JPH07286916A - Method for measuring residual stress - Google Patents

Abstract

PURPOSE:To provide a method capable of measuring the residual stresses of a plate with high accuracy. CONSTITUTION:Bias magnetic fields BV, BV,... are applied parallel to the surface of a steel plate S. Induced magnetic fields BI, BI,... which are perpendicular to the direction of the bias magnetic fields BV, BV,... within the same plane and opposite to each other are applied. Complex magnetic fields BC, BC,... are formed by the combination of the bias magnetic fields BV, BV,... and the induced magnetic fields BI, BI,..., causing magnetostrictions alternately in the direction of the bias magnetic fields BV, BV,... and in the opposite direction. An SH0--mode surface wave is generated by the magnetostrictions generated. Such bias magnetic fields BV, BV,... and induced magnetic fields BI, BI,... can be applied by use of an electromagnet, and therefore sufficient magnetic field strength to measure residual stresses can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a residual stress of a plate material by an SH ₀ mode surface wave whose vibration direction is perpendicular to the traveling direction and which is parallel to the surface of the plate material.

[0002]

2. Description of the Related Art Residual stress of a rolled steel sheet causes, for example, a warp in a cut product steel sheet, which causes deterioration of quality and yield. It is important to measure with high accuracy.

FIG. 6 is a schematic diagram for explaining the principle of a conventional residual stress measuring method by nondestructive inspection. The directions of the principal stresses σ ₁ and σ ₂ remaining in the steel sheet S and the structure anisotropy of the steel sheet S are shown in FIG. It shows the case where the axis of is in agreement. A transverse wave T ₁ that is polarized in the rolling direction of the steel sheet S and a transverse wave T ₂ that is polarized in a direction perpendicular to the ultrasonic wave are generated, and are incident vertically from above the steel sheet S to the thickness direction of the steel sheet S. When the propagation times T _T1 and T _T2 of the steel sheet are measured and the directions of the principal stresses σ ₁ and σ ₂ remaining in the steel sheet S coincide with the axis of the structure anisotropy of the steel sheet S, the following equation (1) is used. Thus, the acoustic birefringence amount B can be obtained. Then, based on the acoustic birefringence amount B, the steel material S is calculated by the following equation (2).
Calculate the internal average stress of. _{B = (V T1 -V T2)} / V T = (T T1 -T T2) / T T ... (1) B = B 0 + C A (σ 1 -σ 2) ∴ σ 1 -σ 2 = (B- _{B 0) / C a ... (} 2) where, V _T1: transverse wave T ₁ of the sound speed V _T2: transverse wave T ₂ of the acoustic velocity V _T: mean of the transverse wave T _1, T ₂ sound speed T _T: transverse wave T _1, T ₂ Average propagation time B ₀ : Effect of tissue anisotropy C _A : Acoustoelastic constant

However, in such a method, the effect B _{0 of the} tissue anisotropy can be measured in the initial state without residual stress, and the value is stable, or it is negligibly small. There is a problem that the internal average stress cannot be accurately measured unless it is a value. Therefore,
In order to measure the residual stress regardless of the tissue anisotropy, a method of using a surface wave of SH ₀ mode in which the vibration direction is perpendicular to the traveling direction and parallel to the plane of the material to be inspected has been proposed.

As a probe for generating SH ₀ mode surface waves, a piezoelectric probe using a piezoelectric element or a Lorentz force type electromagnetic ultrasonic probe using Lorentz force is known. The piezoelectric probe generates transverse ultrasonic waves by utilizing electroacoustic conversion by a Y-cut transducer, and makes this incident on a steel plate at a required angle via a contact medium such as oil, and thereby SH Generates a _0- mode surface wave. However, in sound velocity measurement, the contact medium causes an error and is not suitable for measuring residual stress that requires high accuracy. Also, since the measurement results are affected by the surface condition of the steel plate, the steel plate surface with which the piezoelectric probe contacts must be mirror-finished,
Not practical for measurements on the production line.

On the other hand, the Lorentz force type electromagnetic ultrasonic probe is generated by applying a plurality of magnetic fields parallel to the steel sheet and having directions opposite to each other to the steel sheet and generating eddy currents at right angles to the magnetic fields. Since the SH ₀ mode surface wave is generated by utilizing the Lorentz force, it can be applied to the measurement of residual stress in the manufacturing line.

FIG. 7 shows a Lorentz force type electromagnetic ultrasonic probe.
FIG. 3 is a perspective view showing a main part, in which both the transmitter and the receiver have the same structure.
It Above the steel plate S, a plurality of strip-shaped permanent magnets 21, 2 are provided.
1, ..., N poles or S poles alternately face the steel plate S
Are arranged in two rows like this, and by the permanent magnets 21, 21, ...
A magnetic field B that is upward or downward at right angles to the steel plate S_P, B
_P, ... are applied. Also, at the lower end of the permanent magnets 21, 21, ...
Is opposite to this, the probe coil 22 having the required number of turns
Is provided, and power is supplied to the probe coil 22.
Then, the magnetic field B is applied to the steel plate S._P, B_PEddy current at right angles to
I occurs. As a result, the steel plate S is directly connected with the eddy current I.
Lorentz force R, R, ...
Then, the Lorentz force R, R, ...₀
A mode surface wave is generated.

Then, the steel plate S conveyed in its longitudinal direction
In addition, the SH ₀ mode surface wave is propagated at the same distance in the transport direction and the direction perpendicular to it, the propagation time of both surface waves is measured, and the residual stress of the plate material is measured based on the difference between both propagation times. To do.

[0009]

However, in the conventional method of measuring the residual stress using the Lorentz force type electromagnetic ultrasonic probe, the magnetic field strength that can be applied by the permanent magnet is limited to 2000 to 3000 gauss. Therefore, it is insufficient to measure the residual stress, and there is a problem that sufficient measurement accuracy cannot be obtained. Also, it is not possible to construct a Lorentz force type electromagnetic ultrasonic probe using an electromagnet. The present invention has been made in view of such circumstances, and an object thereof is to accurately measure the residual stress of a plate material by using SH ₀ mode surface waves generated by utilizing magnetostriction. To provide a method.

[0010]

A method for measuring residual stress according to a first aspect of the invention is to propagate a surface wave of SH ₀ mode to a plate material conveyed in its longitudinal direction in the conveying direction and in a direction perpendicular thereto, In the method of measuring the propagation times of both surface waves and measuring the residual stress of the plate material based on the difference between the two propagation times, the SH ₀ mode surface wave is generated by utilizing magnetostriction. .

The residual stress measuring method according to the second invention is the first method.
In the invention, the magnetostriction is generated by applying a bias magnetic field in a direction parallel to the surface of the plate material, and an induction magnetic field orthogonal to the bias magnetic field in the same plane and alternately in opposite directions to the plate material. Characterize.

[0012]

FIG. 1 is a plan view of a steel sheet for explaining the method of the present invention, showing the direction of the applied magnetic field and the direction of the generated magnetostriction. Bias magnetic fields B _V , B _V , ... Are applied to the surface of the steel plate S in parallel with this. Then, the induction magnetic fields B _I , B _I , ... Which are perpendicular to the bias magnetic fields B _V , B _V , ... As a result, the bias magnetic fields B _V , B _V , ... And the induced magnetic fields B _I , B
_I, ... and is combined magnetic field B _C synthesized, B _C, ... are formed, the composite magnetic field B _C, B _C, by ..., the bias magnetic field B _V, B _V, ... direction and the opposite direction of the Magnetostriction occurs alternately. Then, due to the generated magnetostriction, a surface wave of SH ₀ mode is generated. Such a bias magnetic field B _V ,
B _V , ... And the induced magnetic fields B _I , B _I , ... Can be applied using electromagnets, and therefore a magnetic field strength sufficient to measure residual stress can be obtained.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments thereof. FIG. 2 is a block diagram showing the configuration of a measuring apparatus for applying the residual stress measuring method according to the present invention, where S is a steel plate. Above the steel plate S conveyed in the arrow direction, the magnetostrictive transmitters 1a, 2a and the transmitters 1a, 1a,
Sensor 1 including receivers 1b and 2b, which are separated by a predetermined distance from 2a
2 are provided in the transport direction (θ = 0) of the steel plate S and in the direction (θ = 90) perpendicular thereto.

Pulses generated by the pulse generator 3 are alternately applied to the transmitters 1a and 2a by the switch 4, and the transmitters 1a and 2a are applied by the applied pulses.
2a generates SH ₀ mode surface waves by magnetostriction generated by applying a composite magnetic field of a bias magnetic field and an induction magnetic field to the steel plate S. This surface wave propagates through the steel plate S for a predetermined distance and then is received by the receivers 1b and 2b, and the received signal is the preamplifier 5
Also, it is amplified by the main amplifier 6 and given to the propagation time measuring circuit 7.

Then, the propagation time measuring circuit 7 _obtains propagation times T _SH0 (0) and T _SH0 (90) from the time when the pulse generator 3 is driven to the time when the received signal is given, and the following formula (3) is obtained. The time difference ΔT calculated by is given to the arithmetic circuit 8. T _SH0 (0) -T _SH0 (90) = ΔT (3)

The arithmetic circuit 8 calculates the residual stress as follows. Now, assuming that the steel plate S has no residual stress,
The velocity V _SH0 of the SH ₀ mode surface wave propagating in the direction of the angle θ from the transport direction of the steel sheet S is approximated by the following equation (4). V _SH0 (θ) = (μ / ρ) ^1/2 + 1/2 (1 / ρμ) ^1/2 · C (B ₀ · W ₄₀₀ + B ₄ · W ₄₄₀ · cos 4 θ) (4) where μ: Lame Constant ρ: Density of steel C: Elastic constant B ₀ : Constant showing anisotropy due to metallic structure B ₄ : Constant showing anisotropy due to metallic structure W ₄₀₀ : 4 when spherical function expansion of crystal grain orientation distribution function is performed
One of the dimensional expansion coefficients W ₄₄₀ : 4 when the crystal grain orientation distribution function is expanded by a spherical function
One of the dimension expansion factors

From equation (4), the propagation time T _SH0 (θ) required to propagate a unit distance can be obtained by the following equation (5). T _SH0 (θ) = 1 / V _SH0 (θ) ≈ D ₁ + D ₂ · W ₄₀₀ + D ₃ · W ₄₄₀ · cos 4θ (5) where D ₁ , D ₂ , D ₃ : constants

On the other hand, when the residual stress σ (θ) is present in the steel plate S, the propagation time T _SH0 (θ) is _calculated by the following equation (6) in which the influence term of the residual stress is added to the equation (5). be able to. T _SH0 (θ) = D ₁ + D ₂ · W ₄₀₀ + D ₃ · W ₄₄₀ · cos 4θ + D ₄ · σ (θ) ... (6)

Then, T _SH0 (0) and T _SH0 (90)
Is measured and the difference ΔT is obtained, the following (7)
As in the formula, the residual stress is directly obtained regardless of the anisotropy of the metal structure. The constant D ₄ is obtained in advance by experiments. T _SH0 (0) −T _SH0 (90) = ΔT = D ₄ {σ (0) −σ (90)} _∴σ (0) _−σ (90) = ΔT / D ₄ (7)

FIG. 3 is a timing chart showing the operation sequence of the measuring apparatus shown in FIG. As shown in FIG. 3A, the pulse generator 3 generates pulses at predetermined intervals and alternately applies the pulses to the transmitters 1a and 2a. Then, as shown in FIG.
The reception echo corresponding to the pulse and the echo corresponding to the surface wave of SH ₀ mode are measured with a slight delay at the receiver 1a, and the propagation time T _SH0 (0) is obtained. See also FIG.
As shown in (c), the reception echo corresponding to the receiver 2a,
The echo corresponding to the SH ₀ mode surface wave is measured with a slight delay, and the propagation time T _SH0 (90) is obtained.

From the obtained T _SH0 (0) and T _SH0 (90), the difference ΔT between the two is obtained as shown in FIG. 3 (d), and the above equation (7) is obtained as shown in FIG. 3 (e). The residual stress is calculated by substituting ΔT.

FIG. 4 is a graph showing the relationship between the stress measurement value and the tensile stress by the method of the present invention in the tensile test, wherein the vertical axis shows the stress measurement value by the method of the present invention and the horizontal axis shows the tensile stress. As is apparent from FIG. 4, the stress measurement value according to the method of the present invention and the tensile stress have a linear relationship, and it is understood that the residual stress can be accurately measured by the method of the present invention.

Next, the results of comparison between the method of the present invention and the conventional method will be described. FIG. 5 is a graph showing the relationship between the clearance (lift-off) between the plate material and the probe and the height of the received echo, where ● indicates the method of the present invention, and ○ indicates the conventional method utilizing the Lorentz force. . As is clear from the figure, in the conventional method using the Lorentz force, the reception echo height (S / N) is 3 even when the probe is brought into close contact with the material to be inspected. It is as low as about 1/4 of the method. Then, as the lift-off increased, the height of the received echo decreased, and the lift-off became 2 mm and measurement became impossible. On the other hand, in the method of the present invention, the received echo height (S / N) was 5 even when the lift-off was 2 mm, which was higher than the maximum value of the conventional method.

[0024]

As described above in detail, in the residual stress measuring method according to the present invention, since the residual stress of the material to be inspected is measured by utilizing magnetostriction, the residual stress can be measured with high accuracy on the manufacturing line. The present invention has excellent effects such as improvement of product quality and improvement of yield.

[Brief description of drawings]

FIG. 1 is a plan view of a steel plate for explaining the method of the present invention.

FIG. 2 is a block diagram showing a configuration of a measuring apparatus for applying the residual stress measuring method according to the present invention.

3 is a timing chart showing an operation sequence of the measuring apparatus shown in FIG.

FIG. 4 is a graph showing a relationship between a stress measurement value and a tensile stress according to the method of the present invention in a tensile test.

FIG. 5 is a graph showing a relationship between a gap between a plate material and a probe and a reception echo height.

FIG. 6 is a schematic diagram for explaining the principle of a conventional residual stress measuring method by nondestructive inspection.

FIG. 7 is a perspective view showing a main part of a Lorentz force type probe.

[Explanation of symbols]

1 sensor 1a transmitter 1b receiver 2 sensor 2a transmitter 2b receiver 3 pulse generator 4 switch 5 preamplifier 6 main amplifier 7 propagation time measurement circuit 8 arithmetic circuit B _V bias magnetic field B _I induced magnetic field B _C complex magnetic field F magnetostriction S steel sheet

Claims (2)

[Claims] 1. A surface wave of SH ₀ mode is propagated in a plate material conveyed in its longitudinal direction in the conveying direction and in a direction perpendicular thereto, and the propagation times of both surface waves are respectively measured. A method for measuring residual stress of a plate material based on a difference, wherein the SH ₀ mode surface wave is generated by utilizing magnetostriction. 2. The magnetostriction is generated by applying to the plate material a bias magnetic field in a direction parallel to the surface of the plate material and an induction magnetic field orthogonal to the bias magnetic field in the same plane and alternately in opposite directions. The residual stress measuring method according to claim 1.