Description
TECHNICAL FIELD
[0002] These inventions relate to treated iron-based alloys, and
more particularly relate to processes and apparatuses for
transforming and/or shaping the same and the various materials
resulting therefrom obtained by treating low, medium, and high
carbon steel and other iron-based alloys to a complex steel
microstructure which may include portions of bainite, coalesced
bainite, acicular ferrite, retained austenite and/or martensite
and combinations thereof by micro-treating said iron based alloy.
BACKGROUND OF THE INVENTION
[0003] Traditionally, metallurgists have wanted to take low
quality metals, such as low carbon steel, and turn them into high
quality steels and more desirable products through inexpensive
treatments, including annealing, quenching, and tempering to name
a few. Previous attempts have met with limited success in that
they did not always produce a desirable product. Other attempts
have failed on a large scale due to high processing costs or the
need to ultimately incorporate expensive alloying.
[0004] Processing of high strength steel generally takes intense
capital equipment, high expenditures, expensive and dangerous
heated fluids, such as quenching oils and quenching salts, and
tempering/annealing processes which include the use of ovens,
heated equipment, and residual heat from pouring molten steel.
These quenching procedures are intended to raise the hardness of
the steel to a desirable value. Bainite and martensite can be made
by these processes and are very desirable materials for certain
high strength applications as they generally have Rockwell C
hardness of from about 30 and up. The increased hardness
correlates to a comparable increase in tensile strength.
[0005] Typical Advanced High Strength Steels have generally
included bainitic and/or martensitic phases. Bainite is generally
acicular steel structured of a combination of ferrite and carbide
that exhibits considerable toughness while combining high strength
with high ductility. Usually formed by austempering, bainite is a
very desirable product. One practical advantage of bainitic steels
is that relatively high strength levels can be obtained together
with adequate ductility without further heat treatment, after the
bainite reaction has taken place. Such steels, when made as a low
carbon alloy, are readily weldable, and bainite will form in the
heat-affected zone adjacent to the weld metal, thereby reducing
the incidence of cracking. Furthermore, these steels having a
lower carbon content tend to improve the weldability and reduce
stresses arising from transformation. When bainite is formed in
medium and high carbon steels, weldability is reduced due to the
higher carbon content.
[0006] The other conventional high strength steel, martensite, is
another acicular steel made of a hard, supersaturated solid
solution of carbon in a body-centered tetragonal lattice of iron.
It is generally a metastable transitional structure formed during
a phase transformation called a martensitic transformation or
shear transformation in which larger workpieces of austenized
steel may be quenched to a temperature within the martensite
transformation range and held at that temperature to attain
equalized temperature throughout before cooling to room
temperature. Martensite in thinner sections is often quenched in
water. Since chemical processes accelerate at higher temperatures,
martensite is easily destroyed by the application of heat. In some
alloys, this effect is reduced by adding elements such as tungsten
that interfere with cementite nucleation, but, more often than
not, the phenomenon is exploited instead. Since quenching can be
difficult to control, most steels are quenched to produce an
overabundance of martensite, and then tempered to gradually reduce
its concentration until the right structure for the intended
application is achieved. Too much martensite leaves steel brittle,
whereas too little martensite leaves it soft.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention, low grade ferrous
alloys in strips, sheets, bars, plates, tubes, workpieces and the
like are converted into multi-phase high strength steels with a
minimum of cost, time and effort. Dual and multiple phase
materials are achievable by practicing the present invention.
[0008] There are provided methods and apparatuses for extremely
rapid micro-treating of low carbon iron-based alloys and articles
made from and containing those alloys. The iron-based, or ferrous,
alloys/articles start out having a first microstructure prior to
the micro-treating, and are converted into a second microstructure
by rapid heating and rapid cooling into high strength steels on at
least a portion of the alloy/article.
[0009] A method for rapidly micro-treating an iron-based alloy is
disclosed for forming at least one phase of a high strength alloy,
where the method comprises the steps of providing an iron-based
alloy having a first micro-structure with an austenite conversion
temperature. This first microstructure is capable of being
transformed to an iron-based alloy having a second micro-structure
including the above mentioned phases by rapidly heating at an
extremely high rate, such as 600[deg.] F./sec to 5000[deg.]
F./sec. The traditional austenitic conversion temperatures are
elevated for given alloys due to the short duration of the thermal
cycle initiated by the rapid heating.
[0010] This heating step involves nearly immediate heating of the
iron-based alloy to a selected temperature above its austenite
conversion temperature. Then, the alloy is immediately quenched,
also at an extremely fast rate, i.e. 600[deg.] F./sec to
10,000[deg.] F./sec on at least a portion of the iron-based alloy
in a quenching unit adjacent the heating unit. This procedure
forms at least one phase of a high strength alloy in a desired
area, depending upon where the treatment was performed. Extremely
rapid quenching will form at least one phase of a high strength
alloy, as described more fully herein below.
[0011] Quenching may be accomplished nearly instantaneously by
various methods and apparatuses, including water baths, water
sprays, chilled forming dies, air knives, open air convection,
final operation chilled progressive dies, final stage chilled line
dies, chilled roll forming dies, and quenching hydroforms among
others.
[0012] Rapid quenching closely following rapid austenization has
been shown to develop a dual hardness microstructure as
illustrated in the attached drawings, herein incorporated by
reference. Experimentation has shown that flash processing of AISI
4130 yields multiple hardness peaks of approximately 525 and 625
Vickers hardness. The combination of hardnesses has been verified
by single sensor differential thermal analysis showing that two
temperature ranges have transformation occurring during the single
quenching operation. In AISI 4130, transformation occurs from 650C
to 550C and again from 470C to 360C during water cooling.
[0013] While the phenomena for this double cooling transformation
is not fully understood, multiple theories are present. The first
is that since the steel is rapidly heated and carbon leveling has
not occurred that carbon enriched areas transform to martensite
while lesser carbon areas transform to bainite.
[0014] Another possible theory is that the upper transformation
temperature occurs when austenite transforms to nano-scale
platelets. The second transformation occurrence during cooling is
the coalescing of the platelets into larger plates. This leads us
to another embodiment of this invention. Since a double
transformation is occurring, one could allow the first
transformation to occur but halt the second. For example, rapidly
heat the iron based alloy, a few seconds later, quench the iron
alloy in a medium that is below the first transformation finish
temperature but above the second transformation start temperature.
The material would complete the first transformation but never get
to the second transformation.
[0015] This may cause for example, the first stage of Flash
Bainite to form, for example, just the nano coalesced bainite
phase, but then leave a significant amount of another phase,
possibly retained austenite, or some other austenite daughter
product. The material could then be brought down from the
temperature between the first transformation end temperature (i.e.
550C) and the second transformation start temperature (470C).
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For a further understanding of the nature and advantages of
the expected scope and various embodiments of the present
invention, reference shall be made to the following detailed
description, and when taken in conjunction with the accompanying
drawings, in which like parts are given the same reference
numerals, and wherein;
FIG. 1A is a FEGSEM
micrograph of bainite processed in accordance with the present
invention;
FIG. 1B is a FEGSEM
micrograph of bainite processed in accordance with the present
invention;
FIG. 2A is a graph of
typical temperature measurements at the inside wall of the
processed tube;
FIG. 2B is a cooling cycle
time/temperature graph of the process in accordance with the
present invention;
FIG. 3 is a transform
analysis graph of temperature versus differential of temperature;
FIG. 4A is a mechanical
heterogeneity analysis of a raw material;
FIG. 4B is a similar
analysis of the flash processed material;
FIG. 5 is a graph of
elongation versus temperature; and
FIG. 6 is a stress versus
strain graph of various examples of material.
DETAILED DESCRIPTION OF THE
DRAWINGS
[0026] It is a first aspect of the present invention to provide an
inexpensive, quick and easy way to produce a low, medium, or high
carbon iron-based alloy containing an appreciable percentage of
nano-sized platelet bainite while having some of the desirable
mechanical properties of nano-sized laths of martensite. While
other thermo-mechanical processing techniques require lengthy
thermal processing to obtain a complex bainitic-martensitic
microstructure, flash bainite processing does so with a single,
rapid quenching operation which takes less than 10 seconds from
above the lower austenitic temperature to below the lower
martensitic temperature.
[0027] It is a second aspect of the present invention to provide a
method and apparatus for micro-treating low, medium, or high
carbon iron-based alloys to contain a desirable quantity of flash
bainite processed complex microstructural material with bainite
and martensite interspersed within the same prior austenitic
grains. The micro-treated low, medium, or high carbon iron-based
alloy may have varying thicknesses for different applications and
may be readily weldable while having high tensile strength, along
with the ability to save material and reduce weight. One aspect of
the present invention for the elevated interruption temperature is
to use a quench medium at this temperature that could be molten
salts, among others. This aspect causes the first iron based alloy
transformation that is stopped by molten salt. All other iron
alloy transformations are intentionally occurring in molten salt
through either continuous cooling transformation or time
temperature transformation. From this temp, 550-470[deg.] C., the
steel could be cooled in a manner in which the remaining austenite
is brought down to ambient temperature with either no further
transformation occurring or transformation to some other desirable
austenite daughter phases.
[0028] Another aspect of this invention has to do with the heating
and quenching apparatus. Other previously filed patent
applications for apparatus employ single or multiple heating and
quenching heads to cool the material. The present method employs a
single heating unit to heat multiple pieces of material. For
example, a rectangular induction coil could have material passing
by it and heated both inside and outside of the coil. If the coil
were appropriately sized, a rectangular tube could be heated
inside the coil while other pieces, such as pieces of bar stock
could be heated on the outside of the coil simultaneously.
[0029] Another aspect of this invention has to do with heating
interrupted pieces of material. For example, a strip could have
multiple cutouts removed from its shape. These pieces could be
manufacture in the soft state and then Flash Bainite Processed in
their final hardness state. Sometimes, when such a strip is
heated, the edges near the cutouts will concentrate heat and melt
the corners. The present aspect of this invention will allow plugs
of similar material to be held in place of the interruptions to
absorb the heat. This will thus prevent the melted corners.
[0030] The concept of rapid heating, quenching, reheating, and
quenching was discussed in my previous patent applications filed
on Oct. 2, 2006, which is incorporated herein by reference,
referring to an iron based alloy component. The method could be
applied as well to a rolling strip of metal. A similar thermal
technique known as quench and partitioning has been used. Quench
and partitioning technology austenizes steel over many minutes,
quenches to below the martensite start temperature, either holds
temp or reheats below martensitic start temperature and then
quenches to ambient. Another aspect of Quench and partitioning
technology austenizes steel over many minutes, quenches to below
the martensite start temperature, reheats above martensitic start
temperature and bainite finish temperature and then quenches to
ambient after a desired amount of transformation has occurred. The
present innovation is a new technique of Quench/Partition
technology. As before with an iron based alloy part, the heater
will rapidly austenize the steel strip, quench the material to
enact a transformation, hold or reheat with the second heater to a
subaustenitic temperature to stabilize or transform the existing
microstructure, and then quench to room temp with the second
quench.
[0031] The resulting high strength steel may include at least one
portion of the resulting high strength material made of coalesced
bainite, bainite, martensite, ferrite, austenite, pearlite, and/or
dual or complex phase combinations thereof, depending on the
placement of the treatments described and claimed herein below.
[0032] Complex phase materials can be made, such as martensite and
bainite located next to ferrite. These highly desired complex
phase materials are achievable in the same workpiece by quenching
only in various patterns so that a pattern of high strength steel
can be manufactured in desired areas across the surface and/or
cross section of an article after it has been heated. By only
quenching certain areas, various material phases are possible in
various locations where desired.
[0033] Looking first with combined reference to FIGS. 1A and 1B,
there can be seen that the flash bainite includes a bi-modal
distribution of platelets or plates which exhibit highly desirable
combinations of strength, ductility and toughness. The flash
processing of the present invention creates almost distortion free
flat sheets, bars, plates and straight tubing. As can be seen in
these figures, the microstructure produces a fine grain structure
within the bi-modal distribution of microstructures which yields
the surprising strength and ductility.
[0034] With reference to FIG. 2A, a graph is shown charting
temperature in degrees C. versus time in seconds to illustrate the
cooling cycle as processed at the inside wall of one of the test
tubes. Typical temperature measurements of this inside wall are
showing that there is a very low temperature-time history ratio.
In this particular example, utilizing AIS 414130 sheet metal
tubing has a lower temperature to time history ratio.
[0035] Looking now to FIG. 2B, there is shown a graph of
temperature versus time showing the flash processing temperature
to time history ratio in addition to the conventional continuous
annealing temperature to time history. Clearly, the temperature to
time history ratio for the continuous annealing is much greater
than that ratio for the flash processing.
[0036] FIG. 3 illustrates an analysis of temperature in degrees
centigrade versus the change in temperature also in degrees
centigrade. This analysis shows transformations at between 550 and
649 degrees C. and 360 to 459 degrees C. This analysis suggests
that we have two different transformation conditions leading to
very localized microstructural heterogeneity, although
experiencing homogeneity on a macro scale.
[0037] Looking now to FIGS. 4A and 4B collectively, there can be
seen two mechanical heterogeneity analyses showing that there are
two distinct regions of microstructure between the raw material
and the flash processed material in accordance with the present
invention. These findings are consistent with the previous
analyses showing two separate transformations during the flash
processing procedure. Both FIG. A and FIG. B are graphs of
normalized frequencies versus hardness, which illustrates the
distribution of hardness. FIG. 4A shows the base metal hardness
distribution as very slight, while the material that has proceeded
through flash processing illustrates both a high hardness zone as
well as a low hardness zone over a broader distribution of
hardness.
[0038] Looking now to FIG. 5, yet another aspect of the invention
is illustrated with fully strengthened with AISI 1010 material
that has been flash processed. This graph shows elongation versus
peak flash temperatures, to show that the highest flash
temperature, 1180 degrees C. having an elongation of 7.9%. At a
peak flash temperature of 1010 degrees C., the elongation
percentage is 5.6. Optimum elongation is found at larger grain
sizes. The chemistry of this material in percent by weight is 0.10
C, 0.31 Mn, less than 0.01 Si, sulfur, phosphorus, and 99.41 iron.
[0039] Last, we look at FIG. 6, which is a graph of tensile
strength in KSI versus tensile strain in percentage. With an
example of AISI 1020 after heating to various temperatures in a
range from 400 to 700 degrees C., 8 examples are shown with
varying widths in inches. This experiment shows that even after
300 seconds of tempering at 500 degrees C., flash processed AISI
1020 will retain 79% of its "as quenched" tensile strength.
Furthermore, the elongation does not drop to + or -5% with less
than 5 seconds of tempering.
[0040] In summary, numerous benefits have been described which
result from employing any or all of the concepts and the features
of the various specific embodiments of the present invention, or
those that are within the scope of the invention. The foregoing
description of a preferred embodiment of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed. Obvious modifications or variations are possible
in light of the above teachings with regards to the specific
embodiments. The embodiment was chosen and described in order to
best illustrate the principles of the invention and its practical
applications to thereby enable one of ordinary skill in the art to
best utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims which are appended hereto.
