### IMFI Structural Steel Testing Method

In testing, three 32mm structural round Q390B steel specimens were prepared in advance. The internal magnetic field of the body was considered to be uniform at the cross section of the round steel. The IMFI structural steel test was designed at room temperature without electromagnetic interference.

#### Materials

The test round steel bar samples were cut from the same round steel to ensure the stability of the original parameters. In Table 1, the material composition and mechanical properties of Q390B structural steel specimens are provided according to Chinese standard33. These specimens are low alloy high strength structural steels. ‘Q390’ indicates that the yield limit value is 390 MPa. “B” refers to low carbon steel. The steel specimens used in the tests are homogeneous.

#### Test device

The internal magnetic field strength (IMFI) distribution of inherent magnetism for the steel round bar test was carried out with the experimental equipment shown in Fig. 1a. The test device was placed in the shield case to avoid interference caused by external magnetic field. A schematic diagram of the IMFI test is shown in Fig. 1a.

The measuring coil sensor is a winding device as shown in Figs. 1b and c. The device is injured 500 times by a 0.18 mm copper wire with an inner diameter of 40 mm. Its external dimensions are as follows: the inner diameter is 35 mm, the outer diameter is 58 mm and the length is 30 mm, as shown in Figures 1b and c. The measurement parameters of the coil sensor are shown in Table 2. The size of the shield box is 350mm × 350mm × 500mm, as shown in Fig. 1d. The thickness of the shield box is 0.5 mm with the initial permeability µ0= 68.8 mH/m, the maximum permeability µm= 377.5 mH/m, the coercivity Hvs= 0.5 A/m, and the saturation magnetic induction intensity Bs = 0.75 t. The fluxmeter, as shown in Fig. 1e, is a compact instrument, with high accuracy, automatic drift correction and microprocessor control, to allow the operator to configure the flowmeter for maximum resolution and accuracy. The magnetic flux is used to inversely calculate the magnetic field distribution inside the magnet. The magnetic flux value of each measurement point is measured by the fluxmeter used in the experiment. The distance between the measuring points was 50 mm. As shown in Figure 1e, the magnetic flux value was collected by a TD8900 flux meter manufactured by Changsha Tianheng Measurement and Control Technology Co., LTD. The steel bar is fixed in the shielding box. When tested, the diameter of the round steel specimen is 30mm, and the length is 500mm. The steel round bar sample and scale label are shown in Fig. 1f.

### Structural Steel IMFI Test Design

During the test, the measuring coil sensor was laterally wrapped around the steel and moved up and down as shown in Fig. 1a.

#### Test procedure

In the experiment, only the change in relative magnetic flux was considered and measured at a specified point along the axial direction. There are two options for performing this experiment. One option is to first set the fluxmeter to zero, and the initial magnetic flux will be generated when the sensor of the measuring coil starts to move towards the end due to the change in the actual magnetic field at the end of the round steel sample. Another option is to set the flowmeter to zero after determining the starting position of the specified measurement point. In such a case, the value of the first measurement point will be zero.

In this article, the last option was chosen to perform IMFI testing of structural steel. First, the measurement sensor was moved to the specified measurement point position. Then the magnetic flux meter was zeroed and started recording data from the prepared specified point. The magnetic flux values ​​at the specified measurement points were measured along the coordinate position of the sample. As shown in Fig. 1a, the positions of the specified measurement points were 0, 50 mm, 100 mm, 150 mm, 200 mm, 250 mm, 300 mm, 350 mm, 400 mm, 450 mm and 500 mm, i.e. the direct method. After measurement, the coordinate positions along the round steel are: 500 mm, 450 mm, 400 mm, 350 mm, 300 mm, 250 mm, 200 mm, 150 mm, 100 mm, 50 mm and 0 mm, i.e. reverse method. After the two-way measurement, the test was complete. Therefore, the measurement error was reduced by taking the average value of the two-way measurement.

The tests were designed and performed to obtain the internal magnetic field strength distribution of inherent magnetism for round steel specimens with no load or external magnetic field in a room temperature environment. According to the change of magnetic flux inside the round steel body, the distribution of magnetic field strength B inside the round steel body along the length can be calculated by the equation. (2) changed from eq. (1).

$$Phi = n cdot B cdot S$$

(1)

$$B = frac{Phi }{n cdot S}$$

(2)

where (Phi) is the magnetic flux; not is the number of windings for the measuring coil sensor; B is the magnetic field strength inside the round steel body; and S is the area of ​​magnetic field intensity inside the round steel body.

### Structural Steel IMFI Test Results

The experimental tests were carried out at a temperature of 20 ± 3°C. In the IMFI test, three samples of 32 mm round steel bars were tested and the experimental data of the test points were recorded by flowmeter software on a computer. In the tests, the front and back contrast method was used to eliminate the error. Through the test, it was found that the magnetism existing in the round steel was not affected by the external environment. The internal magnetic flux field distribution of structural round steel bar sample at the cross section of Q390B round steel is shown in Fig. 2.

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