ORIGINAL
Hot-Rolled, Cold-Rolled, and Plated Steel Sheet Having
Improved Uniform and Local Ductility at a High Strain Rate
Technical Field
This invention relates to a hot-rolled steel sheet, a cold-rolled steel sheet,
and a plated steel sheet having improved uniform ductility and local ductility at a
high strain rate (under a high velocity deformation).
Background Art
10 In recent years, there have been demands for decreases in the weight of
automotive bodies as one measure to decrease the amount of C02 discharged from
automobiles in order to protect the global environment. Decreases in weight
cannot be allowed to be accompanied by decreases in the strength demanded of
automotive bodies. Therefore, increases in the strength of steel sheets for
15 automobiles are being promoted.
There are also increased societal demands for safety of automobiles in
collisions. For this reason, the properties demanded of steel sheets for
automobiles are not simply a high strength; there is also a desire for improved
impact resistance should a collision occur during driving. Namely, there is a
20 desire for high resistance to deformation when deformation takes place at a high
strain rate. The development of steel sheets which can satisfy these demands is
being studied.
In general it is known that the difference between the static stress and the
dynamic stress of a steel sheet (in this invention, this difference being referred to as
25 the static-dynamic difference) is large in steel sheets made of mild steel and
decreases as the strength of steel sheets increases. An example of a multi-phase
steel sheet having both a high strength and a large static-dynamic difference is a
low-alloy TRIP steel sheet.
As a specific example of such a steel sheet, Patent Document 1 discloses a
20 strain induced transformation-type high-strength steel sheet (TRIP steel sheet)
having improved dynamic deformation properties which is obtained by prestraining
a steel sheet h-a ving a composition comprising, in mass percent, 0.04 -
0.15% C, one or both of Si and A1 in a total of 0.3 - 3.0%: and a remainder of Fe
and unavoidable impurities and having a multi-phase structure comprising a main
phase of ferrite and a second phase which includes at least 3 volume percent of
austenite. The pre-straining is carried out by one or both of temper rolling and a
tension leveling such that the amount of plastic deformation T produced by prestraining
satisfies the following Equation (A). The steel sheet before pre-straining
5 has such a property that the ratio V(l O)N(O) which is the ratio. of the volume
fiaction V(10) of the austenitic phase after deformation at an equivalent strain of
10% to the initial volume fraction V(0) of the austenitic phase is at least 0.3. The
steel sheet is characterized in that the difference (od - os) between the quasi-static
deformation strength os when deformed at a strain rate in the range of 5 x - 5 x
lo 10" (s") and the dynamic deformation strength od when deformed at a strain rate in
the range of 5 x lo2 - 5 x 10) (s-l) after pre-straining in accordance with Equation
(A) below is at least 60 MPa. Steel sheets having a multi-phase structure are
hereinafter referred to collectively as multi-phase steel sheets.
0.5[{(V(lO)N(O))IC} - 31 + 15 2 T 2 0.5 [{(V(l O)N(O))IC} - 31 ... (A)
15 As an example of a multi-phase steel sheet having a second phase which is
primarily martensite, Patent Document 2 discloses a high-strength steel sheet
having an improved balance of strength and ductility and having a static-dynamic
difference of at least 170 MPa. The steel sheet comprises fine ferritic grains in
which the average grain diameter ds of nanocrystalline grains having a grain
20 diameter of at most 1.2 pm and the average grain diameter dL of microcrystalline
grains having a grain diameter exceeding 1.2 pm satisfy dLIds 2 3. In that
document, the static-dynamic difference is defined as the difference between the
static deformation stress obtained at a strain rate of 0.01 s-' and the dynamic
deformation stress obtained when carrying out a tensile test at a strain rate of 1000
25 s . However, Patent Document 2 does not contain any disclosure concerning the
deformation stress in an intermediate strain rate region where the strain rate is
greater than 0.01 s-' and less than 1000 s'l.
Patent Document 3 discloses a steel sheet having a high static-dynamic
ratio having a dual-phase structure consisting of martensite having an average grain
30 diameter of at most 3 pm and ferrite having an average grain diameter of at most 5
pin. In chat document, the static-dynamic ratio is detined as the ratio of the
dynamic yield stress obtained at a strain rate of 103 s- 1 to the static yield stress
obtained at a strain rate of 10" s". However, there is no disclosure concerning the
static-dynamic difference in a region in which the strain rate is greater than 0.01 s"
and less than 1000 s". In addition, the static yield stress of the steel sheet
disclosed in Patent Document 3 is a low value of 3 1.9 kgf7mm2 - 34.7 kgf7mm2.
Patent Document 4 discloses a cold-rolled steel sheet having improved
impact absorbing properties in which the structure comprises at least 75% of a
5 ferritic phase having an average grain diameter of at most 3.5 prn and a remainder
of tempered martensite. The impact absorbing properties of the cold-rolled steel
sheet are evaluated by the absorbed energy when a tensile test is carried out at a
strain rate of 2000 s-'. However, there is no disclosure in Patent Document 4
concerning the absorbed impact energy in a strain rate region of less than 2000 s".
10
Prior Art Documents
Patent Documents
Patent Document 1 : JP 3958842 B
Patent Document 2: JP 2006- 16 1077 A
15 Patent Document 3: JP 2004-84074 A
Patent Document 4: JP 2004-277858 A
Disclosure of Invention
Prior art steel sheets like those described above have the following
20 problems.
In the past, steel sheets for use as impact members for automobiles are
aimed at increasing dynamic strength for the purpose of improving absorption of
impact energy.
However, in order to guarantee safety at the time of a collision, it is
25 necessary to improve not only dynamic strength but also uniform ductility and local
ductility at a high strain rate (or a high-velocity deformation).
With a multi-phase high-strength steel sheet having a ferritic phase as a
main phase and a martensitic phase as a second phase (a DP steel sheet), it is
difficult to achieve both formability and impact absorbing properties. In addition,
30 it is difficult to guarantee local ductility.
Accordingly, the object of the present invention is to provide multi-phase
steel sheets in the form of a hot-rolled steel sheet, a cold-rolled steel sheet, and a
plated steel sheet having improved uniform ductility and local ductility at a high
strain rate and a method for the manufacture of these steel sheets.
The present inventors carried out various investigations concerning a
method of improving the uniform ductility and local ductility of a multi-phase steel
sheet at a high strain rate. As a result, they obtained the following findings.
(I) Toughness at a high strain rate is improved by refining grains.
(2) On the other hand, uniform ductility is worsened by refining grains.
(3) A decrease in uniform ductility is compensated for by dispersing
martensite, bainite, or austenite which are harder than ferrite.
(4) In order to improve uniform ductility, it is necessary to disperse a
second phase which is as hard as possible, and hard martensite which has a high
10 content of dissolved C is preferred.
(5) However, if the second phase is hard martensite, local ductility is
worsened.
(6) If a hardness variation is imparted to the second phase, local ductility
increases.
15 (7) In order to satisfjr above (4) and (6), the difference in nanohardness
between the first phase which is ferrite and the second phase is made large and the
variation of nanohardness is made small in the surface layer of the steel sheet,
while the difference in nanohardness is made small and the variation thereof is
made large in the central portion of the sheet thickness, thereby making it possible
20 to provide a hot-rolled steel sheet having both uniform ductility and local ductility
at a high strain rate.
(8) For a cold-rolled steel sheet manufactured from this hot-rolled steel
sheet, uniform ductility and local ductility at a high strain rate are improved by
maintaining the nanohardness of the hot-rolled steel sheet in the central portion of
25 the sheet thickness of the cold-rolled steel sheet and by making the second phase
rod-shaped or lath-shaped.
Based on these findings, it was found that a steel sheet having improved
uniform ductility and local ductility at a high strain rate can be obtained by refining
grains and controlling the hardness of the ferritic phase and the second phase in the
30 surface layer and in the central portion of the thickness of the steel sheet.
One mode of the present invention which is provided based on the above
findings is a hot-rolled steel sheet having improved uniform ductility and local
ductility at a high strain rate and having a metallurgical structure comprising a main
phase of ferrite with an average grain diameter of at most 3.0 pm and a second
phase including at least one of martensite, bainite, and austenite, characterized in
that in a surface layer which is a region from the surface of the steel sheet to a
position at a depth of 100 pm from the surface, the average grain diameter of the
second phase is at most 2.0 pm, the difference (AnHav) between the average
s nanohardness of ferrite (nH,,) which is the main phase and the average
nanohardness of the second phase (riHzndavi)s at least 6.0 GPa to at most 10.0 GPa,
the difference (AanH) of the standard deviation of the nanohardness of the second
phase from the standard deviation of the nanohardness of ferrite is at most 1.5 GPa,
and in a central portion which is a region between a position at a depth of 114 of the
lo sheet thickness from the surface of the steel sheet to the center of the sheet
thickness, the difference (AnH,) in the average nanohardness is at least 3.5 GPa to
at most 6.0 GPa, and the difference ( A m ) in the standard deviations of the
nanohardness is at least 1.5 GPa.
According to another mode, the present invention provides a cold-rolled
15 steel sheet having improved uniform ductility and local ductility at a high strain
rate and having a metallurgical structure comprising a main phase of ferrite having
an average grain diameter of at most 3.0 pm and a second phase including at least
one of martensite, bainite, and austenite, characterized in that in a central portion
which is a region between a position at a depth of 114 of the sheet thickness from
20 the surface of the steel sheet to the center of the sheet thickness, the second phase
has an average grain diameter of at most 2.0 pm and an aspect ratio (major
axislminor axis ratio) of greater than 2, the difference (AnH,,) between the average
nanohardness of ferrite (dm,) which is the main phase and the average
nanohardness of the second phase (nHznd ,,) is at least 3.5 GPa to at most 6.0 GPa,
25 and the difference (AcmH) of the standard deviation of the nanohardness of the
second phase from the standard deviation of the nanohardness of ferrite is at least
1.5 GPa.
According to yet another mode, the present invention provides a plated
steel sheet having improved uniform ductility and local ductility at a high strain
30 rate and having a rnetallurgical structure comprising a main phase of ferrite having
an average grain diameter of at most 3.0 pm and a second phase including at least
one of martensite, bainite, and austenite, characterized in that in a central portion
which is a region between a position at a depth of 114 of the sheet thickness from
the surface of the steel sheet to the center of the sheet thickness, the second phase
has an average grain diameter of at most 2.0 pm and an aspect ratio (major
axidminor axis ratio) of greater than 2, the difference (Ad&,) between the average
nanohardness of ferrite (nH,,) which is the main phase and the average
nanohardness of the second phase (nH z),nd is at least 3.5 GPa to at most 6.0 GPa,
5 and the difference ( A d ) of the standard deviation of the nanohardness of the
second phase from the standard deviation of the nanohardness of ferrite is at least
1.5 GPa.
The above-described hot-rolled steel sheet, cold-rolled steel sheet, and
plated steel sheet may contain, in mass percent, C: at least 0.1% to at most 0.2%,
lo Si: at least 0.1% to at most 0.6%, Mn: at least 1.0% to at most 3.0%, Al: at least
0.02% to at most 1.0%, Cr: at least 0.1% to at most 0.7%, and N: at least 0.002% to
at most 0.0 15%, and they may further contain one or more elements selected from
the group consisting of Ti: at least 0.002% to at most 0.02%, Nb: at least 0.002% to
at most 0.02%, and V: at least 0.01% to at most 0.1%.
15 According to still another mode, the present invention provides a method of
manufacturing a hot-rolled steel sheet having improved uniform ductility and local
ductility at a high strain rate in which a slab obtained by hot forging of a steel
material with a reduction in area of at least 30% at a temperature of at least 850° C
is reheated to at least 1200' C and then subjected to hot continuous rolling, the steel
20 material comprising, in mass percent, C: at least 0.1% to at most 0.2%, Si: at least
0.1% to at most 0.6%, Mn: at least 1.0% to at most 3.0%, Al: at least 0.02% to at
most 1.0%, Cr: at least 0.1% to at most 0.7%, and N: at least 0.002% to at most
0.015%, one or more elements selected from the group consisting of Ti: at least
0.002% to at most 0.02%, Nb: at least 0.002% to at most 0.02%, and V: at least
25 0.0 1 % to at most 0.1 %, and a remainder of Fe and impurities, characterized in that
the hot continuous rolling comprises a rough rolling step in which the reheated slab
is rolled to obtain a steel sheet having an average austenite grain diameter of at
most 50 pm, a finish rolling step in which the steel sheet obtained by the rough
rolling step is rolled such that the final rolling pass is in the temperature range of
30 from (Ae3 - 50" C) to (Ae3 + 50" C) with a roiling reduction of at least 17%, and a
cooling step in which the steel sheet obtained by the finish rolling step is cooled
within 0.4 seconds of the completion of the finish rolling step to 700' C or below at
a cooling rate of at least 600' C/sec, the steel sheet after cooling is held for at least
0.4 seconds in a temperature range of from 600" C to 700' C, and the steel sheet
after holding is cooled to 400" C or below at a cooling rate of at most 120" Clsec.
The present invention also provides a method of manufacturing a coldrolled
steel sheet in which a hot-rolled steel sheet manufactured by the abovedescribed
method of manufacturing a hot-rolled steel sheet is used as a starting
5 material, and the starting material is subjected to cold rolling and continuous
annealing to obtain a cold-rolled steel sheet, characterized in that the cold rolling is
carried out with a rolling reduction of 50 - 90%, and in the continuous annealing,
the steel sheet after cold rolling is heated and held for from 10 seconds to 150
seconds in a temperature range of from 750" C to 850" C and then cooled to a
lo temperature range of 450" C or below.
The present invention also provides a method of manufacturing a plated
steel sheet characterized in that a cold-rolled steel sheet manufactured by the
above-described method of manufacturing a cold-rolled steel sheet is subjected to
galvanizing (zinc plating) followed by heat treatment for alloying in a temperature
15 range not exceeding 550" C.
According to the present invention, it is possible to stably provide a multiphase
hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet having
improved uniform ductility and local ductility at a high strain rate. If these steel
sheets are applied to components of automobiles and the like, they produce
20 extremely beneficial industrial effects such as an expected marked improvement in
the safety of products in collisions.
Modes for Carrying Out the Invention
The present invention has the following 5 features.
25 (i) Strength, uniform ductility, and local ductility are improved by refining
grains.
(ii) Uniform ductility and local ductility at a high strain rate are both
achieved by imparting a variation to the properties of the second phase.
(iii) In the surface layer of a steel sheet, the work hardening rate is
30 improved by kinely dispersing a hard second phase.
(iv) In rhe center of the thickness of the steel sheet, local ductility is
improved by imparting a variation to the hardness of a slightly soft second phase.
(v) In a cold-rolled steel sheet, the aspect ratio of the second phase is
increased.
8
The properties of the second phase are evaluated by the nanohardness
measured by the nanoindentation method. Specifically, a nanohardness measured
with an indentation load of 500 pN using a Berkovich tip is employed.
Below, the present invention will be explained in detail. In this
5 description, unless otherwise specified, percent with respect to the content of
elements in a chemical composition of steel means mass percent.
1. Metallurgical structure
A steel sheet according to the present invention has a metallurgical
lo structure comprising a main phase of ferrite having an average grain diameter of at
most 3.0 ym and a second phase including at least one of martensite, bainite, and
austenite. Due to the presence of the second phase, the proportion of the overall
structure constituted by ferrite which is the main phase is preferably at most 80%.
If the ferrite grain diameter exceeds 3.0 pm, local ductility decreases.
15 Accordingly, the average grain diameter of ferrite is made at most 3.0 p. A
lower limit is not specified, but when manufacture is carried out by the belowdescribed
manufacturing method according to the present invention, it is normally
at least 0.5 pm.
If only a ferritic phase is present, it is difficult to guarantee strength and
20 ductility, so the second phase includes at least one of martensite, bainite, and
austenite.
(1) Structure of the surface layer in a hot-rolled steel sheet
A hot-rolled steel sheet according to the present invention has the following
25 characteristics in its surface layer (the region from the surface of the steel sheet to a
depth of 100 pm). The average grain diameter of the second phase is at most 2.0
pm, the difference (AnH,) between the average nanohardness of ferrite (nH,,)
which is the main phase and the average nanohardness of the second phase (nHZnd
,) is at least 6.0 GPa to at most 10.0 GPa, and the difference (AonH) of the
30 standard deviation of the nanohardness of the second phase from the standard
deviation of the nanohardness of ferrite is at most 1.5 GPa.
When bending deformation or the like is applied, more deformation strains
are imparted to the surface layer than in the center of the sheet thickness, so it is
necessary to give the surface layer a specialized structure.
By finely dispersing a second phase (martensite, bainite, andlor austenite)
which is harder than the ferrite mother phase in the surface layer, the work
hardening rate is increased, thereby increasing uniform ductility.
When the value of A d a v in the surface layer is less than 6.0 GPa, the
5 work hardening rate becomes inadequate. On the other hand, if the value of
AnH, in the surface layer exceeds 10.0 GPa, cracks easily develop in the interface
between ferrite and the second phase.
When the average grain diameter of the second phase exceeds 2.0 pm,
cracks easily develop in the interface between ferrite and the second phase.
10 In order to guarantee the work hardening rate and uniform ductility, it is
necessary to disperse a second phase which is as uniform as possible.
Specifically, uniform ductility is worsened if the difference in the standard
deviations of the nanohardness (AonH) exceeds 1.5 GPa.
It is not necessary to particularly prescribe the structure of the surface layer
15 of a cold-rolled steel sheet which is obtained by cold rolling of a hot-rolled steel
sheet according to the present invention because a cold-rolled steel sheet is often
used after performing surface treatment such as pickling or plating, so the
properties of the sheet change due to surface treatment.
20 (2) Structure of the central portion in a steel sheet according to the present
invention
In a region from (114)t to (112)t of the sheet thickness of a hot-rolled steel
sheet, a cold-rolled steel sheet, and a plated steel sheet according to the present
invention (collectively referred to as a steel sheet according to the present
25 invention), namely, in a region fiom a location at a depth of 114 of the sheet
thickness from the surface of the steel sheet (in the case of a plated steel sheet, from
the surface of the steel sheet forming a substrate) to the center of the sheet
thickness (referred to below as the central portion), the value of AnH, is at least
3.5 GPa to at most 6.0 GPa and the value of AcmH.is at least 1.5 GPa.
3 o if the entire sheet thicknes has a structure like the above-described surface
!ayer, local ductility decreases. Accordingly, a steel sheet according to the present
invention has a multi-layer structure in which the structure in the central portion is
different from the structure in the surface layer or a gradient structure in which the
properties of the structure continuously varies fiom the surface layer to the central
portion.
In order to improve local ductility, it is necessary to disperse a relatively
sofl second phase. Namely, if the value of An&, in the central portion exceeds
6.0 GPa, local ductility decreases. However, if it is less than 3.5 GPa, strength
5 decreases. In addition, variation in the hardness of the second phase is effective at
improving local ductility. Namely, it is not possible to guarantee ductility after
the occurrence of necking if the value of AanH is less than 1.5 GPa.
(3) Grain diameter and aspect ratio of the second phase in the central portion of a
lo cold-rolled steel sheet and plated steel sheet
In a cold-rolled steel sheet and a plated steel sheet obtained by plating of a
cold-rolled steel sheet, the average grain diameter of the second phase in the central
portion is at most 2.0 pm. If it exceeds 2.0 pm, cracks easily develop in the
interface between ferrite and the second phase. Accordingly, the average grain
15 diameter of the second phase is made at most 2.0 pm. There is no particular lower
limit on the average grain diameter of the second phase. When manufacture is
carried out by a manufacturing method according to the present invention, it is
normally at least 0.5 p.m.
Local ductility is increased by changing the shape of the second phase in
20 the central portion from an isometric shape to a rod shape or a lath shape. If the
aspect ratio (major axislminor axis ratio) of the second phase in the central portion
is 2 or less, local ductility becomes inadequate. Accordingly, the aspect ratio of
the second phase is made greater than 2.
25 (4) Chemical composition of the steel
Below, a preferred chemical composition of a steel sheet according to the
present invention will be explained.
C: at least 0.1% to at most 0.2%
Upper and lower limits on the C content are preferably set in order to adjust
30 the contents of ferrite, bainite, martensite, and austenite and to guarantee the static
strength and the static-dynamic difference. Namely, if the C content is less than
0. I%, there is a concern of an increased possibility that the expected strength
cannot be obtained because solid solution strengthening of ferrite becomes
inadequate and none of bainite, martensite, and austenite is formed. On the other
hand, if the C content exceeds 0.2%, there is a concern of an increased possibility
of a decrease in the static-dynamic difference due to excessive formation of a high
hardness phase. Accordingly, the range for the C content is preferably 0.1% to
0.2%.
5 Si: at least 0.1% to at most 0.6%
Si has the effect of increasing the strength of steel by solid solution
strengthening and increasing ductility, and it also has the effect of increasing the
static-dynamic difference by suppressing the formation of carbides. Therefore,
the Si content is preferably at least 0.1%. However, its effects saturate when it is
lo contained in excess of Oh%, and there is a concern of an increased possibility of
embrittlement of the steel. Accordingly, the range for the Si content is preferably
0.1 - 0.6%.
Mn: at least 1 .O% to at most 3.0%
Mn controls transformation behavior and controls the amount and hardness
1s of a transformed phase which is formed during hot rolling and during a cooling
process after hot rolling, so upper and lower limits on the Mn content are preferably
set. Namely, if the Mn content is less than 1.0%, there is concern of an increased
possibility that a desired strength and static-dynamic difference cannot be obtained
because the amounts of a bainitic ferrite phase and a martensitic phase which are
20 formed are reduced. If Mn is added in excess of 3.0%, there is a concern of an
increased possibility of a decrease in dynamic strength due to the amount of a
martensitic phase which becomes excessive. Accordingly, the range for the Mn
content is 1 .O - 3.0%. More preferably, it is 1.5 - 2.5%.
Al: at least 0.02% to at most 1 .O%
25 A1 acts as a deoxidizer. In addition, it has the effect of increasing the
strength and ductility of steel by controlling the amount and hardness of a
transformed phase which is formed during hot rolling and during a cooling step
after hot rolling. Accordingly, preferably at least 0.02% of A1 is contained.
However, the effects of A1 saturate when it is contained in excess of 1.0%, and
30 there is a concern of an increased possibility of enlbrittlement of steel.
:.\ccordingly, the range lor the A1 content is preferably 0.02% - 1 .O%.
Cr: at least 0.1 % to at most 0.7%
Cr controls the amount and hardness of a transformed phase which is
formed during hot rolling and during a cooling step after hot rolling. 'Therefore,
upper and lower limits on the Cr content are preferably set. Cr has a useful effect
of guaranteeing the amount of bainite. In addition, it suppresses precipitation of
carbides in bainite. Furthermore, Cr itself has a solid solution strengthening
effect.
5 If the Cr content is less than 0.1%, there is a concern of an increased
possibility that a desired strength cannot be obtained. On the other hand, if Cr is
added in excess of 0.7%, the above-described effects saturate, and there is a
concern of an increased possibility of ferritic transformation being suppressed.
Accordingly, the range for the Cr content is preferably 0.1 - 0.7%.
10 N: at least 0.002% to at most 0.01 5%
N is added in order to forms nitrides with Ti or Nb and suppress coarsening
of grains. If the N content is less than 0.002%, there is a concern of an increased
possibility of coarsening of the structure after hot rolling due to coarsening of
grains which may occur at the time of slab heating. On the other hand, if the N
15 content exceeds 0.015%, coarse nitrides are formed, leading to a concern of an
increased possibility of an adverse affect on ductility. Accordingly, the range for
the N content is preferably 0.002% to 0.0 15%.
One or more of Ti, Nb, and V is preferably contained.
Ti: at least 0.002% to at most 0.02%
20 When Ti is added, it forms a nitride. TiN is effective at preventing
coarsening of grains. If the Ti content is less than 0.002%, this effect is not
obtained. On the other hand, if Ti is added in excess of 0.02%, it forms coarse
nitrides and thereby decreases ductility, and there is concern of an increased
possibility of ferritic transformation being suppressed. Accordingly, when Ti is
25 added, the added amount is preferably 0.002 - 0.02%.
Nb: at least 0.002% to at most 0.02%
When Nb is added, it forms a nitride. In the same manner as a Ni nitride,
a Nb nitride is effective at preventing coarsening of grains. In addition, Nb forms
a Nb carbide, which contribute to preventing coarsening of ferritic phase grains.
3o These effects are not obtained, if its content is less than 0.002%. If Nb is added in
excess of 0.02%, there is a coricern of an increased possibility of' a ferritic
transformation being suppressed. Accordingly, when Nb is added. the added
amount is preferably 0.002 - 0.02%.
V: at least 0.01% to at most 0.1%
Carbonitrides of V are effective at preventing coarsening of austenitic
phase grains in a low-temperature austenite region. In addition, carbonitrides of V
contribute to preventing coarsening of ferritic phase grains. Accordingly, V may
be added as necessary. These effects are not achieved if the V content is less than
5 0.01%. On the other hand, if V is added in excess of 0.1%, precipitates increase
and there is a concern of an increased possibility of a decrease in the static-dynamic
difference. Accordingly, the added amount of V when it is added is preferably
made 0.01 - 0.1%.
lo (5) Manufacturing method
(5- 1) Method of manufacturing a hot-rolled steel sheet
Below, a preferred example of a manufacturing method for manufacturing
a hot-rolled steel sheet having the above-described metallurgical structure will be
explained. The following manufacturing method is an example, and a hot-rolled
15 steel sheet having the same structure may be manufactured by other manufacturing
methods.
First, a slab having the above-described chemical composition which was
manufactured by continuous casting undergoes hot forging at a temperature of at
least 850" C. A forging temperature of less than 850" C has a low softening effect
20 of the slab, so forging is carried out at 850" C or above. There is no upper limit
on the forging temperature as long as forging can be carried out, but it is preferably
at most 1100" C. There is no limit on the percent reduction in area, but in order to
decrease the average grain diameter of austenite after rough rolling, it is preferably
at least 30%. The hot forged slab is usually cooled to 700" C or below by natural
25 cooling or accelerated cooling.
In order to sufficiently soften the slab prior to hot rolling, the slab is
reheated to 1200" C or above. By making the slab temperature at least 1200" C,
the structure becomes austenite. During heating, austenite undergoes grain
growth, but the grain diameter decreases due to subsequent hot rolling. Hot
30 rolling is carried out in the following manner.
Fiirst rough rolling is carried out to decrease the average austenite grain
diameter to at most 50 pm. The austenite grain diameter is then further refined by
carrying out tinish rolling. The finish rolling is carried out in such a manner that
the iinal rolling pass of the tinish rolling is in the temperature range of from (Ae3 -
50" C) to (Ae3 + 50" C) with a rolling reduction of at least 17%. When the rolling
reduction is less than 17%, the prescribed grain diameter and nanohardness of the
second phase are not obtained.
Here, Ae3 means the thermal equilibrium temperature at which the steel
starts to transform fiom austenite to ferrite. By carrying out a high degree of
reduction in the vicinity of the Ae3 point in the final rolling pass of the finish
rolling, refinement of the grain diameter of a hot-rolled steel sheet when it is a final
product can be achieved. The Ae3 point is calculated using the thermodynamic
calculation software Thermo-Calc (made by Thermo-Calc Software AB) and is the
calculated value of Ae3 in a paraequilibrium state. Table 1 shows the Ae3 point
for each steel type.
Then, in order to suppress recrystallization of austenite, cooling is started
within 0.4 seconds after rolling. This cooling is performed to a temperature of
700" C or below at a cooling rate of at least 600" Clsec. By carrying out this rapid
cooling, recrystallization of austenite can be suppressed and a fine grain structure in
which the average grain diameter of ferrite is at most 3.0 pm can be obtained.
In order to produce ferrite fiom austenite, holding is carried out in a
temperature range of 600 - 700" C for the length of time necessary for ferritic
transformation, namely, for at least 0.4 seconds. Subsequently, cooling is carried
out to 400' C or below at a cooling rate of less than 100" Clsec, whereby the
remainder which did not undergo ferritic transformation remains as austenite or is
transformed into martensite andlor bainite.
As a result of performing the above-described manufacturing steps, a hotrolled
steel sheet characterized by having the following metallurgical structure can
be obtained.
A) The surface layer has the following characteristics:
the average grain diameter of the second phase is at most 2.0 pm,
the difference (AnH,,) between the average nanohardness of ferrite (nH,,,)
which is the main phase and the average nanohardness of the second phase (nHznd
,) is at least 6.0 GPa to at most 10.0 GPa, and
(he difference (AmH) of ;he standard deviation of the nanohardness of the
second phase from the standard deviation of the nanohardness of the ferrite is at
most 1.5 GPa.
B) The central portion has the following characteristics:
the difference (AnH,) in the average nanohardness is at least 3.5 GPa to at
most 6.0 GPa, and
the difference ( A d ) in the standard deviation of the nanohardness is at
least 1.5 GPa.
5
(5-2) Method of manufacturing a cold-rolled steel sheet
The above-described hot-rolled steel sheet is used as a starting material,
and it is subjected to the below-described cold rolling and continuous annealing to
obtain a cold-rolled steel sheet.
10 The rolling reduction in cold rolling is made 50 - 90%. By making the
rolling reduction in cold rolling at least 50%, it becomes easy to accumulate
sufficient work strains in a steel sheet. The upper limit on the rolling reduction is
set from the standpoints of manufacturing equipment andlor manufacturing
efficiency.
15 In continuous annealing, the steel sheet obtained by cold rolling is heated
and held for at least 10 seconds to at most 150 seconds in a temperature range of
750 - 850" C, and then it is cooled to a temperature range of 450" C or below. By
holding for 10 - 150 seconds in a temperature range of 750 - 850" C to perform
recrystallization, the work strains which are accumulated by the above-described
20 cold rolling obstruct the growth of crystal grains, thereby making it possible to
obtain a steel structure having a refined grain diameter.
By carrying out the above-described cold rolling and continuous annealing
on a hot-rolled steel sheet which is manufactured in the above-described manner, it
is possible to obtain a cold-rolled steel sheet characterized by having the following
25 metallurgical structure.
The central portion has the following characteristics:
it includes a second phase having an average grain diameter of at most 2.0
pm and an aspect ratio (major axislminor axis) of greater than 2,
the difference (AnH,,) between the average nanohardness of ferrite (nH,,,)
30 which is the main phase and the average nanohardness of the second phase (nH2,d
,,) is at least 3.5 GPa to at most 6.0 GPa, and
the above-described dif'ference (AonH) in the standard deviation of the
nanohardness is at least 1.5 GPa.
(5-3) Method of manufacturing a plated steel sheet
A plated steel sheet can be obtained by further performing galvanizing
(zinc plating) on the above-described cold-rolled steel sheet. When employing
galvanizing, the galvanizing is preferably followed by alloying heat treatment in a
temperature range not exceeding 550" C. When performing hot dip galvanizing
and alloying heat treatment, it is desirable from the standpoint of productivity to
perform from continuous annealing to hot dip galvanizing and the like in a single
step using continuous hot dip galvanizing equipment. After plating, it is possible
to fbrther increase corrosion resistance by carrying out suitable chemical
conversion treatment (such as coating with a silicate-based chromium-free
chemical conversion treatment solution followed by drying).
Even if plating like that described above is applied to a cold-rolled steel
sheet manufactured in the above-described manner, the structure of the cold-rolled
steel sheet remains in the resulting plated steel sheet. Therefore, its metallurgical
structure is a structure with the following characteristics.
The central portion has the following characteristics:
it includes a second phase having an average grain diameter of at most 2.0
pm and an aspect ratio (major axislminor axis) of greater than 2,
the difference (AnH,,) between the average nanohardness of ferrite (nH,,)
which is the main phase and the average nanohardness of the second phase (nHZnd
,,) is at least 3.5 GPa to at most 6.0 GPa, and
the above-described difference ( A d ) in the standard deviation of the
nanohardness is at least 1.5 GPa.
Examples
(Hot-rolled steel sheet)
Experiments were carried out using slabs made from steel types A, B, C, D,
and E having the chemical compositions shown in Table 1 (thickness of 35 mm,
width of 160 - 250 mm, length of 70 - 90 mm). Steel types A - C and E had
chemical compositions within the range defined by the present invention, and steel
3 had a chemical composition ourside the range of the present invention.
Table 1
For each of the steels, 150 kg of steel obtained by vacuum melting
underwent hot forging and hot rolling under the conditions shown in Table 2 to
5 obtain a steel sheet sample for testing. The finished thickness of the steel test was
1.6 - 2.0 mm.
Steel
type
A
B
0.15
0.15
Si
0.54
0.53
Mn
2.02
2.04
P
0.001
0.001
S
0.002
0.002
Cr
0.25
0.25
Ti
0.010
0.010
Nb
-
0.008
V
-
-
A1
0.035
0.033
N
0.0025
0.0021
Ae3
845
841
Table 2
Test
No.
1
2
3
4
5
6
7
8
9
Steei
type
A
A
A
A
A
B
C
D
E
Forging Hot rolling
Cooling
temp.
of
forged
steel
RT
RT
RT
RT
RT
RT
RT
RT
RT
'fat-
Ing
temp.
(OC)
1250
1250
1250
1250
1250
1250
1250
1250
1250
Heating
temp.
(OC)
1250
1250
1250
1250
1250
-
1250
1250
1250
1250
% Reduction
in
area at
8500C Or
above
50
50
0
50
50
50
50
0
50
Temp. at Cooling conditions
cornpietion
finish
(OC)
800
790
850
850
850
870
820
850
870
Average
cooling
rate to
400°C
( ~ ~ / s e c )
42
250
45
40
40
62
65
-
62
' Time
until
start of
cooling
(see)
0.1
0.5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
Rough rolling Finish rolling
'
Number
of
passes
4
4
4
1
4
---
4
4
4
4
Temp. at
completion
of
cooling
(OC)
650
650
650
650
650
650
650
-
650
Number
of
passes
3
3
3
3
--
3
3
3
3
3
y grain
diameter
after rough
rolling
(w)
35
35
70
120
35
-
25
30
35
25
Inter
mediate
cooling
time
(set)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
-
0.5
-
Rolling
reduction in
each pass
30%-30%-30%
30%-30%-30%
30%-30%-30%
30%-30%-30%
23%-23%-10%
- --
30%-30%-30%
30%-30%-30%
20%-20%-13%
30%-30%-30%
Test Nos. 1,6,7, and 9 were samples of steel sheets manufactured by a
manufacturing method according to the present invention. In contrast, Test Nos. 2
- 5 and 8 were samples of steel sheets manufactured by a manufacturing method
having conditions outside the range defined by the present invention.
5 Table 3 shows the results of measurement of the structure of each steel test
sample. The grain diameter was determined from a two-dimensional image taken
using a scanning electron microscope (SEM) at a magnification of 3000x. The
nanohardness of ferrite and of the hard phase was determined by the
nanoindentation method. A cross section of a sample steel sheet in the rolling
lo direction was polished with emery paper, and then it was subjected to
mechanochemical polishing with colloidal silica and electropolishing to remove a
deformed layer before it is subjected to measurement. The measurement by the
nanoindentation method was carried out using a Berkovich tip with an indentation
load of 500 pN. The indentation at this time had a diameter of at most 0.1 pm.
15 The nanohardness of each phase was measured at 20 random points positioned at
different depths from the surface in a cross section of the steel sheet, and the result
underwent statistical treatment to obtain the difference (AnH,,) in nanohardness
between ferrite and the second phase and the difference (AonH) in standard
deviation of the nanohardness between them (second phase minus ferrite).

Table 4 shows the properties of the resulting steel sheets.
Test
No.
1
Table 4
Steel
5 The tensile properties were evaluated by a quasistatic tensile test at a strain
rate of 0.01 s-I and a dynamic tensile test at a strain rate of 100 s-' both using a test
piece with a gauge length of 4.8 mm and a gauge width of 2 mm. The dynamic
tensile test was performed using a stress sensing block material testing machine.
Bending properties were evaluated by carrying out 1 80° contact bending at
lo an average strain rate of 0.01 s" and visually observing whether there were cracks.
In Table 4, cases in which cracks were not observed are shown as o and cases in
which cracks were observed are shown as x.
The steel sheets of Test Nos. 1,6, 7, and 9 that were manufactured by a
manufacturing method according to the present invention had a tensile strength of
15 at least 900 MPa, uniform elongation of at least 23%, local elongation of at least
lo%, and good bending properties under both quasistatic deformation and dynamic
deformation. The steel sheets ofTest Nos. 2 - 5 and 8 which were manufactured
by a manufacturing method for which the conditions were outside the range defined
by the present invention had a good tensile strength, but uniform elongation, local
fie
A
A
Quasistatic deformation properties
(strain rate: 0.0 1 s-l)
Dynamic deformation
properties
(strain rate: 100 S-')
Tensile
strength
(MPa)
923
999
Uniform
elongation
(%)
27
23
Local
eiongation
18
7
Bending
properties
o
x
strengm
1027
1017
Uniform
elongation
(%)
28
28
Local
elongat
ion
(%I
19
2
elongation, andlor bending properties were inadequate.
(Cold-rolled steel sheet and plated steel sheet)
The hot-rolled steel sheets which were manufactured by the above-
5 described method were subjected to cold rolling and then to heat treatment which
simulated the heat pattern in continuous hot dip galvanizing equipment using a
continuous annealing simulator.
Table 5 shows the methods of manufacturing hot-rolled steel sheets which
were subjected to cold rolling, and Table 6 shows the rolling conditions for cold
lo rolling and the conditions for heat treatment corresponding to continuous annealing
and alloying treatment after plating. The structure of the resulting steel sheets was
measured in the same manner as for the above-described hot-rolled steel sheets.
The average aspect ratio of the second phase in the central portion was found from
the SEM image used for measurement of the average grain diameter.
Table 5
Test
NO.
10
11
12
13
Steel
type
B
6
D
B
-- Forging
Heating
temp.
('C)
1250
1250
1250
1250
Hot rolling
Heat-.
Ing
temp-
PC)
1250
1250
1250
1250
% Reduction
in
area at
8500C Or
above
50
50
50
50
Cooling
temp.
forged
steel
RT
RT
RT
RT
Rough rolling
Number
of
p a
4
4
4
4
Finish rolling
Y grain
diameter
after rough
rolling
(pm>
25
25
25
25
Temp. at
cornpietion
of
finish
(OC)
870
870
850
870
Number
of
passes
3
3
3
3
Rolling
reduction in
each pass
30%-30%-30%
30%-30%-30%
30%-30%-30%
30%-30%-30%
Cooling conditions
Time
until
statart of
cooling
(set)
0.1
0.1
0.1
0.1
Temp. at
completion
of
cooling
(OC)
650
650
650
650
Inter
mediate
cooling
time
(sec)
0.5
0.5
0.5
0.5
Average'
cooling
rate to
400°C
(OC/S~O)
62
1 20
70
62
Table 6
Table 7 shows the results of measurement of the metallurgical structure of
the steel test samples. Table 8 shows the mechanical properties of the resulting
5 steel sheets. The results shown in Table 8 are the results for steel sheets after
carrying out heat treatment corresponding to alloying heat treatment. It is thought
that even if plating treatment and alloying heat treatment are carried out, the
structure of the original cold-rolled steel sheet remains and the same properties are
11
12
13
exhibited, so measurement of the structure and properties of the steel sheets (cold-
Total time for
alloying heat
treatment
300 sec
10 rolled steel sheets) before carrying out heat treatment corresponding to plating was
Heat
treatment
temperature
for alloying
400 - 450" C
B
D
B
omitted.
Annealing
temp.
800" C
Reduction
in cold
rolling
55%
Test
10
55%
35%
35%
Annealing
time
120 sec
Steel
type
B
Table 7
780" C
900" C
900" C
Remark
Inventive
Inventive
Compar.
Compar.
Test
No.
10
11
12
13
120 sec
120 sec
120 sec
Steel
QPe
B
B
D
B
350 - 400" C
400 - 420" C
400 - 420" C
300 sec
300 sec
300 sec
Central portion
A d
(GPa)
1.9
2.1
2.3
2.1
m a v
(GPa)
4.7
4.4
8.7
6.7
Aspect
ratio
of 2nd
phase
2.5
3.5
1.2
1.9
Average
ferrite
grain
diameter
(pm)
~ H w
(GPa)
Average
grain
diameter
of 2nd
phase
(PI
nH2ndav
(GPa)
7.9
7.5
11.8
9.9
2.3
2.5
3.5
3.1
1.8 1 3.2
1.5 3.1
0.8
1.3
3.1
3.1
Table 8
?f"
The steel sheets of Test Nos. 10 and 11 which were manufactured by the
manufacturing method according to the present invention maintained a tensile
5 strength of at least 900 MPa, uniform elongation of at least 23%, local elongation
of at least 10% under both quasistatic deformation and dynamic deformation, and
had good bending properties. In contrast, the steel sheets of Test Nos. 12 and 13
which were manufactured by manufacturing methods having conditions outside the
range defined by the present invention had good tensile strength, but the uniform
lo elongation, local elongation, and/or bending properties were inadequate.
Test
No-
10
11
12
13
Steel
type
B
B
D
B
Quasistatic deformation properties
(strain rate: 0.0 1 s-l)
Dynamic deformation
properties
(strain rate: 100 s-1)
Tensile
strength
("a)
968
975
1023
945
elongation
(%I
11 11
1022
1026
999
Local
elongation
(%)
18
17
6.1
8.8
Uniform
elongation
(%)
27
23
18.2
20
Bending
properties
o
o
x
x
Tensile
strength
(MPa)
23
2 8
14.3
18.5
Uniform
elongation
(%)
19
14
3
7

We claim:
1. A hot-rolled steel sheet having improved uniform ductility and local
ductility at a high strain rate which comprises a main phase of ferrite having an
5 average grain diameter of at most 3.0 pm and a second phase including at least one
of martensite, bainite, and austenite, characterized in that
in a surface layer of the steel sheet which is a region between the surface of
the steel sheet and a location at a depth of 100 pm from the surface, the second
phase has an average grain diameter of at most 2.0 pm, the difference (AH,)
lo between the average nanohardness of ferrite (nH,) which is the main phase and
the average nanohardness of the second phase (nH z)n,d is at least 6.0 GPa to at
most 10.0 GPa, and the difference (AonH) of the standard deviation of the
nanohardness of the second phase from the standard deviation of the nanohardness
of the ferrite is at most 1.5 GPa, and
15 in a central portion of the steel sheet which is a region from a location at a
depth of 1/4 of the sheet thickness from the surface of the steel sheet to the center
of the sheet thickness, the above-described difference (AnH,,) in the average
nanohardness is at least 3.5 GPa to at most 6.0 GPa and the above-described
difference (AmH) in the standard deviation of the nanohardness is at least 1.5 GPa.
20
2. A cold-rolled steel sheet having improved uniform ductility and local
ductility at a high strain rate which comprises a main phase of ferrite having an
average grain diameter of at most 3.0 pm and a second phase including at least one
of martensite, bainite, and austenite, characterized in that
25 in a central portion of the steel sheet which is a region from a location at a
depth of 1/4 of the sheet thickness from the surface of the steel sheet to the center
of the sheet thickness, the second phase has an average grain diameter of at most
2.0 pm and an aspect ratio (major axislminor axis) of greater than 2, the difference
(AnH,,) between the average nanohardness of ferrite (nH,,,) which is the main
:I phase arid the avernge nanohardness of the :iecond phasz (nt12,,:d,, ) is iit least 3.5
,; Pii to :it 1:losc 43.i) {;Pa, .inti the ,li