U.S. patent application number 15/303658 was filed with the patent office on 2017-02-09 for hot-rolled steel sheet having good cold workability and excellent 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.
Application Number | 20170037496 15/303658 |
Document ID | / |
Family ID | 54324166 |
Filed Date | 2017-02-09 |
United States Patent
Application |
20170037496 |
Kind Code |
A1 |
KAJIHARA; Katsura |
February 9, 2017 |
HOT-ROLLED STEEL SHEET HAVING GOOD COLD WORKABILITY AND EXCELLENT
HARDNESS AFTER WORKING
Abstract
This hot-rolled steel sheet 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. The contents of
solid-solved N, C and N are within specific ranges, and bainitic
ferrite having a specific average crystal grain size and pearlite
have specific area occupancies in the structure, with the balance
occupied by polygonal ferrite. This hot-rolled steel sheet has a
specific hardness distribution in the thickness direction.
Inventors: |
KAJIHARA; Katsura; (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: |
54324166 |
Appl. No.: |
15/303658 |
Filed: |
April 16, 2015 |
PCT Filed: |
April 16, 2015 |
PCT NO: |
PCT/JP2015/061767 |
371 Date: |
October 12, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 2211/005 20130101;
C22C 38/16 20130101; C21D 8/0263 20130101; C21D 9/46 20130101; C21D
2211/009 20130101; B21B 2261/22 20130101; C21D 8/0226 20130101;
C22C 38/06 20130101; C21D 2211/002 20130101; C22C 38/00 20130101;
C22C 38/60 20130101; C22C 38/001 20130101; B21B 2001/225 20130101;
C22C 38/002 20130101; C22C 38/08 20130101; C22C 38/04 20130101;
C22C 38/02 20130101; C22C 38/26 20130101; C21D 8/0273 20130101 |
International
Class: |
C22C 38/26 20060101
C22C038/26; C22C 38/08 20060101 C22C038/08; C22C 38/06 20060101
C22C038/06; C21D 8/02 20060101 C21D008/02; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 9/46 20060101
C21D009/46; C22C 38/16 20060101 C22C038/16; C22C 38/04 20060101
C22C038/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 18, 2014 |
JP |
2014-086747 |
Claims
1. A hot-rolled steel sheet excellent in heavy cold workability and
surface hardness after working, wherein: a sheet thickness is from
3 to 20 mm; a component composition comprises, in mass %, C: more
than 0% and 0.3% or less, Si: more than 0% and 0.5% or less, Mn:
from 0.2 to 1%, P: more than 0% and 0.05% or less, S: more than 0%
and 0.05% or less, Al: from 0.01 to 0.1%, and N: from 0.008 to
0.025%, with the remainder being iron and unavoidable impurities,
wherein solute N: 0.007% or more and the contents of C and N
satisfy the relationship 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; an average grain size of the
bainitic ferrite is in a range of from 3 to 50 .mu.m; and
(Hv.sub.max-Hv.sub.min)/Hv.sub.min.ltoreq.0.3, wherein, in a
hardness distribution in a thickness direction, Hv.sub.max and
Hv.sub.min.ltoreq.0.3, are respectively the maximal value and the
minimal value of Vickers hardness values of three portions that are
a surface portion, a t/4 portion where t is the sheet thickness,
and a central portion.
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 member selected from the
group consisting of Cr: more than 0% and 2% or less and Mo: more
than 0% and 2% or less, in mass %; (b) at least one member selected
from the group consisting of Ti: more than 0% and 0.2% or less, Nb:
more than 0% and 0.2% or less, and V: more than 0% and 0.2% or
less, in mass %; (c) B: more than 0% and 0.005% or less in mass %;
(d) at least one member selected from the group consisting of Cu:
more than 0% and 5% or less, Ni: more than 0% and 5% or less, and
Co: more than 0% and 5% or less, in mass %; and (e) at least one
member selected from the group consisting of Ca: more than 0% and
0.05% or less, REM: more than 0% and 0.05% or less, Mg: more than
0% and 0.02% or less, Li: more than 0% and 0.02% or less, Pb: more
than 0% and 0.5% or less, and Bi: more than 0% and 0.5% or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot-rolled steel sheet
which shows satisfactory cold workability (heavy cold workability)
when cold-worked so as to locally undergo an extremely large
deformation strain and which shows given hardness after the
working.
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, investigations are being
made on the production of parts from steel sheets, in place of
production of forged products from steel bars (hot forging, cold
forging, etc.), for the purpose of weight and cost reductions in
parts. However, since transmission parts have complicated shapes,
the parts produced form steel sheets by cold working (press
forming, forging, etc.) have a drawback in that these parts locally
have portions having an extremely large deformation strain (about 2
or larger in terms of true strain amount) and local cracking is apt
to occur therein.
[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-H10-306345
[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] An object of the present invention, which has been achieved
under these circumstances, is to provide a hot-rolled steel sheet
which shows satisfactory cold workability (heavy cold workability)
when cold-worked so as to undergo an exceedingly high degree of
deformation strains and which shows given hardness after the
working.
Means for Solving the Problems
[0019] In a hot-rolled steel sheet excellent in heavy cold
workability and surface hardness after working in the first
invention of the present invention,
[0020] a sheet thickness is from 3 to 20 mm;
[0021] a component composition comprises, in mass %,
[0022] C: more than 0% and 0.3% or less,
[0023] Si: more than 0% and 0.5% or less,
[0024] Mn: from 0.2 to 1%,
[0025] P: more than 0% and 0.05% or less,
[0026] S: more than 0% and 0.05% or less,
[0027] Al: from 0.01 to 0.1%, and
[0028] N: from 0.008 to 0.025%,
[0029] with the remainder being iron and unavoidable impurities,
wherein
[0030] solute N: 0.007% or more and
[0031] the contents of C and N satisfy the relationship
10C+N.ltoreq.3.0;
[0032] 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;
[0033] an average grain size of the bainitic ferrite is in a range
of from 3 to 50 .mu.m; and
[0034] (Hv.sub.max-Hv.sub.min)/Hv.sub.min.ltoreq.0.3, wherein, in a
hardness distribution in a thickness direction, Hv.sub.max and
Hv.sub.min are respectively the maximal value and the minimal value
of Vickers hardness values of three portions that are a surface
portion, a t/4 portion where t is the sheet thickness, and a
central portion.
[0035] In the hot-rolled steel sheet excellent in heavy cold
workability and surface hardness after working in the second
invention of the present invention according to the first
invention, the component composition further comprises, in mass
%:
[0036] at least one member selected from the group consisting of
Cr: more than 0% and 2% or less and Mo: more than 0% and 2% or
less.
[0037] In the hot-rolled steel sheet excellent in heavy cold
workability and surface hardness after working in the third
invention of the present invention according to the first or second
invention, the component composition further comprises, in mass
%:
[0038] at least one member selected from the group consisting of
Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or
less, and V: more than 0% and 0.2% or less.
[0039] In the hot-rolled steel sheet excellent in heavy cold
workability and surface hardness after working in the fourth
invention of the present invention according to any one of the
first to third inventions, the component composition further
comprises, in mass %.
[0040] B: more than 0% and 0.005% or less.
[0041] In the hot-rolled steel sheet excellent in heavy cold
workability and surface hardness after working in the fifth
invention of the present invention according to any one of the
first to fourth inventions, the component composition further
comprises, in mass %:
[0042] at least one member selected from the group consisting of
Cu: more than 0% and 5% or less, Ni: more than 0% and 5% or less,
and Co: more than 0% and 5% or less.
[0043] In the hot-rolled steel sheet excellent in heavy cold
workability and surface hardness after working in the sixth
invention of the present invention according to any one of the
first to fifth inventions, the component composition further
comprises, in mass %:
[0044] at least one member selected from the group consisting of
Ca: more than 0% and 0.05% or less, REM: more than 0% and 0.05% or
less, Mg: more than 0% and 0.02% or less, Li: more than 0% and
0.02% or less, Pb: more than 0% and 0.5% or less, and Bi: more than
0% and 0.5% or less.
Effects of the Invention
[0045] According to the present invention, the hot-rolled steel
sheet has a microstructure which mainly contains bainitic ferrite
having a given average grain size and polygonal ferrite and in
which a solute N amount has been ensured and the content of C and
the content of N satisfy a given relationship. Because of this, the
hot-rolled steel sheet shows reduced deformation resistance during
cold working, thereby prolonging the life of the die. Furthermore,
since this hot-rolled steel sheet has a hardness distribution in a
thickness direction regulated so as to be in a given range, this
hot-rolled steel sheet is less apt to suffer local cracking even in
cold working which may locally cause an extremely large deformation
strain, and the parts obtained therefrom by working can have given
hardness.
BRIEF DESCRIPTION OF THE DRAWING
[0046] FIG. 1 is a view which diagrammatically shows the
configuration of the wedge type compression tester used in the
Examples for evaluating heavy cold workability.
MODES FOR CARRYING OUT THE INVENTION
[0047] The hot-rolled steel sheet in the present invention
(hereinafter referred to also as "steel sheet of the present
invention" or merely as "steel sheet") is explained below in more
detail. The steel sheet of the present invention has the features
of ensuring a solute N amount and having C and N contents which
satisfy a given relationship, in common with the hot-forged
material described in Patent Document 2. However, the steel sheet
of the present invention differs from the hot-forged material
described in Patent Document 2 in that the steel sheet of the
present invention has a C content permissible to a relatively high
value and has a bainitic ferrite/polygonal ferrite/pearlite
multiphase microstructure, that the bainitic ferrite grains are
refined, and that the steel sheet has a hardness distribution in a
thickness direction regulated so as to be in a given range.
[Thickness of Steel Sheet of the Present Invention: From 3 to 20
mm]
[0048] 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.
[0049] 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]
[0050] <C: More than 0% and 0.3% or Less>
[0051] 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: More than 0% and 0.5% or Less>
[0052] 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%>
[0053] 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: More than 0% and 0.05% or Less>
[0054] 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: More than 0% and 0.05% or Less>
[0055] 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%>
[0056] 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 mass % or less.
<N: From 0.008 to 0.025%>
[0057] 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>
[0058] Furthermore, by ensuring a content of solute N (hereinafter
referred to as "solute N amount") in the steel sheet, the static
strain aging can be promoted without increasing the deformation
resistance very much. For ensuring a required strength after cold
working, a solute N amount of 0.007% or more is necessary. However,
in a case where the solute N amount is too high, not only the cold
workability is deteriorated, but also an amount of the solute N
fixed to working strains is increased and this is prone to result
in a hardness distribution along the thickness direction of the
hot-rolled sheet. This hardness distribution in a thickness
direction cannot be eliminated even when the annealing conditions
which will be described later are applied, and this hot-rolled
steel sheet is prone to crack when subjected to working which may
cause an extremely large local deformation strain. Consequently,
the solute N amount is preferably 0.03% or less. Since the content
of N in the steel material is 0.025% or less, the case where the
solute N amount is 0.025% or more is not substantially
occurred.
[0059] 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)
[0060] 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)
[0061] 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.
[0062] 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.
[0063] 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>
[0064] 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.
[0065] 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.
<At Least One Member Selected from the Group Consisting of Cr:
More than 0% and 2% or Less and Mo: More than 0% and 2% or
Less>
[0066] 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.
[0067] 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:
More than 0% and 0.2% or Less, Nb: More than 0% and 0.2% or Less
and V: More than 0% and 0.2% or Less>
[0068] 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: More than 0% and 0.005% or Less>
[0069] 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:
More than 0% and 5% or Less, Ni: More than 0% and 5% or Less and
Co: More than 0% and 5% or Less>
[0070] 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%)>
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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
mass % or less, furthermore 0.4% or less and in particular 0.3% or
less are recommended.
[0078] The microstructure characterizing the steel sheet of the
present invention is described below.
[Microstructure of Steel Sheet of the Present Invention]
[0079] Although the steel sheet of the present invention is based
on a steel having a bainitic ferrite/polygonal ferrite/pearlite
multiphase microstructure as described above, this steel sheet is
especially characterized in that the size of the bainitic ferrite
grains has been regulated so as to be in a specific range and that
the hardness distribution in a thickness direction has been
regulated.
<Bainitic Ferrite: 5% or More, Pearlite: Less than 20%, and
Remainder: Polygonal Ferrite>
[0080] The microstructure of the steel sheet of the present
invention is constituted of a multiphase microstructure composed of
bainitic ferrite, polygonal ferrite, and pearlite. Bainitic ferrite
not only has, during cold working, the function of enhancing the
workability but also has, after the wording, the function of
increasing the hardness, and suppressing the generation of
stretcher strain marks. From the standpoint of making the bainitic
ferrite effectively perform these functions, the area ratio thereof
is 5% or more, preferably 10% or more, more preferably 15% or more.
The upper limit of the area ratio of bainitic ferrite in the steel
sheet of the present invention is substantially about 90%,
preferably 85%, more preferably 80%. Meanwhile, in a case where
pearlite is present in too large amount, the formability of the
steel sheet is deteriorated. Consequently, the area ratio of
pearlite is 20% or less, preferably 19% or less, more preferably
18% or less, especially preferably 15% or less. The lower limit of
the area ratio of pearlite in the steel sheet of the present
invention is substantially about 0.5%, preferably 1%. Although the
remainder is polygonal ferrite, the area ratio of polygonal ferrite
is preferably 5% or more.
[0081] In the microstructure of the steel sheet of the present
invention, a cementite phase is present besides the microstructures
described above. However, the area ratio thereof is as slight as
about 1% at the most. Consequently, in this description, the area
ratios of bainitic ferrite, polygonal ferrite, and pearlite were
normalized and defined so 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>
[0082] 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.
<Hardness Distribution in Thickness Direction:
(Hv.sub.max-Hv.sub.min)/Hv.sub.min Regulated to 0.3 or Less,
Wherein Hv.sub.max and Hv.sub.min are Respectively the Maximal
Value and the Minimal Value of Vickers Hardness Values of Three
Portions that are a Surface Portion, a t/4 Portion where t is the
Sheet Thickness, and a Central Portion>
[0083] Since transmission parts have complicated shapes, press
forming or forging results in regions which locally have an
extremely large deformation strain (corresponding to a true strain
.epsilon. of about 2 or larger). In the case of steel sheets having
a large hardness distribution in a thickness direction (strength
distribution, stress distribution), such large local deformation
strains undesirably result in uneven plastic deformations. In lowly
worked regions, i.e., regions having a small deformation strain
(less than about 2 in terms of .epsilon.), the hardness
distribution in a thickness direction exerts little influence and
arouses no problem. However, in regions having a large strain
amount (about 2 or larger in terms of .epsilon.), the hardness
distribution in a thickness direction undesirably results in local
cracking. In order to prevent local cracking from occurring even in
such regions having a strain amount as extremely large as about 2
in terms of .epsilon., the value of
(Hv.sub.max-Hv.sub.min)/Hv.sub.min, wherein Hv.sub.max and
Hv.sub.min are respectively the maximal value and the minimal value
of Vickers hardness values of three portions that are a surface
portion, a t/4 portion where t is the sheet thickness, and a
central portion, is regulated, as a hardness distribution in a
thickness direction, to 0.3 or less, preferably 0.2 or less, more
preferably 0.15 or less.
[0084] The mechanism by which a hardness distribution in a
thickness direction arises in a conventional hot-rolled steel sheet
is presumed to be as follows. In a hot-rolled steel sheet having a
large sheet thickness, examples of the causes of the occurrence of
a hardness distribution in a thickness direction include a
difference in the degree of working between each surface portion
and the central portion and a difference in working temperature
(including the heat generated by the working) between each surface
portion and the central portion, these differences unavoidably
occurring during the hot working. Furthermore, phase
transformations, generation of residual stress, and the like which
occur during coil cooling also exert influences. In the present
invention, since the alloying components thereof include solute N
in a large amount, the fixing of N to regions having a large
working strain occurs to undesirably increase the hardness of such
regions having a large working strain, and this increase in
hardness also exerts an influence. A plurality of such complicated
factors cause a hardness distribution in a thickness direction,
which is prone to result in thickness-direction unevenness in
strength.
[0085] The steel sheet of the present invention can hence be
obtained by subjecting a sheet which has just been hot-rolled to
batch annealing under the recommended conditions which will be
described later to thereby reduce the hardness distribution in a
thickness direction.
[Method for Measuring Area Ratio of Each Phase]
[0086] 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.
[0087] 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]
[0088] The average grain size of the bainitic ferrite above can be
measured as follows. That is, the grain sizes of bainitic ferrite
present at three portions, i.e., an outermost layer portion, a
portion at 1/4 of the sheet thickness, and a central portion 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.
[Method for Determining Hardness Distribution in Thickness
Direction]
[0089] A thickness-direction cross-section parallel with the
rolling direction of the hot-rolled sheet was examined for Vickers
hardness (Hv) with respect to each of a surface portion (located at
a depth of 400 .mu.m form a sheet surface), a portion at 1/4 of the
sheet thickness, and a central portion in the sheet shickness
direction, using a micro-Vickers hardness tester under the
conditions of a load of 50 g and the number of measurements of 5
times. An average for the five measurements was taken as the
Vickers hardness of each portion.
[0090] Of these Vickers hardness values for the three portions, the
maximal value Hv.sub.max and the minimal value Hv.sub.min were
determined to calculate (Hv.sub.max-Hv.sub.min)/Hv.sub.min.
[0091] 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]
[0092] 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]
[0093] 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]
[0094] 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]
[0095] 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.
[0096] 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]
[0097] 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]
[0098] 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]
[0099] 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+polygonal 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.
[Batch Annealing after Hot Rolling]
[0100] After the hot rolling, the sheet which has just been
hot-rolled (hot-rolled coil) is subjected to batch annealing under
the following conditions in order to regulate the hardness
distribution in a thickness direction so as to be in the given
range.
[0101] Specifically, this batch annealing is conducted in an
atmosphere having an H.sub.2 concentration of from 15 to 20 vol %
by heating the steel sheet from room temperature to a temperature
which is 400.degree. C. or higher but not higher than Ac1 and then
holding the steel sheet for 1 hour or more and 15 hours or less, in
order to suppress surface scale formation and decarbonization.
[0102] The holding temperature and the holding period vary
depending on the thickness of the sheet which has just been
hot-rolled and on the size of the coil, and are suitably selected
in accordance with the required narrowness of the hardness
distribution in a thickness direction, which corresponds to the
degree of cold working to be required, and with the evenness of the
internal temperature of the coil
[0103] This heat treatment serves not only to remove the residual
stress generated during the hot-rolling, thereby softening the
steel sheet and diminishing strains, but also to release the fixed
N element to accelerate spheroidization of carbides. In addition,
the heat treatment serves to dissolve fine lamellae in the
austenite. The hardness distribution in a thickness direction is
reduced thereby. After the batch annealing, the steel sheet is
cooled to 600.degree. C. at a rate of 10.degree. C./h or less to
thereby accelerate the spheroidization of carbides. Subsequently,
the steel sheet is cooled from 600.degree. C. to 400.degree. C. at
a rate of 15.degree. C./h or less, for the purpose of evenly
cooling the whole coil and thereby preventing coil collapse or the
like to stabilize the shape. Thereafter, cooling from 400.degree.
C. may be performed at a higher cooling rate (e.g., about 50 to
100.degree. C./h or higher) by water cooling, etc. so long as the
coil can be cooled while maintaining an even temperature
distribution within the coil.
[0104] In a case where the holding temperature in the batch
annealing is lower than 400.degree. C., those effects are
insufficient. Meanwhile, in a case where the holding temperature
exceeds the Ac1 point, the microstructure changes undesirably. The
holding temperature is more preferably 450 to 650.degree. C.,
especially preferably 500 to 600.degree. C.
[0105] In a case where the holding period is less than 1 hour,
those effects are insufficient. Meanwhile, holding periods
exceeding 15 hours are undesirable because the effects cannot be
enhanced any more, the production efficiency is impaired, and a
surface scale is prone to generate. The holding period is more
preferably 2 to 14 hours, especially preferably 3 to 12 hours.
[0106] The present invention is described in greater detail below
by referring to 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
[0107] Steels having the component compositions shown in Table 1
below were produced by a vacuum melting method and cast into ingots
having a thickness of 120 mm. These ingots were subjected to hot
rolling and then batch annealing under the conditions shown in
Table 2 and Table 3 below to produce hot-rolled steel sheets. In
each test, the following conditions were used: the rate of cooling
after completion of the finish rolling to a stop of rapid cooling
was 20.degree. C./s or higher, and the cooling after the stop of
rapid cooling was slow cooling conducted for 5 to 20 seconds at a
cooling rate of 10.degree. C./s or less; and after the batch
annealing, the steel sheet was cooled to 600.degree. C. at a
cooling rate of 10.degree. C./h or less, subsequently cooled form
600.degree. C. to 400.degree. C. at a cooling rate of 15.degree.
C./h or less, and further cooled from 400.degree. C. by water
cooling.
[0108] The hot-rolled steel sheets thus obtained were each examined
for solute N amount, area ratio of each phase in the microstructure
of the steel sheet, average grain size of bainitic ferrite, and
hardness distribution in a thickness direction by the measurement
methods explained above in the section "MODES FOR CARRYING OUT THE
INVENTION".
[0109] Furthermore, the hot-rolled steel sheets were evaluated for
heavy cold workability and hardness after working in the following
manners.
(Evaluation of Heavy Cold Workability)
[0110] In order to evaluate cold workability which locally causes
an extremely large deformation strain (heavy cold workability), the
following test was conducted as a test in which a working strain of
4 or larger in terms of true strain was introduced into a surface
portion of a test specimen. An 80-ton pressing test machine was
used to perform a wedge type compression test in which a
cylindrical test specimen and wedge type jigs were used, the
configuration of the test being diagrammatically shown in FIG. 1
(the test specimen was compressed at a compression rate of 1 mm/sec
to a reduction of 80% relative to the diameter thereof). Test
specimens used were as follows. In the case of hot-rolled steel
sheets having a thickness of 10 mm or larger, cylindrical test
specimens having a diameter of 10 mm were cut out therefrom. In the
case of hot-rolled steel sheets having a thickness less than 10 mm,
cylindrical test specimens having a diameter equal to the sheet
thickness were cut out therefrom.
[0111] Prior to this compression test, forging analysis software
FORGE (manufactured by TRANSVALOR S.A.) was used to calculate a
distribution of true strains within a test specimen at the time
when the reduction in the compression test was 80%. It was thus
ascertained that the true strain .epsilon. was 4 or larger in the
position located at a depth of 100 .mu.m from the surface portion
compressed by the R part of the compression jig, among the surface
portions of the test specimen.
[0112] The test specimen which had undergone the wedge type
compression test was visually examined and evaluated for heavy cold
workability in accordance with the following criteria. The case of
"o" was regarded as acceptable. [0113] .smallcircle.: no cracks
occurred in the test specimen [0114] .DELTA.: minute cracks
occurred in the surface of the test specimen [0115] x: cracks
occurred in the test specimen (Evaluation of Hardness after
Working)
[0116] Hardness after working was evaluated by measuring the
Vickers hardness (Hv) of the center of the surface of that portion
of the test specimen which had been compressed by the compression
jig in the wedge type compression test, using a Vickers hardness
tester under the conditions of a load of 500 g and the number of
measurements of 5 times. An average thereof was taken as hardness
after working. The steel sheets having a hardness after working of
250 Hv or more were regarded as acceptable.
[0117] The results of those measurements are shown in Tables 4 to 6
below.
TABLE-US-00001 TABLE 1 Kind of Components (mass %) [remainder: Fe
and unavoidabe impurities] steel C Si Mn P S Al N 10 C + 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, Mb: 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, Mb: 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 Batch annealing
conditions Thickness Produc- Kind Heating Final rolling Finish
rolling cooling stop Coiling Holding Holding of hot- tion of
temperature reduction temperature temperature temperature
temperature period rolled sheet No. steel (.degree.C) (%) (.degree.
C.) (.degree. C.) (.degree. C.) (.degree. C.) (h) (mm) 1-1* a 1250
15 917 612 575 none* none* 10 1-2 a 1250 15 900 599 519 500 5 10
1-3 a 1250 15 930 601 521 500 1 10 1-4 a 1250 15 921 612 569 500 13
10 1-5 a 1250 15 887 585 577 410 7 10 1-6 a 1250 15 894 621 590 650
5 10 1-7* a 1250 15 886 563 548 350* 5 10 1-8* a 1250 15 898 575
518 730* 5 10 1-9* a 1250 15 895 629 597 500 20* 10 1-10* a 1250 15
886 577 517 400 0.5* 10 2 a 1250 17 909 621 560 500 5 4 3 a 1250 15
895 593 508 500 5 18 4* a 1000* 14* 805* 524* 409* 500 5 10 5* a
1250 14* 907 589 530 500 5 25 6* a 1250 9* 921 623 579 500 5 10 7 b
1250 17 924 611 582 500 5 10 8 c 1250 15 898 572 529 500 5 10 9 d
1250 15 918 621 542 500 5 10 10 e 1250 15 924 597 509 500 5 10 11 f
1250 16 928 568 511 500 5 10 12 g 1250 15 930 639 589 500 5 10 13 h
1250 16 890 627 563 500 5 10 14 i 1250 17 903 594 551 500 5 10
(Underlined: outside the scope of the present invention. *: outside
the recommended range.)
TABLE-US-00003 TABLE 3 Hot-rolling conditions Final Rapid Thickness
Heating rolling Finish cooling Batch annealing conditions of hot-
Produc- Kind temper- reduc- rolling stop Coiling Holding Holding
rolled tion of ature tion temperature temperature temperature
temperature period sheet No. steel (.degree. C.) (%) (.degree. C.)
(.degree. C.) (.degree. C.) (.degree. C.) (h) (mm) 15 j 1250 15 907
588 523 500 5 10 16 k 1250 16 914 624 555 500 5 10 17 l 1250 16 924
635 547 500 5 10 18 m 1250 16 895 558 511 500 5 10 19 n 1250 16 910
569 519 500 5 10 20 o 1250 15 914 639 579 500 5 10 21 p 1250 16 913
613 578 500 5 10 22 q 1250 15 927 583 523 500 5 10 23 r 1250 16 915
633 579 500 5 10 24 s 1250 15 893 594 564 500 5 10 25 t 1250 16 918
562 513 500 5 10 26 u 1250 15 913 582 580 500 5 10 27 v 1250 15 901
571 520 500 5 10 28 w 1250 16 908 622 553 500 5 10 29 x 1250 16 924
581 506 500 5 10 (Underlined: outside the scope of the present
invention. *: outside the recommended range.)
TABLE-US-00004 TABLE 4 Microstructure BF Hardness distribution in
thickness average direction Hardness Kind Produc- Solute N Area
ratio grain Surface t/4 Central (Hv.sub.max- Heavy after Steel of
tion amount (%) size portion portion portion Hv.sub.min)/ cold
working No. steel No. (mass %) BF PF P (.mu.m) (Hv) (Hv) (Hv)
Hv.sub.min(-) workability (Hv) Remarks 1-1 a 1-1* 0.0085 11 86 3 27
176 104 104 0.69 x -- Comp. steel 1-2 a 1-2 0.008 12 82 6 31 131
112 113 0.17 .smallcircle. 275 Inventive steel 1-3 a 1-3 0.008 13
85 2 29 141 118 115 0.23 .smallcircle. 283 Inventive steel 1-4 a
1-4 0.007 10 84 6 26 126 115 111 0.14 .smallcircle. 258 Inventive
steel 1-5 a 1-5 0.009 14 80 6 32 136 112 107 0.27 .smallcircle. 271
Inventive steel 1-6 a 1-6 0.008 15 81 4 33 124 109 110 0.14
.smallcircle. 255 Inventive steel 1-7 a 1-7* 0.008 13 83 4 41 165
106 107 0.56 x -- Comp. steel 1-8 a 1-8* 0.007 14 81 5 45 111 107
106 0.05 .smallcircle. 210 Comp. steel 1-9 a 1-9* 0.009 14 79 7 29
126 113 115 0.12 .smallcircle. 229 Comp. steel 1-10 a 1-10* 0.008
15 82 3 27 163 105 104 0.57 x -- Comp. 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 occurred before given rolling
reduction during the wedge type compression test. Inventive steel:
[(Hv.sub.max-Hv.sub.min)/Hv.sub.min .ltoreq. 0.3] and [heavy cold
workability = .smallcircle.] and [(hardness after working) .gtoreq.
250 Hv]. Comp. steel: the case where any of the requirements for
the steel of the present invention is not satisfied.)
TABLE-US-00005 TABLE 5 Microstructure BF Hardness distribution in
thickness average direction Hardness Kind Produc- Solute N Area
ratio grain Surface t/4 Central (Hv.sub.max .sub.- Heavy after
Steel of tion amount (%) size portion portion portion Hv.sub.min)/
cold working No. steel No. (mass %) BF PF P (.mu.m) (Hv) (Hv) (Hv)
Hv.sub.min(-) workability (Hv) Remarks 2 a 2 0.008 27 67 6 15 135
125 127 0.08 .smallcircle. 273 Inventive Steel 3 a 3 0.008 8 88 4
38 126 104 104 0.21 .smallcircle. 251 Inventive steel 4 a 4* 0.003
68 9 23 23 129 106 105 0.23 .smallcircle. 208 Comp. steel 5 a 5*
0.009 4 90 6 55 131 105 104 0.26 .smallcircle. 213 Comp. steel 6 a
6* 0.008 4 88 8 52 127 108 109 0.18 .smallcircle. 205 Comp. steel 7
b 7 0.007 43 52 5 28 128 111 109 0.17 .smallcircle. 292 Inventive
steel 8 c 8 0.018 45 48 7 28 134 119 117 0.15 .smallcircle. 296
Inventive steel 9 d 9 0.008 49 47 4 22 132 118 118 0.12
.smallcircle. 308 Inventive steel 10 e 10 0.008 55 42 3 21 142 127
129 0.12 .smallcircle. 318 Inventive steel 11 f 11 0.009 63 32 5 16
144 126 128 0.14 .smallcircle. 311 Inventive steel 12 g 12 0.008 69
20 11 11 151 137 136 0.11 .smallcircle. 325 Inventive steel 13 h 13
0.009 74 11 15 13 162 142 144 0.14 .smallcircle. 331 Inventive
steel 14 i 14 0.008 79 6 15 10 166 151 149 0.11 .smallcircle. 351
Inventive 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 occurred before given rolling reduction during the wedge
type compression test. Inventive steel:
[(Hv.sub.max-Hv.sub.min)/Hv.sub.min .ltoreq. 0.3] and [heavy cold
workability = .smallcircle.] and [(hardness after working) .gtoreq.
250 Hv]. Comp. steel: the case where any of the requirements for
the steel of the present invention is not satisfied.)
TABLE-US-00006 TABLE 6 Microstructure BF Hardness distribution in
thickness average direction Hardness Kind Produc- Solute N Area
ratio grain Surface t/4 Central (Hv.sub.max- Heavy after Steel of
tion amount (%) size portion portion portion Hv.sub.min)/ cold
working No. steel No. (mass %) BF PF P (.mu.m) (Hv) (Hv) (Hv)
Hv.sub.min(-) workability (Hv) Remarks 15 j 15 0.001 48 49 3 26 129
108 108 0.19 .smallcircle. 181 Comp. steel 16 k 16 0.026 44 51 5 28
129 106 106 0.22 x -- Comp. steel 17 l 17 0.007 45 28 27 11 188 153
151 0.25 x -- Comp. steel 18 m 18 0.008 58 35 7 27 159 127 125 0.27
x -- Comp. steel 19 n 19 0.010 25 71 4 22 119 103 102 0.17 o 221
Comp. steel 20 o 20 0.009 45 52 3 26 131 113 114 0.16 x -- Comp.
steel 21 p 21 0.009 45 50 5 26 128 106 105 0.22 x -- Comp. steel 22
q 22 0.010 47 49 4 29 125 107 107 0.17 x -- Comp. steel 23 r 23
0.011 43 55 2 30 122 108 108 0.13 x -- Comp. steel 24 s 24 0.012 58
38 4 27 131 108 108 0.21 x -- Comp. steel 25 t 25 0.008 55 39 6 17
132 114 116 0.16 .smallcircle. 289 Inventive steel 26 u 26 0.009 58
38 4 16 135 118 118 0.14 .smallcircle. 283 Inventive steel 27 v 27
0.008 50 45 5 19 129 107 107 0.21 .smallcircle. 275 Inventive steel
28 w 28 0.008 59 32 9 17 138 128 127 0.09 .smallcircle. 286
Inventive steel 29 x 29 0.020 68 15 17 11 205 155 154 0.33 x --
Comp. 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 occurred before given rolling reduction during the wedge
type compression test. Inventive steel:
[(Hv.sub.max-Hv.sub.min)/Hv.sub.min .ltoreq. 0.3] and [heavy cold
workability = .smallcircle.] and [(hardness after working) .gtoreq.
250 Hv]. Comp. steel: the case where any of the requirements for
the steel of the present invention is not satisfied.)
[0118] Tables 4 to 6 show the following. Steels Nos. 1-2 to 1-6, 2,
3, 7 to 14, and 25 to 28 each employed a steel kind which satisfied
the requirements regarding the component composition specified in
the present invention, and had been produced under the recommended
production conditions. As a result, these steels were steels of the
present invention which satisfied the requirements regarding the
microstructure specified in the present invention, and were on
acceptable levels with respect to both heavy cold workability and
hardness after working. It was ascertained that hot-rolled steel
sheets which show satisfactory heavy cold workability during the
working that causes extremely large strains and which show a given
hardness (strength) after the working were obtained.
[0119] In contrast, steels Nos. 1-1, 1-7 to 1-10, 4 to 6, 15 to 24,
and 29 are comparative steels which each do not satisfy at least
one of the requirements regarding the component composition and
microstructure specified in the present invention. These steels are
each not on an acceptable level with respect to heavy cold
workability and/or hardness after working.
[0120] Specifically, steel No. 1-1 has not undergone batch
annealing after the hot rolling, although this steel satisfies the
requirements concerning the component composition. This steel has
an increased hardness distribution in a thickness direction and is
poor at least in heavy cold workability.
[0121] Steel No. 1-7, although satisfying the requirements
regarding the component composition, has undergone, after the hot
rolling, batch annealing in which the holding temperature was too
low beyond the recommended range. This steel has an increased
hardness distribution in a thickness direction and is poor at least
in heavy cold workability.
[0122] Meanwhile, steel No. 1-8, although satisfying the
requirements regarding the component composition, has undergone,
after the hot rolling, batch annealing in which the holding
temperature was too high beyond the recommended range. This steel
is poor in hardness after working.
[0123] Steel No. 1-9, although satisfying the requirements
regarding the component composition, has undergone, after the hot
rolling, batch annealing in which the holding period was too long
beyond the recommended range. This steel is poor in hardness after
working.
[0124] Meanwhile, steel No. 1-10, although satisfying the
requirements regarding the component composition, has undergone,
after the hot rolling, batch annealing in which the holding period
was too short beyond the recommended range. This steel has an
increased hardness distribution in a thickness direction and is
poor at least in heavy cold workability.
[0125] Steel No 4 satisfies the requirements concerning the
component composition, but the heating temperature before the hot
rolling was too low beyond the recommended range. This steel has an
insufficient solute N amount and is poor in hardness after
working.
[0126] Steel No. 5, although satisfying the requirements regarding
the component composition, has too large a thickness beyond the
specified range after the hot rolling. This steel has an
insufficient bainitic ferrite content but has too large grain size
and is poor in hardness after working.
[0127] Steel No. 6, although satisfying the requirements regarding
the component composition, has undergone hot rolling in which the
final reduction was too low beyond the recommended range. This
steel has an insufficient bainitic ferrite content but has too
large grain size and is poor in hardness after working.
[0128] Steel No. 15 (steel kind j), although produced under the
conditions within the recommended ranges, has too low N content.
This steel is poor in hardness after working.
[0129] Meanwhile, steel No. 16 (steel kind k), although produced
under the conditions within the recommended ranges, has too high N
content. This steel is poor at least in heavy cold workability.
[0130] Steel No. 17 (steel kind 1), although produced under the
conditions within the recommended ranges, has too high C content
and does not satisfy the requirement 10C+N.ltoreq.3.0. Pearlite has
been excessively formed therein, and this steel is poor at least in
heavy cold workability.
[0131] Steel No. 18 (steel kind m), although produced under the
conditions within the recommended ranges, has too high Si content.
This steel is poor at least in heavy cold workability.
[0132] Steel No. 19 (steel kind n), although produced under the
conditions within the recommended ranges, has too low Mn content.
This steel is poor in hardness after working.
[0133] Meanwhile, steel No. 20 (steel kind o), although produced
under the conditions within the recommended ranges, has too high Mn
content. This steel is poor at least in heavy cold workability.
[0134] Steel No. 21 (steel kind p), although produced under the
conditions within the recommended ranges, has too high P content.
This steel is poor at least in heavy cold workability.
[0135] Steel No. 22 (steel kind q), although produced under the
conditions within the recommended ranges, has too high S content.
This steel is poor at least in heavy cold workability.
[0136] Steel No. 23 (steel kind r), although produced under the
conditions within the recommended ranges, has too low Al content.
This steel is poor at least in heavy cold workability.
[0137] Meanwhile, steel No. 24 (steel kind s), although produced
under the conditions within the recommended ranges except the final
reduction during the hot rolling, has too high Al content. This
steel is poor at least in heavy cold workability.
[0138] Meanwhile, steel No. 29 (steel kind x), although produced
under the conditions within the recommended ranges, does not
satisfy the requirement 10C+N.ltoreq.3.0. This steel is poor at
least in heavy cold workability.
[0139] From these results, the applicability of the present
invention was able to be ascertained.
[0140] While the present 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
thereof.
[0141] This application is based on a Japanese patent application
No. 2014-086747 filed on Apr. 18, 2014, the contents of which are
incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0142] The hot-rolled steel sheet of the present invention shows
satisfactory workability in cold working and shows given hardness
after the working. This hot-rolled steel sheet is useful as a steel
material for use in producing, in particular, various automotive
parts, such as, for example, transmission parts, e.g., gears, and
cases.
* * * * *