KR102424980B1 - Ferritic stainless steel with improved strength, workability and corrosion resistance - Google Patents

Abstract

Disclosed herein is a ferritic stainless steel with improved strength, workability and corrosion resistance that can be applied to various industrial fields such as washing machines, refrigerators, and various home appliances.
According to an embodiment of the disclosed ferritic stainless steel, in wt%, 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% , the remainder may include Fe and unavoidable impurities, the value of the following formula (1) may satisfy 21 or more and 25 or less, the tensile strength may be 470 MPa or more, and the elongation may be 27% or more.
(1) 7*Si + Cr
In Formula (1), Si and Cr mean the content (wt%) of each element.

Description

Ferritic stainless steel with improved strength, workability and corrosion resistance

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

Ferritic stainless steel has a beautiful surface quality and a lower price than austenitic stainless steel, so it is used in various industrial fields such as washing machines, refrigerators, and various home appliances. Recently, as the demand for premium home appliances increases, the demand for ferritic stainless steel with higher corrosion resistance and higher strength is increasing. At the same time, cost reduction is also required, and it is necessary to develop steel materials to satisfy this requirement.

In order to be applied in various industrial fields, it must be able to be processed into complex shapes. In general, when the strength is improved through the solid solution strengthening effect, there is a possibility that the workability may be lowered. Therefore, an appropriate alloy design is required to secure sufficient strength and workability.

In addition, the existing high-strength ferritic stainless steel, STS 430, had high strength, but had high C and N and lack of stabilizing elements such as Ti and Nb, so corrosion resistance was poor. Corrosion resistance was improved by lowering C and N and adding Ti or Nb, but when expensive Nb is added, there is a burden of price increase. difficult to satisfy Therefore, it is necessary to develop a ferritic stainless steel with improved corrosion resistance, strength, and workability while having a low cost.

Korean Patent Publication No. 10-2010-0075190 (published date: July 02, 2010)

In order to solve the above problems, the present invention is to provide a ferritic stainless steel with improved strength, workability and corrosion resistance while having a low cost.

As a means for achieving the above object, the ferritic stainless steel according to an embodiment of the present invention is, by weight, 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%, the remainder contains Fe and unavoidable impurities, the value of the following formula (1) satisfies 21 or more and 25 or less, the tensile strength may be 470 MPa or more, and the elongation may be 27% or more.

(1) 7*Si + Cr

In Formula (1), Si and Cr mean the content (wt%) of each element.

In addition, in each ferritic stainless steel of the present invention, the value of the following formula (2) satisfies 20 or more, and the pitting potential value may be 150 mV or more.

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

In the above formula (2), Cr, Si, Ti, C, and N mean the content (% by weight) of each element.

According to the present invention, it is possible to provide a ferritic stainless steel with improved strength, workability and corrosion resistance while having a low cost through the design of the alloy composition.

According to the present invention, the raw material cost can be reduced by adding Si upward and reducing the amount of Cr added.

According to the present invention, strength and workability can be improved through a new composition parameter that controls Si and Cr content by adding Si upward. The ferritic stainless steel according to an embodiment may have a tensile strength of 470 MPa or more and an elongation of 27% or more.

According to the present invention, corrosion resistance can be improved through a new composition parameter that controls the Si, Cr, Ti, C, and N content. The ferritic stainless steel according to an embodiment may have a pitting potential value of 150 mV or more.

The ferritic stainless steel according to a preferred embodiment may have a tensile strength of 470 MPa or more, an elongation of 27% or more, and a pitting potential value of 150 mV or more.

1 is a graph showing the tensile strength value of each Example according to the value of Equation (1).
2 is a graph showing the elongation value of each Example according to the value of Equation (1).
3 is a graph showing the formula potential value of each embodiment according to the value of Equation (2).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes preferred embodiments of the present invention. However, the embodiments of the present invention may be modified in various other forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, the terms "comprises" or "includes" as used in the present application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that it is not intended to be used to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.

On the other hand, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Accordingly, unless explicitly defined herein, specific terms should not be construed in an unduly idealistic or formal sense. For example, a singular expression herein includes a plural expression unless the context clearly dictates otherwise.

In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used in a precise sense to aid the understanding of the present invention. or absolute figures are used to prevent unreasonable use by unconscionable infringers of the mentioned disclosure.

The existing high-strength ferritic stainless steel for home appliances, STS 430 steel, has a problem with low corrosion resistance, a problem that the cost increases when Nb is added to improve corrosion resistance, and a problem that the strength decreases when Ti is added. have. In order to solve this problem, the present inventors have studied in depth ways to improve strength and corrosion resistance while lowering the cost. As a result, it was confirmed that the above-mentioned problems can be solved by controlling the content of alloy components of Si, Cr, Ti, C, and N with a component relational expression based on a ferritic stainless steel component system to which Ti is added, and the present invention came to completion.

Ferritic stainless steel according to an embodiment of the present invention is, by weight, 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% , the remainder may contain Fe and unavoidable impurities.

The reason for limiting the component range of each alloy element will be described below.

The content of C may be 0.0005 to 0.02% by weight.

If the content of C is less than 0.0005% by weight, the refining price for making a high-purity product is high, and if the content of C exceeds 0.02% by weight, impurities in the material increase, and there is a problem in elongation and corrosion resistance. In order to improve elongation and corrosion resistance, the C content may be preferably 0.01 wt% or less.

The content of N may be 0.005 to 0.02% by weight.

If the content of N is less than 0.005% by weight, TiN crystallization is reduced, and the equiaxed crystallization rate of the slab is lowered. In order to improve elongation and corrosion resistance, the N content may be preferably 0.015 wt% or less.

The content of Si may be 0.7 to 1.0% by weight.

In the existing STS 430 steel grade, the Si content was as low as 0.3 to 0.6 wt%, but in the present invention, the Si content is increased to 0.7 to 1.0 wt% to secure strength and corrosion resistance. If the content of Si is less than 0.7% by weight, the high Si content is not sufficient and the tensile strength and corrosion resistance are lowered. More preferably, the Si content may be controlled in the range of 0.8 to 1.0 wt% to improve strength and corrosion resistance. In this case, the target content of Si may be 0.9 wt%.

The stainless steel according to the present invention has improved corrosion resistance by increasing the Si content compared to the existing STS 430 steel type. Although the pitting potential of STS 430 steel is 145 mV or less, the ferritic stainless steel according to the present invention has a pitting potential of 150 mV or more, and a pitting potential of 160 mV or more can also be secured.

The content of Cr may be 16.0 to 17.0 wt%.

If the content of Cr is less than 16.0 wt%, it is difficult to secure sufficient corrosion resistance and strength, and if the content of Cr exceeds 17.0 wt%, there is a problem in that the burden of price increase increases. The Cr content of the existing STS 430 steel was also in the range of 16.0 to 17.0 wt%, but it had to contain 16.7 wt% or more of Cr to secure corrosion resistance. However, in the present invention, since corrosion resistance and strength can be improved by increasing the Si content, the Cr content can be limited to 16.5% or less, thereby further reducing the manufacturing cost. Accordingly, the preferred Cr content may be 16.0 to 16.5% by weight. The content of Cr may be more preferably 16.1 to 16.3% by weight.

The content of Ti may be 0.05 to 0.3 wt%.

If the content of Ti is less than 0.05 wt%, there is a problem in that the dissolved C and N elements cannot be sufficiently fixed and corrosion resistance is lowered, and when the content of Ti exceeds 0.3 wt%, defects due to Ti-based inclusions increase. For the purpose of improving corrosion resistance, the content of Ti may be preferably 0.18 to 0.25 wt%.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

Among the alloy compositions described above, Si and Cr are components closely related to corrosion resistance, strength, and workability of ferritic stainless steel. Si and Cr are elements that enhance the corrosion resistance by strengthening the passivation film of ferritic stainless steel and, at the same time, are dissolved in the matrix to improve strength. However, since Si and Cr are elements that degrade machinability, it is necessary to determine the correlation between each element and the material to derive the optimal component.

In view of this, the inventors of the present invention studied the relationship between the tensile strength and the alloy composition for improving the elongation within the above-described alloy composition, and when the value of the following formula (1) satisfies 21 or more and 25 or less, the tensile strength of 470 MPa or more , it was derived that the elongation rate of 27% or more can be satisfied.

(1) 7*Si + Cr

In Formula (1), Si and Cr mean the content (wt%) of each element.

When the value of Equation (1) is less than 21, it is difficult to secure a tensile strength of 470 MPa or more because the solid solution strengthening effect of Si and Cr is not sufficiently exhibited. On the other hand, when the value of Equation (1) exceeds 25, workability is lowered and it is difficult to secure an elongation of 27% or more.

In addition, the present inventors studied the correlation between Ti, C, N content and corrosion resistance along with Si and Cr content in order to improve corrosion resistance. C forms Cr carbides at the grain boundaries in areas affected by heat by heat treatment, etc., and Cr concentration decreases and depletion occurs around Cr carbides, which may cause intergranular corrosion. Ti fixes C and N to form a carbonitride of Ti(C,N) that is more stable than Cr carbide, thereby suppressing Cr precipitation and improving corrosion resistance.

In view of this, the inventors of the present invention studied the correlation of the alloy composition for improving corrosion resistance within the alloy composition described above. derived.

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

In the above formula (2), Cr, Si, Ti, C, and N mean the content (% by weight) of each element.

If the value of Equation (2) is less than 20, it is difficult to secure a pitting potential of 150mV or more because sufficient corrosion resistance cannot be secured.

Hereinafter, the present invention 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 invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

{Example}

Steel having the composition shown in Table 1 below was manufactured into a slab through continuous casting, then hot rolled at 800 to 1250 ° C, annealed, cold rolled, and then final annealed at 800 to 950 ° C to prepare a final cold rolled product . In Table 1, the values of equations (1) and (2) are values derived by substituting the composition of Table 1 into the values of equations (1) and (2) described in this specification.

For the manufactured cold-rolled product, the specimen was processed with JIS13B in the 90° direction of the rolling direction to measure the tensile strength (MPa) and elongation (%) at room temperature. The pitting potential (E _pit , mV) was measured. The measured results are summarized in Table 2.

steel grade Alloy composition (wt%) C N Si Cr Ti Formula (1) Equation (2) rain
school
Yes 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 foot
number of people
Yes 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

steel grade Tensile strength (MPa) Official potential (mV) Elongation (%) rain
school
Yes A 422 122 32 B 457 127 31 C 461 139 31 D 567 195 25 E 534 189 26 F 491 142 29 foot
number of people
Yes G 481 162 30 H 491 175 29 I 507 167 29

Referring to Tables 1 and 2, it can be seen that the invention examples have a tensile strength of 470 MPa or more as a result of the alloy composition and Formula (1) value of 21 or more, and as a result of Formula (1) value of 25 or less, elongation It can be seen that this is more than 27%. In addition, as a result of the value of Equation (2) being 20 or more, it can be seen that the pitting potential value is 150 mV or more. In addition, when the value of Equation (1) satisfies all of 21 or more and 25 or less, and the value of Equation (2) satisfies all of 20 or more, the tensile strength is 470 MPa or more, the elongation is 27% or more, and the pitting potential value is 150 mV or more. can be found

On the other hand, in Comparative Example steel grades A and B, the Si content was less than the lower limit of the Si content defined by the present invention of 0.7 wt%, the value of Equation (1) was less than 21, and the value of Equation (2) was less than 20 . As a result, the elongation was as high as 30% or more, but the tensile strength was less than 470 MPa, and the pitting dislocation value was less than 150 mV.

In Comparative Example Steel C, the Cr content was less than 16.0 wt%, which is the lower limit of the Cr content defined by the present invention, the value of Equation (1) was less than 21, and the value of Equation (2) was less than 20. As a result, the elongation was as high as 30% or more, but the tensile strength was less than 470 MPa, and the pitting dislocation value was less than 150 mV.

In Comparative Example steel grades D and E, the Si content exceeded 1.0 wt%, the upper limit of the Si content defined by the present invention, and the value of Equation (1) exceeded 25. As a result, the tensile strength was 470 MPa or more, but the elongation was less than 27%.

In Comparative Example Steel F, the Ti content was less than 0.05 wt%, which is the lower limit of the Ti content defined by the present invention, and the value of Equation (2) was less than 20. As a result, the tensile strength was 470 MPa or more and the elongation was 27% or more, but the pitting dislocation value was less than 150 mV.

1, 2, and 3 attached are graphs visualizing the above results. 1 is a graph showing the tensile strength value of each Example according to the value of Equation (1). According to FIG. 1, it can be seen that when the value of Equation (1) is 21 or more, a tensile strength of 470 MPa or more can be secured. However, according to Figure 2, it can be seen that the value of formula (1) satisfies 21 or more and the tensile strength of comparative example steels D and E of 470 MPa or more, the value of formula (1) exceeds 25, and the elongation is less than 27%. .

3 is a graph showing the formula potential value of each embodiment according to the value of Equation (2). According to FIG. 3, it can be seen that when the value of Equation (2) is 20 or more, a formal potential of 150 mV or more can be secured.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (2)

By weight %, 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%, the balance contains Fe and unavoidable impurities,
The value of the following formula (1) satisfies 21 or more and 25 or less, and the value of the following formula (2) satisfies 20 or more,
Ferritic stainless steels with a tensile strength of 470 MPa or more, an elongation of 27% or more, and a pitting potential value of 150 mV or more:
(1) 7*Si + Cr
(2) Cr + 4*Si + 0.1*Ti/(C+N)
(In the above formulas (1) and (2), Si, Cr, Ti, C, and N mean the content (% by weight) of each element). delete

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