JPH0335614B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention proposes a practical method for measuring the phase transformation point of steel.

When subjecting steel materials to hot working, heat treatment, etc., it is often necessary to accurately grasp the temperature of the steel material, especially the phase transformation point. For example, in hot rolling, it is important to accurately understand the phase transformation point of steel in order to control the finishing temperature of the rolling to a low temperature in order to obtain fine grains. When heating, it is important to know whether the steel temperature exceeds the Ac ₃ point for hypoeutectoid steel, and whether the steel temperature exceeds the Ac ₁ point for hypereutectoid steel. .

However, as a method to measure the phase transformation point of steel, it is often used in the laboratory to create a thermal analysis curve by directly measuring the temperature of the steel using a thermocouple. It is difficult to apply and has little practicality.

In contrast, a transformation point measuring device that utilizes the principle of eddy particle flaw detection is known as a practical method. In this method, the steel transitions from the ferromagnetic region to the paramagnetic region (when the temperature rises) or from the paramagnetic region to the ferromagnetic region (when the temperature cools) at its Curie point, so the impedance change of the coil installed opposite the steel material is This method determines the Kyrie point by detecting the impedance change, and is very useful for steel types such as high manganese steel where the Kyrie point and the _A1 point are approximately equal. However, in the case of steel types where the Curie point and _A1 point are far apart, the above transformation point measuring device cannot be used.
It cannot be applied to measure ₁ point A, and if you want to know the ₁ point A, that is, ₁ point Ar, in a steel that is cooling _{, it} is impossible to apply
It is often meaningless to measure the transformation point using the above-mentioned transformation point measuring device unless the Curie point is higher than the Curie point. Furthermore, when using the eddy current flaw detection method, it is difficult to correct for coil lift-off, temperature drift, and other disturbances. In addition, the lift-off amount of the coil needs to be small in order to ensure sensitivity above a certain level, so it is difficult to apply to hot rolling lines where the steel material is at high temperature and moves up and down. There is.

The present invention has been made in view of the above circumstances, and aims to provide a practical method for measuring the phase transformation point of steel that can be applied online, focusing on the fact that the density of steel changes significantly due to phase transformation. With the goal.

The method for measuring the phase transformation of steel according to the present invention is a method for measuring the phase transformation point of steel, in which radiation is transmitted through the material to be measured during the process of temperature change, and changes in attenuation due to the temperature change are determined. , the result is
It is characterized by measuring _one point A and/or _three points A.

First, the principle of the present invention will be explained. At phase transformation points of steel, for example, _A1 point, _A3 point, etc., the crystal structure changes due to the phase transformation, so the density changes greatly. Figure 1 shows the change in density when a steel material is heated to over 900°C and then cooled, with temperature on the horizontal axis and density on the vertical axis.
The above relationship is well illustrated. In the figure, the broken line indicates ordinary carbon steel, the solid line indicates steel containing Nb, V, and Ca, the one-dot chain line indicates steel containing V, and the two-dot chain line indicates steel containing Nb and V. All steel types are 740
The density increases as the steel is cooled down to ℃, but this is because the steel shrinks as the temperature of the steel decreases, resulting in an increase in its density. When steel is further cooled, the temperature varies depending on the steel type, i.e. 735℃ for ordinary carbon steel and 715℃ for steel containing Nb, V, and Ca.
℃, 650℃ for steel containing V, and 650℃ for steel containing Nb and V.
The first inflection point is formed at 595°C, and the density begins to decrease, and shortly after the second inflection point, the density begins to increase. This is interpreted as follows. In other words, the uniform phase of austenite with a face-centered cubic crystal structure is the first inflection point (corresponding to the Ar ₃ point).
ferrite with a diagonal cubic crystal structure begins to precipitate, and as the precipitation of ferrite, which has a lower density than ausnite, progresses, the density decreases for a while, and then reaches a second inflection point (Ar ₁ point). After the eutectoid reaction occurs and pearlite is produced, the density of the steel increases due to shrinkage of the steel as the temperature of the steel decreases.

However, the following equation (1) holds true between the above-mentioned density and the amount of radiation transmitted by transmitting radiation such as X-rays and γ-rays through a steel material.

N(T)=N ₀ e〓 ^m 〓 ^(T)t ……(1) However, N(T): Radiation detection amount when a steel material is interposed between the radiation source and the detector (function of temperature T) N ₀ : Amount of radiation detected above when not passing through steel μ _n : Absorption coefficient ρ(T) : Density (function of temperature T) t : Plate thickness N(T) is relative to ρ(T) which is a function of temperature Te(1)
Since there is a relationship as shown in the equation, it decreases as ρ(T) increases, and increases as ρ(T) decreases. Figure 2 shows temperature on the horizontal axis and N(T) on the vertical axis.
This is a graph schematically showing the change in (T) due to temperature, and N(T) changes as shown in the figure as the temperature changes. In other words, when a steel material is cooled from, for example, a state of 900°C or higher, as the temperature of the steel material decreases, ρ(T)
increases and N(T) decreases, but after forming the first inflection point at a temperature corresponding to the _three transformation points of steel mentioned above, ρ(T) decreases and N(T) decreases. increases. After that, after forming a _second inflection point at a temperature corresponding to the transformation point Ar of the steel mentioned above, ρ(T)
increases again and N(T) decreases again. Therefore N
By measuring the change in (T), in other words, the change in the amount of radiation attenuation after passing through the steel material, the phase transformation point of the steel can be accurately determined.

The above describes the case where hot steel is cooled, but even when heating steel from a state below its phase transformation point, the phase transformation point, for example, Ac ₁ point or
It goes without saying that the density changes greatly at the _three Ac points, and it goes without saying that the phase transformation point of the steel can be accurately determined by measuring the change in N(T) as described above.

Of course, it can also be used to measure not only the _A1 point and _A3 point, but also the Acm point in hypereutectoid steel.

Next, an embodiment of the method of the present invention will be explained in detail based on the drawings. FIG. 3 is a schematic diagram showing an embodiment of the method of the present invention, in which the temperature of the hot steel material 1 is decreasing moment by moment, and the radiation source 2 installed below the steel material 1 emits radiation from the steel material 1. It is designed to occur in such a way that it penetrates in the thickness direction of the plate. The radiation arrangement source 2
A scintillation probe 3 that quantitatively detects the radiation that has passed through the steel material 1 is installed at a position facing the steel material 1, and the pulse train signal of the radiation obtained thereby is sent to the counter 6 and The pulse train is counted by the counter 6 according to the sampling period set by the timer 7,
The aforementioned N(T) is obtained. Further, a thermometer 4 is installed in a portion of the steel material 1 through which radiation passes to measure its temperature. The signals regarding N(T) and temperature T determined by the counter 6 and the thermometer 4 are input to a computing unit 8, and the computing unit 8 performs arithmetic processing based on these signals. It's becoming like that.

When measuring the phase transformation point of steel using such a device, the calculator 8 generates a signal related to the amount N(T) of the radiation transmitted through the steel material 1 and a signal related to the temperature T of the portion of the steel material 1 through which the radiation has passed. By inputting a signal, the phase transformation point of the steel is determined.

When determining the phase transformation point of steel using this method, unlike when using the eddy grain flaw detection method, the distance between the steel material and the detector must be sufficient, and the technology to compensate for disturbances such as temperature drift must be established. It can be applied online because there are Furthermore, since the phase transformation point is determined based on the amount of radiation transmitted that is related to the density change caused by the phase transformation itself, it is possible to measure the phase transformation point of any steel type or material.

As described in detail above, the present invention allows radiation to pass through the material to be measured during the process of temperature change, determines the change in attenuation due to the temperature change, and uses the results as a basis.
By measuring ₁ point A and/or ₃ points A, it is possible to accurately measure the density of steel, and it is also a practical method that can be applied online, making it an effective method when subjecting steel materials to hot working, heat treatment, etc. provide the means.

[Brief explanation of the drawing]

FIG. 1 is a graph showing changes in density of steel due to temperature, FIG. 2 is a graph schematically showing changes in radiation transmission amount depending on temperature, and FIG. 3 is a schematic diagram showing an example of the method of the present invention. 1... Steel material, 2... Radiation source, 3... Scintillation probe, 4... Thermometer.

Claims (1)

[Claims] 1 In the method of measuring the phase transformation point of steel, radiation is passed through the material to be measured during the process of temperature change, and the change in attenuation accompanying the temperature change is determined, and based on the results, A ₁ point and/or A A method for measuring the phase transformation point of steel, which is characterized by measuring _three points.