[Document Type] Specification
[Title of the Invention] STEEL SHEET
[Technical Field of the Invention]
[0001]
The present invention relates to a steel sheet (steel sheet for hot forming)
which is appropriate for applications^ in which quenching is performed simultaneously
with hot forming or immediately after hot forming, such as hot press. More
specifically, the present invention relates to a steel sheet for hot forming in which, for
example, even in a case where hot forming accompanied with high strain forming,
which is a forming process by which a formed portion receives a plastic strain of 20%
or higher, is performed, strain-induced ferritic transformation in the formed portion is
suppressed, and thus the hardness after the hot forming is uniform, resulting in
excellent toughness and low anisotropy in toughness after the hot forming.
Priority is claimed on Japanese Patent Application No. 2012-187959, filed on
August 28, 2012, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Recently, in the field of steel sheets used for vehicles, in order to enhance the
fuel efficiency or impact resistance of a vehicle, applications of a high strength steel
sheet having a high tensile strength have increased. In general, as the strength of the
steel sheet increases, press formability decreases. Therefore, depending on the
application of the high strength steel sheet, it is difficult to manufacture a product
having a complex shape. Specifically, since ductility decreases as the strength of the
steel sheet increases, breaking may occur from a region which is heavily worked, or a
degree of spring back or wall camber is increased as the strength of the steel sheet
- 1 -
increases. As a result, there is a problem of deterioration in the dimensional acciuacy
of a worked member and the like. Accordingly, it is not easy to manufacture a
product having a complex shape by press forming using a steel sheet having a high
strength, particularly, having a tensile strength of 780 MPa or higher.
[0003]
When roll forming is performed as the forming instead of the press forming, a
high strength steel sheet can be worked to a certain degree. However, roll forming
has limitations in that it can be applied only to a member having a uniform crosssection
in the longitudinal direction, and thus the degree of freedom of a member
configuration is significantly limited.
[0004]
Here, as a teclinique of press-forming a hardly press formable material such as
a high strength steel sheet, for example, in Patent Document 1, a hot forming teclinique
of performing forming after heating a material to be formed (for example, hot pressing)
is disclosed. This technique is a technique of performing quenching simultaneously
with or immediately after forming on a steel sheet which is soft before the forming
such that a formed member having a high strength is obtained through the quenching
performed after the forming while good formability is secured during the forming.
According to this teclinique, a structure mainly including martensite can be obtained
after the quenching, and thus a formed member having excellent local deformability
and toughness can be obtained compared to a case of using a high strength steel sheet
having a dual-phase structure.
[0005]
Currently, hot pressing as described above is being applied to a member
having a relatively simple shape, and in the future, application thereof to a member on
- 2 -
which more difficult forming such as burring is performed is expected. However,
when the hot pressing is applied to a member on which more difficult forming is
performed, there is concern that strain-induced ferritic transformation may occur in a
high strain formed portion and thus the hardness of the member after the hot forming
may be locally reduced.
[0006]
In order to suppress the strain-induced ferritic transformation, the hot forming
may be performed in a higher temperature range. However, an increase in the hot
forming temperature causes a reduction in productivity, an increase in manufacturing
cost, the deterioration of surface property, and the like and thus is not easily applied to
mass production technology. For example, in Patent Document 1, a technique of
performing press work at 850°C or higher is described. However, in actual hot
pressing, there may be a case where temperature of the steel sheet decreases to 850°C
or less while the steel sheet which is heated to about 900°C in a heating furnace is
extracted from the heating furnace and is then transported to and inserted into a press
machine. In this case, it is difficult to suppress the strain-induced ferritic
transformation during the forming.
[0007]
From the viewpoint of increasing the productivity of hot pressing and
increasing the material stability in a member after the forming, in Patent Document 2, a
method of manufacturing a hot-pressed high strength steel member having excellent
productivity, in which a process of cooling a material by heat dissipation from a press
die can be omitted, is disclosed. The method disclosed in Patent Document 2 is an
excellent; however, it is necessary that a large amount of elements having an action of
enhancing hardenability such as Mn, Cr, Cu, and Ni is contained in steel. Therefore,
- 3 -
the technique disclosed in Patent Document 2 has a problem of an increase in cost. In
addition, in the member manufactured by using the technique disclosed in Patent
Document 2, there is concern that deterioration in toughness due to various inclusions
being present and the anisotropy in toughness caused by inclusions (mainly, MnS) that
are stretched in the rolling direction will occur. The actual performance of the
member is constrained by the properties on the low toughness side, and thus original
base metal properties cannot be sufficiently exhibited when there is anisotropy in
toughness. The anisotropy in toughness can be reduced by performing morphology
control on the stretched inclusions with a Ca treatment described, for example, in
Patent Document 3. However, in this case, the toughness value is enhanced in a
direction in which the toughness is lowest. However, the amount of inclusions in the
member is increased and thus there is a problem in that toughness values in the other
directions are reduced.
[Prior Art Document]
[Patent Document]
[0008]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2002-102980
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2006-213959
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2009-242910
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
- 4 -
As described above, in the related ait, the applications of the hot pressing
remain in members having a relatively simple shape. Therefore, technical problems
of a local reduction in the hardness, the anisotropy in the toughness, and a reduction in
the toughness value of the member after the hot forming (a steel sheet subjected to the
hot forming process) caused by the strain-induced ferritic transformation in a high
strain formed portion, which occur in a case where a member on which more difficult
forming such as burring is performed, have not been examined.
[0010]
An object of the present invention is to provide a steel sheet for hot forming in
which, even in the case of the above-described problems, that is, even in the case
where hot forming accompanied with high strain forming is performed, strain-induced
ferritic transformation in a formed portion is suppressed, and thus the hardness after
the hot forming is uniform (a difference in hardness is small), resulting in excellent
toughness and low anisotropy in toughness after the hot forming.
[Means for Solving the Problem]
[0011]
The inventors conducted diligent research in order to solve the abovedescribed
problems.
As a result, it was found that, by controlling the chemical composition of a
steel sheet, the amount of inclusions, and center segregation, a steel sheet for hot
forming in which, even in a case where hot forming accompanied with high strain
forming is performed, strain-induced fenitic transformation is suppressed, and thus the
hardness after the hot forming is uniform, resulting in excellent toughness and low
anisotropy in toughness after the hot forming can be obtained, In addition, in the
following description, there may be a case where uniform hardness is referred to as
- 5 -
stable hardness distribution.
[0012]
The summary of the present invention based on the new findings is as follows.
(1) A steel sheet according to an aspect of the present invention includes as a
chemical composition, by mass%: C : 0.18% to 0.275%; Si: 0.02% to 0.15%; Mn :
1.85% to 2.75%; sol.Al : 0.0002% to 0.5%; Cr : 0.05% to 1.00%; B : 0.0005% to
0.01%; P : 0.1% or less; S : 0.0035% or less; N : 0.01% or less; Ni : 0% to 0.15%; Cu :
0% to 0.05%; Ti : 0% to 0.1%; Nb : 0% to 0.2%; and a remainder including Fe and
impurities, in which a cleanliness of a metal structure is 0.08% or less, a which is an
Mn segregation degree expressed by the following expression a is 1.6 or less, and in a
hot forming, a difference AHv in an average hardness after the hot forming between a
low strain formed portion that undergoes a plastic strain of 5% or less and a high strain
formed portion that undergoes a plastic strain of 20% or higher is 40 or less.
a = (a maximum Mn concentration by mass% in a thickness center portion of
the steel sheet) / (an average Mn concentration by mass% at a position at a depth of 1/4
of a sheet thickness from a surface of the steel sheet): expression a
[0013]
(2) In the steel sheet described in (1), the chemical composition may further
include, instead of a portion of Fe, by mass%, one or two selected from the group
consisting of Ni: 0.02% to 0.15%, and Cu : 0.003% to 0.05%.
[0014]
(3) In the steel sheet described in (1) or (2), the chemical composition may
further include, instead of a portion of Fe, by mass%, one or two selected from the
group consisting of Ti: 0.005% to 0.1%, and Nb : 0.005% to 0.2%.
[0015]
- 6 -
(4) In the steel sheet described in any one of (1) to (3), the surface of the steel
sheet may further include a coated layer.
[Effects of the Invention]
[0016]
According to the aspect of the present invention, even in a case where the hot
forming accompanied with high strain forming such as burring is performed, straininduced
ferritic transformation in the formed portion is suppressed, and thus a steel
sheet having a stable hardness distribution after the hot forming, and excellent
toughness and low anisotropy in toughness after the hot forming, can be obtained.
The steel sheet is appropriate for, for example, a material of a mechanical structure
member including a body structure member, an underbody member, and the like of a
vehicle, and thus the present invention is very useful in industrial fields.
In addition, the hot forming may be performed according to a routine method.
For example, a steel sheet material may be heated to a temperature of an AC3 point or
higher (about 800°C) and the AC3 point + 200°C or less, may be held for 0 second or
longer and 600 seconds or less, may be transported to a press machine and is then
press-formed, and may be held for 5 seconds or longer at the bottom dead center of the
press machine. At this time, a heating method may be appropriately selected, and in a
case of rapid heating, electric heating or high-frequency heating may be performed.
In addition, as typical heating, furnace heating which is set to a heating temperature, or
the like may be used. Air cooling is performed during the transportation to the press
machine, and thus there is a possibility that, when the transportation time is lengthened,
ferritic transformation may occur until pressing is started and softening may occur.
Therefore, the transportation time is preferably 15 seconds or less. In order to prevent
an increase in die temperature, cooling of a die may be performed. In this case, as a
- 7 -
cooling method, appropriate cooling, such as a cooling method of installing cooling
piping in a die and supplying a refrigerant to flow therethough, may be performed.
[Embodiment of the Invention]
[0017]
Hereinafter, a steel sheet according to an embodiment of the present invention
(in some cases, referred to as a steel sheet according to this embodiment) will be
described in more detail. In the following description, % related to the chemical
composition of the steel sheet is mass%.
[0018]
1. Chemical Composition
(1)C: 0.18% to 0.275%
C is an important element for increasing the hardenability of steel,
determining strength after quenching, and further controlling local ductility and
toughness after hot forming. In addition, C is an austenite former, and thus has an
action of suppressing strain-induced ferritic transformation during high strain forming,
thereby facilitating obtaining a stable hardness distribution of a member after the hot
forming. However, when the C content is less than 0.18%, it is difficult to secure a
tensile strength of 1100 MPa or higher, which is a preferable strength after the
quenching, and an effect of obtaining a stable hardness distribution by the above action
cannot be obtained. On the other hand, when the C content is higher than 0.275%,
local ductility and toughness are reduced. Therefore, the C content is 0.18% to
0.275%. The preferable upper limit of the C content is 0.26%, and the more
preferable upper limit thereof is 0.24%.
[0019]
(2) Si: 0.02% to 0.15%
- 8 -
Si is an element which increases hardenability and enhances scale adhesion
after hot forming. However, when the Si content is less than 0.02%, there may be a
case where the above-described effect cannot be sufficiently obtained. Therefore, the
lower limit of the Si content is 0.02%. The preferable lower limit thereof is 0.03%.
On the other hand, when the Si content is higher than 0.15%, a heating temperature
necessary for austenitic transformation during the hot forming is significantly high.
Therefore, there may be a case where the cost necessary for a heat treatment is
increased or quenching is insufficiently performed due to insufficient heating. In
addition, Si is a ferrite forming element. Accordingly, when the Si content is too high,
strain-induced ferritic transformation is likely to occur during high strain forming.
Therefore, there may be a case where the hardness of a member after the hot forming is
locally reduced, and thus a stable hardness distribution is not obtained. Furthermore,
there may be a case where a large amount of Si being contained causes a reduction in
wettability in a case of performing a hot dip coating treatment, resulting in non-coated
parts. Therefore, the upper limit of the Si content is 0.15%.
[0020]
(3)Mn: 1.85% to 2.75%
Mn is an element effective in increasing the hardenability of steel and stably
securing the strength of the steel after the quenching. In addition, Mn is an austenite
former, and thus suppresses strain-induced ferritic transformation during high strain
forming, thereby facilitating obtaining a stable hardness distribution of a member after
hot forming. However, when the Mn content is less than 1.85%, there may be a case
where the above-described effect cannot be sufficiently obtained. Therefore, the
lower limit of the Mn content is 1.85%. On the other hand, when the Mn content is
higher than 2.75%, the above-described effect is saturated, and deterioration in
- 9 -
toughness after the quenching is caused. Accordingly, the upper limit of the Mn
content is 2.75%. The preferable upper limit of the Mn content is 2.5%.
[0021]
(4) soLAl: 0.0002% to 0.5%
Al is an element which deoxidizes molten steel and thus improves the
soundness of the steel. When the sol.Al content is less than 0.0002%, deoxidation is
insufficiently performed. Accordingly, the lower limit of the sol.Al content is
0.0002%. Furthermore, Al is also an effective element in increasing the hardenability
of a steel sheet and stably securing strength after quenching, and thus may be actively
contained. However, even when the Al content is higher than 0.5%, the effect is
saturated, and an increase in cost is caused. Therefore, the upper limit of the Al
content is 0.5%.
Sol.Al indicates acid-soluble Al, and the sol.Al content does not include the
amount of Al contained in AI2O3 and the like which is not dissolved in an acid.
[0022]
(5) Cr: 0.05% to 1.00%
Cr is an element which increases the hardenability of steel. In addition, Cr is
an austenite former, and thus suppresses strain-induced ferritic transformation during
high strain forming, thereby facilitating obtaining a stable hardness distribution of a
member after hot forming. However, when the Cr content is less than 0.05%, there
may be a case where the above-described effect cannot be sufficiently obtained.
Therefore, the lower limit of the Cr content is 0.05%. The preferable lower limit
thereof is 0.1%, and the more preferable lower limit thereof is 0.2%. On the other
hand, when the Cr content is higher than 1.00%, Cr is concentrated in carbides in the
steel. As a result, when the steel is provided in the hot forming, solutionizing of
- 10 -
carbides during a heating process is delayed, and hardenability is reduced.
Accordingly, the upper limit of the Cr content is 1.00%. The preferable upper limit of
the Cr content is 0.8%.
[0023]
(6) B: 0.0005% to 0.01%
B is an element effective in increasing the hardenability of steel and stably
securing strength after quenching. When the B content is less than 0.0005%, there
may be a case where the above-described effect cannot be sufficiently obtained.
Accordingly, the lower limit of the B content is 0.0005%. On the other hand, when
the B content is higher than 0.01%, the effect is saturated, and the deterioration in the
toughness of a quenched portion is caused. Therefore, the upper limit of the B
content is 0.01%. The preferable upper limit of the B content is 0.005%.
[0024]
(7) P: 0.1% or less
P is an element which is generally contained as an impurity. However, P has
an action of increasing the hardenability of steel and stably securing the strength of the
steel after the quenching, and thus may be actively contained. However, when the P
content is higher than 0.1 %, toughness is significantly deteriorated. Accordingly, the
P content is limited to 0.1%. The preferable upper limit of the P content is 0.05%.
The lower limit of the P content does not need to be particularly limited, but an
excessive reduction in the P content causes a significant increase in cost. Therefore,
the lower limit of the P content may be 0.0002%.
[0025]
(8) S: 0.0035% or less
S is an element which is contained as an impurity. In addition, particularly, S
- 11 -
forms MnS, and thus is a main factor in the reduction in toughness and the anisotropy
in toughness. When the S content is higher than 0.0035%, the deterioration in
toughness becomes significant, and thus the S content is limited to 0.0035%. The
lower limit of the S content does not need to be particularly limited, but an excessive
reduction in the S content causes a significant increase in cost. Therefore, the lower
limit of the S content may be 0.0002%.
[0026]
(9) N: 0.01% or less
N is an element which is contained as an impurity. When the N content is
higher than 0.01%, coarse nitrides are formed in steel and local deformability and
toughness are significantly deteriorated. Accordingly, the N content is limited to
0.01%. The lower limit of the N content does not need to be particularly limited, but
an excessive reduction in the N content causes a significant increase in cost.
Therefore, the lower limit of the N content may be 0.0002%. The preferable lower
limit of the N content is 0.0008% or higher.
[0027]
In addition to the above-mentioned elements, the steel sheet according to this
embodiment may contain arbitrary elements described below. Such elements are not
necessarily contained therein. Therefore, the lower limits of the amounts thereof are
not particularly Hmited, and the lower limits thereof are 0%.
[0028]
(10) Ni: 0.15% or less, Cu: 0.05% or less
Ni and Cu are elements effective in increasing the hardenability of steel and
stably securing strength after quenching. Therefore, one or two of the elements may
be contained. However, even when an amount of any of the elements higher than the
- 12 -
upper limit is contained, the above-described effect is saturated, which is
disadvantageous in terms of cost. Accordingly, the amount of each of the elements is
set as described above. Preferably, the Ni content is 0.10% or less, and the Cu
content is 0.03% or less. In order to more reliably obtain the above-described effect,
it is preferable that one or two selected from the group consisting of Ni: 0.02% or
higher and Cu: 0.003% or higher are contained.
[0029]
(11) Ti: 0.1% or less, Nb: 0.2 or less
Ti and Nb are elements which suppress recrystallization and further suppress
grain growth by forming fine carbides, thereby forming fine austenite grains when a
steel sheet is heated to an AC3 point or higher and is provided for hot forming. When
fine austenite grains are formed, the toughness of a hot-formed member is significantly
improved. In addition, Ti is primarily bonded to N in steel to generate TiN, and
thereby the consumption of B due to precipitation of BN is suppressed. As a result,
by including Ti, hardenability through B can be increased. In order to obtain the
above-described effect, one or two of the elements may be contained. When a higher
amount of any of the elements than the upper limit is contained, the precipitation
amount of TiC orNbC is increased and thus C is consumed, therefore, there may be a
case where strength after quenching is reduced. Accordingly, the amount of each of
the elements is set as described above. Preferably, the upper limit of the Ti content is
0.08%, and the upper limit of the Nb content is 0.15%. In addition, in order to more
reliably obtain the above-described effect, it is preferable that one or two selected from
the group consisting of Ti: 0.005% or higher and Nb: 0.005% or higher are contained.
[0030]
The remainder excluding the above-described components includes Fe and an
- 13 -
impurity. The impurity indicates a raw material such as ore or scrap, or a material
incorporated from a manufacturing environment.
The steel sheet according to the present invention may be any of a hot-rolled
steel sheet and a cold-rolled steel sheet, and may be an amiealed hot-rolled steel sheet
or an annealed cold-rolled steel sheet which is obtained by performing annealing on
the hot-rolled steel sheet or the cold-rolled steel sheet.
[0031]
2. Metal Structure
(1) Cleanliness: 0.08% or less
Cleanliness in this embodiment is defined as the sum of the amounts of A
series, B series, and C series inclusions contained in a steel sheet, which are obtained
by an aritlimetic calculation specified in JIS G 0555. When the amounts of inclusions
are increased, crack propagation easily occurs, resulting in the deterioration in
toughness and an increase in the degree of anisotropy in toughness. Therefore, the
upper limit of the cleanliness is 0.08%. The preferable upper limit thereof is 0.04%.
In the steel sheet according to this embodiment, MnS which is the A series inclusion is
a main factor of deterioration of the degree of anisotropy in toughness. Therefore,
particularly, the amount of A series inclusion is preferably 0.06% or less. More
preferably, the amount of A series inclusion is 0.03% or less.
In addition, the cleanliness is preferably as low as possible. However, from
the viewpoint of cost, the lower limit thereof may be 0.003% or 0.005%.
[0032]
(2) Mn Segregation Degree a: 1.6 or less
Mn is likely to segregate to the vicinity of a thickness center portion of a steel
sheet during casting. In a case where center segregation significantly occurs,
- 14 -
inclusions such as MnS are concentrated on a segregated portion, resulting in a
reduction in toughness and an increase in the degree of anisotropy in toughness.
Furthermore, martensite generated in the segregated portion during quenching is hard,
and thus the toughness is deteriorated. In addition, due to the interaction between Mn
and P, P segregation is also increased in degree in the Mn segregated portion, which
also causes the deterioration in toughness. Therefore, an Mn segregation degree a
expressed by the following expression 1 is 1.6 or less. The Mn segregation degree a
is preferably approximately 1.0 (that is, segregation does not occur). However, from
the viewpoint of cost, the lower limit thereof may be 1.03 or 1.05.
a = (the maximum Mn concentration (mass%) in a thickness center portion) /
(the average Mn concentration (mass%) at a position at a depth of 1/4 of the sheet
thickness from the surface) ... (expression 1)
[0033]
3. Coated Layer
A coated layer may be formed on the surface of the steel sheet for hot forming
according to the present invention for the purpose of enhancing corrosion resistance
and the like, and obtaining a surface-treated steel sheet. Even when the coated layer
is provided, the effect of this embodiment is not reduced. The coated layer may be an
electro coated layer, or may be a hot dip coated layer. As the electro coated layer, an
electrolytic zinc-coated layer, an electrolytic Zn-Ni alloy coated layer, and the like may
be exemplified. As the hot dip coated layer, a hot-dip galvanized layer, a
galvannealed layer, a hot-dip aluminium-coated layer, a hot-dip Zn-Al alloy coated
layer, a hot-dip Zn-Al-Mg alloy coated layer, a hot-dip Zn-Al-Mg-Si alloy coated layer,
and the like may be exemplified. A coated amount is not particularly limited, and
may be in a general range.
- 15 -
[0034]
4. Manufacturing Method
Next, a representative method of manufacturing the steel sheet for hot forming
according to the present invention will be described. By using the manufacturing
method including the following processes, the steel sheet according to this embodiment
can be easily obtained.
[0035]
(1) Continuous Casting Process (SI)
Molten steel having the above-described chemical composition is casted into a
slab by a continuous casting method. In this continuous casting process, it is
preferable that the molten steel temperature is higher than a liquidus temperature by
5°C or greater, the amount of molten steel being poured per unit time is 6 ton/min or
less, and a center segregation reduction treatment is performed before a cast piece
completely solidifies.
When the amount of the molten steel being poured per unit time (pouring rate)
of the molten steel during the continuous casting is greater than 6 ton/min, the molten
steel in a mold flows fast, and thus inclusions are easily trapped and the amount of the
inclusions in the slab is increased. When the molten steel temperature is higher than
the liquidus temperature by less than 5°C, the viscosity increases, and thus the
inclusions are less likely to float. Therefore, the amount of inclusions in the steel
increases, and the cleanliness is deteriorated (the value thereof is increased). When
the molten steel is continuously casted, it is more preferable that the temperature of the
molten steel is higher than the liquidus temperature by 8°C or greater, and the poured
amount is 5 ton/min or less.
As the center segregation reduction treatment, for example, by performing
- 16 -
electromagnetic stirring or unsolidified layer reduction on an unsolidified layer before
the cast piece completely solidifies, relieving or extraction of a concentrated portion
can be performed.
[0036]
(2) Slab Homogenization Treatment Process (S2)
As a segregation reduction treatment after the slab is completely solidified, a
slab homogenization treatment of heating the slab to 1150°C to 1350°C and holding
the resultant for 10 hours to 50 hours may farther be performed. By performing the
slab homogenization treatment under the above conditions, the segregation degree can
be further reduced. In addition, the preferable upper limit of the heating temperature
is 1300°C, and the preferable upper limit of the holding time is 30 hours.
[0037]
(3) Hot Rolling Process (S3), Cooling Process (S4), and Coiling Process (S5)
The slab obtained by performing the above-described continuous casting
process and the slab homogenization treatment process as necessary, is heated to
1050°C to 1350°C and is then hot-rolled into a steel sheet. The hot-rolled steel sheet
is held in the above temperature range for 5 seconds to 20 seconds. After being held,
the steel sheet is cooled to a temperature range of 400°C to 700°C by water cooling.
Thereafter, the cooled steel sheet is coiled.
[0038]
There may be a case where the slab contains nonmetallic inclusions which are
a cause of the deterioration in the toughness and the local deformability of a member
after quenching is performed on the steel sheet. Therefore, when the slab is provided
for the hot rolling, it is preferable that such nonmetallic inclusions are sufficiently
solutionized. Regarding the slab having the above-described chemical composition,
- 17 -
by heating the slab to 1050°C or higher to be provided for the hot rolling, soliitionizing
of the nonmetallic inclusions is accelerated. Accordingly, it is preferable that the
temperature of the slab provided for the hot rolling is 1050°C or higher. In addition,
the temperature of the slab provided for the hot rolling may be 1050°C or higher, and
the slab having a temperature of less than 1050°C may be heated to 1050°C or higher.
[0039]
In a case where transformation from worked austenite is allowed after finish
rolling, a rolled texture remains, which causes anisotropy in a final product.
Therefore, in order to allow transformation from recrystallized austenite to occur, it is
preferable that the steel sheet after being rolled is held for 5 seconds or longer in the
above temperature range. In order to hold the steel sheet for 5 seconds or longer in a
manufacturing line, for example, the steel sheet may be transported without being
water-cooled in a cooling zone after the finish rolling.
[0040]
By setting a coiling temperature to be 400°C or higher, a ferrite area ratio in
the metal structure can be increased. When the ferrite area ratio is high, the strength
of the hot-rolled steel sheet is suppressed, and thus load control, steel sheet flattening
control, and sheet thickness control are facilitated during cold rolling in a subsequent
process, resulting in an increase in manufacturing efficiency. Therefore, the coiling
temperature is preferably 400°C or higher.
[0041]
On the other hand, by setting the coiling temperature to be 700°C or less,
scale growth after the coiling is suppressed, and thus the generation of scale defects are
suppressed. In addition, the deformation of a coil due to the weight thereof after the
coiling is suppressed, and the generation of scratches on the coil surface due to the
- 18 -
deformation can be suppressed. Therefore, the coiling temperature is preferably
700°C or less. The deformation is caused by volume expansion due to the ferritic
transformation and subsequent thermal contraction, and disappearing the coiling
tension in the coil in a case where untransformed austenite remains after the coiling
and the untransformed austenite transforms into ferrite after the coiling.
[0042]
(4) Pickling Process (S6)
Pickling may be performed on the steel sheet after the coiling process. The
pickling may be performed according to a routine method. Before the pickling or
after the pickling, in order to accelerate flatness correction or scale exfoliation, skin
pass rolling may be performed, and this does not influence the effect of this
embodiment. An elongation rate in a case of performing the skin pass rolling does
not need to be particularly limited, and for example, may be 0.3% or higher and less
than 3.0%.
[0043]
(5) Cold Rolling Process (S7)
Cold rolling may be performed on the pickled steel sheet obtained through the
pickling process, as necessary. A cold roiling method may be performed according to
a routine method. The rolling reduction of the cold rolling may be in a typical range,
and is generally 30% to 80%.
[0044] .
(6) Annealing Process (S8)
Annealing at 700°C to 950°C can be performed on the hot-rolled steel sheet
obtained by the coiling process (S5) or the cold-rolled steel sheet obtained by the cold
rolling process (S7), as necessary.
- 19 -
[0045]
By performing the annealing of holding the hot-rolled steel sheet and the coldrolled
steel sheet within a temperature range of 700°C or higher, the effect of the hot
rolling conditions can be reduced, and thus further stabilization of properties after the
quenching can be achieved. In addition, regarding the cold-rolled steel sheet, the
steel sheet can be softened through recrystallization, and thus workability before the
hot forming can be improved. Therefore, in the case of performing the annealing on
the hot-rolled steel sheet or the cold-rolled steel sheet, it is preferable that the steel
sheet is held within a temperature range of 700°C or higher.
[0046]
On the other hand, by setting the annealing temperature to be 950°C or less,
the cost necessary for the annealing can be suppressed, and high productivity can be
secured. In addition, since the coarsening of the structure can be suppressed, better
toughness can be secured after the quenching. Therefore, in the case of performing
the annealing on the hot-rolled steel sheet or the cold-rolled steel sheet, it is preferable
that the steel sheet is held within a temperature range of 950°C or less.
[0047]
Cooling to 550°C after the annealing in the case of performing the annealing
is preferably performed at an average cooling rate of 3 °C/s to 20 °C/s. By setting the
average cooling rate to be 3 °C/s or higher, the generation of coarse pearlite or coarse
cementite can be suppressed, and thus properties after the quenching can be improved.
In addition, by setting the average cooling rate to be 20 °C/s or less, the stabilization of
the material is easily achieved.
[0048]
(7) Coating Process (S9)
- 20 -
In a case where a coated layer is formed on the surface of the steel sheet to
obtain a coated steel sheet, electro coating or hot dip coating may be performed
according to a routine method. In the case of the hot dip galvanizing, a continuous
hot dip galvanizing facility may be used and the annealing process and a subsequent
coating treatment may be performed in the facility. Otherwise, the coating treatment
may be performed independently from the annealing process. An alloying treatment
may further be performed in addition to the hot dip galvanizing for galvannealing. In
the case of performing the alloying treatment, an alloying treatment temperature is
preferably 480°C to 600°C. By setting the alloying treatment temperature to be
480°C or higher, unevenness in the alloying treatment can be suppressed. By setting
the alloying treatment temperature to be 600°C or less, manufacturing cost can be
suppressed, and high productivity can be secured. After the hot dip galvanizing, skin
pass rolling may be performed for flatness correction as necessary. The elongation
rate of the skin pass rolling may follow a routine method.
[0049]
The amount of inclusions and the segregation degree in the steel sheet are
mostly determined by the processes to the hot rolling and are not substantially changed
before and after the hot forming. Therefore, when the chemical composition, the
amount of inclusions (cleanliness), and the segregation degree of the steel sheet before
the hot forming satisfy the ranges of this embodiment, a hot-pressed member
manufactured by hot pressing performed thereafter also satisfies the ranges of this
embodiment.
Examples
[0050]
Steels having the chemical compositions shown in Table 1 were melted in a
- 21 -
converter for a test, and continuous casting was performed thereon in a continuous
casting machine for a test. As shown in Table 2, in the continuous casting process,
the pouring rate and the molten steel heating temperature difference (molten steel
temperature - liquidus temperature) were variously changed during the casting. In
addition, in a slab solidification procedure, electromagnetic stirring was performed.
Furthermore, in a final solidified slab portion, extraction of a center segregated portion
was performed by unsolidified layer reduction (extrusion) in which the interval
between a pair of upper and lower rolls in the continuous casting machine was
narrowed. For comparison, slabs on which electromagnetic stirring and/or extrusion
(center segregation reduction treatment) were not performed were partially produced.
Thereafter, a slab homogenization treatment was performed at 1300°C for 20 hours.
The slab homogenization treatment was omitted for some of the slabs. By using the
slabs produced as described above, hot rolling was performed, and then the resultants
were cooled and coiled to obtain hot-rolled steel sheets having a sheet thickness of 5.0
mm or 2.9 mm. As for the hot rolling conditions at this time, the heating temperature
of the slabs was 1250°C, the rolling start temperature was 1150°C, the rolling finish
temperature was 900°C, and the coiling temperature was 650°C. The hot rolling was
performed through multi-pass rollmg, and the holding for 10 seconds was performed
after finishing the rolling. Cooling after the hot rolling was performed by water
cooling. For comparison, parts thereof were not subjected to the holding.
In addition, regarding the pouring rates, a size of an actual production facility
is different from that of the continuous casting machine for a test used in this example.
Therefore, in Table 2, in consideration of size factors, a value which is converted into
the pouring rate in the actual production facility is described. In addition, the molten
steel heating temperature difference in Table 2 is a value obtained by subtracting a
- 22 -
liquidus temperature from a molten steel temperature.
[0051]
The obtained hot-rolled steel sheets were subjected to a pickling treatment
according to a routine method to obtain pickled steel sheets. The pickled steel sheets
having a sheet thickness of 5.0 mm were subjected to cold rolling to obtain cold-rolled
steel sheets having a sheet thickness of 2.9 mm. Parts of the hot-rolled steel sheets
were subjected to electro coating. Parts of the cold-rolled steel sheets were subjected
to recrystallization annealing (at an annealing temperature of 800°C for an annealing
time of 60 seconds) in a continuous annealing facility, and parts of the parts were
thereafter subjected to electrolytic zinc coating. Furthermore, parts of the hot-rolled
steel sheets and the cold-rolled steel sheets were subjected to annealing (at an
annealing temperature of 800°C for an annealing time of 60 seconds) and hot dip
galvanizing in a continuous hot dip galvanizing facility. The temperature of a hot dip
galvanizing bath was 460°C, and parts thereof were subjected to an alloying treatment
at 540°C for 20 seconds, thereby obtaining hot-dip galvanized steel sheets and
galvannealed steel sheets.
- 23 -
[0054]
Hot press forming was performed on the manufactured steel sheets as samples,
by using a hot pressing test apparatus. The steel sheets on which punching was
performed with a blank size of 150 mm square and a punching hole diameter of 36 mm
(clearance 10%) were heated in a heating furnace until the steel sheet surface
temperature had reached 900°C, were held at the temperature for 4 minutes, and were
extracted from the heating furnace. Thereafter, the steel sheets were cooled to 750°C
by air cooling, were subjected to hot burring at the time when the temperature had
reached 750°C, and were held for 1 minute at the bottom dead center of the press
machine. Hot burring conditions are as follows.
Punch shape : conical,
Punch diameter : 60 mm,
Press speed : 40 mm/s,
The cooling after the forming was performed by cooling the die, so that the
steel sheet was held for 1 minute at the bottom dead center.
[0055]
In the cross-section of the hot-pressed steel sheet which is parallel to the
rolling direction, the hardnesses of a burring portion (a high strain formed portion
which had undergone a plastic strain of 20% or higher) and a flange portion (a low
strain formed portion which had undergone a plastic strain amount of 5% or less) at the
positions at a depth of 1/4 of the sheet thickness in the cross-section were measured by
a Vickers hardness meter. The measuring load was 98 kN. A measuring method
was based on JIS Z 2244. The hardness measurement was performed a total of five
times while moving by a pitch of 200 um in the same thickness position. The average
value of the five Vickers hardness values obtained from each of the members was
- 27 -
obtained as an average hardness (Hv). The difference between the average hardness
of the burring portion and the average hardness of the flange portion (AHv = (flange
portion Hv) - (burring portion Hv)) was obtained, and a case where AHv was 40 or less
was determined as being acceptable. The examination results of the hardness are
shown in Table 3.
In addition, the amount of strain was obtained by measuring the sheet
thickness at each of the positions of the worked steel sheet and calculating the amount
of a reduction in the sheet thickness after the work from the sheet thickness before the
work.
[0056]
In addition, on the manufactured steel sheets as samples, an examination of a
toughness value (absolute value of toughness) and the anisotropy in toughness was
conducted.
The examination was conducted in the following manner. First, the steel
sheet having a sheet thickness of 2.9 mm was heated until the steel sheet surface
temperature had reached 900°C in the heating furnace, was held for 4 minutes at the
temperature, and was then extracted from the heating furnace. Next, the steel sheet
was cooled to 750°C by air cooling, was interposed between flat plate dies at the time
when the temperature had reached 750°C and was held for 1 minute. Thereafter, the
front and rear surfaces of the samples were ground to a thickness of 2.5 mm. Charpy
impact test samples were collected so that the longitudinal directions of the samples
were the rolling direction and a direction perpendicular to the rolling. At this time, a
notch was a V-notch at a depth of 2 mm. The impact test was performed on the basis
of JIS Z 2242 at room temperature as the test temperature. The ratio between an
impact value in the rolling direction (absorbed energy / cross-sectional area) and an
- 28 -
impact value in the direction perpendicular to the rolling was used as an index of the
anisotropy.
The results are shown in Table 3. As a result of the test, when the impact
value in the longitudinal rolling direction was 70 J/cm2 or more and the impact value
ratio was 0.65 or higher, good properties were determined.
[0057]
The cleanliness of the steel sheet was examined on the basis of JiS G 0555.
Samples were cut from the steel sheet of each of the Test Nos. at five points, and the
cleanliness of each of positions atl/8, 1/4, 1/2, 3/4, and 7/8 of the sheet thickness was
examined by a point counting method. Among the results at each of the sheet
thickness positions, a value having the highest cleanliness was determined as the
cleanliness of the sample. The cleanliness was the sum of the A series, B series, and
C series inclusions.
[0058]
The Mn segregation degree was obtained by performing component surface
analysis of Mn using an EPMA. Samples were cut from the steel sheet of each of the
Test Nos. at five points, 10 visual fields were measured at each of the positions at 1/4
and 1/2 of the sheet thickness with a magnification of 500 times, and the average value
of the Mn segregation degrees of each of the visual fields was employed.
- 29 -
[0060]
In all of the Test Nos. 16 to 19, 21, and 22, the average hardness of the burring
portion which was the high strain deformed portion was significantly reduced
compared to the average hardness of the flange portion which was the low strain
deformed portion, and the AHv values were increased to 41 to 99. This is because the
burring portion was softened by the strain-induced ferritic transformation caused by
the burring work. In this case, in the manufactured hot-formed product, the hardness
was locally different, and thus the strength of the formed product was not uniform but
was partially reduced. Therefore, reliability as a product was reduced.
In addition, in the Test Nos. 4, 8, 10, 12, 15, 18, 20, 23, and 24, the chemical
compositions, the cleanliness or the segregation degree were out of the ranges of the
present invention, and thus the impact value in the rolling direction and/or the impact
value ratio were insufficient.
[0061]
Contrary to this, in all of the steel sheets having the chemical composition of
the present invention, regardless of the presence or absence of the cold rolling process,
presence or absence of the annealing process, or the coating type, the AHv was -4 to 24,
the difference between the average hardness of the flange portion and the average
hardness of the burring portion was small, and the stability of hardness and strength
during the high strain forming was excellent.
In addition, the toughness after the hot rolling and the anisotropy in toughness
exhibited sufficient values.
[Industrial Applicability]
[0062]
In the steel sheet of the present invention, even in a case where hot forming
- 32 -
accompanied with high strain forming such as burring is performed, strain-induced
ferritic transformation in the formed portion is suppressed. Therefore, a steel sheet
having a stable hardness distribution after the hot forming, excellent toughness and low
anisotropy in toughness after the hot forming can be obtained. The steel sheet is
appropriate for, for example, a material of a mechanical structure member including a
body structure member, an underbody member, and the like of a vehicle, and thus the
present invention is very useful in industrial fields.
- 33 -
[Document Type] CLAIMS
1. A steel sheet comprising as a chemical composition, by mass%,
C: 0.18% to 0.275%;
Si: 0.02% to 0.15%;
Mn: 1.85% to 2.75%;
sol.Al: 0.0002% to 0.5%;
Cr: 0.05% to 1.00%;
B: 0.0005% to 0.01%;
P: 0.1% or less;
S: 0.0035% or less;
N: 0.01% or less;
Ni:0%to0.15%;
Cu:0%to0.05%;
Ti:0%to0.1%;
Nb : 0% to 0.2%; and
a remainder including Fe and impurities,
wherein a cleanliness of a metal structure is 0.08% or less,
a which is an Mn segregation degree expressed by the following expression 1
is 1.6 or less, and
in a hot forming, a difference AHv in an average hardness after the hot
forming between a low strain formed portion that undergoes a plastic strain of 5% or
less and a high strain formed portion that undergoes a plastic strain of 20% or higher is
40 or less.
a = (a maximum Mn concentration, by mass%, in a thickness center portion of
the steel sheet) / (an average Mn concentration, by mass%, at a position at a depth of
1/4 of a sheet thickness from a surface of the steel sheet) ... expression 1
2. The steel sheet according to claim 1, wherein the chemical composition
further includes, instead of a portion of Fe, by mass%, one or two selected from the
group consisting of Ni: 0.02% to 0.15%, and Cu : 0.003% to 0.05%.
3. The steel sheet according to claim 1 or 2, wherein the chemical
composition further includes, instead of a portion of Fe, by mass%, one or two selected
from the group consisting of Ti : 0.005% to 0.1%, andNb : 0.005% to 0.2%.
4. The steel sheet according to any of claims 1 to 3, wherein the surface of
the steel sheet further includes a coated layer.