U.S. patent application number 14/652235 was filed with the patent office on 2015-11-19 for hot-rolled steel sheet exhibiting excellent cold formability and excellent surface hardness after forming.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Katsura KAJIHARA.
Application Number | 20150329932 14/652235 |
Document ID | / |
Family ID | 51167045 |
Filed Date | 2015-11-19 |
United States Patent
Application |
20150329932 |
Kind Code |
A1 |
KAJIHARA; Katsura |
November 19, 2015 |
HOT-ROLLED STEEL SHEET EXHIBITING EXCELLENT COLD FORMABILITY AND
EXCELLENT SURFACE HARDNESS AFTER FORMING
Abstract
Disclosed is a hot-rolled steel sheet that has a thickness of 3
to 20 mm and includes, in a chemical composition in mass percent, C
of 0.3% or less (excluding 0%), Si of 0.5% or less (excluding 0%),
Mn of 0.2% to 1%, P of 0.05% or less (excluding 0%), S of 0.05% or
less (excluding 0%), Al of 0.01% to 0.1%, and N of 0.008% to
0.025%, with the remainder consisting of iron and inevitable
impurities. A solute nitrogen content is 0.007% or more, and the
carbon and nitrogen contents meet a condition as specified by
10C+N.ltoreq.3.0. The microstructure of the steel sheet includes
pearlite of less than 20% in area percentage, with the remainder
approximately consisting of ferrite. The average grain size of the
ferrite is 3 to 35 .mu.m. The steel sheet has good cold formability
during forming and still has predetermined surface hardness after
forming.
Inventors: |
KAJIHARA; Katsura;
(Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
51167045 |
Appl. No.: |
14/652235 |
Filed: |
January 10, 2014 |
PCT Filed: |
January 10, 2014 |
PCT NO: |
PCT/JP2014/050368 |
371 Date: |
June 15, 2015 |
Current U.S.
Class: |
148/331 ;
148/320 |
Current CPC
Class: |
C21D 2211/005 20130101;
C22C 38/08 20130101; C22C 38/004 20130101; C21D 9/46 20130101; C22C
38/04 20130101; C22C 38/26 20130101; C22C 38/02 20130101; C22C
38/002 20130101; C21D 8/02 20130101; C22C 38/60 20130101; C22C
38/001 20130101; C21D 2211/009 20130101; C22C 38/16 20130101; C21D
8/0226 20130101; C22C 38/06 20130101 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 9/46 20060101 C21D009/46; C22C 38/26 20060101
C22C038/26; C22C 38/16 20060101 C22C038/16; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2013 |
JP |
2013-002640 |
Mar 19, 2013 |
JP |
2013-056658 |
Claims
1. A hot-rolled steel sheet having excellent cold formability and
satisfactory surface hardness after forming, the hot-rolled steel
sheet having a thickness of 3 to 20 mm, the hot-rolled steel sheet
comprising, in chemical composition in percent by mass (hereinafter
the same for chemical composition): C in a content of 0.3% or less
(excluding 0%); Si in a content of 0.5% or less (excluding 0%); Mn
in a content of 0.2% to 1%; Pin a content of 0.05% or less
(excluding 0%); S in a content of 0.05% or less (excluding 0%); Al
in a content of 0.01% to 0.1%; N in a content of 0.008% to 0.025%;
with the remainder consisting of iron and inevitable impurities, a
content of solute nitrogen being 0.007% or more, and the contents
of carbon (C) and nitrogen (N) meeting a condition as specified by
expression: 10C+N.ltoreq.3.0, a microstructure of the hot-rolled
steel sheet comprising: pearlite in a content of less than 20% in
area percentage based on the total microstructure, with the
remainder approximately consisting of ferrite, the ferrite having
an average grain size of 3 to 35 .mu.m.
2. The hot-rolled steel sheet according to claim 1, further
comprising, in the chemical composition, at least one element
selected from the group consisting of: Cr in a content of 2% or
less (excluding 0%); Mo in a content of 2% or less (excluding 0%);
Ti in a content of 0.2% or less (excluding 0%); Nb in a content of
0.2% or less (excluding 0%) V in a content of 0.2% or less
(excluding 0%) B in a content of 0.005% or less (excluding 0%); Cu
in a content of 5% or less (excluding 0%); Ni in a content of 5% or
less (excluding 0%); Co in a content of 5% or less (excluding 0%);
Ca in a content of 0.05% or less (excluding 0%); at least one
rare-earth element (REM) in a total content of 0.05% or less
(excluding 0%); Mg in a content of 0.02% or less (excluding 0%); Li
in a content of 0.02% or less (excluding 0%); Pb in a content of
0.5% or less (excluding 0%); and Bi in a content of 0.5% or less
(excluding 0%).
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled steel sheet
that has good cold formability during forming and still has
predetermined surface hardness after forming.
BACKGROUND ART
[0002] Recently, better fuel efficiency is required in automobiles
from the viewpoint of environmental protection. To meet the
requirement, steels for use in automobile parts such as gears and
other gearbox unit parts and casings more and more require lighter
weights, i.e., higher strength. To meet the requirement of lighter
weights and higher strength, hot-forged steels prepared from steel
bars by hot forging have been generally used. Instead of the hot
forged gears and other parts, demands are increasingly made to
provide these parts by cold forging so as to reduce CO.sub.2
emission in parts production processes.
[0003] Advantageously, the cold forming (cold forging) offers
higher productivity and provides both good dimensional accuracy and
good steel yield as compared with hot forming and warm forming.
Disadvantageously, however, the cold forming, when employed to
produce parts, has to essentially use steels having high strength,
i.e., high deformation resistance so as to allow the cold-worked
parts to surely have strength at predetermined levels or higher.
Unfortunately, steels with increasing deformation resistance may
more readily invite shorter lives of cold-forming tools and more
readily cause fracture/cracking upon cold forming.
[0004] To prevent this, some conventional techniques produce
high-strength parts surely having predetermined strength (hardness)
by cold-forging a steel into a predetermined shape, and subjecting
the cold-forged steel to a heat treatment such as quenching and
tempering. However, the parts inevitably change their dimensions in
the heat treatment after cold forging and thereby require secondary
correction by machining such as cutting. Under these circumstances,
demands have been made to provide a solution that can omit the heat
treatment and the subsequent forming.
[0005] As a possible solution to the problems, for example, Patent
Literature (PTL) 1 discloses that a wire rod/steel bar for cold
forging having excellent strain aging hardening properties is
obtained by preparing a low-carbon steel, restraining the progress
of natural aging of the steel using solute carbon, and allowing the
steel to surely undergo age hardening by strain aging hardening at
a predetermined level.
[0006] This technique, however, controls the strain aging hardening
by the solute carbon content alone and hardly gives a steel that
has both sufficient cold formability and required level of
hardness/strength after forming.
[0007] Under the circumstances, the present applicant made various
investigations while focusing on the effect of solute carbon and
solute nitrogen in a steel on deformation resistance and static
strain aging hardening. As a result, the present applicant found
that appropriate control of the amounts of these solute elements
gives a mechanical-structure-use steel that exhibits good cold
formability during forming and still has predetermined surface
hardness (strength) after cold forming (cold forging). The present
applicant has already filed a patent application based on these
findings (see PTL 2).
[0008] The steel achieves both good cold formability and higher
hardness (higher strength) after forming. Disadvantageously,
however, the steel is a hot-forged steel as with the wire rod/steel
bar disclosed in PTL 1 and suffers from high production cost. To
achieve still lower production cost, attempts have been made to
produce automobile parts by cold forming using hot-rolled steel
sheets instead of the conventional hot-forged steels.
[0009] Typically, PTL 3 proposes a technique, according to which a
hot-rolled steel sheet for nitriding can have high surface hardness
and a sufficient hardening depth after nitriding.
[0010] Disadvantageously, however, the technique requires nitriding
as an extra process after cold forming and fails to offer
sufficiently low cost.
[0011] PTL 4 proposes a hot-rolled steel sheet that has a chemical
composition containing C in a content of 0.10% or less, Si in a
content less than 0.01%, Mn in a content of 1.5% or less, Al in a
content of 020% or less, Ti and Nb in a content as specified by
(Ti+Nb)/2 of 0.05% to 0.50%, Sin a content of 0.005% or less, N in
a content of 0.005% or less, O in a content of 0.004% or less, so
that the total content of S, N, and O be 0.0100% or less. The
hot-rolled steel sheet has a microstructure containing 95% or more
of ferrite as approximately a ferrite single-phase. The literature
mentions that the hot-rolled steel sheet has excellent dimensional
accuracy in a finely blanked surface, has extremely high surface
hardness of the blanked surface after forming, and still offers
excellent resistance to red-scale defects.
[0012] The hot-rolled steel sheet, however, is designed to treat
nitrogen as a harmful element and to control the nitrogen content
to an extremely low content and absolutely differs in technical
idea from the hot-rolled steel sheet according to the present
invention in which nitrogen is positively utilized.
CITATION LIST
Patent Literature
[0013] PTL 1: Japanese Unexamined Patent Application Publication
(JP-A) No. Hei10(1998)-306345
[0014] PTL 2: JP-A No. 2009-228125
[0015] PTL 3: JP-A No. 2007-162138
[0016] PTL 4: JP-A No. 2004-137607
SUMMARY OF INVENTION
Technical Problem
[0017] The present invention has been made while focusing these
circumstances, and it is an object of the present invention to
provide a hot-rolled steel sheet that has good cold formability
during forming and still has predetermined surface hardness after
forming.
Solution to Problem
[0018] The present invention provides, in one aspect, a hot-rolled
steel sheet having excellent cold formability and satisfactory
surface hardness after forming. The hot-rolled steel sheet has a
thickness of 3 to 20 mm and contains, in a chemical composition in
mass percent (hereinafter the same for chemical composition), C in
a content of 0.3% or less (excluding 0%), Si in a content of 0.5%
or less (excluding 0%), Mn in a content of 0.2% to 1%, P in a
content of 0.05% or less (excluding 0%), S in a content of 0.05% or
less (excluding 0%), Al in a content of 0.01% to 0.1%, and N in a
content of 0.008% to 0.025%, with the remainder approximately
consisting of iron and inevitable impurities. In the hot-rolled
steel sheet, the content of solute nitrogen is 0.007% or more, and
the contents of carbon and nitrogen meet a condition as specified
by the expression: 10C+N.ltoreq.3.0. The microstructure of the
hot-rolled steel sheet indudes pearlite in a content of less than
20% in area percentage based on the total microstructure, with the
remainder approximately consisting of ferrite. The ferrite has an
average grain size of 3 to 35 .mu.m.
[0019] In an embodiment, the hot-rolled steel sheet according to
the aspect may further contain, in the chemical composition, Cr in
a content of 2% or less (excluding 0%) and/or Mo in a content of 2%
or less (excluding 0%).
[0020] In another embodiment, the hot-rolled steel sheet according
to the aspect may further contain, in the chemical composition, at
least one element selected from the group consisting of Ti in a
content of 0.2% or less (excluding 0%), Nb in a content of 0.2% or
less (excluding 0%), and V in a content of 0.2% or less (excluding
0%).
[0021] In yet another embodiment, the hot-rolled steel sheet
according to the aspect may further contain, in the chemical
composition, B in a content of 0.005% or less (excluding 0%).
[0022] In yet another embodiment, the hot-rolled steel sheet
according to the aspect may further contain, in the chemical
composition, at least one element selected from the group
consisting of Cu in a content of 5% or less (excluding 0%), Ni in a
content of 5% or less (excluding 0%), and Co in a content of 5% or
less (excluding 0%).
[0023] In still another embodiment, the hot-rolled steel sheet
according to the aspect may further contain, in the chemical
composition, at least one element selected from the group
consisting of Ca in a content of 0.05% or less (excluding 0%), at
least one rare-earth element (REM) in a total content of 0.05% or
less (excluding 0%), Mg in a content of 0.02% or less (excluding
0%), Li in a content of 0.02% or less (excluding 0%), Pb in a
content of 0.5% or less (excluding 0%), and Bi in a content of 0.5%
or less (excluding 0%).
Advantageous Effects of Invention
[0024] According to the present invention, a microstructure mainly
containing ferrite having a predetermined average grain size is
controlled so that the microstructure contains solute nitrogen in a
certain amount, and the carbon content and the nitrogen content
meet a predetermined condition. This can provide a hot-rolled steel
sheet that has lower deformation resistance during cold forming,
thereby contributes to longer lives of tools, still resists
fracture/cracking, and gives, after forming, a part surely having
predetermined surface hardness.
DESCRIPTION OF EMBODIMENTS
[0025] The hot-rolled steel sheet according to the present
invention will be illustrated in detail below. The hot-rolled steel
sheet according to the present invention is hereinafter also
referred to as "steel sheet according to the present invention" or
simply referred to as "steel sheet". The steel sheet according to
the present invention has a commonality with the hot-forged steel
disclosed in PTL 2 in that solute nitrogen is contained in a
certain amount and that the carbon content and the nitrogen content
are controlled so as to meet a predetermined condition. The steel
sheet according to the present invention, however, differs from the
hot-forged steel in that the upper limit of the carbon content is
relatively high, the steel sheet is controlled to include a
ferrite-pearlite dual-phase microstructure as the microstructure,
and ferrite grains are refined.
[0026] The steel sheet according to the present invention has a
thickness of 3 to 20 mm.
[0027] First of all, the steel sheet according to the present
invention is directed to one having a thickness of 3 to 20 mm. The
steel sheet, if having a thickness of less than 3 mm, may fail to
surely have rigidity as a structure. In contrast, the steel sheet,
if having a thickness greater than 20 mm, may hardly have the
microstructure in the form as specified in the present invention
and hardly have desired effects. The steel sheet preferably has a
thickness of 4 to 19 mm.
[0028] Next, the chemical composition constituting the steel sheet
according to the present invention will be described. All chemical
elements hereinafter are indicated in mass percent.
[0029] Chemical Composition of Steel Sheet According to Present
Invention
[0030] C in a Content of 0.3% or Less (Excluding 0%)
[0031] Carbon (C) significantly affects the formation of steel
sheet microstructure, and the content thereof may be controlled so
as to form a microstructure that is a ferrite-pearlite dual-phase
microstructure, but mainly contains ferrite and contains pearlite
in a minimized amount. The steel sheet, if containing carbon in
excess, may have a higher pearlite fraction in the microstructure
and might have excessively high deformation resistance due to
pearlite's work hardening. To prevent this, the carbon content in
the steel sheet may be controlled to 0.3 percent by mass or less,
preferably 0.25% or less, more preferably 0.2% or less, and
particularly preferably 0.15% or less. However, the steel sheet, if
having an excessively low carbon content, may hardly undergo
deoxidation during steel ingot making. To prevent this, the carbon
content may be controlled to preferably 0.0005% or more, more
preferably 0.0008% or more, and particularly preferably 0.001% or
more.
[0032] Si in a Content of 0.5% or Less (Excluding 0%)
[0033] Silicon (Si) dissolves in the steel, causes the steel sheet
to have higher deformation resistance, and has to be minimized. To
reduce deformation resistance, the Si content in the steel sheet
may be controlled to 0.5% or less, preferably 0.45% or less, more
preferably 0.4% or less, and particularly preferably 0.3% or less.
However, the steel sheet, if having an extremely low Si content,
may hardly undergo deoxidation during steel ingot making. To
prevent this, the Si content may be controlled to preferably 0.005%
or more, more preferably 0.008% or more, and particularly
preferably 0.01% or more.
[0034] Mn in a Content of 0.2% to 1%
[0035] Manganese (Mn) effectively deoxidizes and desulfurizes in
the steel making process. Assuming that the steel has a higher
nitrogen content, in this case, the steel sheet may be susceptible
to fracture/cracking by dynamic strain aging hardening with the
heat generation by mechanical forming. Manganese, however,
effectively contributes to better formability in this process and
restrains fracture/cracking. To have these activities effectively,
the Mn content in the steel sheet may be controlled to 0.2% or
more, preferably 0.22% or more, and more preferably 0.25% or more.
However, the steel sheet, if containing Mn in excess, may have
excessively high deformation resistance and may suffer from a
heterogeneous microstructure due to segregation. To prevent this,
the Mn content may be controlled to 1% or less, preferably 0.98% or
less, and more preferably 0.95 percent by mass or less.
[0036] P in a Content of 0.05% or Less (Excluding 0%)
[0037] Phosphorus (P) is an impurity element and is inevitably
contained in the steel. Phosphorus, if contained in ferrite,
segregates at ferrite grain boundaries to impair cold formability.
This element also causes ferrite to undergo solute strengthening
and cause the steel sheet to have higher deformation resistance. To
prevent this and to offer good cold formability, the phosphorus
content is preferably minimized. However, extreme minimization of
the phosphorus content may bring about an increase in steel making
cost. To prevent this in consideration of process capability, the
phosphorus content may be controlled to 0.05% or less, and
preferably 0.03% or less.
[0038] S in a Content of 0.05% or Less (Excluding 0%)
[0039] Sulfur (S) is an inevitable impurity as with phosphorus,
precipitates as a film of FeS at grain boundaries, and impairs
formability. This element also causes hot brittleness. To prevent
this and to provide better deformability, the sulfur content herein
may be controlled to 0.05% or less, and preferably 0.03% or less.
It is industrially difficult, however, to control the sulfur
content to zero (0). However, sulfur effectively allows the steel
sheet to have better machinability. For better machinability, it is
recommended for the steel sheet to contain sulfur in a content of
preferably 0.002% or more, and more preferably 0.006% or more.
[0040] Al in a Content of 0.01% to 0.1%
[0041] Aluminum (Al) effectively contributes to deoxidation in the
steel making process. To have the deoxidation effect, the steel
sheet may have an Al content of 0.01% or more, preferably 0.015% or
more, and more preferably 0.02% or more. However, the steel sheet,
if having an excessively high Al content, may have lower toughness
and be susceptible to fracture/cracking. To prevent this, the Al
content may be controlled to 0.1% or less, preferably 0.09% or
less, and more preferably 0.08 percent by mass or less.
[0042] N in a Content of 0.008% to 0.025%
[0043] Nitrogen (N) causes static strain aging hardening after
forming and thereby allows the steel sheet to have predetermined
strength, thus being important. For this reason, the nitrogen
content in the steel sheet may be controlled to 0.008% or more, and
preferably 0.0085% or more, and more preferably 0.009% or more.
However, the steel sheet, if having an excessively high nitrogen
content, may be significantly affected not only by static strain
aging hardening, but also by dynamic strain aging hardening during
forming to have higher deformation resistance, thus being
unsuitable. To prevent this, the nitrogen content may be controlled
to 0.025% or less, preferably 0.023 percent by mass or less, and
more preferably 0.02% or less.
[0044] Solute Nitrogen in a Content of 0.007% or More
[0045] The steel sheet includes solute nitrogen in a predetermined
amount. This accelerates static strain aging hardening with less
increase in deformation resistance. The amount of solute nitrogen
is hereinafter also referred to as "solute nitrogen content". The
steel sheet may have a solute nitrogen content of 0.007% or more so
as to surely have required strength after cold forming. However,
the steel sheet, if having an excessively high solute nitrogen
content, may have inferior cold formability. To prevent this, the
solute nitrogen content is preferably controlled to 0.03% or less.
As the total nitrogen content in the steel sheet is 0.025% or less,
the solute nitrogen content does not approximately exceed
0.025%.
[0046] As used herein the term "solute nitrogen content" refers to
the amount as determined by subtracting the total amount of
nitrogen compounds from the total nitrogen content in the steel
sheet. The determination is performed based on Japanese Industrial
Standard (JIS) G 1228. Practical methods for measuring the solute
nitrogen content are exemplified as follows:
[0047] (a) Inert Gas Fusion-Thermal Conductivity Analysis (Total
Nitrogen Content Measurement)
[0048] A specimen is cut out from a test sample, placed in a
crucible, and fused in an inert gas stream to extract nitrogen. The
extract is transferred to a thermal conductivity cell to measure a
change in thermal conductivity to thereby determine the total
nitrogen content.
[0049] (b) Ammonia Separation by Distillation-Indophenol Blue
Absorptiometry (Total Nitrogen Compound Amount Measurement)
[0050] A specimen is cut out from the test sample, dissolved in a
10% AA electrolytic solution, subjected to constant current
electrolysis to measure the total amount of nitrogen compounds in
the steel (steel sheet). The 10% AA electrolytic solution to be
used is a non-aqueous electrolytic solution that contains 10% of
acetone and10% of tetramethylammonium chloride with the remainder
being methanol and does not form a passive film on the steel
surface.
[0051] About 0.5 g of the specimen sampled from the test sample is
dissolved in the 10% AA electrolytic solution, and formed
undissolved residue (nitrogen compounds) is filtered through a
polycarbonate filter having a pore size of 0.1 .mu.m. The
undissolved residue is heated in sulfuric acid, potassium sulfate,
and pure copper chips and is decomposed, and the decomposed product
is combined with the filtrate. The resulting mixture (solution) is
treated with sodium hydroxide to be basic, subjected to steam
distillation, and distilled ammonia is absorbed by diluted sulfuric
acid. This is further combined with phenol, sodium hypochlorite,
and sodium pentacyanonitrosylferrate(III) to form a blue complex,
and the absorbance of the blue complex is measured using an
absorptiometer to determine the total amount of nitrogen
compounds.
[0052] The total amount of nitrogen compounds determined by the
method (b) is subtracted from the total nitrogen content determined
by the method (a) to give the solute nitrogen content.
[0053] The carbon and nitrogen contents meet the condition as
specified by the expression: 10C+N.ltoreq.3.0
[0054] In the steel sheet according to the present invention, the
solute carbon contributes to significantly better deformation
resistance, but less contributes to static strain aging hardening.
In contrast, the solute nitrogen less contributes to higher
deformation resistance, but can accelerate static strain aging
hardening and can effectively contribute to higher hardness after
forming. Based on this, the steel sheet according to the present
invention essentially has the carbon content and the nitrogen
content meeting the condition as specified by the expression:
10C+N.ltoreq.3.0. The condition between the two elements is
preferably 0.009.ltoreq.10C+N.ltoreq.2.8, more preferably
0.01.ltoreq.10C+N.ltoreq.2.5, and particularly preferably
0.01.ltoreq.10C+N.ltoreq.2.0. The condition is specified so as to
increase the hardness after forming with less causing the increase
of deformation resistance during forming. The steel sheet may have
a carbon content and a solute carbon content at certain levels, for
the grain refinement in the hot-rolled steel sheet and for the
formability of the steel sheet at certain level. However, the steel
sheet, if having carbon and nitrogen contents not meeting the
condition (if 10C+N is greater than 3.0), may have excessively high
deformation resistance due to excessive content(s) of carbon and/or
nitrogen. In the inequality, the coefficient of the carbon content
is set 10 times the coefficient of the nitrogen content. This is
set in consideration that the solute carbon, even contained in the
same content with the solute nitrogen, increases the strength and
deformation resistance of the hot-rolled steel sheet according to
the present invention to a degree greater by an order of magnitude
(10 times) as compared with the solute nitrogen.
[0055] The steel sheet according to the present invention basically
contains the chemical composition (elements) with the remainder
approximately consisting of iron and inevitable impurities. The
steel sheet may further contain any of following allowable elements
within ranges not adversely affecting the operation of the present
invention.
[0056] Cr in a Content of 2% or Less (Excluding 0%) and/or Mo in a
Content of 2% or Less (Excluding 0%)
[0057] Chromium (Cr) increases grain boundary strength to
effectively allow the steel to have better deformability. To have
such activities effectively, the steel sheet preferably contain Cr
in a content of 0.2%. However, the steel sheet, if containing Cr in
excess, may have higher deformation resistance and deteriorated
cold formability. To prevent this, the Cr content may be controlled
to preferably 2% or less, more preferably 1.5% or less, and
particularly preferably 1% or less.
[0058] Molybdenum (Mo) effectively allows the steel sheet to have
higher hardness after forming and better deformability. To have
such activities effectively, the steel sheet may contain Mo in a
content of preferably 0.04% or more, and more preferably 0.08% or
more. However, the steel sheet, if containing Mo in excess, may
have inferior cold formability. To prevent this, the Mo content may
be controlled to preferably 2% or less, more preferably 1.5% or
less, and particularly preferably 1% or less.
[0059] At Least One Element Selected from the Group Consisting of
Ti in a Content of 0.2% or Less (Excluding 0%), Nb in a Content of
0.2% or Less (Excluding 0%), and V in a Content of 0.2% or Less
(Excluding 0%)
[0060] These elements have a high affinity for nitrogen, are
coexistent with nitrogen to form nitrogen compounds, and contribute
to grain refinement of the steel. These elements also allow the
formed product (steel sheet) after cold forming to have better
toughness and better resistance to fracture/cracking. Each of the
elements, however, fails to offer further better properties when
contained in a content greater than the upper limit. To prevent
this, the contents of the elements may each be controlled to
preferably 0.2% or less, more preferably 0.001% to 0.15%, and
particularly preferably 0.002% to 0.1%.
[0061] B in a Content of 0.005% or Less (Excluding 0%)
[0062] Boron (B) acts similarly to Ti, Nb, and V mentioned above.
Specifically, boron has a high affinity for nitrogen, is coexistent
with nitrogen to form nitrogen compounds, and contributes to grain
refinement in the steel. This element also allows the formed
product (steel sheet) after cold forming to have better toughness
and better resistance to fracture/cracking. The steel sheet
according to the present invention, when containing boron, can have
a required solute nitrogen content and have higher strength after
cold forming. The boron content may therefore be preferably 0.005%
or less, more preferably 0.0001% to 0.0035%, and particularly
preferably 0.0002% to 0.002%.
[0063] At Least One Element Selected from the Group Consisting of
Cu in a Content of 5% or Less (Excluding 0%), Ni in a Content of 5%
or Less (Excluding 0%), and Co in a Content of 5% or Less
(Excluding 0%)
[0064] These elements each effectively allow the steel to undergo
strain aging hardening and to be hardened and effectively allow the
steel sheet to have higher strength after forming. To have such
activities effectively, each of these elements may be contained in
a content of preferably 0.1% or more, and more preferably 0.3% or
more. However, these elements, if contained in excess, may have
saturated effects of allowing the steel to undergo strain aging
hardening and to be hardened and allowing the steel sheet to have
higher strength after forming and may accelerate fracture/cracking.
To prevent this, the contents of these elements may each be
controlled to preferably 5% or less, more preferably 4% or less,
and particularly preferably 3% or less.
[0065] At Least One Element Selected from the Group Consisting of
Ca in a Content of 0.05% or Less (Excluding 0%), at Least One
Rare-Earth Element (REM) in a Total Content of 0.05% or Less
(Excluding 0%), Mg in a Content of 0.02% or Less (Excluding 0%), Li
in a Content of 0.02% or Less (Excluding 0%), Pb in a Content of
0.5% or Less (Excluding 0%), and Bi in a Content of 0.5% or Less
(Excluding 0%)
[0066] Calcium (Ca) contributes to spheroiclization of MnS and
other sulfide inclusions and allows the steel to have better
deformability and better machinability. To have such activities
effectively, Ca may be contained in a content of preferably 0.0005%
or more, and more preferably 0.001% or more. However, Ca, if
contained in excess, may have saturated effects and is not expected
to exhibit effects consistent with the content. To prevent this,
the Ca content may be controlled to preferably 0.05% or less, more
preferably 0.03% or less, and particularly preferably 0.01% or
less.
[0067] As with Ca, the rare-earth element(s) (REM) contributes to
spheroidization of MnS and other sulfide inclusions and allows the
steel to have better deformability and better machinability. To
have such activities effectively, REM may be contained in a content
of preferably 0.0005% or more, and more preferably 0.001% or more.
However, REM, if contained in excess, may have saturated effects
and is not expected to exhibit effects consistent with the content.
To prevent this, the REM content may be controlled to preferably
0.05% or less, more preferably 0.03% or less, and particularly
preferably 0.01 percent by mass or less.
[0068] As used herein the term "REM" refers to element or elements
including lanthanoid elements (fifteen elements from La to Lu), as
well as Sc (scandium) and Y (yttrium). Of these elements, the steel
sheet preferably contains at least one element selected from the
group consisting of La, Ce, and Y and more preferably contains La
and/or Ce as REM.
[0069] Magnesium (Mg) contributes to spheroidization of MnS and
other sulfide inclusions and allows the steel to have better
deformability and better machinability, as with Ca. To have such
activities effectively, Mg may be contained in a content of
preferably 0.0002% or more, and more preferably 0.0005% or more.
However, Mg, if contained in excess, may have saturated effects and
is not expected to exhibit effects consistent with the content. To
prevent this, the Mg content may be controlled to preferably 0.02%
or less, more preferably 0.015% or less, and particularly
preferably 0.01% or less.
[0070] Lithium (Li) contributes to spheroidimtion of MnS and other
sulfide indusions and allows the steel to have better
deformability, as with Ca. In addition, this element allows
aluminum oxides to have lower melting points and to be harmless and
contributes to better machinability. To have such activities
effectively, Li may be contained in a content of preferably 0.0002%
or more, and more preferably 0.0005% or more. However, Li, if
contained in excess, may have saturated effects and is not expected
to exhibit effects consistent with the content. To prevent this,
the Li content may be controlled to preferably 0.02% or less, more
preferably 0.015% or less, and particularly preferably 0.01% or
less.
[0071] Lead (Pb) effectively contributes to better machinability.
To have such activities effectively, Pb may be contained in a
content of preferably 0.005% or more, and more preferably 0.01% or
more. However, Pb, if contained in excess, may cause problems upon
production, such as formation of roll marks. To prevent this, the
Pb content may be controlled to preferably 0.5% or less, more
preferably 0.4% or less, and particularly preferably 0.3 percent by
mass or less.
[0072] Bismuth (Bi) effectively contributes to better
machinability, as with Pb. To have such activities effectively, Bi
may be contained in a content of preferably 0.005% or more, and
more preferably 0.01% or more. However, Bi, if contained in excess,
may have saturated effects for better machinability. To prevent
this, the Bi content may be controlled to preferably 0.5 percent by
mass or less, more preferably 0.4% or less, and particularly
preferably 0.3% or less.
[0073] Next, the microstructure featuring the steel sheet according
to the present invention will be described.
[0074] Microstructure of Steel Sheet According to Present
Invention
[0075] As is described above, the steel sheet according to the
present invention is based on a steel including a ferrite-pearlite
dual-phase microstructure, in which the size of ferrite grains is
controlled within a specific range.
[0076] Pearlite in Content of Less than 20%, with Remainder Being
Ferrite
[0077] The steel sheet according to the present invention includes
a ferrite-pearlite dual-phase microstructure as its microstructure.
Pearlite, if present in excess, may cause the steel sheet to have
inferior formability. To prevent this, the content of pearlite may
be controlled to less than 20%, more preferably 19% or less,
furthermore preferably 18% or less, and particularly preferably 15%
or less in area percentage. The remainder is approximately
ferrite.
[0078] Ferrite Having Average Grain Size of from 3 to 35 .mu.m
[0079] Ferrite grains constituting the ferrite phase may have an
average grain size of 3 to 35 .mu.m so as to allow the steel sheet
to have better formability and satisfactory surface quality after
forming. The steel sheet, if containing excessively fine (small)
ferrite grains, may have excessively high deformation resistance.
To prevent this, the average ferrite grain size may be controlled
to 3 .mu.m or more, preferably 4 .mu.m or more, and more preferably
5 .mu.m or more. In contrast, the steel sheet, if containing
excessively coarse ferrite grains, may have inferior surface
quality after forming and may have inferior properties such as
toughness and fatigue properties. To prevent this, the average
ferrite grain size may be controlled to 35 .mu.m or less,
preferably 30 .mu.m or less, and more preferably 25 .mu.m or
less.
[0080] Method for Measuring Area Percentages of Phases
[0081] The area percentages of the individual phases may be
determined by subjecting each test sample steel sheet to Nital
etching, taking photos in five fields of view using a scanning
electron microscope (SEM) at 1000-fold magnification, and
determining proportions of ferrite and pearlite by point
counting.
[0082] Method for Measuring Average Grain Size
[0083] The average ferrite grain size may be measured typically in
the following manner. Specifically, sizes of ferrite grains present
at three points, i.e., points corresponding to an outermost layer,
one-fourth the thickness, and the central part in the thickness
direction are measured. The grain size of one ferrite grain is
measured in the following manner. The side surface in the rolling
direction at each measurement point is subjected to Nital etching,
photos in five fields of view in the portion are taken using a
scanning electron microscope (SEM) at 1000-fold magnification, and
the diameter including the center of gravity of a ferrite grain is
determined by image analysis. The determined grain sizes are
averaged to give an average ferrite grain size.
[0084] Next, a preferred method for producing the steel sheet
according to the present invention will be illustrated below.
[0085] Preferred Method for Producing Steel Sheet According to
Present Invention
[0086] The steel sheet according to the present invention may be
produced by any method, as long as a material steel having the
chemical composition can be formed into a desired thickness.
Typically, the steel sheet may be produced by preparing a molten
steel having the chemical composition in a converter, subjecting
this to ingot making or continuous casting to give a slab, and
rolling the slab into a hot-rolled steel sheet having a desired
thickness, under following conditions.
[0087] Molten Steel Preparation
[0088] The nitrogen content in the molten steel can be adjusted by
adding a raw material containing a nitrogen compound to the molten
steel and/or controlling the atmosphere of the converter to be a
nitrogen (N.sub.2) atmosphere upon melting in the converter.
[0089] Heating
[0090] Heating before hot rolling is performed at a temperature of
1100.degree. C. to 1300.degree. C. The heating is performed at such
a high temperature in order to dissolve nitrogen in an amount as
much as possible while preventing the formation of nitrogen
compounds. The lower limit of the heating temperature is preferably
1100.degree. C., and more preferably 1150.degree. C. In contrast,
heating to a temperature higher than 1300.degree. C. is
operationally difficult.
[0091] Hot Rolling
[0092] Hot rolling is performed so that the finish rolling
temperature be 880.degree. C. or higher. The hot rolling, if
performed at an excessively low finish rolling temperature, may
cause ferrite transformation to occur at a high temperature, may
thereby cause carbide precipitates in ferrite to coarsen, and may
cause the steel sheet to have lower fatigue strength (inferior
fatigue resistance). To prevent this, the hot rolling may be
performed at a finish rolling temperature at a certain level or
higher. The hot rolling may be performed at a finish rolling
temperature of more preferably 900.degree. C. or higher so as to
allow austenite grains to coarsen and allow ferrite grains to have
larger grain sizes to certain extent. The upper limit of the finish
rolling temperature may be 1000.degree. C., because such a high
finish rolling temperature as to exceed 1000.degree. C. is
difficult to attain.
[0093] Hot Rolling Pass Schedule
[0094] The hot-rolled steel sheet according to the present
invention has a thickness of 3 to 20 mm. The refinement of ferrite
grains to control the average ferrite grain size within the
predetermined range requires not only the control of the rolling
temperature, but also the control of tandem rolling in the finish
rolling to provide a final rolling reduction of 15% or more. In
general, the finish rolling is performed as five to seven passes of
tandem rolling. In this process, a pass schedule is determined from
the viewpoint of control of holding fast with the rollers and the
steel sheet, and the final rolling reduction is generally set at
about 12% to 13% or more, preferably 16% or more, and more
preferably 17% or more. With an increasing final rolling reduction
(e.g., 20% to 30%), the hot rolling more effectively contributes to
the grain refinement. However, the upper limit of the final rolling
reduction may be set at about 30% from the viewpoint of rolling
control.
[0095] Rapid Cooling After Hot Rolling
[0096] Within 5 seconds after the completion of the finish rolling,
the workpiece is subjected to rapid cooling at a cooling rate
(first rapid cooling rate) of 20.degree. C./s or more, where the
rapid cooling is stopped at a temperature (rapid cooling stop
temperature) of from 580.degree. C. to lower than 670.degree. C.
The rapid cooling is performed so as to obtain a ferrite-pearlite
dual-phase microstructure having predetermined phase fractions. The
rapid cooling, if performed at a rate (rapid cooling rate) of less
than 20.degree. C./s, may accelerate pearlite transformation. The
rapid cooling, if stopped at a temperature of lower than
580.degree. C., may accelerate pearlite transformation or bainite
transformation. The rapid cooling in both cases may hardly give a
ferrite-pearlite steel having predetermined phase fractions and may
cause the steel sheet to have inferior bendability. In contrast,
the rapid cooling, if stopped at a temperature of 670.degree. C. or
higher, may cause carbide precipitates in ferrite to coarsen and
may cause the steel sheet to have deteriorated fatigue strength.
The rapid cooling may be stopped at a temperature of preferably
600.degree. C. to 650.degree. C., and more preferably 610.degree.
C. to 640.degree. C.
[0097] Slow Cooling After Rapid Cooling Stop
[0098] After the rapid cooling stop, the workpiece is slowly cooled
by natural cooling or air cooling at a cooling rate (slow cooling
rate) of 10.degree. C./s or less for 5 to 20 seconds. This allows
ferrite formation to proceed sufficiently and still contributes to
appropriate refinement of carbide precipitates in ferrite. The slow
cooling, if performed at a cooling rate greater than 10.degree.
C./s and/or for a cooling time shorter than 5 seconds, may cause
insufficient formation of ferrite. In contrast, the slow cooling,
if performed for a time longer than 20 seconds, may fail to allow
carbide precipitates to coarsen and may cause the steel sheet to
have deteriorated fatigue strength.
[0099] Rapid Cooling and Coiling After Slow Cooling
[0100] After the slow cooling, the workpiece is subjected again to
rapid cooling at a cooling rate (second rapid cooling rate) of
20.degree. C./s or more and coiled at a temperature of from higher
than 300.degree. C. to 450.degree. C. This process is performed so
as to allow the microstructure to mainly include ferrite and to
allow the steel sheet to have sufficient bendability at certain
level. The second rapid cooling, if performed at a cooling rate
(second rapid cooling rate) of less than 20.degree. C./s, or the
coiling, if performed at a temperature of higher than 450.degree.
C., may cause the steel sheet to include an excessively large
amount of pearlite. In contrast, the coiling, if performed at a
temperature lower than 300.degree. C., may cause the steel sheet to
include martensite and retained austenite and to have inferior
bendability.
[0101] The present invention will be illustrated in further detail
with reference to several examples (experimental examples) below.
It should be noted, however, that the examples are by no means
intended to limit the scope of the invention; that various changes
and modifications can naturally be made therein without deviating
from the spirit and scope of the invention as described herein and
that all such changes and modifications should be considered to be
within the scope of the invention.
EXAMPLES
[0102] Steels having chemical compositions given in Table 1 below
were made by vacuum melting, cast into ingots having a thickness of
120 mm, subjected to hot rolling under conditions given in Table 2,
and yielded hot-rolled steel sheets. In each test (sample), rapid
cooling after the completion of finish rolling down to the rapid
cooling stop temperature was performed at a cooling rate of
20.degree. C./s or more, and slow cooling after the rapid cooling
stop was performed at a cooling rate of 10.degree. C./s or less for
5 to 20 seconds.
[0103] The hot-rolled steel sheets obtained in the above manner
were examined to determine the solute nitrogen content, area
percentages of individual phases, and average ferrite grain size in
the microstructures of the steel sheets by the measurement methods
described as above in Description of Embodiments.
[0104] As the formability of the hot-rolled steel sheets, 90-degree
bendability was evaluated by a 90-degree V-block test, because the
steel sheets are those having a thickness of about 10 mm. In the
test, a test specimen was pushed into a 90-degree die using a
90-degree punch, retrieved from the die, and the outside of the
bent portion was visually observed. The punch has such a curvature
that the ratio R/t of the punch inside minimum bend radius R to the
steel sheet thickness t be 1. As a result of the visual
observation, a sample suffering from fracture was evaluated as
".times."; a sample not suffering from fracture, but suffering from
an apparent crack was evaluated as ".DELTA."; a sample suffering
from no crack although having fine asperities (wrinkles) was
evaluated as ".largecircle."; and a sample suffering from neither
crack nor wrinkles was evaluated as ".circleincircle.". As used
herein the terms "fracture" and "crack" (cracking) refer
respectively to one having a maximum gap distance of 1 mm or more
and one having a maximum gap distance of less than 1 mm and are
distinguished from each other.
[0105] The hardness of the surface in the bent portion after the
bend test was measured to evaluate surface hardness after forming.
The hardness was measured as a Vickers hardness (Hv) of each test
specimen after forming using a Vickers hardness tester. The
measurement was performed five times with a load of 1000 g, at a
measurement position of the central part corresponding to
one-fourth the diameter (D) of the resulting part.
[0106] The results of the measurements are indicated in Table 3
below.
TABLE-US-00001 TABLE 1 Chemical composition (in mass percent) with
the remainder consisting of Fe and inevitable impurities Steel C Si
Mn P S Al N 10C + N Others a 0.02 0.02 0.40 0.007 0.001 0.025 0.011
0.21 -- b 0.05 0.02 0.40 0.007 0.001 0.022 0.008 0.51 -- c 0.05
0.02 0.40 0.007 0.001 0.022 0.023 0.52 -- d 0.05 0.10 0.30 0.007
0.001 0.023 0.009 0.51 -- e 0.05 0.40 0.20 0.007 0.001 0.024 0.009
0.51 -- f 0.10 0.02 0.40 0.007 0.001 0.022 0.010 1.01 -- g 0.15
0.02 0.40 0.007 0.001 0.024 0.009 1.51 -- h 0.20 0.02 0.40 0.007
0.001 0.022 0.010 2.01 -- i 0.26 0.02 0.40 0.007 0.001 0.023 0.009
2.61 -- j 0.05 0.02 0.40 0.007 0.001 0.025 0.003 0.50 -- k 0.05
0.02 0.40 0.007 0.001 0.025 0.030 0.53 -- l 0.31 0.02 0.40 0.007
0.001 0.025 0.008 3.11 -- m 0.05 0.60 0.40 0.007 0.001 0.025 0.010
0.51 -- n 0.05 0.02 0.15 0.007 0.001 0.025 0.012 0.51 -- o 0.05
0.02 1.10 0.007 0.001 0.025 0.011 0.51 -- p 0.05 0.02 0.40 0.060
0.001 0.025 0.010 0.51 -- q 0.05 0.02 0.40 0.007 0.060 0.025 0.011
0.51 -- r 0.05 0.02 0.40 0.007 0.001 0.005 0.012 0.51 -- s 0.05
0.02 0.40 0.007 0.001 0.11 0.013 0.51 -- t 0.05 0.02 0.40 0.007
0.001 0.025 0.010 0.51 Cr: 0.5, Mb: 0.03 u 0.05 0.02 0.40 0.007
0.001 0.025 0.010 0.51 Cu: 0.06, Ni: 0.15 v 0.05 0.02 0.40 0.007
0.001 0.025 0.009 0.51 Ca: 0.0025, Li: 0.001 w 0.05 0.02 0.40 0.007
0.001 0.025 0.009 0.51 Cr: 0.5, Mb: 0.03 x 0.30 0.02 0.40 0.007
0.001 0.024 0.025 3.03 -- (Element indicated with "--": not added,
underlined data: out of the scope of the present invention)
TABLE-US-00002 TABLE 2 Hot rolling conditions Final Rapid cooling
Thickness Heating reduction Final rolling stop Coiling of
hot-rolled Production temperature rate temperature temperature
temperature sheet number Steel (.degree. C.) (%) (.degree. C.)
(.degree. C.) (.degree. C.) (mm) 1 a 1250 16 920 620 430 10 2 a
1250 18 911 607 405 4 3 a 1250 16 910 590 405 18 4* a 1000* 15 780*
545* 320 10 5* a 1250 15 900 600 410 25 6* a 1250 9* 893 593 414 10
7 b 1250 18 920 649 399 10 8 c 1250 16 922 607 311 10 9 d 1250 16
903 593 311 10 10 e 1250 15 914 659 381 10 11 f 1250 16 923 594 395
10 12 g 1250 16 886 596 369 10 13 h 1250 17 891 634 352 10 14 i
1250 18 901 615 332 10 15 j 1250 16 894 617 346 10 16 k 1250 16 892
607 392 10 17 l 1250 17 911 594 331 10 18 m 1250 16 898 615 416 10
19 n 1250 17 913 612 320 10 20 o 1250 15 922 623 427 10 21 p 1250
16 928 636 325 10 22 q 1250 16 928 640 428 10 23 r 1250 17 910 616
374 10 24 s 1250 15 892 625 357 10 25 t 1250 17 896 613 368 10 26 u
1250 15 896 627 315 10 27 v 1250 15 929 624 391 10 28 w 1250 16 910
641 403 10 29 x 1250 17 907 605 411 10 (Underlined data: out of the
scope of the present invention, asterisked data: out of the
recommended range)
TABLE-US-00003 TABLE 3 Microstructure Surface Solute Ferrite
Pearlite hardness nitrogen area area Average ferrite after Steel
Production content percentage percentage grain size 90-Degree
forming No. Steel number (mass %) (%) (%) (.mu.m) bendability (Hv)
Remarks 1 a 1 0.0085 98 2 29 .circleincircle. 260 Inventive steel
sheet 2 a 2 0.008 94 6 16 .circleincircle. 270 Inventive steel
sheet 3 a 3 0.008 98 2 33 .largecircle. 255 Inventive steel sheet 4
a 4* 0.003 96 4 30 .circleincircle. 180 Comparative steel sheet 5 a
5* 0.009 88 12 45 X 190 Comparative steel sheet 6 a 6* 0.008 92 8
41 .DELTA. 190 Comparative steel sheet 7 b 7 0.007 97 3 24
.largecircle. 280 Inventive steel sheet 8 c 8 0.019 97 3 23
.largecircle. 305 Inventive steel sheet 9 d 9 0.0085 97 3 21
.largecircle. 283 Inventive steel sheet 10 e 10 0.008 96 4 21
.largecircle. 290 Inventive steel sheet 11 f 11 0.009 95 5 19
.largecircle. 307 Inventive steel sheet 12 g 12 0.008 90 10 13
.largecircle. 318 Inventive steel sheet 13 h 13 0.009 87 13 11
.largecircle. 332 Inventive steel sheet 14 i 14 0.008 83 17 9
.largecircle. 351 Inventive steel sheet 15 j 15 0.002 96 4 25
.circleincircle. 171 Comparative steel sheet 16 k 16 0.025 97 3 23
X -- Comparative steel sheet 17 l 17 0.007 75 25 8 X -- Comparative
steel sheet 18 m 18 0.0085 96 4 24 X -- Comparative steel sheet 19
n 19 0.010 97 3 23 .largecircle. 221 Comparative steel sheet 20 o
20 0.009 95 5 21 X -- Comparative steel sheet 21 p 21 0.009 96 4 26
X -- Comparative steel sheet 22 q 22 0.010 97 3 27 X -- Comparative
steel sheet 23 r 23 0.011 97 3 26 X -- Comparative steel sheet 24 s
24 0.012 97 3 24 X -- Comparative steel sheet 25 t 25 0.008 98 2 15
.largecircle. 290 Inventive steel sheet 26 u 26 0.009 97 3 16
.largecircle. 280 Inventive steel sheet 27 v 27 0.008 97 3 17
.largecircle. 270 Inventive steel sheet 28 w 28 0.008 98 2 15
.largecircle. 275 Inventive steel sheet 29 x 29 0.020 81 19 11 X --
Comparative steel sheet (Underlined data: out of the scope of the
present invention, asterisked data: out of the recommended range,
Evaluation in 90-degree bendability: .circleincircle.: very good,
.largecircle.: good, .DELTA.: surface crack, X: fracture, --: Not
measured due to fracture, Inventive steel sheet: one having very
good or good 90-degree bendability and a surface hardness after
forming of 250 Hv or more, Comparative steel sheet: one not meeting
the conditions for the inventive steel sheet)
[0107] As is indicated in Table 3, Steel Sheet Nos. 1 to 3, 7 to
14, and 25 to 28 employed steels having chemical compositions
meeting the conditions specified in the present invention and were
produced under recommended hot rolling conditions. As a result,
these gave inventive steel sheets having microstructures meeting
the conditions specified in the present invention. The steel sheets
had 90-degree bendability and surface hardness after forming both
meeting the acceptance criteria, demonstrating that the hot-rolled
steel sheets have good cold formability during forming and still
have predetermined surface hardness (strength) after forming.
[0108] In contrast, Steel Sheet Nos. 4 to 6, 15 to 24, and 29 are
comparative steel sheets not meeting at least one of the conditions
for the chemical composition and microstructure specified in the
present invention. These steel sheets did not meet at least one of
the 90-degree bendability and surface hardness after forming not
meeting the acceptance criterion.
[0109] Typically, Steel Sheet No. 4 had a chemical composition
meeting the condition, but underwent heating before hot rolling at
an excessively low temperature out of the recommended range, and
included solute nitrogen in an insufficient content. The steel
sheet had poor surface hardness after forming.
[0110] Steel Sheet No. 5 had a chemical composition meeting the
condition, but had an excessively large thickness after hot rolling
out of the specific range. The steel sheet included coarsened
ferrite grains and was inferior both in 90-degree bendability and
in surface hardness after forming.
[0111] Steel Sheet No. 6 had a chemical composition meeting the
condition, but underwent hot rolling with an excessively low final
rolling reduction out of the recommended range. The steel sheet
included coarsened ferrite grains and was inferior both in
90-degree bendability and in surface hardness after forming.
[0112] Steel Sheet No. 15 (Steel j) underwent hot rolling under
conditions within the recommended range, but had an excessively low
nitrogen content, and thereby had poor surface hardness after
forming.
[0113] In contrast, Steel Sheet No. 16 (Steel k) underwent hot
rolling under conditions within the recommended range, but had an
excessively high nitrogen content, and was inferior at least in
90-degree bendability.
[0114] Steel Sheet No. 17 (Steel l) underwent hot rolling under
conditions within the recommended range, but had an excessively
high carbon content and failed to meet the condition as specified
by the expression: 10C+N.ltoreq.3.0. The steel sheet included an
excessively large amount of pearlite and was inferior at least in
90-degree bendability.
[0115] Steel Sheet No. 18 (Steel m) underwent hot rolling under
conditions within the recommended range, but had an excessively
high Si content, and was inferior at least in 90-degree
bendability.
[0116] Steel Sheet No. 19 (Steel n) underwent hot rolling under
conditions within the recommended range, but had an excessively low
Mn content, and was inferior at least in surface hardness after
forming.
[0117] In contrast, Steel Sheet No. 20 (Steel o) underwent hot
rolling under conditions within the recommended range, but had an
excessively high Mn content, and was inferior at least in 90-degree
bendability.
[0118] Steel Sheet No. 21 (Steel p) underwent hot rolling under
conditions within the recommended range, but had an excessively
high phosphorus content, and was inferior at least in 90-degree
bendability.
[0119] Steel Sheet No. 22 (Steel q) underwent hot rolling under
conditions within the recommended range, but had an excessively
high sulfur content, and was inferior at least in 90-degree
bendability.
[0120] Steel Sheet No. 23 (Steel r) underwent hot rolling under
conditions within the recommended range, but had an excessively low
Al content, and was inferior at least in 90-degree bendability.
[0121] In contrast, Steel Sheet No. 24 (Steel s) underwent hot
rolling under conditions within the recommended range, but had an
excessively high Al content, and was inferior at least in 90-degree
bendability.
[0122] In contrast, Steel Sheet No. 29 (Steel x) underwent hot
rolling under conditions within the recommended range, but failed
to meet the condition as specified by the expression:
10C+N.ltoreq.3.0, and was inferior at least in 90-degree
bendability.
[0123] These results demonstrate the applicability of the present
invention.
[0124] While the present invention has been particularly described
with reference to specific embodiments thereof it is obvious to
those skilled in the art that various changes and modifications may
be made without departing from the spirit and scope of the present
invention.
[0125] The present application claims priority to Japanese Patent
Application No. 2013-002640 filed on Jan. 10, 2013 and Japanese
Patent Application No. 2013-056658 filed on Mar. 19, 2013, the
entire contents of which are incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0126] The hot-rolled steel sheets according to the present
invention are suitable for automobile parts such as gears and other
gearbox unit parts, and casings.
* * * * *