WO2023234525A1 - Austenite stainless steel and manufacturing method therefor - Google Patents

Abstract

Disclosed in the specification are: an austenite stainless steel having high strength, high ductility, and improved corrosion resistance while realizing ultra-fine grain characteristics; and a manufacturing method therefor. An austenite stainless steel according to an embodiment of the present invention may comprise, in weight%, C: 0.05% inclusive to 0.1% inclusive, Si: 0.2% inclusive to 0.7% inclusive, Mn: 2.0% inclusive to 4.0% inclusive, P: 0% exclusive to 0.1% exclusive, S: 0% exclusive to 0.01% exclusive, Cr: 17% inclusive to 19% inclusive, Ni: 2.0% inclusive to 4.0% inclusive, Cu: 1.0% inclusive to 2.5% inclusive, N: 0.15% inclusive to 0.25% inclusive, and the balance being iron (Fe) and inevitable impurities, and may be 5 ㎛ or less in average grain diameter of the thickness center.

Description

Austenitic stainless steel and its manufacturing method

The present invention relates to austenitic stainless steel and a manufacturing method thereof, and more specifically, to an austenitic stainless steel and a manufacturing method thereof that simultaneously improve high strength, high ductility and corrosion resistance by realizing ultra-fine grain characteristics. .

304 steel, a commonly used austenitic stainless steel, has a yield strength of 200 to 350 MPa, which limits its application to structural members. In order to obtain a higher yield strength in 304 steel, an additional temper rolling process is required, which not only increases the cost but also causes a rapid decrease in elongation and formability. In addition, 304 steel contains a large amount of expensive alloy components, which has the problem of poor cost competitiveness.

Patent document 0001 relates to austenitic stainless steel and its manufacturing method. It discloses austenitic stainless steel having a tensile strength of 600 MPa or more, but its price competitiveness is low due to the high Ni content.

Meanwhile, grain refinement technology is attracting attention as a technology for improving both strength and ductility. In particular, severe plastic deformation (SPD) is attracting attention as a method of ultrafineizing steel materials for structural members. Rigid plastic processing is a method of creating fine grains by applying strong shear stress to the material to create new grain boundaries within the grain boundaries. However, rigid processing methods have problems such as reduced productivity and product size limitations.

Patent Document 0002 discloses a method of performing long-term heat treatment at 600 to 700°C for more than 48 hours to achieve an average grain size of 10 ㎛ or less. However, the method disclosed in Patent Document 0002 has the problem of low productivity and increased manufacturing costs.

(Prior art literature)

Patent Document 0001: Publication of Patent No. 10-2016-0138277 (Publication Date: 2016.12.02.)

Patent Document 0002: Japanese Patent Publication No. 2020-050940 (Publication Date: 2020.04.02.)

The purpose of the present invention to solve the above-mentioned problems is to provide an austenitic stainless steel and a manufacturing method thereof that can simultaneously realize high strength, high ductility, and high corrosion resistance by realizing ultra-fine grain characteristics while being price competitive.

The austenitic stainless steel according to an embodiment of the present invention has, in weight percentage, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: 0%. Exceeding 0.1% or less, S: Exceeding 0% but less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, It includes the remaining iron (Fe) and inevitable impurities, and the average grain diameter at the center of the thickness may be 5㎛ or less.

In addition, the austenitic stainless steel according to an embodiment of the present invention may have an Austenite Stability Parameter (ASP) value of -30 to 30, expressed by equation (1) below.

Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

In the formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element.

Additionally, the austenitic stainless steel according to an embodiment of the present invention may have a Strength Stability Parameter (SSP) value of 0 or more, expressed by equation (2) below.

Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] - 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

In the formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element.

In addition, the austenitic stainless steel according to an embodiment of the present invention may have a Pitting Resistance Equivalent Number (PREN) of 17 or more, represented by Equation (3) below.

Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

In the above formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element.

Additionally, the austenitic stainless steel according to an embodiment of the present invention may have a yield strength of 600 MPa or more.

Additionally, the austenitic stainless steel according to an embodiment of the present invention may have an elongation of 30% or more.

Additionally, the austenitic stainless steel according to an embodiment of the present invention may have a pitting potential value of 200 mV or more.

Additionally, the austenitic stainless steel according to an embodiment of the present invention may have a thickness of 0.4 to 2.0 mm.

In addition, the method for manufacturing austenitic stainless steel according to an embodiment of the present invention is, in weight percent, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: more than 0% and less than 0.1%, S: more than 0% and less than 0.01%, Cr: more than 17% and less than 19%, Ni: more than 2.0% and less than 4.0%, Cu: more than 1.0% and less than 2.5%, N: more than 0.15% Manufacturing an ingot containing 0.25% or less of the remaining iron (Fe) and unavoidable impurities; Manufacturing a hot rolled steel sheet by hot rolling the ingot; Manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; and final annealing the cold rolled steel sheet.

Additionally, in the method of manufacturing austenitic stainless steel according to an embodiment of the present invention, the ingot may have an Austenite Stability Parameter (ASP) value of -30 to 30, expressed by equation (1) below.

Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

In the formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element.

Additionally, in the method of manufacturing austenitic stainless steel according to an embodiment of the present invention, the ingot may have a Strength Stability Parameter (SSP) value of 0 or more, expressed by equation (2) below.

Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] – 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

In the formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element.

Additionally, in the method of manufacturing austenitic stainless steel according to an embodiment of the present invention, the ingot may have a pitting resistance equivalent number (PREN) of 17 or more, expressed by equation (3) below.

Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

In formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element.

In addition, the method of manufacturing austenitic stainless steel according to an embodiment of the present invention may further include the step of intermediate annealing the hot rolled steel sheet before the cold rolling.

Additionally, in the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, the intermediate annealing temperature may be performed at 1050 to 1150°C.

Additionally, in the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, the final annealing temperature may be performed at 800 to 850°C.

Additionally, in the method of manufacturing austenitic stainless steel according to an embodiment of the present invention, the cold rolling may be performed so that the thickness reduction rate of the hot rolled steel sheet is 50% or more at room temperature.

According to an embodiment of the present invention, an austenitic stainless steel and a manufacturing method thereof that can simultaneously realize high strength, high ductility, and high corrosion resistance by realizing ultra-fine grain characteristics while being price competitive can be provided.

Figure 1 is a photograph taken with a scanning electron microscope (SEM) of a cross-section at the center of the thickness of an austenitic stainless steel according to an example of the present invention.

The austenitic stainless steel according to an embodiment of the present invention has, in weight percentage, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: 0%. Exceeding 0.1% or less, S: Exceeding 0% but less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, It includes the remaining iron (Fe) and inevitable impurities, and the average grain diameter at the center of the thickness may be 5㎛ or less.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to sufficiently convey the idea of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In order to clarify the present invention, the drawings may omit illustrations of parts unrelated to the description and may slightly exaggerate the sizes of components to aid understanding.

Throughout the specification, when a part is said to “include” a certain element, this means that it may further include other elements rather than excluding other elements, unless specifically stated to the contrary.

Singular expressions include plural expressions unless the context clearly makes an exception.

Hereinafter, the reason for limiting the numerical content of alloy components in the embodiments of the present invention will be explained. Hereinafter, unless otherwise specified, the unit is weight%.

The austenitic stainless steel according to an embodiment of the present invention has, in weight percentage, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: 0%. Exceeding 0.1% or less, S: Exceeding 0% but less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, It may contain remaining iron (Fe) and inevitable impurities.

The C (carbon) content may be 0.05% or more and 0.1% or less.

C is an effective element in stabilizing the austenite phase and must be added appropriately to secure the yield strength of steel. Considering this, C may be added in an amount of 0.05% or more. However, if the C content is excessive, cold workability may be reduced due to the solid solution strengthening effect. Additionally, if the C content is excessive, ductility and corrosion resistance may be adversely affected due to grain boundary precipitation of Cr carbide during low-temperature annealing. Considering this, the upper limit of C content may be limited to 0.1%.

The content of Si (silicon) may be 0.2% or more and 0.7% or less.

Si can be added to deoxidize steel and is an effective element in improving corrosion resistance. Considering this, Si may be added in an amount of 0.2% or more. However, if the Si content is excessive, the formation of delta ferrite in the casting material may be promoted due to the ferrite phase stabilization effect. Therefore, if the Si content is excessive, hot workability may be reduced and ductility and impact properties may be adversely affected. Considering this, the upper limit of Si content may be limited to 0.7%. Preferably, Si may be added in an amount of 0.3% or more and 0.4% or less.

The content of Mn (manganese) may be 2.0% or more and 4.0% or less.

In the present invention, Mn is an austenite phase stabilizing element added instead of Ni. Considering this, Mn may be added in an amount of 2.0% or more. However, if the Mn content is excessive, S-based inclusions (MnS) may be formed in excessive amounts, thereby reducing ductility and corrosion resistance. Considering this, the upper limit of Mn content may be limited to 4.0%. Preferably, Mn may be added in an amount of 3.6% or more and 3.9% or less.

The content of P (phosphorus) may be more than 0% and less than 0.1%.

P is an impurity inevitably contained in steel and is an element that causes intergranular corrosion and impedes hot workability. Therefore, it is desirable to control the P content as low as possible. Considering this, the upper limit of P content can be controlled to less than 0.1%.

The S (sulfur) content may be more than 0% and less than 0.01%.

S, like P, is an impurity that is inevitably contained in steel and is an element that segregates at grain boundaries and causes deterioration of hot workability. Therefore, it is desirable to control the S content as low as possible. Considering this, the upper limit of S content can be controlled to less than 0.01%.

The content of Cr (chromium) may be 17% or more and 19% or less.

Cr is an element that is effective in suppressing the formation of martensite phase and ensuring corrosion resistance. Considering this, Cr may be added in an amount of 17% or more. However, if the Cr content is excessive, manufacturing costs may increase and hot workability may be reduced due to the formation of a large amount of delta ferrite in the material. Considering this, the upper limit of Cr content can be limited to 19%. Preferably, Cr may be added in an amount of 17.2% or more and 18% or less.

The content of Ni (nickel) may be 2.0% or more and 4.0% or less.

Ni is a strong austenite phase stabilizing element and is an essential element to ensure good hot and cold workability. Therefore, despite adding a certain amount of Mn, Ni may be added in an amount of 2.0% or more. However, when the Ni content is excessive, the martensite transformation initiation temperature (Ms) becomes too low, making it difficult to generate stress-induced martensite during cold working. Additionally, if the Ni content is excessive, it may result in an increase in raw material costs. Considering this, the upper limit of Ni content may be limited to 4.0%. Preferably, Ni may be added in an amount of 3.4% or more and 3.7% or less.

The content of Cu (copper) may be 1.0% or more and 2.5% or less.

Cu is an austenite phase stabilizing element and is effective in softening materials. Considering this, Cu may be added in an amount of 1.0% or more. However, when the Cu content is excessive, the martensite transformation initiation temperature (Ms) becomes too low, making it difficult to generate stress-induced martensite during cold working. Additionally, if the Cu content is excessive, material costs may increase and hot brittleness may occur. Considering this, the upper limit of Cu content may be limited to 2.5%. Preferably, Cu may be added in an amount of 1.5% or more and 2.0% or less.

The content of N (nitrogen) may be 0.15% or more and 0.25% or less.

N is an effective element in stabilizing the austenite phase and improving corrosion resistance. Considering this, N may be added in an amount of 0.15% or more. However, if the N content is excessive, cold workability may be reduced due to the freezing strengthening effect, and the martensite transformation initiation temperature (Ms) may become too low, making it difficult to generate stress-induced martensite during cold working. Additionally, if the N content is excessive, quality defects may occur due to pore formation during casting. Considering this, the upper limit of N content may be limited to 0.25%. Preferably, N may be added in an amount of 0.16% or more and 0.21% or less.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all of them are not specifically mentioned in this specification.

The austenitic stainless steel according to an embodiment of the present invention implements ultra-fine grain characteristics, and the average grain diameter at the center of the thickness may be 5 μm or less. Here, the thickness center means 1/4t to 3/4t when the stainless steel thickness is t.

Meanwhile, in the present invention, average refers to the average value of values measured at five arbitrary locations.

The austenitic stainless steel according to an embodiment of the present invention may have an Austenite Stability Parameter (ASP) value of -30 to 30, expressed by equation (1) below.

Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

In the formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element.

Equation (1) refers to the temperature at which 50% of austenite transforms into martensite when stainless steel is transformed to 0.3 true strain, and is used as an indicator of austenite phase stability. The lower the value of equation (1), the higher the austenite phase stability, and the smaller the amount of processed martensite transformed during deformation.

If the value of equation (1) is less than -30, the amount of TRIP transformation from austenite to martensite is low, and the amount of processed martensite decreases. Therefore, if the value of equation (1) is less than -30, the rate of turning into a reversed austenite phase due to low-temperature annealing decreases, and it may become difficult to secure ultra-fine grains. However, if the value of equation (1) exceeds 30, the yield strength and elongation may be lowered due to too fast TRIP transformation.

The austenitic stainless steel according to an embodiment of the present invention may have a Strength Stability Parameter (SSP) value of 0 or more, expressed by equation (2) below.

Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] – 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

In the formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element.

If the value of equation (2) is less than 0, it may be difficult to implement ultra-fine microstructure characteristics in a wide range of final annealing temperatures. In other words, in order to realize ultra-fine microstructure characteristics in all areas of 800 to 850°C, which is the final annealing temperature range proposed in the present invention, it is necessary to control the value of equation (2) above 0.

The austenitic stainless steel according to an embodiment of the present invention may have a pitting resistance equivalent number (PREN) of 17 or more, expressed by equation (3) below.

Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

In formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element.

If the value of equation (3) is less than 17, the pitting potential measured with a 3.5% NaCl solution (30°C) may not satisfy 200 mV or more. That is, if the value of equation (3) is less than 17, it may be difficult to achieve the target high corrosion resistance.

Ultra-fine grain characteristics can be realized through the alloy components, parameters, and manufacturing method presented in the present invention. Therefore, the austenitic stainless steel according to an embodiment of the present invention may have a yield strength of 600 MPa or more and an elongation of 30% or more.

Additionally, the austenitic stainless steel according to an embodiment of the present invention may have a pitting potential value of 200 mV or more.

The austenitic stainless steel according to an embodiment of the present invention may have a thickness of 0.4 to 2.0 mm. However, it is not limited to this and can be manufactured in various thicknesses depending on the purpose.

Next, a method for manufacturing austenitic stainless steel according to another aspect of the present invention will be described.

The method for manufacturing austenitic stainless steel according to an embodiment of the present invention is, in weight percentage, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: More than 0% but less than 0.1%, S: More than 0% but less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% Hereinafter, manufacturing an ingot containing the remaining iron (Fe) and inevitable impurities; Manufacturing a hot rolled steel sheet by hot rolling the ingot; Manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; and final annealing the cold rolled steel sheet.

The ingot may have an Austenite Stability Parameter (ASP) value of -30 to 30, expressed by equation (1) below.

Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

In the formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element.

Additionally, in the method of manufacturing austenitic stainless steel according to an embodiment of the present invention, the ingot may have a Strength Stability Parameter (SSP) value of 0 or more, expressed by equation (2) below.

Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] - 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

In the formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element.

Additionally, in the method of manufacturing austenitic stainless steel according to an embodiment of the present invention, the ingot may have a pitting resistance equivalent number (PREN) of 17 or more, expressed by equation (3) below.

Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

In the above formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element.

Meanwhile, depending on the purpose and use, it can also be manufactured as a slab instead of the ingot.

The reason for limiting the numerical range of the component range of each alloy composition and the values of Equation (1), Equation (2), and Equation (3) is as described above, and each manufacturing step will be described in more detail below.

First, after preparing an ingot that satisfies the alloy composition, it can be subjected to a series of hot rolling, cold rolling, and final annealing processes.

First, the ingot can be heated at 1150 to 1300°C and then hot rolled to produce a hot rolled steel sheet.

If the heating temperature is low, it may be difficult to re-decompose the coarse precipitates generated during ingot manufacturing. Considering this, the heating temperature may be 1150°C or higher. However, if the heating temperature is too high, internal crystal grains may become too coarse, and surface oxidation may occur severely, causing surface defects. Considering this, the upper limit of the heating temperature may be limited to 1300°C.

Next, before the cold rolling, the step of intermediate annealing the hot rolled steel sheet may be further included. The intermediate annealing step may be performed as needed or may be omitted.

When performing the intermediate annealing, it can be performed at a temperature of 1000 to 1150°C.

If the intermediate annealing temperature is low, the residual martensite fraction may increase and machinability may be reduced. However, if the intermediate annealing temperature is too high, strength may decrease due to grain coarsening.

The final annealing temperature may be 800 to 850°C.

Like the intermediate annealing temperature, if the final annealing temperature is too low, processability may be poor. However, if the final annealing temperature is too high, the strength may decrease due to grain coarsening.

The cold rolling may be performed so that the thickness reduction rate of the hot rolled steel sheet is 50% or more at room temperature. If the thickness reduction rate during cold rolling is less than 50%, the amount of processing-induced martensite decreases and the ratio of ultra-fine reverse transformation austenite phase decreases during low-temperature annealing, making it difficult to secure strength.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

{Example}

For the various alloy composition ranges shown in Table 1 below, they were cast in the form of a 35 kg ingot with a thickness of 150 mm in a vacuum induction melting furnace. The cast ingot was heated in a furnace at 1250°C for 2 hours, then hot-rolled to a sheet width of 200 mm and a thickness of 4 mm to produce a hot-rolled steel sheet, and then air-cooled. The air-cooled hot-rolled steel sheet was intermediately annealed at 1100°C for 1 minute, then pickled and then cold-rolled to a thickness of 1.2 mm to produce a cold-rolled steel sheet. The cold rolled steel sheet was annealed at the final annealing temperature shown in Table 2 to produce a final product.

division	alloy composition
	C	Si	Mn	Cr	Ni	Cu	N
Example 1	0.1	0.4	3.9	18	3.5	1.5	0.21
Example 2	0.06	0.3	3.7	17.7	3.6	1.9	0.18
Example 3	0.05	0.3	3.6	17.7	3.7	2.0	0.2
Example 4	0.07	0.4	3.6	17.2	3.4	1.7	0.16
Comparative Example 1	0.06	0.6	4.3	18.7	4.2	1.4	0.22
Comparative example 2	0.07	0.6	5.7	16.7	2.4	1.5	0.15
Comparative example 3	0.08	0.4	2.9	17.3	2.8	1.3	0.17
Comparative example 4	0.05	0.5	5.1	16.8	4.1	1.8	0.15

Table 2 below shows the Equation (1) value, Equation (2) value, Equation (3) value, final annealing temperature, average grain diameter, pitting dislocation, yield strength, and elongation. The Equation (1) value is expressed in the equation below ( 1) was calculated and shown.

Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

In the formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element.

The value of equation (2) was calculated by calculating equation (2) below.

Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] – 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

In the formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element.

The value of equation (3) was calculated by calculating equation (3) below.

Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

In the above formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element.

The average grain diameter was measured by scanning the central cross section of the steel using a scanning electron microscope (SEM), model name JSM-7001F.

The formal potential was measured using a potentiostat device. At this time, when the steel was immersed in a NaCl solution and a voltage of 20mV/min was applied, the pitting potential at which the current reached 100㎂ was measured. Here, the temperature of the NaCl solution was 30°C, and the concentration was set to 3.5%. Meanwhile, the higher the pitting potential value, the better the corrosion resistance.

Yield strength and elongation were measured at room temperature using a tensile tester from Zwick Roell on JIS13B tensile test specimens at a tensile speed of 15 mm per minute.

division	Equation (1)	Equation (2)	Equation (3)	final annealing temperature (℃)	average grain diameter (㎛)	official avant-garde (mV)	yield strength (MPa)	elongation (%)
Example 1	-19	1.8	19.4	800	3.2	309	679	36.0
				850	4.1	318	622	38.5
Example 2	5	1.0	18.7	800	2.9	273	678	35.8
				850	4.4	281	613	38.4
Example 3	-4.2	0.7	19.7	800	2.9	276	660	38.6
				850	4.1	284	610	41.4
Example 4	28	2.1	18.0	800	3.8	233	666	35.9
				850	4.6	240	624	37.9
Comparative Example 1	-37	-4	20.1	800	3.9	344	608	41.9
				850	8.6	354	557	42.6
Comparative Example 2	56	-2.4	16.3	800	3.2	143	657	38.7
				850	7.4	147	576	42.0
Comparative Example 3	52	-3.6	18.6	800	4.5	265	614	37.9
				850	8.9	273	562	42.2
Comparative Example 4	11	7.6	16.7	800	3.1	169	762	29.4
				850	4.1	164	676	36.5

Referring to Table 2, Examples 1 to 4 satisfied the alloy composition, component range, parameters, and manufacturing process presented in the present invention. Therefore, Examples 1 to 4 satisfied the average grain diameter of 5 ㎛ or less, yield strength of 600 MPa or more, elongation of 30% or more, and pitting potential value of 200 mV or more. That is, Examples 1 to 4 simultaneously satisfied high strength, high ductility, and high corrosion resistance. However, in the case of Comparative Examples 1 to 3, the value of Equation (1) did not satisfy -30 to 30, and Equation (2) ) did not satisfy the value of 0 or more. Therefore, the grain diameter of 5 μm or less and the yield strength of 600 MPa or more were not satisfied. In particular, it was difficult to realize ultra-fine microstructure characteristics in all areas of 800 to 850°C, which is the final annealing temperature range suggested by the present invention.

In the case of Comparative Examples 2 and 4, the Mn content was excessive, and the value of equation (3) did not satisfy 17 or more. Therefore, in the case of Comparative Examples 2 and 4, the formal potential value did not satisfy 200 mV or more. That is, Comparative Examples 2 and 4 had inferior corrosion resistance.

Figure 1 is a photograph taken with a scanning electron microscope (SEM) of a cross-section at the center of the thickness of an austenitic stainless steel according to an example of the present invention.

Referring to FIG. 1, it can be seen that the austenitic stainless steel according to an example of the present invention has an average grain diameter at the center of the thickness of 5 μm or less. In other words, it can be seen that ultra-fine grain characteristics can be realized by following an example of the present invention.

According to one embodiment of the present invention, it is possible to provide an austenitic stainless steel and a manufacturing method thereof that are price competitive and can simultaneously realize high strength, high ductility, and high corrosion resistance by realizing ultra-fine grain characteristics, and an industrial method thereof. Availability is acknowledged.

Claims (16)

1. By weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: 0% to 0.1%, S: 0% to 0.01%, Cr : 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, including the remaining iron (Fe) and inevitable impurities,

   Austenitic stainless steel with an average grain diameter at the center of the thickness of 5㎛ or less.
2. In claim 1,

   Austenitic stainless steel with an ASP (Austenite Stability Parameter) value of -30 to 30, expressed in equation (1) below:

   Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

   (In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element).
3. In claim 1,

   Austenitic stainless steel with an SSP (Strength Stability Parameter) value of 0 or more, expressed in equation (2) below:

   Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] – 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

   (In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element).
4. In claim 1,

   Austenitic stainless steel with a pitting resistance equivalent number (PREN) of 17 or more, expressed in equation (3) below:

   Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

   (In the above formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element).
5. In claim 1,

   Austenitic stainless steel with a yield strength of 600 MPa or more.
6. In claim 1,

   Austenitic stainless steel with an elongation of 30% or more.
7. In claim 1,

   Austenitic stainless steel with a pitting potential value of 200 mV or more.
8. In claim 1,

   Austenitic stainless steel with a thickness of 0.4 to 2.0 mm.
9. By weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: 0% to 0.1%, S: 0% to 0.01%, Cr : 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, to manufacture ingots containing the remaining iron (Fe) and inevitable impurities. step;

   Manufacturing a hot rolled steel sheet by hot rolling the ingot;

   Manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; and

   A method of manufacturing austenitic stainless steel, comprising the step of final annealing the cold rolled steel sheet.
10. In claim 9,

    The ingot has an ASP (Austenite Stability Parameter) value of -30 to 30, expressed by equation (1) below. Method for manufacturing austenitic stainless steel:

    Equation (1): 551 - 462×([C]+[N]) - 9.2×[Si] - 8.1×[Mn] - 13.7×[Cr] - 29×([Ni]+[Cu])

    (In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (% by weight) of each element).
11. In claim 9,

    The ingot is an austenitic stainless steel with an SSP (Strength Stability Parameter) value of 0 or more, expressed in equation (2) below:

    Equation (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] – 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N]

    (In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] mean the content (% by weight) of each element).
12. In claim 9,

    The ingot is an austenitic stainless steel with a pitting resistance equivalent number (PREN) of 17 or more, expressed in equation (3) below:

    Equation (3): [Cr] - 0.5×[Mn] + 16×[N]

    (In the above formula (3), [Cr], [Mn], and [N] mean the content (% by weight) of each element).
13. In claim 9,

    A method of manufacturing austenitic stainless steel, further comprising the step of intermediate annealing the hot rolled steel sheet before the cold rolling.
14. In claim 13,

    A method of manufacturing austenitic stainless steel, wherein the intermediate annealing temperature is performed at 1050 to 1150°C.
15. In claim 9,

    A method of manufacturing austenitic stainless steel, wherein the final annealing temperature is performed at 800 to 850°C.
16. In claim 9,

    The cold rolling is performed so that the thickness reduction rate of the hot rolled steel sheet is 50% or more at room temperature.

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