Patent Grant
7156151
This invention relates to the casting of steel strip. It has particular application for continuous casting of thin steel strip less than 5 mm in thickness in a roll caster.
In a roll caster, molten metal is cooled on casting surfaces of at least one casting roll and formed in to thin cast strip. In roll casting with a twin roll caster, molten metal is introduced between a pair of counter rotated casting rolls that are cooled. Steel shells solidify on the moving casting surfaces and are brought together at a nip between the casting rolls to produce a solidified sheet product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region in which the casting rolls are closest together. In any case, the molten metal is usually poured from a ladle into a smaller vessel, from where it flow through a metal delivery system to distributive nozzles located generally above the casting surfaces of the casting rolls. In twin roll casting, the molten metal is delivered between the casting rolls to form a casting pool of molten metal supported on the casting surfaces of the rolls adjacent to the nip and extending along the length of the nip. Such casting pool is usually confined between side plates or dams held in sliding engagement adjacent to ends of the casting rolls, so as to dam the two ends of the casting pool.
When casting thin steel 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 above. It is therefore necessary to achieve very high cooling rates over the casting surfaces of the casting rolls. 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. 5,760,336 incorporated herein by reference 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, and in turn, a substantially liquid layer formed at the interface between the molten metal and each casting surface. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, the disclosures of which are incorporated herein by reference, 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 thin strip casting. We have given attention in the past to the oxide inclusions and the oxygen levels in the steel metal and their impact on the quality of the steel strip produced. We have now found that the quality of the steel strip and the production of the thin steel strip is also enhanced by control of the hydrogen levels and nitrogen levels in the molten steel. Controlling hydrogen and nitrogen levels has in the past been the subject of investigation in slab casting, but to our knowledge has not been a focus of attention in thin strip casting. For example, see Control of Heat Removal in the Continuous Casting Mould, by P. Zasowski and D. Sosinsky, 1990 Steelmaking Conference Proceedings, 253–259; and Determination and Prediction of Water Vapor Solubilities in CaO—MgO—SiO2 Slags, by D. Sosinsky, M. Maeda and A. Mclean, Metallurgical Transactions, vol. 16b, 61–66 (March 1985).
Specifically we have found that by controlling the hydrogen and nitrogen levels in the steel melt, with low levels of sulfur in the steel, plain carbon steel strip having unique composition and production qualities can be produced by roll casting. There is provided a method of casting steel strip comprising:
introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a free nitrogen content below about 120 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
forming a casting pool of molten metal on the eastine surfaces of the casting rolls; and
solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
The method of casting steel strip may be carried out by the steps comprising the following:
assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;
introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on casting surfaces of the casting rolls with the end closures confining the pool, with the molten steel having a free nitrogen content below about 120 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres; and
counter-rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to provide for 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 content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
Alternatively, there is provided a method of casting steel strip comprising:
introducing molten plain carbon steel on casting roll surfaces of at least one casting roll having a free nitrogen content below about 100 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
forming a casting pool of molten metal on the casting surfaces of the casting rolls; and
solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
The method of casting steel strip may be carried out by the steps comprising the following:
assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;
introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on casting surfaces of the casting rolls with the end closures confining the pool, with the molten steel having a free nitrogen content below about 100 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
counter-rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to provide for 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 content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
As a further alternative, there is provided a method of casting steel strip comprising:
introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a free nitrogen content below about 85 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
forming a casting pool of molten metal on the casting surfaces of the casting rolls; and
solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
The method of casting steel strip may be carried out by the steps comprising the following:
assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;
introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting surfaces of the casting rolls with the end closure confining the pool, with the molten steel having a free nitrogen content below about 85 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
counter-rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to provide for 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 content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
In any of these methods, the free nitrogen content may be 60 ppm or less, and the free hydrogen content may be 1.0 to 6.5 ppm. The free hydrogen content may, for example, be between 2.0 and 6.5 ppm or between 3.0 and 6.5 ppm.
Plain carbon steel for purpose of the present invention is defined as less than 0.65% carbon, less than 2.5% silicon, less than 0.5% chromium, less than 2.0% manganese, less than 0.5% nickel, less than 0.25% molybdenum and less than 1.0% aluminum, together with of other elements such as sulfur, oxygen and phosphorus which normally occur in making carbon steel by electric arc furnace. Low carbon steel may be used in these methods having 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 0.01% to 2.5% by weight, and low carbon cast strip may be made by the method. The steel may have an aluminum content of the order of 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.
In these methods, the sulfur content of the steel may be 0.01% or less; and the sulfur content of the steel may be 0.007% by weight.
In these methods, the free nitrogen may be measured by optical emission spectrometry, calibrated against the thermal conductivity method a described below. The free hydrogen levels may be determined by a Hydrogen Direct Reading Immersed System (“Hydris”) unit, made by Hereaus Electronite.
The maximum allowable free nitrogen and free hydrogen levels may be for total pressure not to exceed 1.0 atmospheres. Higher pressures may be utilized in certain conditions, and the levels of free nitrogen and free hydrogen can be corresponding higher. For example, as explained below, a ferrostatic head may be 1.15, causing the free nitrogen levels and free hydrogen levels to be higher as shown in
The present invention provides cast steel strip with unique properties that are described by the methods by which it is made. This steel strip is plain carbon steel.
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:
As shown in
Casting rolls 22 are water cooled so that shells solidify on the moving casting surfaces of the rolls. The shells are then brought together at the nip 27 between the casting rolls sometimes 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 an 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 25 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 and is provided with an overflow spout 24 and an emergency plug 25.
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 28 which are adjacent to and held against stepped ends of the rolls when the roll carriage is at the casting station. Side closure plates 28 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 movable at the casting station by actuation of a pair of hydraulic cylinder units (or other suitable means) to bring the side plates into engagement with the stepped ends of the casting rolls to form end closures for the molten pool of metal formed on the casting rolls during a casting operation.
The twin roll caster may be of 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.
Results of the control of the free nitrogen and hydrogen levels in thin cast sheets of plain carbon steel are set out in Table 1 and in
The results shown in
The composition of all heats in Table 1 are in percent by weight, and are shown in
More recently, additional heats have been made with low nitrogen and low hydrogen having compositions shown in Table 2. The nitrogen level range from 42 to 118 ppm and the hydrogen levels ranged from 3.0 to 6.9 ppm. However, the hydrogen level of 6.9 ppm is with a ferrostatic head of more than 1 atmosphere pressure, namely about 1.15 atmospheres, as shown by the right-hand curve in
From the heats reported in Table 2, it is seen that the levels of nitrogen can be up to 120 ppm, and the levels of hydrogen are between 1.0, 2.0 or 3.0 and 6.5 ppm at atmospheric pressure. Moreover, the hydrogen level of 6.9 ppm in heat 1655 is with ferrostatic head of more than 1 atmosphere pressure, namely about 1.15 atmospheres, as shown in
The free nitrogen was determined by analysis with optical emission specometry (“OES”) calibrated against the thermal conductivity (“TC”) method on a scheduled basis. Optical emission spectrometry (OES) using arc and spark excitation is the preferred method to determine the chemical composition of metallic samples. This process is widely used in the metal making industries, including primary producers, foundries, die casters and manufacturing. Due to its rapid analysis time and inherent accuracy, Arc/Spark OES systems are most effective in controlling the processing of alloys. These spectrometers may be used for many aspects of the production cycle including in-coming inspection of materials, metal processing, quality control of semi-finished and finished goods and many other applications where a chemical composition of the metallic material is required.
The Thermal Conductivity (TC) method, used to calibrate the OES, typically employs a microprocessor-based, software controlled instrument that can measure nitrogen, as well as oxygen, in a wide variety of metals, refractories and other inorganic materials. The TC method employs the inert gas fusion principle. A weighed sample, placed in a high purity graphite crucible, is fused under a flowing helium gas stream at temperatures sufficient to release oxygen, nitrogen and hydrogen. The oxygen in the sample, in all forms present, combines with the carbon from the crucible to form carbon monoxide. The nitrogen present in the sample releases as molecular nitrogen and any hydrogen is released as hydrogen gas.
In the TC method, oxygen is measured by infrared absorption (IR). Sample gases first enter the IR module and pass through CO and CO2 detectors. Oxygen present as either CO or CO2 is detected. Following this, sample gas is passed through heated rare-earth copper oxide to convert CO to CO2 and any hydrogen to water. Gases then re-enter the IR module and pass through a separate CO2 detector for total oxygen measurement. This configuration maximizes performance and accuracy for both low and high range.
In the TC method, nitrogen is measured by passing sample gases to be measured through heated rare-earth copper oxide which converts CO to CO2 and hydrogen to water. CO2 and water are then removed to prevent detection by the TC cell. Gas flow then passes through the TC cell for nitrogen detection.
As stated above, the free hydrogen is measured by a Hydrogen Direct Reading Immersed System (“Hydris”) unit, made by Hereaus Electronite. This unit is believed to be described in the following referenced US patents: U.S. Pat. Nos. 4,998,432; 5,518,931 and 5,820,745.
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 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.
This application claims priority to and the benefit of U.S. provisional patent application No. 60/510,479 filed Oct. 10, 2003, the disclosure of which is incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3670400 | Singer | Jun 1972 | A |
| 4006044 | Oya et al. | Feb 1977 | A |
| 5103895 | Furuya et al. | Apr 1992 | A |
| 5106412 | Bogan et al. | Apr 1992 | A |
| 5180450 | Rao | Jan 1993 | A |
| 5320687 | Kipphut et al. | Jun 1994 | A |
| 5720336 | Strezov | Feb 1998 | A |
| 5820817 | Angeliu et al. | Oct 1998 | A |
| 5906791 | Angeliu | May 1999 | A |
| 6290787 | Babbit et al. | Sep 2001 | B1 |
| 6328826 | Iung et al. | Dec 2001 | B1 |
| 6372057 | Fujimura et al. | Apr 2002 | B1 |
| 6478899 | Legrand et al. | Nov 2002 | B1 |
| 6588494 | Mazurier et al. | Jul 2003 | B1 |
| 6622779 | Mazurier | Sep 2003 | B1 |
| Number | Date | Country |
|---|---|---|
| 0 922 511 | Jun 1999 | EP |
| 52-139622 | Nov 1977 | JP |
| 58-45321 | Mar 1983 | JP |
| 64-83339 | Mar 1989 | JP |
| 4-279246 | Oct 1992 | JP |
| 11-179489 | Jul 1999 | JP |
| 2001-220642 | Aug 2001 | JP |
| 2002361372 | Feb 2003 | JP |
| 2003-154441 | May 2003 | JP |
| WO 9828452 | Jul 1998 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20050082031 A1 | Apr 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60510479 | Oct 2003 | US |
