JPS6259254B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for advantageously measuring the amount of austenite in a rolled steel plate, particularly in a rolled steel plate having a two-total mixed structure of a ferrite phase and an austenite phase, while the steel plate is running on a production line. Accurately grasping the amount of austenite in the material during the manufacturing process of this type of steel sheet and controlling it within an appropriate range allows stable production of products with optimal material properties, as well as quality control of the material properties. This is extremely important. For example, in high tensile strength dual-phase alloy cold-rolled steel sheets, which have recently been attracting attention mainly as steel sheets for automobile exteriors, the amount of austenite in the annealing process is controlled with sufficient precision, and the martensite after cooling following annealing is The important point is to accurately bring the amount close to the target value. In other words, in manufacturing the above-mentioned high tensile strength dual-phase alloy cold-rolled steel sheet, the cold-rolled steel sheet after rolling is heated to a temperature between Ac ₁ and Ac ₃ points, that is, a temperature in the coexistence region of ferrite and austenite in the carbon-iron phase diagram. Through heating annealing, a two-phase mixed structure with a predetermined ferrite-austenite ratio is rapidly cooled to cause martensitic transformation, resulting in a two-phase mixed structure steel with a ferrite phase and a dispersed martensite phase. However, in this case, it is clear that the amount of martensite is determined by the amount of austenite before quenching. The characteristics of this type of steel plate are as follows. (1) It exhibits a so-called high yield ratio, with low yield stress and high tensile stress, and has good shape properties after processing. (2) High initial work hardening rate. (3) Uniform elongation and large total elongation. (4) Room temperature aging does not occur and there are fewer problems with stretcher strain-in. (5) Large strain age hardening. etc., and the press formability is good overall. Furthermore, the paintability is not much different from that of conventional cold-rolled steel sheets, and the spot weldability is not inferior, and it is also economically superior. In order to obtain the above-mentioned good properties, it is necessary to limit the amount of martensite within an appropriate range, and for this purpose, it is necessary to accurately measure and control the amount of austenite in the region where austenite and ferrite coexist before quenching. This is an important issue. However, in the conventional manufacturing process, an appropriate method for measuring the amount of austenite in steel sheets while the steel sheet is running on the production line, that is, online, has not been developed. That is, it has been customary to measure off-line and treat this as a representative value for the material. Moreover, steel sheets that are generally used in practical use always exhibit a texture with a preferential orientation, even if there are differences in degree.
Accurate austenite quantification is impossible unless this influence is removed. Therefore, this invention includes correction of such texture, especially for steel plates (including steel strips) running on a line.
The aim is to measure the amount of austenite in the steel sheet as it is, and to use the results to control the amount of austenite in the steel. Here, we will give an overview of conventional techniques regarding the use of X-ray diffraction, which is useful for quantifying austenite. Generally, a crystal is defined as a solid state in which atoms are periodically arranged in three-dimensional space to form a spatial lattice. All the lattice points of these spatial lattices can be arranged on a group of mutually parallel and equidistant planes called lattice planes, and regardless of their position in space, only the orientation of the lattice planes is determined by the so-called Miller index ( Miller index). In Miller index representation, the interval between adjacent planes of a group of planes called (hkl) planes is called the lattice spacing and is denoted by dhkl. In the case of iron, both ferrite and austenite phases
dhkl is expressed as dhkl=a/√ ² + ² + ² (1) where a: lattice constant. Now, the characteristic X-ray of wavelength λ is diagonal θ to the lattice plane where the Miller index (hkl) is (90° − θ to the surface normal)
If the X-rays scattered by the atoms are propagated in a direction forming an angle of 2θ with respect to the incident X-rays, that is, in the direction of the diffraction angle, diffraction will occur if the following condition of equation (2) is satisfied. nλ=2dhkl sinθ ……(2) In equation (2), as known as Black's law, n is the reflection order and is a positive integer greater than or equal to 1, and sin
The values gradually increase in the range of θ1. According to this function, the diffracted X-ray intensity from the sample is measured using an X-ray diffractometer, and the diffracted X-ray intensity from each phase of the multiphase mixed crystal material is determined by the concentration of the phase (or component) contributing to the diffraction. An example of an application in steel is known in which quantitative analysis of austenite is performed using the fact that Let the integrated diffraction intensities of the black angle θ in both the ferrite and austenite phases close to each other be I〓, I〓, and the concentration of each phase be [α],
[γ], the random intensity when each is a single phase (diffraction intensity of a sample without texture) is I〓〓, I〓〓
Then, the following relational expression holds: I = [α] ・I 〓 〓, I 〓 = [γ] ・I 〓 〓, [α] + [γ] = 1 ... (3). Therefore, the following equation (4) is derived. This R is an intensity actor.
factor). Also, from equation (3) is obtained, and if (I〓〓〓/I〓〓) is given in both equations (4) and (5), it is possible to quantify austenite. This random intensity ratio I〓〓/I〓〓 value is practically known. The diffraction X-ray intensity ratios for each crystal lattice plane for disorderly oriented samples of α-iron (ferrite phase) and γ-iron (austenite), that is, samples in which crystal grains have no orientation, are shown in the following table.

[Table] However, since ordinary practical materials usually have some kind of texture, it is necessary to correct the influence of texture when quantifying austenite. It is based on the intensity ratio of the diffraction lines obtained by irradiating a crystalline material with X-rays and the crystal lattice spacing.
The combination of dhkl is unique to that substance, and this fact is used to identify unknown substances by X-ray diffraction, but this is because the above intensity ratio changes if there is a texture in the sample. By the way, the following two are the main methods reported so far for texture correction. One method is to measure the diffraction intensities on as many diffraction grating planes as possible from each phase of α and γ, calculate the sum of the measured intensities for each phase, and distribute them proportionally so that it is equal to the relative intensity ratio of a random sample. This is a method of correction. In this method, the correction strength is determined as follows. If the corrected intensity of the (hkl) plane is I ^c _hkl , and the actually measured integrated intensity of the (hkl) plane and the relative intensity of the random sample are I _hkl and I ^R _hkl , respectively, then the following equation (6) I ^c _hkl = (ΣI _hkl )×(I ^R _hkl /ΣI ^R _h
kl )...(6), and from this equation (6), the following equation (7) can be obtained. γ _ij : Austenite amount I determined from each correction intensity from the (hkl) ⁱ diffraction grating surface of ferrite and the (hkl) ^j diffraction grating surface of austenite ⁱ 〓: Corrected intensity from the (hkl) ⁱ diffraction grating surface of ferrite I ^j 〓: (hkl) of austenite The corrected intensity from ^{the j} diffraction grating plane is derived, and by averaging each γ _ij value, accurate austenite quantification is performed. The other method, similar to the case of positive pole figure measurement, is to vibrate the sample in the same way as a Schierz rotating sample stage while keeping the diffraction angle 2θhkl constant, and then average the diffraction intensity itself to obtain the corrected intensity. It is. However, in all of these methods, a sample is taken from the material to be measured, then quantified using a testing device over a sufficient period of time, and that value is used as the representative value of the material, so online continuous testing is required. It is inappropriate as a quantitative measurement method. On the other hand, for example, there is a method of measuring the diffraction intensity of the {211} plane of the ferrite phase and the {220} plane of the austenite phase using characteristic X-rays of Cr-K, and using these to quantify austenite on the production line. In this case, a mechanism for swinging the X-ray incident angle is required to eliminate the influence of texture. In addition, the above-mentioned measurement method measures a room-temperature specimen, and it is virtually impossible to measure a high-temperature material during annealing or before cooling after annealing. Although the conventional method of varying the X-ray incident angle and measuring the randomized diffracted X-ray intensity to remove the influence of texture is simple and practical as an online measurement, The mechanism is complicated, and it can only be used for intermittent measurements, such as on-line measurements, and is extremely incomplete from the standpoint of equipment maintenance for constant measurements. Furthermore, in the case of online measurements such as those during heat treatment processes, it is necessary to pay sufficient consideration to atmospheric temperature conditions, etc., and the size of the head itself becomes large, so it is virtually impossible to use it unless there is sufficient space for installation. It was hot. This invention solves these conventional difficulties and proposes a method that can continuously measure the amount of austenite in a steel sheet online even during the heat treatment process while performing accurate texture correction. In this invention, a sample is irradiated with continuous (white) X-rays instead of characteristic X-rays, and the diffracted X-rays from the sample are detected by a detector fixed at a fixed angle position. This method has been established to advantageously and appropriately quantify the amount of austenite in steel. That is, in practice, the method based on this invention
Each diffraction intensity for the maximum crystal lattice plane for the measurable ferrite phase and each diffraction intensity for the maximum crystal lattice plane for the austenite phase are counted simultaneously, and each diffraction intensity for each phase is summed. This is a method of accurately quantifying the amount of austenite based on a calibration curve that can be prepared in advance from the ratio of the total diffraction intensity of each phase. Generally, there is a relationship between energy E and wave frequency ν, E=h・ν, and ν=c/λ, so (λ: wavelength, C: speed of light, h: Blank's constant) λ=hc/E...(8) There is a relationship. Also, from equations (2) and (8), the lattice spacing is
There is a relationship between dhkl and energy: E as follows: dhkl=hc/2sinθ・1/E (9). Therefore, white X-rays with continuous wavelengths are incident on a crystal material, the electric pulse height obtained from the diffracted X-rays is analyzed, and each energy is sorted and analyzed, and the crystal lattice spacing dhkl and diffraction are determined from the count number of each energy. Using the so-called "energy dispersion method" to determine the relationship with X-ray intensity, a set of "X
At the same time, the diffraction X-ray intensity of each crystal lattice plane from the ferrite phase and those from the austenite phase are obtained at the ``ray source-detector''. Usually, a semiconductor detection element such as Si (Li) is used as a detector. This invention is a method for determining the amount of austenite in steel using the above-mentioned energy dispersion method without being influenced by the texture. As mentioned above, one of the methods to statically (in the laboratory) remove the influence of texture and quantify austenite is to measure the X-ray intensity on as many diffraction grating planes as possible from each phase, The measured intensities for each phase are summed and proportioned to equal the relative intensity ratio of a random sample.
The method was to correct the intensity. From this, the amount of austenite depends on the total diffraction X-ray intensity of each phase of ferrite and austenite.
The method of such correction is followed in this invention as follows. Now, let the correction intensity of an arbitrary diffraction plane (hkl) of the ferrite phase be I〓, and the correction intensity of an arbitrary diffraction plane (h′k′l′) of the austenite phase be I〓, as specifically shown in Table 1. If the relative strengths of the random samples are I〓 _R and I〓 _R , respectively, then the amount of austenite to be obtained is from equations (5), (7), and (6). becomes. here,

[Formula] Furthermore, ( _I〓R / _ΣI〓R )/( _I〓R / _ΣI〓R ) is a constant, so if we set this as k ₂ , (Here, K=k ₁ k ₂ , total diffraction intensity of ferrite:

[Formula] Total diffraction intensity of austenite:

Let it be [formula]. ). From equation (11), it was demonstrated from the measurement results conducted by the inventors that the amount of austenite in steel depends on the total diffraction intensity from ferrite and each austenite phase. Therefore, if the total diffraction X-ray intensity of each ferrite and austenite phase can be measured quickly, it is possible to advantageously quantify the amount of austenite in steel with high accuracy online without being affected by the adverse effects of texture. be. Therefore, in this invention, the energy dispersion method is used to detect and count the X-ray diffraction intensities from as many diffraction planes as possible from the austenite phase and ferrite phase online for steel sheets running on the production line. At the same time, the influence of texture can be eliminated by adding up each phase to obtain the total diffraction intensity of each phase, and comparing these ratios with a calibration curve created in advance based on equation (11) above. Then, the austenite in the steel is determined. Tables 2 and 3 show θ=30° and θ=
The energy units corresponding to each diffraction grating plane dhkl of the ferrite phase and austenite phase and the degree of their distribution (overlapping) were examined in the case of 15 degrees.

【table】

【table】

[Table] From Table 2, the energy units of (111) and (110), (310) and (400), (331) and (222) are quite close, and (222 )〓 and (220)〓, (422)〓 and (400)〓 are located somewhat close together, but considering the resolution of the X-ray diffractometer (SSD) used, it can be seen that they can be easily separated. There is no problem with other diffraction surfaces. As can be seen from Table 1, the (111) plane of the austenite phase and the (110) plane of the ferrite phase each have the largest intensity ratio, that is, in the case of a random sample, their contribution to the total diffraction intensity is the largest. Therefore, during signal processing, an electronic computer performs, for example, fast Fourier transform FFT (FAST FOURIER transform).
It is easy to separate overlapping waveforms using methods such as TRANSFORM. Another practical method is to measure the same sample by changing the diffraction angle. In other words, when the incident angle of X-rays is reduced to θ = 15°, the energy level between each diffraction plane is further expanded as shown in Table 3, (111)〓,
The diffracted X-ray intensities from (110), etc. are separated.
Therefore, (200)〓, (200)〓, ..., (311)〓, etc.
If the intensity is corrected from the intensity of other clearly separated and duplicately measured diffraction surfaces, (111)
The intensities of 〓, (110)〓, etc. are also corrected to the same level. Using this method with sufficient accuracy, the minimum number of diffraction grating planes required to quantify austenite can be determined as shown in FIG. For the ferrite phase, (110), (200), (211), (310),
(222), (321), ... for the austenite phase, (111), (200), (220), (311), (420)
),
The amount of austenite calculated by taking a predetermined number of pieces in the order of (422), . . . is the white and black points in the figure. From this figure, for the ferrite phase, 5
Regarding the austenite phase, it is found that it is sufficient to take 6 or 7 faces into consideration.
These selected diffractive surfaces are difficult to separate (331)
Even if 〓, (222)〓 were excluded, the same accuracy was obtained. That is, for the ferrite phase,
(110), (200), (211), (310), (321) < (220
)
is equivalent to (110), so exclude the 5 faces of >. For austenite phase, (111), (200), (220),
The diffraction intensities of the six planes (311), (222), and (420) are added for each phase, and the ratio is calculated to obtain the calibration curve shown in Figure 2, which is obtained from random samples of each of the α and γ phases. Austenite can be calculated with high accuracy by calculating from the ratio of each total diffraction X-ray intensity and comparing it with a calibration curve. As described above, in this invention, all diffracted X-rays in the measurement region can be detected and counted simultaneously, so online real-time austenite quantification is possible within 30 seconds, including signal processing, without being affected by texture. Become. A more detailed explanation of specific embodiments will be given below. FIG. 3 shows an example of a measurement system for online real-time measurement of the amount of austenite in a steel plate 7 supported by a plurality of suitable rolls 8, and FIG. 4 shows the specific configuration of the head section 1. shows. The tube current and tube voltage are transmitted from the X-ray generator 2 to the spectroscopic chamber 1.
It is supplied to the X-ray tube 1-1 housed in -0.
In addition to the X-ray tube 1-1, the spectroscopic chamber 1-0 houses slit 1-2 and 1-3 semiconductor detectors 1-4, which will be described later, and protects them from heat and dust from the surroundings.
Shredded to protect against corrosive atmospheres.
Furthermore, it is more desirable to provide a mechanism for creating a vacuum atmosphere in the X-ray passage in order to reduce attenuation of X-rays in the passage. Head part 1 is spectroscopic chamber 1-0
It also includes a vacuum pump and heat-resistant water-cooled pipes, and has an internal explosion-proof structure if necessary, which is advantageous when quantifying austenite in a steel plate during annealing in an atmosphere containing hydrogen gas. Continuous (white) X-rays generated from the X-ray tube 1-1 pass through the incident solar slit (or incident collimator) 1-2 as described above to form a parallel beam, and have a constant incident angle θ with respect to the steel plate 7. incident. Of the incident parallel components, X-rays with an energy value E that satisfies equation (9) are reflected as diffraction lines on the (hkl) plane into the incident solar slit 1- with respect to the sample normal.
The light passes through a detection solar slit (or detection collimator) 1-3 placed symmetrically with 2 and reaches a semiconductor detector 1-4. At this time, continuous X-rays are irradiated onto the steel plate 7 at a constant incident angle with a relatively wide irradiation beam width in a plane including the irradiation optical path and the normal line of the steel plate 7, and the steel plate 7 is emitted from the steel plate 7 according to Bragg's law. Therefore, by detecting the diffracted X-rays in an aperture width narrower than the incident beam width, even if the sample position changes during measurement, the diffracted X-ray intensity remains highly accurate without any fluctuations. It can be measured by Note that as the semiconductor detector 1-4, for example, Si
(Li) detection elements etc. are advantageously suitable. In addition, when setting multiple diffraction angles, such as 30° and 15°, as described above, an X-ray optical system including multiple tubes may be provided, but normally a pair of X-ray sources - detection X so that the system forms an optical system with a predetermined angle
Move and set the radiation source system and detection system. Now, the X-rays that have reached the semiconductor detectors 1-4 are converted into electrical pulses and slightly amplified, and then the peak values of the electrical pulses are sorted by a multiple pulse height analyzer (multiple pulse height selector) 3, and these energy analysis After processing, these data are multi-accumulated in the memory area for each peak value. These calculation functions are normally built into the multiple wave height analyzer 3, but the data after the wave height analysis may be sequentially transferred to the memory of the electronic calculator 5. The main data processing process by the electronic calculator 5 is as shown in FIG. As shown in the figure, after the obtained data is A/D converted at an appropriate location, the waveform is smoothed using techniques such as moving average to remove the background, and the waveform is converted using effective methods such as FFT described above. After separating, the respective diffraction X-ray intensities of the ferrite phase and austenite phase obtained according to their energy levels are added to obtain the total diffraction intensity T・I〓, respectively for the ferrite phase and austenite phase.
Find T・I〓 and calculate the ratio T・I〓/T・I〓
The amount of austenite is determined by comparing it with a calibration curve determined in advance. At this time, the total diffraction X-ray intensity in the ferrite phase and the austenite phase is calculated from the diffraction X-rays from the optimal combination of diffraction crystal lattice planes, 5 for the ferrite phase and 6 for the austenite phase. For example, the profiles of (222) and (331) whose energy levels are close to each other are also sufficiently separated as shown in FIG. 6b. In addition, when the texture in the steel sheet is weak, or when it is not necessary to quantify austenite with high precision, at the stage shown in Fig. 5 after waveform separation (integrated intensity calculation) and before addition of the diffraction intensities of each phase, (a) In the same way as the calculation method using equations (6) and (7) above, calculate the diffraction intensity from any one lattice plane of each of the austenite and ferrite phases using equation (6). , after randomization correction, combine these and quantify, or (b) For the diffraction intensity from any one lattice plane of the austenite phase, either as is or according to equation (6), quantify after randomization correction. Regarding the diffraction intensity, a calibration curve of the diffraction intensity and ferrite volume fraction (α) is created in advance. From this, the amount of austenite is determined as γ = 1-α, (I〓〓: random intensity in the case of γ single phase, I〓: diffraction intensity of the measurement sample), or
In exactly the same way, for the diffraction intensity of any one lattice plane of the ferrite phase, It can also be more quantitatively determined. As described above, according to the present invention, the amount of austenite in a steel plate can be measured online with high accuracy and quickly, so various properties of the steel plate can be estimated online, and quality control and grading can be performed. Furthermore, the obtained information on the amount of austenite enables real-time feedback control of heat treatment conditions such as annealing temperature and annealing time, thereby reducing variations in product properties and dramatically improving product yield. Furthermore, this invention is not limited to the field of manufacturing high tensile strength duplex alloy cold rolled steel sheets, but can be easily applied to the field of manufacturing other types of stainless steel sheets, and the measurement temperature range is It can be applied not only to materials at room temperature, but also to materials at higher temperatures, with some consideration given to heat resistance.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the variation in quantitative austenite values and the number of diffraction grating planes used in calculations, and Figure 2 is a graph showing the relationship between T・I〓/T・I〓 and the amount of austenite. , FIG. 3 is a block diagram showing the embodiment, FIG. 4 is an explanatory diagram showing the configuration of the head section, FIG. 5 is an explanatory diagram of the data processing process by an electronic computer, and FIGS. This is a graph showing the separation state of (111)〓, (110)〓, and (222)〓, (331)〓 when θ=30°. 1...Head part, 1-0...Spectroscopy chamber, 1-1...
...X-ray tube, 1-2...Incidence solar slit, 1-
3...Detection solar slit, 1-4...Semiconductor detector, 2...X-ray generator, 3...Multiple wave height analyzer, 4...Traverse, 5...Electronic calculator, 6...
... Line control section, 7 ... Steel plate, 8 ... Roller.

Claims (1)

[Claims] 1. When measuring the amount of austenite in a rolled steel plate having a two-phase mixed structure of a ferrite phase and an austenite phase while the rolled steel plate is running on a production line, continuous X-rays are applied to the rolled steel plate at a constant angle of incidence. irradiation, detecting diffracted X-rays from a large number of crystal lattice planes of each of the ferrite phase and austenite phase that diffract from the rolled steel sheet according to Black's law, and analyzing the obtained diffracted X-rays by an energy dispersive method. The diffraction X-ray intensity from each crystal lattice plane of both the ferrite and austenite phases is measured, then the diffraction X-ray intensity is added for each phase to determine the total diffraction X-ray intensity of each phase, and the ratio of the two is calculated. A method for measuring the amount of austenite in a rolled steel sheet while running on a production line, the method comprising quantifying the amount of austenite in the rolled steel sheet.