Patent Application
20200255918
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. The elongation, ultimate tensile strength and yield strength are such that they can be maintained or even enhanced upon subsequent heat treatment that may be provided by a galvanization coating operation.
Third Generation Advanced High Strength Steels (AHSS) are currently being developed for automobile uses, and in particular automobile body applications. Advanced High-Strength Steels (AHSS) steels are classified by tensile strengths greater than 700 MPa with elongations from 4% to 30% and include such types as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, and complex phase (CP) steels. Example targets for 3rd Generation AHSS are provided in the banana chart for autobody steels which is published by World Auto Steel (
Tensile properties such as ultimate tensile strength (UTS) and total elongation are important benchmarks for establishing combinations of properties. However, 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.
A component in an automobile that experiences early yielding during normal service and undergoes permanent plastic deformation would be unacceptable based on most design criteria. In a crash event however, lower yield strengths, especially when coupled with a high strain hardening coefficient can be advantageous. This is especially true in the front and back ends of a passenger compartment which are often called the crumple zones. In these areas, a lower yield strength material with higher ductility can deform and strain harden increasing strength during the crash event leading to high levels of energy absorption due to the high starting ductility.
For other areas of the automobile, low yield strength would be unacceptable. Specifically, this would include what is called the passenger cage of an automobile. In the passenger cage, the materials utilized must have high yield strength since only very limited deformation/intrusion into the passenger cage is allowed. Once the passenger cage is penetrated this can lead to injury or death to the occupant(s). Thus, a material with high yield strength is required for these areas.
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. Chemistry modifications to an alloy such as the addition of alloying elements that exist in solid solution can also increase the yield strength of a material, however the addition of these alloying elements must take place while the material is molten and may result in increased costs.
Developing high yield strength in the passenger cage from a low yield strength version of AHSS is a possible route. However, it is difficult in many metalworking operations to strain harden the finished part uniformly. This means that while the heavily cold worked areas of a part are much higher yield, there would still be lower yield strength areas which might then deform and cause an unacceptable intrusion into the passenger space.
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. Cold rolling also can introduce anisotropy into the material which will farther reduce its ability to be cold formed into parts.
Steels, which are not stainless, corrode under normal atmospheric conditions and because the oxide spalls, the corrosion or rusting process often continue until failure. Zinc is reportedly used to coat steels and a zinc coating onto steel is applied through a process called galvanization. Zinc coating prevents the steel from corroding and, unlike for iron, the corrosion byproduct is adherent and provides additional corrosion protection.
A method of forming a metal alloy into sheet comprising:
In the above, the thermal exposure in step (d) can optionally be provided during a zinc or zinc alloy galvanization coating procedure. Accordingly, the method herein may also be summarized also as follows:
A method of forming a metal alloy into sheet comprising:
The metallic alloys produced herein provide particular utility in vehicles, railway cars, railway tank cars/wagons, drill collars, drill pipe, pipe casing, tool joints, wellheads, compressed gas storage tanks or liquefied natural gas canisters. More specifically, the alloys find utility in vehicular bodies in white, vehicular frames, chassis, or panels and can be uncoated or zinc or zinc alloy coated/galvanized.
The detailed description below may be better understood with reference to the accompanying FIG.s which are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
In 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 a temperature of 650° C. to a temperature below the melting point (Tm) of said alloy.
The steps to produce sheet from the cast product can therefore vary depending on specific manufacturing routes and specific targeted goals. As an example, consider thick slab casting as one process route to get to sheet of this targeted thickness. 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. If additional gauge reduction is needed, 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. Typically, cold rolled thickness would be 0.5 to 2.5 mm thick. Preferably, 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. In this case, after casting typically forms 35 to 150 mm in thickness by going through a water-cooled mold, 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. After hot rolling, 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. Note that 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.
Notwithstanding the specific process in going from the cast material in Step 1 to Step 2, once the sheet is formed in the preferred range from 0.5 mm to 5.0 mm, the sheet will then exhibit a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (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.
In Step 3 of Method 1, 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. Rather than doing the traditional cold rolling for final gauge reduction with the sheet starting at ambient temperature, 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. Alternatively, 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.
Notwithstanding the specific process to permanently deform the alloy in the temperature range of 150 to 400° C., two distinct conditions can be formed which are shown in Condition 3a and Condition 3b in
In 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 Y3 is equal to Y1±100 MPa and yield strength Z3 is ≥Z1+200 MPa. Preferred properties for this alloy in Condition 3b would be ultimate tensile strength values (Y3) from 800 to 2150 MPa and yield strength (Z3)≥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 (X3) is defined as follows: X3<X1−7.5%.
As will be shown by various case examples, with normal deformation, a metallic material will strain harden/work harden. This is shown for example by the strain hardening exponent (n) in the relationship σ=K εn between stress (σ) and strain (ε). The ramifications of this is that as a material is permanently deformed the basic material properties change. Comparing the initial condition to the final condition will show the typical and expected behavior where yield strength and tensile strength is increased with commensurate reductions in total ductility. Specific case examples are provided to illustrate this effect and then contrast this with the new material behavior noted in this disclosure.
As presented previously, in the description of
Step 4 involves subjecting the reduced thickness sheet formed in Step 3 to a thermal exposure from ≥400° C. to ≤775° C. and for a time of ≥25 s to ≤225 s (s=seconds). Preferred properties for the alloys herein after Step 4 of Method 3 is a total elongation from 10.0 to 65%, ultimate tensile strength from 1100 to 1600 MPa, and yield strength from 500 to 1500 MPa. This provides an increase in the range of total elongation identified in Step 3 (2.0 to 35.0%) enabling subsequent forming operations including of roll forming, metal stamping, metal drawing, or hydroforming while preserving preferred levels of yield strength (i.e. 500 to 1500 MPa).
Step 4 of Method 3 is unique compared to Method 1 and Method 2, in that the thermal exposure which is applied is done without simultaneously applying stress/permanent deformation. Additionally, the thermal exposure in Step 4 of Method 3 of >400° C. to <750° C. is higher than that of Step 3 of Method 1 and Step 3 of Method 2.
The thermal exposure needed for Step 4 of Method 3 is preferably done in a relatively short continuous annealing manner as opposed to the relatively longer times that are found in batch annealing, such as 8 to 24 hours of time. These relatively long temperature exposures will result in deleterious changes in structure including complete recovery of cold work, recrystallization, and grain growth all of which will reduce ductility to levels below the preferred levels of yield strength (i.e. 500 to 1500 MPa). Preferably, the thermal exposure that is achieved in Step 4 is provided herein during a galvanization coating operation. Reference to a galvanization coating operation is reference to coating of the sheet from Step 3 by exposure to a bath of molten zinc or zinc alloys. Zinc alloys are those that contain additives (≤5.0 wt. % total) such as iron, aluminum, silicon, lead, cadmium, copper, magnesium, tin, or antimony. Such additives may therefore be present at a level of 0.1 wt. % up to 5.0 wt. %. This is often referred to as a hot dip galvanization process. Such hot dip galvanization process can be configured to provide the thermal exposure requirements noted herein (i.e. thermal exposure from ≥400° C. to ≤775° C. for a time of ≥25 seconds to ≤225 seconds. Typical thickness of zinc or zinc alloys applied, is from 5 μm to 100 μm thick which can be applied on one side or both sides of the sheet. As now can be appreciated, developing the aforementioned properties (a total elongation from 10.0 to 65%, ultimate tensile strength from 1100 to 1600 MPa, and yield strength from 500 to 1500 MPa) during a galvanization coating process is efficient from the perspective that two steps (coating and thermal exposure) are achieved in one.
The structures and mechanisms in this application leading to the new process route for developing high yield strength are tied to the following chemistries of alloys provided in Table 1.
As can be seen from Table 1, the alloys herein are iron based metal alloys, having greater than 70 at. % Fe. In addition, it can be appreciated that 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 6.5 at. %); Mn (0 to 15.5 at. %); Cr (0 to 9.0 at. %); Ni (0 to 10.5 at. %); Cu (0 to 2.5 at. %); and C (0 to 4.0 at. %). Most preferably, 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 2000 ppm. With regards to minimum levels of the elements when selected, they would preferably be as follows: Si (1.0 at. %), Mn (3.0 at. %), Cr (0.5 at. %); Ni (0.5 at. %); Cu (0.25 at. %); C (0.5 at. %). In such regard, if Si is selected, it is preferably at a level of 1.0 at. % to 6.5 at. %, if Mn is selected, it is preferably at a level of 3.0 at. % to 15.5 at. %, if Cr is selected, it is preferably at a level of 0.5 at. % to 9.0 at. %, if Ni is selected, it is preferably at a level of 0.5 at. % to 10.5 at. %, if Cu is selected it is preferably at a level of 0.25 at. % to 2.5 at. %, if C is selected it is preferably at a level of 0.5 at. % to 4.0 at. %. It should be appreciated, however, that when selecting, e.g. a minimum level of Si, the levels of the other elements (including Fe) are preferably selected such that the atomic percent of all elements present (i.e. Fe, selected elements, impurities) totals 100 atomic percent. Finally, it should be appreciated that a preferred level of Fe is in the range of 70 atomic percent to 85 atomic percent.
Alloys were weighed out into 3,400 gram charges using commercially available ferroadditive powders and a base steel feedstock with known chemistry according to the atomic ratios in Table 1. As alluded to above, 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.
Charges were loaded into a zirconia coated silica crucible which was placed into an Indutherm VTC800V vacuum tilt casting machine. The machine then evacuated the casting and melting chambers and flushed with argon to atmospheric pressure twice prior to casting to prevent oxidation of the melt. The melt was heated with a 14 kHz RF induction coil until fully molten, approximately from 5 to 7 minutes depending on the alloy composition and charge mass. After the last solids were observed to melt it was allowed to heat for an additional 30 to 45 seconds to provide superheat and ensure melt homogeneity. 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
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
Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace to heat. The furnace set point varies between 1100° C. to 1250° C., depending on alloy melting point and point in the hot rolling process, with the initial temperatures set higher to facilitate higher reductions, and later temperatures set lower to minimize surface oxidation on the hot band. The slabs were allowed to soak for 40 minutes prior to hot rolling to ensure they reach the target temperature and then pushed out of the tunnel furnace into a Fenn Model 061 2 high rolling mill. The 50 mm casts are hot rolled for 5 to 10 passes though the mill before being allowed to air cool. Final thickness ranges after hot rolling are preferably from 1.8 mm to 4.0 mm with variable reduction per pass ranging from 20% to 50%.
After the hot rolling, 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
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
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. With increasing cold reduction as a final step after annealing, a corresponding increase of the material yield strength is demonstrated by tensile stress-strain curves in
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).
To prepare SEM samples, pieces were cut by EDM and mounted in epoxy, and polished progressively with 9 μm, 6 μm and 1 μm diamond suspension solution, and finally with 0.02 μm silica. To prepare TEM specimens, the samples were cut from the sheet with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thick is done by polishing with 9 μm, 3 μm and 1 μm diamond suspension solution respectively. 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. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). 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.
SEM analysis of the hot band structure revealed relatively large austenite grains with straight boundaries (
When the hot band was subjected to cold rolling, the austenite phase in selected areas of the hot band structure transforms to refined ferrite phase under stress. Backscattered SEM images of the cold rolled sheet show the transformed and refined structure, and the presence of deformation twins (
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
The magnetic phases volume percent (Fe %) was measured after rolling, in the tensile gauge at least 10 mm from fracture are reported in Table 7. As it can be seen, the magnetic phases volume percent after rolling at temperature of 100° C. and above is significantly lower in a range from 0.3 to 9.7 Fe % as compared to that after cold rolling Alloy 2 at ambient temperature (18.0 Fe %, Table 4). A significant increase in a magnetic phases volume percent was measured in the Alloy 2 after rolling at temperature and tensile tested (Table 7,
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. Moreover, 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.
The responses of 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). For Alloy 18, tensile elongation ranged from 43.0% to 51.9%, ultimate tensile strength ranged from 1083 MPa to 1263 MPa, and yield strength ranged from 772 MPa to 924 MPa from 150 to 400° C. (Table 10), with 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). For Alloy 34, tensile elongation ranged from 21.1% to 31.1%, ultimate tensile strength ranged from 1080 MPa to 1140 MPa, and 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). For Alloy 37, tensile elongation ranged from 1.5% to 9.0%, ultimate tensile strength ranged from 1537 MPa to 1750 MPa, and 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).
Representative curves for each alloy herein are shown in
This Case Example demonstrates that yield strength in alloys herein can be increased although phase transformation of austenite into ferrite is reduced when rolling at temperatures of 100° C. or greater up to 400° C. Examples of changes in yield strength, ultimate tensile strength, and tensile elongation were provided for both Steps 3a and 3b in
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.
Tensile properties of Alloy 2 after rolling at 200° C. with different rolling reduction (0.0 to 70.0%) are listed in Table 16, which also includes data prior to any rolling experiments.
The total elongation of Alloy 2 is plotted as a function of rolling reduction at 200° C. in
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
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. In the broad context of the present disclosure, the temperature range is contemplated to be between 150° C. to 400° C. as provided in the previous case example for Table 7. During this rolling, 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. Thereby, 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%. Microstructure of the initial structure and after rolling was studied by scanning electron microscopy (SEM). To prepare SEM samples, pieces were cut by EDM and mounted in epoxy, and polished progressively with 9 μm, 6 μm and 1 μm diamond suspension solution, and finally with 0.02 μm silica.
After the rolling reduction is increased to 70%, the bands are no longer visible, and refined structure through the volume can be seen (
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.
Rolling at temperature resulted in significant increase in yield strength of the Alloy 2 while high tensile elongation was maintained. TEM study was conducted on the Alloy 2 rolled at 200° C. to analyze the structural changes during the rolling at 200° C. as a function of rolling strain. In this case example, 50 mm thick laboratory cast slab was hot rolled first, and the resultant hot band was then rolled at 200° C. to different strains. To show structural evolution, microstructures of the rolled sheets were studied by transmission electron microscopy (TEM). To prepare TEM specimens, the samples were cut from the sheet using wire-EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thick samples was done by polishing with 9 μm, 3 μm and 1 μm diamond suspension solutions, respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled by electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens were ion-milled using a Gatan Precision Ion Polishing System (PIPS). 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 that the alloys herein maintain austenite structure during rolling at 200° C. with up to 70% reduction. Structural changes including dislocation cell formation and twinning leads to increase in yield strength after rolling at 200° C.
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.), up to 1571 MPa in Alloy 7 (yield strength in hot band is 333 MPa and 575 MPa after rolling at 200° C.), up to 1080 MPa in Alloy 18 (yield strength in hot band is 390 MPa and 834 MPa after rolling at 200° C.), and up to 1248 MPa in Alloy 34 (yield strength in hot band is 970 MPa and 1120 MPa after rolling at 200° C.).
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° C. temperature. 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
Representative stress-strain curves with property combination achieved at each processing method close to optimal are shown in
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
This Case Example demonstrates that multicomponent alloying of the alloys herein resulted in significant increase of austenite stability and transformation to ferrite during rolling is shown to be suppressed at elevated temperatures as compared to cold rolling as clearly provided in the last column in Table 26. It provides higher ductility during rolling itself and higher formability at subsequent sheet forming operations such as stamping, drawing, etc.
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.
In a case of rolling at ambient temperature, the failure occurred at approximately 42% reduction while a reduction of more than 70% was applied during rolling at 200° C. without the failure when the limit of the mill achieved. Mill limitations occurred when the Fenn Model 061 rolling mill could no longer make significant reductions per pass during cold rolling while the material still has ability for further rolling reduction.
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
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.
Examples of the engineering stress-strain curves for the annealed sheet produced by both cold rolling and rolling at 200° C. are shown in
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. Thus, 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). Subsequently, 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
Hot band was prepared from Alloy 2 with approximately 9 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
This Case Example demonstrates for the alloys herein that the limiting rolling reduction increases as temperature increases. It therefore can be seen that the alloys herein are contemplated to allow for permanent deformation with a reduction in thickness of greater than 20% before failure when heated to a temperature falling in the range of 150° C. to 400° C. More preferably, the alloys herein are such that they are contemplated to be capable of permanent deformation with a reduction in thickness of greater than 40% before failure when heated in such temperature range. This provides much greater potential deformation for rolling operations, including processing of industrial material to reach a target gauge. Greater reductions before cracking means that less steps (i.e. cold rolling and recrystallization annealing) may be required to hit a specific targeted gauge during steel production. Additionally, the greater formability demonstrated at elevated temperatures would be beneficial in making parts from a variety of forming operations including, stamping, roll forming, drawing, hydroforming etc.
Hot band from the alloys listed in Table 1 was cold rolled (i.e. permanent deformation at <150° C. resulting in a thickness reduction without any external heating) in Case Example #1 to provide the cold rolled properties in Table 3. From the same cold rolled sheet, additional tensile specimens were cut using wire EDM and the samples were then used for further targeted annealing studies to demonstrate high yield with ductility. Sets of 3 samples from each alloy were annealed in a Lucifer 7GT-K12 box furnace with a set point at 775° C. and dwell time of 80 seconds followed by air cooling. Note that these parameters were chosen since they simulate the conditions on a hot dip galvanization line for coating. Due to the short annealing time, samples did not reach the furnace set point temperature, thus, an instrumented sample was included to record the peak temperature of the samples during the annealing. 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 and galvanization simulation annealing including peak annealing temperatures are listed in Table 28. Additionally, for each alloy, tensile curves are provided after cold rolling and after galvanization simulation as shown in
This Case Example illustrates that high yield strength from 538 to 1490 MPa with improved ductility, can be achieved in the cold rolled alloys herein during heat exposure during galvanization process at a temperature from 652 to 709° C.
Hot band from Alloy 2 was cold rolled (i.e. permanent deformation at ≤150° C. resulting in a thickness reduction without any external heating) with final cold rolling reductions of 25% and 29% to a final thickness of approximately 1.4 mm. Tensile specimens were cut from each cold rolled sheet using wire EDM. Sets of 3 samples were annealed in a Lucifer 7GT-K12 box furnace with a dwell time of 80 seconds followed by air cooling that simulates the potential conditions on the galvanization line during commercial production. An instrumented sample was included to record the peak temperature of the samples during the annealing. The furnace set point was varied from 500° C. to 850° C. to achieve peak temperatures from 405° C. to 752° C. 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 and galvanization simulation annealing at different temperatures are listed in Table 29. Additionally, for the 25% and 29% reduced samples, the individual tensile curves for various thermal exposures are provided in
This Case Example illustrates that high yield strength from 560 MPa to 1141 MPa with improved ductility, can be achieved in the cold rolled alloys herein during galvanization process simulation in a wide temperature range from 405 to 752° C.
Hot band from Alloys 2 and Alloy 13 were cold rolled with final cold rolling reductions of 32% and 37%, respectively, to a final thickness of approximately 1.2 mm. Tensile specimens were cut from each cold rolled sheet using wire EDM. Sets of 3 samples were annealed in a Lucifer 7GT-K12 box furnace with a furnace set point at 725° C. for variable time followed by air cooling that simulates the potential conditions on the galvanization line during commercial production. An instrumented sample was included to record the peak temperature of the samples during the annealing. The annealing time varied from 50 seconds to 200 seconds to achieve peak annealing temperatures from 532° C. to 709° C. 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 and galvanization simulation annealing with various dwell time are listed in Table 30. Additionally, tensile curves are provided for Alloys 2 and 13 as a function of various time exposures in
This Case Example illustrates that high yield strength from 564 to 1184 MPa with improved ductility can be achieved in alloys herein during galvanization process with a wide range of time at temperature ranging from 50 to 200 s.
The present application claims the benefit of U.S. Provisional Application 62/804,932 filed Feb. 13, 2019, the teachings of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62804932 | Feb 2019 | US |
