U.S. patent application number 11/560147 was filed with the patent office on 2007-06-28 for steel sheet having excellent weldability.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Tetsuo Soshiroda, Reiichi Suzuki, Kei Yamazaki.
Application Number | 20070144620 11/560147 |
Document ID | / |
Family ID | 37671567 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070144620 |
Kind Code |
A1 |
Soshiroda; Tetsuo ; et
al. |
June 28, 2007 |
STEEL SHEET HAVING EXCELLENT WELDABILITY
Abstract
Disclosed is a steel sheet, containing: Si: 0.20-2% (the term
"%" herein means "mass %", the same is true hereinbelow), Mn:
1-2.5%, a total mass of Si and Mn being 1.5% or more, and 0: 0.002%
or less (exclusive of 0%), C: 0.02-0.25%, P: 0.1% or less
(exclusive of 0%), S: 0.05% or less (exclusive of 0%),
Al-0.02-0.2%, and N: 0.0015-0.015%. The steel sheet of the
invention can be advantageously used for forming wide beads even in
high-speed arc welding of 100 cm/min or higher.
Inventors: |
Soshiroda; Tetsuo;
(Kakogawa-shi, JP) ; Suzuki; Reiichi;
(Fujisawa-shi, JP) ; Yamazaki; Kei; (Fujisawa-shi,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
37671567 |
Appl. No.: |
11/560147 |
Filed: |
November 15, 2006 |
Current U.S.
Class: |
148/320 |
Current CPC
Class: |
C21D 9/46 20130101; C22C
38/06 20130101; C22C 38/04 20130101; C21D 8/0426 20130101; C21D
6/008 20130101; B23K 35/3073 20130101; B23K 2101/16 20180801; C21C
7/10 20130101; C22C 38/02 20130101; B23K 35/0261 20130101; Y10T
428/265 20150115 |
Class at
Publication: |
148/320 |
International
Class: |
C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2005 |
JP |
2005-375844 |
Claims
1. A steel sheet, comprising: Si: 0.20-2% (the term "%" herein
means "mass %", the same is true hereinbelow), Mn: 1-2.5%, a total
mass of Si and Mn being 1.5% or more, and O: 0.002% or less
(exclusive of 0%), C: 0.02-0.25%, P: 0.1% or less (exclusive of
0%), S: 0.05% or less (exclusive of 0%), Al: 0.02-0.2%, and N:
0.0015-0.015%.
2. The steel sheet of claim 1, wherein a thickness of an inner
oxide layer existing in a surface layer of the steel sheet is 5
.mu.m or less.
3. The steel sheet of claim 1 further comprising at least one
element selected from the group consisting of Cr: 0.03-2%, Mo:
0.03-1%, and B: 0.0003-0.005%.
4. The steel sheet of claim 1 further comprising at least one
element selected from the group consisting of Nb: 0.005-0.1%, Ti:
0.005-0.3%, V: 0.005-5%, and W: 0.005-0.5%.
5. The steel sheet of claim 1 further comprising Cu: 0.03-0.5% or
less.
6. The steel sheet of claim 1 further comprising at least one of:
Ca: 0.0005-0.005% and REM (rare earth elements): 0.0005-0.005%.
7. The steel sheet of claim 1 further comprising Zr and Mg:
0.0005-0.005% in total.
8. The steel sheet of claim 1 further comprising Co: 0.03-1%.
9. The steel sheet of claim 5 further comprising Ni: 0.015-0.5%.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates in general to a steel sheet
used as a material for automotive parts through welding, more
specifically to a high tensile steel sheet excellent in high-speed
arc weldability, capable of forming wide beads at an arc welding
speed as high as 100 cm/min or above.
[0003] 2. Description of the Related Art
[0004] Diverse welding processes are involved in assembly of
automotive parts, and among them is a gas shielded arc welding
particularly for an assembly of parts that need to have superior
rigidity. This is because arc welding is generally known to
increase joint strength, stabilize welding quality, and improve
rigidity of parts. However, a shortcoming of the arc welding is
that it takes a longer time than other processes such as cutting,
pressing, and coating. Because of this, when arc welding is
employed, production line speed is deteriorated, leading to an
increase in production cost. Therefore, if the arc welding speed of
steel sheets can be increased, the cost of production would be
substantially reduced and it will be very meaningful
industrially.
[0005] For instance, an invention in Japanese Patent Laid-Open No.
S61-007089 tried to achieve high-speed gas shielded arc welding by
incorporating a specific amount of Si and Mn into a steel wire for
gas shielded arc welding. Unfortunately though, the cited invention
is mainly directed to specify the composition of the wire, and does
not necessarily take rigidity of a steel sheet into consideration.
In addition, the cited invention unnecessarily defines the content
of oxygen in the wire as 0.008% or more, so as to attain ripple
uniformity of beads and reduction of tension on the beads.
[0006] Moreover, Japanese Patent Laid-Open No. 2000-167691 takes
the composition of a steel sheet into account for arc welding of a
high tensile steel sheet. The gist of the cited invention lies in a
welding procedure of a high tensile steel sheet by specifying "Si
content in the steel sheet (mass %)+Si content in the wire (mass
%).gtoreq.1.5". However, as the cited invention carries out such
welding process at a speed of 30-60 cm/min, it is evident that the
objective of the invention is not in accomplishment of a high speed
arc welding process. Besides, the cited invention does not have any
description about the influence of oxygen on arc welding.
[0007] In general, width of beads tends to decrease in a high-speed
arc welding process. Thus, in order to put high-speed arc welding
to practical use, it is necessary to ensure the width of beads to a
certain extent particularly because groove position or wire
position is often deviated or route gap is created during an actual
arc welding process. However, none of the above-described cited
inventions studied width of the bead during high speed arc
welding.
[0008] Although there is a strong demand for light (thin) steel
sheets in terms of enhancement of fuel efficiency in automobile
industry, it should not outweigh the collision safety. To satisfy
both sides, i.e., to obtain light and collision-safe steel sheets,
more automobile manufacturers now use high tension steel sheets of
reduced thickness. Such thin steel sheets, however, were easily
melted away during high current arc welding so it was not easy to
have a sufficient amount of welding at high current. Because of
this, the welding bead width of a thin steel sheet has been
narrowed and thus, it became necessary to ensure increased bead
width. Furthermore, since high tensile steel, compared with mild
steels, has poor stability in a pressing process, the route gap is
easily increased. Therefore, there is a need to develop a way for
increasing the bead width.
SUMMARY OF THE INVENTION
[0009] It is, therefore, an object of the present invention to
provide a high tensile steel sheet excellent in high-speed arc
weldability, capable of forming wide beads at an arc welding speed
as high as 100 cm/min or above.
[0010] To achieve the above objects and advantages, there is
provided a steel sheet, containing: Si: 0.20-2% (the term "%"
herein means "mass %", the same is true hereinbelow), and Mn:
1-2.5%, a total mass of Si and Mn being 1.5% or more, and O: 0.002%
or less (exclusive of 0%), C: 0.02-0.25%, P: 0.1% or less
(exclusive of 0%), S: 0.05% or less (exclusive of 0%), Al:
0.02-0.2%, and N: 0.0015-0.015%.
[0011] In the steel sheet of the invention, the thickness of an
inner oxide layer existing in a surface layer is preferably 5 .mu.m
or less.
[0012] To improve hardenability, the steel sheet of the invention
preferably contains at least one element selected from the group
consisting of Cr: 0.03-2%, Mo: 0.03-1% or less, and B:
0.0003-0.005%. In addition, for precipitation strengthening, the
steel sheet of the invention preferably contains at least one
element selected from the group consisting of Nb: 0.005-0.1%, Ti:
0.005-0.3%, V: 0.005-0.5%, and W: 0.005-0.5%.
[0013] From the viewpoint of corrosion resistance, the steel sheet
of the invention may further include Cu: 0.03-0.5% or less. When
the steel sheet contains Cu as defined, it may further contain Ni:
0.015-0.5%. In addition, to enhance workability, the steel sheet of
the invention may contain Ca: 0.0005-0.005% and/or REM (rare earth
elements): 0.0005-0.005%, or Zr and/or Mg: 0.0005-0.005% in total.
Moreover, to improve ductility, the steel sheet of the invention
may contain Co: 0.03-1%.
[0014] After careful analysis and proven results, the inventors
discovered that Si and Mn generate advantageous effects in high
speed arc welding. However, they also found out that if an
excessive amount of Si was used, more slags were formed on a weld
bead, and coating came off easily. Meanwhile, if an excessive
amount of Mn was used, workability of the steel sheet was
deteriorated. Therefore, instead of using Si and Mn singly and
excessively, they should be incorporated together. In so doing, the
weld beads had superior corrosion resistance and workability, and
the beads were sufficiently wide after suppressing so-called
humping of weld beads which is a problem that an irregular bead
shape and width is formed in the welding.
[0015] Nevertheless, even though the total content of Si and Mn
satisfies the defined limit, in high-speed arc welding at a speed
of 100 cm/min or higher, humping of weld beads occurs and bead
width is not sufficient. The inventors later discovered after a
more thorough examination that O (oxygen) contained in the steel
sheet exerted a bad influence on humping. Thus, when the total
content of Si and Mn was controlled not to exceed its defined limit
and when O content was suppressed to 0.002% or less, the inventors
were able to obtain beads having a sufficient width and regular
shape without humping even in a high-speed arc welding process at a
welding speed of 100 cm/min or higher.
[0016] A further examination on O (oxygen) revealed that although
the O content was suppressed to 0.002% or less, if an inner oxide
layer in the surface layer of the steel sheet is thicker than 5
.mu.m, oxygen still could adversely affect the high-speed arc
welding of the steel sheet. This is possibly because although the
oxygen content in the steel sheet may be reduced, if an inner oxide
layer exists in the surface layer, oxygen is supplied from the
oxide layer to the weld metal, causing humping and narrow beads.
However, the present invention is not limited by such estimation
mechanism because, according to a preferred embodiment of the
invention, excellent high-speed weldability could be realized by
suppressing the thickness of the inner oxide layer of the steel
sheet to 5 .mu.m or less.
[0017] Additional and/or other aspects and advantages of the
present invention will be set forth in part in the description
which follows and, in part, will be obvious from the description,
or may be learned by practice of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] A preferred embodiment of the present invention will be
described herein below.
[0019] As described above, the main objective or gist of the
invention is to specify the total amount of Si and Mn in a steel
sheet and to suppress oxygen content. Also, in a preferred
embodiment of the invention, it is important to suppress the
thickness of an inner oxide layer existing in the surface layer of
the steel sheet to 5 .mu.m or less. Thus, the following will
describe desired contents of Si, Mn and O first, followed by a
desired thickness of the inner oxide layer.
Si: 0.20-2% (the term "%" herein means "mass %", the same is true
hereinbelow),
[0020] Si is an essential element in the invention and is directly
involved in increasing bead width in high-speed arc welding. Such
an effect is displayed when the Si content in the steel sheet is
0.20% or more, preferably 0.50% or more, and more preferably 0.80%
or more. However, if an excessive amount of Si is used, viscosity
of molten pool increases too much and thus, humped weld beads are
produced. Therefore, the Si content needs to be suppressed to below
3.0% from the viewpoint of weldability. In addition, when the Si
content is too high, an amount of slag formed on the bead is
increased, coating comes off easily later, and corrosion resistance
after coating is deteriorated. Therefore, the Si content needs to
be suppressed to 2% or less from the viewpoint of corrosion
resistance. In short, the Si content is defined as 2% or less,
preferably 1.8% or less, and more preferably 1.5% or less.
Mn: 1-2.5%
[0021] Mn, similar to Si, contributes to high-speed weldability,
and is an essential element in the invention. Also, Mn plays an
important role in securing strength. Therefore, the Mn content is
defined as 1% or more, preferably 1.2% or more, and more preferably
1.5% or more. However, if an excessive amount of Mn is used,
segregation occurs noticeably and two-ply crack occurs in a
punching unit for example, leading to deterioration in workability.
Therefore, the Mn content is defined as 2.5% or less, preferably
2.0% or less, and more preferably 1.5% or less.
Si+Mn: 1.5% or more
[0022] As aforementioned, if the Si content is too much, coating on
the weld portion is easily released, while if the Mn content is too
much, workability is deteriorated. Thus, if Si or Mn is used
singly, it is difficult to obtain wide beads particularly in a
high-speed arc welding at a welding speed of 100 cm/min or higher.
This explains why Si and Mn should be applied in combination to the
steel sheet. In detail, the lower limit of the total amount of Si
and Mn in the steel sheet is defined as 1.5%, preferably, 1.8%, and
more preferably 2.0%. Meanwhile, the upper limit of the total
amount of Si and Mn is defined as 4.5%, which is a sum of the upper
limit of the Si content and that of the Mn content. Since the
effect does not increase when each of the Si content and the Mn
content is excessive, the total content is preferably defined as
4.2% or less, and more preferably 3.8% or less.
O: 0.002% or less (exclusive of 0%)
[0023] If an excessive amount of O is contained in the steel sheet,
viscosity of the molten pool is lowered too much and as a result,
molten metal flowing grows in intensity and humping tends to occur
more often. In addition, the weld bead width is also narrow.
Therefore, to achieve good high-speed arc weldability, it is
important to suppress the O content in the steel sheet of the
invention to 0.002% or less, preferably 0.0018%, and more
preferably 0.0015% or less. Although it seems to be better off
without oxygen at all to ensure good high-speed arc weldability,
making the O content 0% is considered not really possible
industrially.
[0024] The O content in the steel sheet can be reduced by
performing a degassing treatment on molten steel by using an RH
degassing apparatus (hereinafter referred to as RH degassing
treatment). However, the steel sheet of the present invention and
the molten steel forming the same contain a large amount of Si and
Mn that easily bond with 0, deoxygination rate during RH degassing
treatment tends to decrease. Thus, to fabricate the steel sheet of
the invention having reduced O content of 0.002% or less, RH
degassing treatment needs to be performed longer than usual. In
detail, in the case that about 250 tons of molten steel undergo RH
degassing treatment, although it usually takes about 30 minutes,
the invention recommends to extend it to 50 minutes or longer. In
so doing, the O content can be reduced to at least 1/2-2/3 of that
in the 30-min long treatment.
Thickness of Inner Oxide Layer Existing in the Surface Layer of the
Steel Sheet: 5 .mu.m or Less
[0025] "Thickness of the inner oxide layer" in the present
invention means a maximum distance (maximum depth) of an oxide that
exists from the surface to the sheet thickness direction in a cross
section perpendicular to the rolling direction. This `oxide` mainly
consists of Si or Mn oxide, but it further contains Ti oxide and
the like and exists primarily in ferrite grain boundaries.
Thickness of the inner oxide layer can be obtained by observing the
cross section of the steel sheet in perpendicular to the rolling
direction to a 50 .mu.m depth from the sheet surface on an SEM
image (magnifying power: 1,000.times.). Considering that thickness
of the inner oxide layer varies, an average of at least 5
measurements of the thickness in the width direction of the steel
sheet is used as the "thickness of the inner oxide layer" in the
present invention.
[0026] Meanwhile, no matter how low the O content in the steel
sheet may be, if the content of Si and Mn is relatively high as a
steel sheet of the present invention, the inner oxide layer can
still be formed during winding after finish rolling. Hence, in
order to fabricate a steel sheet with none or few of the inner
oxide layer, it is necessary to reduce the O content in the steel
sheet by performing the RH degassing treatment for a sufficient
period of time and, at the same time, to perform winding after the
finish rolled steel sheet is sufficiently cooled. In particular, to
make the inner oxide layer have a thickness of 5 .mu.m or less, it
is recommended to perform winding at a temperature below
600.degree. C.
[0027] Besides Si, Mn and O, the steel sheet of the invention
further contains C, P, S, Al, and N as its major components.
Descriptions on those elements are as follows.
C: 0.02-0.25%
[0028] C is an effective element for ensuring strength, and the
lower limit thereof is designated as 0.02%, preferably, 0.06%, and
more preferably 0.08%. However, an excessive amount of C is used,
cracks may be formed on solidified weld metals. Therefore, the
upper limit of the C content is designated as 0.25%, preferably
0.22%, and more preferably 0.20%.
P: 0.1% or less (exclusive of 0%)
[0029] If an excessive amount of P is contained in the steel sheet,
cracks might generate during welding. Therefore, the upper limit of
the P content is designated as 0.1%, preferably 0.080%, and more
preferably 0.060%. In particular, when a steel sheet is expected to
be applied for use where there is a tendency for generating welding
cracks, the P content is preferably reduced as low as possible.
However, it is industrially impossible to reduce the P content in
the steel sheet to 0%. In addition, P is an effective element for
ensuring strength by solid solution strengthening; the P content is
preferably designated as 0.010% or more, and more preferably 0.015%
or more.
S: 0.05% or less (exclusive of 0%)
[0030] S is a harmful element forming inclusions, and deteriorates
workability of the steel sheet when added excessively. Therefore,
the upper limit of the S content is designated as 0.05%, preferably
0.03%, and more preferably 0.01%. In addition, it is industrially
impossible to reduce the S content in the steel sheet to 0%.
Al: 0.02-0.2%
[0031] Al is an element required for deoxidization. The Al content
is defined as 0.02% or more, preferably 0.025% or more, and more
preferably 0.0030% or more. If an excessive amount of Al is used,
however, more oxide based inclusions are formed, leading to an
increase in scab formation. Therefore, the upper limit of the Al
content is defined as 0.2%, preferably 0.15%, and more preferably
0.10%.
N: 0.0015-0.015%
[0032] N is an effective element for ensuring strength by solid
solution strengthening, and contributes to fine structure by
forming nitrides with Al and the like. Therefore, the lower limit
of the N content is designated as 0.0015%, preferably 0.0020%, and
more preferably 0.0030%. If an excessive amount of N is used,
however, blowholes are formed during welding. Thus, the upper limit
of the N content is designated as 0.015%, preferably 0.010%, and
more preferably 0.0080%.
[0033] The steel sheet of the present invention contains the
above-described elements as its major composition, and the balance
is essentially Fe and inevitable impurities. Depending on raw
materials, resources, manufacturing facilities and the like, other
inevitable impurities (for example, Sn, Pb, Zn, Sb, and As) may be
included as well. Furthermore, if necessary, the steel sheet of the
invention may further contain the following elements
optionally.
Cr: 0.03-2%
[0034] Cr is an effective element for improving quenchability, and
may be added to the steel sheet if necessary. The lower limit of
the Cr content is 0.03%, preferably 0.10%, and more preferably
0.20%. If an excessive amount of Cr is used, however, coating
adhesion is substantially deteriorated even though coating
substrate treatment where the coating substrate is immersed in a
phosphate bath may have been performed, and it exerts a bad
influence upon the base material steel sheet and the corrosion
resistance after coating. Therefore, the upper limit of the Cr
content is defined as 2%, and preferably 1.5%.
Mo: 0.03-1%
[0035] MO is also effective element for improving quenchability,
and may be added to the steel sheet if necessary. The lower limit
of the Mo content is defined as 0.03%, preferably 0.10%, and more
preferably 0.20%. Similar to Cr, if an excessive amount of Me is
used, however, P treatment efficiency is deteriorated and coating
comes off more easily. Therefore, the upper limit of the Mo content
is defined as 1%, and preferably 0.8%.
B: 0.0003-0.005%
[0036] B is also effective element for improving quenchability, and
may be added to the steel sheet if necessary. The lower limit of
the B content is defined as 0.0003%, and preferably 0.0005%.
However, such an effect does not increase no matter how excessively
B may be added and only oxide based inclusions are produced more,
thereby resulting in deterioration of workability. Therefore, the
upper limit of the B content is defined as 0.005% and preferably
0.003% or less.
Nb: 0.005-0.1%
[0037] Nb contributes to the production of crystalline fine
particles and the precipitation strengthening, and is an effective
element for high strength steel sheets. It may be added to the
steel sheet of the invention if needed. The lower limit of the Nb
content is defined as 0.005%, preferably 0.020%, and more
preferably 0.030%. However, since such effects do not increase no
matter how excessively Nb may be added, its upper limit, from the
viewpoint of economic efficiency, is defined as 0.1%, and
preferably 0.8%.
Ti: 0.005-0.3%
[0038] Likewise, Ti contributes to the production of crystalline
fine particles and the precipitation strengthening, and is an
effective element for high strength steel sheet. It may be added to
the steel sheet of the invention if needed. The lower limit of the
Ti content is defined as 0.005%, preferably 0.010%, and more
preferably 0.020%. If an excessive amount of Ti is added, however,
production of TiN inclusions increases sharply and as a result,
workability of the steel sheet is deteriorated. Therefore, the
upper limit of the Ti content is defined as 0.3%, and preferably
0.20%.
V: 0.005-0.5%
[0039] V also contributes to the production of crystalline fine
particles and the precipitation strengthening, and is an effective
element for high strength steel sheets. It may be added to the
steel sheet of the invention if needed. The lower limit of the V
content is defined as 0.005%, preferably 0.010%, and more
preferably 0.020%. However, since such effects do not increase no
matter how excessively V may be added, its upper limit, from the
viewpoint of economic efficiency, is defined as 0.5%, and
preferably 0.3%.
W: 0.005-0.5%
[0040] W is an effective element for high strength steel sheets by
precipitation strengthening, and may be added to the steel sheet of
the invention if needed. The lower limit of the W content is
defined as 0.005%, preferably 0.010%, and more preferably 0.020%.
However, since such effects do not increase no matter how
excessively W may be added, its upper limit, from the viewpoint of
economic efficiency, is defined as 0.5%, and preferably 0.3%.
Cu: 0.03-0.5%
[0041] Cu is an effective element for improving corrosion
resistance, so it may be added to the steel sheet of the invention
if needed. The lower limit of the Cu content is defined as 0.03%,
preferably 0.05%, and more preferably 0.08%. However, since such
effect does not increase no matter how excessively Cu may be added,
its upper limit, from the viewpoint of economic efficiency, is
defined as 0.5%, and preferably 0.3%.
Ni: 0.015-0.5%
[0042] In the case that the steel sheet contains Cu, it is desired
to add Ni in order to prevent cracks. It is not absolutely required
to add both Cu and Ni, and even if the steel sheet already contains
Cu, Ni is not required to be added. The ratio of Ni mass% in the
steel sheet to Cu mass % in the steel sheet is preferably about
0.5. Therefore, the lower limit of the Ni content is defined as
0.015%, which is 1/2 of the lower limit of the Cu content. However,
since the benefit of the Ni addition does not increase simply by
adding Cu excessively more than the Cu mass %, the upper limit of
the Ni content is defined as the same as the Cu, namely, 0.5%, and
preferably 0.3%.
Ca: 0.0005-0.005%
[0043] Ca is an effective element for improving workability of the
steel sheet by enhancing the shape of inclusions, so it may be
added to the steel sheet if needed. The lower limit of the Ca
content is defined as 0.0005%, preferably 0.0010%, and more
preferably 0.0020%. However, such effect does not increase despite
an excessive amount of Ca present, and oxide based inclusions are
produced more, which only deteriorates workability. Therefore, the
upper limit of the Ca content is defined as 0.005%, and preferably
0.003%.
Rare Earth Elements: 0.0005-0.005%
[0044] Similar to Ca, rare earth elements (also known as REM) are
effective for improving workability of the steel sheet by enhancing
the shape of inclusions, so they may be added to the steel sheet if
needed. The lower limit of the REM content is defined as 0.0005%,
preferably 0.0010%, and more preferably 0.0020%. Like Ca, such
effect does not increase despite an excessive amount of REM
present, and only workability of the steel sheet is deteriorated.
Therefore, the upper limit of the REM content is defined as 0.005%,
and preferably 0.003%.
Zr and/or Mg: 0.0005-0.005% in Total
[0045] Both Zr and Mg contribute to dispersion of inclusions and
size reduction of TiN, and are effective elements for improving
workability of the steel sheet so they may be added if needed. The
lower limit of the total amount is defined as 0.0005%, and
preferably 0.0010%. However, production of oxide based inclusions
is increased and workability of the steel sheet is degraded if Zr
and Mg are added excessively. Therefore, the upper limit of the
total amount is defined as 0.005%, and preferably 0.0030%.
Co: 0.03-1%
[0046] Co is an effective element for reducing an amount of solid
fused carbon in the ferrite and for improving workability of the
steel sheet, particularly, elongation of the steel sheet so it may
be added to the steel sheet if needed. The lower limit of the Co
content is defined as 0.03%, and preferably 0.05%. However, since
such effects do not increase no matter how excessively Co may be
added, its upper limit, from the viewpoint of economic efficiency,
is defined as 1%, and preferably 0.80%.
[0047] The steel sheet of the present invention is characterized by
its chemical composition, especially, Si, Mn and O contents and the
thickness of the inner oxide layer. In effect, in the present
invention, there is no specific limit to the thickness of the steel
sheet itself. However, if the steel sheet is too thin, it may be
melted away in the high-speed arc welding. On the other hand,
increasing the thickness of the steel sheet contracts the demand of
light steel sheets. Thus, a desirable thickness of the steel sheet
ranges 1 to 5 mm.
EXAMPLES
[0048] Although preferred examples of the present invention are
disclosed and described in detail below, it is to be understood
that the invention is not limited thereto, and the invention can be
modified to a certain extent as long as modifications are in
accordance with the objectives of the invention described before
and hereinafter and those modifications are included within the
technical scope of the invention.
1. Fabrication of Steel Sheet
[0049] A molten steel decarburized in a converter was secondarily
refined in a LF (ladle refining furnace), and mixed with alloy
elements and subjected to the RH degassing treatment in an RH
degassing apparatus to prepare the molten steel having the chemical
composition listed in Table 1. Only the molten steels corresponding
to the steel sheet No. 109 having high O content (O: 0.0035%) was
subjected to the RH degassing treatment for 30 minutes (average
treatment time), and the others went through the RH degassing
treatment for 50 minutes and the O content therein was reduced to
0.002% or less.
[0050] Then, continuous casting was performed on the steel, and a
slab thusly obtained was subjected to a hot scarfing treatment,
heated at 1,200-1,250.degree. C. in a furnace, subjected to
descaling using high pressure water, and rolled by a roughing mill
to a thickness of 40 mm. Later, the slab temperature was controlled
to 1,000-1,100.degree. C., and the slab was again descaled under
high pressure water and subjected to finish rolling. For the finish
rolling, the slab was continuously rolled for 7 stands, and the
rolling temperature was controlled so that the temperature on the
finish exit side was in a range of 850 to 900.degree. C.
[0051] 2-4 seconds later, the sheet from the finish roller was then
supercooled (water cooling) at a cooling rate of 20-60.degree.
C./sec, and carried to a coiler to be wound at a winding
temperature specified in Table 2 (indicated as "CT" in Table 2).
When the winding temperature was higher than 600.degree. C. (Steel
sheet Nos. 112 and 113), the thickness of their inner oxide layers
(indicated as "Oxide layer" in Table 2) exceeded 5 .mu.m.
[0052] The wound coils were put aside and cooled at ambient
pressure down to 100.degree. C. or below, and passed through the
skin pass and leveler to form cracks on the scales (elongation
ratio: 0.3-0.5%). Then, the coils were pickled by hydrochloric acid
to remove scales therefrom, and coated with rustproofing oil to
fabricate steel sheet Nos. 1-23 (examples of the invention) and
steel sheet Nos. 101-113 (comparative examples).
TABLE-US-00001 TABLE 1 Steel sheet Si Mn Si + Mn O C P S Al N
Others 1 0.25 1.30 1.55 0.0012 0.02 0.080 0.0020 0.035 0.0020 Cu:
0.30, Ni: 0.30 2 0.25 1.30 1.55 0.0014 0.06 0.018 0.0020 0.040
0.0020 -- 3 0.80 1.50 2.30 0.0017 0.07 0.018 0.0200 0.035 0.0020
Ti: 0.015 4 0.50 1.50 2.00 0.0012 0.06 0.010 0.0010 0.035 0.0020
Nb: 0.030, Ca: 0.0020 5 0.60 1.30 1.90 0.0015 0.03 0.080 0.0010
0.035 0.0020 Cu: 0.20, Ni: 0.10, Nb: 0.025, Ca: 0.0015 6 1.10 1.10
2.20 0.0011 0.07 0.010 0.0030 0.035 0.0019 Cu: 0.30, Ni: 0.20,
Cr0.20, Ca: 0.0012 7 1.50 1.10 2.60 0.0018 0.11 0.010 0.0010 0.035
0.0020 Cr: 0.10, Ca: 0.0020 8 0.90 1.65 2.55 0.0015 0.04 0.080
0.0010 0.035 0.0020 Cu: 0.30, Ni0.30, Ti: 0.150 9 1.00 1.40 2.40
0.0009 0.035 0.007 0.0015 0.030 0.0030 Ti: 0.145 10 0.20 1.50 1.70
0.0012 0.08 0.018 0.0030 0.025 0.0020 Nb: 0.050, Ti: 0.160 11 2.00
1.50 3.50 0.0017 0.13 0.100 0.0005 0.035 0.0036 Cr: 0.10 12 0.20
1.50 1.70 0.0019 0.22 0.010 0.0030 0.032 0.0050 Cu: 0.09, Cr: 0.30,
Ti: 0.030, B: 0.0027 13 0.20 1.30 1.50 0.0012 0.23 0.010 0.0035
0.035 0.0020 Cu: 0.08, Cr: 0.25, Ti: 0.030, B: 0.0003 14 0.20 2.20
2.40 0.0015 0.22 0.010 0.0030 0.035 0.0020 -- 15 1.00 1.40 2.40
0.0011 0.035 0.010 0.0015 0.180 0.0030 -- 16 1.50 1.00 2.50 0.0009
0.22 0.010 0.0030 0.032 0.0050 Cr: 1.30 17 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 Mo: 0.2 18 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 V: 0.015 19 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 W: 0.010 20 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 REM: 0.0010 21 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 Zr: 0.002 22 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 Co: 0.05 23 0.25 1.30 1.55 0.0014
0.06 0.018 0.0020 0.035 0.0020 Mg: 0.0005 101 0.50 1.30 1.80 0.0015
0.0030 0.080 0.0020 0.035 0.0020 -- 102 0.25 1.00 1.25 0.0008 0.06
0.018 0.0020 0.040 0.0020 -- 103 0.80 1.50 2.30 0.0014 0.07 0.018
0.0200 0.035 0.0060 Ti: 0.32 104 0.60 1.30 1.90 0.0019 0.45 0.080
0.0010 0.035 0.0020 Cu: 0.20, Ni: 0.10, Nb: 0.025, Ca: 0.0015 105
0.01 1.50 1.51 0.0013 0.11 0.010 0.0010 0.035 0.0020 Cr: 0.10, Ca:
0.0020 106 3.00 1.65 4.65 0.0011 0.04 0.080 0.0010 0.035 0.0020 Cu:
0.30, Ni0.30, Ti: 0.150 107 0.20 3.00 3.20 0.0011 0.23 0.010 0.0035
0.035 0.0020 Cu: 0.08, Cr: 0.25, Ti: 0.030, B: 0.0003 108 0.20 0.45
0.65 0.0018 0.22 0.010 0.0030 0.032 0.0050 Cr: 1.30 109 0.20 1.30
1.50 0.0035 0.035 0.005 0.0100 0.022 0.0045 -- 110 0.05 1.55 1.60
0.0009 0.10 0.009 0.0100 0.040 0.0030 -- 111 0.15 1.45 1.60 0.0012
0.08 0.012 0.0060 0.033 0.0028 -- 112 1.10 1.10 2.20 0.0011 0.07
0.010 0.0030 0.035 0.0030 -- 113 1.10 1.10 2.20 0.0011 0.07 0.010
0.0030 0.035 0.0030 -- The balance is essentially Fe and inevitably
impurities (Unit: mass %)
[0053] In addition, when the O content was about 0.002%, any of
conventionally employed methods can be used. In the present
invention, O contents were measured in order of ppm by
Combustion-Infrared Absorption Spectrometry.
2. Measurement of Thickness of Inner Oxide Layers of Steel
Sheets
[0054] Using the steel sheets thusly obtained, the cross sections
in perpendicular to the rolling direction were observed to the
depth of 50 .mu.m per each point on SEM images (magnifying power:
1000.times.), and the thickness of the respective inner oxide
layers was measured. The measurement results are shown in Table 2
(indicated as "Oxide layer" in Table 2). It should be noted that
the thicknesses of the inner oxide layers shown in Table 2 are
average values of 5 arbitrarily selected ones in the width
direction of each steel sheet.
3. Measurement of Mechanical Properties of Steel Sheets
[0055] As mechanical properties of each steel sheet, tensile
strength (MPa), elongation (%), and hole expansion ratio (%) were
measured, and their measurement results are shown in Table 2 (In
Table 2, tensile strength is indicated as "TS", elongation "El",
and hole expansion ratio ".lamda."). Here, the definition and test
method of hole expansion followed "Hole Expansion Test Method
JFST1001-1996", the standard of Japan Iron and Steel
Federation.
[0056] Tensile strength, elongation, and hole expansion ratio were
evaluated as good when they are 600 MPa or more, 15% or more, and
50% or more, respectively.
4. Welding of Steel Sheets
[0057] The welding conditions of the steel sheets were as
follows:
[0058] Sheet thickness of steel sheet: 2.9 mm.
[0059] Welder: Pulse welder,
[0060] 1 Joint form: lateral lap fillet welding,
[0061] Torch slope angle: movement angle: 45.degree., and operating
angle: 10.degree. for the forward movement,
[0062] Shielded gas composition: Ar: 80 vol %, and CO.sub.2: 20 vol
%
[0063] (In welding of the steel sheet No. 23, a shielded gas
composition was Ar: 90 vol %, and CO.sub.2: 10 vol %.)
[0064] Arc welding current
[0065] Current was adjusted in accordance with given welding speed
as follows. In addition, an optimal voltage was selected depending
on shielded gas compositions;
[0066] 80 cm/min: 210 A
[0067] 90 cm/min: 230 A
[0068] 100 cm/min: 250 A
[0069] 110 cm/min: 270 A
[0070] 120 cm/min: 290 A
[0071] 130 cm/min: 310 A
[0072] 140 cm/min: 330 A
[0073] 150 cm/min: 350 A
[0074] Welding wire: Copper plated wire
[0075] (In welding of the steel sheet No. 21, a non-copper plated
welding wire was used.),
[0076] Wire diameter: 1.2 mm,
[0077] Wire composition: C: 0.05% (the term "%" herein means "mass
%", the same is true hereinbelow), Si: 0.81%, Mn: 1.25%, P: 0.010%,
and S: 0.015%
[0078] (In welding of the steel sheet No. 22, the welding wire had
the composition of C: 0.06%, Si: 0.25%, Mn: 1.34%, P: 0.012%, S:
0.018%, Ti: 0.05%; and in welding of the steel sheet No. 23, the
welding wire had the composition of C: 0.06%, Si: 0.30%, Mn: 1.25%,
P: 0.007%, and S: 0.010%.), and
[0079] Extruded length (contact-to-work distance) of welding wire:
15 mm
5. Evaluation of Weldability of Steel Sheets
[0080] (1) Speed Limit
[0081] The welding speed was increased from 80 cm/min to 10 cm/min
for welding the steel sheets, and a speed limit that does not cause
humping defect was obtained. The results are shown in Table 2. A
steel sheet was evaluated to have a good high-speed weldability if
the speed limit thereof was 100 cm/min or higher.
[0082] (2) Bead Width
[0083] Bead width was measured when a steel sheet was welded at a
welding speed of 100 cm/min. The results are shown in Table 2. It
should be noted that the bead widths in Table 2 are average values
of 3 arbitrary selected bead widths of typical spots of welding. A
steel sheet was evaluated to have a good high-speed weldability if
the bead width thereof was 7.0 mm or more.
[0084] Fracture Position
[0085] Steel sheets were welded at a welding speed of 100 cm/min,
and test pieces of 25 mm in width were collected from their lap
fillet joints to be subjected to a joint tension test. It was
decided whether the fracture position was in the base material
(steel sheet) or the weld metal. A steel sheet was evaluated to
have a good joint strength if the base material was fractured
Moreover, although it was the weld metal that was fractured, if its
fracture strength was 600 MPa or greater (the target tensile
strength of steel sheets), the subject steel sheet was evaluated to
have a good joint strength.
TABLE-US-00002 TABLE 2-1 Oxide Speed Bead Steel CT layer TS EI
.lamda. limit width Fracture sheet (.degree. C.) (.mu.m) (MPa) (%)
TS .times. EI (%) (cm/min) (mm) position*.sup.1 1 500 0 720 23
16560 100 110 7.3 Base material 2 500 0 810 21 17010 90 110 7.2
Base material 3 500 0 820 22 18040 70 140 8.0 Base material 4 500 0
800 19 15200 85 130 7.7 Base material 5 500 0 770 24 18480 90 130
7.7 Base material 6 500 0 830 22 18260 70 140 8.0 Base material 7
500 0 800 18 14400 65 150 8.1 Base material 8 500 0 780 22 17160 80
140 7.9 Base material 9 500 0 775 22 17050 90 150 8.2 Base material
10 500 0 795 18 14310 70 100 7.1 Base matenal 11 500 0 850 17 14450
70 150 8.0 Base material 12 500 0 845 16 13520 60 100 7.0 Base
material 13 500 0 890 15 13350 55 100 7.2 825 14 500 0 880 17 14960
70 100 7.1 800 15 500 0 800 21 16800 80 150 8.1 Base material 16
500 0 900 15 13500 65 150 8.0 785 17 500 0 810 20 16200 70 110 7.4
Base material 18 500 0 800 21 16800 75 120 7.1 Base material 19 500
0 790 20 15800 70 110 7.2 Base material 20 500 0 780 20 15600 85
110 7.2 Base material 21*.sup.2 500 0 775 21 16275 70 110 7.2 Base
material 22*.sup.3 500 0 780 22 17160 85 110 7.3 Base material
23*.sup.4 500 0 765 19 14535 75 120 7.1 Base material *.sup.1If The
fracture position is in a weld metal not a base material (steel
sheet), the fracture strength at that time is stated (unit: MPa).
*.sup.2In welding of steel sheet No. 21, a non-copper plated
welding wire was used. *.sup.3In welding of steel sheet No. 22, a
welding wire having the composition of C: 0.06, Si: 0.25, Mn: 1.34,
P: 0.012, S: 0.018, Ti: 0.05 (Unit: mass %) was used. *.sup.4In
welding of steel sheet No. a welding wire having the composition
of: 0.06, Si: 0.30, Mn: 1.25, P: 0.007, and S: 0.010 (Unit: mass %)
and shielded gases of Ar: 90 and CO.sub.2: 10 (Unit: vol %) were
used.
TABLE-US-00003 TABLE 2-2 Oxide Speed Bead Steel CT layer TS EI
.lamda. limit width Fracture sheet (.degree. C.) (.mu.m) (MPa) (%)
TS .times. EI (%) (cm/min) (mm) position*.sup.1 101 500 0 470 30
14100 110 130 7.5 Base material 102 500 0 780 21 16380 80 100 6.0
Base material 103 500 0 795 19 15105 40 110 7.8 Base material
104*.sup.2 500 0 965 12 11580 35 130 7.5 805 105 500 0 775 21 16275
80 90 -- Base material 106*.sup.3 500 0 790 22 17380 75 90 -- Base
material 107 500 0 900 10 9000 35 100 7.2 770 108 500 0 510 15 7650
40 100 5.5 Base material 109 500 0 800 20 16000 70 90 -- Base
material 110 500 0 810 20 16200 70 90 -- Base material 111 550 0
810 19 15390 65 100 6.5 Base material 112 600 6 790 17 13430 85 100
6.0 Base material 113 650 8 820 18 14760 100 90 -- Base material
*.sup.1If the fracture position is in a weld metal and not a base
material (steel sheet), the fracture strength at that time is
stated (unit: MPa). *.sup.2In welding of steel sheet No. 104,
cracks were formed on the solid weld metal. *.sup.3In welding of
steel sheet No. 106, a lot of slags were formed on the bead.
[0086] As obvious from the results in Table 2, the steel sheet Nos.
1-23 satisfying the requirements of chemical composition and
thickness of the inner oxide layer had excellent high-speed
weldability, i.e., the arc welding speed limit of 100 cm/min or
higher and bead width of 7.0 mm or more at the arc welding speed of
100 cm/min, and exhibited good joint strengths (their fracture
positions were in base materials or the fracture strengths were 600
MPa or greater even though the weld metals were fractured.) In
addition, steel sheet Nos. 23 and 24 had tensile strengths of 600
MPa or greater, elongation rates of 15% or more, and hole expansion
ratios of 50% or more, proving that each has high strength and good
workability.
[0087] Steel sheet No. 101 had the C content below the lower limit
defined by the invention, and insufficient tensile strength of 471
MPa.
[0088] Steel sheet No. 102 had the content of Si+Mn below the lower
limit defined by the invention, narrow bead widths of 6.0 mm, and
unsatisfactory high-speed weldabilities.
[0089] Steel sheet No. 103 had the Ti content exceeding the upper
limit defined by the invention, and insufficient hole expansion
ratio (workability) of 40% due to an increase in inclusions.
[0090] Steel sheet No. 104 had the C content exceeding the upper
limit defined by the invention, and low workability (elongation and
hole expansion ratio). Also, cracks were formed on the solid weld
metal. This is because the excessive amount of C in the steel sheet
increased the C content in the weld metal as well.
[0091] Steel sheet No. 105 had the Si content below the lower limit
defined by the invention, was easily humped, had a low arc welding
speed limit of 90 cm/min, and exhibited unsatisfactory high-speed
weldability.
[0092] Steel sheet No. 106 had the Si content exceeding the upper
limit defined by the invention, and the coating came off easily due
to a great amount of slag formed on its beads. Also, it exhibited
deteriorated corrosion resistance. Since the Si content was as high
as 3.00%, the viscosity of molten pool was increased too much,
thereby causing humping.
[0093] Steel sheet No. 107 had the Mn content exceeding the upper
limit defined by the invention, and exhibited inferior workability
(elongation and hole expansion ratio).
[0094] Steel sheet No. 108 had the Mn content below the lower limit
defined by the invention and thin bead width of 5.5 mm, and
exhibited unsatisfactory high-speed weldability. Also, it had
inferior tensile strength of 510 MPa.
[0095] Steel sheet No. 109 had the O content exceeding the upper
limit defined by the invention, was easily humped due to the
excessively poor viscosity of molten pool, and exhibited the arc
welding speed limit of 90 cm/min which is inferior high-speed
weldability.
[0096] Steel sheet No. 110 had the Si content below the lower limit
defined by the invention, was easily humped during welding, and
exhibited the arc welding speed limit of 90 cm/min which is
inferior high-speed weldability.
[0097] Steel sheet No. 111 had the Si content below the lower limit
defined by the invention and thin bead width of 6.5 mm due to poor
viscosity of molten pool, and exhibited unsatisfactory high-speed
weldability.
[0098] Steel sheet No. 112 had a 6 .mu.m thick inner oxide layer,
thin bead width of 6.0 mm due to poor viscosity of molten pool, and
exhibited unsatisfactory high-speed weldability.
[0099] Steel sheet No. 113 had an 8 .mu.m thick inner oxide layer,
was easily humped due to poor viscosity of molten pool, and
exhibited the arc welding speed limit of 90 cm/min which is
inferior high-speed weldability.
[0100] Moreover, in Tables 2-1 and 2-2, the oxide layer "0" means
that no oxide layer was observed under given method.
[0101] Although the preferred embodiment of the present invention
has been described, it will be understood by those skilled in the
art that the present invention should not be limited to the
described preferred embodiment, but various changes and
modifications can be made within the spirit and scope of the
present invention as defined by the appended claims.
* * * * *