KR20210015012A - Ferritic steel sheet for exhaust system with excellent corrosion resistance - Google Patents

Ferritic steel sheet for exhaust system with excellent corrosion resistance Download PDF

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KR20210015012A
KR20210015012A KR1020190093218A KR20190093218A KR20210015012A KR 20210015012 A KR20210015012 A KR 20210015012A KR 1020190093218 A KR1020190093218 A KR 1020190093218A KR 20190093218 A KR20190093218 A KR 20190093218A KR 20210015012 A KR20210015012 A KR 20210015012A
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corrosion
content
steel sheet
weight
corrosion resistance
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KR102255111B1 (en
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강형구
하헌재
김영준
조규진
이문수
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주식회사 포스코
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Priority to KR1020190093218A priority Critical patent/KR102255111B1/en
Priority to EP20847115.1A priority patent/EP3981895A4/en
Priority to PCT/KR2020/008863 priority patent/WO2021020757A1/en
Priority to JP2022506185A priority patent/JP7297373B2/en
Priority to CN202080051008.4A priority patent/CN114127321B/en
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C21D8/0284—Application of a separating or insulating coating
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

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Abstract

Disclosed is a ferritic steel sheet having excellent corrosion resistance for an exhaust system while reducing Cr which is a high-priced element. According to one embodiment of the present invention, the ferritic steel sheet for an exhaust system having excellent corrosion resistance includes: no more than 0.02 wt% of C; no more than 0.02 wt% of N; no more than 2.0 wt% of Si; no more than 0.5 wt% of Mn; 3.0-5.5 wt% of Cr; 0.001-0.3 wt% of Ti; 1.0-4.0 wt% of Al; and the remaining of Fe and inevitable impurities. The ferritic steel sheet includes a surface scale layer, and a maximum value of Al content within a depth range of 0.2 μm from a surface satisfies no less than 15.0% and a maximum value of Si content satisfies no more than 3.0%.

Description

내식성이 우수한 배기계용 페라이트계 강판 {FERRITIC STEEL SHEET FOR EXHAUST SYSTEM WITH EXCELLENT CORROSION RESISTANCE}Ferritic steel sheet for exhaust system with excellent corrosion resistance {FERRITIC STEEL SHEET FOR EXHAUST SYSTEM WITH EXCELLENT CORROSION RESISTANCE}

본 발명은 배기계용 페라이트계 강판에 관한 것으로, 보다 상세하게는 배기계 시스템에 적합한 내식성 및 내산화성이 우수한 페라이트계 강판에 관한 것이다.The present invention relates to a ferritic steel sheet for an exhaust system, and more particularly, to a ferritic steel sheet having excellent corrosion resistance and oxidation resistance suitable for an exhaust system.

자동차, 이륜차의 배기계 시스템은 외부에 노출되어 겨울철 제설염에 의한 오염으로부터 쉽게 부식될 수 있는 환경에 놓여 있으며, 또한 화석연료의 배기로부터 발생하는 산성 응축수로부터도 쉽게 부식될 수 있는 환경이다. Exhaust systems of automobiles and motorcycles are exposed to the outside and are in an environment that can be easily corroded from pollution by snow removal salt in winter, and also from acidic condensate generated from fossil fuel exhaust.

배기가스 온도가 점점 높아지는 환경에서, 부식을 방지하기 위해 배기계 시스템에 사용되는 소재는 열용량이 큰 주물에서 열용량이 작은 스테인리스강이 주력이 되어왔다. 특히, 오스테나이트계 스테인리스 강재에 비하여 고가의 합금원소가 적게 첨가되는 페라이트계 스테인리스 강재는 내식성도 뛰어나 가격 경쟁력이 높아, 상온 내지 800℃의 배기가스 온도범위에 대응하는 배기계 부품 등(Muffler, Ex-manifold, Collector cone 등)에 주로 사용되어 왔다.In an environment where the exhaust gas temperature is increasingly high, the material used in the exhaust system system to prevent corrosion has been mainly made of stainless steel with a low heat capacity in a casting with a large heat capacity. In particular, ferritic stainless steels, which contain less expensive alloying elements than austenitic stainless steels, are excellent in corrosion resistance and have high price competitiveness. Exhaust system parts corresponding to the exhaust gas temperature range of room temperature to 800℃ (Muffler, Ex- manifold, collector cone, etc.).

내부식성 및 내산화성을 확보하기 위한 가장 일반적인 방법은 Cr 함량이 높인 스테인리스강을 사용하는 것이지만, Cr을 최소 11중량% 이상 포함하는 페라이트계 스테인리스강은 고가이다. 또한 스테인리스강은 Cr 함유량이 높아 난산세성 및 산세 비용이 크며, Nb 등을 다량으로 포함하기 때문에 냉연 소둔 온도도 높여야 한다. 이에 따라 가격 상승을 초래하는 Cr을 저감하면서도 우수한 내식성을 확보하는 배기계용 강판의 필요성이 대두되고 있다.The most common method for securing corrosion resistance and oxidation resistance is to use stainless steel with a high Cr content, but ferritic stainless steels containing at least 11% by weight of Cr are expensive. In addition, stainless steel has a high Cr content, so it is difficult to pickling and pickling costs, and since it contains a large amount of Nb, the cold rolled annealing temperature must also be increased. Accordingly, there is a need for a steel plate for exhaust system that secures excellent corrosion resistance while reducing Cr, which causes price increase.

본 발명의 실시예들은 고가 원소인 Cr을 저감하면서도 배기계용으로 우수한 내식성을 갖춘 페라이트계 강판을 제공하고자 한다.Embodiments of the present invention aim to provide a ferritic steel sheet having excellent corrosion resistance for exhaust systems while reducing Cr, which is an expensive element.

본 발명의 일 실시예에 따른 내식성이 우수한 배기계용 페라이트계 강판은, 중량%로, C: 0.02% 이하, N: 0.02% 이하, Si: 2.0% 이하, Mn: 0.5% 이하, Cr: 3.0 내지 5.5%, Ti: 0.001 내지 0.3%, Al: 1.0 내지 4.0%, 나머지 Fe 및 불가피한 불순물을 포함하고, 표면 스케일층을 구비하며, 하기와 같이 정의되는 Al 피막지수 15.0 이상 및 Si 피막지수 3.0 이하를 만족한다.The ferritic steel sheet for exhaust system having excellent corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, containing the remaining Fe and inevitable impurities, having a surface scale layer, and having an Al film index of 15.0 or more and a Si film index of 3.0 or less, which are defined as follows Satisfies.

[Al 피막지수]: 표면으로부터 0.2㎛ 깊이 범위에서 Al 함량의 최대값(중량%)[Al coating index]: the maximum value of Al content in the 0.2㎛ depth range from the surface (% by weight)

[Si 피막지수]: 표면으로부터 0.2㎛ 깊이 범위에서 Si 함량의 최대값(중량%)[Si film index]: the maximum value of Si content in the 0.2㎛ depth range from the surface (% by weight)

또한, 본 발명의 일 실시예에 따르면, Al, Cr, Si 각 원소의 함량은 하기 식 (1)을 만족할 수 있다.In addition, according to an embodiment of the present invention, the content of each element of Al, Cr, and Si may satisfy Equation (1) below.

(1) 5*Al - (Cr+Si) > 0(One) 5*Al-(Cr+Si)> 0

또한, 본 발명의 일 실시예에 따르면, 하기 식 (2)로 표시되는 부식 감모율이 20% 미만일 수 있다.In addition, according to an embodiment of the present invention, the corrosion loss rate represented by the following equation (2) may be less than 20%.

(2) 부식 감모율(%) = [(부식시험 전 무게) - (부식시험 후 무게)]/(부식시험 전 무게) X 100(2) Corrosion loss rate (%) = [(Weight before corrosion test)-(Weight after corrosion test)]/(Weight before corrosion test) X 100

여기서, 부식시험 후 무게는 부식시험 후 생성된 부식생성물을 제거한 뒤의 무게(g)이다.Here, the weight after the corrosion test is the weight (g) after removing the corrosion products generated after the corrosion test.

또한, 본 발명의 일 실시예에 따르면, 표면의 L*a*b* 표색계의 L* 값이 50 이상일 수 있다.In addition, according to an embodiment of the present invention, the L* value of the surface L*a*b* color system may be 50 or more.

또한, 본 발명의 일 실시예에 따르면, 표면의 L*a*b* 표색계의 a*값이 -10 내지 +10 및 b* 값이 -10 내지 +10 범위일 수 있다.In addition, according to an embodiment of the present invention, the a* value of the surface L*a*b* color system may range from -10 to +10, and the b* value may range from -10 to +10.

본 발명의 실시예에 따른 페라이트계 강판은 배기계 용도로 사용되는 기존의 스테인리스강에 비하여 원료비 및 공정비용을 크게 절감할 수 있을 뿐 아니라 우수한 내식성을 나타낼 수 있다.The ferritic steel sheet according to the exemplary embodiment of the present invention can significantly reduce raw material cost and process cost compared to the existing stainless steel used for exhaust system applications, as well as exhibit excellent corrosion resistance.

또한, 최종 산세 공정을 거치지 않아도 L*a*b* 표색계의 L* 값이 50 이상, a* 및 b* 값이 -10 내지 +10 범위의 밝은 무채색 금속성을 나타내어 표면성상이 우수하다.In addition, even without going through the final pickling process, the L* value of the L*a*b* color system is 50 or more, and the a* and b* values show a bright achromatic metallicity in the range of -10 to +10, so that the surface properties are excellent.

도 1은 L*a*b* 표색계를 나타내는 색공간(COLOR SPACE)이다.
도 2는 본 발명에 따른 발명강 2의 표면으로부터 깊이 방향으로 0.2㎛ 범위에 대한 글로우 방전 분광분석법으로 분석한 합금성분 분포이다.
도 3은 본 발명에 따른 비교강 5의 표면으로부터 깊이 방향으로 0.2㎛ 범위에 대한 글로우 방전 분광분석법으로 분석한 합금성분 분포이다.
도 4는 본 발명에 따른 발명강 2의 산세 후 표면으로부터 깊이 방향으로 0.2㎛ 범위에 대한 글로우 방전 분광분석법으로 분석한 합금성분 분포이다.
도 5는 본 발명에 따른 비교강 10의 냉연소둔 강판 시편의 표면을 나타내는 사진이다.
도 6은 본 발명에 따른 발명강 2의 냉연소둔 강판 시편의 표면을 나타내는 사진이다.1 is a color space representing an L*a*b* color system.
2 is a distribution of alloy components analyzed by glow discharge spectroscopy for a range of 0.2 μm in the depth direction from the surface of Inventive Steel 2 according to the present invention.
3 is a distribution of alloy components analyzed by glow discharge spectroscopy for a range of 0.2 μm in the depth direction from the surface of Comparative Steel 5 according to the present invention.
4 is a distribution of alloy components analyzed by glow discharge spectroscopy for a range of 0.2 μm in the depth direction from the surface after pickling of Inventive Steel 2 according to the present invention.
5 is a photograph showing the surface of a cold-rolled annealed steel sheet specimen of comparative steel 10 according to the present invention.
6 is a photograph showing the surface of a cold-rolled annealed steel sheet specimen of Inventive Steel 2 according to the present invention.

이하에서는 본 발명의 실시예를 첨부 도면을 참조하여 상세히 설명한다. 이하의 실시예는 본 발명이 속하는 기술분야에서 통상의 지식을 가진 자에게 본 발명의 사상을 충분히 전달하기 위해 제시하는 것이다. 본 발명은 여기서 제시한 실시예만으로 한정되지 않고 다른 형태로 구체화될 수도 있다. 도면은 본 발명을 명확히 하기 위해 설명과 관계없는 부분의 도시를 생략하고, 이해를 돕기 위해 구성요소의 크기를 다소 과장하여 표현할 수 있다.Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited only to the examples presented herein and may be embodied in other forms. In the drawings, in order to clarify the present invention, portions not related to the description may be omitted, and the size of components may be slightly exaggerated to aid understanding.

또한 어떤 부분이 어떤 구성요소를 "포함"한다고 할 때, 이는 특별히 반대되는 기재가 없는 한 다른 구성요소를 제외하는 것이 아니라 다른 구성요소를 더 포함할 수 있는 것을 의미한다.In addition, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary.

단수의 표현은 문맥상 명백하게 예외가 있지 않는 한, 복수의 표현을 포함한다.Singular expressions include plural expressions, unless the context clearly has exceptions.

배기계용 스테인리스강, 특히 페라이트계 스테인리스강에는 고온강도 향상을 위해 Nb를 첨가하거나 이를 대체하여 Sn 등을 첨가하기도 하며, 내산화성 향상을 위해 Cr 함량을 높이는 것이 일반적이다. 그러나 Nb, Sb 등의 고용강화 원소 첨가와 Cr 함량 증가는 제조원가를 상승시키는 원인이 되어 바람직한 개발 방향이 아니다.In the exhaust system stainless steel, especially ferritic stainless steel, Nb is added to improve high temperature strength, or Sn may be added in place of it, and the Cr content is generally increased to improve oxidation resistance. However, the addition of solid solution strengthening elements such as Nb and Sb and the increase of Cr content increase manufacturing costs, and are not a desirable development direction.

배기계용 페라이트계 스테인리스강의 원료비 절감을 위해서는, 상대적으로 함량이 높은 고가 원소인 Cr 함량의 저감이 필수적이다. 그러나 Cr은 배기계용 페라이트계 스테인리스강에 있어서 내식성을 확보하는 핵심원소이므로, Cr 저감을 위해서는 내식성을 확보할 수 있는 다른 방안이 필요하다. 본 발명에서는 스테인리스강으로서 Cr의 최소 함량인 11 중량%보다 낮은 Cr 함량을 갖는 강판이면서도, 종래 스테인리스강과 동등 이상의 내식성을 확보하여 원료비를 절감할 수 있는 페라이트계 강판을 제공하고자 한다.In order to reduce the raw material cost of ferritic stainless steel for exhaust systems, it is essential to reduce the Cr content, which is an expensive element with a relatively high content. However, since Cr is a key element for securing corrosion resistance in ferritic stainless steel for exhaust systems, another method for securing corrosion resistance is required to reduce Cr. In the present invention, it is intended to provide a ferritic steel sheet capable of reducing raw material costs by securing corrosion resistance equal to or higher than that of conventional stainless steel while being a steel sheet having a Cr content lower than 11% by weight, which is the minimum content of Cr as stainless steel.

본 발명의 일 실시예에 따른 내식성이 우수한 배기계용 페라이트계 강판은, 중량%로, C: 0.02% 이하, N: 0.02% 이하, Si: 2.0% 이하, Mn: 0.5% 이하, Cr: 3.0 내지 5.5%, Ti: 0.001 내지 0.3%, Al: 1.0 내지 4.0%, 나머지 Fe 및 불가피한 불순물을 포함한다.The ferritic steel sheet for exhaust system having excellent corrosion resistance according to an embodiment of the present invention is, by weight, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, remaining Fe and unavoidable impurities.

이하, 본 발명의 실시예에서의 합금성분 원소 함량의 수치한정 이유에 대하여 설명한다. 이하에서는 특별한 언급이 없는 한 단위는 중량%이다.Hereinafter, the reason for the numerical limitation of the content of the alloy component element in the examples of the present invention will be described. Hereinafter, unless otherwise specified, the unit is% by weight.

C의 함량은 0 초과 0.02% 이하이다.The content of C is more than 0 and not more than 0.02%.

C 함량이 0.02%를 초과할 경우 용접부 인성이 저하될 수 있으며, Cr과 결합하여 Cr23C6 석출물이 생성되어 기지 내 국부 Cr 고갈로 내식성 및 내산화성이 저하된다. 한편, C는 불가피한 불순물로서 0 초과로 포함되며, 극저 함량 제어를 위해서는 제강 VOD 공정비가 증가하는바 바람직하게는 0.005% 이상 포함될 수 있다.If the C content exceeds 0.02%, the toughness of the weld may be deteriorated, and Cr 23 C 6 precipitates are generated by bonding with Cr, resulting in a decrease in corrosion resistance and oxidation resistance due to local Cr depletion in the matrix. On the other hand, C is an inevitable impurity and is contained in excess of 0, and the steel VOD process cost is increased in order to control the ultra-low content, and thus, it may preferably be contained by 0.005% or more.

N의 함량은 0 초과 0.02% 이하이다.The content of N is more than 0 and not more than 0.02%.

강 중 N이 0.02%를 초과할 경우 고용 N의 농도는 한계에 다다르며, Cr과 결합하며 Cr2N 석출물이 생성되어 기지 내 국부 Cr 고갈로 내식성 및 내산화성이 저하된다. 한편, N은 불가피한 불순물로서 0 초과로 포함되며, 극저 함량 제어를 위해서는 제강 VOD 공정비가 증가하는바 바람직하게는 0.005% 이상 포함될 수 있다.When the N content of the steel exceeds 0.02%, the concentration of solid solution N reaches the limit, it binds to Cr, and Cr 2 N precipitates are generated, resulting in a decrease in corrosion resistance and oxidation resistance due to local Cr depletion in the matrix. On the other hand, N is an inevitable impurity and is included in excess of 0, and the steel VOD process cost is increased in order to control the extremely low content, and thus, it may preferably be included in an amount of 0.005% or more.

Si의 함량은 2.0% 이하이다.The content of Si is 2.0% or less.

Si은 고용강화 원소임과 동시에 표층부에 Si 농화 산화막을 형성하여 내산화성을 증가시킨다. 그러나 본 발명에서는 후술할 '산세의 생략'을 구현하기 위해 소둔 열처리 후 Si 피막지수가 3.0 이하로 제한되어야 하며, 이를 위해 총 함량을 2.0% 이하로 제한한다. 그러나 변색 방지를 더욱 쉽게 제어하기 위한 관점에서 1.5% 이하로 포함할 수 있으며, 1.0% 이하로 포함할 수도 있다.Si is a solid solution strengthening element and increases oxidation resistance by forming a Si-enriched oxide film on the surface layer. However, in the present invention, the Si film index after annealing heat treatment should be limited to 3.0 or less in order to implement'omit pickling' to be described later, and for this purpose, the total content is limited to 2.0% or less. However, from the viewpoint of more easily controlling the discoloration prevention, it may be included at 1.5% or less, or 1.0% or less.

Mn의 함량은 0.5% 이하이다.The content of Mn is 0.5% or less.

Mn은 강 중에 불가피하게 포함되는 불순물이며, 오스테나이트를 안정화시키는 역할을 한다. Mn 함량이 0.5%를 초과할 경우 열연 또는 냉연 후 소둔 열처리 시 오스테나이트 역변태가 발생하게 되어 연신율에 악영향을 미치게 된다. 따라서 Mn의 함량을 위와 같이 제한한다.Mn is an impurity that is inevitably included in steel, and serves to stabilize austenite. When the Mn content exceeds 0.5%, reverse austenite transformation occurs during annealing heat treatment after hot rolling or cold rolling, which adversely affects the elongation. Therefore, the content of Mn is limited as above.

Cr의 함량은 3.0 내지 5.5%이다.The content of Cr is 3.0 to 5.5%.

Cr은 내식성을 향상시키는 원소이지만, 원료비 절감을 위한 본 발명의 취지에 따라 5.5% 이하로 제한한다. 단, 최소한의 내식성을 확보하기 위해 3.0% 이상 첨가한다.Cr is an element that improves corrosion resistance, but is limited to 5.5% or less according to the purpose of the present invention for reducing raw material cost. However, more than 3.0% is added to ensure minimum corrosion resistance.

Ti의 함량은 0.001 내지 0.3%이다.The content of Ti is 0.001 to 0.3%.

Ti은 C, N과 결합해 Ti(C,N) 석출물을 형성하여 고용 C, N의 양을 낮추고 Cr 고갈층 형성을 억제하는 역할을 Ti는 용접부 내식성 및 인성 향상을 위해 필수적으로 0.001% 이상 첨가되어야 한다. Ti은 C, N과 결합해 Ti(C,N) 석출물을 형성하여 고용 C, N의 양을 낮추고, Cr 고갈층 형성을 억제하는 역할을 한다. 하지만 Ti 함량이 과다할 경우 주조에 악영향을 주므로 0.3% 이하로 제한한다.Ti combines with C and N to form Ti(C,N) precipitates, thereby lowering the amount of solid solution C and N and suppressing the formation of a Cr depletion layer.Ti is essentially added by 0.001% or more to improve the corrosion resistance and toughness of the weld. Should be. Ti combines with C and N to form Ti(C,N) precipitates, thereby lowering the amount of solid solution C and N, and suppressing the formation of a Cr depletion layer. However, excessive Ti content adversely affects casting, so it is limited to 0.3% or less.

Al의 함량은 1.0 내지 4.0%이다.The content of Al is 1.0 to 4.0%.

본 발명에서는 Al이 소둔 열처리 시 산화피막을 형성시킬 수 있도록 1.0% 이상 충분히 첨가한다. 그러나 과량 첨가할 경우 주조와 압연이 어려워질 수 있어 상한을 4.0% 이하로 제한한다.In the present invention, 1.0% or more is sufficiently added so that Al can form an oxide film during annealing heat treatment. However, if it is added in excess, casting and rolling may become difficult, so the upper limit is limited to 4.0% or less.

본 발명의 나머지 성분은 철(Fe)이다. 다만, 통상의 제조 과정에서는 원료 또는 주위 환경으로부터 의도되지 않는 불순물들이 불가피하게 혼입될 수 있으므로, 이를 배제할 수는 없다. 상기 불순물들은 통상의 제조 과정의 기술자라면 누구라도 알 수 있는 것이기 때문에 그 모든 내용을 특별히 본 명세서에서 언급하지는 않는다.The remaining component of the present invention is iron (Fe). However, in a typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, and this cannot be excluded. Since the impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.

단, 상술한 합금 성분계만으로는 내식성 확보에 불충분하다. 본 발명자들이 검토한 바에 따르면, 원료비 절감을 위해 Cr 함량을 줄이면 외부에 노출되었을 때 부식이 발생하는 등 부식 저항성이 극히 취약해지는 문제가 있었다. 이에 본 발명에서는 내식성을 확보하기 위하여 특별한 방법을 도입하였다.However, the above-described alloy component system alone is insufficient to ensure corrosion resistance. According to the review by the present inventors, when the Cr content is reduced to reduce the cost of raw materials, there is a problem that corrosion resistance becomes extremely weak, such as corrosion occurs when exposed to the outside. Accordingly, in the present invention, a special method was introduced to ensure corrosion resistance.

배기계용 스테인리스강 냉연강판은, 냉간압연 후 연화를 위해 소둔 열처리를 실시한 다음 표면의 스케일 제거를 위해 산세 처리하여 제품을 출하하는 것이 일반적이다. 본 발명에서는 상술한 합금 성분계 조성의 냉연강판을 소둔 열처리함에 있어, 아래와 같이 정의되는 Al 피막지수 및 Si 피막지수 범위를 만족하는 표면을 갖도록 소둔 열처리한 후, 산세 처리 없이 최종 제품을 제조한다. 즉, 본 발명에 따른 페라이트계 강판은 냉연소둔 강판으로, 표면에 스케일층을 갖는다. For exhaust system stainless steel cold-rolled steel sheets, after cold rolling, annealing heat treatment is performed for softening, and then the product is shipped by pickling treatment to remove scale from the surface. In the present invention, in the annealing heat treatment of the cold-rolled steel sheet having the above-described alloy component composition, after annealing heat treatment to have a surface that satisfies the ranges of the Al film index and the Si film index defined below, a final product is manufactured without pickling treatment. That is, the ferritic steel sheet according to the present invention is a cold rolled annealed steel sheet and has a scale layer on its surface.

종래에는 스케일층이 내식성에 불리한 Fe의 다량 함유로 회피의 대상이었으나, 본 발명에서는 내식성에 유리한 Al 농화 산화피막을 형성하여 의도적으로 포함하는 의미를 가진다. 소둔 열처리를 통해 표층으로 농화되어 산화되는 Al과 Si의 함량을 제어함으로써 3.0 내지 5.5% Cr의 페라이트계 강판에서도 스테인리스강과 동등 수준 이상의 내식성 및 내산화성을 확보할 수 있다.Conventionally, the scale layer has been an object of avoidance due to containing a large amount of Fe, which is disadvantageous in corrosion resistance, but in the present invention, it has the meaning of intentionally including by forming an Al-enriched oxide film that is advantageous in corrosion resistance. By controlling the content of Al and Si that are concentrated and oxidized to the surface layer through annealing heat treatment, even a ferritic steel sheet of 3.0 to 5.5% Cr can secure corrosion resistance and oxidation resistance equal to or higher than that of stainless steel.

본 발명의 일 실시예에 따른 배기계용 페라이트계 강판은 표면으로부터 깊이 방향으로 스케일층을 포함한 0.2㎛ 깊이 범위에서의 Al 피막지수가 15.0 이상, 그리고 Si 피막지수가 3.0 이하를 만족한다. Al 피막지수와 Si 피막지수는 아래와 같이 정의된다.The ferritic steel sheet for exhaust system according to an embodiment of the present invention satisfies an Al film index of 15.0 or more and a Si film index of 3.0 or less in a 0.2 μm depth range including the scale layer from the surface to the depth direction. The Al film index and the Si film index are defined as follows.

[Al 피막지수]: 표면으로부터 0.2㎛ 깊이 범위에서 Al 함량의 최대값(중량%)[Al coating index]: the maximum value of Al content in the 0.2㎛ depth range from the surface (% by weight)

[Si 피막지수]: 표면으로부터 0.2㎛ 깊이 범위에서 Si 함량의 최대값(중량%)[Si film index]: the maximum value of Si content in the 0.2㎛ depth range from the surface (% by weight)

일반적으로 Si는 표층부에 Si 농화 산화막을 형성하여 고온 내산화성을 증가시키는 것으로 알려져 있다. 그런데 산세를 실시하지 않는 본 발명에서는 Si 피막지수가 3.0을 초과하는 경우에 표면에 어두운 갈색 스케일층이 형성되어 표면성상이 불량해지며, 이에 Si 피막지수는 3.0 이하로 제한하여야 한다.In general, Si is known to increase high-temperature oxidation resistance by forming a Si-enriched oxide film on the surface layer. However, in the present invention, in which the pickling is not performed, when the Si coating index exceeds 3.0, a dark brown scale layer is formed on the surface, resulting in poor surface properties. Accordingly, the Si coating index should be limited to 3.0 or less.

Al 또한 표층부의 산소와 반응해 불균일한 산화층을 형성하는데, 본 발명에 따른 Al 함량인 1.0 내지 4.0%를 첨가한 후 소둔 열처리를 실시하는 경우 Si의 표층으로의 이동 및 반응을 방해하여 우선적으로 Al 농화 산화막이 형성된다. Al 산화막이 치밀하게 형성되어 Al 피막지수가 15.0 이상인 경우 밝은 금속성 색상을 나타낼 수 있다.Al also reacts with oxygen in the surface layer to form a non-uniform oxide layer.In the case of annealing heat treatment after adding 1.0 to 4.0% of the Al content according to the present invention, the movement and reaction of Si to the surface layer are hindered. A concentrated oxide film is formed. If the Al oxide film is densely formed and the Al film index is 15.0 or higher, a bright metallic color may be displayed.

소재 표면의 금속성 색상은 국제조명위원회가 제정한 L*a*b* 표색계로 나타낼 수 있다. L*a*b* 표색계는 물체의 색을 표현하는데 있어서 현재 모든 분야에서 가장 대중적으로 사용되는 표색계로서, 도 1에서는 L*a*b* 표색계를 나타내는 색공간(COLOR SPACE)을 보여주고 있다. 이 때 L*은 0일 때 검은색, 100일 때 흰색을 강하게 나타내고, a*는 양수일 때 적색(Red) 방향, 음수일 때 녹색(Green) 방향을 나타내며, b*는 양수일 때 황색(Yellow) 방향, 음수일 때 청색(Blue) 방향을 나타낸다. a*, b*가 모두 0이면 무채색이 된다.The metallic color of the surface of the material can be represented by the L*a*b* color system established by the International Lighting Commission. The L*a*b* color system is currently the most popular color system used in all fields in expressing the color of an object, and FIG. 1 shows a color space representing the L*a*b* color system. At this time, L* represents black when 0, white when 100, a* represents red when positive, green when negative, and b* represents yellow when positive. Direction, when negative, indicates the blue direction. When both a* and b* are 0, it becomes achromatic.

본 발명의 일 실시예에 따르면, Al 농화 산화피막을 형성하여 L*a*b* 표색계의 L* 값이 50 이상인 밝은 금속성 표면을 얻을 수 있다. 더욱 밝은 금속성 색상 표현을 위해 L* 값은 60 이상일 수 있으며, 더 바람직하게는 70 이상일 수 있다. 또한, 50 이상의 L* 값과 함께 a* 값과 b* 값이 동시에 -10 내지 +10 범위의 밝은 무채색 색상의 금속성 표면을 얻을 수 있다.According to an embodiment of the present invention, a bright metallic surface having an L* value of 50 or more in an L*a*b* color system may be obtained by forming an Al-enriched oxide film. For brighter metallic color expression, the L* value may be 60 or more, and more preferably 70 or more. In addition, it is possible to obtain a metallic surface of a bright achromatic color in the range of -10 to +10 at the same time as the a* value and the b* value together with an L* value of 50 or more.

도 2 내지 도 4은 본 발명에 따른 실시예들을 표면으로부터 깊이 방향으로 0.2㎛까지 글로우 방전 분광분석법으로 분석한 합금성분 분포이다. 2 to 4 are alloy component distributions analyzed by glow discharge spectroscopy from the surface to 0.2 μm in the depth direction of the embodiments according to the present invention.

도 2는 본 발명의 실시예에 따른 냉연강판을 소둔 열처리 후 산세를 실시하지 않은 시편의 합금성분 분포를 나타낸다. 깊이 방향으로의 측정값 중 Al 함량의 최대값인 Al 피막지수가 15.0 이상이다.2 shows the distribution of alloy components in a specimen not subjected to pickling after annealing heat treatment of a cold-rolled steel sheet according to an embodiment of the present invention. Among the measured values in the depth direction, the Al film index, which is the maximum value of the Al content, is 15.0 or more.

도 3은 Si와 Al의 함량범위가 일반 배기계용 페라이트계 스테인리스강과 동일하며 원료비 절감을 위해 Cr 함량만 낮춘 냉연강판을, 동일하게 소둔 열처리 후 산세를 실시하지 않은 시편의 합금성분 분포를 나타낸다. 즉, Cr과 Al의 함량범위가 본 발명의 조성범위를 벗어나는 냉연소둔 강판에 해당한다. 도 3을 살펴보면 Al 피막지수가 낮고 최표층에서 Si 피막지수가 5.0 가까이 나타난다. 이러한 경우 내식성 및 내산화성이 불충분하였으며 Si 산화막으로 인해 표면 변색 또한 발생하게 된다.3 shows the distribution of alloy components of a cold-rolled steel sheet in which the content range of Si and Al is the same as that of the ferritic stainless steel for general exhaust system and the Cr content is lowered to reduce the raw material cost, and the specimen is not pickled after the same annealing heat treatment. That is, the content range of Cr and Al corresponds to the cold-rolled annealed steel sheet outside the composition range of the present invention. Referring to FIG. 3, the Al coating index is low and the Si coating index is close to 5.0 in the outermost layer. In this case, corrosion resistance and oxidation resistance are insufficient, and surface discoloration occurs due to the Si oxide film.

도 4는 도 2에서와 동일한 합금 성분계 시편을 본 발명의 실시예에 따라 냉연강판을 소둔 열처리하고, 이후 산세까지 실시한 후의 합금성분 분포를 나타낸다. 동일한 함량의 Cr, Al, Si을 포함하더라도 본 발명에서 제시하는 소둔 열처리 후 산세의 생략을 수행하지 않는 경우 Al 피막지수가 낮게 나타난다.FIG. 4 shows the distribution of alloy components after annealing heat treatment of a cold-rolled steel sheet according to an embodiment of the present invention to the same alloy component-based specimen as in FIG. 2, followed by pickling. Even if the same content of Cr, Al, and Si is included, the Al film index is low when the pickling is not omitted after the annealing heat treatment proposed in the present invention.

또한, 본 발명의 일 실시예에 따르면. Al 피막지수 및 Si 피막지수를 동시에 만족하기 위해, 페라이트계 강판은 하기 식 (1)을 만족할 수 있다.Also, according to an embodiment of the present invention. In order to simultaneously satisfy the Al film index and the Si film index, the ferritic steel sheet can satisfy the following formula (1).

(1) 5*Al - (Cr+Si) > 0(One) 5*Al-(Cr+Si)> 0

Al을 식 (1)과 같이 충분히 함유시키면, 소둔 중에 충분한 Al 농화 산화피막을 형성할 수 있다. 반면 그렇지 못 할 경우, Cr과 Si의 산화로 Al 농화 산화피막을 형성할 산소가 불충분하게 되거나, Cr 또는 Si 산화피막의 형성으로 일부 Al 농화 산화피막 형성에 필요한 산소의 이동이 제한될 수 있기에 피해야 한다.If Al is sufficiently contained as in the formula (1), a sufficient Al-rich oxide film can be formed during annealing. On the other hand, if this is not the case, it should be avoided because oxygen to form an Al-enriched oxide film may become insufficient due to the oxidation of Cr and Si, or the movement of oxygen required for the formation of some Al-enriched oxide film may be restricted due to the formation of a Cr or Si oxide film. do.

한편, 스케일층의 두께는 소둔 열처리 온도 및 시간에 따라 다르게 나타날 수 있지만, 본 발명에서는 Al 피막지수가 절반이 되는 지점에서의 두께로 정의될 수 있다. 예를 들어, 도 1에서의 스케일층 두께는 Al 함량 최대값인 Al 피막지수의 중간값에 해당하는 약 0.1㎛일 수 있다.Meanwhile, the thickness of the scale layer may vary depending on the annealing heat treatment temperature and time, but in the present invention, it may be defined as the thickness at the point where the Al film index becomes half. For example, the thickness of the scale layer in FIG. 1 may be about 0.1 μm, which corresponds to an intermediate value of the Al film index, which is the maximum Al content value.

본 발명에 따른 Al 피막지수 및 Si 피막지수를 만족하기 위한 소둔 열처리는 분위기가스 중 고순도 수소를 75% 이상 사용하는 고가의 광휘소둔(BAL) 공정을 이용하지 않아도 되며, 저가의 가스를 사용하는 연속소둔 공정을 거치는 것으로 가능하다. 예를 들면, 연료가스를 열원으로 사용하고 폐가스의 과잉 산소를 0.1 내지 10% 범위로 제한하는 것으로도 본 발명의 목적을 달성 가능하다. The annealing heat treatment to satisfy the Al film index and the Si film index according to the present invention does not require the use of an expensive bright annealing (BAL) process using 75% or more of high-purity hydrogen among atmospheric gases, and continuous use of low-cost gas. It is possible by going through an annealing process. For example, it is possible to achieve the object of the present invention by using fuel gas as a heat source and limiting the excess oxygen in the waste gas to 0.1 to 10%.

과잉 산소를 0.1% 이상으로 하여 산소를 부여함으로써, 소둔 열처리 중 본 발명 함량범위에 따른 Al이 산소와 반응하여 고내식성을 부여하는 피막을 형성하는 것이다. 과잉 산소가 부족하면 충분한 Al 농화 산화피막이 형성되지 않을 수 있다. 반면 과잉 산소가 10%를 초과하면 소재의 Fe, Cr 또는 Si과 산소가 반응하여 Al 농화 산화피막 외에 Fe, Cr, Si 산화피막이 형성될 수 있으며, 이 경우 변색이 발생할 수 있어 부적절하다. By imparting oxygen with 0.1% or more of excess oxygen, Al according to the content range of the present invention reacts with oxygen during annealing heat treatment to form a film that imparts high corrosion resistance. If excess oxygen is insufficient, a sufficient Al-enriched oxide film may not be formed. On the other hand, when the excess oxygen exceeds 10%, Fe, Cr, or Si of the material reacts with oxygen to form an oxide film of Fe, Cr, and Si in addition to the oxide film enriched in Al, and in this case, discoloration may occur, which is inappropriate.

한편, 용이한 제조를 위하여 산소를 0.1% 이하로 제한하고 싶은 경우, 분위기가스 중의 수소를 0.1% 내지 10% 범위로 혼합하면 Fe, Cr, Si과의 산화가 억제되어 0.1% 이하의 적은 양의 산소로도 Al 농화 산화피막 형성이 가능하다. 10% 이상으로 혼합하는 것은 상술한 바와 같이 비용의 상승을 초래하므로 불필요한 일이며, 0.1% 미만의 수소로는 Fe, Cr, Si의 산화를 억제하는 능력이 불충분하여 Al 농화 산화피막 형성이 불충분해진다.On the other hand, when it is desired to limit oxygen to 0.1% or less for easy production, if hydrogen in the atmosphere gas is mixed in the range of 0.1% to 10%, oxidation with Fe, Cr, and Si is suppressed, resulting in a small amount of 0.1% or less. Al-enriched oxide film can be formed even with oxygen. Mixing more than 10% is unnecessary because it causes an increase in cost as described above, and with less than 0.1% hydrogen, the ability to inhibit oxidation of Fe, Cr, and Si is insufficient, resulting in insufficient Al-enriched oxide film formation. .

소둔 열처리 후 산세는 생략하여야 한다. 산세를 미실시함으로써 Al 및 Si 피막지수를 만족하고 스케일층이 제거되지 않은 최표층을 얻을 수 있으며, 질산 및/또는 불산의 혼산 용액을 사용하는 산세 공정의 생략을 통해 제조비용 또한 절감할 수 있다.Pickling should be omitted after annealing heat treatment. By not performing pickling, it is possible to obtain the outermost layer that satisfies the Al and Si film index and does not remove the scale layer, and also reduces the manufacturing cost by omitting the pickling process using a mixed acid solution of nitric acid and/or hydrofluoric acid.

소둔 열처리 및 산세 생략 이전의 냉연강판은 통상적인 제조공정을 거쳐 제조할 수 있으며, 예를 들어, 상술한 합금성분 조성을 포함하는 슬라브를 열간압연하고, 열간압연된 열연강판을 소둔 열처리하고, 산세 후 냉간압연하여 냉연강판으로 제조할 수 있다. Cold-rolled steel sheet before annealing heat treatment and pickling is omitted can be manufactured through a conventional manufacturing process, for example, a slab containing the above-described alloy component composition is hot-rolled, the hot-rolled hot-rolled steel sheet is annealed and heat treated, and then pickled. It can be cold-rolled to produce a cold-rolled steel sheet.

본 발명의 일 실시예에 따른 내식성이 우수한 배기계용 페라이트계 강판은, 하기 식 (2)로 표시되는 부식 감모율이 20% 미만일 수 있다.The ferritic steel sheet for exhaust system having excellent corrosion resistance according to an exemplary embodiment of the present invention may have a corrosion loss rate of less than 20% represented by the following equation (2).

(2) 부식 감모율(%) = [(부식시험 전 무게) - (부식시험 후 무게)]/(부식시험 전 무게) X 100(2) Corrosion loss rate (%) = [(Weight before corrosion test)-(Weight after corrosion test)]/(Weight before corrosion test) X 100

여기서, 부식시험 후 무게는 부식시험 후 생성된 부식생성물을 제거한 뒤의 무게(g)이다.Here, the weight after the corrosion test is the weight (g) after removing the corrosion products generated after the corrosion test.

내식성, 즉 부식에 대한 저항성은 임의로 조성한 부식환경에 노출시켜 알 수 있다. 예를 들면, 물 중에 5%의 부피비가 되도록 NaCl을 함유하는 용액을 소재에 분무하여 다음 4시간 유지하고, 60℃ 정도로 4시간 동안 가열하여 건조시키는 과정을 총 30회 반복하여 후술하는 방법으로 부식 정도를 평가할 수 있다. 평가 환경은 다양하게 구성할 수 있으므로 본 발명에 한정하는 것은 아니다.Corrosion resistance, that is, resistance to corrosion, can be known by exposure to an arbitrarily constructed corrosive environment. For example, spraying a solution containing NaCl in water to a volume ratio of 5% on the material and holding it for the next 4 hours, heating and drying at about 60℃ for 4 hours, repeating a total of 30 times to corrode in the method described below. The degree can be assessed. Since the evaluation environment can be configured in various ways, it is not limited to the present invention.

본 발명에서는 '[(부식시험 전 무게) - (부식시험 후 무게)]/(부식시험 전 무게)'를 감모율이라 정의하고, 100을 곱하여 % 단위로 표시하였다. 이와 같이 부식시험 후 생성된 부식생성물을 제거한 뒤의 무게, 즉 '부식시험 후 무게'를 측정하여, '부식시험 전 무게'와 비교함으로써 감모율을 측정할 수 있다. 감모율이 부식생성물의 제거가 필요하다는 점에서 용이하지 않다면, 무게 대신 두께로 대체할 수 있다. 이 경우 부식생성물의 제거는 필요 없고, 단면을 광학현미경으로 관찰하여 부식생성물을 제외한 모재 금속 부분의 두께를 비교하면 된다.In the present invention,'[(weight before corrosion test)-(weight after corrosion test)]/(weight before corrosion test)' is defined as the loss ratio, multiplied by 100 and expressed in %. As described above, the weight after removing the corrosion products generated after the corrosion test, that is, the weight after the corrosion test, is measured and compared with the weight before the corrosion test to measure the loss ratio. If the loss rate is not easy in that the removal of corrosion products is required, it can be replaced by thickness instead of weight. In this case, it is not necessary to remove the corrosion products, and the thickness of the base metal parts excluding the corrosion products can be compared by observing the cross section with an optical microscope.

본 발명에서 대체하고자 하는 11% Cr이 함유된 강에서 동시에 Al을 1.0% 이상 첨가하면 가공성이 나빠질 수 있다. Si도 마찬가지인데, 이러한 현상은 Al, Si 뿐만 아니라 Cr이 Fe와 원자 위치를 치환하여 가공성의 대표적인 지표인 연신율을 저해하기 때문이다. 반면, 본 발명이 제시하는 식 (1)을 만족하면 Al을 1.0% 이상 함유하고도 연신율을 28% 이상 확보할 수 있다. 이 점은 Al 농화 산화피막을 위한 효과와 함께 본 발명을 통해 부수적으로 얻을 수 있는 효과이다.In the steel containing 11% Cr to be replaced in the present invention, if more than 1.0% of Al is added at the same time, workability may deteriorate. The same is true of Si, because this phenomenon is because Cr, as well as Al and Si, impairs elongation, which is a representative index of workability by substituting Fe and atomic positions. On the other hand, if the equation (1) presented by the present invention is satisfied, an elongation of 28% or more can be secured even if Al is contained in 1.0% or more. This point is an effect obtained by the present invention as well as an effect for an Al-enriched oxide film.

이하 본 발명의 바람직한 실시예를 통해 보다 상세히 설명하기로 한다.Hereinafter, it will be described in more detail through a preferred embodiment of the present invention.

실시예Example

아래 표 1에 기재된 합금 성분계로 주조 후 3㎜까지 열간압연하였다. 열간압연 개시 온도는 지나친 조직 성장을 방지하고 충분한 열간가공성을 얻기 위해 바람직한 1,200℃ 전후로 조절하였다. 표면 산세 후 1㎜까지 냉간압연한 다음, 과잉 산소가 5%인 분위기 가스에서 900℃ 이상의 온도에서 10초 이상 소둔하였다. 이후 발명강과 비교강에 대하여 산세를 실시 및 미실시한 시편을 각각 마련하고 외부 노출과 동일 환경에서의 부식 발생여부를 평가하여 표 2에 나타내었다. 외부 노출에 대한 모사는 물 중 부피비로 5% NaCl을 함유하는 용액을 분무한 다음 72시간 방치한 후 표면에 점녹의 발생 여부로 판단하였다. 부식 발생은 ○로, 부식 미발생은 ×로 표시하였다.It was hot-rolled to 3 mm after casting with the alloy component system shown in Table 1 below. The hot-rolling start temperature was adjusted to around 1,200°C, which is preferable to prevent excessive tissue growth and obtain sufficient hot workability. After surface pickling, it was cold-rolled to 1 mm, and then annealed for 10 seconds or more at a temperature of 900°C or higher in an atmosphere gas containing 5% excess oxygen. Thereafter, specimens with and without pickling were prepared for the invention steel and the comparative steel, respectively, and the occurrence of corrosion in the same environment as well as external exposure was evaluated, as shown in Table 2. For simulation of external exposure, a solution containing 5% NaCl in water was sprayed and left for 72 hours, and then spot rust was judged on the surface. The occurrence of corrosion was indicated by ○, and the occurrence of corrosion was indicated by ×.

구분
(중량%)division
(weight%) CC SiSi MnMn AlAl CrCr TiTi NN 식 (1)Equation (1) 비교강1Comparative Steel 1 0.008 0.008 0.4 0.4 0.3 0.3 0.0040.004 13.413.4 0.2 0.2 0.006 0.006 -13.8-13.8 비교강2Comparative lecture 2 0.006 0.006 0.4 0.4 0.3 0.3 0.0030.003 11.511.5 0.2 0.2 0.006 0.006 -11.9-11.9 비교강3Comparative lecture 3 0.010 0.010 0.4 0.4 0.3 0.3 0.0040.004 8.98.9 0.2 0.2 0.007 0.007 -9.3-9.3 비교강4Comparative lecture 4 0.008 0.008 0.4 0.4 0.3 0.3 0.0020.002 7.17.1 0.2 0.2 0.007 0.007 -7.5-7.5 비교강5Comparative lecture 5 0.008 0.008 0.4 0.4 0.3 0.3 0.0030.003 5.2 5.2 0.2 0.2 0.006 0.006 -5.6-5.6 비교강6Comparative lecture 6 0.008 0.008 1.61.6 0.3 0.3 0.0050.005 5.3 5.3 0.2 0.2 0.005 0.005 -6.9-6.9 비교강7Comparative Steel 7 0.009 0.009 2.62.6 0.3 0.3 0.0070.007 5.65.6 0.2 0.2 0.006 0.006 -8.2-8.2 비교강8Comparative lecture 8 0.009 0.009 3.53.5 0.3 0.3 0.0070.007 4.9 4.9 0.2 0.2 0.006 0.006 -8.4-8.4 비교강9Comparative lecture 9 0.007 0.007 0.4 0.4 0.3 0.3 0.90.9 5.3 5.3 0.2 0.2 0.010 0.010 -1.2-1.2 비교강10Comparative Steel 10 0.007 0.007 1.61.6 0.3 0.3 1.3 1.3 5.2 5.2 0.2 0.2 0.007 0.007 -0.3-0.3 발명강1Invention Lesson 1 0.008 0.008 0.4 0.4 0.3 0.3 1.4 1.4 5.4 5.4 0.2 0.2 0.007 0.007 +1.2+1.2 발명강2Invention Lesson 2 0.008 0.008 0.5 0.5 0.3 0.3 1.8 1.8 5.2 5.2 0.2 0.2 0.006 0.006 +3.3+3.3 발명강3Invention Lesson 3 0.007 0.007 0.5 0.5 0.3 0.3 3.5 3.5 5.5 5.5 0.2 0.2 0.006 0.006 +11.5+11.5 발명강4Invention Lesson 4 0.007 0.007 0.3 0.3 0.3 0.3 2.4 2.4 3.2 3.2 0.1 0.1 0.006 0.006 +8.5+8.5 발명강5Invention Lesson 5 0.007 0.007 1.61.6 0.3 0.3 1.81.8 3.2 3.2 0.1 0.1 0.006 0.006 +4.2+4.2

구분division 외부 노출 부식 발생여부External exposure corrosion 산세 실시Conduct pickling 산세 미실시No pickling 비교강1Comparative Steel 1 ×× ○○ 비교강2Comparative lecture 2 ×× ○○ 비교강3Comparative lecture 3 ○○ ○○ 비교강4Comparative lecture 4 ○○ ○○ 비교강5Comparative lecture 5 ○○ ○○ 비교강6Comparative lecture 6 ○○ ○○ 비교강7Comparative Steel 7 ○○ ○○ 비교강8Comparative lecture 8 ○○ ○○ 비교강9Comparative lecture 9 ○○ ○○ 비교강10Comparative Steel 10 ○○ ○○ 발명강1Invention Lesson 1 ○○ ×× 발명강2Invention Lesson 2 ○○ ×× 발명강3Invention Lesson 3 ○○ ×× 발명강4Invention Lesson 4 ○○ ×× 발명강5Invention Lesson 5 ○○ ××

표 2는 본 발명에 따른 합금 성분계 조성범위를 만족하더라도, 소둔 열처리 후 산세를 실시한 소재는 외부에 노출되었을 때 부식이 발생함을 보여주고 있다. 단, 비교강 1과 비교강 2는 본 발명에서 저감하고자 고가 원소인 Cr을 다량 함유하는 페라이트계 스테인리스강으로, 외부 노출 환경에서도 부식이 발생하지 않았다. 본 발명에 따른 발명강들은 동일한 합금 성분계의 시편임에도 소둔 열처리 후 산세를 미실시한 결과, 부식이 발생하지 않음을 확인할 수 있었다.Table 2 shows that even if the composition range of the alloy component according to the present invention is satisfied, the material subjected to pickling after annealing heat treatment causes corrosion when exposed to the outside. However, Comparative Steel 1 and Comparative Steel 2 are ferritic stainless steels containing a large amount of Cr, an expensive element, to reduce in the present invention, and corrosion did not occur even in an external exposure environment. Even though the inventive steels according to the present invention were specimens of the same alloy component system, as a result of not performing pickling after annealing heat treatment, it was confirmed that corrosion did not occur.

아래 표 3에는 Al 피막지수 및 Si 피막지수와, 산세를 실시하지 않은 시편들의 부식 감모율과, 부식 감모율 20%를 기준으로 판단한 부식 적합도를 나타내었다. 부식 적합도에 적합한 경우를 ○로, 부적합한 경우를 ×로 표시하였다.Table 3 below shows the corrosion suitability determined based on the Al film index and Si film index, the corrosion loss rate of specimens not subjected to pickling, and the corrosion loss rate of 20%. A case suitable for corrosion suitability was indicated by ○, and an inappropriate case was indicated by x.

Al 피막지수 및 Si 피막지수는 글로우 방전 분광분석법으로 분석할 수 있는데, 이 방법은 본 기술분야에서 널리 알려진 방법으로 학계에서 통용되는 글로우 방전 분광분석법에 준하는 방법으로 분석할 수 있다. 단, 데이터를 충분히 확보하기 위하여 표면으로부터 깊이 방향으로 거리에 따른 성분을 분석할 때, 그 해상도는 10㎚ 이하일 것이 요구된다.The Al film index and the Si film index can be analyzed by a glow discharge spectroscopy method, which is a method well known in the art and can be analyzed by a method similar to the glow discharge spectroscopy method commonly used in academia. However, when analyzing components according to the distance from the surface in the depth direction in order to sufficiently secure data, the resolution is required to be 10 nm or less.

구분division Al 피막지수Al film index Si 피막지수Si film index 변색 발생여부Whether discoloration occurs 부식 감모율Corrosion loss rate 부식 적합도Corrosion suitability 비교강1Comparative Steel 1 00 10.610.6 ○○ 4%4% ○○ 비교강2Comparative lecture 2 00 6.96.9 ○○ 10%10% ○○ 비교강3Comparative lecture 3 00 6.06.0 ○○ 20%20% ×× 비교강4Comparative lecture 4 00 3.33.3 ○○ 28%28% ×× 비교강5Comparative lecture 5 00 4.64.6 ○○ 41%41% ×× 비교강6Comparative lecture 6 0.10.1 4.54.5 ○○ 30%30% ×× 비교강7Comparative Steel 7 0.20.2 6.46.4 ○○ 33%33% ×× 비교강8Comparative lecture 8 00 11.311.3 ○○ 31%31% ×× 비교강9Comparative lecture 9 8.88.8 3.63.6 ○○ 30%30% ×× 비교강10Comparative Steel 10 23.123.1 3.33.3 ○○ 12%12% ○○ 발명강1Invention Lesson 1 16.216.2 1.11.1 ×× 18%18% ○○ 발명강2Invention Lesson 2 24.224.2 0.90.9 ×× 17%17% ○○ 발명강3Invention Lesson 3 24.424.4 0.80.8 ×× 12%12% ○○ 발명강4Invention Lesson 4 22.122.1 0.20.2 ×× 13%13% ○○ 발명강5Invention Lesson 5 23.023.0 1.91.9 ×× 12%12% ○○

비교강 1 내지 5는 C, Si, Mn, Al, Ti, N의 함량은 유사하며 단지 Cr의 함량을 점점 저감한 시편이다. 표 3을 참조하면, 비교강 1 및 2는 Cr 함량이 11% 이상인 페라이트계 스테인리스강에 해당하며, 충분한 내식성을 갖기 때문에 부식 감모율이 낮고 부식 적합도 또한 적합하였다. 다만, 본 발명에 따라 산세를 미실시한 결과 Si 피막지수가 10.6으로 높게 나타나고 이에 따른 표면 변색이 발생하였다.Comparative steels 1 to 5 are specimens with similar contents of C, Si, Mn, Al, Ti, and N, and only gradually decreasing the content of Cr. Referring to Table 3, Comparative Steels 1 and 2 correspond to ferritic stainless steels having a Cr content of 11% or more, and because they have sufficient corrosion resistance, the corrosion loss rate is low and corrosion suitability is also suitable. However, as a result of not performing the pickling according to the present invention, the Si film index was as high as 10.6, resulting in surface discoloration.

비교강 3, 4, 5는 Al 함량이 낮아 산세 미실시에도 Al 피막지수가 낮게 나타났고 감모율이 높아 부식 적합도가 부적합하였다. 또한 Si 함량은 적정량이지만 스케일층을 포함한 산화피막 내 Si 최대값인 Si 피막지수가 높아 변색이 발생한 것을 알 수 있었다. 특히 비교강 5는 Al 함량을 제외한 나머지 합금원소 함량이 본 발명 범위를 만족하지만, 아래 발명강 1 내지 3을 참조할 때 산세 미실시 시 Al 피막지수를 확보하기 위한 Al 함량이 부족한 것을 확인할 수 있었다. Al 함량이 식 (1)을 만족하도록 포함되는 경우 발명강 1 내지 3과 같이 Si 피막지수를 낮추고 Al 피막지수를 높일 수 있음을 알 수 있었다.Comparative steels 3, 4, and 5 had low Al content, so the Al coating index was low even without pickling, and the corrosion suitability was unsuitable due to the high loss rate. In addition, although the Si content was an appropriate amount, it was found that discoloration occurred due to the high Si film index, which is the maximum Si value in the oxide film including the scale layer. In particular, Comparative Steel 5 satisfies the scope of the present invention in the content of the remaining alloy elements excluding the Al content, but when referring to Inventive Steels 1 to 3 below, it was confirmed that the Al content for securing the Al film index was insufficient when pickling was not performed. It was found that when the Al content was included to satisfy Equation (1), it was possible to lower the Si film index and increase the Al film index as in Inventive Steels 1 to 3.

비교강 6, 7, 8은 Si 함량을 높인 시편에 해당한다. 일반적으로 내식성 및 내산화성에 효과적이라고 알려져 있는 Si 함량이 높아도 산세 미실시의 경우에는 Al 함량이 충분하지 못해 부식 평가가 부적합하였으며, 표면 변색 또한 발생한 것을 알 수 있었다.Comparative steels 6, 7, and 8 correspond to specimens with increased Si content. In general, even if the Si content, which is known to be effective in corrosion resistance and oxidation resistance, is high, in the case of not performing pickling, the Al content was insufficient, so the corrosion evaluation was unsuitable, and it was found that surface discoloration also occurred.

비교강 9는 Al을 0.9% 함유하지만 Al 함량범위 및 식 (1)을 만족하지 못해 Al 피막지수가 목표 범위에 미치지 못하였고, 이에 따라 부식 평가가 부적합하였다. 이는 Al 함량이 충분치 못해 Si의 산화막 형성을 방해하지 못한 것으로 판단할 수 있었으며, 이에 따라 Si 산화막이 우세하게 형성되어 변색이 발생하였다.Comparative steel 9 contained 0.9% Al, but the Al film index did not meet the target range because the Al content range and Equation (1) were not satisfied, and the corrosion evaluation was therefore inadequate. This could be determined that the Al content was not sufficient to hinder the formation of the Si oxide film, and accordingly, the Si oxide film was predominantly formed and discoloration occurred.

비교강 10은 Al 함량이 충분히 포함되어 Al 피막지수가 15 이상으로 나타났고 부식 평가도 적합하였지만, 식 (1)을 만족하지 못해 Si 피막지수가 높아지는 결과를 낳았다. 비교강 10은 표면 변색이 발생하였는데, Al 피막지수를 만족하여 부식 평가가 적합하더라도 Si 피막지수가 3.0을 초과하는 경우에는 표면의 변색을 억제할 수 없음을 알 수 있었다.Comparative steel 10 contained sufficient Al content and had an Al film index of 15 or more, and was suitable for corrosion evaluation, but it did not satisfy Equation (1), resulting in a higher Si film index. Comparative steel 10 had surface discoloration, and although it satisfies the Al coating index and thus the corrosion evaluation is suitable, it was found that the surface discoloration could not be suppressed when the Si coating index exceeded 3.0.

발명강 1, 2, 3은 본 발명의 합금 성분계 조성범위를 만족하고 산세 미실시 후 Al 피막지수 15.0 이상 및 Si 피막지수 3.0 이하를 모두 만족하여, 부식 평가가 우수하였으며, 변색 또한 발생하지 않았다.Inventive Steels 1, 2, and 3 satisfies the composition range of the alloy component of the present invention and satisfies both the Al film index 15.0 or higher and the Si film index 3.0 or lower after no pickling, excellent corrosion evaluation, and no discoloration occurred.

발명강 4는 본 발명의 조성범위 내에서 Cr 함량이 다소 낮은 편에 속하지만, 식 (1)을 만족하도록 Si 및 Al 함량을 조절함으로써 Al 피막지수 및 Si 피막지수를 목적범위 내로 제어할 수 있었다.Inventive Steel 4 belongs to a somewhat low Cr content within the composition range of the present invention, but by adjusting the Si and Al content to satisfy Equation (1), the Al film index and the Si film index could be controlled within the target range. .

한편, 발명강 5는 본 발명의 조성범위 내에서 Si 함량이 다소 높은 편에 속하지만, 식 (1)을 만족하도록 Si 및 Al 함량을 조절함으로써 Al 피막지수 및 Si 피막지수를 목적범위 내로 제어할 수 있었다.On the other hand, Inventive Steel 5 belongs to a somewhat higher Si content within the composition range of the present invention, but it is possible to control the Al film index and the Si film index within the target range by adjusting the Si and Al contents to satisfy Equation (1). Could

구분division 변색 발생여부Whether discoloration occurs L*L* a*a* b*b* 비교강1Comparative Steel 1 ○○ 4949 +12 +12 +15 +15 비교강2Comparative lecture 2 ○○ 4949 +13 +13 +13 +13 비교강3Comparative lecture 3 ○○ 47 47 +14 +14 +10 +10 비교강4Comparative lecture 4 ○○ 44 44 +15 +15 +4 +4 비교강5Comparative lecture 5 ○○ 40 40 +15 +15 0 0 비교강6Comparative lecture 6 ○○ 39 39 +14 +14 -3-3 비교강7Comparative Steel 7 ○○ 36 36 +13 +13 -13-13 비교강8Comparative lecture 8 ○○ 34 34 +11 +11 -20-20 비교강9Comparative lecture 9 ○○ 35 35 0 0 -1 -One 비교강10Comparative Steel 10 ○○ 3636 0 0 -15-15 발명강1Invention Lesson 1 ×× 78 78 +5+5 +2 +2 발명강2Invention Lesson 2 ×× 79 79 00 +1 +1 발명강3Invention Lesson 3 ×× 78 78 00 +1 +1 발명강4Invention Lesson 4 ×× 78 78 00 +1 +1 발명강5Invention Lesson 5 ×× 78 78 +5+5 -8 -8

표 4는 비교강 및 발명강의 L*a*b* 표색계 값을 나타낸 것으로, 표 3의 변색에 대하여 보다 자세히 보여준다. 소둔 후 산세를 실시하지 않은 경우, 비교강 1 내지 5는 Cr의 산화에 따른 붉은색을 띈 스케일층을 나타내었다. 또한, 비교강 6 내지 10은 Si 피막을 제어하지 못해 보라색 내지는 파란색을 띈 스케일층을 나타내었다. 반면, 발명강 1 내지 5는 본 발명이 제시하는 제조방법을 통해 밝은 금속성 색상을 띈 스케일층을 나타내었다.Table 4 shows the L*a*b* colorimetric values of the comparative steel and the invention steel, and shows the discoloration in Table 3 in more detail. When the pickling was not performed after annealing, Comparative Steels 1 to 5 exhibited a reddish scale layer due to the oxidation of Cr. In addition, the comparative steels 6 to 10 exhibited a purple to blue scale layer due to the inability to control the Si film. On the other hand, Inventive Steels 1 to 5 exhibited a scale layer with a bright metallic color through the manufacturing method proposed by the present invention.

도 5는 본 발명에 따른 비교강 10의 냉연소둔 강판 시편의 표면을 나타내는 사진이다. 도 5를 통해 통상의 강종과 같이 소둔 열처리에 의해 표면에 어두운 갈색의 스케일이 형성된 것을 확인할 수 있다.5 is a photograph showing the surface of a cold-rolled annealed steel sheet specimen of comparative steel 10 according to the present invention. It can be seen from FIG. 5 that dark brown scales are formed on the surface by annealing heat treatment like a conventional steel type.

도 6은 본 발명에 따른 발명강 2의 냉연소둔 강판 시편의 표면을 나타내는 사진이다. 도 6을 통해 발명강 2의 시편은 산세 미실시에도 밝은 금속성의 광택을 나타내는 것을 확인할 수 있으며, L*a*b* 표색계 값은 L*: 79, a*: 0, b*: +1 이었다.6 is a photograph showing the surface of a cold-rolled annealed steel sheet specimen of Inventive Steel 2 according to the present invention. It can be seen from FIG. 6 that the specimen of Inventive Steel 2 exhibits bright metallic luster even without pickling, and the L*a*b* colorimeter values were L*: 79, a*: 0, b*: +1.

상술한 바에 있어서, 본 발명의 예시적인 실시예들을 설명하였지만, 본 발명은 이에 한정되지 않으며 해당 기술 분야에서 통상의 지식을 가진 자라면 다음에 기재하는 청구범위의 개념과 범위를 벗어나지 않는 범위 내에서 다양한 변경 및 변형이 가능함을 이해할 수 있을 것이다.As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and those of ordinary skill in the art are within the scope not departing from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (5)

중량%로, C: 0.02% 이하, N: 0.02% 이하, Si: 2.0% 이하, Mn: 0.5% 이하, Cr: 3.0 내지 5.5%, Ti: 0.001 내지 0.3%, Al: 1.0 내지 4.0%, 나머지 Fe 및 불가피한 불순물을 포함하고,
표면 스케일층을 구비하며,
하기와 같이 정의되는 Al 피막지수 15.0 이상 및 Si 피막지수 3.0 이하를 만족하는 내식성이 우수한 배기계용 페라이트계 강판.
[Al 피막지수]: 표면으로부터 0.2㎛ 깊이 범위에서 Al 함량의 최대값(중량%)
[Si 피막지수]: 표면으로부터 0.2㎛ 깊이 범위에서 Si 함량의 최대값(중량%)In% by weight, C: 0.02% or less, N: 0.02% or less, Si: 2.0% or less, Mn: 0.5% or less, Cr: 3.0 to 5.5%, Ti: 0.001 to 0.3%, Al: 1.0 to 4.0%, the rest Contains Fe and inevitable impurities,
It has a surface scale layer,
A ferritic steel sheet for exhaust systems having excellent corrosion resistance that satisfies an Al film index of 15.0 or more and a Si film index of 3.0 or less defined as follows.
[Al coating index]: the maximum value of Al content in the 0.2㎛ depth range from the surface (% by weight)
[Si film index]: the maximum value of Si content in the 0.2㎛ depth range from the surface (% by weight) 제1항에 있어서,
하기 식 (1)을 만족하는 내식성이 우수한 배기계용 페라이트계 강판.
(1) 5*Al - (Cr+Si) > 0
(여기서, Al, Cr, Si은 각 원소의 함량(중량%)을 의미한다)The method of claim 1,
A ferritic steel sheet for exhaust systems with excellent corrosion resistance that satisfies the following formula (1).
(1) 5*Al-(Cr+Si)> 0
(Here, Al, Cr, Si means the content (% by weight) of each element) 제1항에 있어서,
하기 식 (2)로 표시되는 부식 감모율이 20% 미만인 내식성이 우수한 배기계용 페라이트계 강판.
(2) 부식 감모율(%) = [(부식시험 전 무게) - (부식시험 후 무게)]/(부식시험 전 무게) X 100
(여기서, 부식시험 후 무게는 부식시험 후 생성된 부식생성물을 제거한 뒤의 무게(g)이다)The method of claim 1,
A ferritic steel sheet for exhaust system excellent in corrosion resistance having a corrosion loss ratio of less than 20% represented by the following formula (2).
(2) Corrosion loss rate (%) = [(Weight before corrosion test)-(Weight after corrosion test)]/(Weight before corrosion test) X 100
(Here, the weight after the corrosion test is the weight (g) after removing the corrosion products generated after the corrosion test) 제1항에 있어서,
표면의 L*a*b* 표색계의 L* 값이 50 이상인 내식성이 우수한 배기계용 페라이트계 강판.The method of claim 1,
Ferritic steel sheet for exhaust system with excellent corrosion resistance with L*a*b* color system L* value of 50 or more on the surface. 제4항에 있어서,
표면의 L*a*b* 표색계의 a*값이 -10 내지 +10 및 b* 값이 -10 내지 +10 범위인 내식성이 우수한 배기계용 페라이트계 강판.The method of claim 4,
A ferritic steel sheet for exhaust systems having excellent corrosion resistance in which the a* value of the surface L*a*b* color system is in the range of -10 to +10 and b* value is in the range of -10 to +10.
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PCT/KR2020/008863 WO2021020757A1 (en) 2019-07-31 2020-07-07 Ferrite-based steel sheet having excellent corrosion resistance for exhaust system
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