EP2398602B1

EP2398602B1 - Method for making a hot rolled thin cast strip product

Info

Publication number: EP2398602B1
Application number: EP10743332.8A
Authority: EP
Inventors: Daniel Geoffrey Edelman; Christopher Ronald Killmore; Mary E. Alwin-Becker
Original assignee: Nucor Corp
Current assignee: Nucor Corp
Priority date: 2009-02-20
Filing date: 2010-02-20
Publication date: 2018-10-31
Anticipated expiration: 2030-02-20
Also published as: AU2010215077B2; AU2017202997A1; EP2398602A1; KR101715086B1; EP2398602A4; MY173389A; JP5509222B2; RU2011138463A; KR20110117142A; PL2398602T3; CN105215299A; CN102325608A; CN102325608B; RU2532794C2; AU2017202997B2; EP3431201A3; US20100215981A1; JP2012518539A; AU2010215077A1; EP3431201A2

Description

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 "nip" 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 comprising 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. The cast strip is typically directed to a hot rolling mill where the strip is hot reduced by 10% or more.
In the past, plain low carbon steels have been continuously cast on a twin roll caster, including plain carbon-manganese steel. The physical properties of these plain carbon-manganese steels typically were affected by increasing hot rolling reduction. For example, yield strength and tensile strength decreased with increasing amount of hot rolling, while total elongation typically increased with increasing amount of hot rolling. As a result, in the past the steel compositions had to be tailored for the amount of hot rolling reduction that was applied to provide desired mechanical properties. This resulted in inefficiency and operational problems as melt shops had to provide different molten compositions for different hot rolled strip thickness to provide desired hot rolled steel properties.
Additionally, the steel compositions may have included copper from scrap products incorporated into the molten steel. In the past, copper levels over about 0.2 weight % were generally avoided because of concerns over "hot shortness" during hot rolling reduction, which causes cracks or extremely roughened surfaces on the strip, sometimes referred to as "checking". In cases where copper levels were higher than 0.2% (such as in steels with improved atmospheric weathering resistance), expensive additions such as nickel had to be added to reduce the risk of hot shortness.
The problem of hot shortness has increased the costs in making low alloy steel using electric arc furnaces to form the molten carbon steel. Approximately 75% of the cost of making steel by electric arc furnaces is the cost of the scrap used as the starting material for charging the electric arc furnaces. Steel scrap has been traditionally separated by copper content to less than 0.15% by weight copper, greater than or equal to 0.15% to up to 0.5% by weight copper, and above 0.5% by weight. Scrap with copper content above 0.5% copper could be mixed with scrap with low copper levels to make an acceptable scrap. In any event, the scrap which was low copper below 0.15% by weight is the highest cost scrap, with the other two grades of scrap being of less cost. Scrap with less than 0.15% copper is generally useful in electric arc furnaces for certain commercial methods of making steel, adding considerably to the cost of the steel sheet produced. Scrap grades with copper content up to 0.5% have been useful in bar mills serviced by electric arc furnaces, or in other processes at considerable expense by mixing with scrap of lower copper content to reduce the overall copper content of the scrap to less than 0.15%.
Dainelli P et AL "Des Aciers De Construction Plis Facilement Soudables" Soudage et Techniques Connexes, Institut De Soudure ISSN 0246-0963, which is considered to represent the closest prior art, discloses a method of making hot rolled steel strip having a composition consisting of, by weight, less than 0.25% carbon, greater than 0.01% and less than or equal to 0.15% phosphorus, between 1.0 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminium.
Other prior art includes US 5901777 , US 5567250 , and US 2004/177945 .
According to the present invention there is provided a method of making hot rolled steel strip according to claim 1.
The mechanical properties are preferably within 10% throughout the range from 15% to 35% reduction for yield strength, tensile strength and total elongation.
The molten steel composition may have a free oxygen content between 30 and 60 ppm. The total oxygen content of the molten metal for the hot rolled steel strip may be between 70 ppm and 150 ppm.
The molten steel may have a composition such that the manganese content of the composition of the hot rolled steel strip is between 0.9 and 1.3% by weight.
The molten steel may have a composition such that the composition of the hot rolled steel strip may have in addition between 0.01% and 0.20% niobium by weight. Alternatively or in addition, the composition of molten steel may have a composition such that the composition of the hot rolled steel strip further comprises at least one element selected from the group consisting of molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof by weight.
A hot rolled steel strip may additionally be provided with a coating of zinc or a zinc alloy or aluminum. The hot rolled steel strip may also have a yield strength of at least 440 MPa after hot rolling reductions of at least 35%.
In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawings, in which:

FIG. 1 illustrates a strip casting installation incorporating an in-line hot rolling mill and coiler;
FIG. 2 illustrates details of the twin roll strip caster;
FIG. 3 is a graph showing the effect of hot rolling reduction on the yield strength for elevated manganese steel;
FIG. 4 is a graph showing the effect of hot rolling reduction on the yield strength and elongation for 0.19% carbon steel,
FIG. 5 is a graph showing the effect of amount of carbon on the tensile strength, yield strength, and elongation for test samples having between 0.88% and 1.1% manganese; and
FIG. 6 is a graph showing the effect of hot rolling reduction on the tensile strength, yield strength, and elongation over reductions between about 15% and 45%.

Detailed Description of the Drawings

FIG. 1 illustrates successive parts of a strip caster for continuously casting steel strip. FIGS. 1 and 2 illustrate a twin roll caster 11 that continuously produces 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 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. 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 "meniscus" level) usually rises above the lower end of the delivery nozzle 26 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. Patent. Nos. 5,184,668 and 5,277,243 or U.S. Patent. No. 5,488,988 , or U.S. Patent Application 12/050,987 . Reference may be made to the patent specifications of those patents and patent application for appropriate construction details of a twin roll caster appropriate for use in an embodiment of the present invention.
By employing rapid solidification rates with control of certain parameters in twin roll strip casting, the steel composition of the present invention generates liquid deoxidation products of MnO and SiO₂ in a fine and uniform distribution of globular inclusions. The MnO.SiO₂ inclusions present are also not significantly elongated by the in-line hot rolling process, due to limited hot reduction. The inclusion/particle populations are tailored to stimulate nucleation of acicular ferrite. The MnO.SiO₂ inclusions may be about 10 µm down to very fine particles of less than 0.1 µm, and a majority being between about 0.5 µm and 5 µm. The larger 0.5-10 µm size non-metallic inclusions are provided for nucleating acicular ferrite, and may include a mixture of inclusions, for example including MnS, and CuS. The austenite grain size is significantly larger than the austenite grain size produced in conventional hot rolled strip steel. The coarse austenite grain size, in conjunction with the population of tailored inclusion/particles, assists with the nucleation of acicular ferrite and bainite.
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 at a temperature between 300 and 700°C. Alternatively, the coiling temperature may be between about 450 and 550°C. The resulting microstructure comprises a majority acicular ferrite and bainite.
The effect of hot reduction on yield strength, tensile strength, and total elongation in the present elevated copper and elevated manganese steels results in a steel properties where the tensile strength, yield strength and total elongation are relatively stable with different levels of hot reduction. In previous such steel products, there is typically a decrease in yield and tensile strengths with increasing hot reduction. In contrast, the effect of hot reduction on yield strength, tensile strength, and total elongation is significantly reduced in the present steel products. A coiling temperature below 550°C may be used in conjunction with a high degree of hot rolling to mitigate the hot reduction affect on the mechanical properties.
Hot reductions larger than about 15% can induce recrystallization of austenite, which reduces the grain size and volume fraction of acicular ferrite and bainite.
We have found that the addition of alloying elements that increase the hardenability of the steel suppressed the recrystallization of the coarse as-cast austenite grain size during the hot rolling process, and resulted in the hardenability of the steel being retained after hot rolling, enabling thinner material to be produced with the desired microstructure and mechanical properties over a wide range of percent hot reduction. This is discussed further below, initially in the context of the steel compositions in TABLE 1. TABLE 1

Steel C Mn Si Nb V N (ppm)

Base 0.02-0.05 0.7-0.9 0.15-0.30 <0.003 <0.003 35-90

J 0.19 0.94 0.21 <0.003 <0.003 85

L 0.033 1.28 0.21 <0.003 <0.003 <100
The molten composition of Steels J and L in TABLE 1 had a free oxygen content between 41 and 54 ppm and the compositions of Steel J and L had a greater than 0.01% and less than or equal to 0.15% phosphorus.
A typical composition for plain carbon-manganese steel, such as the Base composition in TABLE 1, includes a manganese content of about 0.60% - 0.90% by weight. We have developed a steel composition having a substantially elevated manganese content (steel L in TABLE 1) to increase the hardenability of the steel. The elevated manganese content provides desired strength levels due to microstructural hardening. Additionally, manganese in solid solution acted to suppress static recrystallization of the deformed austenite after hot rolling mitigating the affect of hot reduction on mechanical properties. This suppression is made possible by the short time scale and minimal hot reduction relative to conventional slab-based production. The present elevated manganese steel composition is relatively stable with the degree of hot rolled reduction for hot reductions up to at least 35%. This allows the production of thinner gauges, such as steel L having a thickness of 0.9 mm, with desired mechanical properties. As shown in FIG. 3, the yield strength for 1.28% manganese steel is less influenced by hot rolling reduction than a plain 0.8% carbon-manganese grade. Additionally, the yield strength of the 1.28% manganese was significantly higher than that of the base 0.8% manganese steel, exceeding 440 MPa for hot rolling reductions greater than 35%.
After hot rolling, the steel strip is cooled to a coiling temperature between about 300°C and 700°C to provide a majority of the microstructure comprising bainite and acicular ferrite. Alternatively, the steel strip is cooled to a coiling temperature between about 450°C and 550°C to provide a majority of the microstructure comprising bainite and acicular ferrite. The mechanical properties at 15% and 35% reduction are within 10% for yield strength, tensile strength and total elongation of the hot rolled strip. Alternatively, mechanical properties may be within 10% throughout the range from 15% to 35% reduction for yield strength, tensile strength and total elongation of the hot rolled strip.
The composition according to the invention includes, by weight, less than 0.25% carbon, between 0.05 and 0.50% silicon, and less than 0.01% aluminum. The manganese content may be between about 1.0% and 1.3% by weight.
In addition, the composition of the elevated manganese steel may include at least one element selected from the group consisting of niobium between about 0.01% and 0.2%, molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof. The hot rolled steel strip also may be hot dip coated to provide a coating of zinc or a zinc alloy or aluminum.
We have also found the desired microstructural hardening to reduce the effect of the hot rolling reduction on the mechanical properties can be provided by addition of between 0.20 and 0.60% copper and the manganese levels kept the same as the minimum described above or reduced to as low as 0.08%, with less than 0.03% tin and less than 0.20% nickel by weight. This elevated copper steel enables use of steel scrap that is higher in copper, such as used in bar mills, to be used in the steel making without hot shortness. A number of trial heats were cast having copper levels in the range of 0.2% to 0.4%, and one trial heat of about 0.6% copper was cast without incurring hot shortness while also avoiding special practices or alloy additions.
The composition with copper may include, by weight, less than 0.25% carbon, between 0.2 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum less than 0.03% tin, less than 0.10% nickel, and between 0.20 and 0.60% copper. Alternatively, the copper content may be between about 0.2% and 0.5% by weight, and alternatively, may be between about 0.3% and 0.4%. Again, the molten steel cast has a free oxygen content between 20 and 75 ppm and the free oxygen content may be between 30 and 60 ppm. Again, the total oxygen levels were between 70 ppm and 150 ppm.
The hot rolled steel strip may have, in addition, a chromium content between about 0.4% and 0.75% by weight. Alternatively, the chromium content may be between about 0.4% and 0.5%.

The modest increase in hardenability provided by copper was used, with less than 0.03% tin and less than 0.20% nickel, to produce a higher strength grade (Grade SS380) using high cooling rates and low coiling temperatures of between about 500°C and 600°C. Alternatively, lower strength grades may be produced with elevated copper using low cooling rates and high coiling temperatures to offset the effect of the increased copper content. As shown in TABLE 2, tensile properties of grades with copper content between 0.20% - 0.40% produced a range of galvanized structural grades, such as Grade SS275 to Grade SS380.

TABLE 2

Mn level (wt %)	Coiling temp.	Hot reduction	Yield Strength (MPa)	Tensile Strength (MPa)	Total Elongation (%)
0.68-0.74	600-700°C	23-28%	321	428	26.0
0.68-0.74	500-600°C	15-20%	378	480	22.7
0.80-0.85	500-600°C	20-26%	403	499	21.2

To produce lower strength grades with elevated copper, higher coiling temperatures between about 600 and 700°C are used to offset the increased copper content. By coiling at increased temperatures, the present steel with elevated copper may provide physical properties similar to plain carbon-manganese steel with low copper content. The present steel composition having elevated copper levels can be made in electric arc furnaces with high copper scrap, as discussed above, at a considerable cost savings over low copper scrap.
In one alternative, the present elevated copper steel is hot dip coated with one or both of a zinc coating or a zinc alloy coating or an aluminum coating, such as a galvanized coating, Galvalume® and Zincalum® coating, aluminized coating or other coating. The microstructure of the present hot dipped elevated copper steel was not significantly altered as the strip temperatures remained well below the A_c1 temperature of the steel. Consequently, the mechanical properties of uncoated elevated copper steel in the hot rolled condition are similar to the mechanical properties after coating on a continuous hot dip galvanizing line.
Alternatively or in addition, the high copper composition may include at least one element selected from the group consisting of niobium between about 0.01% and 0.2%, molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof.
In any case, carbon levels of about 0.20% and greater may also be used for applications where microalloying is not desired. Additionally, higher carbon levels, in the range of 0.30 - 0.50%, may be used in certain applications for material in the thickness range of 1.0 - 1.5 mm. In the past, these elevated carbon steels required multiple annealing and cold rolling steps to achieve this thickness.
The composition of a 0.19% carbon steel is given in TABLE 1 (steel J) and the mechanical properties are presented in FIG. 4 as a function of the hot rolling reduction applied. The strength levels of the present 0.19% carbon steel are higher than current plain low carbon steels. As shown in FIG. 4, the yield strength is over 380 MPa over the full range of hot reductions applied, while being processed with conventional coiling temperatures. This is in contrast to low carbon steels (0.02-0.05% C), where lower coiling temperatures and limited hot reductions are applied to provide yield strengths over 380 MPa.
Additional samples of the present steel were prepared with manganese between about 0.88% and 1.1% and carbon amount between about 0.02% and 0.04%, shown in FIGS. 5 and 6. As shown in FIG. 5, tensile strength, yield strength and total elongation are relatively stable over different levels of manganese amount between 0.88% and 1.1%
The effect of hot reduction on yield strength, tensile strength, and total elongation in the present steels results in a steel properties where the tensile strength, yield strength and total elongation are relatively stable with different levels of hot reduction, as shown in FIG. 6. As discussed above, in previous such steel products, there is typically a decrease in yield and tensile strengths with increasing hot reduction. In contrast, the effect of different amounts of hot reduction on yield strength, tensile strength, and total elongation is significantly reduced in the present steel products. As shown in FIG. 6, the present steel is relatively stable with the degree of hot rolled reduction for reductions up to at least 45%. The hot rolled cast strip to provide after cooling at a temperature between 300 and 700°C, alternatively between about 450 and 550°C, a microstructure comprising a majority bainite and acicular ferrite and having properties such that mechanical properties at 10% and 35% reduction are within 10% for yield strength, tensile strength and total elongation. Alternatively, mechanical properties are within 10% throughout the range from 10% to 35% reduction for yield strength, tensile strength and total elongation. In yet another alternative, mechanical properties at 15% and 35% reduction are within 10% for yield strength, tensile strength and total elongation. Alternatively, mechanical properties are within 10% throughout the range from 15% to 35% reduction for yield strength, tensile strength and total elongation.

Claims

A method of making hot rolled steel strip, the steps comprising:
assembling an internally cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams, the molten steel having a free oxygen content between 20 and 75 ppm and a composition such that hot rolled thin cast strip produced has a composition consisting of, by weight, less than 0.25% carbon, greater than 0.01% and less than or equal to 0.15% phosphorus, between 1.0 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and the balance Fe and impurities; characterised by:
counter rotating the casting rolls to solidify metal shells on the casting rolls as the casting rolls move through the casting pool,

forming a steel strip from the metal shells moving downwardly through the nip between the casting rolls,

hot rolling the steel strip in a range of 10 to 50%, whereby the hardenability provided by manganese between 1.0 and 2.0% in the steel results in the mechanical properties of steel strip hot rolled at 10% reduction and the mechanical properties of steel strip hot rolled at 35% reduction being within 10% for yield strength, tensile strength and total elongation; and

coiling the hot rolled steel strip at a temperature between 300 and 700°C to provide a microstructure comprising bainite and acicular ferrite.
A method of making hot rolled steel strip as claimed in claim 1, comprising hot rolling the steel strip such that the mechanical properties of steel strip hot rolled at a 15% reduction and the mechanical properties of steel strip hot rolled at a 35% reduction are within 10% for yield strength, tensile strength and total elongation.
A method of making hot rolled steel strip as claimed in claim 1 or claim 2 wherein the molten steel has a free oxygen content between 30 and 60 ppm.
A method of making hot rolled steel strip as claimed in any one of the preceding claims wherein the molten steel has a composition such that the manganese content of the hot rolled steel strip is between 0.9 and 1.3% by weight.
A method of making hot rolled steel strip as claimed in any one of the preceding claims wherein the molten steel has a composition such that the hot rolled steel strip has between 0.01% and 0.20% niobium by weight.
A method of making hot rolled steel strip as claimed in any one of the preceding claims wherein the molten steel has a composition such that the hot rolled steel strip further consisting of at least one element selected from the group consisting of molybdenum between 0.05% and 0.50%, vanadium between about 0.01% and 0.20%, and a mixture thereof by weight.
A method of making hot rolled steel strip as claimed in any one of the preceding claims wherein the steel strip has a yield strength of at least 440 MPa after hot rolling reductions of at least 35%.