KR20150074697A - Low-nickel containing stainless steels - Google Patents

Abstract

The present invention relates to a low-nickel containing stainless steel having improved corrosion resistance and delayed fracture resistance. According to an embodiment of the present invention, the low-nickel containing stainless steel comprises: 0.06-0.12 wt% of C; 2.5-4.5 wt% of Mn; 0.5-2.0 wt% of Ni; 1.5-3.2 wt% of Cu; 0.1-1.5 wt% of Si; 18-20 wt% of Cr; 0.05-0.17 wt% of N; and the rest of Fe and other other unavoidable impurities. Ferrite fraction index (FFI) represented by [chemical formula 1] is within 15-35 range. [Chemical formula 1]: FFI = -114-176C-0.9Mn-10.9Ni-2.8Cu+5.5Si+9.8Cr-125N (C, Mn, Ni, Cu, Si, Cr and N respectively means contents (wt%) of each component.).

Description

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

The present invention relates to low nickel containing stainless steels, and more particularly to low nickel containing stainless steels with improved corrosion resistance and delayed fracture resistance.

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. Disadvantages of these 200 series stainless steels are identified in the November 2005 issue of "New 200-series steels" in the International Stainless Steel Forum (ISSF).

Therefore, in order to improve low corrosion resistance and low delay fracture resistance, which are disadvantages of general austenitic stainless steel 200 by many researchers, steel microstructure is austenite, ferrite structure is formed in austenite matrix Research has been carried out to control alloy components to have austenite-ferrite.

For example, U.S. Patent No. 5,286,310 (Patent Document 1) discloses an austenitic stainless steel containing 2.5 to 5.0% of Ni, 6.4 to 8.0% of Mn, 16.5 to 17.5% of Cr, and 2.0 to 3.0% of Cu, And is characterized in that the amount of ferrite defined as 12.48 + 0.52Mn-54.27N-47.98C-1.57Ni-1.62Cu-0.69C2 in weight% is controlled to less than 9% to secure hot workability do

EP 1431408 A1 discloses an austenitic stainless steel containing 1.0 to 5.0% of Ni, 7.5 to 10.5% of Mn, 14.0 to 16.0% of Cr and 1.0 to 3.5% of Cu in weight% As a patent, the ferrite content index defined as 6.77 (Cr + Mo + 1.5Si) -4.85 (Ni + 30C + 30N + 0.5Mn + 0.3Cu) -52.75 in weight% is controlled to less than 8.5, It is characterized by

Further, EP 1352982 A3 (Patent Document 3) contains 9.05% or less of Ni, 6-12% of Mn, 16-20% of Cr and 3.0% or less of Cu in weight percent and generates a ferrite structure in microstructure Thereby improving delayed cracking.

However, Patent Documents 1 to 3 each have an advantage of securing hot workability and improving delayed breakage, respectively. However, since the content of Mn which inhibits the formal resistance is as high as 6 to 12% by weight, .

U. S. Patent No. 5,286, 310 (Feb. 15, 1994) EP 1431408 A1 (June 23, 2004) EP 1352982 A3 (Oct. 15, 2003)

The present invention relates to a method for adjusting the content of ferrite in austenitic matrix so that the ferrite of the second phase is present in a 15 to 35% volume fraction, thereby minimizing the Mn content which hinders the corrosion resistance in the conventional 200 series stainless steel, Provides a low-nickel-containing stainless steel that can improve both formal resistance and delayed fracture resistance.

The low-nickel-containing stainless steel according to one embodiment of the present invention contains 0.06 to 0.12% of C, 2.5 to 4.5% of Mn, 0.5 to 2.0% of Ni, 1.5 to 3.2% of Cu, 0.1 to 3.2% of Si, The ferrite fraction fraction index (FFI) represented by the following formula 1 is 15 to 35, and the content of Fe and other unavoidable impurities is 1.5 to 1.5%, Cr is 18 to 20%, N is 0.05 to 0.17% Is satisfied.

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 0.06 to 0.12% of C, 2.5 to 4.5% of Mn, 0.5 to 2.0% of Ni, 1.5 to 3.2% of Cu, (NIEQ) represented by the following formula 2 is 5.9 to 7.5, and the following formula (2) is satisfied: 0.1 to 1.5%, Cr: 18 to 20%, N 0.05 to 0.17%, and Fe and other unavoidable impurities. 3] is 18 to 21, and the ratio of NIEQ / CREQ is 0.31 to 0.40.

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.

In this case, the stainless steel satisfies the formula (PRE) (Pitting Resistance) expressed by the following formula (4): 18-22.

PRE = Cr + 3.3Mo + 30N-Mn ... ... ... ... [Formula 4]

In the formula 4, Cr, Mo, N and Mn mean the content (wt%) of each component.

The stainless steel is characterized in that the formula potential measured in a 3.5% NaCl solution at 30 캜 is 200 mV or more.

The stainless steel has a tensile elongation in the rolling direction of 35% or more.

The stainless steel has a tensile strength of 700 MPa or more.

According to the embodiment of the present invention, the corrosion resistance and the delayed fracture resistance can be improved as compared with the conventional 200-series stainless steel, and thus, it is possible to produce high-quality and economical stainless steel.

FIG. 1 is a graph showing the relationship between the tensile elongation and the ferrite fraction index FFI of a cold-rolled annealed plate annealed at 1100 ° C,
2 is a graph showing the relationship between the tensile strength and the ferrite fraction index FFI of the cold-rolled annealed plate annealed at 1100 ° C,
3 is an optical microstructure photograph of a cold-rolled and annealed sheet material (Inventive Example 1) annealed at 1100 ° C,
4 is a photograph of the optical microstructure of the cold-rolled and annealed sheet material (Inventive Example 6) annealed at 1100 ° C,
5 is a photograph showing the results of an experiment comparing the delayed fracture behavior of the inventive example and the comparative example using a cold-rolled 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%, 0.06 to 0.12% of C, 2.5 to 4.5% of Mn, 0.5 to 2.0% of Ni, 1.5 to 3.2% of Ni, 0.1 to 1.5% of Si, 18 to 20% 0.05 to 0.17%, and austenite-ferrite containing ferrite structure in the austenitic matrix containing residual Fe and other unavoidable impurities.

Carbon (C) is an austenite forming element and can be used in place of expensive elements such as nickel (Ni). However, in case of excessive addition, it is easily bonded with carbide forming element such as Cr which is effective for corrosion resistance at the ferrite-austenite phase boundary to lower the Cr content around the grain boundary, thereby reducing the corrosion resistance. Therefore, the content is reduced to 0.06% And more preferably less than 0.12%.

Manganese (Mn) is an element that increases deoxidizing agent and nitrogen solubility. When it is used as an expensive austenite substitute for an austenite forming element, it becomes difficult to secure corrosion resistance if its content is excessive. In addition, when the content of Mn is 2.5% or less, it is difficult to secure a proper austenite phase fraction even when Ni, Cu, N, etc. as the austenite forming elements are controlled. Therefore, it is preferable to limit the Mn content to more than 2.5% and less than 4.5%.

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. In order to reduce the cost, instead of reducing the Ni content which is high in price, it is possible to increase the Mn and N, which are the other austenite phase forming elements, to sufficiently maintain the phase fraction balance by the reduction of Ni. However, Ni should be added in an amount of more than 0.5% in order to suppress the formation of fired organic martensite which occurs during cold working and to secure sufficient austenite stability. However, it is preferable to limit the upper limit of the content in order to prevent the production cost of the product due to the expensive Ni from being raised. Therefore, it is preferable to limit the Ni content to more than 0.5% and less than 2.0%.

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. In order to obtain such an effect sufficiently, it is necessary to secure a Cu content of 0.1% or more. 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 preferably limited to a range of more than 1.5% and less than 3.2%.

Silicon (Si) is added for some deoxidizing effect, and it is an element which is concentrated into ferrite when annealed by a ferrite forming element. Therefore, 0.1% or more is added in order to ensure a proper ferrite phase fraction. However, excessive addition of 1.5% or more causes a sharp increase in the hardness of the ferrite phase to lower the elongation. If it is excessive, the slag fluidity is lowered during steelmaking and the inclusions are formed by binding with oxygen to lower the corrosion resistance. Therefore, the Si content is preferably limited to a range of more than 0.1 to less than 1.5%.

Chromium (Cr) is a ferrite stabilizing element together with Si, which plays a major role in securing the ferrite phase and is an essential element for ensuring corrosion resistance. Increasing the content increases the corrosion resistance, but it is necessary to increase the content of expensive Ni and other austenite forming elements to maintain the phase fraction. Accordingly, the content of Cr is preferably limited to a range exceeding 18% and less than 20%.

Nitrogen (N) contributes greatly to the stabilization of the austenite phase together with C and Ni, and is one of the elements causing enrichment in the austenite phase during annealing. 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 preferably limited to be more than 0.05 and less than 0.17%.

At this time, the low-nickel-containing stainless steel according to the present invention preferably satisfies a ferrite fraction index (FFI) of 15 or more and 35 or less expressed by the following formula 1. Therefore, it is preferable to adjust the volume fraction of the generated ferrite to be 15 wt% or more and 35 wt% or less.

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.

The reason for limiting the FFI value is that if the FFI is less than 15, the amount of chromium that stabilizes the ferrite is insufficient, which not only lowers the corrosion resistance but also facilitates generation of delayed fracture in deep drawing parts . On the other hand, when the FFI exceeds 35, the microstructure thereof becomes similar to the duplex steel, and the elongation rate itself is lowered. Also, when the ferrite phase fraction exceeds 35%, there is a limit to obtain a high tensile strength characteristic of an austenitic microstructure steel. That is, it is preferable to limit the FFI to 35 or less in order to secure a good level of tensile elongation and tensile strength as compared with the duplex microstructure steel.

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 as [Equation 2] is 5.9 to 7.5, the chromium equivalent (CREQ) expressed as [Equation 3] is 18 to 21 and the ratio of NIEQ / CREQ is 0.31 to 0.40 In the satisfactory range, the FFI indicating the ferrite fraction as a function of the component is 15 or more and 35 or less.

In order to ensure corrosion resistance, the stainless steel of the present invention preferably has an average pitting resistance (PRE) of 18 to 22 expressed by the following formula (4).

PRE = Cr + 3.3Mo + 30N-Mn ... ... ... ... [Formula 4]

In the formula 4, Cr, Mo, N and Mn mean the content (wt%) of each component.

[Example]

The following examples illustrate the present invention.

Specimens of stainless steels having the composition range of the component according to the present invention are prepared and subjected to ingot preparation, hot rolling, hot rolling, hot rolling annealing, cold rolling annealing after cold rolling to adjust the phase fraction of the material to obtain tensile material and corrosion resistance Respectively. In order to compare delayed cracking behaviors with austenitic stainless steel with a single phase structure, a deep drawing experiment was conducted under the same conditions.

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

division  C  Mn  Ni Cu  Si  Cr  N Inventory 1 0.070 4.4 1.9 2.1 0.5 19.1 0.130 Inventory 2 0.071 4.2 0.9 1.9 0.5 19.0 0.121 Inventory 3 0.071 2.9 1.0 2.1 0.7 19.0 0.120 Honorable 4 0.074 3.1 1.0 2.6 0.5 19.0 0.121 Inventory 5 0.074 2.8 1.0 3.1 0.5 19.0 0.116 Inventory 6 0.074 3.9 0.7 2.0 0.5 19.2 0.137 Honorable 7 0.110 4.0 1.1 2.1 1.1 19.5 0.080 Comparative Example 1 0.025 1.8 2.2 0.7 0.5 21.8 0.180 Comparative Example 2 0.087 8.9 1.8 1.5 0.5 15.6 0.142

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 plate material was produced with a thickness of 1.2 mm through cold rolling, and the plate material was subjected to cold-rolling annealing at an annealing temperature of 1100 캜 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. 1 and Fig. 2, respectively.

division ferrite
Fraction (%) FFI PREN YS
(MPa) TS
(MPa) EL
(%) CREQ NIEQ NIEQ /
CREQ Inventory 1 16.0 16.9 18.6 469.7 814.5 51.4 19.3 7.3 0.38 Inventory 2 27.4 28.5 18.4 488.7 840.1 51.3 19.2 6.1 0.32 Inventory 3 27.2 29.2 19.7 497.2 925.0 47.3 19.3 6.2 0.32 Honorable 4 24.5 25.6 19.5 491.2 898.9 45.7 19.2 6.5 0.34 Inventory 5 25.4 25.0 19.7 489.8 865.3 47.2 19.2 6.5 0.34 Inventory 6 28.0 29.8 19.4 489.0 874.0 50.0 19.4 6.3 0.32 Honorable 7 31.0 32.3 17.9 501.0 862.0 44.0 20.0 6.8 0.34 Comparative Example 1 48.0 47.9 25.4 525.0 700.0 35.0 22.0 6.6 0.30 Comparative Example 2 0.0 0.0 10.9 375.0 831.0 59.0 15.9 7.6 0.48

FIG. 1 is a graph showing the relation between the tensile elongation and the ferrite fraction index FFI of the cold-rolled annealed plate annealed at 1100 ° C, FIG. 2 is a graph showing the relationship between the tensile strength of the annealed cold rolled sheet at 1100 ° C and the FFI It is a graph showing.

As can be seen from FIG. 1, in terms of tensile elongation, inventive steels (Inventive Example 1 to Inventive Example 7) are lower than conventional 200 stainless steel (Comparative Material 2) but superior to ordinary lean duplex steel have. In particular, all the inventive steels have a tensile elongation in the rolling direction of 35% or more.

As can be seen from FIG. 2, the inventive steel (Inventive Example 1 to Inventive Example 7) has superior properties to ordinary lean duplex steel (Comparative Material 1) in terms of tensile strength. In particular, all inventive steels have a tensile strength of 700 MPa or more.

The increase in the ferrite content and the increase in the tensile elongation and the tensile strength were confirmed in [Fig. 1] and [Fig. 2], respectively.

Meanwhile, the ferrite fraction (%) measured in the sample annealed at 1100 ° C. and the ferrite fraction index (FFI) predicted to change the ferrite fraction as a function of the components are shown in Table 2. As a result, it can be seen that the FFI predicts the change of the actual ferrite fraction (%). For example, the optical microstructure of Inventive Example 1 with an FFI value of 16.9 and Inventive Example 6 with 29.8 is shown in FIG. 3 and FIG. 4, respectively. As a reference, in FIGS. 3 and 4, it is proved that FFI predicts the change of the actual ferrite fraction (%) as a ferrite phase in a metal structure having a dark contrast.

The composition ranges of the inventive steels are shown in Table 2 using CREQ and NIEQ. The greatest difference between the inventive and comparative example components is CREQ, the disclosures are based on a 200 series steel (Comparative Example 2) composed of austenite single phase with CREQ range, a lean duplex consisting of 48% ferrite and 52% austenite phase 1) exists between the lectures.

In order to examine the corrosion resistance characteristics of the stainless steel according to the present invention, critical formaldehyde values of the inventive and comparative examples were measured. The measurement of the critical formal electric potential value was performed in a 3.5% NaCl solution at 30 DEG C after the inventive Inventive Example 1, Inventive Example 2 and Comparative Example 2 were annealed at an annealing temperature of 1050 to 1150 DEG C for 30 seconds, ].

Annealing temperature Inventory 1  Inventory 2 Comparative Example 2 1050 ° C 261 265 107 1100 ℃ 264 269 111 1150 DEG C 289 272 114

From the results of Table 3, it can be seen that the formula potential of the invention steel (Inventive Example 1 and Inventive Example 2) is significantly higher than the critical formula potential value of the 200th Steel of Comparative Example 2. In particular, it was confirmed that the inventive steel had a formula potential of 200 mV or more as measured in a 3.5% NaCl solution at 30 ° C.

Thus, the formula potential of an excellent invention steel of the 200 series stainless steel is the result that can be deduced from the PREN value of [Table 2].

On the other hand, experiments were performed to examine the delayed fracture behavior of the comparative example and the inventive example, and the result thereof is shown in Fig.

As can be seen from FIG. 5, the comparative example 2, which is a 200-series stainless steel, shows a very large phenomenological difference. 200-series stainless steel (Comparative Example 2) exhibited very severe delayed fracture.

Accordingly, it has been confirmed through various experiments that the low corrosion resistance and low delay fracture resistance, which are disadvantages of conventional 200 series stainless steels having austenite single phase structure, can be improved through the alloy composition and ferrite phase fraction control proposed in the present invention.

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 (6)

The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.06 to 0.12% of C, 2.5 to 4.5% of Mn, 0.5 to 2.0% of Ni, 1.5 to 3.2% of Ni, 0.1 to 1.5% of Si, 18 to 20% %, The balance Fe and other unavoidable impurities,
A ferritic stainless steel having a ferrite fraction index (FFI) of 15 to 35 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.
The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.06 to 0.12% of C, 2.5 to 4.5% of Mn, 0.5 to 2.0% of Ni, 1.5 to 3.2% of Ni, 0.1 to 1.5% of Si, 18 to 20% %, The balance Fe and other unavoidable impurities,
The nickel equivalent (NIEQ) represented by the following formula 2 is 5.9 to 7.5,
The chromium equivalent (CREQ) represented by the following formula 3 is 18 to 21,
NiEQ / CREQ ratio of 0.31 to 0.40.
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 the stainless steel satisfies a formula resistance index (PRE) of 18 to 22 expressed by the following formula (4).
PRE = Cr + 3.3Mo + 30N-Mn ... ... ... ... [Formula 4]
In the formula 4, Cr, Mo, N and Mn mean the content (wt%) of each component.
The method according to claim 1 or 2,
The stainless steel is a low-nickel-containing stainless steel having a formula potential of 200 mV or more as measured in a 3.5% NaCl solution at 30 ° C.
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 35% 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 700 MPa or more.

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