EP3559295A1 - An object comprising a duplex stainless steel and the use thereof - Google Patents

An object comprising a duplex stainless steel and the use thereof

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Publication number
EP3559295A1
EP3559295A1 EP17818553.4A EP17818553A EP3559295A1 EP 3559295 A1 EP3559295 A1 EP 3559295A1 EP 17818553 A EP17818553 A EP 17818553A EP 3559295 A1 EP3559295 A1 EP 3559295A1
Authority
EP
European Patent Office
Prior art keywords
stainless steel
duplex stainless
equal
less
content
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP17818553.4A
Other languages
German (de)
French (fr)
Other versions
EP3559295B1 (en
Inventor
Tomas Forsman
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sandvik Intellectual Property AB
Original Assignee
Sandvik Intellectual Property AB
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sandvik Intellectual Property AB filed Critical Sandvik Intellectual Property AB
Publication of EP3559295A1 publication Critical patent/EP3559295A1/en
Application granted granted Critical
Publication of EP3559295B1 publication Critical patent/EP3559295B1/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot 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
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot 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
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0268—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • 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
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
    • 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
    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • 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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C2204/00—End product comprising different layers, coatings or parts of cermet

Definitions

  • An object comprising a duplex stainless steel and the use thereof
  • the present disclosure relates to an object comprising a duplex stainless steel, in particular the object is suitable for use in spring applications or as a spring as such.
  • the present disclosure also relates to a method of producing the object.
  • Spring applications in the form of a wire or a strip may be statically or dynamically loaded.
  • the most important properties for steel grades aimed for static spring applications are high proof or yield strength, well-defined elastic modulus, high corrosion resistance and high stress relaxation resistance.
  • the most important properties for steel grades aimed for dynamic spring applications are high proof or yield strength, well-defined elastic modulus, high corrosion resistance, high stress relaxation resistance and high resistance towards fatigue failure.
  • JP 2010 059541 discloses a process wherein the final step is an annealing process aiming for obtaining the maximum elongation of a stainless steel which is a low alloyed duplex steel.
  • the austenite phase of such a grade is unstable and will partially transform to martensite upon plastic deformation.
  • Stainless spring steel grades of austenitic or martensitic origin typically possess excellent combinations of most of the above properties.
  • one major drawback of the austenitic steel grades is that the elastic modulus tends to decrease almost linearly with increasing load up to the proof stress (Rp 0 . 2 ) and as stated above steel grades aiming for spring applications should have an elastic modulus which remains at a high level also upon increasing load and which does not decrease in a linear fashion.
  • Martensitic steel grades may present an elastic modulus which does not decrease linearly with increasing load.
  • one major drawback of the martensitic steel grades is that these steels have problems with their corrosion resistance. It is therefore an aspect of the present disclosure to provide an object which is suitable for spring applications and which will solve or at least reduce the above-mentioned drawbacks.
  • an aspect of the present disclosure is to provide an object manufactured from a duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in weight :
  • duplex stainless steel consists of 55-75 vol% austenite phase and 25-45 vol% ferrite phase;
  • the object has alternating layers of ferrite phase and austenite phase
  • said alternating layers are essentially parallel with the plane of the object and said alternating layers have an average layer thickness which is less than or equal to about 4.5 ⁇ .
  • the term “about” means plus or minus 5% of the numerical value of the number with which it is being used.
  • the aberration "FCC” means austenite phase and the aberration "BCC” means ferrite phase.
  • the expression “essentially parallel” as used herein is intended to mean that the deviation from the plane is less than 10%.
  • the object comprising the duplex stainless steel as defined hereinabove or hereinafter will have low or no content of sigma phase and/or precipitated chromium nitride. Furthermore, said object will have an elastic modulus that will remain relatively high upon increasing load as compared to the behavior of pure austenitic stainless steel. By low or no content of sigma phase and/or precipitated chromium nitride is meant that the amount present should not seriously deteriorate the corrosion resistance or toughness of the duplex stainless steel.
  • Another aspect of the present disclosure provides a method for manufacturing an object as defined hereinabove or hereinafter, the method comprising the steps of:
  • hot working processes to transform the body to a workpiece and the hot working processes are performed at a temperature of about 1050 to about 1300 °C;
  • the final step of the method must be a cold working process because this process will influence the microstructure of the duplex stainless steel the most and thereby have a great impact on the elastic modulus. Furthermore, the present method will provide the object with a higher strength after cold working and the cold working will also ensure that deformation hardening will happen in the object.
  • the present disclosure relates to an object manufactured from a duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in weight :
  • duplex stainless steel consists of 55-75 vol% austenite phase and 25-45 vol% ferrite phase;
  • the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel with the plane of the object and said alternating layers have an average thickness which is less than or equal to about 4.5 ⁇ .
  • the duplex stainless steel as defined hereinabove or hereinafter will provide the object with high resistance against corrosion. Furthermore, the alternating layers of ferrite phase and austenite phase will provide the object with a well-defined elastic modulus which will remain relatively high upon increasing load.
  • a well-defined elastic modulus means that the elastic modulus will remain at a high level upon increasing load on the material and will not decrease almost linearly with increasing load up to the proof stress (Rp 0 . 2 ). Thus, the object will be suitable for spring applications.
  • the duplex stainless steel has a PRE greater than 28.
  • the duplex stainless steel as defined hereinabove or hereinafter will therefore provide the object with high resistance against corrosion, especially against pitting corrosion due to its high PRE value in both ferrite and austenite phase, i.e. the PRE value for both the ferrite and the austenite phase is greater than about 28.
  • the respective amounts of Cr, Mo and N are chosen so that PRE is greater than about 28 in the austenite and ferrite phase respectively.
  • the duplex stainless steel as defined hereinabove or hereinafter consists of 55-70 vol% austenite phase and 30-45 vol% ferrite phase, such as 65-70 vol% austenite phase and 30-35 vol% ferrite phase. This means that no
  • duplex stainless steel of the present disclosure and thereby in the object composed of the duplex stainless steel.
  • the duplex stainless steel as defined hereinabove or hereinafter is highly alloyed and therefore the object will have the ability of undergoing cold deformation generated by cold working without transformation of its austenitic structure into martensitic structure.
  • the object as defined hereinabove or hereinafter is in the form of a sheet or a strip or a wire.
  • the sheet or the strip or the wire may be used for manufacturing a spring, thus the present disclosure also relates to a spring.
  • the at least one hot working process is hot rolling.
  • the hot working process is performed in a temperature of from 1050 to 1300 °C.
  • the at least one hot working process is performed one time or more than one time, e.g. in one embodiment, the hot working, such as hot rolling, may be performed on the body several times, such as e.g. 6 times or until the desired hot working reduction of the workpiece is obtained.
  • the hot working will also form layers of austenite phase and ferrite phase but the thickness of these layers is higher than in the final object.
  • the workpiece may be heated between the hot working steps.
  • the at least one cold working process is cold rolling. According to another embodiment, the at least one cold working is cold drawing.
  • the cold working process is performed one time or more than one time.
  • the cold working process may be performed on the workpiece several times, e.g. 4 times or until the desired cold deformation of the final object is obtained.
  • the cold deformation of the final object thus meaning the deformation of the object, is at least 10%, such at least 25%, such as at least 50%, such as at least 75%, such as from 75 to 95%.
  • the thickness of the obtained final object in its cold rolled condition is of from 20 ⁇ up to 5 mm.
  • the method comprises one hot working process, one cold working process, one hot working process and one cold working process. According to another embodiment, the method comprises one hot working process, one cold working process, one hot working process, one cold working process and one cold working process.
  • the method as defined hereinabove or hereinafter comprises a step of heat treatment, wherein the heat treatment is annealing the object obtained after a cold working step.
  • Annealing may be performed in order to reduce any formed intermetallic phases, such as sigma phase and chromium nitrides, or to reduce the strength of the object or for changing the content of austenite or ferrite phase in the object.
  • the annealing temperature will depend on both the composition and the thickness of the object. Usually, the annealing temperature is above 1000 °C.
  • the object is subjected to an annealing step at least between the second last and the last cold working step.
  • the object is annealed at a temperature range of from 1050°C to 1250°C for a period of from about 1 to 600 seconds.
  • the temperature ramping may be such that the time for passing said range is below about 2 minutes.
  • the last step in the process is a cold working step.
  • the present method also comprises a step of aging the object obtained either after cold working step or after an annealing step.
  • This step will provide an additional increase of the proof stress of the object and also a further improvement of the elastic modulus behavior.
  • the object Before being subjected to aging, the object may be subjected to a forming operation in which it is formed into a spring.
  • the aging may be performed for 0.25 to 4 hours at a temperature of from 400 to 450°C. As the aging step is performed at low temperatures, it may be performed after the final cold working process step.
  • the alloying elements of the duplex stainless steel as defined hereinabove or hereinafter are discussed. The amounts are given in weight (wt%):
  • Carbon, C is a representative element for stabilizing austenitic phase and is an important element for maintaining mechanical strength. However, if a large content of carbon is present, carbides will be precipitated which will reduce the corrosion resistance.
  • the carbon content is limited to less than 0.040 wt%.
  • Manganese, Mn has a deformation hardening effect and it counteracts the
  • Mn has to be present in at least or equal to 0.80 wt%. Additionally, Mn has an austenite stabilizing effect up to a content of about 10 wt%. Above that level, the stabilization of ferrite will be increased and it will therefore become difficult to add further ferrite stabilizing element, such as Cr and Mo, without obtaining too much ferrite. Thus, the maximum content of Mn should not be above 10 wt% According to one embodiment, the content of Mn is equal to or less than 6.0 wt%. According to yet another embodiment it is equal to or less than 5.0 wt%.
  • the content of Mn is in the range of from 2.0 to 5.0 wt%.
  • Mn When Mn is present in amounts as suggested above, it will increase the deformation hardening ability of the duplex stainless steel to and also prevent the austenite phase from becoming so unstable, i.e. it will prevent the transformation from austenitic structure into martensitic structure upon deformation.
  • Nitrogen, N has a positive effect on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and has also a strong effect on the pitting corrosion resistance equivalent PRE as PRE is defined as Cr+3.3Mo+16. Furthermore, N contributes strongly to the solid solution strengthening and deformation hardening of the duplex stainless steel.
  • N has also a strong austenite stabilizing effect and counteracts transformation from austenitic structure to martensitic structure upon plastic deformation.
  • N is added in an amount of 0.10 wt% or higher.
  • the content of N should therefore be equal to or lower than 0.45 wt%.
  • the content of N is of from 0.30 to 0.42 wt%.
  • Molybdenum, Mo has a strong influence on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and it heavily influences the PRE and contributes strongly to both the solid solution strengthening and the deformation hardening. Therefore, Mo is added in amount of equal to or more than 0.90 wt%.
  • Mo also increases the temperature at which unwanted sigma phase is stable and promotes its generation rate and therefore the content of Mo should be equal to or less than 4.5 wt%. According to one embodiment, the content of Mo is of from 2.0 to 4.0 wt%.
  • Chromium, Cr has strong impact on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter, especially the pitting corrosion. Moreover, Cr improves the yield strength and counteracts transformation of austenitic structure to martensitic structure upon deformation of the duplex stainless steel. Cr also has a ferrite- stabilizing effect on the duplex stainless steel. Therefore, the content of Cr should be equal to or above 21.0 wt%. At high levels, an increasing content of Cr will result in a higher temperature for unwanted stable sigma phase and chromium nitrides and a more rapid generation of sigma phase. Therefore, the content of Cr is equal to or less than 28.0 wt%. According to one embodiment, the content of Cr is of from 24.0 to 28.0 wt%, such as 26.0 to 28.0 wt%.
  • Copper, Cu has a positive effect on the corrosion resistance. However, it is optional to add Cu to the duplex stainless steel as defined hereinabove or hereinafter. Often, Cu is present in scrapped goods used for the production of steel, and is allowed to remain in the steel at moderate levels. The content of Cu may therefore be equal to or less than 0.50 wt%. According to one embodiment, the content of Cu is equal to or less than 0.02 wt%.
  • Nickel, Ni has a positive effect on the resistance against general corrosion. Ni also has a strong austenite-stabilizing effect and counteracts transformation from austenitic to martensitic structure upon deformation of the duplex stainless steel. The content of Ni is therefore equal to or more than 4.0 wt%.
  • Ni will result in austenite levels of above 70 %.
  • the content of Ni should, therefore, not be more than or equal to 9.0 wt%. According to one embodiment, the content of Ni is of from 7.0 to 9.0 wt%.
  • Si is almost always present in duplex stainless steels since it may have been used for deoxidization or is in the scrap used for the duplex stainless steels, even though the aim is to have as low amounts as possible. It has a ferrite- stabilizing effect and, at least partly for that reason, the content of Si should be less than 0.60 wt%, such as between 0.40 to 0.60 wt%. .
  • Vanadium, V may be present as an impurity element in duplex stainless steel and because it usually follows the scrap and it is therefore difficult to control the content.
  • the duplex stainless steel should preferably contain as low amounts as possible due to precipitations of carbide and for the present duplex stainless steel, the content of V should be equal to or less than 0.10 wt%, such as equal to or less than 0.01 wt%.
  • Phosphorous (P) may be an impurity and is contained in the duplex stainless steel as defined hereinabove or hereinafter; an amount of less than 0.010 wt%.
  • Sulfur (S) may be an impurity contained in the duplex stainless steel as defined hereinabove or hereinafter. S may deteriorate the hot workability at low temperatures. Thus, the allowable content of S is less than 0.006 wt%.
  • alloying elements may be added to the duplex stainless steel as defined hereinabove or hereinafter in order to improve e.g. the machinability or the hot working properties, such as the hot ductility.
  • Example, but not limiting, of such elements are As, Ca, Co, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce.
  • the amounts of one or more of these elements are of max 0.5 wt%, such as max 0.1 wt%.
  • the present object comprises an duplex stainless steel consisting of all the elements mentioned hereinabove or hereinafter.
  • maximum or “less than or equal to”
  • the skilled person knows that the lower limit of the range is 0 wt% unless another number is specifically stated.
  • the remainder of elements of the duplex stainless steel as defined hereinabove or hereinafter is iron (Fe) and normally occurring impurities.
  • impurities are elements and compounds which have not been added on purpose, but cannot be fully avoided as they normally occur as impurities in e.g. the raw material or the additional alloying elements used for manufacturing of the duplex stainless steel.
  • the duplex stainless steel consists of the alloying elements and ranges mentioned above.
  • the step providing a body of the duplex stainless steel as defined hereinabove or hereinafter may include providing a melt of said duplex stainless steel and casting said melt in order to obtain a body of the duplex stainless steel as defined hereinabove or hereinafter.
  • the casting may include continuous casting of the melt.
  • the thickness of each layer is between 0.01 to about 4.5 ⁇ , such as about 0.5 to about 4.5 ⁇ , such as about 1.0 to about 4.5 ⁇ , such as about 1.0 to about 4.2 ⁇ , such as 2.0 to 4.2 ⁇ .
  • the thickness of the product in its final cold worked condition may be of from 20 ⁇ up to 5 mm.
  • the body comprising the duplex stainless steel as defined hereinabove or hereinafter is subjected to hot working, in which, according to one embodiment, the thickness of the body is reduced from about 100 to 200 mm to 2-15 mm.
  • the thickness measurement of the BCC and FCC phases respectively is performed by taking a perpendicular cross-section of the object (the strip, sheet or wire) and then polishing and etching in acid (such as HNO3) to obtain a contrast between the two phases. The measurement is then performed in a light optical microscope using a suitable magnification (100-1000 times) so that each phase is visible and so that a large enough number of phase boundaries can be counted to obtain a reasonable statistical certainty (more than 30 phase boundaries).
  • An appropriate cross-sectional position for the measurement in a wire is at 25% of its diameter. A strip or sheet should be measured 25% of the width away from the edge, at the thickness center.
  • the thickness of each BCC and FCC phase, in the diameter direction of a wire or along the thickness direction of a strip or sheet, is measured and from this the average BCC and FCC thickness respectively is calculated.
  • Alloys having the chemical composition as seen in Table 1 were melted and casted to 1 kg ingots. After melting and casting, the obtained ingots were hot rolled to strips at a temperature of about 1250 °C using 9 rolling passes. The samples were reheated 3 times during the hot rolling in order to keep the temperature above 1050°C. The final thickness of the strips varied from 3.7 to 4.0 mm.
  • the hot rolled strips were then cold rolled until a cold reduction of about 75% was obtained. 5 passes were used in the cold rolling mill.
  • the ferrite content was determined by using magnetic scale measurements.
  • the magnetic scale measurement was performed according to IEC 60404-1.
  • the content of magnetic phase was assumed to equal the ferrite content and the remainder was assumed to be austenite. The values are found in Table 2.
  • the obtained objects will have a proof stress Rpo.2 above 1200 MPa after cold rolling thereof.
  • the objects of the present disclosure will have high yield strength in combination with good ductility and also good corrosion resistance and high tensile strength due to the solid solution strengthening and deformation hardening, the phase thickness and content.

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Abstract

The present disclosure relates to an object comprising a duplex stainless steel, in particular the object is suitable for use in spring applications. The duplex stainless steel has the following composition, in weight%: - C less than or equal to 0.040; - Si less than or equal to 0.60; - Mn 0.80 - 10.0; - Cr 21.0-28.0; - Ni 4.0-9.0; - Mo 0.9 - 4.5; - N 0.10-0.45; - Cu less than or equal to 0.50; - V less than or equal to 0.10; - P less than or equal to 0.010; - s less than or equal to 0.006; balance Fe and unavoidable impurities. The present disclosure also relates to a method of producing the object comprising said duplex stainless steel.

Description

An object comprising a duplex stainless steel and the use thereof
TECHNICAL FIELD
The present disclosure relates to an object comprising a duplex stainless steel, in particular the object is suitable for use in spring applications or as a spring as such. The present disclosure also relates to a method of producing the object.
BACKGROUND
Spring applications in the form of a wire or a strip may be statically or dynamically loaded. The most important properties for steel grades aimed for static spring applications are high proof or yield strength, well-defined elastic modulus, high corrosion resistance and high stress relaxation resistance. The most important properties for steel grades aimed for dynamic spring applications are high proof or yield strength, well-defined elastic modulus, high corrosion resistance, high stress relaxation resistance and high resistance towards fatigue failure.
JP 2010 059541 discloses a process wherein the final step is an annealing process aiming for obtaining the maximum elongation of a stainless steel which is a low alloyed duplex steel. The austenite phase of such a grade is unstable and will partially transform to martensite upon plastic deformation.
Stainless spring steel grades of austenitic or martensitic origin typically possess excellent combinations of most of the above properties. However, one major drawback of the austenitic steel grades is that the elastic modulus tends to decrease almost linearly with increasing load up to the proof stress (Rp0.2) and as stated above steel grades aiming for spring applications should have an elastic modulus which remains at a high level also upon increasing load and which does not decrease in a linear fashion. Martensitic steel grades may present an elastic modulus which does not decrease linearly with increasing load. However, one major drawback of the martensitic steel grades is that these steels have problems with their corrosion resistance. It is therefore an aspect of the present disclosure to provide an object which is suitable for spring applications and which will solve or at least reduce the above-mentioned drawbacks. SUMMARY
Thus, an aspect of the present disclosure is to provide an object manufactured from a duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in weight :
c less than or equal to 0.040;
Si less than or equal to 0.60;
Mn 0.80 - 10.0;
Cr 21.0-28.0;
Ni 4.0-9.0;
Mo 0.9 - 4.5;
N 0.10-0.45;
Cu less than or equal to 0.50;
V less than or equal to 0.10;
P less than or equal to 0.010;
s less than or equal to 0.006;
balance Fe and unavoidable impurities;
and wherein the duplex stainless steel consists of 55-75 vol% austenite phase and 25-45 vol% ferrite phase;
and wherein the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel with the plane of the object and said alternating layers have an average layer thickness which is less than or equal to about 4.5 μπι. As used herein, the term "about" means plus or minus 5% of the numerical value of the number with which it is being used. Also, in the present disclosure, the aberration "FCC" means austenite phase and the aberration "BCC" means ferrite phase. Further, the expression "essentially parallel" as used herein is intended to mean that the deviation from the plane is less than 10%. Additionally, the object comprising the duplex stainless steel as defined hereinabove or hereinafter will have low or no content of sigma phase and/or precipitated chromium nitride. Furthermore, said object will have an elastic modulus that will remain relatively high upon increasing load as compared to the behavior of pure austenitic stainless steel. By low or no content of sigma phase and/or precipitated chromium nitride is meant that the amount present should not seriously deteriorate the corrosion resistance or toughness of the duplex stainless steel.
Another aspect of the present disclosure provides a method for manufacturing an object as defined hereinabove or hereinafter, the method comprising the steps of:
- providing a body of the duplex stainless steel as defined hereinabove or hereinafter;
-one or more hot working processes to transform the body to a workpiece and the hot working processes are performed at a temperature of about 1050 to about 1300 °C;
- one or more cold working processes to transform the workpiece into the object.
wherein the final step of said method must be a cold working process
The final step of the method must be a cold working process because this process will influence the microstructure of the duplex stainless steel the most and thereby have a great impact on the elastic modulus. Furthermore, the present method will provide the object with a higher strength after cold working and the cold working will also ensure that deformation hardening will happen in the object.
DETAILED DESCRIPTION
The present disclosure relates to an object manufactured from a duplex stainless steel, wherein the duplex stainless steel comprises the following composition, in weight :
C less than or equal to 0.040;
Si less than or equal to 0.60;
Mn 0.80 - 10.0;
Cr 21.0-28.0; Ni 4.0-9.0;
Mo 0.9 - 4.5;
N 0.10-0.45;
Cu less than or equal to 0.50;
V less than or equal to 0.10;
P less than or equal to 0.010;
S less than or equal to 0.006;
balance Fe and unavoidable impurities;
and wherein the duplex stainless steel consists of 55-75 vol% austenite phase and 25-45 vol% ferrite phase;
and wherein the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel with the plane of the object and said alternating layers have an average thickness which is less than or equal to about 4.5 μπι.
The duplex stainless steel as defined hereinabove or hereinafter will provide the object with high resistance against corrosion. Furthermore, the alternating layers of ferrite phase and austenite phase will provide the object with a well-defined elastic modulus which will remain relatively high upon increasing load. A well-defined elastic modulus means that the elastic modulus will remain at a high level upon increasing load on the material and will not decrease almost linearly with increasing load up to the proof stress (Rp0.2). Thus, the object will be suitable for spring applications.
According to one embodiment, the duplex stainless steel has a PRE greater than 28. PRE is herein defined as PRE=Cr+3.3*Mo+16*N (factors to be multiplied with the respective weight percentage of the respective alloying element). The duplex stainless steel as defined hereinabove or hereinafter will therefore provide the object with high resistance against corrosion, especially against pitting corrosion due to its high PRE value in both ferrite and austenite phase, i.e. the PRE value for both the ferrite and the austenite phase is greater than about 28. Hence, the respective amounts of Cr, Mo and N are chosen so that PRE is greater than about 28 in the austenite and ferrite phase respectively. According to one embodiment, the duplex stainless steel as defined hereinabove or hereinafter consists of 55-70 vol% austenite phase and 30-45 vol% ferrite phase, such as 65-70 vol% austenite phase and 30-35 vol% ferrite phase. This means that no
deformation induced martensite will be present in the duplex stainless steel of the present disclosure and thereby in the object composed of the duplex stainless steel. This is possible because the duplex stainless steel as defined hereinabove or hereinafter is highly alloyed and therefore the object will have the ability of undergoing cold deformation generated by cold working without transformation of its austenitic structure into martensitic structure.
According to one embodiment, the object as defined hereinabove or hereinafter is in the form of a sheet or a strip or a wire. The sheet or the strip or the wire may be used for manufacturing a spring, thus the present disclosure also relates to a spring.
According to one embodiment, the at least one hot working process is hot rolling. The hot working process is performed in a temperature of from 1050 to 1300 °C. Additionally, according to one embodiment, the at least one hot working process is performed one time or more than one time, e.g. in one embodiment, the hot working, such as hot rolling, may be performed on the body several times, such as e.g. 6 times or until the desired hot working reduction of the workpiece is obtained. The hot working will also form layers of austenite phase and ferrite phase but the thickness of these layers is higher than in the final object. According to yet another embodiment, the workpiece may be heated between the hot working steps.
According to one embodiment, the at least one cold working process is cold rolling. According to another embodiment, the at least one cold working is cold drawing.
According to one embodiment, the cold working process is performed one time or more than one time. In one embodiment, the cold working process may be performed on the workpiece several times, e.g. 4 times or until the desired cold deformation of the final object is obtained. According to one embodiment, the cold deformation of the final object, thus meaning the deformation of the object, is at least 10%, such at least 25%, such as at least 50%, such as at least 75%, such as from 75 to 95%. According to one embodiment, the thickness of the obtained final object in its cold rolled condition is of from 20 μπι up to 5 mm.
According to one embodiment, the method comprises one hot working process, one cold working process, one hot working process and one cold working process. According to another embodiment, the method comprises one hot working process, one cold working process, one hot working process, one cold working process and one cold working process.
According to one embodiment, the method as defined hereinabove or hereinafter comprises a step of heat treatment, wherein the heat treatment is annealing the object obtained after a cold working step. Annealing may be performed in order to reduce any formed intermetallic phases, such as sigma phase and chromium nitrides, or to reduce the strength of the object or for changing the content of austenite or ferrite phase in the object. The annealing temperature will depend on both the composition and the thickness of the object. Usually, the annealing temperature is above 1000 °C. According to another embodiment, the object is subjected to an annealing step at least between the second last and the last cold working step. Also, according to another embodiment several annealing steps (such as more than one) between respective cold working steps (such as more than one cold working step) may be applied. According to one embodiment, the object is annealed at a temperature range of from 1050°C to 1250°C for a period of from about 1 to 600 seconds. During the heating of the object to this temperature range, it is important to avoid exposing said product to a temperature of 750°C to 1000°C during too long time as this is the temperature range wherein sigma phase and/or chromium nitrides are most rapidly formed. Therefore, the temperature ramping may be such that the time for passing said range is below about 2 minutes. Further, the last step in the process is a cold working step. According to one embodiment, the present method also comprises a step of aging the object obtained either after cold working step or after an annealing step. This step will provide an additional increase of the proof stress of the object and also a further improvement of the elastic modulus behavior. Before being subjected to aging, the object may be subjected to a forming operation in which it is formed into a spring. The aging may be performed for 0.25 to 4 hours at a temperature of from 400 to 450°C. As the aging step is performed at low temperatures, it may be performed after the final cold working process step. Hereinafter, the alloying elements of the duplex stainless steel as defined hereinabove or hereinafter are discussed. The amounts are given in weight (wt%):
Carbon, C is a representative element for stabilizing austenitic phase and is an important element for maintaining mechanical strength. However, if a large content of carbon is present, carbides will be precipitated which will reduce the corrosion resistance.
Therefore, the carbon content is limited to less than 0.040 wt%.
Manganese, Mn, has a deformation hardening effect and it counteracts the
transformation from austenitic to martensitic structure upon deformation. In order to have these effects, Mn has to be present in at least or equal to 0.80 wt%. Additionally, Mn has an austenite stabilizing effect up to a content of about 10 wt%. Above that level, the stabilization of ferrite will be increased and it will therefore become difficult to add further ferrite stabilizing element, such as Cr and Mo, without obtaining too much ferrite. Thus, the maximum content of Mn should not be above 10 wt% According to one embodiment, the content of Mn is equal to or less than 6.0 wt%. According to yet another embodiment it is equal to or less than 5.0 wt%. According to one embodiment, the content of Mn is in the range of from 2.0 to 5.0 wt%. When Mn is present in amounts as suggested above, it will increase the deformation hardening ability of the duplex stainless steel to and also prevent the austenite phase from becoming so unstable, i.e. it will prevent the transformation from austenitic structure into martensitic structure upon deformation. Nitrogen, N, has a positive effect on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and has also a strong effect on the pitting corrosion resistance equivalent PRE as PRE is defined as Cr+3.3Mo+16. Furthermore, N contributes strongly to the solid solution strengthening and deformation hardening of the duplex stainless steel. N has also a strong austenite stabilizing effect and counteracts transformation from austenitic structure to martensitic structure upon plastic deformation. In order to contribute with all these positive effects, N is added in an amount of 0.10 wt% or higher. However, at too high levels, N tends to form chromium nitrides, which should be avoided due to the negative effects on ductility and corrosion resistance. Thus, the content of N should therefore be equal to or lower than 0.45 wt%. According to one embodiment, the content of N is of from 0.30 to 0.42 wt%.
Molybdenum, Mo, has a strong influence on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and it heavily influences the PRE and contributes strongly to both the solid solution strengthening and the deformation hardening. Therefore, Mo is added in amount of equal to or more than 0.90 wt%.
However, Mo also increases the temperature at which unwanted sigma phase is stable and promotes its generation rate and therefore the content of Mo should be equal to or less than 4.5 wt%. According to one embodiment, the content of Mo is of from 2.0 to 4.0 wt%.
Chromium, Cr, has strong impact on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter, especially the pitting corrosion. Moreover, Cr improves the yield strength and counteracts transformation of austenitic structure to martensitic structure upon deformation of the duplex stainless steel. Cr also has a ferrite- stabilizing effect on the duplex stainless steel. Therefore, the content of Cr should be equal to or above 21.0 wt%. At high levels, an increasing content of Cr will result in a higher temperature for unwanted stable sigma phase and chromium nitrides and a more rapid generation of sigma phase. Therefore, the content of Cr is equal to or less than 28.0 wt%. According to one embodiment, the content of Cr is of from 24.0 to 28.0 wt%, such as 26.0 to 28.0 wt%.
Copper, Cu, has a positive effect on the corrosion resistance. However, it is optional to add Cu to the duplex stainless steel as defined hereinabove or hereinafter. Often, Cu is present in scrapped goods used for the production of steel, and is allowed to remain in the steel at moderate levels. The content of Cu may therefore be equal to or less than 0.50 wt%. According to one embodiment, the content of Cu is equal to or less than 0.02 wt%. Nickel, Ni, has a positive effect on the resistance against general corrosion. Ni also has a strong austenite-stabilizing effect and counteracts transformation from austenitic to martensitic structure upon deformation of the duplex stainless steel. The content of Ni is therefore equal to or more than 4.0 wt%. At levels above 9.0 wt%, Ni will result in austenite levels of above 70 %. The content of Ni should, therefore, not be more than or equal to 9.0 wt%. According to one embodiment, the content of Ni is of from 7.0 to 9.0 wt%.
Silicon, Si, is almost always present in duplex stainless steels since it may have been used for deoxidization or is in the scrap used for the duplex stainless steels, even though the aim is to have as low amounts as possible. It has a ferrite- stabilizing effect and, at least partly for that reason, the content of Si should be less than 0.60 wt%, such as between 0.40 to 0.60 wt%. .
Vanadium, V, may be present as an impurity element in duplex stainless steel and because it usually follows the scrap and it is therefore difficult to control the content. The duplex stainless steel should preferably contain as low amounts as possible due to precipitations of carbide and for the present duplex stainless steel, the content of V should be equal to or less than 0.10 wt%, such as equal to or less than 0.01 wt%. Phosphorous (P) may be an impurity and is contained in the duplex stainless steel as defined hereinabove or hereinafter; an amount of less than 0.010 wt%. Sulfur (S) may be an impurity contained in the duplex stainless steel as defined hereinabove or hereinafter. S may deteriorate the hot workability at low temperatures. Thus, the allowable content of S is less than 0.006 wt%.
Optionally small amounts of other alloying elements may be added to the duplex stainless steel as defined hereinabove or hereinafter in order to improve e.g. the machinability or the hot working properties, such as the hot ductility. Example, but not limiting, of such elements are As, Ca, Co, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce. The amounts of one or more of these elements are of max 0.5 wt%, such as max 0.1 wt%.
According to one embodiment, the present object comprises an duplex stainless steel consisting of all the elements mentioned hereinabove or hereinafter. When the terms "max" or "less than or equal to" are used, the skilled person knows that the lower limit of the range is 0 wt% unless another number is specifically stated.
The remainder of elements of the duplex stainless steel as defined hereinabove or hereinafter is iron (Fe) and normally occurring impurities. Examples of impurities are elements and compounds which have not been added on purpose, but cannot be fully avoided as they normally occur as impurities in e.g. the raw material or the additional alloying elements used for manufacturing of the duplex stainless steel. According to one embodiment, the duplex stainless steel consists of the alloying elements and ranges mentioned above.
The step providing a body of the duplex stainless steel as defined hereinabove or hereinafter may include providing a melt of said duplex stainless steel and casting said melt in order to obtain a body of the duplex stainless steel as defined hereinabove or hereinafter. The casting may include continuous casting of the melt. As a result of the method steps being used, alternating layers of ferrite and austenite will, as mentioned before, be seen in the object, said layers being essentially parallel with the plane of the object. The thickness of the layers will affect the proof stress of the product. In order to obtain sufficient proof stress, the average FCC thickness and the BCC thickness of each layer should be less than or equal to about 4.5 μπι. According to other embodiment, the thickness of each layer is between 0.01 to about 4.5 μπι, such as about 0.5 to about 4.5 μπι, such as about 1.0 to about 4.5 μπι, such as about 1.0 to about 4.2 μπι, such as 2.0 to 4.2 μπι. The thickness of the product in its final cold worked condition (after the last cold worked step) may be of from 20 μπι up to 5 mm. Before being cold worked, the body comprising the duplex stainless steel as defined hereinabove or hereinafter is subjected to hot working, in which, according to one embodiment, the thickness of the body is reduced from about 100 to 200 mm to 2-15 mm. The thickness measurement of the BCC and FCC phases respectively, is performed by taking a perpendicular cross-section of the object (the strip, sheet or wire) and then polishing and etching in acid (such as HNO3) to obtain a contrast between the two phases. The measurement is then performed in a light optical microscope using a suitable magnification (100-1000 times) so that each phase is visible and so that a large enough number of phase boundaries can be counted to obtain a reasonable statistical certainty (more than 30 phase boundaries). An appropriate cross-sectional position for the measurement in a wire is at 25% of its diameter. A strip or sheet should be measured 25% of the width away from the edge, at the thickness center. The thickness of each BCC and FCC phase, in the diameter direction of a wire or along the thickness direction of a strip or sheet, is measured and from this the average BCC and FCC thickness respectively is calculated.
The present disclosure is further illustrated by the following non-limiting examples. Examples
Alloys having the chemical composition as seen in Table 1 were melted and casted to 1 kg ingots. After melting and casting, the obtained ingots were hot rolled to strips at a temperature of about 1250 °C using 9 rolling passes. The samples were reheated 3 times during the hot rolling in order to keep the temperature above 1050°C. The final thickness of the strips varied from 3.7 to 4.0 mm.
The hot rolled strips were then cold rolled until a cold reduction of about 75% was obtained. 5 passes were used in the cold rolling mill.
The ferrite content was determined by using magnetic scale measurements. The magnetic scale measurement was performed according to IEC 60404-1. The content of magnetic phase was assumed to equal the ferrite content and the remainder was assumed to be austenite. The values are found in Table 2.
In order to measure the thickness measurement of the BCC and FCC phases, a sample was taken at a perpendicular cross-section of the strip at 25% of the width away from the edge and then the sample was polished and etched (1 M HNO3). The measurement was performed in a light optical microscope (Nikon) using a suitable magnification (1000 times), i.e. each phase was visible and more than 30 phase boundaries were seen. The thickness of each BCC and FCC phase was measured along the thickness direction and the average BCC and FCC thickness respectively was calculated. The obtained values are shown in table 2. The strength of the cold rolled strip was determined by tensile tests according to SS EN ISO 6891-1 in the rolling direction. Two tensile test specimens were water jet cut from each cold rolled strip specimen. The results are collected in table 3. As can be seen from the table, all samples had good tensile strength. Table 1: Chemical composition of the samples - all values are given in weight (wt%).
Table 2 Phase thickness of the samples and the phase content of the samples
8 2.3 1.5 59 41
9 2.8 2.4 58 42
10 2.9 1.7 72 28
11 4.2 2.7 68 32
12 3.3 1.8 71 29
13 3.2 2.5 66 34
14 3.6 2.0 69 31
15 3.8 2.4 66 34
Table 3. Tensile test results
As can be seen, the obtained objects will have a proof stress Rpo.2 above 1200 MPa after cold rolling thereof. The pitting corrosion resistance of the experimental grades, was assessed using the PRE formula (defined as PRE=Cr+3.3Mo+16N) as described earlier. By entering the total compositions (from table 1) in Thermo-Calc®, the equilibrium compositions in each of the phases at the specific BCC content (from table 2) could be deduced and thereby the PRE of each phase could be calculated according to table 4.
Table 4 Composition and pitting corrosion resistance of each of the two phases
Hence, as can be seen from the experiments above, the objects of the present disclosure will have high yield strength in combination with good ductility and also good corrosion resistance and high tensile strength due to the solid solution strengthening and deformation hardening, the phase thickness and content.
As can be seen from the results above, when the average phase thickness is below 4.5 μπι, the extremely fine microstructure (shown in table 5), which is obtained in the present cold worked duplex stainless steel, will have an impact on the mechanical properties.

Claims

1. An object manufactured from a duplex stainless steel, wherein the duplex
stainless steel comprises the following composition, in weight :
C less than or equal to 0.040;
Si less than or equal to 0.60;
Mn 0.80 - 10.0;
Cr 21.0-28.0;
Ni 4.0-9.0;
Mo 0.9 - 4.5;
N 0.10-0.45;
Cu less than or equal to 0.50;
V less than or equal to 0.10;
P less than or equal to 0.010;
s less than or equal to 0.006;
balance Fe and unavoidable impurities;
and wherein the duplex stainless steel consists of 55-75 vol% austenite phase and 25-45 vol% ferrite phase;
and wherein the object has alternating layers of ferrite phase and austenite phase, said alternating layers are essentially parallel with the plane of the object and said alternating layers have an average layer thickness less than or equal to about 4.5 μπι.
The object according to claim 1, wherein the duplex stainless steel has a PRE greater than 28 and wherein PRE is defined as PRE=Cr+3.3Mo+16N.
The object according to claim 1 or 2, wherein the duplex stainless steel consists of, 55-70 vol% austenite phase and 30-45 vol% ferrite phase, such as 65-70 vol% austenite phase and 30-35 vol% ferrite phase.
4. The object according to any one of claims 1 to 3, wherein the average ferrite or austenite thickness is between about 0.01 to about 4.5 μπι, such as about 0.5 to about 4.5 μπι, such as about 1.0 to about 4.5 μπι, such as about 1.0 to about 4.2 μπι.
5. The object according to any one of claims 1 to 4, wherein the content of Mn in the duplex stainless steel is in the range of from 2 to 5 wt%.
6. The object according to any one of claims 1 to 5, wherein the content of N in the duplex stainless steel is in the range of from 0.3 to 0.42 wt%.
7. The object according to any one of claims 1 to 6, wherein the content of Mo in the duplex stainless steel is in the range of from 2 to 4 wt%.
8. The object according to any one of claims 1 to 7, wherein the content of Cr in the duplex stainless steel is in the range of from 24 to 28 wt%, such as 26 to 28 wt%.
9. The object according to any one of claims 1 to 8, wherein the content of Ni in the duplex stainless steel is in the range of from 7.0 to 9.0 wt%.
10. The object according to any one of claims 1 to 9, wherein said object is a sheet or a strip or a wire.
11. A spring comprising the object according to any one of claims 1 to 10.
12. A method for manufacturing an object according to any one of claims 1 to 11, comprising the steps of:
- providing a body of the duplex stainless steel as defined in anyone of claims 1 to 10; -one or more hot working processes to transform the body to a workpiece and the hot working processes are performed at a temperature of about 1050 to about 1300°C;
- one or more cold working processes to transform the workpiece into the object.
wherein the final step of said method must be a cold working process. 13. The method according to claim 12, wherein the hot working process is hot rolling.
14. The method according to claim 12 or claim 13, wherein the cold working process is cold rolling.
15. The method according to any one of claims 12 to 14, wherein the method also comprises one or more) a heat treatment steps wherein the one or more heat treatment step is annealing which is performed at a temperature of above 1000 to 1250°C.
16. The method according to any one of claims 12 to 15, comprising a further step of aging the object for 0.25 to 4 hours at a temperature of from 400 to 450°C wherein the aging is performed after the final cold working step.
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CN1201028C (en) * 2001-04-27 2005-05-11 浦项产业科学研究院 High manganese deplex stainless steel having superior hot workabilities and method for manufacturing thereof
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JP5546178B2 (en) * 2008-08-04 2014-07-09 新日鐵住金ステンレス株式会社 Ferritic / austenitic stainless steel with excellent ingot crack resistance and workability and method for producing the same
CN102605288B (en) * 2012-03-13 2015-03-25 宝山钢铁股份有限公司 Economical double-phase stainless steel with good welding property and manufacturing method for stainless steel
JP6302722B2 (en) * 2014-03-31 2018-03-28 新日鐵住金ステンレス株式会社 High-strength duplex stainless steel wire excellent in spring fatigue characteristics, its manufacturing method, and high-strength duplex stainless steel wire excellent in spring fatigue characteristics
WO2016162525A1 (en) * 2015-04-10 2016-10-13 Sandvik Intellectual Property Ab A method of producing a tube of a duplex stainless steel

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CN110088323A (en) 2019-08-02
CN110088323B (en) 2022-03-22

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