RU2525584C1 - Flaw control of slabs for production of hot-rolled strip - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: acoustic emission transducers are fitted at cold slab surface in the order that allows the control over the entire material of slab. Slab is mechanically strained by slab natural weight to 20-80% of slab material yield point. Strained slab is kept under stress for at least one minute to register acoustic emission signals for their processing. Acoustic emission source coordinates are determined to define the possibility of further use of the slab for production of hot-rolled strip by comparison of diagnostics parameter WAE with tolerable diagnostics parameter magnitude [WAE]. At WAE >[WAE] slab is considered unfit for further rolling.

EFFECT: comprehensive and accurate control.

1 dwg

Description

The invention relates to acoustic non-destructive testing methods and is intended to control the surface and internal defects of cold slabs for the production of a hot-rolled strip.

A known method for determining violations of the continuity of metal in ingots, using to identify defects in slabs and determine their exact location, the method of ultrasonic testing. The method uses a slab marking to further monitor its transformations during the rolling process [AS No. 1117094]. The disadvantages of this method are the significant time spent on the control, the presence of contact surfaces subjected to intense wear, high demands on the quality of contact of the sensors with the surface, the lack of a clear classification of the degree of danger of defects identified during the control.

A known method of monitoring surface defects of hot slabs, using to control the effect of the defects of the surface layer of metal converting laser energy into an acoustic signal, and to reduce the influence of interference on the recorded signal, the control zone is cooled, which reduces the signal attenuation only in this area [AS No. 1453311] . The disadvantages of this method are the need for monitoring the passage of sensors along the entire surface area of the slab, which greatly complicates the control process and increases its cost. In addition, the method indicates the presence of discontinuities, their coordinates and geometric parameters without revealing other equally important characteristics, indicating the possibility of further development of the defect, and the inability to control the presence of defects in the inner layer of the slab.

The closest selected for the prototype is a method of controlling a slab for the production of hot rolled strip [Patent No. 2404872]. The method consists in initiating acoustic emission in a controlled slab and recording its signals. The signal processing is carried out and the totality of the data obtained determines the possibility of further use of the slab in the production of hot-rolled strip. At the same time, a cold slab is used for control, acoustic emission sensors are permanently installed on its surface in an order that ensures control of all the slab material and determination of the coordinates of the signal sources. The slab is mechanically loaded, using its own weight, to stresses from 20 to 80 percent of the yield strength of the slab material, kept under load for at least 1 minute. The obtained acoustic emission signals are processed using a computer, according to the obtained data, the presence of zones of increased activity of changes in the structure of the material and the possibility of further development of the defect in this zone during the production of a hot-rolled strip are judged.

The disadvantage of this method is: the lack of an algorithm for determining the parameters of the diagnostic loading of slabs; lack of technology to determine the most informative, from the point of view of controlling the defectiveness of the strip, parameters of acoustic emission, which would allow to determine the defectiveness of the slabs, which leads to insufficient efficiency and accuracy of control.

The objective of the invention is to increase the efficiency and accuracy of control.

To solve the problem, a method for controlling a slab for the production of a hot-rolled strip is proposed. Acoustic emission sensors are installed on the surface of the controlled slab in the cold state. The number of sensors and the order of their location provides control of the entire slab material with the determination of the coordinates of the signal sources. Then, emission of acoustic emission signals from the surface of the slab is initiated by its mechanical loading, which is created using the dead weight of the slab, for example, the slab is lifted using crane equipment. The slab is loaded to stresses close in magnitude to the yield strength of the slab material, but not exceeding it — from 20 to 80% of the yield strength of the slab material. The slab is kept in a loaded state for at least 1 minute, then the slab is lowered. In the process of the above technological operations, acoustic emission signals are continuously recorded. At the first stage, the allowable value of the diagnostic parameter is determined, which is calculated by the formula:

$$\left[W_{\mathrm{BUT}E}\right]=\ln \left(\frac{{\tau }_0}{{\theta }_T}\right)+\frac{U_0}{KT\text{'}\text{'}}$$

where τ ₀ = 10 ^-12 ÷ 10 ^-14 s is the period of atomic vibrations; U ₀ / KT = 50 ÷ 59; U ₀ - activation energy of the destruction process; K is the Boltzmann constant; T is the absolute temperature; θ _T = is the time of the technological impact of the roll on the rolling part of the sheet (specified by technological data or by calculation); θ _T - time of the technological impact of the roll on the rolling part of the sheet is calculated by the formula:

, $${\theta }_T=\frac{0.5\cdot \sqrt{\left(H-h\right)\left(2D-H+h\right)}}{\frac{n\cdot D\cdot \pi }{i\cdot 60\cdot 1000}}$$

,

where D is the diameter of the working surface of the rolls; N is the thickness of the slab before processing; h is the thickness of the slab after processing; n is the engine speed; i is the gear ratio of the drive. Based on the data obtained during registration of AE signals, the diagnostic parameter is determined:

, $$W_{\mathrm{BUT}E}=\frac{\ln {\xi }_2-\ln {\xi }_{\mathrm{one}}}{K_{H2}-K_{H\mathrm{one}}}$$

,

where ξ ₁ , ξ ₂ are the values of the informative acoustic emission parameter at maximum stresses in the cross section σ _max1 , σ _max2 in the sample under diagnostic loading at different times; K _H1 and K _H2 - load factors determined by the formulas:

, $$K_{H1}=\frac{{\sigma }_{\max 1}}{{\sigma }_T}$$

, $$K_{H2}=\frac{{\sigma }_{\max 2}}{{\sigma }_T}$$

, $$K_{H\mathrm{one}}=\frac{{\sigma }_{\max \mathrm{one}}}{{\sigma }_T}$$

,

where σ _T is the yield strength of the material. Diagnostic parameter W _{AE is} compared with the permissible value of the diagnostic parameter [W _AE ]. The condition criterion of a slab suitable for further processing is as follows: W _AE <[W _AE ]. According to the data obtained, it is judged that there are zones of increased activity of changes in the metal structure in local zones of the slab and the possibility of developing defects in them during the production of a hot-rolled strip by rolling. With increasing value of the diagnostic parameter W _AE. the probability of occurrence of defects in the slab with the prospect of their development during the processing of the slab increases up to the need to reject the slab. A decrease in value indicates less defective slab material.

Determination of the diagnostic parameter W _AE and comparison of its value with an acceptable value allows us to judge the suitability of the slab and significantly simplify the process of flaw detection. The permissible value of the diagnostic parameter [W _AE ] can be determined independently of the control object on samples made of the same material as the slab, and the method of loading. Thus, there is no need to determine the acceptable diagnostic parameter for each slab and conduct lengthy expensive preliminary calibration tests to evaluate the acceptable value of the diagnostic parameter, which increases the efficiency of the control method.

The dependency experiments showed a high degree of correlation of the W _AE parameter with the strip defectiveness parameters and a small effect on the accuracy of the results of changes in the control conditions. The value of the correlation coefficient of the values of the diagnostic parameter W _AE and the total length of defects for the samples was 0.74, which confirms the high accuracy of the control of the defectiveness of the slab. Thus, the distinguishing features are necessary and sufficient to solve the problem.

A load of less than 20 percent is not sufficient to initiate a clear acoustic emission signal, and a load above 80 percent is not recommended, in order to eliminate the possibility of accidentally exceeding the yield stress and damage to the slab. The slab is kept in a loaded state for at least one minute, and then lowered. This time is necessary and sufficient for recording acoustic emission signals, a longer period of time is not advisable, since with an increase in time spent, the result remains the same. After lowering the slab, the registration of acoustic emission signals is stopped.

Depending on the required accuracy, the number and location methods of piezoelectric sensors can vary (to ensure recognition of spatial, flat, or linear coordinates of the location of defects), while the number of sensors in the range of which any point of the slab enters should be one more than the number coordinate axes along which control is carried out, and their location should provide an unambiguous determination of any point on the slab that is part of their control zone, relative to their location as geometry the place of points having a certain difference in the distances to the sensors. The length of the slab is selected in accordance with the parameters of the material, when lifting the slab using crane equipment, the stresses arising inside the slab are close to the yield strength, but do not exceed it. Thus, loading the slab with its own weight using crane equipment, stresses sufficient to initiate acoustic emission occur in the slab, at the same time, there are no overlapping signals associated with plastic deformations in the material. These stresses are similar to the stresses acting in the slab during the production of the hot-rolled strip, and the acoustic emission signals recorded at the time these forces act give information about the possible zones of formation and development of defects during rolling. In order to prevent the occurrence of interference introducing distortions into the obtained results, crane equipment with an electromagnetic load securing device should not be used to lift the slab during loading. During all of the above operations, acoustic emission signals are recorded. After loading, the sensors are removed from the slab.

The implementation of the method was carried out when diagnosing a suitable slab during industrial experiments. The cold slab was loaded with its own weight using crane equipment, which led to the appearance of maximum tensile stresses on the surface, constituting 40-60% of the yield strength, and provided a high probability of recording acoustic emission signals. Acoustic emission sensors were installed in the middle part of a cold slab at a distance of about 2 m from each other (the largest distance providing a high probability of signal reception for a given object). Acoustic emission pulses were recorded using a two-channel measuring acoustic emission system. At the first stage, the allowable value of the diagnostic parameter was determined:

$$\left[W_{\mathrm{BUT}E}\right]=\ln \left(\frac{{\tau }_0}{{\theta }_T}\right)+\frac{U_0}{KT\text{'}\text{'}}$$

where τ ₀ = 10 ^-12 ÷ 10 ^-14 s is the period of atomic vibrations; U ₀ / KT = 50 ÷ 59; U ₀ - activation energy of the destruction process; K is the Boltzmann constant; T is the absolute temperature; the time of the technological impact of the roll on the rolling part of the sheet θ _T = 0.19 is taken from the technological data.

. $$\left[W_{\mathrm{BUT}E}\right]=\ln \left(\frac{{10}^{-12}÷{10}^{-\mathrm{fourteen}}}{0.19}\right)+\mathrm{fifty}÷59\approx \mathrm{twenty}÷\mathrm{thirty}$$

.

When processing the results of recording AE signals, the value of the diagnostic parameter W _{AE was} determined. The total number of pulses is used as the primary informative parameter of acoustic emission. The figure 1 presents graphs of the time dependences of the logarithm of the total number of signals (1) for a steel slab and the growth of stress values (2); the rectangle marks the portion used to determine the diagnostic parameter W _AE corresponding to uniform failure before local flow occurs. The maximum stresses σ _max1 , σ _max2 are the stresses corresponding to the maximum and minimum stresses of the selected area.

, $$W_{\mathrm{BUT}E}=\frac{\ln {\xi }_2-\ln {\xi }_{\mathrm{one}}}{K_{H2}-K_{H\mathrm{one}}}$$

,

where lnξ ₁ = ln8 = 2.08; lnξ ₂ = ln31 = 3.43 - values of the informative acoustic emission parameter at maximum stresses in the cross section σ _max1 = 180 MPa, σ _max2 = 240 MPa in the sample under diagnostic loading at different times; K _H1 and K _H2 - load factors determined by the formulas:

, $$K_{H1}=\frac{{\sigma }_{\max 1}}{{\sigma }_T}$$

; $$K_{H2}=\frac{{\sigma }_{\max 2}}{{\sigma }_T}$$

, $$K_{H\mathrm{one}}=\frac{{\sigma }_{\max \mathrm{one}}}{{\sigma }_T}$$

;

σ _T = 500 MPa - yield strength of the slab material.

; $$K_{H1}=\frac{{\sigma }_{\max 1}}{500}=\mathrm{0,36}$$

. $$K_{H2}=\frac{240}{500}=0.48$$

; $$K_{H\mathrm{one}}=\frac{{\sigma }_{\max \mathrm{one}}}{500}=0.36$$

.

. $$W_{\mathrm{BUT}E}=\frac{31-8}{0.48-0.36}=11.2$$

.

The obtained value of the diagnostic parameter does not exceed the permissible value of the diagnostic parameter W _AE <[W _AE ], which, according to the proposed diagnostic feature, allowed attributing the diagnosed slab to a workpiece of satisfactory quality.

The method allows to increase the efficiency and accuracy of control, to classify defects not by indirect signs (size, location, shape of the defect) that have an effect on the quality of the rolled products, but by such characteristics as the prospect of the development of the defect in local zones and the effect of changes in existing stresses on the activity of the change slab structures. A distinctive feature of the method is also the use of only mechanical loading, since the use of others can lead to the appearance of acoustic noise, carrying false information about the quality of the slab.

Claims (1)

A method for controlling slab defects for the production of a hot-rolled strip, including initiating acoustic emission by installing acoustic emission sensors on the surface of a cold slab in an order that provides control of all the material of the slab and determining the coordinates of the acoustic emission signal sources, mechanical diagnostic loading of the slab by using the dead weight of the slab to stresses from 20 to 80% of the yield strength of the slab material, holding under load for at least 1 min, processing the results register tion of acoustic emission signals, determining possible future use in the production of slab hot rolled strip, characterized in that in a first step determine the allowable value of the diagnostic parameters:

,
where τ ₀ = 10 ^-12 ÷ 10 ^-14 s is the period of atomic vibrations; θ _T is the time of the technological impact of the roll on the rolled part of the sheet (specified by technological data or by calculation); U ₀ / KT = 50 ÷ 59; U ₀ - activation energy of the destruction process; K is the Boltzmann constant; T is the absolute temperature, and then, when processing signals from sensors, the value of the diagnostic parameter is determined:

,
where ξ ₁ , ξ ₂ are the values of the primary informative acoustic emission parameter at maximum stresses in the cross section σ _max1 , σ _max2 in the slab during diagnostic loading at different times; K _H1 and K _H2 - load factors determined by the formulas:

,

,
where σ _T is the yield strength of the material,
compare the diagnostic parameter W _AE with the permissible value of the diagnostic parameter [W _AE ] and when W _AE > [W _AE ] the slab is considered unsuitable for further rolling.