Note: Descriptions are shown in the official language in which they were submitted.
031690E35 2022-08-12
DESCRIPTION
METHOD FOR PRODUCING STEEL COMPONENT HAVING LOCALLY SOFTENED
PART
Technical Field
[0001]
The present disclosure relates to a method for producing
a steel component having a locally softened part.
Background Art
[0002]
In recent years, there is a need for technology that
allows a specific part to become deformed preferentially
during an automobile collision while maintaining high
strength of a whole automobile frame component in order to
protect occupants during the collision. Therefore, a high-
strength steel component usable in this technology,
specifically, in which a specific part is locally softened,
and/or a production method thereof are required.
[0003]
Patent Document 1 discloses a method of applying a heat
shield cover to a part, which is to be intentionally softened
thereafter, when heating a steel sheet to the austenite
single-phase temperature range. Consequently, the
temperature of the part applied with the heat shield cover
remains under the austenite single-phase temperature range
during heating, which suppresses martensitic transformation
of the part after quenching, making this part softer than
other parts not applied with the heat shield cover.
[0004]
1
CA 03169085 2022-08-12
4
Patent Document 2 discloses a method of providing a part
where a steel sheet and a mold do not contact well when
quenching the steel sheet from the austenite single-phase
temperature range while being in contact with the mold.
Consequently, a soft microstructure (ferrite and/or pearlite)
precipitates in this part, and this part is softened.
Conventional Art Document
Patent Document
[0005]
Patent Document 1: JP 2017-78189 A
Patent Document 2: JP 2011-179028 A
Disclosure of the Invention
Problems to be Solved by the Invention
[0006]
In Patent Documents 1 and 2, it is not possible to
selectively soften only the part to be intentionally softened
due to heat transfer and the like in the steel sheet. For
example, in Patent Document 1, although only the part applied
with the heat shield cover is to be softened by being below
the austenite single-phase temperature range, heat is
transferred to the end of the part applied with the heat
shield cover from an adjacent part not applied with the heat
shield cover. As a result, the end of the part applied with
the heat shield cover cannot be softened sufficiently. In
Patent Document 2, although only the part that does not
contact the mold well is to be intentionally softened without
quenching, heat is transferred from this part to an adjacent
part that contacts the mold well. As a result, this adjacent
2
C A 03169085 2022-08-12
part in contact with the mold is susceptible to the softening
effect. Therefore, it is difficult to selectively soften
only the part to be intentionally softened by methods of
softening a steel sheet through local temperature control
such as the methods disclosed in Patent Documents 1 and 2.
[0007]
The embodiments of the present invention have been made
in view of such a situation, and an object thereof is to
provide a method for producing a high-strength steel
component having a locally softened part without local
temperature control.
Means for Solving the Problems
[0008]
The present invention according to a first aspect
provides a method for producing a steel component, which
includes the steps of:
preparing a steel sheet having a chemical composition
including:
C: 0.05 to 0.40% by mass,
Si: 0 to 2.0% by mass,
Mn: 1.0 to 3.0% by mass,
Al: 0.010 to 1.0% by mass,
P: more than 0% by mass and 0.100% by mass or less,
S: more than 0% by mass and 0.010% by mass or less,
N: more than 0% by mass and 0.010% by mass or less, and
B: 0.0005 to 0.010% by mass, with the balance being iron and
inevitable impurities;
heating the steel sheet to a temperature of Ad l point
( C) or higher and lower than Ac3 point ( C) 10 C;
3
031.6 9085 2422-08-12
A
after the heating step, processing the steel sheet by
applying a strain of 0.5% or more thereto at a processing
temperature of 675 C or higher and lower than Ac3 point ( C)
+ 10 C;
after the processing step, holding the steel sheet at
the processing temperature for 1 second or more and 120
seconds or less, or gradually cooling the steel sheet at an
average cooling rate of more than 0 C/sec and 15 C/sec or
less for 1 second or more and 120 seconds or less; and
after the holding or gradually cooling step, cooling the
steel sheet to a temperature of Ms point ( C) - 50 C,
wherein an average cooling rate from the temperature of
the heating step to the Ms point ( C) - 50 C is controlled to
be 10 C/sec or more.
[0009]
The prevent invention according to a second aspect
provides a method for producing a steel component, which
includes the steps of:
preparing a steel sheet having a chemical composition
including:
C: 0.05 to 0.40% by mass,
Si: 0 to 2.0% by mass,
Mn: 1.0 to 3.0% by mass,
Al: 0.010 to 1.0% by mass,
P: more than 0% by mass and 0.100% by mass or less,
S: more than 0% by mass and 0.010% by mass or less,
N: more than 0% by mass and 0.010% by mass or less, and
B: 0.0005 to 0.010% by mass, with the balance being iron and
inevitable impurities;
4
xCA 03169085 2022-08-12
heating the steel sheet to a temperature of Ac3 point
( C) + 10 C or higher and 1,100 C or lower;
after the heating step, processing the steel sheet by
applying a strain of 10% or more thereto at a processing
temperature of Ms point ( C) + 50 C or higher and lower than
Ac3 point ( C) + 10 C;
after the processing step, holding the steel sheet at
the processing temperature for 1 second or more and 120
seconds or less, or gradually cooling the steel sheet at an
average cooling rate of more than 0 C/sec and 15 C/sec or
less for 1 second or more and 120 seconds or less; and
after the holding or gradually cooling step, cooling the
steel sheet to a temperature of Ms point ( C) - 50 C,
wherein an average cooling rate from the temperature in
the heating step to the Ms point ( C) - 50 C is controlled to
be 10 C/sec or more.
[0010]
In a third aspect, the prevent invention provides the
production method according to the first or second aspect,
wherein the steel sheet further includes one or more selected
from the group consisting of:
Cu: more than 0% by mass and 0.50% by mass or less, and
Ni: more than 0% by mass and 0.50% by mass or less.
[0011]
In a fourth aspect, the prevent invention provides the
production method according to any one of the first to third
aspects, wherein the steel sheet further includes one or more
selected from the group consisting of:
Ti: more than 0% by mass and 0.10% by mass or less,
Cr: more than 0% by mass and 3.0% by mass or less, and
5
,CA 03169085 2022-08-12
=
Nb: more than 0% by mass and 0.10% by mass or less.
[0012]
In a fifth aspect, the present invention provides the
production method according to any one of the first to fourth
aspects, further including applying the strain by stretch
forming.
[0013]
In a sixth aspect, the present invention provides the
production method according to any one of the first to fourth
aspects, further including applying the strain by forging.
[0014]
In a seventh aspect, the present invention provides the
production method according to any one of the first to fourth
aspects, further including applying the strain by return
bending during draw forming.
[0015]
In an eighth aspect, the present invention provides the
production method according to any one of the first to fourth
aspects, further including applying the strain by shearing.
[0016]
In a ninth aspect, the present invention provides the
production method according to any one of the first to eighth
aspects, further including applying the strain by a plurality
of times of processing.
[0017]
In a tenth aspect, the present invention provides the
production method according to the ninth aspect, wherein the
plurality of times of processing includes processing for
applying deformation and processing for restoring the
deformation.
6
[0017a]
In yet another aspect, the present invention provides a
method for producing a steel component, which comprises the
steps of: preparing a steel sheet having a chemical
composition comprising: C: 0.05 to 0.40% by mass, Si: 0 to
2.0% by mass, Mn: 1.0 to 3.0% by mass, Al: 0.010 to 1.0% by
mass, P: more than 0% by mass and 0.100% by mass or less, S:
more than 0% by mass and 0.010% by mass or less, N: more than
0% by mass and 0.010% by mass or less, and B: 0.0005 to
0.010% by mass, with the balance being iron and inevitable
impurities; heating the steel sheet to a temperature of Ac3
point ( C) + 10 C or higher and 1,100 C or lower; after the
heating step, processing a part of the steel sheet by
applying a strain of 10% or more thereto at a processing
temperature of Ms point ( C) + 50 C or higher and lower than
Ac3 point ( C) + 10 C; after the processing step, holding the
steel sheet at the processing temperature for 1 second or
more and 120 seconds or less, or gradually cooling the steel
sheet at an average cooling rate of more than 0 C/sec and
15 C/sec or less for 1 second or more and 120 seconds or
less; and after the holding or gradually cooling step,
cooling the steel sheet to a temperature of Ms point ( C) -
50 C, wherein an average cooling rate from the temperature in
the heating step to the Ms point ( C) - 50 C is controlled to
be 10 C/sec or more.
Effects of the Invention
[0018]
According to an embodiment of the present invention, it
is possible to provide a method for producing a high-strength
7
CA 3169085 2024-01-09
=
steel component having a locally softened part without local
temperature control.
Brief Description of the Drawings
[0019]
FIG. 1 is a graph showing the relationship between the
temperature and displacement of a steel sheet when heating
the steel sheet from a low temperature in a formaster test.
FIG. 2 is a graph showing the relationship between the
temperature and displacement of the steel sheet when cooling
the steel sheet from high temperature in the formaster test,
in addition to the relationship shown in FIG. 1.
FIG. 3 is a schematic diagram showing the locations of
samples taken for evaluation in Examples.
FIG. 4 is a schematic cross-sectional view taken along
the line X-X shown in FIG. 3.
Mode for Carrying Out the Invention
[0020]
The inventors of the present application have made
various investigations in order to achieve a method for
producing a high-strength steel component having a locally
softened part without local temperature control.
[0021]
7a
CA 3169085 2024-01-09
,CA 031690,85 2022-08-1.2
As a result, it has been found that by heating a steel
sheet having a predetermined chemical composition to be in a
state where austenite is relatively unstable, such as in a
two-phase region composed of austenite and ferrite, a slight
strain is applied to a part which is to be intentionally
softened in the steel sheet, thus promoting nucleation of a
soft microstructure (ferrite and/or pearlite) only in the
part to be intentionally softened, and then the steel sheet
is held or gradually cooled for a certain time, allowing the
soft microstructure to grow in this part (hereinafter
referred to as first embodiment of the present invention).
[0022]
As a result, it has also been found at the same time
that even when heating a steel sheet in a state where
austenite is relatively stable, such as in an austenite
single-phase region, nucleation of a soft microstructure can
be promoted only in the part to be intentionally softened by
applying a relatively large strain to the part to be
intentionally softened, in the same manner as in the first
embodiment of the present invention (hereinafter referred to
as second embodiment of the present invention).
[0023]
Hereinafter, the details of requirements specified by
the first and second embodiments of the present invention
will be described. As used herein, the term "steel
component" refers to a steel sheet that has been processed
into a predetermined shape by the processing step in the
first and second embodiments of the present invention.
[0024]
<First Embodiment of The Present Invention>
8
CA 031.69085 2022-08-12
=
1
A production method according to the first embodiment of
the present invention includes the step of:
(a) preparing a steel sheet;
(b) after the step (a), heating;
(c) after the step (b), processing;
(d) after the step (c), holding or cooling gradually; and
(e) after the step (d), cooling.
Hereinafter, each step will be described.
[0025]
(a) Step of preparing steel sheet
The steel sheet according to the first embodiment of the
present invention includes: C: 0.05 to 0.40% by mass, Si: 0
to 2.0% by mass, Mn: 1.0 to 3.0% by mass, Al: 0.010 to 1.0%
by mass, P: more than 0% by mass and 0.100% by mass or less,
S: more than 0% by mass and 0.010% by mass or less, N: more
than 0% by mass and 0.010% by mass or less, and B: 0.0005 to
0.010% by mass, with the balance being iron and inevitable
impurities.
Hereinafter, each element will be described in detail.
[0026]
(C: 0.05 to 0.40% by mass)
The C content determines the strength of a steel
component. In order to obtain a sufficient strength of the
steel component, the C content is set at 0.05% by mass or
more, and is preferably 0.10% by mass or more, and more
preferably 0.20% by mass or more.
[0027]
Meanwhile, the excessive C content remarkably reduce the
toughness of a steel component and tends to cause delayed
fracture of the steel component. Thus, the C content is set
9
,CA 0316 9M45 2022-08-12
at 0.40% by mass or less, and is preferably 0.38% by mass or
less, and more preferably 0.36% by mass or less.
[0028]
(Si: 0 to 2.0% by mass)
Si is an element optionally present in the steel sheet.
Si contributes to the hardness stability of the steel sheet
by increasing the resistance to temper softening. Thus, Si
is preferably contained in an amount of more than 0% by mass
in the steel sheet.
[0029]
Meanwhile, Si facilitates the formation of residual
austenite (y) and contributes to a decrease in the yield
strength (YS) and to Mn segregation. Thus, the Si content is
set at 2.0% by mass or less, and is preferably 1.8% by mass
or less.
[0030]
(Mn: 1.0 to 3.0% by mass)
Mn contributes to an increase in the strength of a steel
component by enhancing the hardenability of the steel sheet.
To exhibit this effect, the Mn content is set at 1.0% by mass
or more, and is preferably 1.2% by mass or more, and more
preferably 1.4% by mass or more.
[0031]
Meanwhile, the excessive Mn content may cause coarse
carbides to precipitate in a steel component. Thus, the Mn
content is set at 3.0% by mass or less, and is preferably
2.8% by mass or less, and more preferably 2.6% by mass or
less.
[0032]
(Al: 0.010 to 1.0% by mass)
CA 03169085 2022-08-12
Al is an element that serves as a deoxidizing agent. To
exhibit this effect, the Al content is set at 0.010% by mass
or more. The Al content is preferably 0.020% by mass or
more, and more preferably 0.025% by mass or more. However,
the excessive Al content leads to an increase in production
costs and causes deterioration of surface quality
(decarburization and thinning) due to an increased heating
temperature of the material because Ac3 point is extremely
increased. Thus, the Al content is set at 1.0% by mass or
less. The Al content is preferably 0.60% by mass or less,
and more preferably 0.70% by mass or less.
[0033]
(P: more than 0% by mass and 0.100% by mass or less)
P is an inevitable element that degrades the weldability
of the steel sheet, but also has the effect of contributing
to the solute strengthening of a ferrite phase. To prevent
the degradation in the weldability of the steel sheet while
exhibiting such an effect, the P content is set at 0.100% by
mass or less. The P is preferably 0.050% by mass or less,
and more preferably 0.020% by mass or less. P is an impurity
trapped inevitably in steel, and it is impossible to suppress
its content to 0% by mass in terms of industrial production.
Thus, the P content can be usually more than 0% by mass, and
can further be 0.00050% by mass or more.
[0034]
(S: more than 0% by mass and 0.010% by mass or less)
S is an inevitable element that degrades the weldability
of the steel sheet. Therefore, the S content is set at
0.010% by mass or less. The S content is preferably 0.0080%
by mass or less, and more preferably 0.0050% by mass or less.
11
,CA03169,013520222
Since the S content should be as low as possible, the lower
limit of the S content is not particularly limited, but it is
impossible to set the S content to 0% by mass in terms of
industrial production, and the S content can usually be more
than 0% by mass, and even 0.00010% by mass or more.
[0035]
(N: more than 0% by mass and 0.010% by mass or less)
N is an inevitable element, and an excess N content
generates AlN, which reduces the deoxidizing effect of Al.
Therefore, the N content is set at 0.010% by mass or less.
The N content is preferably 0.0080% by mass or less, and more
preferably 0.0050% by mass or less. Since the N content
should be as low as possible, the lower limit of the N
content is not particularly limited, but it is impossible to
set the N content to 0% by mass in terms of industrial
production, and the N content can usually be more than 0% by
mass, and even 0.00010% by mass or more.
[0036]
(B: 0.0005 to 0.010% by mass)
B contributes to an increase in the strength of a steel
component by enhancing the hardenability of the steel sheet.
To exhibit this effect, the B content is set at 0.0005% by
mass or more, preferably 0.0010% by mass or more, and more
preferably 0.0015% by mass or more.
[0037]
Meanwhile, excessive B content results in the
precipitation of coarse iron boron compounds, reducing the
toughness of a steel component. Thus, the B content is set
at 0.010% by mass or less, and is preferably 0.0080% by mass
or less, and more preferably 0.0060% by mass or less.
12
CA 03169085 2022-08-12
[0038]
(Balance: iron and inevitable impurities)
In one preferred embodiment, the balance includes iron
and inevitable impurities. The inevitable impurities include
elements brought in steel material, depending on the
circumstances including raw materials, source materials,
production facilities, and the like.
There are some elements, such as P, S, and N, for
example, which are inevitable impurities that are usually
preferred in smaller amounts and whose composition range is
separately specified as mentioned above. For this reason,
"inevitable impurities" constituting the balance as used
herein is the concept excluding an element, the composition
range of which is separately specified.
[0039]
Further, the steel sheet according to the first
embodiment of the present invention may optionally contain
the following arbitrary elements as appropriate, and the
properties of the steel component can be further improved
depending on the contained element.
[0040]
(One or more selected from the group consisting of Cu: more
than 0% by mass and 0.50% by mass or less, and Ni: more than
0% by mass and 0.50% by mass or less)
The inclusion of Cu improves the corrosion resistance of
the steel sheet itself, thereby enabling suppression of
hydrogen generation due to corrosion of the steel sheet and
improvement in the delayed fracture resistance. Cu also has
the effect of promoting the formation of iron oxide: a-
Fe0OH, which is said to be thermodynamically stable and
13
CA 03169085 2022-08-12
protective among rusts formed in the atmosphere. By
promoting the formation of the rust, it is possible to
suppress the penetration of generated hydrogen into the steel
sheet, thereby preventing hydrogen induced cracking under a
severe corrosive environment. Thus, the Cu content is
preferably more than 0% by mass, more preferably 0.05% by
mass or more, and still more preferably 0.10% by mass or
more. Meanwhile, the excessive Cu content degrades
platability in a plating process during steel sheet
production and chemical conversion processability after hot
stamping. Thus, the Cu content is preferably set at 0.50% by
mass or less.
Ni is known to have the same effects as Cu. Thus, the
Ni content is preferably more than 0% by mass, more
preferably 0.05% by mass or more, and still more preferably
0.10% by mass or more. Meanwhile, the Ni content is
preferably 0.50% by mass or less.
[0041]
(One or more selected from the group consisting of Ti: more
than 0% by mass and 0.10% by mass or less, Cr: more than 0%
by mass and 3.0% by mass or less, and Nb: more than 0% by
mass and 0.10% by mass or less)
Ti reduces the amount of BN formed in the steel sheet by
forming TIN. This can increase the amount of a solid
solution B in the steel sheet, thus enhancing the effect of
improving the hardenability of B. To exhibit such an effect,
the Ti content is preferably more than 0% by mass, more
preferably 0.0005% by mass or more, and still more preferably
0.0250% by mass or more, or 0.050% by mass or more.
14
CA 03169.085 2022-08-12
Meanwhile, the excessive Ti content in the steel sheet
causes carbides to precipitate on the grain boundaries, which
deteriorates the hardenability of the steel sheet. Thus, the
Ti content is preferably set at 0.10% by mass or less, more
preferably 0.080% by mass or less, and still more preferably
0.070% by mass or less.
[0042]
Cr contributes to ensuring hardness and suppressing the
precipitation of coarse carbides during cooling. To exhibit
these effects, the Cr content is preferably more than 0% by
mass.
Meanwhile, the excessive Cr content in the steel sheet
may cause cracking or the like of the steel sheet. The Cr
content is preferably set at 3.0% by mass or less, more
preferably 2.5% by mass or less, and still more preferably
2.0% by mass or less.
[0043]
Nb is =a carbide-forming element that contributes to the
microstructure refinement of the steel sheet. Thus, the Nb
content is preferably more than 0% by mass, and more
preferably 0.0050% by mass or more.
Meanwhile, by refinement of the microstructure of the
steel sheet, reverse transformation during heat treatment is
promoted, but ferrite formation is promoted during cooling,
which may lead to a reduced strength of steel components.
Such effects become greater as its content increases. In
addition, an inconvenience such as deteriorated cold-
rollability also occurs. From this aspect, the Nb content is
preferably 0.10% by mass or less. It is preferably 0.070% by
mass or less, and more preferably 0.050% by mass or less.
CA 03169085 2022-08-12
[0044]
(b) Heating step
In the first embodiment of the present invention, the
above steel sheet is heated to the Ac]. point ( C) or higher
and lower than the Ac3 point ( C) + 10 C.
At a temperature of lower than the Acl point, austenite
transformation does not occur, making it difficult to produce
a high-strength steel component after a cooling step (e)
mentioned below. Meanwhile, by keeping the temperature of
the steel sheet lower than the Ac3 point + 10 C, it is easier
to promote the nucleation of ferrite and/or pearlite, which
are soft microstructures, in the processing step (c)
mentioned below.
[0045]
The Ac]. and Ac3 points can be determined by examining
the temperatures of the steel sheet during heating and the
displacement history thereof due to expansion and shrinkage
of the steel as it is heated in the formaster test. FIG. 1
is a graph showing the relationship between the temperature
and displacement of the steel sheet when heating the steel
sheet from a low temperature in the formaster test. At low
temperatures, steel can expand linearly with increasing
temperature at an expansion rate corresponding to the
crystalline structure of ferrite (bcc). As the temperature
of steel further increases, austenite with a denser
crystalline structure (fcc) is formed and may begin to
shrink. The temperature at which the linearity starts not to
be satisfied can be defined as Ac]. point. In a higher
temperature range where the temperature of the steel is
increased even further, all ferrite transforms to austenite,
16
CA 03169085 2022-08-12
which can again expand linearly at an expansion rate
according to the crystalline structure of the austenite. The
temperature at which this expansion starts to occur along the
linear line can be defined as Ac3 point.
[0046]
(c) Processing step
After the above heating step (b), the steel sheet is
processed by applying a strain of 0.5% or more at a
temperature of 675 C or higher and lower than Ac3 point +
10 C.
At the above temperatures, there can be lots of grain
boundaries in the steel sheet that are nucleation sites for
ferrite and/or pearlite, which are soft microstructures. In
such an unstable state, by applying a slight strain (i.e.,
0.5% or more), nucleation of ferrite and/or pearlite, which
are soft microstructures, can be promoted remarkably in a
part where the strain is applied: The applied strain is more
preferably 5.0% or more, and still more preferably 9.0% or
more.
The strain can be calculated by the following equation
(1).
Strain (%) = 1(do - di)/do x 1001 (1)
where do is the sheet thickness of the steel sheet before
processing or the sheet thickness of a non-processed portion
of the steel sheet after the processing, and di is the sheet
thickness of a processed part of the steel sheet after the
processing. Both thicknesses are represented by using a unit
of mm.
The strain may be, for example, equivalent plastic
strain determined by FEM analysis. In other words, if the
17
CA 03169085 2022-08-12
equivalent plastic strain determined by the FEM analysis is
0.5% or more, it can be softened in the same way.
[0047]
The Ms point can be determined by examining the
temperatures of the steel sheet during cooling and the
displacement history thereof due to expansion and shrinkage
of the steel as it is cooled in the formaster test. FIG. 2
is a graph showing the relationship between the temperature
and displacement of the steel sheet when cooling the steel
sheet at a relatively high speed after heating, in addition
to the relationship during the heating shown in FIG. 1. At
medium and high temperatures, steel can shrink linearly with
decreasing temperature at a shrinkage rate corresponding to
the crystalline structure of austenite. As the temperature
of the steel is decreased even further, it can be transformed
into martensite and begin to expand. The temperature at
which the linearity starts not to be satisfied can be defined
as Ms point.
[0048]
When the heating temperature in the above heating step
(b) is set at Ad l point ( C) or higher and lower than Ac3
= point ( C) + 10 C, and the processing temperature is set at
lower than 675 C, the transformation to a soft microstructure
becomes more active, so that the softening of a non-processed
portion also becomes more pronounced, making it difficult to
produce a steel component that is locally softened at the
processed part only.
When the heating temperature in the above heating step
(b) is set at Ad l point ( C) or higher and lower than Ac3
point ( C) + 10 C, and the processing temperature is set at
18
Clik 03169085 2022-08-12
4
Ac3 point + 10 C or higher, the areas of the grain
boundaries, which are the nucleation sites of the soft
microstructure, are reduced, and thus the nucleation of the
soft microstructure cannot be promoted only by applying a
slight strain.
[0049]
The above processing temperature may be the same as or
different from the heating temperature of the heating step
(b) above. When these are different, an additional step of
heating and/or cooling may be included between the above
steps (b) and (c). After the step (b) and before the step
(c), a further step of holding the steel sheet at a certain
temperature may be included.
[0050]
The above processing may be any arbitrary one, but
pressing, stretch forming, forging, return bending during
draw forming, shearing, etc., for example, are all suitable.
[0051]
(d) Step of holding or gradually cooling
After the processing step (c), the steel is held for 1
second or more and 120 seconds or less, or gradually cooled
at an average cooling rate of 0 to 15 C/sec. Specifically,
the steel sheet is held at the processing temperature for 1
second or more and 120 seconds or less, or gradually cooled
at an average cooling rate of more than 0 C/sec and 15 C/sec
or less for 1 second or more and 120 seconds or less. This
allows the growth of ferrite and/or pearlite, nucleated in
the step (c) above, which are soft microstructures.
[0052]
19
CA 03169085 2022-08-12
If the average cooling rate is more than 15 C/sec or if
the holding or gradually cooling time is less than 1 second,
ferrite and/or pearlite, which are soft microstructures,
cannot be sufficiently precipitated and grown. The holding
or gradually cooling time is preferably more than 1 second,
more preferably 3 seconds or more, and still more preferably
6 seconds or more.
If the holding or gradually cooling time is more than
120 seconds, ferrite and/or pearlite, which are soft
microstructures, precipitate and grow even in the non-
processed portion, thus failing to obtain a high-strength
steel component. This time is preferably 12 second or less.
[0053]
(e) Cooling step
After the holding or gradually cooling step (d) above,
the steel sheet is cooled to Ms point ( C) - 50 C. At this
time, the average cooling rate from the heating temperature
in the heating step (b) (i.e., Ad l point ( C) or higher and
Ac3 point ( C) + 10 C or lower) to Ms point ( C) - 50 C is
controlled to 10 C/sec or more. This allows martensitic
transformation to occur at least in the non-processed
portion, ensuring sufficient strength in the non-processed
portion. If cooling at an average cooling rate of 10 C/sec
or more is terminated at higher than Ms point ( C) - 50 C,
martensitic transformation cannot occur sufficiently in the
non-processed portion. Besides, if the average cooling rate
is less than 10 C/sec, the martensitic transformation cannot
occur sufficiently in the non-processed portion.
[0054]
CA 0316 r85 2022-08-12
After the cooling step (e) above, the steel sheet can be
cooled to, for example, room temperature. The cooling rate
from Ms point ( C) - 50 C to room temperature is not
particularly limited.
[0055]
<Second Embodiment of the Invention>
A production method according to a second embodiment of
the present invention differs from the production method
according to the first embodiment of the present invention in
the conditions of the heating step (b) and the processing
step (c). Hereinafter, these steps which are different from
those of the first embodiment of the present invention will
be described as a heating step (b') and a processing step
(c').
[0056]
(b') Heating step
In the second embodiment of the present invention, the
above steel sheet is heated to the Ac3 point ( C) + 10 C or
higher and 1,100 C or lower. Unlike the first embodiment of
the present invention, even though the steel sheet is heated
to a temperature of Ac3 point ( C) + 10 C or higher in the
heating step, the nucleation of ferrite and/or pearlite,
which are soft microstructures, can be remarkably promoted if
a relatively large strain is applied in a processing step
(c') to be mentioned later, similarly to the first embodiment
of the present invention. Meanwhile, if the temperature of
the steel sheet exceeds 1,100 C, decarburization on the steel
surface becomes more pronounced, so that the desired strength
cannot be obtained. In addition, there is a possibility that
oxidation will progress, resulting in thinning. In a case
21
,CA 031.69085 2022-08-12
where the steel sheet is plated, oxidation and alloying will
occur, causing problems of which, for example, the hardness
of the plating becomes extremely high, allowing the plating
to be peeled off in the processing step (leading to oxidation
of the steel sheet, and/or pressing scratches).
[0057]
(c') Processing step
After the above heating step (b'), the steel sheet is
processed by applying a strain of 10% or more thereto at a
temperature of Ms point ( C) + 50 C or higher and lower than
Ac3 point ( C) + 10 C. At the temperature of Ms point ( C) +
50 C or higher and lower than Ac3 point ( C) + 10 C,
austenite becomes relatively unstable. Thus, by applying a
relatively large (10% or more) strain, the nucleation of
ferrite and/or pearlite, which are soft microstructures, can
be remarkably promoted in a part where the strain is applied.
The strain applied is more preferably 15% or more, and still
more preferably 40% or more. The strain can be calculated by
the above equation (1). The strain may be, for example,
equivalent plastic strain determined by FEM analysis. In
other words, if the equivalent plastic strain determined by
the FEM analysis is 10% or more, it can be softened in the
same way.
[0058]
At temperatures of Ac3 point ( C) + 10 C or higher,
austenite becomes relatively stable. Thus, even when a
relatively large strain is applied, the nucleation of
ferrite/or pearlite, which are soft microstructures, are
difficult to promote. Meanwhile, at temperatures of lower
than Ms point ( C) + 50 C, martensitic transformation may
22
CA 03169085 2022-08-12
occur, making it difficult to promote nucleation of ferrite
and/or pearlite, which are soft microstructures.
[0059]
The cooling from the temperature after the heating step
(b') (i.e. Ac3 point ( C) + 10 C or higher to 1,100 C or
lower) to the temperature in the processing step (c') (i.e.
Ms point ( C) + 50 C or higher and lower than Ac3 point ( C)
+ 10 C) is not particularly limited, and may be performed at
any average cooling rate. After the step (b') and before the
step (c'), a further step of holding the steel sheet at a
certain temperature may be included.
(0060)
The above processing step (c') may be any arbitrary one,
but pressing, stretch forming, forging, bending back during
draw forming, shearing, etc., for example, are all suitable.
[0061]
In the first and second embodiments of the present
invention, the strain in the steps (c) and (c') may be
applied through a plurality of times of processing.
When the strain is applied through the plurality of
times of processing in the above steps (c) and (c'), the
strain can be calculated by the following equation (2).
[0062]
[Equation 1]
Strain (%)=EI (d n-1 ¨do) /d_1 X 00 I = = = (2)
where dn is a sheet thickness of a processed part of the
steel sheet obtained after the n-th processing, and the unit
of dn is mm.
23
, CA 03169085 2022-08-12
It is noted that the strain determined by the above
equation (2) may be, for example, the total of equivalent
plastic strains determined by FEM analysis after each
processing.
[0063]
For example, when the step (c) or (c') is a single
process, it may be difficult to apply the predetermined
strain (0.5% or more in the first embodiment, 10% or more in
the second embodiment). In such a case, it is advantageous
to perform the above steps (c) and (c') a plurality of times
to accumulate the strain so that the strain is more likely to
exceed the predetermined value.
[0064]
When the step (c) or (c') is a single process, it may be
difficult to set a delivery time from the above step (c) or
(c') to the above cooling step (e) to less than 1 second, or
to make the time for the above holding or gradually cooling
step (d) (i.e. for 1 second or more). In such a case, it is
advantageous to perform the above steps (c) and (c') a
plurality of times because the delivery time between the
plurality times of processing steps can be used as the time
for the holding or gradually cooling step (d).
[0065]
The plurality of times of processing may include
processing for applying deformation and processing for
restoring the deformation. This allows the above strain to
be applied to the initial steel sheet shape without changing
the final steel component shape.
[0066]
24
CIA 03169085 2022-08-12
When each of the above steps (c) and (c') includes a
plurality of times of processing, the above holding or
gradually cooling step (d) may be performed after each time
of processing. For example, when the processing is performed
twice, the first processing may be performed, followed by the
first holding or gradually cooling step, the second
processing and further the second holding or gradually
cooling step. In this case, the total of the time for the
first holding or gradually cooling step and the time for the
second holding or gradually cooling step may be within a
defined time of the step (d) specified by the first and
second embodiments of the present invention, i.e., 1 second
or more and 120 seconds or less.
[0067]
The temperatures in the above steps (a) to (e), (b') and
(c') above are the surface temperature of the steel sheet (or
steel component) and may be measured using a thermocouple or
radiation thermometer. Alternatively, the correspondence
between the ambient temperature of a heating line, etc., and
the surface temperature of the steel sheet (or steel
component) measured by the thermocouple or the like may be
investigated in advance, and thereby the surface temperature
of the steel sheet (or steel component) may be read off from
the ambient temperature of the heating line, etc.
[0068]
According to the first and second embodiments of the
present invention, it is possible to provide a method for
producing a high-strength steel component in which only a
part applied with a predetermined level or more of strain by
CA 03169085 2022-08-12
=
the processing is locally softened, without any local
temperature control.
EXAMPLES
[0069]
The embodiments of the present invention will be
described in more detail by way of Examples. It is to be
understood that the embodiments of the present invention are
not limited to the following Examples, and various design
variations made in accordance with the purports mentioned
hereinbefore and hereinafter are also included in the scope
of the embodiments of the present invention.
Example 1
[0070]
Steel having the chemical composition shown as steel
type No. A in Table 1, (Ad l point: 778 C, Ac3 point: 875 C,
and Ms point: 385 C) was used to prepare a steel sheet with a
sheet thickness of 1.6 mm and an area of 100 mm x 100 mm, and
the prepared steel sheet was heated to 880 C. Thereafter,
the steel sheet was cooled down to 750 C at about 12 C/sec1
and subjected to stretch forming at 750 C. The stretch
forming was performed by pressing a hemispherical punch with
10 mm diameter against the center of the steel sheet with a
100 mm x 100 mm from its back side. The height due to the
stretch forming was set at 3.0 mm. After the stretch
forming, the steel sheet was gradually cooled for 6 seconds
at an average cooling rate of 10.8 C/sec. The steel sheet
was then water-cooled to Ms point ( C) - 50 C (i.e., 335 C),
so that the average cooling rate from 880 C to 335 C was
39.5 C/sec. Thereafter, the steel sheet was allowed to cool
26
, CA 03169085 2022-08-12
to room temperature. The above procedure is defined as
Production Example 1-2.
The Ad, Ac3 and Ms points above were determined by the
formaster test. The formaster test was performed under the
following conditions.
Formaster testing device: FTM-10, manufactured by Fuji
Electronic Industrial Co., Ltd.
Specimen size: 2.0 mm thickness x 3.0 mm width x 10 mm
length (note that two holes of 0.7 mm diameter x 2.0 mm depth
for thermocouple insertion are formed)
Number of tests: 7 times (only cooling rate was changed,
while other conditions were constant)
Heating rate: 10 C/s (room temperature to heating
temperature)
Heating temperature: 950 C
Holding time at the heating temperature: 180 seconds.
Cooling rate: 2, 5, 10, 15, 20, 30, and 40 C/s (heating
temperature to room temperature)
In Table 1, the Cu content of steel type No. A is listed
as "-" because it was at the inevitable impurity level (less
than 0.01% by mass).
[0071]
[Table 1]
Chemical composition (% by mass) * Balance being iron and inevitable
Steel impurities
type No.
Si Mn Al P S N B Ti Cr Cu
A 0.31 1.2 1.2 0.042 0.01 0.001 0.004 0.002 0.04 0.6
B 0/35 019 1.29 0.041 0.013 0.002 0.00540.0033 0.026 0/3 0.07
[00723
27
CIA 03169085 2022-08-12
To evaluate the strain and hardness of a steel component
obtained by Production Example 1-2, evaluation samples were
taken. The locations where the evaluation samples were taken
are shown in Figure 3. As shown in FIG. 3, a stretch formed
portion A (25 mm in the longitudinal direction x 5 mm in the
lateral direction) at the center of the steel component and a
non-processed portion B (10 mm in the longitudinal direction
x 5 mm in the lateral direction) located longitudinally away
from the stretch forming part A were taken.
[0073]
To evaluate the strain of the samples, the sheet
thickness of the steel sheet was determined by cross-
sectional observation with an optical microscope.
The sheet thickness of the stretch formed portion A was
determined at the center of the steel component, at a
distance of 3.75 mm longitudinally from the center (referred
to as middle section), and at a distance of 7.5 mm
longitudinally from the center (referred to as hem section).
Then, by using the above equation (1), the strains at the
center, the middle section, and the hem section of the steel
component were determined by defining each of the sheet
thicknesses of the center, the middle section, and the hem
section of the steel component as the sheet thickness dl of
the processed part, and also by defining the sheet thickness
of the non-processed portion B as the sheet thickness dO of
the steel sheet before the processing.
[0074]
Vickers hardnesses were measured at three locations (the
center, middle section, and hem section) of the stretch
formed portion A and the non-processed portion B. The
28
CA 03169085 2022-08-12
measurement was performed using a Vickers hardness tester
under conditions of a load of 1 kg and a holding time of 10
seconds. The measurement positions were set at three points
that were located at d/4 from the surface of the steel
component in the thickness direction where d is the sheet
thickness. FIG. 4 is a schematic cross-sectional view taken
along the line X-X shown in FIG. 3 and shows hardness
measurement positions of the stretch formed portion A.
Although the hardness measurement positions of the non-
processed portion B are not shown in the drawings, the
measurement positions were set at three points that were
located at the center of the non-processed portion B in the
longitudinal and lateral directions and at d/4 from the
surface of the steel component in the direction of the sheet
thickness.
[0075]
An average value of Vickers hardnesses at three
locations (the center, the middle section, and the hem
section) of the stretch formed portion A, as well as an
average value of Vickers hardnesses at three points of the
non-processed portion B were adopted as the respective
Vickers hardnesses.
[0076]
Steel components (hereinafter referred to as Production
Examples 1-1 and 1-3 to 1-8) were produced by changing any of
the following conditions of Production Example 1-2:
temperature ( C) at which the stretch forming was performed
(referred to as molding temperature), an height (mm) due to
the stretch forming, a cooling rate ( C/sec) during gradually
cooling, a gradually cooling time (sec), and an average
29
CA 03169085 2022-08-12
cooling rate ( C/sec) from a heating temperature to the Ms
point - 50 C. The strain and Vickers hardness of each steel
component were evaluated in the same manner as the steel
component obtained in Production Example 1-2. The results
are shown in Table 2.
In Table 2, numerical values underlined indicate that
they deviate from the scope of the first embodiment of the
present invention.
[0077]
[Table 2]
(a) Step o( (d) Holding or
(b) Heating
preparing (c) Processing step gradually cooling Hardness
step
steel sheet step Average
cooling rate Difference in Vickers
Height Strain from
heating Vickers hardness hardness between center
Production due to
Gradually temperature and non-processed
Examples Heating Molding the Cooling .
cooling to Ms point portion
Steel type temperature temperature . rate
No. strain
Middle Hem time - 50 C
Middle Hem Non-
Middle Hem
forming Center Center
processed Center .
section section
section section section section
.
portion
. _
[T]
IT] [mm] rol [3191 [3(4 ["C/see] [See] ["C/see] WI
[1111] [HVI [HV] [FM [FIV] IHVI
1-1 A 880 750 0.1 0.6 0.6 0.6 10.8 6
39.5 268 262 271 387 -119 -125 -116
.
.
1-2 A 880 750 3 9.7 18.2 4.8
10.8 6 39.5 288 290 385 412 -124 -122 -27 P
.. -
.
,,
1-3 A 880 750 6 14.1 39.9 4.9
10.8 6 39.5 340 330 435 519 -179 -189 -84 F.
al
= to
- --..-
0
0)
LO 1-4 A 880 750 6 14.0 37.2 4.9 9.6
12 27.5 267 281 337 403 -137 -123 -67 .
I-, .
NO
,,,,
1-5 A 880 650 3 7.3 17.7 4.3 7.1 6 24.8 272 286 265 259 13 27 5
,.,,
.
0
-
i
FA
1-6 A 880 650 6 13.9 44.2 4.8 7.1
6 24.8 277 306 249 251 26 55 -2 "
1-7 A 880 1.59 3 9.1 16.4 3.6 4.7 6
16.0 280 302 235 215 65 87 20
1-8 A 880 550 6 14.1 47.9 6.1 4.7 6
16.0 303 345 233 224 79 121 9
4 CA 03169085 2022-08-12
(0078)
Among Production Examples 1-1 to 1-8, Production Example
in which at least one of the center, the middle section, and
the hem section had a Vickers hardness lower by 20 HV or more
than the Vickers hardness of the non-processed portion while
the hardness of the non-processed portion was 310 HV or
higher was determined to satisfy the criteria of "locally
softened high-strength steel component". A preferred
Production Example as the "locally softened" steel component
is one in which at least one of the center, the middle
section, and the hem section had a Vickers hardness lower by
40 HV or more than the Vickers hardness of the non-processed
portion. A further preferred Production Example is one in
which at least one of the center, the middle section, and the
hem section had a Vickers hardness lower by 100 HV or more
than the Vickers hardness of the non-processed portion.
A more preferred Production Example as the "high-
strength steel component" is one in which the Vickers
hardness of the non-processed portion is 400 HV or more, and
an still more preferred Production Example is one in which
the Vickers hardness of the non-processed portion is 500 HV
or more.
The same goes for Examples 2 and 3 to be mentioned
later.
[0079)
From the results in Table 2, the following can be
discussed. Production Examples 1-1 to 1-4 of Table 2 are
examples satisfying all requirements specified by the first
embodiment of the present invention, and were able to
manufacture high-strength steel components in which only a
32 =
CA 03169085 2022-08-12 =
part applied with a predetermined or more strain (0.5% or
more in the first embodiment of the present invention) by the
processing was locally softened without any local thermal
control.
Meanwhile, Production Examples 1-5 to 1-8 of Table 2 are
example not satisfy any of the requirements specified by the
first embodiment of the present invention and were not able
to manufacture high-strength steel components in which a part
applied with a predetermined or more strain (0.5% or more in
the first embodiment of the present invention) by the
processing was locally softened.
[0080]
In Production Examples 1-5 to 1-8, since the forming
temperature was 650 C or 550 C, and less than 675 C, the
entire steel component including the non-processed portion
was softened, and thus a high-strength steel component
locally softened was not able to be produced.
Example 2
[0081]
Steel having the chemical composition shown as steel
type No. A in Table 1 was used to prepare a steel sheet with
a sheet thickness of 1.6 mm and an area of 100 mm x 100 mm,
and the prepared steel sheet was heated to 880 C.
Thereafter, the steel sheet was cooled down to 750 C at about
12 C/sec, and subjected to the first stretch forming at
750 C. The first stretch forming was performed by pressing a
hemispherical punch with 10 mm diameter against the center of
the steel sheet with a 100 mm x 100 mm from its back side.
The height due to the first stretch forming was set at 3.0
mm. After the first stretch forming, the steel sheet was
33
CA 03169085 2022-08-12
gradually cooled for 6 seconds at an average cooling rate of
10.8 C/sec. After the first gradually cooling step, the
second stretch forming was performed. The second stretch
forming was performed by pressing the hemispherical punch
with 10 mm diameter against the locations of the steel sheet
subjected to the first stretch forming in the opposite
direction of the first stretch forming (i.e., from its front
side). After the second stretch forming, the steel sheet was
gradually cooled for 6 seconds at an average cooling rate of
6.7 C/sec. After the second gradually cooling step, the
steel sheet was then water-cooled to Ms point ( C) - 50 C
(i.e., 335 C) so that the average cooling rate from 880 C to
335 C was 26.2 C/sec. Thereafter, the steel sheet was
allowed to cool to room temperature. The above procedure is
defined as a Production Example 2-1.
[0082]
The strain and Vickers hardness of the steel component
obtained in Production Example 2-1 were evaluated in the same
manner as Example 1. The strain was calculated using the
above equation (2). Since the first stretch forming was
performed in the same way as in Production Example 1-2, the
strain was calculated on the assumption that the sheet
thickness after the first stretch forming was the same as
that in Production Example 1-2. The results are shown in
. Table 3. The second stretch forming was performed in the
opposite direction as the first stretch forming, and thus the
height due to the second stretch forming was a negative
value.
[0083)
34
[Table 3]
(a) Step
of (b) Heating
PreParing (c) Processing step
(d) Holding or gradually cooling step Hardness
step Average
steel
cooling rate
sheet from
heating
Difference in Vickers
Production Height Height Strain due to twice
due to First
Second temperature Vickers hardness and non-processed
hardness between center
Example First due to the processing First
Heating gradually Second
gradually to Ms point
¨ molding the first cooling cooling cooling cooling -
50,,c portion
Steel type temperature temperature stretch second
Non-
rate rate
Non-
No. Middle Hem time time
Middle Hem Middle Hem
Center
billing forming section section
section section Pmeessed Center section section
portion
rcj rC] [min] [mm] [/o]
[54] ['C/sec] [Sec] rCisec] [Sec] [C/sec] [NV] [MV]
[NV] [MV] [NV] [MV] (MV]
2-1 A 880 750 3 -3 11.9 34.6 5.9 10.8 6 6.7 6
26.2 319 340 310 381 -62 -41 -71
No
0
co
01
(a)
(71No
0
NO
CA 03169085 2022-08-12
[0084]
From the results in Table 3, the following can be
discussed. Production Example 2-1 of Table 3 is an example
satisfying all requirements specified by the first embodiment
of the present invention, and was able to manufacture a high-
strength steel component in which only a part applied with a
predetermined or more strain (0.5% or more in the first
embodiment of the present invention) by the processing was
locally softened without any local thermal control.
Example 3
[0085]
Steel having the chemical composition shown as steel
type No. A in Table 1 was used to prepare a steel sheet with
a sheet thickness of 1.6 mm and an area of 100 mm x 100 mm,
and the prepared steel sheet was heated to 950 C and held for
60 seconds. Thereafter, the steel sheet was cooled down to
550 C at about 12 C/sec, and subjected to stretch forming at
550 C. The stretch forming was performed by pressing a
hemispherical punch with 10 mm diameter against the center of
the steel sheet with a 100 mm x 100 mm from its back side.
The height due to the stretch forming was set at 0.1 mm.
After the stretch forming, the steel sheet was gradually
cooled for 6 seconds at an average cooling rate of 4.7 C/sec.
The steel sheet was then water-cooled to Ms point ( C) - 50 C
(i.e., 335 C) so that the average cooling rate from 950 C to
335 C was 12.5 C/sec. Thereafter, the steel sheet was
allowed to cool to room temperature. The above procedure is
defined as Production Example 3-1.
[0086]
36
CA 03169085 2022-08-12
The strain and Vickers hardness of the steel component
obtained in Production Example 3-1 were evaluated in the same
manner as Example 1.
[0087]
Steel components (hereinafter referred to as Production
Examples 3-2 to 3-19) were produced by changing any of the
following conditions of Production Example 3-1: temperature
(6C) at which the stretch forming was performed (referred to
as molding temperature), a height due to the stretch forming
(mm), a cooling rate (6C/sec) during gradually cooling, a
gradually cooling time (sec), and an average cooling rate
(6C/sec) from a heating temperature to the Ms point - 50 C.
The strain and Vickers hardness of each steel component were
evaluated in the same manner as in Production Example 3-1.
The results are shown in Tables 4 and 5. The Ad l point of
the steel having the chemical composition shown in steel type
No. B in Table 1 was 778 C, the Ac3 point was 875 C, and the
Ms point was 385 C.
In Tables 4 and 5, numerical values underlined indicate
that they deviate from the scope of the second embodiment of
the present invention.
[0088]
37
[Table 4]
(a) Step of (d) Holding or
(b) Heating
preparing (c) Processing step gradually cooling
Hardness
steel sheet sreP step Average
,
cooling rate Difference in Vickers
Height frorn Strain
Vickers hardness heating hardness between center
Production due to
Gradually temperature and non-processed
Examples steeõ.__ Heating Molding the Cooling .
cooling to Ms point portion
' vlic temperature temperature stretch time - 50 C rate
No. Middle Hem
Middle Hem Non-
. Middle Hem
forming Center
section section
Center section section processed Center portion section section
_
[ C] rq [mm] [%] [04] rm [Thee] [Sec] 1 C/sec] [HV]
[HV] [HV] [RV] [HV] [HV] [HV]
3-1 A 950 550 0.1 1 0 Q 4.7 6 12.5 637
640 628 622 15 17 6
,
3-2 A 950 650 0.1 2 0 0 7.1 6 16.5 642
623 622 623 19 0 -1 .
_
.
3-3 A 950 750
0.1 2 0 0 10.8 6 21.2 636 630 625 625 11 5 0 0
3-4 A 950 550 6 26 41 9 4.7 6 12.5 564
490 629 645 -80 -154 -15 .
L.,
_
3-5 A 950 650 6 17 39 7 7.1 6
1.65 584 555 636 639 -55 -84 -4 .
.
_
.
id 3-6 , A 950 750 6 21 41 6 10.8 6 21.2 542
427 627 633 -91 -206 -6
co
0
_ . . 3-7 A 950 550 3 7 8
6 _ 16.5 0 14.2 593 604 586 590 3 14 -4 .
_
.
3-8 A 950 550 3 7 5 _ 2 4.7 6 12.5 632
628 627 627 5 1 0 i
.
.
0
,
3-9 A 950 550 3 2 14 0 4.7 12 11.1
617 _ 573 613 607 10 -34 6
_
_
3-10 A 950 600 3 1 6 67 6 14.6 636
615 640 617 20 -1 24
3-11 A 950 600 3 fi 13 7 6.7 12 12.8 608 566 618 613 -5 -46 5
3-12 A . 950 650 3 9 13 7 2 ,Q 0 19.7 593
603 587 589 4 14 -2
_
3-13 A 950 650 3 =1 7 9 8 5
_. _ _ 6 1.65 628
629 629 618 11 11 12
. _
3-14 A 950 650 3 11 16 7.1 12 14.2 582 562 625 615 -33 -53 10
. -
.
3-15 A 950 700 3 10 15 8 9.2 6 18.8 602
582 636 625 -23 -43 11
_ - 3-16 A 950 700 3 10 14 7 9.2
12 15.9 593 587 614 623 -30 -36 -9 _
3-17 A 950 750 3 II 6 4 35.5 _ 0 26.7
590 590 589 590 0 0 -I
3-18 A 950 750 3 8 7 5 10.8 6 21.2 621 626 628 643 -22 -17 -14
3-19 A 950 750 3 8 _ 4 _ 5 10.8 12 17.6 635
634 639 638 -3 -4 2
=
[0089]
[Table 5]
(a) Step of (d) Holding or
(b) Heating
preparing step (c) Processing step gradually cooling
Hardness
steel sheet step Average
cooling rate Difference in Vickers
Height from
heating hardness between center
Strain
Vickers hardness
Production due to
Gradually temperature and non-processed
Cooling
the
Examples Heating Molding cooling
to Ms point portion
Steel type temperature temperature h rate
time - 50 C
Non-
No. Middle Hem
+ Middle Hem Middle Item
forming Center section section Center
c section section Prmessed enter section section
portion
rq
rq [mmi N N N [ C/sec) [Sec] rchecl Ellvi RP/ PI filvi
Inv] [HV1 Rivl _
3-20 A 950 700 3 11 19 2 7.9 6
18.6 567 521 584 594 -27 -73 -10
0
3-21 A 950 700 4 14 24 , 2 7.9 6 18.6
515 474 586 591 -77 -117 -5 0
- .
1-,
3-22 A 950 700 5 17 34 , 1 7.9 6 18.6
497 411 589 603 -106 -192 -14 ,
,
_ 0
3-23 A 950 _ 700 6 17 34 2 7.9
6 18.6 493 3% 585 597 -103 -201 -12 0
U,
IL)
N,
l0 3-24 A 950 700 7 ., 17 39 , 2, 7.9 6
, 18.6 ,. 522 , 413 592 606 -84 -193 -13 0
.
1
3-25 A 950 , 700 6 17 40 1 8.3 4 õ
19.8 490 _ 373 597 603 -112 -230 -6 .
3-26 A , 950 700 6 20 36 3 7.7 9
17.1 506 400 587 599 -93 -199 -13
_
1-,
rs,
3-27 A 950 700 6 19 36 2 7.4
12 ' 15.8 471 371 585 590 -119 -219 -5
_
3-28 A 950 700 6 18 37 Q I 2Q,1 .
_ - _ 22.8
595 602 587 589 6 13 -2
_ .
_
3-29 13 950
750 6 24 38 1 al 0 , 25 498 510 494 495 3 16 -1
3-30 B 950 750 6 24 46 . - I 8.7 12
16.7 350 295 441 449 -100 -154 -8
,
¨ _
3-31 B 950 750 6 22 42 2 9.2 6
18.2 383 352 491 480 -97 -128 -II
.
_ . _ -
3-32 B 950 750 6 22 39 3 9.7 6
20 418 397 488 497 -79 -100 -8
--,
-
3-33 B 950 750 0.1 2 0 0 9.7 6
20 483 481 486 491 -8 -10 -5
,
_ -
_
3-34 B 950 750 3 11 17 1 9.7 6
20 438 429 477 492 -54 -63 -15
_ _
-
3-35 B 950
750 4 16 24 2 9.7 6 . 20 447 408 474 491 -44 -83 -17
_
.
' 3-36 B , 950 750 5 19 31 1 9.7 6 20
418 384 484 486 -68 -102 -2
3-37 ¨ B 950 750 6 19 33 , 1 9.7 6 20 414
355 486 481 -67 -126 5
-
3-38 a 950 750 7 19 37 1 9.7 6 20 423
338 - 485 480 -57 -142 5
CA 03169065 2022-08-12
1
[0090]
From the results in Tables 4 and 5, the following can be
discussed. Production Examples 3-4 to 3-6, 3-9, 3-11, and 3-
14 to 3-16 of Table 4 and Production Examples 3-20 to 3-27,
3-30 to 3-32, and 3-34 to 3-38 of Table 5 are examples
satisfying all requirements specified by the second
embodiment of the present invention, and were able to
manufacture high-strength steel components in which only a
part applied with a predetermined or more strain (10% or more
in the second embodiment of the present invention) by the
processing was locally softened without any local thermal
control.
[0091]
Meanwhile, Production Examples 3-1 to 3-3, 3-7 to 3-8,
3-10, 3-12 to 3-13, 3-17, and 3-19 of Table 4 and Production
Examples 3-28, 3-29, and 3-33 of Table 5 are examples not
satisfying any of the requirements specified by the second
embodiment of the present invention, and were not able to
manufacture high-strength steel components in which only a
part applied with a predetermined or more strain (10% or more
in the second embodiment of the present invention) by the
processing was locally softened.
[0092]
In Production Examples 3-1 to 3-3, 3-8, 3-10, 3-13, and
3-19 of Table 4 and Production Example 3-33 of Table 5, the
strains in all the center, the middle section, and the hem
section were less than 10%, and thus the high-strength steel
component locally softened was not able to be produced.
[0093]
CA 03169085 2022-08-12
In Production Example 3-7 of Table 4, the gradually
cooling rate in the holding or gradually cooling step (d) was
more than 15 C/sec (i.e., a gradually cooling time was less
than 1 sec), and the strains in all the center, the middle
section, and the hem section were less than 10%. As a
result, the high-strength steel component locally softened
was not able to be produced.
[0094]
In Production Examples to 3-12 and 3-17 of Table 4 and
Production Examples 3-28 and 3-29 of Table 5, the gradually
cooling rate in the holding or gradually cooling step (d) was
more than 15 C/sec (i.e., gradually cooling time was less
than 1 sec), and thus the high-strength steel component
locally softened was not able to be produced.
[0095]
In Production Example 3-18 of Table 4, the strain
applied to the center of the steel sheet by the processing
was 8%, and did not satisfy the strain of 10% or more
specified by the second embodiment of the present invention,
but a difference in the hardness between the center and the
non-processed portion was 20 HV or more. There is a
possibility that at the center of the component No. 3-18, the
production conditions other than the strain (heating
temperature, cooling rate, and gradually cooling time, etc.)
were preferable conditions, but the details thereof are
unknown.
Example 4
[0096]
Steel having the chemical composition shown as steel
type No. A in Table 1 was used to prepare a steel sheet with
41
' CA 03169085 2022-08-12
a sheet thickness of 1.6 mm and an area of 100 mm x 100 mm,
and the prepared steel sheet was heated to 950 C.
Thereafter, the steel sheet was cooled down to 750 C at about
12 C/sec, and subjected to the first stretch forming at
750 C. The first stretch forming was performed by pressing a
hemispherical punch with 10 mm diameter against the center of
the steel sheet with a 100 mm x 100 mm from its back side.
The height due to the first stretch was set at 4.0 mm. After
the first stretch forming, the steel sheet was gradually
cooled for 6 seconds at an average cooling rate of 9.7 C/sec.
After the first gradually cooling step, the second stretch
forming was performed. The second stretch forming was
performed by pressing the hemispherical punch with 10 mm
diameter against the locations of the steel sheet subjected
to the first stretch forming in the opposite direction of the
first stretch forming (i.e., from its front side). After the
second stretch forming, the steel sheet was gradually cooled
for 6 seconds at an average cooling rate of 5.3 C/sec. After
the second gradually cooling step, the steel sheet was then
water-cooled to Ms point ( C) - 50 C (i.e., 335 C) so that
the average cooling rate from 950 C to 335 C was 16,6 C/sec.
Thereafter, the steel sheet was allowed to cool to room
temperature. The above procedure is Production Example 4-1.
[0097]
The strain and Vickers hardness of the steel component
obtained in Production Example 4-1 were evaluated in the same
manner as Example 1. The strain was calculated using the
above equation (2). It was confirmed that the thickness of
the steel sheet at the center was 1.39 mm, its thickness at
the middle section was 1.22 mm, and its thickness at the hem
42
CA 03169085 2022-08-12
section was 1.58 mm when the second stretch forming was not
performed in Production Example 4-1. These sheet thicknesses
were used as the sheet thicknesses after the first stretch
forming in Production Example 4-1 to calculate the strains.
The results are shown in Table 6. The second stretch forming
was performed in the opposite direction as the first stretch
forming, and thus the height due to the second stretch
forming was a negative value.
[0098]
43
=
[Table 6]
(a) Step
of.
(b) Heating
preparing (c) Processing step
(d) Holding or gradually cooling step Hardness
Average
step
steel
cooling rate
shed from
Difference in Vickers
Height heating hardn
due to ess between center
Production Height Strain due to twice
First Second temperature Vickers hardness
= Example First due to the processing
First Second
gradually .
gradually to Ms point and non-processed
Heating
Steel type temperature molding the first cooling . coo
cooling - 517C ling _portion
coolmg
Non-
temperature stretch ¨" rate rate Non-
Middle Hem
ter Middle Hem
No. stretch Middle Hem time
time
Center
laming fanning Center section section
section section pprocessed¨ rell¨ section section
portion
rC] rq [mini (mm] rh] rC/sec] [Sec] ('C/sec] [Sec]
rC/sec] [NV] [NV] [NV] [NV] [HV] [HV] [NV]
4-1 A 950 750 4 -4 14 39 2 9.7 6 5.3 6 16.6 553 500 577 586 -33 -86 -9
0
No
ON
NO
0
No
alb
NO
0
CO
NO
' CA 03169085 2022-08-12
=
=
[0099]
From the results in Table 6, the following can be
discussed. Production Example 4-1 of Table 6 is an example
satisfying all requirements specified by the second
embodiment of the present invention, and was able to
manufacture high-strength steel components in which only a
part applied with a predetermined or more strain (10% or more
in the second embodiment of the present invention) by the
processing was locally softened without any local thermal
control.
Industrial Applicability
[0100]
In the embodiments of the present invention, it is
possible to provide a method for producing a high-strength
steel component having a locally softened part without any
local temperature control. Such high-strength steel
component is suitable, for example, for materials of
automobile frames.
[Description of Reference Numerals]
[0101]
1 Steel component
2 First location of hardness measurement at center
3 Second location of hardness measurement at center
4 Third location of hardness measurement at center
5 First location of hardness measurement at middle section
6 Second location of hardness measurement at middle section
7 Third location of hardness measurement at middle section
8 First location of hardness measurement at hem section
9 Second location of hardness measurement at hem section
Third location of hardness measurement at hem section
A Stretch formed portion
B Non-processed portion
46
CA 3169085 2024-01-09
