Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-05-17
2001-10-30
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S648000, C148S602000, C266S103000, C266S113000, C072S203000, C072S365200
Reexamination Certificate
active
06309482
ABSTRACT:
FIELD OF INVENTION
This invention relates to the in-line combination of a reversing roll mill (herein referred to as a Steckel mill) and its associated coiler furnaces with a flying shear and controlled cooling apparatus downstream of the Steckel mill, and a preferred method of operating same. This combination of equipment and the method of operating same would find their utility as part of a hot steel rolling mill and preferred method of operating same.
BACKGROUND OF THE INVENTION
In an as-hot rolled microalloyed steel, optimum strength and toughness are conferred by a fine-grained polygonal ferrite structure. Additional strengthening is available via precipitation hardening and ferrite work hardening, although these can be detrimental to the fracture properties. The development of a suitable fine-grained structure by thermomechanical processing or working such as hot rolling can be considered to occur in three or sometimes four stages or regions. In the first, a fine-grained structure is produced by repeated recrystallization of austenite at high temperatures. This is followed, in the second, by austenite pancaking at intermediate temperatures. The third stage involves working the steel at the still lower temperatures of the intercritical region, i.e. the ferrite/austenite two-phase range. Sometimes, further working below the ferrite/austenite two-phase temperature range can occur. For a given chemistry (alloy composition), the final microstructure is dictated by the amounts of strain applied in each of these temperature ranges and the cooling applied after it leaves the rolling mill.
The first stage occurs at temperatures above temperature T
nr
, being the temperature below which there is little or no austenite recrystallization. The second stage occurs at temperatures below temperature T
nr
but above the temperature Ar
3
, being the upper temperature limit below which austenite begins to transform into ferrite. The third stage occurs at temperatures below temperature Ar
3
but above the temperature Ar
1
, being the lower temperature limit below which the austenite-to-polygonal ferrite transformation is complete. The final stage occurs below temperature Ar
1
. (The designations Ar
3
and Ar
1
are conventionally used to identify the upper and lower temperature limit respectively of the ferrite/austenite two-phase region, as it exists during cooling.) Since only limited improvement in steel quality normally occurs below temperature Ar
1
, steel is frequently not rolled below this temperature, although in some cases further such rolling is desirable to further harden the steel albeit at the expense of ductility.
An objective for obtaining superior strength and toughness of steel is to obtain as much fine-grained bainite as possible in the final product. To this end, a specific amount of reduction should occur above the minimum recrystallization temperature T
nr
.
In-line controlled cooling apparatus is previously known for use in rolling mills in which steel progresses in-line from a caster through a series of reduction stands and eventually is reduced to a finished product thickness, cut to length and offloaded. At an appropriate stage downstream of the reduction roll stands, controlled cooling apparatus may be provided that imparts to the rolled steel a relatively rapid cooling intended to consolidate the grain structure that has been obtained during the preceding sequence of reductions of the intermediate steel sheet product. The purpose of the controlled cooling is to cool the rolled intermediate product quickly while still fully austenitic, and more importantly, to promote transformation of austenite to bainite, which possesses attractive combinations of strength and toughness.
A problem with this conventional technology is that the steel undergoing the series of reductions is continuously losing heat and dropping in temperature. Because reduction of the steel, while the temperature of the steel remains above the T
nr
(the temperature above which recrystallization will occur) imparts fine grain structure to the steel and because the sheet is constantly dropping in temperature, it is desirable to run the steel as rapidly as possible through the series of reduction stands in order to optimize the amount of reduction that can occur above the T
nr
. However, such rapid passage of the steel through the series of reduction stands can have at least some undesirable offsetting counter-effects, including:
1. the absence of sufficient time between sequential passes for the desired amount of recrystallization to occur; and
2. the increased capital expenditure required to provide equipment compatible with high-speed rolling mill operation.
Suitable controlled cooling equipment may comprise water spray devices or laminar flow cooling or a combination of both. While in some situations, an immersion cooling might be appropriate, it is seldom suitable for the production of fine-grain bainite steels that is the objective of the controlled-cooling technology heretofore practised.
A further limitation of a conventional rolling line is that the flow-through capacity is limited by the item of in-line equipment having the smallest flow-through capacity. This is true also of Steckel mill lines, an example of one such being disclosed in U.S. Pat. No. 5,414,923 (Thomas et al.). Such Steckel mill lines typically comprise in downstream sequence: a reheat furnace, a Steckel mill with associated coiler furnaces, a downcoiler (or upcoiler), a cooling station, and a plate table with a shear. Of these items of apparatus, typically the maximum-weight capacity of the downcoiler or coiler furnace is substantially less than other items of apparatus in the line. Therefore, the flow-through capacity of some or most of the items of apparatus is not fully utilized; overall production is limited by one of the items of coiling equipment.
SUMMARY OF THE INVENTION
I have discovered that a superior use of controlled cooling with the objective of obtaining a steel product (coil or plate) characterized by fine-grain-structure bainite can be obtained by appropriately combining controlled cooling with Steckel mill rolling. Steckel mill rolling is inherently slower than in-line sequence reduction rolling, and this slower rolling procedure permits the recrystallization within the steel undergoing processing to occur optimally, whereas in high-speed in-line sequential rolling stand-type steel mills, there may be insufficient time between sequential reductions for the steel to take full advantage of the recrystallization phenomenon.
The conventional wisdom is that the time between sequential reductions has to be kept short because the steel sheet being rolled is constantly losing temperature. However, in a Steckel mill line, this problem is not nearly as acute for at least thinner end products because the Steckel mill is used in conjunction with associated coiler furnaces into which the steel product being rolled can be coiled up following each reduction pass of the product through the Steckel mill. The coiled steel is retained in the coiler furnace, and the coiler furnaces are maintained at a temperature that is typically at least about 1,000° C., a temperature which is above the T
nr
for most grades of steel of interest. Consequently, the rate of temperature decline of steel product coiled between successive rolling passes is significantly slowed, thereby substantially extending the amount of time available for reductions above T
nr
, and thereby substantially increasing the amount of austenite recrystallization. While the foregoing advantage of Steckel mill operation applies only to strip and plate intermediate products that can be coiled in the coiler furnaces, thicker flat plate products also benefit to a limited extent from Steckel mill rolling, since during rolling they retain heat more persistently than thinner steel, and the inevitable pauses while the Steckel mill decelerates, reverses, and accelerates facilitate controlled recrystallization of the steel being rolled. The foregoing benefit may be further enha
Dorricott Jonathan
Frank William R.
Barrigar Robert H.
Yee Deborah
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