KR20150074698A - Low-nickel containing stainless steels - Google Patents

Abstract

The present invention relates to a low-nickel-containing stainless steel which maintains austenite structure even when the nickel content is extremely low, and in particular, a low-nickel-containing stainless steel according to an embodiment of the present invention contains C: 0.09 0.1 to 1.5% of Cr, 15.5 to 17% of Cr, 0.11 to 0.18% of N, 0.1 to 10% of Mn, 0 to 0.6% of Ni, , The balance of Fe and other unavoidable impurities, and satisfies a ferrite fraction index (FFI) expressed by the following formula (1) is less than 3.
FFI = -114-176C-0.9Mn-10.9Ni-2.8Cu + 5.5Si + 9.8Cr-125N ... ... ... ... [Formula 1]
In the formula 1, C, Mn, Ni, Cu, Si, Cr and N mean the content (wt%) of each component.

Description

[0001] Low-nickel containing stainless steels [0002]

The present invention relates to a low nickel containing stainless steel, and more particularly to a low nickel containing stainless steel which maintains austenite structure even when the nickel content is excessively low.

In general, stainless steel is classified according to its constituent or metal structure. According to the metal structure, the stainless steel is classified into an austenitic system, a ferritic system, a martensitic system and an ideal system.

Austenitic stainless steels are excellent in cold workability and corrosion resistance, and various products are used in various applications and environments. However, the 300-series stainless steel representing austenitic stainless steel contains a large amount of expensive Ni.

Alloying elements Ni has traditionally been used to make the microstructure of stainless steels into austenite. However, due to the disadvantage that Ni is expensive, there is a steady increase in interest in 200-series stainless steels in which Ni is replaced by Mn.

In general, 200-series stainless steel is sometimes called Cr-Mn stainless steel because it contains 15.5 ~ 19% Cr, 5.5 ~ 10% Mn and 1.0 ~ 6% Ni in weight percent. In addition to being advantageous in cost, it is also excellent in strength and ductility.

However, the lower the Ni content, the higher the Mn content and the lower the Cr content. Also, the lower the Ni content, the more the delayed carccking occurs.

In spite of these disadvantages, there is an advantage in terms of price, so 200 stainless steel which is less than 2wt% of Ni content is often used for parts requiring strength and ductility.

So, a lot of researchers have conducted research on 200-series stainless steels with low nickel content.

For example, Japanese Unexamined Patent Application Publication No. 1999-092885 (Patent Document 1) discloses a steel having an extremely low content of nickel in an amount of less than 0.1% of C, 0.1 to 1% of Si, 5 to 9% of Mn, : The upper limit of the ferrite index is set for a stainless steel containing 13 to 19% of Cu, 1 to 4% of Cu and 0.1 to 2.0% of Ni and utilizing an austenite stability index related to fired organic martensite production Characterized by limiting the range of ingredients of the invention steel

In order to improve the delayed fracture resistance, WO 2011/138503 (Patent Document 2) discloses a ferrite core comprising 0.02 to 0.15% of C, 7 to 15% of Mn, 14 to 19% of Cr, 3%, and Ni: 0.1 to 4%, with respect to C + N content and firing organic martensitic transformation temperature related parameters

The present inventors also continued research on a technique of lowering the nickel content while maintaining the austenite structure in the 200-series stainless steel.

Japanese Laid-Open Patent Publication No. 1999-092885 (May 04, 1999) WO 2011/138503 (Nov. 10, 2011)

The present invention provides an inexpensive low-nickel-containing stainless steel in which strength and ductility are required by controlling an alloy component so as to maintain an austenite structure even when the Ni content is extremely lowered.

The low-nickel-containing stainless steel according to one embodiment of the present invention contains 0.09 to 0.16% of C, 9.5 to 10.5% of Mn, 0 to 0.6% of Ni (excluding 0%), 0.8 to 1.5% of Cu, 0.1 to 1.5% of Si, 15.5 to 17% of Cr, 0.11 to 0.18% of N and the balance of Fe and other unavoidable impurities. The ferrite fraction fraction index (FFI) represented by the following formula 1 ) Is less than 3.

FFI = -114-176C-0.9Mn-10.9Ni-2.8Cu + 5.5Si + 9.8Cr-125N ... ... ... ... [Formula 1]

In the formula 1, C, Mn, Ni, Cu, Si, Cr and N mean the content (wt%) of each component.

The low-nickel-containing stainless steel according to one embodiment of the present invention contains 0.09 to 0.16% of C, 9.5 to 10.5% of Mn, 0 to 0.6% of Ni (excluding 0%), (NiEQ) represented by the following formula 2 is in the range of 6.2 to 8.2%, Ni: 0.1 to 1.5%, Si: 0.1 to 1.5%, Cr: 15.5 to 17%, N: 0.11 to 0.18%, and balance Fe and other unavoidable impurities. , A chromium equivalence (CREQ) represented by the following formula 3 is 16 to 17, and a ratio of NIEQ / CREQ is 0.37 to 0.50.

NIEQ = Ni + 18N + 30C + 0.33Cu + 0.1Mn-0.01 (Mn ² ) ... ... ... ... [Formula 2]

CREQ = Cr + 0.48 Si ... ... ... ... [Formula 3]

In the formulas 2 and 3, Ni, N, C, Cu, Mn, Cr and Si mean the content (wt%) of each component.

Wherein the stainless steel has a tensile elongation in the rolling direction of 45% or more.

The stainless steel has a tensile strength of 800 MPa or more.

According to the embodiment of the present invention, it is possible to lower the content of nickel, which is high in price, to a level considerably lower than that of the conventional 200 series stainless steel, while securing the strength and ductility characteristics to the extent that it can be used in parts requiring both strength and ductility. It has the effect of producing steel.

FIG. 1 is a graph showing the relationship between the elongation and the nominal stress of a cold-rolled annealed plate annealed at 1100 ° C,
2 is an optical microstructure photograph of a cold-rolled and annealed sheet material annealed at 1100 ° C.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know.

The present invention relates to a steel sheet comprising, by weight%, 0.09 to 0.16% of C, 9.5 to 10.5% of Mn, 0 to 0.6% (excluding 0%) of Ni, 0.8 to 1.5% of Cu, 0.1 to 1.5% of Si, To 17%, N: 0.11 to 0.18%, and austenite-ferrite, which contains Fe and other unavoidable impurities and has a ferritic structure in a fraction of less than 3% in austenitic matrix do.

Carbon (C) is an austenite forming element and can be used in place of expensive elements such as nickel (Ni). However, there is a disadvantage in that the Cr content around the grain boundaries is lowered and the corrosion resistance is decreased by easily bonding with the carbide forming element such as Cr effective for corrosion resistance when the excess amount is added. Therefore, the content of C is limited to a range of 0.09 to 0.16%, preferably more than 0.09% and less than 0.16%.

Manganese (Mn) is an element that increases deoxidizing agent and nitrogen solubility. When it is used as an austenite forming element for replacing expensive Ni with nitrogen, if its content is excessive, it becomes difficult to secure corrosion resistance. However, since it is necessary to suppress the firing organic martensite-producing ability by adding Mn, the Mn content is limited to 9.5 to 10.5%, preferably more than 9.5% and less than 10.5%.

Nickel (Ni) is an austenite stabilizing element together with Mn, Cu and N, and plays a major role in increasing the stability of the austenite phase. Instead of reducing the costly Ni content as much as possible to reduce the cost, it is possible to increase the other austenite phase forming elements, Mn and N, to maintain the phase fraction balance by the reduction of Ni. There is a problem that the manufacturing cost of products due to expensive Ni is increased, so it is preferable to limit the upper limit of the Ni content to an extremely low limit. Therefore, in order to maintain the austenite structure while minimally reducing the Ni content, the Ni content is limited to a range of 0 to 0.6% (exclusive of 0%), preferably more than 0 and less than 0.6%.

Copper (Cu) is an element which inhibits work hardening caused by the formation of the processed organic martensite phase and contributes to softening of the austenitic stainless steel. Since Cu is an austenite forming element, the degree of freedom in setting the Ni content is increased with an increase in the Cu content, so that it is easy to design a component suppressing Ni. Cu also contributes greatly to the improvement of the stress corrosion cracking resistance by suppressing the generation of lamination defects. However, since a large amount of Cu content inhibits hot workability, it is preferable to limit the upper limit of the content. Therefore, the content of Cu is limited within a range of 0.8 to 1.5%, preferably more than 0.8% and less than 1.5%.

Silicon (Si) should be added in excess of 0.1% for the deoxidation effect. However, excessive addition of 1.5% or more causes a sharp increase in the hardness of the ferrite phase, resulting in a decrease in elongation. Therefore, the Si content is limited to a range of 0.1 to 1.5%, preferably, more than 0.1 to less than 1.5%.

Chromium (Cr), together with Si, is an essential element for securing corrosion resistance as a ferrite stabilizing element. Increasing the content increases the corrosion resistance but increases the ferrite stability. Therefore, in order to maintain the austenite structure, the upper limit of the addition amount must be limited. Accordingly, the content of Cr is limited within a range of 15.5 to 17%, preferably more than 15.5% and less than 17%.

Nitrogen (N) is an element that contributes greatly to the stabilization of the austenite phase together with C and Ni. Therefore, the increase of the N content can additionally increase the corrosion resistance and enhance the strength. However, if the N content is excessively high, stable production of the steel becomes difficult due to generation of surface defects due to generation of blow holes, pin holes, etc. during casting due to exceeding nitrogen solubility. Therefore, the content of N is limited to 0.11 to 0.18%, preferably 0.11 to less than 0.18%.

At this time, it is preferable that the low-nickel-containing stainless steel according to the present invention satisfies a ferrite fraction index (FFI) represented by the following formula 1 below 3. Therefore, it is preferable to adjust the volume fraction of the generated ferrite to be less than 3 wt%.

FFI = -114-176C-0.9Mn-10.9Ni-2.8Cu + 5.5Si + 9.8Cr-125N ... ... ... ... [Formula 1]

In the formula 1, C, Mn, Ni, Cu, Si, Cr and N mean the content (wt%) of each component. When the value of FFI is negative, FFI is defined as 0.

When the FFI is 3 or more, the phase fraction of ferrite in the microstructure is greatly increased. When the ferrite phase is excessively introduced into the austenitic matrix structure, the elongation percentage may be lowered, so that it is preferable to limit the FFI to less than 3.

On the other hand, the fraction of the austenite phase and the ferrite phase is also described on the Schaffler Diagram defined as NIEQ (nickel equivalent) expressed in [Equation 2] and chromium equivalent (CREQ) expressed in Equation 3 .

NIEQ = Ni + 18N + 30C + 0.33Cu + 0.1Mn-0.01 (Mn ² ) ... ... ... ... [Formula 2]

CREQ = Cr + 0.48 Si ... ... ... ... [Formula 3]

Ni, N, C, Cu, Mn, Cr and Si mean the content (wt%) of each component in [Formula 2] and [Formula 3].

Thus, in the Schaffler Diagram, the nickel equivalent (NIEQ) expressed in [Formula 2] is 6.2 to 8.2, the chromium equivalent (CREQ) expressed in Formula 3 is 16 to 17, and the ratio of NIEQ / CREQ is 0.37 to 0.50 In the satisfactory range, the FFI representing the ferrite fraction as a function of the component is less than 3.

[Example]

The following examples illustrate the present invention.

Specimens of stainless steels having composition ranges according to the present invention were prepared and subjected to ingot preparation, hot rolling, hot rolling, hot rolling and cold rolling followed by cold rolling and annealing to measure the tensile material of the material.

The following Table 1 shows the main alloy composition (wt%) for the experimental steel grade.

division  C  Mn  Ni Cu  Si  Cr  N Inventory 1 0.155 10.3 0.42 1.0 0.4 15.9 0.139 Inventory 2 0.157 9.8 0.2 1.0 1.0 16.4 0.125 Inventory 3 0.098 10.0 0.01 1.3 1.4 16.0 0.169 Comparative Example 1 0.087 8.9 1.8 1.5 0.5 15.6 0.142 Comparative Example 2 0.104 10.3 0.01 1.5 0.6 16.9 0.155 Comparative Example 3 0.095 10.1 0.01 1.5 0.5 17.9 0.137

The various inventive and comparative examples shown in Table 1 were each cast in the form of a 50 kg ingot with a thickness of about 140 mm in a vacuum induction melting furnace. The cast ingot was subjected to a heat treatment at a temperature of 1250 ° C for 3 hours, followed by hot rolling to a thickness of 3.5 mm, followed by air cooling after hot rolling.

The hot rolled material was subjected to hot - rolled annealing at a temperature of 1100 ° C for 1 minute, and the scale was removed for cold rolling. Thereafter, a specimen was produced through cold rolling at a thickness of 1.2 mm, and the specimen was subjected to cold-rolling annealing at an annealing temperature of 1100 ° C for 30 seconds. The tensile test was carried out using a 50 mm gauge specimen taken in the rolling direction, and the results are shown in Table 2. The tensile test results shown in [Table 2] are shown in Fig.

division Cold annealing
delta(%) FFI YS
(MPa) TS
(MPa) EL
(%) CREQ NIEQ NIEQ /
CREQ Inventory 1 0 0 373 910 58 16.1 7.9 0.49 Inventory 2 0 0 395 941 55 16.9 7.5 0.45 Inventory 3 0 0 429 963 52 16.7 6.4 0.39 Comparative Example 1 0 0 375 831 59 15.8 7.6 0.48 Comparative Example 2 4 3 453 835 54 17.2 6.4 0.37 Comparative Example 3 15 17 498 839 48 18.1 5.8 0.32

FIG. 1 is a graph showing a relationship between an elongation and a nominal stress of a cold-rolled annealed plate annealed at 1100 ° C, wherein Comparative Example 1 is a 200-series stainless steel having austenitic structure added with a nickel content higher than that of the prior art.

Figure 1 shows the effect of the presence of trace amounts of ferrite on the tensile properties. For example, in the case of Comparative Example 2 where FFI is 3, the elongation of the tensile specimen may be slowed and the elongation may be lowered as compared with Comparative Example 1 which is a 200-series stainless steel of austenite structure.

As can be seen from Table 2, the inventive Examples 1 to 3 have excellent tensile strengths as compared with Comparative Examples 1 to 3. In Examples 1 to 3, it is shown that the elongation is 50% or more and that it has good ductility.

In the case of Comparative Example 3, the ferrite phase fraction introduced into the microstructure was high, indicating that the elongation was reduced. In addition, if the ferrite phase fraction increases in the austenitic matrix structure, the material acts as a cause of magnetism, and thus the non-magnetic property inherent to the austenitic stainless steel is lost

In Table 2, the composition ranges of the inventive steels are shown using CREQ and NIEQ. As a result, it was confirmed that the ferrite fraction increased sharply as CREQ increased in comparison with CREQ in the case of Comparative Example 2 and Comparative Example 3, respectively.

2 is a photograph of the optical microstructure of the cold-rolled and annealed sheet material annealed at 1100 ° C.

When comparing the microstructures of Inventive Example 1 to Inventive Example 3 and Comparative Example 1 in which FFI has a value of 0, it is confirmed that almost no ferrite as the second phase is present, and the microstructure is almost composed of austenite single phase have. On the other hand, in the case of Comparative Example 2 and Comparative Example 3 in which the FFI is 3 or more, it can be confirmed that the elongated ferrite of the second phase is present in the austenite matrix structure.

As described above, according to the present invention, even if nickel is added at a very low level in a field where strength and ductility are required, ferrite structure is formed in austenite base structure at a fraction of less than 3% so as to have a structure similar to austenitic single- It is possible to manufacture a stainless steel having a composite structure (austenite-ferrite).

Although the present invention has been described with reference to the accompanying drawings and the preferred embodiments described above, the present invention is not limited thereto but is limited by the following claims. Accordingly, those skilled in the art will appreciate that various modifications and changes may be made thereto without departing from the spirit of the following claims.

Claims (4)

(%), Cu: 0.8-1.5%, Si: 0.1-1.5%, Cr: 15.5-17%, C: 0.09-0.16%, Mn: 9.5-10.5% , N: 0.11 to 0.18%, the balance Fe and other unavoidable impurities,
A ferritic stainless steel having a ferrite fraction fraction index (FFI) of less than 3 expressed by the following formula (1).
FFI = -114-176C-0.9Mn-10.9Ni-2.8Cu + 5.5Si + 9.8Cr-125N ... ... ... ... [Formula 1]
In the formula 1, C, Mn, Ni, Cu, Si, Cr and N mean the content (wt%) of each component.
(%), Cu: 0.8-1.5%, Si: 0.1-1.5%, Cr: 15.5-17%, C: 0.09-0.16%, Mn: 9.5-10.5% , N 0.11 to 0.18%, the balance Fe and other unavoidable impurities,
The nickel equivalent (NIEQ) represented by the following formula 2 is 6.2 to 8.2,
The chromium equivalence (CREQ) represented by the following formula 3 is 16 to 17,
NiEQ / CREQ ratio of 0.37 to 0.50.
NIEQ = Ni + 18N + 30C + 0.33Cu + 0.1Mn-0.01 (Mn ² ) ... ... ... ... [Formula 2]
CREQ = Cr + 0.48 Si ... ... ... ... [Formula 3]
In the formulas 2 and 3, Ni, N, C, Cu, Mn, Cr and Si mean the content (wt%) of each component.
The method according to claim 1 or 2,
Wherein said stainless steel is a low-nickel-containing stainless steel having a tensile elongation in the rolling direction of 45% or more.
The method according to claim 1 or 2,
Wherein said stainless steel is a low-nickel-containing stainless steel having a tensile strength of 800 MPa or more.

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