This nonprovisional application claims priority to U.S. Provisional Applications No. 61/920,470, filed on Dec. 24, 2013, and No. 61/931,997, filed on Jan. 27, 2014.

BACKGROUND AND SUMMARY

This invention relates to the making of high strength high ductility high copper carbon alloy steel thin cast strip, and the method for making such cast strip by a twin roll caster.

In a twin roll caster, molten metal is introduced between a pair of counter-rotated, internally cooled casting rolls 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 between the casting rolls. The term â€œnipâ€ is used herein to refer to the general region at which the casting rolls are closest together. The molten metal is poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal, supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.

Throughout the years, the demand for high strength steels has increased. And generally, there has been a compromise between strength and ductility. However, Transformation Induced Plasticity (TRIP) steel is a type of steel alloy which exhibits both excellent strength and ductility. TRIP steel has a triple phase microstructure consisting of ferrite, bainite, and retained austenite. Transformation induced plasticity refers to the transformation of retained austenite to martensite during plastic deformation. See M. Zhang, Continuous cooling transformation diagrams and properties of micro-alloyed TRIP steels, Materials Science and Engineering A 438-440, 2006. This property allows TRIP steels to have a high formability (i.e. achieve greater elongation), while retaining excellent strength. The transformation of retained austenite to martensite produces a high carbon martensite phase which is very strong. The retained austenite is finely dispersed in the ferrite phase. This fine dispersion allows TRIP steels to retain their strength. See also, William D. Callister, Materials Science and Engineering An Introduction, 7th edition, Wiley, 2007, pg. 292.

One advantage of TRIP steels is that they have higher ductility than other steels with similar strength. As such, TRIP steels are suitable for structural and reinforcement parts of complex shapes. For example, the ductility and strength of TRIP steels make them a good candidate for automotive applications. Structural components can be made thinner because TRIP steels have the ductility necessary to withstand high deformation processes such as stamping, as well as the strength and energy absorption characteristics to meet safety regulations. TRIP steels have high strain hardening capacity. They exhibit good strain redistribution and, thus, good drawability. As a result of strain hardening, the mechanical properties of the finished part are superior to those of the initial blank. High strain hardening capacity and high mechanical strength lend these steels good energy absorption capacity. TRIP steels also exhibit a strong bake hardening (BH) effect following deformation, which further improves their crash performance.

Previous high copper carbon alloy steel sheets are known to provide corrosion resistance. However, when the steel oxidizes at temperatures above 1100Â° C., such carbon alloy steel sheets containing about 0.50% copper or more result in a surface defect known as surface â€œhot shortnessâ€ . See E. Sampson et al., Effect of Silicon on Hot Shortness, Iron & Steel Technology, January 2013, pg. 70-79; see also, The Making, Shaping and Treating of Steel (9th edition), pg. 1154. Copper separates during surface oxidation to a layer adjacent the surface of the produced sheet. Copper enriches at the oxide/metal interface until it exceeds the solubility of copper in austenite, at which point a liquid copper phase forms and infiltrates the grain boundaries. Id. This causes embrittlement of the grain boundaries and causes cracking during rolling; thus, resulting in a commercially unacceptable steel.

The occurrence of these undesirable surface conditions could be reduced by careful control of oxidation during heating and by not overheating during hot working. Also, the addition of nickel in an amount equal to at least one-half the copper content has been known to be beneficial to the surface quality of steels containing copper. However, these procedures and alloying additions are costly causing the resulting corrosion resistant steels to be expensive. Notably, nickel is an expensive alloy addition and causes the resulting corrosion resistant steel to be expensive. Therefore, there is still a need for a high strength high ductility high copper carbon alloy thin cast strip.

Presently disclosed is a high copper carbon alloy steel sheet of less than 10 mm in thickness produced with the composition described below, without the purposeful addition of substantial nickel alloy, by solidification and cooling in a non-oxidizing atmosphere to less than 1080Â° C., i.e., below the solidification temperature of copper, at a cooling rate between 1000 and 2000Â° C./s. Hot shortness is inhibited by the rapid solidification and by reduced oxidation of the sheet surface. A non-oxidizing atmosphere is an atmosphere typically of an inert gas such as nitrogen or argon, or a mixture thereof, which contains less than about 5% oxygen by weight. The present high copper carbon alloy steel sheet can be made by the steps comprising: (a) preparing a molten melt producing an as-cast carbon alloy steel sheet comprising (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminum, and a total oxygen level of at least 50 ppm; (ii) nickel in levels below 0.5% typically found in steel scrap used in steelmaking; (iii) the remainder iron and impurities resulting from melting; (b) solidifying and cooling the molten melt into a steel sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s, and (c) hot rolling the as-cast sheet to between 10% and 50% reduction to form a sheet with a microstructure providing a tensile strength of at least 900 MPa and an elongation of at least 15%. By elongation is meant total elongation here and elsewhere in this disclosure.

The as-cast carbon alloy steel sheet may further comprise by weight between 1.0% and 3.5% silicon. The high copper carbon alloy steel sheet can be made by the additional steps of annealing the hot rolled sheet to a temperature to obtain a microstructure providing by volume at least 70% austenite; and then rapidly cooling to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. Rapidly cooling refers to cooling to between 100 and âˆ’100Â° C. at a rate of more than 3Â° C./s. In an embodiment, the hot rolled sheet may be annealed to 630Â° C. to obtain a microstructure providing by volume at least 70% austenite. Then, it may be rapidly cooled to between 100 and âˆ’100Â° C. at a rate of more than 3Â° C./s to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

In another alternative, the high copper carbon alloy steel sheet may be further made by the additional step of cold rolling the hot rolled sheet up to 5% strain to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. The as-cast sheet may be cold rolled by cold stamping, rolling, or pressing the cast sheet at room temperature.

In some embodiments, the molten melt may be solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1200 and 1700Â° C./s. In other embodiments, the molten melt may be solidified and cooled into a steel sheet less than 1.6 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s or between 1200 and 1700Â° C./s. Also, the molten melt may have a free oxygen content between 5 and 70 ppm.

As disclosed, the molten melt may be solidified and cooled to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s to form a steel sheet with a microstructure by volume of at least 10% bainite, at least 2% ferrite and at least 15% retained austenite.

In some embodiments, the high copper carbon alloy steel sheet after the solidifying and hot rolling steps may have a tensile strength of 900 MPa or more and an elongation of at least 25%, or a tensile strength of 1200 MPa or more and an elongation of at least 20%. Alternatively, the high copper carbon alloy steel sheet after the solidifying and hot rolling steps may have a tensile strength of 1500 MPa or more and an elongation of at least 15%.

In order to produce the steel sheet with high strength and high ductility, an annealing process may be used to provide desirable phase distribution. During annealing, the steel is brought to a temperature above the eutectoid, where the material is composed of a solid austenite phase and a solid ferrite phase. See E. Emadoddin & Al. Effect of cold rolling reduction and annealing temperature on the bulk texture of two TRIP-aided steel sheets, Journal of Materials Processing Technology 203, 293-300, 2008. The material may be then isothermally cooled in order to allow the austenite to form a banitic ferrite phase. During the eutectoid transformation, excess carbon is produced by the formation of the low carbon ferrite phase. In a typical steel alloy, the excess carbon would form carbides. However, alloying elements may be used to prevent the formation of carbides during the transformation. In consequence, the excess carbon diffuses to the remaining austenite phase.

In order to obtain the desirable microstructure, the isothermal transformation may be completed at a temperature where the formation of bainitic ferrite is slow to allow the carbon to diffuse to the austenite. The carbon enriched austenite phase eventually reaches a sufficiently high carbon content that it is stable at room temperature. The result of the annealing process may be a material composed primarily of ferrite, and bainite formed from the austenite phase during annealing, as well as dispersed retained austenite and martensite phases.

The high copper carbon steel sheet may be made by preparing a molten melt producing an as-cast carbon alloy steel sheet (as discussed above), solidifying and cooling the molten melt into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s, and hot rolling the as-cast sheet to between 10% and 50% to form a steel sheet with a microstructure providing a tensile strength of at least 900 MPa and an elongation of at least 15%. Additionally, the high copper carbon alloy steel sheet may be heated at a suitable temperature for a suitable amount of time for annealing. In some embodiments, the high copper carbon alloy steel sheet may be made by annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 100 hours. In other embodiments, the high copper carbon alloy steel sheet may be made by annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 25 hours.

Alternatively, after cooling the steel sheet at a cooling rate between 1000 and 2000Â° C./s and hot rolling the as-cast sheet to between 10% and 50%, a steel sheet may be formed with a microstructure providing a tensile strength of at least 900 MPa and an elongation of at least 15%, the hot rolled cast sheet may be continuously or batch annealed and then if desired, galvanized.

Also disclosed is a high copper carbon alloy steel sheet made by the steps comprising: (a) preparing a molten melt producing an as-cast carbon alloy steel sheet comprising (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminum, and a total oxygen level of at least 100 ppm; (ii) nickel in levels below 0.5% typically found in steel scrap used in steelmaking; (iii) the remainder iron and impurities resulting from melting; (b) solidifying and cooling the molten melt into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s; and (c) hot rolling the as-cast sheet to between 10% and 50% reduction to form a sheet with microstructure providing a tensile strength of at least 900 MPa and an elongation of at least 15%.

Again, the as-cast carbon alloy steel sheet may further comprise by weight between 1.0% and 3.5% silicon. The high copper carbon alloy steel sheet can be made by the additional steps of annealing the hot rolled sheet to a temperature to obtain a microstructure providing by volume at least 70% austenite; and then rapidly cooling to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. In some embodiments, the hot rolled sheet may be annealed to 630Â° C. to obtain a microstructure providing by volume at least 70% austenite. Then, it may be rapidly cooled to between 100 and âˆ’100Â° C. at a rate of more than 3Â° C./s to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

In another alternative, the high copper carbon alloy steel sheet may be made by the additional step of cold rolling the hot rolled sheet up to 5% strain to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

Again, the molten melt may be solidified and cooled into a steel sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1200 and 1700Â° C./s. Alternatively, the molten melt may be solidified and cooled into a sheet less than 1.6 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s or between 1200 and 1700Â° C./s. Also, the molten melt may have a free oxygen content between 5 and 70 ppm.

The molten melt may be solidified and cooled to form a steel sheet with a microstructure by volume of at least 10% bainite, at least 2% ferrite and at least 20% retained austenite. In some embodiments, the high copper carbon alloy steel may have a tensile strength of 900 MPa or more and an elongation of at least 25% or a tensile strength of 1200 MPa or more and an elongation of at least 20%. Alternatively, the high copper carbon alloy steel sheet may have a tensile strength of 1500 MPa or more and an elongation of at least 15%.

The high copper carbon alloy steel sheet may be made by preparing a molten melt producing an as-cast carbon alloy steel sheet (as discussed above), solidifying and cooling the molten melt into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s, and hot rolling the as-cast sheet to between 10% and 50% to form a sheet with microstructure providing a tensile strength of at least 900 MPa and an elongation of at least 15%. Additionally, the high copper carbon alloy steel sheet may be heated at a constant temperature for a suitable amount of time for annealing. In some embodiments, the high copper carbon alloy steel sheet may be made by annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 100 hours. In other embodiments, the high copper carbon alloy steel sheet may be made by annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 25 hours.

Alternatively, after solidifying and cooling the molten melt into a steel sheet and hot rolling the as-cast sheet to between 10% and 50% to form a sheet with microstructure providing a tensile strength of at least 900 MPa and an elongation of at least 15%, the hot rolled cast sheet may be in-line or batch annealed and/or coating the hot rolled cast sheet in a hot bath of molten metal selected from the group consisting of zinc, aluminum and alloys thereof.

Also disclosed is a method of making a high copper carbon alloy steel sheet comprising the steps of: (a) preparing a molten melt producing an as-cast carbon alloy steel sheet comprising (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminum, and a total oxygen level of at least 50 ppm; (ii) nickel at an impurity level found in steel scrap; (iii) the remainder iron and impurities resulting from melting; (b) forming the melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between; (c) counter rotating the casting rolls to form a thin cast sheet of less than 10 mm in thickness extending downwardly from the nip; (d) cooling the cast sheet to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s and (e) hot rolling the thin cast sheet to between 10% and 50% reduction in a non-oxidizing atmosphere to produce a thin cast sheet comprising a desired microstructure as disclosed herein. The as-cast carbon alloy steel sheet may further comprise by weight between 1.0% and 3.5% silicon.

In some embodiments, the method of making a high copper carbon alloy steel sheet may also include the additional steps of annealing the hot rolled sheet to a temperature to obtain a microstructure providing by volume at least 70% austenite; and then rapidly cooling to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. In an embodiment, the hot rolled sheet may be annealed to 630Â° C. to obtain a microstructure providing by volume at least 70% austenite. Then, it may be rapidly cooled to between 100 and âˆ’100Â° C. at a rate of more than 3Â° C./s to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

In another embodiment, the method of making a high copper carbon alloy steel sheet may include the additional step of cold rolling the hot rolled sheet up to 5% strain to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

The molten melt may be solidified and cooled into a steel sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1200 and 1700Â° C./s. Alternatively, the molten melt may be solidified and cooled into a sheet less than 1.6 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s or between 1200 and 1700Â° C./s. Also, the molten melt may have a free oxygen content between 5 and 70 ppm.

The molten melt may be solidified and cooled to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s to form a steel sheet with a microstructure by volume of at least 10% bainite, at least 2% ferrite and at least 15% retained austenite. In some embodiments, the high copper carbon alloy steel sheet may have a tensile strength of 900 MPa or more and an elongation of at least 25%. In other embodiments, the high copper carbon alloy steel sheet may have a tensile strength of 1200 MPa or more and an elongation of at least 20%. In yet another embodiment, the high copper carbon alloy steel sheet may have a tensile strength of 1500 MPa or more and an elongation of at least 15%. By the solidifying and hot rolling steps, the steel sheet may have more than 50% MnSiO₂and MnS inclusions with less than 5 Î¼m in size.

The method of making a high copper carbon alloy steel sheet may additionally comprise the step of annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 100 hours. In other embodiments, the high copper carbon alloy steel sheet may be made by annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 25 hours. Alternatively, the method of making a high copper carbon alloy steel sheet may additionally comprise the step of in-line or batch annealing and/or coating the hot rolled cast sheet in a hot bath of molten metal selected from the group consisting of zinc, aluminum and alloys thereof.

Finally, disclosed is a method of making a high copper carbon alloy steel sheet comprising the steps of: (a) preparing a molten melt producing an as-cast carbon alloy steel sheet comprising (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminum, and a total oxygen level of at least 100 ppm; (ii) nickel at an impurity level found in steel scrap; (iii) the remainder iron and impurities resulting from melting; (b) forming the melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between; (c) counter rotating the casting rolls to form a thin cast sheet of less than 10 mm in thickness extending downwardly from the nip; (d) cooling the cast sheet to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s and (e) hot rolling the thin cast sheet to between 10% and 5% reduction in a non-oxidizing atmosphere to produce a thin cast sheet comprising a bainitic microstructure. The molten melt may further comprise by weight between 1.0% and 3.5% silicon.

Alternatively, the method of making a high copper carbon alloy steel sheet may additionally include the steps of annealing the hot rolled sheet to a temperature to obtain a microstructure providing by volume at least 70% austenite; and then rapidly cooling to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. In an embodiment, the hot rolled sheet may be annealed to 630Â° C. to obtain a microstructure providing by volume at least 70% austenite. Then, it may be rapidly cooled to between 100 and âˆ’100Â° C. at a rate of more than 3Â° C./s to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

In another embodiment, the method of making a high copper carbon alloy steel sheet may include the additional step of cold rolling the hot rolled sheet up to 5% strain to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite.

Again, the molten melt may be solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1200 and 1700Â° C./s. Alternatively, the molten melt may be solidified and cooled into a sheet less than 1.6 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s or between 1200 and 1700Â° C./s. Also, the molten melt may have a free oxygen content between 5 and 70 ppm.

The molten melt may be solidified and cooled to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s to form a steel sheet with a microstructure by volume of at least 10% bainite, at least 2% ferrite and at least 20% retained austenite. In some embodiments, the high copper carbon alloy steel sheet may have a tensile strength of 900 MPa or more and an elongation of at least 25%. In other embodiments, the high copper carbon alloy steel sheet may have a tensile strength of 1200 MPa or more and an elongation of at least 20%. In yet other embodiments, the high copper carbon alloy steel sheet may have a tensile strength of 1500 MPa or more and an elongation of at least 15%.

The method of making a high copper carbon alloy steel sheet may additionally comprise the step of annealing the hot rolled cast sheet with a soak at between 550 and 800Â° C. for between 5 and 100 hours. In other embodiments, the high copper carbon alloy steel sheet may be made by annealing the hot rolled cast sheet with a soak at between 500 and 800Â° C. for between 5 and 25 hours. Alternatively, the method of making a high copper carbon alloy steel sheet may additionally comprise the step of in-line or batch annealing and/or coating the hot rolled cast sheet in a hot bath of molten metal selected from the group consisting of zinc, aluminum and alloys thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully explained, illustrative results of experimental work carried out to date will be described with reference to the accompanying drawings in which:

FIG. 1 illustrates a strip casting installation incorporating an in-line hot rolling mill and coiler; and

FIG. 2 illustrates details of the twin roll strip caster.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates successive parts of strip caster for continuously casting steel strip of the present invention. FIGS. 1 and 2 illustrate a twin roll caster 11 that may continuously produce a cast steel strip 12, which passes in a transit path 10 across a guide table 13 to a pinch roll stand 14 having pinch rolls 14A. Immediately after exiting the pinch roll stand 14, the strip passes into a hot rolling mill 16 having a pair of reduction rolls 16A and backing rolls 16B where the cast strip is hot rolled to reduce a desired thickness. The hot rolled strip passes onto a run-out table 17 where the strip may be cooled by convection and contact with water supplied via water jets 18 (or other suitable means) and by radiation. The rolled and cooled strip then passes through a pinch roll stand 20 comprising a pair of pinch rolls 20A and then to a coiler 19. Final cooling of the cast strip takes place after coiling.

As shown in FIG. 2, twin roll caster 11 comprises a main machine frame 21, which supports a pair of laterally positioned casting rolls 22 having casting surfaces 22A. Molten metal is supplied during a casting operation from a ladle (not shown) to a tundish 23, through a refractory shroud 24 to a distributor or moveable tundish 25, and then from the distributor or moveable tundish 25 through a metal delivery nozzle 26 between the casting rolls 22 above the nip 27. The molten metal delivered between the casting rolls 22 forms a casting pool 30 above the nip supported on the casting rolls. The casting pool 30 is restrained at the ends of the casting rolls by a pair of side closure dams or plates 28, which are pushed against the ends of the casting rolls by a pair of thrusters (not shown) including hydraulic cylinder units (not shown) connected to the side plate holders. The upper surface of casting pool 30 (generally referred to as the â€œmeniscusâ€ level) usually rises above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is immersed within the casting pool 30. Casting rolls 22 are internally water cooled so that shells solidify on the moving roller surfaces as they pass through the casting pool, and are brought together at the nip 27 between them to produce the cast strip 12, which is delivered downwardly from the nip between the casting rolls.

The twin roll caster may be of the kind that is illustrated and described in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243 or U.S. Pat. No. 5,488,988, or U.S. patent application Ser. No. 12/050,987. Reference is made to those patents for appropriate construction details of a twin roll caster that may be used in an embodiment of the present invention.

The in-line hot rolling mill 16 is typically used for reductions of 10% to 50%. On the run-out-table 17, the cooling may include water cooling section and air mist cooling to control cooling rates of austenite transformation to achieve desired microstructure and material properties.

A high copper carbon alloy sheet was made from a molten melt produced in a twin roll caster. The carbon alloy steel sheet comprises (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminum, a total oxygen level of at least 50 ppm or at least 100 ppm; (ii) nickel in levels below 0.5% typically found in steel scrap used in steelmaking; and (iii) the remainder iron and impurities resulting from melting. The molten melt is rapidly solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. and, as-cast sheet is hot rolled to between 10% and 50% reduction to form a steel sheet with microstructure providing a tensile strength of at least 900 MPa and elongation of at least 15%.

In one example, the molten melt was solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s. In another example, the molten melt was solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1200 and 1700Â° C./s. In any case, the rapid solidification ensures that copper remains in solution and also helps inhibit hot shortness issues.

The microstructures of the high copper carbon alloy steel sheet can be observed by measuring the percentage of bainite, ferrite and retained austenite. A microstructure may have at least 10% bainite, at least 2% ferrite and at least 20% retained austenite. Retained austenite is believed to be in a metastable state in the steel sheet that transforms into martensite under local stress and strain. This transformation absorbs energy and improves steel sheet formability and the work hardening rate of the steel, delaying the onset of necking. The high percentage of retained austenite is believed to provide improved strength and ductility. Also, after the solidifying step, the steel sheet has more than 50% MnSiO₂and MnS inclusions with less than 5 Î¼m in size.

The mechanical properties of tensile strength and elongation of the high copper carbon alloy steel sheet may be measured. For example, a high copper carbon alloy steel sheet having a tensile strength of at least 900 MPa and an elongation of at least 25% may be obtained. In another example, a high copper carbon alloy steel sheet having a tensile strength of at least 1200 MPa and an elongation of at least 20% may be obtained. In yet another example, a high copper carbon alloy steel sheet having a tensile strength of at least 1500 MPa and an elongation of at least 15% may be obtained.

A high copper carbon alloy sheet may be made by twin roll caster into thin cast steel strip from a molten melt. The high copper carbon alloy steel sheet comprised (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminum, a total oxygen level of at least 50 ppm or at least 100 ppm; (ii) nickel in levels below 0.5%; and (iii) the remainder iron and impurities resulting from melting. To clarify, this amount of nickel does not involve purposefully additions of nickel to the composition, but is the level of nickel typically found in scrap metal used in making steel in electric arc furnaces. The high copper carbon alloy sheet may further comprise by weight between 1.0% and 3.5% silicon. The molten melt may be rapidly solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. at a cooling rate between 1000 and 2000Â° C./s and then hot rolled to between 10% and 50% reduction. And finally, the hot rolled cast sheet may be annealed.

For example, the hot rolled cast sheet may be annealed by soaking between 550 and 800Â° C. for between 5 and 100 hours. More specifically, the hot rolled cast sheet may be annealed by soaking between 550 and 800Â° C. for between 5 and 25 hours. It is understood soaking means maintaining the strip in the annealing atmosphere at the target temperature for the stated time.

Alternatively, the hot rolled cast sheet may be continuously annealed and/or coated. The continuous annealing may be done by holding the hot rolled strip at an annealing temperature for a generally limited length of time and may be followed by applying desired cooling patterns to the strip. Coating may be done by immersion of the hot rolled strip in a zinc alloy, an aluminium alloy or a zinc aluminium alloy bath resulting in the coating of the strip with the metal alloy. The bath is generally high in zinc or aluminium or a percentage of zinc and aluminium, and includes coatings known as galvanizing, aluminizing and GalvalumeÂ® coatings.

Alternatively, the hot rolled steel strip may be further processed with quenching and partitioning steps. This generally involves rapidly cooling the hot rolled strip from the annealing temperature (typically an inter critical temperature) to a temperature where some of the austenite present transforms in part to martensite (between the martensite start temperature and the martensite finish temperature), and then reheated to a temperature between 200 and 400Â° C. so the carbon in the martensite is partitioned into the remaining autensite.

The precise pattern of annealing, coating and/or quenching and partitioning will determine on the properties specified for the final product desired.

A high copper carbon alloy sheet may also be made from a molten melt produced in a twin roll caster where the carbon alloy steel sheet comprises (i) by weight, between 0.15% and 0.50% carbon, less than 1.0% chromium, between 3.0% and 9.0% manganese, between 0.2% and 3.5% silicon, more than 0.5% copper, less than 0.01% aluminium, a total oxygen level of at least 50 ppm or at least 100 ppm; (ii) nickel in levels below 0.5% typically found in steel scrap used in steelmaking; and (iii) the remainder iron and impurities resulting from melting. The molten melt is rapidly solidified and cooled into a sheet less than 10 mm in thickness in a non-oxidizing atmosphere to below 1080Â° C. and, as-cast sheet is hot rolled to between 10% and 50% reduction to form a steel sheet with microstructure providing a tensile strength of at least 900 MPa and elongation of at least 15%. After hot rolling, the hot rolled sheet is annealed to a temperature to obtain a microstructure providing by volume at least 70% austenite; and then it is rapidly cooled to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. Alternatively, after hot rolling, the hot rolled sheet up to 5% strain to obtain a microstructure providing by volume at least 20% austenite and at least 50% martensite. The in process strain (Îµ) may be determined by measuring the original thickness and the final thickness and given by:

Îµ=1âˆ’((Original thicknessâˆ’Final thickness)/Original thickness)

While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described, and that all changes and modifications that come within the spirit of the invention described by the following claims are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Modifications may be made without departing from the spirit and scope of the invention.