This invention relates to the production and processing of steels to achieve ultrafine microstructures. For example, in a ferrite containing steel, ultrafine microstructures are considered to be those having a significant proportion of grains of a size less than 5 microns in a plain carbon steel, or less than 3 microns in a microalloyed steel. In particular this invention relates to a method for producing ultrafine grains of ferrite in medium carbon manganese steel with a very small amount of niobium.
The present invention is related to high strength high crank resistant low alloy steels adapted to use for the material for, such as, ship building, auto chassis and such applications where high strength coupled with low ratio of yield strength to ultimate tensile strength is required.
The demand for steel products of the nature described above rests to a considerable extent on increasing need for high strength in steel strip, sheet and the like, with a minimum of weight and, understandably, at as little cost as possible.
BACKGROUND OF THE INVENTION
There are several methods for producing ultrafine grained steel known in the prior art. More specifically, the method used for producing ultrafine grained steel heretofore devised and utilized are known to consist basically of familiar, expected and obvious procedural configurations, notwithstanding the myriad of the processes encompassed by the crowded prior art, which have been developed for the fulfillment of countless objectives and requirements.
There are many processes which are prevalent for the refinement of the microstructures in steels.
1. Equal channel angular pressing.
2. Deformation Induced ferrite transformation.
3 Accumulative Roll Bonding (ARB)
4. Warm Annealing of Cold Rolled Martensite.
Equal channel angular pressing
This involves severe plastic deformation by pressing the sample through a die as illustrated schematically in Fig 1. The sample is machined to fit into a channel. Several factors influence the nature of the microstructures attained in ECAP including the processing route by which the sample is rotated between consecutive pressings, the angle subtended by the two channels within the die, the speed and the temperature associated with the pressing.
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The drawbacks associated with the process are
1. Very high strains are required for achieving the desired microstructure.
2. All steel compositions cannot be processed.
Deformation Induced ferrite transformation.
This method of refining the microstructure in steels requires moderate to high levels of deformation at a temperature, which is above the austenite to ferrite transformation temperature during deformation. Such refinement can be obtained by a careful choice of thermo-mechanical processing parameters viz. the reheating temperature, the roll entry temperature, the extent of deformation, the rate of deformation and the cooling rates during as well as after the deformation.
The critical factors that determine the final microstructure are the temperature of deformation, the extent of deformation and the extent of super cooling of austenite, which in turn depends on the cooling rate. Shown in Fig 2 is the schematics of the thermo-mechanical processing.
The drawback associated with the process is
1. Microstructural refinement can be effected only upto a limited depth.
Accumulative Roll Bonding (ARB)
In ARB, the rolled material is cut, stacked to be the initial thickness and rolled again. The strains that can be obtained is unlimited in this process, because, in principle, the process can be repeated an endless number of times. Because of better joinability and workability, this process is carried out at warm temperatures. The temperatures at which the process is carried out are such that no re-crystallization takes place. Fig 3 shows the process schematically.
The drawback associated with the process is
1. Only interstitial free steels can be rolled in the said process.
Warm Annealing of Cold Rolled Martensite
Cold roiling and annealing of low carbon (upto 0.2 wt %) martensite can produce ultrafine grained structure in carbon steels. 50% cold rolling of matensite followed by annealing at temperatures between 723 and 773 K results in multiphase ultrafine structures composed of equiaxed ultrafine ferrite grains with nano carbides distributed uniformly. It is the characteristics of the martensite starting structure, which play an important rote in ultrafine grain subdivision. The solute carbon atoms in martensite also play a role in refining of the grains.
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The drawback associated with the process is
1. It is difficult to obtain full martensitic structure in thicker plain carbon -
manganese steels
Japanese specification JP7034181 to NAGAO MAMORU et al discloses a method for production of high-strength austenitic-ferritic stainless steel. The high-strength austenitic-ferritic stainless steel having an ultrafine-grained structure is produced so that the powder of austenitic-ferritic stainless steel with 40 to 100mum powder size of austenite-ferrite two phases or a ferrite single phase of supersaturated solid solution is subjected to intense working by a mechanical alloying method, then rapidly heated from a room temp, to a solidifying temp, of 1000 to 1020 deg.C, succeedlngly subjected to compacting by executing hot rolling at 40 to 70% draft or hot extrusion at 5 to 15 extrusion ratio, and immediately subjected to rapid cooling to a room temp. In this producing method, heating from a room temp, to a solidifying temp, of 1000 to 1020 deg.C can be regulated to <=15min at the latest.
Japanese specification JP4304314 to ISHIKAWA TADASHI et al discloses a method for production of high toughness steel plate. A cast slab or steel plate having a temp, not higher than the Ar3 point is heated by means of both external heat and heat increase by working or either of the above. Rolling is started from a temp not higher than the Ac1 point and rolling is finished at >=30% reduction of area in the course of temp, rise from the Ac1 point up to a temp, between the Ac3 point and (Ac3 point +50 deg.C). In the course of subsequent cooling until the Ar3 point is reached, rolling is done at >=20% reduction of area. By this method, the steel plate showing, as the toughness of the base material in the steel material, showing <=-120 deg.C by vTrs in the Charpy impact test can be obtained stably and economically with superior productivity.
Japanese specification JP2301540 to AIHARA KENJI et al discloses a method for production for fine-grained ferrite steel. Steel stock such as low carbon steel is heated, e.g. to 700 deg.C and is subjected to hot rolling at 90% rolling reduction; at the end of the rolling, the temp, of the rolled stock is raised to 915 deg.C exceeding the AC1 transformation point Or, in succession to the temp raising, the rolled stock is held to the temp, range of the Ae1 point (A1 transformation point in a perfect equilibrium state) or above for a certain time and a part or whole of a ferrite structure is inversely transformed into austenite for a time to form into a ultrafine austenite grain structure of < 5µm average grain size, which is thereafter subjected to air cooling to transform the austenite into a ultrafine ferrite structure, so that the steel having excellent workability, low temp, toughness, corrosion resistance, etc., can be manufactured.
Japanese specification JP11124655 to KAWASAKI STEEL CORP discloses a method for production of steel wire, wire rod and bar steel having ultrafine grain. At the time of subjecting wire rod to wire drawing, or at the time of subjecting the stock for wire rod or the stock for steel bar to hot working, working in a dynamic
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recrystallized region is continuously repeated for #=5 times. It is effective that this working is executed by rolling. At this time, the temp, difference in the material between that in the inlet side of the initial rolling and that in the outlet side of the final rolling is regulated to=60°C, preferably to =30°C, and the draft is regulated to=60% in total. In this way, the structure of the material is made of one essentially consisting of ferrite, the average ferrite grain size is regulated to =2 urn, and the aspect ratio of the ferrite grains is regulated to =1.5. It is preferable that the componental comps. of the material to be used is composed of the one contg., by weight, 0.05 to 0.15% C=2.0% Si, s1.0% Mn, =0.05% P, =0.05% S, and the balance substantial iron.
Therefore, according to the present invention, there is provided a method of producing steel having one or more zones of ultrafine-grained steel comprising the steps of: -
Reheating of the cast alloy in a hot rolling mill.
Providing of thermo-mechanical simulations / processing to the cast alloy.
Early stage water cold rolling of the thermo-mechanical simulated cast alloy.
The principal object of the invention is to provide a practical process for the production of steels with ultrafine microstructures in any of a variety of phases or mixtures of phases, e.g. bainite.
The second object of the present invention is to provide a practical process for the production of steels with ultrafine ferrite microstructures.
The third objective of the present invention is to provide steel with an ultrafine microstructure, particularly an ultrafine ferrite microstructure.
It is a fourth preferred object of the present invention to provide easy affordable and economical steps for use in the production of steels with ultrafine ferrite microstructure.
The present invention stems from an initial surprising discovery that an austenite to ferrite transformation which achieves ultrafine ferrite grains by the single deformation of steel having large austenite grains,
These together with other objects of the invention, along with the various features of novelty, which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which there is illustrated preferred embodiments of the invention.
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BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings
wherein:
FIG. 1 is a schematic illustration of equal channel angular pressing (ECAP) as prior art to the present invention.
FIG. 2 is a schematic time-temperature cycle for producing an ultrafine grain structure in unalloyed and low alloy steels.
FIG. 3 is a schematic illustration showing the principle of accumulative Roll-Bonding (ARB) process.
FIG. 4 is a schematic depiction of thermo-mechanical processing to obtain ultrafine grains of femte.
FIG. 5 is a scanning electron micrograph showing composite microstructure consisting of ultrafine grains of ferrite in the surface and bainite in the interior.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A first important aspect of present invention resides in that the 10 mm hot rolled plate is reheated at a high temperature (between 1200 degree C & 1300 degree C) in order to obtain coarse austenite grains
A second important aspect of present invention besides in cooling of the reheated plates down to - 820 degree C and rolling with an entry temperature of ~ 800 degree C in a single pass with a rolling reduction of at least 65%.
The third aspect of the invention resides in ensuring a minimum cooling rate of 7 degree C per S immediately, and if feasible, also during the deformation. Experimentally a large number of thermo-mechanical simulations were carried out in the Gleeble 3500 C thermo - mechanical simulator. Various combinations of parameters like strain, strain rate, deformation temperature and post deformation cooling rate were tried in order to arrive at a window of the processing parameters. Based on the results of the simulation, actual thermo-mechanical processing was done in an experimental rolling mill.
These aspects of thermo - mechanical processing, taken together, ensures a core of bainite sandwiched between the surface layers of ultrafine grains of ferrite (1-2 microns in size). These layers are of 0.5 mm depth on each surface.
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A single pass rolling is done at a reduction of at least 65%. The Rolling speed is in the range 0.1 to 5.0 m/s. The deformation is followed by an early cooling preferably water-cooling at the rate of 7 degree Celsius per second. Said high deformation levels are typical of technologies based on conventional casting, and in present invention are very efficiently replaced by causing a controlled, as amount and distribution, phase transformation. Ferrite to Austenite, able to refine and homogenize the microstructure.
Typically, the cast steel composition comprises Carbon content upto 0.8 wt % and Manganese up to 1.0 wt %. Manganese is a recognized element for increasing the hardenability of steel. To this composition an element is added in trace amount is Niobium, and the other constituents in the composition are Silicon, Phosphorus and Sulphur. Niobium has been added as the alloying element to increase the wear resistance. Niobium has the ability to form hard, primary carbides that do not form at the eutectic cell boundaries and, therefore, do not have a negative effect on toughness. Also, Niobium when used as an alloying element, it has been found to reduce the size of the eutectic cell, thereby increasing strength. Niobium metal also has advantages over other strong carbide formers because it does not significantly affect the inoculation process.
In another aspect of the present invention, the invention provides steel with an ultrafine microstructure, for example having ultrafine ferrite grains, which is uniform and at least partially ultrafine (approx 1-2 microns) grains of ferrite in one or more zones in the surfaces (~0.5mm on each surface) of the steel strip The yield strength / ultimate tensile strength ratio is in the range of 0.62 to 0.63 is obtained which is much lower than those obtained by the others in the similar processed steels. The ratio is particularly attractive at a yield strength of -480 -490 Mpa and an ultimate tensile strength between 760 and 770 Mba.
Several experiments have been carried out using the aforesaid process. The following Example is intended solely to illustration purposes not limiting the scope of present invention.
Example 1
100 X 100 mm ingots were cast. The ingots were rough rolled to 10 mm thick plates. The plates were then reheated to 1250 degree C and then were cooled down to ~ 820 degree C and rolled in a single pass at a temperature of -800 degree C, giving a reduction of 65%. The composition of the steel is shown in Table 1.
TABLE1
0.15C - 0.92Mn - 0.01 Si - 0.036S - 0.039P - 0.013Nb
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The formation of ultrafine (~1-2microns) grains of ferrite in the surfaces (~0.5mm on each surface) of the 3.5mm thermo-mechanicaily processed plate was ford in the experimental sample.
The above-described embodiments of the invention are intended to be examples of the present invention. Numerous modifications changes and improvements within the scope of the invention will occur to the reader. Those of skill in the art may effect alterations and modifications thereto,, without departing from the scope of the invention which is defined solely by the claims appended hereto.
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WE CLAIM:
1 A method of producing steel having one or more zones of ultrafine-grained steel comprising the steps of-
reheating of the cast alloy in a hot rolling mill,
providing of thermo-mechanical simulations / processing to the cast alloy, and
early stage water cold rolling of the thermo-mechanical simulated cast alloy.
2. The method as claimed in claim 1, wherein the Reheating of the cast alloy
is at a high temperature ranging from 1200 degree Celsius to 1300 degree
Celsius.
3. The method as claimed in claim 1, wherein the Reheating of the cast alloy
is done in a furnace.
4. The method as claimed in claim 1, wherein the thermo-mechanical
processing is carried out in a hot rolling mill.
5. The method as claimed in claim 4, wherein the thermo mechanical
processing / deformation comprises passing the steel between a pair of contra-
rotating rolls effective to reduce a thickness dimension of the steel by at least
65%.
6. The method as claimed in claim 5, wherein only a single pass of the steel
is performed to achieve desired deformation.
7. The method as claimed in claim 6, wherein the rolling speed is in the
range 0.1 to 5.0 m/s.
8. The method as claimed in claim 1, wherein the zone of the ultrafine
microstructure comprises a whole cross-section of the structure.
9. The method as claimed in claim 1, wherein the zones of the ultrafine
microstructure comprise a surface layer or layers of the steel.
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10. The method as claimed in claim 1, wherein the early stage water cold
rolling is done at the rate on 7 degree Celsius per second.
11. The method as claimed in claim 1, wherein the cast alloy comprises of 0.8
wt % of Carbon, 1.0 wt % of Manganese and trace amount of niobium.
12. A method of producing steel having one or more zones of ultrafine-grained
steel, substantially as herein described with particular reference to the
accompanying drawings.

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A method of producing steel having one or more zones of ultrafine-grained steel comprising the steps of reheating of the cast alloy in a hot rolling mill, providing of thermo-mechanical simulations / processing to the cast alloy, and early stage water cold rolling of the thermo-mechanical simulated cast alloy.