U.S. patent application number 14/913432 was filed with the patent office on 2016-07-14 for hot-rolled steel sheet having excellent cold workability and excellent surface properties and hardness after working.
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, Takuya KOCHI.
Application Number | 20160201172 14/913432 |
Document ID | / |
Family ID | 52628407 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160201172 |
Kind Code |
A1 |
KAJIHARA; Katsura ; et
al. |
July 14, 2016 |
HOT-ROLLED STEEL SHEET HAVING EXCELLENT COLD WORKABILITY AND
EXCELLENT SURFACE PROPERTIES AND HARDNESS AFTER WORKING
Abstract
A hot-rolled steel sheet which has a thickness of 3-20 mm and
contains specific amounts of C, Si, Mn, P, S, Al and N, with the
balance made up of iron and unavoidable impurities. This hot-rolled
steel sheet contains a specific amount of solid-solved N, and the
contents of C and N satisfy the relation 10C+N.ltoreq.3.0. This
hot-rolled steel sheet contains bainitic ferrite and pearlite,
respectively in an area ratio of 5% or more and in an area ratio of
less than 20% relative to the whole structure, with the balance
made up of polygonal ferrite. The average crystal grain size of the
bainitic ferrite is 3-50 .mu.m.
Inventors: |
KAJIHARA; Katsura; (Hyogo,
JP) ; KOCHI; Takuya; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi
JP
|
Family ID: |
52628407 |
Appl. No.: |
14/913432 |
Filed: |
September 2, 2014 |
PCT Filed: |
September 2, 2014 |
PCT NO: |
PCT/JP2014/073075 |
371 Date: |
February 22, 2016 |
Current U.S.
Class: |
428/544 |
Current CPC
Class: |
C21D 2211/009 20130101;
C22C 38/26 20130101; C22C 38/16 20130101; C21D 2211/002 20130101;
C22C 38/00 20130101; C22C 38/08 20130101; C21D 9/46 20130101; C21D
2211/004 20130101; C21D 8/0226 20130101; C22C 38/06 20130101; C22C
38/001 20130101; C21D 2211/005 20130101; C22C 38/02 20130101; C21D
8/0263 20130101; C22C 38/002 20130101; C22C 38/04 20130101; C22C
38/40 20130101 |
International
Class: |
C22C 38/26 20060101
C22C038/26; C22C 38/08 20060101 C22C038/08; C21D 9/46 20060101
C21D009/46; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/16 20060101
C22C038/16; C22C 38/06 20060101 C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 4, 2013 |
JP |
2013-183091 |
Claims
1. A hot-rolled steel sheet excellent in cold workability as well
as in surface property and hardness after working, wherein: a sheet
thickness is from 3 to 20 mm; a component composition comprises, in
mass % (hereinafter, the same applies to chemical components), C:
0.3% or less (exclusive of 0%), Si: 0.5% or less (exclusive of 0%),
Mn: from 0.2 to 1%, P: 0.05% or less (exclusive of 0%), S: 0.05% or
less (exclusive of 0%), Al: from 0.01 to 0.1%, and N: from 0.008 to
0.025%, with a remainder being iron and unavoidable impurities,
wherein solute N: 0.007% or more and the contents of C and N
satisfy the relationship of 10C+N.ltoreq.3.0; a microstructure
comprises, in terms of area ratio relative to an entire
microstructure, bainitic ferrite: 5% or more, pearlite: less than
20%, and remainder: polygonal ferrite; and an average grain size of
the bainitic ferrite is in a range of from 3 to 50 .mu.m.
2. The hot-rolled steel sheet according to claim 1, wherein the
component composition further comprises at least one member of the
following (a) to (e): (a) at least one of Cr: 2% or less (exclusive
of 0%) and Mo: 2% or less (exclusive of 0%), (b) at least one
member selected from the group consisting of Ti: 0.2% or less
(exclusive of 0%), Nb: 0.2% or less (exclusive of 0%) and V: 0.2%
or less (exclusive of 0%), (c) B: 0.005% or less (exclusive of 0%),
(d) at least one member selected from the group consisting of Cu:
5% or less (exclusive of 0%), Ni: 5% or less (exclusive of 0%) and
Co: 5% or less (exclusive of 0%), and (e) at least one member
selected from the group consisting of Ca: 0.05% or less (exclusive
of 0%), REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less
(exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or
less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled steel sheet
exhibiting good cold workability during working and exhibiting,
after working, predetermined surface property (sometimes referred
to as "surface quality") and post-working hardness.
BACKGROUND ART
[0002] In recent years, from the standpoint of environmental
protection, lighter weight, i.e., higher strength, of steel
materials for use in various parts for automotive, for example,
transmission parts such as gear, and casings, is increasingly
required with the purpose of enhancing the fuel efficiency of
automobiles. To meet this requirement for lighter weight and higher
strength, a steel material prepared by hot-forging a steel bar
(hot-forged material) has been used as a commonly-employed steel
material. In addition, in order to reduce CO.sub.2 emission in the
process of producing parts, a requirement for cold forging of parts
such as a gear, which had been heretofore worked by hot forging, is
also more and more increasing.
[0003] Cold working (cold forging) is advantageous in that the
productivity is high compared with hot working and warm working and
moreover, both the dimensional accuracy and the steel material
yield are good. The problem occurring in the case of producing
parts by the cold working is that a steel material having high
strength, i.e., high deformation resistance, must be necessarily
used so as to ensure that the strength of cold-worked parts is
equal to or more than a predetermined value expected. However, a
higher deformation resistance of a steel material used leads to a
shortening of the life of a metal mold for cold working.
[0004] In the field of transmission parts, studies on the
production of parts using a steel sheet instead of a forged product
(hot forging, cold forging, etc.) of a steel bar are ongoing with
an aim to reduce the weight and cost of the parts, but there is a
drawback that in the parts after cold working of a steel sheet, a
surface defect called stretcher strain mark (hereinafter, simply
referred to as "SS mark") is readily generated on the surface.
[0005] Therefore, conventionally, a method where a steel material
is cold-forged into a predetermined shape and then subjected to a
heat treatment such as quenching-tempering to produce a
high-strength part assured of predetermined strength (hardness) is
sometimes conducted. However, the heat treatment after cold forging
inevitably causes a change in part dimension and therefore, it must
be secondarily corrected by machining such as cutting. A possible
solution to omit the step of heat treatment or subsequent working
has been demanded.
[0006] In order to solve the problems above, for example, it is
disclosed that when the progress of natural aging is restrained by
using solute C in a low-carbon steel to ensure a predetermined
amount of age hardening due to strain aging, a wire rod/steel bar
for cold forging, excellent in the strain aging property, can be
obtained (see, Patent Document 1).
[0007] However, in this technique, strain aging is controlled only
by the solute C amount, and a steel material having sufficient cold
workability as well as predetermined surface quality and
hardness/strength after working can be hardly obtained.
[0008] Then, the present applicant had made various studies by
focusing the attention on the difference of the effects of solute C
and solute N contained in a steel material on the deformation
resistance and static strain aging. As a result, it was found that
when the amounts of these solute elements are appropriately
controlled, a steel material for mechanical structure exerting good
cold workability during working and exhibiting predetermined
hardness (strength) after cold working (cold forging) can be
obtained. The present applicant has already filed a patent
application based on this finding (see, Patent Document 2).
[0009] This steel material realizes both cold workability and
higher hardness (higher strength) after working but is a hot-forged
material, similarly to the wire rod/steel bar described in Patent
Document 1, and the production cost is disadvantageously high. In
order to more reduce the production cost, studies are also being
made to produce automobile parts by cold working by using a
hot-rolled steel sheet in place of the conventional hot-forged
material.
[0010] For example, a hot-rolled steel sheet for nitriding
treatment, ensuring that high surface hardness and sufficient
hardening depth are obtained after nitriding treatment, has been
proposed (see, Patent Document 3).
[0011] However, this technique further requires a nitriding
treatment after cold working and has a problem that a sufficient
cost reduction cannot be realized.
[0012] In addition, a hot-rolled steel sheet having a composition
containing C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less,
and Al: 0.20% or less, containing (Ti+Nb)/2: from 0.05 to 0.50%,
and containing S: 0.005% or less, N: 0.005% or less, and O: 0.004%
or less, with the total of S, N and O being 0.0100% or less, where
the microstructure is a substantially ferrite single phase of 95%
or more, has been proposed. This hot-rolled steel sheet is thought
to be excellent in the dimensional accuracy of a finely blanked
surface, ensure very high surface hardness of the blanked surface
after working, and also be excellent in the resistance to red-scale
defect (see, Patent Document 4).
[0013] However, this hot-rolled steel sheet where N is limited to a
very low content as a harmful element utterly differs in the
technical idea from the hot-rolled steel sheet according to the
present invention where N is positively utilized.
RELATED ART
Patent Document
[0014] Patent Document 1: JP-A-10-306345
[0015] Patent Document 2: JP-A-2009-228125
[0016] Patent Document 3: JP-A-2007-162138
[0017] Patent Document 4: JP-A-2004-137607
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0018] By taking notice of these circumstances, the present
invention has been made, and an object thereof is to provide a
hot-rolled steel sheet exhibiting good cold workability during
working and exhibiting predetermined surface property and hardness
after working.
Means for Solving the Problems
[0019] The invention described in claim 1 is a hot-rolled steel
sheet excellent in cold workability as well as in surface property
and hardness after working, in which:
[0020] a sheet thickness is from 3 to 20 mm;
[0021] a component composition contains, in mass% (hereinafter, the
same applies to chemical components),
[0022] C: 0.3% or less (exclusive of 0%),
[0023] Si: 0.5% or less (exclusive of 0%),
[0024] Mn: from 0.2 to 1%,
[0025] P: 0.05% or less (exclusive of 0%),
[0026] S: 0.05% or less (exclusive of 0%),
[0027] Al: from 0.01 to 0.1%, and
[0028] N: from 0.008 to 0.025%,
[0029] with a remainder being iron and unavoidable impurities, in
which solute N: 0.007% or more and
[0030] the contents of C and N satisfy the relationship of
10C+N.ltoreq.3.0;
[0031] a microstructure contains, in terms of area ratio relative
to an entire microstructure,
[0032] bainitic ferrite: 5% or more,
[0033] pearlite: less than 20%, and
[0034] remainder: polygonal ferrite; and
[0035] an average grain size of the bainitic ferrite is in a range
of from 3 to 50 .mu.m.
[0036] The invention described in claim 2 is the hot-rolled steel
sheet according to claim 1, in which the component composition
further contains at least one member of the following (a) to (e):
[0037] (a) at least one of Cr: 2% or less (exclusive of 0%) and Mo:
2% or less (exclusive of 0%), [0038] (b) at least one member
selected from the group consisting of Ti: 0.2% or less (exclusive
of 0%), Nb: 0.2% or less (exclusive of 0%) and V: 0.2% or less
(exclusive of 0%), [0039] (c) B: 0.005% or less (exclusive of 0%),
[0040] (d) at least one member selected from the group consisting
of Cu: 5% or less (exclusive of 0%), Ni: 5% or less (exclusive of
0%) and Co: 5% or less (exclusive of 0%), and [0041] (e) at least
one member selected from the group consisting of Ca: 0.05% or less
(exclusive of 0%), REM: 0.05% or less (exclusive of 0%), Mg: 0.02%
or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb:
0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of
0%).
Advantage of the Invention
[0042] According to the present invention, in a microstructure
mainly composed of bainitic ferrite having a predetermined average
grain size +polygonal ferrite, the solute N amount is ensured and
at the same time, the C content and the N content satisfy a
predetermined relationship, so that a hot-rolled steel sheet which
is reduced in the deformation resistance during cold working,
thereby not only extending the life of a metal mold but also hardly
allowing occurrence of cracking in a steel sheet, and can assure
the parts obtained after working of predetermined surface property
and post-working hardness, can be provided.
MODE FOR CARRYING OUT THE INVENTION
[0043] The hot-rolled steel sheet according to the present
invention (hereinafter, sometimes referred to as "steel sheet of
the present invention" or simply as "steel sheet") is described in
more detail below. The steel sheet of the present invention is
common with the hot-forged material described in Patent Document 2
in that the amount of N dissolved in solid is ensured and at the
same time, the C content and the N content satisfy a predetermined
relationship, but differs in that the C content is allowable up to
a somewhat high range, the microstructure is a bainitic
ferrite-polygonal ferrite-pearlite multi-phase microstructure and
at the same time, the bainitic ferrite particle is refined.
[Thickness of Steel Sheet of the Present Invention: from 3 to 20
mm]
[0044] First, a steel sheet having a thickness of 3 to 20 mm is
targeted by the steel sheet of the present invention. If the sheet
thickness is less than 3 mm, the rigidity as a structure cannot be
ensured. On the other hand, if the sheet thickness exceeds 20 mm,
the microstructure specified in the present invention can be hardly
achieved, and the desired effects cannot be obtained. The sheet
thickness is preferably from 4 to 19 mm.
[0045] Next, the component composition constituting the steel sheet
of the present invention is described. In the following, the units
of chemical components are all mass %.
[Component Composition of Steel Sheet of the Present Invention]
<C: 0.3% or Less (Exclusive of 0%)>
[0046] C is an element greatly affecting the formation of the
microstructure of the steel sheet and although the microstructure
is a bainitic ferrite-polygonal ferrite-pearlite multi-phase
microstructure, in order to form a bainitic ferrite-polygonal
ferrite-based microstructure with as little pearlite as possible,
the content of this element needs to be limited. If C is contained
too much, the pearlite fraction in the steel sheet microstructure
is increased, leaving a fear that the deformation resistance
becomes excessive due to work hardening of pearlite. Therefore, the
C content in the steel sheet is limited to 0.3% or less, preferably
0.25% or less, more preferably 0.2% or less, and still more
preferably 0.15% or less. However, if the C content is too small,
deoxidation during melting of steel is difficult to be achieved and
at the same time, the strength and hardness after cold working can
be hardly satisfied. Therefore, it is preferably 0.0005% or more,
more preferably 0.0008% or more and still more preferably 0.001% or
more.
<Si: 0.5% or Less (Exclusive of 0%)>
[0047] Si forms a solid solution in steel to thereby increase the
deformation resistance of the steel sheet and thus, is an element
needed to be reduced as much as possible. Therefore, in order to
suppress the increase in deformation resistance, the Si content in
the steel sheet is limited to 0.5% or less, preferably 0.45% or
less, more preferably 0.4% or less, and still more preferably 0.3%
or less. However, if the Si content is extremely small, deoxidation
during melting is difficult to be achieved and at the same time,
the strength and hardness after cold working can be hardly
satisfied. Therefore, it is preferably 0.005% or more, more
preferably 0.008% or more, and still more preferably 0.01% or
more.
<Mn: from 0.2 to 1%>
[0048] Mn is an element exerting deoxidation and desulfurization
actions in the process of steel making. Furthermore, when the N
content in the steel material is increased, cracking is readily
generated due to dynamic strain aging by heat generation during
working, but, on the other hand, Mn has an effect of enhancing the
workability on this occasion and inhibiting cracking. In order to
effectively bring out these actions, the Mn content in the steel
sheet is 0.2% or more, preferably 0.22% or more and more preferably
0.25% or more. However, if the Mn content is too large, the
deformation resistance becomes excessive, and segregation occurs to
produce a heterogeneity in the microstructure. Therefore, it is 1%
or less, preferably 0.98% or less and more preferably 0.95% or
less.
<P: 0.05% or Less (Exclusive of 0%)>
[0049] P is an impurity element unavoidably contained in the steel.
It is an element that, if contained in ferrite, segregates at a
ferrite grain boundary to deteriorate cold workability and
contributes to solid-solution hardening of ferrite and thereby
gives rise to an increase in the deformation resistance. Therefore,
the P content is preferably reduced as much as possible in view of
cold workability, but if excessively reduced, the steel making cost
increases. Therefore, in consideration of process capability, the
content is 0.05% or less and preferably 0.03% or less.
<S: 0.05% or Less (Exclusive of 0%)>
[0050] S is also an unavoidable impurity, similarly to P, and is an
element precipitating as FeS at a grain boundary in a film form to
deteriorate the workability. In addition, this also has an action
of causing hot shortness. In the present invention, from the
standpoint of enhancing the deformation performance, the S content
is 0.05% or less and preferably 0.03% or less. However, reduction
of the S content to 0 is difficult in industry. Since S has an
effect of enhancing the machinability, in view of machinability
enhancement, it is recommended to be contained in an amount of
preferably 0.002% or more and more preferably 0.006% or more.
<Al: from 0.01 to 0.1%>
[0051] Al is an element effective for deoxidation in the process of
steel making. In order to obtain this deoxidation effect, the Al
content in the steel sheet is 0.01% or more, preferably 0.015% or
more and more preferably 0.02% or more. However, if the Al content
is excessively large, toughness is reduced and cracking readily
occurs. Therefore, the content is 0.1% or less, preferably 0.09% or
less and more preferably 0.08% or less.
<N: from 0.008 to 0.025%>
[0052] N is an important element for obtaining predetermined
strength by static strain aging after working. Therefore, the N
content in the steel sheet is 0.008% or more, preferably 0.0085% or
more and more preferably 0.009% or more. However, if the N content
is excessively large, the effect of dynamic strain aging during
working, in addition to static strain aging, becomes significant,
and thus the deformation resistance is increased, which is
unsuitable. Therefore, the content is 0.025% or less, preferably
0.023% or less and more preferably 0.02% or less.
<Solute N: 0.007% or More>
[0053] When a predetermined amount of solute N (hereinafter,
referred to as "solute N amount") is ensured in the steel sheet,
the static strain aging can be promoted without increasing the
deformation resistance very much. In order to ensure predetermined
strength after cold working, the solute N amount needs to be 0.007%
or more. However, if the solute N amount is excessively large, not
only the cold workability is deteriorated but also the amount of
the solute N fixed to working strain is increased, and as a result,
SS mark is readily generated and the surface property is
deteriorated as well. Therefore, it is preferably 0.03% or less. In
this connection, since the N content in the steel material is
0.025% or less, the solute N amount is substantially kept from
becoming 0.025% or more.
[0054] Here, the solute N amount in the present invention is an
amount determined by subtracting the amount of total N compounds
from the total N amount in the steel sheet in conformity with JIS G
1228. An example of the practical method for measuring the solute N
amount is described below.
(a) Inert Gas Fusion Method-Thermal Conductivity Method
(Measurement of Total N Amount)
[0055] A sample cut out from a test material is placed in a
crucible and fused in an inert gas stream to extract N, and the
extract is transferred to a thermal conductivity cell and measured
for the change in thermal conductivity to determine the total N
amount. (b) Ammonia Distillative Separation and Indophenol Blue
Absorptiometry (measurement of amount of total N compounds)
[0056] A sample cut out from a test material is dissolved in a 10%
AA-type electrolytic solution and a constant current electrolysis
is performed to measure the amount of total N compounds in the
steel. The 10% AA-type electrolytic solution used is a nonaqueous
solvent-type electrolytic solution composed of 10% acetone and 10%
tetramethylammonium chloride, with the remainder being methanol,
and is a solution not forming a passive film on the steel
surface.
[0057] About 0.5 g of the sample of the test material is dissolved
in the 10% AA-type electrolytic solution, and the insoluble residue
(N compounds) produced is filtered through a polycarbonate-made
filter having a pore size of 0.1 .mu.m. The obtained insoluble
residue is decomposed by heating in sulfuric acid, potassium
sulfate and pure copper-made chips, and the decomposition product
is combined with the filtrate. The resulting solution is made
alkaline with sodium hydroxide and then subjected to steam
distillation, and the distilled ammonia is absorbed by diluted
sulfuric acid. Furthermore, phenol, sodium hypochlorite and sodium
pentacyanonitrosylferrate(III) are added to produce a blue complex,
and the absorbance thereof is measured by using an absorptiometer
to determine the amount of total N compounds.
[0058] The solute N amount can be determined by subtracting the
amount of total N compounds determined by the method (b) from the
total N amount determined by the method (a).
<Contents of C and N Satisfy the Relationship of
10C+N.ltoreq.3.0>
[0059] In the steel material of the present invention, solute C
greatly increases the deformation resistance and does not so much
contribute to static strain aging and, on the other hand, the
solute N can promote the static strain aging without raising the
deformation resistance very much and therefore, has an action of
allowing for an increase in the hardness after working. Therefore,
in the steel material of the present invention, in order to
increase the hardness after working without raising the deformation
resistance during working very much, the C content and the N
content must satisfy the relationship of 10C+N.ltoreq.3.0. It is
preferably 0.009.ltoreq.10C+N.ltoreq.2.8, more preferably
0.01.ltoreq.10C+N.ltoreq.2.5 and still more preferably
0.01.ltoreq.10C+N.ltoreq.2.0. From the standpoint of refining the
grain in the hot-rolled steel sheet and ensuring formability of the
steel sheet, the C content and the solute C amount are needed to
some extent, but if 10C+N>3.0, the amounts of C and/or N are too
large, and the deformation resistance becomes excessive. In the
inequality above, the coefficient of the C content is set to be 10
times the coefficient of the N content by taking into account the
fact that even when the contents are the same, the degree of
increase in the strength and deformation resistance in the
hot-rolled steel sheet of the present invention, which is
attributable to the solute C, is about one digit (10 times) larger
than that attributable to the solute N.
[0060] The steel of the present invention fundamentally contains
the above-described components, with the remainder being iron and
unavoidable impurities, but in addition, the following allowable
components may be added, as long as the action of the present
invention is not impaired.
<Cr: 2% or Less Eexclusive of 0%) and/or Mo: 2% or Less
(Exclusive of 0%)>
[0061] Cr is an element having an action of increasing the grain
boundary strength and thereby enhancing the deformation performance
of the steel. In order to effectively bring out such an action, Cr
is preferably contained in an amount of 0.2% or more, but if Cr is
contained too much, the deformation resistance may be increased to
reduce the cold workability. Therefore, it is recommended that the
content thereof is 2% or less, furthermore 1.5% or less, and in
particular 1% or less.
[0062] Mo is an element having an action of increasing the hardness
of the steel material after working and the deformation
performance. In order to effectively bring out such an action, Mo
is preferably contained in an amount of 0.04% or more, more
preferably 0.08% or more. However, if Mo is contained too much, the
cold workability may be deteriorated. Therefore, it is recommended
that the content thereof is 2% or less, furthermore 1.5% or less,
and in particular 1% or less.
<At Least One Member Selected from the Group Consisting of Ti:
0.2% or Less (Exclusive of 0%), Nb: 0.2% or Less (Exclusive of 0%)
and V: 0.2% or Less (Exclusive of 0%)>
[0063] These elements have a high affinity for N and are elements
fulfilling the role of forming N compounds by coexisting with N,
refining the grain of steel, enhancing the toughness of a processed
product obtained after cold working, and also enhancing the
cracking resistance. Even if each element is contained in an amount
over the upper limit value, an effect of improving the property is
not obtained. Therefore, it is recommended that the content of each
element is 0.2% or less, furthermore from 0.001 to 0.15% and in
particular from 0.002 to 0.1%.
<B: 0.005% or Less (Exclusive of 0%)>
[0064] Similarly to Ti, Nb and V above, B has a high affinity for N
and is an element fulfilling the role of forming a N compound by
coexisting with N, refining the grain of steel, enhancing the
toughness of a processed product obtained after cold working, and
also enhancing the cracking resistance. Therefore, in the case
where the steel sheet of the present invention contains B, a
predetermined solute N amount can be ensured to enhance the
strength after cold working. For this reason, it is recommended
that the content thereof is 0.005% or less, furthermore from 0.0001
to 0.0035% and in particular from 0.0002 to 0.002%.
<At Least One Member Selected from the Group Consisting of Cu:
5% or Less (Exclusive of 0%), Ni: 5% or Less (Exclusive of 0%) and
Co: 5% or Less (Exclusive of 0%)>
[0065] All of these elements have an action of hardening the steel
material by stain aging and are elements effective for enhancing
the post-working strength. In order to effectively bring out such
an action, each of these elements is preferably contained in an
amount of 0.1% or more and furthermore 0.3% or more. However, if
the content of each of these elements is too much large, the effect
of hardening the steel material by stain aging and furthermore the
effect of enhancing the post-working strength may be saturated, or
the cracking may be promoted. Therefore, it is recommended that
each of them is 5% or less, furthermore 4% or less and in
particular 3% or less.
<At Least One Member Selected from the Group Consisting of Ca:
0.05% or Less (Exclusive of 0%), REM: 0.05% or Less (Exclusive of
0%), Mg: 0.02% or Less (Exclusive of 0%), Li: 0.02% or Less
(Exclusive of 0%), Pb: 0.5% or Less (Exclusive of 0%), and Bi: 0.5%
or Less (Exclusive of 0%)>
[0066] Ca is an element spheroidizing a sulfide compound-based
inclusion such as MnS to thereby enhance the deformation
performance of steel and at the same time, contributing to
improvement of the machinability. In order to effectively bring out
such an action, Ca is preferably contained in an amount of 0.0005%
or more and furthermore 0.001% or more. Even if contained too much,
the effect thereof is saturated and an effect consistent with the
content cannot be expected. Therefore, 0.05% or less, furthermore
0.03% or less and in particular 0.01% or less are recommended.
[0067] REM is, similarly to Ca, an element spheroidizing a sulfide
compound-based inclusion such as MnS to thereby enhance the
deformation performance of steel and at the same time, contributing
to improvement of the machinability. In order to effectively bring
out such an action, REM is preferably contained in an amount of
0.0005% or more and furthermore 0.001% or more. Even if contained
too much, the effect thereof is saturated and an effect consistent
with the content cannot be expected. Therefore, 0.05% or less,
furthermore 0.03% or less and in particular 0.01% or less are
recommended.
[0068] The "REM" as used in the present invention means to include
lanthanoid elements (15 elements from La to Lu) as well as Sc
(scandium) and Y (yttrium). Among these elements, it is preferable
to contain at least one element selected from the group consisting
of La, Ce and Y, and it is more preferable to contain La and/or
Ce.
[0069] Mg is, similarly to Ca, an element spheroidizing a sulfide
compound-based inclusion such as MnS to thereby enhance the
deformation performance of steel and at the same time, contributing
to improvement of the machinability. In order to effectively bring
out such an action, Mg is preferably contained in an amount of
0.0002% or more and furthermore 0.0005% or more. Even if contained
too much, the effect thereof is saturated and an effect consistent
with the content cannot be expected. Therefore, 0.02% or less,
furthermore 0.015% or less and in particular 0.01% or less are
recommended.
[0070] Li is, similarly to Ca, an element spheroidizing a sulfide
compound-based inclusion such as MnS to allow for enhancement of
the deformation performance of steel and in addition, contributing
to improvement of the machinability by lowering the melting point
of an Al-based oxide and thereby making it harmless. In order to
effectively bring out such an action, Li is preferably contained in
an amount of 0.0002% or more and furthermore 0.0005% or more. Even
if contained too much, the effect thereof is saturated and an
effect consistent with the content cannot be expected. Therefore,
0.02% or less, furthermore 0.015% or less and in particular 0.01%
or less are recommended.
[0071] Pb is an element effective for enhancing the machinability.
In order to effectively bring out such an action, Pb is preferably
contained in an amount of 0.005% or more and furthermore 0.01% or
more. However, if contained too much, there arises a problem with
production such as formation of a roll mark. Therefore, 0.5% or
less, furthermore 0.4% or less and in particular 0.3% or less are
recommended.
[0072] Bi is, similarly to Pb, an element effective for enhancing
the machinability. In order to effectively bring out such an
action, Bi is preferably contained in an amount of 0.005% or more
and furthermore 0.01% or more. Even if contained too much, the
effect of enhancing the machinability is saturated. Therefore, 0.5%
or less, furthermore 0.4% or less and in particular 0.3% or less
are recommended.
[0073] The microstructure characterizing the steel sheet of the
present invention is described below.
[Microstructure of Steel Sheet of the Present Invention]
[0074] As described above, the steel sheet of the present invention
has the steel of a bainitic ferrite-polygonal ferrite-pearlite
multi-phase microstructure as a base and, particularly, is
characterized in that the size of the bainitic ferrite particle is
controlled to a specific range.
<Bainitic Ferrite: 5% or More, Pearlite: Less Than 20% and
Remainder: Polygonal Ferrite>
[0075] The microstructure of the steel sheet of the present
invention is composed of a multi-phase microstructure of bainitic
ferrite, polygonal ferrite and pearlite. Bainitic ferrite has an
action of enhancing the workability during cold working while
increasing the hardness after working, and at the same time,
suppressing generation of a stretcher stain mark. In order to
effectively bring out such an action, the content thereof is, in
terms of area ratio, 5% or more, preferably 10% or more and more
preferably 15% or more. If an excess of pearlite is present, the
formability of the steel sheet is deteriorated. Therefore, pearlite
is, in terms of area ratio, less than 20%, preferably 19% or less,
more preferably 18% or less and still more preferably 15% or less.
The remainder is polygonal ferrite.
[0076] In addition to the above-described microstructure, a
cementite phase is also present in the microstructure of the steel
sheet of the present invention, but its amount is as very small as
about 1% or less at the most in terms of area ratio. Therefore, in
the description of the present invention, respective area ratios of
bainitic ferrite, polygonal ferrite and pearlite are defined as
ones specified such that the total area ratio of these three phases
is 100%.
<Average Grain Size of Bainitic Ferrite: in a Range of from 3 to
50 .mu.m>
[0077] The average grain size of bainitic ferrite constituting the
bainitic ferrite microstructure must be in a range of from 3 to 50
.mu.m so as to enhance the workability of the steel sheet and
satisfy the surface property after working. If the bainitic ferrite
particles are excessively fine, the deformation resistance becomes
too high. Therefore, the average grain size thereof is 3 .mu.m or
more, preferably 4 .mu.m or more and more preferably 5 .mu.m or
more. On the other hand, if the bainitic ferrite is excessively
coarsened, the surface property after working is deteriorated and
in addition, toughness, fatigue property, etc. are reduced.
Therefore, the average grain size thereof is 50 .mu.m or less,
preferably 45 .mu.m or less and more preferably 40 .mu.m or
less.
[Method for Measuring Area Ratio of Each Phase]
[0078] As for the area ratio of the each phase above, each test
steel sheet is subjected to Nital etching, and five visual fields
are photographed by a scanning electron microscope (SEM,
magnification: 1,000 times), and as a result, respective
percentages of bainitic ferrite, polygonal ferrite and pearlite can
be determined by a point counting method.
[0079] Here, the bainitic ferrite is defined as a ferrite particle
existing in the bainite (collectively referring to upper bainite
and lower bainite) microstructure in which the grain is in an
axially extended shape (see, Tadashi Furuhara, "Current Opinion on
Definition of Bainite Structure in Steels", Netsu Shori, Vol. 50,
No. 1, February 2010, pp. 22-27) and an aspect ratio (major
axis/minor axis ratio) is 2 or more. In addition, the polygonal
ferrite is defined as a ferrite particle in which the grain is in
an equiaxed shape and an aspect ratio (major axis/minor axis ratio)
is less than 2.
[Method for Measuring Average Grain Size]
[0080] The average grain size of the bainitic ferrite above can be
measured as follows.
[0081] That is, the grain sizes of bainitic ferrite present at
three portions, i.e., an outermost layer part, a part at 1/4 of the
sheet thickness, and a central part in the sheet thickness
direction, are measured. As to the grain size of one bainitic
ferrite particle, the side surface part in the rolling direction at
each measurement portion is subjected to Nital etching, five visual
fields of the corresponding region are photographed by a scanning
electron microscope (SEM; magnification: 1,000 times), and the
diameter including the center of gravity of the bainitic ferrite
grain is determined by image analysis and defined as the average
grain size.
[0082] A preferable production method for obtaining the
above-described steel sheet of the present invention is described
below.
[Preferable Method for Producing Steel Sheet of the Present
Invention]
[0083] The production of the steel sheet of the present invention
may be conducted according to any method as long as it is a method
capable of forming a raw material steel having the above-described
chemical composition into a desired thickness. For example, it can
be conducted by a method in which, under following conditions, a
molten steel having the above-described component composition is
prepared in a converter, subjected to ingot making or continuous
casting to form a slab, and then rolled into a hot-rolled steel
sheet having a desired thickness.
[Preparation of Molten Steel]
[0084] The N content in the molten steel can be adjusted by adding
a N compound-containing raw material to the molten steel and/or
controlling the atmosphere of the converter to a N.sub.2
atmosphere, during melting in the converter.
[Heating]
[0085] Heating before hot rolling is performed at 1,100 to
1,300.degree. C. In this heating, a high-temperature heating
condition is necessary so as to produce no N compound and dissolve
as much N as possible in solid. As for the heating temperature,
preferable lower limit is 1,100.degree. C. and more preferable
lower limit is 1,150.degree. C. On the other hand, a temperature
more than 1,300.degree. C. is operationally difficult.
[Hot Rolling]
[0086] Hot rolling is performed such that the finish rolling
temperature is 880.degree. C. or higher. If the finish rolling
temperature is too low, ferrite transformation takes place at a
high temperature, leading to coarsening of the precipitated carbide
in ferrite (collectively referring to bainitic ferrite and
polygonal ferrite), and the fatigue strength is deteriorated.
Therefore, a finish rolling temperature not less than a certain
level is necessary. The finish rolling temperature is more
preferably 900.degree. C. or more so as to coarsen the austenite
particle and thereby increase the grain size of bainitic ferrite to
a certain extent. The upper limit of the finish rolling temperature
is 1,000.degree. C. because temperature ensuring is difficult.
[0087] The thickness of the hot-rolled steel sheet of the present
invention is from 3 to 20 mm. In order to refine the bainitic
ferrite grain and thereby control the average grain size thereof to
fall in a predetermined grain size range, not only the rolling
temperature must be controlled as above but also the final rolling
reduction by tandem rolling in the finish rolling must be
controlled to be 15% or more. Usually, in the finish rolling,
tandem rollings of from 5 to 7 passes are conducted, where the pass
schedule is set from the standpoint of controlling jamming of the
sheet and the final rolling reduction is up to approximately from
12 to 13%. The final rolling reduction is preferably 16% or more
and more preferably 17% or more. As the final rolling reduction is
higher, e.g., 20% or 30%, the effect of more refining the grain is
obtained, but in view of rolling control, the upper limit is
specified to be about 30%.
[Rapid Cooling After Hot Rolling]
[0088] After the completion of the finish rolling, the sheet is
rapidly cooled at a cooling rate (first cooling rate) of 20.degree.
C./s or more within 5 seconds and the rapid cooling is stopped at a
temperature (rapid cooling stop temperature) of 550.degree. C. or
more and less than 650.degree. C. This is performed so as to obtain
a bainitic ferrite-polygonal ferrite-pearlite multi-phase
microstructure having predetermined phase fractions. If the cooling
rate (rapid cooling rate) is less than 20.degree. C./s, pearlite
transformation is promoted, and if the rapid cooling stop
temperature is less than 550.degree. C., bainite transformation is
suppressed. In both cases, a bainitic ferrite-polygonal
ferrite-pearlite steel having predetermined phase fractions can be
hardly obtained, and the cold workability or surface quality after
working is deteriorated. On the other hand, if the rapid cooling
stop temperature is 650.degree. C. or more, the precipitated
carbide in ferrite is coarsened, and the fatigue strength is
reduced. The rapid cooling stop temperature is preferably from 560
to 640.degree. C. and more preferably from 580 to 620.degree.
C.
[Slow Cooling After Stopping of Rapid Cooling]
[0089] After stopping the rapid cooling, the sheet is slowly cooled
by standing to cool or air cooling at a cooling rate (slow cooling
rate) of 10.degree. C./s or less for 5 to 20 seconds. Accordingly,
the precipitated carbide in ferrite is appropriately refined while
allowing polygonal ferrite formation to proceed sufficiently. If
the cooling rate exceeds 10.degree. C./s or the slow cooling time
is less than 5 seconds, the amount of polygonal ferrite formed is
insufficient, whereas if the slow cooling time exceeds 20 seconds,
the precipitated carbide is not coarsened and the fatigue strength
is deteriorated.
[Rapid Cooling and Coiling After Slow Cooling]
[0090] After the slow cooling, the sheet is again rapidly cooled at
a cooling rate (second rapid cooling rate) of 20.degree. C./s or
more and coiled at 500 to 600.degree. C. This is performed so as to
form a bainitic ferrite+ferrite-based microstructure and thus
ensure cold workability. If the cooling rate (second rapid cooling
rate) is less than 20.degree. C./s or the coiling temperature
exceeds 600.degree. C., the cold workability is deteriorated due to
formation of a large amount of pearlite, whereas if it is less than
500.degree. C., the amount of bainitic ferrite formed is
insufficient and the surface quality after working is
deteriorated.
[0091] The present invention is described in greater detail below
by referring to
[0092] Examples, but the present invention is by no means limited
to the following Examples and may be carried out by appropriately
making changes as long as they are in conformity to the gist
described hereinabove and hereinafter, all of which are included in
the technical scope of the present invention.
EXAMPLES
[0093] Steel having a component composition shown in Table 1 below
was melted by a vacuum melting method and cast into an ingot having
a thickness of 120 mm, which was subjected to hot rolling under
conditions shown in Table 2 below to produce a hot-rolled steel
sheet. In each test, the cooling rate until stopping of rapid
cooling after the completion of finish rolling was 20.degree. C./s
or more, and the cooling after stopping the rapid cooling had the
conditions in which a slow cooling is performed at a cooling rate
of 10.degree. C./s or less for from 5 to 20 s.
[0094] The thus-obtained hot-rolled steel sheets were measured for
the solute N content, area ratio of each phase in the steel sheet
microstructure, and average grain size of bainitic ferrite, by
respective measurement methods described in "MODE FOR CARRYING OUT
THE INVENTION" above.
[0095] In addition, those hot-rolled steel sheets were evaluated as
follows for the cold workability as well as the surface quality and
hardness after working.
(Evaluation of Cold Workability)
[0096] The cold workability was evaluated based on the amount of
work hardening defined by the difference in the hardness's at the
sample central part between before and after working. First, the
hot-rolled steel sheet was subjected to a working test by using a
working reproducing tester (manufactured by Fuji Electronic
Industrial Co., Ltd, Thermecmaster Z) under the conditions of
working temperature: room temperature, rolling reduction: 70% and
strain rate: 10/s. The shape of the working specimen was adjusted
according to the sheet thickness such that the diameter/height
ratio became substantially constant, e.g., .phi.8 mm.times.12 mm,
.phi.6 mm.times.10 mm, or .phi.4 mm.times.6 mm. In the measurement
of hardness, the Vickers hardness (Hv) of each working specimen was
measured before and after the working test above by using a Vickers
hardness tester and setting the measurement position at 1/4 portion
(position in the midst between the center and the outer
circumference) in the circle diameter direction at the central part
in the compression direction of the working specimen, under the
conditions of load: 500 g and number of measurements: 5, and
respective averages were designated as the pre-working hardness and
the post-working hardness.
[0097] Then, the cold workability was evaluated, as described
above, based on the amount of work hardening defined by
"post-working hardness-pre-working hardness". A larger amount of
work hardening means better workability, and 80 Hv or more was
judged as passed.
(Evaluation of Hardness After Working)
[0098] As the evaluation of hardness after working, evaluation was
performed based on the above-described post-working hardness
measured after the working test and sample specimen of 250 Hv or
more was judged as passed.
(Evaluation of Surface Quality After Working)
[0099] The surface quality after working was evaluated by the
presence or absence of SS mark generation after a tensile test. For
this purpose, a No. 5 specimen (25 mm.times.50 mm.times.[from sheet
surface to center in the sheet thickness direction (the sheet
thickness was adjusted by taking into account the capability of the
tensile tester)]) of JIS Z 2201 in a direction perpendicular to the
rolling direction was sampled and subjected to a tensile test based
on JIS Z 2241 (1980) (Method for Tensile Test of Metal Material)
under the conditions of room temperature of 20.degree. C. and an
initial crosshead speed of 600 mm/min until the strain amount
reached 15%. The presence or absence of SS mark generation on the
surface of the specimen after the tensile test was made to appear
clear by grinding the specimen surface with a grindstone and
evaluated with an eye. The sample specimen was judged as passed
when SS mark was not generated, and judged as failed when SS mark
was generated.
[0100] These measurement results are shown in Table 3.
TABLE-US-00001 TABLE 1 Components (mass %) Steel [remainder: Fe and
unavoidable impurities] Species C Si Mn P S Al N 10C + N Others a
0.05 0.02 0.40 0.007 0.001 0.025 0.011 0.51 -- b 0.08 0.02 0.40
0.007 0.001 0.022 0.008 0.81 -- c 0.08 0.02 0.40 0.007 0.001 0.022
0.023 0.82 -- d 0.08 0.10 0.30 0.007 0.001 0.023 0.009 0.89 -- e
0.08 0.40 0.20 0.007 0.001 0.024 0.009 0.89 -- f 0.11 0.02 0.40
0.007 0.001 0.022 0.010 1.11 -- 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 0.08 0.02 0.40
0.007 0.001 0.025 0.003 0.80 -- k 0.08 0.02 0.40 0.007 0.001 0.025
0.030 0.83 -- l 0.31 0.02 0.40 0.007 0.001 0.025 0.008 3.11 -- m
0.08 0.60 0.40 0.007 0.001 0.025 0.010 0.81 -- n 0.08 0.02 0.15
0.007 0.001 0.025 0.012 0.81 -- o 0.08 0.02 1.10 0.007 0.001 0.025
0.011 0.81 -- p 0.08 0.02 0.40 0.060 0.001 0.025 0.010 0.81 -- q
0.08 0.02 0.40 0.007 0.060 0.025 0.011 0.81 -- r 0.08 0.02 0.40
0.007 0.001 0.005 0.012 0.81 -- s 0.08 0.02 0.40 0.007 0.001 0.11
0.013 0.81 -- t 0.08 0.02 0.40 0.007 0.001 0.025 0.010 0.81 Cr:
0.5, Nb: 0.03 u 0.08 0.02 0.40 0.007 0.001 0.025 0.010 0.81 Cu:
0.06, Ni: 0.15 v 0.08 0.02 0.40 0.007 0.001 0.025 0.009 0.81 Ca:
0.0025, Li: 0.001 w 0.08 0.02 0.40 0.007 0.001 0.025 0.009 0.81 Cr:
0.5, Nb: 0.03 x 0.30 0.02 0.40 0.007 0.001 0.024 0.025 3.03 -- (--:
not added, underlined: outside the scope of the present
invention)
TABLE-US-00002 TABLE 2 Hot Rolling Conditions Rapid Final Finish
Cooling Thickness Heating Rolling Rolling Stop Coiling of Hot-
Production Steel Temperature Reduction Temperature Temperature
Temperature Rolled No. Species (.degree. C.) (%) (.degree. C.)
(.degree. C.) (.degree. C.) Sheet (mm) 1 a 1250 15 902 627 557 10 2
a 1250 17 908 621 595 4 3 a 1250 15 893 620 545 18 4* a 1000* 14*
803* 526* 412* 10 5* a 1250 14* 905 571 521 25 6* a 1250 9* 899 579
552 10 7 b 1250 17 920 581 514 10 8 c 1250 15 921 609 531 10 9 d
1250 15 915 616 532 10 10 e 1250 15 916 611 532 10 11 f 1250 16 907
587 542 10 12 g 1250 15 906 576 511 10 13 h 1250 16 901 570 520 10
14 i 1250 17 909 634 559 10 15 j 1250 15 912 610 532 10 16 k 1250
16 910 581 533 10 17 l 1250 16 926 639 549 10 18 m 1250 16 889 569
501 10 19 n 1250 16 906 574 523 10 20 o 1250 15 893 570 568 10 21 p
1250 16 919 595 550 10 22 q 1250 15 920 591 521 10 23 r 1250 16 922
599 595 10 24 s 1250 14* 890 596 579 10 25 t 1250 16 892 571 560 10
26 u 1250 15 895 625 591 10 27 v 1250 15 896 564 518 10 28 w 1250
16 911 601 570 10 29 x 1250 16 906 590 507 10 (underlined: outside
the scope of the present invention, *outside the recommended
range)
TABLE-US-00003 TABLE 3 Presence or Post- Amount of Solute N
Microstructure Absence of Working Work Steel Steel Production
Amount Area Ratio (%) BF Average SS Mark Hardness Hardening No.
Species No. (mass %) BF PF P Grain Size (.mu.m) Generation (Hv)
(Hv) Remarks 1 a 1 0.0084 10 87 3 27 none 257 93 steel of invention
2 a 2 0.008 25 68 7 14 none 273 101 steel of invention 3 a 3 0.008
8 89 3 34 none 251 83 steel of invention 4 a 4* 0.003 69 8 23 21
none 192 53 comparative steel 5 a 5* 0.009 3 92 5 53 generated 193
65 comparative steel 6 a 6* 0.008 4 88 8 51 generated 195 71
comparative steel 7 b 7 0.007 45 51 4 23 none 290 103 steel of
invention 8 c 8 0.019 48 47 5 22 none 295 107 steel of invention 9
d 9 0.0085 53 43 4 20 none 303 105 steel of invention 10 e 10 0.008
58 38 4 19 none 311 106 steel of invention 11 f 11 0.009 65 30 5 16
none 307 111 steel of invention 12 g 12 0.008 71 18 11 11 none 322
103 steel of invention 13 h 13 0.009 77 10 13 10 none 335 102 steel
of invention 14 i 14 0.008 83 2 15 8 none 353 91 steel of invention
15 j 15 0.002 46 50 4 24 none 171 71 comparative steel 16 k 16
0.025 49 48 3 22 generated -- -- comparative steel 17 l 17 0.007 43
32 25 9 generated -- -- comparative steel 18 m 18 0.0085 63 32 5 23
none -- -- comparative steel 19 n 19 0.010 22 74 4 22 none 221 71
comparative steel 20 o 20 0.009 43 51 6 21 none -- -- comparative
steel 21 p 21 0.009 41 54 5 26 none -- -- comparative steel 22 q 22
0.010 45 51 4 27 none -- -- comparative steel 23 r 23 0.011 39 57 4
26 none -- -- comparative steel 24 s 24 0.012 55 41 4 24 none -- --
comparative steel 25 t 25 0.008 55 42 3 15 none 290 103 steel of
invention 26 u 26 0.009 58 38 4 16 none 280 105 steel of invention
27 v 27 0.008 49 47 4 17 none 270 104 steel of invention 28 w 28
0.008 63 34 3 15 none 275 98 steel of invention 29 x 29 0.020 75 6
19 10 generated -- -- comparative steel (underlined: outside the
scope of the present invention, *outside the recommended range, BF:
bainitic ferrite, PF: polygonal ferrite, P: pearlite, --: not
measured because cracking was generated before reaching the
predetermined rolling reduction during working test, steel of
invention: [no SS mark generation] and [post-working hardness after
working .gtoreq.250 Hv] and [amount of work hardening .gtoreq.80
Hv], comparative steel: when the above-described conditions of the
steel of invention are not satisfied)
[0101] As shown in Table 3, each of Steel Nos. 1 to 3, 7 to 14 and
25 to 28 was produced by using a steel species satisfying the
requirements specified for the component composition of the present
invention under the recommended hot rolling conditions, and as a
result, it could be confirmed that these are steel of the invention
fulfilling the requirements specified for the microstructure of the
present invention, all of the surface property after working, the
post-working hardness and the amount of work hardening meet the
acceptance standards, and a hot-rolled steel sheet exhibiting good
cold workability during working and exhibiting predetermined
surface quality and hardness (strength) after working can be
obtained.
[0102] On the other hand, Steel Nos. 4 to 6, 15 to 24 and 29 are
comparative steels failing in satisfying at least one of the
requirements specified for the component composition and the
microstructure in the present invention, where at least one of the
surface property after working, the post-working hardness and
therefore the amount of work hardening does not meet the acceptance
standard.
[0103] For example, Steel No. 4 satisfies the requirements for the
component composition, but since the heating temperature before hot
rolling is outside the recommended range and is too low, the solute
N amount is insufficient and therefore the post-working hardness is
inferior.
[0104] Steel No. 5 satisfies the requirements for the component
composition, but since the sheet thickness after hot rolling is
outside the specified range and is too large, the bainitic ferrite
is lacking but, on the other hand, is coarsened, and therefore both
the post-working hardness and the amount of work hardening are
inferior.
[0105] Steel No. 6 satisfies the requirements for the component
composition, but since the final rolling reduction at the time of
hot rolling is outside the recommended range and is too small, the
bainitic ferrite is lacking but, on the other hand, is coarsened,
and therefore both the post-working hardness and the amount of work
hardening are inferior.
[0106] In Steel No. 15 (steel species j) where the hot rolling
conditions are in the recommended range but the N content is too
low, both the post-working hardness and the amount of work
hardening are inferior.
[0107] On the other hand, in Steel No. 16 (steel species k) where
the hot rolling conditions are in the recommended range but the N
content is too high, not only the cold workability but also the
surface property after working is inferior.
[0108] In Steel No. 17 (steel species 1) where the hot rolling
conditions are in the recommended range but the C content is too
high and the requirement of 10C+N.ltoreq.3 0 is not satisfied,
pearlite is excessively formed and not only the cold workability
but also the surface property after working is inferior.
[0109] In Steel No. 18 (steel species m) where the hot rolling
conditions are in the recommended range but the Si content is too
high, at least the cold workability is inferior.
[0110] In Steel No. 19 (steel species n) where the hot rolling
conditions are in the recommended range but the Mn content is too
low, both the post-working hardness and the amount of work
hardening are inferior.
[0111] On the other hand, in Steel No. 20 (steel species o) where
the hot rolling conditions are in the recommended range but the Mn
content is too high, at least the cold workability is inferior.
[0112] In Steel No. 21 (steel species p) where the hot rolling
conditions are in the recommended range but the P content is too
high, at least the cold workability is inferior.
[0113] In Steel No. 22 (steel species q) where the hot rolling
conditions are in the recommended range but the S content is too
high, at least the cold workability is inferior.
[0114] In Steel No. 23 (steel species r) where the hot rolling
conditions are in the recommended range but the Al content is too
low, at least the cold workability is inferior.
[0115] In Steel No. 24 (steel species s) where the hot rolling
conditions except for the final rolling reduction are in the
recommended range but the Al content is too high, at least the cold
workability is inferior.
[0116] On the other hand, in Steel No. 29 (steel species x) where
the hot rolling conditions are in the recommended range but the
requirement of 10C-FN.ltoreq.3.0 is not satisfied, not only the
cold workability but also the surface property after working are
inferior.
[0117] From the above, the applicability of the present invention
could be confirmed.
[0118] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope of the
present invention.
[0119] This application is based on Japanese Patent Application
(Patent Application No. 2013-183091) filed on Sep. 4, 2013, the
contents of which are incorporated herein by way of reference.
INDUSTRIAL APPLICABILITY
[0120] The hot-rolled steel material of the present invention is
useful, e.g., for various parts for automotive (for example,
transmission parts such as gear, and casings) and can realize
lighter weight and higher strength.
* * * * *