EP4403663A1

EP4403663A1 - Ferritic stainless steel and manufacturing method therefor

Info

Publication number: EP4403663A1
Application number: EP22887483.0A
Authority: EP
Inventors: Sooho Park; Jiwoong JUN; Jeehyun YU; Hoihun KIM
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-10-26
Filing date: 2022-10-20
Publication date: 2024-07-24
Also published as: KR20230059481A; EP4403663A4; US20240417834A1; CN118159680A; WO2023075287A1; JP2024541928A

Abstract

Disclosed in the present specification are: a ferritic stainless steel having improved elongation even if batch annealing is omitted; and a manufacturing method therefor. A ferritic stainless steel according to one embodiment of the present invention may comprise, by wt%, 0.01-0.1% of C, 0.01-1.0% of Si, 0.01-1.5% of Mn, P in an amount greater than 0% and less than or equal to 0.05%, S in an amount greater than 0% and less than or equal to 0.005%, 13.0-18.0% of Cr, 0.005-0.1% of N, 0.005-0.2% of Al, 0.05-0.25% of Ni, and the balance of Fe (iron) and other inevitable impurities.

Description

[Technical Field]

The disclosure relates to a ferritic stainless steel and method of manufacturing the same, and more particularly, to a ferritic stainless steel and method of manufacturing the same, by which elongation is improved while omitting box annealing.

[Background Art]

Ferritic stainless steel has less costly alloy elements added thereto, and is thus highly competitive in price as compared to austenitic stainless steel. The ferritic stainless steel has good corrosion resistance, so is widely used for construction materials, vehicles, home appliances, kitchen tools, etc.
In general, 430 series hot-rolled steel material undergoes a box annealing process, and the box annealing process is performed at a temperature of 800 to 850 °C, at which an austenite phase is transformed to a ferrite phase, for 35 to 50 hours. A purpose of the box annealing is to recrystallize the deformed structure formed in hot rolling and decompose the austenite phase into the ferrite phase and carbides. However, the box annealing not only consumes a lot of energy but also reduces productivity due to the long-term heat treatment. Hence, the development of a continuous annealing manufacturing technology that may facilitate reduction of manufacturing costs by reducing energy and improving productivity has been proceeded.
Patent document 1 discloses that hot rolling is performed on 430 steel alloy-designed to enable continuous annealing at least 1 pass with a reduction rate of at least 20% per rough rolling pass, and patent document 2 discloses that when work is done with a starting temperature of finishing rolling being at least 950 °C and a coiling temperature being 650 °C or less, quality characteristics are good without occurrence of sticking defects.
In the meantime, when the box annealing is omitted but continuous annealing is performed and then heat treatment is performed under a normal annealing condition for the 430 series stainless steel, there is a risk of having low elongation due to formation of fine Cr carbides precipitated during cooling after hot rolling.
There have been almost no attempts to improve the elongation by controlling the annealing heat treatment temperature of the hot-rolled steel material and cold-rolled steel material associated with Ac1, which is the phase transformation temperature calculated by an alloy composition to solve the problem.

(Prior Art Literature)

Patent Document 1: JP Patent Publication No. 57- 70230 (published on February 10, 1997 )
Patent Document 2: JP Patent Publication No. 57- 155326 (published on December 22, 1989 )

[Disclosure]

[Technical Problem]

To solve the aforementioned problem, the disclosure aims to provide a ferritic stainless steel and method of manufacturing the same, by which elongation is improved while omitting box annealing but performing continuous annealing.

[Technical Solution]

According to an embodiment of the disclosure, a ferritic stainless steel includes, in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities, wherein an Ac1 value defined in the following equation 1 may be at least 920 and less than 990. Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
In equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements.
In an embodiment of the disclosure, the ferritic stainless steel has 27% or more of elongation.
In an embodiment of the disclosure, a method of manufacturing a ferritic stainless steel includes manufacturing a slab including, in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities, wherein an Ac1 value defined in the following equation 1 is at least 920 and less than 990; reheating the slab; manufacturing a hot-rolled steel material by hot-rolling and coiling the reheated slab; obtaining a hot-rolled and wound hot-rolled sheet by hot-rolled-sheet-annealing the hot-rolled steel material at a hot-rolled sheet annealing heat treatment temperature T (HRA, °C) which satisfies equation (2) below, followed by cooling and winding; manufacturing a cold-rolled sheet by cold-rolling the hot-rolled and wound hot-rolled sheet; and cold-rolled-sheet-annealing the cold-rolled sheet at a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) which satisfies equation (3) below, followed by cooling and winding. Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
In equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements. $840 \leq T (HRA, ° C) \leq Ac 1 - 20$
$870 \leq T (CRA, ° C) \leq Ac 1 - 20$
In an embodiment of the disclosure, in the method of manufacturing the ferritic stainless steel, the reheating may be performed at 1100 to 1250 °C.
In an embodiment of the disclosure, in the method of manufacturing the ferritic stainless steel, the hot-rolling may be performed at a finish rolling completion temperature of 800 to 950 °C.
In an embodiment of the disclosure, in the method of manufacturing the ferritic stainless steel, the coiling may be performed at 750 to 850 °C.
In an embodiment of the disclosure, in the method of manufacturing the ferritic stainless steel, the hot-rolled sheet annealing and winding and the cold-rolled sheet annealing and winding may be performed for 30 seconds to 10 minutes.
In an embodiment of the disclosure, in the method of manufacturing the ferritic stainless steel, the cooling after the hot-rolled sheet annealing and the cooling after the cold-rolled sheet annealing may be performed at a cooling rate of 10 to 50 °C/s.
In an embodiment of the disclosure, in the method of manufacturing the ferritic stainless steel, the cold-rolling may be performed at a reduction rate of 60 to 90%.

[Advantageous Effects]

According to an embodiment of the disclosure, a ferritic stainless steel and method of manufacturing the same, by which elongation is improved by controlling an annealing heat treatment temperature while omitting box annealing and performing continuous annealing, may be provided.
Furthermore, according to an embodiment of the disclosure, manufacturing costs may be saved by omitting the box annealing process that requires long time.

Brief Description of Drawings

FIG. 1 is a graph representing an annealing heat treatment temperature range that may secure 27% or more of elongation.

Best Mode

According to an embodiment of the disclosure, a ferritic stainless steel includes, in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities, wherein an Ac1 value defined in the following equation 1 may be at least 920 and less than 990. Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
In equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements

Modes

Reference will now be made in detail to embodiments, which are illustrated in the accompanying drawings. The following embodiments are provided as examples to convey the full spirit of the disclosure to those of ordinary skill in the art to which the embodiments of the disclosure belong. The disclosure is not limited to the embodiments suggested herein but may be specified in other forms. In the drawings, unrelated part of the description is not shown to clarify the disclosure, and the size of an element may be a little exaggerated to help understanding.
Throughout the specification, the term "include (or including)" or "comprise (or comprising)" is inclusive or open-ended and does not exclude additional, unrecited components, elements or method steps, unless otherwise stated.
It is to be understood that the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.
A reason for numerical limitation of the content of an alloy composition in an embodiment of the disclosure will now be described. A unit of weight(wt)% will now be used unless otherwise mentioned.
In an embodiment of the disclosure, a ferritic stainless steel includes, in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities.
The content of C (carbon) may be 0.01 to 0.1%.
C is a powerful austenite phase stabilizing element, which is effective to increase strength of the material through solid solution strengthening. Considering this, at least 0.01% of C may be added. However, when the content of C is excessive, the strength overly increases, causing deterioration of elongation, toughness, etc., of the steel material. Considering this, the upper limit of the content of C is limited to 0.1%.
The content of Si (silicon) may be 0.01% to 1.0%.
Si is an element added as a deoxidizer in a steelmaking step, which is effective in increasing yield strength and corrosion resistance. Also, Si is an element that may increase stability of the ferrite phase. Considering this, 0.01% or more of Si may be added. However, when the content of Si is excessive, it may cause hardening of the material, thereby deteriorating elongation and toughness. Considering this, the upper limit of the content of Si may be limited to 1.0%.
The content of Mn (manganese) may be 0.01 to 1.5%.
Mn is an element effective in increasing corrosion resistance. Considering this, Mn may be added in at least 0.01% and preferably, at least 0.2%. However, when the content of Mn is excessive, it forms inclusions (MnS), which deteriorates hot workability, ductility and toughness of the steel material. Considering this, the upper limit of the Mn content may be limited to 1.5%, more preferably, 1.0%.
The content of P (phosphorus) may be more than 0 to 0.05%.
P is an impurity unavoidably contained in steel, and is a source element to cause intergranular corrosion in winding or hinder hot workability. Hence, it is desirable to control the P content as low as possible. Considering this, the upper limit of the P content may be limited to 0.05%.
The content of S (sulfur) may be more than 0 to 0.005%.
S is an impurity unavoidably contained in steel, and is a source element that is precipitated on grain boundaries and hinders hot workability. Hence, it is desirable to control the S content as low as possible. Considering this, the upper limit of the S content may be limited to 0.005%.
The content of Cr (chrome) may be 13.0 to 18.0%.
Cr is an element that improves corrosion resistance by forming a passive state film in an oxidizing environment. Considering this, Cr may be added in at least 13.0%. However, when the content of Cr is excessive, it promotes delta (δ) ferrite formation in the slab, reducing the elongation rate and impact toughness, and increases manufacturing costs. Considering this, the upper limit of the content of Cr may be limited to 18.0%.
The content of N (nitrogen) may be 0.005 to 0.1%.
Like C, N is an interstitial element, which is effective in increasing yield strength of the steel material according to the solid solution strengthening effect. Considering this, N may be added in at least 0.005%. However, when the content of N is excessive, impact toughness and formability may deteriorate. Considering this, the upper limit of the content of N may be limited to 0.1%.
The content of Al (aluminum) may be 0.005 to 0.2%.
Al is a powerful deoxidizer, an element that plays a role to reduce the content of oxygen in melted steel. Considering this, Al may be added in at least 0.005%. However, when the content of Al is excessive, non-metal inclusions increase, so that Sliver defects in the cold-rolled strip may occur and at the same time, weldability may deteriorate. Considering this, the upper limit of the Al content may be limited to 0.2%, and more preferably, 0.15% or less.
The content of Ni (nickel) may be 0.05 to 0.25%.
Ni is effective in softening the steel material. Considering this, 0.05% or more of Ni may be added. However, when the content of Ni is excessive, it may increase costs. Considering this, the upper limit of the content of Ni may be limited to 0.25%.
The remaining component is iron (Fe) in the disclosure. However, unintended impurities may be inevitably mixed in from raw materials or surroundings in the normal manufacturing process, so they may not be excluded. These impurities may be known to anyone skilled in the ordinary manufacturing process, so not all of them are specifically mentioned in this specification.
In an embodiment of the disclosure, the ferritic stainless steel may have an Ac1 value defined in the following equation 1, which may be at least 920 and less than 990. Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
In equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements.
Ac1 refers to a temperature at which an austenite phase is transformed to a ferrite phase. The disclosure is characterized by improvement of the elongation rate by controlling an annealing heat treatment temperature based on an Ac1 value calculated by designing an alloy composition and ingredient ranges.
When the calculated value of Ac1 is low, heat treatment is performed at low temperature so that sufficient recrystallization does not occur during continuous annealing of the hot-rolled sheet. Considering this, the alloy composition and ingredient ranges may be designed such that the calculated value of Ac1 is at least 920. However, when the calculated value of Ac1 is overly high, contents of austenite forming elements such as C, N, etc., are reduced, leading to insufficient formation of carbides and nitrides, which deteriorates strength. Considering this, the alloy composition and ingredient ranges may be designed such that the calculated value of Ac1 is less than 990.
In an embodiment of the disclosure, the ferritic stainless steel may have at least 27% of elongation rate by controlling the annealing heat treatment temperature based on the calculated value of Ac1.
Next, a method of manufacturing a ferritic stainless steel according to another aspect of the disclosure will now be described.
In an embodiment of the disclosure, a method of manufacturing a ferritic stainless steel includes manufacturing a slab including, in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities, wherein an Ac1 value defined in equation 1 below is at least 920 and less than 990; reheating the slab; manufacturing a hot-rolled steel material by hot-rolling and coiling the reheated slab; obtaining a hot-rolled and wound hot-rolled sheet by hot-rolled-sheet-annealing the hot-rolled steel material at a hot-rolled sheet annealing heat treatment temperature T (HRA, °C) which satisfies equation (2) below, followed by cooling and winding; manufacturing a cold-rolled sheet by cold-rolling the hot-rolled and wound hot-rolled sheet; and cold-rolled-sheet-annealing the cold-rolled sheet at a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) which satisfies equation (3) below, followed by cooling and winding. Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
In equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements. $840 \leq T (HRA, ° C) \leq Ac 1 - 20$
$870 \leq T (CRA, ° C) \leq Ac 1 - 20$
The reason of numerical limitation of equation 1 and the ingredient range of each alloy composition is as described above, and each manufacturing step will now be described in more detail.
First, a slab that satisfies the alloy composition and equation 1 may be manufactured, and may then undergo a series of hot rolling, hot-rolled sheet annealing and winding, cold rolling, cold-rolled sheet annealing and winding processes.
The slab may be hot-rolled at a reheating temperature of 1100 to 1250 °C.
When the reheating temperature of the slab is too low, the load of the rolling roll may increase. Considering this, the reheating temperature of the slab may be at least 1100 °C. However, when the reheating temperature is too high, the grain diameter of the slab may be coarsened, which may deteriorate the strength. Considering this, the upper limit of the reheating temperature of the slab may be limited to 1250 °C.
Next, the hot rolling may be performed at a finish rolling completion temperature of 800 to 950 °C.
When the finish rolling completion temperature is low, the rolling load may increase, leading to a reduction in productivity. Considering this, the finish rolling completion temperature may be at least 800 °C. However, when the finish rolling completion temperature is too high, the crystal grain size may increase to reduce strength. Considering this, the finish rolling completion temperature may be controlled to be 950 °C or less.
Furthermore, the coiling may be performed at 750 to 850 °C.
When the coiling temperature is low, it may be difficult to control the shape of the coil, and when the coiling temperature is too high, it is likely to cause defects in the post-process due to continuous phase transformation after the coiling. Considering this, the coiling temperature may be set to 750 to 850 °C.
After this, the hot-rolled steel material may be hot-rolled-sheet-annealed at the hot-rolled sheet annealing heat treatment temperature T (HRA, °C) that satisfies the following equation (2): $840 \leq T (HRA, ° C) \leq Ac 1 - 20$
When the hot-rolled sheet annealing heat treatment temperature T (HRA, °C) is low, sufficient recrystallization is not performed. However, as cold-rolled sheet annealing is performed as the post-process, it may proceed at a relatively low temperature. Considering this, the hot-rolled sheet annealing heat treatment temperature T (HRA, °C) may be at least 840 °C. However, when the hot-rolled sheet annealing heat treatment temperature T (HRA, °C) is at least Ac1 temperature, an austenite phase may be formed, and a martensite phase may be formed during quenching after the heat treatment. Considering this, the upper limit of the hot-rolled sheet annealing heat treatment temperature T (HRA, °C) may be limited to (Ac1 - 20).
The heat-rolled sheet annealing may be performed for 30 seconds to 10 minutes.
When the hot-rolled sheet annealing time is short, the elongation may deteriorate due to the high fraction of residual martensite. Considering this, the hot-rolled sheet annealing may be performed for at least 30 seconds. However, when the hot-rolled sheet annealing time is too long, the strength may be reduced due to coarsening of crystal grains, and thickness of a surface oxide layer may increase so that winding hours to remove the oxide layer may be prolonged or the oxide layer may not sufficiently removed. Considering this, the hot-rolled sheet annealing may be controlled to 10 minutes or less.
The cooling after the hot-rolled sheet annealing may be performed at a cooling rate of 10 to 50 °C/s.
When the cooling rate is low, elongation and formability may deteriorate because of non-uniformity of the structure due to softening. On the other hand, when the cooling rate is too high, the elongation is affected adversely due to excessive hardening. Considering this, the cooling rate may be controlled to 10 to 50 °C/s.
The cold rolling may be performed at a reduction rate of 60 to 90%.
When the reduction rate is low, it is difficult to obtain a recrystallized structure because accumulated energy from cold working is not sufficient. However, when the reduction rate is too high, cracks may occur due to rolling. Considering this, the reduction rate may be controlled to 60 to 90%.
After this, the cold-rolled sheet may be cold-rolled-sheet annealed at a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) which satisfies the following equation (3). $870 \leq T (CRA, ° C) \leq Ac 1 - 20$
Like the hot-rolled sheet annealing heat treatment temperature T (HRA, °C), when the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) is low, recrystallization is not sufficiently done. Considering this, the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) may be at least 870 °C. However, when the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) is at least Ac1 temperature, an austenite phase is formed and a martensite phase may be formed during quenching after the heat treatment, so the upper limit of the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) may be limited to (Ac1 - 20).
The cold-rolled sheet annealing may be performed for 30 seconds to 10 minutes, and cooling after the cold-rolled sheet annealing may be performed at a cooling rate of 10 to 50 °C/s.
The reason for limiting the numerical values of the cold-rolled sheet annealing time and cooling rate is as described above.
Embodiments of the disclosure will now be described in more detail. The embodiments may be merely for illustration, and the disclosure is not limited thereto. The scope of the disclosure is defined by the claims and their equivalents.

{Embodiment}

A slab was manufactured with various alloy ingredient ranges shown in table 1 below. The manufactured slab was reheated at 1200 °C, hot-rolled at a finish rolling completion temperature of 800 °C, and then coiled at 750 °C to produce hot rolled steel material.
Ac1 refers to a value defined in the following equation 1: Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

In equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements.

[Table 1]

	alloy ingredients
	C	Si	M n	Cr	Ni	N	Al	P	S
A	0.0 44	0.3 4	0.1 7	16. 18	0.1 4	0.01 29	0.1 22	0.0 26	0.00 05	97 4.3
B	0.0 31	0.2 7	0.8 0	16. 25	0.0 9	0.01 98	0.1 07	0.0 23	0.00 06	93 6.6
C	0.0 41	0.1 9	0.8 1	16. 39	0.1 1	0.02 13	0.0 93	0.0 25	0.00 07	91 0.3

The manufactured hot-rolled steel material was hot-rolled-sheet annealed at a hot-rolled sheet annealing heat treatment temperature T (HRA, °C) for 10 minutes, cooled at a cooling rate of 30 °C/s and wound to obtain a hot-rolled wound hot-rolled sheet. Next, it is cold rolled at a reduction rate of 60%, cold-rolled-sheet annealed at the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) for 10 minutes, cooled at a cooling rate of 30 °C/s and then wound to produce steel. Table 2 below shows the hot-rolled sheet annealing heat treatment temperature T (HRA, °C), the cold-rolled sheet annealing heat treatment temperature T (CRA, °C), thickness and elongation of the manufactured steel.

The elongation (rate) was measured with a tensile tester from Zwick Roell.

[Table 2]

section		Hot-rolled sheet annealing heat treatment temperature T (HRA, °C)	Cold-rolled sheet annealing heat treatment temperature T (CRA, °C)	Thickness (mm)	Elongation (rate) (%)
example	A1	855	900	0.6	30
	B1	910	910	0.6	33
	B2	900	890	0.6	31
	B3	880	880	0.5	28
	B4	845	890	0.6	29
	B5	860	895	0.6	30
	B6	850	900	0.6	31
	B7	855	895	0.6	30
Comparative example	A1		880	845	0.8	24
	A2	850	860	0.5	26
	A3	830	870	0.5	25
	A4	840	840	1.0	23
	A5	830	850	1.2	23
	A6	860	810	1.5	22
	B1	840	920	0.5	25
	B2	830	880	0.5	26
	C1	820	880	0.6	25
	C2	840	910	0.4	25

Referring to table 2, the examples performed annealing at an annealing heat treatment temperature that satisfies both equations 2 and 3 below. Accordingly, all the examples had elongation rates that satisfied at least 27%. Equation 2: 840 ≤ T (HRA, °C) ≤ Ac1 - 20 $870 \leq T (CRA, ° C) \leq Ac1 - 20$
Comparative examples A3, A5, B2 and C1 failed to satisfy equation 2.
In comparative examples A3, A5, B2 and C1, recrystallization was not done sufficiently because the hot-rolled sheet annealing heat treatment temperature T (HRA, °C) did not satisfy at least 840 °C, so the elongation rate did not satisfy at least 27%.
Comparative examples A1, A2, A4, A6, B1 and C2 did not satisfy equation 3.
In comparative examples A1, A2, A4 and A6, recrystallization was not done sufficiently because the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) did not satisfy at least 870 °C, so the elongation rate did not satisfy at least 27%.
In comparative examples B1 and C2, the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) did not satisfy (Ac1 - 20 °C) or less, leading to formation of an austenite phase, so a martensite phase was formed during quenching after heat treatment. Hence, the elongation rate did not satisfy at least 27%.
FIG. 1 is a graph representing an annealing heat treatment temperature range that may secure at least 27% of elongation.
Referring to FIG. 1, it can be seen that when annealing is performed at an annealing heat treatment temperature that satisfies equations (2) and (3) below, an elongation of at least 27% may be secured. $840 \leq T (HRA, ° C) \leq Ac1 - 20$
$870 \leq T (CRA, ° C) \leq Ac1 - 20$

[Industrial Applicability]

According to an embodiment of the disclosure, a ferritic stainless steel and method of manufacturing the same, by which elongation is improved by controlling annealing heat treatment temperature while omitting box annealing and performing continuous annealing, may be provided, so that manufacturing costs may be saved by omitting the box annealing that requires long time, so the industrial applicability is acknowledged.

Claims

A ferritic stainless steel comprising:
in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities,

wherein an Ac1 value defined in equation 1 below is at least 920 and less than 990: Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

(wherein in equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements).
The ferritic stainless steel of claim 1, wherein an elongation rate is at least 27%.
A method of manufacturing a ferritic stainless steel comprising:
manufacturing a slab including, in percent by weight (wt%), 0.01 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, more than 0 to 0.05% of P, more than 0 to 0.005% of S, 13.0 to 18.0% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.05 to 0.25% of Ni, the remaindered Fe (iron) and other unavoidable impurities, wherein an Ac1 value defined in equation 1 below is at least 920 and less than 990;

reheating the slab;

manufacturing a hot-rolled steel material by hot-rolling and coiling the reheated slab;

obtaining a hot-rolled and wound hot-rolled sheet by hot-rolled-sheet-annealing the hot-rolled steel material at a hot-rolled sheet annealing heat treatment temperature T (HRA, °C) which satisfies equation (2) below, followed by cooling and winding;

manufacturing a cold-rolled sheet by cold-rolling the hot-rolled and wound hot-rolled sheet; and

cold-rolled-sheet-annealing the cold-rolled sheet at a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) which satisfies equation (3) below, followed by cooling and winding: Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

(wherein in equation 1, [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] refer to contents (wt%) of the respective elements). $840 \leq T (HRA, ° C) \leq Ac1 - 20$
$870 \leq T (CRA, ° C) \leq Ac1 - 20 .$
The method of claim 3, wherein the reheating is performed at 1100 to 1250 °C.
The method of claim 3, wherein the hot-rolling is performed at a finish rolling completion temperature of 800 to 950 °C.
The method of claim 3, wherein the coiling is performed at 750 to 850 °C.
The method of claim 3, wherein the hot-rolled sheet annealing and the cold-rolled sheet annealing are performed for 30 seconds to 10 minutes.
The method of claim 3, wherein the cooling after the hot-rolled sheet annealing and the cooling after the cold-rolled sheet annealing are performed at a cooling rate of 50 °C/s.
The method of claim 3, wherein the cold rolling is performed with a reduction rate of 60 to 90%.