EP4613906A1

EP4613906A1 - Ferritic stainless steel with improved impact toughness, and manufacturing method thereof

Info

Publication number: EP4613906A1
Application number: EP23907421.4A
Authority: EP
Inventors: Ilchan Jung; Junghyun KONG; Hyunggu KANG
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-19
Filing date: 2023-11-13
Publication date: 2025-09-10
Also published as: EP4613906A4; WO2024136135A1; KR20240096251A

Abstract

The disclosed invention relates to a ferritic stainless steel with an improved impact toughness, and a manufacturing method thereof. A ferritic stainless steel with an improved impact toughness according to an embodiment has, in weight%: C: 0.005% to 0.015%; N: 0.005% to 0.015%; Si: 0.01% to 0.60%; Mn: 0.2% to 0.9%; Cr: 10.5% to 13.0%; Ni: 0.6% to 1.1%; Ti: 0.05% to 0.30%; B: 0.0005% to 0.0070%; P: 0.04% or less; S: 0.01% or less; and the remaining Fe and inevitable impurities, wherein the ferritic stainless steel may have an impact toughness of over 100J at -20°C .

Description

[Technical Field]

The present invention relates to a ferritic stainless steel having improved impact toughness and a method of manufacturing the same.

[Background Art]

The present invention relates to a ferritic stainless steel having improved impact toIn general, stainless steels may be classified based on chemical composition or metallographic structure. When classified by metallographic structure, stainless steels may be classified into austenitic stainless steels, ferritic stainless steels, martensitic stainless steels, and dual phase stainless steels.
Ferritic stainless steels have excellent corrosion resistance while containing a lower amount of expensive alloying elements, offering better cost competitiveness than austenitic stainless steels. In particular, low-Cr ferritic stainless steels containing 10.5 to 14% of Cr exhibit excellent cost competitiveness even among ferritic stainless steels, and are thus widely used as automobile exhaust system flanges and construction structural materials.
For such automobile exhaust system flanges and construction structural materials, thick hot-rolled annealed steels with a thickness of 3 mm or more are mainly used.
In a process of manufacturing the thick hot-rolled annealed steel parts, processing defects due to brittle fracture may occur during stamping. In addition, brittle fracture may occur due to vibration during actual use in automobiles, buildings, and the like.
Accordingly, improving impact toughness characteristics of ferritic stainless steels is of great importance in order to suppress brittle fracture during processing and actual use.

[Disclosure]

[Technical Problem]

To resolve the above-described issues, the present invention is directed to providing a ferritic stainless steel having improved impact toughness and suppressed brittle fracture by controlling an alloy composition and a manufacturing method, and a method of manufacturing the same.

[Technical Solution]

The present invention may provide a ferritic stainless steel having improved impact toughness.
The ferritic stainless steel having improved impact toughness according to an embodiment may include, in percent by weight (wt%), 0.005 to 0.015% of C, 0.005 to 0.015% of N, 0.01 to 0.60% of Si, 0.2 to 0.9% of Mn, 10.5 to 13.0% of Cr, 0.6 to 1.1% of Ni, 0.05 to 0.30% of Ti, 0.0005 to 0.0070% of B, 0.04% or less of P, 0.01% or less of S, and the remainder being Fe and inevitable impurities, and may have an impact toughness of 100J or more at -20°C.
The ferritic stainless steel having improved impact toughness according to an embodiment may satisfy a value of Expression (1) of 4 to 60. $10000 B - 100 (C + N) / Ti .$
In Expression (1), B, C, N, and Ti may represent the content (weight %) of each element.
The ferritic stainless steel having improved impact toughness according to an embodiment may have an impact toughness of 120J or more at 20°C.
The ferritic stainless steel having improved impact toughness according to an embodiment may have an average grain diameter of 65 µm or less.
The ferritic stainless steel having improved impact toughness according to an embodiment may have an average thickness of 3 to 15mm.
The present invention provides a method of manufacturing a ferritic stainless steel having improved impact toughness.
The method of manufacturing a ferritic stainless steel having improved impact toughness according to an embodiment includes: preparing a slab comprising, in percent by weight (wt%), 0.005 to 0.015% of C, 0.005 to 0.015% of N, 0.01 to 0.60% of Si, 0.2 to 0.9% of Mn, 10.5 to 13.0% of Cr, 0.6 to 1.1% of Ni, 0.05 to 0.30% of Ti, 0.0005 to 0.0070% of B, 0.04% or less of P, 0.01% or less of S, and the remainder being Fe and inevitable impurities; reheating the slab, and then performing rough rolling and finish rolling to produce a hot-rolled material; and coiling the hot-rolled material at 700 to 900°C and then cooling the coiled hot-rolled material.
In the ferritic stainless steel having improved impact toughness according to an embodiment, the slab may have a value of Expression (1) in a range of 4 to 60, $10000 B - 100 (C + N) / Ti,$
In Expression (1), B, C, N, and Ti may represent the content (weight %) of each element.
The ferritic stainless steel may have an impact toughness of 100J or more at -20°C.
The ferritic stainless steel may have an impact toughness of 120J or more at 20°C.
The reheating may be performed at 1200 to 1280°C.
The finish rolling may be performed at 900 to 1100°C.
The ferritic stainless steel may have an average grain diameter of 65 µm or less.

[Advantageous Effects]

According to an embodiment of the disclosed invention, a ferritic stainless steel with improved impact toughness capable of suppressing brittle fracture to suppress a processing defect rate and ensuring excellent durability during actual use of parts and a method of manufacturing the same can be provided.

[Description of Drawings]

FIG. 1 is an image of a central portion of a ferritic stainless steel with improved impact toughness according to an example of the disclosed invention, taken using electron backscatter diffraction (EBSD).

[Modes of the Invention]

Hereinafter, examples of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the examples shown herein but may be embodied in other forms. In order to make the description of the present invention clear, unrelated parts are not shown and, the sizes of components are exaggerated for clarity.
Throughout the specification, when a part is referred to as "including", "comprising" and/or "having" a certain element, it is understood that, unless expressed otherwise, the description does not preclude the presence or addition of one or more elements.
The singular form of a noun corresponding to an item may include one or a plurality of the items unless clearly indicated otherwise in a related context.
Hereinafter, the reason for numerically limiting the alloy element contents in the embodiments of the present invention will be described. Unless otherwise specified, the units thereof are expressed in weight percent (wt%).
A ferritic stainless steel having improved impact toughness according to an embodiment includes: in percent by weight (wt%), 0.005 to 0.015% of C, 0.005 to 0.015% of N, 0.01 to 0.60% of Si, 0.2 to 0.9% of Mn, 10.5 to 13.0% of Cr, 0.6 to 1.1% of Ni, 0.05 to 0.30% of Ti, 0.0005 to 0.0070% of B, 0.04% or less of P, 0.01% or less of S, and the remainder being Fe and inevitable impurities.
The content of C (carbon) may be 0.005 to 0.015%.
C is an element effective in increasing strength through solid solution strengthening. However, controlling the C content to an extremely low level may lead to an increase in steelmaking VOD process costs, thereby lowing the cost competitiveness. Considering this, the C content may be 0.005% or more. However, when the C content is excessive, Cr₂₃ C₆ precipitates may be generated, causing local depletion of Cr in the matrix, which may lower corrosion resistance. Considering this, the upper limit of the C content may be 0.015%. Preferably, the C content may be 0.007 to 0.014%.
The content of N (nitrogen) may be 0.005 to 0.015%.
N is an element effective in increasing the strength of steel. However, controlling the N content to an extremely low level may lead to an increase in steelmaking VOD process costs, which may lower the cost competitiveness. Considering this, the N content may be 0.005% or more. However, when the content of N is excessive, the solid solution N concentration may reaches its limit, and Cr₂N precipitates are generated, causing local depletion of Cr in the matrix, which may lower the corrosion resistance. Considering this, the upper limit of the N content may be 0.015%. Preferably, the content of N may be 0.006 to 0.015%.
The content of Si (silicon) may be 0.01 to 0.60%.
Si is an element that may partially contribute to strength improvement through solid solution strengthening. Considering this, Si may be added in an amount of 0.01% or more. However, when the content of Si is excessive, the strength of the steel may increase excessively, which may lower the impact toughness. Considering this, the upper limit of the Si content may be set to 0.60%. Preferably, the content of Si may be 0.20 to 0.40%.
The content of Mn (manganese) may be 0.2 to 0.9%.
Mn is an element that facilitates the formation of an austenite phase at high temperatures and is effective in implementing microstructural refinement through the activation of a phase transformation from an austenite phase to a ferrite phase. Considering this, the content of Mn may be 0.2% or more. However, when the content of Mn is excessive, the corrosion resistance of the material may be significantly lowered. Considering this, the upper limit of the content of Mn may be 0.9%. Preferably, the content of Mn may be 0.2 to 0.7%.
The content of Cr (chromium) may be 10.5 to 13.0%.
Cr is an element that is essential for forming a passive film that suppresses oxidation of the steel. In addition, Cr is an element that is effective in suppressing high-temperature oxidation. Considering this, the content of Cr may be added in an amount of 10.5% or more. However, when the content of Cr is excessive, the formation of an austenite phase at high temperatures may be suppressed, and microstructure refinement through the activation of a phase transformation may be limited. Considering this, the upper limit of the Cr content may be set to 13.0%. Preferably, the Cr content may be 11.3 to 12.5%.
The content of Ni (nickel) may be 0.6 to 1.1%.
Ni is an element that facilitates the formation of an austenite phase at high temperatures and is effective in implementing microstructure refinement through phase transformation activation. Considering this, Ni may be added in an amount of 0.6% or more. However, an excessive Ni content may reduce the cost competitiveness due to increased raw material costs. Considering this, the upper limit of the Ni content may be 1.1%.
The content of Ti (titanium) may be 0.05 to 0.30%.
Ti may improve weldability by combining with C and N to suppress the formation of Cr carbonitrides. Considering this, Ti may be added in an amount of 0.05% or more. However, since Ti is an expensive element, an excessive Ti content may reduce the cost competitiveness. Considering this, the upper limit of the Ti content may be set to 0.30%. Preferably, the content of Ti may be 0.16 to 0.27%.
The content of B (boron) may be 0.0005 to 0.0070%.
B is an element that is segregated at grain boundaries and is an effective element for suppressing the propagation of microcracks and thus improving impact toughness. Considering this, B may be added in an amount of 0.0005% or more. However, when the content of B is excessive, B may combine with N to form a BN precipitation phase, which may lower the impact toughness. Considering this, the upper limit of the B content may be set to 0.0070%. That is, when the content of B exceeds 0.0070%, the room temperature impact toughness and/or the low temperature impact toughness may be lowered. In this case, it is difficult to implement the effect of suppressing brittle fracture. Preferably, the content of B may be 0.0011 to 0.0065%.
The content of P (phosphorus) may be 0.04% or less.
P is an inevitable impurity included in steel and is an element that causes intergranular corrosion during pickling in the steel manufacturing process or inhibits hot workability. Considering this, the upper limit of the P content may be set to 0.04%. Preferably, the upper limit of the P content may be set to 0.02%.
The content of S (sulfur) may be 0.01% or less.
S is an inevitable impurity included in steel and is an element that segregates at grain boundaries and inhibits hot workability. Considering this, the upper limit of the S content may be limited to 0.01%. Preferably, the upper limit of the S content may be set to 0.007%.
The remaining component(s) of the disclosed invention is iron (Fe). However, unintended impurities may inevitably be introduced from raw materials or the surrounding environment in a typical manufacturing process, and thus cannot be excluded. Since such impurities may be well known to those skilled in the art of conventional manufacturing processes, details thereof are not described in this specification.
The ferritic stainless steel with improved impact toughness according to an embodiment may have a ferrite structure fraction of 98 % by volume or more, specifically 99% by volume or more. Such ferritic stainless steel has a relatively low coefficient of thermal expansion and excellent corrosion resistance compared to steel grades having other microstructures. In this case, for example, the steel may realize strength, corrosion resistance, and workability suitable for application to exhaust system parts of internal combustion engine vehicles, home appliance parts, kitchenware, interior and exterior construction items with excellent corrosion resistance, hydrogen fuel cell parts for hydrogen vehicles, and the like.
The inventors of the present invention have conducted various studies on the ferritic stainless steel with improved impact toughness, and as a result, have obtained the following findings.
In general, ferritic stainless steels having a body centered cubic (BCC) crystal structure exhibits significantly low impact toughness compared to austenitic stainless steels having a face centered cubic (FCC) crystal structure. In addition, in the case of Ti-stabilized ferritic stainless steels, there is no phase transformation during the steelmaking-continuous casting-hot rolling process, making it difficult to implement grain refinement, which may lead to further degradation in impact toughness.
Conventionally, in order to improve the impact toughness of Ti-stabilized ferritic stainless steels, large amounts of austenite stabilizing elements such as Mn and Ni have been added. This is intend to implement grain refinement by activating the phase transformation from the austenite phase to the ferrite phase during the manufacturing process of the Ti-stabilized ferritic stainless steel. However, the improvement in impact toughness by addition of large amounts of austenite stabilizing elements was not significant, and there was a drawback in terms of cost competitiveness.
A ferritic stainless steel with improved impact toughness according to an embodiment may satisfy a low-temperature impact toughness of 100J or more -20°C. When a ferritic stainless steel has a low-temperature impact toughness of less than 100J at -20°C, the resistance to brittle fracture in a low-temperature environment may be inferior. For example, when processing for application to an exhaust system flange part, it is difficult to suppress brittle fracture. In this case, the processing may include stamping, milling, and the like.
In addition, the low temperature impact toughness at -20°C may be 110J or more, more specifically 120J or more, and more specifically 130J or more. In this case, the ferritic stainless steel with improved impact toughness according to an embodiment of the present invention may be applied to, for example, exhaust system flange parts, and since these parts are exposed to a low temperature environment of about -20°C for a long time, in which resistance to low-temperature brittleness is critical, the ferritic stainless steel with improved impact toughness according to the embodiment of the present invention may implement more advantageous properties in such a harsh environment.
The ferritic stainless steel with improved impact toughness according to an embodiment may have a room temperature impact toughness of 120J or more at 20°C. When the room temperature impact toughness of the ferritic stainless steel at 20°C is less than 120J, it may be difficult to suppress brittle fracture during processing for application to, for example, an exhaust system flange part. In this case, the processing may include stamping processing, milling processing, and the like.
The ferritic stainless steel with improved impact toughness according to an embodiment may have a value of Expression (1) of 4 to 60, specifically 5 to 55, and more specifically 10 to 40. $10000 B - 100 (C + N) / Ti$
In Expression (1), B, C, N, and Ti represent the content (weight %) of each element.
The inventors of the present invention have prepared various hot-rolled steel sheets having the component range and microstructure according to the disclosed invention and analyzed the impact toughness evaluation at room temperature and low temperature, and have derived Expression (1).
Expression (1) represents the degree to which B is segregated at grain boundaries. Since C and N, like B, may be segregated at grain boundaries, C and N may prevent B from being segregated. Therefore, by adding Ti to form a Ti(C, N) precipitation phase, the segregated of C and N at grain boundaries may be suppressed. Through this, segregation of B may be facilitated, and propagation of microcracks inside the crystal grains may be inhibited, thereby improving impact toughness.
The disclosed invention is intended to improve the impact toughness through segregation of B by controlling the value of Expression (1) in a range of 4 to 60. When the value of Expression (1) is less than 4, the content of B segregated may be too small, resulting in a limited improvement in impact toughness. On the other hand, when the value of Expression (1) exceeds 60, an excessive segregation of B may occur, leading to the formation of a precipitate phase by combining with elements such as Ti, C, and N, thereby lowering the impact toughness, specifically, the low-temperature impact toughness -20°C.
In one example, the value of Expression (1) may be preferably 4.0 to 59.0, more preferably 4.1 to 58.8, and even more preferably 5 to 55, 5.4 to 55, or 5.4 to 53.8. Within the above range, the ferritic stainless steel with improved impact toughness according to an embodiment of the present invention may exhibit further improved impact toughness at both room temperature of 20°C and low temperature of -20°C, and the effect of suppressing brittle fracture may be further improved with higher efficiency.
The ferritic stainless steel with improved impact toughness according to an embodiment may additionally have an average grain diameter of 65 µm or less, preferably 50 µm or less in a central portion of the steel. In this case, by implementing grain refinement through phase transformation activation, the impact toughness value, specifically the room temperature impact toughness at 20°C and the low temperature impact toughness at -20°C may be further improved.
In one example, the value of Expression (1) may preferably be in a range of 10 to 40, more preferably 11.5 to 37.8. At the same time, the average grain diameter in a central portion of the steel may be controlled to 65 µm or less. In this case, the ferritic stainless steel with improved impact toughness according to an embodiment of the present invention may realize a room temperature impact toughness of 150J or more at 20°C and a low temperature impact toughness of 130J or more at -20°C, thereby realizing even more excellent impact toughness.
Here, the average refers to the average value measured at five arbitrary locations. In addition, the central portion of the steel refers to a region between 1/4t and 3/4t when the thickness of the steel is t.
FIG. 1 is an image of a central portion of a ferritic stainless steel with improved impact toughness according to an example of the disclosed invention, taken using electron backscatter diffraction (EBSD).
Referring to FIG. 1, it can be seen that the average grain diameter of the ferritic stainless steel according to an embodiment is 32 µm. In other words, it can be seen that the ferritic stainless steel with improved impact toughness according to an embodiment has achieved grain refinement.
The ferritic stainless steel with improved impact toughness according to an embodiment of the disclosed invention may provide a ferritic stainless steel with improved impact toughness, particularly with improved low-temperature impact toughness, exhibiting an impact toughness of 100J or more at -20°C, by controlling the alloy composition, Expression (1), and a manufacturing method thereof. In addition, the ferritic stainless steel with improved impact toughness according to an embodiment of the disclosed invention may provide a ferritic stainless steel with improved impact toughness, particularly with improved room-temperature impact toughness, exhibiting an impact toughness of 120J or more at 20°C. That is, according to an embodiment of the disclosed invention, despite being a ferritic stainless steel, processing defect rates may be suppressed and durability may be ensured during actual use by improving the impact toughness.
A ferritic stainless steel with improved impact toughness according to an embodiment may have a thickness of 3 to 15 mm in a hot-rolled state. However, the present invention is not limited thereto, and the thickness may be adjusted according to the purpose and function.
Next, a method of manufacturing a ferritic stainless steel with improved impact toughness according to another aspect of the disclosed invention will be described.
A method of manufacturing a ferritic stainless steel having improved impact toughness according to an embodiment may include: preparing a slab comprising, in percent by weight (wt%), 0.005 to 0.015% of C, 0.005 to 0.015% of N, 0.01 to 0.60% of Si, 0.2 to 0.9% of Mn, 10.5 to 13.0% of Cr, 0.6 to 1.1% of Ni, 0.05 to 0.30% of Ti, 0.0005 to 0.0070% of B, 0.04% or less of P, 0.01% or less of S, and the remainder being Fe and inevitable impurities; reheating the slab, and then performing rough rolling and finish rolling to produce a hot-rolled material; and coiling the hot-rolled material at 700 to 900°C and then cooling the coiled hot-rolled material.
In the ferritic stainless steel having improved impact toughness according to an embodiment, the slab may have a value of Expression (1) below in a range of 4 to 60, specifically 5 to 55, and more specifically 10 to 40. $10000 B - 100 (C + N) / Ti$
In Expression (1), B, C, N, and Ti represent the content (weight %) of each element.
The reasons for numerically limiting the component range of each alloy composition and the value of Expression (1) are as described above, and the following provides details of manufacturing operations.
After preparing a slab satisfying the alloy composition and Expression (1), a series of processes including reheating, rough rolling, finish rolling, coiling, and cooling may be performed.
Meanwhile, in order to suppress grain growth of the material after the rolling, additional annealing may be omitted and only pickling may be performed.
The reheating may be performed at 1200 to 1280°C.
By controlling the reheating temperature to 1200 to 1280°C, coarse precipitates generated during the preparing of the slab may be sufficiently re-decomposed. In addition, by controlling the reheating temperature to 1200 to 1280°C, the average grain diameter may be prevented from increasing and surface defects due to surface oxidation may be suppressed.
The finish rolling may be performed at 900 to 1100°C.
By controlling the finish rolling temperature to 900 to 1100°C, cracks may be prevented from occurring on the surface of the steel during rolling, and the structure may be homogenized, thereby improving toughness and strength. In addition, by controlling the finish rolling temperature to 900 to 1100°C, coarsening of austenite grains may be prevented and ferrite grain refinement after transformation may be sufficiently implemented.
The hot-rolled steel sheet may be coiled at 700 to 900°C.
By controlling the coiling temperature to 700 to 900°C, the shape and surface quality may be improved. In addition, by controlling the coiling temperature to 700 to 900°C, the formation of a martensite phase during the cooling process may be suppressed.
The cooling may be performed through air cooling.
Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

{Examples}

(Manufacturing of Examples 1 to 5, 7 to 9 and Comparative Examples 1 to 6)

For the various alloy composition ranges shown in Table 1 below, slabs were prepared by melting in a vacuum melting furnace. In this case, the components were controlled such that P was 0.04% or less and S was 0.01% or less. The slabs were reheated at 1250°C, and then rough-rolled, and finish-rolled to produce a hot-rolled material. The finish rolling was performed at 1000°C. The hot-rolled material was coiled at 800°C and then air-cooled to produce a specimen with a thickness of 8 mm.

(Manufacturing of Example 6)

An 8-mm-thick specimen was produced in the same method as in the manufacturing of Example 5 described above, except that the coiling temperature was changed to 900°C.

[Table 1]

(wt %)	C	N	Si	Mn	Cr	Ni	Ti	B
Example 1	0.013	0.014	0.50	0.3	12.2	0.6	0.21	0.0017
Example 2	0.010	0.007	0.60	0.7	12.5	0.9	0.27	0.0011
Example 3	0.008	0.009	0.20	0.7	12.0	0.8	0.16	0.0016
Example 4	0.011	0.009	0.30	0.4	11.8	0.7	0.21	0.0021
Example 5	0.007	0.006	0.30	0.4	11.9	0.7	0.23	0.0025
Example 6	0.007	0.006	0.30	0.4	11.9	0.7	0.23	0.0025
Example 7	0.012	0.013	0.40	0.2	11.6	0.6	0.19	0.0051
Example 8	0.014	0.015	0.30	0.4	11.3	1.1	0.26	0.0065
Example 9	0.007	0.006	0.30	0.6	11.2	0.8	0.25	0.0064
Comparative Example 1	0.010	0.009	0.20	0.8	12.6	0.7	0.23	0.0005
Comparative Example 2	0.007	0.006	0.10	0.4	11.4	0.8	0.22	0.0006
Comparative Example 3	0.011	0.012	0.40	0.5	12.8	1.0	0.18	0.0015
Comparative Example 4	0.007	0.008	0.20	0.5	11.0	0.9	0.29	0.0069
Comparative Example 5	0.006	0.011	0.50	0.7	10.6	0.7	0.25	0.0085
Comparative Example 6	0.013	0.014	0.20	0.5	10.7	0.9	0.11	0.0077

Table 2 below shows the values of Expression (1), the average grain diameter, the impact toughness at 20°C, and the impact toughness -20°C.
The values of Expression (1) were calculated by Expression (1) below. $10000 B - 100 (C + N) / Ti$
In Expression (1), B, C, N, and Ti represent the content (weight %) of each element.
The average grain diameter was measured at the central portion of the specimen using EBSD.
Here, the average refers to the average value measured at five random locations. In addition, the central portion of the steel refers to a region between 1/4t and 3/4t when the thickness of the steel is t.

The impact toughness was measured by performing Charpy-V notch impact tests at temperatures of 20°C and -20°C using an impact tester manufactured by Zwick Roell. In addition, when the measured impact toughness value at -20°C is 100J or more, the brittle fracture suppression capability is evaluated as "excellent", and when the measured impact toughness value at -20°C is less than 100J, the brittle fracture suppression capability is evaluated as "poor", and the results are shown in Table 2.

Table 21

(wt %)	Expr (1)	Grain size (µm)	20°C impact toughness (J)	-20°C impact toughness (J)	Brittle fracture suppression capability
Example 1	4.1	32	122	101	Excellent
Example 2	4.7	36	125	107	Excellent
Example 3	5.4	33	147	128	Excellent
Example 4	11.5	31	151	131	Excellent
Example 5	19.3	36	155	130	Excellent
Example 6	19.3	65	121	104	Excellent
Example 7	37.8	29	153	133	Excellent
Example 8	53.8	28	146	125	Excellent
Example 9	58.8	37	128	104	Excellent
Comparative Example 1	-3.3	34	117	95	Poor
Comparative Example 2	0.1	32	121	98	Poor
Comparative Example 3	2.2	37	123	99	Poor
Comparative Example 4	63.8	28	119	93	Poor
Comparative Example 5	78.2	33	96	83	Poor
Comparative Example 6	52.5	35	103	88	Poor

As can be seen from Table 1 and Table 2, Examples 1 to 9 according to the present invention achieved a low-temperature impact toughness, particularly an impact toughness - 20°C, of 100 J or more. In such cases, even when exposed to a low-temperature environment for a long time, the resistance to low-temperature brittleness is excellent, and, for example, the brittle fracture suppression capability during stamping processing for application to exhaust system flange parts is excellent.
Example 5 according to the present invention was manufactured in the same method as Example 6, except that the coiling temperature was changed to additionally control the grain size. From the property measurement results of Examples 5 and 6, it can be seen that when the grain size is controlled to 65 µm or less, an additional improvement in impact toughness is achieved.
According to an example of the disclosed invention, a ferritic stainless steel with improved impact toughness, capable of suppressing a processing defect rate and ensuring excellent durability during actual use of parts, and a manufacturing method thereof may be provided.

Claims

A ferritic stainless steel having improved impact toughness, comprising, in percent by weight (wt%), 0.005 to 0.015% of C, 0.005 to 0.015% of N, 0.01 to 0.60% of Si, 0.2 to 0.9% of Mn, 10.5 to 13.0% of Cr, 0.6 to 1.1% of Ni, 0.05 to 0.30% of Ti, 0.0005 to 0.0070% of B, 0.04% or less of P, 0.01% or less of S, and the remainder being Fe and inevitable impurities, and
having an impact toughness of 100J or more at -20°C .
The ferritic stainless steel having improved impact toughness of claim 1, wherein a value of Expression (1) below is 4 to 60, $10000 B - 100 (C + N) / Ti,$ wherein in Expression (1), B, C, N, and Ti represent the content (weight %) of each element.
The ferritic stainless steel having improved impact toughness of claim 1, having an impact toughness of 120J or more at 20°C.
The ferritic stainless steel having improved impact toughness of claim 1, having an average grain diameter of 65 µm or less.
The ferritic stainless steel having improved impact toughness of claim 1, having an average thickness of 3 to 15mm.
A method of manufacturing a ferritic stainless steel having improved impact toughness, the method comprising:
preparing a slab comprising, in percent by weight (wt%), 0.005 to 0.015% of C, 0.005 to 0.015% of N, 0.01 to 0.60% of Si, 0.2 to 0.9% of Mn, 10.5 to 13.0% of Cr, 0.6 to 1.1% of Ni, 0.05 to 0.30% of Ti, 0.0005 to 0.0070% of B, 0.04% or less of P, 0.01% or less of S, and the remainder being Fe and inevitable impurities;

reheating the slab, and then performing rough rolling and finish rolling to produce a hot-rolled material; and

coiling the hot-rolled material at 700 to 900°C and then cooling the coiled hot-rolled material.
The method of claim 6, wherein the slab has a value of Expression (1) below in a range of 4 to 60, $10000 B - 100 (C + N) / Ti,$ wherein in Expression (1), B, C, N, and Ti represent the content (weight %) of each element.
The method of claim 6, wherein the ferritic stainless steel has an impact toughness of 100J or more at -20°C .
The method of claim 6, wherein the ferritic stainless steel has an impact toughness of 120J or more at 20°C.
The method of claim 6, wherein the reheating is performed at 1200 to 1280°C.
The method of claim 6, wherein the finish rolling is performed at 900 to 1100°C.
The method of claim 6, wherein the ferritic stainless steel has an average grain diameter of 65 µm or less.