JPH0949092A - Highly efficient pickling method for austenitic stainless steel plate - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To produce a γ-stainless steel thick steel plate, the solution heat treatment after hot rolling is omitted, or the high temperature of the pickling solution is used in high-speed pickling when light solution heat treatment is carried out. A high-speed pickling method that requires no equipment and has a low equipment cost. SOLUTION: A thick steel plate having a thickness of 4 mm or more after hot-rolling γ-stainless steel or after solution heat treatment for a short time is subjected to ordinary mechanical descaling to obtain a HNO concentration of 50 to 200 g / l.
When pickling using a low temperature nitric hydrofluoric acid solution at 20 to 60 ° C consisting of a mixture of ₃ and HF at a concentration of 20 to 200g / l, immediately before pickling, at a temperature of the following formula or above, and at 150 ° C. The steel sheet is preheated below, and the pickling solution near the surface of the steel sheet is pickled by adjusting the pickling solution in the vicinity of the steel sheet surface at a high temperature of 70 ° C. or more for 30 seconds or more during pickling. Preheated temperature of pickled steel sheet (℃) ≧ [70 × (A + B / 5) − (B / 5 ×
C)] / A A: Steel plate thickness (cm) B: Liquid amount per unit area of steel plate (l / m ² · min) C: Temperature of nitric hydrofluoric acid solution (° C)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thick steel sheet of austenitic stainless steel represented by 18Cr-8Ni stainless steel (hereinafter referred to as 18Cr-8Ni type austenitic stainless steel). The present invention relates to a pickling method for removing scale after hot rolling or solution heat treatment at a high speed and at a low equipment cost for manufacturing stainless steel plate.

[0002]

2. Description of the Related Art 18Cr-8Ni type austenitic stainless steel is hot-rolled, air-cooled and then heated to a temperature of 1000.degree.
It is manufactured by a process of holding for a long time in the heat treatment furnace held as described above, performing recrystallization of the processed structure and adjusting the grain size thereof, and at the same time performing a heat treatment of melting and solidifying the carbide. This solution heat treatment has been carried out in order to prevent the problem that chromium carbide or the like precipitates at the grain boundaries, which significantly deteriorates the corrosion resistance of this portion, and consequently makes stainless steel easily corroded.

As described above, the manufacturing method by the solution heat treatment in which the material is cooled to room temperature after hot rolling and heated to a high temperature again, enormous heat energy and a long time are consumed, and there is a problem in productivity. Due to such a problem, recently, as a manufacturing method in which the solution heat treatment after hot rolling is omitted, JP-A-55-10 is used.
No. 7729, JP-A-60-26619, JP-A-60-197817, etc., quenching from a high temperature after hot rolling to omit the solution heat treatment or to shorten the time. A technology for producing austenitic stainless steel sheets that is becoming more and more popular has been developed.

When the application rate of the method of eliminating or reducing the solution heat treatment is increased, the productivity is limited by the solution heat treatment in the conventional solution heat treatment-pickling process of the hot rolled steel sheet. However, the descaling rate in pickling has become a limitation, and the high-speed pickling and pickling method for hot-rolled stainless steel and heat-treated materials is an important issue. Conventionally, for thick steel plates with a thickness of 4 to 80 mm, due to such solution treatment heat treatment restrictions, a low temperature nitric hydrofluoric acid pickling solution at 20 to 40 ° C. is used as a pickling condition, and batch pickling is repeated for a sufficient time. However, sufficient examination has not been made on speeding up pickling of thick steel plates. High-speed pickling can be achieved by raising the temperature of the pickling solution, but expensive acid-resistant materials must be used as materials for the pickling tank, which increases initial equipment investment and equipment maintenance costs. For,
It was desired to develop a pickling method with low equipment cost.

[0005]

SUMMARY OF THE INVENTION The present invention is a pickling method in which solution heat treatment after hot rolling is omitted or a light solution heat treatment is performed, and the pickling time is shortened and intergranular corrosion is not caused. Therefore, it is an object of the present invention to provide a pickling method in which the temperature of the pickling solution does not need to be raised and the equipment cost and equipment maintenance cost can be reduced.

[0006]

In order to achieve the above object, the gist of the present invention is to provide a thick steel sheet having a thickness of 4 mm or more as hot-rolled austenitic stainless steel or after solution heat treatment for a short time. 20-60 consisting of a mixture of HNO ₃ with a concentration of 50-200 g / l and HF with a concentration of 20-200 g / l, which has been subjected to normal mechanical descaling.
Using a low-temperature nitric hydrofluoric acid solution at ℃, preheat the steel plate to a temperature above formula (1) and below 150 ℃ just before pickling, and heat the pickling solution near the surface of the steel plate to a high temperature above 70 ℃ during pickling. A high-efficiency pickling method for an austenitic stainless steel plate is characterized by holding for 30 seconds or more and pickling. Preheated temperature of pickled steel sheet (° C) ≧ [70 × (A + B / 5) − (B / 5 × C)] / A (1) where A: thickness of pickled steel sheet ( cm) B: Liquid amount per unit area of steel plate surface (l / m ² · min) C: Temperature of nitric hydrofluoric acid pickling solution (° C)

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have studied the start of pickling of 18Cr-8Ni type austenitic stainless steel sheet in a mechanically descaled steel sheet in a nitric hydrofluoric acid pickling solution as a result of the pickling reaction. It was found that the start had a great influence on the plate thickness. That is, in an immersion pickling experiment in a nitric hydrofluoric acid solution containing 120 g / l HNO ₃ and 50 g / l HF at 50 ° C., all of the plate temperatures were 4 ° C., 30 mm, and 80 mm thick at 20 ° C. Four
The reaction does not start after immersion for 20 seconds for mm thick material, 50 seconds for 30 mm thick material, and 90 seconds for 80 mm thick material. After that, hydrogen is generated and pickling proceeds rapidly, and the acid is picked up 8 minutes after the reaction starts. The washing is finished.

Next, when the steel sheet was preheated to 50 ° C. and immersed in the pickling solution, pickling was started immediately after the immersion, and pickling was completed in 8 minutes at any plate thickness. Further, when the steel sheet was preheated to 90 ° C. and immersed in the pickling solution, the pickling of 80 mm thick material was completed in half the pickling time of 30 mm thick material. Thus, when the steel plate preheating temperature is higher than the pickling solution temperature, the reason why the pickling time becomes shorter as the plate thickness becomes thicker is related to the heat capacity stored in the steel plate, and the thicker the plate thickness, the more remarkable .

Therefore, a steel plate having a thickness of 4 to 80 mm is used as the material to be pickled, and the preheating temperature of the steel plate is changed to 50 to 200 ° C.
Next, the temperature of the nitric hydrofluoric acid pickling solution containing 120 g / l HNO ₃ and 50 g / lHF was set to 40 ° C., and the amount of pickling solution per unit area of the steel plate surface was kept constant at 15 l / m ² · min. The results of examining the conditions under which the washing solution can be kept at a high temperature of 70 ° C. or higher for 30 seconds or longer are shown in FIG. As is clear from the figure, it was found that the thicker the plate to be pickled, the lower temperature preheating was sufficient. In addition, the thickness of the pickled steel sheet, the preheated temperature of the pickled steel sheet, the temperature of the nitric acid hydrofluoric acid pickling solution, and the liquid amount per unit area of the steel sheet surface include the pickling solution near the steel sheet surface at a high temperature of 70 ° C or higher. It can be seen that the following equations are established as the conditions for holding for 30 seconds or more, and sufficient preheating can be performed by controlling the preheating temperature of the pickled steel sheet so as to satisfy these relationships. Preheated temperature of pickled steel sheet (° C) ≧ [70 x (A + B / 5)-
(B / 5 × C)] / A where A: plate thickness of pickled steel plate (cm) B: liquid amount per unit area of steel plate surface (l / m ² · min) C: hydrofluoric acid pickling Solution temperature (℃) When the temperature of the pickling solution near the surface of the steel sheet during preheating pickling is less than 70 ° C, it is insufficient for high-speed pickling, and higher temperatures allow faster pickling, but the upper limit is the normal pickling temperature upper limit. 90 ° C. or lower is desirable. If the temperature exceeds 90 ° C, the surface will be washed with acid. If the holding time is less than 30 seconds, the high-speed pickling is insufficient, and the longer the pickling time is, the faster the pickling can be performed.

Regarding the preheating temperature of the steel sheet, even if it is the same temperature as the pickling solution, it has the effect of shortening the pickling time, but it is 70 when aiming at high-speed pickling to bring the pickling solution near the surface of the steel sheet to 70 ° C. or higher. It is effective at temperatures of ℃ or more and is more effective at higher temperatures, but if it exceeds 150 ℃, the temperature distribution on the surface of the steel sheet becomes non-uniform and the effects are saturated.

With respect to the thickness of the steel sheet, if the thickness is less than 4 mm, the effect of preheating is small, and if it is 4 mm or more, the heat storage amount increases as the thickness increases. If the temperature of nitric hydrofluoric acid, which is a pickling solution, exceeds 60 ° C., an expensive material is required for the material of the pickling tank, and the equipment cost increases significantly. On the other hand, if the temperature is lower than 20 ° C., the temperature raising effect by preheating is extremely reduced.

The amount of liquid per unit area on the surface of the steel sheet requires 3 l / m ² · min or more per surface area for pickling the surface of the steel sheet. Below this, the pickling solution becomes insufficient and uneven pickling occurs. For uniform pickling, the larger the volume, the better.
If the amount of liquid supplied exceeds the amount determined by the steel plate thickness, the preheating temperature and the pickling solution temperature in the above calculation formula, the pickling solution temperature in the vicinity of the steel plate surface during preheating pickling cannot be maintained. Regarding this pickling method, the effect of preheating the steel sheet is more remarkable not only when the pickling is dipping but also when the spray pickling is performed, and the pickling time is significantly shortened.

Further, the inventors of the present invention have found that a solution of a high-speed pickling solution obtained by subjecting a hot-rolled 18Cr-8Ni type austenitic stainless steel hot-rolled sheet or a steel sheet after solution heat treatment for a short time to ordinary mechanical descaling and preheating. As a result of examining the composition, the following facts were clarified. When the steel sheet is preheated, pickling proceeds extremely well even in the HNO ₃ -HF liquid, the HNO ₃ concentration is effective in a low condition of 50 to 200 g / l, and the HF is also effective at 20 to 200 g / l. is there. When the concentration of HNO ₃ exceeds 200 g / l, the pickling completion time becomes long, which promotes intergranular corrosion. HNO ₃ is 5
If it is less than 0 g / l, the pickling rate is low. HF is effective for high-speed pickling and is effective at 20 g / l or more, but 200 g / l
If it exceeds 1, the effect is saturated.

As a result of accelerating the dissolution reaction by such a liquid composition and steel plate preheating, no intergranular corrosion is observed on the surface after pickling. In particular, in the case of a material in which the solution heat treatment is omitted as it is hot rolled, it is more effective in preventing intergranular corrosion when it is rapidly cooled in the range of 850 to 550 ° C. after the hot rolling is completed.

The effect of mechanical descaling is also important in these high-speed pickling methods, and it is effective to apply the conventionally used shot blasting, black leather cold rolling method, high pressure water spraying method containing sand iron particles, etc. In particular, it is effective in lighter application than the conventional one in the case of a material for which solution heat treatment for hot-rolled sheet having a small scale thickness is omitted.

[0016]

EXAMPLES Examples of the present invention will be described. Hot rolling by normal method and solution heat treatment for 5 minutes at 1100 ° C, or as-rolled sheet thickness 4 to 80 mm without solution heat treatment by controlling hot rolling reduction rate and cooling method after hot rolling , Width 500
~ 1500mm, length 1000 ~ 6000mm 18% Cr
-8% Ni type austenitic stainless steel, and 1
Mechanical descaling was performed on a thick steel plate of 7% Cr-12% Ni-2% Mo type austenitic stainless steel by shot blasting or high-pressure water spraying containing sand iron grains under normal conditions. Then, the thick steel plate after mechanical descaling is heated in a pickling line in a pickling line using warm water or a high temperature aqueous solution containing a neutral salt, and further steam to raise the temperature. After heating to the preheating temperature to the center of these steel plates in ~ 5 minutes,
The spray pickling of O ₃ -HF and the dipping pickling with enhanced stirring of HNO ₃ -HF were carried out, the time until the completion of pickling was measured, and the intergranular corrosion of the steel sheet surface after pickling was investigated. As a comparative method, the results of pickling without preheating are shown.

The temperature of the pickling solution near the surface of the steel sheet was measured by spot welding a thermocouple to the surface of the steel sheet and coating with a resinous coating material, and measuring the temperature of the steel sheet surface. Thus, 30 seconds after the start of pickling, the thermocouple display temperature is 70 ° C, 80 ° C, 9 ° C.
When the temperature is 0 ° C, the temperature of the pickling solution near the steel plate surface is 70 ° C, 8
Expressed as 0 ° C and 90 ° C.

Thus, compared with the pickling which requires 5 to 20 minutes without preheating, the pickling solution in the vicinity of the steel plate surface is 70%.
It was found that the pickling was completed in about 2 minutes at 80 ° C. and within 1 minute at 80 ° C. or 90 ° C., and that intergranular corrosion did not occur, and the excellent high-speed pickling method of the present invention was confirmed. .

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

INDUSTRIAL APPLICABILITY The present invention uses a conventional pickling device for low-temperature pickling, which does not require a pickling device using an expensive acid-resistant material when manufacturing an austenitic stainless steel thick steel plate,
By simply controlling the preheating of the steel plate and the amount of pickling solution, the actual pickling temperature can be raised and high-speed pickling processing can be performed, and the equipment cost is low, high productivity and excellent products can be provided. Therefore, its industrial effect is enormous.

[Brief description of drawings]

FIG. 1 uses a 4 to 80 mm thick plate as a material to be pickled, and changes the steel plate preheating temperature at 50 to 200 ° C. to continue 120
The solution temperature of nitric hydrofluoric acid pickling containing g / lHNO ₃ and 50 g / lHF was set to 40 ° C., and the amount of pickling solution per unit area of the steel plate surface was kept constant at 15 l / m ² · min. The figure which shows the relationship between the plate thickness of the pickled steel sheet and the preheat temperature of the pickled steel sheet which investigated the condition which can hold | maintain the washing | cleaning solution at high temperature of 70 degreeC or more for 30 seconds or more.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hidehiko Sumitomo 1-1 Hideba-cho, Tobata-ku, Kitakyushu, Kitakyushu, Fukuoka (72) Inside the Yawata Works, Nippon Steel Co., Ltd. Machi 1-1 No. 1 Nippon Steel Co., Ltd. Yawata Works (72) Inventor Kenichi Yano 2-15 Edaemitsu 2-chome, Hachimanto-ku, Kitakyushu, Fukuoka Prefecture Mishima Kosan Co., Ltd.

Claims (1)

[Claims] 1. A mechanical steel sheet having a thickness of 4 mm or more as hot-rolled austenitic stainless steel or after solution heat treatment for a short time is subjected to normal mechanical descaling to obtain HNO _{3 at} a concentration of 50 to 200 g / l and a concentration of 20-200g /
A low temperature nitric hydrofluoric acid solution of 20 to 60 ° C. consisting of a mixed solution of HF of 1 is used, and the temperature is equal to or higher than the temperature shown in the formula (1) immediately before the pickling and is 15
A high-efficiency pickling of an austenitic stainless steel plate, characterized in that the steel plate is preheated to 0 ° C. or lower, and the pickling solution in the vicinity of the steel plate surface is held at a high temperature of 70 ° C. or higher for 30 seconds or longer during pickling. Method. Preheated temperature of pickled steel sheet (° C) ≧ [70 × (A + B / 5) − (B / 5 × C)] / A (1) where A: thickness of pickled steel sheet ( cm) B: Liquid amount per unit area of steel plate surface (l / m ² · min) C: Temperature of nitric hydrofluoric acid pickling solution (° C)