KR20180073915A - Ferritic stainless steel having excellent strength and corrosion resistance to acid and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a ferritic stainless steel having excellent strength and acid resistance and a method of manufacturing the same. An embodiment of the present invention provides a ferritic stainless steel including: 0.1 to 0.2 wt % C, 0.005 to 0.05 wt % N, 0.01 to 0.5 wt % Mn, 12.0 to 19.0 wt % Cr, 0.01 to 0.5 wt % Ni, 0.3 to 1.5 wt % Cu, and the remainder Fe and other unavoidable impurities, wherein the number of carbides having 100 nm diameter or more is 50 to 200 ea / 100 μm2.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a ferritic stainless steel having excellent strength and acid resistance and a method for producing the ferritic stainless steel.

The present invention relates to a ferritic stainless steel and a method for producing the ferritic stainless steel, and more particularly to a ferritic stainless steel excellent in strength and acid resistance and a method for producing the ferritic stainless steel.

Among stainless steels, ferritic stainless steels are widely used for building materials, kitchen containers, household appliances, and automobile exhaust system parts.

Ferritic stainless steels have recently been applied to automotive battery cells. Automotive companies are demanding higher strength and corrosion resistance than conventional ferritic stainless steels to ensure long-term battery performance. I also demand the material of the price.

In order to meet the requirements of such automobile manufacturers, there are methods such as work hardening, solidification of hardening, and precipitation hardening of ferritic stainless steels. However, ferritic stainless steels having no phase transformation have a problem of drastically decreasing workability during hardening, It is difficult to utilize Mo, Nb, etc., which have excellent effects, as expensive elements.

In the past, C was an element that deteriorated the workability of ferritic stainless steels. On the contrary, if a large amount of C is added, the strength of the ferritic stainless steel can be improved due to the precipitation of carbide, and strength and workability can be secured simultaneously if only a certain degree of ductility is secured due to recent development of processing technology.

However, even if a large amount of C is added, if hot rolled at a high temperature or the rolling reduction is low and the coiling temperature is high, carbides can not be precipitated in the deformed microstructure and become coarse, which makes it difficult to miniaturize the grain and secure a desired strength.

Japanese Laid-Open Patent Publication No. 2006-183081

Embodiments of the present invention provide an excellent ferritic stainless steel capable of improving the strength and acid resistance by controlling the alloy components of the ferritic stainless steel to control precipitates and crystal grains of the waste light stainless steels.

In addition, embodiments of the present invention provide a method of manufacturing a ferritic stainless steel capable of improving strength and acid resistance by controlling a slab reheating temperature, a reduction ratio, and a coiling temperature during hot rolling to control precipitates and crystal grains.

The ferritic stainless steel excellent in strength and acid resistance according to an embodiment of the present invention is characterized by containing 0.1 to 0.2% of carbon (C), 0.005 to 0.05% of nitrogen (N), 0.01 to 0.2% of manganese (Mn) (Fe) and other inevitable impurities, and has a diameter of 100 nm or more, preferably 0.5 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.5% The number of carbides per unit area is 50 to 200 ea / 100 탆 ² .

According to an embodiment of the present invention, the average crystal grain diameter may be 10 탆 or less.

Also, according to one embodiment of the present invention, the tensile strength may be 520 MPa or more.

Also, according to one embodiment of the present invention, the elongation percentage may be 20% or more.

Also, according to an embodiment of the present invention, the critical current density I _crit may be 10 mA or less in a 5% sulfuric acid atmosphere.

A method for producing a ferritic stainless steel excellent in strength and acid resistance according to an embodiment of the present invention comprises: 0.1 to 0.2% of carbon (C), 0.005 to 0.05% of nitrogen (N), and manganese (Mn) Ferritic stainless steel comprising 0.01 to 0.5% Cr, 12.0 to 19.0% Ni, 0.01 to 0.5% Cu, 0.3 to 1.5% Cu, and balance of Fe and other unavoidable impurities. Hot rolling a steel slab and cold rolling, and the value of the following formula (1) at hot rolling satisfies 1,000 or less.

15 * RHT / R4 + CT ------ Equation (1)

Here, RHT (° C.) denotes the slab reheating temperature, R4 (%) denotes the reduction rate of the R4 stand of the rough rolling, and CT (° C.) denotes the coiling temperature.

Also, according to an embodiment of the present invention, the value of the formula (1) may satisfy 800 to 1,000.

Also, according to one embodiment of the present invention, RHT is less than 1,250 占 폚, R4 is more than 40%, and CT may be less than 650 占 폚.

According to an embodiment of the present invention, the number of carbides having a diameter of 100 nm or more is 50 to 200 ea / 100 탆 ² and the average crystal grain diameter is 10 탆 or less.

Embodiments of the present invention can improve the strength and acid resistance of a ferritic stainless steel through control of precipitates and crystal grains by controlling an alloy component and a hot rolling condition of a ferritic stainless steel.

1 is a graph for explaining the correlation between hot rolling conditions of a ferritic stainless steel and the number of carbides of a cold-rolled steel sheet.
2 is a photograph of a distribution of precipitates in a ferritic stainless steel cold-rolled steel sheet according to an embodiment of the present invention, taken through a transmission electron microscope (TEM).
3 is a photograph of a distribution of precipitates in a ferritic stainless steel cold-rolled steel sheet according to a comparative example of the present invention, taken through a transmission electron microscope (TEM).
4 is a graph for explaining the correlation between the number of carbides and the tensile strength of a cold-rolled steel sheet of a ferritic stainless steel.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

The ferritic stainless steel excellent in strength and acid resistance according to an embodiment of the present invention is characterized by containing 0.1 to 0.2% of carbon (C), 0.005 to 0.05% of nitrogen (N), 0.01 to 0.2% of manganese (Mn) (Fe), and other inevitable impurities, in an amount of 0.1 to 0.5%, Cr (Cr) 12.0 to 19.0%, Ni (Ni) 0.01 to 0.50%, Cu (Cu)

Carbon (C): 0.1 to 0.2%

The amount of carbon (C) is 0.1 to 0.2%. If the amount of carbon (C) is less than 0.1%, there is a problem that the amount of austenite produced during hot rolling is reduced, the ferrite band structure is not destroyed and remains, thereby increasing the grain size. The strength is lowered to less than 500 MPa. If the amount of carbon (C) is more than 0.2%, the amount of the carbide of the material increases so much that the elongation of the final product is lowered and the carbides fall off, thereby deteriorating the surface quality and corrosion resistance.

Nitrogen (N): 0.005 to 0.05%

The amount of nitrogen (N) is 0.005 to 0.05%. If the amount of nitrogen (N) is less than 0.005%, the manufacturing cost increases due to the increase of the refining time and the shortening of the life of the refractory. Further, the subcooling during casting is low, %, There is a high possibility of occurrence of pinholes due to nitrogen in the slab casting, and the number of Cr ₂ N precipitates per unit area in the final cold rolled product is increased, resulting in a Cr depleted zone formed around the Cr ₂ N precipitate A large number of pits are formed on the surface of the cold-rolled product, thereby deteriorating the surface quality.

Manganese (Mn): 0.01 to 0.5%

The amount of manganese (Mn) is 0.01 to 0.5%. If the amount of manganese (Mn) is less than 0.01%, there is a problem that the refining price is expensive. If the amount of manganese (Mn) exceeds 0.5%, there is a problem that the elongation and corrosion resistance are inferior.

chrome( Cr ): 12.0 to 19.0%

The amount of chromium (Cr) is 12.0 to 19.0%. If the amount of chromium (Cr) is less than 12.0%, there is a problem that corrosion resistance is deteriorated. If the amount of chromium (Cr) is more than 19.0%, the elongation rate is lowered and a hot sticking defect occurs.

nickel( Ni ): 0.01 to 0.50%

The amount of nickel (Ni) is 0.01 to 0.50%. If the amount of nickel (Ni) is less than 0.01%, there is a problem that the refining price becomes expensive. If the amount of nickel (Ni) exceeds 0.5%, the impurities of the material increase and the elongation decreases.

Copper (Cu): 0.3 to 1.5%

The amount of copper (Cu) is 0.3 to 1.5%. If the amount of copper (Cu) is less than 0.3%, the critical current density (I _crit ) exceeds 10 mA in a 5% sulfuric acid atmosphere and sufficient acid resistance can not be ensured. If the amount of copper (Cu) Not only does the price go up too much, but it also results in a drop in hot workability and a decrease in the elongation of the final product.

In order to obtain a desired tensile strength in a final cold-rolled product of a ferritic stainless steel, it is necessary to secure a large number of fine carbides, and the crystal grains are required to be finer.

In the ferritic stainless steel excellent in strength and acid resistance according to an embodiment of the present invention, the number of carbides having a diameter of 100 nm or more is 50 ea / 100 탆 ² or more.

For example, the carbide may be an M ₂₃ C ₆ type carbide-based metal precipitate.

In order to increase the number of carbides per unit area, a sufficient deformed structure should be formed in the hot rolled material during the hot rolling process. If the deformed structure is not sufficiently formed, it is difficult to increase the amount of carbide because the carbide precipitation site is not sufficient.

In order to form a sufficient deformed structure in the hot rolled material, the slab reheating temperature, rough rolling reduction ratio and hot rolled coil winding temperature should be controlled during the hot rolling process, and the details thereof will be described later.

That is, through controlling the hot rolling process conditions, the number of carbides having a diameter of 100 nm or more can be 50 ea / 100 μm ² or more, and by securing a large number of fine carbides, a tensile strength of 520 MPa or more can be secured. If the above-mentioned process conditions are exceeded, the amount of carbide can not be sufficiently obtained because of the coarse carbide.

For example, when the number of carbides having a diameter of 100 nm or more is less than 50 ea / 100 mu m < ^{2 &} gt ;, the amount of carbide is small and coarsening of crystal grains occurs and tensile strength is lowered.

For example, the ferritic stainless steel may have an average grain diameter of 10 mu m or less.

For example, the ferritic stainless steel according to one embodiment of the present invention may have a tensile strength of 520 MPa or more.

For example, the ferritic stainless steel according to an embodiment of the present invention may have an elongation of 20% or more.

For example, the ferritic stainless steel according to an embodiment of the present invention may have a critical current density I _crit of 10 mA or less in a 5% sulfuric acid atmosphere.

The ferritic stainless steel according to one embodiment of the present invention for producing the ferritic stainless steel may contain 0.1 to 0.2% of carbon (C), 0.005 to 0.05% of nitrogen (N) 0.01 to 0.5% of manganese (Mn), 12.0 to 19.0% of chromium (Cr), 0.01 to 0.5% of nickel (Ni), 0.3 to 1.5% of copper (Cu) And hot rolling the ferrite-based stainless steel slab, wherein the value of the following formula (1) satisfies 1,000 or less upon hot rolling.

15 * RHT / R4 + CT ------ Equation (1)

Here, RHT (° C.) denotes the slab reheating temperature, R4 (%) denotes the reduction rate of the R4 stand of the rough rolling, and CT (° C.) denotes the coiling temperature.

A ferritic stainless steel slab is produced through continuous casting of molten steel containing the above components. Thereafter, the slab is hot-rolled and hot-rolled to produce a hot-rolled coil having a thickness of 2 to 10 mm.

For example, the slab reheating temperature RHT is less than 1,250 DEG C, the rolling reduction R4 of the rough rolling is 40% or more, and the coiling temperature CT is less than 650 DEG C, The rolling conditions are performed so that the value of the above formula (1) satisfies 1,000 or less.

1 is a graph for explaining the correlation between hot rolling conditions of a ferritic stainless steel and the number of carbides of a cold-rolled steel sheet.

Referring to FIG. 1, when the value according to the formula (1) is 1,000 or less, the number of carbides having a diameter of 100 nm or more is 50 ea / 100 μm ² or more.

Even if the carbon content is sufficient, when the hot rolling condition of the formula (1) is exceeded, a sufficient deformed structure is not formed in the hot rolled material and the carbide precipitation site is not sufficiently formed.

In particular, if the coiling temperature is as high as 650 ° C. or higher, coarsening of the precipitate occurs, and the desired number of carbides can not be obtained. As a result, the crystal grains become rough and the desired tensile strength can not be obtained in the final product.

For example, the value of the formula (1) may satisfy 800 to 1,000.

When the value of the above formula (1) is less than 800, the temperature during hot rolling may be too low, resulting in poor plate shape.

The hot-rolled plate is subjected to an annealing process, and the carbide is sufficiently precipitated through the annealing at 700 to 900 ° C in the annealing process. For example, the annealing heat treatment may be performed by a BAF annealing process. After the annealing heat treatment, a cold rolled sheet having a thickness of less than 2 mm is produced through cold rolling and a final heat treatment may be performed through a heat treatment at a temperature of 800 to 900 ° C.

For example, the number of carbides having a diameter of 100 nm or more in the cold-rolled sheet is 50 ea / 100 μm ² or more and the average crystal grain diameter is 10 μm or less.

Hereinafter, the present invention will be described in more detail with reference to examples.

Example

Slabs of inventive steels 1 to 4 and comparative steels 1 to 9 satisfying the following Table 1 were produced through continuous casting and reheated according to the hot rolling conditions of the following Table 2 and then hot rolled to obtain a hot- . Annealing heat treatment at 900 占 폚 was performed in the BAF annealing step. Thereafter, a cold rolled steel sheet having a thickness of 1 mm was prepared by cold rolling, heat treatment at 900 ° C was conducted, and the final product was prepared by surface short ball treatment and acid pickling with sulfuric acid and fruit juice.

C N Mn Cr Ni Cu Inventive Steel 1 0.103 0.014 0.13 14.3 0.11 0.67 Invention river 2 0.171 0.016 0.11 17.2 0.09 0.45 Invention steel 3 0.122 0.006 0.24 16.7 0.13 1.21 Inventive Steel 4 0.125 0.008 0.19 16.5 0.12 1.05 Comparative River 1 0.133 0.012 0.23 17.5 0.15 1.79 Comparative River 2 0.147 0.015 0.24 16.9 0.17 0.14 Comparative Steel 3 0.227 0.022 0.15 17.1 0.21 0.84 Comparative Steel 4 0.232 0.017 0.14 17.6 0.11 0.66 Comparative Steel 5 0.042 0.046 0.21 16.2 0.11 0.12 Comparative Steel 6 0.051 0.042 0.15 15.2 0.13 0.23 Comparative Steel 7 0.047 0.041 0.17 16.9 0.14 0.77 Comparative Steel 8 0.062 0.015 0.16 17.3 0.13 0.81 Comparative Steel 9 0.085 0.015 0.25 18.1 0.15 0.67

Steel grade RHT (° C) CT (° C) R4 (%) 15 * RHT / R4 + CT Example 1 Inventive Steel 1 1,130 550 45 927 Example 2 Invention river 2 1,130 550 45 927 Example 3 Invention steel 3 1,180 550 45 943 Example 4 Inventive Steel 4 1,180 580 45 973 Comparative Example 1 Inventive Steel 1 1,250 550 30 1,175 Comparative Example 2 Invention river 2 1,180 650 30 1,240 Comparative Example 3 Invention steel 3 1,180 580 30 1,170 Comparative Example 4 Inventive Steel 4 1,250 550 40 1,019 Comparative Example 5 Comparative River 1 1,130 550 45 927 Comparative Example 6 Comparative River 2 1,130 580 45 957 Comparative Example 7 Comparative Steel 3 1,180 550 45 943 Comparative Example 8 Comparative Steel 4 1,180 580 45 973 Comparative Example 9 Comparative Steel 5 1,180 650 30 1,240 Comparative Example 10 Comparative Steel 6 1,130 550 45 927 Comparative Example 11 Comparative Steel 7 1,180 550 45 943 Comparative Example 12 Comparative Steel 8 1,180 650 45 1,043 Comparative Example 13 Comparative Steel 9 1,250 650 30 1,275

The number of the carbides having a diameter of 100 nm or more of the final cold-rolled steel sheet per unit area, the average crystal grain diameter, the tensile strength, the elongation, and the critical current density in the 5% sulfuric acid atmosphere were measured and shown in Table 3 below.

A TEM replica was produced for the final cold rolled plate and the number of carbide precipitates per unit area (100 μm ² ) was measured.

The number of carbides (ea / 100 탆 ² ) Average grain diameter (占 퐉) Elongation (%) Tensile Strength (MPa) Critical current density (mA) in 5% sulfuric acid atmosphere Example 1 91 6.8 25.7 529 5.6 Example 2 124 5.9 23.9 531 7.2 Example 3 72 7.4 25.3 542 3.7 Example 4 75 8.1 26.7 537 3.5 Comparative Example 1 32 10.8 27.6 498 8.5 Comparative Example 2 45 11.6 25.3 508 5.9 Comparative Example 3 37 10.5 26.7 515 4.2 Comparative Example 4 41 12.1 27.2 498 4.5 Comparative Example 5 81 6.5 18.8 554 3.1 Comparative Example 6 107 5.4 25.7 527 14.5 Comparative Example 7 227 4.8 18.5 567 6.9 Comparative Example 8 305 5.3 19.2 572 7.5 Comparative Example 9 13 18.1 30.1 457 15.6 Comparative Example 10 19 17.3 29.8 453 13.8 Comparative Example 11 28 20.6 28.9 476 6.3 Comparative Example 12 21 14.1 27.7 481 5.4 Comparative Example 13 26 12.0 26.3 491 4.2

2 is a photograph of a distribution of precipitates in a ferritic stainless steel cold-rolled steel sheet according to an embodiment of the present invention, taken through a transmission electron microscope (TEM). 3 is a photograph of a distribution of precipitates in a ferritic stainless steel cold-rolled steel sheet according to a comparative example of the present invention, taken through a transmission electron microscope (TEM).

FIG. 2 is a photograph of a cold-rolled steel sheet according to Example 2, and FIG. 3 is a photograph of a cold-rolled steel sheet according to Comparative Example 2.

Referring to FIGS. 2 and 3, when the value of 15 * RHT / R4 + CT according to the relational expression concerning the reheating temperature of the slab, the reduction ratio of R4, and the coiling temperature during hot rolling exceeds 1,000 and the carbon content Sufficient amorphous structure is not formed in the hot-rolled material and the carbide precipitation site is not sufficient.

In addition, as in Comparative Example 2, when the coiling temperature is high, coarsening of the precipitate occurs and the desired number of carbides can not be obtained.

As in Comparative Example 5, when the content of copper is excessive, the elongation of the final product is 18.8%, which means that the elongation is decreased. When the content of copper is small as in Comparative Example 6, The density (I _crit ) is 14.5 mA and sufficient acid resistance can not be ensured.

As in Comparative Examples 7 and 8, when the carbon content is excessive, the number of carbides increases and the elongation decreases. As in Comparative Examples 9 to 13, when the content of carbon is small, it is confirmed that the grain size is large and the tensile strength is less than 500 MPa and the strength is lowered.

4 is a graph for explaining the correlation between the number of carbides and the tensile strength of a cold-rolled steel sheet of a ferritic stainless steel.

Referring to FIG. 4, the graphs of the number of carbides and the tensile strength of the cold-rolled steel sheet according to the examples and the comparative examples are shown. As the number of carbides increases, the tensile strength also tends to increase.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will readily obviate modifications and variations within the spirit and scope of the appended claims. It will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims (9)

(Ni): 0.01 to 0.5%, manganese (Mn): 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.2% (Fe) and other unavoidable impurities, and the number of carbides having a diameter of 100 nm or more per unit area of 50 to 200 ea / 100 mu m < ² > Excellent ferritic stainless steel. The method according to claim 1,
Ferritic stainless steel excellent in strength and acid resistance with an average grain diameter of 10 탆 or less. The method according to claim 1,
A ferritic stainless steel having a tensile strength of 520 MPa or more and excellent strength and acid resistance. The method according to claim 1,
A ferritic stainless steel having an elongation of 20% or more and excellent strength and acid resistance. The method according to claim 1,
A ferritic stainless steel excellent in strength and acid resistance with a critical current density (I _crit ) of 10 mA or less in a 5% sulfuric acid atmosphere. (Ni): 0.01 to 0.5%, manganese (Mn): 0.01 to 0.5%, chromium (Cr): 12.0 to 19.0%, nickel (Ni): 0.01 to 0.2% Hot rolling a ferritic stainless steel slab containing 0.5 to 0.5% of copper, 0.3 to 1.5% of copper, remaining iron (Fe) and other unavoidable impurities, and cold rolling,
A process for producing a ferritic stainless steel excellent in strength and acid resistance which satisfies the following formula (1) when hot rolling.
15 * RHT / R4 + CT ------ Equation (1)
Here, RHT (° C.) denotes the slab reheating temperature, R4 (%) denotes the reduction rate of the R4 stand of the rough rolling, and CT (° C.) denotes the coiling temperature. The method according to claim 6,
Wherein the value of the formula (1) is in the range of 800 to 1,000, and the ferrite-based stainless steel has excellent strength and acid resistance. The method according to claim 6,
RHT is less than 1,250 占 폚, R4 is not less than 40%, CT is less than 650 占 폚, and strength and acid resistance are excellent. The method according to claim 6,
The cold-rolled sheet material is characterized in that the number of carbides having a diameter of 100 nm or more per unit area is 50 to 200 ea / 100 占 퐉 ² , and the average crystal grain diameter is 10 占 퐉 or less.

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