EP3481972B1 - Verfahren zur herstellung von stahl mit hoher streckgrenze - Google Patents

Verfahren zur herstellung von stahl mit hoher streckgrenze Download PDF

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Publication number
EP3481972B1
EP3481972B1 EP17824946.2A EP17824946A EP3481972B1 EP 3481972 B1 EP3481972 B1 EP 3481972B1 EP 17824946 A EP17824946 A EP 17824946A EP 3481972 B1 EP3481972 B1 EP 3481972B1
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Prior art keywords
alloy
rolling
mpa
sheet
yield strength
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French (fr)
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EP3481972A1 (de
EP3481972A4 (de
Inventor
Daniel James Branagan
Andrew E. Frerichs
Brian E. Meacham
Andrew T. Ball
Grant G. Justice
Kurtis R. CLARK
Sheng Cheng
Scott T. ANDERSON
Scott T. LARISH
Taylor L. Giddens
Logan J. TEW
Alla V. Sergueeva
Jason K. Walleser
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United States Steel Corp
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United States Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/18—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

Definitions

  • This disclosure is related to high yield strength steel. Due to the unique structures and mechanisms, yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation. These new steels can offer advantages for a myriad of applications where relatively high yield strength is desirable along with relatively high UTS and total elongation such as the passenger cage in automobiles.
  • UTS ultimate tensile strength
  • AHSS Third Generation Advanced High Strength Steels
  • MS martensitic steels
  • DP dual phase steels
  • TRIP transformation induced plasticity
  • CP complex phase
  • Example targets for 3 rd Generation AHSS are provided in the banana chart for autobody steels which is published by World Auto Steel ( FIG. 1 ).
  • AHSS ultimate tensile strength
  • AHSS materials are not generally classified by the yield strength (YS). Yield strength of a material is also of large importance to automobile designers since once a part is in service and if the part is stressed beyond yield, the part will permanently (plastically) deform. Materials that have high yield strength resist permanent deformation to higher stress levels than those with lower yield strength. This resistance to deformation is useful by allowing structures made from the material to withstand greater loads before the structure permanently deflects and deforms. Materials with higher yield strength can thereby enable automobile designers to reduce associated part weight through gauge reduction while maintaining the same resistance to deformation in the part. Many types of emerging grades of third generation AHSS suffer from low initial yield strengths, despite having various combinations of tensile strength and ductility.
  • the yield strength of a material can be increased in a number of ways on the industrial scale.
  • the material can be cold rolled a small amount (with a reduction ⁇ 2%) in a process called temper rolling. This process introduces a small amount of plastic strain in the material, and the yield strength of the material is increased slightly corresponding to the amount of strain that the material was subjected to during the temper pass.
  • Another method of increasing the yield strength in the material is through a reduction in the material's crystal grain size, known as Hall-Petch strengthening. Smaller crystal grains increase the required shear stress for the initial dislocation movement in the material, and the initial deformation is delayed until higher applied loads.
  • the grain size can be reduced through process modifications such as altered annealing schedules to limit grain growth during the recrystallization and growth process that occurs during annealing after plastic deformation.
  • Cold working steel from a fully annealed state is a known route to increase yield strength and tensile strength. It can be applied uniformly across a sheet during processing through cold rolling increasing the yield strength and tensile strength. However, this approach results in a decrease in total elongation and often to levels much below 20%. As elongation decreases, the cold forming ability also decreases, reducing the ability to produce parts with complex geometries resulting in a decrease in the usefulness of the AHSS. Higher ductility with a minimum of 30% total elongation is generally needed to form complex geometries through cold stamping processes. While processes such as roll forming can be used to create parts from lower elongation material, the geometric complexity of parts from these processes is limited.
  • the present invention relates to a method to increase yield strength in a metallic alloy as defined in claim 1.
  • the preferred embodiments are defined in the dependent claims 2-14.
  • FIG.s are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
  • FIG. 2 represents a summary of preferred Method 1 to develop high yield strengths from a low yield strength material by a route which results in either of two conditions as provided in conditions 3a or 3b.
  • the starting condition is to supply a metal alloy.
  • This metal alloy will 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 and preferably cooled at a rate of 10 -4 K/s to 10 3 K/s and solidified to a thickness of >5.0 mm to 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 would exhibit a fraction of austenite ( ⁇ -Fe) at least 10 volume percent up to 100 volume percent and all increments in between in the temperature range from 150 to 400°C.
  • Step 2 of Method 1 the alloy is preferably processed into sheet form with thickness from 0.5 to 5.0 mm.
  • This step 2 can involve hot rolling or hot rolling and cold rolling. If hot rolling the preferred temperature range would be at a temperature of 700 °C and below the Tm of said alloy. If cold rolling is employed, such is understood to be at ambient temperature. Note that after hot rolling or hot rolling and cold rolling, the sheet can be additionally heat treated, preferably in the range from at a temperature of 650 °C to a temperature below the melting point (Tm) of said alloy.
  • Tm melting point
  • the steps to produce sheet from the cast product can therefore vary depending on specific manufacturing routes and specific targeted goals.
  • the alloy would preferably 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 are typically from 1.5 to 5.0 mm in thickness.
  • cold rolling can be done at various reductions per pass, variable number of passes and in different mills including tandem mills, Z-mills, and reversing mills.
  • cold rolled thickness would be 0.5 to 2.5 mm thick.
  • the cold rolled material is annealed to restore the ductility lost from the cold rolling process either partially or completely at a temperature range from 650 °C to a temperature below the melting point (Tm) of said alloy.
  • 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 10 in number.
  • the strip is rolled into hot band coils with typical thickness from 1 to 5 mm in thickness. If further processing is needed, cold rolling can be applied in a similar manner as above.
  • 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..
  • the sheet will then exhibit a total elongation of X 1 (%), an ultimate tensile strength of Y 1 (MPa), and a yield strength of Z 1 (MPa).
  • Preferred properties for this alloy would be ultimate tensile strength values from 900 to 2050 MPa, tensile elongation from 10 to 70%, and yield strength is in a range from 200 to 750 MPa.
  • the alloy is permanently (i.e. plastically) deformed in the temperature range from 150°C to 400°C.
  • Such permanent deformation may be provided by rolling and causing a reduction in thickness. This can be done for example during the final stages of the development of a steel coil.
  • elevated temperature rolling is now preferably done in the targeted temperature range of 150 to 400°C.
  • One method would be to heat the sheet to the targeted temperature range prior to going through the cold rolling mill.
  • the sheet could be heated by a variety of methods including going through a tunnel mill, a radiative heater, a resistance heater, or an induction heater. Another method would be to heat directly the reduction rollers.
  • a third example for illustration would be to low temperature batch anneal the sheet and then send this through the cold rolling mill(s) at the targeted temperature range.
  • the sheet may be deformed at the elevated temperature range into parts using a variety of processes providing permanent deformation during the making of parts by various methods including roll forming, metal stamping, metal drawing, hydroforming etc.
  • Condition 3a comparing said alloy in Step 2 and after Step 3, the total elongation and ultimate tensile strength are relatively unaffected but the yield strength is increased. Specifically, the total elongation X 2 is equal to X 1 ⁇ 7.5%, the tensile strength Y 2 is equal to Y 1 ⁇ 100 MPa, and the yield strength Z 2 is ⁇ Z 1 + 100 MPa.
  • Preferred properties for this alloy in Condition 3a would be ultimate tensile strength values (Y 2 ) from 800 to 2150 MPa, tensile elongation (X 2 ) from 2.5% to 77.5%, and yield strength (Z 2 ) ⁇ 300 MPa. More preferably, yield strength may fall in the range of 300 to 1000 MPa.
  • Condition 3b comparing said alloy in Step 2 and after Step 3, the ultimate tensile strength is relatively unaffected but the yield strength is increased. Specifically, the ultimate tensile strength Y 3 is equal to Y 1 ⁇ 100 MPa and yield strength Z 3 is ⁇ Z 1 + 200 MPa. Preferred properties for this alloy in Condition 3b would be ultimate tensile strength values (Y 3 ) from 800 to 2150 MPa and yield strength (Z 3 ) ⁇ 400 MPa. More preferably, yield strength may fall in the range of 400 to 1200 MPa. In addition, unlike Condition 3a, the total elongation drop is greater than 7.5%, that is, in Step B, the total elongation (X 3 ) is defined as follows: X 3 ⁇ X 1 - 7.5%.
  • FIG. 3 identifies a summary of Method 2 of the present disclosure.
  • the first 3 steps in Method 2 are identical to Method 1 with Step 4 being an additional step for Method 2.
  • Step 4 can be applied to the alloys herein in either Condition 3a or Condition 3b.
  • Condition 3a or 3b various combinations of properties (i.e. total elongation, ultimate tensile strength, and yield strength) are provided for each Condition 3a or 3b.
  • alloys in Condition 3a or 3b may be further characterized by their particular structure. This then allows further tailoring of the final properties by the use of a further optional step of permanently deforming the alloys at temperatures from ambient to ⁇ 150°C, or more preferably at a range of temperatures of 0°C to 150°C. This can be done for example by adding another step during the production of steel coils as illustrated in FIG. 3 .
  • Step 4 can be a skin pass (i.e.
  • Step 4 of Method 2 the sheet could be subsequently made into parts using a variety of deformation processes including roll forming, metal stamping, metal drawing, hydroforming etc. Notwithstanding the exact process to activate Step 4 in Method 2, final properties can be developed with the said alloy which are contemplated to exhibit properties with tensile elongation from 10 to 40 %, ultimate tensile strength from 1150 to 2000 MPa, and yield strength from 550 to 1600 MPa).
  • the alloys herein are iron based metal alloys, having greater than 70 at.% Fe.
  • the alloys herein are such that they comprise Fe and at least four or more, or five or more, or six elements selected from Si, Mn, Cr, Ni, Cu or C. Accordingly, with respect to the presence of four or more, or five or more elements selected from Si, Mn, Cr, Ni, Cu or C, such elements are present at the following indicated atomic percents: Si (0 to 6.13 at.%); Mn (0 to 15.17 at. %); Cr (0 to 8.64 at. %); Ni (0 to 9.94 at. %); Cu (0 to 1.86 at. %); and C (0 to 3.68 at. %).
  • the alloys herein are such that they comprise, consist essentially of, or consist of Fe at a level of 70 at.% or greater along with Si, Mn, Cr, Ni, Cu and C, wherein the level of impurities of all other elements is in the range from 0 to 5000 ppm.
  • Impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Al, Co, Mo, N, Nb, P, Ti, V, W, and S which if present would be in the range from 0 to 5000 ppm (parts per million) with preferred ranges of 0 to 500 ppm.
  • the casting machine then evacuated the chamber and tilted the crucible and poured the melt into a 50 mm thick, 75 to 80 mm wide, and 125 mm deep channel in a water cooled copper die and would represent Step 1 in Figs. 2 and 3 .
  • the process can be adapted to a preferred as-cast thickness at a range from >5.0 to 500 mm.
  • the melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure.
  • the alloys herein were preferably processed into a laboratory sheet.
  • Laboratory alloy processing is developed to simulate the hot band production from slabs produced by continuous casting and would represent Step 2 in FIGS. 2 and 3 .
  • Industrial hot rolling is performed by heating a slab in a tunnel furnace to a target temperature, then passing it through a either a reversing mill or a multi-stand mill or a combination of both to reach the target gauge in a preferred temperature range from 700 °C up to the melting point (Tm) of the alloy.
  • Tm melting point
  • the slab thickness has been reduced to a final thickness of the hot band from 1.8 to 2.3 mm.
  • Processing conditions can be adjusted by changing the amount of hot rolling and/or adding cold rolling steps to produce the preferred thickness range from 0.5 to 5.0 mm.
  • 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.
  • Tensile properties of the alloys in the hot rolled condition are listed in Table 2 which have been processed to a thickness from 1.8 to 2.3 mm.
  • the ultimate tensile strength values may vary from 913 to 2000 MPa with tensile elongation from 13.8 to 68.5%.
  • the yield strength is in a range from 250 to 711 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 hot band from alloys herein listed in Table 1 was, for comparison purposes, cold rolled to final target gauge thickness of 1.2 mm through multiple cold rolling passes.
  • Tensile specimens were cut from each cold rolled sheet using wire EDM.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control.
  • Tensile properties of alloys herein after cold rolling are listed in Table 3. As it can be seen, the yield strength is significantly increased over the range in a hot band with maximum at 711 MPa (Table 2). After cold rolling yield strength varies from 1037 to 2000 MPa. The ultimate tensile strength values after cold rolling are in a range from 1431 to 2222 MPa. However, a drop in tensile elongation is recorded for each alloy herein after cold rolling with variation from 4.2 to 31.1%. The general trends in effect of cold rolling on tensile properties of alloys herein are illustrated in FIG. 4 to FIG. 6 .
  • the relative magnetic phases content was measured by Feritscope in both a hot band and after cold rolling for each alloy herein that is listed in Table 4 and illustrated in FIG. 7 for selected alloys.
  • the magnetic phases volume percent of 0.1 to 56.4 Fe% in a hot band increases to the range from 1.6 to 84.9 Fe% after cold rolling confirming a phase transformation during deformation.
  • This comparative Case Example demonstrates that yield strength can be increased in alloys herein by cold rolling (i.e. at ambient temperature). Ultimate tensile strength is also increasing but cold rolling leads to a significant decrease in alloy ductility indicated by a drop in tensile elongation that can be a limiting factor in certain applications. Strengthening, as shown by the increase in ultimate tensile strength, is related to a phase transformation of austenite to ferrite as depicted by measurements of magnetic phases volume percent before and after cold rolling.
  • Alloy 2 was processed into a hot band with a thickness of 4.4 mm.
  • the hot band was then cold rolled with different reduction through multiple cold rolling (i.e. at ambient temperature) passes. After cold rolling the samples were heat treated with intermediate annealing at 850°C for 10 min. This represented a start condition for each sample which represented a fully annealed condition to remove the prior cold work. From this start condition, subsequent cold rolling at different percentages (i.e. 0%, 4.4%, 9.0%, 15.1%, 20.1%, 25.1% and 29.7%) as provided in Table 5 was applied so that the final gauge for tensile testing would be at a targeted constant thickness of 1.2 mm.
  • This Comparative Case Example #2 demonstrates that yield strength in alloys herein can be altered by cold rolling reduction to achieve relatively higher yield strength values with increase in tensile strength but with decrease in ductility. The higher cold rolling reduction that is applied, the higher yield strength achieved and the lower tensile elongation recorded.
  • Hot band from Alloy 2 with thickness of 4 mm was cold rolled to a final thickness of 1.2 mm through multiple cold rolling passes with intermediate annealing at 850°C for 10 min. Microstructures of the hot band and the cold rolled sheet were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
  • 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.
  • This Case Example demonstrates a microstructure evolution from the initial hot band austenitic structure during cold rolling leading to alloy strengthening (increase in ultimate tensile strength) by grain refinement due to phase transformation into ferrite with nanoprecipitation as well as dislocation density increase and deformation twinning.
  • the starting material was a hot band from Alloy 2 with approximately 2.5 mm thickness prepared by hot rolling of 50 mm thick laboratory cast slab mimicking processing at commercial hot band production.
  • the starting material had an average ultimate tensile strength of 1166 MPa, an average tensile elongation of 53.0% and an average yield strength of 304 MPa.
  • the starting material also had a magnetic phases volume percent of 0.9 Fe%.
  • the hot band was media blasted to remove oxide and loaded into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to allow the plate to reach temperature.
  • the hot band was rolled on a Fenn Model 061 rolling mill with steadily decreasing roll gaps, and was loaded into the furnace for at least 10 minutes between passes to ensure a constant starting temperature (i.e. 50, 100, 150, 200, 250°C, 300°C, 350°C, and 400°C) for each subsequent rolling pass for a total targeted 20% reduction.
  • Samples were EDM cut in the ASTM E8 Standard geometry.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tensile tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • Tensile properties of the Alloy 2 after rolling at identified temperatures are listed in Table 6. Depending on rolling temperature, the yield strength is increased to a range from 589 to 945 MPa as compared to the values of 250 to 711 MPa in a hot band (Table 2). The ultimate tensile strength of the Alloy 2 varies from 1132 to 1485 MPa with tensile elongation from 21.2 to 60.5%. An example stress-strain curves are shown in FIG. 15 . As can be seen, rolling at temperature of 200°C of the hot band from Alloy 2 demonstrates the possibility to increase yield strength with minimal changes in ductility and ultimate strength consistent with Step 3a in Fig.3 .
  • This Case Example demonstrates that yield strength in alloys herein can be increased by rolling at elevated temperatures whereby phase transformation of austenite into ferrite is reduced. Significant drops in Fe% occur when rolling temperature is greater than 100°C.
  • rolling of the hot band from alloys herein at temperatures of 150°C to 400°C demonstrates the ability to increase yield strength (e.g. increasing yield strength to a value of at least 100 MPa or more over the original value) without significant change in ductility (i.e. change limited to plus or minus seven and one half percent ( ⁇ 7.5% tensile elongation) and maintain the ultimate tensile strength at about the same level (i.e. ⁇ 100 MPa as compared to the original value).
  • the starting material was a hot band from each of Alloy 7, Alloy 18, Alloy 34, and Alloy 37 with approximately 2.5 mm initial thickness prepared by hot rolling of 50 mm thick laboratory cast slab mimicking commercial processing. Alloys 7, 18, 34, and 37 were processed into hot bands with a thickness of approximately 2.5 mm by hot rolling at temperatures between 1100°C and 1250°C and subsequently media blasted to remove the oxide. The tensile properties of hot band material were previously listed in Table 2. The hot band was media blasted to remove oxide and loaded into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to allow the plate to reach the desired temperature.
  • the resulting cleaned hot band was rolled on a Fenn Model 061 rolling mill with steadily decreasing roll gaps, and was loaded into the furnace for at least 10 minutes between passes to ensure constant temperature.
  • the hot band was rolled to a targeted 20% reduction and samples were EDM cut in the ASTM E8 Standard geometry.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tensile tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • each alloy in particular of their elongation, yield strength, and ultimate tensile strength were monitored across the entire range of temperatures investigated. Each alloy was tested after rolling at temperatures ranging from 100°C at the lowest to 400°C at the highest. For Alloy 7, tensile elongation ranged from 14.7% to 35.5%, ultimate tensile strength ranged from 1218 MPa to 1601 MPa, and yield strength ranged from 557 MPa to 678 MPa across the investigated temperature range (Table 8), with Fe% numbers ranging from 29.9 to 41.7 before tensile testing, and 57.7 to 65.4 after testing (Table 9).
  • tensile elongation ranged from 43.0% to 51.9%
  • ultimate tensile strength ranged from 1083 MPa to 1263 MPa
  • yield strength ranged from 772 MPa to 924 MPa from 150 to 400°C (Table 10)
  • Fe% numbers ranging from 6.8 to 12.3 before tensile testing and from 31.5 to 39.6 after testing in the 150 to 400°C range (Table 11).
  • tensile elongation ranged from 21.1% to 31.1%
  • ultimate tensile strength ranged from 1080 MPa to 1140 MPa
  • yield strength ranged from 869 MPa to 966 MPa in the 150 to 400°C range (Table 12), with Fe% numbers ranging from 0.4 to 1.0 before tensile testing and 0.8 to 2.1 after testing (Table 13).
  • tensile elongation ranged from 1.5% to 9.0%
  • ultimate tensile strength ranged from 1537 MPa to 1750 MPa
  • yield strength ranged from 1384 MPa to 1708 MPa in the 150 to 400°C range (Table 14), with Fe% numbers ranging from 74.5 to 84.3 before tensile testing and 71.1 to 85.6 after testing (Table 15).
  • Alloy 2 was processed into a hot band with thickness of approximately 2.5 mm from the laboratory cast. Following hot rolling, Alloy 2 was rolled at 200°C to varying rolling reductions ranging from approximately 10% to 40%. Between rolling passes, the Alloy 2 sheet material was placed in a convection furnace at 200°C for 10 minutes to maintain the temperature. When the desired rolling reduction was achieved, ASTM E8 tensile samples were cut via wire-EDM and tested.
  • FIG. 21 shows the representative tensile curves for Alloy 2 as a function of rolling reduction at 200°C. It is observed that the yield strength of the material increases rapidly with increasing reduction, without changing the ultimate tensile strength (i.e. a change of plus or minus 100 MPa) up to 30% reduction.
  • FIG. 21 shows the representative tensile curves for Alloy 2 as a function of rolling reduction at 200°C. It is observed that the yield strength of the material increases rapidly with increasing reduction, without changing the ultimate tensile strength (i.e. a change of plus or minus 100 MPa) up to 30% reduction.
  • step 22 provides a comparison of the trends for yield strength and ultimate tensile strength as a function of rolling reduction at 200°C, showing that while the yield strength increase is relatively rapid, the ultimate tensile strength change is consistent with step 3a property changes in Figure 2 up to 30.4% rolling reduction and is consistent with step 3b property changes at 39.0% rolling reduction.
  • the total elongation of Alloy 2 is plotted as a function of rolling reduction at 200°C in FIG. 23 . It demonstrates that while the yield strength of Alloy 2 is increasing with additional reduction during rolling at 200°C, the available ductility does not decrease rapidly until >30% reduction. Note that this is simulated using laboratory rolling and commercial rolling methods including tandem mill rolling, Z-mill rolling, and reversing mill rolling will additionally apply a strip tension during rolling so the exact amount of reduction whereby ductility decreases may change.
  • the magnetic phases volume percent (Fe%) was measured using a Fischer Feritscope FMP30 for the samples after rolling at 200°C and again after tensile testing in the tensile gauge (i.e. the reduced gauge section present in the tensile specimen). These measurements, shown in Table 17, are indicative of the amount of deformation-induced phase transformation that is occurring in the alloy during the rolling process and during subsequent tensile testing.
  • the amount of deformation-induced phase transformation in Alloy 2 after rolling and tensile testing is shown in FIG. 24 . It can be seen that the deformation-induced phase transformation is largely suppressed at 200°C, as the magnetic phases volume percent only increases slightly with increasing rolling reduction.
  • This Case Example demonstrates that the yield strength of the alloys described herein may be tailored by varying the rolling reduction at temperatures greater than ambient as shown here for Alloy 2 by rolling at 200°C.
  • the temperature range is contemplated to be between 150°C to 400°C as provided in the previous case example for Table 7.
  • the deformation pathway is modified such that relatively limited deformation-induced phase transformation is occurring, which results in the ability to retain significant ductility and maintain ultimate tensile strength while increasing yield strength in the cold rolled state.
  • the parameters of the rolling can be optimized to improve the yield strength of the material without sacrificing the ductility or ultimate tensile strength.
  • Alloy 2 was processed into a hot band with thickness of 9 mm from the laboratory cast mimicking processing at commercial hot band production.
  • the hot band was cold rolled with 50% reduction and annealed at 850°C for 10 minutes with air cooling mimicking cold rolling processing at commercial sheet production. Media blasting was used to remove the oxides which formed during annealing. Then the alloys were cold rolled again until failure or the mill limited reduction. Samples were heated to 200°C in a convection oven for at least 30 minutes prior to cold rolling to ensure they were at uniform temperature, and reheated for 10 minutes between passes to ensure constant temperature. Alloy 2 sheet was cold rolled first with reduction of 30% and then to a maximum reduction of 70%.
  • SEM scanning electron microscopy
  • FIG. 25 shows the backscattered SEM images of the microstructure before cold rolling that is mostly austenitic with annealing twins inside micron-sized grains.
  • FIG. 26 shows the bands with similar orientation.
  • the bands with similar orientation are deformation twins in one austenitic grain while bands in different direction are twins in another crystal orientation grain. Some grain refinement can be observed in selected areas.
  • FIG. 27 After the rolling reduction is increased to 70%, the bands are no longer visible, and refined structure through the volume can be seen ( FIG. 27 ). As shown in the high magnification image in FIG. 27b , fine islands with size much smaller than 10 ⁇ m can be discerned. Considering the high deformation exerted in the stable austenite during the rolling process, the austenite could be dramatically refined typically in the range of 100 to 500 nm. Feritscope measurements suggest that the austenite is stable at 200°C with nearly 100% austenite maintained after rolling.
  • This Case Example demonstrates austenite stabilization (i.e. the resistance to transformation to ferrite) in alloys herein during the rolling at 200°C even at high rolling reduction of 70% and microstructural refinement of the austenite in contrast to cold rolling when refinement occurs through austenite transformation to ferrite.
  • FIG. 28 shows the bright-field TEM images of the microstructure in the Alloy 2 rolled at 200°C with 10% reduction. It can be seen that the austenite grains are filled with tangled dislocations, and dislocation cell structure is exhibited. However, due to the relatively low rolling strain, the original austenite grain boundaries are still visible. It is noted that the austenite is stable during the rolling at 200°C. Electron diffraction suggests that austenite is the predominant phase that was also consistent with Feritscope measurement. Rolling at 200°C with 10% reduction increases the average yield strength from 303 MPa in the hot band to 529 MPA (see Table 16).. When the sheet is rolled to 30%, TEM qualitatively shows higher dislocations density in the grains, as shown in FIG.
  • Alloys 2, Alloy 7, Alloy 18, and Alloy 34 were processed into hot band with a thickness of ⁇ 2.7 mm, this was media blasted to remove the oxide and rolled at 200°C to 20% reduction. The material was sectioned and then rolled at a range of reductions at ambient temperature. ASTM E8 tensile samples were cut by wire EDM and tested in an Instron 5984 frame using Instron's Bluehill software.
  • Tensile properties of the selected alloys after combined rolling are listed in Table 18 through Table 21. Significant increase in yield strength after combination of rolling methods was observed in all three alloys as compared to the hot band state or just after rolling with ⁇ 20% reduction in rolling thickness at 200°C and subsequent rolling reduction at ambient temperature.
  • Yield strength up to 1216 MPa recorded for Alloy 2 Yield strength in hot band is 309 MPa and 803 MPa after rolling at 200°C
  • Yield strength in hot band is 309 MPa and 803 MPa after rolling at 200°C
  • 1571 MPa in Alloy 7 Yield strength in hot band is 333 MPa and 575 MPa after rolling at 200°C
  • 1080 MPa in Alloy 18 Yield strength in hot band is 390 MPa and 834 MPa after rolling at 200°C
  • up to 1248 MPa in Alloy 34 Yield strength in hot band is 970 MPa and 1120 MPa after rolling at 200°C.
  • FIG. 31 through FIG. 34 shows the corresponding tensile curves for alloys 2, 7, 18, and 34, respectively.
  • This Case Example demonstrates a pathway to creating a third distinct set of property combinations, which may be achieved by processing the alloy into a sheet at a thickness of 0.5 mm to 5.0 mm, followed by deforming (rolling) and reducing thickness in one pass at a temperature in the range of 150°C to 400°C, and then subsequent reductions in thickness at temperatures ⁇ 150°Ctemperature. This is observed to provide relatively higher yield strength compared to only cold rolling, and higher tensile strengths compared to only rolling at temperature.
  • a hot band from Alloy 2 was processed into a sheet by different methods herein towards higher yield strength and property combination according to the steps provided in FIG. 2 and FIG. 3 .
  • Alloy 2 was first cast and then processed into a sheet via hot rolling which was from 2.5 to 2.7 mm thick.
  • the reference hot band material was hot rolled to ⁇ 1.8 mm to reduce gauge prior to testing.
  • the hot band was rolled with a 20% reduction at 200°C. Prior to rolling, it was heated up to 200°C for 30 minutes before being rolled 20% at 200°C with a 10 minute reheat between rolling passes to maintain temperature.
  • FIG. 3 example i.e.
  • FIG. 35 Representative stress-strain curves with property combination achieved at each processing method close to optimal are shown in FIG. 35 .
  • the yield strength can be significantly increased (i.e. 469 MPa increase) by rolling at 200°C with minimal change in alloy ultimate tensile strength (i.e. 34 MPa increase) and elongation (i.e. 1.8% decrease).
  • This is provided by the example condition 3a in FIG. 2 .
  • this is a route to higher yield strength (i.e. 688 MPa increase) and tensile strength (i.e.
  • Step 4 in FIG. 3 could also be done by for example by cold stamping the part by various processes whereby the areas in the stamped part would experience higher yield strength and tensile strength with commensurate lower ductility which was used up partially in forming the part.
  • This Case Example demonstrates an achievement of high yield strength in alloys herein by various methods or their combination which provides a variety of the strength / elongation combinations in the resultant sheet from alloys herein.
  • Alloy 2 was produced in a sheet form with 1.4 mm thickness from the slab by hot rolling and cold rolling to a targeted thickness with subsequent annealing. Tensile specimens were cut from the Alloy 2 sheet using wire EDM. Tensile properties were measured at different temperatures in a range from -40°C to 200°C.
  • Tensile properties of the Alloy 2 sheet at different temperatures are listed in Table 26.
  • the magnetic phases volume percent was measured in the tensile sample gauge after testing at each temperature using Feritscope that is also listed in Table 26. As it can be seen, yield and ultimate tensile strength are decreasing with increasing test temperature while tensile elongation is increasing.
  • Tensile elongation and magnetic phases volume percent (Fe%) as a function of test temperature are plotted in FIG. 36 showing that despite higher elongation at elevated temperatures, the magnetic phases volume percent in a tensile sample gauge after testing drops significantly and close to zero after testing at 200°C.
  • Alloy 2 was processed into a hot band with thickness of 4.4 mm. Two sections of the hot band were then rolled, one at ambient temperature and one at 200°C. The plate at 200°C was heated in a mechanical convection oven for 30 minutes prior to rolling and reheated for 10 minutes between passes to ensure constant temperature.
  • the magnetic phases volume percent (Fe%) was measured by Feritscope at different levels of reductions during cold rolling and rolling at 200°C. The data are shown in FIG. 37 . As it can be seen, the magnetic phases volume percent (Fe%) increases rapidly with reduction at ambient temperature leading to the material limit for rolling at ⁇ 42%. In a case of the rolling at 200°C, the magnetic phases volume percent (Fe%) remains under 3 Fe% even at maximum rolling reduction of > 70%.
  • a sheet from Alloy 2 with final thickness of 1.2 mm was produced by utilizing both cold rolling and rolling at 200°C. In a case of cold rolling, the rolling was cycled with intermediate annealing to restore the alloy ductility and achieve the targeted thickness with reduction of 29% at final rolling step.
  • Tensile samples were EDM cut from the sheet with 1.2 mm thickness produced by both rolling methods and annealed at 1000°C for 135 sec.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • FIG. 38 Examples of the engineering stress-strain curves for the annealed sheet produced by both cold rolling and rolling at 200°C are shown in FIG. 38 . As it can be seen, despite different rolling methods towards targeted thickness, the final properties of the sheet after annealing are similar.
  • This Case Example demonstrates that rolling where the austenite is stable and does not transfer to ferrite as demonstrated here for Alloy 2 at 200°C significantly improves rolling ability of the alloys herein that will allow reduction in processing steps towards targeted sheet gauges.
  • this elevated temperature rolling can be used to hit a near final targeted gauge with high cold rolling reduction as provided in this example of > 70%.
  • This near final gauge material can then be annealed to restore the starting properties (i.e. the initial condition).
  • the final targeted gauge can be obtained by rolling in the temperature range provided in this application from 150 to 400°C following the steps and procedures in FIG. 2 or FIG. 3 .
  • Hot band was prepared from Alloy 2 with approximately9 mm thickness. It was heated to 200 to 250°C for 60 minutes and rolled to approximately 4.5 mm with 10 minute reheats between rolling passes to ensure consistent temperature. Once at 4.5 mm it was sectioned and annealed at 850°C for 10 minutes and allowed to air cool. The material was media blasted to remove the oxide and heated to the desired temperature for at least 30 minutes prior to rolling, and reheated for 10 minutes between passes to ensure consistent temperature. The material was rolled until failure (visible cracking) characterized by such visible cracks propagating in from the ends of the sheet at least 2 inches. At around 70% reduction the mill had difficulty achieving the loads necessary to reduce the material and rolling was stopped, this is an equipment limitation and not a material limitation. The control material for room temperature rolling was hot band at 4.4 mm thick which was rolled at room temperature until failure. The results of the maximum rolling reduction as a function of rolling temperature are provided in Table 27 and Fig. 39 .

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Claims (14)

  1. Verfahren zum Erhöhen der Streckgrenze in einer metallischen Legierung, das Folgendes umfasst:
    a. Zuführen einer Metalllegierung, die aus wenigstens 70 Atom-% Eisen und wenigstens vier Elementen besteht, die aus Si, Mn, Cr, Ni, Cu oder C und unvermeidlichen Verunreinigungen ausgewählt sind, Schmelzen der Legierung, Abkühlen mit einer Geschwindigkeit von 10-4 K/sec bis 103 K/sec und Erstarren auf eine Dicke von >5,0 mm bis 500 mm, wobei:
    wenn Si in der Legierung vorhanden ist, es in einer Menge vorhanden ist, die von über 0 bis 6,13 Atom-% reicht;
    wenn Mn in der Legierung vorhanden ist, es in einer Menge vorhanden ist, die von über 0 bis 15,17 Atom-% reicht;
    wenn Cr in der Legierung vorhanden ist, es in einer Menge vorhanden ist, die von über 0 bis 8,64 Atom-% reicht;
    wenn Ni in der Legierung vorhanden ist, es in einer Menge vorhanden ist, die von über 0 bis 9,94 Atom-% reicht;
    wenn Cu in der Legierung vorhanden ist, es in einer Menge vorhanden ist, die von über 0 bis 1,86 Atom-% reicht; und
    wenn C in der Legierung vorhanden ist, es in einer Menge vorhanden ist, die von über 0 bis 3,68 Atom-% reicht;
    b. Verarbeiten der Legierung zu einem ersten Blech mit einer Dicke von 0,5 bis 5,0 mm, wobei das erste Blech eine Gesamtdehnung von X1 (%), eine Zugfestigkeit von Y1 (MPa) und eine Streckgrenze von Z1 (MPa) aufweist, wobei X1 von 10,0 bis 70,0 % beträgt, Y1 von 900 bis 2050 MPa beträgt und Z1 von 200 bis 750 MPa beträgt;
    c. dauerhaftes Verformen des ersten Blechs bei einer Temperatur, die von 150 °C bis 400 °C reicht, durch ein Reduzieren der Dicke des ersten Blechs, um ein zweites Blech mit einer der folgenden Zugeigenschaftskombinationen A oder B herzustellen:
    A.
    (1) Gesamtdehnung X2=X1±7,5 %;
    (2) Zugfestigkeit Y2=Y1±100 MPa; und
    (3) Streckgrenze Z2>Z1+100 MPa.
    B.
    (1) Zugfestigkeit Y3=Y1±100 MPa; und
    (2) Streckgrenze Z3>Z1+200 MPa.
  2. Verfahren nach Anspruch 1, wobei die Legierung aus wenigstens 70 Atom-% Eisen und Si, Mn, Cr, Ni, Cu und C und unvermeidlichen Verunreinigungen besteht, wobei:
    Si in der Legierung in einer Menge vorhanden ist, die von über 0 bis 6,13 Atom-% reicht;
    Mn in der Legierung in einer Menge vorhanden ist, die von über 0 bis 15,17 Atom-% reicht;
    Cr in der Legierung in einer Menge vorhanden ist, die von über 0 bis 8,64 Atom-% reicht;
    Ni in der Legierung in einer Menge vorhanden ist, die von über 0 bis 9,94 Atom-% reicht;
    Cu in der Legierung in einer Menge vorhanden ist, die von über 0 bis 1,86 Atom-% reicht; und
    C in der Legierung in einer Menge vorhanden ist, die von über 0 bis 3,68 Atom-% reicht.
  3. Verfahren nach Anspruch 1, wobei das zweite Blech die Zugeigenschaftskombination A vorweist und die Zugeigenschaftskombination A wie folgt ist: X2=2,5 % bis 77,5 %, Y2=800 MPa bis 2150 MPa und Z2>300 MPa.
  4. Verfahren nach Anspruch 1, wobei das zweite Blech die Zugeigenschaftskombination B vorweist und die Zugeigenschaftskombination B wie folgt ist: Y3=800 MPa bis 2150 MPa und Z3>300 MPa.
  5. Verfahren nach Anspruch 1, wobei in Schritt (c) das erste Blech mit einer Reduktion der Dicke von über 20 % vor einem Versagen dauerhaft verformt wird.
  6. Verfahren nach Anspruch 1, wobei in Schritt (c) das erste Blech durch Rollformen, Metallstanzen, Metallziehen oder Hydroformen dauerhaft in das zweite Blech verformt wird.
  7. Verfahren nach Anspruch 1, wobei nach Schritt (a) die Legierung über 10 Vol-% Austenit enthält.
  8. Verfahren nach Anspruch 1, das ferner nach Schritt (c) Folgendes umfasst:
    (d) dauerhaftes Verformen des zweiten Blechs bei einer Temperatur von weniger als oder gleich 150 °C, um ein drittes Blech auszubilden.
  9. Verfahren nach Anspruch 8, wobei das dritte Blech die folgende Zugeigenschaftskombination vorweist:
    (1) Gesamtdehnung = 10,0 bis 40,0 %;
    (2) Zugfestigkeit = 1150 bis 2000 MPa;
    (3) Streckgrenze = 550 bis 1600 MPa.
  10. Verfahren nach Anspruch 9, wobei Schritt (b) bei einer Temperatur von 700 °C bis zu einer Temperatur unter dem Schmelzpunkt (Tm) der Legierung durchgeführt wird.
  11. Verfahren nach Anspruch 9, wobei die Legierung nach Schritt (b) bei einer Temperatur von 650 °C bis zu einer Temperatur unter dem Schmelzpunkt (Tm) der Legierung wärmebehandelt wird.
  12. Verfahren nach Anspruch 9, wobei in Schritt (c) das erste Blech durch Rollformen, Metallstanzen, Metallziehen oder Hydroformen dauerhaft in das zweite Blech verformt wird.
  13. Verfahren nach Anspruch 9, das ferner ein Positionieren des dritten Blechs in einem Fahrzeugrahmen, einem Fahrzeugchassis oder einer Fahrzeugplatte umfasst.
  14. Verfahren nach Anspruch 13, das ferner das Positionieren des dritten Blechs in einem Bohrkragen, einem Bohrgestänge, einem Rohrgehäuse, einer Werkzeugverbindung, einem Bohrlochkopf, einem Speichertank für komprimiertes Gas, einem Eisenbahnkesselwagen/Tankwagen oder Kanister für verflüssigtes Erdgas umfasst.
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WO2019143443A1 (en) * 2018-01-17 2019-07-25 The Nanosteel Company, Inc. Alloys and methods to develop yield strength distributions during formation of metal parts
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