EP3395996A1 - Acier inoxydable duplex pauvre ayant une résistance à la corrosion et une usinabilité améliorées et procédé de fabrication s'y rapportant - Google Patents

Acier inoxydable duplex pauvre ayant une résistance à la corrosion et une usinabilité améliorées et procédé de fabrication s'y rapportant Download PDF

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EP3395996A1
EP3395996A1 EP16879118.4A EP16879118A EP3395996A1 EP 3395996 A1 EP3395996 A1 EP 3395996A1 EP 16879118 A EP16879118 A EP 16879118A EP 3395996 A1 EP3395996 A1 EP 3395996A1
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stainless steel
duplex stainless
steel
less
corrosion resistance
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EP3395996A4 (fr
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Jeom Yong Choi
Hak Kim
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Posco Holdings Inc
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Posco Co Ltd
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    • 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
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present disclosure relates to a lean duplex stainless steel and a method of manufacturing the same, and more particularly, to a lean duplex stainless steel manufactured with low costs by adjusting amounts of high-priced alloying components and having excellent corrosion resistance equal to or greater than those of STS 304 steels and excellent formability by inhibiting formation of thermal martensite via adjustment of cooling conditions during coiling and cooling after hot rolling and by increasing elongation via adjustment of a phase ratio and a method of manufacturing the lean duplex stainless steel.
  • austenitic stainless steels having excellent formability and corrosion resistance include iron (Fe), as a base metal, chromium (Cr) and nickel (Ni), as major raw materials, and other elements such as molybdenum (Mo) and copper (Cu) and have been developed to a variety of steel types suitable for various applications.
  • Fe iron
  • Cr chromium
  • Ni nickel
  • Mo molybdenum
  • Cu copper
  • Austenitic stainless steels a type of steels having excellent corrosion resistance and pitting corrosion resistance, include a low content of carbon (C) and at least 8% of nickel (Ni). Accordingly, an increase in price of nickel (Ni) causes a wide range of fluctuation in price of raw materials, and thus unstable price may result in low price competitiveness. Thus, in order to compensate for this, there is a need to develop a new steel type having corrosion resistance equal to or higher than those of austenitic stainless steels while reducing the content of nickel (Ni).
  • duplex stainless steel which is a stainless steel having a fine structure consisting of a mixture of an austenite phase and a ferrite phase, has properties of both austenitic and ferritic steels.
  • duplex stainless steels such as, a stainless steel disclosed in US Patent No. 6096441 (registered on August 1, 2000), have been suggested.
  • US Patent No. 6096441 discloses "austenoferritic stainless steel having a low nickel content and a high tensile elongation".
  • the duplex stainless steel provides excellent corrosion resistance to various corrosion environments and has better corrosion resistance than AISI 304 and AISI 316 austenite stainless steels.
  • AISI 304 and AISI 316 austenite stainless steels In the case of such duplex stainless steels, not only manufacturing costs increase due to high-priced elements such as nickel (Ni) and molybdenum (Mo), but also price competitiveness decreases in comparison with other steel types due to consumption of nickel (Ni), molybdenum (Mo), and the like.
  • duplex stainless steels including low-priced alloy elements that replace the high-priced alloy elements such as nickel (Ni) and molybdenum (Mo) have drawn a great deal of attention and interest to reduce the manufacturing costs.
  • Lean duplex stainless steels are economical and easy to obtain high strength due to corrosion resistance equal to that of AISI 304 and 316 steels which are conventional austenitic stainless steels and a low Ni content so as to be currently in the spotlight as steel materials for industrial facilities requiring corrosion resistance such as desalination facilities, pulp facilities, paper manufacturing facilities, and chemical facilities.
  • Such lean duplex steels are, for example, S32304 stainless steel standardized in ASTM A240 (main component: 23Cr-4Ni-0.13N) and S32101 stainless steel standardized in ASTM A240 (main component: 21Cr-1.5Ni-5Mn-0.22N).
  • duplex stainless steels are designed to enhance corrosion resistance rather than cold processibilty, i.e., formability, to provide superior corrosion resistance to corrosion resistance required in certain applications.
  • stress corrosion resistance is also better than design requirements to provide a technical solution
  • ductility a factor related to formability, is lower than that of austenitic stainless steels.
  • duplex stainless steels suitable for industrial equipment and various molding processes requiring corrosion resistance equal to or higher than those of AISI 304, 304L, and 316 steels, and particularly, formability, i.e., ductility, equal to that of AISI 304 steel manufactured with reduced manufacturing costs by replacing the high-priced elements.
  • austenitic stainless steels generally having excellent formability, i.e., high elongation include 4% or more of high-priced Ni, manufacturing costs thereof increase and a valuable natural resource of Ni is consumed in a large amount.
  • Patent Document 0001 US Patent No. 6096441 (registered on August 1, 2000)
  • the present disclosure is directed to providing a lean duplex stainless steel having excellent corrosion resistance and excellent formability by increasing elongation and manufactured with reduced costs via adjustment of components of the duplex stainless steel.
  • the present disclosure is directed to providing a method of manufacturing a lean duplex stainless steel having excellent formability by inhibiting formation of thermal martensite and by increasing elongation via adjustment of cooling conditions during coiling and cooling after hot rolling.
  • the stainless steel may further include at least one selected from the group consisting of 1.0% or less of molybdenum (Mo) and 1.0% or less of tungsten (W) and a total content of molybdenum (Mo) and tungsten (W) may be from 0.15 to 1.0%.
  • the stainless steel may further include at least one selected from the group consisting of 0.05% or less of titanium (Ti), 0.09% or less of niobium (Nb), 0.095% or less of vanadium (V), and 0.19% or less of tin (Sn).
  • the stainless steel may further include at least one selected from the group consisting of 0.19% of tin (Sn) and 0.1% of antimony (Sb).
  • the stainless steel may include 40 to 75% of an austenite phase and the remainder of a ferrite phase.
  • the stainless steel may have a fraction of thermal martensite of 10% or less.
  • the stainless steel may have a pitting potential of 360 mV or higher.
  • the stainless steel may have a hot-rolled elongation of 35% or more.
  • the stainless steel may have a cold-rolled elongation of 40% or more.
  • Another aspect of the present disclosure provides a method of manufacturing a lean duplex stainless steel having excellent corrosion resistance and formability including preparing a lean duplex stainless steel slab including, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities, and hot rolling, hot annealing, coiling, cooling, cold rolling, and cold annealing the slab, wherein the stainless steel has a predicted pitting potential of 360 to 440 mV obtained by Equation (1) below.
  • a coiling temperature of a hot annealed steel and a cooling speed after coiling satisfy Equation (3) below.
  • A coiling temperature (°C) and B is cooling speed after coiling (°C/sec).
  • manufacturing costs may be reduced in comparison with austenitic stainless steels by adjusting the content of an alloy component such as Ni, Si, Mn, and Cu among components of the duplex stainless steel, and both of formability and corrosion resistance of the duplex stainless steel may be improved by increasing an elongation of a hot annealed steel to 35% or more and an elongation of a cold annealed steel to 40% or more via adjustment of phase fractions of ferrite and austenite phases and via addition of Mo, W, rare earth elements, and the like to improve corrosion resistance equal to or higher than that of STS 304 steels.
  • an alloy component such as Ni, Si, Mn, and Cu among components of the duplex stainless steel
  • both of formability and corrosion resistance of the duplex stainless steel may be improved by increasing an elongation of a hot annealed steel to 35% or more and an elongation of a cold annealed steel to 40% or more via adjustment of phase fractions of ferrite and austenite phases and via addition of Mo, W,
  • formability may be improved by inhibiting formation of thermal martensite via adjustment of cooling conditions during coiling and cooling after hot rolling and by increasing elongation via adjustment of phase fractions of ferrite and austenite phases.
  • a lean duplex stainless steel having excellent corrosion resistance and formability includes, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities, wherein a predicted pitting potential obtained by Equation (1) below is from 360 to 440 mV.
  • a lean duplex stainless steel having excellent corrosion resistance and formability includes, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities.
  • C carbon
  • Si silicon
  • Mn manganese
  • Cr chromium
  • Cu copper
  • Fe iron
  • the content of C is 0.08% or less (excluding 0).
  • C is an austenite-forming element and an effective element for increasing strength of a material by solid solution hardening.
  • carbide-forming element such as Cr, which is effective for corrosion resistance, at an interface between ferrite and austenite phases resulting in a decrease in the Cr content around crystal grains thereby deteriorating corrosion resistance
  • the C content may be greater than 0% and equal to or less than 0.08% to maximize corrosion resistance.
  • the Si convent is from 0.7 to 1.1%.
  • Si is added in a small amount for deoxidation effects and a ferrite-forming element enriched in ferrite by annealing.
  • the Si content needs to be 0.7% or more to obtain a proper fraction of a ferrite phase.
  • the Si content exceeding 1.1% rapidly increases hardness resulting in a decrease in elongation of the duplex stainless steel and makes it difficult to obtain the austenite phase for sufficient elongation.
  • excess Si atoms lower fluidity of a slag during a steelmaking process and bind to oxygen atoms to form inclusions thereby impairing corrosion resistance.
  • the Si content may be from 0.7% or more to 1.1% or less.
  • the Mn content is from 2.4 to 3.5%.
  • Mn as a deoxidizer and an element increasing a solid solubility of nitrogen, is an austenite-forming element and used to replace the high-priced Ni.
  • Mn content exceeds 3.5%, it is difficult to obtain corrosion resistance at a level similar to that of STS 304 steel. This is because excess Mn atoms may form MnS with S atoms contained in steels thereby impairing corrosion resistance, although Mn may increase the solid solubility of N.
  • the Mn content when the Mn content is less than 2%, it is difficult to obtain a proper faction of the austenite phase even by adjusting the contents of Ni, Cu, N, and the like which are austenite-forming elements and it is also difficult to obtain a sufficient solid solubility of N at an atmospheric pressure due to a low solid solubility of N added thereto.
  • the Mn content may be from 2.4% or more to 3.5% or less.
  • the Cr content is from 17.9 to 20.7%.
  • the Cr as a ferrite-stabilizing element together with Si, not only plays an important role in obtaining the ferrite phase of the duplex stainless steel but also is an essential element to obtain corrosion resistance.
  • an increase in the Cr content improves corrosion resistance, the contents of the austenite-forming elements such as the high-priced Ni also need to be increased to maintain a phase ratio. Thus, manufacturing costs may be increased.
  • the Cr content may be from 17.9% or more to 20.7% or less to obtain corrosion resistance at a level similar to that of STS 304 steel while maintaining a phase ratio of the duplex stainless steel.
  • the Ni content is from 0.05 to 1.15%.
  • Ni as an austenite-stabilizing element together with Mn, Cu and N, plays a main role in obtaining the austenite phase of the duplex stainless steel.
  • the balance of the fractions of the phases may be maintained by increasing the contents of Mn and N, which are austenite-forming elements replacing the high-priced Ni, in order to reduce manufacturing costs.
  • the Ni content may be 0.05% or more to obtain sufficient stabilization of austenite to inhibit formation of strain induced martensite during cold rolling.
  • excess Ni makes it difficult to obtain a proper fraction of austenite due to an increases in the fraction of austenite and particularly lowers price competitiveness to STS 304 steel due to an increase in manufacturing costs by using the high-priced Ni.
  • the Ni content may be from 0.05% or more to 1.15% or less.
  • the N content is from 0.18 to 0.3%.
  • N as an element playing an important role in stabilizing the austenite phase of the duplex stainless steel together with Ni, is enriched in the austenite phase by an annealing process.
  • the increase of the N content may additionally improve corrosion resistance and increase strength.
  • the solid solubility of N may vary in accordance with the content of added Mn.
  • the N content exceeds 0.3% in the case where the Mn content is within the range described above, steels may not be stably manufactured due to surface defects caused by formation of blow holes and pin holes during a steelmaking process due to excessive solid solubility of N.
  • the N content may be 0.2% or more to obtain corrosion resistance at a level similar to that of STS 304 steel.
  • the N content is too low, it is difficult to obtain proper phase fractions.
  • the N content may be from 0.18% or more to 0.30% or less.
  • the Cu content may be from 0.4 to 2.8%.
  • Cu as an austenite-forming element, is an element for the balance of the phase fractions and for replacing Ni.
  • Cu is an element having the same effects as Ni.
  • the Cu content needs to be 0.4% or more in the case where the Ni content is within the range described above to obtain sufficient ductility, i.e., to generate strain induced martensite or mechanical twining during a cold rolling process.
  • the Cu content may be 2.8% or less in consideration of an amount to be solidified.
  • the Cu content may be from 0.4% or more to 2.8% or less.
  • the lean duplex stainless steel having excellent corrosion resistance and formability may further include at least one selected from the group consisting of 1.0% or less of molybdenum (Mo) and 1.0% or less of tungsten (W).
  • Mo molybdenum
  • W tungsten
  • Mo and W as ferrite-forming elements, improve corrosion resistance and are mostly distributed in the ferrite phase.
  • W is an element added in place of Mo.
  • the above-described alloying components promote formation of intermetallic compounds at a temperature of 600 to 1,000°C during heat treatment, resulting in impairing of corrosion resistance and mechanical properties.
  • Mo In the case of Mo, more than 0% may be added to obtain the effect on improving corrosion resistance. However, when the Mo content exceeds 1.0%, intermetallic compounds are formed, resulting in rapid deterioration of corrosion resistance, particularly, elongation.
  • the Mo content may be greater than 0% to 1.0% or less.
  • W more than 0% may be added to obtain the effect on improving corrosion resistance.
  • W content exceeds 1.0%, intermetallic compounds are formed, resulting in rapid deterioration of corrosion resistance, particularly, elongation.
  • the W content may be greater than 0% to 1.0% or less.
  • the lean duplex stainless steel having excellent corrosion resistance and formability may have a total content of molybdenum (Mo) and tungsten (W) of 0.15 to 1.0% to obtain elongation and corrosion resistance of hot rolled and cold rolled materials.
  • Mo molybdenum
  • W tungsten
  • the total of Mo and W may be from 0.15 to 1.0% to obtain excellent corrosion resistance and elongation.
  • the lean duplex stainless steel having excellent corrosion resistance and formability may further include at least one selected from the group consisting of 0.05% or less of titanium (Ti), 0.09% or less of niobium (Nb), 0.095% or less of vanadium (V), and 0.19% or less of tin (Sn).
  • Ti, Nb, and V serve as deoxidizers, bind to oxygen to form inclusions during a steelmaking process and a casting process, and react with C or N to form carbides or carbonitrides while coiling and cooling after a hot rolling process or while hot annealing and cold annealing. These precipitates inhibit formation of Cr carbides and inhibit formation of thermal martensite while cooling, thereby contributing improvement of elongation in a hot rolled state.
  • the Ti content may be more than 0% to 0.05% or less
  • the Nb content may be more than 0% to 0.09% or less
  • the V content may be more than 0% to 0.095% or less.
  • the lean duplex stainless steel having excellent corrosion resistance and formability may further include at least one selected from the group consisting of 0.19% or less of tin (Sn) and 0.1% of antimony (Sb).
  • Sn is known as an element that is enriched on the surface during an annealing process to improve corrosion resistance of an alloy. To obtain the effect of addition of Sn, more than 0% of Sn needs to be added.
  • Sn is a ferrite phase-forming element and causes brittleness during a hot rolling process. While brittleness is caused during the hot rolling process when Sn is added in an amount more than 0.19%, the effect on forming the ferrite phase is not affected when Sn is added in an amount more than 0.19%. Thus, the Sn content may be more than 0% to 0.19% or less.
  • Sb is known as an element that is enriched on the surface during an annealing process to improve corrosion resistance of an alloy. To obtain the effect of addition of Sb, more than 0% of Sn needs to be added. When Sb is added in an amount more than 0.1%, brittleness may be caused during a hot rolling process. Thus, the Sb content may be from more than 0% to 0.1% or less.
  • FIG. 1 is a graph for describing a correlation between predicted pitting potentials and cold rolled elongations of cold annealed duplex stainless steels manufactured according to examples of the present disclosure and comparative examples.
  • the lean duplex stainless steel having excellent corrosion resistance and formability has a predicted pitting potential of 360 to 440 mV which is obtained by Equation (1) below.
  • Creq Cr + 1.37Mo + 0.75W + 1.5Si + 2Nb + 3Ti + 5V + 5.5A1. While pitting potential of a material increases by adding elements capable of improving corrosion resistance such as Mo and W, elongation may decrease by adding these elements.
  • the pitting potential of the stainless steel may be 360 mV or more. Accordingly, the lean duplex stainless steel according to an embodiment may have corrosion resistance equal to or better than those of STS 304 steels.
  • the stainless steel may include, in a volume fraction, 40 to 75% of an austenite phase and the remainder of a ferrite phase.
  • the volume fraction of the austenite phase is less than 40%, austenite-forming elements are excessively enriched in the austenite phase during an annealing process.
  • austenite is sufficiently stabilized to suppress an amount of modified organic martensite transformation that occurs during deformation and tensile strength of the material may be sufficiently obtained due to excessive increase in strength of austenite due to excessive solidification of alloying elements.
  • the fraction of austenite may be 40% or more to improve ductility.
  • the fraction of austenite exceeds 75%, surface cracking occurs during hot rolling, thereby deteriorating hot rolling formability, and thus properties of two-phase steel may be lost.
  • the ferrite phase is rapidly strengthened by accumulation of the ferrite phase-forming elements, yield strength is increased to destruct the ferrite phase, resulting in a rapid decrease in ductility of the stainless steel.
  • the fraction of austenite may be 75% or less.
  • a preferable fraction of the austenite phase to obtain a proper elongation of a hot annealed duplex stainless steel or a cold annealed steel, i.e., an elongation as a result of formation of strain induced martensite from austenite may be from 40 to 75%.
  • the remainder of the stainless steel includes the ferrite phase, that is, the fraction of the ferrite phase may be from 25 to 60%.
  • the fraction of thermal martensite may be 10% or less in the stainless steel.
  • FIG. 2 is a photograph illustrating a microstructure of a hot annealed steel manufactured according to Comparative Example 11 including thermal martensite.
  • FIG. 3 is a graph illustrating stress-strain curves of hot annealed duplex stainless steels manufactured according to Example 3 of the present disclosure and Comparative Example 11.
  • FIG. 4 is a graph for describing a correlation between fractions of thermal martensite and hot rolled elongations of hot annealed duplex stainless steels manufactured according to examples of the present disclosure and comparative examples.
  • FIG. 2 the microstructure of the duplex stainless steel including thermal martensite according to a comparative example is illustrated.
  • FIG. 2 is a photograph illustrating a microstructure of a hot annealed steel manufactured according to Comparative Example 11 including thermal martensite.
  • dark brown indicates ferrite phase 1
  • gray indicates austenite phase 2
  • relatively light brown shown as needles indicates thermal martensite 3 formed while cooling.
  • Example 3 Stress-strain curves of a steel according to Comparative Example 11 in which a large amount of thermal martensite is observed and a steel according to Example 3 in which little thermal martensite is observed are shown in FIG. 3 .
  • the of Example 3 according to an embodiment of the present disclosure has a higher strain rate of a hot annealed steel than that of Comparative Example 11. That is, when thermal martensite is present, a steel may have a relatively low strain rate, i.e., an elongation of 30% or less, in comparison with a steel normally coiled and not including thermal martensite.
  • the lean duplex stainless steel according to an embodiment of the present disclosure has a hot rolled elongation of 35% or more and a cold rolled elongation of 40% or more.
  • a lean duplex stainless steel may be manufactured by preparing a lean duplex stainless steel slab having the composition and hot rolling, hot annealing, coiling, and cooling the slab.
  • A coiling temperature (°C) and B is cooling speed after coiling (°C/sec).
  • duplex stainless steels When duplex stainless steels are coiled and slowly cooled or heat-treated at a temperature of 600 to 900°C according to a conventional method, precipitates having a sigma phase and including Cr nitrides, Mo or W are formed in an interface between the ferrite phase and the austenite phase contained in the duplex stainless steel and grow to an austenite phase.
  • alloying components such as C, N, and Cr, which form solid solution in the austenite phase, around the precipitates are precipitated, and thus the austenite phase around the precipitates forms thermal martensite during cooling due to lack of alloying components.
  • FIG. 5 is a graph for describing formation of thermal martensite in hot annealed duplex stainless steels according to coiling temperature and cooling speed after coiling.
  • thermal martensite tends to be formed in the hot annealed steel according to the coiling temperature and cooling speed after coiling. That is, more than 10% of thermal martensite was observed in coiling temperature and cooling speed conditions shown as "X" in FIG. 5 and no thermal martensite or 10% or less of thermal martensite was observed in coiling temperature and cooling speed conditions shown as "O" in FIG. 5 .
  • the hot annealed steel may have an elongation of 35% or more by inhibiting formation of thermal martensite via adjustment of the coiling temperature and cooling speed conditions to satisfy Equation (3) above.
  • the elongations were obtained by collecting the samples in a direction perpendicular to a rolling direction, i.e., a width direction, and performing measurement at a strain rate of 6.6x10 -3 /s.
  • FIG. 1 is a graph for describing the correlation between predicted pitting potentials and cold rolled elongations of cold annealed duplex stainless steels manufactured according to examples of the present disclosure and comparative examples.
  • FIG. 1 illustrates elongations and predicted pitting potentials of the steels of Examples and the steels of Comparative Examples shown in Table 3 as a graph. Referring to FIG. 1 , as the predicted pitting potential increases, the elongation decreases. To obtain the elongation of the cold annealed steel of 40% or more in the width direction of the cold annealed steel, it may be confirmed that the predicted pitting potential needs to be within the range of 360 to 440 mmV.
  • manufacturing costs may be reduced by adjusting the contents of the alloying components such as Ni, Si, Mn, and Cu among components of the duplex stainless steel in comparison with austenitic stainless steels, and both of formability and corrosion resistance of the duplex stainless steel may be improved by increasing an elongation of the hot annealed steel to 35% or more and an elongation of the cold annealed steel to 40% or more via adjustment of the factions of ferrite and austenite and by improving corrosion resistance to a level equal to or greater than those of STS 304 steels via addition of Mo, W, rare earth elements, and the like.
  • formability may be improved by obtaining a proper elongation by suppressing formation of thermal martensite via adjustment of cooling conditions during coiling and cooling after hot rolling.
  • the lean duplex stainless steel according to embodiments of the present disclosure may be industrially applicable as steel materials for industrial facilities such as desalination facilities, pulp facilities, paper manufacturing facilities, and chemical facilities.

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EP16879118.4A 2015-12-23 2016-08-12 Acier inoxydable duplex pauvre ayant une résistance à la corrosion et une usinabilité améliorées et procédé de fabrication s'y rapportant Pending EP3395996A4 (fr)

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CN108368592A (zh) 2018-08-03

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