[Technical Field]
5 [OOOl]
The present invention relates to a hot-rolled steel sheet which is subjected to
burring work or stretch flanging work, for example, suitable for high-strength structural
parts of an automobile or the like and hardly has a damage occurrence in an end face at the
time of punching of the steel sheet and a method for producing the same. This
10 application is based upon and claims the benefit of priority from the prior Japanese Patent
Application No. 2012-142692, filed on June 26, 2012, the entire contents of which are
incorporated herein by reference.
[Background Art]
[0002]
15 In recent years, there is a te~lde~icthya t weight reduction of automotive members
is emphasized fiom the viewpoint of energy saving and safety and durability thereof are
also additionally emphasized, and thus higher strengthening is rapidly progressing than
ever before. As an example of this tendency, a high-strength steel sheet is adapted to be
applied not only to outer panels of an auton~obileb ut also to structural members.
20 [0003]
The steel sheet to be applied to such structural members also requires workability
such as hole expandability in addition to press formability. For this reason, a
high-strength hot-rolled steel sheet having excellent workability in a burring work, a
stretch flanging work or the like has been developed (for example, see Patent Literatures 1
25 and 2).
However, with the higher strength of the hot-rolled steel slieet, there is a probleln
that peeling or burr-like defects occur in an end face of a hole fornled by a punching work
of the steel sheet. These defects significantly impair a design nature in the end face of the
product and also have a risk of affecting fatigue strength or the like as a stress
concentration portion.
5 [0004]
With respect to the above problems, a hot-rolled steel sheet has been proposed in
which an area ratio of a second hard phase and cementite is restricted and the damage is
suppressed in the punched end face (for example, see Patent Literatures 3 and 4).
However, even though the formation of the second hard phase and cementite is suppressed,
10 w11en a clearance of the punching work is set to the most severe condition to the damage of
the end face, there are cases where the defects occur in the end face of the hole.
[0005]
In contrast, a high-strength hot-rolled steel sheet has been developed in which B is
added or the adding amount of P is limited so as to suppress a fi-acture in crystal grain
15 boundaries during working and thus the damage occurrence in the punched end face is
suppressed (see Patent Literatures 5 and 6). Furthermore, a high-strength hot-rolled steel
sheet has been developed in which the segregation amount of C or C and B is controlled in
large-angle crystal grain boundaries of ferrite and thus the damage occurrence in the
punched end face can be prevented even when the punching work is carried out under the
20 nlost severe conditions (see Patent Literatures 7 and 8). However, the steel sheets
disclosed in Patent Literatures 5 to 8 include a structure mainly containing a ferrite phase.
Accordingly, these steel sheets \yere difficult to achieve high strength of 850 MPa or
higl~el:
[Prior Art Literatures]
25 [Patent Literatures]
[0006]
[Patent Literature 1] JP H10-36917A
[Patent Literature 21 JP 2001-172745A
[Patent Literature 31 JP 2004-3 15857A
[Patent Literature 41 JP 2005-298924A
5 [Patent Literature 51 JP 2004-3 15857A
[Patent Literattire 61 JP 2005-298924A
[Patent Literature 71 JP 2008-261029A
[Patent Literature 81 JP 2008-266726A
[Summary of the I~ivention]
10 [Problems to Be Solved by the Invention]
[0007]
The invention has been made to solve tlie above problems and an object of the
invention is to provide a high-strength hot-rolled steel sheet which achieves both excellent
stretch flange formability and ductility, in particulal; high strength of tensile strength of
15 850 MPa or higher and has excelle~itp unching workability which can prevent damage in
an end face even when punching work is carried out under the most severe conditions.
[Means for Solving the Problems]
[0008]
The inventors have investigated on correlations among the frequency of damage
20 occurrence in tlie ppuched end face, kinds of elements segregated in crystal grain
boundaries, and tlie segregation amount in tlie crystal grain boundaries by setting a
cleara~iceo f punchi~igw ork to the most severe condition. As a result, the i~iventorsfo und
using mainly a bainite structure that tlie damage of the punched end face was reduced
when a ratio of large-angle crystal grain boundaries in which a grain bou~idarya ngle of the
25 steel sheet is 15' or niore to small-angle crystal grain boundaries in which the grain
boundary angle is 5" or more but less than 15" was controlled within a proper range and
tlie appropriate amount of C and B was segregated in the large-angle crystal grain
boundaries.
Tlie invention has been made based on novel findings, and the gist of the
invention is as follows:
5 [0009]
[I1
A high-strength hot-rolled steel sheet including, by mass %,
C: 0.050 to 0.200%;
Si: 0.01 to 1.5%;
Mn: 1.0 to 3.0%;
B: 0.0002 to 0.0030%;
Ti: 0.03 to 0.20%;
P: limited to 0.05%.or less;
S: limited to 0.005% or less;
Al: limited to 0.5% or less;
N: limited to 0.009% or less; and
one or more of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0.20%,
witli the balance being conlposed of Fe and inevitable impurities,
wherein a ratio of a length of small-angle crystal grain boundaries that are
20 boundaries having a crystal orientation angle of 5" or more but less than 15" to a length of
large-angle crystal grain boundaries that are boundaries having a crystal orientation angle
of 15" or more is 1 : 1 to 1 : 4,
a total segregation amount of C and B in the large-angle grain boundaries is 4 to
20 atoms/nni2,
25 tensile strength is 850 MPa or higher, and
a hole expansion ratio is 25% or more.
[21
The high-strength hot-rolled steel sheet according to [I], wherein the content of P
is limited to 0.02% or less by mass %,
the content of P is limited to 0.02% or less by mass % and
5 the segregation amount of P in the large-angle grain boundaries is 1 atomslnm2 or
less.
[OO lo]
[31
A tnetl~od for producing a high-strength hot-rolled steel sheet, the nrethod
10 including:
with respect to a steel slab containing by mass %,
C: 0.050 to 0.200%,
Si: 0.01 to 1.5%,
Mn: 1 .O to 3.0%,
B: 0.0002 to 0.0030%,
Ti: 0.03 to 0.20%,
P: limited to 0.05% or less,
S: limited to 0.005% or less,
Al: limited to 0.5% or less,
N: limited to 0.009% or less, and
one or more of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0.20%,
with the balance being composed of Fe and inevitable impurities,
heating the steel slab to 1200°C or higher;
completing finish rolling at a temperature of 910°C or higher;
performing air cooling for 0.5 to 7 seconds after colnpleting the finish rolling;
subjecting to primary cooling up to a temperature of 550 to 450°C at a cooling
rate of 4O0C/s or more;
subjecting to holding or air cooling at a temperature that is not higher than a stop
temperature of the primary cooling but not lower than 450°C for 7.5 to 30 seconds;
subsequently subjecting to secondary cooling up to a tetnperature of 200°C or
5 lower at a cooling rate of 15"CIs or more; and
subjecting to coiling.
[41
The method for producing the high-strength hot-rolled steel sheet according to 131,
wherein the content of P is limited to 0.02% or less, by mass %, in the steel slab.
10 [Effects of the Invention]
[OOll]
According to the invention, a high-strength hot-rolled steel sheet is provided
which achieves a good balance between stretch flange formability and ductility, in
particular, high strength of tensile strength of at least 850 MPa, and has excellent punching
15 workability in which a damage occurrence in an end face is suppressed regardless of
conditions of a clearance of punching work. The invention remarkably contributes to the
industry.
[Brief Description of the Drawings]
[0012]
20 [FIG. I] FIG. 1 is a diagram illustrating an example of a three-dimensional atomic
distribution image (a) at a position of crystal grain boundaries and a ladder chart analysis
(b) which are obtained by a three-dimensional atom probe nleasuring method.
[FIG. 21 FIG. 2 is a diagram illustrating correlations among a segregation amount
of CI a ratio of a length of large-angle crystal grain boundaries to a length of small-angle
25 crystal grain boundaries, and a datnage occurrence rate in a punched end face.
[FIG. 31 FIG. 3 is a diagra~n illustrating a correlation between a segregation
amount of P and a damage occurrence rate in a punched end face.
[Modes for Carrying out the Invention]
[0013]
The inventors carried out a punching work in various clearances using a
5 high-strength hot-rolled steel sheet having tensile strength of 850 MPa or higher with
excellent ductility and hole expandability to quantitatively examine end face properties
thereof.
[0014]
Specifically, a hole of 10 mm diameter was punched by varying the clearance in
10 accordance with a hole expanding test method disclosed in Japan Iron and Steel Federation
Standard JFS T 1001 - 1996, and a darnage occurrence rate in an entire circun~ferericeo f a
punclied end face (referred to as a damage occurrence rate in a punched end face) was
obtained by dividing a value calculated by measuring and adding up angles in a range to be
visually regarded as the damage among the entire circumference of the end face punched
15 into a round-shape, by 360".
As a result, when the clearance was increased, peeling or burr-like damage
occurred which was not confirmed in the case of being punched with the clearance of
about 12.5% recommended by a general hole expanding test metliod. Therefore, it was
found that the clearance of 16% was the most severe condition.
20 Here, the following examination was carried out with the clearance of 16%.
[0015]
Next, with respect to an influence of a structure on punching workability of a steel
sheet and further the frequency of a damage occurrence in the punched end face, that is,
correlations among the damage occul.l.ence rate in the punched end face, kinds and
25 anlounts of elenients segregated in large-angle crystal grain boundaries, and the ratio of
small-angle crystal grain boundaries to large-angle crystal grain boundaries, tlie
investigation was cal~ied out. Further, in the invention, the large-angle crystal grain
boundaries are defined as a grain boundary at which an angle difference between crystal
orientations of crystal grains adjacent to each other is 15" or more. Furthermore, in the
invention, the small-angle crystal grain boundary is defined as a grain boundary at which
5 an angle difference between crystal orientations of crystal grains adjacent to each other is
5" or more but less than 15".
[0016]
A slab containing, by mass %, C: 0.050 to 0.200%, Si: 0.01 to 1.5%, Mn: 1.0 to
3.0%, B: 0.0002 to 0.0030%, Ti: 0.03 to 0.20%, P: limited to 0.05% or less, S: limited to
10 0.005% or less, Al: limited to 0.5% or less, N: limited to 0.009% or less, and one or more
of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0. 20% with the balance being
conlposed of Fe and inevitable impurities was melted and was subjected to hot rolling to
produce a steel sheet under various heat treatment conditions.
No. 5 test piece of JIS Z 2201 was sampled fro111 the steel sheet and tensile
15 characteristics were evaluated in conformity with JIS Z 2241. In addition, a hole
expanding test was canied out according to a test method disclosed in Japan Iron and Steel
Federation Standard JFS T 1001-1996 and stretch flange formability of the steel sheet was
evaluated. Furtl~eer, the damage occurrence rate in the punclled end face was evaluated
after the punching work and before the hole expanding test.
20 [0017]
Next, amounts of B, C, and P segregated in five or more points of large-angle
crystal grain boundaries in individual steel were measured to obtain an average value.
In order to actively utilize bainite, the steel sheet of the invention includes the
small-angle crystal grain boundaries having an angle less than 15" in addition to the
25 large-angle crystal grain boundaries. In the small-angle crystal grain boundaries, there
was a tendency that the segregation amount was reduced from the difference in the number
of trap sites of the segregated elements compared to the large-angle crystal grain
boundaries. Howevel; since the correlation in the segregation amount between 'the
small-angle crystal grain boundaries and the large-angle crystal grain boundaries was
recognized, the segregation amount in the large-angle crystal grain boundaries was here
5 measured. An angle of the crystal orientation was determined by analyzing a Kikuchi
pattern obtained from a transmission electron microscope observation of the sample.
In the invention, a structure mainly containing the bainite preferably contains the
bainite in which an area ratio exceeds 50% when the end face is observed and may contain
ferrite or a second phase less than 50%.
10 [0018]
As for a method of measuring the amounts of segregation elements, in order to
strictly compare a distribution of the segregation elements in the micro region, it is suitable
to obtain the Excess amounts using a three-dimensional atom probe method as described
below. That is, the ciystal grain boundary portion of the sample to be measured is
15 subjected to cutting and electropolishing to prepare an acicular sample. Further, at this
time, a focused ion-beam processing method may be utilized together with electropolishing.
A region including the crystal grain boundaries and an angle of the grain boundary are
observed in a relatively wide visual field by FIM, and the three-dimensional atom probe
measurement is carried out.
20 In the three-ditnensional atom probe nleasurelnetlt, integrated data can be
reconstructed to obtain an actual distribution image of atoms in a real space. Since an
atomic surface is discontinuous at the position of the grain boundaries, the position of the
grain boundaries can be recognized as a grain boundary surface and it can be visually
observed that various elements are segregated in the position of the grain boundaries.
25 [0019]
Next, in order to estimate the segregation amount of each element, a ladder cllart
was obtained by vertically cutting out in a cuboid shape with respect to the crystal grain
boundaries from an atomic distribution image including the crystal grain boundaries. An
observatiorl example of the crystal grain boundaries and an example of the ladder chart
analysis are illustrated in (a) and (b) of FIG. 1, respectively.
5 From the ladder chart analysis, the segregation amount of each atom is segregated.
That is, the segregation amount of each atom was estimated using an Excess amount
represented by an additional number of atonls per unit area of the grain boundaries fro111 a
solid solution amount. This estimation referred to "Quantitative Observation of Grain
Boundary Carbon Segregation in Bake Hardening Steels", Nippoi1 Steel Techrzicnl Repol.t,
10 No. 381, October (2004): p. 26-30 by Takahashi et al.
[0020]
hl addition, the crystal grain boundaries was originally a surface, but used a length
as an indicator which was estimated in the following manner in the invention.
The sample, which was cut out to obtain the end face parallel to a rolling direction
15 and a sheet thickness direction of the steel sheet, was polished and was further
electro-polished. Subsequently, an EBSP measurement was carried out using an Electron
Back Scatter Diffraction Pattern-Orientation Imaging Microscopy (EBSP-OIMTM) method
under measurement conditiolls of a magnification of 2000 times, an area of 40 pm x 80 pm,
and a measurement step of 0.1 pm.
20 [0021]
The EBSP-OIMTM method is constituted by a device and a software that a highly
inclined sample in a scanning electron microscope (SEM) is irradiated with electron beams,
a Kikuchi pattern formed by backscattering is photographed by a high-sensitive camera,
and an image thereof is processed by a computer, thereby measuring a crystal orientation
25 of an irradiation point for a short time period.
In the EBSP measurement, it is possible to quantitatively analyze a crystal
orientation of a bulk sample surface, and an analysis area is an area which can be observed
by the SEM. It is possible to obsellre clystal orientation distributions within the sample
by performing measurement over several hours and mapping tlie area to be analyzed with
several tens of thousands of points in a grid shape at regular intervals.
5 From the nieasurement result, an area in \vliich an orientation difference between
the crystal grains was not less than 15" appeared on a line, this area was recognized as a
large-angle crystal grain boundary, and a length of the large-angle crystal grain boundaries
was obtained by software. Similarly, an area in which the orientation difference between
the crystal grains was 5" or more but less than 15" was recognized as a small-angle crystal
10 grain boundary and a length of the small-angle crystal grain boundaries was obtained by
software.
[0022]
A relation between the total segregation amount of C and B, tlie ratio of the length
of the large-angle crystal grain boundaries to the length of the small-angle crystal grain
15 boundaries, and the damage occurrence rate in the punched end face of tlie steel is
illustrated in FIG. 2.
As illustrated in FIG. 2, it was obsel-ved that a large anlount of C and B was
segregated in the large-angle crystal grain boundaries of tlie steel sheet in which tlie
damage occurrence rate in the punched end face was small.
20 [0023]
In the steel sheet of the invention, it is possible to maintain the total amount of C
and B segregated in the grain boundaries within an appropriate range by partially
dispersing and precipitating carbides of Ti, Nb, V, and Mo into the crystal grain to ensure a
solid solution C in tlie crystal grain, precipitating nitrides of Ti, Nb, and V to suppress
25 precipitation of BN, and leaving a solid solution B in tlie crystal grain. Thus, it is
possible to maintain excellent resistance to damage of the end face at the time of puncliing
the steel sheet.
As the reason of improving the resistance to damage of the end face of the steel
sheet in this way, it is considered that the crystal grain boundaries are strengthened by the
segregated C and B and a crack growth is suppressed in the crystal grain boundaries at the
5 time of the punching work.
[0024]
On the other hand, even if a large amount of C and B was segregated in the
large-angle crystal grain boundaries, when the ratio of the length of the large-angle crystal
grain boundaries to the length of the small-angle c~ystalg rain boundaries was small, the
10 resistance to damage of the end face was deteriorated at the time of punching the steel
sheet. As the reason for this, it is considered to be related to the fact that when the ratio
of the length of the large-angle crystal grain boundaries is reduced, a unit of the bainite
structure relatively increases, a block grain boundary tends to decrease, and thus toughness
is deteriorated. Further, in an area in which the ratio of the length of the large-angle
15 crystal grain boundaries became very large, the damage occurrence rate in the punched end
face was suppressed to be low, but the strength was reduced because the structure mainly
contained ferrite.
[0025]
I11 addition, FIG. 3 illustrates a relation between the segregation amount of P and
20 the damage occurrelice rate in the punched end face. As illustrated in FIG. 3, in the case
of increasing the segregation amount of P by intentionally adding P while maintaining the
segregation amount of C and B to a certain amount or more in the crystal grain boundaries,
it was found that a punching damage occurrence rate was being increased.
[0026]
25 From the above results, it was found that when the carbides and BN were
excessively precipitated during cooling after hot rolling, the solid solution C and the solid
solution B was reduced, a small amount of C and B was segregated in the grain boundaries,
and tlme damage occurred in the punched end face. Therefore, a method was further
examined in which a large amount of C and B was segregated in tlme large-angle crystal
grain boundaries to improve the puncliing workability, as compared to tlme nornlal steel.
5 Consequently, it was found that when the carbides and BN were suppressed to be
precipitated into the crystal grain, the darmmage of the punched end 'face was suppressed.
On the otlier hand, unlike C and B, it was found that tliere were elements to reduce the
grain boundary strengthening amount when being segregated in the grain boundaries.
[0027]
10 Details of the invention defined in claims are described in the following.
(Segregation amount)
If the damage occurrence rate in the punched end face is 0.3 or less at the
clearance of the most severe condition, the range is allowable as practical steel. In the
exatnination of the invention, the clearance of 16% is the most severe condition, but can be
15 varied due to tlme nmaterial of the steel sheet and a tool. Thus, it is necessary to confirm
the most severe clearance condition by performing the punching work while varying the
clearance froni 12.5% to 25% to confirm tlme end face properties. In order to make the
end face damage to be 0.3 or less in the case of carrying out the punching work of the steel
sheet under tlme most severe clearance co~idition, it is necessary to opti~iiizet he amount of
20 element to be segregated in tlie grain boundaries of the crystal grain boundaries as
described below.
[0028]
As illustrated in FIG. 2, if the total segregation atnount of C and B in tlie
large-angle crystal grain boundaries is 4 atoms/nm2 or more, tlme damage occurrence rate in
25 the punched end face can be confined to be 0.3 or less when the steel sheet is subjected to
the punching work under tlie nmost severe clearance condition. If the total segregation
amount of C and B is below 4 atorns/nm2, the grain boundary strengthening amount is
insufficient and the damage significantly occurs in the punched end face.
Meanwhile, there was no preferred upper limit of the total segregation amount of
C and B in the crystal grain boundaries, but it was considered that the upper limit of the
5 amount, which can be substantially segregated in the steel sheet of the invention, was about
20 atorns/nm2. The total segregation amount of C and B in the crystal grain boundaries is
more preferably in the range of 6 to 15 atoms/nm2 in which the damage hardly occurs in
the punched end face.
[0029]
10 Further, in order to prevent the segregation amount of C in the grain boundaries
from being reduced by the precipitation of the segregated C as a carbide such as cementite,
the steel sheet is rapidly cooled down to 200°C or lower after a desired segregation is
achieved by cooling after hot rolling. Thus, the total segregation amount of C and B can
range from 4 to 20 atoms/nm2.
15 [0030]
Meanwhile, the segregation amount of P is preferably small. The reason for this
is because it is corisidered that P has an effect of embritting the grain boundaries. I11
addition, the reason is that the crack growth is facilitated at the time of the punching work
and the damage occurrence rate is increased \vllen the segregation amount of P increases.
20 Further, there is also a concern that the segregation amounts of C and B are reduced as P
occupies segregation sites. The segregation amount of P is preferably 1 atomslmn2 or less.
In order for the segregation amount of P to be 1 aton~slnno~r l ess, the content of P may be
limited to 0.02% or less.
[003 11
25 (Ratio of length of large-angle crystal grain boundaries to length of small-angle
crystal grain boundaries)
As illustrated in FIG. 2, when the total segregation amount of C and B is 4 to 20
atorns/nm2 and further the ratio of the length of the large-angle crystal grain boundaries to
the length of the small-angle crystal grain boundaries is 1 or more and 4 or less, the
damage occurrence rate in the punched end face can be confined to be 0.3 or less when the
5 steel sheet is subjected to the punching work under the most severe clearance condition.
It is considered to be related to the fact that when the ratio of the length of the large-angle
crystal grain boundaries to the length of the small-angle crystal grain boundaries is less
than 1, a block grain size of bainite tends to increase and toughness is deteriorated thereby
increasing the damage occurrence rate in the punched end face. In addition, when the
10 ratio of the length of the large-angle crystal grain boundaries to the length of the
small-angle crystal grain boundaries is more than 4, the damage occurrence rate in the
punched end face is suppressed to be low, but the strength is reduced because the structure
mainly contains ferrite. Thus, in this case, it will not satisfy the steel sheet of the
invention having the tensile strength of 850 MPa or higher.
15 [0032]
(Composition)
In the invention, the steel sheet is preferably defined to have the following
component colnpositions such that a structure of the steel sheet has the segregation amount
in the grain boundaries and the ratio of the length of the large-angle crystal grain
20 boundaries to the length of the small-angle crystal grain boundaries which are described
above as the steel sheet composition, the steel sheet has elongation of 15% or more, hole
expansion ratio of 25% or more, tensile strength of 850 MPa or highel; and the damage
occurrence rate in the punched end face is 0.3 or less when the punching work of the steel
sheet is carried out under the most severe clearance condition. Further, "%" to be
25 described below represents "% by mass" values unless otherwise specified.
In addition, the intended effects of the invention are sufficiently exhibited by basic
conlponents to be described below, but other components may be contained within the
range of not inhibiting the intended properties of the steel sheet of the invention. For
example, Cr of less than 0.2% and Cu of less than 0.15% may be contained.
[0033]
5 C: C is an element contributing to improve strength, and the content of C is
necessary to be 0.050% or more to obtain the st~ucturem ainly containing bainite of the
invention and sufficiently ensure the segregation amount of C in the grain boundaries.
On the other hand, when the content of C exceeds 0.200%, the formation of cenlentite or
the formation of a transfornlation structure such as pearlite or martensite is unnecessarily
10 promoted, and thus elongation or hole expandability is reduced. Therefore, the content of
C is set to be in the range of 0.050 to 0.200%.
[0034]
B: B is an important element in the invention, and the damage of the punched end
face is prevented by the segregation of B even when the segregation of C in the grain
15 boundaries is insufficient. The content of B is necessary to be 0.0002% or more to obtain
the above effect. On the other hand, when the content of B exceeds 0.0030%, workability
such as ductility is reduced. Accordingly, the content of B is set to be in the range of
0.0002 to 0.0030%.
[0035]
20 Si: Si serves as a solid solution strengthening element, which is effective for
improvement of the strengtl~. The content of Si is necessary to be 0.01% or more to
obtain such an effect. On the other hand, when the content of Si exceeds 1.5%, the
workability is deteriorated. Accordingly, the content of Si is set to be in the range of 0.01
to 1.5%.
25 [0036]
Mn: Mn is necessary for deoxidation and desulfurization, wl~ichis also effective
as a solid solution strengthening element. Further, the content of Mn is necessary to be
1 .O% or more to stabilize austenite and easily obtain bainite structure. On the other hand,
wlien tlie content of Mn exceeds 3.0%, the segregation easily occurs and the workability is
deteriorated. Accordingly, the content of Mn is set to be in the range of 1.0 to 3.0%.
5 [0037]
Ti: Ti is an element used to precipitate carbides and nitrides into clystal grains of
ferrite or bainite and increase the strength of the steel sheet by precipitation strengthening.
In order to sufficiently generate the carbides and nitrides, the content of Ti is set to be
0.03% or more. On the other hand, when the content of Ti exceeds 0.20%, the carbides
10 and nitrides become coarse. Accordingly, the content of Ti is set to be in the range of
0.03 to 0.20%.
[0038]
P: P is an impurity, and the content of P is necessary to be limited to 0.05% or less.
In addition, the content of P is preferably limited to 0.02% or less to suppress the
15 segregation of P in the grain boundaries and prevent cracks of the grain boundaries.
[0039]
Further, in the invention, one or more of V, Nb, and Mo, which are elements used
to precipitate the carbides into the crystal grains, may be contained to achieve the high
strength of the steel sheet. In order to promote the grain boundary segregation of B,
20 fi~rtherniore, one or two kinds of V and Nb as a nitride precipitating element may be
preferably contained, thereby suppressing the precipitation of BN.
[0040]
V and Nb: V and Nb are elenients used to precipitate carbides and nitrides into
crystal grains of ferrite or bainite and increase the strength of the steel sheet by
25 precipitation strengthening. In order to sufficiently generate the carbides and nitrides, the
each content of V and Nb is preferably 0.01% or more. On the other hand, wlien the each
content of V and Nb exceeds 0.20%, the carbides and nitrides tnay beconle coarse.
Accordingly, the each content of V and Nb is preferably set to be in the range of 0.01 to
0.20%.
[0041]
5 Ma: Mo is a carbide forming element and may be contained for the purpose of
precipitating the carbides into crystal grains and contributing to precipitation strengthening.
In order to sufficiently generate the carbides, the content of Mo is preferably 0.01% or
more. On the other hand, when tlie adding amount of Mo exceeds 0.20%, coarse carbides
may be generated. Accordingly, the content of Mo is preferably set to be in the range of
10 0.01 to 0.20%.
[0042]
~uttherlilore,t he content of N, S, and A1 is preferably limited to the following
upper limit.
N: N forms nitrides and reduces the workability of the steel sheet, and thus the
15 content thereof is preferably limited to 0.009% or less.
S: S is present as an inclusion such as MIIS and deteriorates stretch flange
formability to fi~~thecaru se cracking during hot rolling. Tlierefore, it is preferable to
reduce the content of S as much as possible. Pa~ticularlyt,h e content of S is preferably
litnited to 0.005% or less to prevent the cracking during the hot rolling and to improve the
20 workability.
Al: A1 forlns precipitates such as nitrides and impairs the workability of the steel
sheet, and thus the content thereof is preferably limited to 0.5% or less. Furtlies, A1 of
0.002% or more is preferably added for the purpose of deoxidation of molten steel.
[0043]
25 In the invention, W as a solid solution strengthening element may be also added
for the purpose of improving the strength of the steel sheet, in addition to the above basic
con~ponents.
[0044]
(Producing conditions)
A steel slab obtained by melting and casting the steel consisting of the above
5 conlponent co~npositionsi n a convelitional manner is subjected to hot rolling. The steel
slab is preferably produced in colitinuous casting equipment from the viewpoint of
productivity. A heating temperature of hot rolling is 1200°C or higher to sufficiently
decompose and dissolve carbide forming ele~iientsa nd carbon it1 steel. When the heating
temperature is excessively high, it is not economically preferred. Therefore, the upper
10 limit of the heating temperature is preferably 1300°C or lower. After the casting, the steel
slab is cooled down and may be subjected to initial rolling at a temperature of 1200°C or
higher. In the case of heating the steel slab cooled to 1200°C or lower, it is preferable to
hold for one or more hours.
[0045]
15 A finishing temperature of finis11 rolling in the hot rolling is necessary to be 910°C
or higher to suppress the formatiori of coarse carbides. The upper limit of the finishing
temperature of the finish rolling needs not to be specifically limited in order to obtain the
effects of the invention, but is preferably 1000°C or lower because there is a possibility
that scale defects occur at the time of working.
20 Furthelmore, the finish rolling is preferably performed at a total reduction ratio of
60% or more in three stands from a final stand to make crystal grain sizes of austenite fine.
The reduction ratio is preferably as high as possible, but the upper limit thereof is
substantially 95% fro111 the viewpoint of productivity or equipment loads.
[0046]
25 After completing the hot rolling, it is preferable to perfom] air cooli~igfo r 0.5 to 7
seconds. Tliis is because of promoting recrystallizatioll of austenite to easily obtain the
structure of the invention mainly containing bainite. When the air cooling is perfor~ned
for a period below 0.5 seconds, the transformation occurs fron~n on-recrystallized austenite
grains, which may lead to the ferrite formation during the cooling. When the air cooling
is performed for a period above 7 seconds, TiC precipitation proceeds in the austenite and
5 effective precipitation may become few in the bainite or ferrite.
[0047]
Subsequently, in order to suppress the precipitation of the carbides in the austenite
region, the ferrite transformation, and the pearlite transforn~ationa s much as possible, it is
necessary that cooling rate of pri~nary cooling is 4O0Cls or more and a finishing
10 temperature of the primary cooling ranges from 550°C or lower to 450°C or higher.
When the cooling rate of the primary cooling is less than 4O0C/s, coarse carbides
are precipitated during the cooling, the segregation amount of C in the grain boundaries is
reduced, and thus there is a concern that the damage of the punched end face increases.
The upper limit of the cooling rate of the primary cooling is not particularly limited, but a
15 reasonable cooling rate is 30O0Cls or less in consideration of capacity of cooling
equipment. In addition, when the finishing temperature of the primary cooling exceeds
550°C, the bainite is formed at a high temperature and the ratio of the length of the
large-angle crystal grain boundaries is reduced. Moreover, when the finishing
temperature exceeds 600°C, the ferrite transformation is promoted and thus the strength is
20 reduced, and the hole expansion ratio is reduced by the formation of pearlite. Meanwhile,
when the finishing temperature is lower than 450°C, a large a~nount of tnartensite is
fornled and the hole expansion ratio is reduced.
[0048]
Subsequently, it is necessary to hold or air-cool from a stop temperature or lower
25 of the primary cooling to a temperature higher than 450°C for 7.5 seconds or longer to
realize a bainite transformation. In the case of a period shorter than 7.5 seconds, the
bainite transformation becomes insufficient, a large anlount of martensite is formed by
subsequent cooling, and the workability is deteriorated. The holding or air cooling period
is preferably 10 seconds or longer and more preferably 15 seconds or longer, From the
viewpoint of productivity, the air cooling is preferred and the upper limit period of the air
5 cooling is 30 seconds.
[0049]
Subsequently, secondary cooling is carried out up to a temperature of 200°C or
lower at 15OC/s or more. The reason is that when the temperature higher than 200°C is
held after the bainite transformation, carbides such as cementite are precipitated, C to be
10 segregated becomes insufficient, and thus it is difficult to obtain the segregation amount of
C in the grain boundaries according to the invention. The upper limit of the cooling rate
of the secondary cooling is not particularly limited, but a reasonable cooling rate is
20O0C/s or less in consideration of the capacity of the cooling equipment. In the case of
perfonning coiling after the cooling is carried out from 200°C or lower to a room
15 temperature or highel; the precipitation of cementite or the like is less likely to occur and C
segregated in the large-angle crystal grain boundaries of the bainite is held. More
preferably, when the coiling is performed at 100°C or highel; a solid solution C in the
crystal grain may migrate to more stable crystal grain boundaries to increase the
segregation amount.
20 [Examples]
[0050]
Examples of the invention will be described together with Conlparative Exainples.
Materials having coinponent compositions (the balance is Fe and inevitable
impurities) indicated in Table 1 were variously dissolved. Component values indicated in
25 the Table are chemical analysis values, and the unit thereof is mass %. In Table 1, a mark
"-" means the case of not being intentionally added.
[0051] [Table 11
Steel
type
A
B
C
D
E
-F
E
-H
Test
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A mark "-" indicates the case of not being intentionally added.
[0052]
Next, a hot-rolled steel sheet was produced by hot rolling carried out under
producing conditions as shown in Table 2. Primary cooling is a cooling to be performed
immediately after the cotnpletion of the hot rolling, and secondary cooling is a cooling to
be performed prior to coiling.
[0053] [Table 21
Chemical composition (~nass% )
c
0.052
0.064
0.070
0.103
0.166
0.069
0.067
&&I
Steel
type
A
A
A
B
B
C
C
D
D
E
E
E
P
H
Si
1.5
0.8
1.1
0.8
0.02
1.2
1.1
0.95
Note
1"~c"tivo
Ervllplc
cot" L~YC *
~~n"a~m"pltci ~ e
-&
*Em& corn x'rtive
~ n v e " ~ i ~ e
cxmpto
C O ~ * amtive
~nvintive
Exartlple
*I~nxvamntlipvele CMN rratirc
CXMi"E &tiv e
c m ~am tire * cmw&s
Elrmn~lle
Mn
2.2
2.5
2.3
1.8
1.1
2.4
2.5
1.2
Heating
temperature
'c
1240
1250
1230
1250
1250
1230
1250
1240
1250
1260
1240
1250
1230
1240
conditions
Finishing
tcrnperatsrs
afpritnarp
cooling
OC
520
530
540
550
3s
520
550
640
500
550
!m
520
530
550
P
0.030
0.008
0.009
0.007
0.009
0.055
0.009
0,008
Finisbisg
te.,Frat,,re
I.
hat rolling
"c
960
970
910
970
970
950
960
860
930
870
950
960
950
950
~oldiwoiair
'm'i"g*"*
%=
malng
. S
20
8
15
15
10
18
22
20
25
30
25
15
20
20
S
0.001
0.002
0.001
0.W2
0.003
0.001
0.001
0.001
~ i ~ . ~ ~ ~ l i ~ ~
period
hof "Iling
S
2
0.5
a2
7
2
5
2
3
1
4
4
2
2
3
Producing
. nrnary
cooling rate
%/s
-30
50
40
40
50
50
40
40
40
50
40
40
40
50
Secondary
cooling rate
'c/8
20
15
15
20
15
15
20
15
20
20
15
15
15
20
Coiling
ternpnt.re
*C
cIw
1%
,30
1 40
130
180
<,@,
<,@J
<$@
A1
O.WO
0.31
0.026
0.031
0.034
0.025
0.032
0.030
Mo
-
-
-
0.1
-
-
-
-
N
0.001
0.006
0.002
0.002
0.003
0.002
0.002
0.001
B
0.0015
0.0024
0.0012
0.0015
0.0003
0,0009
0.0001
0.001
V
0.05 -
-
-
0.03
-
-
0.05
Ti
0.17
0.06
0.15
0.09
0.05
0.16
0.13
0.14
Nb
-
0.08
0.03
-
0.06
-
0.02
-
[0054]
Frorn these steel sheets, No. 5 test piece disclosed in JIS Z 2201 was worked and
tensile characteristics were evaluated in conformity with a test method disclosed in JIS Z
2241. As one of stretch flange fornlability, a hole expanding test was evaluated according
5 to a test method disclosed in Japan Iron and Steel Federation Standard JFS T 1001-1996.
Further, a damage occurrence rate in a punched end face was obtained in such a manner
that a hole of 10 nlm diameter was punched as in the hole expanding test, the shape of end
face was visually observed, and angles in a range to be regarded as the damage was
measured among the end faces punched into circle-shapes. In addition, tlie hole
10 expansion ratio was tested in accordance with a hole expanding test method of a metallic
material disclosed in JIS Z 2256, and it was evaluated to pass the test when tlie hole
expansion ratio was 25% or more.
[0055]
In addition, a columnar sample of 0.3 tnm x 0.3 mm x 10 mnm was cut out from
15 the steel sheet, and a purpose grain boundary portion was prepared to have a sharp
acicular-shape by electropolishing or focused ion-beam processing method and then was
subjected to a three-dimensional atom probe measurement. In order to estimate the
segregation amount of each element in the grain boundaries, a ladder chart was obtained by
vertically cutting out in a cuboid shape with respect to the grain boundaries from an atomic
20 distribution image including the grain boundaries. From the ladder chart analysis, the
segregation amount of each element was estimated using an Excess amount. In individual
steel, the segregation amount of each element in five or more grain boundaries was
examined to obtain an average value. The obtained average value was set as the
segregation amount of each element in the individual steel.
25 [0056]
Furthermore, the sample, which was cut out to obtain the end face parallel to a
rolling direction and a sheet thickness direction of the steel sheet, was polished and was
further electro-polished. Subsequently, an EBSP nleasuren~ent was performed on the
satnple using the above-described EBSP-01~~m"e thod under ~neasurementc onditions of
a magnification of 2000 times, an area of 40 pm x 80 pm, and a measurenlent step of 0.1
5 pm. From the measurement result of the individual steel, an area in which an orientation
difference between tlie crystal grains was not less than 15" was recognized as a large-angle
crystal grain boundary, an area in which tlie orientation difference between the crystal
grains was not less than 5" and below 15" was recognized as a small-angle crystal grain
boundary, and lengths of the large-angle crystal grain boundaries and the small-angle
10 crystal grain boundaries were obtained by software.
[0057]
Each of test results described above is indicated in Table 3. Next, each of data
indicated in Table 3 will be schematically described.
Test Nos. 2, 4, 7, 9, and 10 are exanlples in which components and producing
15 conditions of the steel sheet are within the scope of the invention, in which the strength is
high, hole expandability is excellent, and the damage rate of the punched end face is also
small.
[0058]
Meanwliile, No. 1 is an example in which a cooling rate of the primary cooling is
20 slow and the damage of the punched end face occurs, and No. 6 is an exanlple in which a
coiling temperature is high, the total segregation amount of C and B in the grain
boundaries is insufficient, and the damage of the punched end face occurs.
No. 5 is an example in which a finishing temperature of the pritnary cooling is low,
a large amount of martensite is fol.med, and the hole expansion ratio is reduced.
25 No. 3 is an example in whicli an air cooling period after the hot rolling is short
and tlie strength is reduced, No. 8 is an example in which the finishing temperature of the
primary cooling is high and tlie strength is reduced, and No. 14 is an example in which tlie
content of C is insufficient and the strength is reduced.
No. 1 I is an example in which the finishing temperature of the prinlary cooling is
slightly high, the ratio of the large-angle grain boundaries is reduced, and tlie damage of
5 the punched end face occurs.
No. 13 is an exarnple in which the content of B is insufficient, the segregation
amount in the grain boundaries is not attained, and the damage of tlie end face occurs
during the punching.
No. 12 is an example in which the content of P is large and the damage of the
10 punched end face occurs.
[0059] [Table 31

WE CLAIMS:-
A high-strength hot-rolled steel sheet conlprising: by Inass %,
C: 0.050 to 0.200%;
Si: 0.01 to 1.5%;
Mn: 1.0 to 3.0%;
B: 0.0002 to 0.0030%;
Ti: 0.03 to 0.20%;
P: limited to 0.05% or less;
S: limited to 0.005% or less;
Al: limited to 0.5% or less;
N: limited to 0.009% or less; and
one or more of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0.20%,
with the balance being composed of Fe and inevitable impurities,
wherein a ratio of a length of small-angle crystal grain boundaries that are
boundaries having a crystal orientation angle of 5" or more but less than 15" to a length of
large-angle crystal grain boutidaries that are boundaries having a crystal orientation angle
of15"ormoreis1:1to1:4,
a total segregation a~iiounto f C and B in the large-angle grain boundaries is 4 to
20 20 a t o m s / d ,
tensile strength is 850 MPa or highel; and
a hole expansion ratio is 25% or more.
[Claim 2]
The high-strength hot-rolled steel sheet according to claim 1, wherein the content
25 of P is limited to 0.02% or less by mass %, and
the segregation amount of P in the large-angle grain boundaries is 1 atoms/nm2 or
less.
[Claim 3]
A method for producing a high-strength hot-rolled steel sheet, the method
comprising:
5 with respect to a steel slab containing by mass %,
C: 0.050 to 0.200%,
Si: 0.01 to 1.5%,
Mn: 1 .O to 3.0%,
B: 0.0002 to 0.0030%,
Ti: 0.03 to 0.20%,
P: limited to 0.05% or less,
S: limited to 0.005% or less,
Al: limited to 0.5% or less,
N: lirnited to 0.009% or less, and
one or more of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0.20%,
with the balance being composed of Fe and inevitable impurities,
heating the steel slab to 1200°C or higher;
completing finish rolling at a temperature of 910°C or higher;
perfornling air cooling for 0.5 to 7 seconds after completing the finish rolling;
20 subjecting to primary cooling up to a temperature of 550 to 450°C at a cooling
rate of 4O0Cls or more;
subjecting to holding or air cooling at a temperature that is not higher than a stop
temperature of the primary cooling but not lower tha11450"C for 7.5 to 30 seconds;
subseq~~entlsyu bjecting to secondary cooling up to a temperature of 200°C or
25 lower at a cooling rate of 15OCIs or more; and
subjecting to coiling.
[Claim 4]
The method for producing the high-strength hot-rolled steel sheet according to
claim 3, wherein the content of P is limited to 0.02% or less, by mass %, in the steel slab.