DESCRIPTION
Title of Invention: Heat Treated Steel Product Having
High Strength and Excellent Chemical Conversion Coating
5 Ability and Method of Production of Same
Technical Field
[0001] The present invention relates to a heat treated
steel product excellent in chemical conversion coating
10 ability for priming a coating surface and excellent in
corrosion resistance after coating even after heat
treatment 1vithout going through a special step for
removal of oxidation scale and to a method of production
of the same. . ..
15 Background Art
[0002] In recent years, from the viewpoint of global
environmental issues and crash safety performance,
thinner and higher strength structural parts have been
sought for automobiles. To meet with these demands,
20 structural parts for automobiles made using high strength
steel sheet as materials have been increasing. However,
if using high strength steel sheet as the material for
press-forming operations to produce structural parts for
automobiles, defects such as wrinkles and springback 1·1ill
25 easily occur in the shaped products. Therefore, it is not
easy to use high strength steel sheet as the material for
press-forming operations to produce structural parts for
automobiles.
[0003] As the means for solving such a problem, the
30 technique of hot v/Orking the steel material and rapidly
cooling it for hardening so as to produce a high strength
part has been commercialized. For example, hot pressing
1vorks steel sheet at a high temperature where it is soft
and high in ductility, so can form complicated shapes
35 with good dimensional precision. Furthermore, by heating
steel sheet to the austenite region and rapidly cooling
it in a die, it is possible to simultaneously achieve
- 2 -
higher strength of the steel sheet through martensite
transformation.
[0004] However, 1-1i th such a working method, the steel
is heated to a high temperature of 800 to 1000°C, so the
5 problem arises that the surface of the steel sheet
oxidizes. If such scale remains, when coating the surface
at the next step, the adhesion between the steel sheet
and the coating film will become inferior and a drop in
the corrosion resistance will be invited. Therefore,
10 after the press-forming, shot blasting or other treatment
for scale removal is necessary.
[0005] PLT 1 discloses the technique of bending of a
metal material during 1-1hich making a heating device and
cooling device move relative to the metal material while
15 using the heating device to locally heat the metal
material, giving a bending moment to the portion greatly
falling in deformation resistance due to the heating to
bend the material to a desired shape bent tlvOdimensionally
or three-dimensionally, and next using the
20 cooling device to cool the material to harden it (below,
referred to as "hot 3D bending") .
[0006] Hot 3D bending is a working technique used
mainly for automobile members. It was developed as a
technique for simultaneously meeting the two
25 contradictory needs of lighter 1·1eight of the car body and
improved crash safety. Hot bending locally heats a steel
tube while hardening it by water cooling and
simultaneously gives a bending moment to bend the tube
and thereby enables production of a complicatedly shaped
30 closed cross-section structural member by a single
process. Shaping an auto part in cross-sectional
structure at over 1470 MPa becomes possible.
[0007] However, this method also heats the steel
material to the austenite region and rapidly cools it by
35 a cooling medium so as to try to raise the strength of
the steel material by martensite transformation, so there
are the problems that oxidation scale forms on the
- 3 -
surface, the adhesion of the steel material and coating
film becomes inferior when coating the surface in the
next step, and a drop in corrosion resistance is invited.
[0008] To deal with these problems, the inventors
5 disclosed by PLT 2 an invention relating to a method of
production and production apparatus of hardened steel
material conveying steel material in its longitudinal
direction while heating the conveyed steel material to
the hardenable temperature region, then cooling to harden
10 the steel material to thereby suppress or eliminate the
formation of oxidation scale.
[0009] According to this invention, the steel material
is conveyed in its longitudinal direction while using a
heating device arranged at a first position separated
15 from the conveyed steel material so as to heat the steel
material to the hardenable temperature region and using a
cooling device arranged at a second position d01-mstream
from the first position in the direction of conveyance of
the steel material to spray the steel material with a
20 cooling medium and thereby harden the steel material.
During this, at the steel material, an inert gas or
reducible gas is filled in the space around the heated
part from the heating device so as to produce a hardened
steel material suppressed in oxidation scale.
25 Citation List
Patent Literature
[0010]
83304A
PLT 1: Japanese Patent Publication No. 2007-
PLT 2: Japanese Patent Publication No. 2011-89150A
30 Summary of Invention
Technical Problem
[0011] The inventors engaged in intensive studies to
further improve the invention disclosed by PLT 2. As a
result, the inventors discovered that even if using a
35 heating device to blow and fill an inert gas into a space
around the part where the steel material is heated, it is
extremely difficult to completely eliminate oxidation
- 4 -
scale and scale (oxidation film) is unavoidably formed
and that, depending on the heating and cooling
conditions, due to that unavoidable scale, sometimes the
chemical conversion coating ability is inferior.
5 [0012] If scale is thick, it easily peels off and
therefore the steel material is impaired in chemical
conversion coating ability and electrodeposition coating
ability after being worked. Further, if the scale is
uneven in thickness, the chemical conversion coating or
10 electrodeposition coating becomes uneven. However,
providing an oxidation scale removal step to remove the
oxidation scale after working leads to a rise in cost, so
this is not preferable.
[0013] The present invention \vas made in consideration
15 of this new problem and has ~as its object the provision
of a heat treated steel product excellent in chemical
conversion coating ability for priming a coating surface
and excellent in corrosion resistance after coating even
after heat treatment \vithout going through a special step
20 for removal of oxidation scale and the provision of a
method of production of the same. Furthermore,
specifically, it has as its object the provision of a
heat treated steel product which is produced by heat
treating or bending with heat treatment an unplated steel
25 material and which has high strength and excellent
chemical conversion coating ability and corrosion
resistance after coating, so for example, can be suitably
used as an automobile member and the provision of a
method of production of the same.
30 Solution to Problem
[0014] The inventors investigated in what cases the
chemical conversion coating ability deteriorates and as a
result learned that by heating in an atmosphere in which
an inert gas is blm·m, even if a small amount of scale is
35 formed, if the scale dissolves and iron ions are supplied
at the time of formation of the chemical conversion
coating or if the base material dissolves and iron ions
- 5 -
are supplied, a sound chemical conversion coating is
formed and that, on the other hand, if the formed scale
does not sufficiently dissolve at the time of formation
of the chemical conversion coating, the chemical
5 conversion coating ability will be inferior.
[0015] The inventors engaged in further intensive
studies and as a result learned that if the scale has a
thickness of l ~m or less and the FeO contained in the
scale is 90% or more, Fe ions are sufficiently supplied
10 at the time of formation of the chemical conversion
coating and a good chemical conversion coating becomes
possible. Further, they learned that for realizing such
scale, it is sufficient to use a working apparatus having
a gas chamber, heating device, and cooling device and
15 perform 3D bending·while running the inert gas and during
that time making the time period during which the steel
material dwells in a 600°C or more temperature region less
than 1 second. The present invention was made based on
this discovery and has as its gist the following:
20 [0016] (1) Heat treated steel product having high
strength and excellent chemical conversion coating
ability comprising a scale with FeO content of 90% or
more, having a thickness of 1 ~m or less on the surface.
[0017] (2) The heat treated steel product having high
25 strength and excellent chemical conversion coating
ability according to (1), wherein the steel has a
structure consisting of martensite, or martensite and
tempered martensite.
[0018] (3) The heat treated steel product according to
30 (1) or (2), wherein the steel product is a hollow member
having a closed horizontal cross-sectional shape.
[0019] (4) The heat treated steel product according to
any one of (1) to (3), wherein a maximum value and a
minimum value of the thickness of the scale are within
35 ±10% of an average value of the thickness.
[0020] (5) A method of producing a heat treated steel
- 6 -
product using a working apparatus having a gas chamber, a
heating device, and a cooling device from an upstream
side, the method of producing the heat treated steel
·product comprising introducing an inert gas into a·gas
5 chamber and filling the inert gas into a space including
the heating device and the cooling device while making
the working apparatus move relative to a steel material
so that the steel material is locally heated by the
heating device then the steel material is cooled by the
10 cooling device, .wherein a time period during which the
steel material dwells in a 600°C or more temperature
region is less than 1 second and between the heating and
cooling, a bending operation is performed at a portion of
the steel material greatly dropping in deformation
15 resistance due to
[0021] ( 6) The
heating.
method of producing a heat treated
steel product according to ( 5) 1vherein in the step of
cooling, a time period during which the steel material
dwells in a 600°C to 300°C temperature region is within 3
20 seconds.
Advantageous Effects of Invention
[0022] According to the present invention, at the time
of chemical conversion, the scale is dissolved and a
sound chemical converted film is formed, so a heat
25 treated steel product is provided which is excellent in
chemical conversion coating ability and in turn excellent
in corrosion resistance after coating as well even if
supplied to the chemical conversion coating and other
coating steps without going through a shot blasting or
30 other scale removal step and therefore is suitable for
use for applications in which a certain degree of
corrosion resistance is necessary even if heavy corrosion
resistance of an extent requiring sacrificial protection
by plating is not sought.
35 [0023] As a portion for application of the heat
treated steel product according to the present invention,
in the case of an auto part, making the strength higher
- 7 -
enables the vehicle to be made lighter in weight. A
portion where corrosion resistance is demanded is
preferable. For example, a pillar, door beam, roof,
bumper, or other reinforcements, frames, arms, etc. may
5 be mentioned.
Brief Description of Dra\vings
[0024] FIG. 1 is a view showing one example of a
working apparatus able to be used in the present
invention.
10 Description of Embodiments
15
20
[0025] The reasons for limitation of the heat
treatment use product and the method of production of the
same according to the present invention will be explained·
below.
[0026] The·heat treated steel product of the present
invention is produced using as a material a steel
material which has not been plated. The surface of the
product after heat treatment has a very thin scale (oxide
film). The thickness has to be 1 ~m or less.
[0027] If the thickness of the scale exceeds 1 ~m, a
lot of scale will remain without being dissolved at the
time of chemical conversion, the supply of iron ions will
become insufficient, and the chemical conversion coating
ability will become degraded. Further, if the scale
25 becomes thicker, even if a chemical conversion coating is
formed on the scale, the scale and base iron will easily
peel apart and the coating adhesion will become inferior.
Therefore, the thickness of the scale is 1 ~m or less,
preferably 0.5 ~m or less.
30 [0028] Further, scale has to include FeO: 90% or more.
This ratio can be found by finding the X-ray intensities
of FeO, Fe30 4 , and Fe20 3 by analyzing the product surfaces
by X-ray diffraction and calculating the ratio of the Xray
intensity of FeO with respect to the total of the X-
35 ray intensities of FeO, Fe30 4 , and Fe20 3 •
[0029] If the ratio of FeO is less than 90%, a lot of
- 8 -
scale will remain without dissolving at the time of
chemical conversion, the supply of iron ions will become
insufficient, and the chemical conversion coating ability
will become inferior. The reason is not necessary clear,
5 but is believed to be like the following:
[0030] In scale, first, FeO is formed at a high
temperature. Along with the progression of oxidation,
Fe30 4 is formed or, during the cooling process, some of
the FeO undergoes eutectoid transformation and Fe30 4 is
10 formed. If, in the·product, the ratio of FeO iri the scale
decreases and the ratio of Fe30 4 increases, the chemical
conversion coating ability deteriorates since Fe30 4 is
harder to dissolve in a chemical conversion coating
solution compared 1·1ith FeO.
15 [0031]' The steel product of the present invention has
to have the high strength obtained by heat treatment
while being excellent in chemical conversion coating
ability, so the steel structure is comprised of
martensite. However, depending on the required strength
20 and performance, part of the martensite may also be
replaced 1vi th tempered martensite. Further, carbides and
residual austenite which unavoidably remain in the
process of heat treatment may also be contained.
[0032] Note that, the steel is not limited in
25 structure in the non-heat treated parts provided any1.,here
in a heat treated steel product as needed and the
boundary region between a heat treated part and non-heat
treated part. Such a part may be provided at part of the
product.
30
35
[0033] The heat treated steel product of the present
invention is not particularly limited in shape, but a
hollow member having a closed horizontal cross-sectional
shape is suitable. A heat treated steel product can be
produced for example by hot 3D bending. Hot 3D bending is
suitable for obtaining a high strength, high rigidity
hollo1-1 member having any bent shape.
[0034] The heat treated steel product of the present
- 9 -
invention is produced using a working apparatus having a
gas chamber, heating device, and cooling device from the
upstream side. Below, this \vill be explained more
specifically using FIG. 1.
5 [0035] FIG. 1 shows one example of the \vorking
apparatus used in the present invention. The steel
material 11 is made to move with respect to the working
apparatus 10 to work it. The working apparatus has a gas
chamber 12, heating device 13, and cooling device 14 from
10 the upstream side. In FIG. 1, for assisting understanding
of the structure, the cross-section is dra\vn, but the gas
chamber 12, heating device 13, and cooling device 14 are
provided so as to cover the entire circumference of the
steel material 11.
15 [0036] Inside the gas chamber 12 I. argon, nitrogen, or
another inert gas is introduced. The inert gas is filled
in the space containing the heating device 13 and cooling
device 14. The steel material 11 is heated locally by the
heating device 13 (lla), then is cooled by the cooling
20 device 14. Here, in the process of heating and cooling,
the time period during \vhich the steel material 11 d\vells
in the 600°C or more temperature region is made less than
1 second.
[0037] If performing the heat treatment in the state
25 where air is contained in the space around the heated
part of the steel material, thick scale is formed and the
chemical conversion coating ability and corrosion
resistance after coating deteriorate. On the other hand,
even if blmving and filling the inert gas in the space
30 around the heated part, if the dwell time in the 600°C or
higher temperature region where the steel material
rapidly oxidizes exceeds 1 second, the scale is formed
thickly or the scale advances in degree of oxidation and
the ratio of Fe30 4 increases, so the chemical conversion
35 coating ability deteriorates.
[0038] Therefore, in the present invention, a working
apparatus provided \·lith a gas chamber at an upstream side
10 -
of a heating device is used, the inert gas is introduced
into the gas chamber, and the space around the heated
part and cooled part of the steel material including the
space around the steel material before heating is filled
5 1vith the inert gas. Furthermore, in the heating and
cooling process, the time period during which the steel
material dwells at 600°C or more is made less than 1
second, preferably is made 0.5 second or less.
[0039] Furthermore, in the process where the steel
10 material is cooled, the time period during which the
steel material dwells at 600°C to -300°C in temperature
region is preferably made within 3 seconds. If scale is
formed at a high temperature, then becomes near 600°C or
less in the cooling process, the F~O undergoes eutectoid
15 transformation and Fe30 4 is formed. For this reason,
making the steel material quickly pass through the 600°C
to 300°C temperature region \•/here the reaction easily
proceeds so as to suppress the formation of Fe30 4 and
return the steel material to a low temperature with the
20 FeO as is is preferable for obtaining an excellent
chemical conversion coating ability.
[0040] Furthermore, in the present invention, by
sufficiently filling the inert gas around the heated part
of the steel material, it becomes possible to make the
25 thickness of the scale uniform. Preferably, the maximum
value and minimum value of thickness of the scale can be
made ±10% or less of the average value of the thickness.
[0041] It is also possible add tempering or other heat
treatment in accordance with the strength and performance
30 required from the product. In this case, it is effective
to make the dwell time at 600°C or more through all of the
heat treatment less than 1 second, more preferably make
the dwell time from 600°C to 300°C less than 3 seconds.
[0042] Note that, the positioning devices 21a, 22b,
35 industrial robot 32, chuck 33, etc. drawn in FIG. 1 show
preferable examples of a working apparatus able to be
- 11 -
used in the present invention. The present invention is
not limited by this drawing needless to say. Further,
while not shown, it is also possible to provide a shield
plate at the dovmstream side of the cooling device 14 to
5 make it easier for the inert gas to fill the space
including the gas chamber 12, heating device 13, and
cooling device 14.
Examples
[0043] To confirm the effects of the present
10 invention, rectangular cross-section electric resistance
welded steel tubes having the chemical composition sh01m
in Table 1 (40 mm x 40 mm x thickness 1.6 mm) as
materials were prepared.
15
[0044] Table 1
(mass%, balance: Fe and unavoidable impurities)
C Si Nn P S sol. Al N Cr Ti Nb B
0.22 0.20 0.75 0.014 0.003 0.04 0.004 0.30 0.030 0.025 0.015
[0045] These steel tube materials 1~ere heat treated
under the conditions shown in Table 2 using the hot 3D
bending apparatus sh01~n in FIG. 1 to obtain heat treated
steel products. Note that, No. 3 of Table 2 was tempered
20 by control of the cooling process of the hot 3D bend{ng
apparatus.
[0046] The obtained heat treated steel products were
examined for cross-sectional structure after Nital
etching using a scanning electron microscope at powers of
25 500X for four fields to confirm the steel structure.
[0047] Further, the surfaces of the steel tubes 1~ere
measured for thickness of the scale by X-ray
photoelectron spectrometry. X-ray diffraction was used
for analysis of the scale composition. The X-ray
30 intensities of the FeO, Fe30 4 , and Fe20 3 were found and the
ratio of the X-ray intensity of FeO with respect to the
total of the X-ray intensities of the FeO, Fe30 4 , and Fe20 3
was calculated. This was used as the FeO ratio in the
scale.
35 [0048] Here, the ratio of the X-ray intensity
- 12 -
evaluates the X-ray diffraction peaks of FeO, Fe30 4 , and
Fe203 at the time of an X-ray source of CuKa (40kV-50mA)
by the Rietveld method.
[0049] Further, the obtained heat treated steel
5 products Here formed Hith similar chemical conversion
coatings as above, then were formed Hith
electrodeposition coatings by a PN-110 made by Nippon
Paint aiming at a coating thickness of 20 MID to obtained
coated products. The coated products Here evaluated for
10 cross-cut tape peeling after immersion in 40°C warm v1ater
for 240 hours as a coating film adhesion test. Further,
they Here evaluated for rust and blistering of the cut
parts after 180 cycles of a JASO cyclic corrosion test.
[0050] In the coating film adhesion test, samples
15 where there were no greatly peeling pieces and 1vhere
small peeling of the cut cross parts accounted for· 5
area% or less were judged as "good''. In the evaluation of
rust and blistering of the JASO test, samples with a
maximum width of rust or blistering at the two sides of
20 the cuts of 12 mm or less were judged as "good".
[0051] The results are shown together in Table 2. Note
that, in the column of "Steel structure" of Table 2, "M"
indicates martensite, while "TM" indicates tempered
martensite. In the results of evaluation of the corrosion
25 resistance after coating, good is indicated by ''G", while
poor is indicated by ''P''.
[0052] Table 2
Corrosion
Production conditions Scale resistance
after coating
Dv;ell DHell Steel
No. Class
Heated Heating time at time at structure Thickness FeO Coating
part temp. 600°C 600 to ratio adhesion
JASO
(llffi) test atmosphere (oC) or more 300°C (%) test
{sec) {sec)
~ 1000 0.4 1 J.! 0.3 98 G G Inv. ex.
r2- 1000 0.9 1 H 0.8 90 G G Inv. ex.
Blown
H+
3
nitrogen
1000 0.4 2.8 partial 0.4 92 G G Inv. ex.
TH f-;j- 1000 2.0 1 H 1.5 85 G p Comp. ex. fT 1000 1.0 10 H 1.0 80 G p Comp. ex.
6 Air 1000 0.4 1 J.! 3.0 70 p p Comp. ex.
- 13 -
[0053] As shown in Table 2, it could be confirmed that
by satisfying the ranges prescribed in the present
invention, a heat treated steel product excellent in
chemical conversion coating ability and in turn excellent
5 also in corrosion resistance after coating is provided
even if used for a chemical conversion coating step
without going through a shot blasting or other scale
removal step.
" I - 14 -
CLAIMS
Claim 1. Heat treated steel product having high
strength and excellent chemical conversion coating
ability comprising a scale with FeO content of 90% or
5 more, having a thickness of 1 ~m or less on the surface.
Claim 2. The heat treated steel product having high
st.rength and excellent chemical conversion coating
ability according to claim 1, wherein the steel has a
structure consisting of martensite, or martensite and
10 tempered martensite.
Claim 3. The heat treated steel product according to
claim 1 or 2, wherein the steel product is a hollow
member having a closed horizontal cross-sectional shape.
Claim 4. The heat treated steel product according to
·15 any one of claims 1 to 3, wherein a maximum value and a
minimum value of the thickness of the scale are \vithin
±10% of an average value of the thickness.
Claim 5. A method of producing a heat treated steel
product using a \olorking apparatus having a gas chamber, a
20 heating device, and a cooling device from an upstream
side, said method of producing the heat treated steel
product comprising
introducing an inert gas into a gas chamber and
filling the inert gas into a space including the heating
25 device and the cooling device while
making the working apparatus move relative to a
steel material so that the steel material is locally
heated by the heating device then the steel material is
cooled by the cooling device, wherein
30 a time period during which the steel material
dwells in a 600°C or more temperature region is less than
1 second and
between the heating and cooling, a bending
operation is performed at a portion of the steel material
greatly dropping in deformation resistance due to heating.
Claim 6. The method of producing a heat treated
steel product according to claim 5, wherein in the step
of cooling, a time period during which the steel material dwells in a 600°C to 300°C temperature region is within 3 seconds.