EP4249622A1

EP4249622A1 - Ferritic stainless steel with improved strength, workability, and corrosion resistance

Info

Publication number: EP4249622A1
Application number: EP21894831.3A
Authority: EP
Inventors: Jieon PARK; Jong-Su Paek
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-11-19
Filing date: 2021-08-26
Publication date: 2023-09-27
Also published as: CN116490628A; KR20220068743A; JP2023550410A; KR102424980B1; WO2022108058A1; US20230416885A1

Abstract

Disclosed is a ferritic stainless steel with improved strength, workability, and corrosion resistance, which may be applied to various industrial fields, such as washing machines, refrigerators, and all kinds of electric home appliances. An embodiment of the disclosed ferritic stainless steel comprises, in percent by weight (wt%), 0.0005 to 0.02% of C, 0.005 to 0.02% of N, 0.7 to 1.0% of Si, 16.0 to 17.0% of Cr, 0.05 to 0.3% of Ti, and the balance being Fe and inevitable impurities, wherein the ferritic stainless steel has a value of Formula (1) below satisfying 21 to25, a tensile strength of 470 MPa or more, and an elongation of 27% or more. 7*Si+CrIn Formula (1), Si and Cr represent the contents (wt%) of the respective elements.

Description

[Technical Field]

The present disclosure relates to a ferritic stainless steel with improved strength, workability, and corrosion resistance.

[Background Art]

Ferritic stainless steels have been applied in various industrial fields, such as washing machines, refrigerators, and all kinds of electric home appliances due to fine surface quality and lower manufacturing costs compared to austenitic stainless steels. In recent years, with the increased demand for premium home appliances, the need for ferritic stainless steels with improved corrosion resistance and strength is increasing. In addition, cost reduction has been required, and thus there is a need to develop a steel material to satisfy these requirements.
For application to various industrial fields, materials should be processed into complex shapes. Since the risk of deteriorating workability generally increases in the case of increasing strength by solid solution strengthening effect, it is necessary to appropriately design alloying elements to obtain sufficient strength and workability.
Also, although STS 430 steel type, as an existing high-strength ferritic stainless steel, has high strength, corrosion resistance is low due to the high contents of C and N and lack of stabilizing elements such as Ti and Nb. Although attempts have been made to improve corrosion resistance by lowering the contents of C and N and adding Ti or Nb, manufacturing costs may increase in the case of adding the high-priced Nb and strength may decrease in the case of adding Ti despite no risk of increasing manufacturing costs, and therefore it is difficult to satisfy strength requirements. Therefore, there is a need to develop a ferritic stainless steel having improved corrosion resistance, strength, and workability with low manufacturing costs.
(Patent Document 0001) Korean Patent Laid-open Publication No. 10-2010-0075190 (Published on July 2, 2010 )

[Disclosure]

[Technical Problem]

To solve the above-described problems, provided is a ferritic stainless steel having improved strength, workability, and corrosion resistance with low manufacturing costs.

[Technical Solution]

In accordance with an aspect of the present disclosure, a ferritic stainless steel according to an embodiment of the present disclosure includes, in percent by weight, 0.0005 to 0.02% of C, 0.005 to 0.02% of N, 0.7 to 1.0% of Si, 16.0 to 17.0% of Cr, 0.05 to 0.3% of Ti, and the balance being Fe and inevitable impurities, wherein a value of Formula (1) below satisfies 21 to25, a tensile strength is 470 MPa or more, and an elongation is 27% or more. $7 * Si + Cr$
In Formula (1), Si and Cr represent the contents (wt%) of the respective elements.
In addition, in the ferritic stainless steel of the present disclosure, a value of Formula (2) below may satisfy 20 or more and a pitting potential may be 150 mV or more. $Cr + 4 * Si + * Ti / (C + N)$
In Formula (2), Cr, Si, Ti, C, and N represent the contents (wt%) of the respective elements.

[Advantageous Effects]

According to the present disclosure, a ferritic stainless steel having improves strength, workability, and corrosion resistance with low manufacturing costs may be provided by designing an alloy composition.
According to the present disclosure, the manufacturing costs may be reduced by increasing the Si content and the Cr content.
According to the present disclosure, strength and workability may be improved by increasing the Si content and using a new composition parameter to adjust the contents of Si and C. A ferritic stainless steel according to an embodiment has a tensile strength of 470 MPa or more and an elongation of 27% or more.
According to the present disclosure, corrosion resistance may be improved by using a new composition parameter to adjust the contents of Si, Cr, Ti, C, and N. A ferritic stainless steel according to an embodiment may have a pitting potential of 150 mV or more.
A ferritic stainless steel according to an embodiment of the present disclosure may have a tensile strength of 470 MPa or more, an elongation of 27% or more, and a pitting potential of 150 mV or more.

[Description of Drawings]

FIG. 1 is a graph illustrating tensile strengths of embodiments with respect to values of Formula (1).
FIG. 2 is a graph illustrating elongations of embodiments with respect to values of Formula (1).
FIG. 3 is a graph illustrating pitting potentials of embodiments with respect to values of Formula (2).

[Best Mode]

A ferritic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.0005 to 0.02% of C, 0.005 to 0.02% of N, 0.7 to 1.0% of Si, 16.0 to 17.0% of Cr, 0.05 to 0.3% of Ti, and the balance being Fe and inevitable impurities, wherein a value of Formula (1) below satisfying 21 to25, a tensile strength is 470 MPa or more, and an elongation is 27% or more. $7 * Si + Cr$
In Formula (1), Si and Cr represent the contents (wt%) of the respective elements.

[Modes of the Invention]

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
Also, the terms used herein are merely used to describe particular embodiments. An expression used in the singular encompasses the expression of the plural, unless otherwise indicated. Throughout the specification, the terms such as "including" or "having" are intended to indicate the existence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, operations, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
The terms "about", "substantially", etc. used throughout the specification means that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the present invention or to prevent an unconscious infringer from illegally using the disclosure of the present invention.
STS 430 steel type that has been conventionally used as high-strength ferritic stainless steel for home appliances has problems of low corrosion resistance and an increase in manufacturing costs in the case of adding Nb to improve the corrosion resistance and also has a problem of a decrease in strength in the case of adding Ti. In order to solve such problems, the present inventors have deeply studied methods for improving strength and corrosion resistance while reducing manufacturing costs. As a result, it was confirmed that the above-mentioned problems could be solved by adjusting the contents of alloying elements Si, Cr, Ti, C, and N using an expression of the relation therebetween based on the chemical composition of a ferritic stainless steel containing Ti, thereby completing the present disclosure.
A ferritic stainless steel according to an embodiment of the present disclosure may include, in percent by weight (wt%), 0.0005 to 0.02% of C, 0.005 to 0.02% of N, 0.7 to 1.0% of Si, 16.0 to 17.0% of Cr, 0.05 to 0.3% of Ti, and the balance being Fe and inevitable impurities.
Hereinafter, reasons for numerical limitations on the contents of alloying elements in the embodiment of the present disclosure will be described.
The content of C may be from 0.0005 to 0.02 wt%.
When the C content is less than 0.0005 wt%, refining costs for obtaining high-purity products increase. When the C content exceeds 0.02 wt%, the content of impurities increases, resulting in deterioration of elongation and corrosion resistance. In order to improve elongation and corrosion resistance, the C content may be 0.01 wt% or less.
The content of N may be from 0.005 to 0.02 wt%.
When the N content is less than 0.005 wt%, an equiaxed crystal ratio of a slab decreases due to reduced TiN crystallization. When the N content exceeds 0.02 wt%, the content of impurities increase, resulting in deterioration of elongation and corrosion resistance. In order to improve elongation and corrosion resistance, the N content may be 0.015 wt% or less.
The content of Si may be from 0.7 to 1.0 wt%.
Although conventional STS 430 steel type has a low Si content of 0.3 to 0.6 wt%, the Si content is increased to a range of 0.7 to 1.0 wt% in the present disclosure to obtain strength and corrosion resistance. When the Si content is less than 0.7 wt%, the amount of solute Si is insufficient to deteriorate tensile strength and corrosion resistance. When the Si content exceeds 1.0 wt%, the strength of a material excessively increases to cause a problem of deterioration in elongation. In order to improve strength and corrosion resistance, the Si content may be controlled to a range of 0.8 to 1.0 wt%. In this case, a target content of Si may be 0.9 wt%.
A stainless steel according to the present disclosure has improved corrosion resistance compared to conventional STS 430 steel type by increasing the Si content. Although a pitting potential of the STS 430 steel type is 145 mV or less, the ferritic stainless steel according to the present disclosure has a pitting potential of 150 mV or more and may also have a pitting potential of 160 mV or more.
The content of Cr may be from 16.0 to 17.0 wt%.
When the Cr content is less than 16.0 wt%, it is difficult to obtain sufficient corrosion resistance and strength. When the Cr content exceeds 17.0 wt%, there is a problem of rising prices. Although the Cr content of the conventional STS 430 steel type is also in the range of 16.0 to 17.0 wt%, the Cr content should be at least 16.7% to obtain corrosion resistance. However, because corrosion resistance and strength may be improved by increasing the Si content in the present disclosure, manufacturing costs may further be reduced by controlling the Cr content to 16.5% or less. Accordingly, a preferable Cr content may be from 16.0 to 16.5 wt%. More preferably, the Cr content may be from 16.1 to 16.3 wt%.
The Ti content may be from 0.05 to 0.3 wt%.
When the Ti content is less than 0.05 wt%, the solute elements C and N could not be sufficiently fixed, resulting in deterioration of corrosion resistance. When the Ti content exceeds 0.3 wt%, defects caused by Ti-based inclusions increase. The Ti content for the purpose of improving corrosion resistance may be from 0.18 to 0.25 wt%.
The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments, and thus addition of other alloying elements is not excluded. The impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.
In the above-described alloy composition, Si and Cr are elements closely related to corrosion resistance, strength, and workability of a ferritic stainless steel. Si and Cr are elements that strengthen a passivated layer of a ferritic stainless steel to improve corrosion resistance and also are incorporated into a matrix structure to improve strength. However, because Si and Cr are elements deteriorating workability, it is necessary to derive optimal components by identifying the relationship between each of the elements and the material.
Based on this, the present inventors have studied the relationship among the alloying elements in the above-described alloy composition to improve tensile strength and elongation and found that a tensile strength of 470 MPa or more and an elongation of 27% or more may be obtained in the case where a value of Formula (1) below satisfies a range of 21 to 25. $7 * Si + Cr$
In Formula (1), Si and Cr represent the contents (wt%) of the respective elements.
When the value of Formula (1) is less than 21, solid solution strengthening effects of Si and Cr cannot be sufficiently obtained making it difficult to obtain a tensile strength of 470 MPa or more. On the contrary, when the value of Formula (1) exceeds 25, workability deteriorates making it difficult to obtain an elongation of 27%.
Also, the present inventors have studied the relationship between the contents of Ti, C, and N, as well as the contents of Si and Cr, and corrosion resistance to improve corrosion resistance. Since C forms a Cr carbide at grain boundaries of a region thermally affected by heat treatment, and Cr concentration reduction and Cr depletion occurring around the Cr carbide may cause grain boundary corrosion. Because Ti fixes C and N to form a Ti(C,N) carbonitride that is stabler than the Cr carbide, Cr precipitation may be inhibited, thereby improving corrosion resistance.
Based these properties, the present inventors have studied the relationship among the alloying elements in the above-described the alloy composition to improve corrosion resistance and found that a pitting potential of 150 mV or more may be obtained in the case where a value of Formula (2) below satisfies 20 or more. $Cr + 4 * Si + 0.1 * Ti / (C + N)$
In Formula (2), Cr, Si, Ti, C, and N represent the contents (wt%) of the respective elements.
When the value of Formula (2) is less than 20, sufficient corrosion resistance cannot be obtained making it difficult to obtain a pitting potential of 150 mV or more.
Hereinafter, the present disclosure will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by matters described in the claims and able to be reasonably inferred therefrom.

{Example}

After steels having compositions of alloying elements of Table 1 below were prepared into slabs by continuous casting, the slabs were hot-rolled at a temperature of 800 to 1250°C and annealed, and then cold-rolled and finally annealed at a temperature of 800 to 950°C to prepare cold-rolled products. In Table 1, values of Formulae (1) and (2) are values obtained by substituting the compositions of Table 1 into Formulae (1) and (2).

Samples of the prepared cold-rolled products were processed according to the JIS13B standards at a right angle (90°) to a rolling direction and tensile strength (MPa) and elongation (%) of the samples were measured, and pitting potential (E_pit, mV) thereof was measured in a 3.5% NaCl solution at room temperature after polishing the surfaces of the samples with a #600 paper. The measurement results are shown in Table 2 below.

Table 1

	Stee type	Alloying elements (wt%)
	Stee type	C	N	Si	Cr	Ti	Formula (1)	Formula (2)
Comparati ve Example	A	0.0124	0.0157	0.24	16.8	0.25	18.5	18.6
	B	0.0074	0.0153	0.51	16.2	0.23	19.8	19.3
	C	0.0085	0.0092	0.78	14.9	0.21	20.4	19.2
	D	0.0067	0.0119	1.45	16.8	0.22	27.0	23.8
	E	0.0098	0.0087	1.31	16.2	0.24	25.4	22.7
	F	0.0123	0.0102	0.85	16.3	0.02	22.3	19.8
Inventive Example	G	0.0072	0.0090	0.78	16.7	0.23	22.2	21.2
	H	0.0075	0.0095	0.89	16.5	0.22	22.7	21.4
	I	0.0066	0.0110	0.95	16.1	0.19	22.8	21.0

Table 2

	Steel type	Tensile strength (MPa)	Pitting potential (mV)	Elongation (%)
Comparative Example	A	422	122	32
	B	457	127	31
	C	461	139	31
	D	567	195	25
	E	534	189	26
	F	491	142	29
Inventive Example	G	481	162	30
	H	491	175	29
	I	507	167	29

Referring to Tables 1 and 2, it was confirmed that the inventive examples had tensile strengths of 470 MPa or more since the chemical composition defined in the present disclosure was satisfied and the values of Formula (1) were 21 or more. Elongations of 27% or more were obtained since the values of Formula (1) were 25 or less. In addition, pitting potentials of 150 or more were obtained since the values of Formula (2) were 20 or more. Also, when the value of Formula (1) was in the range of 21 to 25 and the value of Formula (2) was 20 or more, all of the tensile strength of 470 MPa or more, the elongation of 27% or more, and the pitting potential of 150 mV or more were satisfied. On the contrary, in the case of Steel Types A and B according to the comparative examples, the Si contents were below the lower limit of 0.7 wt% defined in the present disclosure, the values of Formula (1) were below 21, and the values of Formula (2) were below 20. As a result, although high elongations of 30% or more were obtained, the tensile strengths were below 470 MPa, and the pitting potentials were below 150 mV.
In Steel Type C according to the comparative examples, the Cr content was below the lower limit of 16.0 wt% defined in the present disclosure, and the value of Formula (1) was below 21, and the value of Formula (2) was below 20. As a result, although a high elongation of greater than 30% was obtained, the tensile strength was below 470 MPa and the pitting potential was below 150 mV.
In Steel Types D and E according to the comparative examples, the Si content was greater than the upper limit of 1.0 defined in the present disclosure and the values of Formula (1) exceeded 25. As a result, although the tensile strengths were greater than 470 MPa, the elongations were less than 27%.
In Steel Type F according to the comparative example, the Ti content was below the lower limit of 0.05 wt% defined in the present disclosure and the value of Formula (2) was below 20. As a result, although the tensile strength was greater than 470 MPa and the elongation was greater than 27%, the pitting potential was below 150 mV.
FIGS. 1, 2, and 3 provided herein are graphs visualizing the above-described results. FIG. 1 is a graph illustrating tensile strengths of embodiments with respect to values of Formula (1). Referring to FIG. 1, when the value of Formula (1) is 21 or more, a tensile strength of 470 MPa or more may be obtained. However, referring to FIG. 2, Steel Types D and E according to the comparative example having tensile strengths of 470 or more due to the value of Formula (1) of 21 or more had elongations below 27T because the value of Formula (1) exceeded 25.
FIG. 3 is a graph illustrating pitting potentials of the embodiments with respect to the values of Formula (2). Referring to FIG. 3, it may be confirmed that a pitting potential of 150 mV or more may be obtained when the value of Formula (2) is 20 or more.
While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

[Industrial Applicability]

According to an embodiment of the present disclosure, a ferritic stainless steel having reduced manufacturing costs may be provided by adjusting the contents of Si and Cr. Also, according to an embodiment of the present disclosure, a ferritic stainless steel having improved strength, workability, and corrosion resistance may be obtained by composition-related parameters. Therefore, the ferritic stainless steel may be applied to various industrial fields.

Claims

A ferritic stainless steel comprising, in percent by weight (wt%), 0.0005 to 0.02% of C, 0.005 to 0.02% of N, 0.7 to 1.0% of Si, 16.0 to 17.0% of Cr, 0.05 to 0.3% of Ti, and the balance being Fe and inevitable impurities,
wherein a value of Formula (1) below satisfies 21 to25, a tensile strength is 470 MPa or more, and an elongation is 27% or more: $7 * Si + Cr$

(wherein in Formula (1), Si and Cr represent the contents (wt%) of the respective elements).
The ferritic stainless steel of claim 1, wherein a value of Formula (2) below satisfies 20 or more, and a pitting potential is 150 mV or more: $Cr + 4 * Si + 0.1 * Ti / (C + N)$
(wherein in Formula (2), Cr, Si, Ti, C, and N represent the contents (wt%) of the respective elements).