Patent Application
20090047536
This invention relates generally to steelmaking, and particularly carbon steels formed by continuous casting of thin strip.
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.
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° C., and usually 1600° 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.
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. In the past, in U.S. Pat. No. 7,048,033 attention has been given controlling 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. Pat. No. 7,156,151, hydrogen levels and nitrogen levels have been regulated in the molten metal to enhance the casting and quality of the steel strip. In U.S. Pat. No. 6,547,849, a method is disclosed of providing silicon/manganese killed molten steel having a sulfur content of less than 0.02% by weight for casting. Finally, in U.S. patent application Ser. No. 11/622,754, filed Jan. 12, 2007, and published as U.S. 2007/0175608 on Aug. 2, 2007, a thin cast strip with reduced microcracks and method of making the same is disclosed by controlling the sulfur content of the cast strip to between about 0.003% and about 0.008% by weight, along with the carbon content to between about 0.010% and about 0.065% by weight.
In these prior disclosures, the teachings are generally to have low sulfur levels, such 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 to reduce or eliminate microcracking, or for any other purpose, except for U.S. application Ser. No. 11/622,754, filed Jan. 12, 2007. There has been no suggestion to our knowledge of controlling the ratios of manganese/sulfur or manganese/silicon for any reason in the casting of thin strip, or any other steelmaking.
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. 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.
On the other hand, thin cast strip formed by twin roll casting has been known to have a tendency to form microcracks in the strip surface. One cause has been the formation of an oxide layer on the surface of the casting rolls that acts as a thermal barrier causing irregular solidification of the cast strip and formation of microcracks in the strip surface.
We have found that microcracking is related to the steel chemistry and certain process parameters. That the “strength” of newly formed shells can be made resistant and reduce the formation of microcracks in the cast strip surface. We have also observed that sulfur is a surface active element in liquid steel. From these observations, we have found that microcracking in the cast strip of low carbon steel can be controlled by regulating the ratio of sulfur to manganese in the molten metal, oxygen and free-oxygen and also to a lesser degree the ratio of manganese to silicon in the molten metal.
The present disclosure describes a thin cast steel strip produced by continuous casting by steps comprising:
The average manganese to silicon ratio in the molten low carbon steel introduced to produce the cast strip may be greater than 3.5.
The thin steel strip produced by continuous casting may have a carbon content between about 0.025% and about 0.065% by weight, or alternatively, a carbon content below about 0.035% by weight.
The thin cast strip may have a chromium content less than 1.5% by weight or less than 0.5% by weight and/or the thin cast strip may have titanium content less than 0.005% by weight.
The thin steel strip may be less than 5 mm in thickness, or less than 2.5 mm in thickness.
The molten metal in the casting pool may have a total oxygen content of at least 100 ppm and a free oxygen content between 30 and 50 ppm. Alternatively or in addition, the thin steel strip produced by continuous casting may be from the molten metal in the casting pool having a nitrogen content less than about 52 ppm. Alternatively or in addition, the sum of the partial pressures of the hydrogen and nitrogen is less than 1.15 atmospheres.
Alternatively, disclosed is a method of casting thin steel strip comprising:
The average manganese to silicon ratio in the molten low carbon steel introduced in the method to produce cast strip may be greater than 3.5.
A thin steel strip produced by the method casting steel strip may have a carbon content between about 0.010% and about 0.065% by weight.
The thin cast strip produced by the method may have a chromium content less than 1.5% by weight or less than 0.5% by weight and/or the thin cast strip may have titanium content less than 0.005% by weight.
The thin steel strip may be less than 5 mm in thickness, or less than 2.5 mm in thickness.
We have also found that additional variables that effect solidification and ‘strength’ of the newly formed shells are the temperature of the molten metal in the tundish and casting speed. Reduced temperature of the molten metal in tundish and cast speeds allows time for shell growth to larger thickness and more strength reducing microcracking adjacent to the surface of the cast strip. We have found that the thin steel strip produced by continuous casting may be cast at a tundish temperature for the molten metal below 1612° C. (2933.7° F.) and a casting speed less than 76.88 meters per minute. These additional variables are relevant to both the thin cast strip produced as well as the method by which the thin cast strip is produced.
Microcracking (generally referred to as “cracking”) is a defect that may appear in the surface portions of thin cast strip. Cracking may result from the formation of voids, surface cavities or depressions, or inclusions adjacent the surface of the strip. Cracking may occur during the formation and cooling process.
Referring to
As shown in
Casting rolls 22 are internally water cooled so that shells solidify the moving casting surfaces of the rolls. The shells are then brought together at the nip 27 between the casting rolls sometime with molten metal between the shells, to produce the solidified strip 12 which is delivered downwardly from the nip.
Frame 21 supports a casting roll carriage which is horizontally movable between as assembly station and a casting station.
Casting rolls 22 may be counter-rotated through drive shafts (not shown) driven by an electric, hydraulic or pneumatic motor and transmission. Rolls 22 have copper peripheral walls formed with a series of longitudinally extending and circumferentially spaced water cooling passages supplied with cooling water. The rolls may typically be about 500 mm in diameter and up to about 2000 mm long in order to produce strip product of about 2000 mm wide.
Tundish 23 is of conventional construction. It is formed as a wide dish made of a refractory material such as for example magnesium oxide (MgO). One side of the tundish receives molten metal from the ladle.
Delivery nozzle 26 is formed as an elongate body made of a refractory material such as for example alumina graphite. Its lower part is tapered so as to converge inwardly and downwardly above the nip between casting rolls 22.
Nozzle 26 may have a series of horizontally spaced generally vertically extending flow passages to produce a suitably low velocity discharge of molten metal throughout the width of the rolls and to deliver the molten metal between the rolls onto the roll surfaces where initial solidification occurs. Alternatively, the nozzle may have a single continuous slot outlet to deliver a low velocity curtain of molten metal directly into the nip between the rolls and/or the nozzle may be immersed in the molten metal pool.
The pool is confined at the ends of the rolls by a pair of side closure plates that are adjacent to and held against stepped ends of the rolls when the roll carriage is at the casting station. Side closure plates are illustratively made of a strong refractory material, for example boron nitride, and have scalloped side edges to match the curvature of the stepped ends of the rolls. The side plates can be mounted in plate holders which are moveable at the casting station by actuation of a pair hydraulic cylinder units (or other suitable means) to bring the side plates into engagement with the stepped ends of the casting rolls during a casting operation.
The twin roll caster may be the kind illustrated and described in some detail in, for example, U.S. Pat. Nos. 5,184,668; 5,277,243; 5,488,988; and/or 5,934,359; U.S. patent application Ser. No. 10/436,336; and International Patent Application PCT/AU93/00593, the disclosures of which are incorporated herein by reference. Reference may be made to those patents for appropriate constructional details but forms no part of the present invention.
Referring to
During crack assessment the top and bottom surfaces of the strip are each divided into 7 areas (14 areas for 2 sides) and a crack rating is given for each area. The crack rating for each area may range from “0” (for essentially defect free strip) to “5”, where “1” is less than 5 microcracks, “2” is between 5 and 24 microcracks, “3” is between 24 and 42 microcracks, “4” is between 42 and 60 microcracks, and “5” is greater than 60 microcracks in the strip. The overall crack rating “CR” is the sum of the crack rating of all 14 areas of the strip. As shown in
This analysis verified that the microcracking in the cast thin strip, and the method of making the same, was much reduced in different steel compositions with a manganese/silicon ratio above 250.
Referring to
The benefits of the present cast strip, and method of making the same, are also illustrated in the heats 175404, 175406 and 175408 reported in Table I below in percent by weight. Heats 175404 and 175406 produced steel with surface microcracks and heat 175408 produced steel without surface microcracks.
The values given in Table I are percent by weight, as are other values of element content given this application unless otherwise stated.
As shown by Table I, considerably improved results in microcracking of the surfaces of the thin strip in heat 175408 were obtained when the manganese to sulfur ratio was 316 and the manganese to silicon ratio was 4.06. The manganese, sulfur and silicon, like oxygen levels described above, were measured in the tundish 23 by known techniques.
From Heats 175404, 1754406 and 175408 we found it was possible to turn microcracks on and off between campaigns by varying the ratios of Mn/S and Mn/Si. When the ratio of Mn/S was below 250 and the ratio of Mn/Si was below 3.5, both the bottom and top surfaces of the cast strip showed microcracks across of the entire width of the strip as shown in
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We also did statistical tests on the interrelationships between the variable studied, particularly on manganese/sulfur ratio, manganese/silicon ratio, casting speed, carbon content, nitrogen content, and tundish temperature. These are reported in Table II below.
As shown in Table II, statistical correlations were found the particular levels of each of the parameters reposted above, namely manganese/sulfur ratio, manganese/silicon ratio, casting speed, carbon content, nitrogen content, and tundish temperature.
The continuously thin cast strip may be of low carbon steel, which may include 2.5% or less silicon, 0.5% or less chromium, less than 0.005% by weight titanium, 2.0% or less manganese, 0.5% or less nickel, 0.25% or less molybdenum, and 1.0% or less aluminum, together with sulfur between 0.003 and 0.008% and phosphorus and other impurities at levels that normally occur in making carbon steel by electric arc furnace. Low carbon steel, for example, may vary to have manganese content in the range 0.01% to 2.0% by weight, and silicon content in the range 0.01% to 2.5% by weight. In any event, the steel may have aluminum content of the order of 0.1% or less by weight, and may be 0.06% or less by weight. In addition to or in the alternative, the steel may have a vanadium content of the order of 0.02% or less and a niobium content on the order of 0.01% or less.
While this invention has been described and illustrated with reference to various embodiments, it shall be understood that such description is by way of illustration and not by way of limitation. Accordingly, the scope and content of the present invention are to be defined only by the terms of the appended claims.
