KR20190077672A - Ferritic stainless steel excellent in ridging property - Google Patents

Abstract

A ferritic stainless steel which exhibits excellent liquefaction properties by controlling addition of Nb and Ti to form precipitates and solid solution Nb.
The ferritic stainless steel excellent in ridging properties according to one embodiment of the present invention is characterized by containing 0.003 to 0.1% of C, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P, 0.05% or less of S : 0.005% or less, Cr: 10-30%, Ti: 0.01-0.3%, Nb: 0.01-0.6%, Al: 0.01-0.15%, N: 0.003-0.03%, balance Fe and other unavoidable impurities, The following formula (1) is satisfied.
(1) Nb - [3.2 * C + 0.8 * N + 0.5 * (Cr + Fe)] 0.15
Here, Nb, C and N mean the content (wt%) of each element, and Cr and Fe mean the precipitate content (wt%) according to ICP analysis.

Description

FERRITIC STAINLESS STEEL EXCELLENT IN RIDGING PROPERTY "

The present invention relates to a ferritic stainless steel having excellent ridging properties, and more particularly to a ferritic stainless steel exhibiting excellent ridging characteristics by addition of Nb and Ti to control precipitate formation and solid Nb content.

Generally, stainless steel is classified according to chemical composition or metal structure. According to the metal structure, the stainless steel is classified into an austenitic system, a ferritic system, a martensitic system and an ideal system. Among these stainless steels, ferritic stainless steels are mainly used for various kitchen appliances, automobile exhaust system parts, building materials, household appliances and the like because they are excellent in corrosion resistance while adding a small amount of expensive alloying elements. Required steel grade.

However, ferritic stainless steels have a problem of ridging defects, which are surface defects in the form of wrinkles parallel to the rolling direction during molding. The ridging defect not only deteriorates the appearance of the product, but also causes a problem that when the ridging is severe, a polishing process is added after molding to increase the manufacturing time and increase the manufacturing cost. For this reason, in order to expand the use of ferritic stainless steels, it is necessary to improve ridging characteristics and ensure excellent surface quality.

The cause of ridging is rooted in the development of columnar structure in casting. That is, when the main phase having a constant orientation remains without being broken in the rolling or annealing process, it is expressed as a ridging defect due to the width and thickness direction deformation behavior different from the surrounding recrystallized structure at the time of tensile processing. Various attempts have been made to remove the tissue causing ridging to overcome such ridging defects. Attempts have been made to improve the ridging property by improving the equilibrium constant and reducing the fraction of the main phase, or to reduce the ridging by controlling the process parameters such as hot rolling temperature, hot rolling reduction ratio and annealing temperature control during the production process. There is little meaningful research result related to alloy composition control.

Embodiments of the present invention are to provide a ferritic stainless steel material having excellent ridging characteristics by adding Nb and Ti in combination to form precipitates and control the solid content of Nb.

The ferritic stainless steel excellent in ridging properties according to one embodiment of the present invention is characterized by containing 0.003 to 0.1% of C, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P, 0.05% or less of S : 0.005% or less, Cr: 10-30%, Ti: 0.01-0.3%, Nb: 0.01-0.6%, Al: 0.01-0.15%, N: 0.003-0.03%, balance Fe and other unavoidable impurities, The following formula (1) is satisfied.

(One) Nb - [3.2 * C + 0.8 * N + 0.5 * (Cr + Fe)]? 0.15

Here, Nb, C and N mean the content (wt%) of each element, and Cr and Fe mean the precipitate content (wt%) according to ICP analysis.

According to an embodiment of the present invention, the stainless steel may include [Nb, Ti] [C, N] -dependent precipitates in the microstructure and [Nb, Ti] [C, And may include sidewall precipitates.

According to an embodiment of the present invention, the grain size of the [Nb, Ti] [C, N] independent precipitate is 0.01 to 1.0 탆, and the grain size of the TiN crystallized product is 2.0 to 10.0 탆.

According to an embodiment of the present invention, the [Nb, Ti] [C, N] independent precipitation and the [Nb, Ti] [C, And may include any one or more selected.

According to an embodiment of the present invention, the ridging height of the stainless steel may be 15 탆 or less.

According to the embodiment of the present invention, it is possible to provide a ferritic stainless steel excellent in ridging characteristics by merely adding Nb and Ti and controlling alloy element composition.

Further, the ferrite stainless steel according to the embodiment of the present invention may have a ridging height of 15 탆 or less.

FIG. 1 is a transmission electron microscope (TEM) photograph of a TiN crystallization product of Inventive Steel 1 according to an embodiment of the present invention. FIG.
2 is a transmission electron microscope (TEM) photograph showing the carbonitride of Inventive Steel 1 according to an embodiment of the present invention.

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.

Also, when an element is referred to as "comprising ", it means that it can include other elements as well, without departing from the other elements unless specifically stated otherwise.

The singular forms " a " include plural referents unless the context clearly dictates otherwise.

The inventors of the present invention conducted various studies to improve the ridging characteristics of the ferritic stainless steel, and the following findings were obtained.

Generally, a large amount of Nb is added to ferritic stainless steels for home appliances and heat resistance exhaust systems in order to secure the corrosion resistance and high temperature strength of welds. In the case of Nb-containing ferritic stainless steels inevitably, Nb carbonitride (Nb [C, N ]) And Laves Phase precipitates, and these various precipitates and solid solution Nb are the main causes for suppressing recrystallization during hot rolling and annealing annealing of the steel. When the recrystallization is suppressed, a band structure causing ridging is left, resulting in a ridging defect.

On the other hand, when Nb and Ti are added in combination, the Nb and Ti carbonitride precipitates independently precipitated in the ferrite matrix (hereinafter referred to as' [Nb, Ti] [C, N] Nb and Ti carbonitride precipitates (hereinafter referred to as '[Nb, Ti] [C, N] -dependent precipitates') precipitated with coarse TiN as nuclei. [Nb, Ti] [C, N] independent precipitation and [Nb, Ti] [C, N] Among these, [Nb, Ti] [C, N] -dependent precipitates do not significantly affect recrystallization inhibition as compared with [Nb, Ti] [C, N] independent precipitates. That is, when Nb and Ti are added in combination, the amount of [Nb, Ti] [C, N] independent precipitate decreases and the inhibition of recrystallization can be reduced.

Therefore, as one means for improving the ridging of the existing Nb-added ferritic stainless steels, Nb and Ti are added together as much as possible, and as much Nb and Ti carbonitride as possible are added to the [Nb, Ti] C, N] -dependent precipitates so as to reduce the number of [N, Ti] [C, N] independent precipitates.

The ferritic stainless steel excellent in ridging properties according to one embodiment of the present invention is characterized by containing 0.003 to 0.1% of C, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P, 0.05% or less of S : 0.005% or less, 10 to 30% of Cr, 0.01 to 0.3% of Ti, 0.01 to 0.6% of Nb, 0.01 to 0.15% of Al, 0.003 to 0.03% of N and balance Fe and other unavoidable impurities.

Hereinafter, the reasons for limiting the numerical values of the alloy element content in the examples of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.003 to 0.1%.

C is an element that greatly affects the strength of the steel. When the content is excessive, the strength of the steel is excessively increased to deteriorate the ductility. However, if the content is too low, the strength is lowered, and the lower limit can be limited to 0.003%.

The content of Si is 0.01 to 2.0%.

Si is an element added for deoxidation of molten steel during steelmaking and stabilization of ferrite. In the present invention, 0.01% or more Si is added. However, if the content is excessive, the material is hardened and the ductility of the steel is lowered.

The content of Mn is 0.01 to 1.5%.

Mn is an element effective for improving the corrosion resistance. In the present invention, 0.01% or more is added, and more preferably 0.1% or more is added. However, if the content is excessive, the occurrence of Mn-based fumes is increased so that the weldability is deteriorated and the ductility of the steel is deteriorated due to the formation of excessive MnS precipitates, which is limited to 1.5% or less, more preferably 1.0% do.

The content of P is 0.05% or less.

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, so that it is preferable to control the content as low as possible. In the present invention, the upper limit of the P content is controlled to 0.05%.

The content of S is 0.005% or less.

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, the upper limit of the S content is controlled to 0.005%.

The Cr content is 10 to 30%.

Cr is an element effective for improving the corrosion resistance of steel. In the present invention, Cr is added in an amount of 10% or more. However, if the content is excessive, not only the manufacturing cost is increased but also the intergranular corrosion occurs, which is limited to 30% or less

The content of Ti is 0.01 to 0.3%.

Ti is an element that fixes C and N to reduce the amount of solid solution C and solid solution N in the steel and is effective in improving the corrosion resistance of steel. In the present invention, 0.01% or more, and more preferably 0.1% or more is added. However, if the content thereof is excessive, not only the production cost increases but also surface defects are caused by the formation of Ti inclusions. The content is limited to 0.3% or less, more preferably 0.2% or less.

The content of Nb is 0.01 to 0.6%.

When the amount of Nb is less than 0.01%, there is a problem that the high temperature strength of the material is lowered because the amount of Nb contained in the material is small. When the amount of Nb is less than 0.01% If it is more than 0.6%, the cost of raw material is increased.

The content of Al is 0.01 to 0.15%.

Al is a strong deoxidizing agent and serves to lower the content of oxygen in molten steel. In the present invention, Al is added in an amount of 0.01% or more. However, if the content thereof is excessive, a sleeve defect of the cold-rolled strip occurs due to an increase in nonmetallic inclusions, and at the same time, the weldability deteriorates, and is limited to 0.15% or less, more preferably 0.1% or less.

The content of N is 0.003 to 0.03%.

N is an element which plays a role of accelerating recrystallization by precipitation of austenite during hot rolling. In the present invention, 0.003% or more is added. However, if the content is excessive, the ductility of the steel is deteriorated, and it is limited to 0.03% or less.

As described above, as one means for reducing the number of [N, Ti] [C, N] independent precipitates, it is possible to induce precipitation of as many [Nb, Ti] [C, N] .

In the ferritic stainless steel excellent in ridging property according to an embodiment of the present invention, the solid content of solid solution Nb expressed by the following formula (1) is 0.15% or less.

(One) Nb - [3.2 * C + 0.8 * N + 0.5 * (Cr + Fe)]? 0.15

Here, Nb, C and N mean the content (wt%) of each element, and Cr and Fe mean the precipitate content (wt%) according to ICP analysis.

According to an embodiment of the present invention, the grain size of the [Nb, Ti] [C, N] independent precipitate is 0.01 to 1.0 탆, and the grain size of the TiN crystallized product is 2.0 to 10.0 탆.

FIG. 1 is a transmission electron microscope (TEM) image of a TiN crystallization product of Inventive Steel 1 according to an embodiment of the present invention. (TEM) photographs. Referring to FIGS. 1 and 2, the coarse crystals located at the center correspond to TiN inclusions and have a particle diameter of 2 탆 or more. It can be seen that a large amount of [Nb, Ti] [C, N] -dependent precipitates are precipitated around the crude TiN precipitate.

The ferritic stainless steel according to the present invention exhibits a ridging height of 15 占 퐉 or less and has excellent ridging characteristics.

The ferritic stainless steel of the present invention having excellent ridging properties as described above can be produced by various methods, and the production method thereof is not particularly limited. However, it can be produced by the following method as one embodiment thereof.

A method of manufacturing a ferritic stainless steel according to an embodiment of the present invention includes the steps of casting a molten steel having the above composition into a slab, hot rolling the slab to produce a hot rolled sheet, hot rolling the hot rolled sheet, And a step of cold-rolling and cold-annealing the hot-rolled and annealed sheet to produce a cold-rolled steel sheet.

Hereinafter, preferred embodiments of the present invention will be described in detail.

Example

The slabs having the compositions shown in Table 1 below were prepared, hot rolled and hot-rolled and annealed, and cold-rolled and cold-annealed to obtain cold-rolled and annealed steel sheets.

division Composition (% by weight) C Si Mn Cr Ti Nb Al N Comparative River 1 0.007 0.37 0.25 19.47 0 0.435 0 0.008 Comparative River 2 0.010 0.37 0.25 19.43 0.087 0.480 0.060 0.011 Inventive Steel 1 0.006 0.18 0.26 18.22 0.124 0.146 0.019 0.009

Table 2 shows the results of analyzing the composition of the precipitates by ICP emission spectroscopy and the solid content of Nb calculated according to equation (1), by extracting the precipitates from the cold-rolled annealed steel sheets used in the tests. The cold-rolled steel sheet was heat-treated at a temperature of 880 to 1,080 占 폚 at 20 to 30 占 폚 intervals by cold rolling, and then the structure was observed to show the temperature at which the recrystallization was completed.

The ridging height was expressed as the Wt value by measuring the surface roughness of the cold-rolled annealed steel sheet after 15% tensile test of the specimen in the rolling direction in accordance with JIS 13B standard.

division ICP analysis precipitate (% by weight) Employment Nb
(weight%) Redetermination
Temperature (℃) Rising height
(탆) Nb Ti Cr Fe Comparative River 1 0.140 0.003 0.011 0.046 0.304 1,000 28 Comparative River 2 0.100 0.057 0.012 0.062 0.402 1,050 25 Inventive Steel 1 0.025 0.039 0.005 0.018 0.108 920 13

As shown in Table 2, the comparative steel 1 and comparative steel 2 had a high recrystallization completion temperature of 1,000 to 1,050 ° C, which was high as 0.3% and 0.4%, respectively, as calculated by the formula (1). On the other hand, Inventive Steel 1 was found to have a low recrystallization temperature of 920 ° C due to the low solids Nb content calculated by Equation (1) to be about 0.1%.

The high recrystallization temperature of the cold-rolled steel sheet means that recrystallization does not occur even when hot-rolled. As a result, it was confirmed that the ridging height of the comparative steels having a high recrystallization completion temperature of the cold-rolled steel sheet was as high as 25 μm or more.

Further, precipitates were extracted from the produced cold-rolled annealed steel sheet and a transmission electron microscope (TEM) photograph was taken. FIG. 1 is a transmission electron microscope (TEM) image of a TiN crystallization product of Inventive Steel 1 according to an embodiment of the present invention. (TEM) photographs. Referring to FIGS. 1 and 2, the coarse crystals located at the center correspond to TiN inclusions having a particle diameter of 2 μm or more, and a large amount of [Nb, Ti] [C, N] -dependent precipitates And it was confirmed that it was precipitated.

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 recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

Claims (5)

0.003 to 0.1% of Si, 0.01 to 2.0% of Si, 0.01 to 1.5% of Mn, 0.05% or less of P, 0.005% or less of S, 10 to 30% of Cr, 0.01 to 0.3% of Ti, 0.01 to 0.6% of Nb, 0.01 to 0.15% of Al, 0.003 to 0.03% of N, balance Fe and other unavoidable impurities,
A ferritic stainless steel having excellent ridging properties satisfying the following formula (1).
(1) Nb - [3.2 * C + 0.8 * N + 0.5 * (Cr + Fe)] 0.15
(Where Nb, C and N mean the content (wt%) of each element, and Cr and Fe mean the precipitate content (wt%) according to ICP analysis) The method according to claim 1,
In the stainless steel,
A ferritic stainless steel having excellent ridging properties including [Nb, Ti] [C, N] independent precipitates in microstructure and [Nb, Ti] [C, N] 3. The method of claim 2,
The grain size of the [Nb, Ti] [C, N] independent precipitate is 0.01 to 1.0 탆,
Wherein the TiN crystallized product has a grain size of 2.0 to 10.0 占 퐉 and excellent ridging properties. 3. The method of claim 2,
The [Nb, Ti] [C, N] independent precipitation and the [Nb, Ti] [C,
TiC, TiN, NbC, and NbN. The ferritic stainless steel has excellent ridging properties. 3. The method of claim 2,
Wherein the ridging height of the stainless steel is 15 占 퐉 or less and the ridging property of the ferritic stainless steel is excellent.

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