JP5089038B2 - A pickling method for hot-rolled steel strip of stainless steel plate, Fe-Ni base alloy or Ni base high alloy. - Google Patents

Description

  The present invention relates to a pickling method for stainless steel, Fe—Ni base alloy and Ni base high alloy having excellent surface quality without using nitric acid or hydrofluoric acid.

Stainless steel plates, Fe-Ni base alloys and Ni base high alloy plates, usually called hot rolled steel strips, are first annealed in an air atmosphere in a hot rolled plate made by hot rolling a cast slab material. The surface scale and internal oxide layer generated at this time are removed through a two-stage pickling process after one or two of the scale breaker and shot blasting that cause cracks on the scale surface. Is obtained.
In this pickling treatment, the former treatment is mainly aimed at removing strong oxide scale on the surface and removing the internal oxide layer, and the latter treatment is generally referred to as finish pickling, and the appearance surface texture The main purpose is to homogenize the passivation film.
In the pickling treatment, sulfuric acid is generally used in the first stage treatment. Furthermore, in the final pickling treatment, a mixed acid of hydrofluoric acid and nitric acid is used.

  However, the method of using sulfuric acid in the previous treatment is, for example, as disclosed in Patent Document 1, particularly when a very strong oxide scale layer is formed on a surface such as a Ni-based alloy, and the thickness thereof is uneven. As a result, the scale layer remains in the previous pickling treatment, and the scale residue cannot be removed even in the final pickling, resulting in uneven glossiness and whiteness, and as a quality defect with poor appearance. The value is greatly impaired.

Moreover, when there is such a quality defect in the material before cold rolling, that is, the remaining oxide scale or unevenness in the hot-rolled steel strip is re-annealed after the cold rolling-2 stage. Even after the final finishing after the pickling process, it may become a quality defect with poor appearance.
A thin plate material having such a defect, that is, a thick strip material, cannot be applied to an application having severe design requirements such as a roof and an exterior material.
In such a case, as in Patent Document 2, a final finishing must be performed by adding a step of removing the scale layer by a mechanical grinding method such as a belt grinder in the step before and after cold rolling, The efficiency is drastically reduced.

In addition, a sol-like retentate consisting mainly of a low-solubility metal hydroxide adheres to the surface of the stainless steel plate, Fe-Ni base alloy and Ni-base high alloy plate during pickling treatment, and the entire surface is adhered to the surface. By clothing, the surface after pickling may turn black, resulting in poor appearance.
In general, this smut is often dealt with by providing a smut removing tank as disclosed in Patent Document 3. However, this method requires a separate pickling tank dedicated to smut removal, which results in a larger facility. In addition, the sol-like smut coated on the surface of the steel plate during the pickling treatment has no acid permeability. Therefore, the steel sheet surface where scale did not dissolve in the previous pickling was dissolved in the next step, unless smut was removed and subjected to finish pickling, unless it was subjected to strong pickling in this finish pickling. An appearance defect due to unevenness is unavoidable.
However, performing strong pickling in finish pickling not only impairs efficiency and stability of operation, but here it is common to use a mixed acid containing nitric acid, so there is an environmental problem It can also promote the generation of NO _X gas.
Japanese Patent Publication No. 3-36891 JP 2000-073192 A JP 11-172499 A

As described above, the descaling ability in the pickling process is limited. In particular, in order to prevent the surface layer scale and scale residue after hot rolling, 10 to 20 μm per side including the base layer portion (= internal oxide layer). The above peeling performance is required. For this reason, the pickling process alone cannot complete the treatment, and a final finishing process must be performed by adding a process of removing the scale layer by a mechanical grinding method, resulting in a problem that the efficiency is extremely reduced.
In addition, regarding the removal of smut generated during pickling, separately providing a smut removal tank not only impairs efficiency, but also causes a new appearance defect due to dissolution unevenness.

In addition, nitric acid and hydrofluoric acid, which have a serious impact on the environment, are often used in pickling solutions that have been used in the past, and the use of dangerous hydrochloric acid in handling is also a problem.
Optimum in the hot rolling-anneal pickling process of stainless steel, Fe-Ni base alloy and Ni base high alloy to reduce or reduce the amount of acid used that has a serious impact on quality and environment It is an object of the present invention to provide a first-stage pickling method.

In order to solve these problems, the inventors have intensively studied and, among the two-stage pickling in the annealing pickling process after hot rolling, the sulfuric acid-chloride salt-Fe ³⁺ in the preceding pickling. -By performing descaling with a pickling solution containing hydrogen peroxide as a main component at a temperature of 70 to 85 ° C, the steel plate, Fe-Ni base alloy and Ni base height are improved in pickling ability and surface quality. The knowledge which obtained the alloy plate was found.

  The present invention eliminates the generation of acid gas that has a serious impact on the environment in the previous pickling step in the pickling step after annealing of austenitic stainless steel, improves the descaling property, and has excellent surface quality. Steel plates and Fe—Ni based alloy plates can be obtained.

A preferred embodiment of the present invention will be described in detail.
Sulfuric acid constituting the pickling solution is an important acid for descaling and surface passivation of stainless steel, Fe—Ni-based alloys and Ni-based high alloys, and the concentration control of sulfuric acid is extremely important. When the amount is less than 2.0 mol / L, scale residue is observed, leading to deterioration of the corrosion resistance of stainless steel, Fe—Ni base alloy and Ni base high alloy. On the other hand, when it exceeds 3.5 mol / L, the surface roughness becomes remarkable and the basic unit of acid is remarkably increased. Therefore, the concentration control of sulfuric acid is preferably 2.0 to 3.5 mol / L, more preferably 2.5 to 3.5 mol / L.

Sodium chloride and magnesium chloride perform some functions as oxidizing agents similar to hydrogen peroxide and Fe ³⁺ described later. Therefore, when the amount of the chloride salt containing one or two kinds of sodium chloride and magnesium chloride is less than 0.2 mol / L, the descaling property is lowered, and when it is less than 0.1 mol / L, a scale residue is observed. If it exceeds 0.8 mol / L, the surface of the steel material is excessively dissolved, and the surface becomes rough, resulting in a decrease in yield. Therefore, the concentration control of chloride is preferably 0.3 to 0.8 mol / L, and more preferably 0.3 to 0.7 mol / L.

Hydrogen peroxide undergoes a reaction represented by the following formula to form an effect as an oxidizing agent, and H ₂ O ₂ + 2H ⁺ + 2Fe ²⁺ → 2Fe ³⁺ + 2H ₂ O (2), Fe ^When the increase of ³⁺ is promoted and 0.30 mol / L or less is added to the pickling tank, the reaction is promoted. Therefore, the addition of hydrogen peroxide is 0.003 or more and 0.30 mol / L or less, more preferably 0.005 or more and 0.20 mol / L or less.

Fe ³⁺ is generated by adding hydrogen peroxide in sulfuric acid, and increases the cathode reaction and promotes the anodic reaction, resulting in an amount of cutting that is approximately doubled. That is, the current rapidly increases in the active region and the natural potential difference becomes 300 mV or more, and as a result, the amount of cutting is increased.
Therefore, the preferable range of the amount of Fe ³⁺ is 10 to 50 g / L, and it is necessary to set the upper limit to 50 g / L in order to prevent the acid from being lost due to the increase of metal ions (Fe ³⁺ ). is there. Moreover, since the effect of the oxidizing agent of Fe ³⁺ itself cannot be obtained, the lower limit is set to 10 g / L. Actually, the total iron concentration is obtained by collecting a sample of the pickling solution. In accordance with this value, sulfuric acid and sodium chloride are added, and the Fe ³⁺ concentration is controlled not to exceed 50 g / L.

In the present invention, sulfuric acid concentration [H ₂ SO ₄ ] (mol / L), hydrogen peroxide concentration [H ₂ O ₂ ] (mol / L), hydrogen peroxide δ wt% {= [H ₂ O ₂ ] / ( 34/10)} and chloride salt / sulfuric acid concentration ratio X {= ([NaCl] + [MgCl ₂ ]) / [H ₂ SO ₄ ]}, and expressed as a stabilization conditional expression (1) during pickling (FIG. 1), the value obtained from this equation is controlled as the control index value CI _H2SO4 (mol / L) for the input of hydrogen peroxide and chloride salt, and this control deviation ΔH ₂ SO ₄ becomes 0.06 or less. In this case, it is preferable that optimum control can be performed and a particularly excellent effect can be exhibited.
(In the above formula, 2.0 ≦ [H ₂ SO ₄ ] ≦ 3.5, 0.003 ≦ [H ₂ O ₂ ] ≦ 0.30, 0.005 ≦ X ≦ 0.4, 10 ≦ [Fe ^{3 +} ] ≦ 50.)

Sodium chloride and magnesium chloride in the pickling solution have an action of weakening the covering power of the smut. As the Fe ³⁺ concentration increases, the natural potential difference rises, and as a result, the activation of sodium chloride and / or magnesium chloride progresses to the excited state from the ground state, and the surface of the steel sheet mainly due to hydroxyl bonding of the smut. It works to weaken the covering power.
By combining the chemical action that weakens the covering power between the smut and the steel sheet surface with the pickling solution flow in the tank and the physical smut removal action by increasing the relative speed of the steel sheet, the residual scale and uneven dissolution Can be suppressed. The relative speed of the pickling liquid flow and the steel plate in this tank is preferably 35 m / min or more, more preferably 40 m / min or more.

In addition, the oxidizing power of Fe ³⁺ in the pickling solution greatly affects the smut removing action, and the natural potential difference increases as the concentration of Fe ³⁺ increases. That is, the natural potential difference in the pickling solution is that Fe ³⁺ is reduced and Fe ²⁺ is increased, and the potential is lowered accordingly. As the natural potential difference decreases, the adhesion of the smut increases, so it is necessary to add sulfuric acid or hydrogen peroxide to increase Fe ³⁺ to keep the natural potential difference stable.
Therefore, a preferable natural potential difference is 300 to 500 mV, and more preferably 350 to 450 mV.
Further, it is more preferable that the natural potential difference in the pickling treatment liquid and the relative speed between the pickling liquid flow in the tank and the steel plate have a relationship as shown in the following formula.

Of the Fe-Ni base alloy plate, stainless steel plate and Ni base high alloy which are the objects of the pickling method of the present invention, a hot rolled material with a thickness of 3.0 mm of SUS304 austenitic stainless steel which is difficult to descal. used. Using a steel strip with scale after continuous annealing at 1100 ° C (plate temperature) for 80 seconds in an annealing furnace, first the previous pickling treatment is performed to check whether there is any remaining scale, and then finish under normal pickling conditions The pickling treatment was performed, and the presence or absence of appearance defects such as glossiness and streaky unevenness was evaluated. The results are shown in [Table 1]. In Examples 1-7, there is no scale residue and glossiness failure, and excellent results are shown.
In addition, the control value of the pickling solution shows the result of the sequential calculation. Two control output values of ΔH ₂ O ₂ representing the amount of H ₂ O ₂ added and ΔClr representing the amount of chloride added are shown. When added, a stable pickling solution composition is obtained. In other words, there is a difference between the pickling solution composition shown in Examples 1 to 7 and the stable pickling solution composition by the amount of the control output value. Regarding Examples 1 to 7, the control output is within the range in which the optimum control is possible, but in Comparative Examples 2 and 4, the sulfuric acid concentration is outside the range in which the optimum control is possible, so there is no control output.

The graph showing the optimal controllable range of a pickling liquid composition. The graph which shows smut removal conditions. It shows that it is preferable to have the relationship of Formula (2) in the relative velocity of the natural potential difference in the pickling treatment liquid and the pickling liquid flow in the tank.

Claims (4)

In the preceding pickling after annealing of the hot rolled steel strip of stainless steel, Fe-Ni base alloy or Ni base high alloy, the temperature is 70 to 50, which is mainly composed of sulfuric acid-Fe ³⁺ -hydrogen peroxide-chloride salt. The composition of the pickling solution at 85 ° C. is as follows: sulfuric acid: 2.0 to 3.5 mol / L, Fe ³⁺ : 10 to 50 g / L, chloride salt containing one or two kinds of sodium chloride and magnesium chloride: 0.2 to 0.8 Stainless steel sheet , characterized in that it is mol / L, and at the same time hydrogen peroxide is added continuously and controlled in the range of 0.003 to 0.3 mol / L, descaling is performed, followed by finish pickling. , Fe-Ni based alloy or Ni Motodakago gold hot pickling method rolled steel strip. The sulfuric acid concentration [H ₂ SO ₄ ] (mol / L), hydrogen peroxide concentration [H ₂ O ₂ ] (mol / L), hydrogen peroxide δ wt% {= [H ₂ O ₂ ] × (34/10)} and chloride salt / sulfuric acid concentration ratio X {= ([NaCl] + [MgCl ₂ ]) / [H ₂ SO ₄ ]} The control index value CI _H2SO4 (mol / L) for introducing hydrogen peroxide and chloride salt is a value obtained from this equation, and the control deviation | ΔH ₂ SO ₄ | calculated from the control index value is 0. It controls so that it may become 06 or less, The pickling method of the hot-rolled steel strip of Claim 1 characterized by the above-mentioned.

CI _H2SO4 (mol / L) =-2.7 · ln (X) + 10.0δ ³ -21.7δ ² + 19.5δ-7.4 (1)

_{ΔH 2 SO 4 = [H 2} SO 4] -CI H2SO4, | ΔH 2 SO 4 | ≦ 0.06

(In the above formula, 2.0 ≦ [H ₂ SO ₄ ] ≦ 3.5, 0.003 ≦ [H ₂ O ₂ ] ≦ 0.30, 0.005 ≦ X ≦ 0.4, 10 ≦ [Fe ^{3 +} ] ≦ 50.) The relative speed of the hot-rolled steel strip of stainless steel plate, Fe-Ni base alloy or Ni-base high alloy and the flow in the tank of the pickling solution is 35 m / min or more, 3 or 3 Pickling method for hot rolled steel strip. Relative speed between hot rolled steel strip of stainless steel plate, Fe-Ni base alloy or Ni base high alloy and pickling liquid in tank: υ ≧ 35 m / min, stainless steel plate, Fe—Ni base alloy or Ni base The hot-rolled steel according to claim 3, wherein the relationship of the natural potential difference between the hot-rolled steel strip of high alloy and the pickling solution: 300≤E≤500mV satisfies the following formula (2). Pickling method of the belt.

υ [m / min] ≧ 515 × e ⁽⁻² × ^{ln (E) +9)} + 0.05 × E−5 (2)

υ [m / min]: Relative speed of pickling solution in tank and steel plate E [mV]: Natural potential difference (300 ≦ E ≦ 500 mV)