AU2001291499B2 - A method of producing steel - Google Patents

A method of producing steel Download PDF

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AU2001291499B2
AU2001291499B2 AU2001291499A AU2001291499A AU2001291499B2 AU 2001291499 B2 AU2001291499 B2 AU 2001291499B2 AU 2001291499 A AU2001291499 A AU 2001291499A AU 2001291499 A AU2001291499 A AU 2001291499A AU 2001291499 B2 AU2001291499 B2 AU 2001291499B2
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Prior art keywords
carbon steel
low carbon
strip
mpa
sec
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AU2001291499A
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AU2001291499A1 (en
Inventor
Walter Blejde
Rama Mahapatra
Kannappar Mukunthan
Lazar Strezov
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Nucor Corp
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BlueScope Steel Ltd
IHI Corp
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Priority to AUPR0479A priority Critical patent/AUPR047900A0/en
Priority to AUPR0479 priority
Application filed by BlueScope Steel Ltd, IHI Corp filed Critical BlueScope Steel Ltd
Priority to PCT/AU2001/001215 priority patent/WO2002026422A1/en
Priority to AU2001291499A priority patent/AU2001291499B2/en
Publication of AU2001291499A1 publication Critical patent/AU2001291499A1/en
Assigned to BLUESCOPE STEEL LIMITED, ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES COMPANY reassignment BLUESCOPE STEEL LIMITED Request for Assignment Assignors: BHP STEEL (JLA) PTY LTD, ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES COMPANY
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Publication of AU2001291499B2 publication Critical patent/AU2001291499B2/en
Assigned to NUCOR CORPORATION reassignment NUCOR CORPORATION Request for Assignment Assignors: BLUESCOPE STEEL LIMITED, ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES COMPANY
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Description

WO 02/26422 PCT/AU01/01215 1 A METHOD OF PRODUCING STEEL STRIP This application claims priority to Australian Patent Application No. PR0479, filed September 29, 2000.
Background and Summary of the Invention The present invention relates to a method of producing steel strip and the cast steel strip produced according to the method.
In particular, the present invention relates to producing steel strip in a continuous strip caster.
The term "strip" as used in the specification is to be understood to mean a product of 5mm thickness or less.
The applicant has carried out extensive research and development work in the field of casting steel strip in a continuous strip caster in the form of a twin roll caster.
In general terms, casting steel strip continuously in a twin roll caster involves introducing molten steel between a pair of contra-rotated horizontal casting rolls which are internally water cooled so that metal shells solidify on the moving rolls surfaces and are brought together at the nip between them to produce a solidified strip delivered downwardly from the nip between the rolls, the term "nip" being used 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 so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length 2 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, although alternative means such as electromagnetic barriers have also been proposed.
The casting of steel strip in twin roll casters of this kind is for example described in United States Patents 5,184,668, 5,277,243 and 5,934,359.
The applicant has determined that steel strip of a given composition that has a wide range of microstructures, and therefore a wide range of yield strengths, can be produced by continuously casting the strip and thereafter selectively cooling the strip to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0 C. It is understood that the transformation range is within the range between 850 0 C and 400 0 C and not that entire temperature range. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.
Specifically, from work carried out by the applicant on low carbon steel, including low carbon steel that has been silicon/manganese killed or aluminum killed, it has been determined that selecting cooling rates in the range of 0.01 0 C/sec to greater than 100 0 C/sec to transform the strip from austenite to ferrite in a temperature range between 850°C and 400°C, can produce steel strip that has yield strengths that range from 200 MPa to 700 MPa or greater. This is a significant development since, unlike conventional slab casting/hot rolling processes where chemistry changes are necessary to produce a broad range of properties, it has been determined that the same outcome can be achieved with a single chemistry.
Accordingly, there is provided a method of N 'Melboumc\Case\Paten 3U9 DOOO39999P3973 LAU. I SpcisP3973 I AU. I Specificafion 2007-I-I Sdoc 16101/07 3 producing steel strip which includes the steps of: continuously casting molten low carbon steel into a strip of no more than 5mm thickness having austenite grains that are coarse grains of 100-300 micron width; and cooling the strip to transform the austenite grains to ferrite in a temperature in a temperature range between 850 0 C and 400 0 C at a selected cooling rate of at least 0.01 0 C/sec to produce a microstructure that includes at least some polygonal ferrite that provides a strip having a yield strength from between 200 MPa The term "low temperature transformation products" includes Widmanstatten ferrite, acicular ferrite, bainite and martensite.
The method may include passing the strip onto a run-out table and step includes controlling cooling of the strip on the run-out table to achieve the selected cooling rate to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0
C.
The method may include the additional step of inline hot rolling the cast strip prior to cooling the strip to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0 C. This inline N \Melboume\Cases\Paient\39000-39999\P3973 [AU. Specis\P3973 I AU.I Specificadon 2007-1.15.doc 16/01/07 WO 02/26422 PCT/AU01/01215 4 hot rolling step reduces the strip thickness up to The cast strip produced in step (a) illustratively has a thickness of no more than 2mm.
The coarse austenite grains produced in step (a) of 100-300 micron width have a length dependent on the thickness of the cast strip. Generally, the coarse austenite grains are up to slightly less than one-half the thickness of the strip. For example, for cast strip of 2mm thickness, the coarse austenite grains will be up to about 750 microns in length.
The cast strip produced in step may have austenite grains that are columnar.
The upper limit of the cooling rate in step (b) is at least 100 C/sec.
The term "low carbon steel" is understood to be mean steel of the following composition, in weight percent: C: 0.02-0.08 Si: 0.5 or less; Mn: 1.0 or less; residual/incidental impurities: 1.0 or less; and Fe: balance The term "residual/incidental impurities" covers levels of elements, such as copper, tin, zinc, nickel, chromium, and molybdenum, that may be present in relatively small amounts, not as a consequence of specific additions of these elements but as a consequence of standard steel making. By way of example, the elements may be present as a result of using scrap steel to produce low carbon steel.
5 The low carbon steel may be silicon/manganese killed and may have the following composition by weight: Carbon Manganese Silicon Sulphur Aluminium 0.02 0.08% 0.30 0.80% 0.10 0.40% 0.002 0.05% less than 0.01% The low carbon steel may be calcium treated killed and may have the following composition by aluminum weight: Carbon Manganese Silicon Sulphur Aluminum 0.02 0.08% 0.40% max 0.05% max 0.002 0.05% 0.05% max The aluminum killed steel may be calcium treated.
The yield strength of aluminum killed steel is generally 20 to 50 MPa lower than that of silicon/manganese killed steel.
Illustratively, the cooling rate in step is less than l°C/sec to produce a microstructure that is predominantly polygonal ferrite and has a yield strength less than 250 MPa.
Illustratively, the cooling rate in step is in the range of 1-15°C/sec to produce a microstructure that is a mixture of polygonal ferrite, Widmanstatten ferrite and acicular ferrite and has a yield strength in the range of 250-300 MPa.
Illustratively, the cooling rate in step is N:\MelboumNc\CasesPatcrnU9000-39999\3973 IAU. I\Spci%\P3973 AU I Specificaon 2007.. 1.doc 16/01/07 6 in the range of 15-100 0 C/sec to produce a microstructure that is a mixture of polygonal ferrite, bainite and martensite and has a yield strength in the range of 300- 450 MPa.
Illustratively, the cooling rate in step is at least 100 0 C/sec to produce a microstructure that is a mixture of polygonal ferrite, bainite and martensite and has a yield strength at least 450 MPa.
The continuous caster may be a twin roll caster.
There is provided a low carbon steel produced by the method described above having desired microstructure and yield strength.
Brief Description of the Drawings In order that the invention may be more fully explained, an example will be described with reference to the accompanying drawings, of which: Figure 1 illustrates a strip casting installation incorporating an in-line hot rolling mill and coiler; and Figure 2 illustrates details of the twin roll strip caster; and Figures 3(a) to 3(d) are photomicrographs of cast strip that illustrate the effect on final microstructure of cooling rates during the austenite to ferrite transformation temperature range.
Detailed Description of the Invention The following description of the described embodiments is in the context of continuous casting steel strip using a twin roll caster. The present invention is N Melboumc\Case\PatentU9000-9999P3973 I AU I\Specrs\P39731 AU. I Specificaton 2007-I-5.doc 16/01/07 WO 02/26422 PCT/AU01/01215 7 not limited to the use of twin roll casters and extends to other types of continuous strip casters.
Figure 1 illustrates successive parts of a production line whereby steel strip can be produced in accordance with the present invention. Figures 1 and 2 illustrate a twin roll caster denoted generally as 11 which produces a cast steel strip 12 that passes in a transit path 10 across a guide table 13 to a pinch roll stand 14 comprising pinch rolls 14A. Immediately after exiting the pinch roll stand 14, the strip passes into a hot rolling mill 16 comprising a pair of reduction rolls 16A and backing rolls 16B by in which it is hot rolled to reduce its thickness. The rolled strip passes onto a runout table 17 on which it may be cooled by convection by contact with water supplied via water jets 18 (or other suitable means) and by radiation. The rolled strip then passes through a pinch roll stand 20 comprising a pair of pinch rolls 20A and thence to a coiler 19. Final cooling (if necessary) of the strip takes place on the coiler.
As shown in Figure 2, twin roll caster 11 comprises a main machine frame 21 which supports a pair of parallel casting rolls 22 having a 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 25 and thence through a metal delivery nozzle 26 into the nip 27 between the casting rolls 22. Molten metal thus delivered to the nip 27 forms a pool 30 above the nip and this pool is confined at the ends of the rolls by a pair of side closure dams or plates 28 which are applied to the ends of the rolls by a pair of thrusters (not shown) comprising hydraulic cylinder units connected to the side plate holders. The upper surface of pool 30 (generally referred to as the "meniscus" level) may rise above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is immersed WO 02/26422 PCT/AU01/01215 8 within this pool.
Casting rolls 22 are water cooled so that shells solidify on the moving roll surfaces and are brought together at the nip 27 between them to produce the solidified strip 12 which is delivered downwardly from the nip between the rolls.
The twin roll caster may be of the kind which is illustrated and described in some detail in United States Patents 5,184,668 and 5,277,243 or United States Patent 5,488,988 and reference may be made to those patents for appropriate constructional details which form no part of the present invention.
The above-described twin roll caster continuously casts strip 12 of no less than 2mm thickness with a microstructure of columnar austenite grains of 100- 300 micron width.
In accordance with the illustrated embodiment of the method described, the cooling rate of the cast strip to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0 C is selected to control transformation of austenite into a ferrite microstructure that is required to provide specified yield strength of the cast strip.
In accordance with the illustrated embodiment, the cooling rate is at least 0.01°C/sec and may be in excess of 100 0 C/sec and is selected to transform the steel strip from austenite to ferrite until austenite transformation is completed.
In the case of low carbon steels, such a range of microstructures can produce yield strengths in the range of 200 MPa to in excess of 700 MPa.
WO 02/26422 PCT/AU01/01215 9 With such cooling rates for low carbon steel it is possible to produce cast strip having microstructures including: predominantly polygonal ferrite; (ii) a mixture of polygonal ferrite and low temperature transformation products, such as a widmanstatten ferrite, acicular ferrite, and bainite; and (iii) predominantly low temperature transformation products.
In the case of low carbon steels, such a range of microstructures can produce yield strengths in the range of 200 MPa to in excess of 700 MPa.
The present disclosure is based in part on experimental work carried out on silicon/manganese killed low carbon steel.
The table set out below summarises the effect of cooling rate to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0 C on the microstructure and resultant yield strength of silicon/manganese killed low carbon steel strip. The strips were cast in a twin roll caster of the type described above.
Cooling Rate Coiling Microstructure Yield
O
C/sec) Temperature Constituents Strength (oC) (MPa) 0.1 >800 Polygonal ferrite, 210 Pearlite 13 670 Polygonal ferrite, 320 Widmanstatten ferrite, acicular WO 02/26422 PCT/AU01/01215 10 ferrite 580 Polygonal ferrite, 390 Bainite 100 <400 Polygonal ferrite, 490 Bainite, Martensite Figures 3(a) to 3 are photomicrographs of the final microstructure of the cast strip.
It is clear from the table and the photomicrographs that selection and control of the cooling rate had a significant impact on the microstructure and yield strength of the single chemistry cast strip. As noted above, in conventional slab casting/hot rolling processes, a range of different chemistries would be required to achieve the range of yield strength. The range of chemistries was in the past achieved by adding differing amounts of alloys that add considerable cast to the steel production process.
Control of the cooling rate to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0 C is achieved by controlling cooling on the run-out table 17 and/or the coiler 19 of the strip casting installation.
The production of soft materials (yield strength 350 MPa) requires relatively slow cooling rates through the austenite to ferrite transformation temperature range.
In order to achieve the slow cooling rates, it is necessary to complete austenite transformation on the coiler 19.
The production of harder materials (yield strength 400 MPa) requires higher cooling rates to transform the strip from austenite to ferrite in a temperature range between 850 0 C and 400 0 C. In order to WO 02/26422 PCT/AU01/01215 11 achieve the higher cooling rates the austenite transformation is completed on the run-out table.
Figures 3(a) to are photomicrographs of the final microstructures of the cast strip.
Although the invention has been illustrated and described in detail in the foregoing drawings and description with reference to several embodiments, it should be understood that the description is illustrative and not restrictive in character, and that the invention is not limited to the disclosed embodiments. Rather, the present invention covers all variations, modifications and equivalent structures that come within the scope and spirit of the invention. Additional features of the invention will become apparent to those skilled in the art upon consideration of the detailed description, which exemplifies the best mode of carrying out the invention as presently perceived. Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.

Claims (22)

1. A method of producing steel strip including the steps of: continuously casting molten low carbon steel into a strip of no more than thickness having austenite grains that are coarse grains of 100-300 micron width; and cooling the strip to transform the austenite grains to ferrite in a temperature range between 850 0 C and 400 0 C at a selected cooling rate of at least 0.01 0 C/sec to produce a microstructure that includes at least some polygonal ferrite that provides a strip having a desired yield strength of at least 200 MPa.
2. The method of in claim 1 wherein the cast strip produced in step has a thickness of no more than 2mm.
3. The method of in claim 1 or claim 2 wherein the austenite grains produced in step are columnar.
4. The method of any one of the preceding claims wherein the cooling rate in step is at least 100 0 C/sec. The method of any one of the preceding claims wherein the low carbon steel is silicon/manganese killed.
6. The method of any one of the preceding claims wherein the silicon/manganese killed low carbon steel has the following composition by weight: N:\Melbournc\Cascs\Patent\3900.3999\P3973I AU. \Specis\P3973 IAU I Specificaon 2007-1-l5.doc 16/01/07 WO 02/26422 PCT/AU01/01215 13 Carbon Manganese Silicon Sulphur Aluminium The method carbon steel
7. the low 0.02 0.08% 0.30 0.80% 0.10 0.40% 0.002 0.05% less than 0.01% of any one of claims 1 to 4 wherein is aluminum killed. of claim 7 wherein the aluminum killed the following composition by weight: 0.02 0.08% 0.40% max 0.05% max 0.002 0.05% 0.05% max
8. The method low carbon steel has Carbon Manganese Silicon Sulphur Aluminum
9. The method of any one of the preceding claims wherein the cooling rate in step is less than 1 0 C/sec to produce a microstructure that has a yield strength in the range of 200-250 MPa. The method of any one of claims 1 to 8 wherein the cooling rate in step is in the range of 1-15 0 C/sec to produce a microstructure that has a yield strength in the range of 250-300 MPa.
11. The method of any one of claims 1 to 8 wherein the cooling rate in step is in the range of 100 0 C/sec to produce a microstructure that has a yield strength in the range of 300-450 MPa.
12. The method of any one of claims 1 to 8 wherein the cooling rate in step is at least 100 0 C/sec to produce a microstructure that has a yield strength at least 450 MPa. 14
13. The method of any one of the preceding claims further includes passing the strip onto a run-out table and step includes controlling cooling of the strip on the run-out table to achieve the selected cooling rate to transform the austenite grains to ferrite in a temperature range between 850 0 C and 400 0 C.
14. The method of any one of the preceding claims further includes the step of in-line hot rolling the cast strip produced in step to reduce the strip thickness up to The method of any one of the preceding claims wherein the continuous casting is done with a twin roll caster.
16. The method of claim 1 wherein the yield strength is 200 MPa to 700 MPa.
17. A low carbon steel produced by a process including the steps of: continuously casting molten low carbon steel into a strip of no more than thickness with austenite grains that are coarse grains of 100-300 micron width; and providing desired mechanical properties in the cast strip by cooling the strip to transform the austenite grains to ferrite in a temperature range from 850 0 C to 400 0 C at a selected cooling rate of at least 0.01 0 C/sec to produce a microstructure that includes at least some polygonal ferrite that provides a strip having a yield strength between 200 and in excess of 700 N:\Mclboure\Cases\Patent\39000-3999 9P3973 I AU. I\Specis\P3973IAU I Specilicaton 2007-I-15.doc 16101/07 15 MPa.
18. The low carbon steel of in claim 17 wherein the cast strip produced in step has a thickness of no more than 2mm.
19. The low carbon steel of in claim 17 or claim 18 wherein the austenite grains produced in step are columnar. The low carbon steel of any one of claims 17 to 19 wherein the cooling rate in step is at least 100 0 C/sec.
21. The low carbon steel of any one of claims 17 to wherein the low carbon steel is silicon/manganese killed.
22. The low carbon steel of claim 21 wherein the low carbon steel has the following composition by weight: Carbon 0.02 0.08% Manganese 0.30 0.80% Silicon 0.10 0.40% Sulphur 0.002 0.05% Aluminum less than 0.01%
23. The method of any one of claims 17 to 20 wherein N \Melbome\Cases\Paten\39000-39999\P3973 IAU I\SpecisP3973 I AU I Spcificadwi 2007-I-I5.doc 16101/07 WO 02/26422 PCT/AU01/01215 16 the low carbon steel is aluminum killed.
24. The low carbon steel of claim 23 wherein the low carbon steel has the following composition by weight: Carbon 0.02 0.08% Manganese 0.40% max Silicon 0.05% max Sulphur 0.002 0.05% Aluminum 0.05% max The low carbon steel of any one of claims 17 to 19 wherein the cooling rate in step is less than l°C/sec in order to produce a microstructure that is predominantly polygonal ferrite and has a yield strength between 200 and 250 MPa.
26. The low carbon steel of any one of claims 17 to 19 wherein the cooling rate in step is in the range of 1-15 0 C/sec in order to produce a microstructure that is a mixture of polygonal ferrite, Widmanstatten ferrite and acicular ferrite and has a yield strength in the range of
250-300 MPa. 27. The low carbon steel of any one of claims 17 to 19 wherein the cooling rate in step is in the range of 15-100°C/sec in order to produce a microstructure that is a mixture of polygonal ferrite and bainite and has a yield strength in the range of 300-450 MPa. 28. The low carbon steel of any one of claims 17 to 19 wherein the cooling rate in step is at least 100 0 C/sec in order to produce a microstructure that is a mixture of polygonal ferrite, bainite and martensite and has a yield strength of at least 450 MPa.
AU2001291499A 2000-09-29 2001-09-28 A method of producing steel Ceased AU2001291499B2 (en)

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AUPR0479A AUPR047900A0 (en) 2000-09-29 2000-09-29 A method of producing steel
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PCT/AU2001/001215 WO2002026422A1 (en) 2000-09-29 2001-09-28 A method of producing steel
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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9149868B2 (en) 2005-10-20 2015-10-06 Nucor Corporation Thin cast strip product with microalloy additions, and method for making the same
US9999918B2 (en) 2005-10-20 2018-06-19 Nucor Corporation Thin cast strip product with microalloy additions, and method for making the same
US10071416B2 (en) 2005-10-20 2018-09-11 Nucor Corporation High strength thin cast strip product and method for making the same

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH03274231A (en) * 1990-02-06 1991-12-05 Nippon Steel Corp Manufacture of thin strip steel sheet
WO1995003155A1 (en) * 1993-07-21 1995-02-02 Bmd Badische Maschinenfabrik Durlach Gmbh Workpiece blasting process and device
WO1998026882A1 (en) * 1996-12-19 1998-06-25 Hoogovens Staal B.V. Process and device for producing a steel strip or sheet
GB2334464A (en) * 1998-02-19 1999-08-25 Kvaerner Metals Cont Casting Low cost apparatus and method for manufacturing of light gauge steel strip
US6062055A (en) * 1997-04-10 2000-05-16 Danieli & C. Officine Meccaniche Spa Rolling method for thin flat products and relative rolling line
WO2001012184A1 (en) * 1999-08-16 2001-02-22 Revaax Pharmaceuticals, Llc Neurotherapeutic composition and method

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH03274231A (en) * 1990-02-06 1991-12-05 Nippon Steel Corp Manufacture of thin strip steel sheet
WO1995003155A1 (en) * 1993-07-21 1995-02-02 Bmd Badische Maschinenfabrik Durlach Gmbh Workpiece blasting process and device
WO1998026882A1 (en) * 1996-12-19 1998-06-25 Hoogovens Staal B.V. Process and device for producing a steel strip or sheet
US6062055A (en) * 1997-04-10 2000-05-16 Danieli & C. Officine Meccaniche Spa Rolling method for thin flat products and relative rolling line
GB2334464A (en) * 1998-02-19 1999-08-25 Kvaerner Metals Cont Casting Low cost apparatus and method for manufacturing of light gauge steel strip
WO2001012184A1 (en) * 1999-08-16 2001-02-22 Revaax Pharmaceuticals, Llc Neurotherapeutic composition and method

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9149868B2 (en) 2005-10-20 2015-10-06 Nucor Corporation Thin cast strip product with microalloy additions, and method for making the same
US9999918B2 (en) 2005-10-20 2018-06-19 Nucor Corporation Thin cast strip product with microalloy additions, and method for making the same
US10071416B2 (en) 2005-10-20 2018-09-11 Nucor Corporation High strength thin cast strip product and method for making the same

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