JPH09296285A - Method for pickling austenitic stainless hot rolled steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a mirror finished face good in glass as the the surface of a product after cold rolling annealing using an inexpensive sulfuric acid in pickling for a hot rolled steel strip in the production of an austenitic stainless steel thin sheet. SOLUTION: This pickling method is the one in which, at the time of pickling an austenitic stainless steel hot rolled strip in a soln. of sulfuric acid of 50 to 600g/l sulfuric acid concn. heated at 50 to 120 deg.C, in the soln. of sulfuric acid, the steel strip is used as the cathode, and power is supplied to the space between the anode provided opposite to the cathode at 30 to 200A/dm<2> current density, then, the pickling is started.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pickling method for descaling the surface oxide scale of austenitic stainless steel hot rolled steel strip, which is capable of performing stable pickling and providing good surface properties after pickling. The present invention relates to a pickling method for a hot rolled sheet.

[0002]

2. Description of the Related Art A method of manufacturing a stainless steel plate and a steel strip is as follows. A continuously cast slab is hot-rolled to manufacture a hot-rolled plate, and annealing is omitted or annealed to descale and cold-rolled to a product sheet thickness, and then finished. It consists of a step of solution treatment followed by pickling, or bright annealing and temper rolling.

As a method for removing the scale formed on the surface of an austenitic stainless steel hot-rolled sheet which has undergone heat history such as annealing in addition to hot rolling, generally, after mechanical descaling by shot blasting, A method of pickling by pickling with a mixed acid of nitric acid-hydrofluoric acid is widely adopted. In the case of austenitic stainless steel, the commonly used acid concentrations are 5 to 15% nitric acid and 1 hydrofluoric acid.
~ 5% ("Stainless Steel Handbook" 1973 edition P.
842).

The pickling method using a mixed acid of nitric acid and hydrofluoric acid has a good surface finish, but it generates NOx gas and requires expensive equipment for treating this gas. Further, this mixed acid is greatly affected by the ions dissolved in the solution as the pickling progresses, and the capacity becomes insufficient due to an increase in the dissolved ions, so that the liquid needs to be frequently renewed. Therefore, there is a drawback that liquid cost and waste acid treatment are expensive. Furthermore, an expensive acid-resistant material peculiar to nitric acid-hydrofluoric acid is required as the material for the pickling tank, and it is an expensive pickling method from the viewpoint of equipment installation costs and maintenance costs.

On the other hand, the sulfuric acid pickling method is applied to ferritic stainless steel. This sulfuric acid pickling method does not generate NOx gas and requires no gas treatment equipment. Further, the effect of dissolved ions in the solution accompanying the progress of pickling is small, and liquid cost and waste acid treatment are inexpensive. Furthermore, an inexpensive material is sufficient as an acid resistant material for the pickling tank. From this, there has been a strong demand for a pickling method which uses sulfuric acid as a basic pickling solution and has excellent pickling performance as an alternative to the nitric acid-hydrofluoric acid mixed acid.

As a method of pickling an austenitic stainless hot-rolled steel strip using such a sulfuric acid solution, for example, in Japanese Patent Publication No. 61-25783, a sulfuric acid treatment is performed after shot blasting, and a sulfuric acid first solution is used as the pickling main solution. There is a method in which pickling with a diiron-sulfuric acid solution is performed, and as a post-treatment, pickling is performed in nitric acid or hydrochloric acid and then nitric acid. However, even if this pickling method is applied, the operating concentration range of ferric sulfate-sulfuric acid is narrow, so that it often goes out of the range, making pickling impossible. Even when the pickling is sufficiently performed, pitting corrosion of a diameter of about 3 to 7 μm and a similar depth occurs on the surface after the pickling, and the product plate is not damaged by subsequent cold rolling and has a flaw. There was a problem that it remained and became a macroscopically opaque steel plate.

Also, an electrolytic pickling method in which electricity is applied during pickling has been studied. For example, in JP-A-1-316500, a hot-rolled steel strip is used as a cathode in a solution containing sulfuric acid and Cr ⁶⁺ ions. A method of obtaining high-speed dissolution by applying electricity and pickling is disclosed. However, in this pickling method, it is necessary to continuously apply a large current during pickling, and since an expensive compound is added to contain Cr ⁶⁺ ions in the solution, the equipment cost for energizing is increased. There is a problem that the operating cost required for electric power, solution, etc. is increased, and development of an inexpensive pickling method has been desired.

[0008]

SUMMARY OF THE INVENTION The present invention relates to an inexpensive pickling method for hot-rolled austenitic stainless steel sheets by using a sulfuric acid solution to stabilize pickling operations and improve the surface properties after pickling. It is an object of the present invention to make it possible to manufacture a high-quality austenitic stainless steel thin plate with low acid solution cost and equipment cost.

[0009]

Means for Solving the Problems The inventors of the present invention have made earnest studies on a method of pickling an austenitic stainless hot-rolled steel strip using a sulfuric acid solution, and as a result, pickling does not proceed during sulfuric acid pickling. That is, pitting corrosion that occurs on the surface after sulfuric acid pickling is caused by Fe ions from ferric sulfate, Fe ions, Cr ions, and Ni that are concentrated in the sulfuric acid solution as pickling progresses.
It was determined that the ions were affected and that the passivation caused by these ions was the cause. Furthermore, by electrically energizing the steel strip as the cathode, the active solution can be easily started, the pickling time for descaling can be shortened, the surface roughness after pickling can be reduced, and pitting corrosion can be prevented. They have found that they can be prevented, and have completed the present invention based on their findings.

That is, according to the present invention, the hot rolled steel strip obtained by hot rolling an austenitic stainless steel slab has a sulfuric acid concentration of 50 to 600 g / l (liter) and a temperature of 50 to 600 g / l.
A steel strip is used as a cathode in a sulfuric acid solution at 120 ° C., and a current density between the anode and the anode provided facing the cathode is 30 to 200 A /
A method for pickling an austenitic stainless hot rolled steel strip is characterized in that dm ² is energized to start pickling.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been found that when Fe pickling, Cr ions and Ni ions increase in the pickling solution as pickling progresses, pickling does not start just by immersing the mechanically descaled steel sheet. Therefore, Cr content is 17%
An experiment was carried out by bringing the following ferritic stainless steels or ordinary steels into which active materials were previously dissolved and bringing them into contact with each other to start pickling. Shot blast average particle size 400μ
m, projection speed 50 m / sec, projection amount per unit area 7
The steel strip after mechanical descaling at 5 kg / m ² is 300 g / l (liter) H ₂ SO ₄ and the total amount of Fe, Cr, and Ni ions dissolved is 50 g / l.
It was pickled in a 90 ° C. solution designated as 1.

As a result, the pickling time required for descaling is 270 seconds or more, and the surface roughness after pickling is R.
The maximum roughness was more than 50 μm, which was more than twice the value of the conventional pickled surface with nitric acid-hydrofluoric acid. Furthermore, the pickled surface had pitting corrosion with a diameter of 3 to 7 μm and a similar depth. After cold rolling and bright annealing and temper rolling, the product surface has a scab-like flaw and a macroscopically opaque surface, which gives a glossy mirror surface obtained by conventional nitric hydrofluoric acid pickling. The surface was markedly different from.

Such inactivation of pickling, prolongation of pickling time, surface roughness and pitting corrosion after pickling are all problems. The method of applying current to start the active dissolution of pickling was examined, and an experiment was conducted to examine the correspondence between the current density and the frequency of active dissolution. As shown in FIG. 1, the current density is 30 A
/ Dm ² or more, active dissolution occurs stably. Furthermore, as shown in FIGS. 2 and 3, the result of the surface roughness at the time of completion of descaling and the time of completion of descaling is 30 A / dm.
^By applying a current of ² or more for 3 seconds or more, the pickling time required for descaling can be shortened to 90 seconds or less, and the surface roughness can be reduced to Rmax 25 μm or less, and even 20 μm or less due to higher current density and longer energization time. Can be adjusted. Further, no pitting corrosion was observed on the surface to which current of 30 A / dm ² or more was applied.

The surface roughness thus obtained is 25 μm.
The pickled sheet of m or less was cold-rolled from 3 mm to 0.7 mm in thickness, bright-annealed and temper-rolled, and then the surface properties of the product were examined. As a result, it was found that a glossy mirror surface can be manufactured. A pickled plate having a surface roughness of more than 25 μm, which has been subjected to immersion pickling or pickling at a current density of less than 30 A / dm ² and having pitting corrosion on the surface, was similarly cold-rolled, bright-annealed and temper-rolled. However, it became a cloudy surface macroscopically, and became a problem surface as a product surface.

By applying a cathodic current in the initial stage of such pickling, active dissolution easily occurs, the pickling time is shortened, the surface roughness is reduced, and the occurrence of pitting corrosion can be prevented. This is considered to be because the surface potential of the steel strip is changed to the active dissolution potential in a solution containing easy Fe ions, Cr ions and Ni ions, and the surface of the steel strip is uniformly active dissolved at this potential.

Next, the reason for limitation will be described. Regarding the cathode current density, as shown in FIG. 1, FIG. 2 and FIG. 3, when a cathode current of 30 A / dm ² or more is applied, the active dissolution generation rate is stabilized, the pickling time is shortened, and the surface roughness after pickling and Can reduce pitting corrosion. These effects act effectively higher the current density, but, 200A / dm ² by weight, the rapid rise of liquid temperature occurs, 30~200A / dm ² and the surface temperature control Mutsukashiku also becomes excessive pickling conditions And The energization method is obtained by connecting a direct current power supply between a pair of electrodes with a length of 300 to 500 mm and flowing a current through indirect energization with the same electrode width as the coil width provided in parallel with the steel plate facing the solution. To be Further, it can also be obtained by directly supplying electricity to the surface of the steel strip by using one of the electrodes with a conductor roll. Particularly in the case of indirect energization, it is necessary to limit the current density of the electrode whose surface is the steel strip to be 80 A / dm ² or less. This is because when an anodic current is passed through the steel strip exceeding 80 A / dm ² , the steel strip at the current-carrying position of the anode has a function of shifting from the active dissolution potential to the passivation potential to stop the active dissolution, and stable pickling can be performed. This is because it cannot be obtained.

[0017] For the H ₂ SO ₄ solution, if secure the pickling initial current density Fe ions, Cr ions, the pickling without problems H ₂ SO ₄ solution containing a large amount of Ni ions proceeds, H ₂ SO ₄ Effective in a concentration range of 50 to 600 g / l. Increasing the concentration of H ₂ SO ₄ is effective for high-speed pickling and is effective at 50 g / l or more, but becomes saturated when it exceeds 600 g / l. Further, the stable operation can be performed in the range of 100 g / l or more and 400 g / l or less.
It should be noted that the pickling temperature is within the range up to the boiling point (120 ° C.), so that the higher the temperature, the higher the speed of the pickling is possible, but if it is less than 50 ° C., the dissolving ability becomes small, so it was set to 50 to 120 ° C.

Also in this pickling method, the effect of mechanical descaling is important, and when it is insufficient, it adversely affects the start of pickling, so sufficient mechanical descaling is required. Shot blasting, high pressure water spraying containing iron sand particles, grinding with an abrasive brush, tension leveler, etc. are used.

There is a black pickling product on the surface immediately after the H ₂ SO ₄ pickling.
50 g / l (liter) HNO ₃ and 10-400 g
It is necessary to immerse or spray in a mixed solution of 1 / l (liter) HCl and 50 to 100 ° C. to whiten it. If the amount of HNO ₃ is less than 10 g / l, it is insufficient for whitening.
When it exceeds 0 g / l, the surface roughness is increased. Also HNO ₃
The amount of HCl coexisting with is effective for high-speed whitening and is 10 g /
The effect is exhibited at 1 or more, but the effect is saturated at more than 400 g / l. If the temperature exceeds 100 ° C, the surface becomes rough, and if it is less than 50 ° C, the whitening effect is small. From the viewpoint of high speed whitening and operation, 80 to 90 ° C is desirable.

For whitening, it is also useful to whiten by dipping or spraying in a mixed solution of 50 to 200 g / l (liter) HNO ₃ and 10 to 250 g / l (liter) HF at 40 to 100 ° C. Is. HNO ₃ amount of less than 50 g / l is insufficient for whitening, and 200 g / l
When it exceeds, the surface roughness is increased. The amount of HF coexisting with HNO ₃ is effective for high-speed whitening, and the effect is exhibited at 10 g / l or more, but the effect is saturated when it exceeds 250 g / l. When the temperature exceeds 100 ° C, the surface becomes rough, and 4
If it is less than 0 ° C, the whitening effect is small. From the viewpoint of high-speed whitening and operation, 70 to 90 ° C is desirable.

[0021]

Embodiments of the present invention will be described below. 1200 ° C
The slab of 250mm thickness, 1000mm width, and 6000mm length heated to 30mm thickness is finished by 30mm thickness, rough rolling at 1100 ° C, then 7mm by 3mm thickness at 980 ° C and then 450 ° C. Rolled 18% Cr-8% Ni steel with hot-generated scale on the surface (SUS30
4) is subjected to shot blasting, a high-pressure water spraying method including sand iron particles, a method of grinding with an abrasive grain brush, and mechanical descaling by a tension leveler, and H ₂ SO having different (Fe ion + Cr ion + Ni ion) concentrations.
Immersed in ₄ solution, descaling time after energization pickled pickled pickling initial, surface irregularities, the results of investigating the product surface texture after rolling cold rolled bright-annealed temper FIG
It was shown to. All conditions were 120g after H ₂ SO ₄
/ L (liter) HNO ₃ , 60 g / l (liter) H
After being soaked in Cl, 70 ° C., 20 seconds for whitening, evaluation was performed after pickling.

The pickling time can be shortened by using the steel strip as a cathode and passing a current density of 30 to 200 A / dm ² between the cathode and the anode provided facing the cathode to start pickling. ,
It was confirmed that the surface roughness after pickling was reduced, pitting corrosion was eliminated, and good results were obtained for the product surface properties after cold rolling, bright annealing and temper rolling.

[0023]

[Table 1]

[0024]

INDUSTRIAL APPLICABILITY The present invention enables stable pickling even in inexpensive sulfuric acid pickling and a pickling method for improving the surface quality after pickling when producing an austenitic stainless steel thin plate, and cold rolling annealing is performed. This is what makes it possible to obtain a high-quality mirror surface with high gloss as the surface of the later product plate, and its industrial effect is enormous.

[Brief description of drawings]

FIG. 1 shows a 300 g / l steel strip after mechanical descaling on a hot rolled steel strip of SUS304 with a shot blasting average grain size of 400 μm, a projection speed of 50 m / sec, and a projection amount per unit area of 75 kg / m ^2. Liter) H ₂ S
O ₄ in Fe ions, Cr ions, the penetration amount of Ni ions energized in a solution of 90 ° C. which was 50 g / l, diagram showing the relationship between the current density and the active dissolution incidence.

FIG. 2 is a diagram showing a relationship between a current density, an energization time, and a descaling completion time, which are supplied with electricity in the initial stage of pickling when pickling using a shot blasting and pickling solution under the same conditions as in FIG. The results are shown for pickling in which all active solubilization including less than dm ² was started.

FIG. 3 shows surface roughness immediately after completion of descaling corresponding to FIG. 2 with respect to current density and energization time.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yu Tadokoro No. 1-1 Tobata-cho, Tobata-ku, Kitakyushu, Fukuoka Prefecture Nippon Steel Corporation Yawata Works

Claims (1)

[Claims] 1. A hot rolled steel strip obtained by hot rolling an austenitic stainless steel slab has a sulfuric acid concentration of 50 to 6
A steel strip is used as a cathode in a sulfuric acid solution of 00 g / l (liter) and a temperature of 50 to 120 ° C., and a current density of 30 to 200 A / dm ² is passed between the steel strip and an anode provided facing the cathode to generate an acid. A method for pickling an austenitic stainless hot-rolled steel strip, which comprises starting washing.