Note: Descriptions are shown in the official language in which they were submitted.
CA 02218154 1997-10-14
A METHOD FOR MANUFAC~JRlNG A STAINLESS STEEL STRIP
TECHNICAL FlELD
5 The present invention relates to a method for mqnllfaGt lring a ~qinl~ steel strip, particularly
an a~ .niti~ stainless steel strip, with desired final thickness and a yield strength of at least
250 N/mm2.
BACKGROUND ART
10 The mqmlfqc~lring of stqinl~c~ steel strips having a yield strength of at least 250 N/mm2
conventionally includes casting an elongated product to obtain a strand, cutting the strand
into slabs, and hot-rolling the slabs to form strips. A~er surface conditioning the strips,
including among other things pickling the strip, the hot-rolled strips can be used without further
thickness redll~tinn in certain appli~q~inn~ However, subsequent cold-rolling ofthe hot-rolled
5 strips is required in many other apphcations. This subsequent cold-rolling process is intended to
achieve one or more or all ofthe following effects, viz. to further reduce the thickness ofthe
strips, to enhance the mechanical strength andlor to improve the surfaces ofthe strips.
Before being cold-rolled, the hot-rolled strips are qnnPql~d and pickled, and scrap-ends are
20 welded onto both ends ofthe strips. The actual cold-rolling process is carried out conv~ntion-q-lly
in several passes through a cold-rolling mil1, therewith enabhng the thickness to be reduced by up
to about 80%, normally 10-60%, for instance for cold-rolled strips which are intended for use as
construction m~~eriql.c after having been sht into narrower strands. The scrap-ends must be
removed before the strip can finally be coiled.
The above briefly described hot-rolling and cold-rolling operations are expensive operations and
are performed in hot-rolling mills and cold-rolling mills which require very large investment
costs.
30 Cold-rolling drm~ qlly increases the mel hqni~ql strength ofthe steel, which is in itsehf desirable
for many app1 -qtion~ and this particularly concerns cold-rolling of ,q,llct~nitic stainless steeL
However, the strips become practically impossible to work, e.g. to bend, stamp, emboss, etc.;
properties which are in many cases necessary in order to enable the strips to be used as
constIuction mqt~riql.c It is therefore necessary to anneal the strips upon completion ofthe cold-
35 rolling process, by heating the strips to a temperature above the recrystallization temperature ofthe steel, ie. to a te_perature above 1,050~C. This ll~al~ l greatly reduces the mer.llqni~ ql
strength ofthe strip. Accor&g to cu~ent standards, a yield strength of 190-220 N/mm2 must
be clql~ lqted for in construction work.
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The properties obtained with conventional terlm~ e~ for instance a relatively low yield point,
are desirable properties in the majority of cases, although conventional terlnni-lues are irrational
in several aspects. However, improvements have been proposed with the intention of
rationa]ising manufacture. For instance, it is proposed in SE 467 055 (WO 93/19211) to reduce
5 thir.~ec~ in conjunction with an annea]ing process by stretching the hot strip. However, a higher
? mechanical strength is a desirable property in certain applications, such as for constructional
appli~ n.c The properties ofthe final cold-rolled strip are not improved in this latter respect
when practising the aforesaid method, and nor is such improvement intended.
10 SUM~ARY OF THE INVENTION
The object ofthe invention is to produce, in a rational and cost efficient way, ~t~inlf~c~ steel
strips, particularly ~t~inl~ ct~nitir steel strips, having a desired thin thickness and a higher
m~rhqnirql strength than that achieved in the conventional m ~llfachlre of hot- and cold-rolled
.ct~inl~c~ ~llst~nitir steel strips while obtaining an acceptable surface finish at the same time.
15 These and other objects can be achieved, according to a first aspect ofthe invention, by a
method which is initiqte(l by continuously casting the stainless steel into a strip having a thickness
of at least 1 mm and at most 10 mm, suitably at least ll/2 mm and at maximum 6 mm and
coo]ing said cast strip to room temperature. This process step, which can be performed
through a technique known per se, provides a fast so]idification of the steel which in turn
may give a cast structure of the steel which can promote the achievement of desirable
properties at the end product ofthe method. Typically, due to the fast solidification, the cast
structure will be fine grained as compared to structures obtained by conventional ingot
casting or conventional continuos strand casting. If; for example, as is recommended in the
method of the invention, use is made of a twin-roll strip caster, the strip cast structure
25 would normally contain regions of columnar grains adjacent to the surfaces ofthe strip and
a central equiaxed region. The following steps ofthe method ofthe invention are performed
in a mode such that the advantage ofthe fine grained cast structure can be taken care ofto
achieve desirable features for the end product ofthe method. The strip casting may also
allow for mqmlf~cturing strips of stainless steel alloys which are very difficult or even
30 impossible to mqmlf~cture according to conventional techniques because of embrhtlement
or other problems attributed to the segregation or formation of undesirable phases in the
steel or other phenomena. Also the cooling ofthe cast strip, which can be performed
comparatively fast due to the thin gange ofthe cast strip, can co~ il)ule to the desired
results.
Next, according to the first aspect of the invention, the cast strip is cold-rolled with at least
a 10% thickness reduction to a thickness which is at least 2% and at most 20% greater than
the intended final thickness of the f~nished product, whereupon follows annealing the thus
cold-rolled strip at a temperature of between 1,050~C and 1,250~C; and cold-working the strip
CA 02218154 1997-10-14
after said annealing process so as to permanently P1onga~e the strip and therewith reduce its
thir~nP~ by 2-20%.
According to another aspect ofthe invention, at least some ofthe said objects can be achieved by
5 continuously casting the st~ml~s~ steel to obtain an P1~)nga~ed cast product, hot-rolling said
r1~nga~ed cast product to the shape of a strip, coohng said hot-rolled strip to room temperature,
cold-rolling the hot-rolled strip with at least a 10% thickness reduction o a thickness which
is at least 2% and at most 20% greater than the intended final thickness of the finished
product, anneahng the thus cold-rolled strip at a temperature of between 1,050~C and 1,250~C;
o and cold-working the strip after said annealing process so as to p~ ly elongate the strip
and therewith reducnng its thir~es~ by 2-20%. More particularly, according to said other aspect,
the ~slinl~ steel is continuously cast into a strip having a thickness of at least 1 mm and at most
10 mm, suitably a thickness of ll/2 to 6 mm, for the achievement of a cast structure suitable for
the subsequent steps ofthe process, and hot-rolling said cast strip with at least 5% and at most
50% thir.Ln~c~ reduction, suitably at least 10% and suitably at most 30% thickness redllction, so
as to break down the cast structure ofthe strip material prior to coohng the hot rolled strip to
room temperature.
The strip which is subjected to initial cold-rolling nn accordance with any ofthe said aspects of
20 the invention typically consists of a cast and/or hot-rolled strip that has not undergone any
descaling l~ but has been cooled and coiled a~er said casting and/or hot-rolling.
Optionally, however, the cast strip may be subjected to heat-~edtl..~l.l at a temperature in the
temperature range of 900-1200~C for up to 3 minutes, plerel~bly for at least 30 seconds prior to
coohng and/or coiling. Thus,whether the heat~ ..L...~ option is used or not, cold-rolling most
25 advantageously is performed on a strip on which oxide scale still remains on the surfaces thereof.
In principle, the initial cold-rolling process performed on said cast strip and/or on said hot-rolled
strip can be carried out in several passes through a corresponding number of mutually sequential
roll stands, although it will preferably be carried out in one single pass. The ,~l ~xi~ red~lçtion
30 in thickness that can be achieved nn one single pass will depend on the steel grade, the initial
flimrn~ions ofthe strip, and the capacity ofthe rolling mill. It can be said generally that one single
pasg will result in a rna~mllm thirl~nP~.c relhlction of about 30%, normally at rnaximum 25%.
This means that nn the majority of cases, the thir.1~nr.s.c ofthe hot rolled strip will be reduced by
10 to 60%, pl~r~l~ly by 10 to 40% when practising the invention, this redll~ion being
35 dependent on the initial thickness ofthe strip and the final thickness desired. The strip is annealed
at a temperature of between 1,050~C and 1,200~C and then cooled to room temperature before
being cold-stretched.
The strip, after anneahng it, may be cold-worked by stretching it in a strip stretching mill which
may be of any known kind, for instance the kind used to de-scale the surfaces of hot-rolled strips
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prior to pick1ing. The strip is preferably cold-stretched by a combination of high stretches and
bending the strip around rolls. The cold-stretching process is carried out to a degree such as to
permqnPntly P1ong,qte the strip and therewith obtain a thickness redllcti--n of 2-20%, plt;r~bly
2-10%, normally 3-5%. As a result ofthe combination of high stretches and bending ofthe strip
around rolls of relat*ely small lliqmpter~ the decrease in width will be mmimal and practically
negligible. The rechlctil)n in strip thickness will therefore correspond Pc~ntiqlly to the degree of
P1~)nga1ion achieved. The mq-tPriq1 is plasticized as a result ofthe cold-stretching process, the
yield strength increasing in the order of l00 MPa, and still higher in the case of certain steel
grades. As an qhprnqtive7 the strip may, after anneahng it, be cold-worked by cold-rolling to a
0 degree such as to permanently P1r~ngate the strip and therewith obtain a thickness reduction of 2-
20%, preferably 3-10%.
A chara~Pri~tir feature ofthe invent*e method is that it takes place continuously, by which is
meant that the method does not include any reversing steps, for instance reverse rolling, re-
coiling between the various steps or like reverses. In order to make a continuous process
possible, the manllffic~lring }ine pl~;r~l~tbly includes, in a known manner, strip mqg~ es, so
called loopers, at the beginning and at the end ofthe m m-ffic~lring chain, ie. prior to the initial
cold-rolling and subsequent to cold-working ofthe strip by cold-stretching or cold-working.
20 The invent*e method will normally also inc~de pickling of the annealed strip. The strip is
plererably pickled prior to being cold-worked after the annealing operation, although it is also
conceivable to pickle the strip after the final cold-working process. The strip is plt;r~l~bly shot-
blasted prior to being pickled.
25 BRlEF DESCRIPIION OF THE DRAW~GS
The invention will now be described in more detail with reference to the accompanying
drawings, in which
Fig. 1 illustrates very s~hPn~ ,q11y the principles ofthe invention according to a first
embodiment, inchuding an initial strip casting;
30 Fig. 2 illustrates in more detail the mqnllfqc~lring hne, following the initial strip casting,
according to the first embodiment;
Fig. 3 illustrates in larger scale and in more detail a cold-stretching mill used in the first
ernbodiment ofthe method;
Fig. 4 schem tirqlly ilhl~r,qtP,c a modification ofthe first embodiment ofthe method ofthe
invention;
Fig. 5 ~rl-emqtirqlly iL~strates the principals ofthe invention according to a second
embodiment, inchuding an initial strip casting;
Fig. 6 schemqtirq11y illustrates the principa1s ofthe invention according to a third embodiment,
inrhl(ling an initial strip casting and hot rolling operation; and
CA 022181F74 1997- 10- 14
Fig. 7 srhpmqtirql1y ilhlctr~tes the principals ofthe invention according to a fourth embodiment,
which hkewise inr.1~Idec an initial StTip casting and hot-rolling operation.
DETAILED DESCRIPIION OF THE INVENTION
In all the m nIlfi~chlring hnes which are schpnlqtirqlly illustrated in Fig. 1, Fig. 4, Fig. 5, Fig. 6,
and Fig. 7, a cast strip 100 is initially m~nllf~lred by continuos strip casting in a casting
machine 101, which preferably is a twin-roll strip caster, although also other continuously
operating strip casters can be employed. The molten stainless steel is teemed into the caster 101
from a ladle 102 via a tundish 103. As the steel passes between the twin-rollers in caster 101 it
0 sohdifies in a manner known per se to form the cast strip 100, which according to the
embodiments illustrated in Fig. 1, Fig. 4 and Fig. 5 is cooled and coiled 104. Pnor to coiling, the
cast strip 100 is - optionally in the embodiments illustrated in Fig. 1, Fig. 4 and Fig. 5 - subjected
to heat~ ,at~l in a furnace 105 at 900-1200~C for a period of up to 3 minutes, suitably for at
least 30 seconds.
The first embodiment ofthe method will nowbe fwther descnbed with reference to Fig. 1. The
coiled cast strip 100, which has the form ofthe coil 104, is transported to a plant for fu~ther
processing ofthe cast strip 100. This plant can be located in connection to the strip casting
f~IhiPs or at a distance therefrom, and comprises an uncoiling capstan 1, which contains the coil
20 104, a cold-rolling mill 2 cl)n~cting of one single roll stand 2 ofthe so-called Z-high type, an
anneahng furnace 3, a coohng box 4, a shot-blasting machine 16, a pickhng bath 5, a cold-
stretching mill 6 and a recoiler 7 which takes up the finished steel strip.
Fig. 2 shows the m ~nIf~lring hne, following the initial strip casting line, in more detail,
25 wherein the same reference mImPr~I~ have been used for units that have correspondence in Fig.
1. In adllition to the aforesaid units, the m nIlf:lctllring hne also inchudes a ~hPqrmg unit 8, a
welding machine 9, a strip feeder 10 which feeds the cast strip 100A to the ~hPqrm~ unit 8 and
the welding machine 9, a cast strip looper generally referenced 12, a thickness mPq~lring means
13 which measures the thickness ofthe cast strip 100A upstream ofthe rolling mill 2, and a
30 thicknPc~ mPq~Irin~ means 14 which measures the thirlrnPc.c ofthe cast strip 100B downstream
from the cold-rolling mill 2, the shot-blasting machine 16, a wiping and rinsing box 17
downstreamfromthe picl~ng bath 5, a pair of guide rollers 18, the cold-stretc~ng mill 6, a
looper generally referenced 20 for the storage of cold-rolled and cold-stretched f_ished strip
100F, a front feeder 21, and a drive motor and power tran~mic~ion means together referenced 22
35 for operating the recoiler 7.
The mannf:~chIrin~ line also includes a large number of g ude rollers, direction ~h ngin~; rollers,
and a bridle roll arrangement that comprises two or four rolls. The bridle roll arrangement is thus
comprised of a two-roll bridle roll unit 25 downstream from the welding machine 9, a two-roll
~ CA 022l8lF,4 l997- lO- l4
bridle roll unit 26 upstream from the cold-rolling rnill 2, a four-roll bridle roll unit 27 between the
cold-rolling rnill 2 and the annealing furnace 3, a four-roll bridle roll unit 28 upstream from the
cold-stretching rnill 6, a two-roll bridle roll unit 29 downstream from the cold-stretching mill 6, a
strip centre guide 19, the strip mag~ e 20, and a terminating two-roll bridle roll unit 31
5 between the looper 20 and the recoiler 7. The prin~y function ofthe bridle rolls is to increase or
decrease the tension in the strip and to keep the strip in tension.
The cast strip looper 12 mcludes direction rh n~n~; rollers 34, 35, 36 and 37, of which the roller
35 iS coupled to a strip t~n~ioning unit m a known manner. Correspondingly, the cast strip looper
10 20 inrhldec direction rh n ing rollers 39, 40, 41, 42, 43 and 44, of which the roller 40 is
connected to a strip t~n.cioning unit, also m a known manner.
The m~nllf-~lrin~; Ime illustrated m Fig. 2 operates m the following manner. It is ~ mrd that
m nllfachlring is m the phase illustrated m the Figure, ie. that the cast strip looper 12 and the
cold-rolled strip looper 20 contain a given amount of strip, that cast strip lOOA is bemg uncoiled
from the rewmder 1, and that the mished strip lOOF is being coiled on the recoiler 7. The Ime is
driven by several driven rollers, plil-lalily driven by bridle rolls m a known manner. After having
passed through the cast strip looper 12, the thickness ofthe strip is measured by means ofthe
thickness mP~ring means 13 upstream from the cold-rollmg ill 2 and is cold-rolled m the mill
2 m one single pass, whereafter the thickness ofthe cold-rolled strip lOOB is measured by the
thickness mrq~lting means 14. The cast strip lOOA will normally have an initial thickness of 2 to
4 mm and is reduced by 10-30% m the cold-rolling mill 2. The roll gap is adhl~ted m accordance
with the results ofthe thickness mea~u~e~ L~ so as to obtain a cold-rolled strip lOOB of desired
thickness, corresponding to 2-20%, p-~r~.ably 2-10%, normally 3-5% greater than the mtended
mished llimrn~ion after cold-stretching the strip in the terminating part ofthe mqmlf~c~lring hme.
The cold-rollmg process imparts a high degree of hardness to the strip lOOB, and the strip is
therefore passed mto the annealmg furnace 3 after having passed the four-roller bridle roll unit
27. The strip lOOB is heated throughout its thickness in the ~mr~ling furnace 3 to a temperature
of between 1,050~C and 1,200~C, ie. to a temperature above the re-crystallisation temperature
ofthe ~ nitir steel, and is ... -;..I;.;..ed at this temperature long enough for the steel to re-
crystallise completely. The strip is then cooled m the coohmg box 4. When heating the strip m the
annealing furnace 3, which in accordance with the present embodiment does not take place in a
protectmg gas atmosphere (som~thing which would be possible per se), oxides form on the sides
35 ofthe strip, partially m the form of oxide scale. The strip is substantially de-scaled m the shot-
blasting _achine 6, and then pickled in the pickling bath 5 comprised of applopliale picklmg
chemicals, wherein the picl~ng process can be effected m a known manner. The cold-rolled,
annealed and pickled strip lOOE is led through the wiping and rinsing box 17 and thereafter
1 CA 02218154 1997-10-14
.
through the cold-stretching mill 16 between the four-roller bridle roll unit 28 and the two-roller
bridle roll unit 29 which function to hold the strip in tension and prevent it from sliding.
Fig. 3 illustrates the design ofthe cold-stretching mill 6. The cold-stretching rnill 6 comprises
5 three strip-stretching units 47, 48 and 49. Each stretching unit mrhldes a respective lower roller
50, 51, 52 journalled in a statinn~ry base 53, 54, 55, and a respective upper stretching roller 56,
57, 58 journalled in a respective roller holder 59, 60, 61. The positions ofthe roller holders in
relation to the strip and in relation to the lower stretching rollers 50, 51, 52 can be adju~ted by
means of jacks 62, 63, 64 respectively. The upper strip-stretching rollers 56, 57, 58 are initially in
o upper positions (not shown), so that the strip lOOE, which is held stretched between the bridle
roll units 28 and 29, will extend straight through the cold-stretching mill 6. Starting from this
initial position, the upper stretching rollers 56, 57 and 58 are lowered by means ofthe jacks 62,
63, 64 to the positions shown in Fig. 3, whereby the strip lOOE- lOOF will form a winding
passway, as shown in Fig. 3, while at the same time is stretched in its cold state to a degree of
such high mqgnh~lde as to plasticize the strip. According to the illustrated embodiment, the lower
stretching rollers 50, 51 and 52 have lliqm~ters of 70, 200 and 70 mm respectively, whereas the
upper stretching rollers 56, 57 and 58 have ~iqm~tPrs of 70, 70 and 200 mmrespectively. As a
result ofthe chosen setting ofthe adjustable upper strip-stretching rollers 56, 57, 58 and by
virtue ofthe chosen ~1iqm~t~rs ofthe rollers, the paTt ofthe strip which passes through the cold-
20 stretching mill will be pl~cti~i7~d as the strip continues to be drawn through said mill 6 and to be
bent around the stretching rollers, therewith obtaining permanent t 1~)ngation ofthe strip and
therewith a reducti--n in strip thickness of 2-20%, preferably 2-10%, normally 3-5%. The width
ofthe strip is also reduced slightly at the same time, even though the reduetion is only one-tenth
ofthe ~1~ nga1ion and can çe~ntiqlly be ignored. The permanent ~ ng;ltion ofthe strip also
25 results in a thickness reduc~ion which corresponds es.~ntiqlly to the ~1rmga1il)n ofthe strip. A
finished strip lOOF with desired final thickness can be obtained by adapting the redllctinn of strip
thickness, achieved by cold-rolling the strip in the cold-rolling mill 2 to the thickness re~ )n
obtained by cold-stretching the strip in the cold-stretching mill 6, or vice versa, said strip being
coiled onto the recoiler 7 after having passed through the cold-rolled strip looper 20. The dlive
30 machinery ofthe integrated m~mlffict lring line descnbed above consists ofthe drive machinery
22 coupled to the StIip recoiler 7.
When greater reductinns are desired than those achievable with a cold-rolling mill that comprises
only one roll stand and only one cold-stretching mill, a phlrahty of roll stands 2A, 2B, etc., can
35 be coupled sequentially in a series, as illustrated in Fig. 4. This Figure also illustrates the possibili-
ty of placing the pickling bath 5 downstream from the cold-stretching mill 6. In this case, the
cold-stretching mil1 may also function to de-scale the strip sur~ces, therewith possibly
eliminating the need for a shot-blasting m~hime upstream from the pickling bath.
CA 02218154 1997-10-14
In the hne schematically shown in Fig. 5, the cold-stretching mill 6 of Fig. l-Fig. 4, and the bridle
rolls on both sides thereofis replaced by a cold-rolling ill 6'. The cold-rolled and pickled strip
lOOE (as far as the mamlffi~lring ofthe cold-rolled strip lOOE is concerned, see the rolego~--g
description) is passed through the cold rolling ill 6' to obtain a permanent e1nng~tion ofthe
stnp and therewith a re~ cti-)n in strip thickness of 2-20%, preferably 3-10%. The pI~ ~t
eIong~tinn ofthe strip also results in a thir1~nec~ re~ )n which corresponds to the elongation
ofthe strip. A fmished strip lOOF with desired final thickness can be obtained by adapting the
redIlctinn in strip thickness achieved by cold-rolling the strip in the cold-rolling mill 2 to the
thickness re~hIcti-)n obtained by cold-rolling the strip in the cold-rolling rnill 6, or vice versa,
0 whereafter the strip is coiled onto the recoiler 7. As far as the details ofthe line following the
stnp-casting is concerned, reference is made to Fig. 2 and the foregoing description in
connection with Fig. 2.
Turning now to Fig. 6 and Fig. 7, the cast strip 100 is passed through a holding furnace 105
15 which ,-,-;.,I;.;.I~ the temperature ofthe cast strip 100 at or raises it to a temperature between 900
and 1200~C suitable for hot-rolling. After having passed the holding furnace 105, the cast strip is
hot-rolled in a hot-rolling mill 106 which works at the same rate as the caster 101. The cast and
hot-rolled strip 100' is then coiled 104'.
20 The hnes following the strip casting and hot-rolling hne, Fig. 6 and Fig. 7, are designed and
operate in the same way as have been descnbed with reference to the corresponding hnes of Fig.
1 and Fig. 5, respectively. These hnes therefor shall not be further descnbed here, instead
reference is made to the roregO-Ilg description with reference to Fig. 1, Fig. 2, Fig. 3 and Fig. 5.
25 By the method of the invention, in principle any stainless steel material can be processed,
but in the first place m~mIf~cturing of allst~niti~ stainless steel strips are contemplated.
Since the method has advantages in terms of economical production as well as in terms of
desirable material improvements, it can be employed for mass production of strips of
standard grades oftype 304 and 316 and variations thereofbut also for m~mIf~chlring of
30 strips of special stainless steels w_ich contain very high contents of nickel and/or
molybdenum, e.g. 5-15% molybdenum, or other alloy eIem~nts which may cause problems
in connection with conventional stainless steel strip manufacturing.
