DESCRIPTION
Title of Invention: HIGH-STRENGTH HOT-FORMED STEEL SHEET
MEMBER
Technical Field
[0001] The present invention relates to a high
strength hot formed steel sheet member, more particularly
relates to a high strength hot formed steel sheet member
10 excellent in delayed fracture resistance.
Background Art
[0002] In the field of steel sheets for automobile
use, to achieve both lighter weight for improved fuel
efficiency and improvement of the impact resistance,
15 there has been growing use of high strength steel sheet
having a high tensile strength. However, along with
higher strength, the press formability of steel sheet
falls, so production of complicated shapes of products
has become difficult.
20 [0003] As a result, for example, along with the higher
strength of steel ~heet, the problem of the ductility
falling and fracture occurring at portions with a high
working degree and the problem of the springback and wall
camber becoming greater and therefore the dimensional
25 precision deteriorating arise. Therefore, it has not been
easy to press-form steel sheet having a high strength, in
particular 780 MPa or more tensile strength, into a
product having a complicated shape.
[0004] Therefore, in recent years, as disclosed in PLT
30 1, as art for press-forming high strength steel sheet and
other such hard-to-shape materials, hot stamping has been
employed. "Hot stamping" is a hot forming technique which
heats a material used for forming and then forms it. With
this technique, the sheet is hardened simultaneously with
35 the forming process, so at the time of the forming
process, the steel sheet is soft and has good
shapeability while after the forming process, the shaped
5
- 2 -
member can be given a strength higher than steel sheet
for cold forming use.
[0005] PLT 2 discloses a steel member having a 980 MPa
tensile strength.
[0006] PLT 3 discloses to lower the cleanliness and
segregation ratios of P and S to obtain a hot pressed
steel sheet member excellent in strength and toughness.
Citation List
Patent Literature
10 [0007] PLT 1. Japanese Patent Publication No. 2002-
102980A
PLT 2. Japanese Patent Publication No. 2006-213959A
PLT 3. Japanese Patent Publication No. 2007-314817A
Summary of Invention
15 Technical Problem
[0008] The metal material of PLT 1 is insufficient in
hardenability at the time of hot pressing, so there is
the problem of inferior stability of hardness as a
result. PLTs 2 and 3 disclose steel sheets excellent in
20 tensile strength and toughness, so room remains for
improvement in terms of the delayed fracture resistance.
[0009] The present invention was made for solving the
above problem and has as its object the provision of high
strength hot formed steel sheet member realizing both
25 hardness stability and delayed fracture resistance. Note
that, a hot formed steel sheet member is in many cases
not a flat sheet, but a shaped member. In the present
invention, this will be referred to as a "hot formed
steel sheet member" including also the case of a shaped
30 member.
Solution to Problem
[0010] The inventors engaged in intensive studies on
the relationship of the chemical composition and metal
structure for satisfying both hardness stability and
35 delayed fracture resistance. As a result, they obtained
the following discoveries.
[0011] (a) By refining the prior y-grains, it is
- 3 -
possible to improve the fracture resistance and suppress
delayed fracture. To refine the priory-grains, it is
necessary to include a prescribed amount of Nb.
[0012] (b) If the steel contains a large amount of
5 inclusions, hydrogen is trapped at the interfaces of the
inclusions. This easily becomes the starting points of
delayed fracture. For this reason, in particular in the
case of such a hot formed steel sheet member having a 1.7
GPa or more tensile strength, it is necessary to lower
10 the value of the cleanliness of the steel prescribed in
JIS G 0555 (2003).
[0013] (c) By being able to reduce the center
segregation of Mn, it becomes possible to suppress the
concentration of MnS acting as the starting points of
15 delayed fracture and suppress the formation of hard
structures at the center part of sheet thickness. To
reduce the center segregation of Mn, it is necessary to
limit the Mn content to a certain value or less and to
lower the segregation ratio of Mn.
20 [0014] (d) If limiting the Mn content, the
hardenability falls and the hardness stability
deteriorates, so it is necessary to supplement the
hardenability by including mainly Cr and B.
[0015] (e) If the number density of the residual
25 carbides is high, they become hydrogen trapping sites in
the same way as inclusions and become starting points for
delayed fracture. For this reason, it is necessary to
lower the number density.
[0016] (f) By hot forming steel sheet adjusted in
30 chemical composition, reduced in inclusions, and reduced
in center segregation of Mn in the above way while
reducing the residual carbide density, it is possible to
obtain a steel sheet member excellent in hardness
stability and delayed fracture resistance.
35 [0017] The present invention was made based on the
above discoveries and has as its gist the following.
[0018] (1) A high strength hot formed steel sheet
- 4 -
member having: a chemical composition comprising, by
mass%, C: 0.25 to 0.40%, Si: 0.005 to 0.14%, Mn: 1.50% or
less, P: 0.02% or less, S: 0.005% or less, sol. Al:
0.0002 to 1.0%, N: 0.01% or less, Cr: 0.25 to 3.00%, Ti:
5 0.01 to 0.05%, Nb: 0.01 to 0.50%, and B: 0.001 to 0.01%,
a balance of Fe and unavoidable impurities, a total of
content of Mn and cbntent of Cr of 1.5 to 3.5%, an Mn
segregation ratio a represented by the following formula
(i) of 1.6 or less, a value of cleanliness of steel
10 prescribed by JIS G 0555(2003) of 0.08% or less, an
average grain size of prior y-grains of 10 ~ or less, and
a number density of residual carbides present of 4xl03/mm2
or less:
a~[Maximum Mn concentration at center part in sheet
15 thickness (mass%)]/
[Average Mn concentration at position of 1/4 sheet
thickness depth from surface (mass%)] ... (i)
[0019] (2) The high strength hot formed steel sheet
member according to (1) wherein the chemical composition
20 further includes, by mass%, one or more elements selected
from Ni: 0 to 3.0%, Cu: 0 to 1.0%, Mo: 0 to 2.0%, V: 0 to
0.1%, and Ca: 0 to 0.01%.
[0020] (3) The high strength hot formed steel sheet
member according to (1) or (2) having a plating layer at
25 the surface of the steel sheet.
[0021] (4) The high strength hot formed steel sheet
member according to any one of (1) to (3) wherein the
steel sheet member has a 1.7 GPa or more tensile
strength.
30 Advantageous Effects of Invention
[0022] According to the present invention, it is
possible to obtain a high strength hot formed steel sheet
member having a 1.7 GPa or more tensile strength and able
to realize both hardness stability and delayed fracture
35 resistance. The high strength hot formed steel sheet
member of the present invention is particularly suitable
- 5 -
for use. as an impact resistant part of an automobile.
Brief Description of Drawings
[0023] FIG. l is a schematic view showing the shape of
a die set in forming a hat shape in an example.
5 FIG. 2 is a schematic view showing the shape of a shaped
article obtained by hot forming in an example.
Description of Embodiment
[0024] Below, the requirements of the present
invention will be explained in detail.
10 [0025] (A) Chemical Composition
The reasons for limitation of the elements are as
follows. Note that in the following explanation, the "%"
in the content means "mass%".
[0026] C: 0.25 to 0.40%
15 C is an important element for raising the hardenability
of steel and securing the strength after hardening.
Further, C is an austenite-forming element, so has the
action of suppressing the strain-induced ferrite
transformation at the time of high strain formation. For
20 this reason, obtaining a stable hardness distribution in
the hot formed steel sheet member is facilitated. If the
C content is less than 0.25%, it becomes difficult to
secure a 1100 MPa or more tensile strength after
hardening and to obtain the above effect. Therefore, the
25 C content is made 0.25% or more. On the other hand, if
the C content exceeds 0.40%, the strength after hardening
excessively rises and the toughness deteriorates.
Therefore, the C content is made 0. 40% or less .. The C
content is preferably 0.37% or less, more preferably
30 0.35% or less.
[0027] Si: 0.005 to 0.14%
Si is an element having the action of suppressing the
formation of scale at the time of high temperature
heating at the time of hot forming. If the Si content is
35 less than 0.005%, the above effect can no longer be
sufficiently obtained. Therefore, the Si content is made
0.005% or more. On the other hand, if the Si content is
- 6 -
over 0.14%, the heating temperature required for
austenite transformation at the time of hot forming
becomes remarkably high. For this reason, a rise in the
cost required for heat treatment is invited and
5 insufficient heating causes the hardening to become
insufficient.
[0028] Further, Si is a ferrite-forming element, so if
the Si content is too high, strain-induced ferrite
transformation easily occurs at the time of high strain
10 formation, so at the hot formed steel sheet member, a
local drop in hardness is caused and a stable hardness
distribution can no longer be obtained. Furthermore, if
including a large amount of Si, sometimes the wettability
drops when performing hot dip coating and gives rise to
15 nonplating defects. Therefore, the Si content is made
0.14% or less. An Si content of 0.01% or more is
preferable, while 0.03% or more is more preferable.
Further, the Si content is preferably 0.12% or less.
[0029] Mn: 1.50% or less
20 Mn is an element useful for raising the hardenability of
steel sheet and stably securing the strength after hot
forming. However, in the present invention, to reduce the
center segregation of Mn, the content has to be limited.
If the Mn content is over 1.50%, the segregation of Mn
25 causes the toughness to deteriorate. Therefore, the Mn
content is made 1.50% or less. An Mn content of 0.5% or
more is preferable, and 1.3% or less is preferable.
[0030] P: 0.02% or less
P is an element contained as an impurity, but has the
30 action of raising the hardenability of the steel and
furthermore stably securing the strength of the steel
after hardening, so may be proactively included. However,
if the P content exceeds 0.02%, the toughness remarkably
deteriorates. Therefore, the P content is made 0.02% or
35 less. A P content of 0.01% or less is preferable. A lower
limit of the P content does not have to be particularly
set. However, excessive reduction of the P content causes
- 7 -
the cost to remarkably rise, so the P content is
preferably 0.0002% or more.
[0031] S: 0.005% or less
S is an element contained as an impurity, but forms MnS
5 and degrades the delayed fracture property. If the S
content exceeds 0.005%, the toughness and delayed
fracture property remarkably deteriorate. Therefore, the
S content is made 0.005% or less. A lower limit of the S
content does not have to be particularly set. However,
10 excessive reduction of the S content causes the cost to
remarkably rise, so the S content is preferably 0.0002%
or more.
[0032] Sol. Al: 0.0002 to 1.0%
Al is an element having the action of deoxidizing the
15 molten steel and making the steel sounder. If the sol. Al
content is less than 0.0002%, the deoxidation is not
sufficient. Furthermore, Al is also an element which has
the action of raising the hardenability of the steel
sheet and stably securing the strength after hardening,
20 so may be proactively included. Therefore, the sol .. ·Al
content is made 0.0002% or more. However, even if over
1.0% is included, the effect obtained by that action is
small and the cost increases. For this reason, the Al
content is made 1.0% or less. AnAl content of 0.01% or
25 more is preferable, an 0.2% or less is preferable.
[0033] N: 0.01% or less
N is an element contained as an impurity and degrades the
toughness. If the N content exceeds 0.01%, coarse
nitrides are formed in the steel and the local
30 deformation ability and toughness are remarkably
degraded. Therefore, the N content is made 0.01% or less.
An N content of 0.008% or less is preferable. A lower
limit of the N content does not have to be particularly
set. However 1 excessive reduction of the N content causes
35 the cost to remarkably rise, so the N content is
preferably 0.0002% or more. 0.0008% or more is more
preferable.
- 8 -
[0034] Cr: 0.25 to 3.00%
Cr is an element having the action of raising the
hardenability of the steel. For this reason, in the
present invention, which limits the Mn content to 1.5% or
5 less, it is a particularly important element. Further, Cr
is an austenite-forming element and has the action of
suppressing the strain-induced ferrite transformation at
the time of high strain formation. For this reason, by
including Cr, it becomes easy to obtain a stable hardness
10 distribution in the hot formed steel sheet member.
[0035] If the Cr content is less than 0.25%, the above
effect cannot be sufficiently obtained. Therefore, the Cr
content is made 0.25% or more. On the other hand, if the
Cr content exceeds 3.00%, the Cr concentrates at the
15 carbides in the steel to thereby delay the dissolution of
carbides in the heating process when supplied for hot
forming and to lower the hardenability. Therefore, the Cr
content is made 3.00% or less. A Cr content of 0.3% or
more is preferable, while 0.4% or more is more
20 preferable. Further, a Cr content of 2.5% or less is
preferable.
[0036] Ti: 0.01 to 0.05%
Ti is an element having the action of suppressing the
recrystallization of the austenite grains when heating a
25 hot-forming use steel sheet to the Ac3 point or more and
supplying it for hot forming. Furthermore, it has the
action of forming fine carbides and suppressing the
growth of austenite grains to thereby obtain fine grains.
For this reason, it has the action of greatly improving
30 the toughness of the hot formed steel sheet member.
Further, Ti preferentially bonds with the N in the steel,
so suppresses the consumption of B due to the
precipitation of BN and as a result has the action of
raising the hardenability due to B.
35 [0037] Therefore, the Ti content is made 0.01% or
more. However, if over 0.05% is included, the amount of
precipitation of TiC increases, C is consumed, and the
- 9 -
strength after hardening falls. For this reason, the Ti
content is made 0.05% or less. A Ti content of 0.015% or
more is preferable, and 0.04% or less is preferable.
[ 0038] Nb: 0.01 to 0.50%
5 Nb, like Ti, is an element having the action of
suppressing the recrystallization when heating the hotforming
use steel sheet to the Ac3 point or more for hot
forming and, furthermore, forming fine carbides to
suppress grain growth and make the austenite grains
10 finer. For this reason, it has the action of greatly
improving the toughness of the hot formed steel sheet
member.
[0039] Therefore, the Nb content is made 0.01% or
more. However, if over 0.50% is included, the amount of
15 precipitation of NbC increases, C is consumed, and the
strength after hardening falls. For this reason, the Nb
content is made 0.50% or less. A Nb content of 0.015% or
more is preferable, and 0.45% or less is preferable.
[0040] B: 0.001 to 0.01%
20 B is an element having the action of enabling raising of
the hardenability of steel and stable securing of the
strength after hardening. For this reason, in the present
invention, which limits the Mn content to 1.5% or less,
it is a particularly important element. If the B content
25 is less than 0.001%, it is not possible to sufficiently
obtain the above effect. Therefore, the B content is made
0.001% or more. On the other hand, if the B content
exceeds 0.01%, the above effect becomes saturated and
furthermore deterioration of the toughness of the
30 hardened part is invited. Therefore, the B content is
made 0.01% or less. A B content of 0.005% or less is
preferable.
[0041] Mn+Cr: 1.5 to 3.5%
As explained above, Mn and Cr are elements which raise
35 the hardenability of the steel sheet and stably secure
the strength after hardening, so are extremely effective.
However, if the total content of Mn and Cr is less than
- 10 -
1.5%, the effect is not sufficient 1 while if over 3.5%,
the effect becomes saturated and conversely securing
stable strength becomes difficult. Therefore, the total
content of Mn and Cr is made 1.5 to 3.5%. A total content
5 of Mn and Cr of 2.0% or more is preferable, and 3.0% or
less is preferable.
[0042] The high strength hot formed steel sheet member
of the present invention has a chemical composition
comprised of the elements from the above C to B and of a
10 balance of Fe and impurities.
[0043] Here, "impurities 11 mean components mixed in at
the time of industrial production of steel sheet due to
the ore, scraps, and other raw materials and various
factors in the production process and allowed in a range
15 not detrimentally affe~ting the present invention.
20
[0044] The high strength hot formed steel sheet member
of the present invention may contain, in addition to the
above elements, one or more elements selected from the
amounts of Ni, Cu, Mo, V, and Ca shown below.
[0045] Ni: 0 to 3.0%
Ni is an element effective for increasing the
hardenability of steel sheet and stably securing strength
after hardening, so may be included in accordance with
need. However, even if over 3.0% of Ni is included, the
25 effect is small and the cost increases. For this reason,
if including Ni, the content is made 3.0% or less. An Ni
content of 1.5% or less is preferable. If desiring to
obtain the above effect, an Ni content of 0.01% or more
is preferable, while 0.05% or more is more preferable.
30 [0046] Cu: 0 to 1.0%
Cu is an element effective for increasing the
hardenability of steel sheet and stably securing strength
after hardening, so may be included in accordance with
need. However, if over 1.0% of Cu is included, the effect
35 is small and the cost increases. For this reason, if
including Cu, the content is made 1.0% or less. A Cu
content of 0.5% or less is preferable. If desiring to
- 11 -
obtain the above effect, a Cu content of 0.01% or more is
preferable, while 0.03% or more is more preferable.
[0047] Mo: 0 to 2.0%
Mo is an element having the action of forming fine
5 carbides and suppressing the growth of grains when
heating the hot forming-use steel sheet to the Ac3 point
or more for hot forming. For this reason, it has the
action of greatly improving the toughness of the hot
formed steel sheet member. For this reason, Mo may be
10 included in accordance with need.
[0048] However, if the Mo content is over 2.0%, the
effect becomes saturated and the cost increases.
Therefore, when including Mo, the content is made 2.0% or
less. An Mo content of 1.5% or less is preferable, while
15 1.0% or less is more preferable. To obtain the above
effect, an Mo content of 0.01% or more is preferable,
while 0.04% or more is more preferable.
[0049] V: 0 to 0.1%
V is an element effective for increasing the
20 hardenability of steel sheet and stably securing strength
after hardening, so may be included in accordance with
need.
However, if over 1.0% of V is included, the effect is
small and the cost increases. For this reason, if
25 including V, the content is made 0.1% or less. A V
content of 0.05% or less is preferable. If desiring to
obtain the above effect, a V content of 0.001% or more is
preferable, while 0.005% or more is more preferable.
[0050] Ca: 0 to 0.01%
30 Ca is an element having the effect of refining the
inclusions in the steel and improving the toughness after
hardening, so may be included in accordance with need.
However, if the Ca content exceeds 0.01%, the effect
becomes saturated and the cost increases. Therefore, if
35 including Ca, the content is made 0.01% or less. A Ca
content of 0.005% or less is preferable. If desiring to
obtain the above effect, a Ca content of 0.001% or more
- 12 -
is preferable, while 0.002% or more is more preferable.
[0051] (B) Microstructure
Mn segregation ratio a: 1.6 or less
a~[Maximum Mn concentration at center part of sheet
5 thickness (mass%)]/
[Average Mn concentration at position of 1/4 sheet
thickness depth from surface (mass%)] ... (i)
[0052] At the center part of the cross-section of
sheet thickness of the steel sheet, the occurrence of
10 center segregation would cause Mn to concentrate.
Therefore, MnS would concentrate at the center as
inclusions, hard martensite would easily form, a
difference would arise in hardness with the surroundings,
and the toughness would deteriorate.
15 [0053] In particular, if the value of the segregation
ratio a of Mn represented by the above formula (i)
exceeds 1.6, the toughness would remarkably deteriorate.
Therefore, to improve the toughness, the value of a of
the hot-forming use steel sheet has to be made 1.6 or
20 less. To further improve the toughness, the value of a is
preferably made 1.2 or less.
[0054] Note that, the value of a does not greatly
change due to hot forming, so if making the value of a of
the hot forming-use steel sheet the above range, it is
25 possible to make the value of a of the hot formed steel
sheet member 1.6 or less.
[0055] The maximum Mn concentration at the center part
of sheet thickness is found by the following method. An
electron probe microanalyzer (EPMA) was used for line
30 analysis at the center part of sheet thickness of the
steel sheet. From the results of analysis, three
measurement values were selected in the order of the
highest down and the average value was calculated.
Further, the average Mn concentration at a position of
35 1/4 sheet thickness depth from the surface was found by
the following method. Using the same EPMA, 10 loca1cions
- 13 -
at positions of 1/4 steel sheet depth were analyzed. The
average value was calculated.
[0056] The segregation of Mn in the steel sheet is
mainly controlled by the composition of the steel sheet,
5 in particular the contents of impurities, and the
conditions of the continuous casting. It does not
substantially change before and after hot rolling and hot
forming. Therefore, if the state of segregation of the
hot forming-use steel sheet satisfies the requirements of
10 the present invention, the inclusions and segregated
state of the hot formed steel sheet member produced by
hot forming after that similarly satisfy the requirements
of the present invention.
[0057] Cleanliness: 0.08% or less
15 If the steel sheet member has large amounts of the Abased,
B-based, and C-based inclusions described in JIS G
0555 (2003), the inclusions will easily become starting
points for delayed fracture. If the inclusions increase,
fracture propagation will easily occur, so the delayed
20 fracture resistance will deteriorate and the toughness
will deteriorate. In particular, in the case of a hot
formed steel sheet member having a 1.7GPa or more tensile
strength, it is necessary to keep the proportion of the
inclusions low.
25 [0058] If the value of the cleanliness of the steel
prescribed in JIS G 0555 (2003) exceeds 0.08%, since the
amount of the inclusions is large, it becomes difficult
to secure a practically sufficient toughness. For this
reason, the value of the cleanliness of the hot-forming
30 use steel sheet is made 0.08% or less. To much further
improve the toughness, the value of cleanliness is
preferably made 0.04% or less. Note that, the value of
the cleanliness of the steel was calculated by the
percent area occupied by the above A-based, B-based, and
35 C-based inclusions.
[0059] Note that, the hot forming does not cause the
value of the cleanliness to greatly change, so by making
5
- 14 -
the value of cleanliness of the hot-forming use steel
sheet the above range enables the value of the
cleanliness of the hot formed steel sheet member to also
be made 0.08% or less.
[0060] In the present invention, the value of
cleanliness of the hot formed steel sheet member is found
by the following method. Test samples were cut out from
five locations of the hot formed steel sheet member. At
the positions of thickness 1/Bt, 1/4t, 1/2t, 3/4t, and
10 7/Bt of each test sample, the point count method was used
to investigate the cleanliness. Further, the numerical
value of the largest value of cleanliness at the sheet
thicknesses (the lowest cleanliness) was made the value
of cleanliness of that test sample.
15 [0061]
less
Average Grain Size of Priory-Grains: 10 ~or
As explained above, if making the grain size of the prior
y-grains in the hot formed steel sheet member smaller, the
delayed fracture resistance is improved. In steel sheet
20 mainly comprised of martensite, if delayed fracture
occurs, sometimes the sheet breaks at the prior y-grain
boundaries. However, by making the priory-grains finer,
it is possible to keep the prior y-grain boundaries from
becoming starting points of cracking and delayed fracture
25 from occurring and the delayed fracture resistance can be
improved. If the average grain size of the prior y-grains
exceeds 10 ~' this effect cannot be exhibited.
Therefore, the average grain size of the prior y-grains in
the hot formed steel sheet member is made 10 ~or less.
30 [0062] The average grain size of the prior y-grains can
be measured using the method prescribed in IS0643. That
is, the number of crystal grains in a measurement field
are counted. The area of the measurement field is divided
by the number of crystal grains to find the average area
35 of the crystal grains, then the crystal grain size is
- 15 -
calculated by the circle equivalent diameter. At that
time, a grain at the boundary of the field is counted as
1/2. The magnification is preferably adjusted to cover
200 or more crystal grains. Further, to improve the
5 precision, measurement of a plurality of fields is
preferable.
[0063] Residual Carbides: 4x10 3/mm2 or less
In the case of hot forming, the redissolution of the
carbides generally present in the steel enables
10 sufficient hardenability to be secured. However,
sometimes part of the carbides will not re-dissolve, but
will remain. Residual carbides have the effect of
suppressing y-grain growth due to pinning when heating and
holding the steel during hot forming. Therefore, during
15 heating and holding, the presence of residual carbides is
desirable. At the time of hot forming, the smaller the
amount of these residual carbides, the more improved the
hardenability and the more a high strength can be
secured. Therefore, when finishing the heating and
20 holding operation, it is preferable that the number
density of residual carbides can be reduced.
[0064] If a large amount of residual carbides are
present, not only is the hardenability after hot forming
liable to fall, but also the residual carbides will
25 sometimes deposit at the prior y-grain boundaries and
cause the grain boundaries to become brittle. In
particular, if the number density of residual carbides
exceeds 4x103/mm2
, the hardenability after hot forming is
liable to deteriorate. Therefore, the number density of
30 residual carbides in the hot formed steel sheet member is
preferably made 4x10 3/mm2 or less.
[0065] If a large amount of residual carbides are
present, hydrogen is trapped at the carbide interfaces,
so easily becomes starting points for hydrogen
35 embrittlement cracking and the delayed fracture
resistance also becomes poor.
- 16 -
[0066] (C) Plated/Coated Layer
The high strength hot formed steel sheet member of the
present invention may have a plated or coated layer on
its surface for the purpose of improving the corrosion
S resistance etc. The plated/coated layer may be an
electroplated layer or a hot dip coated layer. For the
electroplated layer, electrogalvanization, electro Zn-Ni
alloy plating, electro Zn-Fe alloy plating, etc. may be
mentioned. Further, as the hot dip coated layer, hot dip
10 galvanization, hot dip galvannealing, hot dip aluminum
coating, hot dip Zn-Al alloy coating, hot dip Zn-Al-Mg
alloy coating, hot dip Zn-Al-Mg-Si alloy coating, etc.
may be mentioned. The amount of plating/coating
deposition is not particularly limited and may be
lS adjusted within general ranges.
[0067] (D) Method of Production of Hot Forming-Use
Steel Sheet
The hot forming-use steel sheet used for the high
strength hot formed steel sheet member of the present
20 invention can be produced by the method of production
shown below.
[0068] Steel havi~g each above chemical composition is
smelted in a furnace, then is cast to prepare a slab. To
make the cleanliness of the steel sheet 0.08% or less,
2S when continuously casting the molten steel, preferably
the heating temperature of the molten steel is made a
temperature S°C or more higher than the liquidus
temperature of the steel and the amount of casting of
molten steel per unit time is kept to 6t/min or less.
30 [0069] If the amount of casting per unit time of the
molten steel at the time of continuous casting exceeds
6t/min, the fluid motion of the molten steel in the mold
is fast, so inclusions are easily trapped in the
solidified shell and the inclusions in the slab increase.
3S Further, if the molten steel heating temperature is less
than a temperature S°C higher than the liquidus
temperature, the viscosity of the molten steel becomes
- 17 -
higher and it becomes difficult for inclusions to float
up inside the continuous casting machine resulting in an
increase in inclusions in the slab and easy deterioration
of the cleanliness.
5 [0070] By casting while making the molten steel
heating temperature from the liquidus temperature of the
molten steel 5°C or more and making the amount of casting
of molten steel per unit time 6t/min or less, it becomes
difficult for inclusions to be brought into the slab. As
10 a result, the amount of inclusions at the stage of
preparing a slab can be effectively reduced and a steel
sheet cleanliness of 0.08% or less can be easily
achieved.
[0071] When continuously casting molten steel, the
15 molten steel heating temperature is preferably made a
temperature of 8°C or more higher than the liquidus
temperature, Further, the amount of casting of molten
steel per unit time is preferably made 5t/min or less. By
making the molten steel heating temperature a temperature
20 8°C or more higher than the liquidus temperature and
making the amount of casting of molten steel per unit
time 5t/min or less, the cleanliness can be easily made
0.04% or less, so this is preferable.
[0072] Further, to suppress the concentration of MnS
25 forming starting points of delayed fracture, it is
preferable to reduce the center segregation of Mn by
center segregation reduction treatment. As center
segregation reduction treatment, the method of
discharging the molten steel at which Mn has concentrated
30 at the unsolidified layer before the slab becomes
completely solidified can be mentioned.
[0073] Specifically, by electromagnetic stirring,
reduction of the unsolidified layer, or other treatment,
the molten steel at which Mn has concentrated before
35 complete solidification can be discharged. Note that the
electromagnetic stirring treatment can be performed by
- 18 -
giving fluid motion to the unsolidified steel by 250 to
1000 Gauss, while the unsolidified layer rolling
treatment can be performed by rolling the finally
solidified part by a gradient of about 1 mm/m.
5 [0074] A slab obtained by the above method may if
necessary be treated by soaking. By performing the
soaking treatment, it is possible to make the
precipitated Mn disperse and lower the segregation ratio.
The preferable soaking temperature when performing
10 soaking treatment is 1200 to 1300°C, while the soaking
time is 20 to SOh.
[0075] After that, the slab is hot rolled. The hot
rolling conditions, from the viewpoint of enabling
carbides to be more uniformly formed, are preferably made
15 a hot rolling starting temperature of 1000 to 1300°C in
temperature range and a hot rolling end temperature of
850°C or more. The coiling temperature is preferably high
from the viewpoint of the processability, but if too
high, scale formation will cause the yield to fall, so
20 500 to 650°C is preferable. The hot rolled steel sheet
obtained by the hot rolling may be treated to remove the
scale by pickling etc.
[0076] In the present invention, to refine the prior ygrain
size after hot forming and lower the number density
25 of the residual carbides, it is important to anneal the
descaled hot rolled steel sheet to obtain hot rolled
annealed steel sheet.
[0077] To refine the prior y-grain size after hot
forming, it is necessary to suppress the growth of the y-
30 grains by the carbides in the solution. However, to
improve the hardenability and secure high strength in a
hot formed steel sheet member, it is necessary to reduce
the number density of the residual carbides.
[0078] To refine the prior y-grain size in the hot
35 formed steel sheet member and lower the number density of
the residual carbides, the form of the carbides present
- 19 -
in the steel sheet before hot forming and the degree of
concentration of elements in the carbides become
important. It is desirable that the carbides be finely
dispersed, but in that case, the carbides dissolve more
5 quickly, so the effect of grain growth cannot be
expected. If making the Mn, Cr, and other elements
concentrate in the carbides, it becomes harder for the
carbides to form solid solutions. Therefore, the degree
of concentration of elements in the carbides is
10 preferably high.
[0079] The form of the carbides can be controlled by
adjusting the annealing conditions after the hot rolling.
Specifically, the annealing is performed at an annealing
temperature of the Ac1 to the Ac1 point-100°C for 5h or
15 less.
[0080] If making the coiling temperature after the hot
rolling 550°C or less, the carbides easily finely
disperse. However, the degree of concentration of the
elements in the carbides also falls, so annealing is
20 performed to make the elements concentrate more.
[0081] If the coiling temperature is 550°C or more,
pearlite forms and elements increasingly concentrate in
the carbides in the pearlite. In this case, annealing is
performed to break up the pearlite and disperse the
25 carbides.
[0082] As the steel sheet for high strength hot formed
steel sheet member use in the present invention, it is
possible to use hot rolled annealed steel sheet, cold
rolled steel sheet, or cold rolled annealed steel sheet.
30 The treatment process may be suitably selected in
accordance with the demanded level of sheet thickness
precision of the product. Note that, carbides are hard,
so even if performing cold rolling, they are not changed
in form. Their form before the cold rolling is maintained
35 even after the cold rolling.
[0083] The cold rolling may be performed using an
- 20 -
ordinary method. From the viewpoint of securing excellent
flatness, the reduction rate at the cold rolling is
preferably made 30% or more. On the other hand, to avoid
the load from becoming excessive, the reduction rate at
5 the cold rolling is preferably 80% or less.
[0084] When annealing the cold rolled steel sheet, it
is preferable to degrease and otherwise treat it in
advance. The annealing is performed for removing strain
relief by cold rolling and is preferably performed by
10 annealing at the Acl point or less for 5h or less,
preferably 3h or less.
[0085] (E) Method of Forming Plated/Coated Layer
The high strength hot formed steel sheet member of the
present invention may have a plated/coate3d layer at its
15 surface for the purpose of improving the corrosion
resistance etc. The plated/coated layer is preferably
formed at the steel sheet before hot forming.
[0086] When galvanizing the surface of the steel
sheet, from the viewpoint of the productivity, hot dip
20 galvanization is preferably performed on a continuous hot
dip galvanization line. In that case, the steel sheet may
be annealed before the plating treatment on the
continuous hot dip galvanization line or the heating and
holding temperature may be lowered and just coating
25 treatment and not annealing performed.
[0087] Further, it is also possible to perform hot dip
galvanization, then alloying heat treatment to obtain a
hot dip galvannealed steel sheet. The galvanization may
also be performed by electroplating. Note that
30 galvanization need only be performed on part of the
surface of a steel material, but in the case of steel
sheet, it is generally performed on the entire surfaces
of one or both surfaces.
[0088] (F) Method of Production of High Strength Hot
35 Formed Steel Sheet Member
By hot forming the above hot-forming use steel sheet, it
is possible to obtain a high strength hot formed steel
- 21 -
sheet member.
[0089] The h~ating speed of the steel sheet at the
time of hot forming is preferably 20°C/s or more from the
viewpoint of suppressing grain growth. More preferable is
5 50°C/s or more. The heating temperature of the steel sheet
is preferably over the Ac3 point and not more than the Ac3
point+l50°C. If the heating temperature is the Ac3 point
or less, the structure will not become an austenite
single phase before the hot forming and ferrite,
10 pearlite, or bainite will remain in the steel sheet. As a
result, after hot forming, sometimes the structure will
not become a martensite single-phase structure and the
desired hardness cannot be obtained. Further, the
hardness of the hot formed steel sheet member will
15 greatly vary. Furthermore, the delayed fracture
characteristic deteriorates. If the heating temperature
exceeds the Ac3 point+l50°C, the austenite coarsens and
the steel sheet member will sometimes deteriorate in
toughness.
20 [0090] The heating time of the steel sheet at the time
of hot forming is preferably 1 to 10 min. If the heating
time is less than 1 min, even if heating, sometimes
conversion to a single phase of austenite is
insufficient. Further, the carbides are insufficiently
25 dissolved, so even if the y-grain size becomes fine, the
number density of the residual carbides will become
greater. If the heating time exceeds 10 min, the
austenite will coarsen and the hot formed steel sheet
member will deteriorate in hydrogen embrittlement
30 resistance.
[0091] The hot forming start temperature is preferably
made the Ar3 point or more. If the hot formed start
temperature is a temperature of less than the Ar3 point,
ferrite transformation starts, so even with forced
35 cooling after that, the structure will not become a
martensite single-phase structure in some cases. After
- 22 -
hot forming, rapid cooling by a 10°C/s or more cooling
speed is preferable, while rapid cooling by a 20°C/s or
more speed is more preferable. The upper limit of the
cooling speed is not particularly prescribed.
5 [0092] To obtain a high strength hot formed steel
sheet member with a single-phase martensite structure
with little variation in hardness, it is preferable to
cause rapid cooling after hot forming until the surface
temperature of the steel sheet becomes 350°C or less. The
10 cooling end temperature is preferably made 100°C or less,
more preferably is made room temperature.
Examples
[0093] Below, examples will be used to more
specifically explain the present invention, but the
15 present invention is not limited to these examples.
[0094] Steel having each of the chemical compositions
shown in Table 1 was smelted in a test converter and
continuously cast by a continuous casting test machine to
obtain a width 1000 mm, thickness 250 mm slab. Here, at
20 the conditions shown in Table 2, the heating temperature
of the molten steel and amount of casting of molten steel
per unit time were adjusted.
[0095] The cooling speed of the slab was controlled by
changing the amount of water at the secondary cooling
25 spray zone. Further, the center segregation reduction
treatment was performed at the end part of solidification
using a roll mill to softly reduce the thickness by a
gradient of 1 mm/m and discharge the concentrated molten
steel of the final solidified part. In some slabs, after
30 that, a soaking treatment was performed under conditions
of 1250°C and 24 h.
[0096] Table 1
Steel Chemical composition (mass%, balance:
type c Si Mn p s sol. Al N Cr
A 0.31 0.10 1. 30 0.005 0.002 0.04 0.002 0.50
B 0.28 0.05 1.10 0.005 0.002 0.04 0.002 1. 00
c 0.35 0.05 1. 30 0.005 0.002 0.04 0.002 0.50
0 0.32 0.05 1. 4 0 0.005 0.002 0.04 0.002 0.40
E 0.34 0.05 1. 20 0.005 0.002 0.04 0.002 0.60
F 0.31 0.05 1. 30 0.005 0. 002 0.04 0.002 0.70
G 0.30 0.05 1. 30 0.005 0.002 0.04 0.002 0. 60
H 0.29 0.05 1. 30 0.005 0. 002 0.04 0.002 1. 00
I 0.31 0.13 2.40* 0.005 0. 002 0.04 0.002 0.20*
J 0.21* 0.10 1. 30 0.005 0. 002 0.04 0.002 0.10*
K 0.35 0 .. 10 0. 4 0 0.005 0. 002 0.04 0.002 0.30
L 0.32 0.10 1. 30 0.005 0. 002 0.04 0.002 0.40
M 0.30 0.10 1. 30 0.005 0.003 0.04 0.002 0.30
N 0.31 0.10 1. 4 0 0.005 0.008* 0.04 0.002 0.40
0 0.32 0.50* 1. 00 0.005 0.002 0.04 0.002 0.60
* Outside range of present invention
Fe and unavoidable impurities)
Ti Nb B Cu Ni Mo
0. 02 0.08 0.0030 - - -
0.02 0.08 0.0015 - - -
0.02 0.08 0.0015 - - -
0.02 0.08 0.0015 0. 1 - -
0.02 0.08 0.0015 - 0.5 -
0.02 0.08 0.0015 - - 0.1
0.02 0.08 0.0015 -
0.02 0.08 0.0015
0.02 0.08 0.0020 -
0.02 0.08 0.0018 - -
0.02 0.08 0.0015 - -
0.02 - * 0.0020 - - -
0.02 0.08 0.0003* - - -
0.02 0.08 0.0015 - - -
0. 02 0.08 0.0015 - - -
v Ca
- -
- -
- -
- -
- -
- -
0.01 -
- 0.005
- -
- -
- -
-
-
- -
- -
---------
Mn+Cr
1.8
2.1
1.8
1 . 8
1.8
2.0
1.9
2.3
2.6
1. 4 *
0.7*
1.7
1.6
1.8
,_ ____ ).__ E)_
N
w
- 24 -
[0097] The obtained slab was hot rolled by a hot
rolling mill to obtain a thickness 3.0 hot rolled steel
sheet. This was coiled up, then the hot rolled steel
sheet was pickled and further annealed.
5 [0098] After that, part of the steel sheet was cold
rolled by a cold rolling machine to obtain thickness 1.5
mm cold rolled steel sheet. Furthermore, part of the cold
rolled steel sheet was annealed at 600°C for 2h to obtain
steel sheet for hot-forming use.
10 [0099] After that, as shown in FIGS. 1 and 2, a hot
press apparatus was used to hot press the above hotforming
use steel sheet 1 by die set (punch 11 and die
12) (forming hat shape) to obtain a hot formed steel
sheet member 2. More specifically, the steel sheet was
15 heated inside a heating furnace by 50°C/s until reaching
the target temperature, was held at that temperature for
various times, then was taken out from the heating
furnace and immediately hot pressed by a die set with a
cooling system attached so as to form and anneal it
20 simultaneously. The hot formed steel sheet member was
evaluated as follows:
[0100] Evaluation of Mechanical Characteristics of Hot
Formed Steel Sheet Member
The hot formed steel sheet member was measured for
25 tensile strength (TS) by taking a JIS No. 5 tensile test
piece from a direction perpendicular to the rolling and
performing a tensile test based on JIS Z 2241 (2011).
[0101] Evaluation of Cleanliness
Test samples were cut out from five locations of the hot
30 formed steel sheet member. At the positions of thickness
1/St, l/4t, 1/2t, 3/4t, and 7/St of each test sample, the
point count method was used to investigate the
cleanliness. Further, the numerical value of the largest
value of cleanliness at the sheet thicknesses (the lowest
35 cleanliness) was made the value of cleanliness of that
test sample.
- 25 -
[0102] Measurement of Mn Segregation Ratio a
At the center part of sheet thickness of the hot formed
steel sheet member, an EPMA was used for line analysis.
Three measurement values were selected from the results
5 of analysis in order from the highest one down, then the
average value was calculated to find the maximum Mn
concentration at the center part of sheet thickness.
Further, at a position of 1/4 sheet thickness depth from
the surface of the hot formed steel sheet member, an EPMA
10 was used to analyze 10 locations. The average value was
calculated to find the average Mn concentration at a
position of 1/4 sheet thickness depth from the surface.
Further, the maximum Mn concentration at the center part
of sheet thickness was divided by the average Mn
15 concentration at the position of 1/4 sheet thickness
depth from the surface to find the Mn segregation ratio
a.
[0103] Measurement of Average Grain Size of Prior Y=
Grains
20 The average grain size of the prior y-grains in the hot
formed steel sheet member was found by counting the
number of crystal grains in the measurement field,
dividing the area of the measurement field by the number
of crystal grains to find the average area of the crystal
25 grains, and calculating the crystal grain size by the
circle equivalent diameter. At that time, a grain at the
boundary of the field was counted as 1/2 and the
magnification was suitably adjusted to cover 200 or more
crystal grains.
30 [0104] Number Density of Residual Carbides
The surface of the hot formed steel sheet member was
corroded using a picral solution. A scanning electron
microscope was used to examine this enlarged to 2000X.
Several fields were examined. At that time, the number of
35 fields in which carbides were present were count and the
number of 1 mm2 was calculated.
- 26 -
[0105] Evaluation of Delayed Fracture Resistance
The delayed fracture resistance was evaluated by cutting
out a test piece of a length 68 mm and width 6 mm having
the rolling direction as the longitudinal direction,
5 applying strain to the test piece by four point bending,
dipping it into 30°C, pH 1 hydrochloric acid in that
state, observing any cracks after the elapse of 100
hours, and converting the lower limit strain at which
cracking occurs to a stress value from a stress-strain
10 curve of the test piece.
[0106] Variation in Hardness
The following test was performed to evaluate the hardness
stability. Hot forming-use steel sheets were heated by a
heat treatment simulator by 50°C/s until the target
15 temperatures, then were held in various ways. After that,
the sheets were cooled by cooling speeds of about 80°C/s
and 10°C/s until room temperature. These samples were
tested for Vicker's hardness at positions of 1/4
thickness of the cross-section. The hardness was measured
20 based on JIS Z 2244 (2009). The test force was made 9.8N,
the hardnesses at five points were measured, the average
values of the hardnesses at the five points when the
cooling speed was about 80°C/s and 10°C/s were made HS 80
and HS 1o, and the difference ~Hv was used as an indicator
25 of the hardness stability.
[0107] Table 2
Molten Molten
steel steel Amount of Slab center
Test Steel liquidus heating casting of segregation Soaking
no. type temp. temp. molten steel reduction treatment
(t/min) treatment
('C) ('C)
1 A 1506 1536 6.0 Yes No
2 A 1506 1531 7.0 No 1250'Cx24h
3 B 1508 1543 5.1 Yes 1250'Cx24h
4 B 1508 1506 4.5 No No
5 c 1503 1540 3.2 Yes 1250'Cx24h
6 c 1503 154 0 3.2 No No
7 c 15 03 1540 3.2 Yes No
8 D 1505 1530 3.3 Yes 1250'Cx24h
9 D 1505 1530 3.3 Yes 1250'Cx24h
10 D 1505 1530 3.3 Yes 125 O'Cx2 4h
11 D 1505 1530 3.3 Yes 1250'Cx24h
12 E 1504 1521 2.8 Yes No
13 F 1506 1532 3.4 Yes No
14 G 1507 1537 2.5 Yes No
15 H 1506 1546 3.0 Yes 1250'Cx24h
16 I* 1500 1532 3.5 Yes No
17 J* 1514 1567 4. 3 Yes No
18 K* 1508 1525 5.5 Yes No
19 L* 15 05 1547 3.5 Yes No
20 M* 1507 1538 4 .1 Yes No
21 N* 1505 1517 2.5 Yes No
22 0* 1501 1517 3.5 Yes No
Annealing
after hot
Coiling
rolling
Annealing
temp. Cold
after cold
I 'C) Temp. Time
rolling
rolling
('C) (h)
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 No No
510 650 1 Yes Yes
620 650 1 Yes No
510 650 1 Yes Yes
650 - - Yes No
510 650 1 Yes Yes
510 620 10 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
620 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
510 650 1 Yes Yes
Tensile
strength
(MPa)
1925
1912
17 62
1993
2118
2095
2083
197 6
1905
1872
1965
2049
1915
1879
1823
2070
1462
1969
1971
1884
1950
. 1945
Hot forming
Heating Heating
target and
temp. holding
time
('C)
Is I
880 90
880 90
880 90
880 10
880 90
880 90
880 70
880 90
880 90
1000 120
880 70
880 90
880 90
880 90
880 90
880 90
880 90
880 90
880 90
880 90
880 90
880 90
N
-.j
[0108] Table 3
Variation in hardness Prior y- Segregation
Test no. grain size ratio
HSso HS1o l>Hv
I >'ITl I a
1 553 482 71 6 1.1
2 542 456 86 7 1.2
3 502 458 44 6 0.8
4 562 482 80 3 1. 9*
5 583 507 76 7 1.1
6 581 503 78 7 1. 8*
7 578 496 82 6 1.2
8 551 472 79 6 1.1
9 535 432 103 4 1.1
10 545 470 75 2 0* 1.2
11 548 462 86 5 1.1
12 567 563 5 6 1.1
13 537 500 37 6 1.1
14 529 523 5 6 1.1
15 516 511 5 6 0.7
16 552 515 37 6 1. 8*
17 441 340 101 6 1.1
18 557 14 6 411 6 1.1
19 549 4 61 88 13* 1.1
20 530 229 301 6 1.1
21 545 474 71 6 1.1
22 544 43 9 105 6 1.1
* Outside range of present invention
Density of residual Cleanliness
carbides I% I 1/mm'l
0.02 1.25x 103
0.09* 1.752x 103
0.02 2.253x 103
0.09* 7.12x 10'*
0. 02 2. 789x 103
0. 02 3.2x 103
0. 02 4. ?x 1 03*
0.02 3.437x 103
0.02 5.12x 103*.
0. 02 0.05x 103
0.02 3.78x103
0. 02 2. 019x 103
0. 02 2.293x 103
0. 02 2.058x 103
0.02 2. 251x 103
0.02 3. 015x 103
0.02 3. 248x 103
0. 02 3.75x103
0.02 3.015x 103
0.02 2.75x 103
0.09* 2.514x 103
0.02 2.3x 103
Delayed fracture
breaking stress
IMP a I
1460
1210
1620
1195
1310
1180
1190
1490
1100
1160
1340
1300
1460
1520
1550
1050
22 60
1750
1150
1230
1050
1070
Inv. ex.
Comp. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Camp. ex.
Camp. ex.
Inv. ex.
Camp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Camp. ex.
Camp. ex.
Comp. ex.
Camp. ex.
Camp. ex.
Camp. ex.
Camp. ex.
N
co
- 29 -
[0109] Samples with a delayed fracture resistance and
hardness stability of respectively a delayed fracture
cracking stress of 1250MPa or more and a ~Hv of 100 or
less were judged as good.
5 [0110] Table 3 shows the results.
[0111] Test No. 2 had a composition of the steel
satisfying the requirements of the present invention, but
had a large amount of casting of molten steel per unit
time, so the result was the value of the cleanliness
10 exceeded 0.08% and the delayed fracture strength was
inferior.
[0112] Test No. 4 had a composition of steel
satisfying the requirements of the present invention, but
had a low molten steel heating temperature, so the value
15 of the cleanliness exceeded 0.08%. Further, no center
segregation treatment and soaking treatment were
performed, so the Mn segregation ratio exceeded 1.6.
Furthermore, the heating and holding time at the time of
hot forming was short, so the residual carbide density
20 became high. As a result, the result was the delayed
fracture strength was inferior.
[0113] Test No. 6 did not include center segregation
treatment and soaking treatment, so the result was that
the Mn segregation ratio exceeded 1.6 and the delayed
25 fracture strength was inferior.
30
[0114] Test No. 7 did not include annealing after hot
rolling, so the result was that the dissolution of the
carbides was delayed and the delayed fracture strength
was inferior.
[0115] Test No. 9 had a long annealing time after hot
rolling, so the result was that the dissolution of the
carbides was insufficient and the number density of the
residual carbides became high, so the delayed fracture
strength was inferior.
35 [0116] Test No. 10 had a high heating temperature at
the time of hot forming, so the result was the austenite
grains coarsened and the fracture strength was inferior.
5
- 30 -
[0117] Test No. 16 had an Mn content exceeding the
prescribed upper limit value, so the result was that the
Mn segregation ratio exceeded 1.6 and the delayed
fracture strength was inferior.
[0118] Test Nos. 17 and 18 were low in total contents
of Mn and Cr, so the result was that the hardness
stability was inferior.
[0119] Test No. 19 did not contain Nb, so the result
was that the prior y-grain size become larger and the
10 delayed fracture strength was inferior.
[0120] Test No. 20 was low in B content, so the result
was that the hardness stability was inferior.
[0121] Test No. 21 had an S content exceeding the
prescribed upper limit value, so the result was that the
15 value of the cleanliness exceeded 0.08% and the delayed
fracture strength was inferior.
[0122] Test No. 22 had an Si content exceeding the
prescribed upper limit valuB, so the result was that the
A3 point rose, the structure did not become a martensite
20 single-phase structure after hot forming, and the
hardness stability and delayed fracture strength were
inferior.
[0123] Test Nos. 1, 3, 5, 8, and 11 to 15 satisfying
the requirements of the present invention were excellent
25 in both hardness stability and delayed fracture
resistance in the results.
Industrial Applicability
[0124] According to the present invention, it is
possible to obtain a high strength hot formed steel sheet
30 member having a 1.7GPa or more tensile strength and
realizing both hardness stability and delayed fracture
resistance. The high strength hot formed steel sheet
member of the present invention is particularly suitable
for use as impact resistant parts of an automobile.
35 Reference Signs List
[0125] 1. hot forming-use steel sheet
2. hot formed steel sheet member
- 31 -
11. punch
12. die

CLAIMS
Claim 1. A high-strength hot-formed steel sheet
member having
a chemical composition comprising, by mass%,
C: 0.25 to 0.40%,
Si: 0.005 to 0.14%,
Mn: 1. 50% or less,
P: 0.02% or less,
s: 0.005% or less,
sol. Al: 0.0002 to 1.0%,
N: 0.01% or less,
Cr: 0.25 to 3.00%,
Ti: 0.01 to 0.05%,
Nb: 0.01 to 0.50%,
B: 0.001 to 0.01%, and
a balance of Fe and unavoidable impurities;
a total of content of Mn and content of Cr of
1.5 to 3.5%;
an Mn segregation ratio a represented by the
20 following formula (i) of 1.6 or less;
25
a value of cleanliness of steel prescribed by
JIS G 0555 (2003) of 0.08% or less;
having an average grain size of prior y-grains
of 10 ~ or less; and
a number density of residual carbides present
of 4xl03/mm2 or less:
a~[Maximum Mn concentration at center part in
sheet thickness (mass%)]/
[Average Mn concentration at position of 1/4
30 sheet thickness depth from surface (mass%)] ... ( i)
35
Claim 2. The high-strength hot-formed steel sheet
member according to claim 1 wherein said chemical
Composition further includes, by mass%, one or more
elements selected from
Ni: 0 to 3.0%,
Cu: 0 to 1.0%,
---- ·---
- 33 -
Mo: 0 to 2.0%,
V: 0 to 0.1%, and
Ca: 0 to 0.01%.
Claim 3. The high-strength hot-formed steel sheet
5 member according to claim 1 or 2 having a plating layer
at the surface of said steel sheet.
Claim 4. The high-strength hot-formed steel sheet
member according to any one of claims 1 to 3 wherein said
steel sheet member has a tensile strength of 1.7 GPa or
10 more.