or the chill volume ratio of that portion.

(6) The molten metal chilling tendency evaluation device
Simultaneous casting in multiple molds that have temperature means and can change the cooling rate
According to the chilling tendency evaluation method for molten metal of the present invention, in which the molten iron is injected, it is necessary to pour the same molten metal at the same time and collect a plurality of test pieces solidified at various cooling rates. This is because the tendency of chilling of the molten metal property changes with the passage of time and a decrease in temperature.

(7) A method for evaluating the chilling tendency of molten metal
Based on the critical cooling rate of chill crystallization in the cooling process of hot water
The chilling tendency is evaluated and the meltability is evaluated based on the evaluation result.
Adjusting the shape and using the molten metal, chill-free cast iron
The chilling tendency is evaluated based on the critical cooling rate of chill crystallization in the cooling process of the cast iron molten metal by utilizing the method for evaluating a chilling tendency of the molten metal of (1) to (5) for manufacturing The molten metal properties can be adjusted based on the results, and the chill-free cast iron can be manufactured using the molten metal.

[0014]

EXAMPLES An example of a molten metal chilling tendency evaluation method and apparatus of the present invention and a cast iron manufacturing method using the molten metal chilling tendency evaluation method will be described below. (Example 1) An example of an apparatus for evaluating the tendency of molten metal to chill is shown in FIG.
And FIG. 7 demonstrates. FIG. 6 is a sketch of a molten metal chilling tendency evaluation device, and FIG. 7 is a sectional view of FIG. Reference numeral 1 denotes a mold, which has a molten metal injection port 2, a weir 3, and voids 4 concentrically arranged at eight positions. In addition, the shell cups 5 are inserted into the eight hole portions 4 respectively,
Inside each shell cup 5, a cooling metal 6 having a different thickness t is provided. Then, thermocouples 7 are inserted from the bottom of the mold 1 toward the respective shell cups 5, and the temperature detecting portion 8 of the thermocouple 7 is arranged so as to be at the center position in the shell cups 5. The thermocouple 7 is a platinum-platinum-rhodium thermocouple that withstands high temperatures and has a good response speed. In addition, each thermocouple 7
Are connected together and connected to a temperature measuring device (not shown) by a data logger.

When the molten metal is poured into the pouring port 2, the molten metal is simultaneously poured into the shell cups 5 at eight locations via the weir 3. Next, the molten metal in each shell cup 5 starts to solidify by the cooling metal 6 with the thickness t changed. Then, the temperature detected by the temperature detecting unit 8 provided in each shell cup 5 is analyzed by the temperature measuring device to obtain the time-temperature diagram (cooling curve) shown in FIG. In this way, it is possible to simultaneously inject molten cast iron and simultaneously obtain a large number of test pieces with different cooling rates from the molten metal having the same properties.

After the molten metal in the shell cup 5 is solidified, a test piece is cut out from the portion in the vicinity of the temperature detecting portion 8 and mechanically polished to # 2400 with emery paper, and then buffed to a mirror finish and nital corrosion to a radius of about 2. Examine within 3 mm. For the test pieces in which chill crystallization was observed,
Further, the test piece is immersed in an ammonium sulfide solution for about 60 S, and the test piece is observed at a magnification of 200 times. Any 5 fields of view in the vicinity of the temperature detecting portion are measured by an image analyzer, and the average value is taken as the chill area ratio. FIG. 9 shows about 6 for ammonium sulfide solution.
It is a metallographic micrograph of the test piece after being immersed in 0S. In FIG. 9, the white part is the crystallized chill and the black part is the part other than the chill.

(Embodiment 2) The fact that the cooling rate and the amount of chill crystallization are correlated in the same molten metal will be explained in an embodiment applied to spheroidal graphite cast iron. The molten metal is a spheroidal graphite cast iron molten metal shown in Table 1 (including Fe and unavoidable impurities except for the indication)
Is. High-frequency induction melting is used for melting, and tapping temperature is 1
550 ± 10 ° C, pouring temperature is 1410 ± 10 ° C.
The spheroidizing treatment and the primary inoculation are carried out by a sandwich method in a ladle for 50 kg, and the secondary inoculation is carried out by adding the mixture to the ladle immediately before pouring and stirring. Casting is performed 3 times with the above molten metal, and the chilling tendency is measured by using the molten metal chilling tendency evaluation device.

[0018]

[Table 1] Chemical composition of molten metal (mass%) C Si Mn P S Mg CE value Molten metal A 3.73 2.46 0.28 0.017 0.009 0.035 4.55 Molten metal B 3.74 2.55 0.29 0.015 0.010 0.039 4.59 Molten metal C 3.82 2.42 0.25 0.015 0.013 0.039 4.63

The results are shown in the relationship diagram between the maximum cooling rate after solidification and the chill area ratio in FIG. Strictly speaking, it is conceivable that some variations will occur between the charges even if the molten metal is made under the same conditions. However, in all of molten metal A, molten metal B, and molten metal C, the relationship between the maximum cooling rate after solidification and the chill area ratio is almost the same, and the broken line appears, and the critical cooling rate (P) is about 7 ° C./s. . From the above, it is understood that the method for evaluating the tendency of molten metal chilling of the present invention has reproducibility.

(Example 3) When the CE (carbon equivalent) value changes, the chilling tendency changes. Utilizing this, the result of confirming whether the chill crystallization amount changes with the cooling rate following the change of the CE value will be described by way of examples. Casting is performed by adjusting the components of a molten metal D having a high Si value and a large Si% relative to the molten metal C and a molten metal E having a low CE value of a small Si% relative to the molten metal C. Table 2 shows the chemical composition of each charge (including Fe and unavoidable impurities, except for the indication).

[0021]

TABLE 2 Chemical composition of the molten metal (mass%) C Si Mn P S Mg CE value molten C 3.82 2.42 0.25 0.015 0.013 0.039 4.63 melt D 3.84 2.57 0.24 0.014 0.010 0.038 4.70 melt E 3.82 2.09 0.27 0.010 0.009 0.033 4.52

FIG. 4 is a diagram showing the results of the maximum cooling rate and the chill area ratio after solidification of the components of Table 2 having different CE values. The critical cooling rate (Pd) of molten metal D is about 16 ° / s,
Compared with the molten metal C of Example 1, a value 10 ° / s larger was shown, indicating that the molten metal has a smaller tendency to chill.
On the other hand, the melt E has a critical cooling rate (Pe) of about 4 ° /
s, which is 3 ° / s smaller than that of the molten metal C and has a large tendency of chilling. Thus, it was confirmed that the higher the CE value, the higher the critical cooling rate and the more the critical cooling rate changes in accordance with the CE value. The present invention makes clear the difference in the chilling tendency.

(Example 4) Next, by utilizing the method for evaluating the chilling tendency of the molten metal of the present invention, the amount of Bi added to the spheroidal graphite cast iron molten metal which is effective in preventing chill crystallization is examined, and chilling is performed. An example of producing a molten metal having a low tendency will be described. Bi
Is a strong spheroidization-inhibiting element, but is known to exhibit an effect of increasing the number of graphite particles with the addition of an extremely small amount, and is expected as one effective measure for reducing the chilling tendency of spheroidal graphite cast iron. However, the Bi addition amount that maximizes the number of graphite particles is 5
0 ppm (0.005%) to 100 ppm (0.010
%). However, the optimum addition amount is not clear. Therefore, the present invention is carried out to examine the correlation between the Bi addition amount and the chilling tendency. The molten metal treatment is F as a spheroidizing agent.
e-46Si-5.8Mg-3.5Ca-1.2REM
The same as Example 1 except that the inclusion of was used. For Bi addition, a method of adding metal Bi to a ladle during tapping from a high-frequency melting furnace is used, and the addition amount is 25 ppm, 50 ppm, 10
It is set to 0 ppm and 400 ppm. Table 4 shows the chemical components of each charge (however, Bi is in ppm, and Fe and unavoidable impurities are included except for the indication).

[0024]

[Table 4] Chemical composition (mass%) of molten metal, Bi is addition amount C Si Mn P S S Mg Bi (ppm ) CE value Molten metal F 3.78 2.46 0.27 0.016 0.007 0.041 25 4.60 Molten metal G 3.74 2.44 0.28 0.013 0.007 0.041 50 4.55 Molten metal H 3.80 2.40 0.28 0.014 0.009 0.037 100 4.60 Molten metal I 3.82 2.39 0.25 0.014 0.008 0.034 400 4.62

The obtained results are shown in FIG. For comparison, the relationship between the maximum cooling rate of the molten metal C and the chill area in Example 1 is shown by the broken line in the figure. As shown in FIG. 5, the critical cooling rate is about 7 ° / mol for molten metal F (Bi: 25 ppm).
s, molten metal G (Bi: 50 ppm) is approximately 12 ° / s, molten metal H (Bi: 100 ppm) is approximately 7 ° / s, and molten metal I
(Bi: 400 ppm) is about 2 ° / s. The critical cooling rate of the molten metal C (Bi: no addition, indicated by a broken line) is about 7 ° / s. From FIG. 5, the melt I (B
i: 400 ppm) has an extremely small critical cooling rate, and B
It can be seen that chilling is greatly promoted if the amount of i added is unnecessarily large. On the other hand, molten metal G (Bi: 50p
pm) has a high critical cooling rate, and addition of Bi: 50 ppm significantly suppresses chilling, and Bi: 50 ppm is an appropriate addition amount. If attention is paid only to the critical cooling rate, the molten metal F (Bi: 25 ppm) and the molten metal H (Bi: 100 ppm) are almost the same as the molten metal C, but the molten metal added with Bi: 25 to 100 ppm is Molten metal C when the maximum cooling rate is in the range of 10-40 ° C / s
On the other hand, since the amount of chill is small, it is understood that the addition of Bi in this range can suppress chilling. As described above, in the material having spheroidal graphite cast iron, when Bi is added at 50 ppm, the critical cooling rate is the largest and the chilling tendency is small, and chilling is suppressed by proper addition of Bi. As a result, it is possible to manufacture sound spheroidal graphite cast iron without chilling.

[0026]

As described above in detail, according to the method and apparatus for evaluating the chilling tendency of molten metal of the present invention, the critical cooling of chill crystallization can be performed by analyzing the cooling curves with various cooling rates. The rate can be obtained, and the chilling tendency can be determined from the difference in the critical cooling rate. Since the obtained critical cooling rate is a quantitative and universal one expressed in units of ° C / s, the chilling tendency can be objectively compared and examined. Further, since the measured portion of the cooling curve and the observed portion of the structure are made to coincide with each other, it is possible to perform an accurate evaluation in which the relationship between the cooled state and the chill crystallization is correctly associated.
In addition, according to the method for producing cast iron using the molten metal chilling tendency evaluation method of the present invention, it becomes possible to efficiently produce chill-free cast iron with the molten metal whose properties are optimally adjusted.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a chill area ratio of crystallization and a maximum cooling rate in an example of the present invention.

FIG. 2 is a diagram illustrating a maximum cooling rate after solidification of a cast iron molten metal.

FIG. 3 is a diagram showing the relationship between the maximum cooling rate after solidification and the chill area ratio in another example of the present invention.

FIG. 4 is a diagram showing the results of maximum cooling rate and chill area ratio after solidification of the components shown in Table 2 in which the CE value was changed in still another example of the present invention.

FIG. 5 is a diagram showing the results of the maximum cooling rate and the chill area ratio after solidification in the case where the amount of Bi added is changed in still another embodiment of the present invention.

FIG. 6 is a sketch of an embodiment of the chilling tendency evaluation device of the present invention.

7 is a cross-sectional view of the chilling tendency evaluation device of FIG.

FIG. 8 is a diagram showing, for comparison, the relationship between the average cooling rate from 1200 ° C. to 1170 ° C. above the eutectic temperature of the spheroidal graphite cast iron melt and the area ratio of crystallized chill.

FIG. 9 is a metallographic micrograph of a test piece after being immersed in an ammonium sulfide solution.

[Explanation of symbols]

1: mold, 2: injection port, 3: weir,
4: hole part, 5: shell cup, 6: chilled part, 7:
Thermocouple, 8: temperature detection part, t: thickness.

Claims (7)

[Claims] 1. A method for evaluating a chilling tendency of a molten metal, which comprises evaluating a chilling tendency based on a critical cooling rate of chill crystallization in a cooling process of a cast iron molten metal. 2. The chilling tendency evaluation method for molten metal according to claim 1, wherein the critical cooling rate is obtained from the maximum cooling rate after solidification of the cast iron molten metal. 3. The chilling tendency is evaluated based on the slope of the chill crystallization amount-maximum cooling rate function from the critical cooling rate of chill crystallization in the cooling process of the cast iron molten metal. 2. The chilling tendency evaluation method for molten metal according to 2. 4. The same molten metal is solidified at various cooling rates,
The chilling tendency evaluation method for molten metal according to claim 1, wherein the critical cooling rate is determined based on the presence or absence of chilling at that time. 5. The method according to claim 1, wherein in determining the critical cooling rate, the presence or absence of chill crystallization and the amount thereof are measured as a chill area ratio or a chill volume ratio of the temperature measuring section. Method for evaluating chilling tendency of molten metal. 6. An apparatus for evaluating the tendency of molten metal chilling, wherein molten cast iron is simultaneously injected into a plurality of molds having temperature measuring means and capable of changing cooling rates. 7. The chilling tendency is evaluated based on the critical cooling rate of chill crystallization during the cooling process of the cast iron molten metal, and the molten metal properties are adjusted based on the evaluation result. A method for producing cast iron using a method for evaluating the chilling tendency of molten metal, which comprises producing cast iron that does not exist.