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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method of detecting a surface defect or
flaw of metallic material, and specifically a method of detecting designed
to carry out high precision detection of the depth of a surface defect or
flaw of metallic material at any temperature.
Generally, various sorts of surface defects or flaws take shape in the
manufacturing processes of metallic material. The surface flaws of these
sorts are what must be detected and removed in a proper manner.
Given below will be a statement of surface flaws taking shape on iron and
steel members constituting a sort of metallic material. In general
practice, intermediate products of such iron and steel members as billets,
blooms, slabs and the like are manufactured in the iron and steel
manufacturing processes through a continuous casting process or a blooming
or slabbing process. These intermediate products of iron and steel members
have various shapes and various depths of surface flaws caused during in
the course of the manufacturing processes.
In the conventional practice, each one of the above-mentioned iron and
steel members manufactured by either a continuously casting apparatus or a
blooming apparatus is subjected to cooling down to the normal temperature
level, and then an inspection of whether or not surface flaws are present
is conducted. Surface flaws, if any, are removed, and reheating is
conducted, then the iron or steel member is properly rolled into a
finished product, such as a steel plate, a hoop, a strip steel member, or
the like.
The said intermediate products of iron and steel members of the normal
temperature level have surface flaws removed by such means as melting
and/or grinding, when the surface flaws are detected directly through
visual inspection by an inspecting worker or when information with regard
to the presence and the position of the flaws is given by such a surface
flaw detection system as detects the presence of surface flaws of the iron
and steel members and the positions of the surface flaws of the iron and
steel members. However, the visual inspection by an inspecting worker has
proved that measurement of the depth of a flaw is not practicable, and
mere location of the presence of a flaw has been conducted. Even the said
surface flaw detection system could only obtain information with regard to
the presence and the position of the flaw, and it was not possible to
obtain information with regard to the depth of the flaw. For this reason,
in the flaw removal processes of scarfing and grinding, surface flaws have
been removed in a manner of repeating the trial-and-error method wherein a
melting workman and a grinding workman conducted scarfing and grinding to
such depth and over such an area as were regarded intuitively by them to
be appropriate. The scarfing workman and the grinding then workman
conducted inspection once again thereafter with regard to whether or not
the flaws had been removed. If some flaws remain untreated, the scarfing
and grinding processes were repeated. On the other hand, in the case of
introducing an apparatus for automatically removing surface flaws and
combining the same with the above-mentioned surface flaw detection system,
it cannot be helped but to statistically find in advance the maximum depth
of flaws created, and to remove all of the detected surface flaws by as
much as to the said maximum depth by the application of automatic scarfing
and grinding processes. In this case, melting and grinding are often
conducted to unrequired depth, to thus result in a gross metal loss.
Besides, in the case of conducting removal a surface flaws in a manner of
repeating the trial-and-error method for the purpose of reducing the said
metal loss, considerable impairment of efficiency entails, which makes it
imperative to increase the manhours for treatment of the flaws and the
number of automatic flaw removal apparatuses as well, thus resulting in an
increase in labor cost and equipment cost.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of detecting a
surface flaw of metallic material, featuring that the said problematical
points are properly removed by detecting the depth of the surface flaw, so
that the flaw is enabled to be removed at the minimum level of metal loss
and efficiently enough, and tht a considerable economic effect can be
achieved.
Another object of the present invention is as set forth below. The recent
trend characterized by a few methods specifically designed for saving
energy is such that iron and steel members manufactured by the application
of a continuous casting process or a blooming process are charged in place
into a heating oven while the iron and steel member are still hot enough,
without cooling the same down to the normal temperature level or the
vicinity thereof. This saves the fuel cost required for the heating oven
and improves the capacity of the heating oven. This method is called an
iron/steel member hot charge method). There is also a method wherein iron
and steel members manufactured by the application of the said continuous
casting process or the said blooming process are subjected to hot rolling,
without being subjected to reheating at all, to also attain saving of
energy. This method is called an iron/steel member direct rolling method.
For the application of the iron/steel hot charge method and the iron/steel
member direct rolling method, it is imperative that surface flaws of the
iron/steel members be properly detected in an intermediate process between
either the continuous casting process or the blooming process and the said
heating oven process or the said hot rolling process, and that the surface
flaws thus detected be removed in a proper manner. In the execution of the
said hot charging and the said direct rolling, the said surface flaw
detection system is arranged in place in the intermediate process between
either the continuous casting process or the blooming process and the
product rolling process. A difference between an intermediate product
manufacturing plant and a finished product rolling plant is defined in
terms of the capacity thereof, and depends upon such conditions of
manufacture as the classification of steel and the sizes of products.
Furthermore, the surface temperature of the iron/steel members, as the
material for the intermediate products to be subjected to treatment by a
hot surface flaw detecting means, is subjected to dispersion within a wide
range of the normal temperature levels through approximately 1,200.degree.
C, according to such unexpected troubles as various irregularities that
take shape in respective processes.
Therefore, in order to achieve not only the effect of energy saving but
also the improvement of the yield through the reduction in metal loss, it
is necessary that the surface flaw detecting means for such iron/steel
members as the material for intermediate products set forth in the
preceding paragraph be capable of detecting the depth of the surface
flaws, as well as detecting the presence of the surface flaws,
irrespective of the surface temperature of the iron/steel members.
To meet the above-mentioned requirement, still another other object of the
present invention is to provide a method of detecting the surface flaws of
the iron/steel members, that is well capable of detecting the depth of the
surface flaws, irrespective of the level of the surface temperature of the
iron/steel members.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram to show the method of detecting a surface flaw
introduced in the present invention;
FIGS. 2, 3 are diagrams to show the interrelation between the depth of
surface flaws and the values of rise in temperature;
FIG. 4 is a flow chart to show an example of the constitution of the
surface flaw detecting apparatus to be employed in the case of applying
the present invention to a continuously casting slab;
FIG. 5 is a diagram to show examples of output signals of a memory
reproducer and a deviation operator during one scanning period by a
temperature distribution detector;
FIG. 6 is a diagram to show the types of such surface flaws as take shape
on a continuously casting slab, the shapes of the said surface flaws, and
the results of visual observation at the slab surface temprature of
600.degree. C or over after induction heating;
FIG. 7 is a perspective to show another example of the constitution of the
surface flaw detecting apparatus for materializing the method introduced
in the present invention;
FIG. 8 is a diagram to show the output signals of a scanning type radiation
dosimeter;
FIGS. 9, 10 are digrams to show respectively the relation between the slab
surface temperature before induction heating at 100.degree. C and
50.degree. C in mean set value of temperature rise and the actualizing
function of the surface flaw;
FIGS. 11, 12 are diagrams to show the relation between the actualizing
function obtained at the slab surface temperature before induction heating
at 100.degree. C and 50.degree. C in mean set value of temperature rise
and the measured depth of a flaw;
FIG. 13 shows a series of the surface flaw detecting apparatuses for
iron/steel members with such iron/steel members of 20.degree. -
1,200.degree. C as have strong magnetic properties;
FIG. 14 is a diagram of .lambda. heat distribution in the direction of the
thickness of the material by induction heating;
FIG. 15 is a diagram of rising temperature in the direction of the
thickness of the material under specific conditions;
FIG. 16 is a diagram of temperature distribution in the direction of the
thickness of the material immediately after cooling the surface
temperature by as much as 200.degree. C at the surface temperature of
1,000.degree. C;
FIG. 17 is a diagram to show the distribution of rising temperature in the
direction of the thickness of the material at the time of subjecting the
same to induction heating immediately after cooling the surface thereof;
and
FIG. 18 is a diagram to show an example of the constitution of the surface
flaw detecting apparatus with either a hot metallic material or warm
metallic material specifically selected as an object of detection.
DETAILED DESCRIPTION OF THE INVENTION:
A detailed description of the method of detection according to the present
invention will be given below. To start with, in the subsequent paragraph
there will e given a description of the principle of the detection of the
position of a surface flaw, as well as that of the presence of the surface
flaw.
When an induction current is caused to run through the surface layer to
approximately the same depth as a flaw present on the metallic material by
using an induction heating coil, or when the said surface layer is
directly subjected to electrification, the surface of the metallic
material is subjected to heating by the thermal action of the electric
current. In this case, the flaw-bearing portion grows higher in
temperature than other normal portions. FIG. 6, for example shows the
types of such surface flaws as take shape on the iron/steel metallic
material for an intermediate product. The figure shows, on a continuously
casting slab, the shapes of the surface flaws, and the results of visual
inspection at 600.degree. C or over in surface temperature of the slab
after being subjected to induction heating. In FIG. 6 "Crack-small"
represents a case wherein a slight hot spot is observed in the peripheral
area of the flaw, "Crack-large" represents a case wherein a considerable
hot spot is observed in the peripheral area of the flaw, "Crack -
hexagonal type" represents a case wherein a fairly remarkable hot spot is
observed in the peripheral area of the flaw, "Oscillation crack"
represents a case wherein high temperature is observed in a linear shape
and considerable hot spots are overlapped, and "Blow hole crack"
represents a case wherein the flow as a whole becomes high in temperature
(hot surface). Therefore, when the said portion whereof the temperature
rises high is taken as a surface flaw-bearing portion, and the said
portion whereof the temperature rises high is subjected to detection by
the employment of a thermometer, including an infrared ray thermometer or
the like, which scans in the direction of the width of the metallic
material along the induction heating coil, the position of the flaw can be
detected from the position of the portion whereof the temperature has
risen high in the scanning range.
The inventor investigated the point where the surface flaw-bearing portion
is raised in terms of temperature by induction heating, and examined the
machanism whereby the surface flaw-bearing portion is raised in terms of
the temperature thereof. To put it in specific terms, when an induction
current is caused to run through the surface layer of such a sort of
metallic material as bears a surface flaw (hereinafter referred to as a
flaw in a simplified term), by the employment of an induction heating
coil, the low of the electric current along the induction heating coil is
inhibited in the flaw-bearing portion, and a turbulence zone of the
electric current is produced in the said flaw-bearing portion.
The electric current thus inhibited in the flaw-bearing portion is shunted
to the lower portion (in the direction of the depth) of the flaw, and to
the both ends of the flaw as well, in a manner conforming with the
electrical resistance of the paths of the respective electric currents. At
this time, the electric current thus shunted runs through such a shunt
path or by-pass as has the minimum electrical resistance. To put it
otherwise, the electric current shunted to the lower portion of the flaw
is concentrated at the lower end in the direction of the depth of the
flaw, and the electric current shunted to the both ends of the flaw runs
in a manner of being concentrated at the end of the flaw. The quantity of
the electric current thus subjected to shunt to the ends of the flaw is
related to the depth and the length of the surface flaw.
In case a slight flaw present in the width of an opening is observed in a
planar direction, the both ends of the flaw are higher in terms of current
density than a flawless portion (hereinafter referred to as a normal
portion) in the induction current path.
The surface portion of the material is heated by virtue of the thermal
action of the induction current, and the temperature is further raised, in
excess of the normal portion, at the both ends of the flaw where the
current density has been raised to a higher level. With respect to a flow
large enough in terms of the width of the opening thereof, the bottom
portion of the flaw is also observable in a plane view, and the lower
portion (the bottom) of the flaw, in addition to the both ends of the said
flaw, likewise has the temperature raised by the concentration of the
electric current.
In the method of detecting a surface flaw, wherein a continuous linear
induction current is produced in the vicinity of the surface of metallic
material 1 by means such as a high-frequency induction heating apparatus 7
as comprises an induction heating coil 2 and a high-frequency power source
3, the surface layer thereof is subjected to induction heating, and,
immediately after the induction heating, distribution of surface
temperature is detected by the employment of a temperature distribution
detector 4. Additionally, the state of temperature distribution before the
induction heating is found in advance by the employment of another
temperature distribution detector 5 and a signal processing apparatus.
Then, whether or not a surface flaw is present is detected from deviation
signals between the detecting signals of the said both detectors 4, 5, in
such a manner as is shown in FIG. 1. The inventor examined the possibility
of detecting the depth of the surface flaw in various ways, as set forth
in the preceding paragraphs, and conducted a series of experiments on the
basis of the said examination, which resulted in finding that a certain
interrelation was present between the depth of the surface flaw and such
values of temperature rise as are shown in FIGS. 2 and 3, which shows an
example of the results of the said experiments.
Shown in FIG. 2 is an arrangement of such test data as were obtained as the
results of a series of tests conducted in a repeated manner by such
apparatus and such test conditions as are shown in FIG. 1 and Table 1,
respectively, and FIG. 2 reveals the relation between with measured depth
of the flaw specifically taken as a parameter. Herein, the effective
length of the flaw represents the length of the orthogonal constituents of
the surface flaw to the coil; the width of the coil represents the length
of the coil in the longitudinal direction of the material, and the mean
value of temperature rise represents the difference in temperature
distribution between that before induction heating and that after
induction heating, to put it otherwise, a mean value of deviations of
temperature distribution.
Table 1
______________________________________
Induction heating apparatus 7
1. Frequency 30KHz, constant
2. Dimensions of induction heating coil
Inner size in the direction of
the thickness of material under test
75 [mm]
Inner size in the direction of
the width of material under test
110 [mm]
Size in the direction of the length
of material under test 10 [mm]
______________________________________
Material under test 1
1. Material Such a continuously cast slab piece as
makes it impossible to observe the
presence of a surface flaw by visual
observation from the surface
2. Dimensions Thickness 50 (mm)
Width 90 (mm)
Length (in the direction of
200 (mm)
the material)
3. Speed of 10 [mm/sec]
transfer
4. Temperature
Initial temperature (tempera-
ture before induction heating)
550 [.degree. C]
Mean temperature after
heating 650 [.degree. C]
______________________________________
Temperature distribution detectors 4, 5
1. Detectors Of the linear scanning type; provided with
built-in infrared ray thermometer of
0.5 [.degree. C] in sensitivity at 700 [.degree. C] in
temperature of substance to be measured
2. Position of scanning of material under test by
detector 4
Linear scanning of surface under coil
edge in the direction of transfer of
material under test
______________________________________
Method of measurement of depth of surface flaw
The depth of a surface flaw was found by sub-
jecting the material under test at every 1 mm
thereof in the direction of its thickness.
______________________________________
As clarified through FIG. 2, [the value of temperature rise at the end of
the surface flaw/the mean value of temperature rise] becomes virtually
constant in the range of [the effective length of the flaw/the width of
the coil] .gtoreq. 1, and the larger in value the measured depth of the
flaw is, the larger [the value of temperature rise at the end of the
surface flaw/the means value of temperature rise] becomes. To put it
otherwise, in case the effective length of the flaw is longer than the
width of the induction current path (the width of the induction heating
coil), the temperature at the end of the surface flaw is related virtually
only to the depth of the flaw.
Shown in FIG. 3 is a plot of the data of the surface flaw given as
[measured depth of the flaw] wherein [the effective length of the flaw/the
width of the coil] .gtoreq. 1, and the data of [the value of temperature
rise at the end of the surface flaw/the mean value of temperature rise].
FIG. 3 reveals that [the value of temperature rise at the end of the
surface flaw/the mean value of temperature rise] and the depth of the flaw
are virtually proportionate to each other.
To put it otherwise, when the points shown in FIG. 3 and Table 1 are taken
as criteria, the depth of the surface flaw d [mm] can be expressed by the
formula of
##EQU1##
Here, K.sub.1 is approximately 8.7, and K.sub.2 is 1. Tm represents the
mean value of temperature rise, and T represents the value of temperature
rise at the end of the surface flaw. This reveals that, in case the length
of the flaw is larger in value than the width of the electric current path
(the width of the induction heating coil) among others, the value of the
temperature rise at the end of the surface flaw is related virtually only
to the depth of the flaw. As to the surface flaw to be expressed by [the
effective length of the flaw/the width of the coil] < 1 shown in FIG. 2,
the value of the temperature rise at the end of the surface flaw can be
made proportionate virtually to the depth of the flaw by rendering the
electric current path only minute in terms of the width thereof in a
reverse manner. Therefore, the depth of the flaw can be found by taking
the level of the value of temperature rise of the material as a criterion
therefor.
Now, the present invention is specifically designed for measuring the depth
of the surface flaw on the basis of such a consideration as is set forth
above and a knowledge obtained through a series of experiments. The
subject matter thereof is carried out in that some metallic material is
subjected to the continuous transfer at constant speed in an induction
heating coil or along the said coil. Alternately the induction heating
coil is subjected to continuous transfer at constant speed along some
static metallic material in a reverse manner. A high-frequency linear
induction current is thereby caused to be generated in the said metallic
material, whereby the temperature of the surface layer of the said
material in the coil projection portion is caused to rise in a squential
manner. Such non-uniformity of temperature as is produced in the portion
bearing a surface flaw is detected, to thus carry out detection of the
surface flaw. The depth of the surface flaw is found in a proper manner by
taking the degree of the said non-uniformity of temperature as a guide
criterion therefor.
A description of the method of detecting a surface flaw introduced in the
present invention will be given below by making reference to an
illustration wherein the said method is applied to a continuously casting
slab, as shown in FIG. 4. Reference 8 is a table roller to be rotated at
constant speed; 9 is a continuously casting slab to be transferred at
constant speed by the table roller 8; 10 is a roll of high-frequency
induction heating coil arranged in place between the table rollers in such
a manner that the slab can be placed through the said coil; 11 is a
high-frequency power source for the said coil 10; 12 is a temperature
distribution detector that is arranged in place on the input side of the
said coil 10, and scans the surface of the slab 9 before its being heated
by scanning in the direction of the width thereof, and detects
distribution of temperature in a sequential manner; and 13 is a
temperature distribution detector that is arranged on the output side of
the said coil 10 and detects the temperature distribution in the direction
of the width thereof immediately after linear induction heating.
Now, with regard to the induction heating coil 10, there is preferably used
a type of coil whereof the width is small-dimensioned and an induction
current of a minute width can be produced. With regard to a surface flaw
in the direction of casting the said slab 9, for instance an oscillation
crack, it is likewise recommended that the slab 9 be arranged in such a
manner as to be inclined by an appropriate angle in the direction making a
right angle with said direction of casting, for the purpose of obtaining
sufficient effective length of the surface flaw.
Reference 14, 15 are amplifiers designed for amplifying temperature
distribution signals transmitted from the said detectors 12, 13,
respectively. Reference 16, 17 are such memory reproducers as keep the
temperature distribution signals in memory temporarily in a sequential
manner, and transmit the temperature distribution signals likewise in a
sequential manner by delaying the detecting signal before heating either
of such a length of time as is required for transferring a steel member of
the distance between the positions for observing the said detectors 12,
13, or by an amount dependent on the frequencies of scanning within the
said length of time. Reference 18 is a deviation operator that feeds as an
output such a deviation signal as is corresponding to the value of
temperature rise. Reference 19 is a memory reproducer that keeps a
derivation signal in memory and reproduces the same. Reference 20 is a
mean value operator that operates a mean value of temperature rise by
taking a deviation signal as a criterion thereof. Reference 21 is a memory
reproducer that keeps in memory and reproduces a mean value of temperature
rise. Reference 22 is an operator that conducts operation of [the value of
temperature rise/the mean value of temperature rise] for each and every
time of scanning by the said detector. Reference 23 is a subtractor.
Reference 24 is a coefficient setter. Reference 25 is a multiplier.
Reference 26 is a coefficient setter.
Shown in FIG. 5 are exemplified output signals 27, 28, 29 of the memory
reproducer 16, 17 and the deviation operator 18 at one and the same time.
In the case of the surface flaw detecting apparatus constituted in such a
manner as set forth in the preceding paragraphs, only the surface layer of
the slab of the coil projecting portion is subjected to high-frequency
induction heating in a sequential manner, in the course of continuous
transfer, at constant speed, of the slab 9. The temperature of the slab 9
before its being heated and the temperature of the slab 9 immediately
after its being heated are measured by the detectors 12, 13, and the
temperature thus measured are kept in memory in the memory reproducers 16,
17 in a sequential manner. The memory reproducer 16 reproduces and
transmits necessary signals corresponding to the results of measurement of
the temperature before heaing either for such a length of time as is
required for transferring the slab 9 over the observation distance of the
surface of the slab 9 for the reproduction output of the memory reproducer
17, or by staggering the results of meaurement of the temperature before
heating by as often as the frequencies of scanning within the said length
of time. The difference between the detecting signals for respective times
of scanning is provided by the deviation operator 18, and the value of the
temperature rise on the surface of the slab 9 is thus found.
However, in case the temperature of the slab before heating is uniform over
the whole surface, measurement of the temperature by scanning is not
required. In this case measurement of the temperature at one typical spot
is sufficient. To put it otherwise, operation of the deviation can be
conducted simply enough by merely subtracting some certain value (the
value of the temperature measured at the said typical spot) from the
result of the measurement of the temperature conducted by the detector 13
arranged on the side of the output.
The said deviation signal is kept in memory in the memory reproducer 19 and
the mean value operator 20 determines the mean value of temperature rise,
as well as the deviation signal, and causes the results of the operation
to be kept in memory in the memory reproducer 21.
The reproducers 19, 21 the value of temperature rise and the mean value of
temperature rise for each and every case of scanning. The division
operator 22 finds a proportion of the value of temperature rise to the
mean value of temperature rise (the value of temperature rise/ the mean
value of temperature rise). The subtractor 23 carries out substraction of
such a coefficient 1 as is set in the coefficient setter 24 from [the
value of temperature rise/the mean value of temperature rise]. The
multiplier 25 carries out multiplication of [(the value of temperature
rise/the mean value of temperature rise)-1] by such a coefficient (8.7) as
is set in the coefficient setter 26. A signal 30 designating the depth of
a flaw is thus generated as an output. The signal thus generated is
subjected to a-c/d-c conversion, and is displayed on a line printer, a
cathode-ray tube, and/or the like. It goes without saying that a part or
the whole of the said signal processing portion can be subjected to proper
processing by a computer for information processing.
Additionally, it goes without saying that the position of a surface flaw on
the surface of the slab can be detected by selecting the speed of transfer
of a steel member, the frequency of scanning and the speed of scanning by
the temperature distribution detector as the guide criteria therefor.
The gist of the description given in the preceding paragraphs lies in that
a continuous linear induction current is so caused as to be provided on
the surface layer of a metallic material by the employment of a
high-frequency induction heating apparatus. The difference in the heat
release value attributable to the difference in electrical resistance or
in degree of electrical current concentration between a surface flaw and a
normal surface portion in a continuous linear induction current path is
properly detected by the employment of a temperature distribution detector
immediately after induction heating, the surface region before conduction
induction heating and the corresponding surface region of material
immediately after conducting heating are detectected by said temperature
distribution detectors. Deviation signals for the detecting signals
generated by the detectors is properly found, and a signal for a mean
value of temperature rise is formed by taking the said deviation signal as
a criterion. Then the depth of a surface flaw is found by taking the ratio
of the said deviation signals to the signal for a mean value of
temperature rise as a criterion therefor. To put it otherwise, now that
distribution of temperature of the material before and after induction
heating is properly detected and the depth of a flaw is found by taking
the deviation of temperature distribution as a criterion therefor, the
depth of the flaw can be properly found in a satisfactory manner, even in
case non-uniformity of temperature is present on the surface of the
material before conducting induction heating.
While coming up with the above-mentioned method, the inventor examined a
method of detecting the depth of a surface flaw by the application of a
process of detecting distribution of surface temperature immediately after
conducting induction heating.
In the description given above, the mean value of the difference in
temperature distribution in the directon of a current path, before and
after induction heating, and in the induction current path, is defined to
the mean value of temperature rise. The said mean value of temperature
rise provide a marked difference depending upon whether or not a surface
flaw is present on the said current path.
With regard to the value of temperature rise in the normal portion of the
material by means of induction heating, the said value of temperature rise
can be found by conducting proper operation of required power by taking
the dimensions of a coil, the dimensions of the material, the speed of
material feed, and the said value of temperature rise in the normal
portion of the material as the criteria therefor. By setting the feed rate
and the power in such a manner as to enable the said predetermined value
of temperature rise to be obtained in a proper manner. The set value
.theta. of temperature rise and the aforementioned mean value T of
temperature rise may be so regarded as to be virtually equal, since the
area of a surface flaw present in an electrical current path is only quite
small, compared with the area of the normal portion. Accordingly, the
value of temperature rise set in such a manner as is described in the
preceding paragraphs may be defined to be a mean set value .theta. of
temperature rise. Furthermore, when the hot spot temperature to be
generated in the portion of a flaw by induction heating is expressed to be
.theta.d, the experimental formula of (1) given above can be modified as
shown below.
##EQU2##
Suppose that .DELTA..theta. = .theta.d - .theta.
##EQU3##
To put it otherwise, the depth d of the surface flaw is proportionate to
the ratio of such hot spot temperature .theta.d as is generated in the
portion of a surface flaw by induction heating to the mean set value
.theta. of temperature rise.
The original and primary object of the induction heating to be applied in
the case of such a method of detecting a surface flaw as employs an
induction heating process does not always rest with raising the
temperature of the metallic material by heating. The said object rests
when that temperature difference is caused to be produced between the
normal portion and the portion bearing a surface flaw in such an operating
process wherein the surface portion of the metallic material is subjected
to heating for raising the temperature thereof from some certain initial
level of temperature up to some certain preset level of temperature. To
put it otherwise, the said object rests when a surface flaw-bearing
portion is actualized thermally as a portion wherein the temperature is
raised up to a high enough level.
Therefore, when the mean set value of temperature rise by the application
of an induction heating process is expressed to be .theta., and the
temperature at such a hot spot as takes shape in a surface flaw-bearing
portion is expressed to be .theta.d, the thermally actualizing power K of
the surface flaw-bearing portion by induction heating on the surface of
metallic material, (the value of K), is to be defined to be what is
expressed by the following formula.
##EQU4##
When the surface portion of material is subjected to heating at such
heating speed as can neglect diffusion of heat, then the surface
temperature of the material is caused to rise virtually according to the
distribution of electric current density, and the distribution of electric
current density is determined by the size (including length and depth),
the shape, and/or the like of a flaw. Then the said actualizing power K is
what is inherent and determined by taking the size and the shape of a
flaw. Therefore, once the surface flaw is thus determined, the temperature
difference .DELTA..theta. is proportionate to the mean set value of
temperature rise .theta..
According to the results of a series of experiments conducted with such a
continuously cast low-carbon steel slab as is described in a paragraph
given later, the actualizing power K (the value of K) was determined by
the primary function of the depth of a surface flaw d under some certain
condition. As to the surface flaw of d = 1 - 10mm in depth, and wherein
the mean set value of temperature rise .theta. was 50.degree. C. When the
surface temperature T before induction heating of a slab was T .ltoreq.
650.degree. C, then T + .theta. 700.degree. C. When T .gtoreq. 750.degree.
C, then T + .theta. .gtoreq. 800.degree. C. When the mean set value
.theta. of temperature rise is 100.degree. C, then with the surface
temperature T before induction heating of a slab after T .ltoreq.
550.degree. C, then T + .theta. .ltoreq. 650.degree. C. With T .gtoreq.
750.degree. C then T + .theta. .gtoreq. 850.degree. C. In these tests,
the relation between the depth d of the surface flaw and the actualizing
power K was
K = d/10 (5)
When the relation shown in the formula (5) is established, other conditions
are as shown below.
Instantaneous field of view of radiation dosimeter 1mm .times. 2mm
Conditions of induction heating
Frequency 50 KHz
Heating rate 270.degree. C/sec
Now that the time required of the material to pass under the width of the
coil is the heating time, and the material has the temperature raised by
heating by as much as the mean value of temperature rise thereof in the
course of time required for passage, the heating rate is defined in such a
manner as is shown in the following formula.
##EQU5##
Even in case the material is free from a surface flaw, the temperature
thereof still has more or less non-uniformity even a radiation dosimeter
of the line scanning type has more or less noise generated in the system
thereof. The output side of the radiation dosimeter has more or less
non-uniformity of temperature even in a flawless portion thereof. When the
said non-uniformity of temperature (and the noise level as well) is
expressed to be .DELTA..theta.m, and the said temperature difference
.DELTA..theta. is of the same level and has virtually the same frequency
composition as the non-uniformity of temperature .DELTA..theta.m, it is
difficult to discriminate .DELTA..theta. from .DELTA..theta.m. To put it
otherwise, it is nothing easy to conduct detection thereof as a surface
flaw.
For discriminating the said .DELTA..theta. as a flaw signal from the said
noise leve .DELTA..theta.m, .DELTA..theta./.DELTA..theta.m, i.e., S/N
(flaw signal level/noise level) is required to be 1.5 - 2 or over.
The noise level .DELTA..theta.m is determined by such noise composition as
is characteristic of a radiation dosimeter of the linear scanning type,
and by the level of the non-uniformity of temperature on the surface of
the material in a flawless portion. The said factors to determine the
noise level .DELTA..theta.m has nothing to do with the mean set value of
temperature rise .theta. by induction heating, and is virtually constant.
Therefore, in case the relation between various surface flaws and the
thermal activating power K (value of K) of a surface flaw-bearing portion
on the surface of the material, also the noise level .DELTA..theta.m, is
known. As a result the minimum mean set value of temperature rise
.theta.min, required for detecting by the application of the said formula
of S/N = 15 can be established, with regard to a flaw to be detected. To
put it otherwise, the required flaw signal level .DELTA..theta. can be
determined by taking the noise level .theta..DELTA.m and the S/N ratio as
the criteria therefor, and the minimum mean set value of temperature rise
.theta.min can be determined by taking the value of K characteristic of a
flaw to be detected as a criterion therefor.
It goes without saying that the mean value of temperature rise may be so
set as to be in excess of the minimum mean set value of temperature rise
.theta.min. For all that, however, when the rate of processing the
material [ton/hour] is the same, the capacity of the induction heating
apparatus is virtually proportionate to the mean set value of temperature
rise, and the equipment cost is thus virtually proportionate to the
capacity of the equipment. An increase in the mean set value of
temperature rise up to an extremely hig level makes it imperative to rise
excessive equipment investments. Hence, it is far from being economical,
although detection of surface flaws can be facilitated thereby.
On the part of the end of a surface flaw and/or the bottom of a surface
flaw, created by the concentration of induction currents, the area of a
high-temperature spot, that is to say, a so-called hot spot, is subjected
to fluctuation, according to the size (length, depth) and the shape of the
surface flaw.
Therefore, it is recommendable that the instantaneous field of view of a
radiation dosimeter of the scanning type be so selected as to be virtually
the same as the minimum dimensions of a hot spot taking shape on a surface
flaw.
Furthermore, to enable diffusion of heat to be neglected, the higher the
said heating rate at the time of obtaining the mean set value of
temperature rise is, the more desirable.
As set forth above, the heat actualizing power (value of K) of a surface
portion bearing a flaw attributable to induction heating is what is
determined by the depth d of the surface flaw. This occurs when the width
of th | | |
