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Thin cast steel strip with reduced microcrackingRelated Patent Categories: Metal Founding, Process, Shaping Liquid Metal Against A Forming Surface, Continuous Or Semicontinuous Casting, Having Continuously Advancing Shaping Surface, Utilizing Roll Couple MoldThin cast steel strip with reduced microcracking description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070175608, Thin cast steel strip with reduced microcracking. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATION [0001] This application is filed as the utility application of the provisional application converted from application Ser. No. 11/306,910, filed Jan. 16, 2006, to which priority is claimed. BACKGROUND AND SUMMARY OF THE INVENTION [0002] This invention relates generally to steelmaking, and particularly carbon steels formed by continuous casting of thin strip. [0003] Thin steel strip may be formed by continuous casting in a twin roll caster. In twin roll casting, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls, which are cooled, so that metal shells solidify on the moving roll surfaces and are brought together at the nip between the rolls to produce a solidified strip product delivered downwardly from the nip. The term "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 to form a casting pool of molten metal supported on the casting surfaces of the rolls 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. [0004] When casting thin strip with a twin roll caster, the molten metal in the casting pool will generally be at a temperature of the order of 1500.degree. C. and above. A high heat flux and extensive nucleation on initial solidification of the metal shells on the casting surfaces is needed to form the steel strip. U.S. Pat. No. 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial portion of the metal oxides formed are liquid at the initial solidification temperature. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, nucleation of the steel on initial solidification can be influenced by the texture of the casting surface. In particular, International Application AU 99/00641 discloses that a random texture of peaks and troughs in the casting surfaces can enhance initial solidification by providing substantial nucleation sites distributed over the casting surfaces. [0005] Attention has been given in the past to the steel chemistry of the melt, particularly in the ladle metallurgy furnace before the casting of the thin strip. We have given attention in the past, in U.S. application Ser. No. 10/761,953, published as US20040177945A1, to the oxide inclusions and the oxygen levels in the steel metal and their impact on the quality of the steel strip produced. In U.S. application Ser. No. 10/961,300, published as US20050082031A1, we have also regulated hydrogen levels and nitrogen levels in the molten metal to enhance the casting and quality of the steel strip. Finally, in U.S. Pat. No. 6,547,849, we have provided a method of providing silicon/manganese killed molten steel having a sulfur content of less than 0.02% by weight for casting. [0006] In these prior disclosures, the teachings are generally to have low sulfur levels, say as less than 0.025 or 0.02%. See, e.g., International Application AU 99/00641 and U.S. Pat. No. 6,547,849. There is no suggestion of purposely providing very low levels of sulfur levels to reduce or eliminate microcracking, or for any other purpose. [0007] Generally, sulfur has been an undesirable impurity in steelmaking, including in continuous casting of thin strip. Steelmakers generally go to great lengths and expense to minimize sulfur content in making steel. Sulfur is primarily present as sulfide inclusions, such as MnS inclusions. Sulfide inclusions may provide sites for voids and/or surface cracking. Sulfur may also decrease ductility and notch impact toughness of the cast steel, especially in the transverse direction. Further, sulfur creates red shortness, or brittleness in red hot steel. Sulfur also reduces weldability. [0008] Sulfur is generally removed from molten steel by a desulphurization process. Steel for continuous casting may be subjected to a deoxidation and then desulphurization in the ladle metallurgy, prior to casting. One such method involves stirring the molten steel by injecting inert gases, such as argon or nitrogen, while the molten metal is in contact with slag having a high calcium content. See U.S. Pat. No. 6,547,849. [0009] On the other hand, thin cast strip formed by twin roll casting has been known to have a tendency to form of microcracks in the strip surface. One cause has been the formation of an oxide layer upon the surface of the casting rolls which acts as a thermal barrier causing irregular solidification of the cast strip and formation of microcracks in the strip surface. We have discovered that another cause of microcracks has been the absence of particular levels of sulfur. [0010] We have found that steel having a sulfur content between about 0.003% and about 0.008% by weight or less is generally desired for casting of thin strip. Just as we have determined that a minimum oxygen content is necessary in steel for casting of thin strip, to improve contact between the casting rolls and the steel and generate high heat flux and rapid initial solidification in formation of thin strip, we have found that a minimum sulfur content is necessary to significantly reduce or eliminate microcracking in continuously strip cast steel. [0011] The present invention relates to specific sulfur content in steel to substantially reduce or eliminate microcracking in steel. Disclosed is a thin cast steel strip produced by continuous casting by the steps comprising: [0012] a. assembling a pair of internally cooled casting rolls having a nip between them and with confining closures adjacent the ends of the nip; [0013] b. introducing molten carbon steel having a sulfur content of between about 0.003% and about 0.008% by weight and a carbon content of between about 0.010% and about 0.065% by weight between the pair of casting rolls to form a casting pool between the casting rolls; [0014] c. counter rotating the casting rolls to form solidified metal shells on the surfaces of the casting rolls; and [0015] d. forming from the solidified metal shells thin steel strip downwardly through the nip between the casting rolls. [0016] The sulfur content of the molten steel in the casting pool may not exceed about 0.006% by weight and may have a sulfur content of no less than about 0.003% by weight. In any event, the thin strip produced has a substantially improved surface quality with less tendency for microcracks. [0017] The molten steel may be a low carbon steel. Again, the molten steel has a carbon content between about 0.01% and about 0.065%. [0018] The molten metal in the casting pool may have a total oxygen content of at least 70 ppm and a free oxygen content between 20 and 60 ppm. The steel strip may be formed of a solidified steel containing oxide inclusions distributed corresponding to a total oxygen content in the range 100 ppm to 250 ppm and free oxygen content between 30 and 50 ppm in the molten metal in the casting pool. [0019] The thin strip may contain fine oxide particles of silicon and iron distributed through the microstructure having an average particle size less than 50 nanometers. The steel strip may also contain oxide particles that increase the resistance to austenite grain coarsening up to at least 1000.degree. C. The steel strip may contain fine oxide particles capable of producing an average austenite grain size of less than 50 microns up to at least 1000.degree. C. for a holding time of at least 20 minutes. The steel strip may be capable of restricting ferrite recrystallization for strain levels up to 10% and temperatures up 750.degree. C. with hold times up to 20 minutes. [0020] The thin steel strip may contain, by weight, less than 0.06% aluminum, less than 0.01% titanium, less than 0.01% niobium, and less than 0.02% vanadium. [0021] The steel strip may be made from a molten melt in the casting pool containing a nitrogen content below about 120 ppm or 100 ppm or 85 ppm, and a free hydrogen content below about 6.9 ppm at atmospheric pressure, and the sum of the partial pressures of hydrogen and nitrogen below 1.15 atmospheres. The molten steel forms thin steel strip having nitrogen and hydrogen levels reflected by the content of the molten steel to provide for the formation of thin steel strip. [0022] Alternatively, disclosed is a cast thin steel strip, and a method of making thereof, comprises: [0023] a. introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a sulfur content between about 0.003% and about 0.008% by weight and a carbon content between about 0.010% and about 0.065% by weight; and [0024] b. solidifying the molten steel to form metal shells on the casting rolls and to form thin steel strip there from. [0025] The steel strip may have a sulfur content between about 0.003% and 0.006%. [0026] The steel strip may be less than 5 mm or 2.5 mm in thickness. The steel strip may contain solidified oxide inclusions distributed such that surface regions of the strip to a depth of about 2 microns from the surface contain such inclusions a per unit area density of at least 120 inclusions/mm.sup.2. [0027] Alternatively, disclosed is a method of casting thin steel strip is provided comprising the steps of: [0028] a. assembling a pair of cooled casting rolls having a nip there between and confining end closures adjacent to ends of the casting rolls; [0029] b. introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on casting surfaces of the casting rolls confined by the end closures, with the molten steel having a sulfur content between about 0.003% and about 0.008% by weight and a carbon content between about 0.010% and about 0.065% by weight; [0030] c. counter-rotating the casting rolls to form solidified metal shells on casting surfaces of the casting rolls; and [0031] d. forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip. [0032] The steel strip may have a sulfur content between about 0.003% and 0.006%. 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