KR101795948B1 - Electrolytic pickling method for ferrite-base heat treated cold-rolled stainless steel sheet - Google Patents

Abstract

The present invention relates to a technique of pickling a medium and low chrome ferrite-based stainless steel sheet without a mixed acid depositing process. The medium and low chrome ferrite-based stainless steel sheet, comprises: 0.01 wt% or less of C (excluding 0 wt%); 0.55-0.90 wt% of Si; 0.20-0.40 wt% of Mn; 0.03 wt% or less of P; 0.001 wt% or less of S; 11.0-14.3 wt% of Cr; 0.005-0.01 wt% of N; 0.15-0.25 wt% of Ti; 0.005-0.040 wt% of Al; 0.04-0.08 wt% of Sn; and the remaining consisting of Fe and unavoidable impurities. Provided is a method of pickling a medium and low chrome ferrite-based stainless steel sheet, wherein the method comprises: a step of annealing and heating a medium and low chrome ferrite-based cold-rolled steel sheet satisfying formula (1), 28 <= Si(%) Sn(%) 1000 <= 48; a step of performing neutral salt electrolytic pickling on the annealed and heated cold-rolled steel sheet by using an electrolyte liquid including sodium sulfide as an electrolyte; and a step of performing sulfuric acid electrolytic pickling on the cold-rolled steel sheet after the neutral salt electrolytic pickling using a sulfuric acid electrolyte liquid including sulfuric acid as an electrolyte.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a cold-rolled stainless steel sheet,

The present invention relates to a method for pickling a medium- and low-chrome ferritic stainless steel containing 11 to 14.3% by weight of chromium, and more particularly, to a method for pickling a pickled ferrous stainless steel using only electrolytic pickling of neutral salt electrolysis and sulfuric acid electrolysis will be.

Generally, the pickling of medium-low-chromium ferritic stainless steels can be roughly classified into two types. Firstly, cold rolling, annealing annealing, molten salt immersion and nitric acid electrolysis are carried out in this order. Secondly, cold annealing, annealing, Neutral salt electrolytic pickling, sulfuric acid electrolytic pickling, and mixed acid deposition.

An example of the second method is KR2012-0075344. This patent document relates to a method of immersing and pickling in a mixed acid containing hydrofluoric acid, and since a mixed acid is used, there is a risk due to hydrofluoric acid contained in mixed acid.

Foshan is a very dangerous chemical substance. Five to seven people were killed, eight to ten tons of Foshan leaked, and three thousand people were injured, plants were destroyed, groundwater and drinking water source Contaminated. In addition, two or three liters of FOSHAN were leaked from the FOSC leak in Samsung Electronics (January 27, 2013), killing one person and injuring four. Therefore, in order to prevent similar accidents, possibility of conversion into alternative treatment technology that does not use hydrofluoric acid is examined as much as possible.

In addition, when mixed acid is used, the amount of hydrogen peroxide contained in the mixed acid is very large. The use of such a large amount of hydrogen peroxide is pointed out as a cause of lowering the economical efficiency of the pickling process. Accordingly, a method of operating a heat exchanger and a chiller to reduce hydrogen peroxide consumption has been proposed, and the operating cost due to the operation of the additional facility is increased.

On the other hand, in the case of using a molten salt, the mixing can be omitted but there is a disadvantage that the productivity is limited and the surface is damaged by the reaction between the molten salt at a high temperature and the surface of the strip.

The present invention relates to a new pickling method capable of pickling at high speed while omitting a mixed deposition process involving hydrofluoric acid in the pickling process of the steel surface in the manufacture of medium to low-chrome ferritic stainless steels containing 11 to 14.3% by weight of chromium .

The present invention relates to a pickling method capable of pickling at high speed while omitting a mixed acid deposition process. The present invention relates to a pickling method capable of pickling at high speed, wherein C is 0.01% or less (excluding 0%), 0.55 to 0.90% of Si, 0.20 to 0.40% , P: not more than 0.03%, S: not more than 0.001%, Cr: 11.0 to 14.3%, N: 0.005 to 0.01%, Ti: 0.15 to 0.25%, Al: 0.005 to 0.040%, and Sn: 0.04 to 0.08% , The remainder being Fe and inevitable impurities, and the relationship (1)

28? Si (%) 占 Sn (%) 占 1000? 36 (1)

Annealing the intermediate-low-chromium ferrite-based cold rolled steel sheet; Subjecting the cold-rolled steel sheet subjected to annealing to a neutral salt electrolytic pickling treatment using an electrolyte solution containing sodium sulfate as an electrolyte; And a step of subjecting the neutralized salt electrolytic pickled cold-rolled steel sheet to a sulfuric acid electrolytic pickling treatment using a sulfuric acid electrolytic solution containing sulfuric acid as an electrolyte, thereby yielding a low-chromium ferritic stainless steel cold rolled steel sheet pickling method.

The cold-rolled steel sheet may further contain 0.3 to 0.4% by weight of Nb.

The annealing heat treatment is preferably performed at a temperature of 960 ° C to 1040 ° C for 30 seconds to 2 minutes.

The neutral salt electrolytic pickling and the sulfuric acid electrolytic pickling can be performed by applying a current density of 0.12 A / cm 2 or more and 0.25 A / cm 2 or less for 10 seconds to 90 seconds, respectively.

It is preferable that the temperature of the electrolytic solution containing sodium sulfate as an electrolyte is 70 ° C to 85 ° C and the temperature of the sulfuric acid electrolytic solution containing sulfuric acid as an electrolyte is 40 ° C to 55 ° C.

It is preferable that the neutral salt electrolyte has a concentration of sodium sulfate of 180 to 250 g / L and a sulfuric acid concentration of 60 to 100 g / L of the sulfuric acid electrolytic solution.

According to the present invention, the risk of hydrofluoric acid can be reduced by omitting the mixed acid deposition process using hydrofluoric acid, and the cost reduction effect can be obtained.

Also, it is possible to provide a low-to-medium-chromium ferritic stainless steel cold-rolled steel sheet excellent in acidity and having excellent surface quality after pickling even though the mixed acid deposition process is omitted.

1 is a graph showing the relationship between cracks occurring on the surface after cold rolling according to the product of Si and Sn amount according to the embodiment.
Fig. 2 is a graph showing the relationship between scale area ratios remaining after pickling according to the product of Si and Sn.

The present inventors have devised a method of omitting the acid precipitation process using an acid in the pickling treatment of the intermediate-low-chromium ferritic stainless steel cold-rolled steel sheet and securing excellent surface quality after pickling by electrolytic pickling process alone.

As a result, it is possible to optimize the alloy composition of the low-and-low-chromium ferritic stainless steel cold-rolled steel sheet, and to control not only the thickness of the scale formed on the surface of the cold-rolled steel sheet but also the silicon and pearlite, It has been confirmed that excellent surface characteristics can be obtained after the completion of the present invention.

Hereinafter, the present invention will be described in detail.

A method for pickling a medium-low-chromium ferritic stainless steel cold-rolled steel sheet according to an aspect of the present invention comprises the steps of preparing a low-chromium ferritic stainless steel cold-rolled steel sheet and a step of subjecting the cold-rolled steel sheet subjected to annealing to a neutral salt electrolytic pickling treatment and a sulfuric acid electrolytic pickling treatment .

The intermediate low-chromium ferritic stainless steel cold-rolled steel sheet according to claim 1, wherein the steel sheet comprises 0.01% or less of C, 0.55 to 0.90% of Si, 0.20 to 0.40% of Mn, 0.03% or less of P, It is preferable that the steel sheet be a cold rolled steel sheet comprising 11.0 to 14.3% of Cr, 0.005 to 0.01% of N, 0.15 to 0.25% of Ti, 0.005 to 0.040% of Al, 0.04 to 0.08% of Sn and Fe and unavoidable impurities Do. In addition, the cold-rolled steel sheet may further contain 0.3 to 0.4% of Nb.

Hereinafter, the reasons for limiting the alloy composition and component relationship of the intermediate-low-chromium ferritic stainless steel cold-rolled steel sheet of the present invention will be described in detail. Here, the content of each component means weight% unless otherwise specified.

C: 0.01% or less (excluding 0%)

Carbon (C) is an element which is advantageous for improving the strength of steel, but when added excessively, there is a problem that toughness, corrosion resistance and the like are lowered. Therefore, in the present invention, 0.01% or less desirable.

Si: 0.55 to 0.9%

Silicon (Si) is an element which enables the non-hydrofluoric acid production by deoxidation, high-temperature oxidation resistance, and interaction with Sn when molten steel is produced, and when the content is 0.55% or less, the acidity is lowered. The moldability remarkably decreases. Therefore, as described above, it is preferable that Si contains 0.55% to 0.9%.

Mn: 0.2 to 0.4%

Manganese (Mn) is an effective element for improving the corrosion resistance, and in the present invention, it is preferably contained at 0.2% or more. However, if the content exceeds 0.40%, the generation of Mn fumes is rapidly increased at the time of welding, so that the weldability is deteriorated and the precipitation of MnS is excessively excessive, so that ductility and corrosion resistance are lowered. desirable.

P: not more than 0.03%

Phosphorus (P) is an impurity inevitably contained in the steel, and is an element that causes intergranular corrosion at the time of pickling or deteriorates hot workability. Therefore, it is preferable to control the content to be as low as possible. In the present invention, it is preferable to control the P content to 0.03% or less.

S: not more than 0.005%

Sulfur (S) is an impurity inevitably contained in the steel, and is an element which is segregated in the grain boundaries and is a main cause of inhibiting hot workability. Therefore, it is preferable to control the content to be as low as possible. In the present invention, it is preferable to control the content of S to 0.005% or less.

Cr: 11.0 to 14.3%

Chromium (Cr) is an effective element for improving the corrosion resistance of steel, and classified as stainless steel means Cr content of 11% or more. However, when the content exceeds 14.3%, the Cr-depleted layer is present after pickling because the base metal is not dissolved in the electrolytic pickling process. If the Cr-depleted layer is present, the corrosion resistance is lowered, which is undesirable.

N: 0.005 to 0.01%

Nitrogen (N) is an element that promotes recrystallization by precipitating austenite in a ferrite phase during hot rolling, and it is preferable that it contains 0.005% or more. However, if the content is excessive and exceeds 0.01%, there is a problem that the ductility of the steel is lowered, which is not preferable.

Ti: 0.15 to 0.35%

Titanium (Ti) is an element effective in reducing the amount of solid carbon and nitrogen employed in the steel by fixing carbon and nitrogen, and improving the corrosion resistance of steel. In the present invention, it is preferable that titanium (Ti) is included at 0.15% or more. However, when the content is excessively over 0.35%, not only is the production cost increased, but also surface defects are caused by forming Ti inclusions, which is not preferable.

Al: 0.001 to 0.040%

Aluminum (Al) is a strong deoxidizer, which serves to lower the content of oxygen in the molten steel, and is preferably added in an amount of 0.005% or more. However, if the content exceeds 0.040%, a sleeve defect of the cold-rolled strip occurs due to an increase in nonmetallic inclusions, and the weldability deteriorates, which is not preferable.

Sn: 0.04 to 0.08%

In the present invention, tin (Sn) is an element essential for controlling the scale and oxide thickness, and is not concentrated on the surface during high temperature annealing, and suppresses the surface diffusion of oxidizing elements (e.g., Si, Cr, etc.) It is a valid element. In order to sufficiently obtain such an effect, Sn is preferably added in an amount of 0.04% or more, but if the content exceeds 0.08%, surface cracking due to Sn may occur.

Nb: 0.3 to 0.4%

Niobium (Nb) is an element effective in reducing the amount of solid carbon and nitrogen employed in the steel by fixing carbon and nitrogen, and improving the corrosion resistance of steel. In the present invention, it is preferable that Nb is contained at 0.3% or more. However, if the content is excessive and exceeds 0.4%, not only the production cost increases but also surface defects are caused due to the formation of Nb inclusions.

The other component of the invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

On the other hand, in the low chromium ferritic stainless steel cold-rolled steel sheet of the present invention satisfying the above-described alloy component composition, it is preferable that the content of Si and Sn affecting the scale formation among the above-mentioned constituent compositions satisfy the component relation expressed by the following formula (1).

28? Si (%) 占 Sn (%) 占 1000? 36 (1)

When the above formula 1 is satisfied, pickling can be carried out without generating a crack on the surface of the steel sheet after pickling even with the electrolytic solution process alone without including the mixed acid deposition step. On the other hand, when the relation of Si and Sn according to the above formula (1) is 28 or less, the content of Sn serving as a barrier functioning to prevent oxidation due to migration of Si in the oxidation process is insufficient, The strength is increased, which may cause surface cracking during the rolling process.

As described above, by controlling the alloy composition of the intermediate-low-chromium ferritic stainless steel cold-rolled steel sheet, the scale can be easily removed by neutral salt and sulfuric acid electrolytic pickling alone, and the residual scale after pickling can be zeroed, An excellent surface quality can be secured for the steel sheet.

As described above, it is preferable that the low chromium ferritic stainless steel cold-rolled steel sheet of the present invention in which the composition of the alloy component is optimized is subjected to annealing heat treatment in a certain temperature range, followed by electrolytic pickling treatment.

The annealing heat treatment is preferably performed at a temperature in the range of 960 to 1040 캜. If the temperature is lower than 960 占 폚 in the annealing heat treatment, recrystallization may be inhibited or the ferrite in a compressed form in the rolling direction may exist as it is, which may deteriorate ductility characteristics. Therefore, it is preferable to conduct the annealing heat treatment at 960 DEG C or higher. The upper limit of the annealing heat treatment temperature is not particularly limited, but it is preferable that the upper limit of the annealing heat treatment temperature does not exceed 1040 占 폚 in consideration of energy efficiency, manufacturing cost, and the like.

At this time, it is preferable that the annealing heat treatment is performed within the above temperature range within a holding time of 2 minutes, preferably 30 seconds to 2 minutes. The target working speed (line speed, m / min) is 125 / thickness (mm), which is changed according to the thickness of the material. For example, when a material of 1.25 mm is heat-treated, heat treatment is performed at a speed of 100 mpm by 125 / 1.25. Therefore, when the length of the heat treatment furnace is 100 m, the time required for the heat treatment is one minute. When the material thickness is increased to 2.5 mm, the time required for the heat treatment to be 50 mpm by 125 / 2.5 corresponds to 2 minutes. From this relationship, the heat treatment time can be increased up to 2 minutes.

After completing the annealing heat treatment in accordance with the above-described procedure, electrolytic pickling is carried out to conduct pickling treatment. At this time, the electrolytic pickling includes neutral salt electrolytic pickling through which the neutral salt electrolytic bath is passed, and sulfuric acid electrolytic pickling through the sulfate electrolytic bath. The electrolytic pickling is a pickling process for removing the oxide scale existing on the surface of the intermediate-low-chrome ferritic stainless steel cold-rolled steel sheet.

Preferably, the neutral salt electrolytic acid is carried out using an electrolytic solution containing sodium sulfate as an electrolyte, and the neutral salt electrolytic bath containing the electrolytic solution preferably includes a temperature of 70 to 85 ° C and a sodium sulfate concentration of 180 to 250 g / L Do. When the temperature is lower than 70 ° C, the reaction rate is too slow to pick up the product at high speed. When the temperature is higher than 85 ° C, the stability of the reaction tank is lowered. When the concentration of the neutral salt is lower than 180 g / L, the conductivity is lowered to lower the pickling efficiency. When the concentration exceeds 250 g / L, the same efficiency is exhibited.

The neutral salt electrolytic pickling can be performed by immersing the low-chrome ferritic stainless steel cold-rolled steel sheet of the present invention in the thus-controlled neutral salt electrolytic bath, and then applying a current with a current density of 0.12 A / cm 2 or more. Preferably, the current density is 0.12 A / cm 2 or more.

If the amount of current applied to the electrolytic cell during the neutral salt electrolytic pickling is less than 0.12 A / cm 2, there is a problem that the surface potential capable of dissolving the scale formed in the surface layer portion of the cold rolled steel sheet is not formed and the effect of neutral salt electrolytic pickling is insufficient. Although the upper limit of the amount of the current is not particularly limited, it is preferable that the upper limit of the amount of current is 0.25 A / cm 2 or less.

At this time, the current density can be applied within a few tens of seconds, preferably 10 to 90 seconds. Considering the pickling time allowed in the high-speed electrolytic pickling process, in which the working speed (TV, (thickness) x (velocity)) is usually 100 or more, it should be performed for 10 to 90 seconds as described above. .

After completion of the neutral salt electrolytic pickling, sulfuric acid electrolytic pickling is carried out. The sulfuric acid electrolytic acid is preferably carried out using a sulfuric acid electrolytic solution containing sulfuric acid as an electrolyte, and the sulfuric acid electrolytic bath containing the electrolytic solution preferably satisfies a temperature of 40 to 55 ° C and a sulfuric acid concentration of 60 to 100 g / L. If the temperature is lower than 40 ° C, the reaction rate is slowed down. If the temperature is higher than 55 ° C, the temperature of the reaction tank rises sharply due to the heat of reaction, so that a non-uniform pickling operation can be performed. On the other hand, when the sulfuric acid concentration of the sulfuric acid electrolytic solution is as low as less than 60 g / L, the conductivity is lowered and the pickling efficiency is lowered. When the concentration is more than 100 g / L,

The sulfuric acid electrolytic pickling can be carried out by immersing the neutral salt electrolytic-acid treated intermediate-low-chromium ferritic stainless steel cold-rolled steel sheet of the present invention in the thus controlled sulfuric acid electrolytic bath and applying a current density of 0.12 A / cm 2 or more.

When the amount of current applied to the electrolytic cell during the electrolytic pickling of sulfuric acid is less than 0.12 A / cm 2, there is a problem that the surface formation potential is lowered during electrolytic pickling and the surface quality is lowered. On the other hand, the upper limit of the amount of the current is not particularly limited, but is preferably 0.25 A / cm 2 or less in consideration of damaging the base material in the lower part of the scale.

At this time, the current density can be applied within a few tens of seconds, preferably 10 to 90 seconds. Considering the pickling time allowed in a high-speed electrolytic pickling process in which the working speed (TV, (thickness) x (velocity)) is usually 100 or more, it is preferably performed for 10 to 90 seconds as described above.

Particularly, when the applied current density in the neutral salt electrolytic pickling process does not satisfy the range of the present invention, it is possible to cause insolubility or substrate damage irrespective of the applied current density in the subsequent sulfate electrolysis pickling process, The applied current density during the neutral salt electrolytic pickling and the applied current density during the sulfuric acid electrolytic pickling in the present invention are not limited to the present invention Is satisfied.

The neutral salt electrolytic acid pickling and the sulfuric acid electrolytic acid pickling can be performed continuously or intermittently, and are not particularly limited, but are preferably performed continuously.

On the other hand, in general, the above-mentioned neutral salt electrolytic pickling treatment and the sulfuric acid electrolytic pickling treatment are carried out to carry out the mixed acid deposition step of immersing and passing through the hydrofluoric acid-containing mixed tank. It is omitted. In other words, the low chromium ferritic stainless steel cold rolled steel sheet of the present invention can have excellent surface quality after neutral salt and sulfuric acid electrolytic pickling even if the mixed acid deposition process is omitted.

Example

Hereinafter, the present invention will be described more specifically by way of examples. The following examples are intended to further illustrate the present invention and are not intended to limit the present invention thereby.

Each of the intermediate-low-chromium ferritic stainless steel cold-rolled steel sheets having the composition shown in Table 1 below and cold-rolled to 1.2 mm was annealed at 1000 캜 for 60 seconds, and then a neutral salt electrolytic solution having a sodium sulfate concentration of 230 g / Electrolytic pickling was performed by applying a current density of 0.17 A / cm &lt; 2 &gt; in the neutral salt electrolytic bath.

Then, electrolytic pickling was performed by applying a current density of 0.17 A / cm 2 in a sulfuric acid electrolytic bath containing a sulfuric acid electrolytic solution having a sulfuric acid concentration of 80 g / L and a temperature of 50 ° C.

Table 1 shows the surface crack length and the residual scale amount after pickling of the cold-rolled steel sheet obtained by neutral salt electrolytic pickling and sulfuric acid electrolytic pickling, and the results are shown in Table 1.

The relationship between the surface crack length and the residual scale amount after pickling and the result of Equation 1 are shown in Figs. 1 and 2, respectively. FIG. 1 is a graph showing the surface crack length according to the result of Equation (1), and FIG. 2 shows the residual scale amount according to the result of Equation (1).

[Table 1]

As can be seen from Table 1 and Fig. 1, when the product of Si and Sn according to the formula (1) was 36 (Comparative Examples 1 to 14 and Inventive Examples 1 to 4), surface cracks While the product of Si and Sn exceeded 36, indicating that the surface cracked during the rolling process (Comparative Examples 15 to 22).

On the other hand, as can be seen from Table 1 and Fig. 2, when the product of Si and Sn according to the formula (1) was 28 (Comparative Examples 1 to 14), the residual scale Respectively. On the other hand, it can be seen that the scale does not remain on the surface after pickling as the product of Si and Sn exceeds 28 (Examples 1 to 4 and Comparative Examples 15 to 22).

Specifically, as can be seen from Table 1, in Comparative Example 11, the product of Si and Sn was 16 according to the formula 1, and surface cracking did not occur after cold rolling and annealing pickling. However, 4% were observed. On the other hand, in Inventive Example 3, the product of Si and Sn was 36, and surface cracking did not occur after cold rolling and annealing, and no residual scale was observed.

Claims (6)

C: not more than 0.01% (excluding 0%), Si: 0.55 to 0.90%, Mn: 0.20 to 0.40%, P: not more than 0.03%, S: not more than 0.001%, Cr: 11.0 to 14.3% , 0.005 to 0.01% of Ti, 0.15 to 0.25% of Ti, 0.005 to 0.040% of Al, 0.3 to 0.4% of Nb and 0.04 to 0.08% of Sn, the balance being Fe and unavoidable impurities, )
28? Si (%) 占 Sn (%) 占 1000? 36 (1)
Annealing the intermediate-low-chromium ferrite type cold rolled steel sheet at a temperature of 960 to 1040 占 폚 for 30 seconds to 2 minutes;
The annealed heat-treated cold rolled steel sheet is subjected to heat treatment at a current density of 0.12 A / cm 2 or more and 0.25 A / cm 2 or less for 10 seconds to 90 seconds using an electrolyte having a concentration of 180 to 250 g / L of sodium sulfate as an electrolyte at a temperature of 70 to 85 캜 A neutral salt electrolytic pickling treatment which is carried out by being applied for a predetermined period of time; And
The cold-rolled steel sheet subjected to the neutral salt electrolytic pickling is subjected to a current density of 0.12 A / cm 2 or more and 0.25 A / cm 2 or less for 10 seconds or more using a sulfuric acid electrolytic solution having a concentration of 60 to 100 g / L of sulfuric acid as an electrolyte at 40 to 55 ° C. For a period of 90 seconds,
Wherein the low-chromium ferrite-based stainless steel cold-rolled steel sheet pickling method is carried out by the method.
delete delete delete delete delete

Images