JPH0688794A - Quality control system for descaling and pickling stainless steel material - Google Patents

Abstract

PURPOSE:To obtain a quality control system for stably obtaining stainless steel material having excellent corrosion resistance and no quality variation by accurately detecting corrosion resistance (pickling degree) of the material after descaling and/or pickling step and feeding back it. CONSTITUTION:A quality control system for stainless steel material 1 descaled and/or pickled after heat treating comprises fluorescent X-ray analyzers 7, 8, 9, 10 for detecting 'Si and/or Cr concentration of the surface of the material' or 'the ratio of the Si and/or Cr concentration of the surface of the material to that of base metal', and a controller 11 for regulating conditions of the descaling or pickling based on the detected result.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a descaling after heat treatment.
The present invention relates to a corrosion resistance quality control system for a stainless steel material for maintaining the original corrosion resistance of a stainless steel material that has been pickled and / or pickled.

[0002]

2. Description of the Related Art Generally, JIS standard 2
As with B specification stainless steel plate, "scale removal after annealing.
The stainless steel material whose surface is finished by pickling is
If the pickling conditions such as pickling time are inappropriate, the oxide scale produced by annealing remains on the surface, leading to deterioration of corrosion resistance. Further, even if the oxide scale is removed, a Cr-deficient layer below the oxide scale (a metal with a low Cr concentration caused by the preferential oxidation of Cr and transfer to the scale) It is also known that if the removal of the layer) is insufficient, the original corrosion resistance of the stainless steel material cannot be exhibited. on the other hand,
If the pickling conditions are too strict, not only will the surface become rough due to overpickling, but valuable resources (stainless steel, acid, energy) will be wasted.

Therefore, in stainless steel annealing / pickling materials,
It is desirable to perform pickling without excess or deficiency from both aspects of quality and cost, but until now, the extent of this pickling has been rapid.
There was no established method for making an accurate judgment.

Of course, if the stainless steel material manufactured through the pickling process is brought into a laboratory and the surface roughness, gloss, and corrosion resistance are examined in this laboratory, the excess or deficiency of the pickling can be determined, but this takes time and labor. Since it takes too much time to meet the demands of the manufacturing site immediately, it cannot be said that the contribution to the stabilization of product quality in the pickling line is sufficient.

In view of the above, the object of the present invention is to "finish" the stainless steel material which has undergone the steps of descaling and / or pickling, that is, "the original corrosion resistance of the stainless steel is guaranteed. Corrosion resistance can be determined by quickly and accurately determining whether or not descaling or pickling has been performed to the extent that it is carried out, and by rapidly incorporating the results of this determination as an adjustment factor for descaling and pickling conditions. The goal is to establish a quality control system in the descaling and pickling processes that ensures stable and stable production of stainless steel products with excellent quality.

[0006]

Means for Solving the Problems The inventors of the present invention have obtained the following findings as a result of earnest studies from various viewpoints in order to achieve the above object. a) The Si concentration or Cr concentration on the surface of stainless steel and the degree of pickling finish (that is, corrosion resistance) correlate very well,
By detecting the Si concentration or Cr concentration on the steel surface, it is possible to accurately grasp the finish degree (corrosion resistance) of descaling and pickling.

B) As the method for measuring the Si concentration or Cr concentration on the surface of the stainless steel material, for example, SIMS (Secondary Ion Mass Spectroscopy), method of melting and wet analysis (ICP), SE
M scanning electron microscope) and EDX (energy-dispersion analyzer) are used together, fluorescent X-ray analysis method, inductive plasma emission analysis method, and the like. Among them, the fluorescent X-ray analysis method is quick and inexpensive. It can be said that it is an excellent method for online application because it can analyze and measure

C) Therefore, in the descaling and / or pickling line of the heat-treated stainless steel material, the "concentration of Si or Cr on the surface of the steel material after descaling and / or pickling by a fluorescent X-ray analyzer is used. By detecting the "concentration" and evaluating the degree of its corrosion resistance, and adjusting the treatment conditions such as pickling conditions based on this result, the stainless steel material with excellent corrosion resistance and luster inherent in stainless steel can be made extremely stable. It becomes possible to manufacture.

The present invention has been completed by further studies based on the above findings and the like, and "is descaled or pickled (that is, descaled and / or pickled after heat treatment). ) A stainless steel quality control system for detecting "Si and / or Cr concentration on the steel surface" or "ratio of Si and / or Cr concentration on the steel surface and Si and / or Cr concentration in the metal" A fluorescent X-ray analyzer and a controller that adjusts the conditions of descaling or pickling based on the detection result are used to stabilize a stainless steel material with excellent quality such as corrosion resistance. It has a major feature in "the point that it has been obtained".

By the way, "Si and / or Cr concentration on steel surface" or "Si and / or Cr concentration on steel surface and
By using the ratio of Si and / or Cr concentration as an index,
"Stainless steel material that has been descaled or pickled after heat treatment"
The principle by which the corrosion resistance of can be evaluated is as follows.

That is, when manufacturing a stainless steel material,
A process of softening a cold-rolled steel material by annealing is often performed. For example, in the case of SUS304, 1100 to 1150 in the combustion atmosphere of hydrocarbon gas.
It is a general method to heat at a temperature of about ° C for 1 to several minutes. However, in this process, an oxide scale is generated on the surface of the stainless steel material, and at the same time, a metal layer having a low Cr concentration is formed under the scale (metal side) as described above.

When the Cr-deficient layer site was investigated in detail, this layer became a metal layer in which Si was clearly enriched in comparison with the bare metal (hereinafter simply referred to as "Si enriched layer"). I know that

That is, FIG. 2 shows the results of the investigation of the “change in the depth direction” of the chemical composition on the surface portion of the SUS304 annealed material.
An oxide scale mainly composed of oxides of Cr, Mn, and Fe exists up to the depth of μm, and a thickness of about 1.5 μm exists under the scale.
It is recognized that the Si concentrated layer is present.

Such stainless steel annealed material is descaled and pickled by a neutral salt electrolysis method (Lusner method) and a nitric-hydrofluoric acid dipping method in the usual manner. Oxides of Cr and Mn mainly in the layer are dissolved, and then the Si concentrated layer is dissolved by nitric hydrofluoric acid.

What is important here is that the present inventors have confirmed by experiments. "If the dissolution of the Si concentrated layer by nitric hydrofluoric acid is insufficient, local corrosion such as pitting corrosion easily occurs. , The original corrosion resistance of the stainless steel cannot be guaranteed. " Moreover, according to the above experiment, the degree of dissolution of this Si-enriched layer correlates well with the measured value of Si concentration on the surface of the stainless steel material, and the Si concentration on the surface of the stainless steel material and the corrosion resistance of the stainless steel material have a very strong correlation, It was also confirmed that the Si concentration on the surface is a sufficient index for evaluating the corrosion resistance of the stainless steel material, since the corrosion resistance is correspondingly improved when the Si concentration is lowered.

The reason for this is that the diffusion of oxygen from the surface oxidation scale side probably produces oxides such as SiO ₂ .
Therefore, it is presumed that local defects or compositional nonuniformity of the metal structure occur in the vicinity thereof and become a starting point of local corrosion such as pitting corrosion. Then, with the progress of pickling, the Si concentrated layer is removed and the starting point of local corrosion is reduced, and since the starting point of local corrosion also disappears when sufficient pickling is performed, it is considered that the corrosion resistance of the stainless steel material is improved. To be

By comparing the Si concentration analysis results with the separately analyzed Si concentration value in the metal of the stainless steel, the degree of Si enrichment on the surface can be determined more finely, and therefore the "surface Si The corrosion resistance can be evaluated more accurately by using the "ratio of the concentration and the Si concentration in the metal" as an index.

By the way, as is clear from the analysis results shown in FIG. 2, since the Si-enriched layer directly below the oxidation scale of the heat-treated stainless steel is also a Cr-deficient layer, the "Cr concentration" on the surface of the stainless steel is It can be seen that can also be an index for corrosion resistance evaluation, like “Si concentration”. However, since Si has a higher degree of concentration on the surface, it can be said that Si concentration is a better indicator of corrosion resistance.

Thus, by analyzing the surface concentration of Si or Cr, the corrosion resistance of the annealed stainless steel material (degree of pickling)
However, in order to perform a more accurate evaluation, it is more desirable to analyze and measure both Si and Cr on the surface of the stainless steel material and use both of them as indexes. This is because the single analysis value of Si or Cr is easily affected by fluctuations in analysis conditions (variations in primary X-ray intensity, variations in steel surface roughness, variations in X-ray tube or dispersive crystal and strip surface, angles, etc.). This is because if both analysis values of Si, Cr and Si are used as indicators, these influences can be offset, so that the corrosion resistance (about pickling) can be evaluated more accurately. Also, since the Si concentration decreases and the Cr concentration increases as the depth from the surface increases, the “Si concentration / Cr concentration ratio” has a larger change rate than the concentration of Si or Cr alone,
Enables higher sensitivity judgment.

Next, the quality control system for descaling and pickling stainless steel materials according to the present invention will be described in more detail based on examples.

FIG. 1 is a schematic explanatory view of an example (based on detection of surface Si concentration) of a quality control system for descaling and pickling stainless steel materials according to the present invention.

In FIG. 1, a stainless steel strip 1 which has been subjected to heat treatment such as annealing is shown in an acid bath (eg 10% HN).
O ₃ -1% HF is stored) 2, and desalted and / or pickled, and after passing through the washing tank 4 and the drying device 5, tension release
The product is rolled up by reel 6. Here, the surface of the strip 1 is irradiated with X-rays by the fluorescent X-ray analyzer 10 between the acid tank 2 and the tension reel 6, and the characteristic X-rays (fluorescent X-rays) of Si emitted from the surface are detected. The intensity is detected by the detector 9 and fluorescent X-ray analysis is performed.

As a method for measuring the Si concentration on the surface of the stainless steel material, SIMS (secondary ion mass spectrometry), a method of performing wet analysis by melting (ICP), an emission analysis method of inductive plasma, etc. are conceivable, but they are rapid and From the viewpoint of inexpensive analysis and measurement, it is essential to adopt the fluorescent X-ray analysis method.

That is, since the fluorescent X-ray analysis method is a) nondestructive analysis method, the product after pickling can be checked and analyzed on-site as it is. B) Since high vacuum is not required, the steel strip is continuously annealed with acid. It has the advantages that online analysis can be performed in the washing production line, c) the analysis time is short, etc., and by making use of these advantages, online analysis can be performed to enable highly efficient production of sound products. Is.

X-ray tube 7 for irradiating the strip surface with X-rays
For this, one using Pt, Au, W, Rh, etc. as an anticathode and a Be thin film as a window is used. The dispersive crystal 8 functions as a "diffraction grating" for extracting only the X-rays having a wavelength unique to Si from the characteristic X-rays having various wavelengths emitted from the strip surface. In the case of an electromagnetic wave having a short wavelength, crystals such as LiF and EDDT (ethylenediamine ditertarate) are used. As the detector 9, a scintillation counter or a proportional coefficient tube is used.

Since the characteristic X-ray of Si (Kα ₁ : 7.125Å) has a relatively long wavelength, the absorption and attenuation by air is relatively large. Therefore, if the distance from the strip surface to the detector 9 is long (for example, 30 cm), most of it is absorbed by the time it enters the detector 9, and the detection sensitivity deteriorates. The following method can be considered as a countermeasure.

(1) As shown in FIG. 1, the X-ray passage is covered with a shield 14 to create a vacuum, or the air is replaced with He or H ₂ gas (a vacuum is generated near the surface of the running strip). Since it is quite difficult, it is realistic to replace the air with He or H ₂ gas). (2) Make the distance between the strip surface and the detector as short as possible (the shorter the distance, the higher the detection sensitivity, but it is necessary to consider the flatness of the strip, etc. so that the dispersive crystal and the detector do not contact the strip). ).

By the way, when the incident angle of the primary X-rays irradiating the surface of the strip 1 is close to a right angle, the X-rays penetrate deeply into the surface of the stainless steel, and more characteristic X-rays are emitted from the inside. It becomes difficult to understand the Si concentration in the area close to. Conversely, the smaller the angle of incidence of the primary X-ray on the strip surface, the more sensitively the Si concentration near the surface can be analyzed. Therefore, the incident angle of the primary X-ray is preferably 10 ° or less. However, when the flatness of the strip is poor, the irradiation position of the X-rays is likely to move, and the detection of the characteristic X-rays becomes unstable. Therefore, it may be necessary to increase the incident angle to some extent.

Information on the Si concentration obtained by the fluorescent X-ray analysis is sent to the control device 11 and compared with the Si concentration in the strip metal which has been analyzed and measured in advance by an appropriate means. And the difference (the value of "[Surface Si%]-[Si% in metal]]"
Is larger than the target value (preferably 0% from the viewpoint of corrosion resistance), the controller 11 gives an instruction to increase the temperature and concentration of the acid in the acid tank 12, and the acid adjusted in this way is used as the acid. It is sent to the tank 2. In this case, it is also possible to take a measure to extend the immersion time by lowering the immersion roll 3 in the acid tank 2.

Even if the above operation is performed, "[surface Si
%] − [Si% in metal] ”does not fall below the target value, an instruction is sent to the Lusner-electrolysis power supply control system 13 to increase the electrolysis current and accelerate the dissolution of the oxide scale.

On the contrary, the above-mentioned "[Surface Si%]-[Si in metal]
%] ”Is smaller than the target value, the temperature or concentration of the acid in the acid tank 12 is lowered, or the dip roll 3 in the acid tank 2 is used.
Is raised to shorten the immersion time.

In the above-mentioned example, the case where the analytical element on the surface of the stainless steel material is Si has been described. However, as described above, the Si concentration layer immediately below the oxidation scale of the annealed stainless steel material is also a Cr-deficient layer, Descaling and pickling conditions can also be controlled by fluorescent X-ray analysis of the Cr concentration of. However, Cr has a disadvantage that the rate of change in surface concentration is smaller than that of Si, but on the other hand, characteristic X-rays (Kα ₁ : 2.2
Since the wavelength of 90Å) is short, it has the advantage that absorption and attenuation by air is small. As described above, it is more desirable to analyze and measure both the Si concentration and the Cr concentration on the surface of the stainless steel including these points and use both of them as indexes.

Next, the analysis value of, for example, the Si concentration on the surface of the stainless steel material by fluorescent X-ray analysis has a strong correlation with the corrosion resistance (pickling degree) of the stainless steel material, and serves as an index for sufficiently evaluating the corrosion resistance (pickling degree). Here is an example of a confirmation test for obtaining.

First, SUS304 cold rolled steel sheet (sheet thickness:
0.5mm) in propane gas combustion atmosphere 1100 ℃ × 1.5
It was annealed for min and used as a test material. Next, a test piece of 50 mm × 100 mm was cut out from the above-mentioned test material, subjected to descaling and pickling by the “Lusner method” and “nitric hydrofluoric acid immersion” under the following conditions, and then under the following conditions. X-ray fluorescence analysis ",
"Rusting test" and "Measurement of pitting potential" were conducted.

Lusner method conditions In 20% Na ₂ SO ₄ at 80 ° C., alternating electrolysis is carried out by repeating anodic electrolysis for 2 seconds and cathodic electrolysis for 1 second at a current density of 80 mA / cm ² . Nitrofluoric acid immersion condition Immerse in 8% HNO ₃ -0.7% HF aqueous solution at 50 ° C.

Fluorescent X-ray analysis conditions An X-ray canometer "VXQ-150 type" manufactured by Shimadzu Corporation was used. The X-ray tube is "OEG75H-" manufactured by MACHLETT of the United States.
2A type ", and the tube voltage was 40 kV and the tube current was 60 mA. The detector of Si is Ne Exatron.

Scoring test conditions: An artificial seawater solution was sprayed and deposited on a test piece in a fixed amount, and then dried by irradiation with an infrared lamp (however, the amount of the artificial seawater salt deposited after drying was 0.5 ± 0.05 mg). Adjust to be / cm ^2. ) Then, put this in a thermo-hygrostat (temperature 30 ° C, relative humidity 50%), hold it for 3 days to rust, and after washing and drying, evaluate the degree of rust by the rating number specified in JIS H8502. (The larger the number from 0 to 10, the less rusting).

Measurement conditions of pitting potential : Measured in 3.5% NaCl aqueous solution at 30 ° C. according to JIS G0577. However, the passivation treatment of the test piece and the polishing before the measurement were not performed, and the measurement was performed with the pickled skin as it was. The measurement was repeated twice for each condition.

The test results are shown in Table 1. Although Table 1 shows an example in which the treatment time of the Lusner method is only 48 seconds, the dissolution of the oxide scale is completed by the treatment of 48 seconds, and even if the treatment time is further extended, This is because it was confirmed by a separate experiment that nothing dissolved.

[0039]

[Table 1]

As is clear from the results shown in Table 1,
The quantitative analysis value of Si by fluorescent X-ray analysis became smaller as the nitric-hydrofluoric acid immersion time became longer, and became 0.55% after 30 seconds (Here, the Si content of this sample is 0.55%. Has been confirmed by a wet analysis method separately performed).
This is because the Si-enriched layer that is not soaked with nitric-hydrofluoric acid (immersion time 0 sec.) Has a Si-enriched layer formed by annealing on the surface, so that the Si-enriched layer is dissolved, so that the Si analysis value is It is considered that when it gradually decreases and the Si-enriched layer is completely dissolved, it becomes 0.55%, which is the same as the Si content of the ingot.

Needless to say, Table 1
The X-ray fluorescence analysis value of Si indicates the average content of Si from the surface of the stainless steel plate to a certain depth, and the content of Si on the outermost surface should be higher. Of course, the shallower this "depth" is, the more accurately the detection can be performed, but the change in the Si concentration in the depth direction could be detected sufficiently even under the fluorescent X-ray analysis conditions used this time.

By the way, the rating number of the rusting test shows a large value as the nitric acid / hydrofluoric acid immersion time becomes longer, and shows that the rusting is reduced, which is due to the tendency that the Si concentration on the surface of the stainless steel plate is lowered and that it is extremely low. Is in good agreement with. Further, the pitting corrosion potential becomes a noble value as the nitric-hydrofluoric acid immersion time becomes longer, indicating that pitting corrosion does not easily occur, which also corresponds well to the tendency for the Si concentration on the surface of the stainless steel plate to decrease.

From these results, it can be confirmed that the Si analysis value on the surface of the stainless steel sheet by the fluorescent X-ray analysis has a strong correlation with the rust resistance and the pitting corrosion resistance, and can be sufficiently used as an index for evaluating the corrosion resistance. Further, in the test based on the Cr analysis value on the surface of the stainless steel sheet, it is needless to say that almost the same result was obtained although the magnitude of the analysis value (concentration) was opposite.

[0044]

[Summary of Effects] As described above, according to the present invention, a stainless steel material that has undergone heat treatment such as annealing is accurately scaled and / or pickled, and is excellent in corrosion resistance and has uniform quality. It is possible to provide industrially useful effects such as the ability to provide a stable quality control system for descaling and pickling.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an example of a quality control system for descaling and pickling stainless steel according to the present invention.

FIG. 2 is a graph showing the results of investigating the “change in the depth direction” of the chemical composition in the surface portion of the SUS304 annealed material.

[Explanation of symbols]

 1 Strip 2 Acid Tank 3 Immersion Roll 4 Water Wash Tank 5 Drying Device 6 Tension Roll 7 X-ray Tube 8 Spectroscopic Crystal 9 Detector 10 Fluorescent X-ray Analyzer 11 Controller 12 Acid Tank 13 Lusner-Electrolytic Power Supply Control System 14 shields

Claims (3)

[Claims] 1. A quality control system for stainless steel material that has been descaled or pickled after heat treatment, comprising:
"Si concentration" or "ratio of Si concentration on steel surface and Si concentration in metal"
A fluorescent X-ray analyzer for detecting the presence of a corrosion-resistant material, and a controller for adjusting the conditions of descaling or pickling based on the detection result. . 2. A quality control system for stainless steel material that has been descaled or pickled after heat treatment, comprising:
"Cr concentration" or "ratio of Cr concentration on steel surface to Cr concentration in metal"
A fluorescent X-ray analyzer for detecting the presence of a corrosion-resistant material, and a controller for adjusting the conditions of descaling or pickling based on the detection result. . 3. A quality control system for stainless steel material that has been descaled or pickled after heat treatment, comprising:
X-ray fluorescence analyzer for detecting "Si and Cr concentration" or "ratio of Si and Cr concentration on steel surface to Si and Cr concentration in metal", and descaling or acid removal based on this detection result A corrosion resistance quality control system for stainless steel, comprising a control device for adjusting washing conditions.