U.S. patent number 3,640,114 [Application Number 04/835,809] was granted by the patent office on 1972-02-08 for method of hot rolling metal.
This patent grant is currently assigned to Teledyne, Inc.. Invention is credited to William R. Foley, Jr..
United States Patent |
3,640,114 |
Foley, Jr. |
February 8, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
METHOD OF HOT ROLLING METAL
Abstract
In the art of moving metal at hot forgeable temperature through
and reducing such metal in a hot rolling mill, engaging the metal
by a roll and applying to the roll coolant at a temperature below
the boiling point of water so that the roll is subjected to
repeated heating and cooling cycles, the roll having a surface
contacting the metal which surface is of chromium alloy steel which
has simultaneously exceptional resistance to thermal fatigue
cracking and good resistance to wear at the temperatures produced
by contact with metal at hot forgeable temperature and which has
the following composition: the sum of the percentages of the
chromium, molybdenum, tungsten and vanadium being between about 4
and about 9, the balance being substantially iron. The roll may
comprise a ferrous metal body to which the metal contacting surface
of chromium alloy steel is applied. The metal contacting surface
may be and preferably is applied to the roll body by weld
deposition. The metal moved through the rolling mill is preferably
iron or steel above its critical temperature.
Inventors: |
Foley, Jr.; William R. (Allison
Park, PA) |
Assignee: |
Teledyne, Inc. (Los Angeles,
CA)
|
Family
ID: |
27097182 |
Appl.
No.: |
04/835,809 |
Filed: |
June 23, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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656365 |
Jul 27, 1967 |
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Current U.S.
Class: |
72/199; 420/111;
420/104; 420/105 |
Current CPC
Class: |
B21B
27/00 (20130101); C22C 38/38 (20130101); C22C
38/22 (20130101); C21D 9/38 (20130101); B23K
35/308 (20130101) |
Current International
Class: |
B23K
35/30 (20060101); C22C 38/38 (20060101); C21D
9/38 (20060101); B21B 27/00 (20060101); C22C
38/22 (20060101); B21b 001/00 () |
Field of
Search: |
;75/126C ;29/196.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bizot; Hyland
Parent Case Text
This application is a continuation of my copending application Ser.
No. 656,365, filed July 27, 1967 now abandoned.
Claims
I claim:
1. A method for improving the performance and efficiency of a hot
rolling mill which accepts metal at hot forging temperature, moves
it through the mill and plastically reduces the metal, the method
consisting of the steps of providing the mill with a roll having a
metal contacting surface of chromium alloy steel having the
following composition:
the sum of the percentages of chromium, molybdenum, tungsten and
vanadium being between about 4 and about 9, the balance being
substantially iron, whereby to simultaneously greatly improve the
resistance to thermal fatigue cracking while maintaining a good
level of resistance to wear of the roll at hot forging temperature,
engaging the metal at hot forging temperature by such roll and
applying to the roll a coolant at a temperature below the boiling
point of water so that the roll is subjected to repeated heating
and cooling cycles.
2. A method for improving the performance and efficiency of a hot
rolling mill which accepts metal at hot forging temperature, moves
it through the mill and plastically reduces the metal, the method
consisting of the steps of providing the mill with a roll having a
ferrous metal core and a metal contacting surface of chromium alloy
steel having the following composition:
3. A method for improving the performance and efficiency of a hot
rolling mill which accepts metal at hot forging temperature, moves
it through the mill and plastically reduces the metal, the method
consisting of the steps of providing the mill with a roll having a
ferrous metal core and a weld deposited metal contacting surface of
chromium alloy steel having the following composition:
the sum of the percentages of chromium, molybdenum, tungsten and
vanadium being between about 4 and about 9, the balance being
substantially iron, whereby to simultaneously greatly improve the
resistance to thermal fatigue cracking while maintaining a good
level of resistance to wear of the roll at hot forging temperature,
engaging the metal at hot forging temperature by such roll and
applying to the roll a coolant at a temperature below the boiling
point of water so that the roll is subjected to repeated heating
and cooling cycles.
4. A method as claimed in claim 1 in which the metal contacting
surface has the following composition:
5. A method as claimed in claim 1 in which the metal contacting
surface has the following composition:
6. A method as claimed in claim 1 in which the metal moved through
the rolling mill is iron or steel about its critical temperature.
Description
This invention relates to a method of hot rolling metal and
particularly to operations performed on metal in its hot forgeable
temperature ranges, particularly in a hot rolling mill.
The invention is concerned with the drastic temperature extremes,
necessarily cyclic in nature, to which certain working parts of the
mill, especially rolls, are subjected and the damage which thermal
shocks caused by the cyclic temperature changes do to the working
parts of the mill and to improvement of the performance and
efficiency of the mill which can be effected by importantly, in
accordance with the invention, improving the resistance of such
working parts of the mill to damage by thermal shock.
Contact with hot metal being rolled in a hot rolling mill is made
principally by rolls. These may be the rolls which transport the
hot metal and on which it may rest for a period while awaiting the
next processing step or the work rolls which produce plastic
deformation by applying squeezing pressure to the metal. During the
engagement of a roll with hot metal heat is rapidly transferred to
the surface of the roll by conduction and radiation. In some
locations even convection may contribute to the transfer but in any
case the roll surface receives an intense local heating.
Immediately after engaging hot metal a roll surface is cooled to
prevent too much build up of heat in the roll which would reduce
its strength and cause the sticking of hot metal on subsequent
contacts. Roll cooling is normally done by water sprays although
there are a few locations in a mill where a blast of air at ambient
temperature may be adequate.
The effect on a roll of the alternaTe heating by metal and cooling
by water is to produce a deterioration of the roll surface known as
"fire cracking" in which the surface develops cracks due to thermal
fatigue in a pattern of small checks or alligatoring. These fire
cracks steadily progress and finally render the roll useless by
marking the hot metal which the roll engages. Normally a roll wears
away at the same time that it fire cracks and depending upon the
function performed either form of deterioration may predominate and
force down time on the mill for roll replacement. A roll removed
from the mill may be redressed. This is accomplished by metal
removal which restores the roll contour and if possible establishes
a new working surface below the fire crack damage. A roll can stand
a limited number of redressings and more metal may be lost in
dressing out fire cracks than is ever worn away in use.
The rate of formation and propagation of fire cracks in a mill roll
varies with the hot working temperature of the metal being
processed; steel is commonly handled above its critical temperature
in the range of 1,450.degree. to 2,200.degree. F., nickel at about
1,200.degree. to 2,300.degree. F., copper at about 1,200.degree. to
1,600.degree. F. and aluminum at about 900.degree. to 950.degree.
F. Because of the large tonnage rolled and the high-hot working
temperature the rolling of steel is responsible for most fire crack
damage to mill rolls. While the fire cracking of rolls cannot be
stopped it can be slowed down as this invention teaches. The
conduct of the rolling operation can thereby be greatly improved by
reducing the rate at which fire cracking spreads into extensive and
insupportable damage whereby the mill product becomes increasingly
marked and the hazard of roll breakage increases with the deepening
cracks which act as fatigue notches. At this point mill operation
becomes a gamble of additional tonnage before a roll change against
breakage in the mill. Roll failure by wear is bad enough but wear
can be observed and followed and a replacement part scheduled for a
time when a mill is down. Roll failure by breakage during mill
operation is the most expensive type of failure and minimization of
the risk of roll breakage is of the utmost importance to mill
operations.
Through long years of experience the manufacturers of new mill
rolls for hot work uses have settled on roll composition ranges
which they offer as the optimum combination of mechanical
properties and economy in service. These rolls are a high carbon
and relatively low-alloy composition and are used in the dead soft
annealed condition. The rolls must be at least soft enough for
machining and although the dead soft annealing treatment is very
expensive because of the weeks of furnace time and heavy losses of
scaled and decarburized metal it does put the roll in condition to
deliver reasonable wear while having maximum ductility which is
known to be favorable to fire cracking resistance. While fire
cracking of such rolls is suppressed somewhat there are many
locations in a mill where rolls do crack and spall and become unfit
for continued use long before wear can become a problem. Thus the
operation of a rolling mill with the best rolls available has not
been satisfactory.
There has also been a long and checkered history of efforts to
rebuild by welding hot work rolls which have been repeatedly
redressed after failures resulting from some combination of wear
and thermal cracking. Individual cases have been cited as
successful but the larger number of expensive failures have given
the process the reputation of a losing gamble throughout the steel
industry. The practice of reclamation by weld surfacing has been
further retarded by the lack of weld deposits with high-thermal
fatigue resistance and good resistance to wear at elevated
temperatures. This combination of properties has not been
attainable in a single material because those characteristics that
promote wear resistance, for example, high hardness, are usually
incompatible with the characteristics that favor fire crack
resistance such as high ductility. Furthermore, the working surface
of a welded roll must be used substantially as deposited since the
finished bearings at either end will not tolerate exposure to a
heat treatment such as the annealing treatment commonly used on a
new roll body while still in the unfinished condition. Thus a roll
produced by applying a weld deposit to a supporting structure such
as a used roll body represents the more restricted situation and a
material which performs satisfactorily in the weld deposited form
without benefit of heat treatment can therefore also be employed as
a new roll material where heat treatments can be added.
The problem of improving mill roll performance with a material
resistant to fire cracking and also to wear directed attention to
the tool steels, particularly the family of 5 percent chromium hot
work die steels which satisfy one requirement in having good
resistance to wear at elevated temperatures.
Since the wear resistance of materials generally changes as the
hardness changes, and in the case of equipment operating at
elevated temperatures the wear occurs at some high temperature, it
is necessary to provide for wear resistance by utilizing materials
of good high-temperature hardness and resistance to softening at
elevated temperatures. Some help is available in the control of
these properties because long experience with the performance of
the family of 5 percent chromium tool steels used under hot working
conditions has led to appreciation of the contributions to
performance which various alloy additions can impart to such
steels. For example, it has been proposed to add tungsten in an
amount of about 1.3 to 2 percent as an aid to grain refinement, hot
hardness and tempering resistance and it has been stated that when
dissolved in the matrix tungsten is extremely reluctant to
precipitate on tempering, and when it does, at temperatures of
950.degree. to 1,100.degree. F., probably in the form of W.sub.2 C,
it accounts for the phenomenon of secondary hardness and, in the
high-speed steel tools, for red hardness. It has also been said
that because molybdenum has a smaller atomic weight than tungsten
it will produce twice as many atoms for alloying in the steel; thus
1 percent molybdenum can be substituted for 1.6 to 2 percent
tungsten. It has further been said that molybdenum also imparts
high-temperature strength to steels when present in amounts as low
as about 0.5 percent, that molybdenum when present alone, however,
promotes decarburization and that small amounts of chromium
minimize this effect. It has still further been said that vanadium
present in virtually all the chromium-molybdenum steels is highly
effective in promoting hot hardness at 1,100.degree. F. and that
0.5 percent vanadium is as effective as 1.5 percent vanadium.
The effects of chromium itself are well known. It imparts oxidation
resistance to steel and greatly increases its hardenability. In
conjunction with molybdenum an optimum increase in high-temperature
strength is accomplished at about 2 percent chromium.
The relative effectiveness of various alloying elements in
imparting hot hardness to tool and die steels has been said to be,
in descending order of effectiveness, tungsten, molybdenum, cobalt
(with tungsten or molybdenum), vanadium, chromium and
manganese.
Such metallurgical conclusions and generalizations as those cited
above have grown out of the very extensive study of tool steels,
particularly those used for hot work applications. While there is a
great difference between forged and heat-treated tool steels and
cast hard facing weld deposits, the same basic metallurgical
principles have been found to apply with regard to alloying
elements and their effective ranges in regard to high-temperature
properties. However, with regard to the vital property of thermal
fatigue, no guidance has been available from any source.
Drawing upon the known metallurgical principles for managing
high-temperature hardness and resistance to softening a number of
chromium alloy steel weld deposit compositions were produced with
contents of other alloy metals selected from the ranges known to be
effective with respect to hot hardness and softening resistance.
These weld deposits were then subjected to a test for evaluating
resistance to thermal fatigue by which metal samples are subjected
to alternate exposures to oxyacetylene flame impingement and cold
water quench. This drastic test simulates the temperature cycle
produced by contact with hot metal and roll cooling water. The
number of cycles of heating and quenching required for visible
crack initiation is used as a measure of the relative thermal
fatigue resistance of a metal.
These thermal fatigue tests were carried out in collaboration with
a principal manufacturer of mill rolls which has for some years
done investigation work on the suitability of various steel
compositions as materials resistant to the heat and wear
encountered by steel mill rolls in a hot mill. This fire cracking
test is so severe that not even the best of this manufacturer's
roll materials had ever survived as many as 500 thermal cycles
before showing fire cracking. Such thermal fatigue tests showed
that while some of the chromium alloy steel analyses failed after
as few as 16 cycles other analyses survived 500 cycles and still
were satisfactory after enduring a second 500 cycle program. Table
I lists a group of weld deposit analyses, all except No. 10 being
chromium alloy steels, together with the number of cycles before
failure in the fire cracking test.
Hot hardness tests and hardness measurements after exposure to
elevated temperatures were made on many of the weld deposits and
the results are shown in Table II. Alloy 13 was included in the
tests as a steel of minimum alloy content. Its poor hot hardness
eliminated it as an alloy of interest and its softening tendency
and fire cracking resistance were not determined. The data in Table
II confirm that in general the various alloying elements impart
characteristics to weld deposits similar to those imparted to
wrought tool steels. ##SPC1##
As shown for example by alloys No. 2, No. 3, No. 6 and No. 8 I have
achieved in a single weld deposit both unprecedented resistance to
fire cracking and a good level of the properties which produce
resistance to wear at elevated temperatures.
While, as expected, the alloy content of my improved roll materials
was found to be important to the development of hot hardness and
hardness retention, I was not able to relate fire cracking
resistance to such alloy content. Unexpectedly I discovered carbon
to be the vital controlling factor. I developed through testing
that whatever the metallic alloy analysis all compositions of
exceptional resistance to thermal fatigue had a matrix carbon level
of less than 0.2 percent. With carbon above 0.2 percent failure due
to thermal fatigue always occurred within 500 test cycles (actually
within 278 cycles) and in some instances failure was noted after as
few as 10 cycles.
I have established the lower practical limit of carbon at about
0.03 percent by weight. Alloys below this carbon level may have
excellent resistance to thermal fatigue but they are soft and their
wear resistance is so seriously impaired that they are unsuitable
for the uses intended such as the reclamation of steel mill
rolls.
The data in Table I also indicate that care must be exercised in
developing alloys for service in conditions of temperature cycling
to prevent the formation of either low-melting eutectic-type phases
or hard, brittle intermetallic grain boundary phases. These will
cause premature failure by fire cracking due to a mechanism
analogous to that which occurs in high-carbon alloys described
previously. These phases are of the type that occur in a weld
deposit when high concentrations of alloying elements such as
molybdenum and nickel are present in the alloy. This condition is
not present in wrought tool steels because of the hot work
performed in the forging of the ingots and therefore imposes a
limitation upon weld deposits only. Such an alloy is listed in
Table I as alloy No. 10. It will be noted that although the carbon
level is well below the threshold value of 0.2 percent, failure by
fire cracking occurred after only 278 cycles. Microscopic
examination showed that cracking had initiated at grain boundaries
in a high-alloy intermetallic phase and then propagated in the
usual manner. Consequently, limitations must be placed on the
combination of alloying elements and the concentration of each as
well as on the carbon level to maintain the resistance to fire
cracking.
The data in Table II confirm that the information present in the
literature concerning the effects of chromium, molybdenum, tungsten
and vanadium on the hot hardness and softening resistance of
wrought steels is generally applicable to weld deposits providing
the amounts of alloying elements are kept low enough to prevent the
formation of second phases. The presence of chromium is required
for oxidation resistance and the data show that a 2 percent
chromium steel, alloy No. 13, has insufficient hardenability and
hot hardness to have utility in a hot mill roll. Increasing the
chromium to about 3.5 percent without any other alloying addition,
alloy No. 12, resulted in sufficient hardenability to obtain
satisfactory hardness at room temperature and 800.degree. F. but
the hardness at 1,000.degree. F. and 1,200.degree. F. and the
softening resistance were too low. With as much as about 8 percent
chromium and about 1 percent molybdenum, alloy No. 6, the softening
resistance and the hardness at 1,200.degree. F. were marginal.
Alloy No. 11 illustrates the help which can be gained from use of
low levels of vanadium in amounts up to 1 percent in a composition
which otherwise would be unsatisfactory in hot hardness or
softening resistance. Although in alloy No. 11 the addition of
about 0.7 percent molybdenum and 0.3 percent vanadium increased the
hardenability, hot hardness and softening resistance to an
acceptable level in the presence of only 2 percent chromium, it is
believed more conservative to take 3 percent chromium as the
minimum which should be used.
Alloy No. 9, an alloy with about 7 percent chromium and about 1.5
percent molybdenum, has satisfactory hot hardness with marginal
softening resistance. It may be noted that molybdenum above about 4
percent did cause premature failure by fire cracking due to a grain
boundary phase. To insure against the possibility of a detrimental
grain boundary condition, the upper limit for molybdenum is
established at 2 percent.
All the chromium alloys that contained tungsten in amounts from
about 1 percent to over 7 percent had satisfactory hot hardness and
softening resistance. This was accomplished without lowering the
fire cracking resistance by the formation of a grain boundary
phase. Thus, an upper limit for tungsten is established at about 8
percent although this is becoming an expensive alloy. The alloys
with the lower levels of tungsten were fortified by about 1.5
percent molybdenum with some benefit.
The data in Table II indicate that weld deposits containing
chromium in amounts from about 3 to about 8 percent were
satisfactory for use for hot mill rolls when fortified by additions
of tungsten, vanadium and molybdenum or combinations of the three.
In any case, however, a total of the chromium, molybdenum, tungsten
and vanadium of 4 percent was the minimum acceptable level
necessary for resistance to high temperature wear and fire
cracking.
Since it has been established that a minimum of 3 percent chromium
is required for oxidation resistance and hardenability, at least 1
additional alloy or a total of 4 percent is required to develop
adequate high temperature properties.
Alloy No. 8, which had a chromium and tungsten total of about 9
percent, exhibits good fire cracking properties and excellent
elevated temperature behavior. There is danger of the formation of
intergranular phases in deposits with too high an alloy content as
exemplified by alloy No. 10. Therefore, a value of 9 percent of the
total of chromium, molybdenum, tungsten and vanadium is established
as a maximum.
Good elevated temperature properties are achieved in weld deposits
by the addition of chromium, molybdenum, tungsten and vanadium in
the amount stated without the formation of any constituent that
would be detrimental to the fire cracking resistance of the alloy.
Good high-temperature properties may be obtained in weld deposits
containing the above stated elements at percentages higher than
those given. However, the contributions which those elements can
confer are well developed within the limits stated and the use of
higher concentrations would normally be considered as wasteful. In
addition, the danger of impairing the fire cracking resistance of
weld deposits by the use of higher percentages of alloying elements
is substantial.
Certain residual elements normal to ferrous hard-facing deposits
may also be present; the range of silicon is commonly from 0.2
percent to 2 percent and of manganese from 0.3 percent to 3
percent. Phosphorus and sulphur may be present in typically
commercial amounts. The balance is substantially iron.
Thus I have discovered that a wear resistant hard-facing weld
deposit resistant to fire cracking should contain
Broad Narrow Element Range (%) Range (%) Preferred (%)
__________________________________________________________________________
carbon 0.03-0.2 0.05-0.2 about 0.15 chromium 3-8 4.5-6 about 5
molybdenum 0-2 0.5-1.5 about 1 tungsten 0-7 0.5-4 about 1.5
vanadium 0-1 0-0.5 about 0 manganese 0.3-3 0.3-2.5 about 2 silicon
0.2-2 0.2-2 about 1
__________________________________________________________________________
and with the sum of the percentages of chromium, molybdenum,
tungsten and vanadium between about 4 and about 9, the balance
being substantially iron.
In order to evaluate mill performance under actual operating
conditions a roll weld surfaced with alloy No. 2 of Table I was
installed and used for a full campaign in the universal structural
mill of a leading steel producer. During the campaign the roll was
teamed with a standard roll and both were employed in the primary
stand of the mill while it was hot rolling heavy I-beam steel
sections. Steel at a temperature of about 2,300.degree. F. was in
contact with the roll during the rolling operation. Cold water at
ambient temperature and in any event at a temperature below the
boiling point of water to quench the heated surface after contact
with the plastic hot steel is continuously sprayed on the roll. The
roll was redressed at normal intervals and was visually examined
prior to each redressing. Such examination evidenced a reduction of
75 percent in fire cracking as compared with the standard steel
roll currently employed by the steel mill for the same purpose. The
operating test proved conclusively that my composite roll has
significantly superior resistance to thermal fatigue as compared
with standard rolls while maintaining good resistance to wear.
Chromium steels with alloy metal contents equal to those of my
chromium steels but with carbon contents over 0.20 percent and
therefore unsuitable for rolling mill use have been used for many
tool steels, for welding electrodes and also for steel mill rolls.
Chromium steels with analyses which fall within the limits of my
invention have been made previously. They have been shown to have
properties and have been demonstrated as useful for purposes none
of which remotely anticipates or foreshadows their useful
employment as a working surface of a roll in the hot rolling of
metal.
A steel within the analysis which I require for rolling mill use
has been proposed as a valve surfacing material resistant to the
corrosive attack of hot refinery oil. This is a very special type
of corrosive attack by the sulfur and other compounds present in
oil. In refinery use the service is continuous and the temperature
is maintained constant by the hot oil. This service is not
characterized by the rapid and extreme temperature cycles which by
alternate heating and quenching produce fire cracking by fatigue in
rolling mill rolls. The property of resistance to corrosion by hot
oil teaches nothing about resistance to fire cracking under violent
thermal cycles.
Since carbon content contributes to wear resistance in plastic
molding dies but renders shaping of the die difficult a low carbon
5 percent chromium die steel made soft by an annealing heat
treatment has been used for ease of shaping by cold hobbing. When
shaping is finished the physical properties of the steel which
prevailed up to this point now become a liability and before
employment in the final and intended use as a plastic molding die a
new set of properties must be produced. This is accomplished by a
hardening heat treatment preceded by a carburization to introduce
high-surface carbon and good wear resistance. The value of
low-carbon content for ease of machining in no way points to the
property of resistance to fire cracking under rolling mill use
conditions upon which my invention depends.
While I have described certain present preferred methods of
practicing the invention it is to be distinctly understood that the
invention is not limited thereto but may be otherwise variously
practiced within the scope of the following claims.
* * * * *