Method for on-line detection of spheroidization rate and shrinkage porosity index of nodular cast iron

Abstract

The invention provides a method for on-line detection of spheroidization rate and shrinkage porosity index of nodular cast iron, which comprises the following steps: pouring molten iron subjected to spheroidization into a thermal analysis sample cup, recording the change rule of the temperature of the molten iron in the thermal analysis sample cup along with time, and drawing a cooling curve of solidification time-temperature; according to the shape of the cooling curve, determining which of a hypoeutectic solidification mode, a near-eutectic solidification mode and a hypereutectic solidification mode the solidification mode of the spheroidized iron liquid belongs to; different calculation formulas are adopted for predicting the nodularization rate and the shrinkage porosity index of the nodular cast iron according to the three different solidification modes. The method comprises the following steps: drawing a cooling curve between solidification time and temperature of molten iron, carrying out numerical differential treatment on the curve to obtain a temperature value of a feature point, and judging which solidification mode of the molten iron belongs to a hypoeutectic solidification mode, a near-eutectic solidification mode and a hypereutectic solidification mode according to the shape of the cooling curve. And further calculating the spheroidization rate and shrinkage index of the spheroidized molten iron through a formula.

Description

Technical Field

[0001] The present invention belongs to the technical field of molten iron metallurgical quality detection, and particularly relates to a method for on-line detecting the spheroidization rate and shrinkage porosity index of ductile iron. Background Art

[0002] Ductile iron is a commonly used structural material. The output of ductile iron in China is 15.05 million tons, accounting for more than 50% of the world's total output of ductile iron. However, compared with the advanced foreign level, the defect rate of ductile iron castings is relatively high. The metallurgical quality of ductile iron molten iron has a great influence on the internal quality of ductile iron castings, such as spheroidization rate, shrinkage cavity, shrinkage porosity, tensile strength, elongation, impact toughness, etc. At present, most of the molten ductile iron is melted by medium-frequency induction furnaces. To ensure the internal quality of ductile iron, usually, technical indicators such as controlling the chemical composition of the molten iron, the treatment temperature of the molten iron, the pouring temperature, and controlling the purity and stability of the furnace charge must be within the required technical specifications. Currently, traditional methods for detecting the chemical composition of ductile iron include using a spectral analyzer to detect alloy elements, an infrared carbon-sulfur analyzer to detect carbon and sulfur contents, a common thermal analyzer to detect carbon and silicon in the front-of-furnace molten iron, and using a rapid thermocouple to control the tapping temperature and pouring temperature. It can be said that the melting equipment and detection instruments of foundry enterprises in China are currently complete and advanced. However, the common problem existing in current foundry enterprises in China is that: after the optimization design of the ductile iron casting process and practice, it is proved that the designed gating and risering system is reasonable, and the chemical composition and temperature of the molten iron also meet the requirements of the process technical specifications. However, defects such as shrinkage cavity, shrinkage porosity, or unqualified spheroidization rate often occur in the ductile iron castings poured from such spheroidized molten iron without any warning, resulting in unqualified mechanical properties or leakage, and the internal quality not meeting the requirements. In addition, the current traditional method for detecting the spheroidization rate is to pour specimens or take samples from the casting body. Such a "postmortem" inspection method, even if problems are found, the molten iron has already been poured into waste castings, resulting in a very large loss. Thus, the scrap rate of ductile iron castings increases, the yield rate of molten iron decreases, the production cost increases, the profit rate decreases, and the market competitiveness of products declines. This shows that the current conventional technologies and instruments for detecting the metallurgical quality of ductile iron molten iron cannot fully and accurately characterize the quality of the molten iron. That is to say, for ductile iron, accurate chemical composition does not necessarily mean that the metallurgical quality of the molten iron is high-quality molten iron. Because the main factor determining the internal quality of cast iron parts is that the solidification process after pouring the ductile iron molten iron into the mold should be reasonable and stable, and currently, there is no method or instrument for detecting and controlling the solidification process of ductile iron.

[0003] Thermal analysis method for spheroidizing effect of ductile iron (CN 117538368 A) and the article "Prediction of Spheroidization Rate and Shrinkage Characteristics of Ductile Iron Based on Thermal Analysis Technology" (Foundry, 2024, Vol73(7)) proposed a method for predicting the spheroidization rate and shrinkage rate of ductile iron based on thermal analysis. The prediction formula of this invention patent and paper is based on hypoeutectic ductile iron. In fact, in addition to hypoeutectic, there are also near-eutectic and hypereutectic types of ductile iron, and most of the materials of ductile iron are of the near-eutectic type. The method of this invention patent is not applicable to the latter two types of ductile iron. In addition, the prediction model of this invention patent includes two variable factors, the lowest eutectic temperature TEU and the highest eutectic temperature TER. In fact, the measured values of TEU and TER are not only affected by the spheroidization rate, but often the changes in chemical composition, fluctuations in melting process, fluctuations in inoculation process, volatility of thermocouple wire quality, and operating factors of pouring sample cups all affect the measured values of TEU and TER. Therefore, this invention method affects the test accuracy of the spheroidization rate and shrinkage rate of ductile iron in actual production. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for on-line detecting the spheroidization rate and shrinkage porosity index of ductile iron in view of the deficiencies of the above-mentioned prior art. This method plots the cooling curve between the solidification time and temperature of the molten iron after spheroidizing treatment, performs numerical differentiation and integration on this curve to obtain the characteristic point temperature values, and determines which one of the hypoeutectic, near-eutectic and hypereutectic solidification modes the solidification mode of the spheroidized molten iron belongs to according to the shape of the cooling curve, and further calculates the spheroidization rate and shrinkage index of the spheroidized molten iron through formulas.

[0005] To solve the above technical problem, the technical solution adopted by the present invention is: A method for on-line detecting the spheroidization rate and shrinkage porosity index of ductile iron, characterized in that the method includes:

[0006] Pour the molten iron after spheroidizing treatment into a thermal analysis sample cup, record the change law of the molten iron temperature in the thermal analysis sample cup with time, and plot the cooling curve of time-temperature;

[0007] Determine which one of the hypoeutectic solidification mode, near-eutectic solidification mode and hypereutectic solidification mode the solidification mode of the spheroidized molten iron belongs to according to the shape of the cooling curve of solidification time-temperature;

[0008] If the solidification mode of the spheroidized molten iron belongs to the hypoeutectic solidification mode, calculate and predict the spheroidization rate of ductile iron according to Formula 1; Formula 1 is:

[0009] SGR1 = c0 + c1×△T1 + c2×△T2 + c3×△T3 + c4×K1 + c5×K2

[0010] Among them, SGR1 is the spheroidization rate of the hypoeutectic solidification mode ductile iron melt; c0 is the constant phase; c1, c2, c3, c4, c5 are all coefficients; △T1 is the temperature difference between the primary austenite precipitation temperature and the eutectic minimum temperature, △T1=(TAL - TEU); △T2 is the eutectic reheat temperature, △T2=(TER - TEU); △T3 is the temperature difference between the eutectic maximum temperature and the eutectic end temperature, △T3=(TER - TES); K1 is the ratio of the primary hypoeutectic austenite precipitation time to the entire solidification time, K1 = t (TEU-TAL) / t (TES-TAL) , K2 is the ratio of the time from the eutectic maximum temperature to the end of solidification to the entire solidification time, K2 = t (TES-TER) / t (TES-TAL) ; TAL is the primary austenite precipitation temperature, TEU is the eutectic minimum temperature, TER is the eutectic maximum temperature, and TES is the eutectic end temperature;

[0011] If the solidification mode of the spheroidizing melt belongs to the near-eutectic solidification mode, the spheroidization rate of ductile iron is predicted according to Formula Two; Formula Two is:

[0012] SGR2 = c0 + c2·△T2 + c3·△T3 + c5·K2;

[0013] Among them, SGR2 is the spheroidization rate of the near-eutectic solidification mode ductile iron melt;

[0014] If the solidification mode of the spheroidizing melt belongs to the hypereutectic solidification mode, the spheroidization rate of ductile iron is predicted according to Formula Three; Formula Three is:

[0015] SGR3 = c0 + c1×△T1' + c2×△T2' + c3×△T2 + c4×△T3 + c5×K1' + c6×K2';

[0016] Among them, SGR3 is the spheroidization rate of the hypereutectic solidification mode ductile iron melt; c6 is the coefficient; △T1' is the temperature difference between the primary graphite precipitation temperature and the lowest temperature of austenite precipitation before eutectic, △T1'=(TGL - TGU); △T2' is the reheat temperature of austenite precipitation before eutectic, △T2'=(TGR - TGU); K1' is the ratio of the time from the start of primary graphite precipitation to the highest temperature of austenite precipitation before eutectic to the entire solidification time, K1' = t( TGR-TGL) / t (TES-TGL) , K2' is the ratio of the time from the eutectic maximum temperature to the end of solidification to the entire solidification time, K2' = t (TES-TER) / t (TES-TGL) ; TGL is the primary graphite precipitation temperature, TGU is the lowest temperature of austenite precipitation before eutectic, and TGR is the highest temperature of austenite precipitation before eutectic;

[0017] Calculate the shrinkage porosity index of the spheroidized iron liquid according to Formula Four, and Formula Four is: PI = 1 - S3 / S, where PI is the shrinkage porosity index of the spheroidized iron liquid, S is the integral area of the cooling curve during the entire solidification process; S3 is the integral area of the cooling curve between the eutectic highest temperature and the eutectic end temperature.

[0018] Preferably, if the solidification mode of the spheroidized iron liquid belongs to the hypoeutectic solidification mode, based on the cooling curve of solidification time - temperature, obtain TAL, TEU, TER, and TES of the ductile iron;

[0019] If the solidification mode of the spheroidized iron liquid belongs to the near - eutectic solidification mode, based on the cooling curve of solidification time - temperature, obtain TEU, TER, and TES of the ductile iron;

[0020] If the solidification mode of the spheroidized iron liquid belongs to the hypereutectic solidification mode, based on the cooling curve of solidification time - temperature, obtain TGL, TGU, TGR, TEU, TER, and TES of the ductile iron.

[0021] Preferably, when the solidification mode of the spheroidized iron liquid is the hypoeutectic solidification mode, the entire solidification process is between the primary austenite precipitation temperature and the eutectic end temperature;

[0022] When the solidification mode of the spheroidized iron liquid is the near - eutectic solidification mode, the entire solidification process is between the eutectic lowest temperature and the eutectic end temperature;

[0023] When the solidification mode of the spheroidized iron liquid is the hypereutectic solidification mode, the entire solidification process is between the primary graphite precipitation temperature and the eutectic end temperature.

[0024] Preferably, a thermocouple is installed in the center of the thermal analysis sample cup to collect the temperature during the solidification process of the iron liquid.

[0025] Preferably, according to the drawn cooling curve of solidification time - temperature, perform differentiation on the cooling curve to obtain the extreme values on the curve, and after analysis, obtain TAL, TEU, TER, TES, TGL, TGU, TGR.

[0026] The present invention has the following advantages compared with the prior art:

[0027] In the early stage of research of the present invention, a large number of experimental studies are carried out, and the results show that there are three common modes in the solidification cooling curve of ductile iron liquid:

[0028] The first is the hypoeutectic ductile iron solidification mode curve: such as Figure 1As shown in the figure, under non-equilibrium solidification conditions, when the active carbon equivalent is less than the carbon equivalent at the eutectic point, the first inflection point TAL of the solidification curve is the temperature at which primary austenite begins to precipitate, the second inflection point TEU is the lowest temperature at which austenite and graphite spheroids undergo eutectic, the third inflection point TER is the highest temperature at which austenite and graphite spheroids grow eutectically, and the fourth inflection point TES is the end temperature of the eutectic growth of austenite and graphite spheroids. This type of solidification cooling curve can characterize the precipitation temperature and growth of each phase during the solidification process of hypoeutectic ductile iron. There are 4 typical characteristic points on the curve, namely TAL, TEU, TER, and TES.

[0029] The second is the solidification mode curve of near-eutectic ductile iron: As Figure 2 shown in the figure, under non-equilibrium solidification conditions, when the active carbon equivalent is equal to or slightly higher than the active carbon equivalent at the eutectic point, the first inflection point TEU of the solidification curve is the lowest temperature at which austenite and graphite spheroids grow eutectically, the second inflection point TER is the highest temperature at which austenite and graphite spheroids grow eutectically, and the third inflection point TES is the end temperature of the eutectic growth of austenite and graphite spheroids. This type of solidification cooling curve can characterize the precipitation and growth of each phase during the solidification process of near-eutectic ductile iron. There are 3 typical characteristic points on the curve: TEU, TER, and TES.

[0030] The third is the solidification mode curve of hypereutectic ductile iron: As Figure 3 shown in the figure, under non-equilibrium solidification conditions, when the active carbon equivalent is much higher than the active carbon equivalent at the eutectic point, the first inflection point TGL of the solidification curve is the temperature at which primary austenite precipitates, the second inflection point TGU is the lowest temperature at which austenite precipitates before eutectic, the third inflection point is the highest temperature at which austenite grows before eutectic, the fourth inflection point TEU is the lowest temperature at which austenite and graphite spheroids grow eutectically, the fifth inflection point TER is the highest temperature at which austenite and graphite spheroids grow eutectically, and the sixth inflection point TES is the end temperature of the eutectic growth of austenite and graphite spheroids. This type of solidification cooling curve can characterize the precipitation and growth of each phase during the solidification process of hypereutectic ductile iron. There are 6 typical characteristic points on the curve: TGL, TGU, TGR, TEU, TER, and TES.

[0031] The solidification mode of ductile iron has a very significant impact on the spheroidization rate and the tendency to shrinkage porosity. Since the solidification of ductile iron liquid poured into the cavity is non-equilibrium solidification, the current method of determining the solidification mode of ductile iron by calculating the carbon equivalent and comparing it with the eutectic point of the equilibrium iron-carbon phase diagram is very inaccurate, with a very large deviation in guiding actual production, or even having a counterproductive guiding effect.

[0032] Therefore, a cooling curve of solidification time - temperature is plotted, and the solidification mode is determined according to the cooling curve. The computer measurement and control system can judge the non - equilibrium solidification mode of the detected ductile iron liquid according to the types and quantities of characteristic points of the above - mentioned different solidification mode curves, and then predict the spheroidization rate and shrinkage porosity index of the spheroidized iron liquid.

[0033] For hypoeutectic ductile iron, the TAL temperature value is greater than the TEU value. If the difference between (TAL - TEU) is larger, it means that the chemical composition of the ductile iron deviates more to the left from the eutectic point, that is, the hypoeutectic degree is greater, the volume fraction of primary austenite is more, the liquid shrinkage before eutectic and the volume shrinkage of austenite precipitation are larger, and the casting is more likely to produce shrinkage cavities. At this time, a larger riser needs to be designed for liquid feeding to prevent the casting from producing shrinkage cavities. However, the casting process yield is reduced. In addition, the more primary austenite dendrites there are, the smaller the volume fraction of the iron liquid undergoing eutectic solidification, that is, the smaller the volume fraction of eutectic graphitization, and the smaller the volume of graphitization expansion. It will also cause the iron liquid in the later stage of solidification not to be compensated by enough spheroidized iron liquid during crystallization, so the casting is prone to produce shrinkage porosity. Moreover, during eutectic, graphite spheres nucleate and grow at the front of primary austenite dendrites. Under the influence of austenite dendrites, the carbon diffusion amounts from all directions during the growth of graphite spheres are different, resulting in inconsistent growth rates of graphite spheres in all directions, making the roundness of graphite spheres not high, and thus the spheroidization rate is reduced. At the same time, the number of graphite spheres is small.

[0034] For near - eutectic ductile iron, before the characteristic point TEU appears on the solidification curve, there are no TAL and TGL characteristic points, which means that there is no precipitation of primary phase before the start of eutectic of austenite and graphite spheres, that is, there is no solidification shrinkage of primary austenite before eutectic, and no formation of primary coarse graphite spheres. This shows that all the spheroidized iron liquid crystallizes in the eutectic solidification mode. Therefore, the proportion of graphite sphere cores generated in the spheroidized iron liquid is high. When the graphite sphere cores grow to a certain size and are surrounded by austenite shells, the graphite spheres grow under the surrounding of austenite shells. During the growth process, the carbon diffusion amounts from all directions are basically the same, and the growth rates of graphite spheres in all directions are also the same. So, the number of eutectic graphite spheres is large, the shape of graphite spheres is relatively round, and the spheroidization rate is relatively high. In addition, due to the large number of eutectic graphite spheres and their relatively high volume fraction, the volume expansion of eutectic graphitization is large, which is beneficial to compensating the liquid shrinkage and solidification shrinkage during eutectic solidification. As a result, the casting is not easy to produce shrinkage cavities or shrinkage porosity.

[0035] For hypereutectic ductile iron, the first inflection point on the solidification curve is TGL. Primary graphite balls first precipitate from the liquid phase. As the temperature of the spheroidized molten iron decreases, this type of graphite ball grows freely in the liquid phase very quickly, so the size of the graphite ball growth is relatively large. In addition, when solidification reaches the eutectic start stage, the volume fraction precipitation ratio of this part of the graphite balls is also relatively large, sometimes reaching 50-60% of the total volume fraction of graphite balls in ductile iron. Of course, a relatively large graphitization volume expansion will also occur. However, at this time, the casting has not solidified yet, and the riser neck or the ingate has not solidified and frozen. This primary graphitization volume expansion will cause the molten iron to return to the gating and risering system. In actual production, it is often found that the riser does not shrink, which belongs to this situation. As more primary graphite balls gradually grow, the molten iron around the graphite balls becomes carbon-depleted. At the same time, as the temperature decreases, the carbon equivalent and temperature of this part of the molten iron drop below the left of the stable eutectic point, that is, the left of the pseudo-eutectic zone, which is reflected as the TGU point on the solidification curve. At this time, an austenite shell forms around the primary graphite balls, and some austenite shells will also grow into dendritic shapes during the growth process. The growth of this type of austenite is accompanied by the release of a relatively large amount of latent heat of crystallization, causing the temperature of the molten iron to rise to the TGR point. The larger the difference (TGR - TGU), the more the volume fraction of primary graphite balls and the accompanying austenite volume fraction before eutectic. The formation of austenite is accompanied by a relatively large shrinkage of the spheroidized molten iron. At this time, if external liquid cannot be replenished, the cavity volume will have a deficit, resulting in the casting being prone to external shrinkage depression defects. Since the carbon content of austenite is relatively low, the carbon content at its growth front increases, and the carbon equivalent of the liquid phase increases, moving to the right of the pseudo-eutectic zone, resulting in a decrease in the growth rate of austenite before eutectic. After that, the remaining liquid phase enters the co-growth stage of eutectic graphite balls and austenite, that is, reaching the characteristic point TEU on the solidification curve. The eutectic growth of austenite and graphite balls in the remaining liquid phase causes a certain eutectic temperature to rise. When it reaches the highest point TER, the eutectic growth rate decreases, the released latent heat of crystallization decreases, and the temperature gradually decreases until it reaches the solidification end characteristic point TES. Since a relatively large volume fraction of primary graphite balls are formed before eutectic TEU, the volume fraction of graphite balls formed in the later stage of eutectic solidification is greatly reduced, and the eutectic graphitization volume expansion is very small, resulting in insufficient compensation for the shrinkage of the spheroidized molten iron in the later stage of solidification, thus being prone to porosity. In addition, the size distribution of graphite balls in this hypereutectic solidification mode of ductile iron is very uneven, and the proportion of large and coarse graphite balls is relatively large, resulting in a significant reduction in the mechanical properties of ductile iron.

[0036] Based on the above research on the crystallization theory of ductile iron with different solidification modes, according to the method of the present invention, the ductile iron can be controlled to solidify within the range of near-eutectic solidification mode. Such a solidification mode can enable the ductile iron to obtain a high nodularity, with uniformly distributed graphite nodules, small graphite nodule diameters, and at the same time having a low shrinkage tendency, which is very beneficial to improving the internal quality of ductile iron castings, that is, having higher mechanical properties, and the castings are not prone to shrinkage porosity defects, solving the problems that cannot be solved by the current existing technologies.

[0037] The technical solution of the present invention will be further described in detail below with reference to the drawings and embodiments. Description of the Drawings

[0038] Figure 1 is the solidification cooling curve of hypoeutectic ductile iron;

[0039] Figure 2 is the solidification cooling curve of near-eutectic ductile iron;

[0040] Figure 3 is the solidification cooling curve of hypereutectic ductile iron

[0041] Figure 4 is the schematic diagram of the integral area of the entire solidification process curve in the solidification cooling curve of the hypoeutectic ductile iron of the present invention.

[0042] Figure 5 is the schematic diagram of the integral area of the entire solidification process curve in the solidification cooling curve of the near-eutectic ductile iron of the present invention.

[0043] Figure 6 is the schematic diagram of the integral area of the entire solidification process curve in the solidification cooling curve of the hypereutectic ductile iron of the present invention.

[0044] Figure 7 is the picture of the graphite nodule distribution in the hypoeutectic ductile iron of QT600 grade in Example 2 of the present invention.

[0045] Figure 8 is the picture of the graphite nodule distribution in the near-eutectic ductile iron QTD1050-6 in Example 3 of the present invention.

[0046] Figure 9 is the picture of the graphite nodule distribution in the hypereutectic ductile iron QT450 in Example 4 of the present invention. Detailed Embodiments

[0047] Example 1

[0048] A method for on-line rapid detection of the nodularity and shrinkage porosity index of nodular iron liquid disclosed in this embodiment is as follows:

[0049] Step 1: Pour the spheroidized molten iron into a thermal analysis sample cup. The computer control system records the variation law of the temperature of the molten iron in the sample cup with time and plots a cooling curve of solidification time - temperature. A K-type thermocouple is installed at the center of the sample cup. The temperature signal of the spheroidized molten iron during solidification in the sample cup is converted into a voltage signal, and the voltage signal is sent to the serial port of the computer control system through a sampler and a data cable. The computer then converts the voltage signal into a temperature value. Then, the computer system records the variation value of the temperature of the spheroidized molten iron with time and plots a thermal analysis curve of the solidification cooling process, that is, a cooling curve of solidification time - temperature.

[0050] Step 2: The computer automatically determines the solidification mode of the spheroidized molten iron according to the shape of the cooling curve of solidification time - temperature of the spheroidized molten iron.

[0051] Generally, ductile iron has thermal analysis curves of three solidification modes, as Figure 1 , Figure 2 , Figure 3 shown. Figure 1 The curve shown is the hypoeutectic solidification mode, Figure 2 The curve shown is the near - eutectic solidification mode, Figure 3 The one shown is the hypereutectic solidification mode. The computer differentiates the cooling curve of solidification time - temperature obtained in Step 1 and obtains the temperature of the corresponding characteristic points on the solidification curve by detecting the extreme values on the differential curve.

[0052] For the solidification thermal analysis curve of hypoeutectic ductile iron as Figure 1 shown, the computer can obtain the temperature values of characteristic points TAL, TEU, TER, and TES. Among them, TAL is the primary austenite precipitation temperature, TEU is the eutectic minimum temperature, TER is the eutectic maximum temperature, and TES is the eutectic end temperature.

[0053] For the solidification thermal analysis curve of near - eutectic ductile iron as Figure 2 shown, the computer can obtain the temperature values of characteristic points TEU, TER, and TES. The physical meanings of the characteristic values are the same as those of the hypoeutectic solidification curve.

[0054] For the solidification curve of hypereutectic ductile iron as Figure 3 shown, the computer can obtain the temperature values of characteristic points TGL, TGU, TGR, TEU, TER, and TES. Among them, TGL is the primary graphite precipitation temperature, TGU is the lowest temperature of austenite precipitation before eutectic, TGR is the highest temperature of austenite precipitation before eutectic, and the physical meanings of other characteristic values are the same as those of the hypoeutectic solidification curve.

[0055] Step 3: Predict the spheroidization rate of the spheroidized molten iron:

[0056] The computer measurement and control system determines the solidification mode based on the characteristic point temperature values of the cooling curve tested in the second step, and further calculates the technical characteristic parameters of the relevant solidification curve according to the solidification mode.

[0057] For the hypoeutectic solidification mode curve, the following are calculated: ΔT1, ΔT2, ΔT3, K1, K2. Among them, ΔT1 is the difference between the primary austenite precipitation temperature and the eutectic minimum temperature, ΔT1 = (TAL - TEU); ΔT2 is the eutectic reheat temperature, ΔT2 = (TER - TEU); ΔT3 is the difference between the eutectic maximum temperature and the eutectic end temperature, ΔT3 = (TER - TES); K1 is the ratio of the primary austenite precipitation time of hypoeutectic to the entire solidification time, K1 = t (TEU-TAL) / t (TES-TAL) ; K2 is the ratio of the time from the eutectic maximum temperature to the end of solidification to the entire solidification time, K2 = t (TES-TER) / t ( TES-TAL ) .

[0058] For the near-eutectic solidification mode curve, the following are calculated: ΔT2, ΔT3, K2. Among them, ΔT2 is the eutectic reheat temperature, ΔT2 = (TER - TEU); ΔT3 is the difference between the eutectic maximum temperature and the eutectic end temperature, ΔT3 = (TER - TES); K2 is the ratio of the time from the eutectic maximum temperature to the end of solidification to the entire solidification time, K2 = t (TES-TER) / t (TES-TAL) .

[0059] For the hypereutectic solidification mode curve, the following are calculated: ΔT1, ΔT2, ΔT3, ΔT4, K1, K2. Among them, ΔT1' is the temperature difference between the primary graphite precipitation temperature and the lowest temperature of austenite precipitation before eutectic, ΔT1' = (TGL - TGU); ΔT2' is the reheat temperature of austenite precipitation before eutectic, ΔT2' = (TGR - TGU); ΔT2 is the eutectic reheat temperature, ΔT2 = (TER - TEU); ΔT3 is the difference between the eutectic maximum temperature and the eutectic end temperature, ΔT3 = (TER - TES); K1' is the ratio of the time from the start of primary graphite precipitation to the highest temperature of austenite precipitation before eutectic to the entire solidification time, K1' = t( TGR-TGL) / t (TES-TGL) , K2' is the ratio of the time from the eutectic maximum temperature to the end of solidification to the entire solidification time, K2' = t (TES-TER) / t (TES-TGL) ;

[0060] The computer measurement and control system automatically selects the following corresponding formula to calculate the predicted nodularity of ductile iron according to the mode of the ductile iron solidification cooling curve:

[0061] Hypoeutectic: SGR1 = c0 + c1×△T1 + c2×△T2 + c3×△T3 + c4×K1 + c5×K2 Eutectic: SGR2 = c0 + c2×△T2 + c3×△T3 + c5×K2

[0062] Hypereutectic: SGR3 = c0 + c1×△T1' + c2×△T2' + c3×△T2 + c4×△T3 + c5×K1' + c6×K2'

[0063] Among them, SGR1, SGR2 and SGR3 represent the spheroidization rates of hypoeutectic, near-eutectic and hypereutectic nodular cast iron melts respectively; c0 is the constant phase, and c1, c2, c3, c4, c5, c6 are all coefficients of each item in the prediction formula; the above coefficients are obtained by optimizing multiple linear regression of big data.

[0064] Step 4: Predict the shrinkage porosity index of the spheroidized melt:

[0065] The shrinkage porosity index is a technical parameter that characterizes the tendency of the spheroidized melt to produce shrinkage porosity. Whether it is hypoeutectic, eutectic or hypereutectic nodular cast iron, the computer control system can obtain the shrinkage porosity index of the nodular cast iron melt based on the integral area ratio of the cooling curve, as Figure 4 shown.

[0066] According to the cooling curve obtained in the first step, the computer system automatically integrates it to obtain the integral area S of the entire solidification process curve and the integral area S3 of the curve between the eutectic highest temperature TER and the eutectic end temperature TES. The shrinkage porosity index is calculated using the integral areas S and S3 during the entire solidification period. The calculation formula is as follows: PI = 1 - S3 / S, where PI is the shrinkage porosity index. The smaller the shrinkage porosity index, the smaller the tendency of the nodular cast iron melt to shrink during solidification, and the less likely the casting is to form shrinkage cavities or shrinkage porosity.

[0067] As Figure 4 shown, when the solidification mode of the spheroidized melt is hypoeutectic solidification mode, the integral area S of the entire solidification process curve is the integral area of the curve between the primary austenite precipitation temperature TAL and the eutectic end temperature TES, that is, the area enclosed by TAL - a - TES; the integral area S3 of the curve between the eutectic highest temperature TER and the eutectic end temperature TES is the area enclosed by TER - b - TES.

[0068] As Figure 5 shown, when the solidification mode of the spheroidized melt is near-eutectic solidification mode, the integral area S of the entire solidification process curve is the integral area of the curve between the eutectic lowest temperature TEU and the eutectic end temperature TES; that is, the area enclosed by TEU - a - TES; the integral area S3 of the curve between the eutectic highest temperature TER and the eutectic end temperature TES is the area enclosed by TER - b - TES.

[0069] As Figure 6 shown, when the solidification mode of the spheroidizing molten iron is hypereutectic solidification mode, the integral area S of the entire solidification process curve is the integral area of the curve between the primary graphite precipitation temperature TGL and the eutectic end temperature TES; that is, the area enclosed by TGL-a-TES; the integral area S3 of the curve between the eutectic maximum temperature TER and the eutectic end temperature TES is the area enclosed by TER-b-TES.

[0070] In this embodiment, the thermal analysis sample cup is the in-furnace thermal analysis temperature acquisition sample cup disclosed in the prior art CN206002472U; the computer measurement and control system is the QTMCS-2023 ductile iron on-line measurement and control system developed by Hebei University of Technology, and other operating processes are well-known in the technical field.

[0071] Example 2

[0072] Prepare a differential case casting of hypoeutectic ductile iron QT600 grade, and the product requirements are that the spheroidization rate is greater than 80% and the shrinkage porosity index is less than 0.5. Therefore, during the process of preparing a differential case casting of hypoeutectic ductile iron QT600 grade by the method disclosed in Example 1, the spheroidization rate and shrinkage porosity index of ductile iron QT600 grade are detected online. The specific process is as follows:

[0073] The first step: Pour the molten iron of ductile iron QT600 grade into the thermal analysis sample cup, and the computer control system records the change law of the molten iron temperature in the sample cup with time and draws a cooling curve of solidification time-temperature, as Figure 1 shown.

[0074] The second step: Determine the solidification mode of the spheroidizing molten iron

[0075] According to the shape of the cooling curve of solidification time-temperature, it is judged that the solidification mode of the spheroidizing molten iron belongs to hypoeutectic solidification mode;

[0076] The computer differentiates the cooling curve obtained in the first step above, and obtains the characteristic point temperatures corresponding to the solidification curve by detecting the zero values on the differential curve: TAL, TEU, TER, TES and the solidification time corresponding to the characteristic points, as shown in Table 1, where TAL is the primary austenite precipitation temperature, TEU is the eutectic minimum temperature, TER is the eutectic maximum temperature, and TES is the eutectic end temperature. Further determine that the solidification mode of the spheroidizing molten iron is hypoeutectic solidification mode.

[0077] Table 1 Characteristic values of the solidification curve in Example 2

[0078] Characteristic point temperature TAL TEU TER TES Temperature / °C 1165.00 1147.86 1150.92 1131.84 Solidification time / 0.5 s 34 75 121 225

[0079] The third step: Predict the spheroidization rate of the spheroidizing molten iron:

[0080] Formula 1 for calculating the nodularity of hypoeutectic ductile iron:

[0081] SGR1 = c0 + c1×△T1 + c2×△T2 + c3×△T3 + c4×K1 + c5×K2;

[0082] = 117.705 + 0.71988 + 9.847 - 0.36252 - 11.3767 - 17.364

[0083] Wherein, c0 = 117.705, c1 = 0.042, c2 = -3.197, c3 = -0.019, c4 = -73.398, c5 = -9.017; △T1 = (TAL - TEU) = 17.14, △T2 = (TER - TEU) = 3.08; △T3 = (TER - TES) = 19.08; K1 = t (TEU-TAL) / t (TES-TAL) = 0.2147; K2 = t (TES-TER) / t (TES-TAL) = 0.5445;

[0084] After calculation, the nodularity of the hot analysis sample is predicted to be 88%. After inspection, as Figure 7 shown, the nodularity of the as-cast ductile iron differential case casting body is 87%. Therefore, using Formula 1 to calculate and predict the nodularity is close to the actual detected value, meeting the requirements of the product for the distribution of graphite nodules.

[0085] The fourth step is to predict the microporosity index of the nodularized molten iron

[0086] According to PI = 1 - S3 / S, calculate the microporosity index PI of the nodularized melt to be 0.399, which is less than the requirement of 0.5. Dissect the gear differential case blank casting body, and there is no microporosity defect.

[0087] Example 3

[0088] Prepare the blank gear casting QT450 of near-eutectic ductile iron QTD1050-6. During the preparation process using the method disclosed in Example 1, on-line detect the nodularity and microporosity index of ductile iron QTD1050-6. The specific process is as follows:

[0089] The first step: Pour the nodularized molten iron into the hot analysis sample cup, and the computer control system records the change law of the molten iron temperature in the sample cup with time and draws the time-temperature cooling curve, as Figure 2 shown.

[0090] The second step: Determine the solidification mode of the nodularized molten iron:

[0091] According to the shape of the solidification time-temperature cooling curve of the spheroidized iron melt, it is determined that the solidification mode of the spheroidized iron melt is near-eutectic solidification mode. The computer measurement and control system differentiates the solidification cooling curve obtained in the above first step to obtain the characteristic points TEU, TER, and TES of the solidification curve and their corresponding solidification times, as shown in Table 2, where TEU is the eutectic minimum temperature, TER is the eutectic maximum temperature, and TES is the eutectic end temperature, as Figure 2 shown. The computer system determines that the spheroidized iron melt is in the near-eutectic solidification mode according to the characteristic points TEU, TER, and TES of the near-eutectic ductile iron solidification thermal analysis curve.

[0092] Table 2 Characteristic values of the solidification curve of Example 2

[0093] Characteristic point temperature TEU TER TES Temperature / °C 1133.95 1141.46 1126.05 Solidification time / 0.5 s 65 129 298

[0094] Step 3: Predict the spheroidization rate of the spheroidized iron melt:

[0095] Calculate the spheroidization rate of the near-eutectic ductile iron according to Formula 2:

[0096] SGR2 = c0 + c2·△T2 + c3·△T3 + c5·K2

[0097] where SGR2 is the spheroidization rate of the near-eutectic solidification mode ductile iron melt; c0 = 83.642, c2 = 0.339, c3 = -0.079, c5 = 6.311. △T2 = (TER - TEU) = 7.51; △T3 = (TER - TES) = 15.41; K2 = t (TES-TER) / t (TES-TAL) = 0.7253;

[0098] After calculation, the predicted spheroidization rate is 90%. After inspection, as Figure 8 shown, the spheroidization rate of the cast ductile iron QT450 blank gear casting body is 90%, meeting the requirements of the product for the distribution of graphite nodules.

[0099] Step 4: Predict the shrinkage porosity index of the spheroidized iron melt:

[0100] Calculate the shrinkage porosity index PI of the spheroidized melt according to PI = 1 - S3 / S, and PI = 0.256, which is less than the requirement of 0.5. Dissect the gear blank casting body, and there is no shrinkage porosity defect.

[0101] Example 4

[0102] Prepare a hypereutectic ductile iron QT450 pipe clamp casting. During the preparation process using the method disclosed in Example 1, online detect the spheroidization rate and shrinkage porosity index of the ductile iron QT450. The specific process is as follows:

[0103] Step 1: Pour the spheroidized ductile iron QT450 into a thermal analysis sample cup. The computer control system records the variation law of the molten iron temperature in the sample cup with time and plots a time-temperature cooling curve, as Figure 3 shown.

[0104] Step 2: Determine the solidification mode of the spheroidized molten iron.

[0105] According to the shape of the solidification time-temperature cooling curve of the spheroidized molten iron, it is judged that the solidification mode of the spheroidized molten iron is hypereutectic solidification mode. The computer differentiates the solidification cooling curve obtained in Step 1, and obtains the characteristic point temperatures TGL, TGU, TGR, TEU, TER, and TES and their corresponding solidification time points on the solidification curve by detecting the extreme values on the differential curve, as shown in Table 3, where TGL is the primary graphite precipitation temperature, TGU is the lowest temperature of austenite precipitation before eutectic, TGR is the highest temperature of austenite precipitation before eutectic, TEU is the lowest eutectic temperature, TER is the highest eutectic temperature, and TES is the eutectic end temperature. According to the characteristic points of the hypereutectic ductile iron solidification curve, it is further determined that Figure 3 the curve shown is the solidification mode of hypereutectic ductile iron.

[0106] Table 3 Characteristic values of the solidification curve of Example 4

[0107]

[0108] Step 3: Predict the spheroidization rate of the spheroidized molten iron:

[0109] Calculate the predicted spheroidization rate of hypereutectic ductile iron according to Equation 3:

[0110] SGR3 = c0 + c1×ΔT1' + c2×ΔT2' + c3×ΔT2 + c4×ΔT3 + c5×K1' + c6×K2';

[0111] where SGR3 is the spheroidization rate of the molten iron of hypereutectic ductile iron in the solidification mode; c0 = 74.37, c1 = -0.28, c2 = -4.85, c3 = -5.07, c4 = 0.048, c5 = 135.9, c6 = 16.94. ΔT1' = (TGL - TGU) = 7.15; ΔT2' = (TGR - TGU) = 1.595; ΔT2 = (TER - TEU) = 0.62; ΔT3 = (TER - TES) = 52.37; K1' = t( TGR-TGL) / t (TES-TGL) = 0.1276, K2' = t (TES-TER) / t (TES-TGL) = 0.6291;

[0112] After calculation, the predicted spheroidization rate is 92%. After inspection, asFigure 9 As shown, the spheroidization rate of the as-cast ductile iron QT450 blank gear casting body is 90%, meeting the requirements of the product for the distribution of graphite nodules.

[0113] Step 4: Predict the shrinkage porosity index of the hypereutectic spheroidized iron liquid:

[0114] According to the formula PI = 1 - S3 / S, calculate the shrinkage porosity index of the hypereutectic spheroidized iron liquid. After calculation, PI = 0.419, which is less than the requirement of 0.5. After dissecting the hypereutectic ductile iron QT450 pipe clamp blank casting body, there are no shrinkage porosity defects.

[0115] The above are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent variations made to the above embodiments according to the technical essence of the invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for online detection of spheroidization rate and shrinkage index of ductile iron, characterized in that: The method includes: The spheroidized molten iron is poured into a thermal analysis sample cup, the temperature variation of the molten iron in the thermal analysis sample cup over time is recorded, and a cooling curve of solidification time-temperature is drawn; According to the shape of the cooling curve of solidification time-temperature, determine whether the solidification mode of the spheroidized iron liquid belongs to the hypoeutectic solidification mode, the near-eutectic solidification mode and the hypereutectic solidification mode; If the solidification mode of the spheroidized iron liquid belongs to the hypoeutectic solidification mode, the spheroidization rate of ductile iron is predicted by formula 1; formula 1 is: SGR1=c0+c1×△T1+c2×△T2+c3×△T3+c4×K1+c5×K2; Among them, SGR1 is the spheroidization rate of ductile iron liquid in hypoeutectic solidification mode; c0 is a constant phase; c1, c2, c3, c4, and c5 are all coefficients; △T1 is the difference between the precipitation temperature of primary austenite and the lowest temperature of eutectic, △T1 = (TAL-TEU); △T2 is the eutectic recalescence temperature, △T2 = (TER-TEU); △T3 is the difference between the highest temperature of eutectic and the end temperature of eutectic, △T3 = (TER-TES); K1 is the ratio of the precipitation time of hypoeutectic primary austenite to the entire solidification time, K1 = t (TEU-TAL) / t (TES-TAL) , K2 is the ratio of the time from the highest eutectic temperature to the end of solidification to the entire solidification time, K2 = t (TES-TER) / t (TES-TAL) ; TAL is the primary austenite precipitation temperature, TEU is the lowest eutectic temperature, TER is the highest eutectic temperature, and TES is the eutectic end temperature; If the solidification mode of the spheroidized iron liquid belongs to the near-eutectic solidification mode, the spheroidization rate of ductile iron is calculated and predicted according to Formula 2; Formula 2 is: SGR2=c0+c2·△T2+c3·△T3+c5·K2; Among them, SGR2 is the spheroidization rate of ductile iron liquid in near-eutectic solidification mode; If the solidification mode of the spheroidized iron liquid belongs to the hypereutectic solidification mode, the spheroidization rate of ductile iron is predicted by formula 3; formula 3 is: SGR3=c0+c1×△T1'+c2×△T2'+c3×△T2+c4×△T3+c5×K1'+c6×K2'; Among them, SGR3 is the spheroidization rate of ductile iron liquid in hypereutectic solidification mode; c6 is the coefficient; △T1' is the temperature difference between the precipitation temperature of primary graphite and the lowest precipitation temperature of pre-eutectic austenite, △T1'=(TGL-TGU); △T2' is the recalescence temperature of pre-eutectic austenite precipitation, △T2'=(TGR-TGU); K1' is the ratio of the time from the beginning of primary graphite precipitation to the highest precipitation temperature of pre-eutectic austenite to the entire solidification time, K1'=t( TGR-TGL) / t (TES-TGL) , K2' is the ratio of the time from the highest eutectic temperature to the end of solidification to the entire solidification time, K2' = t (TES-TER) / t (TES-TGL) ; TGL is the precipitation temperature of primary graphite, TGU is the minimum precipitation temperature of austenite before eutectic, and TGR is the maximum precipitation temperature of austenite before eutectic; The shrinkage index of the spheroidized iron liquid is calculated and predicted according to formula 4, which is: PI=1-S3 / S, wherein PI is the shrinkage index of the spheroidized iron liquid, S is the integral area of ​​the cooling curve of the entire solidification process; S3 is the integral area of ​​the cooling curve between the highest eutectic temperature and the end temperature of the eutectic.

2. The method for online detection of spheroidization rate and shrinkage index of ductile iron according to claim 1, characterized in that: If the solidification mode of the spheroidized iron liquid belongs to the hypoeutectic solidification mode, the TAL, TEU, TER and TES of the ductile iron are obtained based on the cooling curve of solidification time-temperature; If the solidification mode of the spheroidized iron liquid belongs to the near-eutectic solidification mode, the TEU, TER and TES of the ductile iron are obtained based on the cooling curve of solidification time-temperature; If the solidification mode of the spheroidized iron liquid belongs to the hypereutectic solidification mode, the TGL, TGU, TGR, TEU, TER and TES of the ductile iron are obtained based on the solidification time-temperature cooling curve.

3. The method for online detection of spheroidization rate and shrinkage index of ductile iron according to claim 1, characterized in that: When the solidification mode of the spheroidized iron liquid is the hypoeutectic solidification mode, the entire solidification process is between the primary austenite precipitation temperature and the eutectic end temperature; When the solidification mode of the spheroidized iron liquid is a near-eutectic solidification mode, the entire solidification process is between the lowest eutectic temperature and the end temperature of the eutectic; When the solidification mode of the spheroidized iron liquid is the hypereutectic solidification mode, the entire solidification process is between the primary graphite precipitation temperature and the eutectic end temperature.

4. The method for online detection of spheroidization rate and shrinkage index of ductile iron according to claim 1, characterized in that: A thermocouple is installed at the center of the thermal analysis sample cup to collect the temperature of the molten iron during the solidification process.

5. The method for online detection of spheroidization rate and shrinkage index of ductile iron according to claim 1, characterized in that: According to the cooling curve of solidification time-temperature, the cooling curve is differentiated, the extreme value on the cooling curve is taken, and TAL, TEU, TER, TES, TGL, TGU, and TGR are obtained after analysis.

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