LASER WELDING METHOD FOR CONTINUOUS PRODUCTION PROCESS FOR COILS
CLAIM OF PRIORITY
[0001] This appl ication claims the benefit of Korean Patent App 1 i cat i on No. 2005-130102 tiled on December 26, 2005 in the Korean Intel loctual Property Office, the disclosure of which is incorporated heroin by reference.
BACKGROUND OF THE INVENTION Field of the Invention
[0002] The present invention relates to a laser welding method lor hot-rolled steel sheets in a continuous production process (or co i Is, in which two hot-rolled steel sheets are welded together. More: particularly, the present invention relates to a laser welding method for hot rolled steel sheets in a continuous production process • or co i i s which ensures less hardening in a weld of steel subject to low-temperature transformation in a laser welding process involving abrupt heating and cooling, thereby guaranteeing stable welding quality.
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Description of the Related Art
[0003] A technological field of producing sheet meta L has faced a strong
demand for a continuous production process which leads to higher
productivity and quality, and bigger-sized products.
[0004] Such a continuous production process has found its appl ication
broadened to high quality steel such as electrical steel or ferrit.i>:
stainless steel .
[0005] The continuous production process for colls is represented by

pickling and tandem cold rol ling mil 1 (PCM) processes that are conducted in synchronization. A cold coil obtained from a hot coi I may be manufactured by conducting pickling and tandem cold rol 1 inq mill (TCM) , respectively. However, alternatively, the cold rolled coil can be produced by PCM. The PCM process enhances productivity significantly over the respective implementation of the pickling and TCM processes, thus growing in its application recently.
[0006] A great importance in the TCM field lies in a techn i quo o f bond i ng a rear end of a preceding rolled sheet and a following rolled sheet together. The bonding technology for the TCM includes sol id at,a to bonding and welding.
[0007] In the case of welding, the rear end of the preceding rolled sheet is welded with the following rolled sheet at an entrance of a TCM line to form a weld and then passes through a following cold rol 1 i ng mill line. Here, the weld, i f in poor quality, is fractured wh i le passing through the following cold rolling mill line, potential ly halting an overall process. Therefore, for the TCM line, it is pivotal to attain a high quality weld in hot and cold coils. Especially, the PCM 1 ino is longer in the production line and greater in the number of loopers compared with the existing pickling and TCM lines, thereby requiring more rigorous quality standard for welding than the existing lines. [0008] Examples of welding for the TCM line includes f I.ash butt wo Id i ng in which short circuit and flashing are repeated, and a laser weldinq for utilizing a high-density heat source.
[0009] The flash butt welding is big in heat input, thus facing an obstacle in selecting a welding material. For example, electrical steel, ferritic stainless steel, and high carbon steel are not sufficient, in

bonding strength, also occasionally ruptured during cold rolling. Especially, the high carbon steel is high in C content and thus cons idered ill-suited for the flash butt welding. Also, when the welding is repeated under given schedules and conditions, welds each demonstrate} non-uniform quality and thus pose a problem to reproducibi 1 ity. [0010] Laser welding has higher energy density regu i ring sma 1 ler heat-input than the existing flash butt welding, thus ach 1 ev i ng supe rb we 1 d i rig guality.
[0011] However the laser welding, when conducted on the high carbon steel for the TCM process, causes pores and pin holes in weld metal and cracks in the weld metal and a heat-affected /.one (HA/,). [0012] The pores and pin holes are closely related -to the C content. of the welding material . It is known that carbon in mo 1 ten met a I react s with oxygen in the air during welding, creating CO gas. Thus the CO gas trapped inside remains during solidification of the molten rnetal, thus resulting in pores.
[0013] Therefore, it is of importance to lower C content of the molten metal and notably a suitable welding material may be adopted to d i rn i n i sh occurrence of pores.
[0014] Rupture of the welds is associated with hardening of microstructures thereof. The high carbon s tee U suf: fa rs rupture in the weld chiefly owing to martensite microstructures created in the abrupt heating and cooling processes during welding. Here, the weld motai and heat-affected zone are concurrently hardened in the! r microstruci ures, which thus complicates and varies ways to remedy this problem. [0015] The following is conventional technologies for conducting TCM for steel which is hardened it its microstructure.

[0016] First, Japanese Laid-Open Patent Application No. 1! b-bO?'/6 discloses heat treatment ot a weld, in which a fixed heat source; is employed to keep the weld at a specific heat: treatment tempo ratu re du r i nq a certain duration according to C content of a hot-rolled steel sheet. However, this increases an overall welding time depending on the heat treatment duration.
[0017] Another conventional technology is taught i.n Japanese ha id-Open Patent Application No. H 5-132719. This technology perta.ins to laser-welding a weld and then heat treating the same at a temperature of at least 400°C and up to Acl point within a minute. However, welding should be performed during quite a long time to fully e 1 imlnate a ha rdened microstructure at a temperature of at least 400°C and up to Ad point. Also, in case of abrupt coo] ing which is required in the laser welding, the weld should be abruptly heated within few minutes after welding for heat treatment, thereby considerably complicating the heat. treatment, process.
[0018] Japanese Laid-Open Patent Application No. H 8~b7b02 discloses further another conventional welding technology, in which low carbon steel with superior weldability is inserted between -j o i nts of hi qh ca rbon steel. This more than doubles the number of weldi.ng processes over other welding methods arid requires preparation oi a leader strip every t i rrio, thereby not suitable for mass-production.
[0019] Japanese Laid-Open Patent Application No, H 8-21b872 teaches further another technology, in which a weld i.s cooled while passing through a mixed area of ferrite and pearlite to be heat treated. In this laser welding, heating and cooling occur more abruptly than in Arc welding, and thus the weld is hardly transformed into the mixed

area of ferrite and pearlite. Especially, the hiqh carbon st.ecl, i: welded, suffers hardening severely.
[0020] Further another conventional technology is taught, in Japanese Laid-open Patent Application No. 2000-317642, in which bonding portions are laser welded and then a hot-rolled steel sheet is heat treated. However, here, due to laser welding, a weld is abruptly cooled and transformed into a martensite microstructure before heat treatment, thereby leading to microcracks. Therefore, this technology is hardly applicable to a production line such as PCM that needs high qua I ify welding.
[0021] Japanese Laid-open Patent Application No. 2001-353587 proposes further another conventional technology, i.n which a filler wi re is utilized in a joint between heterogeneous materials of h igh carbon stee I and low carbon steel . In this method, heat treatment is not employed, and a laser beam is irradiated onto the low carbon to prevent cracks in the weld. Yet this technology fails to eliminate a hardened microstructure produced in a heat-affected zone of the hi qh carbon wh i ch is not melted.
[0022] Further another conventional technology is disclosed in Japanese Laid-open Patent Application No . 2000-317642 . Th i s techno I ogy concerns flash butt welding for heat treating a weld. Similar technologies for heat treating welds, however by different met.hods, are suggested in Japanese Patent Application Publicaf i on Nos . 11 5~ ! 3'-"/ 1 9 and 2000-317642 and 2004-76159. However, these methods do not ensure stable quality for the weld of high carbon steel which contains at. least. 0.5% C. [0023] The aforesaid technologies for bonding steel sheets for TCM,

although significant in their number, are merely applicable to high carbon steel with relatively low C content or a production lino not requiring high welding quality.
[0024] Consequently, there has been a demand for a technoloqy for securing good quality for a weld joint to carry out TCM on steel which is transformed in its microstructure in a laser weld i nq i nvo 1 v i ng abrupt, heating and cooling. Such steel is exemplified by high carbon sloe I dual phase (DP) steel, transformation induced plast.i ci ty (TRIP) steel and composite phase (CP) steel.
SUMMARY OF THE INVENTION
[0025] The present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present i riven I i on is to provide a laser welding method which ensures a weld to be less hardened in its microstructure to guarantee stable qua.l. i ty of the weld, thereby improving productivity for a continuous product-ion process. [0026] According to an aspect of the invention, the invention provides a laser welding method for hot-rolled steel sheets in a continuous production process for coils, including:
butting two hot-rolled steel sheets subject to 1 ow-tempe ra t.u re transformation against each other; and
laser-welding butted portions of the hot-rolled steel sheets wi Ih a welding material containing up to lwt% C and 0 to 1.22 wt% Or. [0027] The welding material comprises a carbon steel or an Ni al loy, and preferably comprises one selected from a group consisting of a wi re, a powder and a film. [0028] According to an embodiment of the invention, preferably, before

the laser-welding, the butted portions of the hot-rol led steel sheet;;
are pre-heated to a temperature of 600°C to 800 °C.
[0029] According to further another embodiment of the invention,
preferably, after the laser-welding, a weld of the butted portions is
post-heated to a temperature of 800°C to 1100°C. Most preferably, both
of the pre-heatirig and post-heating are conducted.
[0030] The hot-rolled steel sheets subject to low-temperature
transformation comprise one selected from a group consisting of a high
carbon steel containing at least 0.5wt% C, a duaJ phase (DP) steel,
a transformation induced plasticity (TRIP) steel and a composite phase
(CP) steel.
[0031] The high carbon steel consists of at least 0 . Swt% C, 0.1 to 0 . bwt%
Si, 0.3 to 0.6wt% Mn, up to 0.05wt% P, up to 0.05wf% S, up t.o O.bwt'*
Cu, up to 3wt% Ni, 0.05 to 0.5wt% Cr, at least 0.05wt% Al, the balance
being Fe and unavoidable impurities.
[0032] The continuous production process for coils comprises one
selected from a group consisting of picking and tandem cold rol1 ing
mill line, pickling and oiling line, annealing and pickling line,
pickling line and tandem cold rolling mill line.
BRIEF DESCRIPTION OF THE DRAWINGS [0033] The above and other objects, features and other advantages of
the present invention will be more clearly understood from the fol lowing
detailed description taken in conjunction with the accompanying
drawings, in which:
[0034] FIG. 1 is a conceptual view illustrating an exemplary laser
welding apparatus according to the invention;

[0035] FIG. 2 is a graph illustrating a thermal cycle of a laser weld according to the invention; and
[0036] FIG. 3 is a picture illustrating a laser weld after a PCM process for SK85 steel according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] Exemplary embodiments of the present invention will now be described in detail with reference to the accompany ing drawings. [0038] In this specification, "hot-rolled steel sheets subject to low-temperature transformation" denote hot-rolled steel sheets whose welds are transformed in their microstructures at a low temperature when the hot-rolled steel sheets are bonded together to form the welds by laser welding and cooled down. The low-temperature transformation microstructure denotes a microstructure containing martens it.e and bainite. Also, the hot-rolled steel sheets subject to Low-temperature transformation after laser welding include high carbon steel or high strength steel.
[0039] The high carbon steel designates steel having at least O.bwt% C. By way of a representative example, the high carbon steel consists of at least 0.5wt% C, 0.1 to 0.5wt% Si, 0.3 to 0.6wt% Mn, up to O.Obwtv P, up to 0.05wt% S, up to 0.5wt% Cu, up to 3wt% Ni_, O.Ob to O.bwtV; Cr, at least 0.05wt% Al, the balance being Fe and unavoidable impurities. In the present invention, % represents wt% unless spoci Nod otherwise. Steel containing at least 0.5% C is construed to fa.1.1 within the high carbon steel according to the invention. Here, an alloy element such as Mo, V, Ti, W, B, Nb and Sb may be added to impart a spoci fie function to the high carbon steel.

[0040] The high strength steel is designed to have a tensi le strength of 450MPa, and exemplified by steel subject low-temperature transformation. Examples of the steel subject to low-temperature transformation include a dual phase (DP) steel, a trans forma t i on induced plasticity (TRIP) steel, and a composite phase (CP) steel. The high strength steel. is typically referred to as steel subject, to low-temperature transformation. The DP steel has a dual phase with two characteristics of ductile ferrite and strong martens!te. The DP steel ensures superior processabilty and high strength with a small alloy element. Meanwhile, the TRIP steel is composed of ductile ferrite, strong bainite and austenite meta-stable at a high temperature. In the TRIP steel, the meta-stable austenite phase is transformed into very strong martensite phase. The CP steel has precipitation in the martensite or bainite microstructure . The steel w.i th trans format.! on microstructure as just described is phase-transformed in its microstructure after laser welding.
[0041] As described above, in the high carbon steel or steel subject to low-temperature transformation, a weld is phase-transformed in its microstructure at a low-temperature into e.g., martensite or banite with strong brittleness, when the steel is laser welded and coo I ed down . In this fashion, the low-temperature transformation microstructure causes crack or rupture in the weld.
[0042] Also, in this specification, "a weld" refers to a bond i nq port i on formed by laser welding a rear end of a preceding hot co:i 1 and a f o.l 1 ow i nq hot coil in a continuous production process for colls. Here, the weld includes molten metal which is melted by laser and solidified, and a heat affected zone (HAZ) Influenced by a heat source of the laser.

[0043] Moreover, in this specification, "a continuous production
process for coils" denotes continuously producing coils in a hot. or
cold rolling line. Here, the continuous production line includes a
polishing process in which the hot-rolled steel sheets are bonded
together to be polished, or all continuous production processes such
as a molten zinc plating process or an annealing process.
[0044] First, according to an aspect of the invention, the reason for
hardening in a weld will be examined based oru-a micro-structure test
conducted on a Laser weld with low temperature transformation
microstructure and an Erichsen test.
[0045] The inventors of this invention confirmed that the wold metal
and heat-affected zone, respectively, suffered hardening for which
low-temperature transformation micro-structures were mainly
responsible.
[0046] Such low-temperature transformation micro-structures can be
inhibited or diminished by chemical composition of the weld. To this
end, a welding material with less C content may be employed to reduce
C content of the weld metal. More preferably, heat treatment aiong with
the welding material can serve to diminish hardness.
[0047] The quality of the weld is affected not only by hardness of the
weld as just described but also an overall hardness dis t r i but i on the reo f .
[0048] Thus, the invention further proposes heat-treatment of the weld
in order to lower hardness of the weld and alleviate an ovcral I hardness
distribution thereof. This allows the steel subject to low-ternperature
transformation to achieve a stable quality laser weld.
[0049] Also, in a continuous production process such as PCM, welding
and heat treatment, if carried out independently, prolong a welding

time, thereby decreasing an overall production rate. Moreover, an increase in C content in the hot-rolled steel sheets leads to an i ncrease in heat treatment time.
[0050] As a result, according to the invention, to solve these problems, as shown in FIG. 1, a heat treatment device is configured integral with a welding device so that a weld of hot-rolled steel sheets can be welded and heat treated as well.
[0051] In an explanation with reference to FIG. 1, the we id i ng appa ra tus of the invention is largely constructed of the welding device 10 and the heat treatment device 20. These two devices are most, preferably formed integral with each other but not limited thereto. KIG. 1 is a schematic view illustrating the welding apparatus according to the invention, in which thus these devices are not shown configured integrally. But a technique for configuring these devices integrally can be worked by a known art and thus will be explained i n no more del.a i 1 . [0052] The welding device 10 of the invention includes a laser generator 12 for generating laser and a filler supplier 16 for supplying a welding material . The heat treatment device 20 adopts as a heat source a 1 1 means for heating a moving hot coil speedily. A preferable heat source is a high frequency induction coil. The heat treatment device 20 is constructed of a pre-heater 22 installed upstream of the welding device 10 to heat hot-rolled steel sheets 40 prior to welding and a post-heater 24 for heating hot-rolled steel sheets 50 after we]ding. [0053] In a case where moving hot-rolled steel sheets are welded together via the welding apparatus of the invention, first a front end of the following hot coil 40 and a rear end of a preceding hot coi 1 50 are butted against each other and welded by the welding device 10

after being heated by the pre-heater 22. A we Id" 60 with the hot-rol led
steel sheets bonded together is post-heated by the post, heater 24.
[0054] In FIG. I, reference numeral 30 which has not been explained
denotes a guide roll.
[0055] The moving hot-rolled steel sheets, when welded via the welding
apparatus of the invention, also move the welding apparatus. Here, the
hot-rolled steel sheets may move in the same or opposite di reel, ion with
respect to the welding apparatus. In a case where the hot-rol led st.ee I
sheets move in the same direction as the welding apparatus, preferably,
the hot-rolled steel sheets move at the same or faster rate wi th respect
to the welding apparatus.
[0056] FIG. 2 is a schematic view illustrating a thermal cycle history
for welds of hot-rolled steel sheets and a subsequent microstructure
status thereof when welding is performed by a movable welding apparatus
according to an embodiment of the invention.
[0057] The invention suggests a method for controlling hardening of
the welds to conduct continuous production process for the hot-rolled
steel sheets with low-temperature transformation microst ructu res . The
method involves controlling chemical composition of the welds and heat
treating the welds.
[0058] These methods will be explained hereunder.
[0059] First, a method for controlling chemical compos i t i on o f the we Ids
will be explained.
[0060] This method pertains to controlling a welding material which
constitutes most of molten metal. Preferably, the wolding materia 1 is
supplied from a filler suppler 16 to a weld 60 and bas i ca 1 ly contains
up to 0.1% C and 0 to 1.22% Cr. Such a welding material adopts carbon

steel and an Ni alloy with high tensile strength. The carbon steel is preferable since it assures more stable welding quality. [0061] A welding material composed of stainless steel and an NJ al1oy is degraded in wettability with high carbon steel, i.e., the base material, and has base material elements not completely di luted therein in a case where an optimum welding parameter is not derived. This may render the weld brittle occasionally. However, the invention is not. limitative of the Ni alloy.
[0062] Preferably, according to the invention, the weld metal is regulated to contain up to 0.4% C. This is because in the laser welding, a tiny amount of the welding material is melted and fi 1 led in the weld so that the base material, dilutes at a greater rate compared wi th fypica 1 Arc welding.
[0063] For example, in a case where the hot-rolled steel sheets adopt, high carbon steel containing at least 0.85% C, the welding material should contain up to about O.lwt% C in order to keep C content of the weld metal at up to 0.4% with a dilution rate set to maximum 3()'f>. [0064] Further, Cr reacts with carbon in the hot-rolled steel .sheets to form chromium carbides in the vicinity of the weld metal and heat-affected zone. Thus preferably, the welding material should contain up to 1.22 % Cr.
[0065] As a result, according to the invention, preferably, thowolding material supplied from the welding device is carbon steel or an Ni a I I oy containing up to 0.1 % C and up to 0 to 1.22 % Cr. Main composition of the carbon steel and Ni alloy is FeandNi, respectively. The welding material applicable according to the invention can be typical carbon steel or Ni alloy that satisfies the aforesaid C and Cr conditions.

The welding material is preferably shaped as a wire, but. a powder or a film is also applicable.
[0066] Then an explanation will be given about a method for heat I-real i rig the weld.
[0067] In this invention, heat treatment of the weld breaks down into pre-heating and post-heating. The pre-heating is carried out prior to welding to prevent cracks in a weld joint and the post-heating is performed after welding to relieve hardening of the weld joint. [0068] In a case where the hot-rolled steel sheets subject, to low-temperature transformation is laser welded and the weld undergoes only post-heating, the weld after we.l ding is abrupt! y coo.l ed down before being post-heated, thus potentially causing fracture in the wold. [0069] Consequently, it is preferable to pre-heat the welding material subject to low temperature transformation in order to al leviato abrupt. cooling resulting from laser welding.
[0070] In a case where the movable heat treatment device is employed as in an embodiment of the invention, the welding material docs not achieve sufficient pre-heating effect just above a martcnsite transformation temperature (Ms), and thus preferably should be pre-heated to a temperature higher than that.
[0071] Therefore, according to an embodiment of the invention, preferably, the weld of the high carbon steel is pre-heated to a temperature of 600 °C to 800 °C. A preheating temperature of up to 600 °C does not ensure a suf f i cient time for pre-heating the mov i ng hot- ro 1 I ed steel sheets, thereby leading to unsatisfactory quality for the weld. On the other hand, a preheating temperature of at least 800 "C deforms the weld due to excessive heat input, thus rendering the weld far from

solid.
[0072] Moreover, according to the invention, post-heatinq of the wold is carried out by two concepts.
[0073] The first concept pertains to tempering in which the weld is post-heated at Acl or less during a relatively long time to change its martensite microstructure into a tempered martens! to , the roby a tta i n i ng ductility.
[0074] The second concept is to actively control a cool i.nq therrna 1 eye: lo in laser welding to transform the weld into ferrito and pear lite microstructures.
[0075] The tempering method requires relatively long heat treatment and subsequently ensures sufficient ductility. Yet, in a coi I production line wi th a fast yield rate, this type of tedj ous post-heat i ng may degrade productivity. Therefore, in the laser welding system using a movable heat source, the second concept of relieving a cool ing cycle after laser welding is more preferable.
[0076] The weld is post-heated preferably to a temperature of 800 'C to 1100 °C, and more preferably 950 °C to 1100 °C. Also, according to an embodiment of the invention, preferably, the wold is post-heated without holding time and naturally cooled down.
[0077] A post-heating temperature of up to 800°C creates a martensite microstructure in the weld after cooling due to Jack of heat input, thus ineffective in reducing hardness . On the other hand, a post-heating temperature of at least 1100°C coarsens the microstructure of the weld due to excessive heat input or partially regenerates the martensite that is a hardened microstructure, thereby deteriorating physical properties of the weld.

[0078] The laser welding of the invention is applicable to al 1 methods for continuously producing coils which include, for example, pick I ing and tandem cold rolling mill (PCM) line, pickling and oi I i nq line (POL), annealing and pickling line (APL), pickling line (Ph) and tandem cold rolling mill (TCM) line.
[0079] Preferred embodiments of the invention will be explained hereunder by way of Examples.
Examples
[0080] The Examples adopted hot-rolled steel sheets (mainly composed
of Fe) of high carbon steel having a composition noted in Table ! . The
hot-rolled steel sheets were 2.0 mm thick and welded to each other via
a CO2 laser welding device having a maximum output of 12KW.
Table 1 (Table Remove)

[0081] Also, the Examples utilized as a welding material a wi re filler ($0 . 9mm) , The welding material which had chemical compos i t i on as shown in Table 2 included carbon steel-based ER70S-G and KR80S-G, stainless steel-based ER308, and nickel alloy-based ERNiCrMo03 and KRNi. The carbon steel-based ER70S-G and ER80S-G had Fe as a main chemical composition, which however is not designated in Table 2. Table 2

(Table Remove)
[0082] The hot-rolled steel sheets were laser weJdcd via the laser welding device under the condition that the weld was f roc from welding defects such as a pore underfill. Here, the laser had an output of B.lkw and a welding rate of 4.5m/min, and a joint had a spacing of 0.1bmm. [0083] To heat treat the weld, a high frequency induct i on f'u rnaco hav i nq a heat source of 2 Ow* 2 001mm moved along a weld line by chang i ng an out put. [0084] The weld was heat treated at a heating rate of about TOOoC/s, with pre-heating and post-heating conducted at" varying temperatures, and then naturally cooled down (air-cooled).
[0085] In heat treating the weld, an R-type pyrometer was spot-welded on a melting boundary to measure the temperature hj story of the weld heated by the high wave induction furnace. A highest attainable temperature was obtained from the temperature history curve to detormi no as a heat treatment temperature.
[0086] The weld obtained according to above-mentioned conditions passed the standard for the PCM line that was Erlchsen height, of 4mm or more. Thus, quality was measured with an hiri onsen tester. To evaluate the quali ty of the weld, measurement was taken on a pi ast i ca 1 i y
deformed height of.' the weld until the occurrence of cracks. [0087] First, Table 2 shows quality evaluation results of SK8b stoc containing 0.85% C according to welding materials and heat tredtment conditions. Table 3
(Table Remove)

Notes)
WM* Welding material
HT** Heat treatment
EH*** Erichsen height (mm)
[0088] As seen in Table 3, in a case where the weld of SK8b stool was
neither applied with a welding material nor heat treated, the wold
suffered cracks just after welding, thereby failing to achieve a wold
joint that could satisfy the standard.
[0089] Moreover, in a case where only one of pro-heat inq and
post-heating was carried out, the weld could not attain a qual i t.y enough
to be passable .
[0090] In contrast, in a case where pre-heating and post-hea t.i rig were
both performed as in this invention, the weld was found to be improved
in its quality compared with when only one of them was performed.
[0091] Also, in a case where the weld was preheated to a temperature
of 600°C to 800°C and post-heated to a temperature of 9bO°C to 1100°C
as seen in Table 3, the weld exhibited stable welding qua! i ty exceeding
the Erichsen height of 4.0mm, which is a threshold Cor determining
passability for the PCM .line.
[0092] Furthermore, when it comes to the welding material, KR'/OS-G,
ER80S-G, and ERNi with small Cr content exceeded the Krichson height
of 4.0mm. Meanwhile, ER308, and ERNiCrMo-3 containing 1 9 . 8 6% and 71 . }'/'*.
Cr content, respectively did not achieve satisfactory welding qua I i ty
even with the same heat treatment technique.
[0093] This is because Cr element contained in the welding material
reacted with a carbon element in the hot-rolled steel, sheets to generate
chromium carbides at a grain boundary, thereby rendering the grain
boundary brittle.
[0094] Table 4 demonstrates quality evaluation results of SbOC steel
containing 0.5% C according to the welding material and heat treatment
«
conditions.
[0095] The Examples shown in Table 4 were carried out under the heat treatment temperature conditions which produced the highest Krichsen value as in Table 3, that was, preheated to a temperature of 723°C and post-treated to a temperature of 1005°C.
Table 4(Table Remove)

WM* Welding material
HT** Heat treatment
EH*** Erichsen height (mm)
[0096] As seen in Table 4 , the laser weld of S5°C s tee I overall exhibited
«*'
superior quality to SK85 steel regardless of the welding material:-; and
heat treatment method.
[0097] In addition, the laser weld of S5°C steel, even though only
post-heated, obtained a value exceeding the Erichsen height of -1.0 mm,
i.e., the threshold. This is apparently attributed to less hardening
resulting from lower C content in the steel.
[0098] In addition, as seen in Table 4, the weld, when pro-heated and
post-heated at the same time, demonstrated much higher qua I ity.
[0099] Preferred embodiments of the invention- were explained
hereinabove. However this invention is not limited to welding

conditions of the continuous production process lor high carbon stool as manifested in the Examples. The invention is app 1 icable to various welding conditions necessary for the continuous production process which encompasses the scope of the invention.
[00100] As set forth above, according to exemplary embodiments of the invention, a laser welding for a continuous production process provides welding conditions that have not been adopted, thereby continuously producing steel sheets subject to low temperature transformation. [00101] Also, to continuously produce the hot-rolJod steel sheets subject to low temperature transformation, the invention ensures a secure laser weld joint free from welding imperfections. Moreover, as shown in FIG. 3, the invention enables continuous production without causing fracture in a laser weld.
[00102] In addition, according to the invention, the welding time is shortened to 25 seconds, which are required for general steel, irrespective of C content of steel. This noticeably enhances productivity in the continuous production process.
[00103] Further, steel subject to low temperature transformation under the welding conditions according to the invention can withstand strong compressive .load applied in the continuous production process and tensile load imposed between stands, thereby carrying out continuous production with the weld free from rupture.
[00104] While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made wi thout departing from the spirit and scope of the invention as defined by the appended claims.

What Is Claimed Is:
1. A laser welding method for hot-rolled steel sheetjs in a
continuous production process for coils, comprising:
butting two hot-rolled steel sheets subject to 1ow-temptrature transformation against each other; and
laser-welding butted portions of the hot-roiled s tee 1 shoots w i t.h a welding material containing up to lwt% C and 0 to 1 .22 wt%: Cr.
2. The laser welding method according to claim 1, wherein the
welding material comprises a carbon steel or an Ni al l.oy.
3. The laser welding method according to claim 1, whorpin the
welding material comprises one selected from a group consisting of a
wire, a powder and a film.
4. The laser welding method according to claim 1 , whore i nj before
the laser-welding, the butted portions of the hot-rol led steel! sheets
are pre-heated to a temperature of 600°C to 800°C.
5. The laser welding method according to claim 1 or 4, wherein
after the laser-welding, a weld of the butted portions is posti-heated
to a temperature of 800°C to 1100°C.
6. The laser welding method according to claim 6, wherein the
hot-rolled steel sheets subject to low-temperature transformation
comprise one selected from a group consisting of a high carbojn steel
containing at least 0.5wt% C, a dual phase (DP) steel, a transformation

induced plasticity (TRIP) steel and a composite phase (CP) sjtoo I .
7. The laser welding method according to claim 6, wherein tjho high
carbon steel consists of at least 0.5wt% C, 0.1 to O.bwt% Si,: 0.3 t.o
0.6wt% Mn, up to 0.05wt% P, up to 0.05wt% S, up to O.bwt.% Cu, up t.o
3wt% Ni, 0.05 to 0.5wt% Cr, at least 0.05wt% At, the balance beinq Ko
and unavoidable impurities.
8. The laser welding method according to claim 1, wherein the
continuous production process for the coils comprises one selected from
a group consisting of pickling and tandem cold ro.l .1 ing mil) I ine,
pickling and oiling line, annealing and pickling line, pick! ing I ine
and tandem cold rolling mill line.
9. A laser welding method for hot-rolled steel sheets in a
continuous production process for coils substantial ly as j herein
described with reference to the foregoing description, examples,! tables
and the accompanying drawings.