WO2007045038A1 - A steel product with a high austenite grain coarsening temperature, and method for making the same - Google Patents
A steel product with a high austenite grain coarsening temperature, and method for making the same Download PDFInfo
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- WO2007045038A1 WO2007045038A1 PCT/AU2006/001554 AU2006001554W WO2007045038A1 WO 2007045038 A1 WO2007045038 A1 WO 2007045038A1 AU 2006001554 W AU2006001554 W AU 2006001554W WO 2007045038 A1 WO2007045038 A1 WO 2007045038A1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
- B22D27/20—Measures not previously mentioned for influencing the grain structure or texture; Selection of compositions therefor
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
- B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/10—Supplying or treating molten metal
- B22D11/11—Treating the molten metal
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/10—Supplying or treating molten metal
- B22D11/11—Treating the molten metal
- B22D11/116—Refining the metal
- B22D11/117—Refining the metal by treating with gases
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/041—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular fabrication or treatment of ingot or slab
- C21D8/0415—Rapid solidification; Thin strip casting
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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12993—Surface feature [e.g., rough, mirror]
Definitions
- the present invention relates to steel products .
- Refinement of the ferrite grain size has led to improvement in the strength and toughness of steels.
- the final ferrite grain size of the steel can be determined, in large part, by the austenite grain size prior to cooling and transformation to ferrite grains.
- austenite grain growth also occurs during the processing of the steel, e.g., during hot rolling, thermomechanical processing, normalizing, welding, enamelling or annealing. If coarse austenite grains are formed during such processing, they are often difficult to refine in subsequent processing operations, and such refinement comes at added cost in processing of the steel.
- Coarsening of austenite grains during processing can result in the steel having poor mechanical properties .
- This invention relates to carbon steel products that exhibit a high austenite grain coarsening — O — temperature, without the necessity for additions of conventional austenite grain refining elements such as Al 7 Nb, Ti, and V. These elements form nitride or carbo- nitride particles , which act to provide a high austenite grain coarsening temperature, whereas the steel of this invention utilizes precipitated, fine oxide particles comprising Si, Fe and O to achieve similar high austenite coarsen temperatures .
- the steel composition presently disclosed has high levels of oxygen and a dispersion of silicon and iron oxide particles of less than 50 nanometers and generally ranging from ranging in size from 5 to 30 nanometers .
- the ability to restrict austenite grain growth during heat treatment cycles and welding processes facilitates the achievement of a fine final microstructure on cooling to ambient temperature .
- a high austenite grain coarsening temperature provides a wide temperature range from which a known and reliable austenite grain size will be produced, which aids in achieving the desired final microstructure .
- the resultant fine ferrite grain size is conducive to achieving an attractive combination of strength, toughness and formability.
- the steel product presently disclosed also exhibits a high ferrite recrystallization temperature. Such an attribute can restrict or even prevent the extent of critical strain grain growth of ferrite. This phenomenon is induced through heating lightly plastically strained areas in cold formed steel products to subcritical temperatures .
- the resultant large ferrite grain size can provide a low strength region in the formed product, which may be deleterious to the performance of the product.
- the nucleation rate of new recrystallised ferrite grain size is low, which leads to the growth of large ferrite grains .
- the steel product of the present invention may be made by continuous casting strip steel in a twin roll caster.
- twin roll casting molten metal is introduced between a pair of counter-rotated horizontal casting rolls, which are cooled, so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip.
- nip is used herein to refer to the general region at which the rolls are closest together.
- the molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip.
- This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
- the molten steel in the casting pool will generally be at a temperature of the order of 1500 to 1600° C, and above, and therefore high cooling rates are needed over the casting roll surfaces . It is important to achieve a high heat flux and extensive nucleation on initial solidification of the steel on the casting surfaces to form the metal shells during casting.
- United States Patent 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry so that a substantial proportion of the metal oxides formed as deoxidation products are liquid at the initial solidification temperature so as to form a substantially liquid layer at the interface between the molten metal and the casting surface.
- nucleation of the steel on initial solidification can be influenced by the texture of the casting surface .
- International Application AU 99/00641 discloses that a random texture of peaks and troughs can enhance initial solidification by providing potential nucleation sites distributed throughout the casting surfaces .
- nucleation is also dependent on the presence of oxide inclusions in the steel melt and that, surprisingly, it is not advantageous in twin roll strip casting to cast with "clean" steel in which the number of inclusions formed during deoxidation has been minimized in the molten steel prior to casting.
- the molten steel contains a distribution of oxide inclusions (typically MnO, CaO, SiO 2 and/or Al 2 O 3 ) sufficient to provide an adequate density of nucleation sites on the casting roll surfaces for initial and continued high solidification rates and the resulting strip product exhibits a characteristic distribution of solidified inclusions and surface characteristics.
- oxide inclusions typically MnO, CaO, SiO 2 and/or Al 2 O 3
- a steel product with a high austenite grain coarsening temperature comprising less than 0.4% carbon, less than 0.06 % aluminium, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium by weight and having fine-size oxide particles containing silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers in size, or less than 40 nanometers in size.
- the average oxide particle size may be between 5 and 30 nanometers.
- the aluminium content may be less than 0.05% or 0.02% or 0.01%.
- the molten steel used to produce the steel product may include oxide inclusions comprising any one or more of MnO, SiO 2 and Al 2 O 3 distributed through the steel at an inclusion density in the range 2 gm/cm 3 to 4 gm/cm 3 .
- the oxide inclusions in the molten steel may range in size between 2 and 12 microns .
- the steel product with a high austenite grain coarsening temperature may comprise less than 0.4% carbon, less than 0.06% aluminium, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium by weight and having fine-size oxide particles capable of producing austenite grains through the microstrueture resistant to coarsening at high temperature.
- the steel microstructure has an average austenite grain size of less than 50 microns, or less than 40 microns, up to at least 1000° C, or even greater than 1050 0 C, for a holding time of at least 20 minutes.
- the average austenite grain size may be between 5 and 50 microns up to least 1000 0 C, or at least 1050 0 C, for a holding time of at least 20 minutes.
- the fine particles may be oxides of silicon and iron less than 50 nanometers in size.
- the aluminium content may be less than 0.05% or 0.02% or 0.01% by weight.
- the steel product with a high austenite grain coarsening temperature is a carbon steel of less than 0.4% carbon, less than 0.06% aluminium, less than 0.01 % titanium, less than 0.01% niobium, and less than 0.02% vanadium by weight may be capable of resisting ferrite recrystallization up to temperatures of 750° C for strain levels up to at least 10% (for conventional processing heating rates and holding times up to at least 30 minutes) .
- the steel product with a high austenite grain coarsening temperature may have a carbon content less than 0.01%, or less than 0.005%, and aluminium content less than 0.01% or less than 0.005%.
- the steel product with a high austenite grain coarsening temperature may be made in a twin roll caster with the molten steel having total oxygen content in the casting pool of at least 70 ppm, usually less than 250 ppm, and a free-oxygen content of between 20 and 60 ppm.
- the molten steel may have total oxygen content in the casting pool of at least 100 ppm, usually less than 250 ppm, and a free-oxygen content between 30 and 50 ppm.
- the closely controlled chemical composition of the molten steel, particularly the soluble oxygen content, and the very high solidification rate of the process, provide conditions for the formation of fine-sized, generally spheroid-shaped oxide particles distributed through the steel microstructure, which restrict the average austenite grain size, on subsequent reheating to less than 50 microns for temperatures up to least 1000° C for a holding time of at least 20 minutes.
- the austenite grain coarsening properties exhibited by the present steel product are similar to or better than those generally observed with conventional normalized aluminium killed steels, where the presence of aluminium nitride particles in the steel microstructure act to restrict austenite grain growth.
- the austenite grain coarsening properties of the steel in fact approach the grain coarsening properties observed with titanium treated aluminium killed- continuously slab cast steels. See, JP
- the steel product presently described with a high austenite grain coarsening temperature has a microstrueture with austenite grain growth inhibition better than aluminium killed fine grained steels in the absence of the conventional grain refining elements , aluminium, titanium, niobium and vanadium: Unique steel with different microstructure and resulting strength properties is thus provided by the present cast steel, and without the added costs associated with such fine grained steels in the past.
- the austenite grain coarsening properties of the present cast steel confers benefits as refinement of the microstructure of the heat affected zone associated with welding processes and other heat treatments such as normalizing, enamelling and annealing. In the past, excessive coarsening of austenite grains during heat treatment has been found to lead to coarse microstructure in the steel on cooling and an associated loss of strength and toughness in the steel at ambient temperatures .
- titanium, niobium and vanadium levels in the steel products presently disclosed are generally those observed as impurities introduced by using scrap as a starting material for making the steel in an electric arc furnace.
- purposeful introduction of titanium, niobium and vanadium may be made without avoiding the presently claimed invention where the levels are so low that they do not provide the fine grain features by alternative means as discussed above.
- a low carbon steel strip with a high austenite grain coarsening temperature may be made by the steps comprising: assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip; introducing molten carbon steel between said pair of casting rolls to form a casting pool between the casting rolls with said closures confining the pool adjacent the ends of the nip, with the molten steel having a total oxygen content in the casting pool of at least 70 ppm, usually less than 250 ppm, and a free- oxygen content of between 20 and 60 ppm; counter rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.
- a carbon steel strip with a high a ⁇ stenite grain coarsening temperature may also be made by the steps comprising: assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip; introducing molten carbon steel between said pair of casting rolls to form a casting pool between the casting rolls with said closures confining the pool adjacent the ends of the nip, with the molten steel having a total oxygen content in the casting pool of at least 100 ppm, usually less than 250 ppm, and a free- oxygen content between 30 and 50 ppm; counter rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.
- the total oxygen content of the molten steel in the casting pool may be about 200 ppm or about 80-150 ppm.
- the total oxygen content includes free oxygen content between 20 and 60 ppm or between 30 and 50 ppm.
- the free oxygen may be measured at a temperature between 154O 0 C and 1600 0 C, which is the typical temperature of the molten steel in the metal delivery system where the oxygen content is typically measured.
- the total oxygen content includes, in addition to the free oxygen, the deoxidation inclusions present in the molten steel at the introduction of the molten steel into the casting pool.
- the free oxygen is formed into solidification inclusions adjacent to the surface of the casting rolls during formation of the metal shells and cast strip.
- the low carbon steel here is defined as steel with a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.01% to 2.0% by weight and a silicon content in the range 0.20% to 10% by weight.
- the steel may have aluminum content of the order of 0.02% or 0.01%, or less, by weight. The aluminum may for example be as little as 0.008% or less by weight.
- the molten steel may be a silicon/manganese killed steel .
- the oxide inclusions are solidification inclusions and deoxidation inclusions .
- the solidification inclusions are formed during cooling and solidification of the steel in casting, and the oxidation inclusions are formed during deoxidation of the molten steel before casting.
- the solidified steel may contain oxide inclusions usually comprised of any one or more of MnO, Si ⁇ 2 and AI 2 O 3 distributed through the steel at an inclusion density in the range 2 gm/cm 3 and 4gm/cm 3 .
- the molten steel may be refined in a ladle prior to introduction between the casting rolls to form the casting pool by heating a steel charge and slag forming material in the ladle to form molten steel covered by a slag containing silicon, manganese and calcium oxides.
- the molten steel may be stirred by injecting an inert gas into it to cause desulphurization, and then injecting oxygen, to produce molten steel having the desired total oxygen content of at least 70 ppm, usually less than 250 ppm, and a free oxygen content between 20 and 60 ppm in the casting pool .
- the total oxygen content of the molten steel in the casting pool may be at least 100 ppm and the free oxygen content between 30 and 50 ppm.
- the total oxygen and free oxygen contents in the ladle are generally higher than in the casting pool, since both the total oxygen and free oxygen contents of the molten steel are directly related to its temperature, with these oxygen levels reduced with the lowering of the temperature in going from the ladle to the casting pool.
- the desulphurization may reduce the sulphur content of the molten steel to less than 0.01% by weight.
- the thin steel strip produced by continuous twin roll casting as described above has a thickness of less than 5 mm and is formed of cast steel containing solidified oxide inclusions .
- the distribution of the inclusions in the cast strip may be such that the surface regions of the strip to a depth of 2 microns from the outer faces contain solidified inclusions to a per unit area density of at least 120 inclusions/mm 2 .
- the solidified steel may be a silicon/manganese killed steel and the oxide inclusions may comprise any one or more of MnO, SiO 2 and Al 2 O 3 inclusions.
- the inclusions typically may range in size between 2 and 12 microns, so that at least a majority of the inclusions are in that size range.
- the method described above produces a unique steel high in oxygen content distributed in oxide inclusions.
- the combination of the high oxygen content in the molten steel and the short residence time of the molten steel in forming steel strip has resulted in unique steel with improved ductility and toughness properties .
- Figure 1 shows the effect of inclusion melting points on heat fluxes obtained in twin roll casting trials using silicon/manganese killed steels
- Figure 2 is an energy dispersive spectroscopy (EDS) map of Mn showing a band of fine solidification inclusions in a solidified steel strip;
- EDS energy dispersive spectroscopy
- Figure 3 is a plot showing the effect of varying manganese to silicon contents on the liquidus temperature of inclusions ;
- Figure 4 shows the relationship between alumina content (measured from the strip inclusions) and deoxidation effectiveness
- Figure 5 is a ternary phase diagram for
- Figure 6 shows the relationship between alumina content inclusions and liquidus temperature
- Figure 7 shows the effect of oxygen in a molten steel on surface tension
- Figure 8 is a plot of the results of calculations concerning the inclusions available for nucleation at differing steel cleanliness levels .
- Figures 9-13 are plots showing the total oxygen content of production steel melts in the tundish immediately above the casting pool of molten steel during casting of thin strip with a twin-roll caster;
- Figures 14-18 are plots of the free oxygen content of the same productions steel melts reported in Figures 9-
- Figure 19 is a TEM photomicrograph showing dispersion of the fine-sized particles in a thin cast strip of the present invention
- Figure 20 is the energy dispersive spectroscopy
- FIG. 19 is a graph of average austenite grain size as a function of temperature for a holding time of 20 minutes for a steel product of the present invention
- Figure 22 shows photomicrographs of the microstructure of a steel product of the present invention and a conventional hot rolled A1006 strip steel after bending and heating to 600 0 C, 650 0 C, 700 0 C, 750 0 C, 800 0 C, and 85O 0 C; and
- Figure 23 is a graph showing the critical strain levels required to induce ferrite iron recrystallization in a high temperature steel product of the present invention and a conventional hot rolled A1006 strip steel.
- Liquid inclusions are not produced when their melting points are higher than the steel temperature in the casting pool. Therefore, there is a dramatic reduction in heat transfer rate when the inclusion melting point is greater than approximately 1600° C. With casting trials, we found that with aluminum killed steel; the formation of high melting point alumina inclusions (melting point 2050°C) could be limited if not avoided by, calcium additions to the composition to provide liquid CaO-Al 2 ⁇ 3 inclusions.
- the solidification oxide inclusions are formed in the solidified metal shells. Therefore, the thin steel strip comprises inclusions formed during cooling and solidification of the steel, as well as deoxidation inclusions formed during refining in the ladle.
- Mn+Si+30 MnO-SiO 2
- the comparative levels of the solidification inclusions are primarily determined by the Mn and Si levels in the steel .
- Figure 3 shows that the ratio of Mn to Si has a significant effect on the liquidus temperature of the inclusions.
- a manganese silicon killed steel having a carbon content in the range of 0.001% to 0.1% by weight, a manganese content in the range 0.1% to 2.0% by weight and a silicon content in the range 0.1% to 10% by weight and an aluminum content of the order of 0.01% or less by weight can produce such solidification oxide inclusions during cooling of the steel in the upper regions of the casting pool .
- the steel may have the following composition, termed MO6 : Carbon 0.06% by weight
- Deoxidation inclusions are generally generated during deoxidation of the molten steel in the ladle with Al, Si and Mn .
- the composition of the oxide inclusions formed during deoxidation is mainly MnO- Si ⁇ 2 -Al 2 O 3 based. These deoxidation inclusions are randomly located in the strip and are coarser than the solidification inclusions near the strip surface formed by reaction of the free oxygen during casting.
- the alumina content of the inclusions has a strong effect on the free oxygen level in the steel and can be used to control the free oxygen levels in the melt.
- Figure 4 shows that with increasing alumina content, the free oxygen levels in the steel is reduced.
- the free oxygen reported in Figure 4 was measured using the Celox® measurement system made by Heraeus Electro-Nite, and the measurements normalized to 1600 0 C to standardize reporting of the free oxygen content as in the following claims .
- MnO-SiO 2 inclusions are diluted with a subsequent reduction in their activity, which in turn reduces the free oxygen level, as seen from the following reaction:
- Both solidification and deoxidation inclusions are oxide inclusions and provide nucleation sites and contribute significantly to nucleation during the metal solidification process, but the deoxidation inclusions may be rate controlling in that their concentration can be varied and their concentration affects the concentration of free oxygen present.
- the deoxidation inclusions are much bigger, typically greater than 4 microns, whereas the solidification inclusions are generally less than 2 microns and are MnO-SiO 2 based, and have no Al 2 O 3 whereas the deoxidation inclusions also have Al 2 O 3 present as part of the inclusions.
- the total oxygen content may be measured by a ⁇ Leco" instrument and is controlled by the degree of "rinsing" during ladle treatment, i.e., the amount of argon bubbled through the ladle via a porous plug or top lance, and the duration of the treatment.
- the total oxygen content was measured by conventional procedures using the LECO TC-436 Nitrogen/Oxygen Determinator described in the TC 436 Nitrogen/Oxygen Determinator Instructional Manual available from LECO (Form No. 200-403, Rev. Apr. 96, Section 7 at pp. 7-1 to 7-4.
- the free oxygen levels in Ca-Si grades were lower, typically 20 to 30 ppm compared to 40 to 50 ppm with M06 grades .
- Oxygen is a surface-active element and thus reduction in free oxygen level is expected to reduce the wetting between molten steel and the casting rolls and causes a reduction in the heat transfer rate between the metal and the casting rolls.
- free oxygen reduction from 40 to 20 ppm may not be sufficient to increase the surface tension to levels that explain the observed reduction in the heat flux.
- the relationship of the oxygen content of the liquid steel on initial nucleation and heat transfer has been examined using a model described in Appendix 1.
- This model assumes that all the oxide inclusions are spherical and are uniformly distributed throughout the steel.
- a surface layer was assumed to be 2 microns and it was assumed that only inclusions present in that surface layer could participate in the nucleation process on initial solidification of the steel .
- the input to the model was total oxygen content in the steel, inclusion size, strip thickness, casting speed, and surface layer thickness.
- the output was the percentage of inclusions of the total oxygen in the steel required to meet a targeted nucleation per unit area density of 120/mm 2 .
- Figure 8 is a plot of the percentage of oxide inclusions in the surface layer required to participate in the nucleation process to achieve the target nucleation per unit area density at different steel cleanliness levels as expressed by total oxygen content, assuming a strip thickness of 1.6 mm and a casting speed of ⁇ Om/min.
- the oxygen content of the steel can be controlled to produce a total oxygen content in the range 100 to 250 ppm and typically about 200 ppm.
- the two micron deep layers adjacent the casting rolls on initial solidification will contain oxide inclusions having a per unit area density of at least 120/mm 2 .
- These inclusions will be present in the outer surface layers of the final solidified strip product and can be detected by appropriate examination, for example by energy dispersive spectroscopy (EDS) .
- EDS energy dispersive spectroscopy
- the total oxygen is at least about 70 ppm (except for one outlier) and typically below 200 ppm, with the total oxygen level generally between about 80 ppm and 150 ppm.
- the free oxygen levels were above 25 ppm and generally clustered between about 30 and about 50 ppm, which means the free oxygen content should be between 20 and 60 ppm. Higher levels of free oxygen will cause the oxygen to combine in formation of unwanted slag, and lower levels of free oxygen will result in insufficient formation of solidification inclusions for efficient shell formation and strip casting.
- the chemical composition and processing conditions used in making product with a high austenite grain coarsening temperature of the present invention results in the formation of a distribution of precipitated, fine- sized oxide particles of silicon and iron with an average particle size less than 50 nanometers in size throughout the steel microstructure.
- the chemical composition and the specific total oxygen and free oxygen content in the molten steel, and the very high solidification rate of the present twin roll casting method can cause the formation of a generally uniform distribution of such fine particles through the steel product. This distribution of fine oxide particles has been found to confer particular, previously unknown properties to product of a high austenite grain coarsening temperature.
- austenite grain growth behaviour of the steel product was unique in that the austenite grains resist coarsening to relatively high temperatures up to least 1000° C.
- An example of the austenite grain growth behaviour for a 0.05% carbon steel product is shown in Figure 21.
- the austenite grain size was measured using the linear intercept method as described in AS1733-1976.
- the austenite grain boundaries were etched using a saturated picric acid based etchant. It can be seen that the austenite grain size remains fine for temperatures up to at least 1050° C, , for a holding time at temperature of 20 minutes. Similar studies have been conducted on steels covering different carbon levels with similar results .
- the austenite grain coarsening temperatures, for a holding time of 20 minutes were in excess of 1050° C for the 0.02%C steel and 1000° C for the 0.20% C steel.
- Table 3 Table 3
- the austenite grain coarsening temperatures exhibited by the present steels are in the order of that usually observed in the past with other aluminium killed steels, where the presence of aluminium nitride particles in the steel microstructure acts to restrict austenite grain growth.
- the austenite grain coarsening temperatures of the present steels in fact approach the grain coarsening temperatures observed with titanium treated aluminium killed, continuously slab cast steels.
- the cooling rate of continuously cast slabs can produce fine TiN particles, with particle sizes ranging down to 5-10 microns.
- the ability of aluminium to form a suitable dispersion of AlN particles when the appropriate levels of aluminium and nitrogen are present in the steel has lead to the concept of aluminium killed fine grained steels.
- the ultra fine particles less than 50 nanometers produced in the present steels confer similar or better austenite grain growth inhibition to aluminium, killed fine grained steels .
- the present steels thus produce a fine grained steel in the absence of the conventional grain refining elements Al , Ti , Nb and V.
- the fine oxide particles in the present steel product which act to resist austenite grain growth, can be beneficial to products that undergo welding, enamelling or full annealing. Avoided is excessive coarsening of austenite grains during heat treatment, which can lead to a coarse microstructure on cooling, and an associated loss of strength and toughness at ambient temperatures .
- the photomicrographs also illustrate the strain required to initiate ferrite grain coarsening.
- the through thickness strain distribution was calculated and applied to the photomicrographs to determine the strain- temperature combinations where ferrite grain coarsening recrystallization began.
- the results of this analysis are given in Figure 23.
- the results show that significantly higher strains are required in the present steel product to induce coarsening of the ferrite than for conventional A1006. In fact, only very small strains are required in conventional AlO06 strip to produce coarsening of the ferrite grains .
- This behaviour of the present steel product is similar to steels with the presence of a substantially uniform distribution of fine-sized oxide particles as described above. This attribute can be relevant where heating could be applied to formed products, such as joining processes like brazing.
- the controlled chemical composition of the liquid steel, particularly the total and free oxygen content, and the very high solidification rate of the process provide the conditions for the precipitation and formation of the uniform dispersion of nano-sized particles of less than 50 nanometer size particles. These fine oxide particles act to inhibit austenite grain growth during high temperature heating and raise the strain to induce ferrite recrystallisation. These attributes are important in fabrication of the steel product. It is clear that the present steel product with these properties may be produced by twin-roll continuous casting of thin steel strips as described above.
- D 3 density of steel
- kg/m 3 Di density of inclusions
- kg/m 3 O t total oxygen in steel
- ppm d inclusion diameter
- m Vi volume of one inclusions
- m 3 itii mass of inclusions
- microns N 3 total number of inclusions present in the surface (that can participate in the nucleation process)
- u casting speed
- m/min L 3 strip length
- m A 3 strip surface area
- Nreq Total number of inclusions required to meet the target nucleation density
- NC t target nucleation per unit area density, number/mit ⁇ 2 (obtained from dip testing)
- N av % of total inclusions available in the molten steel at the surface of the casting rolls for initial nucleation process . b . Equations
- Mn-Si killed steel 0.42kg of oxygen is needed to produce 1 kg of neclusions with a composition of 30% MnO, 40% SiO 2 and 30% Al 2 O 3 .
- 0.38 kg of oxygen is required to produce 1 kg of inclusions with a composition of 50% Al 2 O 3 and 50% CaO.
- N s (2.0 t s x 0.001 x N t /t) (5)
- L 3 (ItI 3 X 1000)/(D s x w x t/1000)
- N req A S x 10 6 x NC t ( 8 )
- N av % (N req /N s ) x 100 . 0
- Eg. 1 calculates the mass of inclusions in steel.
- Eq. 2 calculates the volume of one inclusion assuming they are spherical .
- Eq. 3 calculates the total number of inclusions available in steel.
- Eq. 4 calculates the total number of inclusions available in the surface layer (assumed to be 2 ⁇ m on each side) . Note that these inclusions can only participate in the initial nucleation.
- Eq. 5 and Eq. 6 are used to calculate the total surface area of the strip.
- Eq. 7 calculates the number of inclusions needed at the surface to meet the target nucleation rate .
- Eq. 8 is used to calculate the percentage of total inclusions available at the surface which must participate in the nucleation process . Note if this number is great than 100%, then the number of inclusions at the surface is not sufficient to meet target nucleation rate .
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- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Continuous Casting (AREA)
- Treatment Of Steel In Its Molten State (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
Description
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Priority Applications (7)
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NZ568183A NZ568183A (en) | 2005-10-20 | 2006-10-19 | A steel product with a high austenite grain coarsening temperature, and method for making the same |
JP2008535849A JP6078216B2 (en) | 2005-10-20 | 2006-10-19 | Steel material with high austenite grain roughening temperature and method for producing the same |
CN2006800480159A CN101340990B (en) | 2005-10-20 | 2006-10-19 | A steel product with a high austenite grain coarsening temperature, and method for making the same |
EP06790405.2A EP1945392B1 (en) | 2005-10-20 | 2006-10-19 | A steel product with a high austenite grain coarsening temperature, and method for making the same |
KR1020087012078A KR101322703B1 (en) | 2005-10-20 | 2006-10-19 | A steel product with a high austenite grain coarsening temperature, and method for making the same |
PL06790405T PL1945392T3 (en) | 2005-10-20 | 2006-10-19 | A steel product with a high austenite grain coarsening temperature, and method for making the same |
AU2006303818A AU2006303818B2 (en) | 2005-10-20 | 2006-10-19 | A steel product with a high austenite grain coarsening temperature, and method for making the same |
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US11/255,604 | 2005-10-20 | ||
US11/255,604 US7485196B2 (en) | 2001-09-14 | 2005-10-20 | Steel product with a high austenite grain coarsening temperature |
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US (3) | US7485196B2 (en) |
EP (1) | EP1945392B1 (en) |
JP (2) | JP6078216B2 (en) |
KR (1) | KR101322703B1 (en) |
CN (1) | CN101340990B (en) |
AU (1) | AU2006303818B2 (en) |
DE (1) | DE102006049629A1 (en) |
MY (1) | MY145404A (en) |
NZ (1) | NZ568183A (en) |
PL (1) | PL1945392T3 (en) |
RU (2) | RU2421298C2 (en) |
UA (1) | UA96580C2 (en) |
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2006
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- 2006-10-19 KR KR1020087012078A patent/KR101322703B1/en active IP Right Grant
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Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN105543683A (en) * | 2007-05-06 | 2016-05-04 | 纽科尔公司 | A thin cast strip product with microalloy additions and method for making the same |
CN105543683B (en) * | 2007-05-06 | 2018-09-11 | 纽科尔公司 | Thin strip slab product containing microalloy additions and its manufacturing method |
JP2010535634A (en) * | 2007-08-13 | 2010-11-25 | ニューコア・コーポレーション | Thin cast steel strip with reduced microcracking |
KR101555229B1 (en) | 2007-08-13 | 2015-09-23 | 누코 코포레이션 | Thin cast steel strip with reduced microcracking |
Also Published As
Publication number | Publication date |
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JP2009511749A (en) | 2009-03-19 |
US7485196B2 (en) | 2009-02-03 |
US20060144553A1 (en) | 2006-07-06 |
US20070212249A1 (en) | 2007-09-13 |
US20090191425A1 (en) | 2009-07-30 |
US8002908B2 (en) | 2011-08-23 |
RU2008119827A (en) | 2009-11-27 |
AU2006303818B2 (en) | 2011-11-03 |
EP1945392B1 (en) | 2022-01-26 |
JP6078216B2 (en) | 2017-02-08 |
DE102006049629A1 (en) | 2007-05-03 |
CN101340990A (en) | 2009-01-07 |
PL1945392T3 (en) | 2022-05-02 |
NZ568183A (en) | 2011-08-26 |
UA96580C2 (en) | 2011-11-25 |
EP1945392A4 (en) | 2015-12-02 |
AU2006303818A1 (en) | 2007-04-26 |
CN101340990B (en) | 2011-08-03 |
RU2011104055A (en) | 2012-08-10 |
EP1945392A1 (en) | 2008-07-23 |
MY145404A (en) | 2012-02-15 |
JP2015083717A (en) | 2015-04-30 |
RU2421298C2 (en) | 2011-06-20 |
KR101322703B1 (en) | 2013-10-25 |
KR20080065294A (en) | 2008-07-11 |
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