KR101379079B1 - Lean duplex stainless steel - Google Patents

Abstract

The present invention relates to lean duplex stainless steel having excellent elongation and corrosion resistance. The lean duplex stainless steel according to the present invention comprises, by weight%, 0.08% or less of C, 0.2 to 3.0% or less of Si, 2 to 4% of Mn, 19 to 23% of Cr, 0.3 to 2.5% : 0.2 to 0.3%, Cu: 0.5 to 2.5%, the balance Fe and other unavoidable impurities. As a result, it can be sufficiently used as an alternative to 304 steel used for molding by securing corrosion resistance of equivalent to or higher than 304 steel and excellent elongation of 50% or more.

Description

Lean duplex stainless steel

The present invention relates to a lean duplex stainless steel, and more particularly to a lean duplex stainless steel excellent in elongation and corrosion resistance.

In general, austenitic stainless steel having good workability and corrosion resistance contains iron (Fe) as a base metal and contains Cr and Ni as main raw materials. In addition, austenitic stainless steels have been developed into various steel types suitable for various applications by adding other elements such as Mo and Cu.

These austenitic stainless steels are steel grades excellent in corrosion resistance and corrosion resistance, and contain 8% or more of Ni component by weight in terms of low carbon. For this reason, there is a problem in that the price is unstable due to the large fluctuation in cost caused by the rise in Ni price, resulting in inferior competitiveness. Therefore, in order to compensate for this, it is necessary to develop a new steel grade that can secure corrosion resistance equal to or higher than that of the austenitic stainless steel grade while lowering the Ni content.

Accordingly, the duplex stainless steel, which is a stainless steel having a microstructure composed of a mixture of an austenite phase and a ferrite phase, exhibits both the austenitic and ferritic characteristics. Various duplex stainless steels have been proposed to date, and examples thereof include US Pat. Nos. 5,624,504 and 61,641.

In the case of the duplex stainless steel, it provides excellent corrosion resistance in various corrosive environments, and exhibits superior corrosion resistance than that of the austenitic stainless steel such as AISI's 304 and 316. In the case of such a duplex stainless steel, not only the manufacturing cost is increased by expensive elements such as Ni and Mo, but also the Ni and Mo are consumed, which leads to a decrease in price competitiveness with other steel grades.

Accordingly, in recent years, lean duplex stainless steel, which has eliminated expensive alloying elements such as Ni and Mo and added low-cost alloying elements in place of these elements, has been further increased. Interest in the trend is increasing.

An object of the present invention is to provide a lean duplex stainless steel that can reduce the cost by adjusting the content of components such as Ni, Si, and Cu so as to secure an appropriate corrosion resistance or more than austenitic stainless steel.

In addition, by controlling the phase ratio of austenite and ferrite, the present invention secures high ductility of at least 50% and at the same time ensures corrosion resistance of 304 steel, thereby significantly improving workability and greatly saving expensive alloying elements. It is an object of the present invention to provide lean duplex stainless steel.

Lean Duplex stainless steel according to the present invention is a weight%, C: 0.08% or less, Si: 0.2-3.0% or less, Mn: 2-4%, Cr: 19-23%, Ni: 0.3% -2.5%, N: 0.2-0.3%, Cu: 0.5-2.5%, balance Fe and other unavoidable impurities.

In addition, the stainless steel may further include W: 0.1 to 1.0% by weight.

In addition, the stainless steel may be made of 45 to 75% austenite and 55 to 25% ferrite in a volume fraction.

Furthermore, the stainless steel may have a content of calcined organic martensite of 5% or less.

In addition, the stainless steel may have an elongation of 50% or more.

According to the present invention, by controlling the content of Ni, Si, Cu, and Mo alloy components of expensive elements, it is possible to remarkably improve resource savings and raw material costs. In particular, it can be sufficiently used as an alternative to 304 steel used for molding by ensuring corrosion resistance of 304 steel or more and excellent elongation of 50% or more.

In addition, by controlling the phase fraction and the composition range of the alloy component to secure an elongation of 50% or more, the workability can be improved to be used for forming and bending purposes. In addition, by reducing the specific gravity of the material by manufacturing the material can be made of a lightweight material.

1 is a graph showing the nominal strain and nominal stress of the material according to the present invention heat-treated at 1100 ℃.
Figure 2 is a graph comparing the critical formula potential of the inventive steel and the comparative steel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to embodiments of the present invention and other details necessary for those skilled in the art to understand the present invention with reference to the accompanying drawings. However, the present invention may be embodied in various different forms within the scope of the claims, and thus the embodiments described below are merely exemplary, regardless of expression.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

The present invention is a lean duplex stainless steel in which the content of expensive alloying elements such as Ni, Mo, Si, Cu and the like is reduced among duplex stainless steel having two phases of austenite phase and ferrite phase. Lean duplex stainless steel can maintain corrosion resistance more than equivalent to 304 steel, which is a common austenitic stainless steel. In addition, it is possible to secure elongation of at least austenitic stainless steel and to secure elongation of at least 304 steel. Due to the excellent elongation and corrosion resistance of the present invention, lean duplex stainless steel, which is used as an alternative to 304 steel, can be used, for example, in corrosive environments or general products for molding. In addition, it may be manufactured and used as a product such as a strip, a bar, a plate, a sheet, a pipe, or a tube.

The lean duplex stainless steel is economical because it has a low Ni content while ensuring corrosion resistance equivalent to 304 steel and 316 steel, which are austenitic stainless steels. In addition, it has been spotlighted as a steel material for industrial facilities such as freshwater, pulp, paper, and chemical facilities requiring high strength and easy corrosion resistance. Such lean duplex stainless steels are disclosed in Japanese Patent Laid-Open Nos. 61-056267, WO 02/027056 and WO 96/18751. Among them, lean duplex stainless steel disclosed in Japanese Patent Application Laid-Open No. 61-56267 and WO 02/027056 is standardized by ASTM A240, the former is S32304 (representative component 23Cr-4Ni-0.13N), and the latter is S32101 (representative component 21Cr-1.5Ni-5Mn-0.22N).

In addition, disclosed in Korean Patent Laid-Open Publication No. 2006-0074400, S81921 steel, which is standardized by ASTM A240, contains expensive alloying elements such as 2.5% and 2.4% by weight of Ni and Mo, respectively.

These duplex stainless steels are designed for cold workability, i.e., strengthening of corrosion resistance rather than formability, providing superior corrosion resistance than that required in certain applications. In addition, the stress corrosion resistance is also superior to the design requirements to provide a technical solution, but ductility, which is a factor related to workability, is inferior to that of austenitic stainless steel. This causes a lot of restrictions on applications in various industrial fields that require molding, bending, and the like, and there are aspects that are not economically feasible. Therefore, while eliminating these expensive elements, while reducing the manufacturing cost, it ensures corrosion resistance equivalent to or higher than that of 304 steel, 304L steel and 316 steel. Lecture development is needed.

In addition, austenitic stainless steel having excellent moldability, elongation, contains 4% or more of expensive Ni, and thus has a problem in that the material cost is very high during manufacturing and a large amount of Ni, which is a valuable resource, is consumed.

In addition, a large amount of Mn greatly increases the nitrogen solubility of steel for securing corrosion resistance of lean duplex stainless steel, but easily forms inclusions such as MnS, which are detrimental to corrosion resistance, thereby inhibiting corrosion resistance. In addition, the operation of the electric furnace causes environmental problems due to the generation of Mn dust and the like. Therefore, a two-phase tissue steel in which a ferrite phase and an austenite phase coexist in a manner of securing Ni and Mn while reducing elongation and corrosion resistance equivalent to that of an austenite series.

In Japanese Patent Laid-Open No. 11-071643, an austenitic and ferritic stainless steel having excellent elongation is controlled by limiting the amount of Ni added to less than 0.1 to 1% and controlling the stability index of austenite in the two-phase structure steel in the range of 40 to 115. The steel sheet manufacturing method is presented. In order to secure the excellent workability of the austenitic ferritic stainless steel, Korean Laid-Open Patent Publication No. 2010-0097741 uses N-0.05 to 0.15% and calcined organic martensite generated during a tensile test was used. In addition, Korean Laid-Open Patent No. 2006-0127107 contains 0.05 to 0.6% of N and adjusts the stability of austenite present in two-phase steel during cold working to utilize a phase change generated during processing.

Hereinafter, the lean duplex stainless steel which consists of an austenite phase and a ferrite phase of this invention is demonstrated in detail.

According to the present invention, it is possible to secure high elongation of not less than 50% and corrosion resistance of 304 steel at the same time while being excellent in general properties of duplex stainless steel made of austenitic ferrite. That is, the present invention is a low-carbon chromium stainless steel, containing high nitrogen, and while optimizing the content of Mn, expensive alloy elements such as Ni, Si, Mo, Cu, etc. were adjusted to an optimal level. The austenitic ferrite duplex stainless steel with excellent elongation and corrosion resistance is controlled by adjusting the phase ratio of austenite and ferrite to adjust the distribution of the appropriate alloy element to minimize the formation of calcined organic martensite to 5% or less during cold working. Manufacture steel.

Lean duplex stainless steel according to the present invention can significantly reduce the raw material cost in the manufacturing cost, greatly improve the price competitiveness, improve the elongation can be used for a variety of applications, such as molding, complex bending processing other than simple bending. In addition, it is possible to replace the 304 steel used for molding by ensuring the elongation and corrosion resistance.

Hereinafter, the reason for component limitation of the present invention will be described.

C is an austenite forming element and is an element effective for increasing the material strength by solid solution strengthening. However, when excessively added, it easily bonds with carbide-forming elements such as Cr, which is effective for corrosion resistance at the ferrite-austenite phase boundary, thereby lowering the Cr content around grain boundaries to reduce corrosion resistance. It is preferable to add in the range of% or less.

Si is partially added for the deoxidation effect and is an element that is concentrated in ferrite during annealing heat treatment as a ferrite forming element. Therefore, 0.2% or more is added in order to ensure proper ferrite phase fraction. However, excessive addition of 3.0% or more rapidly increases the hardness of the ferritic phase, affecting the reduction of the elongation of the two-phase steel, and makes it difficult to secure the austenite phase to secure sufficient elongation. In addition, when excessively, the slag fluidity is reduced during steelmaking, and in combination with oxygen, inclusions are formed to lower the corrosion resistance. Therefore, the Si content is preferably limited to 0.2 to 3.0%.

N is an element that greatly contributes to stabilization of the austenite phase together with Ni in duplex stainless steel, and is one of the elements that thicken on the austenite phase during annealing heat treatment. Therefore, the increase in N content may result in increased corrosion resistance and higher strength. However, the solubility of N changes with the amount of Mn added. In the Mn range of the present invention, if the N content exceeds 0.3% or more, the stable production of the steel is caused by the occurrence of surface defects caused by blow holes, pin holes, etc. during casting due to the nitrogen solubility of Becomes difficult. On the other hand, in order to ensure the corrosion resistance of the 304 steel level, N is added 0.2% or more, and if the N content is too low, it is difficult to secure the proper phase fraction. Therefore, the N content is preferably limited to 0.20 to 0.30%.

Mn is an element that increases the deoxidizer and the nitrogen solubility, and is an austenite forming element, and when used for replacing expensive Ni, it is difficult to secure the corrosion resistance at the level of 304 when the content is added in excess of 4%. When Mn is added in a large amount, it has an effect on the solubility of nitrogen but combines with S in steel to form MnS and worsen corrosion resistance. When the content of Mn is less than 2%, it is difficult to secure a proper austenite phase fraction even when Ni, Cu, N, etc. as the austenite forming elements are controlled, and the solubility of N added is low, Can not be obtained. It is preferable to limit the content of Mn to 2% to 4%.

Cr is a ferrite stabilizing element together with Si, which plays a major role in securing ferrite phase of two-phase stainless steel and is an essential element for securing corrosion resistance. Increasing the content increases the corrosion resistance, but the content of expensive Ni or other austenite forming elements should be increased to maintain the phase ratio. Accordingly, the content of Cr is limited to 19 to 23% to ensure corrosion resistance of STS304 or more while maintaining the phase ratio of the two-phase stainless steel.

Ni is an austenite stabilizing element together with Mn, Cu and N, and plays a major role in securing the austenite phase of the duplex stainless steel. Instead of reducing the costly Ni content as much as possible to reduce cost, the austenitic phase forming elements Mn and N may be increased to sufficiently maintain the balance of phase fraction due to the reduction of Ni. However, 0.3% or more should be added to secure sufficient austenite stability in order to suppress the formation of calcined organic martensite generated during cold working. Adding a large amount of Ni increases the austenite fraction, making it difficult to secure an appropriate austenite fraction. In particular, it is difficult to secure competitiveness compared to 304 due to an increase in manufacturing cost of products due to expensive Ni. Therefore, it is preferable to limit the content of Ni to 0.3% to 2.5%.

W is an element that improves corrosion resistance as an austenite forming element and replaces Mo, but is an element that promotes the formation of intermetallic compounds at 700 ~ 1000 ℃ during heat treatment to cause corrosion resistance and deterioration of mechanical properties. If the content exceeds 1%, the formation of an intermetallic compound causes a sharp decrease in corrosion resistance and especially elongation. In addition, in order to exhibit the effect of improving the corrosion resistance, it should be added at least 0.1%. Therefore, it is preferable to limit the content of W to 0.1% to 1.0%.

At this time, it is preferable that the austenitic ferrite duplex stainless steel according to the present invention satisfies the constitution with a 75 to 45% austenite phase and a 25 to 55% ferrite phase by volume fraction.

This results in excessive concentration of austenite forming elements on the austenite phase during annealing when the austenite phase fraction is less than 45%. As a result, the austenite is sufficiently stable to suppress the amount of strained organic martensite transformation occurring during deformation, and the tensile strength of the material can also be sufficiently secured due to excessive increase in austenite strength due to excessive solid solution of alloying elements. However, a phenomenon in which ductility decreases occurs, and desired elongation and strength cannot be obtained sufficiently. Therefore, from the viewpoint of high ductility, the austenite fraction is preferably 45% or more.

However, when the austenite fraction is 75% or more, surface cracking or the like occurs during hot rolling, resulting in deterioration of hot workability and loss of properties as a two-phase structure steel. Therefore, the austenite fraction is preferably 75% or less.

In addition, the present invention is characterized in that the amount of calcined organic martensite formed during cold working or tensile deformation is 5% or less. Calcined organic martensite is a hard phase formed when unstable austenite is deformed, causing work hardening and contributing to an increase in elongation of steel. In the case of the present invention steel, which is a duplex stainless steel consisting of austenite and ferrite, the stability of the austenite phase can be controlled by appropriate distribution of alloying elements. As a result, plastic organic martensite was formed before and after local necking during tensile deformation.

As shown in FIG. 1, when the calcined organic martensite is formed rapidly, hardening of the raw material due to rapid work hardening causes a rapid decrease in elongation. Therefore, in the case of the duplex stainless steel made of austenitic and ferrite of the present alloy system, when the calcined organic martensite is 5% or less, it is possible to secure 50% or more, which is an elongation comparable to that of 304 steel. Accordingly, the amount of calcined organic martensite formed during cold working is preferably 5% or less.

EMBODIMENT OF THE INVENTION Hereinafter, the austenitic lean duplex stainless steel excellent in the elongation and corrosion resistance of this invention is demonstrated in detail. Specimens of lean duplex stainless steels for the composition range of the component according to the present invention were prepared by hot rolling, hot rolled annealing, cold rolled annealing, and then adjusting the phase fraction of the material to measure elongation and corrosion resistance. Table 1 below shows the alloy composition (wt%) for the experimental steel.

Steel grade C Cr Mn Ni Si Cu N Mo W Comparative River 1 0.065 18.14 1.37 8.06 0.45 0.2 0.042 0.1 - Comparative River 2 0.025 21.84 1.76 2.51 0.54 0.47 0.19 0.58 - Comparative Steel 3 0.03 21 5.05 1.5 0.217 - 0.22 0.3 - Comparative Steel 4 0.021 20.30 5.05 0.198 0.217 - 0.102 - - Comparative Steel 5 0.048 19.97 3.02 - 0.201 1.0 0.284 - - Inventive Steel 1 0.054 19.93 3.03 0.35 2.0 - 0.202 - - Invention river 2 0.50 20.12 3.03 2.05 2.0 0.8 0.234 - - Invention steel 3 0.019 19.98 3.05 - 0.194 1.04 0.261 - - Inventive Steel 4 0.052 20.03 3.10 0.5 1.95 2.0 0.251 - - Invention steel 5 0.051 20.05 3.02 1.02 1.95 2.03 0.252 - - Invention steel 6 0.05 20.0 3.0 1.51 1.95 2.02 0.253 - - Invention steel 7 0.049 19.95 3.0 1.95 1.97 2.02 0.251 - - Inventive Steel 8 0.051 19.87 2.91 0.5 0.865 1.0 0.24 - - Invention river 9 0.05 19.95 2.97 1.01 2.6 1.0 0.235 - - Invented Steel 10 0.051 19.93 2.96 1.04 1.53 1.0 0.232 - 0.9 Invention steel 11 0.047 21.33 3.04 1.02 1.53 1.0 0.23 - 0.48

Table 2 shows the phase fractions of ferrite and austenite according to the heat treatment temperature of some of the experimental steels of Table 1 above.

Steel grade Heat treatment temperature (캜) Ferrite fraction (%) Austenitic fraction (%)
Comparative Steel 4
950 71 30 1050 78 22 1100 83 17
Comparative Steel 5
950 45 55 1000 32 68 1100 35 65 Inventive Steel 1 1000 35 65 1100 36 64 Invention river 2 1000 33 67 1100 37 63 Invention steel 3 1000 42 58 1100 40 60 Inventive Steel 4 1000 53 47 1100 38 62 Invention steel 5 1000 34 66 1100 28 72 Invention steel 6 1000 38 62 1100 33 67 Invention steel 7 1000 43 57 1100 42 58 Inventive Steel 8 1000 47 53 1100 45 55 Invention river 9 1000 51 49 1100 47 53 Invented Steel 10 1000 47 53 1100 42 58 Invention steel 11 1000 49 51 1100 48 52

In the case of duplex stainless steel, the phase fraction changes depending on the alloy composition and the heat treatment temperature. Accordingly, Table 2 shows the ferrite and austenite phase fractions when the comparative steel 4, the comparative steel 5, and the inventive steels 1 to 11 having different alloy components were heat-treated at 950 ° C, 1050 ° C, 1100 ° C, and 1200 ° C, respectively. It is shown. Inventive steel 1 to the invention steel 11 it can be seen that the phase fraction of ferrite is included in the range of about 25 to 55%, the austenite phase fraction is 75 to 45%. In Comparative steel 4, when the heat treatment at 1050 ° C and 1100 ° C, the ferrite phase percentages were 78% and 83%, respectively, and the austenite phase percentages were 22% and 17%, respectively. That is, it can be seen that the comparative steel 4 is not included in the range of the ferrite and austenite phase fractions of the present invention.

1 is a representative nominal strain-nominal stress comparison curve obtained in the present invention.

Referring to FIG. 1, a tensile test is performed after heat treatment of each material at 1100 ° C. FIG. In the case of the austenitic 304 steel of Comparative Steel 1, the elongation was about 70%. In particular, in the case of Comparative Steel 2, which is a duplex stainless steel that is similar to the inventive steel, the elongation is inferior to about 30%. However, in the case of Comparative Steel 5 in which the austenite stability of the duplex stainless steel is not controlled, the elongation decreases due to the rapid work hardening caused by the rapid formation of the calcined organic martensite (see Table 3).

As shown in FIG. 1, it can be seen that in the case of the inventive steel, there is little work hardening rate in the curve of stress-strain. This controls the transformation of austenite into calcined organic martensite during cold working, and the elongation is at least 50%. This value is comparable to the elongation of 304 steel to be replaced in the present invention, showing that the elongation is very excellent compared to the duplex stainless steel of the same grade.

Steel grade Heat treatment temperature
(℃) Austenite
Fraction (%) Elongation
(%) Plasticity maximum
Martensite (%)
Comparative Steel 4
950 30 33 4 1050 22 36 5 1000 17 48 7 Comparative Steel 5 1000 68 32 35 1100 65 35 28 Inventive Steel 1 1000 65 50.5 0 1100 64 56 0 Invention river 2 1000 67 61 5 1100 63 61 4 Invention steel 3 1000 58 54.5 0 1100 60 59.7 0 Inventive Steel 4 1000 47 67 0 1100 62 63 0 Invention steel 5 1000 66 56 0 1100 72 61 0 Invention steel 6 1000 62 56 0 1100 67 67 0 Invention steel 7 1000 57 61 0 1100 58 61 0 Inventive Steel 8 1000 53 57 3 1100 55 55 2 Invention river 9 1000 49 51 5 1100 53 50.5 2 Invented Steel 10 1000 53 53 3 1100 58 52 2 Invention steel 11 1000 51 52.5 0 1100 52 54 2

Table 3 shows the elongation after heat treatment of the comparative steel and the inventive steel used for 5 minutes at various heat treatment temperatures, and the amount of calcined organic martensite formed upon stretching. As shown in Table 3, when the elongation is very excellent, most of the calcined organic martensite is 5% or less. In the case of the comparative steel 4, the austenite phase fraction is insufficient, and in the case of the comparative steel 5, as shown in FIG. It can be seen.

2 is a critical formula current value of the corrosion resistance characteristics of the inventive steels and the comparative steels measured in a 3.5% NaCl solution, and shows corrosion resistance at least equal to the critical corrosion current value of the 304 steel of Comparative Steel 1. FIG.

As described above, according to the present invention, an elongation of 50% or more can be secured by controlling the alloy component composition and the phase fraction.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention.

The scope of the present invention is defined by the following claims. The scope of the present invention is not limited to the description of the specification, and all variations and modifications falling within the scope of the claims are included in the scope of the present invention.

Claims (5)

By weight%, C: 0.08% or less (greater than 0), Si: 0.2-3.0%, Mn: 2-4%, Cr: 19-23%, Ni: 0.3% -2.5%, N: 0.2-0.3%, Cu: Lean duplex stainless steel containing 0.5-2.5%, balance Fe and other unavoidable impurities, consisting of 45-75% austenite and 55-25% ferrite in volume fraction. The method of claim 1,
The stainless steel is a weight%, W: Lean duplex stainless steel further comprises 0.1 to 1.0%. delete The method of claim 1,
The stainless steel is a lean duplex stainless steel having a content of calcined organic martensite of 5% or less. The method of claim 1,
The stainless steel is a lean duplex stainless steel having an elongation of 50% or more.

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