KR102959091B1 - Ferritic stainless steel and the method for manufacturing the same - Google Patents

Abstract

This specification discloses a ferritic stainless steel with improved elongation while omitting annealing, and a method for manufacturing the same.
A ferritic stainless steel according to one embodiment of the present invention may comprise, in weight%, C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities.

Description

Ferritic Stainless Steel and Method for Manufacturing the Same

The present invention relates to a ferritic stainless steel and a method for manufacturing the same, and more specifically, to a ferritic stainless steel and a method for manufacturing the same that improves elongation while omitting annealing.

Ferritic stainless steel is a material with high price competitiveness compared to austenitic stainless steel because it contains fewer expensive alloying elements. Due to its excellent corrosion resistance, ferritic stainless steel is widely used in applications such as construction materials, transportation equipment, home appliances, and kitchenware.

Generally, 430 series hot-rolled steel undergoes a box annealing process, which is performed for 35 to 50 hours at 800 to 850°C, the temperature at which the austenite phase transforms into the ferrite phase. The purpose of the box annealing is to recrystallize the deformation structure formed during hot rolling and to decompose the austenite phase into the ferrite phase and carbides. However, the box annealing process has problems such as high energy consumption and reduced productivity due to the long heat treatment time. Therefore, the development of continuous annealing type manufacturing technology has been pursued to reduce manufacturing costs by reducing energy consumption and improving productivity.

Patent document 0001 discloses that for 430 steel alloyed to enable continuous annealing, at least one pass is performed with a reduction rate of 20% or more per rough rolling pass during hot rolling, and patent document 0002 discloses that when the finishing rolling start temperature is 950°C or higher and the coiling temperature is 650°C or lower, the quality characteristics are good without the occurrence of sticking defects.

Meanwhile, for 430 series stainless steel, if continuous annealing is omitted and heat treatment is performed under normal annealing conditions, there is a risk that the elongation will be reduced due to the formation of fine Cr carbides precipitated during cooling after hot rolling.

To address this, there are currently almost no attempts to improve elongation by controlling the annealing heat treatment temperature of hot-rolled and cold-rolled steel in conjunction with Ac1, the phase transformation temperature calculated based on the alloy composition.

Japanese Patent Publication No. 57-70230 (Date of publication: Feb. 10, 1997) Japan Patent Publication No. 57-155326 (Date of publication: Dec. 22, 1989)

The objective of the present invention, which is to solve the aforementioned problem, is to provide a ferritic stainless steel with improved elongation and a method for manufacturing the same, while omitting phase annealing and performing continuous annealing.

A ferritic stainless steel according to one embodiment of the present invention comprises, in weight%, C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities, and the Ac1 value defined in Equation (1) below may be 920 or more and less than 990.

Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

In the above equation (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.

In addition, the ferritic stainless steel according to one embodiment of the present invention may have an elongation of 27% or more.

In addition, a method for manufacturing a ferritic stainless steel according to one embodiment of the present invention comprises, in weight%, a step of manufacturing a slab comprising C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities, wherein the Ac1 value defined in Equation (1) below is 920 or more and less than 990; and a step of reheating the slab. The method may include the steps of: hot rolling the above-mentioned reheated slab and then coiling it to produce hot-rolled steel; annealing the above-mentioned hot-rolled steel at a hot-rolled steel annealing heat treatment temperature T (HRA, °C) satisfying the following equation (2), then cooling and pickling it to obtain a hot-rolled pickled hot-rolled plate; cold rolling the above-mentioned hot-rolled pickled hot-rolled plate to produce a cold-rolled plate; and annealing the above-mentioned cold-rolled plate at a cold-rolled steel annealing heat treatment temperature T (CRA, °C) satisfying the following equation (3), then cooling and pickling it.

Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

In the above equation (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.

Equation (2): 840 ≤ T(HRA, ℃) ≤ Ac1 - 20

Equation (3): 870 ≤ T(CRA, ℃) ≤ Ac1 - 20

In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the reheating step may be performed at 1100 to 1250°C.

In addition, in a method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the hot rolling may be performed at a finishing rolling completion temperature of 800 to 950°C.

In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the coiling may be performed at 750 to 850°C.

In addition, in a method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the annealing and pickling of the hot-rolled plate and the annealing and pickling of the cold-rolled plate may be performed for 30 seconds or more and 10 minutes or less.

In addition, in a method for manufacturing ferritic stainless steel according to one embodiment of the present invention, cooling after annealing the hot-rolled plate and cooling after annealing the cold-rolled plate can be performed at a cooling rate of 10 to 50℃/s.

In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the cold rolling may be performed at a reduction rate of 60 to 90%.

According to one embodiment of the present invention, a ferritic stainless steel with improved elongation and a method for manufacturing the same can be provided by omitting phase annealing and performing continuous annealing, while controlling the annealing heat treatment temperature.

In addition, according to one embodiment of the present invention, manufacturing costs can be reduced by omitting the long-duration annealing process.

Figure 1 is a graph showing the annealing heat treatment temperature range capable of securing an elongation of 27% or more.

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following embodiments are presented to sufficiently convey the concept of the present invention to those skilled in the art to which the present invention pertains. 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 the illustration of parts unrelated to the description and may slightly exaggerate the size of components to aid understanding.

Throughout the specification, when a part is described as "including" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

Singular expressions include plural expressions unless there is an obvious exception in the context.

The reasons for the numerical limitations on the alloy component content in the embodiments of the present invention are explained below. Unless otherwise specified, the units are weight percent.

A ferritic stainless steel according to one embodiment of the present invention may comprise, in weight%, C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities.

The content of C (carbon) may be 0.01% or more and 0.1% or less.

C is a strong austenite phase stabilizing element and is an effective element for increasing material strength through solid solution strengthening. Considering this, C can be added in an amount of 0.01% or more. However, if the C content is excessive, the strength increases excessively, which lowers the elongation and toughness of the steel. Considering this, the upper limit of the C content may be restricted to 0.1%.

The content of Si (silicon) may be 0.01% or more and 1.0% or less.

Silicon is an element added as a deoxidizer during the steelmaking process and is effective in improving yield strength and corrosion resistance. Additionally, silicon is an element that can enhance the stability of the ferrite phase. Considering this, silicon may be added in an amount of 0.01% or more. However, if the silicon content is excessive, it may cause material hardening, which may result in inferior elongation and toughness. Considering this, the upper limit of the silicon content may be restricted to 1.0%.

The content of Mn (manganese) may be 0.01% or more and 1.5% or less.

Mn is an element effective in improving corrosion resistance. Considering this, Mn can be added in an amount of 0.01% or more, and preferably 0.2% or more. However, if the Mn content is excessive, it forms inclusions (MnS), which lowers the hot workability, ductility, and toughness of the steel. Considering this, the upper limit of the Mn content can be limited to 1.5%, and more preferably to 1.0%.

The content of P (phosphorus) may be greater than 0% and less than or equal to 0.05%.

P is an impurity inevitably contained in steel that causes intergranular corrosion during pickling or impairs hot workability. Therefore, it is desirable to control the P content to be as low as possible. Considering this, the upper limit of the P content may be restricted to 0.05%.

The content of S (sulfur) may be greater than 0% and less than or equal to 0.005%.

S is an impurity inevitably contained in steel that segregates at grain boundaries and is an element that causes a hindrance to hot workability. Therefore, it is desirable to control the S content to be as low as possible. Taking this into consideration, the upper limit of the S content may be restricted to 0.005%.

The content of Cr (chromium) may be 13.0% or more and 18.0% or less.

Cr is an element that improves corrosion resistance by forming a passivation film in an oxidizing environment. Considering this, Cr can be added in an amount of 13.0% or more. However, if the Cr content is excessive, it promotes the formation of delta (δ) ferrite in the slab, which reduces elongation and impact toughness and increases manufacturing costs. Considering this, the upper limit of the Cr content may be restricted to 18.0%.

The content of N (nitrogen) may be 0.005% or more and 0.1% or less.

Like C, N is an interstitial element that is effective in improving the yield strength of steel through solid solution strengthening. Considering this, N can be added in an amount of 0.005% or more. However, if the N content is excessive, impact toughness and formability may be inferior. Considering this, the upper limit of the N content may be restricted to 0.1%.

The content of Al (aluminum) may be 0.005% or more and 0.2% or less.

Al is a powerful deoxidizer and an element that lowers the oxygen content in molten steel. Considering this, Al can be added in an amount of 0.005% or more. However, if the Al content is excessive, non-metallic inclusions increase, which can cause sliver defects in cold-rolled strips and simultaneously degrade weldability. Considering this, the upper limit of the Al content can be limited to 0.2%, and more preferably to 0.15% or less.

The content of Ni (nickel) may be 0.05% or more and 0.25% or less.

Ni has the effect of softening steel. Considering this, Ni can be added in an amount of 0.05% or more. However, if the Ni content is excessive, there is a problem of increased costs. Considering this, the upper limit of the Ni content may be limited to 0.25%.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

A ferritic stainless steel according to one embodiment of the present invention may have an Ac1 value defined in Equation (1) below of 920 or higher and less than 990.

Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

In the above equation (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.

Ac1 refers to the temperature at which the austenite phase transforms into the ferrite phase. A feature of the present invention is that the elongation is improved by controlling the annealing heat treatment temperature based on the Ac1 value calculated through the design of the alloy composition and component range.

If the calculated Ac1 value is low, heat treatment is performed at a low temperature, and sufficient recrystallization does not occur during continuous annealing of hot-rolled sheets. Considering this, the alloy composition and component range can be designed so that the calculated Ac1 value is 920 or higher. However, if the calculated Ac1 value is excessively high, the content of austenite-forming elements such as C and N decreases, and the formation of carbides and nitrides is insufficient, which may result in inferior strength. Considering this, the alloy composition and component range can be designed so that the calculated Ac1 value is less than 990.

By controlling the annealing heat treatment temperature based on the above-calculated Ac1 value, the ferritic stainless steel according to one embodiment of the present invention may have an elongation of 27% or more.

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

A method for manufacturing ferritic stainless steel according to one embodiment of the present invention comprises the steps of: manufacturing a slab having, in weight%, C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities, wherein the Ac1 value defined in Equation (1) below is 920 or more and less than 990; and reheating the slab. The method may include the steps of: hot rolling the above-mentioned reheated slab and then coiling it to produce hot-rolled steel; annealing the above-mentioned hot-rolled steel at a hot-rolled steel annealing heat treatment temperature T (HRA, °C) satisfying the following equation (2), then cooling and pickling it to obtain a hot-rolled pickled hot-rolled plate; cold rolling the above-mentioned hot-rolled pickled hot-rolled plate to produce a cold-rolled plate; and annealing the above-mentioned cold-rolled plate at a cold-rolled steel annealing heat treatment temperature T (CRA, °C) satisfying the following equation (3), then cooling and pickling it.

Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

In the above equation (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.

Equation (2): 840 ≤ T(HRA, ℃) ≤ Ac1 - 20

Equation (3): 870 ≤ T(CRA, ℃) ≤ Ac1 - 20

The reason for limiting the component range of each alloy composition and the numerical value of the above formula (1) is as described above, and each manufacturing step is explained in more detail below.

First, after manufacturing a slab satisfying the above alloy composition and the above formula (1), a series of processes such as hot rolling, hot rolling plate annealing and pickling, cold rolling, cold rolling plate annealing and pickling may be performed.

The above slab can be hot-rolled at a reheating temperature of 1100 to 1250°C.

If the reheating temperature of the slab is too low, the load on the rolling rolls may increase. Considering this, the reheating temperature of the slab may be 1100°C or higher. However, if the reheating temperature is too high, the grain diameter of the slab may coarsen, and the strength may be compromised. Considering this, the upper limit of the reheating temperature of the slab may be limited to 1250°C.

Next, the hot rolling can be performed at a finishing rolling completion temperature of 800 to 950°C.

If the finish rolling completion temperature is low, the rolling load increases, which may reduce productivity. Considering this, the finish rolling completion temperature may be 800℃ or higher. However, if the finish rolling completion temperature is too high, the grain size may increase, which may reduce strength. Considering this, the finish rolling completion temperature can be controlled to 950℃ or lower.

In addition, the above winding can be performed at 750 to 850℃.

If the winding temperature is low, it may be difficult to control the shape of the coil, and if the winding temperature is too high, there is a risk of causing defects in subsequent processes due to continuous phase transformation after winding. Considering this, the winding temperature can be set to 750 to 850℃.

After that, the above hot-rolled steel can be annealed at a hot-rolled plate annealing heat treatment temperature T (HRA, °C) satisfying the following equation (2).

Equation (2): 840 ≤ T(HRA, ℃) ≤ Ac1 - 20

If the hot-rolled plate annealing temperature T (HRA, °C) is low, recrystallization does not occur sufficiently. However, since cold-rolled plate annealing is performed as a subsequent process, it can be carried out at a relatively low temperature. Considering this, the hot-rolled plate annealing temperature T (HRA, °C) may be 840 °C or higher. However, if the hot-rolled plate annealing temperature T (HRA, °C) is above the Ac1 temperature, an austenite phase may be formed, and a martensite phase may be formed upon rapid cooling after heat treatment. Considering this, the upper limit of the hot-rolled plate annealing temperature T (HRA, °C) may be limited to Ac1 - 20.

The annealing of the hot-rolled plate above can be performed for 30 seconds or more and 10 minutes or less.

If the annealing time of the hot-rolled plate is short, the fraction of residual martensite may be high, resulting in inferior elongation. Considering this, the annealing of the hot-rolled plate may be performed for 30 seconds or more. However, if the annealing time of the hot-rolled plate is too long, the strength may decrease due to grain coarsening, and the thickness of the surface oxide layer may increase, leading to a longer pickling time for removing the oxide layer or insufficient removal of the oxide layer. Considering this, the annealing of the hot-rolled plate may be controlled for 10 minutes or less.

After annealing the above hot-rolled plate, cooling can be performed at a cooling rate of 10 to 50℃/s.

If the cooling rate is low, the elongation and formability may decrease due to non-uniformity of the structure caused by softening. However, if the cooling rate is too high, it has an adverse effect on the elongation due to excessive hardening. Taking this into consideration, the cooling rate can be controlled to 10 to 50℃/s.

The above cold rolling can be performed with a reduction rate of 60 to 90%.

When the reduction ratio is low, the accumulated energy from cold working is insufficient, making it difficult to obtain a recrystallized structure. However, when the reduction ratio is too high, cracks may occur due to rolling. Taking this into consideration, the reduction ratio can be controlled to 60 to 90%.

After that, the above cold-rolled sheet can be annealed at a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) satisfying the following equation (3).

Equation (3): 870 ≤ T(CRA, ℃) ≤ Ac1 - 20

Similar to the hot-rolled plate annealing temperature T (HRA, °C), if the cold-rolled plate annealing temperature T (CRA, °C) is low, recrystallization does not occur sufficiently. Taking this into consideration, the cold-rolled plate annealing temperature T (CRA, °C) may be 870 °C or higher. However, if the cold-rolled plate annealing temperature T (CRA, °C) is above the Ac1 temperature, an austenite phase may be formed, and a martensite phase may be formed upon rapid cooling after heat treatment; therefore, the upper limit of the cold-rolled plate annealing temperature T (CRA, °C) may be limited to Ac1 - 20.

The annealing of the cold-rolled sheet can be performed for 30 seconds or more and 10 minutes or less, and the cooling after annealing the cold-rolled sheet can be performed at a cooling rate of 10 to 50℃/s.

The reason for limiting the numerical values of the annealing time and cooling rate of the cold-rolled sheet is as described above.

The present invention will be explained in more detail below through examples. However, the description of these examples is merely for illustrating the implementation of the present invention and does not limit the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

{Example}

Slabs were manufactured for various alloy composition ranges shown in Table 1 below. The manufactured slabs were reheated at 1200°C, then hot-rolled at a finishing rolling temperature of 800°C, and then coiled at 750°C to produce hot-rolled steel.

Ac1 refers to the value defined in equation (1) below.

Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])

In the above equation (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.

Gangjong Alloy composition Ac1
(℃) C Si Mn Cr Ni N Al P S A 0.044 0.34 0.17 16.18 0.14 0.0129 0.122 0.026 0.0005 974.3 B 0.031 0.27 0.80 16.25 0.09 0.0198 0.107 0.023 0.0006 936.6 C 0.041 0.19 0.81 16.39 0.11 0.0213 0.093 0.025 0.0007 910.3

The above-mentioned hot-rolled steel was annealed for 10 minutes at a hot-rolled plate annealing heat treatment temperature T (HRA, °C), then cooled at a cooling rate of 30 °C/s and pickled to obtain a hot-rolled pickled hot-rolled plate. Next, the steel was manufactured by cold rolling with a reduction rate of 60%, then annealing the cold-rolled plate for 10 minutes at a cold-rolled plate annealing heat treatment temperature T (CRA, °C), then cooling at a cooling rate of 30 °C/s and pickling.

Table 2 below shows the hot-rolled plate annealing temperature T (HRA, °C), cold-rolled plate annealing temperature T (CRA, °C), and the thickness and elongation of the manufactured steel.

Elongation was measured using a tensile testing machine from Zwick Roell.

division Hot-rolled plate annealing heat treatment temperature
T(HRA, ℃) Cold-rolled sheet annealing heat treatment temperature
T(CRA, ℃) thickness
(mm) elongation rate
(%) Examples 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, all examples underwent annealing at an annealing heat treatment temperature satisfying Equations (2) and (3) below. Accordingly, all examples satisfied an elongation of 27% or more. Equation (2): 840 ≤ T(HRA, ℃) ≤ Ac1 - 20

Equation (3): 870 ≤ T(CRA, ℃) ≤ Ac1 - 20

Comparative examples A3, A5, B2 and C1 did not satisfy Equation (2).

Comparative examples A3, A5, B2 and C1 did not satisfy the requirement of an annealing heat treatment temperature T (HRA, °C) of 840 °C or higher, so recrystallization was not sufficiently achieved, and thus the elongation did not satisfy the requirement of 27% or higher.

Comparative examples A1, A2, A4, A6, B1 and C2 did not satisfy Equation (3).

Comparative examples A1, A2, A4, and A6 did not satisfy a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) of 870 °C or higher, so recrystallization was not sufficiently achieved, and thus the elongation did not satisfy 27% or higher.

Comparative Examples B1 and C2 did not satisfy the cold-rolled sheet annealing heat treatment temperature T (CRA, °C) of Ac1 - 20 °C or lower, so an austenite phase was formed, and a martensite phase was formed upon rapid cooling after heat treatment. Therefore, the elongation did not satisfy 27% or more.

Figure 1 is a graph showing the annealing heat treatment temperature range capable of securing an elongation of 27% or more.

Referring to Fig. 1, it can be seen that when annealing is performed at an annealing heat treatment temperature satisfying the following equations (2) and (3), an elongation rate of 27% or more can be secured.

Equation (2): 840 ≤ T(HRA, ℃) ≤ Ac1 - 20

Equation (3): 870 ≤ T(CRA, ℃) ≤ Ac1 - 20

Claims (9)

In weight percent, C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities,
Ferritic stainless steel with an Ac1 value of 920 or more and less than 990 as defined in Equation (1) below:
Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
(In the above formula (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.) In claim 1,
Ferritic stainless steel with an elongation of 27% or more. A step of manufacturing a slab comprising, in weight%, C: 0.01% or more and 0.1% or less, Si: 0.01% or more and 1.0% or less, Mn: 0.01% or more and 1.5% or less, P: greater than 0% and 0.05% or less, S: greater than 0% and 0.005% or less, Cr: 13.0% or more and 18.0% or less, N: 0.005% or more and 0.1% or less, Al: 0.005% or more and 0.2% or less, Ni: 0.05% or more and 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities, wherein the Ac1 value defined in Equation (1) below is 920 or more and less than 990;
Step of reheating the above slab;
A step of manufacturing hot-rolled steel by hot-rolling the above-mentioned reheated slab and then coiling it;
A step of obtaining a hot-rolled and pickled hot-rolled plate by annealing the above hot-rolled steel at a hot-rolled plate annealing heat treatment temperature T (HRA, °C) satisfying the following equation (2), then cooling and pickling;
A step of manufacturing a cold-rolled plate by cold-rolling the hot-rolled plate described above; and
A method for manufacturing ferritic stainless steel comprising the step of annealing the above cold-rolled sheet at a cold-rolled sheet annealing heat treatment temperature T (CRA, °C) satisfying the following equation (3), and then cooling and pickling:
Equation (1): Ac1 = 36*[Cr] + 90*[Si] + 760*[Al] + 350 - (800*[C] + 1300*[N] + 150*[Ni] + 50*[Mn])
(In the above formula (1), [Cr], [Si], [Al], [C], [N], [Ni], and [Mn] represent the content (weight%) of each element.)
Equation (2): 840 ≤ T(HRA, ℃) ≤ Ac1 - 20
Equation (3): 870 ≤ T(CRA, ℃) ≤ Ac1 - 20. In claim 3,
A method for manufacturing ferritic stainless steel, wherein the above-mentioned reheating step is performed at 1100 to 1250℃. In claim 3,
A method for manufacturing ferritic stainless steel, wherein the above hot rolling is performed at a finishing rolling completion temperature of 800 to 950°C. In claim 3,
The above-mentioned coiling is a method for manufacturing ferritic stainless steel, performed at 750 to 850°C. In claim 3,
A method for manufacturing ferritic stainless steel, wherein the annealing of the hot-rolled plate and the annealing of the cold-rolled plate are performed for 30 seconds or more and 10 minutes or less. In claim 3,
A method for manufacturing ferritic stainless steel, wherein the annealing of the hot-rolled plate followed by cooling and the annealing of the cold-rolled plate followed by cooling are performed at a cooling rate of 10 to 50℃/s. In claim 3,
A method for manufacturing ferritic stainless steel, wherein the above cold rolling is performed at a reduction rate of 60 to 90%.