US20180371575A1

US20180371575A1 - Austenitic stainless steel having excellent orange peel resistance and manufacturing method therefor

Info

Publication number: US20180371575A1
Application number: US15/775,335
Authority: US
Inventors: Hyung Gu KANG; Sang Hoon Kim; Gyu Jin JO; Byoung Jun SONG
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2015-11-12
Filing date: 2016-11-09
Publication date: 2018-12-27
Also published as: KR20170056047A; CN108348990A; JP2018536089A; WO2017082631A1

Abstract

An austenitic stainless steel having excellent orange peel resistance and a method for producing the same are disclosed. In the austenitic stainless steel having excellent orange peel resistance, according to an embodiment of the present disclosure, a ratio Gs/Gi of an average crystal grain size Gs of surface crystal grains included in a first area corresponding to a depth of 10% or less of a total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel with respect to an average crystal grain size Gi of internal crystal grains included in a second area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel may be 0.5 or smaller. Therefore, it is possible to prevent deterioration of surface roughness due to orange peel of the steel surface even after post-processing of the austenitic stainless steel while increasing the sizes of crystal grains in order to reduce strength of the austenitic stainless steel, and also to reduce cost by replacing copper pipes

Description

TECHNICAL FIELD

The present disclosure relates to austenitic stainless steel having excellent orange peel resistance and a manufacturing method thereof, and more particularly, to austenitic stainless steel having excellent orange peel resistance, which is capable of preventing deterioration of surface roughness due to orange peel of the steel surface even after post-processing of the stainless steel by controlling average crystal grain sizes at different depths of the stainless steel while increasing the sizes of crystal grains in order to reduce strength, and a method of manufacturing the austenitic stainless steel.

BACKGROUND ART

There was a trial for applying stainless steel to refrigerant pipes for household and car air conditioners, since stainless steel has excellent corrosion resistance and a low price.
However, since refrigerant pipes for air conditioner are limited in installation space, in many cases, constructing pipes essentially accompanies the work of bending the pipes with human power. Copper pipes or aluminum pipes used typically do not make any problem since they are sufficiently soft, but general stainless steel has a problem that it does not have flexibility essentially required for construction of pipes.
Metal materials are work hardened when they are subject to deformation, such as extension or compression, and as they are subject to higher deformation, they become stronger. Bending pipes is a combined action of extension and compression, and as the degree of bending is greater, the material will be more hardened. Particularly, when 304 steel which is most widely used as austenitic stainless steel is used in pipes for air conditioner, it is very difficult to bend the pipes with human power in space for installing the pipes since the degree of work hardening is high.
In order to overcome the problem, a method of reducing the strength of stainless steel by increasing the size (that is, grain size) of crystal grains is used. The strength of stainless steel decreases as the grain size increases. However, as the size of crystal grains increases, orange peel being uneven defects appears on the surface after processing. Therefore, it is difficult to reduce the strength of stainless steel only by increasing the size of crystal grains. The orange peel should be removed since it impairs the beauty, and removing the orange peel through polishing requires additional cost and time.

(Patent Document 0001) Korean Laid-open Patent Application No. 10-2010-0020446

DISCLOSURE

Technical Problem

The embodiments of the present disclosure are directed to providing austenitic stainless steel having excellent orange peel resistance, which is capable of preventing deterioration of surface roughness due to orange peel of the steel surface even after post-processing of the stainless steel by controlling average crystal grain sizes at different depths of the stainless steel while increasing the sizes of crystal grains in order to reduce strength.
Further, the embodiments of the present disclosure is directed to providing a method of manufacturing austenitic stainless steel having excellent orange peel resistance, which is capable of preventing deterioration of surface roughness due to orange peel, wherein the austenitic stainless steel is casted through strip casting to control a delta ferritic phase content and thereby suppress the growth of surface crystal grains.

Technical Solution

In austenitic stainless steel having excellent orange peel resistance, according to an embodiment of the present disclosure, a ratio Gs/Gi of an average crystal grain size Gs of surface crystal grains included in a first area corresponding to a depth of 10% or less of a total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel with respect to an average crystal grain size Gi of internal crystal grains included in a second area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel may be 0.5 or smaller.
According to an embodiment of the present invention, the austenitic stainless steel may include silicon (Si) of 0.1 to 0.65 wt % (weight percentage), manganese (Mn) of 1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0 wt %, chrome (Cr) of 16.5 to 18.5 wt %, copper (Cu) of 6.0 wt % or less (except for zero), carbon (C) and nitrogen (N) of 0.13 wt % or less (except for zero), iron (Fe), and other inevitable impurities.
According to an embodiment of the present invention, the average crystal grain size Gs of the surface crystal grains may be 100 μm or less.
According to an embodiment of the present invention, the austenitic stainless steel may be manufactured by strip casting.
According to an embodiment of the present invention, when the austenitic stainless steel is casted by strip casting, a delta ferritic phase content remaining upon solidification may be 5 weight % or more.
According to an embodiment of the present invention, a delta ferrite phase content of a cold-rolled structure of the austenitic stainless steel may be 0.5 weight % or more.
In a method of manufacturing austenitic stainless steel having excellent orange peel resistance by strip casting of passing stainless steel between a pair of rotating rolls to freeze the stain steel into a solid, according to an embodiment of the present disclosure, the method comprises; casting austenitic stainless steel, and controlling elements of the austenitic stainless steel according to Equation (1) such that a content Delta of delta ferritic phase remaining upon solidification is 5 wt % or more to manufacture a hot-rolled steel sheet:
Delta=((Cr+Mo+1.5Mn+0.5Nb+2Ti+18)/(Ni+0.3Cu+30*(C+N)+0.5Mn+36)+0.262)×161−161, Equation (1)

- Herein, element symbols of Equation (1) represent weight percentages (weight %) of the corresponding elements.

According to an embodiment of the present invention, the austenitic stainless steel may include silicon (Si) of 0.1 to 0.65 wt % (weight percentage), manganese (Mn) of 1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0 wt %, chrome (Cr) of 16.5 to 18.5 wt %, copper (Cu) of 6.0 wt % or less (except for zero), carbon (C) and nitrogen (N) of 0.13 wt % or less (except for zero), iron (Fe), and other inevitable impurities.
According to an embodiment of the present invention, the method further comprises; performing heat-treatment on the hot-rolled steel sheet, and then performing cold rolling on the hot-rolled steel sheet at a total reduction ratio of 50% or higher, thereby manufacturing a cold-rolled steel sheet, wherein a delta ferritic phase content of a cold-rolled structure of the cold-rolled steel sheet is 0.5 weight % or more.

Advantageous Effects

The embodiments of the present disclosure may prevent deterioration of surface roughness due to orange peel of the surface of austenitic stainless steel even after post-processing of the austenitic stainless steel, while increasing the sizes of crystal grains in order to reduce the strength of the austenitic stainless steel, and may reduce cost by replacing copper pipes or aluminum pipes used as air conditioner refrigerant pipes with stainless pipes.
Also, the embodiments of the present disclosure can prevent deterioration of surface roughness due to orange peel by casting austenitic stainless steel through strip casting to control a delta ferritic phase content and thereby suppress the growth of surface crystal grains.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an apparatus provided to describe strip casting for manufacturing austenitic stainless steel according to an embodiment of the present disclosure.

FIG. 2 is a cross-sectional view for describing an austenitic stainless cold-rolled sheet according to an embodiment of the present disclosure.

FIG. 3 is a picture showing an austenitic stainless steel pipe according to an embodiment of the present disclosure, bent 90 degrees.

FIG. 4 is a picture showing the surface of an austenitic stainless steel pipe of FIG. 1.

FIG. 5 is a picture showing the surface of a typical austenitic stainless steel pipe.

FIG. 6 is a picture showing a section microstructure of the austenitic stainless steel pipe of FIG. 1.

FIG. 7 is a picture showing a section microstructure of the typical austenitic stainless steel pipe.

FIG. 8 is a graph for describing the size of a crystal grain of austenitic stainless steel according to an embodiment of the present disclosure and surface roughness when an austenitic stainless steel pipe is bent 90 degrees.

BEST MODE

In austenitic stainless steel having excellent orange peel resistance, according to an embodiment of the present disclosure, a ratio Gs/Gi of an average crystal grain size Gs of surface crystal grains included in a first area corresponding to a depth of 10% or less of a total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel with respect to an average crystal grain size Gi of internal crystal grains included in a second area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel may be 0.5 or smaller.

MODES OF THE INVENTION

Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to transfer the technical concepts of the present disclosure to one of ordinary skill in the art. However, the present disclosure is not limited to these embodiments, and may be embodied in another form. In the drawings, parts that are irrelevant to the descriptions may be not shown in order to clarify the present disclosure, and also, for easy understanding, the sizes of components are more or less exaggeratedly shown.
Austenitic stainless steel having excellent orange peel resistance, according to an embodiment of the present disclosure, may include silicon (Si) of 0.1 to 0.65 wt % (weight percentage), manganese (Mn) of 1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0 wt %, chrome (Cr) of 16.5 to 18.5 wt %, copper (Cu) of 6.0 wt % or less (except for zero), carbon (C) and nitrogen (N) of 0.13 wt % or less (except for zero), iron (Fe), and other inevitable impurities.
Hereinafter, a reason for numerical limitation of elements constituting austenitic stainless steel having excellent flexibility according to the present disclosure will be described.
Silicon (Si) may be added within the range of 0.1 to 0.65 wt %.
Since silicon (Si) is an element essentially added for deoxidation, silicon (Si) of 0.1 wt % or more may be added.
However, if the silicon (Si) content is excessively high, the material may be hardened, and combined with oxygen to form inclusions, resulting in a deterioration of corrosion resistance. Therefore, the maximum silicon (Si) content may be limited to 0.65 wt %.
Manganese (Mn) may be added within the range of 1.0 to 3.0 wt %.
Since manganese (Mn) is an element essentially added for deoxidation and increasing the stabilization of the austenite phase, manganese (Mn) of 1.0 wt % or more may be added to maintain the austenite balance. However, if the manganese (Mn) content is excessively high, the corrosion resistance of the material may deteriorate, and accordingly, the maximum manganese (Mn) content may be limited to 3.0 wt %.
Nickel (Ni) may be added within the range of 6.5 to 10.0 wt %.
If nickel (Ni) and chrome (Cr) are added together, corrosion resistance such as pitting corrosion resistance may be improved, and also if the nickel (Ni) content increases, the austenitic steel may be softened.
Also, since nickel (Ni) is an element contributing to an improvement of phase-stabilization of the austenitic stainless steel, nickel (Ni) of 6.5 wt % or more may be added to maintain the austenite balance. However, if the nickel (Ni) content is excessively high, manufacturing cost of the austenitic stainless steel may increase, and accordingly, the maximum nickel (Ni) content may be limited to 10.0 wt %.
Chrome (Cr) may be added within the range of 16.5 to 18.5 wt %.
Since chrome (Cr) is an element essentially added for improving corrosion resistance, chrome (Cr) of 16.5 wt % or more may need to be added for general use. However, if the chrome (Cr) content is excessively high, the austenite phase may be hardened, and manufacturing cost may increase. Accordingly, the maximum chrome (Cr) content may be limited to 18.5 wt %.
Copper (Cu) may be added within the range of 6.0 wt %.
Copper (Cu) may soften the austenitic stainless steel. However, if the copper (Cu) content is excessively high, hot workability may deteriorate, and the austenitic phase may be hardened. Accordingly, the maximum copper (Cu) content may be limited to 6.0 wt %.
Carbon (C) and nitrogen (N) may be added by 0.13 wt % or less.
Carbon (C) and nitrogen (N), which are interstitial solid solution hardening elements, may harden the austenitic stainless steel, and if the Carbon (C) and nitrogen (N) content is high, strain induced martensite generated during processing may be hardened to increase work hardness of the material. Accordingly, the Carbon (C) and nitrogen (N) content may need to be limited, and in the present disclosure, the Carbon (C) and nitrogen (N) content may be limited to 0.13 w % or less.
In the austenitic stainless steel having excellent orange peel resistance, according to an embodiment of the present disclosure, a ratio Gs/Gi of an average crystal grain size Gs of surface crystal grains included in a first area corresponding to a depth of 10% or less of a total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel with respect to an average crystal grain size Gi of internal crystal grains included in a second area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel may be 0.5 or smaller.
That is, by maintaining the size of the surface crystal grains distributed in the first area adjacent to the surface of the austenitic stainless steel smaller than that of the internal crystal grains of the austenitic stainless steel, a pipe made of the austenitic stainless steel may prevent the generation of orange peel on the surface when it is bent.
For example, the average crystal grain size Gs of the surface crystal grains may be 100 μm or smaller. Meanwhile, the average crystal grain size Gi of the internal crystal grains may be larger than 100 μm.
The austenitic stainless steel having excellent orange peel resistance, according to an embodiment of the present disclosure, may be manufactured by strip casting of passing stainless steel between a pair of rotating rolls to freeze it into a solid.
FIG. 1 is a schematic diagram of an apparatus provided to describe strip casting for manufacturing austenitic stainless steel according to an embodiment of the present disclosure.
The strip casting is a steel casting process for producing hot-rolled thin strips directly from molten steel. The strip casting can innovatively reduce manufacturing cost, equipment investment cost, energy consumption, and pollutant gas emissions, etc. by omitting a hot-rolling process.
In FIG. 1, a twin roll strip caster used for the strip casting is shown. Referring to FIG. 1, molten steel may be put in a ladle 2, and the molten steel may enter a turn dish 3 along a nozzle. The molten steel entered the turn dish 3 may be supplied between edge dams 6 installed at both ends of casting rolls 1 and between the casting rolls 1 through a molten injecting nozzle 4, and then solidified. In a molten metal area between the casting rolls 1, the surfaces of the molten metal may be protected by a meniscus shield 5 for preventing oxidation, and proper gas may be injected to adjust an atmosphere appropriately. When the molten steel escapes from a roll nip at which the casting rolls 1 meet, a thin sheet 7 may be produced and drawn. Then, the thin sheet 7 may pass through a rolling mill 8 to be rolled, and then cooled and wound in winding equipment 9.
The strip casting may be performed to cast liquid molten steel directly into a sheet having a thickness of 1 to 5 mm, and then to apply high cooling speed to the sheet. That is, the strip casting may be used to manufacture a hot coil using the twin roll strip caser. The twin roll strip caster may supply molten steel between the casting rolls 1 rotating in opposite directions and between the edge dams 6 installed at the sides of the casting rolls 1, and emit a large amount of heat through the surfaces of the casting rolls cooled with water, thereby casting steel. At this time, solidified shells may be formed on the surfaces of the casting rolls due to high cooling speed, and after casting, in-line rolling may be sequentially performed to manufacture a thin hot-rolled steel sheet having a thickness of 1 to 5 mm. Since the strip casting casts a thin sheet directly, the strip casting may omit a process of manufacturing a slab through continuous casting and a hot-rolling process.
Particularly, when austenitic stainless steel is manufactured through typical continuous casting, delta ferritic phase may be generated in an initial time of solidification in order to secure stability of solidification, and then solidification to austenitic phase may be performed. A content of delta ferritic phase remaining during casting may be within the range of about 1 wt % to about 10 wt % according to the kind of steel. That is, when a typical slab is casted, delta ferritic phase may remain in the slab.
However, the slab may be heated in a reheating furnace for two hours or more for hot rolling. At this time, most of the delta ferritic phase may be resolved to austenitic phase by solid state transformation, and then hot rolling may be performed at high temperature so that most of the delta ferritic phase existing in the slab casting structure may be resolved. Accordingly, a delta ferrite content of a typical hot coil made of austenitic stainless steel may be 0.5 wt % or less.
Since the strip casting casts a thin sheet directly from molten steel using water-cooling rolls, a process of reheating slabs and a process of hot rolling may be omitted, and accordingly, a high delta ferritic phase content of 5 wt % or more may be obtained. Also, most of the delta ferritic phase may remain on the surface of the stainless steel sheet, so that the delta ferritic phase remaining on the surface of the stainless steel sheet may suppress the growth of crystal grains.
Accordingly, by controlling the content of the delta ferritic phase remaining in the structure of the austenitic stainless hot-rolled steel manufactured according to strip casting to 5 wt % or more, it is possible to increase the content of in-structure delta ferritic phase remaining in a final product. Accordingly, the surface crystal grains distributed in the area adjacent to the surface of the austenitic stainless steel may be formed with a size that is smaller than that of the internal crystal grains of the austenitic stainless steel.
That is, the content of delta ferritic phase remaining in the austenitic stainless hot-rolled steel according to an embodiment of the present disclosure may be 5 wt % or more.
In order to control the content Delta of delta ferritic phase remaining in the casted austenitic stainless steel upon solidification to 5 wt % or more, elements may be controlled according to Equation (1) below:
Delta=((Cr+Mo+1.5Mn+0.5Nb+2Ti+18)/(Ni+0.3Cu+30*(C+N)+0.5Mn+36)+0.262)×161−161, Equation (1)
Herein, element symbols of Equation (1) represent weight percentages (wt %) of the corresponding elements.
Thereafter, the hot-rolled steel sheet may be subject to heat treatment, and then to cold rolling at a total reduction ratio of 50% or higher, thereby manufacturing an austenitic stainless cold-rolled steel sheet.
A cold-rolled structure of the austenitic stainless cold-rolled sheet, according to an embodiment of the present disclosure, cold-rolled by the cold-rolling process described above may have a delta ferritic phase content of 0.5 wt % or smaller.
Hereinafter, the present disclosure will be described in more detail through embodiments.
As shown in Table 1, by controlling a content Delta of delta ferritic phase of a hot-rolled steel sheet casted by strip casting to 5 wt % or more, austenitic stainless steel was manufactured according to the present disclosure. Then, cold-rolling was performed at a total reduction ratio of 50% or higher to manufacture a cold-rolled steel sheet, and the cold-rolled steel sheet extended by 15%. Thereafter, surface roughness (orange peel) of the resultant steel sheet was measured. For tension, standard tension pieces such as JIS13B are manufactured, or the sheet material may extend as it is. 15% was used for comparison under the same condition, and the characteristics of the present disclosure are not necessarily obtained only upon extension of 15%.
An average crystal grain size Gs of surface crystal grains included in an area corresponding to a depth of 10% or less of a total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel, and an average crystal grain size Gi of internal crystal grains included in an area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel were measured, and a ratio Gs/Gi of the average crystal grain size Gs with respect to the average crystal grain size Gi was represented.
The average crystal grain size Gs of the surface crystal grains means an average crystal grain size of crystal grains included in an area corresponding to a depth of 10% or less of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel. Also, the average crystal grain size Gi of the internal crystal grains means an average crystal grain size of crystal grains included in an area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel.

TABLE 1

			Existence or
			Absence of
Delta	Process	Gs/Gi	Orange Peel

Embodiment

1	5.1	Strip Casting	0.47	◯
Embodiment 2	6.2	Strip Casting	0.42	◯
Embodiment 3	7.2	Strip Casting	0.37	◯
Embodiment 4	8.5	Strip Casting	0.42	◯
Comparative	5.5	Continuous	1.03	X
Example 1		Casting
Comparative	6.4	Continuous	0.95	X
Example 2		Casting
Comparative	4.1	Strip Casting	0.74	X
Example 3
Comparative	6.2	Continuous	0.97	X
Example 4		Casting

Referring to Table 1, when austenitic stainless steel was manufactured by strip casting, and the content of delta ferritic phase was controlled to 5 wt % or more, no orange peel was generated in the final product. Also, when the content of the delta ferritic phase was controlled to 5 wt %, the ratio Gs/Gi of the average crystal grain size Gs of the surface crystal grains with respect to the average crystal grain size Gi of the internal crystal grains was 0.5 or smaller.
Table 3 shows measured surface grain sizes of Embodiments 1 to 4 and Comparative Examples 1 to 4.

	TABLE 2

	Surface Grain size
	(Gs, μm)	Orange Peel

Embodiment

1	14	◯
	Embodiment 2	40	◯
	Embodiment 3	58	◯
	Embodiment 4	85	◯
	Comparative	105	X
	Example 1
	Comparative	145	X
	Example 2
	Comparative	192	X
	Example 3
	Comparative	280	X
	Example 4

Referring to Table 2, excellent orange peel appeared in a stainless steel sheet having a surface grain size Gs of 100 μm or smaller, whiled orange peel appeared significantly in a stainless steel sheet having a surface grain size Gs that is larger than 100 μm, like the Comparative Examples.
The surface grain sizes and internal grain sizes of the Embodiments 1 to 4 and Comparison Examples 1 to 4 were measured, and ratios of the surface grain sizes with respect to the internal grain sizes are shown in Table 3.

TABLE 3

Surface	Internal		Existence or
Grain size	Grain size		Absence of
(Gs, μm)	(Gi, μm)	Gs/Gi	Orange Peel

Embodiment

1	14	30	0.47	◯
Embodiment 2	40	95	0.42	◯
Embodiment 3	58	155	0.37	◯
Embodiment 4	85	201	0.42	◯
Comparative	105	102	1.03	X
Example 1
Comparative	145	153	0.95	X
Example 2
Comparative	192	260	0.74	X
Example 3
Comparative	280	290	0.97	X
Example 4

As shown in Table 3, under the condition that the surface grain size Gs is 100 μm or smaller, and the ratio Gs/Gi of the surface grain size Gs to the internal grain size Gi is 0.5 or smaller, excellent orange peel resistance appeared.
In Table 4, by controlling the content Delta of delta ferritic phase of a hot-rolled steel sheet casted by strip casting to 5 wt % or more, austenitic stainless steel was manufactured.
Then, cold-rolling was performed at a total reduction ratio of 50% to manufacture an austenitic stainless steel pipe. The austenitic stainless steel pipe was bent 90 degrees using a hand bender, and then surface roughness (orange peel) of the bent portion was measured. The hand bender was used to apply deformation with a uniform radius of curvature, and another apparatus may be used instead of the hand blender. Also, the angle of 90 degrees was used to apply uniform deformation, and the characteristics of the present disclosure may be obtained although another angle is used.
FIG. 2 is a cross-sectional view for describing an austenitic stainless cold-rolled sheet according to an embodiment of the present disclosure. FIG. 3 is a picture showing an austenitic stainless steel pipe according to an embodiment of the present disclosure, bent 90 degrees.
An average crystal grain size Gs of surface crystal grains included in a first area A1 corresponding to a depth of 10% or less of a total thickness of austenitic stainless steel 100 from the surface of the austenitic stainless steel 100, and an average crystal grain size Gi of internal crystal grains included in a second area A2 corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel 100 from the surface of the austenitic stainless steel 100 were measured, and a ratio Gs/Gi of the average crystal grain size Gs with respect to the average crystal grain size Gi was represented.

	TABLE 4

	Surface Grain size	Existence or Absence of
	(Gs, μm)	Orange Peel

Embodiment

1	35	◯
Embodiment 2	50	◯
Embodiment 3	69	◯
Embodiment 4	95	◯
Comparative	110	X
Example 1
Comparative	150	X
Example 2
Comparative	170	X
Example 3
Comparative	250	X
Example 4

As shown in the Embodiments of Table 4, in a pipe having a surface grain size of 100 μm or smaller, excellent orange peel appeared, and as shown in the Comparative Examples of Table 4, in a pipe having a surface grain size that is larger than 100 μm, orange peel appeared significantly.
That is, orange peel appeared in the pipe having the surface grain size that is larger than 100 μm. Therefore, by maintaining the surface grain size Gs at 100 μm or smaller, and increasing the size of the internal crystal grains Gi, the strength of the stainless steel may be reduced through an increase in size of the crystal grains of the stainless steel.
For example, the average crystal grain size Gi of the internal crystal grains of the austenitic stainless steel may be larger than 100 μm.

TABLE 5

Surface	Internal		Existence or
Grain size	Grain size		Absence of
(Gs, μm)	(Gi, μm)	Gs/Gi	Orange Peel

Embodiment

1	35	101	0.35	◯
Embodiment 2	50	152	0.33	◯
Embodiment 3	69	169	0.41	◯
Embodiment 4	95	210	0.45	◯
Comparative	110	102	1.08	X
Example 1
Comparative	150	153	0.98	X
Example 2
Comparative	170	260	0.65	X
Example 3
Comparative	250	201	1.24	X
Example 4

If the surface grain size Gs is 100 μm or smaller as in the Embodiments of Table 5, and the ratio Gs/Gi of the surface grain size Gs to the internal grain size Gi is 0.5 or smaller, excellent orange peel resistance appeared.
FIG. 4 is a picture showing the surface of an austenitic stainless steel pipe of FIG. 1. FIG. 5 is a picture showing the surface of a typical austenitic stainless steel pipe. FIG. 6 is a picture showing a section microstructure of the austenitic stainless steel pipe of FIG. 1. FIG. 7 is a picture showing a section microstructure of the typical austenitic stainless steel pipe.
More specifically, FIGS. 4 and 6 show the surface and section microstructure of an austenitic stainless steel pipe according to the Embodiment 3, and FIGS. 5 and 7 show the surface and section microstructure of an austenitic stainless steel pipe according to the Comparative Example 3.
Referring to Table 4, Table 5, and FIGS. 5 to 7, it can be seen that no orange peel is generated on the surface of the austenitic stainless steel pipe according to an embodiment of the present disclosure, the size of crystal grains adjacent to the surface of the stainless steel is smaller than that of the other crystal grains since in the section microstructure, crystal grains of an area adjacent to the surface of the stainless steel have an average size of 100 μm or smaller, and crystal grains of the inside of the stainless steel have an average size that is larger than 100 μm. Accordingly, the size of the crystal grains of the area adjacent to the surface of the stainless steel is smaller than that of the crystal grains of the other area. Also, in the surface of the austenitic stainless steel pipe according to the Comparison Example, orange peel was generated so that the outer appearance deteriorates, and in the section microstructure, the size of crystal grains of an area adjacent to the surface of the stainless steel was a little smaller than that of internal crystal grains of the stainless steel, wherein the crystal grains of the area adjacent to the surface of the stainless steel averagely have a size that is larger than 100 μm.
FIG. 8 is a graph for describing the size of a crystal grain of austenitic stainless steel according to an embodiment of the present disclosure and surface roughness when an austenitic stainless steel pipe is bent 90 degrees.
Referring to FIG. 8, it can be seen that by limiting the size Gs of surface crystal grains, a stainless steel pipe capable of suppressing orange peel while increasing an average crystal grain size of the stainless steel in order to reduce the strength of the stainless steel is manufactured. That is, it can be seen from FIG. 8 that in the Comparative Examples, the surface roughness Rz exceeds 10 μm so that orange peel is generated, while in the Embodiments, the surface roughness Rz is 10 μm or smaller so that orange peel is suppressed although the size Gi of internal crystal grains increases like the Comparative Examples.
More specifically, as shown in FIG. 8, Embodiment in which the average crystal grain size Gi of internal crystal grains is 100 μm corresponds to an austenitic stainless steel pipe manufactured according to the Embodiment 1 in which the surface roughness Rz is about 4.5 μm so that no orange peel is generated, Embodiment in which the average crystal grain size Gi of internal crystal grains is 150 μm corresponds to an austenitic stainless steel pipe manufactured according to the Embodiment 2 in which the surface roughness Rz is about 4.8 μm so that no orange peel is generated, and Embodiment in which the average crystal grain size Gi of the internal crystal grains is 200 μm corresponds to an austenitic stainless steel pipe manufactured according to the Embodiment 4 in which the surface roughness Rz is about 5.2 μm so that no orange peel is generated.
Unlike this, Comparative Example in which the average crystal grain size Gi of internal crystal grains is 100 μm corresponds to an austenitic stainless steel pipe manufactured according to the Comparative Example 1 in which the surface roughness Rz is about 12 μm so that orange peel is generated, Comparative Example in which the average crystal grain size Gi of internal crystal grains is 150 μm corresponds to an austenitic stainless steel pipe manufactured according to the Comparison Example 2 in which the surface roughness Rz is about 19 μm so that orange peel is generated, and Embodiment in which the average crystal grain size Gi of the internal crystal grains is 200 μm corresponds to an austenitic stainless steel pipe manufactured according to the Embodiment 4 in which the surface roughness Rz is about 22 μm so that orange peel is generated.
While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The austenitic stainless steel according to the embodiments of the present disclosure may be applied to refrigerant pipes for household or car air conditioners.

Claims

1. Austenitic stainless steel having excellent orange peel resistance, wherein a ratio Gs/Gi of an average crystal grain size Gs of surface crystal grains included in a first area corresponding to a depth of 10% or less of a total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel with respect to an average crystal grain size Gi of internal crystal grains included in a second area corresponding to a depth that is deeper than 10% of the total thickness of the austenitic stainless steel from the surface of the austenitic stainless steel is 0.5 or smaller.

2. The austenitic stainless steel of claim 1, comprising silicon (Si) of 0.1 to 0.65 weight %, manganese (Mn) of 1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0 weight %, chrome (Cr) of 16.5 to 18.5 weight %, copper (Cu) of 6.0 weight % or less (except for zero), carbon (C) and nitrogen (N) of 0.13 weight % or less (except for zero), iron (Fe), and other inevitable impurities.

3. The austenitic stainless steel of claim 1, wherein the average crystal grain size Gs of the surface crystal grains is 100 μm or less.

4. The austenitic stainless steel of claim 1, manufactured by strip casting.

5. The austenitic stainless steel of claim 4, wherein when the austenitic stainless steel is casted by strip casting, a delta ferritic phase content remaining upon solidification is 5 weight % or more

6. The austenitic stainless steel of claim 5, wherein a delta ferrite phase content of a cold-rolled structure of the austenitic stainless steel is 0.5 weight % or more.

7. A method of manufacturing austenitic stainless steel having excellent orange peel resistance by strip casting of passing stainless steel between a pair of rotating rolls to freeze the stain steel into a solid, the method comprising,

casting austenitic stainless steel, and controlling elements of the austenitic stainless steel according to Equation (1) such that a content Delta of delta ferritic phase remaining upon solidification is 5 wt % or more to manufacture a hot-rolled steel sheet:

Delta=((Cr+Mo+1.5Mn+0.5Nb+2Ti+18)/(Ni+0.3Cu+30*(C+N)+0.5Mn+36)+0.262)×161−161, Equation (1)

wherein element symbols of Equation (1) represent weight percentages (weight %) of the corresponding elements

8. The method of claim 7, wherein the austenitic stainless steel comprises silicon (Si) of 0.1 to 0.65 weight %, manganese (Mn) of 1.0 to 3.0 weight %, nickel (Ni) of 6.5 to 10.0 weight %, chrome (Cr) of 16.5 to 18.5 weight %, copper (Cu) of 6.0 weight % or less (except for zero), carbon (C) and nitrogen (N) of 0.13 weight % or less (except for zero), iron (Fe), and other inevitable impurities.

9. The method of claim 7, further comprising performing heat-treatment on the hot-rolled steel sheet, and then performing cold rolling on the hot-rolled steel sheet at a total reduction ratio of 50% or higher, thereby manufacturing a cold-rolled steel sheet,

wherein a delta ferritic phase content of a cold-rolled structure of the cold-rolled steel sheet is 0.5 weight % or more.