JPH0136524B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a method and apparatus for measuring the graphite nodularity rate of molten cast iron subjected to graphite nodularization treatment over a wide range with high precision and high speed. Regarding.

The present invention quickly and accurately determines the graphite spheroidization rate before casting product cast iron and before pouring molten cast iron, so it can be used to improve cast iron manufacturing efficiency and product quality control. .

[Prior art]

In the process of manufacturing cast iron, controlling the nodularization rate of graphite with high precision is extremely important in terms of product quality control. Conventionally, the following methods are generally known as methods for measuring the graphite nodularity of cast iron.

The first method is to determine the spheroidization rate using a microscopic photograph of a cross section of a cast iron product manufactured from molten metal that has been subjected to the same spheroidization treatment, or a test material cast under the same conditions as the cast iron product. It is. Although this method is reliable in measuring the spheroidization rate, it has the disadvantage that it takes a long time to measure. Moreover, since the test is performed after pouring the molten metal, even if the quality is determined to be poor, many products have already been cast, and all products formed from this molten metal will be considered defective. Therefore, there is a drawback that manufacturing efficiency is extremely low.

The second method is to take out a part of the molten metal after graphite has been spheroidized, measure the cooling curve of this molten metal, extract various features of this cooling curve, and calculate these features and A method of estimating the spheroidization rate of graphite using a relational expression obtained by multiple regression analysis with the spheroidization rate of graphite measured in advance on a large number of samples, that is, a thermal analysis method is known. There is. However, conventional thermal analysis methods use a single empirical formula, and graphite The problem is that the measurement accuracy of the spheroidization rate is poor. That is, it has not been possible to measure the spheroidization rate of graphite over a wide range with high precision.

[Purpose of the invention]

Therefore, the present invention has been made to improve these drawbacks, and takes into account the characteristic values of the cooling curve of the original melt before spheroidizing graphite, and calculates the cooling curve of the molten metal after spheroidizing graphite. Based on the characteristic values of the cooling curve, the measured cooling curve is classified into multiple predetermined classes, and the spheroidization rate of graphite can be calculated over a wide range with high precision using the linear function specified by each class. It is an object of the present invention to provide a method and apparatus for measuring the graphite nodularity of molten cast iron that can be measured easily and at high speed.

[Structure of the invention]

The first invention involves taking out a part of the source water before subjecting it to graphite spheroidization treatment, measuring the cooling curve of the extracted source water, and determining the primary crystal temperature of the source water from the cooling curve. The crystal temperature is detected, and then the source melt is subjected to graphite spheroidization treatment, a part of the molten metal after this treatment is taken out, a cooling curve of the taken out molten metal is measured, and from the cooling curve, it is determined that Detecting the primary crystal temperature, supercooling temperature, and eutectic temperature of the subsequent molten metal, classifying the cooling curve of the molten metal after the treatment according to the presence or absence of these temperatures, and detecting the primary crystal temperature, eutectic temperature of the source metal. A linear function with temperature, primary crystal temperature, subcooling temperature, and eutectic temperature of the molten metal after treatment as variables, and whose coefficients differ depending on the classification, and the coefficients of the linear function for each classification are set in advance in a large number. The method consists of measuring the graphite nodularity of the molten metal after the above treatment using a linear function determined experimentally based on the correlation between the above variables and the graphite nodularity of the sample.

The method of the present invention measures the cooling curve of the source water before graphite spheroidization treatment as a first characteristic point, and measures the spheroidization rate of graphite from the primary crystal temperature and eutectic temperature obtained from the cooling curve. This is a variable for The second feature is that the primary crystal temperature, subcooling temperature, and eutectic temperature are detected from the cooling curve of the molten metal after the graphite spheroidization treatment, and these are used as variables to measure the graphite spheroidization rate. It is. Furthermore, the third
The feature is that the cooling curve is classified according to the presence or absence of the above-mentioned primary crystal temperature, supercooling temperature, and eutectic temperature, and the spheroidization rate of graphite is determined by the linear function specified by the classification. be. That is, the spheroidization rate of graphite can be determined by the following equation.

SG=A・TL0+B・TE0+C・TL +D・TC+E・TE+F …(1) Here, SG is the spheroidization rate of graphite, TL0,
TE0 is the primary crystal temperature and eutectic temperature in the cooling curve of the source water, respectively. Further, TL, TC, and TE are the primary crystal temperature, supercooling temperature, and eutectic temperature obtained from the cooling curve of the molten metal after graphite spheroidization treatment, respectively. Furthermore, A, B, C, D, E, and F are coefficients of each variable, respectively. These coefficients differ depending on the pre-classified class. Furthermore, each coefficient also includes 0. These coefficients are correlated with the graphite spheroidization rate measured by other methods (for example, the Japan Foundry Foundry Association method [NIK method]) for cast iron products with different cast iron components or spheroidization treatments, and each of the above variables. The relationship is measured and determined for each class using multiple regression analysis.

The classification of the cooling curve of the present invention is generally preferably comprised of the following four classifications, but is not necessarily limited to the number of classifications.

The first type is a case where primary crystal, supercooled, and eutectic temperatures appear significantly in the cooling curve of the molten metal subjected to spheroidization treatment. The second type is a case where the supercooling temperature is not detected but the primary crystal temperature and the eutectic temperature are detected. The third type is a case where the primary crystal temperature is not detected, but the subcooling temperature and the eutectic temperature are detected. In the fourth category, neither the primary crystal temperature nor the supercooling temperature is detected,
This is a case where only the eutectic temperature is detected.

However, the second class and the fourth class have similar corollaries of the calculation formula for the spheroidization rate, and can be combined into one class.

Here, the detection of the primary crystal temperature, supercooling temperature, and eutectic temperature is performed by determining the stagnation point of the cooling curve. This stagnation point is a point that exists in a section where the differential coefficient of the cooling curve exists within a certain range. Hereinafter, this section will be referred to as a stagnation section. The distinction between primary, supercooled, and eutectic is determined by the order in which the stagnation zone appears, the stagnation time, and whether the cooling curve shifts upward or downward after exiting the stagnation zone. For example, under the conditions of 250 g of molten metal taken out and a shell-shaped round bar of 35 mmφ x 40 mmh, the stagnation zone is defined as the range in which the differential coefficient of the cooling curve exists in the range of ±2.5〓/sec for 2.4 seconds or more. Also, the primary temperature is the first stagnation zone, and the stagnation zone is 16
It is detected as the intermediate value of the stagnation section where the curve moves downward after escaping the stagnation section. In addition, the supercooling temperature is the minimum value of the first or second stagnation section in which the curve moves upward after escaping the stagnation section, or if the above conditions are not satisfied, , is detected as the minimum value that appears earlier than the eutectic temperature in the stagnation section where the eutectic temperature is detected.

In addition, the eutectic temperature is the second or third stagnation section, and is the maximum value of the stagnation section in which the curve changes downward after escaping the stagnation section, or the first and 16 second stagnation section. It is detected as the maximum value of the continuous stagnation section.

The above conditions are just examples, and are not limited to these, but when the above values are met,
Primary, supercooled, and eutectic temperatures are detected.

Next, a device for measuring graphite nodularity of molten cast iron, which is a second invention, includes a temperature measuring device that measures the temperature in a time series during the cooling process of the molten metal, and a signal from the temperature measuring device is inputted to a predetermined value. A measuring device comprising: a control device that outputs a signal to an output device after the processing; and an output device that inputs the signal from the control device and performs display based on the signal, the control device comprising: , a stagnation point detection unit that detects a temperature stagnation point based on a signal from the temperature measurement device, and determines whether the detected stagnation point is a primary temperature, a supercooled temperature, or a eutectic temperature. a stagnation point determination unit that determines the stagnation point determination unit; a class determination unit that determines the class to which the cooling curve belongs according to the determination result of the stagnation point determination unit; a calculation unit that calculates the spheroidization rate; and stores the series of each of the linear functions, the primary crystal temperature of the base water, and the eutectic temperature of the base water measured from the cooling curve of the base water before the graphite spheroidization process. The graphite nodularity measuring device is characterized by comprising a storage section.

This device has a temperature measuring device that measures a cooling curve, performs processing according to the first invention described above based on data obtained from the temperature measuring device, classifies the cooling curve, and produces graphite according to the classification. This is a device that calculates the spheroidization rate using an empirical formula for calculating the spheroidization rate and outputs the result. Further, each coefficient of the above-mentioned calculation formula is stored in this device.

Hereinafter, the present invention will be explained in detail based on examples.

〔Example〕

The method of the present invention can be implemented by a computer device. Furthermore, the device of the present invention can be similarly configured by a computer device, temperature detection means, output device, and the like. Therefore, specific embodiments of the method and apparatus of the present invention will be described below.

FIG. 1 is a block diagram showing the configuration of a measuring device according to a specific embodiment of the present invention. 2 is a cup for taking out a part of the molten metal and measuring its cooling curve; a thermocouple 4 made of alumel-chromel is installed at the bottom of the cup 2; Electric power is input to the thermometer 6 via a conductor. The thermometer 6 samples the analog electromotive force every 0.4 seconds,
Convert to digital signal, binary code + base number (BCD)
It is output to the parallel/serial converter 8 as an encoded code expressed as . Parallel/serial converter 8 converts BCD data to serial data,
It is output to the serial data input port of the microcomputer 10. The microcomputer 10 includes a printer 12 for outputting predetermined measurement results;
CRT16 is connected, predetermined program and
A floppy disk device 14 storing calculation formulas and their coefficients is connected.

FIG. 2 is a flowchart showing the process performed by the microcomputer 10 used in the present invention. The microcomputer 10 is a step
Start execution from 100 and perform various initial settings.
In step 102, a framework for outputting data to the CRT 16, graph axes, numerical values, scales, etc. are output. Step 104 is a subroutine for detecting that the molten metal has been poured into the cup 2 and detecting the measurement start time. Step 106 determines the return parameters from subroutine A and starts measurement when G=1. Step 108 includes a stagnation point detection subroutine C, which is the central processing, and executes a subroutine B, which performs various calculations. In step 110, return parameters for subroutine B are determined. If D=1, it means that a disconnection occurred in the temperature measuring device system during the measurement, and the process moves to step 114, erases the CRT screen, returns to step 102, and restarts the measurement from the beginning. If the process ends normally, the process moves to step 112 and the CRT1 is connected to the printer 12.
A hard copy of No. 6 was taken to complete the measurement of one cooling curve and the graphite nodularity rate.

FIG. 3 is a flowchart showing the processing of subroutine A. In step 200 an initial set is performed. Here, B is a parameter that stores the number of temperature measurements. Temperature TP in step 202
In step 204, it is determined that TP=2472〓. In the case of that value, it indicates that the input from the thermocouple 4 of the thermometer 6 is in an open state, which means that the thermocouple 4 is not connected normally, so the program returns to the main program in step 212. Then, temperature reading and determination are repeated until the thermocouple 4 is connected to the thermometer 6.

Next, the thermocouple 4 is connected to the thermometer 6, and when its output is less than 2472〓, in step 206, the TP
>1999 judgment is made. If this condition is met, it indicates that molten metal has been poured into cup 2. Therefore, in step 208, G is set to 1 to indicate the start of measurement, and in step 210, the temperature at that time is determined.
Store TP in TP(0) and return to the main program. If the conditions are not met in step 206, this indicates that molten metal has not been poured into the cup, so the process returns to the main program in step 214 and repeats the same routine until the cup is filled with molten metal.

FIG. 4 is a flowchart showing the processing of subroutine B. Initial settings are performed in step 300. As for the values set here, D, F, and N are each set to 0. D is a flag for wire breakage detection, F is a flag for measurement completion, and N is a parameter indicating the classification of the cooling curve. F= in step 302
If it is determined to be 1, the process moves to step 304 and returns to the main program, ending the measurement. step
In step 306, the value of B is updated by 1 and the number of temperature measurements is counted. Read temperature in step 308, TP
(B) and store the value on the CRT16 in step 310.
Display the plot above. Next, in step 312, the difference ΔT from the previously measured temperature is determined. In other words, the rate of change of the cooling curve for 0.4 seconds has been determined.
Next, in step 313, the value of N is determined, and since it is initially 0, the process moves to step 314, and the maximum value of the cooling curve is detected. That is, molten metal is poured into the cup 2, the temperature rises, the maximum temperature is recorded, and the maximum temperature during the cooling process is detected. If the maximum temperature is detected, the value of N is set to 1 in step 316.
Set to . If N is 0, step 302
The maximum temperature is detected by the repeating routine of ~313~314~315~418~302, and N=1 is set.
In step 320, it is determined that ΔT≦15. If the conditions are not met, the process moves to step 322 and it is determined that the wire is broken. That is, it is considered that normally there will not be a temperature change of more than 15〓 between two adjacent measurement intervals. If a wire breakage is detected, the value of D is determined in step 110 of the main program, and predetermined processing for wire breakage detection is performed. Further, step 322 is a step for determining the end of the measurement point. That is, the measurement is completed when TP(B)<2000〓 or B>310 is satisfied. The temperature of the molten metal in the cup is
If the number of measurement points falls below 2000〓, or if the number of measurement points
It will end if it reaches 310 or more. This program reads data every 0.4 seconds, so 124
The measurement will be completed in seconds.

Step 326 is a step for executing subroutine C for detecting a stagnation point on the curve. Fifth
The figure is a flowchart showing the flow of this subroutine C. Further, FIG. 7 is a diagram showing the classification of cooling curves for determining stagnation points by this program. Step 500 is a step for setting initial values, where N1 is set to B-4 and N3 is set to 0.
N3 is a parameter for counting the number of stagnation points. Proceeding to step 502, the width of the temperature change four times before the current measurement point is determined. The width of this temperature change is determined by whether it is 1 or less. In other words, specifically, since it is measured at 0.4 second intervals, it is expressed as an absolute value.
When the temperature gradient is less than 2.5〓/sec, it is determined that the point is within the stagnation zone. If it is the stagnation point, the process moves to step 504 and returns to subroutine B. If stagnation points are detected continuously,
Repeat the routine of steps 302-502-504-328-418-302. If it is not the stagnation point, the routine of steps 506-328-418-302-506 is repeated. Further, when escaping from a stagnation section which is a collection of stagnation points, step 506 determines whether the curve after escaping is upward (U=1) or downward (U=1).
0) is determined. Next, in step 516, the temperature difference is determined by looking up the sequentially input data, and if it is a stagnation point, in step 518, the number of stagnation points (N3), the starting point of the stagnation point (N1), and the stagnation point are determined. The end point (N7) of is detected. Steps 512 and 514 show corresponding processing when the stagnation section starts from the first measurement. Next, in step 520,
Determine whether or not it is a stagnation section. step 520
If the result is YES, step 522 sets N=2, and step 524 returns.
In other words, when the stagnation point continues for 2.4 seconds or more and the vehicle escapes from the stagnation point, that section is determined to be a stagnation section. N=
2 is the first stagnation section. In other words, the first stagnation section has been detected as shown in FIGS. 7a to 7e. In the case of the first stagnation section, the process passes through the following steps and returns to step 328 of subroutine B. Step 328 is the first stagnation section, and if the curve rises after exiting the stagnation section, N=10 is set at step 330.
Then, in step 332, the minimum temperature in the stagnation section is read, and in step 334, the subcooling temperature TC is determined by the CRT16.
displayed on the screen. That is, the supercooled temperature is detected under the above conditions. In this case, the primary crystal temperature
This shows the case where TL is not detected, and the cooling curve becomes one of the curves shown in FIG. 7 f to j.

Step 336 is the first stagnation section, and after exiting the stagnation section, the curve descends and the stagnation time is 16.
If it is less than 40 seconds, that is, the number of measurement points, the value of N is set to 20 in step 338.
Then, in step 339, the average temperature of the stagnation section is determined, and in step 340, the TL is displayed. In this case, the first stagnation section indicates the primary crystal temperature. That is, the cooling curve has the characteristics shown in any one of FIGS. 7a to 7e.

In step 342, it is determined that the curve is in the first stagnation zone, and after exiting the stagnation zone, the curve descends and the stagnation time is determined to be 16 seconds or more.In step 344, the value of N is set to 30, and in step 346, the curve is determined to be the highest in the stagnation zone. Read the temperature and display TE in step 348. In other words, in this case, there is no primary crystal temperature or supercooling temperature, and only the eutectic temperature exists, and the cooling curve is
The characteristics are shown in any one of n, i, and j in FIG. 7.
Next, subroutine D shown in step 350 detects only the eutectic temperature, but it is a subroutine for categorizing the curves in more detail during the classification. Figure 6 shows this subroutine D.
This is a flowchart showing the flow of the process. step
600 detects the minimum temperature from the first point of the stagnation zone until the TE temperature is detected. Then, this temperature is set as TC, which is the supercooling temperature.

If TC is smaller than TE in step 602, the temperature of this TC is determined to be the supercooling temperature in step 604, TC is displayed, and the process returns in step 606.
In this case, it means that the cooling curve has the characteristics shown in FIG. 7n.

If the determination at step 602 is no, then at step 608 the lowest temperature from the eutectic temperature TE to the last point of the stagnation section is read and set as TE2. If TE is greater than TE2+3 in step 610, the process moves to step 612, sets the value of N to 90, and returns in step 614. In this case, it means that the cooling curve has the characteristics shown in FIG. 7i. In the opposite case, the process moves to step 616, sets the value of N to 80, and returns to step 618.
This means that the cooling curve has the characteristics shown in FIG. 7j.

Next, in step 360, the spheroidization rate of graphite is calculated according to the value of N.

Next, the value of SG calculated in step 362 is displayed.

Next, when the first stagnation section is 16 seconds or less, the value of N is set to 10 or 20. Therefore, until the second stagnation area is detected by the same routine as described above, the same processing as that for detecting the first stagnation area described above is performed, and eventually in step 526, when the second stagnation area is detected, In step 528, the value of N is set to 21. Further, when the value of N is 10, if the second stagnation section is detected, the value of N is set to 11 in step 534. Second
When the detection of the stagnation area is completed, subroutine C is exited, and the nature of the stagnation area is determined in step 364. That is, in step 364, if the cooling curve rises after leaving the second stagnation zone, in step 368, the minimum temperature in that stagnation zone is read, and in step 370
The value of N is set to 50 in step 372, and the minimum temperature is displayed as TC. That is, in this case, it is determined that the second stagnation section is at a subcooled temperature.

Next, in step 374, after completing the second stagnation section, if the curve is descending, the maximum temperature during the stagnation section is read in step 376, and the value of N is set to 60 in step 378.
Set it to , and move to step 380. In this case, the subcooled temperature is not detected and the second stagnation section is assumed to be the eutectic temperature, and the process moves to step 380, which corresponds to FIG. 7c, d, and e. Subroutine D is a program for analyzing the properties of the eutectic point in more detail, as explained above when N=30, and is processed according to the value of N shown in parentheses in Figure 6. Ru. Therefore, the explanation thereof will be omitted.

When moving to step 382, the first stagnation section,
and a second stagnation zone, and this is a case where at least the primary crystal temperature and the eutectic temperature are detected. At step 382, the spheroidization rate is calculated corresponding to the value of N, and at step 380, the spheroidization rate is displayed.

Next, in step 386, if the value of N is 11 and the curve falls when exiting the second stagnation zone, the maximum temperature in the stagnation zone is read in step 388, and the value of N is set to 12 in step 390. . in this case,
The first stagnation section is determined to be a supercooled temperature because the curve rises after the stagnation section, and the second stagnation section is determined to be a eutectic temperature. That is, this is the case of the characteristic shown in FIG. 7f. At step 394, the values of TE and SG are displayed.

Next, in step 396, if the value of N is 11 and the curve rises after exiting the second stagnation section, step 396
398, abnormality processing is performed. Such a case is considered impossible.

In this manner, processing was performed on the curve having the first and second stagnation sections.

Next, if there is a third stagnation zone, the detection of that stagnation zone is similar to the processing for the first and second stagnation zones, and the value of N is set to 50 in step 370. That is, this is a case where the crystal has a primary crystal temperature and a subcooling temperature. Therefore, in step 538, the third stagnation interval is detected, and in step 540, the value of N is set to 51.

Therefore, in step 400, if the curve drops after the third stagnation interval, then in step 402 the maximum temperature in the third stagnation interval is read. That is, read the eutectic temperature. At step 402, the value of N is set to 52, at step 406 the spheroidization rate is calculated, and at step 408 the value is displayed. In this case, the cooling curve is represented by the characteristic diagram shown in FIG. 7a. That is, it has primary, supercooled, and eutectic temperatures. After the third stagnation section is detected in step 410, if the cooling curve rises, it is determined to be abnormal and processing is performed in step 412.

In this way, all the cooling curves shown in FIG. 7 are classified.

Step 414 detects a wire breakage. If the wire is broken, a flash display is given to the CRT 16 in step 416, and step 302 is performed via step 418.
and returns to the main program at step 304.

The formula used to calculate the spheroidization rate is: Class 1...When primary crystal, supercooling, and eutectic temperatures exist, (N = 52, 60) SG = -0.33514 x TL + 3.65415 x TC - 4.2933 x
TE＋2112.4 Class 2...When only primary crystal and eutectic temperature exist,
(N=70, 40) SG=0.95477×TLO+10.41974×TEO−6.10634×TL+
6.79197×TE−24543.1 Class 3…When only supercooling and eutectic temperature exist,
(N=12, 30) SG=-0.49921×TLO+0.13294×TEO+0.63908×TC
-1.49792×TE+2617.8 Class 4...When only the eutectic temperature exists, (N=90,
80) SG=0.09701×TLO+1.23095×TEO−2.66802×T
E+2928.3 The results measured using the above program are
It is shown in Fig. 14. Of these, FIG. 8 shows the measurement results of the cooling curve of the original melt, and FIGS. 9 to 14 show the measurement results of the cooling curve of the molten metal after treatment.
In addition, Fig. 15 to Fig. 19 are respectively Fig. 9 to Fig. 19.
14 is a micrograph showing the composition of a cross section of cast iron corresponding to the cooling curve of FIG. 13. From these results, the spheroidization rate of graphite was determined by the Japan Foundry Institute (NIK) method and compared with the results of the present invention, and the measurement error of the graphite spheroidization rate was within 5%.

〔Effect of the invention〕

In summary, the present invention extracts a part of the molten metal after spheroidizing graphite, measures its cooling curve, detects primary crystal, supercooling, and eutectic temperatures from the cooling curve,
The cooling curves are classified according to their presence or absence or their temperature relationship, and the classification is done by taking into account the primary crystal and eutectic temperatures obtained from the cooling curve measured by taking out a part of the source water before spheroidizing. The graphite spheroidization rate is measured using a corresponding linear function.

Therefore, according to the method and apparatus of the present invention, the spheroidization rate can be measured extremely accurately and quickly over a wide range. In addition, since the spheroidization rate can be measured accurately over a wide range, it is suitable for high-mix, low-volume production. Furthermore, since the measurement is performed in the state of the molten metal before casting, it is possible to prevent the occurrence of defective products, which has the effect of improving manufacturing efficiency.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a measuring device according to a specific embodiment of the present invention. FIG. 2 is a flowchart of the main program showing the processing of the computer used in the apparatus of this embodiment. Figure 3, Figure 4, Figure 5
6 are flowcharts showing the processing of the subprograms. FIG. 7 is a diagram showing classified patterns of cooling curves. Figure 8 ~ 1st
FIG. 4 is an output diagram of experimental results measured using the apparatus of the same example. FIGS. 15 to 19 are micrographs showing the cross-sectional structure of sample cast iron corresponding to FIGS. 9 to 13, which are the output results of the apparatus of the same example.

Claims (1)

[Claims] 1. Take out a part of the source water before subjecting it to graphite spheroidization treatment, measure the cooling curve of the extracted source water, and determine the primary crystal temperature of the source water and the primary crystal temperature of the source water from the cooling curve. The eutectic temperature is detected, and then the source melt is subjected to graphite spheroidization treatment, a portion of the molten metal after this treatment is taken out, the cooling curve of the taken out molten metal is measured, and from the cooling curve, the Detecting the primary crystal temperature, supercooling temperature, and eutectic temperature of the molten metal after the treatment, classifying the cooling curve of the molten metal after the treatment according to the presence or absence of these temperatures, and detecting the primary crystal temperature, supercooling temperature, and eutectic temperature of the source metal. A linear function whose coefficients vary depending on the classification, with the crystallization temperature, the primary crystal temperature of the molten metal after treatment, the supercooling temperature, and the eutectic temperature as variables, and the coefficients of the linear function for each classification are determined in advance. A method of measuring the graphite spheroidization rate of the molten metal after the treatment, using a linear function experimentally determined by the correlation between the above variables and the graphite spheroidization rate for a large number of samples. 2. A temperature measuring device that measures the temperature in the cooling process of molten metal in time series; A control device that inputs a signal from the temperature measuring device and outputs a signal to an output device after predetermined processing; and the control device. and an output device that inputs a signal from the temperature measuring device and performs display based on the signal, the control device detecting a temperature stagnation point based on the signal output from the temperature measuring device. a stagnation point detection unit that determines whether the detected stagnation point is at a primary crystal temperature, a supercooled temperature, or a eutectic temperature; a class discriminator that determines the class to which the cooling curve belongs; an arithmetic unit that computes the graphite nodularity rate based on the linear function specified for each class according to the classification; and a system and system of each of the linear functions. 1. A graphite spheroidization rate measuring device comprising: a storage unit that stores the primary crystal temperature of the source water and the eutectic temperature of the source water measured from the cooling curve of the source water before graphite spheroidization treatment.