KR20010061641A - A Nondestructive Measurement Method for Grain Size of Ferrite Crystal by Ultrasonic Method - Google Patents

Abstract

PURPOSE: A non-destructive measuring method is provided to measure the crystal grain size of a ferrite from a damping change and the correlation of the average crystal grain size by using an ultrasonic damping coefficient. CONSTITUTION: A damping coefficient(alpha) of a carbon steel is measured by using an ultrasonic probe. Herein, the carbon steel has a two-phase complex structure with ferrite and perlite. The measured damping coefficient is put into a formula to calculate the crystal grain size of the ferrite. The formula is as follow. D = "1.79" + 22.97 * alpha¬1/b. Herein, the b is 2.02 to 2.04. A standard deviation is 0.95.

Description

A nondestructive measurement method for grain size of ferrite crystal by ultrasonic method

The present invention is a method for non-destructively measuring the ferrite grains at room temperature after cooling the steel sheet, and more specifically, the size of the ferrite average grain size is determined from the correlation between the attenuation change and the ferrite average grain size in carbon steel by measuring the ultrasonic attenuation coefficient. It relates to a method that can be measured nondestructively.

Conventionally, in order to measure the ferrite average grain size of a steel sheet, the microstructure is subjected to a series of processes such as mechanical cutting, mounting, and polishing of the steel sheet, followed by observing the microstructure using an optical microscope or a scanning electron microscope, and mobilizing an image analyzer. Was calculated. Therefore, a large amount of manpower and time required for preparation of the specimen and measurement of the ferrite grain size have inefficient characteristics, and the ferrite average grain size of the desired point in the steel sheet cannot be measured without damaging the product.

Accordingly, the present inventors conducted research and experiments to solve the above problems, and proposed the present invention based on the results. The present invention measures the size of the ferrite average grain size by measuring the ultrasonic attenuation coefficient, It is an object of the present invention to provide a method for non-destructively measuring ferrite grain size from the correlation between the change and the ferrite average grain size in carbon steel.

1 is a graph showing a natural logarithmic-natural logarithmic relationship between attenuation coefficient and ferrite average grain size.

Figure 2 is a graph showing the correlation between the ferrite average grain size measured by an optical microscope with the ferrite average grain size calculated by the method of the present invention

The present invention for achieving the above object, after measuring the attenuation coefficient (α) of the carbon steel having a two-phase mixed structure of ferrite + pearlite by using an ultrasonic probe, by substituting the measured α in the following formula (1) Non-destructive measurement method of carbon steel ferrite grain size by the ultrasonic method, characterized in that obtained by calculating the ferrite grain size.

D _av = 1.79 + 22.97 x α ^{1 / b}

[b: 2.02-2.04, R ² : 96.31%, σ (standard deviation): 0.95]

Hereinafter, the present invention will be described in detail.

The present invention is to measure the attenuation coefficient using the ultrasonic transducer and to measure the grain size of the ferrite non-destructively by using it. Next, after explaining the theoretical background to which the present invention has been proposed, the configuration of the present invention proposed by such a background will be described in detail. The ultrasonic probe is preferably immersion type, but is not limited thereto.

It is already known that the energy decreases exponentially by the scattering of ultrasonic waves at grain boundaries when ultrasonic waves propagate the material. Given a small amplitude plane wave, the energy intensity I at the distance x from the ultrasonic source is given by the following equation (2).

I = Io exp (-αx)

Where Io represents the initial energy intensity and α represents the attenuation coefficient, respectively. Attenuation correlates with frequency and is more easily measured since it depends on frequency rather than phase velocity. In general, attenuation measurements can be usefully applied when the frequency dependence is closely related to material properties. The attenuation coefficient of the ultrasonic wave depends on the wavelength λ and the crystal grain size D of the ultrasonic wave. It is called Rayleigh scattering region having the case of λ << D. In case of using ultrasonic transducer having the same frequency in this region, the correlation between grain size and attenuation coefficient is expressed by the following equation (3). 3), it is possible to calculate the grain size.

AD ⁿ

Where A is a proportionality constant and n is given a value of 3.

However, the above Equation (3) is applicable when the following conditions are satisfied, and the actual Equation (3) cannot be applied because it is difficult to satisfy the following assumption in the steel sheet produced in the actual field.

1) There should be no anisotropy of grains.

2) Priority defenses should not exist.

3) The grain should be equiaxed.

4) have a single phase determinant with no public or inclusions.

Therefore, the present inventors have obtained the following results through laboratory and field-produced steel sheets through heat treatment to have various grain sizes of carbon steel sheets.

That is, the steel sheet having the composition of carbon steel as shown in Table 1 was changed in various rolling conditions and heat treatment conditions to change the average grain size of ferrite, and then the damping coefficient was measured using a submerged longitudinal wave probe having a center frequency of 15 MHz. The correlation between grain size and attenuation coefficient is shown in FIG. 1. When the ultrasonic center frequency is 15MHz, the wavelength is about 400μm and the size of ferrite grains is 50μm or less, so it belongs to Rayleigh scattering (λ << D) region.

Chemical composition (wt%) C Mn Si P S Ti Nb sol. Al N ₂ D 0.16 1.11 0.26 0.019 0.004 0.011 - 0.030 0.0035 N 0.16 1.10 0.26 0.019 0.004 0.012 0.010 0.028 0.0029 E 0.10 1.40 0.22 0.015 0.006 0.011 - 0.042 0.0025

In fact, the industrial carbon steel has a complex phase of ferrite and perlite, so it is not a single phase and its grain size is not equiaxed, so it is necessary to obtain a new relational relationship to measure the average grain size.

In Fig. 1, the slope of the relationship between the attenuation coefficient and the average size of the ferrite grains from the natural logarithm-natural logarithmic graph shows the n value in Equation (3), which is about 2.03. As a result of several times, the value was about 2.02-2.04, and 2.03 was the best approximation. The correlation between the gradient value obtained in FIG. 1, the measured ultrasonic attenuation coefficient α, the ferrite grain size, and (D _av ) is given by the following Equation (1).

[Equation 1]

D _av = 1.79 + 22.97 x α ^{1 / b}

((R ² : 96.31%),

σ (standard deviation): 0.95,

b: 2.02-2.04, unit of attenuation coefficient α is Nepers / cm, and unit of ferrite grain is μm.]

Therefore, the magnitude of the ferrite grains can be calculated by substituting the α value measured by the attenuation coefficient ultrasonic wave into the above equation (1).

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

The typical attenuated and accelerated-cooled steels having a tensile strength of 40-60kg / mm2 produced in the field were taken from 28 to measure ultrasonic attenuation coefficients, and the grain size was calculated using Equation (1). In the present Example, b value was set to 2.03 in the said Formula (1).

Steel grades and chemical compositions of the steel sheets collected at the site are shown in Table 2 below. In Table 2, SS400 represents a carbon steel for general structural strength of 40kgf / mm2, and SM41B, SM50B, and SM490 steels represent 40 ~ 60kgf / mm2 of welded steel for tensile strength. NVA, LRA, AH & EH steel refers to 40 ~ 60kgf / mm2 shipbuilding steel, API X65 is yield strength 65ksi steel sheet, and S45C refers to mechanical structural carbon steel containing 0.45% of carbon content. .

The C content of these steel sheets can be seen that the steel sheet is distributed from 0.1% to 0.49%. The thickness of the steel sheet used in the present embodiment had a thickness range of 6 to 40 mm and the ultrasonic probe used a central frequency of 15 MHz for immersion.

Under the above conditions, ultrasonic attenuation coefficients were measured using the field-produced steel, and ferrite average grain size and phase fraction were measured by an optical microscope. The measured values are shown in Table 3 below, and the distribution of the pearlite fraction was about 7% to 73%.

In addition, the measured ultrasonic attenuation coefficient was substituted into Equation (1) to calculate the size of the ferrite average grain size. It is shown in Figure 2 using the calculated value and the ferrite average grain size measured by the optical microscope.

turn Steel grade Chemical composition (wt%) C Mn Si Cr Mo Ni Nb Cu Ti V One SS400 0.126 0.70 0.24 0.024 0.009 0.020 - 0.016 2 SS400 0.162 0.54 0.27 0.026 0.007 0.025 - 0.016 3 SS400 0.150 0.81 0.25 0.021 0.011 0.019 - 0.020 4 SS400 0.199 0.68 0.23 0.012 0.007 0.013 - 0.019 5 SS400 0.193 0.85 0.26 0.025 0.013 0.025 - 0.037 - 6 SS490 0.181 1.45 0.27 0.021 0.018 0.018 0.017 0.020 0.001 7 SM41B 0.146 1.05 0.26 0.037 0.014 0.020 - 0.019 0.015 8 SM41B 0.204 1.13 0.26 0.026 0.015 0.031 0.026 0.004 0.005 9 SM50B 0.130 1.42 0.40 0.021 0.020 0.041 0.006 0.028 0.001 10 SM50B 0.135 1.32 0.39 0.015 0.016 0.018 0.021 0.009 0.001 - 11 API X65 0.104 1.58 0.28 0.042 0.017 0.029 0.041 0.032 0.001 0.056 12 NVA 0.156 1.50 0.41 0.021 0.020 0.031 - 0.021 0.013 0.005 13 NVA 0.162 0.57 0.25 0.016 0.008 0.018 0.014 0.002 - 14 NVA 0.127 0.63 0.24 0.015 0.009 0.030 - 0.022 - 15 NVA 0.184 0.70 0.24 0.017 0.010 0.031 - 0.021 - 16 LRA 0.166 1.49 0.40 0.020 0.021 0.031 0.019 - 17 LRA 0.152 1.52 0.40 0.017 0.019 0.030 - 0.011 0.002 18 LRA 0.117 0.64 0.25 0.013 0.010 0.023 0.022 - - 19 LRA 0.140 1.01 0.25 0.018 0.013 0.024 - 0.039 0.001 - 20 AH32 0.115 1.37 0.28 0.027 0.015 0.037 - 0.028 0.013 - 21 SM490 0.166 0.29 0.31 0.023 0.016 0.021 0.005 0.046 0.001 0.005 22 API X65 0.080 1.40 0.22 0.031 0.018 0.028 0.042 0.021 0.010 0.005 23 S45C 0.485 0.69 0.24 0.020 0.007 0.012 0.005 0.007 0.001 0.005 24 S45C 0.472 0.67 0.23 0.020 0.007 0.016 0.005 0.007 0.001 0.005 25 S45C 0.474 0.70 0.24 0.020 0.008 0.015 0.005 0.007 0.002 0.050 26 EH32 0.147 1.45 0.38 0.020 0.018 0.021 0.005 0.020 0.001 0.048 27 AH32 0.112 0.90 0.26 0.022 0.011 0.022 0.005 0.019 0.001 0.005 28 S45C 0.500 0.81 0.24 0.022 0.009 0.016 0.005 0.012 0.002 0.005

turn Attenuation Coefficient (Nepers / cm) D _av (μm) Percentage of% Ferrite Pearlite One 0.0644 7.93 88.9 11.1 2 0.0699 8.80 86.4 13.6 3 0.1105 10.46 85.1 14.9 4 0.3010 15.32 83.0 17.0 5 0.2456 13.81 82.1 17.0 6 0.1201 10.83 77.6 22.4 7 0.2296 13.81 84.6 15.4 8 0.2853 14.80 68.8 31.2 9 0.1445 11.21 75.5 24.5 10 0.1030 10.10 81.5 18.5 11 0.1000 3.83 87.9 12.1 12 0.0981 9.43 78.1 21.9 13 0.2194 13.34 86.1 13.9 14 0.1299 11.21 90.2 9.8 15 0.5095 18.86 82.8 17.2 16 0.2719 14.29 74.9 25.1 17 0.1250 11.21 78.0 22.0 18 0.1980 12.88 89.8 10.2 19 0.3927 17.00 86.0 14.0 20 0.0265 6.22 91.8 8.2 21 0.1001 10.10 80.2 19.8 22 0.0403 6.90 92.8 7.2 23 0.3520 15.86 26.6 73.4 24 0.2331 13.34 30.0 70.0 25 0.0510 7.66 33.8 66.2 26 0.0560 7.93 87.7 12.3 27 0.0490 7.93 94.7 5.3 28 0.1205 10.46 29.7 70.3

Table 3 shows a comparison between the ferrite average grain size measured using an optical microscope and an image analyzer and the ferrite average grain size calculated by Equation (1) by measuring the ultrasonic attenuation coefficient. 2 shows the correlation between the ferrite grain size calculated by the present invention and the measured ferrite grain size.

As can be seen from Table 3 and Figure 2, it can be seen that satisfactory linear relationship between the optical microscope measurement value and the value calculated from the ultrasonic attenuation coefficient, which is a destructive method. In addition, the pearlite fraction of the steel grade shown in the said Table 3 was 5-75%.

Therefore, it can be seen that the non-destructive method according to the present invention is a method capable of measuring ferrite grains with a very high degree of economy compared to the conventional methods of destructive use of cutting, mounting, polishing, microscopy and an image analyzer.

According to the present invention as described above, by using the ultrasonic attenuation coefficient by measuring the size of the ferrite average grain size, which is a metallurgical factor that greatly affects the strength, fracture toughness, fatigue and creep properties of the steel sheet, It can be measured.

Claims (2)

After the attenuation coefficient (α) of the carbon steel having the two-phase mixed structure of ferrite + perlite is measured by using an ultrasonic probe, the measured α is substituted into the following formula (1) to calculate the ferrite grain size. Nondestructive Measurement of Carbon Steel Ferrite Grain Size by Ultrasonic Method [Equation 1] D _av = 1.79 + 22.97 x α ^{1 / b} (b: 2.02-2.04) The method of claim 1, B is a non-destructive measurement method of carbon steel ferrite grain size by ultrasonic method, characterized in that 2.03.