LEAN DUPLEX STAINLESS STEEL AND METHOD OF MANUFACTURING THE SAME

Abstract

A lean duplex stainless steel and a method of manufacturing the same are disclosed. A lean duplex stainless steel according to an embodiment of the present invention comprises, by weight percent, 0.05 to 0.1% of carbon (C), 2.0 to 4.0% of silicon (Si), 4.0 to 8.0% of manganese (Mn), 13.0 to 15.0% of chromium (Cr), 0.05 to 0.15% of nitrogen (N), with the remainder being iron (Fe) and other unavoidable impurities. Therefore, it is possible to minimize the manufacturing costs by minimizing or excluding the alloy components of Cr, Ni, Mo, and the like. in the component system of the duplex stainless steel, to secure an elongation of 30% or more and to secure a corrosion resistance level of 400 series general steel.

Claims

1. A lean duplex stainless steel comprising: by weight percent, 0.05 to 0.1% of carbon (C), 2.0 to 4.0% of silicon (Si), 4.0 to 8.0% of manganese (Mn), 13.0 to 15.0% of chromium (Cr), 0.05 to 0.15% of nitrogen (N), with the remainder being iron (Fe) and other unavoidable impurities.

2. The lean duplex stainless steel according to claim 1, wherein the ferrite fraction(FF(%)) according to the following formula (1) is 60 to 80%, and modified Md30(MM( C.)) according to the following formula (2) is 110 C. or less.
FF(%)=398146*C+9.07*Si-0.58*Mn-22.5*Cr-416N (1)
MM( C. )=551[462*(C+N)/(10.01*FF)]9.2*Si-8.1*Mn-13.7*Cr (2)

3. The lean duplex stainless steel according to claim 1, wherein the stainless steel comprises 13.5 to 14.5% of chromium (Cr).

4. The lean duplex stainless steel according to claim 1, wherein the stainless steel has a Cr equivalent of 13.0 to 16.0 according to the following formula (3).
Cr equivalent=Cr+0.48Si+1.5Mo (3)

5. The lean duplex stainless steel according to claim 1, wherein the stainless steel comprises 0.05% or less of nickel (Ni).

6. The lean duplex stainless steel according to claim 1, wherein the stainless steel has a Ni equivalent of 5.0 or less according to the following formula (4).
Ni equivalent=Ni+18N+30C+0.1Mn-0.01Mn.sup.2 (4)

7. The lean duplex stainless steel according to claim 1, wherein the elongation of the stainless steel is 30% or more.

8. A method of manufacturing a lean duplex stainless steel comprising: a step of hot rolling a lean duplex stainless steel slab comprising, by weight percent, 0.05 to 0.1% of carbon (C), 2.0 to 4.0% of silicon (Si), 4.0 to 8.0% of manganese (Mn), 13.0 to 15.0% of chromium (Cr), 0.05 to 0.15% of nitrogen (N), with the remainder being iron (Fe) and other unavoidable impurities; a step of annealing the hot-rolled steel sheet at a temperature of 1,050 to 1,150 C.; and a step of water-cooling.

9. The lean duplex stainless steel according to claim 8, wherein the hot-rolled steel sheet is annealed for 10 to 60 minutes.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 is a Schaeffler's diagram for explaining the component system of a lean duplex stainless steel according to an embodiment of the present invention.

[0030] FIG. 2 is a photograph showing microstructure of a lean duplex stainless steel according to an embodiment of the present invention, using a transmission electron microscope (TEM).

[0031] FIG. 3 is a graph for explaining the correlation between stress and elongation of a lean duplex stainless steel according to an embodiment of the present invention.

[0032] FIG. 4 is a graph showing the pitting potential value of a lean duplex stainless steel according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] A lean duplex stainless steel according to an embodiment of the present invention comprises, by weight percent, 0.05 to 0.1% of carbon (C), 2.0 to 4.0% of silicon (Si), 4.0 to 8.0% of manganese (Mn), 13.0 to 15.0% of chromium (Cr), 0.05 to 0.15% of nitrogen (N), with the remainder being iron (Fe) and other unavoidable impurities.

MODE FOR INVENTION

[0034] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0035] The following embodiment is presented to fully convey the idea of the present invention to those skilled in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. In order to clarify the present invention, the drawings may omit the showing of parts not related to the description, and the size of the components may be slightly expressed in a somewhat exaggerated manner to facilitate understanding.

[0036] A lean duplex stainless steel according to an embodiment of the present invention comprises, by weight percent, 0.05 to 0.1% of carbon (C), 2.0 to 4.0% of silicon (Si), 4.0 to 8.0% of manganese (Mn), 13.0 to 15.0% of chromium (Cr), 0.05 to 0.15% of nitrogen (N), with the remainder being iron (Fe) and other unavoidable impurities.

[0037] The amount of carbon (C) in a lean duplex stainless steel is 0.05 to 0.1%. Carbon (C) is an element for forming the austenite phase and is an effective element for increasing the strength of a material by solid solution strengthening. Carbon (C) should be added in an amount of 0.05% or more to contribute to the austenite phase stability. When carbon (C) is excessively added in the production of a material, segregation and coarse carbides are formed at the center portion, which adversely affect the hot rolling-annealing-cold rolling-cold annealing as post processing, and it easily bonds with an element forming a carbide such as chromium (Cr) effective for corrosion resistance at the ferrite-austenite phase boundary, thereby reducing the chromium (Cr) content around the grain boundaries and reducing the corrosion resistance, so it is preferable to add carbon (C) in the range of 0.1% or less in order to maximize the corrosion resistance.

[0038] The amount of silicon (Si) in a lean duplex stainless steel is 2.0 to 4.0%. Silicon (Si) is an element for forming the ferrite phase and is an element concentrated in ferrite during annealing. The content of chromium (Cr) in the lean duplex stainless steel according to an embodiment of the present invention is lower than that of general lean duplex stainless steel, and it is preferable to add of silicon (Si) 2.0 to 4.0% in order to secure an appropriate ferrite phase fraction. However, when silicon (Si) is added in an amount of more than 4.0%, the hardness of the ferrite phase is rapidly increased, and the workability and impact properties are lowered. Therefore, it is preferable to limit the content of silicon (Si) to 2.0 to 4.0%.

[0039] The amount of manganese (Mn) in a lean duplex stainless steel is 4.0 to 8.0%. Manganese (Mn) is an element for forming the austenite phase, as an element for controlling the fluidity of molten metal, and deoxidizing and increasing the solid solubility of nitrogen. Manganese (Mn) is added in place of high-priced nickel (Ni). When manganese (Mn) is added in an amount of less than 4%, the austenite stability is lowered at room temperature and transformed into martensite during the cooling process, so it is difficult to maintain a dual-phase structure. When manganese (Mn) is added in an amount of more than 8%, it is difficult to control the phase fraction as the austenite phase fraction is excessive. Therefore, it is preferable to limit the content of manganese (Mn) to 4.0 to 8.0%

[0040] The amount of chromium (Cr) in a lean duplex stainless steel is 13.0 to 15.0%. Chromium (Cr) is minimized in order to reduce the manufacturing costs of duplex stainless steel, and is preferable to limit the amount of chromium (Cr) 15.0% or less so as to deviate from the component range of the general lean duplex stainless steel. However, it is preferable to add at least 13.0% of chromium (Cr) in order to secure the corrosion resistance of the duplex stainless steel. Therefore, it is preferable to limit the content of chromium (Cr) to 13.0 to 15.0%. More preferably, the lean duplex stainless steel may contain 13.5 to 14.5% of chromium (Cr).

[0041] The amount of nitrogen (N) in a lean duplex stainless steel is 0.05 to 0.15%. Nitrogen (N) contributes greatly to the stabilization of the austenite phase together with nickel (Ni) in duplex stainless steels, and is one of the elements concentrated in the austenite phase during annealing.

[0042] Therefore, by increasing the content of nitrogen (N), the corrosion resistance can be improved and the strength can be increased incidentally. However, since the solubility of nitrogen (N) may be changed depending on the content of manganese (Mn) added, controlling its content is needed.

[0043] If the nitrogen (N) content exceeds 0.15% in the component range of manganese (Mn) of the present invention, due to exceeding the solubility of nitrogen, blow holes and pin holes are formed during casting, which causes surface cracks in the product and edge cracks during rolling. Therefore, it is preferable to limit the content of nitrogen (N) to 0.05 to 0.15%.

[0044] For example, a lean duplex stainless steel according to an embodiment of the present invention may contain up to 0.05% of nickel (Ni). Nickel (Ni) is an element that contributes greatly to the stabilization of the austenite phase together with nitrogen (N) in duplex stainless steels.

[0045] If the amount of nickel (Ni) exceeds 0.05%, the content of nickel (Ni), which is a high-priced metal, increases, and thus the manufacturing costs increase.

[0046] For example, a lean duplex stainless steel according to an embodiment of the present invention may have the ferrite fraction(FF(%)) of 60 to 80% according to the following formula (1), and the modified Md30( C. )) of 110 C. or less according to the following formula (2).

FF(%)=398146*C+9.07*Si-0.58*Mn-22.5*Cr-416N (1)

MM( C.)=551[462*(C+N)/(10.01*FF)]9.2*Si-8.1*Mn-13.7*Cr (2)

[0047] For example, when the austenite phase transforms into the martensite phase during the cooling process after the annealing process, or rapidly transforms into the martensite phase during the transformation process, the modified Md30 according to formula (2) is higher than 110 C. and the elongation of the lean duplex stainless steel is less than 30%, specifically about 10 to 15%, thus the workability is very weak.

[0048] For example, the stainless steel may have a Cr equivalent of 13.0 to 16.0 according to the following formula (3).

Cr equivalent=Cr+0.48Si+1.5Mo (3)

[0049] For example, the stainless steel may have a Ni equivalent of 5.0 or less according to the following formula (4).

Ni equivalent=Ni+18N+30C+0.1Mn-0.01Mn.sup.2 (4)

[0050] FIG. 1 is Schaeffler's diagram for explaining the component system of a lean duplex stainless steel according to an embodiment of the present invention.

[0051] Referring to FIG. 1, in other words, in the lean duplex stainless steel according to an embodiment of the present invention, the Cr equivalent and the Ni equivalent decrease as the contents of Cr and Ni decrease, and it can be seen that Cr and Ni equivalents are located at a lower region than those of existing lean duplex steel.

[0052] Generally, the austenite phase stability at room temperature is lowered in such that region. As a result, after the annealing heat treatment, the austenite phase is transformed into the martensite phase during the cooling process, thus the workability and impact characteristics are rapidly decreased.

[0053] However, in the present invention, the stability of the austenite phase is secured by controlling the content of silicon (Si) to 2.0% or more in order to compensate for the lower contents of Cr and Ni and suppress the transformation phenomenon, and additionally, the nitrogen (N) content is controlled to 0.15% or less to suppress the driving force of the martensitic transformation.

[0054] Therefore, by increasing the stability of the austenite phase at room temperature of the lean duplex stainless steel of the present invention, the elongation can be secured at 30% or more, and the corrosion resistance can be secured at the level of 400 series general steel.

[0055] A method of manufacturing a lean duplex stainless steel according to an embodiment of the present invention comprises hot rolling a lean duplex stainless steel slab comprising, by weight percent, 0.05 to 0.1% of carbon (C), 2.0 to 4.0% of silicon (Si), 4.0 to 8.0% of manganese (Mn), 13.0 to 15.0% of chromium (Cr), 0.05 to 0.15% of nitrogen (N), with the remainder being iron (Fe) and other unavoidable impurities, annealing the hot-rolled steel sheet at a temperature of 1,050 to 1,150 C., and water-cooling.

[0056] The lean duplex stainless steel slab of the above component can be rolled by a conventional method, and the hot-rolled steel sheet may have a thickness of 5 to 20 mm.

[0057] For example, the hot-rolled steel sheet is annealed at a temperature of 1,050 to 1,150 C. for 10 to 60 minutes.

[0058] Under the above component system and heat treatment conditions, in microstructure of the lean duplex stainless steel, martensite phase transformation does not occur during the cooling process and the an austenite-ferrite dual phase structure is maintained, and the ferrite phase fraction may be maintained at 60 to 80% and the modified Md30 may have a value of 110 C. or less.

[0059] Through the following examples, the present invention will be described in more detail.

[0060] Invention Steel and Comparative Steel

[0061] Lean duplex stainless steel slabs containing the component system of the invention steels and comparative steels of Table 1 were produced, and then rolled thick plate specimens were produced by plate rolling.

[0062] As shown in Table 1, the component of low-priced lean duplex stainless steel which is a target steel of the present invention is shown. Particularly, chromium (Cr) was fixed within the range of 13.5 to 14.5% and manganese (Mn) was fixed within the range of 5.5 to 6.5%, which have a great influence on securing the corrosion resistance level of STS 409 or more, and the contents of silicon (Si) and nitrogen (N) were changed.

TABLE-US-00001 TABLE 1 C Si Mn Cr N Invention 0.048 2.47 5.93 13.87 0.051 Steel 1 Invention 0.048 3.04 6.12 14.02 0.053 Steel 2 Invention 0.047 3.07 6.1 13.94 0.097 Steel 3 Comparative 0.064 1.02 6.01 13.98 0.048 Steel 1 Comparative 0.047 1.99 5.87 13.95 0.047 Steel 2 Comparative 0.05 4.1 6.08 14.1 0.052 Steel 3 Comparative 0.051 3.03 6.04 13.87 0.151 Steel 4

[0063] Then, the above rolled thick plate specimens were maintained at a temperature of 1,100 C. for 30 minutes and then water-cooled, and materials, characteristics change, tensile properties and corrosion resistance properties were evaluated.

TABLE-US-00002 TABLE 2 Martensite phase Ferrite Modified Elongation transformation fraction (%) Md30 ( C.) (%) Etc. Invention X 76.7 94.2 32.5 Steel 1 Invention X 77.5 73.8 31.9 Steel 2 Invention X 61.4 109.8 31.2 Steel 3 Comparative 59.9 172.4 10.2 Steel 1 Comparative X 72.4 136.9 15.3 Steel 2 Comparative X 85.5 53.7 Brittle Steel 3 fracture occurred Comparative X 39.6 129.3 30.8 Edge crack Steel 4 occurred

[0064] As shown in Table 2, the structure of invention steel 1 to 3 and comparative steel 1 to 4 was observed to confirm whether the steel was transformed into the martensite phase, and if it was transformed, it was indicated by and if it was not transformed, it was indicated by . The ferrite fraction and the modified Md30 value were calculated with reference to the component of Table 1 and formula (1), (2). With respect to elongation, ASTM sub-size tensile specimens were extracted in the rolling direction, and a tensile test was performed at room temperature at a strain rate of 20 mm/min.

[0065] FIG. 2 is a photograph showing microstructure of a lean duplex stainless steel according to an embodiment of the present invention, using a transmission electron microscope (TEM).

[0066] FIG. 2 shows the microstructure of invention steel 1 of the present invention. As shown in the photograph of FIG. 2, when invention steel 1 is manufactured in the component range of the present invention, it can be seen that the austenite phase is not transformed into the martensite phase but is maintained in the austenite phase.

[0067] Referring to Table 1 and Table 2, when the content of silicon (Si) was less than 1.0 to 2.0%, it was confirmed that the austenite phase transforms into the martensite phase during the cooling process, or rapidly transforms into the plasticity-induced martensite phase during the transformation (Md30 exceeding 110 C.), thus the elongation was 10 to 15% and the workability was very weak. Further, when the content of silicon (Si) exceeded 4.0%, brittle fracture was caused during specimen processing due to an increase in ferrite phase fraction and hardness, thus it is preferable to be limited to 4.0% or less.

[0068] Therefore, when the content of silicon (Si) is maintained at 2.0 to 4.0%, the phase stability of austenite is increased to maintain modified Md30 at 110 C. or lower, and workability can be secured at an elongation of 30% or more.

[0069] When the content of nitrogen (N) exceeds 0.15%, the percentage yield decreases due to the frequent occurrence of edge cracks during rolling, and therefore it is preferable to limit the nitrogen (N) to 0.15% or less.

[0070] FIG. 3 is a graph for explaining the correlation between stress and elongation of a lean duplex stainless steel according to an embodiment of the present invention.

[0071] FIG. 3 shows a graph of tensile characteristics for invention steel 1 and comparative steel 2 of the present invention.

[0072] ASTM sub-size tensile specimens were extracted in the rolling direction, and a tensile test was performed at room temperature at a strain rate of 20 mm/min.

[0073] When the content of silicon (Si) was less than 2.0%, it could be seen that it was impossible to secure a desired elongation due to rapid transformation into the plasticity-induced martensite phase during transformation. Therefore, it was found that when the content of silicon (Si) was maintained at 2.0 to 4.0%, the stability of the austenite phase was increased and it was possible to secure an elongation of 30% or more.

[0074] FIG. 4 is a graph showing the pitting potential value of a lean duplex stainless steel according to an embodiment of the present invention.

[0075] FIG. 4 is a graph comparing the pitting potential value of invention steel 1 of the present invention with the pitting potential value of STS 409 and STS 430 of 400 series general steel.

[0076] FIG. 4 shows the pitting potential of each specimen of invention steel 1, STS 409, and STS 430 in a 1.0% NaCl solution. Accordingly, it could be confirmed that the lean duplex stainless steel according to an embodiment of the present invention has a corrosion resistance between STS 409 and STS 430.

[0077] In the above description, although example embodiments of the invention have been described, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications may be made without departing from the concept and scope of the following claims.

INDUSTRIAL APPLICABILITY

[0078] Lean duplex stainless steel and method of manufacturing the same according to the embodiments of the present invention are applicable to steels for industrial facilities such as desalination facilities, pulp, paper manufacture, and chemical facilities.