KR102255102B1 - Manufacturing method of austenitic stainless steel with improved surface quality - Google Patents

Abstract

Disclosed is a method for manufacturing austenitic stainless steel with improved surface quality. The method comprises the steps of: hot-rolling and annealing a hot-rolled steel sheet including, wt%, C: 0.04 to 0.07%, Si: 0.3 to 0.5%, Mn: 1.0 to 1.5%, Ni: 8.0 to 10.0%, Cr: 17.0 to 19.0%, P: 0.1% or less (excluding 0), S: 0.01% or less (excluding 0), and the remainder of Fe and the inevitable impurities; immersing the hot-rolled and annealed steel sheet in a sulfuric acid solution to perform an AC electrolytic treatment thereon for 20 to 45 seconds; and immersing the treated steel sheet in a mixed acid solution containing nitric acid and hydrofluoric acid for 5 seconds to 15 seconds.

Description

Manufacturing method of austenitic stainless steel with improved surface quality {MANUFACTURING METHOD OF AUSTENITIC STAINLESS STEEL WITH IMPROVED SURFACE QUALITY}

The present invention relates to a method of manufacturing an austenitic stainless steel, and in particular, to a method of manufacturing an austenitic stainless steel capable of improving the surface gloss uniformity of a final cold-rolled steel sheet by optimizing pickling conditions after hot rolling annealing.

In general, STS 304 is a general-purpose austenitic stainless steel widely used in various applications due to its excellent corrosion resistance, workability and weldability.

However, there is a problem in that the surface quality cannot be secured due to the uneven gloss generated on the surface of the STS 304 cold rolled steel sheet. This gloss unevenness is caused by a difference in frequency of surface defects in the width direction of the steel sheet formed after hot rolling annealing and then through the pickling process, and remains after cold rolling. Therefore, there is a need to develop a method for manufacturing an austenitic stainless steel that can secure a uniform surface gloss.

On the other hand, conventionally, it has been proposed a method of diffusing and removing delta-ferrite by reducing the deviation in the width direction of the steel sheet of delta-ferrite, which causes uneven microstructure of the surface, and increasing the heating temperature of the slab before hot rolling. Even in the rolling and annealing process, there is a problem that it is not completely removed and remains unevenly.

Embodiments of the present invention are intended to provide a method of manufacturing an austenitic stainless steel capable of securing a uniform surface gloss even in a final cold-rolled annealed material by preventing local intergranular erosion of a hot-rolled annealed pickling steel sheet.

A method of manufacturing an austenitic stainless steel with improved surface quality according to an embodiment of the present invention is in weight %, C: 0.04 to 0.07%, Si: 0.3 to 0.5%, Mn: 1.0 to 1.5%, Ni: 8.0 to 10.0 %, Cr: 17.0 to 19.0%, P: 0.1% or less (excluding 0), S: 0.01% or less (excluding 0), and the rest is hot rolling annealing a hot-rolled steel sheet containing Fe and inevitable impurities ; Immersing the steel sheet subjected to hot rolling annealing in a sulfuric acid solution and performing alternating current electrolytic treatment for 20 to 45 seconds; And immersing the steel sheet subjected to alternating current electrolysis in a mixed acid solution containing nitric acid and hydrofluoric acid for 5 seconds to 15 seconds.

Further, according to an embodiment of the present invention, the AC electrolytic treatment may be performed by applying a current density ^{of 35 to 45 A/dm 2.}

In addition, according to an embodiment of the present invention, the concentration of the sulfuric acid solution may be 50 to 300 g/L.

Further, according to an embodiment of the present invention, the temperature of the sulfuric acid solution may be 60 to 80°C.

In addition, according to an embodiment of the present invention, the concentration of hydrofluoric acid in the mixed acid solution may be 10 to 50 g/L.

In addition, according to an embodiment of the present invention, the temperature of the mixed acid solution may be 40 to 80°C.

According to an embodiment of the present invention, after hot rolling annealing, the surface quality of the final cold rolled annealed steel sheet can be secured by optimizing the conditions of alternating current electrolysis and mixed acid immersion in a sulfuric acid solution.

1 is a photograph showing the gloss unevenness occurring on the surface of an existing STS 304 cold-rolled steel sheet.
2 is an electron microscope photograph of the microstructure of the white and black portions of the surface of the STS 304 cold-rolled steel sheet.
3 is an electron microscope photograph of an erosion part of a grain boundary (Grian Boundary) of an STS 304 hot-rolled annealed steel sheet.
Fig. 4 is a schematic diagram showing a principle in which non-uniform surface gloss occurs in a final cold-rolled steel sheet due to non-uniform erosion of grain boundaries.
5 is a flow chart showing a pickling process of an austenitic stainless steel hot-rolled annealed steel sheet according to the disclosed embodiment.
6 is an electron microscope photograph of a microstructure of an austenitic hot-rolled annealed pickling steel sheet according to Example 1 and Comparative Example 1. FIG.
7 is a graph showing pickling according to the application time of AC electrolysis and the mixed acid immersion time of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited only to the embodiments presented herein, but may be embodied in other forms. In the drawings, in order to clarify the present invention, portions not related to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.

Throughout the specification, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary. Singular expressions include plural expressions, unless the context clearly makes exceptions.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Typically, in manufacturing a stainless steel sheet, in order to obtain a beautiful surface quality by removing the oxide scale formed on the steel sheet, and to improve the corrosion resistance of the steel sheet, physical descaling such as brush treatment or shotball blasting, sodium sulfate, sulfuric acid or nitric acid Various methods, such as electrolytic descaling using an electrolyte or the like, chemical descaling using a salt bath or mixed acid, are being performed, and these processes are collectively referred to as a'pickling process'.

In this pickling process, a one-stage electrolytic pickling such as electrolytic descaling and a two-stage mixed acid immersion process such as chemical descaling are distinguished. At this time, hydrofluoric acid (HF) contained in the mixed acid solution locally erodes the grain boundaries, which have relatively low corrosion resistance than the inside of the grains, and these eroded parts remain even after cold rolling, reducing the surface quality of the final cold-rolled steel sheet. there is a problem.

FIG. 1 is a photograph showing the gloss unevenness occurring on the surface of an existing STS 304 cold-rolled steel sheet, and FIG. 2 is an electron microscope photograph of the microstructure of the white and black parts of the surface of the STS 304 cold-rolled steel sheet.

Referring to FIG. 2, it can be seen that the microstructure of the white portion is uniform, whereas the microstructure of the black portion has a large amount of defects indicated by arrows.

3 is an electron microscope photograph of the erosion part of the grain boundary (Grian Boundary) of the STS 304 hot-rolled annealed steel sheet, and the erosion part indicated by the arrow in FIG. 3 is generated in the black part of FIG. 2 through the process of cold rolling and cold rolling annealing. It remains as a defect.

Fig. 4 is a schematic diagram showing a principle in which non-uniform surface gloss is generated in a final cold-rolled steel sheet due to non-uniform erosion of grain boundaries. Fig. 4(a) is a cross-sectional view of a steel plate, and Fig. 4(b) is a plan view of the steel plate. Referring to FIG. 4A, it can be seen that grain boundary erosion has occurred on the surface of the steel sheet. The part in which such intergranular erosion is present locally appears in the form of segregation in (b) of FIG. 4 and is recognized as a black band by the naked eye.

Therefore, in order to ensure uniform surface quality in the final cold-rolled annealing material, it is required to induce erosion of grain boundaries uniformly over the entire surface area in the pickling process after hot rolling annealing, or to suppress surface erosion of grain boundaries.

After hot rolling annealing, the present inventors introduced sulfuric acid alternating current electrolytic treatment to suppress local erosion of grain boundaries, while attempting to secure pickling properties of the hot-rolled annealed steel sheet through a mixed acid immersion process later.

The method of manufacturing an austenitic stainless steel with improved surface quality according to an embodiment of the present invention is, in weight%, C: 0.04 to 0.07%, Si: 0.3 to 0.5%, Mn: 1.0 to 1.5%, Ni: 8.0 to 10.0%, Cr: 17.0 to 19.0%, P: 0.1% or less (excluding 0), S: 0.01% or less (excluding 0), and the rest is hot-rolled and annealing a hot-rolled steel sheet containing Fe and inevitable impurities. step; Immersing the steel sheet subjected to hot rolling annealing in a sulfuric acid solution and performing alternating current electrolytic treatment for 20 to 45 seconds; And immersing the alternating current electrolytic-treated steel sheet in a mixed acid solution containing nitric acid and hydrofluoric acid for 5 seconds to 15 seconds.

Hereinafter, the reason for limiting the numerical value of the content of the alloy component in the examples of the present invention will be described. Hereinafter, unless otherwise specified, the unit is% by weight.

The content of C is C: 0.04 to 0.07%.

Carbon (C) is an element that is effective for stabilizing the austenite phase, and can be added by 0.04% or more to secure the yield strength of the austenitic stainless steel. However, if the content is excessive, the upper limit may be limited to 0.07% because it may adversely affect ductility, toughness, and corrosion resistance by inducing grain boundary precipitation of Cr carbide as well as lowering cold workability due to the solid solution strengthening effect.

The content of Si is 0.3 to 0.5%.

Silicon (Si) serves as a deoxidizer during the steelmaking process and is an element effective in improving corrosion resistance, and can be added by 0.3% or more. However, Si is an effective element for stabilizing the ferrite phase, and when excessively added, it promotes the formation of delta ferrite in the casting slab, thereby lowering the hot workability and lowering the ductility/toughness of the steel due to the solid solution strengthening effect. It can be limited.

The content of Mn is 1.0 to 1.5%.

Manganese (Mn) is an austenite-phase stabilizing element added instead of nickel (Ni) in the present invention, and may be added by 1.0% or more in order to suppress the generation of processing organic martensite to improve cold-rollability. However, if the content is excessive, it may reduce the ductility, toughness and corrosion resistance of austenitic stainless steel by forming an excessive amount of S-based inclusions (MnS). It can be limited to 1.5%.

The content of Ni is 8.0 to 10.0%.

Nickel (Ni) is a strong austenite phase stabilizing element, and as its content increases, the austenite phase is stabilized to soften the material and can be added by 8.0% or more to suppress work hardening caused by the occurrence of deformed organic martensite. . However, since Ni is an expensive element, it causes an increase in raw material cost when a large amount is added, so the upper limit can be limited to 10.0%.

The content of Cr is 17.0 to 19.0%.

Although chromium (Cr) is a ferrite stabilizing element, it is effective in suppressing the formation of martensite phase, and as a basic element that secures corrosion resistance required for stainless steel, it can be added by 17% or more. However, if the content is excessive, the manufacturing cost increases, and the formation of delta (δ) ferrite in the slab causes deterioration in hot workability, so the upper limit may be limited to 19.0%.

The content of P is less than 0.1% (excluding 0).

Phosphorus (P) is an impurity that is inevitably contained in steel, and is an element that causes grain boundary corrosion or impairs hot workability, so it is desirable to control its content as low as possible. In the present invention, the upper limit of the P content is managed to be 0.1% or less.

The content of S is not more than 0.01%.

Sulfur (S) is an impurity that is inevitably contained in steel, and is an element that segregates at grain boundaries and is the main cause of impairing hot workability, so it is desirable to control its content as low as possible. In the present invention, the upper limit of the S content is managed to be 0.01% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from the raw material or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.

Hereinafter, in the method of manufacturing an austenitic stainless steel having improved surface quality according to the present invention, a method of pickling after hot rolling annealing will be described in detail.

The austenitic stainless steel containing the above composition can be produced as a cast piece by continuous casting or steel ingot casting, and after performing a series of hot rolling and hot rolling annealing, a final product can be manufactured through a pickling process.

Here, hot rolling and hot rolling annealing may be performed by a conventional method, and are not particularly limited. For example, the slab may be hot rolled at a temperature of 1,100 to 1,200°C, which is a typical rolling temperature, and the hot rolled steel sheet may be hot rolled and annealed at a temperature of 800 to 1,100°C. At this time, hot rolling annealing may be performed for 10 seconds to 10 minutes.

The hot-rolled annealing material is subjected to a pickling process including a sulfuric acid alternating current electrolytic treatment step and a mixed acid immersion step.

5 is a flow chart showing a pickling process of an austenitic stainless steel hot-rolled annealed steel sheet according to the disclosed embodiment.

Referring to FIG. 5, the hot-rolled annealed steel sheet was pickled by a method of immersing in a mixed acid solution containing nitric acid and hydrofluoric acid after hot rolling annealing, followed by immersion in a sulfuric acid solution. At this time, hydrofluoric acid (HF) contained in the mixed acid solution locally erodes the grain boundaries, which have relatively low corrosion resistance than the inside of the grains, and these eroded parts remain even after cold rolling and cause uneven surface gloss, resulting in the final cold-rolled steel sheet. There is a problem of reducing the surface quality of the.

In order to eliminate uneven surface gloss in the final cold-rolled steel sheet, it is necessary to uniformly arrange grain boundary erosion portions in the hot-rolled annealed steel sheet or to prevent erosion of grain boundaries on the surface. In the present invention, in the sulfuric acid immersion process, by applying AC power instead of DC power to simultaneously erode not only the grain boundaries but also the inside of the grains, it was intended to prevent local erosion of the grain boundaries.

Since AC power has a characteristic that the magnitude and direction of voltage changes with time, if the applied current density and time are properly controlled, the pickling effect of the surface of stainless steel is secured while simultaneously eroding the grain inside and grain boundaries of the hot-rolled annealed steel sheet. can do.

In the present invention, all waveforms such as a sine wave, a square wave, a triangle wave, and a sawtooth wave may be applied as the waveform of the AC power source.

In addition, the current of the present invention may be applied with a pulse of 10 or more per minute, and it is preferable that a pulse of 60 (1 Hz) or more is applied. In the case of less than 10 pulses per minute, since the degree of change of current is slow, there is a problem that a passivation film is additionally generated on the surface of the material and erosion does not occur.

According to the disclosed embodiment, the current density may be applied in the range ^{of 35 to 45A/dm 2 in the AC electrolytic treatment.} When the applied current density is ^{less than 35A/dm 2} , the surface potential capable of dissolving the scale formed after hot rolling annealing is not formed, so the pickling effect cannot be secured, and the applied current density is more than ^{45A/dm 2} There is a problem in that side reactions such as oxygen generation reaction or the like or surface erosion due to over-pickling occurs, and the applied current density is preferably controlled to be ^{35 to 45 A/dm 2.}

Further, according to the disclosed embodiment, the alternating current electrolytic treatment may be performed for 20 seconds to 45 seconds. When the AC electrolysis treatment time is less than 20 seconds, the pickling effect of the hot-rolled annealed steel sheet cannot be secured, and when the AC electrolysis treatment time exceeds 45 seconds, the line speed is lowered and process efficiency cannot be ensured.

In the AC electrolysis treatment step, the concentration of the sulfuric acid solution may be 50 to 300 g/L. When the concentration of the sulfuric acid solution is less than 50 g/L, the pickling effect of the hot-rolled annealed steel sheet cannot be secured. On the other hand, when the concentration of the sulfuric acid solution exceeds 300 g/L, the pickling effect is saturated. Therefore, the upper limit of the concentration of the sulfuric acid solution may be limited to 300 g/L in consideration of economical efficiency.

It is preferable that the alternating current electrolytic treatment is performed at a temperature of 60 to 80°C. At this time, when the temperature is less than 60° C., it is difficult to exhibit the effect of pickling, such as removing foreign substances, and it is difficult to secure pickling efficiency. When the temperature exceeds 80° C., there is a problem in loss due to volatilization of sulfuric acid and durability of surrounding structures.

On the other hand, since the pickling effect of the alternating current electrolytic treatment performed in the sulfuric acid solution is not sufficient, a complete pickling effect can be secured through a process of immersion in a mixed acid solution containing nitric acid and hydrofluoric acid.

According to the disclosed embodiment, immersion in the mixed acid solution may be performed for 5 seconds to 15 seconds. When the immersion time is less than 5 seconds, the pickling effect of the hot-rolled annealed steel sheet cannot be ensured, and when the immersion time exceeds 15 seconds, there is a problem that local erosion of the grain boundaries occurs.

The concentration of hydrofluoric acid in the mixed acid solution may be 10 to 50 g/L. Hydrofluoric acid removes the Si oxide from the surface of the steel sheet by dissolving the Si oxide _{and bonding in the form of H 2} SiF _{6 or the like.} At this time, if the concentration of hydrofluoric acid is less than 10g/L, the problem of micro-pickling may occur on the surface of the steel sheet due to insufficient dissolving power for the Si oxide layer.If the concentration of hydrofluoric acid exceeds 50g/L, the erosion rate of the base material increases, resulting in the surface of the steel sheet after pickling. There is a problem of getting rough. In addition, the temperature of the mixed acid solution is preferably maintained in the range of 60 to 80 ℃.

In this way, by optimizing the conditions of alternating current electrolysis and mixed acid immersion in the sulfuric acid solution, it is possible to secure a uniform surface gloss even in the final cold-rolled annealed material that has undergone general cold-rolling and cold-rolling annealing without performing additional processes.

Hereinafter, the present invention will be described in more detail through examples.

For the range of STS 304 steel components distributed on the market, molten steel was manufactured through an electric furnace and AOD process, and the slab was manufactured by continuous casting, heated at 1,250°C for 2 hours, and then hot-rolled. After hot rolling, hot rolling annealing was performed at 1,100°C for 90 seconds.

Subsequently, the hot-rolled annealed material specimens were subjected to a shot ball blast process, followed by a first step sulfuric acid alternating current electrolytic pickling, a brush treatment, and a second step mixed acid immersion pickling under the conditions of Table 1 below.

The sulfuric acid alternating current electrolytic pickling was applied at 65° C. and 40 A/cm _{2 in a} 200 g/L H 2 SO _{4 solution.} Mixed acid immersion Pickling was immersed in a solution of 65°C and a mixture of 100 g/L of nitric acid and 30 g/L of hydrofluoric acid.

The pickling evaluation was evaluated as pass when there was no surface residual scale through the observation of an optical microscope after hot rolling annealing and pickling, and in Table 1 below, the pass was marked as ○ and the failing was indicated as X.

If the grain boundary erosion is clearly distinguished by observation with an electron microscope after hot rolling annealing, the grain boundary erosion was judged as fail, and if the grain boundary erosion was difficult to distinguish in the X2000 magnification of the electron microscope, it was judged as pass because there was almost no grain boundary erosion. In, pass is indicated by ○, and rejection is indicated by X.

Steel grade Sulfuric acid exchange
Delivery time (seconds) Mixed acid immersion time (sec) Pickling Whether grain boundary erosion Example 1 30 10 ○ ○ Example 2 45 10 ○ ○ Example 3 20 10 ○ ○ Example 4 30 5 ○ ○ Example 5 45 5 ○ ○ Comparative Example 1 0 10 X X Comparative Example 2 0 30 X X Comparative Example 3 0 60 ○ X Comparative Example 4 45 30 ○ X Comparative Example 5 60 30 ○ X Comparative Example 6 30 30 ○ X Comparative Example 7 30 60 ○ X Comparative Example 8 45 60 ○ X Comparative Example 9 60 60 ○ X Comparative Example 10 60 10 ○ ○ Comparative Example 11 20 30 ○ X Comparative Example 12 20 60 ○ X

6 is an electron microscope photograph of a microstructure of an austenitic hot-rolled annealed pickling steel sheet according to Example 1 and Comparative Example 1. FIG. Referring to FIG. 6, it can be seen that in Example 1, erosion of grain boundaries occurred relatively less than in Comparative Example 1.

7 is a graph showing pickling according to the application time of AC electrolysis and the mixed acid immersion time of the present invention.

Referring to Figure 7 and Table 1, Examples 1 to 4 satisfies the sulfuric acid alternating current electrolysis treatment time and the mixed acid immersion time ranges defined by the present invention, thereby ensuring sufficient pickling properties while preventing local erosion of grain boundaries Could.

On the other hand, Comparative Examples 1 to 3 are cases of immersion in a mixed acid solution through an immersion process in a sulfuric acid solution without AC electrolysis as in the prior art, and local grain boundary erosion occurred, as well as mixed acid. Except for the case of Comparative Example 3 in which the immersion time was 60 seconds, sufficient pickling properties could not be secured.

Comparative Examples 4 to 9, 11, and 12 were subjected to alternating current electrolytic treatment, but the immersion time in the mixed acid solution exceeded 15 seconds, and local grain boundary erosion occurred.

Comparative Example 10 is a case in which sufficient pickling properties are secured while preventing local erosion of grain boundaries, but the sulfuric acid alternating current electrolysis treatment time is 60 seconds, and the process efficiency is considered as a comparative example.

According to the disclosed embodiment, by optimizing the conditions of alternating current electrolysis and mixed acid immersion in a sulfuric acid solution after hot rolling annealing, local grain boundary erosion of the hot rolled annealing pickling steel sheet can be prevented, thereby ensuring uniform surface gloss even in the final cold rolled annealed material. have.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those of ordinary skill in the art are within the scope of not departing from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (6)

In% by weight, C: 0.04 to 0.07%, Si: 0.3 to 0.5%, Mn: 1.0 to 1.5%, Ni: 8.0 to 10.0%, Cr: 17.0 to 19.0%, P: 0.1% or less (excluding 0), S: Hot rolling annealing a hot-rolled steel sheet containing 0.01% or less (excluding 0), and the rest containing Fe and inevitable impurities;
Immersing the steel sheet subjected to hot rolling annealing in a sulfuric acid solution and performing alternating current electrolytic treatment for 20 to 45 seconds; And
The method of manufacturing an austenitic stainless steel with improved surface quality comprising: immersing the steel sheet subjected to alternating current electrolysis in a mixed acid solution containing nitric acid and hydrofluoric acid for 5 seconds to 15 seconds. The method of claim 1,
The AC electrolytic treatment is a method of manufacturing an austenitic stainless steel with improved surface quality performed by applying a current density of 35 to 45 A/dm ^2. The method of claim 1,
The concentration of the sulfuric acid solution is 50 to 300 g/L, a method for producing an austenitic stainless steel with improved surface quality. The method of claim 1,
The temperature of the sulfuric acid solution is 60 to 80 ℃ method for producing an austenitic stainless steel with improved surface quality. The method of claim 1,
The hydrofluoric acid concentration of the mixed acid solution is 10 to 50 g/L, a method for producing an austenitic stainless steel with improved surface quality. The method of claim 1,
The temperature of the mixed acid solution is 60 to 80 ℃ method for producing an austenitic stainless steel with improved surface quality.

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