KR101903181B1 - Duplex stainless steel with improved corrosion resistance and formability and method of manufacturing the same - Google Patents
Duplex stainless steel with improved corrosion resistance and formability and method of manufacturing the same Download PDFInfo
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- KR101903181B1 KR101903181B1 KR1020160177374A KR20160177374A KR101903181B1 KR 101903181 B1 KR101903181 B1 KR 101903181B1 KR 1020160177374 A KR1020160177374 A KR 1020160177374A KR 20160177374 A KR20160177374 A KR 20160177374A KR 101903181 B1 KR101903181 B1 KR 101903181B1
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- 229910001039 duplex stainless steel Inorganic materials 0.000 title claims abstract description 79
- 230000007797 corrosion Effects 0.000 title claims abstract description 63
- 238000005260 corrosion Methods 0.000 title claims abstract description 63
- 238000004519 manufacturing process Methods 0.000 title abstract description 15
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 38
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 23
- 229910052721 tungsten Inorganic materials 0.000 claims abstract description 21
- 238000000034 method Methods 0.000 claims abstract description 20
- 239000012535 impurity Substances 0.000 claims abstract description 7
- 229910001220 stainless steel Inorganic materials 0.000 claims description 19
- 229910000859 α-Fe Inorganic materials 0.000 claims description 17
- 238000005096 rolling process Methods 0.000 claims description 13
- 238000001816 cooling Methods 0.000 claims description 8
- 239000010935 stainless steel Substances 0.000 claims description 8
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 238000005266 casting Methods 0.000 claims description 5
- 229910052802 copper Inorganic materials 0.000 claims description 5
- 238000005097 cold rolling Methods 0.000 claims description 4
- 229910000831 Steel Inorganic materials 0.000 description 52
- 239000010959 steel Substances 0.000 description 52
- 230000000052 comparative effect Effects 0.000 description 16
- 229910052804 chromium Inorganic materials 0.000 description 14
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 13
- 239000000463 material Substances 0.000 description 12
- 238000001556 precipitation Methods 0.000 description 12
- 229910000765 intermetallic Inorganic materials 0.000 description 10
- 229910052757 nitrogen Inorganic materials 0.000 description 10
- 239000013535 sea water Substances 0.000 description 10
- 238000009749 continuous casting Methods 0.000 description 6
- 238000010438 heat treatment Methods 0.000 description 5
- 230000000087 stabilizing effect Effects 0.000 description 5
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 3
- 229910000963 austenitic stainless steel Inorganic materials 0.000 description 3
- 229910052801 chlorine Inorganic materials 0.000 description 3
- 239000000460 chlorine Substances 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 239000002994 raw material Substances 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 239000010960 cold rolled steel Substances 0.000 description 2
- 239000000470 constituent Substances 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 239000012467 final product Substances 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 229910052748 manganese Inorganic materials 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000008021 deposition Effects 0.000 description 1
- 238000004945 emulsification Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910000734 martensite Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000003014 reinforcing effect Effects 0.000 description 1
- 230000035939 shock Effects 0.000 description 1
- 238000010583 slow cooling Methods 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 238000005507 spraying Methods 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 239000003381 stabilizer Substances 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
- B21B3/02—Rolling special iron alloys, e.g. stainless steel
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
- Heat Treatment Of Steel (AREA)
Abstract
A duplex stainless steel excellent in corrosion resistance and moldability and a method for manufacturing the same are disclosed. The duplex stainless steel according to one embodiment of the present invention is characterized by containing C: 0.05% or less (excluding 0), Cr: 24.5 to 32.5%, Mo: 2.0 to 4.0%, W: 2.5 to 4.5% 2.0% or less (excluding 0), Mn: 2.0% or less (excluding 0), N: 0.25 to 0.45%, Si: 1.0% or less (excluding 0), Nb: 0.2% ), The balance Fe and other unavoidable impurities, and an austenite phase having an aspect ratio of 1.0 to 3.0 among the microstructures.
Description
The present invention relates to a duplex stainless steel and a method of manufacturing the duplex stainless steel, and relates to a duplex stainless steel excellent in corrosion resistance and moldability and a method of manufacturing the duplex stainless steel.
Generally, austenitic stainless steel, which is generally good in workability and corrosion resistance, contains Fe and Cr as a major raw material, and Ni and other elements such as Mo and Cu are added to develop various steel types .
300 series stainless steel excellent in corrosion resistance and workability include Ni and Mo which are expensive raw materials, and 400 series stainless steel is discussed as an alternative to this, but the problem that the formability is less than 300 series stainless steel Lt; / RTI > It can be applied to the corrosion resistance of 400 grade according to the use environment in the plate which is relatively less molded than the heat / cold laminate with large molding amount. However, it is used as the thick plate due to the shock characteristic and thermal deterioration of 400 stainless steel There are many limitations to this.
On the other hand, duplex stainless steel mixed with austenite phase and ferrite phase has all the advantages of austenitic and ferritic stainless steels, and various types of duplex stainless steels have been developed to date.
Duplex stainless steel having two phases of austenite phase and ferrite phase has excellent strength and corrosion resistance, and is more than twice as high in yield strength as austenitic stainless steel, and has a formula in chlorine atmosphere, crevice corrosion and stress corrosion cracking The resistance is very good, and its use is increasing steadily as industrial equipment related to seawater.
There are several types of duplex stainless steels and they are classified as follows according to their corrosion resistance level.
The representative steel of Standard Duplex stainless steels based on 22% Cr by weight is UNS32205 (22Cr-5Ni-3Mo-0.18N), similar to 317L austenitic stainless steels with low Mo content, Low-grade duplex stainless steel, which has corrosion resistance similar to knight stainless steel, is called Lean Duplex stainless steel.
UNS32750 (25Cr-7Ni-3.5Mo-0.28N) is the representative steel of Super Duplex stainless steel which has higher corrosion resistance based on 25% Cr by weight. As described above, the corrosion resistance of the duplex stainless steel is determined depending on the contents of Cr, Mo, W, and N, so that the higher the content of these elements, the higher the corrosion resistance. Of the duplex stainless steels currently available in sheet form, the highest corrosion resistant steels are super duplex stainless steels.
However, when used in a seawater atmosphere, the seawater is concentrated depending on the location of the equipment and is often exposed in a very harsh environment. In such a circumstance, even in the case of super duplex stainless steel, it may be difficult to use stably. In order to increase the content of Cr, Mo, W and N, a super corrosion resistant duplex stainless steel sheet having corrosion resistance is required.
However, since the increase in the content of the corrosion resistant strengthening alloying elements such as the above-mentioned components not only deteriorates the hot workability but also increases the deposition rate on the sigma (s) phase and the chi (c) phase, the continuous casting-hot rolling- In the case of a general plate material manufacturing process, if the temperature control between the processes is not easy, there is a disadvantage that the mechanical properties and corrosion resistance of the steel are lowered due to the failure of precipitation control. Therefore, there is a limit in improving the corrosion resistance of the super corrosion resistant duplex stainless steel due to the increase of the content of the reinforcing element.
The embodiments of the present invention can control the component system of the duplex stainless steel to secure excellent corrosion resistance even in a seawater atmosphere and to secure mechanical properties such as strength and elongation through control of microstructure and to provide corrosion resistance And a process for producing the duplex stainless steel.
A duplex stainless steel excellent in corrosion resistance and moldability according to an embodiment of the present invention is characterized by containing, by weight%, C: not more than 0.05% (excluding 0), Cr: 24.5 to 32.5%, Mo: 2.0 to 4.0%, W: 2.5 (Excluding 0), Mn: not more than 2.0% (excluding 0), N: 0.25 to 0.45%, Si: not more than 1.0% (excluding 0), Nb: 0.2% or less (excluding 0), the balance Fe and other unavoidable impurities, and an austenite phase having an aspect ratio of 1.0 to 3.0 among the microstructures.
According to an embodiment of the present invention, there is provided a ferritic stainless steel comprising 0.03% or less of C (excluding 0), 26.0 to 28.0% of Cr, 2.3 to 2.8% of Mo, 3.0 to 3.5% of W, : Not more than 1.2% (excluding 0), Mn: 0.9 to 1.1%, N: 0.30 to 0.45%, and Si: 0.15 to 0.7%.
Further, according to one embodiment of the present invention, the microstructure may contain, in a volume fraction, 40 to 65% of ferrite phase and 45 to 55% of austenite phase.
Also, according to one embodiment of the present invention, the duplex stainless steel may have a PREC value of 45.0 or more.
According to an embodiment of the present invention, the duplex stainless steel may have a yield strength (YS) of 800 MPa or more and a tensile strength (TS) of 1 GPa or more.
Also, according to an embodiment of the present invention, the duplex stainless steel may have an elongation of more than 20%.
Further, according to an embodiment of the present invention, the duplex stainless steel may satisfy the following formula (1).
EI RD - EI TD ≤ 3.0 ------ Equation (1)
Here, EI RD means elongation in the rolling direction and EI TD means elongation in the rolling direction.
According to an embodiment of the present invention, the duplex stainless steel may be a cold rolled thin plate having a thickness of 1.0 to 1.5 mm.
A method for producing duplex stainless steel excellent in corrosion resistance and moldability according to an embodiment of the present invention is characterized by comprising, by weight%, C: not more than 0.05% (excluding 0), Cr: 24.5 to 32.5%, Mo: 2.0 to 4.0% (Excluding 0), Mn: 2.0% or less (excluding 0), N: 0.25 to 0.45%, Si: 1.0% or less (excluding 0) , Nb: not more than 0.2% (excluding 0), the balance Fe and other unavoidable impurities, and cold-rolling the duplex stainless steel sheet.
Also, according to an embodiment of the present invention, the molten steel may be strip casted to continuously cast the duplex stainless steel sheet.
Also, according to an embodiment of the present invention, the duplex stainless steel sheet may be cast to a thickness of 3.0 to 5.0 mm.
According to an embodiment of the present invention, the step of water-cooling the duplex stainless steel sheet and the step of heat-treating the duplex stainless steel sheet at a temperature of 1,100 to 1,150 ° C for 30 to 180 seconds may be further included.
According to an embodiment of the present invention, the duplex stainless steel sheet can be cold-rolled at a total reduction ratio of 50% or more.
Further, according to an embodiment of the present invention, the cold-rolled cold rolled thin plate may be 1.0 to 1.5 mm thick.
According to an embodiment of the present invention, the cold-rolled cold rolled thin plate may further include a heat treatment at a temperature of 1,100 to 1,150 ° C for 10 to 30 seconds.
Also, according to an embodiment of the present invention, the cold-rolled cold rolled thin plate may include an austenite phase having an aspect ratio of 1.0 to 3.0 among microstructures.
Further, according to one embodiment of the present invention, the microstructure may contain, in a volume fraction, 40 to 65% of ferrite phase and 45 to 55% of austenite phase.
Embodiments of the present invention can control the content of elements such as Cr, Mo, W, and N in the constituent system of the duplex stainless steel to ensure excellent corrosion resistance even in a seawater atmosphere.
Also, by adding a trace amount of Nb, the microstructure can be secured and the strength can be ensured through microstructure.
In addition, the aspect ratio of the austenite phase in the microstructure of the cold rolled steel is minimized by optimizing the manufacturing process to secure the steel in an equiaxed form, and the anisotropy of the microstructure can be minimized to improve the formability.
1 is a photograph of microstructure of a duplex stainless steel cold rolled thin plate according to an embodiment of the present invention.
2 is a photograph showing microstructure of a duplex stainless steel cold rolled thin plate according to a comparative example of the present invention.
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill 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. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.
A duplex stainless steel excellent in corrosion resistance and moldability according to an embodiment of the present invention is characterized by containing, by weight%, C: not more than 0.05% (excluding 0), Cr: 24.5 to 32.5%, Mo: 2.0 to 4.0%, W: 2.5 (Excluding 0), Mn: not more than 2.0% (excluding 0), N: 0.25 to 0.45%, Si: not more than 1.0% (excluding 0), Nb: 0.2% or less (excluding 0), the balance Fe and other unavoidable impurities.
C: 0.05% or less (excluding 0)
C is an element effective for increasing the material strength by solid solution strengthening, but it is easily combined with a carbide forming element such as Cr which is effective for corrosion resistance at the ferrite-austenite phase boundary in an excessive amount, thereby lowering the Cr content around the grain boundary to reduce the corrosion resistance Therefore, the content of C is limited to 0.05% or less. In order to minimize the risk of precipitation of carbide which may hinder corrosion resistance, the content of C is preferably 0.03% or less.
Cr, Mo, and W are essential stabilizers for ferrite phase stabilization, and are essential elements for securing high corrosion resistance as well as securing the ferrite phase of duplex stainless steels. Increasing the content increases the corrosion resistance but increases the content of stabilizing elements of austenite in order to maintain the phase fraction. Thus, there is a disadvantage that the cost increases and the hot workability deteriorates, and the content range is limited for each element as follows.
Cr : 24.5 to 32.5%
Cr is the most element among the elements improving the corrosion resistance of the duplex stainless steel and is a basic element, but has a relatively small influence on the deterioration of hot workability. Considering the Mo and W content, the range of Cr content that can control the seawater corrosion resistance and the precipitate for plate production is limited to 24.5 to 32.5%. For example, Cr is more preferably 26.0 to 28.0%.
Mo : 2.0 to 4.0%, W: 2.5 to 4.5%
W and Mo are more resistant to corrosion than Cr. W, which is twice as heavy as Mo, is known to exhibit the same effect as Mo in improving corrosion resistance when calculated by atomic ratio. However, when W is contained in combination with Mo, , It is necessary to add a compound in order to maximize the effect of improving the corrosion resistance.
Since the chi (c) phase precipitated preferentially according to the addition of W is advantageous in delaying the interfacial emulsification rate by delaying the precipitation on the sigma (s) with time, it is required to add at least 2.5% or more and is mainly involved in the precipitation of sigma phase At least 2% of Mo should be added to maximize the W addition effect.
However, if the content of W and Mo is too large, it is impossible to control the precipitate, so there is a limit to the increase of the content. Therefore, Mo is limited to 2.0 to 4.0% and W is limited to 2.5 to 4.5% in consideration of the range 35 to 55 of the crevice corrosion constant K value. For example, it is more preferable that Mo is 2.3 to 2.8% and W is 3.0 to 3.5%.
Since Ni, Cu, Mn and N play an important role in ensuring austenite phase fraction of duplex stainless steel as an austenite phase stabilizing element and can improve corrosion resistance or degrade workability and corrosion resistance depending on the alloy composition, Considering the addition amount of Cr, Mo and W based on ensuring the resistance, the content range is defined as follows for each element so that the volume fraction of ferrite phase is 40 to 65%.
Ni : 6.0 to 10.0%
Ni is the most powerful element among the austenite phase stabilizing elements, and has an advantage of improving the front corrosion as well as the main role for balancing the phase fraction of the duplex stainless steel. The increase in Ni content is directly related to the increase in raw material price, so it needs to be minimized. The range of the Ni content is set to 6.0 to 10.0% in consideration of the contents of Cu, Mn and N in the steel having a crevice corrosion constant K value in the range of 35 to 55. For example, Ni is more preferably 6.5 to 7.5%.
Cu: 2.0% or less (excluding 0)
Cu has an advantage of improving the corrosion resistance in a sulfuric acid atmosphere. However, in the chlorine atmosphere, the Cu content is limited to 2.0% or less because it has a disadvantage in reducing the formal resistance and lowering the hot workability. For example, Cu is more preferably 1.2% or less (excluding 0).
Mn: 2.0% or less (excluding 0)
Mn is a useful element that uses an appropriate amount for improving the fluidity of the molten metal, but is also widely used as an austenite phase stabilizing element that can replace expensive Ni in duplex stainless steel in order to secure a phase fraction. However, the increase of Mn content is not desirable when the corrosion resistance is required because it is involved in the formation of inclusions such as MnS. Therefore, the Mn content is limited to 2.0% or less in order to secure the resistance to seawater corrosion. For example, Mn is more preferably 0.9 to 1.1%.
N: 0.25 to 0.45%
For example, N is preferably set to 0.30 to 0.45%.
N is a useful element that can improve the corrosion resistance in a chlorine atmosphere, as well as to make the strength of duplex stainless steel strong. However, if the N content is too high, the hot workability is reduced and the yield rate is lowered. Therefore, it is desirable to limit the N content to 0.25 to 0.45% in consideration of Cr, Mo, and W contents for securing corrosion resistance against seawater. For example, it is more preferable to set N to 0.30 to 0.45%.
Si : 1.0% or less (excluding 0)
Si is an important element used as a de-oxidizing material in the production of duplex stainless steel and is essentially contained in a certain amount.
Si, which is also used as a ferrite phase stabilizing element, is limited to 1.0% or less because it promotes the precipitation of an intermetallic compound to deteriorate the mechanical properties associated with impact toughness. For example, Si is more preferably 0.15 to 0.7%.
Nb : Not more than 0.2% (excluding 0)
Nb is strong in affinity with C and N to easily form carbides and nitrides, and is formed at a high temperature to inhibit the growth of grains during heat treatment of steel, thereby finer texture and thereby yield strength. However, it is possible to promote precipitation of an intermetallic compound upon overdosing, and it is preferable to limit the production to 0.2% or less because it causes an increase in manufacturing cost with an expensive element.
The duplex stainless steel excellent in corrosion resistance and moldability according to an embodiment of the present invention includes an austenite phase having an aspect ratio of 1.0 to 3.0 among microstructures.
Generally, duplex stainless steels coexist in ferrite phase and austenite phase, and both phases are elongated in the rolling direction during rolling. That is, the aspect ratio, which is the ratio of the length to the thickness of each phase, increases. Since each of these phases has different materials, the anisotropy of the structure due to the difference in material between the phases increases, It is disadvantageous.
Therefore, according to the embodiments of the present invention, the tissue anisotropy can be improved by finely controlling the tissue and controlling the aspect ratio of each phase by optimizing the component control and the manufacturing process. That is, it is possible to improve the tissue anisotropy by controlling the elongated structure in equiaxed crystal form. In addition, the elongation of the material can be improved by improving the anisotropy of the structure.
For example, the microstructure may comprise, in volume fractions, from 40 to 65% ferrite phase and from 45 to 55% austenite phase. This is necessary to maintain the characteristics of the duplex structure steel.
The present invention relates to a ferrite-austenitic stainless steel. The ferrite-austenitic structure referred to in the present invention means that the ferrite phase and the austenite phase occupy most of the structure, and the ferritic phase and the austenite phase And the like. For example, when the ferrite phase and the austenite phase occupy most of the structure, it means that the sum of the ferrite phase and the austenite phase accounts for 90% or more in the structure forming the stainless steel, and the ferrite phase and the austenite phase May be occupied by the martensite phase in which the austenite phase is transformed.
For example, the duplex stainless steel may have a formal equivalent index (PREN) value of 45.0 or greater.
Generally, the highest corrosion resistance of duplex stainless steels applicable in seawater atmosphere is the Super Duplex S32750 steel with 25Cr - 7Ni, which is composed of Cr, Mo, W, and N contents, Has a value of 40. Therefore, in order to secure a corrosion resistance level that can be used for a long time in a seawater atmosphere, the PREN value should be higher than that of super duplex stainless steel. To this end, the present invention increases the content of Cr, Mo, W and N, Accordingly, the value of the formula-internal-equivalent-index (PREN) may be 45.0 or more.
For example, the duplex stainless steel may have a yield strength (YS) of 800 MPa or more and a tensile strength (TS) of 1 GPa or more.
The duplex stainless steel contains a trace amount of Nb to suppress the growth of crystal grains during the heat treatment of the steel, thereby miniaturizing the structure and thereby improving the yield strength.
For example, the duplex stainless steel may have an elongation greater than 20%. Further, the duplex stainless steel can satisfy the following formula (1).
EI RD - EI TD ≤ 3.0 ------ Equation (1)
Here, EI RD means elongation in the rolling direction and EI TD means elongation in the rolling direction.
By controlling the aspect ratio of the austenite phase in the microstructure of the duplex stainless steel of the present invention, it is possible to reduce the difference in the elongation value according to the rolling direction, thereby improving the tissue anisotropy. Therefore, the formability of the steel can be improved by improving the anisotropy of the structure.
For example, the duplex stainless steel may be a cold rolled thin plate having a thickness of 1.0 to 1.5 mm.
Cr, and Mo is added, it is easy to precipitate intermetallic compounds that cause brittleness of steel in the microstructure. As the content of the elements increases, the temperature required for precipitation increases and the time is advanced. The slow cooling rate during the continuous casting in slabs prevents precipitation of intermetallic compounds.
Accordingly, in the present invention, a continuous casting thickness is cast in the form of a thin plate of 3.0 to 5.0 mm, and water is sprayed during cooling to obtain a rapid cooling rate, and the material is cooled before the time required for precipitation of the intermetallic compound. However, precipitation of intermetallic compounds is remarkably reduced compared to materials having the same composition through slab casting and hot rolling through a conventional production process, such as continuous casting. However, intermetallic compounds including Cr, Mo, and Nb The precipitation of the precipitate did not completely disappear. Thereafter, the intermetallic compound formed in the continuous casting process can be reused through the heat treatment process of the continuous casting plate material.
A method of producing a nickel-reduced duplex stainless steel according to an embodiment of the present invention comprises: a step of preparing a nickel-reduced duplex stainless steel having a C content of 0.05% or less (excluding 0), 24.5 to 32.5% of Cr, 2.0 to 4.0% (Excluding 0), Mn: 2.0% or less (excluding 0), N: 0.25 to 0.45%, Si: 1.0% or less (excluding 0), Nb: 0.2% % Or less (excluding 0), the balance Fe and other unavoidable impurities, and cold-rolling the duplex stainless steel sheet.
Here, the duplex stainless steel sheet may be continuously cast by strip casting the molten steel.
For example, the duplex stainless steel sheet may be cast to a thickness of 3.0 to 5.0 mm.
In one embodiment of the present invention, the step of water-cooling the duplex stainless steel sheet and the step of heat-treating the duplex stainless steel sheet at a temperature of 1,100 to 1,150 ° C for 30 to 180 seconds may be further included.
The duplex stainless steel thin plate continuously cast can be water-cooled by spraying water during cooling. Thus, rapid cooling rate of the duplex stainless steel sheet can be obtained, so that the material is cooled before the time required for precipitation of the intermetallic compound can do.
Thereafter, the intermetallic compound formed in the continuous casting can be reused by heat-treating the duplex stainless steel sheet.
The duplex stainless steel sheet is cold-rolled, and at this time, it can be cold-rolled at a total reduction ratio of 50% or more.
For example, the cold-rolled cold rolled sheet may be 1.0 to 1.5 mm thick.
In one embodiment of the present invention, the method may further include a step of heat-treating the cold-rolled thin plate at a temperature of 1,100 to 1,150 ° C for 10 to 30 seconds.
As a result of this heat treatment, all the rolled microstructure in the cold rolled thin plate can be recrystallized.
For example, the cold-rolled cold rolled thin plate may include an austenite phase having an aspect ratio of 1.0 to 3.0 among microstructures. For example, the microstructure may comprise, in volume fractions, from 40 to 65% ferrite phase and from 45 to 55% austenite phase. The description thereof is as described above.
Hereinafter, the present invention will be described in more detail with reference to examples.
Invention river
The molten steel containing the components according to the inventive steels of the following Table 1 was continuously cast through strip casting and cooled to a thickness of 3 mm by water cooling. The continuous cast thin plate was heat treated at 1,100 ° C for 60 seconds. The cold rolled thin sheet was produced by cold rolling at a thickness of 1.0 mm and then heat treated at a temperature of 1,100 ° C for 10 seconds to prepare a final product.
Comparative steel
Molten steel containing the component system according to the respective comparative steels shown in the following Table 1 was usually continuously cast to prepare a slab having a thickness of 150 mm. Then, ordinary hot rolling was performed, and the hot-rolled sheet was heat-treated at 1,100 ° C for 60 seconds. The hot rolled sheet was cold rolled at a thickness of 1.0 mm to prepare a cold rolled thin plate, and then heat treated at a temperature of 1,100 ° C for 10 seconds to prepare a final product.
Tensile test specimens of JIS 13B standard were prepared for each steel material in a direction parallel and perpendicular to the rolling direction and subjected to tensile test at room temperature according to the ASTM E8 method. The yield strength, tensile strength, elongation, The difference in elongation according to the direction of sample collection was measured and is shown in Table 2 below.
Here, PREN is a value calculated as Cr + 3.3 (Mo + 0.5W) + 16N which is commonly used, and the elongation difference by direction (EI RD - EI TD ) obtained by subtracting the rolling vertical elongation from the rolling elongation it means.
The method of measuring the aspect ratio of the austenite phase is to estimate the length and thickness of each austenite phase with respect to 10 austenite phases in the circle area, assuming a virtual circle having a diameter of 100 m at any point in the microstructure The ratio was then averaged.
1 is a photograph of microstructure of a duplex stainless steel cold rolled thin plate according to an embodiment of the present invention. 2 is a photograph showing microstructure of a duplex stainless steel cold rolled thin plate according to a comparative example of the present invention.
FIG. 1 is a photograph of the microstructure of the inventive steel 1, and FIG. 2 is a photograph of the microstructure of the comparative steel 1.
1 and 2, the microstructure of the inventive steel 1 has an equiaxed microstructure in which the ratio of length and thickness of the austenite phase is small, while in the microstructure of the comparative steel 1, the austenite phase is in the rolling direction And it can be confirmed that it has a shape elongated long.
That is, in the inventive steels, the aspect ratio of the austenite phase was measured to have a value of 1.0 to 3.0, and in the case of the comparative steels, the aspect ratio of the austenite phase was measured to have a value of 3.5 to 9.0, It can be seen that it has a large aspect ratio as compared with the invention steel.
In case of the inventive steels, the contents of Cr, Mo, W, N and the like in the constituent system of the duplex stainless steel are controlled so that the PREN value is not less than 45.0 and not only the comparative steels but also the inner formula equivalence index ) Value of 40 is much higher than that of Super Duplex S32750 steel.
In the inventive steels, the yield strength and the tensile strength can be improved by adding a trace amount of Nb, and it can be seen that the steels have improved strength compared with the comparative steels without Nb added.
In the elongation ratios, it is found that the inventive steels have a small difference in elongation per direction and the formability is improved by minimizing the anisotropy of the microstructure. In the case of the comparative steels, the formability is decreased due to the large aspect ratio.
That is, it was confirmed that the inventive steels were superior in yield strength and tensile strength to those of comparative steels, and had excellent elongation. In addition, the inventive steels have a lower elongation ratio than the comparative steels according to the direction of the tensile test, and the anisotropy according to this direction can be expected to improve the formability of the steel.
Therefore, it is possible to expand the application range by improving the elongation difference according to the direction due to the tissue anisotropy which has been conventionally pointed out in the duplex stainless steel cold rolled steel sheet conventionally. By securing high strength and high corrosion resistance, duplex stainless steel can be used for off- It can be used as an industrial structural material.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will readily obviate modifications and variations within the spirit and scope of the appended claims. It will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.
Claims (17)
The microstructure contains, in a volume fraction, 40 to 65% ferrite phase and 45 to 55% austenite phase,
The aspect ratio of the austenite phase is 1.0 to 3.0,
Duplex stainless steel with excellent corrosion resistance and moldability with a formal equivalent index (PREN) value of at least 45.0.
Cu: not more than 0.03% (excluding 0), Cr: 26.0 to 28.0%, Mo: 2.3 to 2.8%, W: 3.0 to 3.5% 0.9 to 1.1% of N, 0.30 to 0.45% of N, and 0.15 to 0.7% of Si, which is excellent in corrosion resistance and moldability.
The duplex stainless steel has a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 1 GPa or more, and excellent corrosion resistance and moldability.
The duplex stainless steel is excellent in corrosion resistance and moldability with an elongation of more than 20%.
The duplex stainless steel satisfies the following formula (1) and is excellent in corrosion resistance and moldability.
EI RD - EI TD ≤ 3.0 ------ Equation (1)
Here, EI RD means elongation in the rolling direction and EI TD means elongation in the rolling direction.
The duplex stainless steel is a cold rolled thin plate having a thickness of 1.0 to 1.5 mm, and is excellent in corrosion resistance and moldability.
And a step of cold-rolling the thin duplex stainless steel sheet to produce a duplex stainless steel having excellent corrosion resistance and moldability.
Water-cooling the duplex stainless steel sheet; And
And heat treating the duplex stainless steel sheet at a temperature of 1,100 to 1,150 캜 for 30 to 180 seconds.
Wherein said stainless steel sheet is cold-rolled at a total reduction ratio of 50% or more, and wherein said stainless steel sheet is excellent in corrosion resistance and moldability.
Wherein the cold-rolled cold rolled thin plate has a thickness of 1.0 to 1.5 mm and is excellent in corrosion resistance and moldability.
And heat treating the cold-rolled thin plate at a temperature of 1,100 to 1,150 캜 for 10 to 30 seconds.
Wherein the cold-rolled cold rolled thin plate comprises an austenite phase having an aspect ratio of 1.0 to 3.0 among microstructures, and having excellent corrosion resistance and moldability.
Wherein the microstructure has a volume fraction of 40 to 65% of a ferrite phase and 45 to 55% of an austenite phase, and has excellent corrosion resistance and moldability.
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