JP2017090083A - Radiation thickness measuring device, and its calibration method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly-accurate radiation thickness measuring device.SOLUTION: There is provided a thickness measuring device utilizing radiation. The radiation thickness measuring device has a function for calibrating a density correction formula used for linear absorption rate calculation on the basis of a thickness measurement result in a hot state using the device, a surface temperature measurement result at the thickness measuring time in the hot state, a band organization estimation result at the hot thickness measuring time, a back stress estimation result of a material at the hot thickness measuring time, and thickness data obtained by measuring the same material in a cold state.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a hot radiation thickness measuring device and a calibration method thereof, and more particularly to a steel plate thickness measuring device and its calibration method for use in shipbuilding, construction, automobiles, industrial machines and the like.

  In the hot rolling line, as a device for measuring the plate thickness, the linear absorptance is calculated from the output radiation dose and the radiation dose received through the plate thickness, and from this linear absorptivity, density correction, mutual interference correction, linear A method of calculating the plate thickness through the correction of the property is taken.

  Since the density of steel sheets varies greatly depending on temperature, steel type, phase change, etc., this information is stored in the radiation thickness measurement device in the form of a table or correction formula, and correction can be performed using these data for each measurement. It is common.

  For example, Patent Document 1 discloses a method of correcting a measurement value of a gamma ray plate thickness meter using a correction table stratified by a steel type and temperature prepared in advance.

  As a method for calibrating this correction table, Patent Documents 2 to 4 show a method for correcting density type correction data of a radiation plate thickness meter using the result of a laser plate thickness meter in the cold state. . In order to convert a plate thickness from a cold value to a hot value, or from a hot value to a cold value, it is necessary to take into account thermal strain associated with temperature change and transformation strain associated with phase transformation.

  However, in these conventional methods, when converting the plate thickness in the hot state from the plate thickness in the cold state, the influence of the back stress on the transformation strain and the thermal strain and the influence of the band structure cannot be considered. It was the cause of variation in the plate thickness measurement.

  Here, the back stress is a virtual stress introduced when the material is plastically deformed, and is a second-order tensor that represents the movement of the center portion of the yield surface when considering the kinematic hardening phenomenon. That is, when a back stress is introduced, it is possible to express anisotropy such that plastic deformation tends to occur in a certain direction but plastic deformation hardly occurs in a certain direction. FIG. 1 schematically shows the back stress.

  The band structure is a layered heterogeneous structure caused by segregation of alloy components generated during casting, and is characteristically expressed in a material that has undergone casting and rolling. This structure can be avoided by holding it at a high temperature (1300 ° C or higher) for a long time (about 3 days). Usually, the rolling line is maintained while keeping the band structure in consideration of the effects of cost increase. Sent to.

JP 2012-093314 A JP 2003-35530 A JP 2001-147116 A JP 2005-83820 A JP-A-6-2891

T. A. Kop, “Anisotropic dilatation behavior during transformation of hot rolled steels showing banded structure”, Mater. Sci. And Technol., 17 (2001), pp. 1569-1574 T. Siwecki, “Effect of Micro-Segregation on phase transformation and residual stress”, Mater. Sci. Forum Vols. 539-547 (2007), pp. 4596-4601 A. Jablonka, K. Harster and K. Schwerdtfeger, Thermomechanical properties of iron and iron-carbon alloys: density and thermal contraction, Steel Research, 62, 1, pp. 24-33 (1991) J. Miettinen, Calculation of solidification-related thermo physical properties for steels, Metallurgical and Materials Transactions B, 28B, pp.281-297 (1997)

  In view of the above circumstances, the present invention relates to a hot radiation thickness measuring apparatus having a calibration function in consideration of the influence of back stress and band structure on transformation strain and thermal strain.

In developing a high-precision hot thickness measuring apparatus, the present inventors examined the influence of solidification processes and rolling / working processes up to the measurement on transformation strain and thermal strain.
In the process, we focused on the back stress introduced by the rolling process, the solidification process, and the band structure formed by the rolling process.
As a result, we found a phenomenon in which transformation strain and thermal strain change greatly due to the influence of back stress, and transformation strain changes greatly depending on the band structure.

  The present invention is a density model that takes into account the effects of back stress and band structure in converting between hot and cold values when calibrating hot radiation thickness measurement values based on the above examination results. By using, the thickness measurement variation can be greatly reduced, and the gist thereof is as follows.

(1) In a hot thickness measurement device using radiation, the hot thickness measurement result using the device, the surface temperature measurement result during hot thickness measurement, and the band structure prediction result during hot thickness measurement And a function of calibrating a correction formula used for linear absorptance calculation based on a back stress prediction result of the material at the time of hot thickness measurement and thickness data obtained by measuring the same material in the cold. Radiation thickness measuring device.

(2) The hot radiation thickness measurement apparatus according to (1), wherein the thickness measurement apparatus uses gamma rays as radiation.
(3) In the hot radiation thickness measuring apparatus according to (1), a thickness measuring apparatus using X-rays as radiation.
(4) In the hot radiation thickness measurement apparatus according to any one of (1) to (3), a linear absorptance calculation is performed based on at least one of residual strain, cumulative rolling reduction, and dislocation density instead of back stress. A hot radiation thickness measuring apparatus having a function of calibrating a correction formula used in the invention.
(5) In the hot radiation thickness measurement apparatus according to any one of (1) to (4), a density correction expression or a density correction table is stored on a level 2 process computer, and the radiation sent from the thickness measurement apparatus A hot radiation thickness measuring apparatus having a function of converting an absorptance into a plate thickness on a process computer.
(6) In the hot radiation thickness measurement apparatus according to any one of (1) to (5), a temperature correction formula or other correction formula using a calculated thickness average temperature instead of a surface temperature measurement result during thickness measurement. A hot radiation thickness measuring apparatus having a function of calibrating

(7) In the calibration method of the hot thickness measuring device using radiation, the temperature at the time of measurement of the material for which thickness measurement was performed, the back stress prediction result of the material at the time of thickness measurement, and the band structure prediction result at the time of thickness measurement And a calibration method for a hot radiation plate thickness measuring apparatus, wherein the correction equation is calibrated based on one or more kinds of thickness data obtained by measuring the same material in the cold.

  According to the present invention, it is possible to provide a high-accuracy hot radiation thickness measuring apparatus with little measurement variation.

It is a schematic diagram of back stress. It is an example of the band structure | tissue observed with the optical microscope. This is an example showing the rolling reduction and transformation strain before transformation. It is a comparison between the conventional density model and the density model of the present invention. It is a comparison figure of thickness accuracy.

First, the basic configuration of the hot radiation thickness measuring apparatus of the present invention will be described.
The present inventors have found that transformation strain and thermal strain vary greatly depending on the casting conditions and rolling conditions of the material being measured.

  Many studies have been conducted on the phenomenon that the transformation start temperature changes due to pre-processing, the free energy state due to the dislocation density, and the transformation mode changes, but this has been linked to the back stress, Furthermore, it has not been known about a phenomenon in which a large difference in transformation strain or thermal stress occurs depending on the processing mode of rolling.

Although there are studies on the band structure and anisotropic transformation expansion (Non-Patent Document 1 and Non-Patent Document 2), how it changes depending on the casting and rolling conditions, and finally affects the transformation strain. There has been no research on whether or not.
Therefore, the inventors conducted experiments under various casting conditions, rolling conditions, and cooling conditions, and considered these effects.

  First, it will be investigated how the band structure is affected by casting conditions and rolling conditions.

  In general, the band structure is a phenomenon in which solidification segregation that occurs during casting remains in a band shape after rolling, and this solidification segregation is closely related to the cooling rate during casting. Further, since this solidification segregation is deformed by rolling, the solidification segregation is arranged according to casting conditions (especially cooling rate) and rolling conditions (especially rolling reduction).

  Here, solidification segregation can be uniformly diffused by heat treatment at high temperature called soaking. Therefore, the case where the material obtained by rolling is heated to 950 ° C. and the reduction is applied at 900 ° C., the case where the material is heated to 1300 ° C. (soaking treatment), and the reduction is applied to 900 ° C. will be described below.

  FIG. 2 shows a case where a thick steel plate having a tensile strength of 400 MPa is heated to 950 ° C. and rolled at 900 ° C. (a), and when heated to 1300 ° C. and rolled at 900 ° C. (b), perpendicular to the plate width direction. It shows the microstructure in various aspects. For tissue imaging, a 50% optical microscope was used after corroding with a nital corrosive solution. As is clear from this result, the band structure remains in the case of heating at 950 ° C., but the band structure disappears in the case of heating at 1300 ° C., and a uniform structure is obtained. This is because the alloy elements are made uniform by diffusion under high temperature.

  Next, after rolling this steel plate at 900 ° C., the strain change in the thickness direction during cooling was measured. A laser displacement meter was used to measure the strain change. The relationship between this strain measurement result and temperature is shown in FIG. However, FIG. 3 shows only the transformation strain by removing the thermal strain from the total strain amount.

  FIG. 3 shows that the transformation strain increases according to the amount of reduction. This is a phenomenon that occurs due to deformation having anisotropy when microscopic plastic deformation occurs in the phase transformation because back stress is introduced into the material by reduction.

  Here, the back stress introduced by rolling decreases with time due to the effects of recovery and recrystallization. Therefore, considering the recovery and recrystallization depending on the component and temperature, changes in back stress are considered. Although there is no limitation on the calculation method of the decrease amount, it is preferable to use an equation according to the component system, the temperature history, or the like. In particular, the influence of alloy components such as Nb and Ti called microalloy is large, and it is more preferable to consider these influences. The effect of cumulative reduction can also be introduced as back stress. Here, the back stress is usually expressed by a tensor on the second floor. However, the introduction of the back stress by rolling is only considered as a scalar of the back stress by considering only the compression component, so that there is no great influence on accuracy. As the compression back stress develops, the apparent thickness-direction transformation strain increases. Here, the transformation of this material was the ferrite-pearlite transformation, but it was found that the back stress basically has the same effect on the transformation strain in phases such as bainite and martensite.

Next, thermal strain will be described. The thermal strain may be given linearly with respect to temperature, but varies greatly before and after the phase transformation. Depending on the components, in the case of steel, the austenite phase takes a value of about 2.2 × 10 ⁻⁵ [K ⁻¹ ], and the ferrite-pearlite phase takes a value of about 1.5 × 10 ⁻⁵ [K ⁻¹ ]. It is known.

  On the other hand, the inventors considered the influence of deformation on thermal strain. Table 1 shows the linear expansion coefficients in the austenite region and ferrite region after processing from the same test results as above. From the results in Table 1, it was found that the linear expansion coefficient in the austenite region is hardly affected by the processing, but the linear expansion coefficient in the ferrite region tends to increase with the rolling reduction. This cause is considered to be related to texture formation by processing and transformation.

  The inventors of the present invention have reached the present invention through the above examination process, and the requirements and preferable requirements defined in the present invention will be sequentially described below.

First, the measurement principle of the hot radiation thickness measuring apparatus is shown.
The hot radiation thickness measurement device has a radiation generator that uses a radioactive element as a generation source, and a receptor that measures the radiation emitted from the generator on the opposite side of the measurement target, and the temperature during thickness measurement. Means for calculating at level 1 or level 2 including in a measurement or process computer, and has functions of density correction (including temperature correction) and mutual interference correction and linearity correction, and the amount of radiation absorbed and these The thickness of the measurement object is measured from the correcting means.

  Here, any kind of radiation may be used, but it is preferable to change the intensity according to the thickness to be measured. For example, in the case of a thick steel plate having a thickness exceeding 6 mm, it is usual to perform plate thickness measurement using gamma rays.

  Next, a density formula for density correction will be described. Here, there are cases where temperature correction and density correction are described separately. However, if density changes caused by temperature changes are taken into account, they can be treated uniformly as density correction. To do.

  The density of the hot material to be measured varies depending on the temperature and crystal structure of the target material. For example, in the case of a steel material, it is an austenite phase in the hot state, and becomes a phase such as ferrite and pearlite in the cold state, and the density changes depending on the crystal structure, temperature, and alloy components. Density formulas that take these effects into account have been proposed (Non-patent Document 3 and Non-Patent Document 4), and these values can be used for steel materials, but preferably for each steel type in each temperature range in advance. Measure the density.

  On the other hand, according to the present invention, since the transformation strain and thermal strain have anisotropy due to the band structure (segregation structure) and back stress, the above density formula or density data is insufficient. That is, in addition to the density formula or density data as a base, correction is performed by the influence of anisotropy according to process conditions represented by casting conditions and rolling conditions.

  For this correction, the influence of the back stress due to rolling and the influence of the band structure due to casting / rolling are obtained in advance through experiments and numerical calculation methods for each steel type. Preferably, a unified correction formula is created in consideration of the influence of alloy components.

  It is possible to calibrate the hot value using the thickness measurement data in the cold using this density formula.

  Next, a method for calculating the thickness at the time of actual measurement will be described taking a hot rolling line for thick steel plates as an example.

  For hot thickness measurement, temperature data at the time of thickness measurement is required, but this data should use temperature measurement data using a radiation thermometer or laser ultrasonic wave close to the plate thickness measuring instrument. Can do. Also, measured data or calculated temperature received from a process computer at level 2 can be used.

  Furthermore, it is possible to transmit the value measured by the thickness measuring device (level 1) to the process computer (level 2), and to perform complicated correction on the process computer. In this case, the transmission system and the process computer An integrated system including the above can be called a thickness measuring device.

  The back stress and band structure prediction during thickness measurement are described. The back stress at the time of thickness measurement is set to 0 on the exit side of the heating furnace, and the back stress progress due to the subsequent rolling, and the back stress reduction due to recovery and recrystallization are sequentially calculated. The band structure prediction is calculated from the cooling conditions, the component system and the machining history during continuous casting. Here, although the back stress and the band structure prediction method are not limited, for example, the following formula can be used as a development formula of the back stress.

However, preferably a relational expression is obtained in advance for each steel type.

  By the way, the back stress and the band structure at the time of measurement can be obtained by the above method. However, when obtaining the plate thickness of the product, it is necessary to consider the influence of rolling after the measurement. For this influence, the result of the rolling reduction calculation at the stage before the start of rolling called the setup calculation can be used.

  However, since the result of the setup calculation may differ from the actual rolling reduction rate, it is preferable to update the rolling history sequentially with the actual result when measuring the thickness.

Next, the transformation start temperature will be described.
The transformation start temperature does not depend on the direction of the back stress and changes to the high temperature side according to the introduction of processing strain. This is because an increase in dislocation density due to processing brings about an increase in free energy and promotes transformation.

  The rise in transformation start temperature due to processing can also be determined from the experimental results shown in FIG. Here, since the increase in the transformation start temperature is caused by the increase in the dislocation density, it is preferable to consider the influence of the decrease in the dislocation density due to the effects of recovery and recrystallization.

Next, thermal strain will be described.
Since the thermal strain is also affected by the machining history, it is necessary to grasp the influence on the thermal strain in advance. Table 1 above shows the changes in thermal strain values (pre-expansion coefficients) arranged for each rolling reduction. From Table 1, it can be seen that the linear expansion coefficient in the ferrite region is increased by processing (rolling down).

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
In mass%, C: 0.1%, Mn: 1.0%, Si: 1.0%, Al: 0.03%, N: 0.004%, P: 0.001%, S: 0.00. 001%, Ti: 0.0%, Nb: 0.0%, Cr: 0.0%, Cu: 0.1%, Ni: 0.0%, B: 0.0%, Mo: 0.0 A plurality of steel pieces having a thickness of 250 mm casted by a continuous casting machine containing%, W: 0.0%, and V: 0.0% were prepared.

  A sample for microstructural imaging was cut out from the steel piece, corroded with 3% nital solution, and 50-fold optical microscopic imaging was performed to observe the band structure (FIG. 2). From this result, it can be seen that the bandwidth is about 40 to 50 μm and substantially satisfies the expression (3).

  Next, a compression test piece sample was cut out from the steel piece. This compression test piece was heated to 950 ° C. at 10 ° C./s using an induction heating apparatus and held for 5 minutes. After holding the temperature, air cooling is performed, and the transformation start temperature is measured from the length measurement result. At this time, the transformation start temperature was about 780 ° C.

  Similarly, the cut compression test piece was heated to 950 ° C. at 10 ° C./s, held for 5 minutes, and then 10%, 20% and 30% compression strains were added at 0.2 s before 5 s to 780 ° C., Thereafter, the load was unloaded at 0.2 s, and the amount of transformation strain in an unloaded state was measured (FIG. 3 (a)).

  Next, a sample of a compression test piece was cut out from the steel piece as described above. The compression test piece is heated to 1300 ° C. at 10 ° C./s using an induction heating device, held for 5 minutes, air-cooled, and the transformation start temperature is measured from the length measurement result. At this time, the transformation start temperature was about 780 ° C. as in the case of heating at 950 ° C.

  Similarly, the cut compression test piece was heated to 1300 ° C. at 10 ° C./s, held for 5 minutes, and then 10%, 20% and 30% compression strains were added in 0.2 s before 5 s to be 780 ° C., Thereafter, the load was unloaded at 0.2 s, and the transformation strain amount in an unloaded state was measured (FIG. 3B).

  From the above results, the influence on the transformation start temperature due to the reduction is obtained as follows.

  Next, these steel pieces were heated to 1200 ° C. in a heating furnace, then rolled to 20 mm in a hot rolling mill, and cooled from 900 ° C. to room temperature in a cooling bed. At this time, when the rolling was completed, the radiation absorption rate was measured using a gamma ray plate thickness meter installed on the delivery side of the rolling mill, and stored together with the average cross-sectional temperature of the plate calculated by the process computer. As a gamma ray thickness gauge, TOSHIGE 141A manufactured by TOSHIBA was used.

  The plate cooled to room temperature was measured for cold plate thickness using a laser plate thickness meter installed in the inspection line. The laser thickness gauge used was TOSHIBA-TOSAGE-LD.

  When calibrating the gamma ray thickness gauge from the laser thickness gauge data, taking into account the effects of back stress and band structure, and ignoring these effects, calibrating by the conventional method FIG. 5 shows the density correction data when the above is performed.

  Using the density correction data obtained above, 40 steel grades were rolled, and the thickness accuracy was compared. The result is shown in FIG. From this result, the plate thickness error average value was improved from -0.09 mm to 0.006 mm, and the plate thickness error rate standard deviation was improved from 0.006 to 0.0018.

  ADVANTAGE OF THE INVENTION According to this invention, the radiation thickness measuring device which measures hot thickness with high precision is provided, and the precision of the thickness which is the most important one of the quality of a product improves. Therefore, the present invention has high industrial applicability.

Claims (7)

  In a hot thickness measuring device using radiation, the hot thickness measurement result using the device, the surface temperature measurement result during hot thickness measurement, the band structure prediction result during hot thickness measurement, Hot radiation thickness measurement, which has a function to calibrate the correction formula used for linear absorption rate calculation based on the back stress prediction result of the material at the time of measuring the thickness and the thickness data of the same material measured in the cold apparatus.   The hot radiation thickness measurement apparatus according to claim 1, wherein gamma rays are used as radiation.   The hot radiation thickness measurement apparatus according to claim 1, wherein the thickness measurement apparatus uses X-rays as radiation.   The hot radiation thickness measurement apparatus according to any one of claims 1 to 3, wherein the linear absorptance is based on at least one of a residual strain amount, a cumulative reduction ratio, and a dislocation density instead of a back stress prediction result. A hot radiation thickness measuring apparatus having a function of calibrating a correction formula used for calculation.   The hot radiation thickness measurement apparatus according to any one of claims 1 to 4, wherein a density correction formula or a density correction table is stored on a process computer, and the radiation absorption rate sent from the thickness measurement apparatus is processed. A hot radiation thickness measuring apparatus having a function of converting into a plate thickness on a computer.   The hot radiation thickness measurement apparatus according to any one of claims 1 to 5, wherein a temperature correction formula or other correction formula is calculated using a calculated thickness average temperature instead of a surface temperature measurement result during thickness measurement. A hot radiation thickness measuring apparatus having a function of calibrating.   In the hot thickness measurement method using radiation, the hot thickness measurement result, the surface temperature measurement result during the hot thickness measurement, the band structure prediction result during the hot thickness measurement, and the hot thickness measurement A calibration method for a radiation thickness meter, comprising: calibrating a correction formula used for linear absorptivity calculation based on a back stress prediction result of a material and thickness data obtained by measuring the same material in a cold state.