6
PRIOR ART DOCUMENT
Patent Document
[0008]
5 Patent Document 1: Japanese Examined Patent Application, Publication No.
H06-3 5647
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
10 [0009]
In order to solve the above-described problems, the present invention aims to
provide a high-strength steel sheet, a hot-dipped steel sheet, an alloyed hot-dipped steel
sheet, and production methods thereof, and these steel sheets have a tensile strength in a
range of 590 MPa or more, and are excellent in fatigue properties, elongation, and
15 collision properties.
Means for Solving the Problems
[0010]
The high-strength steel sheet of the present invention having excellent fatigue
20 properties, elongation and collision properties, includes: in terms of percent by mass, 0.03
to 0.10% of C; 0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of
S; 0.01 to 1.2% of Al; 0.06 to 0.15% of Ti; and 0.01% or less of N; and contains as the
balance, iron and inevitable impurities. A tensile strength is in a range of 590 MPa or
more, and a ratio of a yield strength to the tensile strength is in a range of 0.80 or
25 more. A niicrostructure includes bainite at an area ratio of 40% or more and the balance
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being either one or both of ferrite and martensite. A density of Ti(C,N) precipitates
having sizes of 10 nm or smaller is in a range of 10 precipitates/mm or more. A ratio
(Hvs/Hvc) of a hardness (Hvs) at a depth of 20 (xm from a surface to a hardness (Hvc) at a
center of a sheet thickness is in a range of 0.85 or more.
5 In the high-strength steel sheet of the present invention having excellent fatigue
properties, elongation and collision properties, a fatigue strength ratio may be in*a range of
0.45 or more.
An average dislocation density may be in a range of 1 x lO'"* m"^ or less.
The high-strength steel sheet may further include one or more selected from the
10 group consisting of: in terms of percent by mass, 0.005 to 0.1% of Nb; 0.005 to 0.2% of
Mo; 0.005 to 0.2% of V; 0.0005 to 0.005% of Ca; 0.0005 to 0.005% of Mg; 0.0005 to
0.005% of B; 0.005 to 1% of Cr; 0.005 to 1% of Cu; and 0.005 to 1% Ni.
[0011]
The hot-dipped steel sheet of the present invention having excellent fatigue
15 properties, elongation and collision properties, includes: the high-strength steel sheet of
the present invention described above; and a hot-dipped layer provided on the surface of
the high-strength steel sheet.
In the hot-dipped steel sheet of the present invention having excellent fatigue
properties, elongation and collision properties, the hot-dipped layer may consist of zinc.
20 The alloyed hot-dipped steel sheet of the present invention having excellent
fatigue properties, elongation and collision properties, includes: the high-strength steel
sheet of the present invention described above; and an alloyed hot-dipped layer provided on
the surface of the high-strength steel sheet.
[0012]
25 The method for producing the high-strength steel sheet of the present invention
18
galvanizing properties and chemical conversion properties. Therefore, the Al content is
set to be in a range of 1.2% or less and is preferably set to be in a range of 0.6% or less.
[0028]
Ti is an important element important in the present invention. Ti is an important
5 element for precipitation strengthening of the steel sheet during annealing after hot rolling.
In the production process, it is necessary to maintain a solid solution state while
suppressing the amount of formed precipitates as low as possible in a hot rolling stage (a
stage from hot rolling to coiling); and therefore, a coiling temperature during the hot
rolling is set to be in a range of 600°C or less at which Ti precipitates are less likely to be
10 generated. In addition, skin pass rolling is performed before annealing; and thereby,
dislocations are introduced. Next, in an annealing stage, Ti(C,N) is finely precipitated on
the introduced dislocations. In particular, at or in the vicinity of the surface layer of the
steel sheet where a dislocation density is increased, the effect (fine precipitation of
Ti(C,N)) becomes notable. Due to this effect, it becomes possible to attain Hvs/Hvc >
15 0.85, and high fatigue properties can be achieved. In addition, by precipitation strengthening
due to an addition of Ti, a yield ratio which is a ratio of a yield strength to a tensile strength
can be in a range of 0.80 or more. Among many precipitation strengthening elements, Ti has
the highest precipitation strengthening ability. This is because a difference between the
solubility of Ti in a y phase and the solubility of Ti in an
20 a phase is large. In order to achieve a tensile strength of 590 MPa or more,
Hvs/Hvc>0.85, and a yield ratio of 0.80 or more, it is necessary to set the Ti content to be
in a range of 0.06% or more as shown in FIGS. 8 and 9. In the case where the Ti content
is less than 0.06%, as shown in FIG. 10, a precipitate density of Ti(C,N) having sizes of 10
nm or smaller becomes less than 10'° pieces/mm^ and thereby, a high yield ratio is not

1 t ,
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WE CLAIM:
1. A high-strength steel sheet having excellent fatigue properties, elongation and
collision properties, comprising: in terms of percent by mass,
0.03 to 0.10% of C;
0.01 to 1.5% of Si;
1.0 to 2.5% of Mn;
0.1% or less of P;
0.02% or less of S;
0.01 to 1.2% of Al;
0.06 to 0.15% of Ti; and
0.01% or less of N,
with the balance being iron and inevitable impurities,
wherein a tensile strength is in a range of 590 MPa or more, and a ratio of a yield
strength to the tensile strength is in a range of 0.80 or more,
a microstructure comprises bainite at an area ratio of 40% or more and the
balance being either one or both of ferrite and martensite,
a density of Ti(C,N) precipitates having sizes of 10 nm or smaller is in a range of
1 A O
10 precipitates/mm or more, and
a ratio (Hvs/Hvc) of a hardness (Hvs) at a depth of 20 (xm from a surface to a
hardness (Hvc) at a center of a sheet thickness is in a range of 0.85 or more.
2. 'Rie high-strength steel sheet having excellent fatigue properties, elongation and
collision properties as claimed in claim 1,
wherein a fatigue strength ratio is in a range of 0.45 or more.
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3. The high-strength steel sheet having excellent fatigue properties, elongation and
collision properties as claimed in claim 1,
wherein an average dislocation density is in a range of Ixio''* m"^ or less.
4. The high-strength steel sheet having excellent fatigue properties, elongation and
collision properties as claimed in claim 1,
wherein the high-strength steel sheet further comprises one or more selected from
the group consisting of: in terms of percent by mass,
0.005 to 0.1% of Nb;
0.005 to 0.2% of Mo;
0.005 to 0.2% of V;
0.0005 to 0.005% of Ca;
0.0005 to 0.005% of Mg; and
0.0005 to 0.005% of B.
5. A hot-dipped steel sheet having excellent fatigue properties, elongation and collision
properties, comprising:
the high-strength steel sheet as claimed in claim 1; and
a hot-dipped layer provided on a surface of the high-strength steel sheet.
6. The hot-dipped steel sheet having excellent fatigue properties, elongation and
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collision properties as claimed in claim 5,
wherein the hot-dipped layer consists of Zn.
7. An alloyed hot-dipped steel sheet having excellent fatigue properties, elongation and
collision properties, comprising:
the high-strength steel sheet as claimed in claim 1; and
an alloyed hot-dipped layer provided on a surface of the high-strength steel sheet.
8. A method for producing the high-strength steel sheet having excellent fatigue
properties, elongation and collision properties as claimed in claim 1, the method
comprising:
heating a slab comprising: in terms of percent by mass%, 0.03 to 0.10% of C; 0.01 to
1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06
to 0.15% of Ti; and 0.01% or less of N, with the balance being iron and inevitable impurities,
at a temperature in a range of 1,150 to 1,280°C and performing hot rolling under conditions
where a finish rolling is finished at a temperature in a range of not less than an Ars point,
thereby obtaining a hot-rolled material;
coiling the hot-rolled material in a temperature range of 600°C or less, thereby
obtaining a hot-rolled steel sheet;
subjecting the hot-rolled steel sheet to acid pickling;
subjecting the pickled hot-rolled steel sheet to first skin pass rolling at an elongation
rate in a range of 0.1 to 5.0%;
aimealing the hot-rolled steel sheet under conditions where a maximiim heating
temperature (Tmax°C) is in a range of 600 to 750°C and a holding time (t seconds) in a
temperature range of 600°C or higher fulfills expressions (1) and (2) as follows; and
, I
1 \ 1
• t
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subjecting the annealed hot-rolled steel sheet to second skin pass rolling,
530 - OJxTmax < t < 3,600 - 3.9xTmax.. .(1)
t>0...(2).
9. The method for producing the high-strength steel sheet having excellent fatigue
properties, elongation and collision properties as claimed in claim 8,
wherein an elongation rate is set to be in a range of 0.2 to 2.0% in the second skin
pass rolling.
10. The method for producing the high-strength steel sheet having excellent fatigue
properties, elongation and collision properties as claimed in claim 8,
wherein 1/2 or more of the amount of Ti contained in the hot-rolled steel sheet
after the coiling exists in a solid-solution state.
11. A method for producing the hot-dipped steel sheet having excellent fatigue properties,
elongation and collision properties as claimed in claim 5, the method comprising:
heating a slab comprising: in terms of percent by mass%, 0.03 to 0.10% of C; 0.01 to
1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06
to 0.15% of Ti; and 0.01% or less of N, with the balance being iron and inevitable impurities,
at a temperature in a range of 1,150 to 1,280°C and performing hot rolling under conditions
where a finish rolling is finished at a temperature in a range of not less than an Ars point,
thereby obtaining a hot-rolled material;
coiling the hot-rolled material in a temperature range of 600°C or less, thereby
obtaining a hot-rolled steel sheet;
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subjecting the hot-rolled steel sheet to acid pickling;
subjecting the pickled hot-rolled steel sheet to first skin pass rolling at an elongation
rate in a range of 0.1 to 5.0%;
annealing the hot-rolled steel sheet under conditions where a maximum heating
temperature (Tmax°C) is in a range of 600 to 750°C and a holding time (t seconds) in a
temperature range of 600°C or higher fulfills expressions (1) and (2) as follows, and
performing hot dipping to form a hot-dipped layer on a surface of the hot-rolled steel sheet,
thereby obtaining a hot-dipped steel sheet; and
subjecting the hot-dipped steel sheet to second skin pass rolling,
530 - 0.7xTmax < t < 3,600 - 3.9xTmax...(1)
t>0...(2).
12. The method for producing the hot-dipped steel sheet having excellent fatigue
properties, elongation and collision properties as claimed in claim 11,
wherein an elongation rate is set to be in a range of 0.2 to 2.0% in the second skin
pass rolling.
13. A method for producing the alloyed hot-dipped steel sheet having excellent fatigue
properties, elongation and collision properties as claimed in claim 7, the method
comprising:
heating a slab comprising: in terms of percent by mass%, 0.03 to 0.10% of C;
0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2%
of Al; 0.06 to 0.15% of Ti; and 0.01% or less of N, with the balance being iron and
inevitable impurities, at a temperature in a range of 1,150 to 1,280°C and performing hot
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rolling under conditions where a finish rolling is finished at a temperature in a range of not
less than an Ars point, thereby obtaining a hot-rolled material;
coiling the hot-rolled material in a temperature range of 600°C or less, thereby
obtaining a hot-rolled steel sheet;
subjecting the hot-rolled steel sheet to acid pickling;
subjecting the pickled hot-rolled steel sheet to first skin pass rolling at an elongation
rate in a range of 0.1 to 5.0%;
annealing the hot-rolled steel sheet under conditions where a maximum heating
temperature (Tmax°C) is in a range of 600 to 750°C and a holding time (t seconds) in a
temperature range of 600°C or higher fiilfiUs expressions (1) and (2) as follows,
performing hot dipping to form a hot-dipped layer on a surface of the hot-rolled steel sheet so
as to obtain a hot-dipped steel sheet, and subjecting the hot-dipped steel sheet to an alloying
treatment to convert the hot-dipped layer into an alloyed hot-dipped layer; and
subjecting the hot-dipped steel sheet on which the alloying treatment is performed to
second skin pass rolling,
530 - O.VxTmax 0...(2).
14. The method for producing the alloyed hot-dipped steel sheet having excellent fatigue
properties, elongation and collision properties as claimed in claim 13,
wherein an elongation rate is set to be in a range of 0.2 to 2.0% in the second skin
pass rolling.
Dated this 19* day of October 2011 t^ ^ \^
(DEBASHISH BANERJEE)
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]