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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
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  • 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
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  • 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
  • C21D8/0247—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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
  • C21D2211/002—Bainite
• • 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

Definitions

• the invention relates to a hot rolled steel strip or sheet and manufacturing method thereof.
• micro-alloyed hot-rolled strips having yield strength up to 700 MPa are typically produced on the hot wide strip mill (HWSM) at moderate cooling rates of ⁇ 20-30°C/s, from the hot rolling finish temperature to cooling temperatures of ⁇ 550-600°C.
• HWSM hot wide strip mill
• the microstructure of such steels typically contains ferrite, coarse cementite and perlite.
• the global ductility is good, e.g. elongation.
• the local ductility is worse, e.g. formability of punched edges: quantifiable by means of Hole Expansion Ratio HER.
• micro-alloyed hot-rolled strip With conventional micro-alloyed hot-rolled strip, complex forming on stamped edges often cannot be realised. There is a need for a micro-alloyed hot-rolled strips having yield strength up to 700 MPa which have an improvement in the formability of punched edges (HER).
• EP3715491A1 disclose a hot rolled sheet and a manufacturing method thereof.
• the tensile strength is 440 MPa or higher.
• the microstructure comprises 30 - 70% of a first ferrite and another structure of at least one among bainite and a second ferrite such that the total sum is 95 % or more.
• the average grain size of the first ferrite is less than 5 ⁇ m and the average gran size of the other structure is less than 10 ⁇ m.
• the hot rolling finishing temperature is below Ae3.
• EP3516085 B1 disclose a method of manufacturing a high-strength hot-rolled steel strip.
• the tensile strength is at least 570 MPa, preferably 780 MPa or higher.
• the method includes cooling the hot-rolled steel strip with a primary cooling rate between 50 to 150 °C/s to an intermediate temperature between 600 and 720 °C and coiling between 580 and 660 °C.
• the steel has a substantially single-phase ferritic microstructure.
• the strip or sheet has a composition consisting of the following alloying elements (in wt. %): C 0.03 - 0.12 Mn 0.3 - 2.2 Al 0.01 - 0.1 V 0.0001 - 0.08 Nb 0.01 - 0.10 Ti 0.001 - 0.2 Si ⁇ 0.60 Cr ⁇ 0.4 Ni ⁇ 0.25 Mo ⁇ 0.2 Cu ⁇ 0.4 balance Fe apart from impurities
• composition is excluding any coatings applied to the strip or sheet.
• C carbon
• Fe3C cementite
• Mn manganese
• the lower limit may be set to 0.3, 0.35, or 0.4 %.
• the upper limit may be set to 2.2, 2.1, 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, or 1.0 %.
• a preferred range is 0.35 - 2.1 %, a more preferred range is 0.35 - 1.6 %.
• Al has an action of deoxidizing molten steel in a refining process of the steel to sound the steel.
• Al further has, similarly to Si, an action of suppressing the precipitation of the iron-based carbides such as cementite and thereby improving the strength and Hole expansion ratio.
• An upper limit of 0.1 % is set to suppress the generation of a non-metallic inclusion in the steel thereby improving the ductility and the low-temperature toughness.
• the lower limit may be set to 0.01 or 0.02 %.
• the upper limit may be set to 0.1, 0.08, or 0.06 %.
• a preferred range is 0.02 - 0.08 %.
• V 0.0001- 0.08 %
• V vanadium
• the upper limit may be set to 0.08, 0.07, 0.06, or 0.05 %.
• the lower limit may be set to 0.0001 or 0.001 %.
• Nb(niobium) improves the strength of the steel sheet by precipitation strengthening or solid-solution strengthening. Furthermore, the addition of Nb can lead to a finer initial structure (the prior austenite grain size). A finer prior austenite grain size can increase the elongation at fracture and the Hole expansion ratio.
• the upper limit may be set to 0.10, 0.09, 0.08, 0.07, or 0.06 %.
• the lower limit may be set to 0.01 or 0.02 %.
• a preferred range is 0.02 - 0.06 %.
• Ti can improve the strength of the steel sheet by precipitation strengthening or solid-solution strengthening.
• the addition of Ti can lead to a finer initial structure (the prior austenite grain size).
• a finer prior austenite grain size can increase the elongation at fracture and the Hole expansion ratio.
• too much Ti can cause coarse titanium nitrides, which are bad for the Hole expansion ratio.
• the upper limit of Ti may be set to 0.2, 0.15, 0.1, 0.05, 0.01, or 0.005 %.
• the lowest amount of Ti may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %.
• Si is an element having an action of improving the strength of the steel sheet, but also functions as a deoxidizer in the steel melt.
• the upper limit may be 0.6, 0.5, 0.4,0.3, 0.25, 0.2, 0.15, 0.1, 0.08 %.
• a lower limit may be 0.01 %.
• a deliberate addition of Si is not necessary according to the present invention.
• Cr(chromium), Mo (molybdenum) and Ni (nickel) can improve the strength of the steel sheet by solid-solution strengthening or quench-hardening strengthening. Cr, Mo, and Ni affect the Ac3 temperature, and they also promote transformation behaviour, and facilitate obtaining a martensitic microstructure. Too much of Cr, Mo, and Ni can affect the Hole expansion ratio and the forming properties negatively.
• the upper limit of Cr may be set to 0.4, 0.3, 0.2, 0.15, 0.1, or 0.05%.
• the lowest amount of Cr may be set to 0.001, 0.005, 0.01, 0.02 or 0.03 %.
• a deliberate addition of Cr is not necessary according to the present invention.
• the upper limit of Ni may be set to 0.25, 0.2, 0.15, 0.1, or 0.05%.
• the lowest amount of Ni may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %.
• a deliberate addition of Ni is not necessary according to the present invention.
• the upper limit of Mo may be set to 0.2, 0.15, 0.1, 0.05, 0.01 %.
• the lowest amount of Mo may be set to 0.001, or 0.002%.
• a deliberate addition of Mo is not necessary according to the present invention.
• Cu improves scale removal and thus has an influence on a better surface quality. If the steel is made from scrap, as is common when using an electric arc furnace, Cu can be up to 0.4 %.
• the upper limit of Cu may be set to 0.4, 0.3, 0.2, 0.15, 0.1, or 0.05 %.
• the lowest amount of Cu may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %.
• a deliberate addition of Cu is not necessary according to the present invention.
• V+ Nb + Ti ⁇ 0.2 %
• the combined content of Ti, Nb and V is preferably restricted to ⁇ 0.2%, more preferably ⁇ 0.15%. Higher amounts of these elements are not necessary to achieve the desired properties of the steel.
• N nitrogen
• nitrogen is an element generally contained as an impurity.
• the N content is set to 0.015 % or less, preferably 0.10 % or less.
• a nominal amount may be in the range of 0.001 - 0.008 %.
• P phosphorus
• P is an element generally contained as an impurity.
• P causes cracking in the hot rolling, and is segregated at a grain boundary to decrease the low-temperature toughness and also decrease the workability and the weldability. Therefore, the P content is set to 0.10 % or less.
• the upper limit may be set to 0.10, 0.05, 0.04, 0.03 or 0.02 %.
• S is an element generally contained as an impurity.
• S content is set to 0.010 or less.
• the upper limit may be set to 0.010, 0.05, 0.03, 0.01, 0.005 or 0.001 %
• Other impurity elements may be comprised in the steel in normal occurring amounts.
• Oxygen and hydrogen can further be limited to
• the elements should preferably be balanced such that a parameter Q expressed by the relation below is less than 30, preferably less than 20, more preferably less than 10.
• Q Ti / 48 / S / 32 wherein [Ti] indicates the Ti content (mass%) and [S] indicates the S content (mass%).
• the steel should fulfil the following conditions: YS, Yield strength (R p0.2 ) 250 - 700 MPa, preferably 300 - 650 MPa, more preferably 300 - 600 MPa HER, Hole expansion ratio ( ⁇ ) ⁇ 70 %, preferably 80 - 200 %
• TS Tensile strength (R m ) 300 - 900 MPa, preferably 400 - 850 MPa, TE, Total elongation ⁇ 15 %, preferably 20 - 40 %
• TS, YS, TE are examples of properties related global ductility.
• HER is a property related to local ductility.
• the R m , R p0.2 values are derived in accordance with the Industrial Standard DIN EN ISO 6892-1:2019, wherein the samples are taken in the longitudinal direction of the strip.
• the Total Elongation is determined according to EN10149-2 2013 (D).
• D The Total Elongation is determined according to EN10149-2 2013 (D).
• t ⁇ 3mm A 80mm used, and from t ⁇ 3mm A 5 (proportional measuring length related to the sample cross-section) is used.
• the Hole expansion ratio ( ⁇ ) is determined by the Hole expansion test according to ISO 16630:2017. In this test a conical punch having an apex of 60 ° is forced into a 10 mm diameter punched hole made in a steel sheet having the size of 100 ⁇ 100 mm 2 . The test is stopped as soon as the first crack is determined, and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean value is used for the calculation.
• the lower limit of the Hole expansion ratio ( ⁇ ) can be 70, 80, 90 or 100 %.
• the upper limit may be 150 or 200 %.
• the strip or sheet thickness of the final product is 2.0 - 6.5 mm, preferably 2.5 - 5 mm, more preferably 3-4 mm.
• the strip or sheet width may be 500 - 2000 mm, preferably 700 - 1750 mm in non-slit condition.
• the microstructure of the steel contains the main phases ferrite and bainite. High elongation is associated with ferrite, and bainite is associated with good edge stretchability expressed as HER.
• the steel balance strength and formability and is suitable for applications requiring improved edge stretch capability.
• microstructural constituents are in the following expressed in volume % (vol. %).
• the phases a) or b) in the list above may balance the microstructure.
• Bainite represents a carbon-rich second phase of the present steel.
• Bainite can be upper bainite and/or lower bainite and/or granular bainite.
• Quasi polygonal ferrite looks similar to granular bainite (irregular, undulating grain boundaries and a dislocation substructure) and is therefore included in the range.
• Pearlite and/or cementite may be present in lesser amounts.
• the content of pearlite and/or cementite can be 0%.
• Retained austenite should preferably not be present but may be present at smaller amount.
• the content may be 0%.
• the microstructure including the amount of each phase, can be identified in scanning electron microscope (SEM) using 20000 times magnification. Preferably by cutting out a sample from a steel plate and polishing a cross section of a plate parallel to the rolling direction. The microstructure should be taken from 1 ⁇ 4 of the thickness. The surface can be etched to make the phases easier to identify.
• SEM scanning electron microscope
• the amount of retained austenite can e.g. be determined by means of the saturation magnetization method such as described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61 - 64 .
• the coiled strip can be produced by the following steps:
• the cooling rate of 40 - 60 °C/s in the temperature range from 800 to 600 °C and the coiling temperature between Bs -200 °C and Bs -100 °C facilitates a ferritic matrix without coarse cementite and without or at most small amounts of pearlite. If the cooling rate is too high and/or the coiling temperature is too low, martensite may form.
• the coiling temperature is between Bs - 175 °C and B S - 125 °C.
• Table 1 Steel C Si Mn P S Al Cr Ni Mo Cu V Nb Ti V+Nb+Ti N 1 0.039 0.024 0.384 0.0089 0.0015 0.044 0.025 0.012 0.003 0.011 0.003 0.024 0.001 0.028 0.0034 2 0.059 0.013 0.421 0.0081 0.0019 0.048 0.023 0.011 0.003 0.009 0.003 0.03 0.001 0.034 0.0044 3 0.065 0.017 0.521 0.0086 0.011 0.054 0.032 0.021 0.005 0.034 0.002 0.036 0.003 0.041 0.0031 4 0.071 0.016 0.8 0.011 0.0019 0.043 0.032 0.013 0.005 0.014 0.003 0.039 0.001 0.043 0.0027 5 0.078 0.018 0.
• the steels #1-11 were reheated to a reheating temperature TRH at a heating rate of about 0.1 °C/s.
• TRH reheating temperature
• the reheated samples where thereafter hot rolled in austenitic range to a hot rolled strip.
• the hot rolling finishing temperature FRT ranged from 887 °C to 929 °C and was above Ae3.
• the hot rolled strip were thereafter cooled and coiled at a coiling temperature CT.
• the inventive steels were cooled at a temperature in the range 40 to 60 °C/s between 600 and 800 °C whereas the comparative steels were cooled at lower cooling rates. By cooling the strip faster in the range 600 to 800 °C, the formation of coarse cementite and perlite can be avoided or supressed.
• the inventive steels were all coiled at a coiling temperature within Bs -200 °C and Bs -100 °C, whereas the comparative examples were coiled at temperatures closer to Bs.
• the coiling temperatures CT for the inventive steels were chosen to avoid or supress formation of coarse cementite and perlite and to achieve a high fraction of bainite in the microstructure. After coiling phase transformation is finished for these steels.
• the mechanical properties of the produced strips #1-11 were determined using the measurement methods defined in the description. Mechanical properties are shown in Table 2. As can be seen, the inventive steels #1, 2, 4, 7, 9, and 11 met all mechanical criterions, particular in having a Tensile strength YS of 250 - 700 MPa while having a Hole expansion ratio HER ⁇ 70 %. However, the comparative examples failed to meet the requirements of the Hole expansion ratio HER.
• microstructures of the inventive steels all fulfilled in vol. %: polygonal ferrite 20 - 55 bainite and quasi polygonal ferrite 45 - 80 pearlite ⁇ 1 cementite ⁇ 3 retained austenite ⁇ 1
• Specifically inventive steel 11 had the following microstructure in vol. %: polygonal ferrite 23 bainite and quasi polygonal ferrite 76 pearlite 0 cementite 1
• Fig. 1 is metallographic image of steel 11.
• the comparative steel 10 had the following microstructure in vol. %: polygonal ferrite 90 bainite and quasi polygonal ferrite 0 pearlite 9 cementite 1
• Fig. 2 is metallographic image of steel 10.

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• 2024
  • 2024-06-14 EP EP24182350.9A patent/EP4663804A1/fr active Pending
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