Technical Field
[0001] The present invention relates to steel sheet for a hot stamped member which is suitable
for the hot stamping method, one of the shaping methods giving a high strength member,
and a method of production of the same.
Background Art
[0002] In the field of automobiles, construction machinery, etc., vigorous efforts are being
made to reduce weight by use of high strength materials. For example, in automobiles,
the amount of use of high strength steel sheet has been steadily increasing for the
purpose of cancelling out the increase in vehicle weight accompanying the improvements
in impact safety and performance and furthermore improving fuel efficiency to reduce
the amount of emission of carbon dioxide.
[0003] In the trend toward expanded use of such high strength steel sheet, the biggest problem,
unavoidable when raising the strength of steel sheet, is the rise of the phenomenon
called "degradation of the shape fixability". This phenomenon is the general term
for loss of ease of obtaining a target shape due to the increase in the amount of
springback after shaping accompanying higher strength. To solve this problem, working
steps which were unnecessary with low strength materials (materials with shape fixabilities
which are excellent or not a problem) (for example, restriking) have been performed
or the product shapes have been changed.
[0004] As one method for dealing with this situation, the hot shaping method called the
"hot stamping method" has come under attention. This heats a steel sheet (worked material)
to a predetermined temperature (generally, the temperature resulting in an austenite
phase) to lower the strength (that is, facilitate shaping), then shapes it by a die
of a lower temperature than the worked material (for example room temperature) to
thereby easily impart a shape and simultaneously utilize the temperature between the
two for rapid cooling heat treatment (quenching) so as to secure the strength of the
shaped product.
[0005] Several arts relating to steel sheet suitable for such a hot stamping method and
method of shaping the same have been reported.
[0006] PLT 1 shows steel sheet obtained by controlling the amounts of elements which the
steel sheet contains and the relationship among the amounts of the elements to predetermined
ranges so as to give a member which is excellent in impart characteristics and delayed
fracture characteristic after hot shaping (synonymous with hot stamping).
[0007] PLT 2, in the same way as the above, discloses a method comprising making the amounts
of elements which the steel sheet contains and the relationship among the amounts
of the elements to predetermined ranges and heating before shaping the steel sheet
in a nitriding atmosphere or a carburizing atmosphere so as to obtain a high strength
part.
[0008] PLT 3 describes means for prescribing the composition and microstructure of steel
sheet and limiting the heating conditions and shaping conditions so as to obtain hot
pressed parts with a high productivity.
[0009] Recently, the hot stamping method has become widely recognized for its usefulness.
Members for which its application has been studied have become much more diverse.
Among these, for example, there are parts, such as underbody parts of automobiles,
where not only the strength of the parts, but also the fatigue characteristic is an
important, necessary characteristic.
[0010] The fatigue characteristic of steel sheet is improved together with the static strength,
so steel sheet (product) made high in strength by the hot stamping method also can
be expected to exhibit a commensurate fatigue characteristic, if compared with steel
sheet of the same strength not using the hot stamping method (high strength steel
sheet produced by controlling the composition or method of production of the strength
steel sheet, below, called "ordinary high strength steel sheet"), it became clear
that depending on the production conditions, the fatigue characteristics of the former
were inferior to the latter.
[0011] Studied in detail, it was discovered that compared with the deviation in hardness
of the surfacemost part of "ordinary high strength steel sheet", the deviation in
hardness of the surfacemost part of steel sheet (product) raised in strength by using
the hot stamping method is larger. It was concluded that this deviation in hardness
might be related to the fatigue characteristic.
[0012] The relationship between the deviation in hardness and the fatigue characteristic
is not necessarily clear, but in a high strength member which is produced by the hot
stamping method (for example, a tensile strength of 1500 MPa or more), the effect
of the notch sensitivity on the fatigue characteristic is extremely large, so it is
guessed that this deviation in hardness might be an indicator comparable to the flatness
of a surface layer.
[0013] Therefore, the inventors studied the art for reducing as much as possible the deviation
in hardness after hot stamping and as a result discovered that the deviation in surface
layer hardness of the steel sheet before hot stamping has an impact. No literature
has been found which studies steel sheet for hot stamping use from such a perspective.
[0014] PLT 1 discusses steel sheet for hot shaping use where all of Ni, Cu, and Sn are essential,
wherein the impact characteristics and the delayed fracture characteristic are improved,
but does not allude to the fatigue characteristic or the deviation in surface layer
hardness before hot stamping.
[0015] PLT 2 relates to the art of heating in a carburizing atmosphere so as to raise the
strength of a shaped part, but does not allude to the fatigue characteristic or the
deviation in surface layer hardness before hot stamping. Heating in a carburizing
atmosphere is essential. Compared with heating in the air, the production costs rise.
Further, when using carbon monoxide as the source of carbon, there is a concern that
tremendous costs would be required for securing the safety of operations. It is believed
that this art is not easily workable.
[0016] PLT 3 also does not allude to the fatigue characteristic and the deviation in surface
layer hardness before hot stamping.
[0017] As opposed to this, as art for obtaining steel sheet for hot stamping use which has
a fatigue characteristic of the same extent as "ordinary high strength steel sheet",
there is PLT 4. Further, while as art inherent to the case of use of steel sheet which
has been galvanized, PLT 5 is known as art for improving the fatigue characteristic
of a member which is produced by the hot stamping method.
[0018] PLT 4 discloses to make fine particles which contain Ce oxides disperse slight inward
from the steel sheet surface so as to improve the fatigue characteristic after hot
stamping, but advanced steelmaking art is required, so there is the problem that even
a person skilled in the art would not necessarily find it easy to work it.
[0019] The art of PLT 5 relates to facilities for hot stamping technology. There is the
problem that without new capital investment, even a person skilled in the art could
not enjoy its benefits.
In this way, steel sheet for hot stamping use for obtaining steel sheet (product)
made high in strength by hot stamping, which enables fatigue characteristics of the
same extent as "ordinary high strength steel sheet" of the same strength to be secured
relatively easily, has been sought, but no art which solves this problem has been
found.
Citations List
Patent Literature
Summary of Invention
Technical Problem
[0021] The present invention, in view of the above situation, has as its object the provision
of steel sheet for a hot stamped member which enables the production of a product
of high strength steel sheet which has an excellent fatigue characteristic of the
same extent as high strength steel sheet which is produced by controlling the composition
of the steel sheet or method of production ("ordinary high strength steel sheet")
when producing a product by applying the hot stamping method to steel sheet and of
a method of production of the same. Solution to Problem
[0022] The inventors engaged in intensive research to solve this problem. As a result, they
discovered that making the deviation in hardness near the surface layer of steel sheet
before hot stamping within a predetermined range is extremely effective for improving
the fatigue characteristic of the steel sheet after hot stamping (product). They discovered
that such steel sheet can be obtained by controlling the conditions when recrystallization-annealing
the cold rolled steel sheet, conducted repeated tests, and thereby completed the present
invention.
[0023] The gist of the invention is as follows:
- (1) Steel sheet for a hot stamped member which includes composition which contains,
by mass%,
C: 0.15 to 0.35%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.3%,
Al: 0.01 to 0.5%, and
a balance of Fe and unavoidable impurities, and limit the impurities to
P: 0.03% or less,
S: 0.02% or less, and
N: 0.1% or less,
wherein a standard deviation of Vicker's hardness at a position of 20 µm from the
steel sheet surface in the sheet thickness direction is 20 or less.
- (2) The steel sheet for a hot stamped member as set forth in (1) which further contains,
by mass%, one or more of elements selected from
Cr: 0.01 to 2.0%,
Ti: 0.001 to 0.5%,
Nb: 0.001 to 0.5%
B: 0.0005 to 0.01%,
Mo: 0.01 to 1.0%
W: 0.01 to 0.5%,
V: 0.01 to 0.5%,
Cu: 0.01 to 1.0%, and
Ni: 0.01 to 5.0%.
- (3) The steel sheet for a hot stamped member as set forth in (1) or (2) which has
on the surface of the steel sheet one of a 5 µm to 50 µm thick Al plating layer, a
5 µm to 30 µm thick galvanized layer, or a 5 µm to 45 µm thick Zn-Fe alloy layer.
- (4) A method of production of steel sheet for a hot stamped member comprising recrystallization-annealing
cold rolled steel sheet which includes composition which contains, by mass%,
C: 0.15 to 0.35%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.3%,
Al: 0.01 to 0.5%, and
a balance of Fe and unavoidable impurities, and limit the impurities to
P: 0.03% or less,
S: 0.02% or less, and
N: 0.1% or less,
in which step, including
a first stage of heating by an average heating rate of 8 to 25°C/sec from room temperature
to a temperature M (°C) and
then a second stage of heating by an average heating rate of 1 to 7°C/sec to a temperature
S (°C),
wherein the temperature M (°C) is 600 to 700 (°C) and the temperature S (°C) is 720
to 820 (°C).
- (5) The method of production of steel sheet for a hot stamped member as set forth
in (4) wherein the steel further contains, by mass%, one or more of Cr: 0.01 to 2.0%,
Ti: 0.001 to 0.5%,
Nb: 0.001 to 0.5%
B: 0.0005 to 0.01%,
Mo: 0.01 to 1.0%
W: 0.01 to 0.5%,
V: 0.01 to 0.5%,
Cu: 0.01 to 1.0%, and
Ni: 0.01 to 5.0%.
- (6) The method of production of steel sheet for a hot stamped member as set forth
in (4) or (5) wherein a hot rolling rate in the hot rolling step is 60 to 90%, while
a cold rolling rate of the cold rolling step is 30 to 90%.
- (7) The method of production of steel sheet for a hot stamped member as set forth
in any one of (4) to (6) which further includes, after the recrystallizationannealing
step, a step of dipping the steel sheet in an Al bath to form an Al plating layer
on the surface.
- (8) The method of production of steel sheet for a hot stamped member as set forth
in any one of (4) to (6) which further includes, after the recrystallizationannealing
step, a step of dipping the steel sheet in a galvanization bath to form a galvanized
layer on the surface.
- (9) The method of production of steel sheet for a hot stamped member as set forth
in any one of (4) to (6) which further includes, after the recrystallizationannealing
step, a step of dipping the steel sheet in a Zn bath to form a galvanized layer on
the surface, then further heating to 600°C or less to form a Zn-Fe alloy layer on
the surface.
Advantageous Effects of Invention
[0024] The steel sheet for a hot stamped member of the present invention can be produced
by a known steelmaking facility. Further, a shaped part which is obtained using the
steel sheet for a hot stamped member of the present invention for shaping by widespread
hot stamping facilities (hot stamped members) has a fatigue characteristic equal to
"ordinary high strength steel sheet" of the same strength, so has the effect of expanding
the scope of application of hot stamped members (parts).
Brief Description of Invention
[0025]
FIG. 1 is perspective view which shows a sheet press die for hot stamping which is
used for the examples of the present invention. FIG. 2 is a view which shows fatigue
test pieces.
FIG. 3 is a perspective view which shows locations of measurement of hardness in a
test piece for hardness measurement use of the same dimensions as the crack growth
region of the fatigue test piece which is shown in FIG. 2.
FIG. 4 is a graph which shows the correlation between the fatigue limit ratio and
standard deviation of hardness before hot stamping of steel sheet for a hot stamped
member of Example 1.
FIG. 5 is a perspective view which schematically shows steel sheet (member) which
is formed into a hat shape by the hot stamping method.
FIG. 6 is a graph which shows the correlation between the fatigue limit ratio and
standard deviation of hardness before hot stamping of steel sheet for a hot stamped
member of Example 2.
Description of Embodiments
[0026] The inventors engaged in research using steel sheet which contains, by mass%, C:
0.23%, Si: 0.5%, and Mn: 1.6% to prepare a hot stamped member and evaluated its characteristics.
They discovered that the fatigue characteristic is one of the same but that there
are hot stamped members which are the same in composition of the steel sheet and almost
the same in tensile strength, but differ in fatigue characteristic. Therefore, they
investigated the differences of these in detail, whereupon they learned that there
are differences in the deviation in hardness near the surface layers of hot stamped
members. Accordingly, they further changed the composition and recrystallization conditions
of cold rolled steel sheet over a broad range to investigate the fatigue characteristic
of hot stamped members and discovered that there is a strong correlation between the
fatigue characteristic of hot stamped members and the deviation in surface hardness
of the same and that to obtain a hot stamped member which is excellent in fatigue
characteristic, it is effective to make the various in surface hardness of steel sheet
before hot stamping within a predetermined range and that further to obtain such steel
sheet, it is possible to control the conditions when recrystallization-annealing cold
rolled steel sheet to a predetermined range.
[0027] Details will be explained in the examples, but the inventors used these test findings
as the basis to experimentally clarify the suitable range of deviation in hardness
and the annealing conditions and thereby completed the present invention.
Composition of Steel Sheet
[0028] First, the composition of steel sheet will be explained. Here, the "%" in the composition
mean mass%.
C: 0.15 to 0.35%
[0029] C is the most important element in increasing the strength of steel sheet by hot
stamping. To obtain a 1200 MPa or so strength after hot stamping, 0.15% or more has
to be included. On the other hand, if over 0.35% is included, deterioration of toughness
is a concern, so 0.35% is made the upper limit.
Si.: 0.01 to 1.0%
[0030] Si is a solution strengthening element. Up to 1.0% can be effectively utilized. However,
if more than that is included, trouble is liable to occur at the time of chemical
treatment or coating after shaping, so 1.0% is made the upper limit. The lower limit
is not particularly limited. The effect of the present invention can be obtained.
However, reduction more than necessary just raises the steelmaking load, so the content
is made the level of inclusion due to deoxidation, that is, 0.01% or more.
Mn: 0.3 to 2.3%
[0031] Mn is an element which functions as a solution strengthening element in the same
way as Si and also is effective for raising the hardenability of steel sheet. This
effect is recognized at 0.3% or more. However, even if over 2.3% is included, the
effect becomes saturated, so 2.0% is made the upper limit.
P: 0.03% or less, S: 0.02% or less
[0032] The two elements are both unavoidable impurities. They affect the hot workability,
so have to be limited to the above ranges.
Al: 0.01 to 0.5%
[0033] Al is suitable as a deoxidizing element, so 0.01% or more should be included. However,
if included in a large amount, coarse oxides are formed and the mechanical properties
of the steel sheet are impaired, so the upper limit is made 0.5%.
N: 0.1% or less
[0034] N is an unavoidable impurity. It easily bonds with Ti or B, so has to be controlled
so as not to reduce the targeted effect of these elements. 0.1% or less is allowable.
The content is preferably 0.01% or less. On the other hand, reduction more than necessary
places a massive load on the production process, so 0.0010% should be made the target
for the lower limit.
Cr: 0.01 to 2.0%
[0035] Cr has the effect of raising the hardenability, so can be suitably used. This effect
becomes clear at 0.01% or more. On the other hand, even if over 2.0% is added, this
effect becomes saturated, so 2.0% is made the upper limit.
Ti: 0.001 to 0.5%
[0036] Ti is an element which acts to stably draw out the effect of B, explained later,
through the formation of its nitride, so can be effectively used. For this reason,
0.001% or more has to be added, but if excessively added, the nitrides become excessive
and deterioration in toughness or shear surface properties is invited, so 0.5% is
made the upper limit.
Nb: 0.001 to 0.5%
[0037] Nb is an element which forms carbonitrides and raises the strength, so can be effectively
used. This effect is recognized at 0.001% or more, but if over 0.5% is included, the
controllability of the hot rolling is liable to be impaired, so 0.5% is made the upper
limit.
B: 0.0005 to 0.01%
[0038] B is an element which raises the hardenability. The effect becomes clear at 0.0005%
or more. On the other hand, excessive addition leads to deterioration of hot workability
and a drop in the ductility, so 0.01% is made the upper limit.
Mo: 0.01 to 1.0%, W: 0.01 to 0.5%, V: 0.01 to 0.5%
[0039] These elements all have the effect of raising the hardenability, so can be suitably
used. The effect becomes clear in each case at 0.01% or more. On the other hand, it
is an expensive element, so the concentration where the effect becomes saturated is
preferably made the upper limit. For Mo, this is 1.0%, while for W and V, it is 0.5%.
Cu: 0.01 to 1.0%
[0040] Cu has the effect of raising the strength of the steel sheet by addition of Cu in
0.01% or more. However, excessive addition detracts from the surface quality of the
hot rolled steel sheet, so 1.0% is made the upper limit.
Ni: 0.01 to 5.0%
[0041] Ni is an element which has the effect of raising the hardenability, so can be effectively
used. The effect becomes clear at 0.01% or more. On the other hand, it is an expensive
element, so 5.0% where the effect becomes saturated is made the upper limit. Further,
it also acts to suppress the drop in the surface quality of the hot rolled steel sheet
due to Cu, so inclusion simultaneously with Cu is desirable.
[0042] Note that in the present invention, the composition other than the above consist
of Fe, but unavoidable impurities which enter from the scrap and other melting materials
or the refractories etc. are allowed.
[0043] Deviations in Steel Sheet Surface Hardness The deviations in steel sheet surface hardness will be explained.
[0044] First, the method of determining (measuring) the hardness of the steel sheet surface
will be explained.
[0045] The hardness of the steel sheet surface ideally should be measured by a hardness
meter (for example Vicker's hardness meter) with the steel sheet surface facing upward
and with the sheet thickness direction matched with the vertical direction, but to
clearly determine indentations (measure dimensions of indentations precisely), the
surface (measurement surface) has to be polished or other certain work is necessary.
In such work (for example, mechanical polishing), at least several dozen µm or so
are removed from the original surface. Further, even if removing part of the surface
using an acid etc. to chemically polish it, there is no difference. Rather, the smoothness
is often degraded. Therefore, using such a technique to determine (measure) the hardness
of the steel sheet surface is not practical.
[0046] Therefore, the inventors decided to determine the hardness at a cross-section parallel
to the sheet thickness direction of the steel sheet. By doing so, the steel sheet
surface can be measured without working it (without removing the steel sheet surface).
However, in this case as well, the position able to be measured by a hardness meter
in this way is inside from the surface a slight amount in the sheet thickness direction.
For this reason, as a next best solution, the inventors attempted to obtain information
on a portion close to the surface by making an indentation by as low a load as possible.
[0047] Specifically, refer to FIG. 3. First, the measurement surface (steel sheet cross-section)
was polished to a mirror finish. A Vicker's hardness meter was used with a test load
(load pushing in indenter) of 10 gf, a pushing time of 15 seconds, and a measurement
position in the sheet thickness direction of 20 µm from the steel sheet surface. The
"hardness of the steel sheet" as used in the Description indicates the hardness determined
based on the above technique.
[0048] Further, the hardness of the steel sheet surface in steel sheet which has as a surface
layer of the steel sheet either an Al plating layer, galvanized layer, and Zn-Fe alloy
layer was measured at a position 20 µm from the boundary (interface) between the plating
layer and the steel sheet.
[0049] For example, the Al plating layer of the steel sheet which is used in the examples
is deemed to be comprised of an outside layer which has Al as its main composition
and an inside (steel sheet side) layer which is believed to be a reaction layer of
Al and Fe, so the hardness was measured at a position 20 µm from the boundary of the
inside layer and the steel sheet in the sheet thickness direction and this was used
as the surface hardness of the steel sheet.
[0050] Next, the galvanized layer of the steel sheet which is used in the examples is deemed
to be comprised of two layers of an outside layer which has Zn as its main composition
and an inside layer which is a reaction layer of Al which was added in a fine amount
in the Zn bath and Fe, so the hardness was measured at a position 20 µm from the boundary
of the inside layer and the steel sheet in the sheet thickness direction and this
was used as the surface hardness of the steel sheet.
[0051] Further, the Zn-Fe alloy layer of the steel sheet which is used in the examples is
deemed to be comprised of a plurality of alloy layers which are comprised of Zn and
Fe, so the hardness was measured at a position 20 µm from the boundary of the inside-most
layer and the steel sheet in the sheet thickness direction and this was used as the
surface hardness of the steel sheet.
[0052] For the purpose of finding the deviation in hardness, the above measurement was performed
in the region corresponding to the fatigue crack growth region (21) of the fatigue
test piece which is shown in FIG. 2. FIG. 3 is a perspective view which shows the
location of measurement of the hardness. The indenter of the Vicker's hardness meter
was pushed in at a position of 20 µm from the surface or the steel sheet or the interface
of the steel sheet and the plating layer in the sheet thickness direction. This operation,
as shown in FIG. 3, was performed at indentation intervals of 0.1 mm in a direction
parallel to the surface of the steel sheet at 300 points per measurement sample (over
30 mm by measurement length) (first measurement surface). Further, the same operation
was performed at another location 5 mm from the first measurement surface taken in
advance (second measurement surface).
[0053] The hardnesses were found for the total 600 points in this way. The standard deviation
using this as the population was calculated and used as an indicator of the deviation.
[0054] Note that the above measurement length of 30 mm and the two locations 5 mm apart
were determined so as to match with the crack growth region of the fatigue test piece
which is explained later.
[0055] In the experiment which is explained in the examples, samples with a fatigue limit
ratio after hot stamping of 0.4 or more and ones with a ratio below that were compared
for deviation in hardness of the steel sheet surface, whereupon in the former, the
standard deviation was 40 or less. Therefore, the inventors proceeded with more detailed
investigations, whereupon it became clear that the deviation in hardness after hot
stamping has a standard deviation of 40 or less when the deviation in hardness of
the steel sheet before hot stamping, determined by a similar technique, has a standard
deviation of 20 or less.
[0056] In the present invention, the standard deviation of the Vicker's hardness at a position
20 µm from the steel sheet surface in the sheet thickness direction was defined as
20 or less based on such experimental findings.
Method of Production of Steel Sheet for Hot Stamped Member
[0057] Finally, the method of production of steel sheet for a hot stamped member of the
present invention will be explained.
[0058] The steel sheet for a hot stamped member of the present invention is processed in
the accordance with the usual methods by the steps of steelmaking, casting, hot rolling,
pickling, and cold rolling to obtain cold rolled steel sheet. The composition is adjusted
to the above-mentioned scope of the present invention in the steelmaking step, the
steel is cast to a slab in the continuous casting step, then the slab is started to
be hot rolled at for example a 1300°C or less heating temperature. The rolling is
ended around 900°C. The coiling temperature can be selected as, for example 600°C
etc. The hot rolling rate may be made 60 to 90%. The cold rolling is performed after
the pickling step. The rolling rate can be selected from 30 to 90% in range.
[0059] The annealing step for recrystallizing the cold rolled steel sheet which was produced
in this way is extremely important. The annealing step is performed using a continuous
annealing facility and is comprised of two stages of a first step of heating by an
average heating rate of 8 to 25°C/sec from room temperature to the temperature M (°C)
and a second stage of then heating by an average heating rate of 1 to 7°C/sec down
to a temperature S (°C). Here, the temperature M has to be 600 to 700(°C), and the
temperature S has to be 720 to 820(°C). These conditions are determined based on the
results of the experiment which is explained in the examples which are described below.
[0060] The reason why, when recrystallization-annealing under these conditions, the standard
deviation of the Vicker's hardness which was measured at a position of 20 µm from
the steel sheet surface in the sheet thickness direction is 20 or less, that is, steel
sheet with a small deviation in hardness is obtained, is not necessarily clear, but
the distribution of crystal grain size is preferably as uniform as possible and the
dimensions and distribution of carbides are also preferably similarly as uniform as
possible, so the following may be guessed from the viewpoint of the distribution of
recrystallized particle size and the dimensions and distribution of carbides.
[0061] The recrystallization process of cold rolled steel sheet is complicated, so it is
not suitable to separate and independently discuss the meanings of the heating rate
for the phenomenon called recrystallization and the highest heating temperature at
that heating rate. Therefore, first, regarding the first stage, for example, consider
the case where the heating rate is small and where it is large with respect to a certain
single temperature M (°C). It is believed that in the former case, that is, when the
heating rate is small, the density of recrystallization nuclei is (relatively) low
and the individual recrystallized grains freely grow, but in the high temperature
region near M (°C), fine recrystallized grains are produced from the remaining non-recrystallization
region and, at the stage where the temperature of the steel sheet reaches M(°C), (relatively)
large crystal grains and small crystal grains are mixed.
[0062] On the other hand, it is believed that in the case of the latter, that is, when the
heating rate is large, the density of recrystallized grain nuclei is high, a large
number of recrystallized grains grow at a fast rate, and the grain boundaries become
closer and further, in the high temperature region near M (°C), the recrystallized
grains compete in growth and as a result crystal grains which have specific crystal
orientations grow while eating away at crystal grains which have other crystal orientations,
so at the stage when reaching M (°C), it is believed there are large crystal grains
and small crystal grains mixed together. Therefore, a combination of the suitable
heating rate and M (°C) whereby the recrystallized grains become close in grain boundaries
at the stage where the temperature reaches M (°C) becomes necessary for achieving
a more uniform distribution of recrystallized particle sizes. The 8 to 25°C/sec of
the average heating rate of the first stage and the 600 to 700°C of the temperature
M (°C) are believed to correspond to these suitable conditions.
[0063] Next, to control competition of growth of recrystallized grains after the temperature
of the steel sheet reaches M (°C), the heating rate of the second stage has to be
made smaller than the first stage. Further, in the temperature region from the temperature
M (°C) to the temperature S (°C), reformation of carbides due to the diffusion of
carbon becomes active, so the combination of the setting of the highest temperature
S (°C) of the annealing step and the heating rate up to that temperature has important
meaning.
[0064] When the heating rate is small for one S (°C), the carbides which were present at
the temperature M (°C) uniformly grow, so it may be that a steel sheet results in
which carbides of various dimensions which were present in the stage when reaching
the temperature M (°C) are present in various ways. On the other hand, when the heating
rate its large, small carbides disappear and large carbides grow and therefore the
dimensions of the carbides become closer to uniform ones relatively speaking, but
the density becomes small. Therefore, unevenness of hardness of the steel sheet is
caused due to the carbides. As opposed to these, when the combination of the heating
rate and the temperature S (°C) of the second stage is suitable, the small carbides
grow preferentially and it may be that a steel sheet results in which relatively uniform
dimension carbides are dispersed at a suitable density, so the unevenness of hardness
of the steel sheet due to carbides becomes uneven. The 1 to 7°C/sec of the heating
rate of the second stage and the 720 to 820°C of the temperature S (°C) correspond
to such suitable conditions.
[0065] After reaching the temperature S, the temperature S may be held for a short time
or the next cooling step may be immediately shifted to. When holding the temperature
S, from the viewpoint of coarsening of the crystal grains, the holding time is preferably
180 seconds or less, more preferably 120 seconds or less.
[0066] The cooling rate from the temperature S in the cooling step is not particularly limited,
but 30°C/sec or more rapid cooling is preferably avoided. Therefore, the cooling rate
from the temperature S is less than 30°C/sec, preferably 20°C or less, more preferably
10°C or less. Steel sheet for hot stamping use is often sheared to a predetermined
shape and then used for hot stamping. This is because it is feared that rapid cooling
raises the shear load and lowers the production efficiency.
[0067] After annealing, the sheet may be cooled down to room temperature. During cooling,
it may be dipped in a hot dip Al bath to form an Al plating layer.
[0068] The hot dip Al bath may contain 0.1 to 20% of Si.
[0069] The Si which is contained in the Al plating layer affects the reaction of Al and
Fe which occurs during heating before hot stamping. Excessive reaction is liable to
detract from the press formability of the plating layer itself. On the other hand,
excessive control of the reaction is liable to invite adherence of Al on the press
forming die. To avoid such a problem, the content of Si in the Al plating layer is
preferably 1 to 15%, more preferably 3 to 12%.
[0070] Further, during the cooling after annealing, the sheet was dipped in a hot dip galvanization
bath to form a galvanized layer.
[0071] Furthermore, the sheet was dipped in a hot dip galvanization bath to form a galvanized
layer, then was heated to 600°C or less to form a Zn-Fe alloy layer.
[0072] The hot dip galvanization bath could contain 0.01 to 3% of Al.
[0073] The existence of Al has a strong affect on the reaction of Zn and Fe. When forming
a galvanized layer, the reaction layer of the Fe and Al becomes an obstacle and suppresses
mutual dispersion of Zn and Fe. On the other hand, a Zn-Fe alloy layer is comprised
of a Zn-rich alloy layer (ζ-phase, δ
1-phase) and Fe-rich alloy layer (Γ
1-phase, Γ-phase), but the former is rich in adhesion with the base iron, but the workability
is degraded, while the latter is excellent in workability, but is insufficient in
adhesion. Therefore, it is necessary to suitably control the ratio of composition
of these four phases to satisfy the targeted properties (giving preference to adhesion,
giving preference to workability, or balancing the two etc.) This can be performed
by including in the hot dip galvanization bath 0.01 to 3% of Al so as to enable control
of the diffusion of Fe. What sort of concentration to use may be selected by the manufacturer
in accordance with the ability or objective of the production facility.
[0074] The thicknesses of the Al plating layer, galvanized layer, and Zn-Fe alloy layer
do not influence the fatigue characteristic of the steel sheet after hot stamping
or the fatigue characteristic of the parts, but if excessively thick, the press formability
is liable to be affected. As shown in the examples, when the thickness of the Al plating
layer is over 50 µm, the phenomenon of galling is recognized. When the thickness of
the Zn plating layer exceeds 30 µm, adhesion of the Zn to the die frequently occurs.
When the thickness of the Zn-Fe alloy layer is over 45 µm, scattered cracking of the
alloy layer is seen, and the productivity is otherwise impaired. Therefore, the thicknesses
of the layers are preferably made Al plating layer: 50 µm or less, galvanized layer:
30 µm or less, and Zn-Fe alloy layer: 45 µm or less.
[0075] When these plating layers are thin, there is no problem at all in shapeability, but
from the viewpoint of the corrosion resistance, which is aimed at imparting these
plating layers, the lower limits of the plating layers are preferably made as follows:
That is, the limits are the Al plating layer: preferably 5 µm or more, more preferably
10 µm or more, the galvanized layer: preferably 5 µm or more, more preferably 10 µm
or more, and the Zn-Fe alloy layer: preferably 5 µm or more, more preferably 10 µm
or more.
Examples
[0076] Below, examples will be used as the basis to explain the present invention in detail.
Example 1
[0077] Steels "a" to "f" which have the composition which is shown in Table 1 were produced
and cast. The slabs were heated to 1250°C and supplied to a hot rolling step where
they were hot rolled at a final temperature of 900°C and a coiling temperature of
600°C to obtain thickness 3.2 mm steel sheets. These hot rolled steel sheets were
pickled, then cold rolled to obtain thickness 1.6 mm cold rolled steel sheets.
[0078] The cold rolled steel sheets were recrystallized and annealed under the conditions
of i to xviii described in Table 2 to obtain the steel sheets for hot stamped members
1 to 32 which are shown in Table 3. From part, two test pieces for measurement of
the hardness before hot stamping were obtained. The positions for sampling the test
pieces were made positions 5 mm separated in the width direction of the obtained steel
sheet for hot stamped member.
[0079] The average heating rate 1 (first stage) and average heating rate 2 (second stage)
in Table 2 respectively show the average heating rates from room temperature to temperature
M (°C) and the average heating rate from temperature M (°C) to the temperature S (°C).
[0080] These steel sheets for hot stamped members were held at 900°C for 10 minutes, then
were sandwiched by the test-use sheet press die which was shown in FIG. 1 and hot
stamped. Each type of steel sheet for a hot stamped member was used hot stamping 10
pieces. From one among these, two tensile test pieces based on the provisions of JIS
No. 5 and two test pieces for measurement of hardness (same procedure as with hot
stamping) were obtained. From the remaining nine, two fatigue test pieces which are
shown in FIG. 2 each, for a total of 18, were obtained. The method of working for
obtain test pieces was electrodischarge machining.
[0081] A tensile test was performed to find the tensile strength σ
B (average value of two tensile test pieces). On the other hand, 18 test pieces were
used to run a plane bending fatigue test and determine the 1×10
7 cycle fatigue strength σ
W. The test conditions were a stress ratio of -1 and a repetition rate of 5Hz.
[0082] The test pieces for measurement of hardness were polished to a mirror finish at cross-sections
parallel to the rolling directions of cold rolled steel sheets both before and after
hot stamping.
[0083] The hardness at 20 µm inside from the surfaces of these test pieces in the sheet
thickness direction was measured using a Vicker's hardness meter (HM-2000 made by
Mitsutoyo). The pushing load was made 10 gf, the pushing time was made 15 seconds,
and the measurement interval in the direction parallel to the surface made 0.1 mm
for measurement of 300 points.
[0084] Two test pieces were measured in the same way. The standard deviation of hardness
was calculated from the data of the Vicker's hardness of a total of 600 points.
[0085] Table 3 shows the steel number, processing conditions, standard deviation of hardness
before hot stamping, tensile strength σ
B (average of two), strength σ
W, fatigue limit ratio σ
W/σ
B, and standard of hardness after hot stamping. The correlation between the fatigue
limit ratio σ
W/σ
B and the standard deviation of hardness before hot stamping is shown in FIG. 4.
[0086] It was learned that the tensile strength σ
B of steel sheet after hot stamping is almost entirely unaffected by the recrystallization-annealing
conditions in steel sheet of the same composition (code "b"). On the other hand, the
fatigue characteristics (σ
W/σ
B) were strongly affected by the recrystallization-annealing conditions.
[0087] In steel sheets using the annealing conditions i, iii, iv, vii, viii, xv, and xviii
of the present invention, relatively high fatigue characteristics, that is, a 0.4
or more fatigue limit ratio (σ
W/σ
B), could be obtained in the range of about 1200 to 1500 MPa in tensile strength. As
opposed to this, in steel sheets which were annealed under conditions outside the
scope of the present invention, the obtained fatigue limit ratio was a low level of
about 0.3.
[0088] This difference is due to the fact that the fatigue limit ratio is correlated with
the standard deviation of hardness after hot stamping. Simultaneously, it clearly
depends on the standard deviation of the hardness before hot stamping. As shown in
Nos. 1 to 6, 8, 9, 12, 13, 16, 17, 20, 21, and 23 to 28, it became clear that when
the standard deviation of the hardness is 2 or less, a hot stamped member which has
an excellent fatigue characteristic (high fatigue limit ratio) is obtained.
[0089] Further, as the conditions of recrystallization-annealing for obtaining steel sheet
with a standard deviation of hardness before hot stamping of 20 or less, there are
a first stage of heating by an average heating rate of 15 to 25°C/sec from room temperature
to a temperature M (°C) and a second stage of then heating by an average heating rate
of 2 to 5°C/sec to the temperature S (°C). It became clear that M is 620 to 680 (°C)
and S is 780 to 820 (°C).
Table 1
Steel no. |
C |
Si |
Mn |
P |
S |
Al |
N |
Others |
a |
0.25 |
0.7 |
1.9 |
0.02 |
0.002 |
0.03 |
0.004 |
Ti : 0.03, B: 0.003 |
b |
0.23 |
0.5 |
1.6 |
0.02 |
0.002 |
0.03 |
0.003 |
|
c |
0.21 |
0.3 |
1.4 |
0.02 |
0.002 |
0.03 |
0.002 |
B: 0.004 |
d |
0.20 |
0.2 |
1.2 |
0.02 |
0.002 |
0.03 |
0.004 |
Cr: 0.2, Ti: 0.02, B: 0.002 |
e |
0.18 |
0.2 |
1.3 |
0.02 |
0.002 |
0.03 |
0.003 |
Cr: 1.4, Ti: 0.02, B: 0.002 |
f |
0.15 |
0.3 |
1.1 |
0.02 |
0.002 |
0.03 |
0.003 |
Cr: 0.1, B: 0 004 |
Table 2
Condition no. |
Average heating rate 1 (°C/sec) |
Temp. M (°C) |
Average heating rate 2 (°C/sec) |
Temp. S(°C) |
Cooling conditions |
|
i |
20 |
650 |
|
800 |
No holding Cooling by average cooling rate 6°C/sec to 670°C, holding for 10 seconds,
then air cooling to room temperature. |
Inv. ex |
ii |
25 |
590 |
3 |
800 |
Ditto |
Comp. ex. |
iii |
25 |
600 |
3 |
800 |
Ditto |
Inv. ex. |
iv |
8 |
700 |
3 |
800 |
Ditto |
Inv. ex. |
v |
8 |
710 |
3 |
800 |
Ditto |
Comp. ex. |
vi |
15 |
650 |
7 |
830 |
Ditto |
Comp. ex. |
vii |
15 |
650 |
7 |
820 |
Ditto |
Inv. ex. |
viii |
15 |
650 |
2 |
720 |
Ditto |
Inv. ex. |
ix |
15 |
650 |
2 |
710 |
Ditto |
Comp. ex. |
x |
7 |
600 |
4 |
800 |
Ditto |
Comp. ex. |
xi |
8 |
600 |
4 |
800 |
Ditto |
Inv. ex. |
xii |
25 |
700 |
3 |
800 |
Ditto |
Inv. ex. |
xiii |
26 |
700 |
3 |
800 |
Ditto |
Comp. ex. |
xiv |
20 |
650 |
0.5 |
720 |
Ditto |
Comp. ex. |
xv |
20 |
650 |
1 |
720 |
Ditto |
Inv. ex. |
xvi |
20 |
650 |
7 |
820 |
Ditto |
Inv. ex. |
xvii |
20 |
650 |
8 |
820 |
Ditto |
Comp. ex. |
xviii |
20 |
650 |
3 |
800 |
Holding for 10 sec., then air cooling to room temperature |
Inv. ex. |
Underlined figures indicate outside scope of present invention. |
Table 3
No. |
Steel no. |
Processing conditions |
Standard deviation of hardness before hot stamping |
σB (MPa) |
σW (MPa) |
σW/σB (fatigue limit ratio) |
Standard deviation of hardness after hot stamping |
|
1 |
a |
i |
10 |
1510 |
619 |
0.41 |
27 |
Inv. ex. |
2 |
b |
i |
9 |
1508 |
603 |
0.40 |
22 |
Inv. ex. |
3 |
c |
i |
6 |
1501 |
630 |
0.42 |
20 |
Inv. ex. |
4 |
d |
i |
8 |
1498 |
614 |
0.41 |
21 |
Inv. ex. |
5 |
e |
i |
11 |
1503 |
646 |
0.43 |
27 |
Inv. ex. |
6 |
f |
i |
7 |
1422 |
597 |
0.42 |
24 |
Inv. ex. |
7 |
b |
ii |
30 |
1512 |
484 |
0.32 |
46 |
Comp. ex. |
8 |
b |
iii |
12 |
1506 |
602 |
0.40 |
20 |
Inv. ex. |
9 |
b |
iv |
16 |
1489 |
610 |
0.41 |
23 |
Inv. ex. |
10 |
b |
v |
29 |
1502 |
451 |
0.30 |
42 |
Comp. ex. |
11 |
b |
vi |
24 |
1499 |
465 |
0.31 |
44 |
Comp. ex. |
12 |
b |
vii |
1 |
1505 |
647 |
0.43 |
19 |
Inv. ex. |
13 |
b |
viii |
11 |
1516 |
637 |
0.42 |
22 |
Inv. ex. |
14 |
b |
ix |
24 |
1511 |
453 |
0.30 |
43 |
Comp. ex. |
15 |
b |
x |
32 |
1522 |
502 |
0.33 |
51 |
Comp. ex |
16 |
b |
xi |
16 |
1518 |
638 |
0.42 |
24 |
Inv. ex. |
17 |
b |
xii |
19 |
1512 |
650 |
0.43 |
26 |
Inv. ex. |
18 |
b |
xiii |
33 |
1507 |
452 |
0.30 |
49 |
Comp. ex. |
19 |
b |
xiv |
29 |
1500 |
480 |
0.32 |
46 |
Comp. ex. |
20 |
b |
xv |
12 |
1496 |
598 |
0.40 |
22 |
Inv. ex. |
21 |
b |
xvi |
11 |
1506 |
617 |
0.41 |
25 |
Inv. ex. |
22 |
b |
xvii |
27 |
1503 |
496 |
0.33 |
45 |
Comp. ex. |
23 |
a |
xviii |
10 |
1510 |
631 |
0.42 |
19 |
Inv. ex. |
24 |
b |
xviii |
6 |
1512 |
605 |
0.40 |
12 |
Inv. ex. |
25 |
c |
xviii |
8 |
1503 |
601 |
0.40 |
14 |
Inv. ex. |
26 |
d |
xviii |
13 |
1509 |
649 |
0.43 |
24 |
Inv. ex. |
27 |
e |
xviii |
18 |
1499 |
600 |
0.40 |
27 |
Inv. ex. |
28 |
f |
xviii |
11 |
1418 |
610 |
0.43 |
22 |
Inv. ex. |
Underlined figures indicate outside scope of present invention. |
Example 2
[0090] Steels 2a to 2h which have the composition which is shown in Table 4 were produced
and cast The slabs were hot rolled under the same conditions as Example 1 to obtain
thickness 3.0 mm steel sheets These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0091] These steel sheets were recrystallized and annealed under conditions of i, ix, and
xviii of Table 2 to obtain steel sheets for hot stamped members.
[0092] From these steel sheets, test pieces for measurement of hardness were obtained by
the same procedure was in Example 1
[0093] These steel sheets for a hot stamped member were held at 900°C for 5 minutes, then
were formed to hat shapes which are shown in FIG. 5 by the hot stamping method. As
shown in this figure, fatigue test pieces which are shown in FIG. 2 and JIS No. 5
tensile test pieces were obtained from the top parts of the hats.
[0094] These test pieces were used by the same procedure as in Example 1 to find the standard
deviation of hardness before hot stamping and the tensile strength σ
B (average of two) and 1×10
7 cycle fatigue strength σ
W of the steel sheet after hot stamping (member).
[0095] Table 5 should these results. The correlation between the fatigue limit ratio σ
W/σ
B and the standard deviation of the hardness before hot stamping is shown in FIG. 6.
[0096] In steel sheets for a hot stamped member which were recrystallized and annealed using
conditions i and xviii in the scope of the present invention, even if steel sheets
which contain Mo, W, V, Cu, and Ni, the deviation in hardness of the surface layer
before hot stamping had a standard deviation of 20 or less. Further, if using these,
it became clear that a hot stamped member with a fatigue limit ratio of 0.4 or more,
that is, excellent in fatigue characteristic, was obtained.
[0097] On the other hand, in steel sheets which were recrystallized and annealed using the
condition ix which is outside the scope of the present invention, the deviation in
hardness of the surface layer before hot stamping has a standard deviation of over
20. The fatigue limit ratio of the hot stamped members obtained by using these was
0.26 to 0.31. It became clear the fatigue characteristic was inferior.
Table 4
Steel no. |
Composition (mass%) |
C |
Si |
Mn |
P |
S |
Al |
N |
Others |
2a |
0.35 |
0.3 |
1.0 |
0.02 |
0.004 |
0.03 |
0.004 |
Cr: 0.2 Ti: 0.01, B. 0.002, Cu 0.1, Ni : 0.1 |
2b |
0.31 |
1.5 |
1.2 |
0.02 |
0.004 |
0.03 |
0.004 |
Cr: 0.5, Ti: 0.02, B: 0.004, Nb 0.02, Mo: 0.2 |
2C |
0.28 |
1.0 |
1.7 |
0.02 |
0.004 |
0.03 |
0.004 |
W: 0.2 Ni: 2.0 |
2d |
0.25 |
0.8 |
1.9 |
0.02 |
0.004 |
0.03 |
0.004 |
Ti: 0.03, B. 0.003, Mo: 0.2 Ni: 1.0 |
2c |
0.23 |
0.8 |
1.6 |
0.02 |
0.004 |
0.03 |
0.003 |
Mo 0.1 W: 0.5, V: 0.5 |
2f |
0.21 |
0.4 |
1.4 |
0.02 |
0.004 |
0.03 |
0.002 |
B: 0.004, Mo: 0.1, V: 0.5 |
2g |
0.20 |
0.3 |
1.2 |
0.02 |
0.004 |
0.03 |
0.004 |
Cr 0.2, Ti: 0.02, Mo: 0.2, W: 0.4 |
2h |
0.18 |
0.3 |
1.3 |
0.02 |
0.004 |
0.03 |
0.003 |
Cr: 1 4 Ti: 0.02, R. 0.002, Mo: 0.1, V:0.2 |
Table 5
No |
Steel no. |
Processing conditions |
Standard deviation of hardness before hot stamping |
σB (MPa) |
σW (MPa) |
σW/σB, (fatigue limit ratio) |
|
29 |
2a |
i |
18 |
1794 |
718 |
0.40 |
Inv. ex. |
30 |
2a |
ix |
40 |
1790 |
465 |
0.26 |
Comp. ex. |
31 |
2a |
xviii |
19 |
1802 |
721 |
0.40 |
Inv. ex. |
32 |
2b |
i |
16 |
1706 |
682 |
0.40 |
Inv. ex. |
33 |
2b |
ix |
37 |
1696 |
441 |
0.26 |
Comp ex. |
34 |
2b |
xviii |
18 |
1711 |
702 |
0.41 |
Inv. ex. |
35 |
2C |
i |
15 |
1598 |
639 |
0.40 |
Inv. ex. |
36 |
2C |
ix |
30 |
1592 |
430 |
0.27 |
Comp. ex. |
37 |
2C |
xviii |
14 |
1590 |
636 |
0.40 |
Inv. ex. |
38 |
2d |
i |
15 |
1492 |
612 |
0.41 |
Inv. ex. |
39 |
2d |
iv |
26 |
1500 |
435 |
0.29 |
Comp. ex. |
40 |
2d |
xviii |
5 |
1498 |
614 |
0.41 |
Inv. ex. |
41 |
2e |
i |
9 |
1492 |
597 |
0.4 |
Inv. ex. |
42 |
2e |
ix |
31 |
1502 |
421 |
0.28 |
Comp. ex. |
43 |
2e |
xviii |
10 |
1516 |
622 |
0.41 |
Inv ex. |
44 |
2f |
i |
12 |
1508 |
603 |
0.4 |
Inv. ex. |
45 |
2f |
ix |
36 |
1512 |
462 |
0.31 |
Comp. ex. |
46 |
2f |
xviii |
19 |
1522 |
609 |
0.4 |
Inv. ex. |
47 |
2g |
i |
14 |
1496 |
613 |
0.41 |
Inv. ex. |
48 |
2g |
ix |
33 |
1504 |
406 |
0.27 |
Comp. ex. |
49 |
2g |
xviii |
13 |
1526 |
641 |
0.42 |
Inv. ex. |
50 |
2h |
i |
14 |
1506 |
602 |
0.4 |
Inv. ex. |
51 |
2h |
ix |
32 |
1512 |
454 |
0.3 |
Comp. ex. |
52 |
2h |
xviii |
15 |
1528 |
64 |
0.42 |
Inv. ex. |
Underlined figures indicate outside scope of present invention. |
Example 3
[0098] Steels 3a to 3d which have the composition which is shown in Table 6 were produced
and cast. The slabs were hot rolled under the same conditions as Example 1 to obtain
thickness 2.5 mm steel sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0099] These steel sheets were heated by an average heating rate of 19°C/sec up to 655°C,
then were heated by an average heating rate of 2.5°C to 800°C, then were immediately
cooled by an average cooling rate of 6.5°C/sec. Further, they were dipped in a 670°C
hot dip Al bath (containing 10% of Si and unavoidable impurities), taken out after
5 seconds, adjusted in amount of deposition by a gas wiper, then air cooled down to
room temperature.
[0100] From the obtained steel sheets, the same procedure as in Example 1 was used to obtain
test pieces for measurement of hardness. To measure the hardness, the hardness at
a position 20 µm from the boundary of the inside layer of the Al plating layer (reaction
layer of Al and Fe) and the steel sheet was measured by the same procedure as in Example
1. At the time of this measurement, the thickness of the Al plating layer (total of
two layers) was also measured. The range of measurement of thickness was made the
same length 30 mm as the range of measurement of hardness. Seven points were measured
at measurement intervals of 5 mm at each of the first measurement surface and second
measurement surface for a total of 14 measurement positions. The average value was
found.
[0101] These steel sheets were hot stamped into hat shapes by the same procedure as in Example
2. The heating conditions were holding at 900°C for 1 minute.
[0102] From the top parts of the hats, fatigue test pieces which are shown in FIG. 2 and
JIS No. 5 tensile test pieces were obtained.
[0103] These test pieces were used to find the tensile strength σ
B (average of two) and 1×10
7 cycle fatigue strength σ
w. Table 7 shows the results.
[0104] In all examples, excellent steel sheet for a hot stamped member with a fatigue limit
ratio of 0.4 or more was obtained, but in Nos. 57, 62, 67, and 72 where the thickness
of the Al plating layer exceeded 50 µm, a galling phenomenon occurred at a high frequency
at the long wall parts of the hat shape. In examples of 50 µm or less, no galling
phenomenon occurred at all Therefore, it was judged that the upper limit of thickness
when Al plating the steel sheet surface is 50 µm or less.
Table 6
Steel no. |
C |
Si |
Mn |
P |
S |
Al |
N |
Others |
3a |
0.33 |
0.09 |
1.8 |
0.01 |
0.004 |
0.04 |
0.003 |
Cr: 0.2, Mo: 0.2, Cu: 0.1, Ni: 0.05 |
3b |
0.25 |
0.18 |
1.4 |
0.01 |
0.004 |
0.04 |
0.003 |
Cr: 0.002 Ti: 0.02. B: 0.003, Mo: 0.2, W: 0.1 V: 0.1 |
3C |
0.22 |
0.12 |
1,3 |
0.02 |
0.008 |
0.03 |
0.004 |
Cr: 0.13, Ti: 0.03 Nb: 0.02, B. 0.002 |
3d |
0.15 |
0.33 |
1.0 |
0.02 |
0.008 |
0.03 |
0.004 |
B: 0.0005 |
Table 7
No |
Steel no. |
Standard deviation of hardness before hot stamping |
σB (MPa) |
σW (MPa) |
σW/σB (fatigue limit ratio) |
Thickness of Al plating layer (µm) |
|
53 |
3a |
17 |
1784 |
714 |
0.40 |
16.0 |
Inv. ex. |
54 |
3a |
18 |
1789 |
716 |
0.40 |
22.2 |
Inv. ex. |
55 |
3a |
16 |
1801 |
720 |
0.40 |
33.9 |
Inv. ex. |
56 |
3a |
14 |
1792 |
717 |
0.40 |
48.6 |
Inv. ex. |
57 |
3a |
14 |
1790 |
716 |
0.40 |
51.0 |
Comp. ex. |
58 |
3b |
12 |
1516 |
652 |
0.43 |
1.1 |
Inv. ex. |
59 |
3b |
15 |
1520 |
638 |
0.42 |
19, 6 |
Inv. ex. |
60 |
3b |
19 |
1524 |
671 |
0.44 |
34.2 |
Inv. ex. |
61 |
3b |
18 |
1522 |
685 |
0.45 |
49.6 |
Inv. ex. |
62 |
3b |
20 |
1534 |
614 |
0.40 |
54.7 |
Comp. ex. |
63 |
3C |
11 |
1502 |
631 |
0.42 |
14.5 |
Inv. ex. |
64 |
3C |
14 |
1509 |
649 |
0.43 |
20.1 |
Inv. ex. |
65 |
3C |
9 |
1513 |
635 |
0.42 |
34.6 |
Inv. ex. |
66 |
3C |
13 |
1519 |
668 |
0.44 |
49.2 |
Inv. ex. |
67 |
3C |
18 |
1524 |
610 |
0.40 |
55.3 |
Comp. ex. |
68 |
3d |
10 |
1318 |
554 |
0.42 |
17.2 |
Inv. ex. |
69 |
3d |
10 |
1326 |
557 |
0.42 |
20.4 |
Inv. ex. |
70 |
3d |
8 |
1320 |
554 |
0.42 |
10.2 |
Inv. ex. |
71 |
3d |
14 |
1314 |
539 |
0.41 |
42.0 |
Inv. ex. |
72 |
3d |
15 |
1310 |
537 |
0.41 |
53.6 |
Comp. ex. |
Underlined figures indicate outside scope of present invention |
Example 4
[0105] Steels 3a to 3d which have the composition which is shown in Table 6 were produced
and cast. The slabs were hot rolled under the same conditions as Example 1 to obtain
thickness 2.5 mm steel sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0106] These steel sheets were heated by an average heating rate of 19°C/sec up to 655°C,
then were heated by an average heating rate of 2.5°C to 800°C, then were immediately
cooled by an average cooling rate of 6.5°C/sec. Further, they were dipped in a 460°C
hot dip galvanization bath (containing 0.15% of Al and unavoidable impurities), taken
out after 3 seconds, adjusted in amount of deposition by a gas wiper, then air cooled
down to room temperature.
[0107] From the obtained steel sheets, the same procedure as in Example 1 was used to obtain
test pieces for measurement of hardness. To measure the hardness, the hardness at
a position 20 µm from the boundary of the inside layer of the Zn plating layer (reaction
layer of Al and Fe) and the steel sheet was measured by the same procedure as in Example
1. At the time of this measurement, the thickness of only the Zn plating layer may
also be measured. The range of measurement of thickness was made the same length 30
mm as the range of measurement of hardness. Seven points were measured at measurement
intervals of 5 mm at each of the first measurement surface and second measurement
surface for a total of 14 measurement positions. The average value was found.
[0108] These steel sheets were hot stamped into hat shapes by the same procedure as in Example
2. They were heated to 880°C and held for 5 seconds, then air-cooled down to 700°C
and pressed.
[0109] From the top parts of the hats, fatigue test pieces which are shown in FIG. 2 and
JIS No. 5 tensile test pieces were obtained.
[0110] These test pieces were used to find the tensile strength σ
B (average of two) and 1×10 cycle fatigue strength σ
W Table 8 shows the results
[0111] In all examples, excellent steel sheet for a hot stamped member with a fatigue limit
ratio of 0.4 or more was obtained, but in Nos 77, 82, 87, and 92 where the thickness
of the galvanized layer exceeded 30 µm, adhesion of Zn was observed at a high frequency
in the die In examples of 30 µm or less, no adhesion of Zn occurred at all. Therefore,
it was judged that the upper limit of thickness when galvanizing the steel sheet surface
is 30 µm or less
Table 8
No. |
Steel no. |
Standard deviation of hardness before hot stamping |
σB (MPa) |
σW (MPa) |
σW/σB (fatigue limit ratio) |
Thickness of galvanized layer (µm) |
|
73 |
3a |
17 |
1785 |
714 |
0.40 |
6.1 |
Inv. ex. |
74 |
3a |
17 |
1788 |
715 |
0.40 |
12.5 |
Inv. ex. |
75 |
3a |
16 |
1802 |
721 |
0.40 |
23.8 |
Inv. ex. |
76 |
3a |
13 |
1794 |
718 |
0.40 |
28.6 |
Inv. ex. |
77 |
3a |
5 |
1793 |
717 |
0.40 |
31.0 |
Comp. ex. |
78 |
3b |
12 |
1516 |
652 |
0.43 |
11.1 |
Inv. ex. |
79 |
3b |
15 |
1522 |
639 |
0.42 |
19.6 |
Inv. ex. |
80 |
3b |
19 |
1534 |
675 |
0.44 |
24.8 |
Inv. ex. |
81 |
3b |
18 |
1532 |
689 |
0.45 |
29.0 |
Inv. ex. |
82 |
3b |
20 |
1543 |
618 |
0.40 |
33.7 |
Comp. ex. |
83 |
3c |
10 |
1518 |
638 |
0.42 |
10.3 |
Inv. ex. |
84 |
3c |
14 |
1 36 |
660 |
0.43 |
17.2 |
Inv. ex. |
85 |
3c |
9 |
1524 |
640 |
0.42 |
19.6 |
Inv. ex. |
86 |
3c |
14 |
1539 |
677 |
0.44 |
29.3 |
Inv. ex. |
87 |
3c |
13 |
1544 |
618 |
0.40 |
32.3 |
Comp. ex. |
88 |
3d |
10 |
1336 |
561 |
0.42 |
11.2 |
Inv ex. |
89 |
3d |
12 |
1342 |
564 |
0.42 |
17.4 |
Inv. ex. |
90 |
3d |
8 |
1318 |
554 |
0.42 |
20.2 |
Inv. ex. |
91 |
3d |
13 |
1320 |
541 |
0.41 |
28.0 |
Inv. ex. |
92 |
3d |
15 |
1330 |
545 |
0.41 |
33.4 |
Comp. ex. |
Underlined figures indicate outside scope of present invention. |
Example 5
[0112] Steels 3a to 3d which have the composition which is shown in Table 6 were produced
and cast The slabs were hot rolled under the same conditions as Example 1 to obtain
thickness 2.5 mm steel sheets. These hot rolled steel sheets were pickled, then cold
rolled to 1.2 mm.
[0113] These steel sheets were heated by an average heating rate of 19°C/sec up to 655°C,
then were heated by an average heating rate of 2.5°C to 800°C, then were immediately
cooled by an average cooling rate of 6.5°C/sec. Further, they were dipped in a 460°C
hot dip galvanization bath (containing 0.13% of Al, 0.03% of Fe, and unavoidable impurities),
taken out after 3 seconds, adjusted in amount of deposition by a gas wiper, then heated
to 480°C to form an Zn-Fe alloy layer, then air cooled down to room temperature.
[0114] From the obtained steel sheets, the same procedure as in Example 1 was used to obtain
test pieces for measurement of hardness. To measure the hardness, the hardness at
a position 20 µm from the boundary of the inner-most layer of the Zn-Fe alloy layer
(reaction layer of Zn and Fe) and the steel sheet was measured by the same procedure
as in Example 1. At the time of this measurement, the total thickness of the Zn-Fe
alloy layer (which was comprised of four layers) was also measured. At the time of
this measurement, the thickness of the Al plating layer (total of two layers) was
also measured. The range of measurement of thickness was made the same length 30 mm
as the range of measurement of hardness. Seven points were measured at measurement
intervals of 5 mm at each of the first measurement surface and second measurement
surface for a total of 14 measurement positions. The average value was found.
[0115] These steel sheets were hot stamped into hat shapes by the same procedure as in Example
2. They were heated to 880°C and held for 5 seconds, then air-cooled down to 700°C
and pressed.
[0116] From the top parts of the hats, fatigue test pieces which are shown in FIG. 2 and
JIS No. 5 tensile test pieces were obtained.
[0117] These test pieces were used to find the tensile strength σ
B (average of two) and 1×10
7 cycle fatigue strength σ
W. Table 9 shows the results.
[0118] In all examples, excellent steel sheet for a hot stamped member with a fatigue limit
ratio of 0 4 or more was obtained, but in Nos 97, 102, 107, and 112 where the thickness
of the Zn-Fe alloy layer exceeded 45 µm, fine cracks occurred in the alloy layer after
pressing In examples of 45 µm or less, no fine cracks formed at all. Therefore, it
was judged that the upper limit of thickness when forming a Zn-Fe alloy layer on the
steel sheet surface is 45 µm or less
Table 9
No. |
Stell no. |
Standard deviation of hardness before hot stamping |
σB (MPa) |
σW (MPa) |
σW/ σB (fatigue limit ratio) |
Thickness of Zn-Fe alloy layer (µm) |
|
93 |
3a |
17 |
1773 |
727 |
0.41 |
15.0 |
Inv. ex. |
94 |
3a |
16 |
1777 |
711 |
0.40 |
22.2 |
Inv. ex. |
95 |
3a |
17 |
1802 |
739 |
0.41 |
31.5 |
Inv. ex. |
96 |
3a |
14 |
1786 |
714 |
0.40 |
39.9 |
Inv. ex. |
97 |
3a |
13 |
1772 |
109 |
0.40 |
46.0 |
Comp. ex. |
98 |
3b |
12 |
1505 |
632 |
0.42 |
15.7 |
Inv. ex. |
99 |
3b |
18 |
1519 |
638 |
0.42 |
21.6 |
Inv. ex. |
100 |
3b |
19 |
1513 |
651 |
0.43 |
39.2 |
Inv. ex. |
101 |
3b |
18 |
1502 |
661 |
0.44 |
44.6 |
Inv. ex. |
102 |
3b |
14 |
1518 |
622 |
0.41 |
49.7 |
Comp. ex. |
103 |
3C |
11 |
1506 |
633 |
0.42 |
14.5 |
Inv. ex. |
104 |
3C |
14 |
1503 |
646 |
0.43 |
20.8 |
Inv. ex. |
105 |
3C |
9 |
1500 |
645 |
0.43 |
34.6 |
Inv. ex. |
106 |
3C |
12 |
1506 |
633 |
0.42 |
42.2 |
Inv. ex. |
107 |
3C |
19 |
1510 |
619 |
0.41 |
45.3 |
Comp. ex. |
108 |
3d |
17 |
1307 |
523 |
0.40 |
15.2 |
Inv. ex. |
109 |
3d |
11 |
1313 |
551 |
0.42 |
18.4 |
Inv. ex. |
110 |
3d |
8 |
1320 |
554 |
0.42 |
30.6 |
Inv. ex. |
111 |
3d |
14 |
1314 |
539 |
0.41 |
42.9 |
Inv. ex. |
112 |
3d |
15 |
1310 |
537 |
0.41 |
48.6 |
Comp. ex. |
Underlined figures indicate outside scope of present invention |
Preference Signs List
[0119]
- 11a
- top die
- 11b
- bottom die
- 12
- steel sheet
- 21
- fatigue crack growth region
- 51
- test piece sampling position