EP3645763A1 - Rétention de propriétés mécaniques dans des alliages d'acier après traitement et en présence de sites de concentration de contraintes - Google Patents

Rétention de propriétés mécaniques dans des alliages d'acier après traitement et en présence de sites de concentration de contraintes

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Publication number
EP3645763A1
EP3645763A1 EP18824168.1A EP18824168A EP3645763A1 EP 3645763 A1 EP3645763 A1 EP 3645763A1 EP 18824168 A EP18824168 A EP 18824168A EP 3645763 A1 EP3645763 A1 EP 3645763A1
Authority
EP
European Patent Office
Prior art keywords
alloy
mpa
sheet
thickness
atomic percent
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.)
Withdrawn
Application number
EP18824168.1A
Other languages
German (de)
English (en)
Other versions
EP3645763A4 (fr
Inventor
Daniel James Branagan
Alla Sergueeva
Brian E. Meacham
Andrew E. Frerichs
Sheng Cheng
Scott Larish
Grant G. Justice
Andrew T. Ball
Jason K. Walleser
Logan J. TEW
Scott T. ANDERSON
Kurtis Clark
Taylor L. GIDDENS
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.)
United States Steel Corp
Original Assignee
Nanosteel Co Inc
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 Nanosteel Co Inc filed Critical Nanosteel Co Inc
Priority to EP21206699.7A priority Critical patent/EP3971313A1/fr
Priority to EP21206702.9A priority patent/EP3971314A1/fr
Publication of EP3645763A1 publication Critical patent/EP3645763A1/fr
Publication of EP3645763A4 publication Critical patent/EP3645763A4/fr
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B2001/225—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by hot-rolling

Definitions

  • This disclosure is related to retention of mechanical properties in high strength steel at reduced thicknesses and which mechanical property performance is also retained at relatively high strain rates. These new steels can offer advantages for a myriad of applications where reduced sheet thickness is desirable.
  • the alloys herein are those that retain useful mechanical properties after introduction of a geometric discontinuity and an accompanying stress concentration.
  • LSS Low Strength Steels
  • HSS High Strength Steels
  • AHSS Advanced High Strength Steels
  • Advanced High Strength Steels are of primary interest for advanced engineering applications, and are classified by tensile strengths greater than 700 MPa and include such types as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, and complex phase (CP) steels.
  • MS martensitic steels
  • DP dual phase
  • TRIP transformation induced plasticity
  • CP complex phase
  • tensile elongation of LSS, HSS and AHSS ranges from 25% to 55%, 10% to 45%, and 4% to 30%, respectively.
  • the body-in-white structure of an automobile is designed to provide a rigid structure that will protect the passenger while traveling at speed and in the case of a collision.
  • dynamic loading, rapid deformation, and energy dissipation occurs throughout the automobile and body-in- white structure in particular.
  • the time frame over which this occurs can be 100 ms. High strain rates are observed throughout the body-in-white structure during this time, and materials need to be able to withstand complex loads across a range of strain rates.
  • a low speed collision that occurs in a parking lot would result in a lower strain rate for body-in- white than would a collision at highway speeds.
  • the mechanical properties of materials for the body-in-white structure are measured by many means, including uniaxial tensile testing, across this range of strain rates such that their response during a collision can be predicted and design considerations taken into account.
  • High strain rates can result in a change in mechanical properties, limiting the maximum lightweighting that automobile designers are able to achieve by requiring additional thickness to maintain safety under high strain rate conditions.
  • Martensitic steels provide excellent strength yet require a quench as a final processing step to create the necessary micro structure. Quenching is difficult to control at a small scale and may potentially cause unacceptable distortion in small parts. Final processing may not be performed on the final part geometry but rather on sheet or foils in some applications. For thermally sensitive materials such as martensitic steels, thermal exposure during cutting to produce the final part may detrimentally alter the micro structure and compromise properties. Geometry effects also play a greater role in mechanical properties of ductile materials at the small scale, with the effects of stress concentrators, grain size, and thickness adversely changing the material's mechanical response to stress.
  • the present invention is directed at a method to retain mechanical properties in a metallic sheet alloy at reduced thickness comprising supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C, melting said alloy, cooling at a rate of ⁇ 250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm. This is followed by processing the alloy into sheet form with thickness Ti with the sheet having a total elongation of Xi (%), an ultimate tensile strength of Yi (MPa), and a yield strength of Zi (MPa).
  • the present invention relates to a method to retain mechanical properties in a metallic sheet alloy at relatively high strain rates comprising supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C and melting said alloy and cooling at a rate of ⁇ 250 K/s and solidifying to a thickness of 25.0 mm up to 500 mm. This is then followed by processing the alloy into sheet form with thickness from 1.2 mm to 10.0 mm with the sheet having a total elongation of Xi (%), an ultimate tensile strength of Yi (MPa), and a yield strength of Zi (MPa) when tested at a strain rate Si.
  • the present invention is directed at A method to retain mechanical properties in a metallic sheet alloy
  • a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C and melting said alloy and cooling at a rate of ⁇ 250 K/s and solidifying to a thickness of 25.0 mm up to 500 mm.
  • This is then followed by processing the alloy into sheet form with thickness from 1.2 mm to 10.0 mm with the sheet having a total elongation of Xi (%), an ultimate tensile strength of Yi (MPa), and a yield strength of Zi (MPa).
  • FIG.s are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
  • FIG. 1 Summary of novel ductility achievement in alloys herein at reduced length scales.
  • FIG. 2 Summary of novel ductility achievement in the alloys herein at high strain rates.
  • FIG. 3 Summary of maintained ductility in the alloys herein with introduced stress concentration sites such as edge notches.
  • FIG. 4 Yield strength and ultimate tensile strength as a function of Alloy 2 sheet thickness.
  • FIG. 5 Tensile elongation as a function of Alloy 2 sheet thickness.
  • FIG. 6 Comparison of stress - strain curves for Alloy 2 sheet with different thicknesses.
  • FIG. 7 Effect of sheet thickness on tensile elongation of samples from various alloys.
  • FIG. 8 Effect of sheet thickness on yield strength in samples from various alloys.
  • FIG. 9 Effect of sheet thickness on ultimate tensile strength in samples from various alloys.
  • FIG. 10 SEM images of the microstructure in the center of Alloy 1 sheet samples with various thicknesses; a) 0.7 mm thick cold rolled sheet, b) 0.7 mm thick cold rolled and annealed sheet, c) 0.5 mm thick cold rolled sheet, and d) 0.5 mm thick cold rolled and annealed sheet.
  • FIG. 11 SEM images of the microstructure in the center of Alloy 2 sheet samples with various thicknesses; a) 1.0 mm thick cold rolled sheet, b) 1.0 mm thick cold rolled and annealed sheet, c) 0.5 mm thick cold rolled sheet, d) 0.5 mm thick cold rolled and annealed sheet, e) 0.2 mm thick cold rolled sheet, and f) 0.2 mm thick cold rolled and annealed sheet.
  • FIG. 12 SEM images of the microstructure in the center of Alloy 27 sheet samples with various thicknesses; a) 0.8 mm thick cold rolled sheet, b) 0.8 mm thick cold rolled and annealed sheet, c) 0.5 mm thick cold rolled sheet, d) 0.5 mm thick cold rolled and annealed sheet, e) 0.4 mm thick cold rolled sheet, and f) 0.4 mm thick cold rolled and annealed sheet.
  • FIG. 13 SEM images of the microstructure in the center of Alloy 37 sheet samples with various thicknesses; a) 1.4 mm thick cold rolled sheet, b) 1.4 mm thick cold rolled and annealed sheet, c) 0.5 mm thick cold rolled sheet, d) 0.5 mm thick cold rolled and annealed sheet, e) 0.3 mm thick cold rolled sheet, and f) 0.3 mm thick cold rolled and annealed sheet.
  • FIG. 14 Schematic illustration of the ASTM D 638 Type V tensile specimen geometry; all dimensions are in millimeters.
  • FIG. 15 Schematic diagram of the direct tension split Hopkinson bar (SHB) device.
  • FIG. 16 Effect of strain rate on the tensile elongation at fracture for Alloy 2 sheet.
  • FIG. 17 Bright-field TEM micrographs of microstructure in gauge section of the sample from
  • FIG. 18 Bright- field TEM micrographs of microstructure in gauge section of the sample from
  • FIG. 19 Bright- field TEM micrographs of micro structure in gauge section of the sample from Alloy 2 sheet tested at strain rate of 100 s "1 ; a) lower magnification image, b) higher magnification image.
  • FIG. 20 Bright-field TEM micrographs of micro structure in gauge section of the sample from
  • FIG. 21 Bright- field TEM micrographs of micro structure in gauge section of the sample from
  • FIG. 22 Bright-field TEM micrographs of micro structure in gauge section of the sample from
  • FIG. 23 Feritscope measurements at the gauge section of the samples from Alloy 2 sheet tested at different strain rates.
  • FIG. 24 Schematic illustration of the notched tensile sample.
  • FIG. 25 Notch diameter with a constant depth of 0.5 mm effect; a) on tensile elongation and b)
  • FIG. 26 Half circle notch diameter effect; a) on tensile elongation and b) on ultimate tensile strength of the sheet from Alloy 2.
  • FIG. 27 SEM images of the fracture surface in the Sample 1 from Alloy 2 with a notch of 1 mm in diameter; a) in the center of the fracture cross section, b) near the edge of the fracture cross section.
  • FIG. 28 SEM images of the fracture surface in the Sample 2 from Alloy 2 with a notch of 6 mm in diameter; a) in the center of the fracture cross section, b) near the edge of the fracture cross section.
  • FIG. 1 represents a summary on mechanical property retention in the alloys herein when reduced in thickness.
  • the starting condition is to supply a metal alloy.
  • This metal alloy will preferably comprise at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C.
  • the alloy chemistry is melted, cooled at a rate of ⁇ 250 K/s, and solidified to a thickness of 25.0 mm and up to and including 500 mm.
  • the casting process can be done in a wide variety of processes including ingot casting, bloom casting, continuous casting, thin slab casting, thick slab casting, thin strip casting, belt casting etc.
  • Preferred methods would be continuous casting in sheet form by thin slab casting, thick slab casting, and thin strip casting.
  • Preferred alloys exhibit a fraction of austenite ( ⁇ -Fe) at least 10 volume percent up to 100 volume percent and all increments in between.
  • the alloy is then processed into sheet form with a thickness Ti that is in the range of 1.2 mm to 10.0 mm, and therefore includes thicknesses of 1.2 mm, 1.3 mm. 1.4 mm 1.5 mm.
  • the steps to produce this sheet at thickness Ti from the cast product can vary depending on specific manufacturing routes and specific targeted goals.
  • thick slab casting as one process route to get to sheet of this targeted thickness.
  • the alloy would be cast going through a water cooled mold typically in a thickness range of 150 to 300 mm in thickness.
  • the cast ingot after cooling would then be preferably prepared for hot rolling which may involve some surface treatment to remove surface defects including oxides.
  • the ingot would then go through a roughing mill hot roller which may involve several passes resulting in a transfer bar slab typically from 15 to 100 mm in thickness. This transfer bar would then go through successive / tandem hot rolling finishing stands to produce hot band coils which have a thickness Ti in the above referenced range from 1.2 mm to 10.0 mm.
  • Another example would be to preferably process the cast material through a thin slab casting process.
  • the newly formed slab goes directly to hot rolling without cooling down with auxiliary tunnel furnace or induction heating applied to bring the slab directly up to targeted temperature.
  • the slab is then hot rolled directly in multi-stand finishing mills which are preferably from 1 to 7 in number.
  • the strip is rolled into hot band coils with thickness Ti in the above referenced range of 1.2 mm to 10.0 mm in thickness.
  • bloom casting would be similar to the examples above but higher thickness might be cast typically from 200 to 500 mm thick and initial breaker steps would be needed to reduce initial cast thickness to allow it to go through a hot rolling roughing mill.
  • Strip casting would be similar but lower thickness might be cast of Ti having a value of 1.2 mm to 10.0 mm in thickness with preferably only one hot rolling stand directly after casting.
  • the specific process in going from the slab material in Step 1 to a preferred thickness Ti of 1.2 mm to 10 mm and then in Step 2 to a preferred thickness in the range of 0.2 mm to less than 1.2 mm may include hot rolling, cold rolling, and/or cold rolling followed by annealing.
  • the alloy thickness may preferably be 0.2 mm, 0.3 mm, 0.4 mm. 0.5 mm. 0.6 mm. 0.7 mm. 0.8 mm. 0.9 mm, 1.0 mm 1.1 mm up to by not including 1.2 mm.
  • Hot rolling is generally used to provide a preferred thickness from 1.2 mm to 10.0 mm and is typically done in roughing mills, finishing mills, and/or Steckel mills.
  • Cold rolling is preferred in Steps 1 and/or Step 2 and is generally done using tandem mills, Z-mills, and/or reversing mills.
  • the cold rolled material depending on property targets may be annealed to restore the ductility lost from the cold rolling process either partially or with restoration of ductility.
  • ductility is reduced and cold rolling will continue until or just before cracking is observed.
  • Restoration of the tensile ductility of the cold rolled sheet generally occurs with heat treatments at 700°C and above. Once the sheet is formed with thickness Ti specified in Step 2, the sheet will then exhibit a total elongation of Xi (%), an ultimate tensile strength of Yi (MPa), and a yield strength of Zi (MPa).
  • Preferred properties for alloys herein in Step 2 would be tensile elongation from 12 to 80%, ultimate tensile strength values from 700 to 2100 MPa, and yield strength is in a range from 250 to 1500 MPa.
  • Step 3 the alloy is preferably cold rolled and annealed in similar manner as in Step 2 to thickness T 2 ⁇ Ti.
  • the thickness of the alloy in Step 3 is identified as T 2 and is less than the thickness Ti in Step 2.
  • FIG. 2 shows a summary on ductility retention of the present disclosure in the alloys herein at relatively high strain rates, that is where the alloys experience a strain rate of S 2 of >0.007 to 1200 s "1 .
  • Step 1 and Step 2 are identical to that described above in relation to FIG. 1.
  • the sheet will then exhibit a total elongation of Xi (%), an ultimate tensile strength of Yi (MPa), and a yield strength of Zi (MPa) when tested at strain rate Si, which is preferably at or below 0.007 s "1 and in the range from 0.007 to 0.0001 s "1
  • Preferred properties for this alloy would be tensile elongation from 12 to 80%, ultimate tensile strength values from 700 to 2100 MPa, and yield strength is in a range from 250 to 1500 MPa.
  • Alloys herein are also shown to avoid brittle fracture when stress concentration sites are introduced such as notches at the sheet edge.
  • a stress concentration site herein is a location on the alloy sheet where stress can be concentrated, including but not limited to geometric discontinuities, such as a notch, hole, cut in the surface, crack, chipped portion, dent, etc.
  • FIG. 3 shows a summary on how changes in mechanical properties are retained in the alloys herein with the introduction of stress concentration sites such as edge notches.
  • Preferred properties for this alloy would again be tensile elongation from 12 to 80%, ultimate tensile strength values from 700 to 2100 MPa, and yield strength is in a range from 250 to 1500 MPa.
  • the sheet that experiences a stress concentration is capable of exhibiting the following in response to a deformation: X 4 ⁇ 0.2Xi (%), an ultimate tensile strength Y 4 ⁇ 0.5Yi (MPa), and a yield strength Z 4 > O.6Z1 (MPa).
  • the preferred properties of the alloy in Step 3 are as follows: X 4 > 2.4%; Y 4 > 350 MPa, and Z 4 > 150 MPa.
  • the alloys herein comprise, consist essentially of, or consist of iron based metal alloys, having greater than 70 at. % Fe, and at least four or more elements selected from the following six (6) elements: Si, Mn, Cr, Ni, Cu, and C.
  • the level of impurities of other elements are in the range of 0 to 5000 ppm. Accordingly, if there is 5000 ppm of an element other than the selected elements identified, the level of such selected elements may then in combination be present at a lower level to account for the 5000 ppm impurity, such that the total of all elements present (selected elements and impurities) is 100 atomic percent.
  • the preferred levels of the elements may fall in the following ranges: Si (1.14 to 6.13), Mn (3.19 to 15.17), Cr (0.78 to 8.64); Ni (0.9 to 11.44), Cu (0.37 to 1.87), and C (0.67 to 3.68). Accordingly, it can be appreciated that if four (4) elements are selected, two of the six elements are not selected and may be excluded.
  • a particularly preferred level of Fe is in the range of 73.95 to 84.69 at. %.
  • the level of impurities of other elements are preferably controlled in the range of 0 to 5000 ppm (0 to 0.5 wt%).
  • Impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Al, Co, N, P, Ti, Mo, W, Ga, Ge, Sb, Nb, Zr, O, Sn, Ca, and S which if present would be in the range from 0 to 5000 ppm (parts per million) (0 to 0.5 wt%) at the expense of the desired elements noted above. Preferably, the level of impurities is controlled to fall in the range of 0 to 3000 ppm (0.3 wt%).
  • Step 1 in FIG. 1, FIG. 2 and FIG. 3 The melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure.
  • Laboratory casting corresponds to Step 1 in FIG. 1, FIG. 2 and FIG. 3 and provides slabs with thickness of 50 mm. Depending on equipment capability, slab thickness in Step 1 can vary from 25.0 to 500 mm.
  • a sample of between 50 and 150 mg from each alloy herein was taken in the as-cast condition. This sample was heated to an initial ramp temperature between 900°C and 1300°C depending on alloy chemistry, at a rate of 40°C/min. Temperature was then increased at 10°C/min to a max temperature between 1425°C and 1515°C depending on alloy chemistry. Once this maximum temperature was achieved, the sample was cooled at a rate of 10°C/min back to the initial ramp temperature before being reheated at 10°C/min to the maximum temperature.
  • DSC Differential Scanning Calorimetry
  • Laboratory Hot Rolling The alloys herein were preferably processed into a laboratory hot band by hot rolling of laboratory slabs at high temperatures.
  • Laboratory alloy processing is developed to simulate the hot band production from slabs produced by continuous casting.
  • Industrial hot rolling is performed by heating a slab in a tunnel furnace to a target temperature, then passing it through either a reversing mill or a multi-stand mill or a combination of both to reach the target gauge. During rolling on either mill type, the temperature of the slab is steadily decreasing due to heat loss to the air and to the work rolls so the final hot band is formed at a reduced temperature. This is simulated in the laboratory by heating in a tunnel furnace to between 1100°C and 1250°C, then hot rolling.
  • the laboratory mill is slower than industrial mills causing greater loss of heat during each hot rolling pass so the slab is reheated for 4 minutes between passes to reduce the drop in temperature, the final temperature at target gauge when exiting the laboratory mill commonly is in the range from 800°C to 1000°C, depending on furnace temperature and final thickness.
  • Tensile specimens were cut from laboratory hot band using wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. Samples were tested under displacement control at a constant displacement rate of 0.036 mm/s, which resulted in sample strain rates, calculated from video strain measurements, ranging from 4.4xl0 "4 s "1 to 6.8xl0 "3 s "1 , depending on several factors including, but not always limited to mechanical compliance, sample slippage, and settling of the wedge action grips used.
  • Tensile properties of the alloys in the hot rolled condition with a thickness from 1.8 to 2.3 mm are listed in Table 3 including magnetic phases volume percent (Fe%) that was measured by Feritscope.
  • the ultimate tensile strength values may vary from 913 to 2011 MPa with tensile elongation from 13.0 to 69.5%. The yield strength is in a range from 250 to 1313 MPa.
  • Mechanical properties of the hot band from steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions.
  • the relative magnetic phases volume percent was measured by Feritscope with the magnetic phases volume percent of 0.1 to 64.9 Fe% in a hot band depending on alloy chemistry. Note that the Table 3 properties correspond to Step 2 of FIG. 1, FIG. 2, and FIG. 3. Further processing of the hot band can additionally occur through cold rolling and annealing as shown for example in Case Example 1.
  • the hot band from alloys herein listed in Table 1 was cold rolled to final target gauge thickness of 1.2 mm through multiple cold rolling passes.
  • Cold rolling is defined as rolling at ambient temperature.
  • Hot band material was media blasted prior to cold rolling to remove surface oxides which could become embedded during the rolling process.
  • the resultant cleaned sheet material was rolled using a Fenn Model 061 2 high rolling mill. Sheet was fed through the rolls, and the roll gap is reduced for each subsequent pass until the desired thickness is achieved or the material hardens to the point where additional rolling does not achieve significant reduction in thickness. Annealing was applied before next rolling to recover ductility. Multiple cycles of cold rolling and annealing might be applied. Once the final gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM.
  • Tensile properties of 1.2 mm thick sheet from alloys herein after cold rolling are listed in Table 4.
  • the ultimate tensile strength values after cold rolling is in a range from 1360 to 2222 MPa; yield strength varies from 1006 to 2073 MPa and tensile elongation is recorded in the range from 4.2 to 37.2%.
  • the magnetic phases volume percent was measured by Feritscope in a range from 1.6 to 84.9 Fe% in a cold rolled sheet depending on alloy chemistry.
  • Table 4 Tensile Properties of 1.2 mm Thick Sheet from the Alloys After Cold Rolling
  • the samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process representing final treatment of sheet material in Step 2 in FIG. 1, FIG. 2 and FIG. 3.
  • Samples were loaded into a furnace preheated to 850°C, and held at temperature for 10 minutes, wrapped in foil and held under a steady argon flow to minimize oxidation damage. Samples were removed at temperature and allowed to air cool to ambient temperature before testing.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron' s Bluehill control and analysis software.
  • Tensile properties of 1.2 mm sheet from alloys herein after annealing are listed in Table 5.
  • the ultimate tensile strength values of the annealed sheet from alloys herein is in a range from 725 to 2072 MPa; yield strength varies from 267 to 1428 MPa and tensile elongation is recorded in the range from 12.8 to 76.9%.
  • the relative magnetic phases volume percent was measured by Feritscope with the magnetic phases volume percent of 0.2 to 68.2 Fe% depending on alloy chemistry.
  • Properties of cold rolled and annealed sheet from Alloys herein corresponds to Step 2 in FIG. 1, FIG. 2 and FIG. 3.
  • This Case Example demonstrates properties of the sheet material from alloys herein with thickness of 1.2 to 1.4 mm and tested at strain rates from 4.4xl0 "4 s "1 to 6.8xl0 "3 s "1 .
  • the hot band from Alloy 2 was cold rolled into sheets with different thicknesses through multiple cold rolling passes. Once the targeted gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process. Samples were wrapped in stainless steel foil to prevent oxidation and loaded into a preheated furnace at 850°C. Samples were left in the furnace for 10 minutes while the furnace purged with argon before being removed and allowed to air cool. The only exception was the final anneal for the 4.8 mm material. This anneal was an 850°C 20 min air cooled anneal, as opposed to the 10 minute anneal used for every other thickness.
  • FIG. 4 and FIG. 5 show the tensile properties of Alloy 2 sheet as a function of the thickness. Average tensile elongation is 53.7% for Alloy 2 sheet with thickness varying from 0.20 to 1.03 mm (as compared to average of 61.5% in Alloy 2 sheet with thickness of 1.2 mm). Slightly higher elongation is observed up to 66.4% in thicker sheet samples above 1.2 mm. The stress - strain curves in FIG. 6 also demonstrate consistent properties and stress-strain behavior in sheet samples with different thicknesses.
  • This Case Example demonstrates that high ductility maintained in the sheet with thickness in a wide range from 4.8 mm down to as small as 0.2 mm. Reduction in sheet thickness below 1.2 mm results in an average total elongation that is no less than that in the sheet with 1.2 mm thickness and above minus 7.8%. An average ultimate tensile strength is 25 MPa less than that in the corresponding sheet with 1.2 mm thickness and above and average yield strength is 67 MPa less.
  • the hot band from Alloy 1, Alloy 27, and Alloy 37 was cold rolled in to sheets with different thicknesses less than 1.2 mm through multiple cold rolling passes. Once the targeted gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process representing final treatment at sheet processing in Step 2 in FIG. 1. Samples were wrapped in stainless steel foil to prevent oxidation and loaded into a preheated furnace at 850°C. Samples were left in the furnace to 10 minutes while the furnace purged with argon before being removed and allowed to air cool.
  • tensile elongation is measured in the range from 44.9 to 51.1%, for Alloy 27 in the range from 63.8 to 73.8%, and for Alloy 37 in the range from 6.0 to 7.0%.
  • Tensile elongation as a function of the sheet thickness is illustrated in FIG. 6 for the selected alloys.
  • FIG. 8 and FIG. 9 show the yield strength and ultimate tensile strength of the sheet with different thicknesses for the selected alloys.
  • the ultimate tensile strength is in a range from 1203 to 1269 MPa in Alloy 1 sheet, from 972 to 1067 MPa in Alloy 27 sheet, and from 1493 to 1614 MPa in Alloy 37 sheet.
  • Yield strength varies from 375 to 444 MPa in Alloy 1 sheet, from 367 to 451 MPa in Alloy 27 sheet, and from 612 to 820 MPa in Alloy 37 sheet.
  • This Case Example demonstrates that tensile ductility of alloys herein is maintained event thickness as small as 0.2 mm demonstrating an average total elongation no less than that in the corresponding sheet with 1.2 mm thickness and above minus 7.3%.
  • An average ultimate tensile strength is a range of ⁇ 35 MPa of that in the corresponding sheet with 1.2 mm thickness and above with the yield strength in a range of ⁇ 98 MPa.
  • the hot band from Alloy 1, Alloy 2, Alloy 27, and Alloy 37 was cold rolled in to sheets with different thicknesses less than 1.2 mm through multiple cold rolling passes. Once the targeted gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process. Samples were wrapped in stainless steel foil to prevent oxidation and loaded into a preheated furnace at 850°C. The microstructures of the cold rolled and annealed state were studied by SEM to show the structural change during processing.
  • FIG. 10 shows microstructures in Alloy 1 sheet samples with different thicknesses.
  • Cold rolled structure is shown in FIG. 10a and FIG. 10c in the center of the sheet with thickness of 0.7 and 0.5 mm, respectively.
  • the cold rolled sample bears the highly deformed microstructure in which grain boundaries are difficult to see.
  • the microstructure in these sheet samples after annealing is shown in FIG. 10b and FIG. lOd represented by the recrystallized structure with equiaxed grains and clear grain boundaries.
  • FIG. 11 shows microstructures in Alloy 2 sheet samples with different thicknesses.
  • Cold rolled structure is shown in FIG. 11a, FIG. 11c and FIG. l ie in the center of the sheet with thickness of 1.0, 0.5 and 0.2 mm, respectively.
  • the cold rolled sample bears the highly deformed microstructure in which grain boundaries are difficult to see.
  • the microstructure in these sheet samples after annealing is shown in FIG. l ib, FIG. l id and FIG. 1 If represented by recrystallized structure with equiaxed grains and clear grain boundaries.
  • the structure in the sheet samples from Alloy 27 is similar to Alloy 1 and Alloy 2 and is shown in FIG. 12.
  • the recrystallized microstructure in the sheet from Alloy 27 has fewer twins as compared to other studied alloys, as shown in FIG. 12b, FIG. 12d and FIG. 12f.
  • Alloy 37 is a different type of the alloy in which the annealing does not lead to the typical recrystallized structure formation.
  • FIG. 13 shows the structures at the center of the sheet from Alloy 37 with different thicknesses after cold rolling and after cold rolling and annealing. Only a small difference between the cold rolled and the annealed structures is observed. Corresponding samples at different thicknesses have effectively identical structures.
  • This Case Example demonstrates that microstructure is maintained in alloys herein after annealing of cold rolled sheet independently from the final sheet thickness.
  • the tensile properties of the material were characterized as a function of strain rate. Tensile samples were tested at 0.0007 s "1 , 0.7 s “1 , 10 s “1 , 100 s “1 , 500 s “1 and 1200 s “1 nominal strain rates in the ASTM D638 Type V tensile geometry shown in FIG. 14. Tensile samples tested at strain rates from 0.0007 s "1 to 500 s "1 were tested on an MTS servo-hydraulic test frame. Samples were inserted into grips and load was applied by raising the crosshead at speeds necessary to produce the nominal strain rates.
  • a slack adapter consisting of a cup and cone rod assembly was used at strain rates greater than 1 s "1 to allow the test frame to achieve the targeted constant strain rate prior to applying load to the specimen.
  • An instrumented bar was used at 500 s "1 to mitigate the effects of standing waves in the test apparatus that occurred during high strain rate testing.
  • a split Hopkinson bar (SHB) was used.
  • the SHB device was composed of 25.4 mm diameter 7075 Al incident and transmission bars, with the test specimen tightly gripped between the Al bars. Strain gauges were used on the transmission and incident bars to measure strain in the bars.
  • a striker tube was launched around the incident tube towards the striker plate to generate the tensile strain pulse and the strain within the sample was recorded.
  • a schematic diagram of the SHB is provided in FIG. 15.
  • Measured strain at failure is provided in Table 8. The measured strain is plotted as function of strain rate in FIG. 16. Table 9 provides the average ductility as measured by tensile elongation at failure for each nominal strain rate. Note that the average tensile elongation measured at all strain rates is close to the overall average of 55.5% across all strain rates. At strain rates from 0.0007 s "1 to 500 s "1 , the average tensile elongation at failure is within approximately +3% of the total average of all tests. Tests at 1200 s "1 were measured to possess higher tensile elongation at failure than all other tests, however due to the nature of this test methodology these values may be measured slightly higher than actual values. Ultimate tensile strength is measured in a range from 944 to 1187 MPa with yield strength from 347 to 512 MPa (Table 10).
  • Tensile properties in Tables 8 through 10 represents sheet material in Step 3 in FIG. 2.
  • microstructures of the samples from sheet from Alloy 2 tested at five different strain rates ranging from 0.0007 s "1 to 1200 s "1 (see Case Example #5) were studied by TEM.
  • pieces are cut from the gauge section of deformed samples by diamond saw. Grinding and polishing are then undertaken to make thin foils from the cut pieces. The polishing was conducted progressively with 9 ⁇ , 6 ⁇ and 1 ⁇ diamond suspension solution, and finally with 0.02 ⁇ silica. Foils with thickness of 70 to 80 ⁇ were obtained after the polishing. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% Nitric acid mixed in Methanol base.
  • the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS).
  • PIPS Gatan Precision Ion Polishing System
  • the ion-milling usually is done at 4.5 keV, and the inclination angle is reduced from 4° to 2° to open up the thin area.
  • the TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
  • FIG. 17 shows the bright-field TEM images of the sample tested at 1200 s "1 . It can be seen that deformation twins are prominent in the high rate deformed sample which are a forming of twinning which does not occur through mechanical deformation but during heat treatment. The twins are distinct and sharp, suggesting that they are newly formed from the deformation. With twinning being a deformation mode in the sample, phase transformation is reduced since the deformation twins maintain austenitic structure. Twinning as a method of deformation can be seen in the sample deformed at strain rates of 500, 100, 10,' and 0.7 s "1 , as shown in FIG. 18 through FIG. 21. The sample deformed at strain rate of 0.0007 s "1 has different structure as can be seen in FIG.
  • the magnetic phases volume percent which correlates to the transformed product phases, is highest in the case of deformation at low strain rate of 0.0007 s "1 .
  • This Case Example demonstrates the alteration of deformation mechanisms during deformation of the alloys herein with higher occurrence of twinning with increasing strain rate.
  • Deformation by twinning at high strain rates suppresses the phase transformation (i.e. means that the total amount of ferrite produced is reduced) allowing to the retention of relatively high tensile ductility of the sheet material in a wide range of strain rates.
  • Tensile specimens were cut from the sheet via wire EDM. The specimens had two notches, symmetric at about the center of the width and the length as showed in FIG. 24. Samples were tested in tension with one grip fixed and the other moving at a fixed rate of 0.125 mm/s displacement rate. Tensile properties were measured on an Instron mechanical testing frame, utilizing Instron' s Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. A 50 mm gauge length was used centered on the notch. Stresses were calculated based on the nominal width not the notched width (FIG. 24).
  • Tensile properties of the Alloy 2 sheet samples as a function of notch diameter and notch depth are listed in Table 11. Tensile elongation of notched samples ranged from 12.4% to 40.7%, yield strength ranged from 298 to 420 MPa, and ultimate tensile strength ranged from 636 to 1123 MPa. Effect of notch diameter with constant depth of 0.5 mm on tensile properties of the sheet from Alloy 2 is illustrated in FIG. 25. Changes in tensile properties of the sheet with half circle notches as a function of notch diameter are shown in FIG. 26. This data represents sheet in Step 3 in FIG. 3.
  • This Case Example demonstrates an increase in tensile elongation of the notched samples from alloys herein with increasing notch diameter at constant depth. In the case of increasing depth, average elongation is shown to be independent of the notch depth (half circle).
  • FIG. 27 and FIG. 28 SEM images of fracture surface after tensile testing are shown for Sample 1 and Sample 2, respectively. Images are taken from the center of the fracture cross section and close to the edge. Both samples demonstrated ductile fracture. There is no difference in fracture mode between the center and the edge of the fracture cross section although finer structure is found closer to the edge.
  • the alloys herein may be utilized in variety of applications.
  • the alloys herein may be positioned in vehicular frame, vehicle chassis or vehicle panel.
  • the alloys herein may be utilized for a storage tank, freight car, or railway tank car.
  • Railway tank cars may specifically include tanks, jacketed tanks or tanks with a headshield.
  • Other applications include body armor, metallic shield, military vehicles, and armored vehicle Such applications apply to the alloys produced according to any one of FIG. 1, FIG. 2 and/or FIG. 3.

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Abstract

Cette invention concerne la rétention de propriétés mécaniques dans un acier à haute résistance à des épaisseurs réduites et dont les performances de propriété mécanique sont également conservées à des taux de déformation relativement élevés. Ces nouveaux aciers peuvent offrir des avantages pour une multitude d'applications où une épaisseur de feuille réduite est souhaitable. De plus, les alliages de l'invention sont ceux qui conservent des propriétés mécaniques utiles après l'introduction d'une discontinuité géométrique et d'une concentration de contraintes associée.
EP18824168.1A 2017-06-30 2018-06-28 Rétention de propriétés mécaniques dans des alliages d'acier après traitement et en présence de sites de concentration de contraintes Withdrawn EP3645763A4 (fr)

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EP21206702.9A EP3971314A1 (fr) 2017-06-30 2018-06-28 Conservation des propriétés mécaniques des alliages d'acier après traitement et en présence de sites de concentration de contraintes

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EP3971314A1 (fr) 2022-03-23
CA3068303A1 (fr) 2019-01-03

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