KR102120695B1 - Ferritic stainless steel excellent in pickling property - Google Patents

Abstract

The present invention is to provide a ferritic stainless steel having excellent pickling properties that can reduce the generation of holes on the surface of the steel generated in the pickling process by suppressing the generation of internal oxides. Ferritic stainless steel having excellent pickling properties according to an embodiment of the present invention is weight %, C: 0.02% or less, Si: 0.5 to 0.6%, Mn: 0.4 to 0.6%, P: 0.03% or less, S: 0.005 % Or less, Ti: 0.15 to 0.25%, Cr: 17.0 to 26.0%, Ni: 0.6% or less, Nb: 0.1 to 0.6%, N: 0.05% or less, the rest include Fe and unavoidable impurities, and the following formula (1 ).
2.14 ≤ Si/Ti≤3.0---Equation (1)

Description

Ferritic stainless steel with excellent pickling properties{FERRITIC STAINLESS STEEL EXCELLENT IN PICKLING PROPERTY}

The present invention relates to a ferritic stainless steel having excellent pickling ability, and more particularly, to a ferritic stainless steel having excellent pickling ability to suppress the generation of internal oxides to reduce the occurrence of holes on the surface of the steel generated during pickling. will be.

In general, ferritic stainless steels are classified into low-chromium ferritic stainless steels and high-chromium ferritic stainless steels according to the chromium content. Usually, the case where the chromium content is 11 to 14% by weight is called low-chromium ferritic stainless steel, and the case where the chromium content is 17 to 26% by weight is called high-chromium ferrite stainless steel.

Since the properties of the scale formed during annealing heat treatment change depending on the chromium content, the pickling method according to the chromium content must be performed differently. In general, in the case of low chromium ferrite stainless steel, the scale is thickened during annealing heat treatment, and in the case of high chromium ferrite stainless steel, the thickness of the scale is relatively thin compared to low chrome ferrite stainless steel.

In particular, internal oxidation is the diffusion of oxygen into the alloy to form alloy elements and precipitates. For the internal oxide, the change in free energy for oxidation of the solute element must be less than the change in free energy for oxidation of the base metal. In addition, the internal oxidation occurs when the base metal is highly soluble in oxygen and easily diffuses so that oxygen and the base metal can react, and the concentration of the solute element is low, so that external oxidation does not occur.

In the case where Ti is included in the alloying component, there is a problem in that internal oxides are formed and holes are formed on the surface of the steel after pickling.

The present invention is to provide a ferritic stainless steel having excellent pickling properties that can suppress the formation of internal oxides and thereby suppress the generation of holes formed by the internal oxides after pickling.

Ferritic stainless steel having excellent pickling properties according to an embodiment of the present invention is weight %, C: 0.02% or less, Si: 0.5 to 0.6%, Mn: 0.4 to 0.6%, P: 0.03% or less, S: 0.005 % Or less, Ti: 0.15 to 0.25%, Cr: 17.0 to 26.0%, Ni: 0.6% or less, Nb: 0.1 to 0.6%, N: 0.05% or less, the rest include Fe and unavoidable impurities, and the following formula (1 ). 2.14 ≤ Si/Ti≤3.0---Equation (1)

In addition, according to an embodiment of the present invention, Al: 0.06% or less may be further included.

In addition, according to an embodiment of the present invention, the depth of the hole formed on the surface. It may be less than 0.15㎛.

According to the ferritic stainless steel having excellent pickling properties according to an embodiment of the present invention, the generation of internal oxides can be suppressed to suppress the generation of holes generated after pickling, thereby improving the pickling quality.

1 is a photograph of the surface after pickling of the steel according to Comparative Example 12 using an optical microscope.
2 is a photograph taken by cross-section of the steel according to Comparative Example 12 using an electron microscope.
3 is a photograph taken by using an optical microscope on the surface after pickling of the steel according to Example 1 of the present invention.
FIG. 4 is a photograph of a cross section after pickling of a steel according to Example 1 of the present invention using an electron microscope.
FIG. 5 is a photograph obtained by analyzing the scale generated after heat treatment of the steel according to Comparative Example 12 using an electron microscope.
FIG. 6 is a photograph obtained by analyzing the scale generated after heat treatment of the steel according to Example 1 of the present invention using an electron microscope.
7 is a photograph of the Si oxide layer generated after heat-treating the steel according to Comparative Example 12 of the present invention using an electron microscope.
8 is a photograph showing a Si oxide layer formed after heat treatment of the steel according to Example 1 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 to sufficiently convey the spirit of the present invention to those of ordinary skill in the art. The present invention is not limited only to the embodiments presented herein, but may be embodied in other forms. In order to clarify the present invention, the drawings may omit the illustration of parts irrelevant to the description, and the size of components may be exaggerated to help understanding.

Ferritic stainless steel having excellent pickling properties according to an embodiment of the present invention is weight %, C: 0.02% or less, Si: 0.5 to 0.6%, Mn: 0.4 to 0.5%, P: 0.03% or less, S: 0.005 % Or less, Cr: 17.0 to 26.0%, Ni: 0.6% or less, Nb: 0.1 to 0.6%, N: 0.05% or less, the rest contain Fe and unavoidable impurities, and satisfy the following formula (1).

2.14 ≤ Si/Ti≤3.0---Equation (1)

Here, Si and Ti mean the content (% by weight) of each element.

In addition, according to an embodiment of the present invention, Al: 0.06% or less may be further included.

Hereinafter, the role and content of each component included in the ferritic stainless steel of the present invention will be described as follows, and% for the following components means weight percent.

The content of C (carbon) is 0.02% or less.

C is an element that greatly affects the strength of the steel material, and when the content is excessive, the strength of the steel material is excessively increased, and thus the ductility is reduced, so it is limited to 0.02%. Accordingly, according to an embodiment of the present invention, the content of C is 0.02% or less.

The content of Si (silicon) is 0.5 to 0.6%.

Si is an element added for deoxidation and ferrite stabilization of molten steel during steelmaking. In the present invention, 0.5% or more is added to form a continuous Si oxide between the outer layer scale and the base material. However, when Si is excessively added, ductility decreases and moldability decreases, so the upper limit is limited to 0.6%. Accordingly, according to an embodiment of the present invention, the content of Si is 0.5 to 0.6%.

The content of Ti (titanium) is 0.15 to 0.25%.

Ti is an element that preferentially combines with C and N to form a precipitate that suppresses corrosion resistance. If Ti is less than 0.15%, the amount required for bonding with C and N dissolved in the material is insufficient, and thus, sensitization may occur. Therefore, in the present invention, Ti is added at least 0.15%. On the contrary, when Ti is added in excess of 0.25%, the formation of internal oxides becomes easy and causes an occurrence of holes on the steel surface after pickling. Accordingly, according to an embodiment of the present invention, the content of Ti is set to 0.15 to 0.25%.

According to an embodiment of the present invention, the content of Si and Ti is controlled by the following equation (1).

2.14 ≤ Si/Ti≤3.0---Equation (1)

When Si/Ti is less than 2.14, Ti is excessively included to generate internal oxide due to Ti. Accordingly, after pickling, holes are formed at a predetermined depth or more on the surface of the steel. When Si/Ti exceeds 3.0, ductility and formability may deteriorate.

Thus, by controlling the contents of Si and Ti, it is possible to suppress the internal oxide generated by Ti and to form a continuous Si oxide layer at the interface between the scale and the base material to suppress the internal oxidation formed in the steel.

The content of Mn (manganese) is 0.4 to 0.6%.

Mn is an element that forms a uniform scale in the surface layer portion during heat treatment. Therefore, in the present invention, 0.4% or more is added. However, when Mn is added in excess of 0.6%, since the Mn-Cr spinel oxide may be thickly formed on the outer layer of the surface of the steel, the upper limit is limited to 0.5%. Accordingly, according to an embodiment of the present invention, the content of Mn is 0.4 to 0.6%.

The content of P (phosphorus) is 0.03% or less.

P is an inevitably contained impurity in steel and is an element that causes grain boundary corrosion during pickling or inhibits hot workability. Therefore, it is desirable to control the P content as low as possible. However, since it is inevitable to be added during the manufacturing process, the upper limit is set to 0.03% in the present invention.

The content of S (sulfur) is 0.005% or less.

S is an inevitably contained impurity in steel and is an element that segregates at grain boundaries and inhibits hot workability. Therefore, it is desirable to control the content of S as low as possible. However, since it is inevitable to be added during the manufacturing process, the upper limit is 0.005% in the present invention.

The content of Cr (chromium) is 17.0 to 26.0%.

Cr is an effective element for improving corrosion resistance of steel. In the present invention, it is limited to high Cr ferritic stainless steel in the range of 17.0 to 26.0%.

The content of Ni (nickel) is 0.6% or less.

Ni may be treated as an impurity in ferritic stainless steel, but may be partially incorporated during scrap melting.

The content of Nb (niobium) is 0.1 to 0.6%.

Nb is an element that preferentially combines with C and N to form a precipitate that suppresses corrosion resistance. When Nb is added at less than 0.1%, the Nb employed in the material is small, and the high temperature strength of the material is lowered. Conversely, when Nb is added in excess of 0.6%, the oxide layer thickens at the interface during annealing, resulting in poor acidity. Accordingly, according to an embodiment of the present invention, the content of Nb is 0.1 to 0.6%.

The content of N (nitrogen) is 0.05% or less.

N is an element that serves to promote recrystallization by depositing austenite during hot rolling. However, according to an embodiment of the present invention, the content of N is limited to 0.05% or less because the ductility of steel is reduced when the content is excessive.

Al (aluminum) may further include 0.06% or less.

Al is a powerful deoxidizer and is an element that plays a role in lowering the oxygen content in molten steel. However, the excessive addition causes a decrease in room temperature ductility, so the upper limit is made 0.06% or less, and it is not necessary to contain it.

Ferritic stainless steel according to an embodiment of the present invention is manufactured by the following method.

In weight percent, C: 0.02% or less, Si: 0.5 to 0.6%, Mn: 0.4 to 0.6%, P: 0.03% or less, S: 0.005% or less, Ti: 0.15 to 0.25%, Cr: 17.0 to 26.0%, Ni: 0.6% or less, Nb: 0.5 to 0.6%, N: 0.05% or less, the remainder contains Fe and unavoidable impurities, and a slab having a thickness of 200 to 220 mm that satisfies the following formula (1) is prepared. Is hot rolled at 5-8 mm at the hot rolling temperature. Thereafter, hot rolling annealing is performed at 1020 to 1040°C, and cold rolling is performed to a thickness of 0.8 to 2 mm. Thereafter, cold rolling annealing is performed at 1020 to 1040°C. Then, pickling is performed.

2.14 ≤ Si/Ti≤3.0---Equation (1)

Hereinafter, the present invention will be described in detail through examples, but the following examples are only intended to illustrate the present invention in more detail, and the scope of the present invention is not limited to these examples.

Example

A slab having the composition of the following [Table 1] was prepared, and the produced slab was hot rolled and hot rolled annealed, then cold rolled and cold rolled and annealed at 1,050°C for 2 minutes and 30 seconds to obtain a 2.0mm cold rolled annealed steel sheet. After that, pickling is performed. The depth of the hole formed in the pickling process of the cold-rolled annealed steel sheet was measured using an electron microscope after polishing the section. [Table 2] is a table describing the depth of holes formed in the pickling process.

Division C SSi Mn SS Ni Cr Nb Ti N Al Si/Ti Comparative Example 1 0.008 0.42 0.25 <0.003 0.15 17.9 0.48 0.29 0.0096 0.056 1.45 Comparative Example 2 0.009 0.52 0.52 <0.003 0.15 18 0.51 0.27 0.0097 0.057 1.93 Comparative Example 3 0.008 0.43 0.24 <0.003 0.16 17.9 0.48 0.26 0.01 0.048 1.65 Comparative Example 4 0.008 0.31 0.27 <0.003 0.15 17.9 0.53 0.28 0.01 0.06 1.11 Comparative Example 5 0.008 0.21 0.5 <0.003 0.15 17.8 0.48 0.26 0.0097 0.046 0.81 Comparative Example 6 0.008 0.42 0.51 <0.003 0.15 17.9 0.5 0.27 0.01 0.048 1.56 Comparative Example 7 0.009 0.43 0.5 <0.003 0.15 17.9 0.48 0.26 0.0099 0.05 1.65 Comparative Example 8 0.009 0.15 0.24 <0.003 0.17 18.1 0.49 0.23 0.0096 0.048 0.65 Comparative Example 9 0.008 0.1 0.23 <0.003 0.15 17.8 0.5 0.24 0.0098 0.046 0.42 Comparative Example 10 0.008 0.05 0.26 <0.003 0.15 17.9 0.5 0.26 0.0096 0.055 0.19 Comparative Example 11 0.008 0.2 0.26 <0.003 0.16 17.9 0.5 0.2 0.0097 0.046 1.00 Comparative Example 12 0.009 0.36 0.49 <0.003 0.15 17.9 0.5 0.21 0.0098 0.053 1.71 Example 1 0.008 0.55 0.5 <0.003 0.15 18 0.5 0.25 0.0099 0.054 2.20 Example 2 0.008 0.6 0.45 <0.003 0.15 17.8 0.5 0.27 0.011 0.052 2.22 Example 3 0.008 0.5 0.51 <0.003 0.15 17.9 0.5 0.22 0.0093 0.053 2.27

division Hole depth
(㎛) Comparative Example 1 0.63 Comparative Example 2 0.52 Comparative Example 3 0.61 Comparative Example 4 0.65 Comparative Example 5 0.71 Comparative Example 6 0.62 Comparative Example 7 0.60 Comparative Example 8 0.75 Comparative Example 9 0.8 Comparative Example 10 0.64 Comparative Example 11 0.69 Comparative Example 12 0.58 Example 1 0.15 Example 2 0.13 Example 3 0.12

Examples 1 to 3 are cases where the Si/Ti ratio is 2.20, 2.22, and 2.27, and the depths of the holes are 0.15 μm, 0.13 μm, and 0.12 μm, respectively. According to the embodiment of the present invention, since the depth of the hole is shallow, it does not cause uneven reflection on the surface.

In Comparative Examples 1 to 12, when the ratio of Si/Ti was less than 2.14, the depth of the hole was 0.52 μm or more, and thus it was confirmed that the depth of the hole was formed deeper than that of the embodiment of the present invention. The depth of the hole causes non-uniform reflection of the steel surface, making it difficult to use as an exterior material.

FIG. 1 is a photograph of the surface after pickling of the steel according to Comparative Example 12 using an optical microscope, and FIG. 2 is a photograph of a cross section after pickling of the steel according to Comparative Example 12 using an electron microscope. Figure 3 is a picture taken using an optical microscope on the surface after pickling of the steel according to Example 1 of the present invention, Figure 4 is a picture taken using an optical microscope on the surface after pickling of the steel according to Example 1 of the present invention to be.

As can be seen in FIGS. 1 to 4, in Comparative Example 12, it was confirmed that the hole was deeply digged due to pickling, but in Example 1, it was confirmed that the hole was not deeply formed even after pickling.

FIG. 5 is a photograph obtained by analyzing the scale generated after heat treatment of the steel according to Comparative Example 12 using an electron microscope, and FIG. 6 shows an electron microscope of the scale generated after heat treatment of the steel according to Example 1 of the present invention. It is a picture analyzed using. 7 is a photograph showing a Si oxide layer formed after heat treatment of the steel according to Comparative Example 12. 8 is a photograph showing a Si oxide layer formed after heat treatment of the steel according to Example 1 of the present invention.

In the case of Figure 5 it can be seen that the internal oxide is generated due to Ti after heat treatment. In addition, it can be seen from FIG. 7 that the Si oxide layer was discontinuously formed. On the contrary, in the case of FIG. 6 according to the embodiment of the present invention, it was confirmed that a hole was not formed on the surface of the steel, and the Si oxide layer was continuously formed in FIG. 8. Accordingly, it can be confirmed that a Si oxide layer was formed by controlling the ratio of Si/Ti to prevent Ti oxide from being formed, thereby preventing formation of holes on the surface of the steel after pickling.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and a person skilled in the art does not depart from the concept and scope of the following claims. It will be understood that various modifications and variations are possible.

Claims (3)

In weight percent, C: 0.02% or less, Si: 0.5 to 0.6%, Mn: 0.4 to 0.6%, P: 0.03% or less, S: 0.005% or less, Ti: 0.15 to 0.25%, Cr: 17.0 to 26.0%, Ni: 0.6% or less, Nb: 0.1 to 0.6%, N: 0.05% or less, Al: 0.06% or less, the rest contains Fe and unavoidable impurities,
The depth of the hole formed on the surface is 0.15 μm or less,
Ferritic stainless steel with excellent pickling properties satisfying the following formula (1).
2.14 ≤ Si/Ti≤3.0---Equation (1)
(Here, Si and Ti mean the content (% by weight) of each element.)


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