WO2019132190A1 - Ferritic stainless steel with improved orange peel resistance and formability - Google Patents

Abstract

Disclosed is ferritic stainless steel with improved orange peel resistance and formability. Ferritic stainless steel according to one embodiment of the present invention comprises, in weight percentage: 0.015-0.040% of C; 0.015-0.040% of N; 0.01-0.40% of Si; 0.01-0.30% of Mn; 0.005-0.020% of P; 0.0001-0.010% of S; 15-22% of Cr; 0.05-0.35% of Ti; 0.05-0.35% of Nb; 0.005-0.05% of Al; and the balance being Fe and other unavoidable impurities, in which the ferritic grain size (FGS) is 8.5 or more, and the <111> orientation index of the inverse pole figure (IPF {111}) of a rolled surface is 5.0 or more.

Description

Ferritic stainless steel with improved orange fill resistance and formability

The present invention relates to a ferritic stainless steel excellent in orange peel resistance and moldability.

Ferritic stainless steels are used in a wide range of fields such as home appliances, kitchen appliances, electronic devices, and automobile exhaust systems. In particular, ferritic stainless steels are more economical than austenitic stainless steels containing a large amount of Ni, and their use is gradually increasing.

However, ferritic stainless steels have lower elongation than austenitic stainless steels and their applications are limited in order to open up moldability. In recent years, improvement of refining technology has made it possible to achieve extremely low carbon and nitrogen, and ferritic stainless steels having improved moldability and corrosion resistance by addition of ferrite stabilizing elements such as Ti and Nb have been developed and applied to various fields.

Ferritic stainless steels to which ferrite stabilizing elements such as Ti and Nb are added exhibit an orange peel defect during molding compared to austenitic stainless steels or semi- ferritic stainless steels undergoing austenite-ferrite phase transformation during hot rolling. The orange peel defect means a defect which causes unevenness of roughness on the surface when the steel is formed by coarse crystal grains so as to degrade the beautiful surface. Such an orange peel is an object to be removed because it is not aesthetically pleasing, There is a problem that additional cost and time are consumed by removing it through polishing.

Therefore, ferritic stainless steels containing ferrite stabilizing elements such as Ti and Nb due to orange peel defects have limitations in application to household appliances and kitchen appliances where surface quality is important after molding.

Embodiments of the present invention provide a ferritic stainless steel having improved orange fill resistance and formability by controlling components, grain size, and texture.

According to an embodiment of the present invention, ferritic stainless steels having improved orange fill resistance and formability include 0.015 to 0.040% of C, 0.015 to 0.040% of N, 0.01 to 0.40% of Si, 0.01 to 0.40% of Si, 0.001 to 0.020% of S, 0.0001 to 0.010% of Cr, 15 to 22% of Cr, 0.05 to 0.35% of Ti, 0.05 to 0.35% of Nb, 0.005 to 0.05% of Al, And other inevitable impurities, and the ferrite grain size (FGS) is 8.5 or more, and the <111> direction index (IPF {111}) of the rolling interface pole figure may be 5.0 or more.

According to an embodiment of the present invention, the ferrite stainless steel having improved orange fill resistance and formability may have a C + N ratio of 0.03 to 0.045%.

According to an embodiment of the present invention, the ferritic stainless steel having improved orange fill resistance and formability has a Mo content of 1% or less (excluding 0), a Ni content of 1% or less (excluding 0), a Cu content of 3 % Or less (excluding 0) and B: 0.005% or less (excluding 0).

According to an embodiment of the present invention, the ferrite stainless steel having improved orange fill resistance and formability may have an Rt value of less than 4.0 mm.

Here, the Rt value refers to the surface roughness after 90 DEG bending deformation with respect to the vertical direction in the rolling direction.

According to an embodiment of the present invention, the ferrite stainless steel having improved orange fill resistance and formability may have an average r value of 1.5 or more.

Here, the average r value = {r (0) + 2 x r (45) + r (90)} / 4.

The ferritic stainless steel according to the disclosed embodiment controls the grain size and texture, and the formability is improved, and orange peel generation can be suppressed.

FIG. 1 is a graph showing the correlation between the grain size (FGS) and the orange fill height according to an embodiment of the present invention.

FIG. 2 is a graph showing a correlation between a set index and an average r value according to an embodiment of the present invention.

3 is a graph showing a correlation between a C + N content and a grain size (FGS) according to an embodiment of the present invention.

4 is a graph showing a correlation between the C + N content and the texture index according to an embodiment of the present invention.

According to an embodiment of the present invention, ferritic stainless steels having improved orange fill resistance and formability include 0.015 to 0.040% of C, 0.015 to 0.040% of N, 0.01 to 0.40% of Si, 0.01 to 0.40% of Si, 0.001 to 0.020% of S, 0.0001 to 0.010% of Cr, 15 to 22% of Cr, 0.05 to 0.35% of Ti, 0.05 to 0.35% of Nb, 0.005 to 0.05% of Al, And other inevitable impurities, and the ferrite grain size (FGS) is 8.5 or more, and the <111> direction index (IPF {111}) of the rolling interface pole figure may be 5.0 or more.

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.

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

The singular forms &quot; a &quot; include plural referents unless the context clearly dictates otherwise.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings. First, the ferritic stainless steel will be described, and then the ferritic stainless steel will be described.

The inventors of the present invention have studied a method for simultaneously improving the orange fill resistance and formability of a ferritic stainless steel and have found that it is necessary to control the grain size and texture as well as controlling the steel composition to an appropriate range And led to the present invention.

The orange peel is strongly generated during the forming of the coarse metal sheet, particularly the bending deformation. In order to obtain a ferritic stainless steel surface that does not pose a practical problem visually or tactually, it can be achieved by securing the orange fill height to 4.0 mm or less.

The present inventors evaluated the orange peel with the surface roughness after bending deformation (maximum peak to valley roughness height indicated by Rt). In this case, in order to minimize the influence of the ridging, which greatly affects the surface roughness of the ferritic stainless steel, an example in which the orange fill is evaluated by performing bending deformation of 90 degrees about the vertical direction in the rolling direction will be described as an example.

The plastic anisotropy (r value) is defined as the width strain / thickness strain (ew / et). The higher the r value, the greater the degree of forming freedom. In general, to have a high r value, .

The formability of a ferritic stainless steel is generally expressed by an average r value defined by the following formula (1).

(1): average r value = {r (0) + 2 x r (45) + r (90)} / 4

Here, x of r (x DEG) means the measurement direction relative to the rolling direction.

Specifically, r (0) is the r value in the 0 degree direction, r (45) is the r value in the 45 degree direction, and r (90 degrees) is the r value in the 90 degree direction.

The r values are obtained by applying a 15% deformation to the rolling direction r (0), rolling direction 45 ° r (45 °) and rolling direction 90 ° r (90 °) 2).

(2): r (x) = ln (W0 / W) / ln (t0 / t)

In this case, W0 is plate width before stretching, W is plate width after stretching, t0 is plate thickness before stretching, and t is plate thickness after stretching.

As the r values increase, the formability increases. As the value increases, it is advantageous. In the present invention, the average r value is set to 1.5 or more to secure the moldability of the ferritic stainless steel.

Generally, the r value of the polycrystalline material is determined by the texture of the material. Texture refers to an array having a constant plane and orientation created inside the crystal. This texture can be quantified by measuring the inverse pole figure of the material. The < 111 > direction of the rolling interface inverse poles shows the texture that improves the r value. Therefore, in the present invention, the case where the texture is evaluated by the <111> direction index (IPF {111}) of the rolling pole pole figure is explained as an example.

Hereinafter, a ferritic stainless steel having improved orange fill resistance and formability only by controlling the alloy element component, grain size and texture is described.

According to one aspect of the present invention, ferritic stainless steels having improved orange fill resistance and formability contain 0.015% to 0.040% of C, 0.015% to 0.040% of N, 15% to 22% of Cr, 0.001 to 0.30%, P: 0.005 to 0.020%, S: 0.0001 to 0.005%, the balance Fe and other unavoidable elements Impurities.

Hereinafter, the reason for limiting the numerical value of the content of the component component in the embodiment of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C and N is 0.015 to 0.040%.

Carbon (C) and nitrogen (N) improve the strength of ferritic stainless steels as interstitial solid solution strengthening elements. In addition, it is combined with an element such as titanium (Ti) or niobium (Nb) to form carbonitride to inhibit grain growth, which is an indispensable element for improving the orange peel resistance through grain refinement. Therefore, in the present invention, it is preferable to add 0.015% or more. However, when the content is excessive, an austenite phase is formed at a high temperature to form acid-labile surface defects such as gold dust, and in addition, there is a problem of lowering the elongation of the material, and the upper limit can be limited to 0.040%.

Preferably, the C + N content is 0.03 to 0.045%.

Considering the lower limit of C and N, the content of C + N is 0.03% or more. However, if C + N is excessively added, it is necessary to limit the content of r, because the r value is decreased by suppressing the development of aggregate tissue favorable for moldability. Therefore, in the present invention, the upper limit of the C + N content can be limited to 0.045%.

Specifically, when the C + N content is lower than the lower limit of 0.03%, the crystal grain size is obtained to be FGS 8.5 or lower, so that the orange fill resistance is lowered. On the other hand, when the content of C + N is 0.045% or more, which is the upper limit, the aggregate index is obtained to be 5.0 or less and the formability is lowered. Therefore, in order to obtain a ferritic stainless steel having improved orange peel resistance and formability, the C + N content should be 0.03 to 0.045%.

The Cr content is 15 to 22%.

Chromium (Cr) is the element which is the largest element among the elements improving the corrosion resistance of stainless steel and is the basic element. It is preferable to add Cr by more than 15% in order to exhibit corrosion resistance. However, if the content is excessive, intergranular corrosion may occur in the ferritic stainless steel containing carbon and nitrogen, and the manufacturing cost may increase, so that the upper limit can be limited to 22%.

The content of Ti is 0.05 to 0.35%.

Titanium (Ti) is an element that fixes carbon and nitrogen, and forms carbonitride to suppress grain growth. Therefore, it is preferable to add Ti in an amount of 0.05% or more for grain refinement. However, if the content is excessive, there is a problem in the manufacturing process due to Ti inclusions, and there is a problem that the manufacturing cost is increased, so that the upper limit can be limited to 0.35%.

The content of Nb is 0.05 to 0.35%.

Niobium (Nb) is an element that fixes carbon and nitrogen, and forms carbonitride to suppress grain growth. Therefore, it is preferable that Nb is added in an amount of 0.05% or more for grain refinement. However, when the content thereof is excessive, Laves precipitates are formed to lower moldability and brittle fracture, and the production cost increases, so that the upper limit can be limited to 0.35%.

The content of Si is 0.01 to 0.40%.

Silicon (Si) is an element which is essentially added for deoxidation. As the element for forming a ferrite phase, it is preferable to add 0.01% or more in order to increase the stability of the ferrite phase. However, when the content is excessive, the material is hardened and bonds with oxygen to form inclusions, thereby causing problems such as deterioration of corrosion resistance and surface defects, so that the upper limit can be limited to 0.40%.

The content of Al is 0.005 to 0.05%.

Aluminum (Al) is an element which is essentially added for deoxidation and can lower the content of oxygen in molten steel, so that it is preferable to add at least 0.005%. However, when the content is excessive, there is a fear that the inclusion is formed by binding with oxygen, so that the corrosion resistance is lowered and the formability is lowered. The upper limit can be limited to 0.05%.

The content of Mn is 0.01 to 0.30%.

Manganese (Mn) is an element which is essentially added for deoxidation and can lower the content of oxygen in molten steel, so that it is preferable to add Mn of 0.01% or more. However, if the content is excessive, precipitates such as manganese sulfide (MnS) are formed to lower the corrosion resistance, and the upper limit can be limited to 0.30%.

The content of P is 0.005 to 0.020%.

Phosphorus (P) is an impurity element and because it is a solid solution strengthening element, it is preferable that the content is as small as possible. However, if the content is excessive, the elongation rate should be reduced. Therefore, the content is restricted to 0.020% or less and it is impossible to completely remove it, and the refining cost increases, so it is preferable to control the content to 0.005% or more.

The content of S is 0.0001 to 0.005%.

Sulfur (S) forms manganese sulfide (MnS) precipitate or the like to lower hot workability and corrosion resistance, so the smaller the content, the better. Therefore, the content is restricted to 0.005% or less, and it is impossible to completely remove it, and the refining cost increases, so it is preferable to control the content to 0.0001% or more.

In addition, the ferritic stainless steel having improved orange fill resistance and formability according to an embodiment of the present invention has a Mo content of 1% or less (excluding 0), a Ni content of 1% or less (excluding 0), a content of Cu of 3% (Excluding 0) and B: 0.005% or less (excluding 0).

Molybdenum (Mo) is a strong corrosion resistance improving element, but when it is excessive, the workability is lowered, and the upper limit can be limited to 1%.

The content of Ni is 1% or less.

Nickel (Ni) is an element which is essentially used for the production of stainless steel. The increase of the Ni content is directly related to the rise of the raw material price, so it needs to be minimized, and the upper limit can be limited to 1%.

The content of Cu is 3% or less.

Copper (Cu) is an element which improves corrosion resistance, but when it is excessive, workability and weldability are lowered, and the upper limit can be limited to 3%.

The content of B is 0.005% or less.

Boron (B) segregates in grain boundaries and strengthens the grain boundaries. However, when the content of boron (B) is excessive, the heat processability is lowered, and the upper limit can be limited to 0.005%.

The remainder of the present invention is iron (Fe). However, in the ordinary 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 manufacturing.

According to an embodiment of the present invention, the ferrite grain size (FGS) of the ferritic stainless steel having improved orange fill resistance and formability satisfying the above-described alloy composition may be 8.5 or more.

FIG. 1 is a graph showing the correlation between the grain size (FGS) and the orange fill height according to an embodiment of the present invention. Referring to FIG. 1, it can be seen that the orange fill height decreases as the FGS value increases, that is, as the grain size decreases.

The surface roughness (maximum peak to valley roughness height) defined by the present invention indicates the degree of orange peel after 90 占 bending deformation is applied about the vertical direction in the rolling direction. When this value is less than 4.0 mm, the orange peel is reduced to a level that does not cause a visual or tactile problem. In the present invention, carbonitride is formed through the addition of carbon (C) and nitrogen (N) to refine the crystal grains to control the orange fill height to be less than 4.0 mm.

According to an embodiment of the present invention, the <111> direction index (IPF {111}) of the rolled region poles of the ferritic stainless steels having improved orange fill resistance and formability satisfying the above alloy composition may be 5.0 or more .

FIG. 2 is a graph showing a correlation between a set index and an average r value according to an embodiment of the present invention. Referring to FIG. 2, it can be seen that the average r value increases as the <111> direction index (IPF {111}) of the rolled region poles increases.

In the present invention, intrinsic elements are formed of carbonitride through the addition of titanium (Ti) and niobium (Nb), which are stabilizing elements of ferrite, to develop an aggregate structure to control the average r value to 1.5 or more.

It was confirmed that the deep drawing workability can be improved when the average r value is 1.5 or more.

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

Example

Experiments were conducted to produce final products according to the production conditions of commercially produced ferritic stainless steels, and ferritic stainless steels as shown in Table 1 were melted into 250 mm thick castings. The cast pieces were hot-rolled to prepare hot-rolled steel sheets having a thickness of 3 to 6 mm. The hot-rolled steel sheet was pickled, cold-rolled, and finally annealed to produce a cold-rolled annealed steel sheet having a thickness of 0.4 to 1.2 mm.

weight%	C	Mn	Cr	Ti	Nb	P	S	N	Si	Al	C + N
Example 1	0.027	0.24	18.3	0.15	0.26	0.0005	0.0005	0.011	0.21	0.034	0.038
Example 2	0.019	0.25	18.3	0.16	0.25	0.0005	0.0005	0.021	0.19	0.039	0.040
Example 3	0.029	0.24	18.3	0.15	0.43	0.0005	0.0005	0.009	0.23	0.034	0.038
Example 4	0.030	0.19	16.5	0.16	0.01	0.0005	0.0005	0.0075	0.22	0.035	0.038
Comparative Example 1	0.065	0.6	16.1	0.01	0.01	0.0005	0.0005	0.03	0.15	0.1	0.095
Comparative Example 2	0.005	0.23	18.4	0.16	0.16	0.0005	0.0005	0.011	0.23	0.021	0.016
Comparative Example 3	0.005	0.2	16.2	0.17	0.01	0.0005	0.0005	0.0075	0.20	0.04	0.013
Comparative Example 4	0.018	0.2	15.8	0.19	0.01	0.0005	0.0005	0.0067	0.22	0.035	0.025
Comparative Example 5	0.045	0.2	16.3	0.18	0.01	0.0005	0.0005	0.0075	0.20	0.041	0.053

The grain size of the cold-rolled annealed steel sheet was determined by the ferrite grain size measurement method specified in JIS G0552.

In addition, the aggregate texture fraction was measured using Electron Backscatter Diffraction (EBSD) and the area including the entire thickness direction of the sheet material was measured by EBSD data. Respectively. In the present invention, since the formability is defined by the average plastic anisotropy ratio of the rolled surface, the rolled surface reverse pole degree is used as a formability index.

The orange peel was subjected to a 90 ° bending deformation about the vertical direction in the rolling direction, and the surface roughness (maximum roughness expressed by Rt: Maximum Peak to Valley Roughness Height).

	FGS	IPF {111}	Orange Fill Height (mm)	R-bar
Example 1	8.7	7.95	3.49	1.91
Example 2	8.7	8.77	3.53	1.88
Example 3	9.0	5.96	3.43	1.64
Example 4	8.9	6.56	3.90	1.47
Comparative Example 1	9.7	2.73	2.48	0.83
Comparative Example 2	8.1	9.46	4.23	1.98
Comparative Example 3	7.0	8.31	5.74	1.91
Comparative Example 4	8.0	7.44	5.10	1.82
Comparative Example 5	9.4	3.35	3.30	1.22

As described above, as the grain size decreases, the orange peel height decreases and the < 111 > direction of the rolling interface poles exhibits a texture that improves the r value. Therefore, in order to improve orange peel resistance and formability, And aggregate organization.

As compared with the comparative examples, the FGS had a grain size of 8.5 or more, and the Rt value was less than 4 mm, and the <111> direction index (IPF {111}) of the rolling interface pole figure was 5.0 or more r value is 1.5 or more, it can be confirmed that the ferritic stainless steel according to the present invention has both improved moldability and surface characteristics.

FIG. 1 is a graph showing the correlation between the grain size (FGS) and the orange fill height according to an embodiment of the present invention. Referring to FIG. 1, it can be seen that the orange fill height decreases as the grain size decreases. That is, in order to obtain a target Rt value of less than 4 mm in the present invention, the FGS should have a grain size of not less than 8.5.

On the other hand, in Comparative Examples 2 to 4, the lower limit of the C + N content was less than the lower limit, and the orange fill height exceeded 4.0 mm.

Specifically, the IPF _{111} of Comparative Example 2 was 9.46, and the texture thereof satisfied the range of the present invention, but the grain size (FGS) of 8.1 did not satisfy the range of the present invention. As a result, the average r value satisfies 1.5 or more at 1.98, but the orange fill height exceeds 4.0 mm at 4.23 mm, thereby failing to meet the surface property requirement.

Referring to Table 2 and FIG. 1, the IPF _{111} of Comparative Example 3 was 8.31, and the texture of the aggregate satisfied the range of the present invention, but the grain size (FGS) As a result, the average r value was 1.91, which was more than 1.5 but the orange fill height exceeded 4.0 mm at 5.74 mm and thus did not satisfy the surface property requirement.

Referring to Table 2 and FIG. 1, the IPF _{111} of Comparative Example 4 was 7.44, and the texture thereof satisfied the range of the present invention, but the grain size (FGS) of 8.0 did not satisfy the range of the present invention. As a result, the average r value was 1.82, which was more than 1.5 but the orange fill height exceeded 4.0 mm at 5.10 mm and thus did not satisfy the surface property requirement.

FIG. 2 is a graph showing a correlation between a set index and an average r value according to an embodiment of the present invention. Referring to FIG. 2, it can be seen that the average r value increases as the <111> direction index (IPF _{111} ) of the rolled region poles increases. That is, in order to obtain a desired average r value of 1.5 or more in the present invention, it should have IPF _{111} of 5.0 or more.

On the other hand, in Comparative Example 1 and Comparative Example 5, the upper limit of the C + N content exceeded, and the texture development favorable for moldability was suppressed, and the r value was less than 1.5.

Specifically, the grain size (FGS) of Comparative Example 1 was 8.7, which satisfied the range of the present invention, but IPF {111} was 2.73, and the texture did not satisfy the scope of the present invention. As a result, the orange peel height was 2.48 mm and the orange peel height was 4.0 mm or less. However, the average r value was 0.83, which was less than 1.5, and thus the formability requirement was not satisfied.

Referring to Table 2 and FIG. 2, the grain size (FGS) of Comparative Example 5 was 9.4, which satisfied the range of the present invention, but IPF {111} was 3.35, and the texture did not satisfy the scope of the present invention. As a result, the orange peel height was 3.30 mm and the orange peel height was less than 4.0 mm, but the average r value was 1.22, which was less than 1.5, and thus the formability requirement was not satisfied.

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.

The ferrite stainless steel improved in orange peel resistance and formability according to the embodiments of the present invention is applicable to materials for household appliances and kitchen appliances where surface quality is important.

Claims (5)

1. 0.001 to 0.020% of P, 0.0001 to 0.010% of S, 0.0001 to 0.010% of Cr, 0.01 to 0.20% of Cr, 0.01 to 0.40% of Cr, 0.015 to 0.040% of C, 0.015 to 0.040% , 0.05 to 0.35% of Ti, 0.05 to 0.35% of Nb, 0.005 to 0.05% of Al, the balance Fe and other unavoidable impurities,

   A ferritic stainless steel having a ferrite grain size (FGS) of not less than 8.5 and an orange fill resistance of not less than 5.0 in the <111> direction index (IPF {111}) of the rolling interface pole point and improved formability.
2. The method according to claim 1,

   C + N is 0.03 to 0.045%, and the orange fill resistance and formability of the ferritic stainless steel are improved.
3. The method according to claim 1,

   Mo: not more than 1% (excluding 0), Ni: not more than 1% (excluding 0), Cu: not more than 3% (excluding 0) and B: not more than 0.005% A ferritic stainless steel having improved orange fill resistance and formability, further containing at least two elements.
4. The method according to claim 1,

   Ferritic stainless steel with improved Rt value of less than 4.0 mm and orange fill resistance and formability.

   (Here, the Rt value refers to the surface roughness after 90 DEG bending deformation with respect to the vertical direction in the rolling direction.)
5. The method according to claim 1,

   Ferritic stainless steel with improved orange fill resistance and formability with an average r value of 1.5 or higher.

   (Where the average r value is {r (0) + 2 x r (45) + r (90)} / 4.

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