HOT-DIP GALVANNEALED HOT-ROLLED STEEL SHEET AND PROCESS
FOR PRODUCING SAME
Technical Field
5 The present invention relates to a hot-dip galvannealed hot-rolled steel sheet
and a process for producing the same. More particularly, the present invention
relates to a high strength hot-dip galvanized hot-rolled steel sheet which is suitable
for an automobile steel sheet, in particular a chassis (suspension) part of an
automobile to be formed into various shapes by a press forming or the like and
10 which is excellent in a hole expansibility. and a process for producing the same.
3ackground Art
A hot-rolled steel sheet produced in a comparatively inexpensive way is
widely used for various industrial equipment including an automobile and others.
15 Since in recent years, from the viewpoint of regulating an amount of emission of
carbon dioxide in order to take measures against global warming, an improvement
in the f~teel fficiency of an automobile has been required, a high strength hot-rolled
steel sheet has been widely applied to the automobile so as to reduce the weight of a
vehicle body and to secure a crashworthiness. Further, here recently, a high
20 strength hot-dip galvannealed hot-rolled steel sheet for which a high strength hotrolled
steel sheet is a base steel sheet has been demanded for a chassis part such as a
suspension arm which requires especially a corrosion resistance.
In a steel sheet adopted as an automobile part, not only strength but also
various working properties, which are required when a part is formed, such as press
2 5 formability and weldability also need to be satisfied. As to the press forming of
the chassis part. a stretch flanging and a barring are extremely frequently used and
hence the high strength hot-dip galvannealed hot-rolled steel sheet supplied for
producing the chassis part is required to have an excellent hole expansibility.
Further, there is a case where the high strength steel sheet is applied for a part
30 of which a high crashworthiness is required and for a part which needs to avoid
plastic deformation when a large load is applied thereto. In such case, the high
strength steel sheet is required to have a high yield ratio. Hence. there is a case
where also the high strength hot-dip galvannealed hot-rolled steel sheet is required
to have the high yield ratio.
In general, in the high strength hot-dip galvannealed hot-rolled steel sheet, in
order to satisfy both of the high yield ratio and the excellent hole expansibility, a
5 steel structure of a single phase structure tends to have ferrite, bainitic ferrite, or
bainite as a main phase, and finely precipitate carbide of Ti, Nb, V or the like and
Cu to thereby uniformly strengthen the main phase. For examples, the high
strength hot-dip galvannealed hot-rolled steel sheets have been developed as shown
below.
10 In a Patent Document 1 discloses a high strength hot-dip galvannealed hotrolled
sheet which has a steel structure mainly comprising bainite and which has the
contents of not only Ti, Nb, V but also P, Cu, Cr, Mo, Ni suitably controlled,
thereby being improved in a fatigue resistance property of a welded portion under a
corrosive environment. However, this steel sheet needs to have a large amount of
15 expensive alloying elements such as Cu, Ni, Mo added thereto and hence is not
suitable for a mass production from the perspective of economy. Further, the steel
sheet may have the slightly poor hole expansibility.
Patent Document 2 discloses a high strength hot-dip galvannealed hot-rolled
steel sheet in which a hot-rolled steel sheet having a (ferrite + bainite) structure is
20 subjected to a thermal history of an optimum hot-dip galvannealing process to
suitably control a structure, the shape of carbide, and solid solution C. thereby being
improved in the hole expansibility. However, in more than 650 MPa of a tensile
strength of a product made of this steel sheet is, the product cannot have a sufficient
hole expansibility.
2 5 Document 3 discloses a hot-dip galvanized hot-rolled steel sheet in which a
steel structure substantially comprising a single phase of ferrite has Ti carbide finely
dispersed, the Ti carbide containing hlo andlor FV. However, this steel sheet needs
to have extremely expensive alloying element such as Mo and W added thereto and
hence is not suitable for the mass production from the perspective of economy.
30 Document 4 discloses a hot-dip galvanized hot-rolled steel sheet in which a
structure mainly comprising ferrite and having a dispersion state of pearlite and
cementite optimally controlled has Nb, V, Ti added thereto to increase precipit~ito' n
strengthening. thereby being improved in the hole expansibility. However, in at
least 650 MPa of a high strength of the steel sheet, it is possible that the steel sheet
does not have a sufficient hole expansibility.
Citation List
5 Patent Document
Patent Document 1: Japanese Patent Laid-Open No. 5-33 1596
Patent Document 2: Japanese Patent Laid-Open No. 5- 1 17834
Patent Document 3: Japanese Patent Laid-Open No. 2003-321736
Patent Document 4: Japanese Patent Laid-Open No. 2002- 12947
10
Summary of Invention
An objective of the present invention is to provide a high strength hot-dip
galvanized hot-rolled steel sheet u~hichh as an excellent hole expandability suitable
for a stretch flanging widely used for forming an automobile part and, in particular,
15 a chassis part and which preferably has a high yield ratio, and a process for
producing the high strength hot-dip galvanized hot-rolled steel sheet.
The present inventors first assumed that a steel structure should mainly
comprise ferrite in order to achieve an excellent hole expansibility and a high yield
ratio in addition. Further. the present inventors paid attention to Ti, which is
30 comparatively inexpensive and increases remarkably precipitation strengthening
even by a small amount of addition, and earnestly studied a method for improving a
hole expansibility of a Ti-added hot-dip galvannealed hot-rolled steel sheet having a
structure mainly comprising ferrite. As a result, the present inventors have
obtained the following findings.
2 5 The present inventors have found that the hole expansibility of a Ti-added
high strength hot-dip galvannealed hot-rolled steel sheet, which mainly comprises
ferrite and to which Ti is added, can be remarkably increased by facilitating a ferrite
transformation in a high temperature range of at least not less than 650°C on a run
out table after hot rolling. It is thought that this is because coherent precipitate of
30 Ti carbide produced in a low temperature range after coiling of the hot-rolled steel
sheet is restrained.
Further. the present inventors have found that the above-described result can
he achieved by greatly reducing an Mn content as compared with an conventional
steel, although it has been thought that a particular amount of Mn is essential for
achieving a high strength in a high strength hot-dip galvannealed hot-rolled steel
sheet.
Further, the present inventors have found that a reduction in the Mn content
5 exhibits not only the effects described above but also the effects of restraining
austenitizing during a period in which the hot-rolled steel sheet is being reheated in
a continuous hot-dip galvannealing line and an incidental composite structure,
which are achieved by homogenization of a steel structure due to a reduction in Mn
micro-segregation and expansion of a ferrite area and hence is extremely effective
10 for improving the hole expansibility. By combination of these effects, the present
inventors have successfully obtained an excellent hole expansibility surpassing an
conventional Ti-added high strength hot-dip galvannealed hot-rolled steel sheet.
The present invention based on the findings described above is a hot-dip
galvannealed hot-rolled steel sheet having a hot-dip galvannealed layer on a surface
15 of a steel sheet, the steel sheet having a chemical composition comprising, in
mass%, C: at least 0.01 and at most 0.20%; Si: at most 0.50%; Mn: at least 0.01 %
and at most 1.30%; P: at most 0.05%; S: at most 0.01%; N: at most 0.01%: A!: at
most 0.50%; and Ti: at least 0.05% and at most 0.50%, and a steel structure
containing a polygonal ferrite having at least 80 area % and the remainder
20 consisting of one kind or two or more kinds selected from bainitic ferrite, bainite,
pearlite, and cementite, wherein the hot-dip galvannealed hot-rolled steel sheet has
a mechanical property of at least 650 MPa of a tensile strength.
The preferable aspects of the present invention are as follows:
- the chemical composition further comprises one element or two or more
25 elements selected from in mass%, Cr: at most 0.80%; Ni: at most 0.50%; Cu: at
most 0.50%; Mo: at most 0.50%; and B: at most 0.0050%,
- the chemical composition further comprises one element or two elements
selected from in mass%, V: at most 0.5%: and Nb: at most 0.170,
- the chemical compositior~fu rther comprises of one element or two elements
30 selected from in mass%, Ca: at most 0.01%; and Bi: at most 0.01%, and
- the hot-dip galvannealed hot-rolled steel sheet has the mechanical property
in which a product of a limiting hole expansion ratio and a tensile strength is at least
60,000 MPa.%, the hole expandability being obtained by a hole expanding test
specified by the Japan Iron and Steel Federation Standards, and in which a yield
ratio is at least 80%, the yield ratio being a ratio of 0.2 proof stress to the tensile
strength.
The present invention also provides a process for producing a hot-dip
5 galvannealed hot-rolled steel sheet, the process comprising the following steps (A)
to (C):
(A) a hot rolling step comprising the steps of:
after heating a slab having the chemical composition to a temperature of at
least 1100°C and at most 1350°C, subjecting the slab to hot rolling; completing the
10 hot rolling within a ternperature range of at least 850°C and at most 980°C to
thereby produce a hot-rolled steel sheet; subjecting the hot-rolled steel sheet to a
primary cooling treatment, a holding treatment, and a secondary cooling treatment
in sequence, the primary cooling treatment cooling the hot-rolled steel sheet to a
temperature range of at least 650°C and at most 800°C by a water cooling unit, the
15 holding treatment holding the hot-rolled steel sheet for a period of at least At
seconds defined by the following formula in a temperature range of at least 650°C
and at most 800°C. the secondary cooling treatment cooling the hot-rolled steel
sheet to a temperature range of at least 400°C and at most 650°C; and coiling the
hot-rolled steel sheet in a ternperature range of at least 400°C and at most 650°C.
20 At (second) = 5.Wfn4 (1
where hln in the formula (1) means a Mn content (unit: mass%) in steel,
(B) a pickling step for subjecting the hot-rolled steel sheet produced by the
hot roiling step to a pickling treatment; and
(C) a continuous hot-dip galvanizing step comprising the steps of: heating the
25 hot-rolled steel sheet produced by the pickling step to a temperature range of at least
650°C and at most 800°C; then cooling and subjecting the hot-rolled steel sheet to a
hot-dip galvanizing treatment; and further holding the hot-rolled steel sheet in a
temperature range of at least 460°C and at most 600°C to thereby subject the hotrolled
steel sheet to an alloying treatment.
3 0 According to the present invention, it is possible to produce a hot-dip
galvannealed hot-rolled steel sheet having a high strength and an excellent hole
expansibility that can be produced at the appropriate cost for a mass production.
The hot-dip galvannealed hot-rolled steel sheet according to the present invention
has the hole expansibility adequate to be applied to a stretch flanging and a barring
and hence can be used widely industrially, in particular, in an automobile field.
Brief Description of Drawings
5 Figure 1 shows a heat pattern (thermal history) in a hot rolling step employed
in an example.
Figure 2 shows a heat pattern in a continuous hot-dip galvanizing step
employed in the example.
10 Description of Embodiments
-4n alloyed hot-dip galvannealed hot-rolled steel sheet according to the
present invention will be described in more detail. In the present specification,
any symbol "%" to define a chemical composition of steel is "a mass%".
1. Chemical composition of steel sheet
A chemical composition of a steel sheet that is a plating base material of the
hot-dip galvannealed hot-rolled steel sheet according to the present invention is as
foilows.
[C: at least 0.01% and at most 0.20%]
C has the effect of improving the strength of the steel sheet. If a C content
20 is less than 0.01%. it is difficult for the steel sheet to obtain a tensile strength of at
least 650 MPa. Hence, the C content is made at least 0.01%. and preferably at
least 0.05%. On the other hand, if the C content is more than 0.20%, hole
expandability and weldability of the steel sheet are extremely deteriorated. Hence,
the C content is made at most 0.20% and, preferably, at most 0.12%.
[Si: at most 0.50%]
Si is a solid solution strengthening element and has a function of enhancing
the strength of the steel sheet. However, if an Si content is more than 0.5056,
wettability of the steel sheet to a hot-dip galvanizing liquid is extremely deteriorated.
Hence, the Si content is made at most 0.50%. and preferably at most 0.2096, and
30 still more preferably, at most 0.10%. In order to obtain an effect by the function
described above, it is preferable that the Si content is at least 0.001 %.
[kfn: at least 0.01 5% and at most 1.30%]
Mn has a function to fix S which can cause hot brittleness as MnS to avoid
the hot brittleness caused by S. When an Mn content is less than 0.01%, it is
difficult for the steel sheet to obtain the effect caused by the function described
above. Hence, the Mn content is made at least 0.01 %, and preferably at least 0. 1 %.
5 On the other hand, when the Mn content is more than 1.30%, a ferrite
transformation temperature is lowered, which hence makes it difficult for the hole
expansibility to be enhanced by facilitating a ferrite transformation in a high
temperature range of at least 650°C. Hence, the Mn content is made at most
1.30% and, preferably, at most 0.8%.
[P: at most 0.05%]
P is an element contained generally as an impurity. However, P is a solid
solution strengthening element and has an function of enhancing the strength of the
steel sheet, so P may be actively contained. However, if a P content is more than
0.05%. weldability and toughness of the steel sheet is extremely deteriorated.
15 Hence, the P content is made at most 0.05% and preferably at most 0.02%.
[S: at most O.O1%]
S is an element contained generally as an impurity and forms MnS in the
steel and deteriorates the stretch flcangeability. If the S content is more than 0.01%,
the stretch flangeability of the steel sheet is extremely deteriorated. Hence, the S
9 -0 content is made at most 0.01%. and preferably at most 0.005%. and more preferably.
at most 0.002%.
EN: at most 0.01%]
N is an element generally contained as an impurity. If an N content is more
than 0.01 96, N forms coarse nitride in the steel and extremely deteriorates the
2 5 stretch flangeability. Hence, the N content is made at most 0.01%, and preferably
at most 0.005%.
[Al: at most 0.50%]
A1 has a function to deoxidize the steel to increase the soundness of the steel
sheet. However, even if the steel shzet contains Al of more than 0.50%, the effect
30 caused by the function described above is saturated and a cost push is just caused.
Hence, the A1 content is made at most 0.50%, and preferably, at most 0.20%, and
more preferably, at most 0.10%. In order to obtain the effect caused by the
function described above, it is preferable that the A1 content is made at least 0.001%.
The A1 content in the steel means the content of acid soluble A1 (sol. Al).
[Ti: at least 0.05% and at most 0.50961
Ti is an important element in the present invention and has a function to form
5 carbide in the steel and strengthen ferrite uniformly. If a Ti content is less than
0.05%, it is difficult for the steel sheet to obtain the effect caused by the function
described above. Hence, the Ti content is made at least 0.05%, and preferably at
least 0.10%. On the other hand, even if the Ti content is made more than 0.50%,
the effect caused by the function described above is saturated and a cost push is just
10 caused. Hence, the Ti content is made at most 0.50% and preferably, at most
0.30%.
In addition to the elements described above. the hot-rolled steel sheet of the
plating base material may further contain arbitrary elements to be described below.
[One element or two or more elements selected from Cr: at most 0.80%: Ni:
15 at most 0.50%; Cu: at most 0.50%; Mo: at most 0.50%; and B: at most 0.0050%]
Each of Cr, Ni, Cu, Mo, and B is an element which has a function of
improving the hardenability of the steel and which is effective in improving the
strength of the steel sheet. Hence, the steel sheet may contain one element or two
or more elements of these elements. However, if the contents of these elements
20 are excessive. a ferrite transformation temperature is lowered similar to Mn and
hence it is difficult for the steel sheet to improve the hole expansibility which can
be improved by facilitating a ferrite transformation in a high temperature range of at
least 650°C. Hence, the contents of these elements are made those described
above. Here. B is especially strong in a function of increasing a hot rolling load.
25 so a B content is preferably at most 0.0009% from the viewpoint of productivity.
In this regard, in order to more surely obtain the effect caused by the operation
described above, any one of conditions of: Cr: at least 0.001 $6: Ni: at least 0.001 %;
Cu: at least 0.001%; Mo: at least 0.001%; and B: at least 0.0001% is preferably
satisfied.
3 0 [One element or two elements selected from V: at most 0.5%: and Nb: at
most O.1%]
Each of V and Nb has a function to form carbide in the steel and strengthen
ferrite uniformly similar to Ti. Although V and Nb are elements more expensive
than Ti, one or two or more elements of these elements may be contained.
However, even if V of more than 0.5% is contained or even if Ni of more than 0.1 %
is contained, the effect caused by the function described above is saturated and a
cost push is just caused. Hence, a V content is made at most 0.5% and a Nb
5 content is made at most 0.1 %. In this regard, in order to more surely obtain the
effect caused by the function described above, either of the elements of less than
0.00 1 % is preferably contained.
[One element or two elements selected from Ca: at most 0.01%; and Bi: at
most 0.01 %]
10 Ca has an function to finely disperse inclusions in the steel and Bi has an
function to reduce the micro segregation of a substitutional alloy element such as
Mn and Si in the steel, so both of Ca and Bi have the function of improving the hole
expansibility of the steel sheet. Hence, one element or two elements of Ca and Bi
may be contained. However, if the content of either element is more than 0.01%,
15 ductility is deteriorated. Hence, the content of either element is made at most
0.017~. In this regard, in order to surely obtain the effect caused by the function
described above, either of the elements is preferably at least 0.0001%.
In this regard, C* defined by the following equation (2) it preferably satisfies
the following equation (3). In this way, the steel sheet can have a more excellent
20 hole expandability.
C" = C - 12.01 x (TP47.88 + Nb92.91 + 0.5 ~ ~ 5 0 . 9 4(2)
- 0.020 I C* !: 0.050 (3)
Here, C* means the amount of C not fixed in the steel other than C existing
as carbide (Tic, NbC, VC, (Ti, V) C, (Ti, Nb) C, (Ti, Nb, V) C) containing Ti, Nb.
2 5 and V in the amount of C in the steel. Further, Ti, Nb, and V in the equation (2)
shows the contents of the respective elements in the steel (unit: mass%).
By making C* at least - 0.020%, it is possible to suppress the exhaustion of C
in a ferrite grain boundary and hence to improve the hole expansibility. It is more
preferable that C* is at least 0.010%. On the other hand, by making C* at most
30 0.050%, it is possible to suppress formation of a second phase such as cementite
and pearlite and hence to improve the hole expansibility. It is more preferable that
C'!' is made at most 0.030%.
2. Steel structure of steel sheet
The hot-rolled steel sheet that is the plating base material of the hot-dip
galvannealed hot-rolled steel sheet according to the present invention has a steel
structure containing polygonal ferrite having at least 80%area% and the remainder
comprising one or two or more kinds selected from bainitic ferrite, bainite, pearlite,
5 and cementite.
In order to achieve the excellent hole expandability and a high yield ratio, the
hot-rolled steel sheet has the steel structure mainly comprising polygonal ferrite.
If an area fraction of the polygonal ferrite of a main phase is less than 8096, it is
difficult for the hot-rolled steel sheet to achieve an excellent hole expandability.
10 Further, it is also difficult for the steel sheet to achieve an excellent ductility.
Hence, the area fraction of the polygonal ferrite is made at least 80%. The area
fraction is preferably at least 90%, and more preferably at least 95%. The upper
limit of the area fraction of the polygonal ferrite is not defined but is preferably at
most 99.9%. More preferably, the upper limit of the area fraction of the polygonal
15 ferrite is at most 99.5% and especially preferably at most 99%.
Martensite and retained austenite greatly deteriorate the hole expansibility
and also reduce the yield ratio. Hence, a remaining structure excluding the
polygonal ferrite does not contain the martensite and the retained austenite but
contains one kind or two or more kinds selected from the bainitic ferrite. the bajnite.
20 the pearlite. and the cementite. The ratio of these phases and structures is not
limited to a particular ratio. Generally, the remaining structure contains the
cementite and further contains the bainitic ferrite in some cases. However, the
remaining structure is not necessarily limited to the structures.
The area fraction of the steel structures is found by observing a section of the
25 steel sheet indicating a typical structure of the steel sheet, the section being at a
position of a depth of 11'4 of a sheet thickness from a surface of the steel sheet.
3. rvlechanical properties of hot-dip galvannealed hot-rolled steel sheet
A steel sheet having a tensile strength of less than 650 MPa hardly meats
needs for higher strength in recent years. Hence, the hot-dip galvannealed hot-
30 rolled steel sheet according to the present invention has a mechanical property of a
tensile strength of at least 650 MPa. The tensile strength is preferably at least 680
MPa, and more preferably at least 700 MPa, and still more preferably at least 750
MPa.
In this regard, as described above, a high-strength hot-dip galvannealed hotrolled
steel sheet is required to have an excellent hole expansibility, so that it is
preferable that the high-strength hot-dip galvannealed hot-rolled steel sheet has a
mechanical property in which a product of a limiting hole expansion ratio which is
5 found according to a hole expansibility test specified by JFS (The Japan Iron and
Steel Federation Standards) T1001, and a tensile strength is at least 60,000 MPa.%.
The product of the limiting hole expansion ratio and the tensile strength becomes an
index of the balance of strength - formability in the stretch flangeability. The
limiting hole expansion ratio itself is preferably at least 70% and more preferably at
10 least 75%.
Further, as described above, in the case that a high-strength hot-dip
galvannealed hot-rolled steel sheet is applied to a part which needs to avoid the
plastic deformation, the high-strength hot-dip galvannealed hot-rolled steel sheet
may also be required to have a high yield ratio. Hence, the high-strength hot-dip
15 galvannealed hot-rolled steel sheet more preferably has a mechanical property in
which a yield ratio which is a ratio of 0.2% proof stress to the tensile strength is at
least 80%. The yield stress is especially preferably at least 85%.
4. Hot-dip galvannealed layer
A hot-dip galvannealed layer is not especially limited but is similar to a
20 galvanized layer in a conventional hot-dip galvannealed hot-rolled steel sheet. A
coating weight of the hot-dip galvannealed layer and a Fe concentration will be
described in the description relating to the following production method.
5. Production method
The hot-dip galvannealed hot-rolled steel sheet according to the present
25 invention is produced by a method including (A) a hot rolling step, (B) a pickling
step, and (C) a continuous hot-dip galvanizing step. Production conditions will be
described for each of the steps.
(A) Hot rolling process
[Slab reheating temperature: at least 1 100°C and at most 1 350°C]
30 A slab heating temperature when a slab having the chemical composition
described above is subjected to hot rolling is made at least 1100°C and at most
1350°C. In order to achieve the high strength and the excellent hole expansibility
in a final product, elements such as Ti, Nb, and V for forming carbide needs to be
provided in the solid solution state during the hot rolling. If the slab reheating
temperature is less than 1 100°C, the elements are not provided in the solid solution
state and hence coarse carbide is formed, which makes it difficult to achieve a
desired strength in a final product. Hence, the slab reheating temperature is made
5 at least 1100°C. On the other hand, if the slab heating temperature is more than
1350°C, not only the effect described above is saturated but also the scale loss is
increased, which hence results in disadvantages in the cost. Hence, the slab
heating temperature is made at most 1350°C.
[Finish rolling temperature of hot rolling: at least 850°C and at most 980°C]
10 If a finish rolling temperature of hot rolling is less than 850°C, a deformation
resistance of the slab is excessively increased, which makes it difficult to roll the
slab. Hence, the finish rolling temperature of hot rolling is made at least 850°C.
On the other hand, if the finish rolling temperature of hot rolling is more than 98032,
a ferrite grain after cooling is made coarse, which makes it difficult to achieve a
15 desired strength in a final product. Hence, the finish rolling temperature of hot
rolling is made at most 980°C.
[Primary cooling stop temperature: at least 650°C and at most 80O0G]
After the hot rolling described above, a primary cooling treatment is
performed by a water cooling unit. If a primary cooling stop temperature is less
20 than 65Q0C, carbide coherent to ferrite parent phase, which xnakes it difficult to
achieve an excellent hole expansibility in a final product. Hence, the primary
cooling stop temperature is made at least 650°C. On the other hand, if the primary
cooling stop temperature is more than 800°C, the carbide precipitated in the ferrite
is made excessively coarse, which makes it difficult to secure the desired strength in
25 the final product. Hence, the primary cooling stop temperature is made at most
800°C. In this regard. a primary cooling rate is not especially defined b~rits
preferably made at least 10°C/sec and less than 20OoC/sec from the restriction of an
actual water cooling unit.
[Holding time in temperature range of at least 650°C and at most 800°C: at least At
30 (second)]
At (second) = 5-r\/ln4( Mn: Mn content (mass%) in steel)
A hot-rolled steel sheet obtained by the primary cooling treatment, is held for
a period of at least At (second) defined as a function of the Nln content in a
temperature range of at least 650°C lo at most QOOcC. A specific aspect of holding
may be achieved by keeping heat or heating but preferably is achieved by air
cooling from a perspective of productivity. Hence, a holding time will be also
referred to as "an intermediate air cooling time" in the following.
5 If the holding time is less than At (second), polygonal ferrite cannot be
sufficiently formed in some cases, which hence makes it difficult to achieve an
excellent hole expansibility in the final product. An upper limit of the hold'?g
time does not need to he especially specified but is preferably made at least 30
seconds from a perspective of productivity.
10 [Secondary cooling stop temperature/coiling temperature: at least 30O0C and at
most 650°C]
After the holding treatment described above, the hor-rolled steel sheet is
subjected to a secondary cooling treatment by a water cooling unit and then is
coiled, thereby being brought into a hot-rolled coil. If a secondary cooling stop
15 temperature and a coiling temperature are more than 650°C, Ti carbide is made
excessively coarse while the hot-rolled steel sheet is being coiled, which makes it
difficult to achieve the desired strength in the final product. Hence, the secondary
cooling stop temperature and the coiling temperature are made at most 650°C. On
the other hand, if the secondary cooling stop temperature and the coiling
20 tenaperature are less than 400°C. the interior of the hot-rolled coil is non-uniformly
cooled and hence a variation in properties in the coil is inade significant, which
hence decrease the yield in some cases. Hence, the secondary cooling stop
temperature and the coiling temperature are made at least 400°C. In this regard, a
qecorzdary cooling rate is not zspecially specified hut is preferably made at least
2 5 IO0C/sec and less than 20O0C/sec from the restriction of an actual water cooling
unit.
It is recomme~dedto perform the hot rolling step according to a conventional
method except for the conditions described above. For example, it is
recornmended to make the slab for the hot rolling by melting steel having the
30 chemica! composition described above and then by corttinuously casting the steel or
by casting and blooming. A continuous casting step is preferably employed from a
perbpective of productivity. Further. in the case of rrnpioying the continuous
casting step. in order to improve cracking resistance by controlling inclusions. it is
preferable to stir molten steel in a mold by using an external magnetic field or a
mechanical stirring unit. The slab produced in this way may be subjected directly
to the hot rolling or may be thermally held or be reheated and then be subjected to
the hot rolling.
5 The hot rolling step is usually performed in multiple passes. It is preferable
that a rolling reduction per one pass is at least 10% and at most 60%. If the rolling
reduction pe; one pass is at least 10%, much strain can be introduced into austenite
and hence the crystal grains of ferrite produced by transformation can be made fine
and hence the structure of the hot-rolled steel sheet is refined, which can more
10 improve the ductility and the hole expansibility. Further, if the rolling reduction
per one pass is made at most GO%, the formation of a texture caused by
unrecrystallized austenite can be suppressed, which can still more improve the
ductility and the hole expandability. The thickness of the hot-rolled steel sheet
may be set according to the use but usually ranges from 1.6 mm to 4.5 mm.
15 (B) Pickling step
The hot-rolled steel strip produced by the hot rolling step is subjected to a
pickling treatment in a pickling step so as to remove scale. It is recommended to
perform the pickling treatment according to an ordinary method. Before or after
the pickling step, in order to flatten or straighten the hot-rolled steel sheet and to
20 facilitate the removal of the scale, it is also recommended to subject the hot-rolled
steel sheet to a skin-pass rolling. An elongation percentage in the case that the
hot-rolled steel strip is subjected to the skin-pass rolling is not especially specified
at a particular value but is preferably at least 0.1% and less than 3.0%.
(C) Contini.rous hot-dip galvanizing step
25 The hot-rolled steel sheet pickled by the pickling step is subjected to a
continuous hot-dip galvanizing step for performing treatments of heating, hot-dip
galvani~inga, nd alloying in sequence, whereby a hot-dip galvanilealed hot-rolled
steel sheet is produced.
[Maximum heating temperature: at least 650°C and at most 80OCC]
30 In a continuous hot-dip galvanizing line. the hot-rolled steel sheet is
subjected to an annealing treatment before the hot-roiled steel sheet is subjected to a
hot-dip galvanizing treatment so as to achieve an excellent platability. An
ordinary in-line annealing unit iilcludes at least an oxidatit furnace (or nonoxidation
furnace having a weakly oxidizing property) and a reducing furnace. By
this annealing treatment, the surface of the hot-rolled steel sheet is oxidized and
reduced, thereby being activated. If a maximum heating temperature is l e s ~th an
650°C, the surface of the hot-rolled steel sheet cannot be sufficiently oxidized and
5 reduced and hence the platability is deteriorated. Hence, the maximum heating
temperature ic made at least 650°C. On the other hand, if the maximum heating
temperature IS more than 800°C, austenitizing of the hot-rolled steel sheet is
promoted and hence deteriorates the strength. Hence. the maximum heating
temperature is made at most 800°C. A holding time in a temperature range of at
10 least 650°C and at most 800°C is not especially specified but it is preferable to hold
the hot-rolled steel sheet for a holding time of at least 10 seconds to at most 200
seconds.
After heating the hot-rolled steel sheet to the maximum heating temperature,
the hot-rolled steel sheet is cooled to a temperature range near a bath temperature of
15 a hot-dip galvanizing bath for hot-dip galvanized treatment. A cooling rate at that
time is not especially specified but it is preferable to set the cooling rate at a value
of at least 1°C/sec to at most 50°C/sec from the restriction of an actual cooling unit.
Further, it is preferable to make a cooling stop temperature at least 400°C to at most
550°C.
20 The hot-rolled steel sheet cooled to the temperature range is dipped in the
hot-dip galvanizing bath, thereby being subjected to a hot-dip galvanizing treatment.
It is recommended to perform the hot-dip galvanizing treatment by an ordinary
method. For example, it is recommended to perform the hot-dip galvanizing
treatment under the following hot-dip galvanizing conditions: temperature of
25 galvanizing bath = at least 320°C and at most 500°C; temperature of steel sheet to
he dipped = at least 420°C to at most 500°C; and dipping time = at most 5 seconds.
It is preferable that the hot-dip galvanizing bath has a composition containing A1 of
at least 0.08 mass% and at most 0.2 mass%. In addition, even if the galvanizing
bath contains Fe, Si, Mg, Mn, Cr. Ti and Pb, which are unavoidable impurities,
30 these elements do not affect the present invention. It is preferable that a coating
weight is controlled by a well-known method such as a gas wiping method after the
hot-rolled steel sheet is dipped in the hot-dip galvanizing bath. It is preferable that
the coating weight per one side is made at least 25 mg/m2 anti at most 75 g/m2.
[Alloying treatment temperature: at least 460°C to at most 600°C]
If an alloying treatment temperature is less than 460°C, an alloying speed is
made excessively slow and hence productivity is deteriorated. Further, there is a
case where unevenness is occurred in the alloying treatment. Hence, the alloying
5 treatment temperature is made at least 460°C. On the other hand, if the alloying
treatment temperature is more than 600°C, the alloying treatment is excessively
promoted and hence the powdering resistance of the steel sheet significantly
deteriorates. Hence, the alloying treatment temperature is made at most 600°C.
An alloying treatment time is not especially specified but is preferably made 5 to 60
10 seconds.
Although an Fe concentration in the hot-dip galvannealed layer is different
depending on the alloying heat treatment conditions and the coating weight, it is
preferable that the Fe concentration ranges from 7 to 14 mass%.
After the hot-rolled steel sheet is passed through the hot-dip galvannealing
15 line, in order to flatten and straighten the steel strip and to control the surface
roughness of the steel sheet, the steel sheet may be subjected to a temper rolling.
In this case, in order to avoid decrease of ductility of the steel sheet, it is preferable
that an elongation percentage is made at most 2%.
20 Example
Steel having a chemical composition shown in Table 1 was melted in a
laboratory and was cast into a steel ingot and then a steel slab was obtained by
forging the steel ingot. Next, the obtained steel slab was hot-rolled by a hot rolling
unit for test under heating and cooling conditions shown in Table 2, whereby a hot-
25 rolled steel sheet having a thickness of 3.2 rnm was obtained. A heat pattern in the
hot rolling is shown in Figure 1. Temperatures at respective points are surface
temperatures measured by a radiation thermometer. A cooling rate in a primary
cooling and a secondary cooling, which were performed by water cooling, was
approximately 40°C/sec.
30 The hot-rolled steel sheet cooled to a room temperature was subjected to a
pickling treatment using an ordinary hydrochloric acid pickling liquid as a descaling
treatment. Then, the hot-rolled steel sheet was not subjected to a cold rolling but
was subjected to a heat treatment simulating a hot-dip galvannealing line shown in
Figure 2 under conditions shown in Table 2 by using a continuous heat treatment
simulator.
[Table I]
5 c*=c-1 2 01 *(T1/47 88+Nbi92 91 +0 5*V:50 94)- Underl~ne means cutsfide of t h e range of the present ~nvention
Hot rolling conditions Hot-d~pg alvarieal~ngc ond~tions
I I I I I I Slab
- 1 - 0. t e e 1 ~irlishr o l l w 1 Pr1m.v c o o n 1 rtermed~atea w 1 At 1 ~o111ng 1 1 LOW .;OI~IR~ 1 Ga':zlng 1 t t 1 Note
temperatlire stop temperature cooilng time temperature
temperature temperature temperature
JIS No. 5 tensile test specimens were obtained in a direction
perpendicular to a rolling direction from the hot-rolled steel sheet subjected to
the same thermal history as a hot-dip galvannealing step and were subjected to
a tensile test. In the tensile test, a yield stress (0.2% proof stress), a tensile
5 strength, and a total elongation were measured and a yield ratio (yield
stress/tensile strength) was calculated for each test specimens. Then, a hole
expanding test was performed according to a JFS T 1001 hole expanding test
method of the Japan Iron and Steel Federation Standards and a limiting hole
expansion ratio, which is a hole &pansion ratio when a crack extended
10 through the sheet thickness, was measured and a value of (the tensile strength
x the limiting hole expansion ratio) was calculated.
A steel structure was observed in the following manner: a longitudinal
cross section of the steel sheet was subjected to a Nital etching: a photograph
of the section was taken at a position of a I/4 depth of thickness from the
15 surface by using an optical microscope or a scanning electron microscope: and
an area fraction of each of structures was calculated from the photograph by a
point counting method. Results obtained in this manner will be shown in
Table 3.
[Table 31
1 8 1 B / 75 1 B F , ~ 1 758 1 700 192.31 19.9 1 53 1 40174 1 ~omparative example I
1 18 1 F 1 95 1 BF.0 1 751 1 677 )90.1 1 20.2 1 83 1 62333 I Example of invention I
9
10
1 1
12
13
14
15
16
17
1 19 1 G 1 99 1 0 I 760 I 669 I 880 1 20.9 1 88 I 66880 I Example of invention I
B
B
Q
Q
C
D
D
D
E
20
21
22
23
24
25
26
27
98
99
30
95
55
99
99
98
98
H
1
J
K
L
M
N
0
0
0
BF,O,M
BF.0.M
BF,8 ,@
0
0
0
0
98
93
94
97
96
98
99
91
697
712
860
816
834
860
868
787
709
0
BF,0
BF,0
BF,0
BF,8
0
0
P,0
615
623
793
739
742
792
789
674
621
739
716
738
734
741
71 1
715
-512
88.2
87.5
92.2
90.6
89.0
92.1
90.9
85.6
87.6
643
651
660
658
679
635
627
448
20.0
20.4
19.2
20.8
18.2
19.4
18.8
18,8
21.0
87.0
90.9
89.4
89.6
91.6
89.3
87.7
87.5
93
90
43
45
50
8 1
79
95
91
20.5
19.8
20.5
20.2
18.5
20,4
20.8
31.6
64821
64080
36980
36720
41 700
69660
68572
74765
64519
Example of invention
Example of invention
Comparative example
Comparative example
Comparative example
Example of invention
Example of invention
Example of invention
Example of invention
83
85
83
85
90
102
104
120
61337
60860
61254
62390
66690
72522
74360
61440
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Comparative example
Test No. 1 to 4, 6, 7, 9, 10, 14 to 26 are examples of invention in which
all of the cliemical composition, the production conditions, and the steel
structure corresponded to ranges defined by the present invention and in which
desired mechanical properties were achieved.
5 On the other hand, in test No. 5, the maximum heating temperature in
the continuous hot-dip galvanizing step was more than a temperature defined
by the present invention and hence a tensile strength was insufficient. In test
No. 8. an intermediate air cooling time after stopping the primary cooling did
not satisfy the time At defined by the present invention and the volume
10 fraction of ferrite was smaller than a range defined by the present invention, so
that a strength - hole expansibility balance was deteriorated. In test No. 11 to
13, a Mn content was more than a value defined by the present invention and
hence the hole expansibility was deteriorated. In test No. 27, a Ti content did
not satisfy a range defined by the present invention and hence a tensile
15 strength was insufficient.
We claim:
1. A hot-dip galvannealed hot-rolled steel sheet having a hot-dip
galvannealed layer on a surface of a hot-rolled steel sheet, characterized by
5 having a chemical composition comprising, in mass%, C: at least 0.0 1 and at
most 0.20%; Si: at most 0.50%; Mn: at least 0.01% and at most 1.30%; P: at
most 0.05%; S: at most 0.01%; N: at most 0.01%; Al: at most 0.50%; and Ti:
at least 0.05% and at most 0.50%, and
by having a steel structure containing a polygonal ferrite having at least
10 80 area % and the remainder consisting of one kind or two or more kinds
selected from bainitic ferrite, bainite, pearlite, and cementite,
wherein the hot-dip galvannealed hot-rolled steel sheet has a
mechanical property of at least 650 MPa of a tensile strength.
I5 2. The hot-dip galvannealed hot-rolled steel sheet as set forth in claim 1,
wherein the chemical composition further comprises one element or two or
more elements selected from in mass%, Cr: at most 0.80%; Ni: at most 0.50%;
Cu: at most 0.50%; Mo: at most 0.50%; and B: at most 0.0050%.
20 3. The hot-dip galvannealed hot-rolled steel sheet as set forth in claim 1 or
claim 2, wherein the chemical composition further comprises one element or
two or more elements selected from in mass%, V: at most 0.5%; and Nb: at
most 0.1%.
25 4. The hot-dip galvannealed hot-rolled steel sheet as set forth in any one of
claims I to 3, wherein the chemical composition further comprises one
element or two or more elements selected from in mass%, Ca: at most 0.01 %;
and Bi: at most 0.01%.
5. The hot-dip galvannealed hot-rolled steel sheet as set forth in any one of
5 claims 1 to 4, wherein the hot-dip galvannealed hot-rolled steel sheet has the
mechanical property in which a product of a limiting hole expansion ratio and
a tensile strength is at least 60,000 MPa.%, the limiting hole expansion ratio
being obtained by a hole expanding test specified by the Japan Iron and Steel
Federation Standards JFST 1001, and in which a yield ratio is at least 8096, the
10 yield ratio being a ratio of 0.2 proof stress to the tensile strength.
6. A process for producing a hot-dip galvannealed hot-rolled steel sheet.
the process comprising the following steps (A) to (C):
(A) a hot rolling step comprising the steps of: after reheating a slab
15 having the chemical composition as set forth in any one of claims 1 to 4 to a
temperature of at least 1100°C and at most 1350°C; subjecting the slab to hot
roiling: completing the hot rolling within a temperature range of at least 850°C
and at most 980°C to thereby produce a hot-rolled steel sheet; subjecting the
hot-rolled steel sheet to a primary cooling treatment, a holding treatment, and
20 a secondary cooling treatment in sequence, the primary cooling treatment
cooling the hot-rolled steel sheet to a temperature range of at least 650°C and
at most 800°C by a water cooling unit, the holding treatment holding the hotrolled
steel sheet for a period of at least At seconds defined by the following
formula in a temperature range of at least 650°C and at most 800°C, the
2 5 secondary cooling treatment cooling the hot-rolled steel sheet to a temperature
range of at least 400°C and at most 650°C; and coiling the hot-rolled steel
sheet in a temperature range of at least 400°C and at most 650°C,
At (second) = 5.Nln4
where Mn in the formula (1) means a Mn content (unit: mass%) in steel,
(B) a pickling step for subjecting the hot-rolled steel sheet produced by
the hot rolling step to a pickling treatment; and
(C) a continuous hot-dip galvanizing step comprising the steps of:
heating the hot-rolled steel sheet produced by the pickling step to a
temperature range of at least 650°C and at most 800°C; cooling and subjecting
the hot-rolled steel sheet to a hot-dip galvanizing treatment; and holding the
hot-rolled steel sheet in a temperature range of at least 460°C and at most
600°C to thereby subject the hot-rolled steel sheet to an alloying treatment.