Ferritic stainless steels with improved strength, processability and corrosion resistance

Abstract

Disclosed in the present specification is a ferritic stainless steel having improved strength, workability, and corrosion resistance, which can be applied to various industrial fields, such as washing machines, refrigerators, and all kinds of home appliances. One embodiment of the disclosed ferritic stainless steel contains, in mass%, C: 0.0005% to 0.02%, N: 0.005% to 0.02%, Si: 0.7% to 1.0%, Cr: 16.0% to 17.0%, Ti: 0.05% to 0.3%, and the balance of Fe and unavoidable impurities, wherein the ferritic stainless steel can have a value satisfying the following formula (1) of 21 to 25, a tensile strength of 470 MPa or more, and an elongation of 27% or more.(1) 7*Si + Cr, wherein Si and Cr represent the content (mass%) of each element.

Description

Technical Field

[0001] This disclosure relates to ferritic stainless steels with improved strength, machinability, and corrosion resistance. Background Technology

[0002] Ferritic stainless steel has been widely used in various industrial sectors, such as washing machines, refrigerators, and all kinds of household appliances, due to its superior surface quality and lower manufacturing cost compared to austenitic stainless steel. In recent years, with the increasing demand for high-quality household appliances, the demand for ferritic stainless steel with improved corrosion resistance and strength has also increased. Furthermore, the requirement for cost reduction necessitates the development of steels that meet these requirements.

[0003] For application in various industrial fields, materials need to be processed into complex shapes. Since the risk of machinability degradation is usually increased when strength is improved through solid solution strengthening, it is necessary to properly design alloying elements to obtain sufficient strength and machinability.

[0004] Furthermore, although STS 430, as an existing high-strength ferritic stainless steel, possesses high strength, its corrosion resistance is low due to its high C and N content and the lack of stabilizing elements such as Ti and Nb. While attempts have been made to improve corrosion resistance by reducing C and N content and adding Ti or Nb, the addition of expensive Nb can increase manufacturing costs, and the addition of Ti can reduce strength (although without the risk of increased manufacturing costs), making it difficult to meet strength requirements. Therefore, there is a need to develop ferritic stainless steels with improved corrosion resistance, strength, and machinability, as well as lower manufacturing costs.

[0005] (Patent Document 0001) Korean Patent Publication No. 10-2010-0075190 (Published on July 2, 2010) Summary of the Invention

[0006] Technical issues

[0007] To address the aforementioned issues, ferritic stainless steels with improved strength, machinability, corrosion resistance, and low manufacturing costs have been developed.

[0008] Technical solution

[0009] According to one aspect of this disclosure, a ferritic stainless steel according to one embodiment of this disclosure comprises, by weight percentage, 0.0005% to 0.02% C, 0.005% to 0.02% N, 0.7% to 1.0% Si, 16.0% to 17.0% Cr, 0.05% to 0.3% Ti, and the balance being Fe and unavoidable impurities, wherein the value of the following formula (1) satisfies 21 to 25, the tensile strength is 470 MPa or greater, and the elongation is 27% or greater.

[0010] (1) 7*Si+Cr

[0011] In equation (1), Si and Cr represent the content (wt%) of the corresponding elements.

[0012] Furthermore, in the ferritic stainless steel of this disclosure, the value of the following formula (2) can be 20 or greater, and the pitting potential can be 150mV or higher.

[0013] (2) Cr + 4*Si + 0.1*Ti / (C + N)

[0014] In equation (2), Cr, Si, Ti, C and N represent the content (by weight%) of the corresponding elements.

[0015] Beneficial effects

[0016] According to this disclosure, ferritic stainless steels with improved strength, machinability, corrosion resistance, and low manufacturing costs can be provided by designing alloy compositions.

[0017] According to this disclosure, manufacturing costs can be reduced by increasing the Si and Cr content.

[0018] According to this disclosure, strength and machinability can be improved by increasing the Si content and using new compositional parameters to adjust the Si and Cr contents. A ferritic stainless steel according to one embodiment has a tensile strength of 470 MPa or greater and an elongation of 27% or greater.

[0019] According to this disclosure, corrosion resistance can be improved by using new compositional parameters to adjust the contents of Si, Cr, Ti, C, and N. A ferritic stainless steel according to one embodiment can have a pitting potential of 150 mV or higher.

[0020] According to one embodiment of this disclosure, the ferritic stainless steel may have a tensile strength of 470 MPa or greater, an elongation of 27% or greater, and a pitting potential of 150 mV or higher. Attached Figure Description

[0021] Figure 1A graph showing the tensile strength of the embodiment relative to the value of equation (1).

[0022] Figure 2 A graph showing the elongation of the implementation scheme relative to the value of equation (1).

[0023] Figure 3 A graph showing the pitting potential of the embodiment relative to the value of equation (2). Detailed Implementation

[0024] According to one embodiment of the present disclosure, the ferritic stainless steel comprises, by weight percent (wt%), 0.0005% to 0.02% C, 0.005% to 0.02% N, 0.7% to 1.0% Si, 16.0% to 17.0% Cr, 0.05% to 0.3% Ti, and the balance being Fe and unavoidable impurities, wherein the value of the following formula (1) satisfies 21 to 25, the tensile strength is 470 MPa or greater, and the elongation is 27% or greater.

[0025] (1) 7*Si+Cr

[0026] In equation (1), Si and Cr represent the content (wt%) of the corresponding elements.

[0027] Invention Embodiments

[0028] In the following, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, embodiments of the present disclosure may be implemented in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be exhaustive and will fully convey the inventive concept to those skilled in the art.

[0029] Furthermore, the terminology used herein is for the purpose of describing specific embodiments only. Unless otherwise stated, singular expressions encompass plural expressions. Throughout the specification, terms such as “comprising” or “having” are intended to indicate the presence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to exclude the possibility of the presence or addition of one or more other features, operations, functions, components, or combinations thereof.

[0030] Furthermore, unless otherwise specified, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Therefore, these terms should not be interpreted in an idealized or overly formal sense unless expressly stated otherwise herein. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise.

[0031] The terms “about,” “substantially,” etc., used throughout this specification mean, when referring to permissible errors in the manufacture and materials, that such permissible error corresponds to a value or similar to that value, and that such value is intended for clear understanding of the invention or to prevent unintentional infringers from unlawfully using the disclosure of the invention.

[0032] STS 430 steel, conventionally used as a high-strength ferritic stainless steel for household appliances, suffers from low corrosion resistance and increased manufacturing costs when Nb is added to improve corrosion resistance. It also exhibits a decrease in strength when Ti is added. To address these issues, the inventors have conducted in-depth research on methods to improve strength and corrosion resistance while reducing manufacturing costs. As a result, it has been demonstrated that the aforementioned problems can be solved by adjusting the contents of the alloying elements Si, Cr, Ti, C, and N using an expression based on the relationship between the chemical composition of Ti-containing ferritic stainless steel, thus completing this disclosure.

[0033] According to one embodiment of this disclosure, the ferritic stainless steel may contain, by weight percentage (wt%), 0.0005% to 0.02% C, 0.005% to 0.02% N, 0.7% to 1.0% Si, 16.0% to 17.0% Cr, 0.05% to 0.3% Ti, and the balance being Fe and unavoidable impurities.

[0034] The reasons for imposing numerical limits on the alloy element content in the embodiments of this disclosure will be described below.

[0035] The content of C can be from 0.0005% by weight to 0.02% by weight.

[0036] When the carbon content is below 0.0005% by weight, the refining cost for obtaining high-purity products increases. When the carbon content exceeds 0.02% by weight, the impurity content increases, leading to a deterioration in elongation and corrosion resistance. To improve elongation and corrosion resistance, the carbon content can be 0.01% by weight or lower.

[0037] The nitrogen content can be from 0.005% by weight to 0.02% by weight.

[0038] When the nitrogen (N) content is below 0.005 wt%, the equiaxed crystal ratio of the billet decreases due to reduced TiN crystallization. When the N content exceeds 0.02 wt%, the impurity content increases, leading to a deterioration in elongation and corrosion resistance. To improve elongation and corrosion resistance, the N content can be 0.015 wt% or lower.

[0039] The Si content can be from 0.7% by weight to 1.0% by weight.

[0040] Although conventional STS 430 steel has a low Si content of 0.3 wt% to 0.6 wt%, this disclosure increases the Si content to a range of 0.7 wt% to 1.0 wt% to achieve improved strength and corrosion resistance. When the Si content is below 0.7 wt%, the amount of solute Si is insufficient, resulting in deterioration of tensile strength and corrosion resistance. When the Si content exceeds 1.0 wt%, the strength of the material increases excessively, leading to a deterioration in elongation. To improve both strength and corrosion resistance, the Si content can be controlled within a range of 0.8 wt% to 1.0 wt%. In this case, the target Si content can be 0.9 wt%.

[0041] By increasing the Si content, the stainless steel according to this disclosure exhibits improved corrosion resistance compared to conventional STS 430 steel. Although the pitting potential of STS 430 steel is 145 mV or lower, the ferritic stainless steel according to this disclosure has a pitting potential of 150 mV or higher, and may also have a pitting potential of 160 mV or higher.

[0042] The Cr content can be from 16.0% to 17.0% by weight.

[0043] When the Cr content is below 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 price increase. Although the Cr content of conventional STS 430 steel is also in the range of 16.0 wt% to 17.0 wt%, in order to obtain corrosion resistance, the Cr content should be at least 16.7%. However, since corrosion resistance and strength can be improved by increasing the Si content in this disclosure, manufacturing costs can be further reduced by controlling the Cr content to 16.5% or lower. Therefore, the preferred Cr content can be 16.0 wt% to 16.5 wt%. More preferably, the Cr content can be 16.1 wt% to 16.3 wt%.

[0044] The Ti content can be from 0.05% by weight to 0.3% by weight.

[0045] When the Ti content is below 0.05 wt%, the solute elements C and N cannot be adequately fixed, leading to a deterioration in corrosion resistance. When the Ti content exceeds 0.3 wt%, defects caused by Ti-based inclusions increase. For the purpose of improving corrosion resistance, the Ti content can be from 0.18 wt% to 0.25 wt%.

[0046] The remaining component in the composition of this disclosure is iron (Fe). However, the composition may contain unintended impurities that are unavoidably incorporated from the raw materials or the surrounding environment, and therefore the addition of other alloying elements is not excluded. Since such impurities are known to any person skilled in the art of manufacturing, they are not specifically mentioned in this disclosure.

[0047] In the aforementioned alloy composition, Si and Cr are elements closely related to the corrosion resistance, strength, and machinability of ferritic stainless steel. Si and Cr strengthen the passivation layer of ferritic stainless steel to improve corrosion resistance and also incorporate into the matrix structure to enhance strength. However, because Si and Cr degrade machinability, it is necessary to determine the optimal composition by defining the relationship between each element and the material.

[0048] Based on this, the inventors have studied the relationship between the alloying elements in the above alloy composition to improve tensile strength and elongation, and found that when the value of the following formula (1) is in the range of 21 to 25, tensile strength of 470 MPa or greater and elongation of 27% or greater can be obtained.

[0049] (1) 7*Si+Cr

[0050] In equation (1), Si and Cr represent the content (wt%) of the corresponding elements.

[0051] When the value of Equation (1) is less than 21, the solid solution strengthening effect of Si and Cr cannot be fully obtained, making it difficult to obtain a tensile strength of 470 MPa or greater. Conversely, when the value of Equation (1) exceeds 25, the processability deteriorates, making it difficult to obtain an elongation of 27%.

[0052] Furthermore, the inventors have investigated 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 Cr carbides at the grain boundaries of the heat-affected zone after heat treatment, and the decrease in Cr concentration and Cr depletion occurring around the Cr carbides can cause grain boundary corrosion. Because Ti fixes C and N to form more stable Ti(C,N) carbonitrides than Cr carbides, Cr precipitation can be suppressed, thereby improving corrosion resistance.

[0053] Based on these characteristics, the inventors have studied the relationship between alloying elements in the above alloy composition to improve corrosion resistance, and found that when the value of the following formula (2) is 20 or greater, a pitting potential of 150 mV or higher can be obtained.

[0054] (2) Cr + 4*Si + 0.1*Ti / (C + N)

[0055] In equation (2), Cr, Si, Ti, C and N represent the content (by weight%) of the corresponding elements.

[0056] When the value of equation (2) is less than 20, sufficient corrosion resistance cannot be obtained, making it difficult to obtain a pitting potential of 150mV or higher.

[0057] The present disclosure will be described in more detail below through embodiments. However, it should be noted that the following embodiments are intended only 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 the matters described in the claims and that can be reasonably inferred from them.

[0058] {Example}

[0059] After steel billets with the alloy element compositions shown in Table 1 below are prepared by continuous casting, the billets are hot-rolled and annealed at a temperature of 800°C to 1250°C, then cold-rolled and finally annealed at a temperature of 800°C to 950°C to prepare cold-rolled products. In Table 1, the values ​​of equations (1) and (2) are obtained by substituting the compositions of Table 1 into equations (1) and (2).

[0060] Samples of cold-rolled products prepared according to JIS 13B standard at a right angle (90°) to the rolling direction were analyzed. The tensile strength (MPa) and elongation (%) of the samples were measured. After polishing the sample surface with #600 paper, the pitting potential (E) was measured at room temperature in a 3.5% NaCl solution. pit (mV). The measurement results are shown in Table 2 below.

[0061] Table 1

[0062]

[0063] Table 2

[0064]

[0065] Referring to Tables 1 and 2, it is confirmed that the inventive examples, due to satisfying the chemical composition defined in this disclosure and having a value of 21 or greater for formula (1), possess a tensile strength of 470 MPa or greater. Since a value of 25 or less for formula (1) is obtained, an elongation of 27% or greater is achieved. Furthermore, since a value of 20 or greater for formula (2) is obtained, a pitting potential of 150 mV or greater is achieved. Moreover, when the value of formula (1) is in the range of 21 to 25 and the value of formula (2) is 20 or greater, all of the following are satisfied: tensile strength of 470 MPa or greater, elongation of 27% or greater, and pitting potential of 150 mV or greater. Conversely, in the cases of steel grades A and B according to comparative examples, the Si content is below the lower limit of 0.7 wt% defined in this disclosure, the value of formula (1) is below 21, and the value of formula (2) is below 20. As a result, although a high elongation of 30% or greater is obtained, the tensile strength is below 470 MPa, and the pitting potential is below 150 mV.

[0066] In steel grade C according to the comparative example, the Cr content is below the lower limit of 16.0% by weight as defined in this disclosure, and the value of formula (1) is below 21, and the value of formula (2) is below 20. As a result, although a high elongation of more than 30% is obtained, the tensile strength is below 470 MPa, and the pitting potential is below 150 mV.

[0067] In steel grades D and E according to the comparative examples, the Si content is greater than the upper limit of 1.0 defined in this disclosure, and the value of equation (1) exceeds 25. As a result, although the tensile strength is greater than 470 MPa, the elongation is less than 27%.

[0068] In steel grade F according to the comparative example, the Ti content is below the lower limit of 0.05% by weight as defined in this disclosure, and the value of formula (2) is below 20. As a result, although the tensile strength is greater than 470 MPa and the elongation is greater than 27%, the pitting potential is less than 150 mV.

[0069] This article provides Figure 1 , 2 Figures 3 and 4 are used to visualize the above results. Figure 1 A graph showing the tensile strength of the embodiment relative to equation (1) is provided. (Refer to...) Figure 1 When the value of equation (1) is 21 or greater, a tensile strength of 470 MPa or greater can be obtained. However, referring to... Figure 2 Since the value of Equation (1) is 21 or greater, steel grades D and E according to the comparative example have an elongation of less than 27% because the value of Equation (1) is greater than 25.

[0070] Figure 3 A graph showing the pitting potential of the embodiment relative to the value of equation (2). (Refer to...) Figure 3 It can be confirmed that when the value of equation (2) is 20 or greater, a pitting potential of 150mV or higher can be obtained.

[0071] Although this disclosure has been specifically described with reference to exemplary embodiments, those skilled in the art will understand that various changes in form and detail may be made without departing from the spirit and scope of this disclosure.

[0072] Industrial applicability

[0073] According to embodiments of this disclosure, ferritic stainless steel with reduced manufacturing costs can be provided by adjusting the Si and Cr content. Furthermore, according to one embodiment of this disclosure, ferritic stainless steel with improved strength, machinability, and corrosion resistance can be obtained by adjusting the relevant compositional parameters. Therefore, ferritic stainless steel can be applied in various industrial fields.

Claims

1. A ferritic stainless steel comprising, by weight percentage, 0.0005% to 0.02% C, 0.005% to 0.02% N, 0.7% to 1.0% Si, 16.0% to 17.0% Cr, 0.05% to 0.3% Ti, and the balance being Fe and unavoidable impurities. The values ​​of equation (1) below satisfy 21 to 25, the values ​​of equation (2) below satisfy 21 or greater, the pitting potential is 150 mV or higher, the tensile strength is 470 MPa or greater, and the elongation is 27% or greater. (1) 7*Si+Cr (2) Cr + 4*Si + 0.1*Ti / (C + N) In Equations (1) and (2), Cr, Si, Ti, C and N represent the content of the corresponding elements in weight percent.

Images