U.S. patent application number 14/764637 was filed with the patent office on 2015-12-24 for high-strength hot-rolled steel sheet and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Chikara KAMI, Katsumi NAKAJIMA, Kazuhiko YAMAZAKI.
Application Number | 20150368739 14/764637 |
Document ID | / |
Family ID | 51261978 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150368739 |
Kind Code |
A1 |
NAKAJIMA; Katsumi ; et
al. |
December 24, 2015 |
HIGH-STRENGTH HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING
THE SAME
Abstract
There are provided a high-strength hot-rolled steel sheet having
high burring formability and a method for manufacturing the
high-strength hot-rolled steel sheet. The high-strength hot-rolled
steel sheet having high burring formability contains, on a mass
percent basis, C: 0.013% or more and less than 0.08%, Si: less than
0.5%, Mn: more than 0.8% and less than 1.2%, P: 0.05% or less, S:
0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03%
or more and 0.15% or less such that C, S, N, and Ti satisfy
0.05.ltoreq.Ti*<0.1 and C.times.(48/12)-0.16<Ti* (wherein
Ti*=Ti-N.times.(48/14)-S.times.(48/32), and C, S, N, and Ti denote
the amounts (% by mass) of the corresponding elements), the
remainder being Fe and incidental impurities, wherein the
high-strength hot-rolled steel sheet has a microstructure in which
a ferrite phase fraction is more than 90%, a carbide containing Ti
is precipitated, and 70% or more of the carbide has a grain size of
less than 9 nm.
Inventors: |
NAKAJIMA; Katsumi; (Tokyo,
JP) ; YAMAZAKI; Kazuhiko; (Tokyo, JP) ; KAMI;
Chikara; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
51261978 |
Appl. No.: |
14/764637 |
Filed: |
January 23, 2014 |
PCT Filed: |
January 23, 2014 |
PCT NO: |
PCT/JP2014/000336 |
371 Date: |
July 30, 2015 |
Current U.S.
Class: |
148/533 ;
148/334; 148/336; 148/337; 148/602 |
Current CPC
Class: |
C21D 8/0426 20130101;
C22C 38/06 20130101; C23C 2/02 20130101; C22C 38/14 20130101; C21D
9/46 20130101; C22C 38/02 20130101; C21D 2211/004 20130101; C21D
8/0463 20130101; C22C 38/001 20130101; Y02P 10/20 20151101; C22C
38/04 20130101; C22C 38/28 20130101; C22C 38/12 20130101; C21D
2211/005 20130101; C22C 38/00 20130101; C23C 2/06 20130101; C22C
38/08 20130101; C22C 38/002 20130101; C22C 38/005 20130101; Y02P
10/212 20151101; C22C 38/16 20130101; C23C 2/28 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/16 20060101 C22C038/16; C22C 38/14 20060101
C22C038/14; C22C 38/00 20060101 C22C038/00; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/28 20060101
C22C038/28; C22C 38/12 20060101 C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2013 |
JP |
2013-016455 |
Claims
1. A high-strength hot-rolled steel sheet comprising, on a mass
percent basis: C: 0.013% or more and less than 0.08%, Si: less than
0.5%, Mn: more than 0.8% and less than 1.2%, P: 0.05% or less, S:
0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03%
or more and 0.15% or less such that C, S, N, and Ti satisfy the
following formulae (1) and (2), the remainder being Fe and
incidental impurities, wherein the high-strength hot-rolled steel
sheet has a microstructure in which a ferrite phase fraction is
more than 90%, a carbide containing Ti is precipitated, and 70% or
more of the carbide has a grain size of less than 9 nm:
0.05.ltoreq.Ti*<0.1 (1) C.times.(48/12)-0.16<Ti* (2) wherein
Ti*=Ti-N.times.(48/14)-S.times.(48/32), and C, S, N, and Ti denote
the amounts (% by mass) of the corresponding elements.
2. The high-strength hot-rolled steel sheet according to claim 1,
wherein 50% by mass or more of Ti is precipitated as precipitates
containing Ti having a grain size of less than 20 nm.
3. The high-strength hot-rolled steel sheet according to claim 1,
further comprising at least one of V: 0.002% or more and 0.1% or
less and Nb: 0.002% or more and 0.1% or less on a mass percent
basis.
4. The high-strength hot-rolled steel sheet according to claim 1,
further comprising at least one of Cu: 0.005% or more and 0.2% or
less, Ni: 0.005% or more and 0.2% or less, Cr: 0.002% or more and
0.2% or less, and Mo: 0.002% or more and 0.2% or less on a mass
percent basis.
5. The high-strength hot-rolled steel sheet according to claim 1,
further comprising B: 0.0002% or more and 0.003% or less on a mass
percent basis.
6. The high-strength hot-rolled steel sheet according to claim 1,
further comprising at least one of Ca: 0.0002% or more and 0.005%
or less and REM: 0.0002% or more and 0.03% or less on a mass
percent basis.
7. A method for manufacturing a high-strength hot-rolled steel
sheet, comprising: heating steel having a composition according to
claim 1 to 1100.degree. C. or more, hot-rolling the steel at a
finish-rolling temperature of (Ar.sub.3+20.degree. C.) or more and
at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, and coiling the hot-rolled
steel sheet at a coiling temperature in the range of 560.degree. C.
to 720.degree. C.
8. A method for manufacturing a high-strength hot-rolled steel
sheet, comprising: heating steel having a composition according to
claim 1 to 1100.degree. C. or more, hot-rolling the steel at a
finish-rolling temperature of (Ar.sub.3+20.degree. C.) or more and
at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, coiling the hot-rolled
steel sheet at a coiling temperature in the range of 500.degree. C.
to 660.degree. C., annealing the hot-rolled steel sheet at a
soaking temperature of 750.degree. C. or less after pickling, and
plating the hot-rolled steel sheet by immersing the hot-rolled
steel sheet in a molten zinc bath.
9. The method for manufacturing a high-strength hot-rolled steel
sheet according to claim 8, wherein the plating treatment is
followed by alloying treatment.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of PCT
International Application No. PCT/JP2014/000336, filed Jan. 23,
2014, which claims priority to Japanese Patent Application No.
2013-016455, filed Jan. 31, 2013, the disclosures of each of these
applications being incorporated herein by reference in their
entireties for all purposes.
FIELD OF THE INVENTION
[0002] Aspects of the present invention relate to a high-strength
hot-rolled steel sheet having high burring formability and a method
for manufacturing the high-strength hot-rolled steel sheet. A
high-strength hot-rolled steel sheet according to aspects of the
present invention may be used in automotive body components, for
example, structural parts, such as members and frames of automotive
bodies, and chassis parts, such as suspensions. However, the
present invention is not limited to these applications.
BACKGROUND OF THE INVENTION
[0003] In recent years, for weight saving of automotive bodies,
high-strength steel sheets have been actively used as materials for
automotive parts. High-strength steel sheets are widely used as
automotive structural parts. For further weight saving of
automotive bodies, there is a strong demand for application of
high-strength steel sheets not only to structural parts but also to
chassis parts in which hot-rolled steel sheets are generally
used.
[0004] Most of automotive parts made of steel sheets are formed
into predetermined shapes by press forming or burring forming.
However, in general, higher-strength steel sheets have lower
workability. Thus, high-strength steel sheets for use in automotive
parts must have high workability as well as desired strength. For
example, because chassis parts are formed by severe processing,
both high strength and workability must be satisfied. In
particular, applicability of high-strength steel sheets to these
parts and mass productivity of these parts often depend on burring
formability.
[0005] Various microstructure control and reinforcement methods
have been used to improve the workability of high-strength
hot-rolled steel sheets. For example, these methods include use of
a complex structure of ductile ferrite and hard martensite, use of
a bainite microstructure, and precipitation strengthening of a
ferrite microstructure. However, high-strength hot-rolled steel
sheets having sufficient workability that can be applied to parts
to be subjected to severe burring forming, such as chassis parts,
cannot be manufactured in the related art. Thus, there is a demand
for high-strength hot-rolled steel sheet having high
workability.
[0006] Patent Literature 1 describes a hot-rolled steel sheet that
has a composition including, on a weight percent basis, C: 0.05% to
0.2%, Si: 0.01% to 0.5%, Mn: 0.01% or more and less than 0.5%, P:
0.05% or less, S: 0.01% or less, Al: 0.005% to 0.1%, N: 0.007% or
less, and Ti: 0.05% to 0.3%, and has a microstructure including a
limited amount of cementite precipitation. In the technique
proposed in Patent Literature 1, a decrease in austenite former Mn
and extension of an a region promote TiC precipitation after hot
rolling and before coiling, thereby securing the strength of the
steel sheet due to precipitation strengthening of TiC. Furthermore,
a decrease in the amount of cementite significantly improves the
hole expandability of the steel sheet. As a result, 400 to 800
N/mm.sup.2 high-strength hot-rolled steel sheets having high
workability can be manufactured.
[0007] Patent Literature 2 describes a technique for manufacturing
a hot-rolled steel sheet having a structure consisting essentially
of ferrite having an average grain size of 5 .mu.m or less by
heating, rolling, and cooling steel having a composition including,
on a mass percent basis, C: 0.01% to 0.10%, Si: 1.0% or less, Mn:
2.5% or less, P: 0.08% or less, S: 0.005% or less, Al: 0.015% to
0.050%, and Ti: 0.10 to 0.30%, and coiling the steel sheet at a
coiling temperature outside the temperature range in which TiC is
coherently precipitated in a matrix phase. In the technique
proposed in Patent Literature 2, a single phase structure of
ferrite having controlled grain size and morphology can impart high
stretch-flangeability to the hot-rolled steel sheet without
reducing the high strength of the hot-rolled steel sheet.
Furthermore, coherent precipitation of TiC in the main phase matrix
impairs ductility or stretch-flangeability.
[0008] Patent Literature 3 describes a hot-rolled steel sheet that
has a composition including, on a mass percent basis, C: 0.005% or
more and 0.050% or less, Si: 0.2% or less, Mn: 0.8% or less, P:
0.025% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.06% or
less, Ti: 0.05% or more, and 0.10% or less and has a matrix
consisting essentially of ferrite and a microstructure including
finely precipitated Ti carbide. This technique is based on the
knowledge that solute strengthening elements Mn and Si adversely
affect stretch-flangeability. Thus, the Mn and Si contents are
minimized, and fine Ti carbide is utilized to secure the strength.
The technique proposed in Patent Literature 3 can be used to
manufacture a high-strength hot-rolled steel sheet having a tensile
strength of 590 MPa or more and high stretch-flangeability.
Furthermore, the addition of B to the composition can suppress
coarsening of Ti carbide.
PATENT LITERATURES
[0009] PTL 1: Japanese Unexamined Patent Application Publication
No. 9-209076 [0010] PTL 2: Japanese Unexamined Patent Application
Publication No. 2002-105595 [0011] PTL 3: Japanese Unexamined
Patent Application Publication No. 2012-26034
SUMMARY OF THE INVENTION
[0012] In the technique proposed in Patent Literature 1, a decrease
in Mn content results in a high ferrite transformation temperature
and coarsening of TiC precipitated in the hot-rolled steel sheet.
In the manufacture of the hot-rolled steel sheet, TiC is mainly
formed during austenite.fwdarw.ferrite transformation in cooling
and coiling steps after hot rolling. Thus, a high ferrite
transformation temperature results in precipitation of TiC in a
high-temperature region and tends to result in coarsening of TiC.
When coarsening of TiC occurs in the hot-rolled steel sheet, high
burring formability cannot be achieved.
[0013] In the technique proposed in Patent Literature 2, the steel
sheet is coiled at a temperature at which TiC is not coherently
precipitated in the main phase matrix in the hot-rolled steel sheet
manufacturing process. Fine TiC that contributes to increased
strength of a steel sheet is not precipitated in a hot-rolled steel
sheet manufactured under such conditions. Thus, high strength and
burring formability cannot be simultaneously satisfied.
[0014] With respect to the technique proposed in Patent Literature
3, because of the low Mn content, it is difficult to uniformly
decrease the ferrite transformation temperature. This results in
low production stability and makes it impossible to finely control
the size of Ti carbide precipitated in the hot-rolled steel sheet.
It is also stated that the addition of B can suppress coarsening of
TiC. However, the addition of B tends to elongate ferrite grains,
and it is impossible to achieve high ductility. Thus, it is
difficult to manufacture a hot-rolled steel sheet having high
strength and burring formability.
[0015] Burring formability required for mass production of
automotive parts is not referred to in these techniques.
[0016] Burring formability of steel sheets has been principally
evaluated in a hole-expanding test by a method according to the
Japan Iron and Steel Federation standard. However, it is difficult
to say that the hole-expanding test accurately simulates a punching
process and a hole-expanding process in mass production of
automotive parts in actual production lines. Thus, there is a
problem that steel sheets that are experimentally shown to have
good burring formability according to the standard often suffer
from processing defects in mass production of automotive parts.
[0017] For example, evaluation of workability in a laboratory alone
is insufficient for mass production of parts. It is necessary to
ensure workability of materials also in consideration of variations
in processing conditions in mass production. Such problems are not
investigated in the related art. Thus, the resulting high-strength
hot-rolled steel sheets do not necessarily have desired strength
and workability required for mass production of automotive parts,
particularly burring formability (hereinafter also referred to as
mass production burring formability). Techniques that utilize Ti
carbide in known structures consisting essentially of ferrite, for
example, the techniques proposed in Patent Literatures 1 to 3,
cannot realize high production stability and mass production
burring formability of high-strength hot-rolled steel sheets.
[0018] As described above, many studies have been made on
hot-rolled steel sheets having high stretch-flangeability (burring
formability). However, high-strength hot-rolled steel sheets that
satisfy mass production burring formability, that is, severe
burring formability required in actual automotive part production
lines are not necessarily manufactured in the related art.
[0019] It is an aim of aspects of the present invention to
advantageously provide a high-strength hot-rolled steel sheet
having a tensile strength (TS) of 540 MPa or more and having high
burring formability, particularly high mass production burring
formability, and a method for manufacturing the high-strength
hot-rolled steel sheet.
[0020] The "mass production burring formability" herein is
evaluated as a burring ratio measured in a hole-expanding test
using a 60-degree conical punch after punching with a 50-mm.phi.
punch (clearance of stamping: 30%) and is different from burring
formability evaluated as a .lamda. value determined by a known
hole-expanding test method, for example, a hole-expanding test
method according to the Japan Iron and Steel Federation
standard.
Solution to Problem
[0021] The present inventors studied methods for evaluating Mass
production burring formability. Burring formability has been
evaluated as a .lamda. value, for example, measured by a
hole-expanding test method according to the Japan Iron and Steel
Federation standard. In this case, the punch diameter is 10
mm.phi.. However, the present inventors found that burring
formability in actual mass production settings for parts is not
correlated with the .lamda. value measured in laboratories
according to the Japan Iron and Steel Federation standard. It was
found that burring formability evaluated in a new hole-expanding
test that includes hole-expanding using a 60-degree conical punch
after punching with a 50-mm.phi. punch (clearance of stamping: 30%)
is closely correlated with mass production punchability and mass
production burring formability.
[0022] The present inventors also extensively studied various
factors that contribute to high strength and workability,
particularly mass production burring formability, of hot-rolled
steel sheets by evaluating mass production burring formability in
the new hole-expanding test.
[0023] Moreover, extensive studies have been made on means for
improving mass production burring formability of a hot-rolled steel
sheet based on a structure consisting essentially of a ductile
ferrite phase while reinforcing the hot-rolled steel sheet, with
consideration given to all the precipitates that can be
precipitated in the hot-rolled steel sheet, such as nitrides,
sulfides, carbides, and complex precipitates thereof (for example,
carbonitride).
[0024] It was found that a hot-rolled steel sheet having a tensile
strength of 540 MPa or more that satisfies severe mass production
burring formability required in actual automotive part production
lines can be manufactured by optimizing the balance between the
amount of C in the hot-rolled steel sheet and the amount of Ti
(Ti*) that contributes to the formation of carbide and increasing
the percentage of carbide having a grain size of less than 9 nm in
carbide precipitated in the hot-rolled steel sheet. It was also
found that mass production burring formability can be further
improved by controlling the size of not only carbide but also all
the precipitates that can be precipitated in a hot-rolled steel
sheet (nitrides, sulfides, carbides, and complex precipitates
thereof).
[0025] The present inventors also studied means for achieving a
desirable size of precipitates that are precipitated in a
hot-rolled steel sheet (nitrides, sulfides, carbides, and complex
precipitates thereof), that is, a size required to impart desired
strength (e.g., tensile strength of 540 MPa or more) and good mass
production burring formability to the hot-rolled steel sheet. As a
result, it was found that it is desirable to properly control the
Mn content and C, S, N, and Ti contents of a hot-rolled steel sheet
and optimize the hot rolling conditions and the cooling and coiling
conditions after hot rolling.
[0026] The following are non-limiting embodiments of the present
invention.
[1] A high-strength hot-rolled steel sheet, containing: on a mass
percent basis, C: 0.013% or more and less than 0.08%, Si: less than
0.5%, Mn: more than 0.8% and less than 1.2%, P: 0.05% or less, S:
0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03%
or more and 0.15% or less such that C, S, N, and Ti satisfy the
following formulae (1) and (2), the remainder being Fe and
incidental impurities, wherein the high-strength hot-rolled steel
sheet has a microstructure in which a ferrite phase fraction is
more than 90%, a carbide containing Ti is precipitated, and 70% or
more of the carbide has a grain size of less than 9 nm.
0.05.ltoreq.Ti*<0.1 (1)
C.times.(48/12)-0.16<Ti* (2)
wherein Ti*=Ti-N.times.(48/14)-S.times.(48/32), and C, S, N, and Ti
denote the amounts (% by mass) of the corresponding elements. [2]
The high-strength hot-rolled steel sheet according to [1], wherein
50% by mass or more of Ti is precipitated as precipitates
containing Ti having a grain size of less than 20 nm. [3] The
high-strength hot-rolled steel sheet according to [1] or [2],
further containing at least one of V: 0.002% or more and 0.1% or
less and Nb: 0.002% or more and 0.1% or less on a mass percent
basis. [4] The high-strength hot-rolled steel sheet according to
any one of [1] to [3], further containing at least one of Cu:
0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or
less, Cr: 0.002% or more and 0.2% or less, and Mo: 0.002% or more
and 0.2% or less on a mass percent basis. [5] The high-strength
hot-rolled steel sheet according to any one of [1] to [4], further
containing B: 0.0002% or more and 0.003% or less on a mass percent
basis. [6] The high-strength hot-rolled steel sheet according to
any one of [1] to [5], further containing at least one of Ca:
0.0002% or more and 0.005% or less and REM: 0.0002% or more and
0.03% or less on a mass percent basis. [7] A method for
manufacturing a high-strength hot-rolled steel sheet, including:
heating steel having a composition according to any one of [1] and
[3] to [6] to 1100.degree. C. or more, hot-rolling the steel at a
finish-rolling temperature of (Ar.sub.3+20.degree. C.) or more and
at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, and coiling the hot-rolled
steel sheet at a coiling temperature in the range of 560.degree. C.
to 720.degree. C. [8] A method for manufacturing a high-strength
hot-rolled steel sheet, comprising: heating steel having a
composition according to any one of [1] and [3] to [6] to
1100.degree. C. or more, hot-rolling the steel at a finish-rolling
temperature of (Ar.sub.3+20.degree. C.) or more and at a total
reduction ratio of 60% or less at last two finish rolling stands,
cooling the hot-rolled steel sheet at an average cooling rate of
40.degree. C./s or more, coiling the hot-rolled steel sheet at a
coiling temperature in the range of 500.degree. C. to 660.degree.
C., annealing the hot-rolled steel sheet at a soaking temperature
of 750.degree. C. or less after pickling, and plating the
hot-rolled steel sheet by immersing the hot-rolled steel sheet in a
molten zinc bath. [9] The method for manufacturing a high-strength
hot-rolled steel sheet according to [8], wherein the plating
treatment is followed by alloying treatment.
[0027] Aspects of the present invention provide a high-strength
hot-rolled steel sheet having a tensile strength of 540 MPa or more
and high burring formability such that the high-strength hot-rolled
steel sheet can be subjected to processing in mass production of
automotive parts. Thus, in one embodiment, a high-strength
hot-rolled steel sheet can be applied to structural parts, such as
members and frames of automotive bodies, and chassis parts, such as
suspensions. Embodiments of the present invention may contribute
greatly to weight saving of these parts.
[0028] Aspects of the present invention can provide a hot-rolled
steel sheet having a tensile strength of 540 MPa or more and good
mass production burring formability. Thus, the high-strength
hot-rolled steel sheet can be applied not only to automotive parts
but also to other applications. Thus, the present invention has
industrially advantageous effects.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Non-limiting embodiments of the present invention will be
further described below.
[0030] A high-strength hot-rolled steel sheet according to one
embodiment contains, on a mass percent basis, C: 0.013% or more and
less than 0.08%, Si: less than 0.5%, Mn: more than 0.8% and less
than 1.2%, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less,
Al: 0.1% or less, and Ti: 0.03% or more and 0.15% or less such that
C, S, N, and Ti satisfy the following formulae (1) and (2), the
remainder being Fe and incidental impurities, wherein the
high-strength hot-rolled steel sheet has a microstructure in which
a ferrite phase fraction is more than 90%, a carbide containing Ti
is precipitated, and 70% or more of the carbide has a grain size of
less than 9 nm.
0.05.ltoreq.Ti*<0.1 (1)
C.times.(48/12)-0.16<Ti* (2)
[0031] wherein Ti*=Ti-N.times.(48/14)-S.times.(48/32), and C, S, N,
and Ti denote the amounts (% by mass) of the corresponding
elements.
[0032] First, examples of the composition of a hot-rolled steel
sheet according to aspects of the present invention will be
described below. Unless otherwise specified, the percentages of the
components are on a mass percent basis.
C: 0.013% or More and Less than 0.08%
[0033] C is an important element that forms an appropriate carbide
in a hot-rolled steel sheet and secures the strength of the steel
sheet. In order to achieve the desired tensile strength (540 MPa or
more), the C content is 0.013% or more. However, a C content of
0.08% or more results in poor workability and undesired burring
formability of a hot-rolled steel sheet. Thus, the C content is
0.013% or more and less than 0.08%, preferably 0.03% or more and
0.07% or less.
Si: Less than 0.5%
[0034] A Si content of 0.5% or more results in very low surface
quality of a hot-rolled steel sheet, which adversely affects
fatigue characteristics, chemical conversion treatability, and
corrosion resistance. Si increases the ferrite transformation
temperature and thereby adversely affects the formation of fine
precipitates. Thus, the Si content is less than 0.5%, preferably
0.001% or more and less than 0.1%, more preferably 0.001% or more
and less than 0.05%.
Mn: More than 0.8% and Less than 1.2%
[0035] Mn is an important element. Mn significantly influences
precipitation of a carbide containing Ti, through control of
austenite-to-ferrite transformation temperatures.
[0036] In the case of a hot-rolled steel sheet containing Ti, a
carbide containing Ti is mainly precipitated by
austenite.fwdarw.ferrite transformation in cooling and coiling
steps after finish rolling in a hot-rolled steel sheet
manufacturing process. Among carbides precipitated in a hot-rolled
steel sheet, fine carbide contributes to high strength of the
hot-rolled steel sheet, but coarse carbide does not contribute to
high strength and adversely affects the workability of the
hot-rolled steel sheet.
[0037] A high austenite-ferrite transformation temperature results
in precipitation of a carbide containing Ti in a high-temperature
region and consequently coarsening of the carbide containing Ti.
Thus, in order to decrease the size of the carbide containing Ti,
it is preferable to decrease the austenite-ferrite transformation
temperature.
[0038] Mn is an element that has an effect of decreasing the
austenite-ferrite transformation temperature. A Mn content of 0.8%
or less results in an insufficient decrease in the
austenite-ferrite transformation temperature. As a result, it is
difficult for a carbide containing Ti to have a desirable size.
Thus, it is difficult to provide a high-strength hot-rolled steel
sheet having high mass production burring formability. On the other
hand, a Mn content of 1.2% or more results in saturation of the
effect and increased costs. An excessively high Mn content of 1.2%
or more also results in increased Mn segregation in the central
portion in the thickness direction. This center segregation impairs
a punched surface before burring forming and is therefore
responsible for low mass production burring formability. Thus, the
Mn content is more than 0.8% and less than 1.2%, preferably more
than 0.8% and less than 1.0%.
P: 0.05% or Less
[0039] P promotes low workability of a hot-rolled steel sheet due
to segregation. Thus, the P content is 0.05% or less, preferably
0.001% or more and 0.03% or less. In the case of a galvanized steel
sheet formed by galvanizing treatment of a hot-rolled steel sheet,
the P content is preferably 0.005% or more, more preferably 0.01%
or more, still more preferably 0.015% or more, in terms of
platability.
S: 0.005% or Less
[0040] S forms a sulfide and decreases the workability of a
hot-rolled steel sheet. Thus, the S content is 0.005% or less,
preferably 0.0001% or more and 0.003% or less, more preferably
0.0001% or more and 0.0015% or less.
N: 0.01% or Less
[0041] An excessively high N content of more than 0.01% results in
the formation of a large amount of nitride in a hot-rolled steel
sheet manufacturing process, low hot ductility, and very low
burring formability of a hot-rolled steel sheet due to coarsening
of nitride. Thus, the N content is 0.01% or less, preferably
0.0001% or more and 0.006% or less, more preferably 0.0001% or more
and 0.004% or less.
Al: 0.1% or Less
[0042] Al is a useful element as a deoxidizing agent for steel.
However, an Al content of more than 0.1% makes casting of steel
difficult and results in a large amount of residual inclusion in
steel and low surface quality and workability of a hot-rolled steel
sheet. Thus, the Al content is 0.1% or less, preferably 0.001% or
more and 0.06% or less.
Ti: 0.03% or More and 0.15% or Less
[0043] Ti is an important element in the present invention. Ti
forms fine carbide and contributes to increased strength of a
hot-rolled steel sheet. In order to achieve the desired strength of
a hot-rolled steel sheet (tensile strength of 540 MPa or more), the
Ti content is 0.03% or more. However, a Ti content of more than
0.15% tends to result in the remaining coarse carbide in a
hot-rolled steel sheet. Coarse carbide has no strength increasing
effect and greatly impairs the workability, toughness, and
weldability of a hot-rolled steel sheet. Thus, the Ti content is
0.03% or more and 0.15% or less, preferably 0.04% or more and 0.12%
or less.
[0044] A hot-rolled steel sheet according to one embodiment
contains C, S, N, and Ti in the ranges described above so as to
satisfy the formulae (1) and (2). The formulae (1) and (2) are
desirably satisfied in order to achieve high strength and good mass
production burring formability of a hot-rolled steel sheet and are
beneficial indicators. In the formulae (1) and (2),
Ti*=Ti-N.times.(48/14)-S.times.(48/32), and C, S, N, and Ti denote
the amounts (%) of the corresponding elements.
0.05.ltoreq.Ti*<0.1 (1)
[0045] As described below, in one embodiment, a predetermined
amount of Ti is added to steel, and carbide in steel is dissolved
by heating before hot rolling. A carbide containing Ti is mainly
precipitated during coiling after hot rolling. However, Ti added to
steel does not entirely contribute to the formation of carbide and
is partly consumed by forming nitride or sulfide. This is because
Ti is likely to form nitride or sulfide rather than carbide in a
higher temperature region than the coiling temperature. Thus, Ti
forms nitride or sulfide before the coiling step in the production
of a hot-rolled steel sheet. Thus, the minimum amount of Ti that
can contribute to the formation of carbide out of Ti added to steel
can be represented by Ti*
(=Ti-N.times.(48/14)-S.times.(48/32)).
[0046] A hot-rolled steel sheet cannot have the desired strength
(tensile strength of 540 MPa or more) at Ti* of less than 0.05.
Thus, in one embodiment, Ti* is 0.05 or more, preferably 0.055 or
more. However, a hot-rolled steel sheet has a tensile strength of
700 MPa or more at Ti* of 0.1 or more. A hot-rolled steel sheet
having such excessively high strength has poor workability.
Furthermore, an excessively high Ti* results in coarsening of
precipitates containing Ti, such as Ti carbide, Ti carbonitride, Ti
nitride, and Ti sulfide, which results in low mass production
burring formability. Thus, in in one embodiment, Ti* is less than
0.1.
C.times.(48/12)-0.16<Ti* (2)
[0047] The formula (2) represents the relationship between the
amount of Ti* and the amount of C. An excessively higher amount of
C than the amount of Ti* results in coarsening of Ti carbide and Ti
carbonitride, precipitation of coarse cementite or pearlite, and a
significant reduction in the workability of a hot-rolled steel
sheet, such as mass production burring formability. In one
embodiment, C.times.(48/12)-0.16<Ti*, preferably
C.times.(48/12)-0.15<Ti*. However, an excessively high amount of
Ti than the amount of C tends to result in low toughness and
weldability of a hot-rolled steel sheet. Thus,
Ti*<C.times.(48/12)+0.08 is preferred. More preferably,
Ti*<C.times.(48/12)+0.06.
[0048] The following are base components of a hot-rolled steel
sheet according to non-limiting embodiments of the present
invention. A hot-rolled steel sheet according to one embodiment may
contain at least one of V: 0.002% or more and 0.1% or less and Nb:
0.002% or more and 0.1% or less, if desirable.
[0049] V and Nb are effective in decreasing the size of crystal
grains and improving the toughness of a hot-rolled steel sheet.
Thus, V and Nb may be added as desired. Part of added V and/or Nb
is precipitated together with Ti as fine complex carbide or complex
precipitates and thereby contributes to precipitation
strengthening. In order to produce such an effect, the V content is
preferably 0.002% or more, and the Nb content is preferably 0.002%
or more. However, a V or Nb content of more than 0.1% is not worth
the cost. Thus, the V content is preferably 0.002% or more and 0.1%
or less, more preferably 0.002% or more and 0.08% or less. The Nb
content is preferably 0.002% or more and 0.1% or less, more
preferably 0.002% or more and 0.08% or less.
[0050] A hot-rolled steel sheet according to one embodiment may
contain at least one of Cu: 0.005% or more and 0.2% or less, Ni:
0.005% or more and 0.2% or less, Cr: 0.002% or more and 0.2% or
less, and Mo: 0.002% or more and 0.2% or less, if desirable.
[0051] Cu and Ni are elements that contribute to high strength of a
hot-rolled steel sheet and may be added, if desirable. In order to
produce such an effect, the Cu content is preferably 0.005% or
more, and the Ni content is preferably 0.005% or more. However, a
Cu or Ni content of more than 0.2% may result in surface layer
cracking during hot rolling in the production of a hot-rolled steel
sheet. Thus, the Cu content is preferably 0.005% or more and 0.2%
or less, more preferably 0.005% or more and 0.1% or less. The Ni
content is preferably 0.005% or more and 0.2% or less, more
preferably 0.005% or more and 0.15% or less.
[0052] Cr and Mo are carbide formation elements, contribute to high
strength of a hot-rolled steel sheet, and may be added, if
desirable. In order to produce such an effect, the Cr content is
preferably 0.002% or more, and the Mo content is preferably 0.002%
or more. However, a Cr or Mo content of more than 0.2% is not worth
the cost. Thus, the Cr content is preferably 0.002% or more and
0.2% or less, more preferably 0.002% or more and 0.1% or less. The
Mo content is preferably 0.002% or more and 0.2% or less, more
preferably 0.002% or more and 0.1% or less.
[0053] A hot-rolled steel sheet according to one embodiment may
contain B: 0.0002% or more and 0.003% or less, if desirable.
[0054] B is an element that retards austenite-ferrite
transformation of steel. B decreases the precipitation temperature
of a carbide containing Ti by suppressing austenite-ferrite
transformation and contributes to a reduction in the size of the
carbide. In order to produce such an effect, the B content is
preferably 0.0002% or more. However, a B content of more than
0.003% results in a strong bainite transformation effect of B,
making it difficult for a hot-rolled steel sheet to have a
structure consisting essentially of a ferrite phase. The B content
is preferably 0.0002% or more and 0.003% or less, more preferably
0.0002% or more and 0.002% or less.
[0055] A hot-rolled steel sheet according to one embodiment may
contain at least one of Ca: 0.0002% or more and 0.005% or less and
REM: 0.0002% or more and 0.03% or less, if desirable.
[0056] Ca and REM are elements that are effective in morphology
control of an inclusion in steel and contribute to improved
workability of a hot-rolled steel sheet. In order to produce such
an effect, the Ca content is preferably 0.0002% or more, and the
REM content is preferably 0.0002% or more. However, a Ca content of
more than 0.005% or a REM content of more than 0.03% may result in
an increased inclusion in steel and low workability of a hot-rolled
steel sheet. Thus, the Ca content is preferably 0.0002% or more and
0.005% or less, more preferably 0.0002% or more and 0.003% or less.
The REM content is preferably 0.0002% or more and 0.03% or less,
more preferably 0.0002% or more and 0.003% or less.
[0057] The remainder may be Fe and incidental impurities. Examples
of the incidental impurities include W, Co, Ta, Sn, Sb, Zr, and O.
The amount of each of the incidental impurities may be 0.1% or
less.
[0058] Next, the microstructure of a hot-rolled steel sheet
according to non-limiting embodiments of the present invention will
be described below.
[0059] A hot-rolled steel sheet according to one embodiment has a
microstructure in which the ferrite phase fraction is more than
90%, a carbide containing Ti is precipitated, and 70% or more of
the carbide has a grain size of less than 9 nm. A hot-rolled steel
sheet preferably has a microstructure in which 50% by mass or more
of Ti is precipitated as precipitates having a grain size of less
than 20 nm.
Ferrite Phase Fraction: More than 90%
[0060] The burring formability of a hot-rolled steel sheet can be
effectively improved when the hot-rolled steel sheet has a
microstructure including a ductile ferrite phase. In order to
achieve a preferable high mass production burring formability, the
ferrite fraction in the microstructure of a hot-rolled steel sheet
is more than 90%, preferably more than 94%, more preferably more
than 96% by area. It is desirable that the ferrite grains have a
polygonal shape from the perspective of burring formability. It is
also desirable that the ferrite grain size be as small as possible.
The hot-rolled steel sheet preferably has a single phase structure
of ferrite in terms of burring formability. In order to improve
punchability, the ferrite fraction is preferably 99% by area or
less.
[0061] A hot-rolled steel sheet according to one embodiment may
have a microstructure other than the ferrite phase, such as
cementite, pearlite, bainite, martensite, and/or retained
austenite. Although these microstructures in the hot-rolled steel
sheet impair burring formability, these microstructures may
constitute approximately less than 10%, preferably less than 6%,
more preferably less than 4% by area in total.
Carbide Containing Ti
[0062] In one embodiment, the desired strength (tensile strength of
540 MPa or more) of a hot-rolled steel sheet is achieved by
precipitation of a carbide containing Ti in the hot-rolled steel
sheet. The carbide containing Ti is mainly precipitated carbide
resulting from austenite.fwdarw.ferrite transformation in the
cooling and coiling steps after finish rolling in a hot-rolled
steel sheet manufacturing process.
[0063] In order to make the maximum use of the precipitation
strengthening effect and optimize the balance between strength and
workability (mass production burring formability), it may be
preferable to reduce the size of a carbide containing Ti
precipitated in a hot-rolled steel sheet. As a result of extensive
studies, the present inventors found that 70% by number or more,
preferably 80% or more, of a carbide containing Ti has a grain size
of less than 9 nm in order to achieve the desirable
characteristics. The "carbide containing Ti" includes complex
carbides containing Ti and at least one of Nb, V, Cr, and Mo as
well as Ti carbide.
Precipitate Containing Ti
[0064] The size of precipitates containing Ti may be controlled to
further improve the mass production burring formability of a
hot-rolled steel sheet.
[0065] As described above, in the case of a hot-rolled steel sheet
made of steel containing Ti, in addition to carbide (carbide
containing Ti) that contributes to high strength of a hot-rolled
steel sheet, nitride, carbonitride, and sulfide containing Ti are
precipitated. In the production of a hot-rolled steel sheet, these
nitride, carbonitride, and sulfide are precipitated faster than
carbide containing Ti. Thus, nitride, carbonitride, and sulfide
containing Ti are precipitated in a higher temperature range than
carbide and are therefore easily coarsened and tend to impair mass
production burring formability.
[0066] As a result of extensive studies, the present inventors
found that the control of the amount and size of these precipitates
is very effective in improving mass production burring formability.
Preferably, 50% by mass or more, more preferably 60% by mass or
more and 85% by mass or less, of Ti in a hot-rolled steel sheet is
preferably precipitated as precipitates containing Ti having a
grain size of less than 20 nm. The precipitates containing Ti
having a grain size of less than 20 nm are mostly carbide
containing Ti and also include nitride, carbonitride, and sulfide
containing Ti.
[0067] The precipitates containing Ti may be precipitates of Ti
carbide, Ti nitride, Ti sulfide, and/or Ti carbonitride, and/or
complex precipitates, such as complex carbide, complex nitride,
complex sulfide, and/or complex carbonitride containing Ti and at
least one of Nb, V, Cr, and Mo.
[0068] Even when precipitates having a grain size of 20 nm or more
out of precipitates containing Ti are precipitated, it is surmised
that a proper amount of precipitates contribute to improved
punchability before burring forming and consequently contribute to
improved burring formability.
[0069] Formation of a coated layer on a surface of a hot-rolled
steel sheet in order to impart corrosion resistance does not reduce
the advantages of the present invention. The type of coated layer
formed on a surface of a hot-rolled steel sheet is not particularly
limited and may be galvanic electroplating or hot-dip plating. The
hot-dip plating may be hot-dip galvanization. The coated layer may
also be galvannealed steel, which was subjected to alloying
treatment after plating.
[0070] Methods for manufacturing a hot-rolled steel sheet will be
described below with reference to non-limiting embodiments.
[0071] One embodiment includes heating steel having the composition
described above to 1100.degree. C. or more, hot-rolling the steel
at a finish-rolling temperature of (Ar.sub.3+20.degree. C.) or more
and at a total reduction ratio of 60% or less at last two finish
rolling stands, cooling the hot-rolled steel sheet at an average
cooling rate of 40.degree. C./s or more, and coiling the hot-rolled
steel sheet at a coiling temperature in the range of 560.degree. C.
to 720.degree. C.
[0072] The steel may be melted by any method, for example, in a
converter, electric furnace, or induction furnace. After that,
secondary smelting is preferably performed with vacuum degassing
equipment. Subsequent casting is preferably performed in a
continuous casting process in terms of productivity and quality. A
blooming method can also be used. A slab (steel) to be casted may
be a general slab having a thickness in the range of approximately
200 to 300 mm or a thin slab having a thickness of approximately 30
mm. In the case of a thin slab, rough rolling may be omitted. A
slab after casting may be subjected to hot direct rolling or may be
subjected to hot rolling after reheating in a furnace.
Steel Heating Temperature: 1100.degree. C. or More
[0073] Steel thus produced is subjected to hot rolling. In one
embodiment, it is desirable to heat the steel (slab) before hot
rolling and redissolve carbide in the steel. At a steel heating
temperature of less than 1100.degree. C., carbide is not
redissolved in the steel, and desirable fine carbide cannot be
formed in the cooling and coiling steps after hot rolling. Thus,
the steel heating temperature is 1100.degree. C. or more,
preferably 1200.degree. C. or more, more preferably 1240.degree. C.
or more.
[0074] However, an excessively high steel heating temperature
results in excessively accelerated oxidation of the surface of a
steel sheet and very poor surface quality and adversely affects the
workability of a hot-rolled steel sheet. Thus, the steel heating
temperature is preferably 1350.degree. C. or less.
[0075] After heating of steel, the steel is subjected to hot
rolling, which is composed of rough rolling and finish rolling. The
rough rolling conditions are not particularly limited. As described
above, when steel is a thin slab, rough rolling may be omitted. In
the finish rolling, the finish-rolling temperature is
(Ar.sub.3+20.degree. C.) or more, and the total reduction ratio at
last two stands of a finish rolling mill is 60% or less.
Finish-Rolling Temperature: (Ar.sub.3+20.degree. C.) or More
[0076] At a finish-rolling temperature of less than
(Ar.sub.3+20.degree. C.), austenite.fwdarw.ferrite transformation
in the cooling and coiling steps after hot rolling is ferrite
transformation from unrecrystallized austenite grains. In such a
case, desired fine carbide cannot be formed, and a hot-rolled steel
sheet cannot have a desirable strength (e.g., tensile strength of
540 MPa or more). Thus, the finish-rolling temperature is
(Ar.sub.3+20.degree. C.) or more, preferably (Ar.sub.3+40.degree.
C.) or more. However, an excessively high finish-rolling
temperature results in coarsening of crystal grains and adversely
affects the punchability of a hot-rolled steel sheet. Thus, the
finish-rolling temperature is (Ar.sub.3+140.degree. C.) or
less.
[0077] The Ar.sub.3 transformation point herein refers to a
transformation temperature at a change point of a thermal expansion
curve measured in a thermecmastor test (thermo-mechanical
simulation test) at a cooling rate of 5.degree. C./s.
Total Reduction Ratio at Last Two Finish Rolling Stands: 60% or
Less
[0078] When the total reduction ratio at last two finish rolling
stands exceeds 60%, this results in increased residual strain and
accelerates ferrite transformation from unrecrystallized austenite
grains. Thus, the total reduction ratio at last two stands of a
finish rolling mill is 60% or less, preferably 50% or less.
Average Cooling Rate: 40.degree. C./s or More
[0079] When the average cooling rate in cooling after hot rolling
is less than 40.degree. C./s, this results in a high ferrite
transformation temperature. As a result, carbide is precipitated in
a high temperature region, desired fine carbide cannot be formed,
and a hot-rolled steel sheet cannot have the desired strength
(e.g., tensile strength of 540 MPa or more). Thus, the average
cooling rate is 40.degree. C./s or more, preferably 50.degree. C./s
or more. However, at an excessively high average cooling rate, it
may be difficult to achieve the desired ferrite microstructure.
Thus, the average cooling rate is 150.degree. C./s or less.
[0080] The average cooling rate herein refers to the average
cooling rate between the finish-rolling temperature and the coiling
temperature.
[0081] In one embodiment, a carbide containing Ti is precipitated
in a period from immediately before coiling to the beginning of the
coiling step by decreasing the ferrite transformation temperature
so as to be close to the coiling temperature at the average cooling
rate. This can prevent precipitation and coarsening of the carbide
containing Ti in a high temperature region. Thus, the resulting
hot-rolled steel sheet can contain precipitated fine carbide.
Coiling Temperature: 560.degree. C. to 720.degree. C.
[0082] As described above, in one embodiment, fine carbide
containing Ti is mainly precipitated in a period from immediately
before coiling to the beginning of the coiling step. Thus, in order
to precipitate a large amount of fine carbide containing Ti, the
coiling temperature is controlled in a temperature range suitable
for precipitation of the carbide containing Ti. At a coiling
temperature of less than 560.degree. C. or more than 720.degree.
C., fine carbide that contributes to high strength of steel is not
sufficiently precipitated, and the hot-rolled steel sheet may not
have a desirable strength. For these reasons, the coiling
temperature ranges from 560.degree. C. to 720.degree. C.,
preferably 600.degree. C. to 700.degree. C.
[0083] In one embodiment, a hot-rolled steel sheet after coiling
may be subjected to pickling and annealing treatment and then to
plating treatment by immersion in a molten zinc bath. After the
plating treatment, the hot-rolled steel sheet may be subjected to
alloying treatment. When the plating treatment is performed, the
coiling temperature ranges from 500.degree. C. to 660.degree. C.,
and the soaking temperature for the annealing treatment is
750.degree. C. or less.
Coiling Temperature: 500.degree. C. to 660.degree. C.
[0084] A higher coiling temperature facilitates the formation of an
internal oxidation layer in a hot-rolled steel sheet. The internal
oxidation layer may promote plating defects. For example, a coiling
temperature of more than 660.degree. C. results in low plating
quality. On the other hand, a low coiling temperature is preferred
in order to prevent plating defects. However, a coiling temperature
of less than 500.degree. C. results in insufficient precipitation
of a carbide containing Ti, and a hot-rolled steel sheet may not
have a desirable strength. Thus, when plating treatment is
performed after coiling, the coiling temperature ranges from
500.degree. C. to 660.degree. C., preferably 500.degree. C. to
600.degree. C.
Soaking Temperature: 750.degree. C. or Less
[0085] As described above, when the coiling temperature is lowered
for plating treatment, fine carbide that contributes to high
strength of a hot-rolled steel sheet (carbide containing Ti) may be
insufficiently precipitated during coiling. Thus, in one
embodiment, the desired strength (tensile strength of 540 MPa or
more) of a hot-rolled steel sheet after plating treatment is
achieved by precipitating fine carbide (carbide containing Ti)
during annealing treatment before the plating treatment. When the
soaking temperature for annealing treatment exceeds 750.degree. C.,
precipitated carbide (carbide containing Ti) is coarsened, and a
hot-rolled steel sheet has low strength. Thus, the soaking
temperature for annealing treatment is 750.degree. C. or less,
preferably 720.degree. C. or less. In order to promote
precipitation of fine carbide (carbide containing Ti), the soaking
temperature for annealing treatment is preferably 600.degree. C. or
more. The holding time at the soaking temperature preferably ranges
from 10 to 1000 seconds.
[0086] After the annealing treatment, the steel sheet is immersed
in a hot-dip galvanizing bath to form a hot-dip galvanized layer on
the surface of the steel sheet. After immersion in the hot-dip
galvanizing bath, the steel sheet may be subjected to alloying
treatment. The annealing treatment and plating treatment are
preferably performed in a continuous hot-dip galvanizing line.
[0087] The type of plating is not limited to hot-dip galvanization
or galvannealing described above and may be electrogalvanizing.
[0088] The plating treatment conditions, the alloying treatment
conditions, and other manufacturing conditions are not particularly
limited and may be general conditions.
EXAMPLES
[0089] Steel slabs (Nos. A to P) containing the components listed
in Table 1 and having the Ar.sub.3 transformation point listed in
Table 1 were heated to a temperature in the range of 1180.degree.
C. to 1290.degree. C., and hot-rolled steel sheets (Nos. 1 to 22)
were formed under the hot-rolling conditions listed in Table 2. The
hot-rolled steel sheets had a thickness in the range of 2.0 to 4.5
mm. The Ar.sub.3 transformation points listed in Table 1 were
determined as described above. Part of the hot-rolled steel sheets
(Nos. 3, 4, 9 to 11, 16, 18, and 19) were subjected to pickling and
were then subjected to annealing treatment at a soaking temperature
listed in Table 2 and hot-dip galvanizing treatment in a hot-dip
galvanization line. In the hot-dip galvanizing treatment, each of
the hot-rolled steel sheets subjected to the annealing treatment
was immersed in a galvanizing bath (0.1% by mass Al--Zn) at
480.degree. C., and a hot-dip galvanized layer was formed on both
faces of the steel sheet at 45 g/m.sup.2. Part of the hot-rolled
steel sheets (Nos. 9 to 11, 16, 18, and 19) were subjected to the
hot-dip galvanizing treatment and alloying treatment. The alloying
treatment temperature was 520.degree. C.
[0090] Test specimens were taken from the hot-rolled steel sheets
(Nos. 1 to 22) and were subjected to microstructure observation, a
tensile test, and a hole-expanding test. The microstructure
observation method and various test methods were as follows:
(i) Microstructure Observation
Ferrite Phase Fraction
[0091] Scanning electron microscope (SEM) test specimens were taken
from the hot-rolled steel sheets. A vertical cross section of each
of the test specimens parallel to the rolling direction was
polished and was subjected to nital etching. SEM photographs were
taken in 10 visual fields at a quarter thickness in the depth
direction and at a magnification ratio of 3000. A ferrite phase and
a non-ferrite phase were separated by image analysis. The fraction
of each of the phases (area fraction) was determined.
Carbide Containing Ti
[0092] Thin film samples were prepared from the hot-rolled steel
sheets (at a quarter thickness in the depth direction). Photographs
were taken in 10 visual fields with a transmission electron
microscope at a magnification ratio of 200,000.
[0093] The number of carbide grains containing Ti (N.sub.0) was
determined from the photographs. The grain size of each carbide
grain containing Ti was determined as an equivalent circle diameter
by image processing. The number of carbide grains having a grain
size of less than 9 nm (N.sub.1) out of the carbide grains
containing Ti was determined. The ratio of the number of carbide
grains having a grain size of less than 9 nm to the number of
carbide grains containing Ti (N.sub.1/N.sub.0.times.100(%)) was
calculated from these numbers (N.sub.0 and N.sub.1).
Precipitates Containing Ti
[0094] Precipitates were extracted from the hot-rolled steel sheets
by constant-current electrolysis using an AA electrolyte solution
(an ethanol solution of acetylacetone-tetramethylammonium
chloride). The extract was passed through a filter having a pore
size 20 nm. Precipitates having a size of less than 20 nm were
separated in this manner, and the amount of Ti in the precipitates
having a size of less than 20 nm was measured by
inductively-coupled plasma optical emission spectrometry (ICP). The
ratio (percentage) of Ti in the precipitates having a size of less
than 20 nm was determined by dividing the amount of Ti in the
precipitates having a size of less than 20 nm by the amount of Ti
in the hot-rolled steel sheet.
(ii) Tensile Test
[0095] Three JIS No. 5 test pieces for tensile test were taken from
each of the hot-rolled steel sheets such that the tensile direction
is a direction perpendicular to the rolling direction. The tensile
strength and total elongation were measured in a tensile test
(strain rate: 10 mm/min) according to JIS Z 2241 (2011). Each of
the hot-rolled steel sheets were subjected to measurements three
times in the tensile test. The tensile strength (TS) and total
elongation (El) were averages of the three measurements.
(iii) Hole-Expanding Test (Evaluation of Mass Production Burring
Formability)
[0096] Test specimens (size: 150 mm.times.150 mm) were taken from
the hot-rolled steel sheets. A hole having an initial diameter
d.sub.0 was formed in each of the test specimens by punching with a
50-mm.phi. punch (clearance of stamping: 30%). The hole was then
expanded by inserting a conical punch having a vertex angle of 60
degrees into the hole from the punched side. A hole diameter
d.sub.1 at which a crack penetrated the steel sheet (test specimen)
in the thickness direction was measured. The burring ratio (%) was
calculated using the following equation.
Burring ratio (%)={(d.sub.1-d.sub.0)/d.sub.0}.times.100
[0097] A burring ratio of 60% or more was considered to be high
mass production burring formability.
[0098] Table 3 shows the results.
TABLE-US-00001 TABLE 1 Chemicalcomposition (mass %) Ar.sub.3
Formulae (1) Steel C Si Mn P S Al N Ti Others Ti* (.degree. C.) and
(2)*1 Remarks A 0.031 0.11 1.19 0.045 0.0035 0.049 0.0018 0.062 --
0.051 875 .smallcircle. Inventive steel B 0.042 0.45 1.04 0.029
0.0004 0.019 0.0036 0.072 V: 0.009 0.059 878 .smallcircle.
Inventive steel C 0.039 0.03 0.84 0.021 0.0017 0.039 0.0033 0.088
B: 0.0009 0.074 861 .smallcircle. Inventive steel D 0.030 0.19 0.81
0.033 0.0005 0.033 0.0023 0.089 -- 0.080 861 .smallcircle.
Inventive steel E 0.049 0.01 0.99 0.009 0.0009 0.044 0.0037 0.096
-- 0.082 882 .smallcircle. Inventive steel F 0.034 0.03 1.08 0.012
0.0014 0.023 0.0072 0.085 Ni: 0.19, REM: 0.001 0.058 844
.smallcircle. Inventive steel G 0.023 0.48 0.96 0.008 0.0026 0.067
0.0011 0.104 Cr: 0.04, Ca: 0.0014 0.096 877 .smallcircle. Inventive
steel H 0.055 0.03 0.88 0.011 0.0009 0.041 0.0044 0.088 Mo: 0.07
0.072 845 .smallcircle. Inventive steel I 0.019 0.02 0.85 0.017
0.0011 0.045 0.0055 0.118 Nb: 0.011 0.097 869 .smallcircle.
Inventive steel J 0.064 0.12 0.93 0.015 0.0006 0.039 0.0014 0.105
Cu: 0.04, Ni: 0.08 0.099 847 .smallcircle. Inventive steel K 0.025
0.03 0.31 0.011 0.0031 0.041 0.0050 0.076 B: 0.0020 0.054 878
.smallcircle. Comparative steel L 0.061 0.04 1.37 0.019 0.0017
0.039 0.0033 0.109 -- 0.095 834 .smallcircle. Comparative steel M
0.079 0.03 0.96 0.013 0.0013 0.047 0.0041 0.069 Nb: 0.009 0.053 835
x Comparative steel N 0.037 0.03 0.68 0.012 0.0029 0.042 0.0045
0.076 -- 0.056 860 .smallcircle. Comparative steel O 0.032 0.17
0.83 0.033 0.0045 0.033 0.0048 0.098 -- 0.075 864 .smallcircle.
Inventive steel P 0.034 0.22 0.84 0.033 0.0047 0.033 0.0055 0.105
-- 0.079 867 .smallcircle. Inventive steel Ti*= Ti--N .times.
(48/14) - S .times. (48/32) Formula (1): Ti* .gtoreq. 0.1, Formula
(2): C .times. (48/12) - 0.14 < Ti* < C .times. (48/12) +
0.08; C, S, N, and Ti in Formulae (1) and (2) denote the amounts
(mass %) of the correspond *1"0" means that both Formulae (1) and
(2) are satisfed. "X" means that one or both of Formulae (1) and
(2) are not satisfied.
TABLE-US-00002 TABLE 2 Manufacturing conditions for hot-rolled
steel sheet Finish- Total reduction Soaking Hot-rolled Plating
Sheet Heating rolling ratio at last Average Coiling temperature
steel treatment thickness temperature temperature two finish
cooling rate temperature for annealing sheet No. Steel *2 (mm)
(.degree. C.) (.degree. C.) rolling passes (%) (.degree. C./s)
(.degree. C.) treatment (.degree. C.) Remarks 1 A -- 3.2 1180 900
55 70 700 -- Example 2 B -- 2.6 1250 950 50 80 645 -- Example 3 B
GI 2.6 1250 915 45 95 590 695 Example 4 B GI 2.6 1250 920 45 85 595
785 Comparative example 5 C -- 2.9 1200 890 40 60 560 -- Example 6
D -- 2.0 1220 935 50 105 640 -- Example 7 D -- 2.0 1220 840 60 130
650 -- Comparative example 8 D -- 2.0 1220 905 75 100 660 --
Comparative example 9 E GA 3.2 1240 910 60 95 630 645 Example 10 E
GA 3.2 1240 915 50 85 455 660 Comparative example 11 E GA 3.2 1240
920 50 30 660 715 Comparative example 12 F -- 2.3 1280 900 40 115
635 -- Example 13 G -- 2.0 1290 920 35 140 630 -- Example 14 H --
4.5 1250 865 55 50 695 -- Example 15 I -- 3.4 1270 890 45 85 575 --
Example 16 J GA 2.0 1260 910 50 70 510 740 Example 17 K -- 2.6 1250
900 55 75 595 -- Comparative example 18 L GA 2.9 1250 900 40 80 560
700 Comparative example 19 M GA 2.9 1200 880 45 45 590 685
Comparative example 20 N -- 2.6 1250 910 60 65 690 -- Comparative
example 21 O -- 2.0 1220 935 50 105 640 -- Example 22 P -- 2.0 1220
935 50 105 640 -- Example *2 "--" represents no plating treatment.
"GI" represents hot-dip galvanizing treatment. "GA" represents
hot-dip galvanizing treatment and alloying treatment.
TABLE-US-00003 TABLE 3 Microstructure of hot-rolled steel sheet
Number percentage of carbide having grain Percentage of Mechanical
properties of hot-rolled steel sheet Hot-rolled Ferrite phase size
of less than 9 Ti precipitate Tensile Total steel fraction nm out
of carbide having grain strength elongation Burring sheet No. (area
%) *3 containing Ti (%) size of 20 nm (%) TS (MPa) El (%) ratio (%)
Remarks 1 91(P: 9) 70 85 545 34 70 Example 2 97(P: 3) 85 80 563 33
81 Example 3 .sup. 98(B: 2) 75 50 568 31 82 Example 4 92(P: 5, B:
3) 55 40 525 24 46 Comparative example 5 91(P: 2, B: 7) 75 55 593
30 73 Example 6 98(P: 2) 90 85 631 33 88 Example 7 91(P: 9) 40 40
530 23 31 Comparative example 8 93(P: 7) 50 50 533 21 37
Comparative example 9 97(P: 3) 85 75 633 32 84 Example 10 .sup.
91(B: 5, M: 4) 65 35 534 21 46 Comparative example 11 79(P: 21) 25
50 516 23 43 Comparative example 12 92(P: 8) 85 80 564 33 67
Example 13 98(P: 2) 80 85 689 25 65 Example 14 94(P: 6) 70 80 604
32 77 Example 15 .sup. 99(B: 1) 75 65 699 26 75 Example 16 95(P: 1,
B: 4) 80 50 678 22 80 Example 17 95(P: 5) 70 40 533 23 44
Comparative example 18 .sup. 93(B: 7) 70 35 642 18 25 Comparative
example 19 55(P: 40, B: 5) 25 40 529 17 28 Comparative example 20
96(P: 4) 55 55 581 25 47 Comparative example 21 98(P: 2) 90 45 605
34 63 Example 22 98(P: 2) 90 40 594 32 60 Example *3 The
percentages of microstructures other than ferrite phase are shown
in parentheses. P: pearlite (including cementite), B: bainite, M:
martensite (including retained austenite).
[0099] The hot-rolled steel sheets according to examples (Nos. 1 to
3, 5, 6, 9, 12 to 16, 21, and 22) had the desired tensile strength
(540 MPa or more) and good mass production burring formability. By
contrast, the hot-rolled steel sheets according to comparative
examples (Nos. 4, 7, 8, 10, 11, and 17 to 20), which were outside
the scope of the present invention, did not have a desired high
strength or sufficient burring ratios.
* * * * *