[Document Type] Specification
[Title of the Invention] HOT-FORMED MEMBER AND MANUFACTURlNG
METHOD OF SAME
[Technical Field of the I~nvention]
[OOOl]
The present invention relates to a hot-formed member used in mechanical
structure coi~~ponenstusc h as body structure components and underbody components
of a vehicle, for example, and a manufacturing mnethod thereof. Specifically, the
present invention relates to a hot-formed member having excellent ductility in which
the total elongatioll obtained by a tensile test is equal to or greater than 15% ~vl~ile
maintaining a tensile strength of 900 MPa to 1300 MPa, and excellent impact
properties in \vhicli an inlpact value obtained by a Charpy test at O°C is equal to or
greater than 20 ~lcrna~n,d a manufacturing method thereof.
[Related Art]
[0002]
In recent years, in order to reduce the weight of a vehicle, eKorts to reduce the
v~eiglo~ft s teel products used by realizing high-strengthening of the steel products used
in a car body have been made. In steel sheets whch are widely used in technical
fields relating to vehicles, press for~llabilithja~s decreased due to an increase in the
strength of the steel sheets, and accordingly, it is difficult to manufacture a member
having a cotnplicated shape. Specifically, ductility of the steel sheets is decreased
due to an increase in the strength of the steel sheets, and accordingly, breaking occurs
in a region of the nle~nbers ubjected to working with high working ratio andlor
springback and wall warp of the member becomes significantly large and causes
deterioration in the shape accuracy of the tnenlbee heref fore, it is not easy to
manufacture a metnber having a cotnplicated shape by applying press fomiing to a
steel sheet having high strength, particularly a tensile strength equal to or greater than
the lcvel of 900 MPa. According to roll forming instead of the press forming, a steel
sheet having high strength can be worked, but the roll forming can only be applied to a
manufacturing metliod of a metnber having a unifonn cross section in a lotigitudirial
direction.
[0003]
Meanwhile, as disclosed in Patent Document 1, in a method called hot
pressing of performing press forming of a heated steel sheet, it is possible to form a
tnetilber having a complicated shape fiom a high-strength steel sheet with excellent
shape accuracy. This is because, in the hot pressing step, the steel sheet is worked in
a state of being heated at a high temperature, and thus the steel sheet at the time of
working is softened and has high ductility. In the hot pressing, it is also possible to
obtain a higli strength member by maltensitic transfortnation, by heating the steel sheet
to an austenite single phase region before the pressing and rapidly cooling (quenching)
the steel sheet in a die after the pressing. Thcrefore, the hot pressing method is an
excellent forming method which secures the high strength of the member and the
formability of tlie steel slieet at tlie same time.
[0004]
Patent Document 2 discloses a pre-press quencliing metliod for obtaining a
higli strength member by forming a steel sheet in a predetermined shape at room
temperature, heating the obtained member to an austenite region, and rapidly cooling
the member in a die. In the pre-press quenching method wliicli is one embodiment of
the hot pressing, it is possible to prevent deformation of a member due to distortion by
heating, with restraining the member by the die. The pre-press quenching method is
an excellent forming method for achieving high strength of a nlember and high shape
accuracy.
[OO05]
However, in recent years, excellent impact absorbing properties are also
required to be achieved in the hot-formed member. That is, it is required that both
excellent ductility and excellent in~pacpt roperties are achieved in the hot-fonned
menlbel: It is difficult to achieve such requirements by technologies in the related art
represented by Patent Document 1 and Patent Document 2. This is because the
rnetallographic microstructure of a men~bero btained by technologies in the related art
has substantially a martensite single phase.
[OOO6]
Therefore, Patent Document 3 discloses a technology of obtaining a member
having high strength and excellent ductility by heating a steel sheet to a dual-phase
temperature region of a ferrite and an austenite to perform pressing of the steel sheet in
a state where the metallographic microstructure of the steel sheet has a ferritemartensite
dual phase microst~ucture,r apid cooling the steel sheet in a die, and
changing the metallograpliic nlicrostrncture of the steel sheet into a ferrite-austenite
dual phase microstructure. However, since elongation of the nlember obtained by the
technology is equal.to or-silaller than approximately lo%, the ductility of the member
disclosed in Patent Docunlent 3 is not sufficiently high. It is necessary that such a
metilber which is required in the technical field related to vehicles and required to have
excellent impact absorbing properties has better ductility than the member described
above, specifically, has an elongation equal to or greater than 15%. The elongation
thereof is preferably equal to or greater than 18% and is more preferably equal to or
greater than 21%.
[0007]
It is possible to significantly increase the ductility of a member obtained by
the hot pressing method by applying a microstt~~ctucreo ntrol method for
transformation induced plasticity steel (TIUP steel) and quench & partitioning steel
(Q&P steel) to the hot pressing method. This is because the residual austenite is
generated in the metallographic microstructure of the member due to a specific thermal
treatment which will be described later.
[OOOS]
Patent Document 4 discloses a technology of obtaining a member having high
strength and excellent ductility by heating a steel sheet obtained by actively adding Si
and Mn to a dual-phase temperature region of a ferrite and an austenite in advance,
performing press-forming and rapid cooling simultaneously with respect to the steel
sheet using a deep drawing apparatus, to hansform the metallographic tnicrosttucture
of the obtained member into a complex-phase microstt~tcturec ontaining ferrite,
martensite, and austenite. It is necessary to perfonn an isothermal holding treatment
at 300°C to 400°C, that is, an austenlpering treatment with respect to the steel sheet, in
order to cause austenite to be contained in the metallograpl~cm icroshuctt~reo f the
metnbes. Accordingly, it is necessary that a die of the deep drawing apparatus in
Patent Document 4 is heated at 300°C to 400°C. In addition, as disclosed in
examples of Patent Document 4, it is necessary that the member be held in a die for
approximately 60 seconds. However, in a case of perfortiling the austempering
treatment, not only the tensile strength of the steel sheet, but also the elongation of the
steel sheet significantly changes depending 011 the holding temperature and the holding
time. Accordingly, in a case of performing the austempering treatment, it is difficult
to ensure stable mechanical properties. In a case of performing the austempering
treatment with respect to a steel containing a large amount of Si, such as a kind of steel
corresponding to a target of the present invention, a significantly hard martensite is
easily generated in the metallographic microstructure and the impact properties of the
member is significantly deteriorated due to this martensite.
[0009]
Patent Docunlent 5 discloses a technology of obtaining a member having high
strength and excellent ductility by heating a steel sheet obtained by actively adding Si
and Mn to a dual-phase temperature region or an austenite single-phase region in
advance, performing forming and rapid cooling to a predetermined temperature wit11
respect to the steel sheet at the same time, and heating the obtained member again, to
change the metallographic microstructure of the member into a conlplex-phase
microstructure containing mattensite and austenite. However, in the manufacturing
method by the technology described above, the tensile strength of the member
significantly changes depending on a rapid-cooling condition, specifically, a
temperature at which the cooling stops. A problem in a step such as significant
difficulty in controlling a cooling stop temperature is inevitable in the manufacturing
method described above. Unlike the manufacturing method of the hot-formed
member of the related art, it is necessary that a further heat treatment step such as reheating
is performed in the manufacturing method disclosed in Patent Document 5.
Therefore, in the mat~ufacturingm ethod disclosed in Patent Document 5, the
productivity is significantly low, co~nparedto that in the manufacturing method of the
hot-formed member of the related art. In addition, as disclosed in exalnples of Patent
Doc~unen5t , it is necessary to heat the steel sheet at a high temperature in the
manufacturing method disclosed in Patent Document 5, and accordingly, second
phases such as martensite are sparsely distributed in the metallographic inicrostructure
of the member. This causes a problelu such as a significant deterioration in the
impact properties of the member.
[OO 101
Thus, it is necessary to newly investigate a hot forming 111ethod of obtaining a
steel sheet member containing residual austenite, withotd using a microstructture
controlling method for the TRIP steel and the Q&P steel.
[OOll]
Meanwhile, a steel which has both of excellent strength and excellent ductility
is obtained by performing a heat treatment with respect to a low carbon steel obtained
by actively adding Mn at the vicinity of A1 tenlperature. For example, Non-Patent
Document I discloses a steel containing several tens % of residual austenite and
having high strength and excellent ductility, which is obtained by performing hot
rolling of a 0.1% C-5% Mn alloy and further performing re-heating.
[Prior At Document]
[Patent Document]
[0012]
[Patent Document 11 Great Britain Patent No. 1490535
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. Hl0-9603 1
[Patent Document 31 Japanese Unexarnined Patent Application, First
Publication No. 201 0-65292
[Patent Document 41 Published Japanese Translation No. 2009-508692 of
tie PCT International Publication
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 201 1-184758
Non-Patent Document
[0013]
won-Patent Document 11 Jour~laol f the Japan Society for Heat Treatment,
Vol. 37 No. 4 (1997), p. 204
[Disclosure of the Invention]
[Problen~sto be Solved by the Invention]
[0014]
Like the method disclosed in Non-Patent Document 1, it is possible to
manufacture a hot-formed menlber containing residual austenite, by optimizing a
chemical composition of the hot-formed member and strictly controlling the heat
treatment temperature in the hot forming step at the vicinity of A1 temperature.
However, in the method disclosed in Non-Patent Document 1, the heating time
significantly affects the tensile strength and the elongation. It is necessaly to perform
the heating for 30 minutes or longel; in order to limiting a change in the obtained
tensile strength and elongation. Such a microstr~~ctucreo ntrolling operation by
performing the heating for a long period of time cannot be applied to a production
technology of a hot-fonned member, when considering the productivity and surface
quality of a men~ber. In addition, in the method disclosed in Non-Patent Docutnent 1,
cementite tends to beAardly dissolved, and accordingly, it is easily assunled that the
impact properties of the hot-formed member obtained by this technology are not
sufficient.
[OOI 51
As described above, a mass production technology of providing a nlelnber
which is manufactured by the hot forming, has a tensile strength equal to or greater
than 900 MPa, and has excellent ductility and impact properties has not yet been
established.
100 161
The present invention is to provide a hot-fonned member having a tensile
strength equal to or greater than 900 MPa and having excellent ductility and impact
properties, which could not be mass-produced in the related art as described above, and
a nlam~factt~rinmge thod thereof.
[Means for Solving the Problenl]
[0017]
The inventors have conducted extensive studies in order to improve the
ductility and impact properties of a hot-formed member having a tensile strength equal
to or greater than 900 MPa, and have found that ductility and impact properties of the
hot-fanned member are significantly improved by (1) increasing the Si content in the
hot-formed member to be higher that1 that of a typical steel sheet for hot fonning, and
(2) changing a metallograpl~cm icrostructure of the hot-formed member into the
metallographic microst~x~ctuirne which a predetermined amount of austenite is
contained and fine austenite and fine martensite are entirely present. In addition, the
inventors found that such a metallographic microstructure is achieved by using a base
steel sheet having the same chemical conlposition as the chemical composition of the
hot-for~nedm ember described aboveaud having a metallographic nlicrostructure in
which one or both of bainite and maltensite are contained and in which particles of
cementite are present at a predetermined number density, as a raw material of a hotformed
member, and optinlizing the heat treatment co~lditionsa t the time of the hot
forming.
[00 181
The present invention is made based on the above-mentioned findings and
details are as follows.
(1) An aspect of the present invention is a hot-formed member llaving a
clien~icacl omposition contprising, by mass%, C: 0.05% to 0.40%, Si: 0.5% to 3.0%,
Mn: 1.2% to 8.0%, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.001% to 2.0%, N:
0.01% or less, Ti: 0% to 1.0%, Nb: 0% to 1.0%, V: 0% to 1.0%, Cr: 0% to 1.0%, Mo:
0% to 1.0%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, Ca: 0% to 0.01%, Mg: 0% to 0.01%,
REM: 0% to 0.01%, Zr: 0% to 0.01%, B: 0% to 0.01%, Bi: 0% to 0.01%, and the
balance of Fe and inlpurities, wherein the hot-formed member has a metallograpl~ic
tnicrostructure which contains an austenite of 10 area% to 40 area% and in ~vltichth e
total number density of particles of the austeriite and particles of a martensite is equal
to or greater than 1.0 piece/pn12, and wherein a tensile strength is 900 MPa to 1300
MPa.
[0019]
(2) In the hot-fotmed member according to (I), the chemical composition may
include one or two or more selected fiom the group consisting of, by mass%, Ti:
0.003% to 1.0%, Nb: 0.003% to 1.0%, V: 0.003% to 1.0%, Cr: 0.003% to 1.0%, Mo:
0.003% to 1.0%, Cu: 0.003% to 1.0%, andNi: 0.003% to 1.0%.
[0020]
(3) In the hot-fonned tnetnber according to (1) or (2), the chemical
composition may include one or two or more selected from the group consisting of, by
mass%, Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, REM: 0.0003% to 0.01%, and
Zr: 0.0003% to 0.01%.
[0021]
(4) In the hot-formed member according to any one of (1) to (3), the chemical
composition may include, by mass%, B: 0.0003% to 0.01%.
roo221
(5) In the liot-fornied iiieniber according to any one of (1) to (4), tlie cl~eemical
composition may include, by mass%, Bi: 0.0003% to 0.01%.
[0023]
(6) Another aspect of the present invention is a manufacturing method of a
hot-fortiied menlber including: heating a base steel sheet having a cheniical
composition which is same as the cliemical co~npositiono f the hot-formed tnetnber
according to any one of (1) to (5) and in which a Mn content is 2.4 mass% to 8.0
mass%, and having a metallographic microstructure in which the total area ratio of one
or both of a bainite and a maltensite is equal to or greater than 70 area%, and particles
of a cementite are present at a number density equal to or greater than 1.0 numberfPm2,
to a temperature region which is equal to or higher than 670°C and lower than 780°C
and is lower than an Ac3 temperature; then holding the temperature of the base steel
sheet in the temperature region which is equal to or higher than 670°C and lower than
780°C and is lower than anAc3 tenlperature for 2 minutes to 20 minutes; then
performing a hot formit~gw ith respect to tlie base steel sheet; and then cooling the base
steel sheet under conditions in \vhich an average cooling rate in a temperature region of
600°C to 150°C is from S0C/sec to 500°C/sec.
[0024]
(7) Still another aspect of the present invention is a ma~lufacturingn iethod of
a hot-formed member including: Ileating a base steel sheet having a chemical
composition which is same as the chemical composition of the hot-formed lnember
according to any one of (I) to (5) and in which a Mn content is equal to or more than
1.2 mass% and less than 2.4 mass%, and having a metallographic microstructure in
which the total area ratio of one or both of a bainite and a martensite is equal to or
greater than 70 area%, and particles of a ce~lientitea re present at a nuniber density
equal to or greater than 1.0 numbcr/pm2, to a temperature region which is equal to or
higher than 670°C and lower than 780°C and is lower than an ACJ temperature; then
holding the temperature of the base steel sheet in the temperature region which is equal
to or higher than 670°C and lower than 780°C and is lower than anAc3 temperature for
2 minutes to 20.1ninutes; then performing a hot forniing with respect to tlie base steel
sheet; and then cooling the base steel sheet under conditions in which an average
cooling rate in a temperature region of 600°C to 500°C is from S°C/sec to 500°C/sec
and the average cooling rate in a temperature region lower than 500°C and equal to or
higher than 150°C is from S°C/sec and 2O0C/sec.
[Effects of the Invention]
[0025]
According to the present invention, effects having technical advantage in
which a hot-formed member having a tensile strength equal to or greater than 900 MPa,
having excellent ductility, and having excellent impact properties can be practicalized
for practical use are achieved.
[Brief Description of the Dra\ving]
[0026]
FIG. 1 k a flowchart showing a manufacturing method according to the
present invention.
[Embodiment of the Invention]
[0027]
Hereinafter, a hot-formed member according to one etnbodi~nento f the
present invention and a manufacturing metliod thereof, which are acliie\red based on
the findings described above will be described. In tlie following description, as the
hot fornling, hot pressing which is a specific embodiment will be described as an
example. Ho\vcver, a forming method otl~etrh an the hot pressing, such as, for
example, roll forming may be used as the hot forming method, as long as
manufacturing conditions which are substantially the same as the manufacturing
conditions disclosed in the following description are achieved.
[0028]
1. Chemical Composition
First, a chemical conipositioll of the hot-formed member according to one
embodiment of the present invention ~vilbl e described. In the following description,
"%" representing the amount of each alloy element means "mass%, unless othel~vise
stated. The chemical compositioll of steel does not change even when the hot
forming is performed, and therefore, the amount of each element in a base steel sheet
before being subjected to the hot forming is equivalent to the amount of each element
in a hot-formed member after the liot fortning.
[0029]
(C: 0.05% to 0.40%)
C is a significantly important eletnetlt which increases the liardenability of
steel and most strongly affects the strength of a hot-formed me~nbear fter quenching.
When the content is less than 0.05%, it is difficult to ensure the tensile strength equal
to or greater than 900 MPa after quenching. Therefore, the C content is set to be
equal to or more than 0.05%. Meanwhile, when the C content exceeds 0.40%, impact
properties of the hot-formed member are significantly deteriorated. Therefore, the C
content is set to be equal to or less than 0.40%. The C content is preferably equal to
or less than 0.25%, in order to improve weldability of the hot-fonned member. The C
cotltetlt is preferably equal to or more than 0.08%, in order to stably ensure the strength
of the hot-formed member.
[0030]
(Si: 0.5% to 3.0%)
Si is an element which is significantly effective for stably ensuring the
strength of steel after quenching. I11 addition, the amount of austenite in a
metallographic n~icrostructurein creases and ductility of the hot-formed member is
itnproved by adding Si. %hen the Si content is less than 0.5%, it is difficult to obtain
the above-mentioned effects. Particularly, in the embodiment, when the amount of
austenite is insufficient, necessary ductility is not obtained, and accordingly, it is
extremely disadvantageous for industrial application. Thus, the Si content is set to be
equal to or more than 0.5%. When the Si content is equal to or more than 1.0%, the
ductility is further improved. Therefore, the Si content is preferably equal to or more
than 1.0%. Meanwhile, when the Si content exceeds 3.0%, it is economically
disadvantageous due to saturated effects obtained by the actions described above and
surface quality of the hot-formed member is significantly deteriorated. Therefore, the
Si content is set to be equal to or less than 3.0%. The Si content is preferably equal to
or less than 2.5% in order to more properly prevent a deterioration in surface quality of
the hot-formed member.
[003 I] . . ,.
(Mn: 1.2% to 8.0%)
hhl is an element which is significantly effective for increasing the
hardenability of steel and stably ensuring the strength of steel after quenching. In
addition, h4n is also effective for increasing ductility of the hot-fornled after quenchitig.
However, when the Mn content is less than 1.2%, these effects are not sufficiently
obtained and it is significantly difficult to ensure the tensile strength equal to or greater
than 900 MPa after q~~enching. Therefore, the MII content is set to be equal to or
more than 1.2%. When the Mn content is equal to or more than 2.4%, the ductility of
the hot-forn~ed~ nell~biesr f urther increased, and accordingly mild cooling after hot
forming which will be described later is not a necessary a mallufacturing step and
productivity is significantly improved. Therefore, the Mn content is preferably equal
to or more than 2.4%. Meanwhile, when the Mn content exceeds 8.0%, austenite is
excessively generated in the hot-fonned member and delayed fracture easily occurs.
Therefore, the Mn content is set to be equal to or less than 8.0%. When the tensile
strength of the base steel sleet before applying the hot forming is decreased,
productivity in a hot forming step which will be described later is improved. In order
to obtain this effect, the Mn content is preferably equal to or less than 6.0%.
[0032]
(P: 0.05% or less)
P is generally an imnpurity unavoidably contained in steel. However, in the
embodiment, P has an effect on increasing strength of steel by solid solution
strengthening, and accordingly P may be actively contained. However, when the P
content exceeds 0.05%, the weldability of the hot-formed member may be significantly
deteriorated. Therefore, the P content is set to be equal to or less than 0.05%. The P
content is preferably equal.to or less than 0.02%, in order to more properly prevent a
deterioration in weldability of the hot-fornled member. The P content is preferably
equal to or Illore than 0.003%, in order to more properly obtain the above-mentioned
strength improvement action. Ho\vever, even when the P co~ltellits 0%, properties
\vhich are necessary for solving the problems can he obtained, and therefore, a lower
limit value of the P content is not necessary to be specified. That is, the lo\ver liniit
value of the P content is 0%.
[0033]
(S: 0.01% or less)
S is an impurity cotitait~edi n steel and it is preferable that a S content is as
stnall as possible, in order to improve weldability. When the S content exceeds
0.01%, weldability is sigtiificantly decreased to an unacceptable level. Therefore, tlie
S contetit is setto be equal to or less than 0.01%. The S content is preferably equal to
or less tlian 0.003% and more preferably equal to or less than 0.0015%, in order to
more properly prevent a decrease in weldability. Since it is preferable that the S
content is as small as possible, a lower limit value of the S content is not necessary to
be specified. That is, the lower limit value of the S content is 0%.
[0034]
(sol. Al: 0.001% to 2.0%)
sol. A1 indicates solution A1 present in steel in a solid solution state. A1 is an
element which has an effect on deoxidation of steel and is also an element which
prevents oxidization of carbonitride forming elements such as Ti and promotes tlie
forming of carbonitride. With such effects, it is possible to prevent ge~ierationo f
surface defects in a steel and improve the manufach~ringy ield of the steel. When the
sol. A1 content is less than 0.001%, it is difficult to obtain the effects described above.
Therefore, the sol, ,A1 content is set to be equal to or more than 0.001%. The sol. A1
content is preferably equal to or more than 0.01%, in order to Inore properly obtain the
effects described above. Meanwhile, when the sol. A1 content exceeds 2.0%,
weldability of tlie hot-formed member is significantly decreased, tlie amoutit of osidcbased
inclusio~lsi s illcreased in the hot-formed iiienibel; atid tlie surface quality of the
hot-formed ~iienlbeirs significantly deteriorated. Therefore, the sol. A1 content is set
to be equal to or less thau 2.0%. The sol. A1 content is preferably equal to or less tlian
1.5%, in order to more properly avoid the phenomenon described above.
[0035]
@I: 0.01% or less)
N is an impurity unavoidably contained in steel and the N content is
preferably as small as possible, in order to improve the weldability. When the N
content exceeds 0.01%, weldability of a hot-formed member is significantly decreased
to an unacceptable level. Therefore, the N content is set to be equal to or less than
0.01%. TheN content is preferably equal to or less than 0.006%, in order to more
properly avoid a decrease in weldability. Since it is preferable that the N content is as
small as possible, the lower limit value of the N content is not necessary to be specified.
That is, the lower limit of the N content is 0%.
[0036]
The chemical composition of the hot-formed member according to the
embodiment includes the balance of Fe and impurities. The impurities are
components mixed from raw materials such as ores or scraps when industrially
manufacturing a steel or due to various reasons of the manufacturing step and means
con~ponentsa llowed to be contained in a range not negatively affecting tie properties
of the hot-formed member according to the en~bodi~nent. Howvevel; the hot-formed
member according to the embodiment may further contain the following elements as
arbitrary components. Even when the following arbitrary elements are not contained
in the hot-formed men~berp, roperties which are necessary for solving the proble~nsc an
be obtained, and therefore, a lower limit value of the arbitrary elelnent content is not
necessary to be specified. That is, the lower limit value of the arbitrary element
content is 0%.
[0037]
(One or Two or More Selected From Gronp Consisting of Ti: 0% to 1.0%, Nb:
0% to 1.0%, V: 0% to 1.0%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 1.0%, and
Ni: 0% to 1.0%)
All of these elements are elements which are effective for increasing the
hardenability of the hot-formed metnber and stably ensuring the strength of the hotformed
member after quenching. Accordingly, one or more selected these elements
may be contained. However, when each amount of Ti, Nb, and V exceeds 1.0%, it is
difficult to perform hot rolling and cold rolling in the manufacturing step. In addition,
when the amount of Cr, Mo, Cu, and Ni exceeds 1.0%, it is economically
disadva~itageousd ue to saturated effects obtained by the actions described above.
Therefore, when each element is contained, the amount of each element is as follows.
In order to more properly obtain the effects obtained by the actions, it is preferable to
satisfy at least one of Ti: 0.003% or more, Nb: 0.003% or more, V: 0.003% or more,
Cr: 0.003% or more, Mo: 0.003% or more, Cn: 0.003% or more and Ni: 0.003% or
more.
[0038]
(One or Two or More Selected From Group Consisting of Ca: 0% to 0.01%,
Mg: 0% to 0.01%, REM: 0% to 0.01%, and Zr: 0% to 0.01%)
These elements are elements \vhicl~a re effective for contributing to the
control of inclusions, particularly fine dispersing of inclnsions and increasing low
temperature tonghness of the hot-formed member. Accordingly, otle or two more
selected froln these elements may be contained. However, when an amount of any
eletnent exceeds 0.01%, the surface qnality of the hot-for~nedm ember may be
deteriorated. Therefore, whet1 each element is contained, the amount of each elenlent
is as follo\vs. The amount of each element to be added is preferably equal to or nlore
than 0.0003%, in order to tilore properly obtain the effects obtained by tlie actions.
Herein, the term "REM" means a total of 17 eletnents formed of Sc, Y, and
lanthanoid and the expression "amount o f REM tileatis a total arnount of these 17
elements. In a case of using lanthanoid as the REM, the REM is added with misch
metal industrially.
[0039]
(B: 0% to 0.01%)
B is an element which has an effect of increasing the low temperature
toughness of the hot-formed meniber. Accordinglj: B may be contained in the hotformed
member. However, when the B content exceeds 0.01%, the hot workability of
the base steel sheet is deteriorated and it beconles difficult to perform hot rolling.
Therefore, when B is contained in the hot-formed niember, the B content is set to be
equal to or lower than 0.01%. In order to more properly obtain the effects obtained
by the actions, the B content is preferably equal to or more that1 0.0003%.
[0040]
(Bi: 0% to 0.01%)
Bi is an element which has an effect of preventing cracks generated when the
hot-formed member is deformed. Accordingly, Bi may be contained in the hotformed
member. . Howevel; wliehen tlie Bi content exceeds 0.01%, the hot workability
of the base steel sheet is deteriorated atid it becomes difficult to perfor111 hot rolling.
Therefore, when Bi is contained in the hot-fonned member, the Bi content is set to he
equal to or lower than 0.01%. I11 order to more properly obtain the effects obtained
by tlie actions, the Bi content is preferably equal to or more than 0.0003%.
[0041]
2. Metallographic Microst~~~ctourfe H ot-Fornied Member
Next, t11c inctallograpl~ic~ ~~icrostructoufr teh e hot-formed member according
to tlie e~nbodin~ewntil l be described. I11 the following description, " % r epresenting
the amourit of each metallographic microstructure means "area%, unless otherwise
stated.
The configuration of the following metallographic microstructure is a
configuration of a portion from an approxinnately 112t thickness position to an
approximately 1/4t thickness position and a position which is not located in a center
segregation portion. The center segregation portion may have a metallographic
microstix~cturew hich is different fiom the representative metallographic microstructure
of the steel. However, the center segregation portion is a minor area with respect to
the entire sheet thickness and does not substantially affect the properties of the steel.
That is, the metallographic microstructure of the center segregation portion is not a
representative of the metallographic microst~x~ctuoref the steel. Accordingly, the
metallographic microst~x~ctuoref the hot-formed member according to the embodiment
is defined as the microstructure of a portion from an approximately 112t thickness
position to an approximately 1/4t thickness position and a position which is not located
in the center segregation portion. The expression "1/2t thickness position" indicates a
position which is at a depth of 112 of a member thickness t from the surface of tlie hotformed
member and.the expression "1/4t tl~icknessp osition" indicates a position wliich
is at a depth of 114 of the member thickness t from the surface of tlie hot-formed
member.
[0042]
(Area Ratio of Austenite: 10% to 40%)
The ductility of the hot-formed member is significantly improved by
containing an appropriate amount of austetiite in the steel. When the area ratio of
austenite is less than lo%, it is difficult to ensure excellent ductility. Accordingly, the
area ratio of austenite is set to be equal to or more than 10%. When the area ratio of
austenite is equal to or nlore than 18%, elongation of t he hot-for~nedm ember is set to
be equal to or more than 21% aud extremely excellent ductility is exhibited in the hotformed
membec Therefore, the area ratio of austenite is preferably equal to or more
than 18%. Meanwhile, when the area ratio of austenite exceeds 40%, delayed
fracture easily occurs in the hot-formed member. Accordingly, the area ratio of
austenite is set to be equal to or less than 40%. The area ratio of austenite is
preferably equal to or lower than 32%, in order to properly prevent occurretlce of
delayed fracture.
[0043]
A measuring method of the area ratio of austenite is well known for a person
skilled in the art and the area ratio thereof can be measured by a comtnon method in
the enlbodiment. In the examples which will be described later, the area ratio of the
austenite is obtained by X-ray diffraction.
[0044]
(Distribution of Austenite and Martensite: Total Number Deusity of Particles
ofA ustenite aud Martensite: 1.0 n~mber/~tmor* m ore)
It is possible to prevent microscopic localizatio~ol f plastic defornlation of the
hot-formed rllernber when performing hot forming, by allowing a large amount of a
fine hard lnicrostructure to be present in the metallographic n~icrostructuret,h at is, by
increasing the number deusity of austenite and lllartensite in the metallographic
microstructure. Accordingly, it is possible to prevent cracks generated it1 austenite
and martensite at the time of deformation and to inlprove the impact properties of the
hot-formed ~ne~nbeeIn order to obtain a hot-fonned member having a tensile
strength equal to or niore than 900 MPa and having excellent impact properties, the
metallographic microstructure of the hot-formed 111ernber is a ~netallographic
microstructure in wl~icltih e total amount of austenite and mai-tensite is present at tlie
number density of 1.0 nut~iberlCnno2r more. In order to more properly obtain the
effect of improving the impact properties described above, tlie lower limit value of tlie
total number density of particles of austenite and martensite is more preferably 1.3
numberlPm2. It is preferable that the total number density of austenite particles and
martensite particles be as large as possible. This is because, as the total number
density of austeriite particles and martensite patticles becomes larger, localization of
deformation is prevented and itnpact properties are fu~zheirm proved. Accordingl~:
the upper limit value of the total number density of austenite particles and martensite
patticles is not necessary to be specified. However, when considering tlie capability
of manufacturing equipment, the substantial upper limit value of the total number
density of austenite particles and maltensite patticles is approxin~ately3 .0 nut~~ber/pn~.
The ratio of the number of austenite particles and the number of martensite
particles is not necessary to be specified. Even when the martensite particles are not
contained in the metallographic microstructure, it is possible to obtain the effect for
preventing cracks described above.
The nurnber density of.the austenite particles and the martensite particles can
be obtained by the following method. First, a test piece is prepared fsom the hotformed
member along a rolling direction and a direction o~-thogonatlo the rolling
direction of the base steel sheet wvhich is a raw material of tlie hot-forlned member.
Then, tlie metallographic microstructures of a cross section of the test piece along the
rolling direction and a cross section thereof orthogot~atlo tlie rolling direction are
imaged by an electron microscope. Tile electro~ml icrographs of a region having a
size of 800 pm x 800 pm obtained as described above are subjected to image ar~alysis
to calculate the number density of the austenite particles and the martensite particles.
It is easy to distinguish the austenite particles and the tnartensite particles from the
surrounding microstruch~resth rough use of an electron microscope.
It is not ~~ecessatroy s pecify an average grain size of the austenite particles
and the maltensite particles. In general, when the average grain size is large, this may
negatively affect the strength of steel. However, as long as when the number density
described above is achieved, the grain size of the austenite particles and the martensite
particles are not coarsened.
[0045]
(Other Microstructr~res)
As a metallographic microstructure other than the austenite and the martensite
described above, one or two or more of ferrite, bainite, cementite, and pearlite may be
contained in the hot-formed member. The amount of ferrite, bainite, cementite, and
the pearlite is not particularly specified, as long as the amount of austenite and
martensite is within the range described above.
[0046]
(Tensile Strength: 900 MPa to 1300 MPa)
The tensile strength of the hot-fom~edm ember according to the embodiment
is equal to or greater than 900 MPa. When the hot-formed member has such a tensile
strength, it is possible to achieve weight saving of various members using the steel
sheet according to the embodiment. Ho\vever, when the tensile stret~gthis greater
than 1300 MPa, brittle fsacture easily occurs on the steel sheet. Therefore, the upper
limit value of the tensile strength of the steel sheet is set to be 1300 MPa. Such
tensile strength can be obtained by the chemical components described above and by
~nanufacturingm ethod ~vhichw ill be described latel:
[0047]
3. Manufacturing Method
Next, a preferred ~nanufacturingn ~ethodo ft he hot-fom~edm ember according
to the embodime~lht aving the above-me~ltionedp roperties will be described.
[0048]
In order to ensure both of the tensile strength equal to or greater than 900 MPa
and excellent ductility and impact properties, it is necessary that the microstructure
after quenching is set as a metallographic microstructure in which the area ratio of
austenite is 10 area% to 40 area% and the total number density of particles of austenite
and maltensite is equal to or greater than 1.0 n~mber/~mas* d escribed above.
[0049]
In order to obtain such a metallographic microstructure, a base steel sheet
having the same chemical conlposition as the cl~emical conlposition of the hot-formed
member described above and having a metallographic microstructure in which total
area ratio of one or both of bainite and mastensite is equal to or greater than 70 area%,
and particles of cementite are present at a number density equal to or greater than 1.0
numberlPm2, is heated to a temperature region \vhich is equal to or higher than 670°C
and lower than 780°C and is lower than an Ac3 temperature in a heating step, and
holding the temperature oft he base steel sheet in the tenlperature region wl~ichis equal
. to or higher that] 67OoC and lower than 780°C and is lower than the Ac3 tenlperature
for 2 minutes to 20 minutes in a holding step, and performing hot pressing of the base
steel sheet in a hot forming step. The expression "temperature region which is equal
to or higher than 670°C and lower than 780°C artd is lo\ver than the A c ~te~ nperature"
indicates a "tenlperature region which is equal to higher than 670°C and lower than
780°C" when the Ac3 temperature is equal to or higher than 780°C, and indicates a
"temperature region which is equal to higher than 670°C and lower than the Ac3
temperature" when the Ac3 temperature is lower than 780°C.
In a case where the Mn content of the base steel sheet is 2.4 mass% to 8.0
mass%, the base steel sheet is cooled under conditions in which an average cooling
rate in a temperature region of 600°C to 150°C is from S0C/sec to 500°C/sec in a
cooling step, after the hot forming step. In a case where the Mn content of the base
steel sheet is equal to or more than 1.2 mass% and less than 2.4 mass%, the base steel
sheet is cooled under conditions in which the average cooling rate in a tenlperature
region of 600°C to 500°C is from S°C/sec to 500°C/sec and the average cooling rate in
a temperature region lower than 500°C and equal to or higher than 150°C is fsotn
S°C/sec and 20°C/sec in a cooling step, after the hot forming step.
[0050]
As a base steel sheet to be subjected to the hot pressing, the base steel sheet
having the same chen~icacl omposition as the che~nicacl omposition of the hot-formed
member described above and having a metallographic microst~~~ctiunr we hich one or
both of bainite and martensite are contained to have a total area ratio equal to or greater
than 70 area% and particles of cementite are present at a number density equal to or
greater than 1.0 numbe~./~itsn u~s ed. This base steel sheet is, for example, a hot
rolled steel sheet, a cold rolled steel sheet, a hot-dip galvanized cold rolled steel sheet,
or a galvannealed cold rolled steel sheet. The base steel sheet having the
metallographic microstructure is subjected to hot pressing under heat treatment
conditions which will be described latel; and accordingly, a hot-formed menlber having
the metallographic microstructure described above, a tensile strength equal to or
greater than 900 MPa, and excelle~d~ut ctility and impact properties is obtained.
The ~netallograpliicm icrostructure of thc base steel sheet described above is
specified in a portio~fir om an approximately 1/2t thickness positio~tio an
approximately 1/4t thickness positio~al nd a position \vl~ichis not located in the center
segregation portion. A reason for specifying the configuration of the metallographic
n~icrostructureo f the base steel sheet in this position is same as the reason for
specifying the configuration of the metallographic microstructure of tlie hot-formed
member of a portion from an approximately 1/2t thickness position to an
approximately 1/4t thickness position and a position which is not located in the center
segregation portion.
roo5 11
(One or Both of Bainite and Maltensite: 70 area% or more in total)
When the total area ratio of bainite and martensite in the base steel sheet is
equal to or greater than 70%, the metallographic microstructure of the hot-formed
member described above is formed in the heating step of the hot pressing which will be
described later and it becomes easy to stably ensure the strength after quenching.
Accordingly, the total area ratio of bainite and martensite in the base steel sheet is
preferably equal to or greater than 70%. It is not necessary to set the upper limit bf
the total area ratio of bainite and martensite. However, the upper limit of the total
area ratio is substantially approximately 99.5 area%, in order to allow particles of
cementite to be present at a rlumber density equal to or greater than 1.0 nu~nber/~nl~.
A method of measuring of each area ratio of bainite and martensite is \\re11
known for a person skilled in the art and the area ratio thereof can be measured by a
common method it1 the embodiment. In tlie examples which \\ill be described latel;
the area ratio of each of bainite and martensite is measured by performing image
analysis of electron ~nicrographso f the ~netallograpl~mici crost~ucture.
[0052]
(Number density of particles of cementite: 1.0 immber/pn2 or more)
The particles of cementite in the base steel sheet are precipitation nuclei of
austenite and maltensite, at the time of heating and cooling during the hot pressing.
In the n~etallographic inicrostiucture of the hot-fonned component, the total nulnber
density of austenite and martensite is necessarily equal to or greater than 1.0
nutnber/pm2, and in order to obtain such a nietallographic microstiuctt~ret,h e particles
of cementite are necessarily present in the metallographic microsttucture of the base
steel sheet at a number density equal to or greater than 1.0 numberll~m2. In a case
diere the number density of cementite in the base steel sheet is smaller than 1.0
numberlPm2, the total number density of austenite and martensite in the hot-formed
member may be smaller than 1.0 number/pin2. As the number density of particles of
cementite in the base steel sheet be large, the total number density of the austenite
patticles and the maltensite particles it1 the hot-formed member increase, thus it is
preferable that tlie tluniber density of particles of cementite in the base steel sheet is
large. However, when considering the upper limit of the capability of the equipment,
the substantial upper liinit of the liulnber density of the particles of cementite is
approxitnately 3.0 ~lumberlpm~.
The number density of cementite can be obtaitied by the following method.
First, a test piece is prepared from tlie base steel sheet along a rolling direction of the
base steel sheet and a direction orthogonal to the rolling direction. Theti, the
metallographic ~nicrostructureso f a cross section of tlie test piece along the rolling
direction and a cross section thereof o~thogonatlo the rolling direction are imaged by
an electron microscope. The electron micrographs of a region having a size of 800
pin x 800 pn~im aged as described above are subjected to image analysis to calculate
the nunlber density of cementite. It is easy to distinguish the cementite particles from
the surrounding n~icrostructuresu sing an electron microscope.
It is not necessary to specify the average grain size of the cementite particles.
As long as the number density described above is achieved, the cenientite which is
coarse and negatively affect the steel is not precipitated.
[0053]
The hot rolled steel sheet satisfying the conditions necessary for the base steel
sheet of the embodinlent can be manufactured, for example, by perforining finish
rolling with respect to an ingot having the same chemical composition as the chemical
cotnposition of the hot-formed member described above in a temperature region equal
to or lower than 900°C, and rapidly cooling the steel sheet after the finish rolling to a
temperature region equal to or lower than 600°C at a cooling rate equal to or faster
than S°C/sec. The cold rolled steel sheet satisfying the conditions necessary for the
base steel sheet of the embodiment can be manufactored, for example, by annealing the
hot rolled steel sheet at a temperature equal to or higher thanAc3 temperature and
performing rapid cooling to a temperature region equal to or lower than 600°C at an
average cooling rate of equal to or faster than S0C/sec. By performing the rapid
cooli~lgu nder the conditions described above, a large anlount of precipitation nuclei of
cementite is generated in the base steel sheet, and as a result, it is possible to obtain the
base steel sheet containing cementite having the nuriiber density equal to or greater
than I .O ~mmber/~rnT~h. e hot-dip galvanized cold rolled steel sheet and the
galvannealed cold rolled steel sheet satisfiing the conditions necessary for the base
steel sheet of the embodiment can be manufactured, for exan~pleb, y performing hot
dip galvanizing and galvantiealitig with respect to the cold rolled steel sheet.
[0054]
(Heating Temperature of Base Steel Sheet: Temperature Region Which is
Equal to or Higher Than 670°C and Lower Than 780°C and is Lower 'Il~anA c3
Temperature)
(Holding Temperature and Holding Time of Base Steel Sheet: Holding in
Temperature Region Which is Equal to or Higher Than 670°C and Lower Than 780°C
and is Lower Than Ac3 Temperature for 2 Millutes to 20 Minutes)
In the heating step of the base steel sheet to be subjected to the hot pressing,
the base steel sheet is heated to the temperature region which is equal to or higher than
670°C and lower than 780°C and is lower than the Ac3 temperature ("C). In the
holding step of the base steel sheet, the temperature of the base steel sheet is held in the
temperature region, that is a temperature region which is equal to or higher than 670°C
and lower than 780°C and is lower than the Ac3 temperature ("C) for 2 minutes to 20
minutes. The Ac3 temperature is a temperature represented by the following
Expression (i) obtained by an experiment. In a case where the steel is heated to a
temperature region equal to or higher than the Ac3 temperature, the ~netallographic
microstrocture of the steel becomes an austenite single phase.
[0055]
A ~ ~ = 9 1 0 - 2 0 3 x ( ~ ~ ~ ~ ) - 1 5 . 2 x N i + 4 4 . 7 x S i + 1 0 4 x V + 3 1 . 5 x M o - 3 0 x
Mn-11 xCr-20xCu+700xP+400xsol.Al+50xTi ...( i)
Herein, an element symbol in the expression represents the a~noun(tu nit:
mass%) of each elen~enitn the chemical cotllposition of the steel sheet. "sol. AI"
represents concentration (unit: mass%) of solution Al.
[0056]
In a case where the holding temperature in the holding step is lower than
670°C and the base stcel sheet contains a large amount of Si, the area ratio of the
austenitc in the base steel sheet before the hot pressing beconles too s~lialal nd tlie
shape accuracy of the hot-fornled member after the hot forming is significantly
deteriorated. Accordingly, the holding tenlpcrature in the holding step is set to be
equal to or higher than 670°C. Meanwhile, when the holding temperature is equal to
or higher than 780°C or equal to or higher than the Ac3 temperature, the sufficient
amount of austenitc is not contained in the ~netallographicm icrost~-uctureo f the hotformed
member after quenching and the ductility of the hot-formed member is
significantly deteriorated. In addition, in a case where the holding temperature is
equal to or higher than 780°C or equal to or higher than the Ac3 temperature, fine hard
microsti~~ctnirse n ot present in the metallograpl~icm icrost~uctureo f the hot-formed
member, and this causes a deterioration in impact properties of the hot-formed member.
Accordingly, the holding temperature is set to be lower than 780°C and lower than the
Ac3 temperature. The holding temperature is preferably from 680°C to 760°C in
order to more properly avoid the unpreferred phenomenon described above.
When the holding time in the holding step is shorter than 2 minutes, it is
difficult to stably ensure the strength of the hot-formed member after quenching.
Accordingly, the holding time is set to be equal to or longer than 2 tninutes.
Meanwhile, when the holding time exceeds 20 minutes, not only the productivity is
suppressed, but the surface quality of the hot-formed member is deteriorated due to
generation of scales or zinc based oxides. Accordingly, tlie holding time is set to be
equal to or shorter than 20 minutes. The holding time is preferably from 3 minutes to
15 minutes in order to more properly avoid the unpreferred phenomenon described
above.
[0057]
A heating rate in the heating step for heating to the telnperature region which
is equal to or higher than 670°C and lowvcr than 780°C and is lower than the A c ~
temperature is not particularly necessary to be limited. I~Iowevel; it is preferable to
heat the steel sheet at an average heating rate of 0.2'C/sec to 100°C/sec. When the
average heating rate is set to be equal to or faster than 0.2"C/sec, it is possible to
ensure higher productivity. In addition, when the average heating rate is set to be
equal to or slower than 100°C/sec, the heating temperature is easily controlled in a case
of performing the heating using a typical furnace. However, when high frequency
heating or the like is used, it is possible to contml the heating temperature with
excellent accuracy, even when the heating is performed at a heating rate exceeding
1 OO°C/sec.
[0058]
(Average cooling rate in cooling step in a case where Mn content of base steel
sheet is 2.4 mass% to 8.0 mass%: S0C/sec to SOO°C/sec in temperature region of 60OoC
to 150°C)
(Average cooling rate in cooling step in a case where Mn content of base steel
sheet is equal to or tilore than 1.2 mass% and less than 2.4 mass%: S0C/sec to
500°C/sec in a temperature 1.egion of 600°C to SOO°C and S°C/sec to 20°C/sec in
temperature region \\rhicli is lower than 500°C and equal to or higher than 150°C)
In the cooling step, the cooling is pcrforlned in the temperature region of
150°C to 600°C so that diffusion type transforlnation does not occur in the hot-fonned
member. When the average cooling rate in the temperature regionof 150°C to 600°C
is slower than S°C/sec, soft ferrite and pearlite are excessively generated in the hotformed
member and it is difficult to ensure the tensile strength equal to or greater than
900 MPa after quenching. Accordingly, the average cooling rate in the temperature
region is set to beequal to or faster than S°C/sec.
Tlie upper liniit value of the average cooling rate in the cooling step changes
depending on the Mn content of the base steel sheet. In a case where the Mn content
of the base steel sheet is 2.4 mass% to 8.0 mass%, it is not necessary to particularly
limit the upper limit value of the average cooling rate. However, the average cooling
rate in the temperature region of 150°C to 600°C hardly exceeds 500°C/sec, in the
typical equipment. . Accordingly, the average cooling rate in the tenlperature region of
150°C to 600°C in a case where the Mn content of the base steel sheet is 2.4 mass% to
8.0 mass% is set to be equal to or slower than 500°C/sec. In a case where the average
cooling rate is excessively high, the production cost increases due to energy related to
coolitlg, and accordingly, the average cooling rate in the temperature region of 150°C
to 60OoC in a case where the Mn content of the base steel sheet is 2.4 mass% to 8.0
mass% is preferably equal to or slower than 200°C/sec.
[0059]
In a case where the Mn content of the base steel sheet is equal to or more than
1.2% and less than 2.4%, it is necessary to perform mild cooling in the temperature
region which is lower than 500°C and equal to or higher than 1 50°C, in order to
iniprove the ductility of the hot-formed member. In a case where the Mn content of
the base steel sheet is equal to or niore than 1.2% and less than 2.4%, specificallp, it is
necessary to perforln cooling in the temperature region which is lower than 500°C and
equal to or higher than 150°C at the average cooling rate of S0C/sec to 20°C/sec, and
Inore specifically, it is preferable to control the cooling rate as described later,
[0060]
In the hot pressing, generally, a die having rootn temperature or several
tens "C inmediately before the hot pressing takes heat fiom the hot-formed member,
and accordingly, the cooling of the hot-fonned n~e~nbies rp erformed. Accordingly, a
size of the die may be changed to change heat capacity of a steel die, in order to
change the cooling rate. In a case wvliere the die size cannot be changed, it is also
possible to change the cooling rate by changing a flow rate of a cooling mediutm using
a fluid cooling type die. In addition, it is also possible to change the cooling rate by
allowing a cooling medium (water or gas) to flow through grooves during pressing
osing a die having a plurality of grooves provided in advance. In addition, it is also
possible to change the cooling rate by operating a pressing machine during the pressing
to separate the die and the hot-formed nie~nbera nd by allowing gas flow between both
items. Furthermore, it is also possible to change the cooling rate by die clearance to
change a contact area between the die and the steel sheet (hot-formed member). With
the above description, the following measures are considered as a way which changes
the cooling rate at approximately 50OoC.
[0061]
(1) A way in which the cooling rate is changed by moving the hot-formed
member into a die having different heat capacity or a die heated to a temperature
exceeding 100°C, immediately after the temperature reaches 500°C;
(2) a way in wliich the cooling rate is changed by changing a flow rate of a
cooling medium in a die immediately after the temperature reaches 50OoC, in a case of
a fluid coolirig type die;- and
(3) To change the cooling rate by operating a pressing ~ilachineto separate the
die and the hot-formed n~enlbera nd by allowing gas flow between both items and
changing the flow rate of the gas, immediately after the temperature reaches 500°C.
[0062]
The type of the forming performed by the hot pressing method of the
embodiment is not particularly limited. Exemplary examples of the forming iticlude
betiding, drawing, stretcliing, hole expending, or flanging. Tlie forming type
described above may be preferably selected depending on the desired type or shape of
tlie hot-formed mernbes. Reprcse~itativee xa~llpleso f the hot-fonned nietnber can
include a door guard bar and a bumper reinforcenient, which are reinforci~ig
components for a vehicle. For exati~plei,n a case where the hot-formed tilember is a
bunlper reillforcement, the hot-formed member which is a galvannealed steel sheet
having a predetermined length may be prepared and may be sequentially subjected to
bending or the like in a die under the conditions described above.
[0063]
In the above description, tlie hot forming has been described as an example of
the hot pressing w11ich is a specific type, but the tnanufacturitig method accordi~lgto
the embodimetlt is not limited to hot pressing. The manufacturing method according
to the embodimetit can be applied to various hot forming including means for cooling
the steel sheet at the same time as the forming or immediately after the forming, in fhe
same manner as in the case of the hot pressing. As such liot forming, roll fanning is
used, for example.
[0064]
The hot-formed member according to the etnboditnent has excellent ductility
and impact properties. It is preferable that the hot-formed member according to the
etilbodiment have ductility so tliat the total elongation obtained by a tensile test is
equal to or greater than 15%. It is more preferable that the total elongation of tlie liotfortned
metnber according to tlie etnbodimetit obtained by a tensile test is equal to or
greater than 18%. It is most preferable tliat tlie total elotigation of the hot-formed
member according to the embodiment obtained by a tensile test is equal to or greater
than 21%. Meanwhile, it. is preferable that the hot-formed member according to the
embodime~iht as impact properties so that an impact value obtained by a Charpy test at
O°C is equal to or greater than 20 ~/cnl~T. he hot-formed member having such
properties is realized by satisfying the configuration described above relating to the
chemical composition and the metallographic microst~ucture.
[0065]
After perfor~ningh ot forming such as hot pressing, shot blast treatnierit is
gerierally performed with respect to the hot-formed member in order to relnove scales.
This shot blast treatment has an effect of introducing compressive stress to the surface
of a treated material. Accordingly, the shot blast treatment performed with respect to
the hot-fanned member is advantageous for preventing delayed fracture in the hotformed
member and improving fatigue strength of the hot-formed member.
[Examples]
[0066]
Hereinafter, examples of the present invention will be described.
Steel sheets having chemical composition shown in Table 1 and the sheet
thickness and the nietallographic microstructure shown in Table 2 were used as base
steel sheets.
[0067]
[Table 11

[Table 21
[0069]
These base steel sheets are steel sheets manufactured by performing hot
rolling of a slab welded in a laboratory (slio~vna s hot rolled steel sheet it1 Table 2) or
steel sheets nlanufactured by performing cold rolling and recrystallization annealing of
the hot rolled steel sheet (showti as cold rolled steel sheet in Table 2). Using a plating
simulator, some steel sheets were subjected to a hot-dip galvanizing treatnlcnt (plating
deposition atnount per one surface is 60 g/m2) or galvatu~ealingt reatnlent (plating
deposition amount per one surface is 60 g/m2, the Fe content in the plated film is 15
mass%). In Table 2, the steel sheets are respectively shown as a hot-dip galvanized
steel sheet and a galvannealed steel sheet. In addition, steel sheets as cold rolled
(shown as "full-hard" in Table 2) steel sheets are also used.
[0070]
These steel sheets were cut to have a width of 100 mm and a length of 200
mn and heated and cooled under the conditions shown in Table 3. A thermocouple
was attached to the steel sheet and the cooling rate was measured. The "average
heating rate" of Table 3 indicates the average heating rate in a temperature region from
room temperature to 670°C. The "l~oldingt ime" sho\nl of Table 3 indicates time for
which the steel sheet was held in the temperature region equal to or higher than 670°C.
The "cooling rate *I" of Table 3 indicates the average cooling rate in the temperature
region from 600°C to 150°C and the "cooling rate *2" indicates the average cooling
rate in the temperature region from 500°C to 150°C. The steel sheets obtained under
various manufacturing conditions were subjected to metallographic microstructure
obsewation, X-ray diffraction measurement, a tensile test, and a Charpy test.
[0071]
[Table 31
+I Average cooling rate from 600" C to 500" C.
*2 Average cooling rate from.500" C to 150" C.
Samples prepared in the examples and comparative examples were not
subjected to the hot pressing using a die, but subjected to the same therrilal history as
that of the hot-folmed tnembee Accordingly, the mechanical properties of the
sa~nplesa re substantially the same as those of the hot-formed tnember having tlle same
thermal history.
[0073]
(Microstructure of Base Steel Sheet)
A test piece was prepared fro~ltlh e heat-treated sample along the rollit~g
direction of the base steel sheet and the direction orthogonal to the rolling direction of
the base steel sheet. Then, the rnetallographic microstructures of a cross section of
the test piece along the rolling direction and a cross section thereof orthogonal to the
rolling direction were imaged by an electron microscope. The electron micrographs
of a region having a total size of 0.01 1nm2 obtained as described above are subjected
to image analysis to identi@ the metallographic microstmcture and measure the total
area ratio of bainite and martensite. In addition, the electron micrographs of a region
having a size of 800 pm x 800 prn obtained by imaging the samples described above
with an electron microscope were subjected to image analysis to calculate the number
density of the cementite particles.
[0074]
(Distribution State of Austenite and Martensite of Heat-Treated Sample)
A test piece was prepared from the heat-treated satnple along the rolling
direction of the base steel sheet and the direction orthogonal to the rollit~gd irection of
the base steel sheet. Then, the ~netallographicm icrostructures of a cross section of
the test piece along the rolling direction and a cross section thereof orthogot~atlo the
rolling direction are i~nagcdb y an electron microscope. The electron micrographs of
a region ltaving a size of 800 11ni x 800 1nn obtained as described above were subjected
to image analysis to calculate the number density of tlie austenite particles and the
martensite pat-ticles.
[0075]
(Area Ratio of Austenite of Heat-Treated Sample)
A test piece having a width of 25 nlm and a length of 25 mm was cut frotom
each heat-treated sample and a thickness thereof is reduced by 0.3 mm by performing
chemical polishing with respect to the surface of the test piece. The X-ray diffraction
was performed with respect to the surface of the test piece after tlie chemical polishing
and a profile obtained as described above was analyzed to obtain the area ratio of
residual austenite. This X-ray difiaction was repeated total three times and a value
obtained by averaging the obtained area ratios is shown in the table as the "area ratio of
austenite".
[0076]
(Tensile Test)
JIS No. 5 tensile tcst piece was prepared from each heat-treated sanlple so that
the load axis was orthogonal to tlie rolling direction and the tensile strength (TS) and
the total elongation (EL) was measured. The samples it1 which the tensile strength
was smaller than 900 MPa and the samples in which the total elongation was less than
15% were determined to be "poor".
1.00771
(Inipact Properties)
A V notch test piece having a thickness of 1.2 tnm was manufactured by
machining the heat-treated sample. The four notch test pieces were laminated,
screwed, and subjected to a Charpy impact test. A V notch direction \x7as parallel to
the rolling direction. When the impact value at O°C was equal to or greater than 20
JICIII~,t lie impact properties were determined to be "excellent".
[0078]
(Other Propesties)
Descaling of the heat-treated samples is performed, and then, presence or
absence of residual scales in the surface of the sample was confirmed. The sample in
which the residual scales were present, was determined as the comparative example in
which surface quality is not good. In addition, the heat-treated samples were dipped
in 0.1 N hydrochloric acid to confirm whether or not the delayed fracture occurred.
The sample in wlich the delayed fracture occ~~rrewda, s determined as the cotnparative
example in which delayed fracture resistance is not good.
[0079]
(Description of Test Results)
Results of the test obtained by simulating the hot pressing are sl~oo\vnin Table
4.
[OOSO]
The underlined numerical values in Tables 1 to 4 indicate that the content,
conditions, or the mechanical properties showvn by the nu~nericavl alues are beyond the
range of the present invention.
[0081]
[Table 41
*1 Scales are not peeled.
*2 Delayed fracture occurs while being dipped in 0.1 N hydrochloric acid.
SanipleNos. 1 to 3, 8, 9, 11, 13, 15, 18,20,21, 25,26,30, and 32 which are
present invention exaniples of Table 4 have a high tensile strength equal to or greater
than 900 MPa and excellent ductility and impact properties. In the samples wl~iclai re
present invention examples, no residual scales were present after descaling, that is,
excellent surface quality was obtained, and cut cross section was not cracked during
tlie dipping in hydrochloric acid, that is, excellent delayed frach~rer esistance was
obtained.
[0083]
Meanwhile, regarding tlie sample No. 4,a cooling rate was beyond the range
regulated in the present invention, thus the desired tensile strength was not obtained.
Regarding the sample Nos. 5 and 6, a metallographic microstructure of a base steel
sheet is beyond the range regulated in the present invention, thus impact properties are
poor.
Regarding the sample Nos. 7 and 24, a chemical composition was beyond the
range regulated in the present invention, thus desired tensile strength was not obtained.
Regarding the sample No. 10, a metallographic microstructure of a base steel
sheet was beyond the range regulated in the present in\rention, thus the desired tensile
strength was not obtained.
Regarding tlie sample No. 12, a cooling rate was beyond the range regulated
in the present invention, thus the ductility mas poor. Regarding the sample Nos. 14
and 16, a heating temperature was beyond the range regulated in the present invention,
thus the ductility and the impact properties were poor.
Regarding the sample No. 17, a heating temperature was beyond tlie range
regulated in the present invention, thus the ductility is poor.
Regarding the sample No. 19, a chemical con~position\v as beyond the range
regulated in the present invention, thus the impact property was poor.
Regarding the sample No. 22, a holding time was beyond the range regulated
in the present invention, thus tlie desired tensile strength was not obtained.
Regarding the sample No. 27, a chemical composition was beyond the range
regulated in the present invention, tlius the ductility was poor.
The sample No. 23 is an example in which a holding time was beyond the
range regulated in the present invention and the sample Nos. 28 and 3 1 are examples in
which chemical conlpositions were beyond the range regulated in the present invention.
In these samples, the tensile strength, the total elongation, and the impact properties
were excellent, but residual scales were present after descaling and surface qualities
were poor. Since tlie sample No. 29 had a chemical composition which was beyond
the range regulated in the present invention, the delayed fracture occurs when
performing dipping in 0.1 N hydrochloric acid and it was determined that the delayed
fracture resistance was poor.
[0084]
In addition, among the steel sheets of the present invention examples, the
sample Nos. 1 to 3,7 to 9, 11, 13, 15, 17, 19, and 21 have a Si content in the preferred
range and the ductility thereof ware more excellent. Among those, the sample Nos. 2,
8, 11, 17, 19, and 21 have an area ratio of aostenite in the preferred range and tlie
ductility thereof was more excellent.
[Docotnent Type] CLAIMS
1. A hot-fom~ed member having a chemical conlposition comprising, by
mass%,
C: 0.05% to 0.40%,
Si: 0.5% to 3.0%,
A h 1.2% to 8.0%,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 2.0%,
N: 0.01% or less,
Ti: 0% to 1.0%,
Nb: 0% to 1.0%,
V: 0% to 1.0%,
Cr: 0% to 1.0%,
Mo: 0% to 1.0%,
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
E M : 0% to 0.01%,
Zr: 0% to 0.01%,
B: 0% to 0.01%,
Bi: 0% to 0.01%, and
the balance of Fe and impurities,
\vherein the hot-fornled inernber has a lnetallographic microstructure \vhich
contains an austenite of 10 area% to 40 area% and in \vhiclich the total number density of
particles of the austenite and particles of a ltla~tensiteis equal to or greater than 1.0
piece/pin2, and
\vlicherein a tensile strength is 900 MPa to 1300 MPa.
2. The hot-formed member according to claiin 1,
\vherein the che~nicacl omposition includes one or two or more selected from
the group consisting of, by mass%,
Ti: 0.003% to 1.0%,
Nb: 0.003% to 1.0%,
V: 0.003% to 1.0%,
Cr: 0.003% to 1.0%,
Mo: 0.003% to 1.0%,
Cu: 0.003% to 1.0%, and
Ni: 0.003% to 1.0%.
3. The hot-formed member according to claim 1 or 2,
wherein the chemical composition includes one or two or Inore selected from
the group co~lsistingo f, by inass%,
Ca: 0.0003% to 0.01%,
Mg: 0.0003% to 0.01%,
REM: 0.0003% to 0.01%, and
Zr: 0.0003% to 0.01%.
4. The hot-formed ~neilichbera ccording to any one of claims 1 to 3,
wherein the chelnical conipositioti includes, by mass%, B: 0.0003% to 0.01%.
5. The hot-formed member according to any one of claims 1 to 4,
wherein the chemical conlposition includes, by tilass%, Bi: 0.0003% to 0.01%.
6; A manufacturing metliod of a hot-formed member, the method
comprising:
heating a base steel sheet having a chemical conlposition which is same as the
chenlical conlposition of the hot-formed member according to any one of claims 1 to 5
and in \vIiich a Mn content is 2.4 mass% to 8.0 mass%, and having a tnetallographic
microstructure in wvhich the total area ratio of one or both of a bainite and a martensite
is equal to or greater than 70 area%, and particles of a cementite are present at a
number density equal to or greater than 1.0 nu~nber/~ntol ~a, t emperature region wvhich
is equal to or higher than 670°C and lower than 78OoC and is lower than an Ac3
tetnperature ;
then holding the temperature of the base steel sheet in the tetnperature region
which is equal to or higher than 670°C and lower than 780°C and is lowver than an Ac3
temperature for 2 minutes to 20 minutes;
theen performing a hot forming with respect to the base steel sheet; and
then cooling the base steel sheet under conditions in wl~icha n average cooling
rate it1 a temperature region of 600°C to 150°C is from S°C/sec to SOO°C/sec.
7. A manufacturing rnethod of a hot-formed membel; the liiethod
comprising:
heating a base steel sheet having a chemical composition which is same as the
clieinical conlposition of the hot-formed member according to any one of claims 1 to 5
and in which a Mn content is 2.4 mass% to 8.0 mass%, and having a metallographic
tiiicrostructure in which the total area ratio of otie or both of a bainite and a maitensite
is equal to or greater than 70 area%, and particles of a cementite are present at a
number density equal to or greater than 1.0 n~mber/~unto* ,a temperature region which
is equal to or higher-than 670°C and lower than 780°C and is lower than an Ac3
temperature;
then l~oldingth e temperature of the base steel sheet in the temperature region
which is equal to or higher than 670°C and lower than 780°C and is lower than anAc3
temperature for 2 minutes to 20 minutes;
then performing a hot forming with respect to the base steel sheet; and
then cooling the base steel sheet under conditions in which an average cooling
rate in a temperature region of 600°C to 500°C is from S°C/sec to 500°C/sec and the
average cooling rate in a temperature region lower than 500°C and equal to or higher
than 150°C is from S°C/sec and 20°C/sec.