ORIGINAL
-5
HOT-DIP GALVANIZED COLD-ROLLED STEEL SHEET AND
PROCESS FOR PRODUCING SAME
Technical Field
The present invention relates to a hot-dip galvanized cold-rolled steel sheet.
More particularly, it relates to a high strength hot-dip galvanized cold-rolled steel
sheet that is excellent in ductility, work hardenability, and stretch flangeability,
and a process for producing the same.
Background Art
In these days when the industrial technology field is highly fractionalized,
a material used in each technology field has been required to deliver special and
high performance. For example, for a steel sheet that is press-formed and put in
use, more excellent formability has been required with the diversification of
press shapes. In addition, as a high strength has been required, the use of a high
strength steel sheet has been studied. In particular, concerning an automotive
steel sheet, in order to reduce the vehicle body weight and thereby to improve the
fuel economy from the perspective of global environments, a demand for a high
strength steel sheet having thin-wall high formability has been increasing
remarkably. In press forming, as the thickness of steel sheet used is smaller,
cracks and wrinkles are liable to occur. Therefore, a steel sheet further
excellent in ductility and stretch flaneability is required. However, the press
formability and the high strengthening of steel sheet are characteristics contrary
to each other, and therefore it is difficult to satisfy these characteristics at the
same time.
As a method for improving the press formability of a high strength coldrolled
steel sheet, many techniques concerning grain refinement of microstructure
have been proposed. For example, Patent Document 1 discloses a
method for producing a very fine grain high-strength hot-rolled steel sheet that is
subjected to rolling at a total reduction of 80% or higher in a temperature region
in the vicinity of Ar3 point in the hot-rolling process. Patent Document 2
discloses a method for producing an ultrafine ferritic steel that is subjected to
continuous rolling at a reduction of 40% or higher in the hot-rolling process.
By these techniques, the balance between strength and ductility of hotrolled
steel sheet is improved. However, the above-described Patent
Documents do not at all describe a method for making a fine-grain cold-rolled
steel sheet to improve the press formability. According to the study conducted
by the present inventors, if cold rolling and annealing are performed on the finegrain
hot-rolled steel sheet obtained by high reduction rolling being a base metal,
the crystal grains are liable to be coarsened, and it is difficult to obtain a coldrolled
steel sheet excellent in press formability. In particular, in the
manufacturing of a composite-structure cold-rolled steel sheet containing a low
temperature transformation phase or retained austenite in the structure, which
must be annealed in the high-temperature region of Acl point or higher, the
coarsening of crystal grains at the time of annealing is remarkable, and the
advantage of composite-structure cold-rolled steel sheet that the ductility is
excellent cannot be enjoyed.
Patent Document 3 discloses a method for producing a hot-rolled steel
sheet having ultrafine grains, in which method, rolling reduction in the dynamic
recrystallization region is performed with a rolling reduction pass of five or more
stands. However, the lowering of temperature at the hot-rolling time must be
decreased extremely, and it is difficult to carry out this method in a general hotrolling
equipment. Also, although Patent Document 3 describes an example in
which cold rolling and annealing are performed after hot rolling, the balance
between tensile strength and hole expandability is poor, and the press formability
is insufficient.
Concerning the cold-rolled steel sheet having a fine structure, Patent
Document 4 discloses an automotive high-strength cold-rolled steel sheet
excellent in collision safety and formability, in which retained austenite having
an average crystal grain size of 5 pm or smaller is dispersed in ferrite having an
average crystal grain size of 10 pm or smaller. The steel sheet containing
retained austenite in the structure exhibits a large elongation due to
transformation induced plasticity (TRIP) produced by the martensitizing of
austenite during working; however, the hole expandability is impaired by the
formation of hard martensite. For the cold-rolled steel sheet disclosed in Patent
Document 4, it is supposed that the ductility and hole expandability are improved
by making ferrite and retained austenite fine. However, the hole expanding
ratio is at most 1.5, and it is difficult to say that sufficient press formability is
provided. Also, to enhance the work hardening index and to improve the
collision safety, it is necessary to make the main phase a soft ferrite phase, and it
is difficult to obtain a high tensile strength.
Patent Document 5 discloses a high-strength steel sheet excellent in
elongation and stretch flaneability, in which the second phase consisting of
retained austenite and/or martensite is dispersed finely within the crystal grains.
However, to make the second phase fine to a nano size and to disperse it within
the crystal grains, it is necessary to contain expensive elements such as Cu and
Ni in large amounts and to perform solution treatment at a high temperature for a
long period of time, so that the rise in production cost and the decrease in
productivity are remarkable.
Patent Document 6 discloses a high strength hot dip galvanized steel sheet
excellent in ductility, stretch flaneability, and fatigue resistance property, in
which retained austenite and low temperature transformation phase are dispersed
in ferrite having an average crystal grain size of 10 pm or smaller and in
tempered martensite. The tempered martensite is a phase that is effective in
improving the stretch flaneability and fatigue resistance property, and it is
supposed that if grain refinement of tempered martensite is performed, these
properties are hrther improved. However, in order to obtain a structure
containing tempered martensite and retained austenite, primary annealing for
forming martensite and secondary annealing for tempering martensite and further
for obtaining retained austenite are necessary, so that the productivity is impaired
significantly.
Patent Document 7 discloses a method for producing a cold-rolled steel
sheet in which retained austenite is dispersed in fine ferrite, in which method, the
steel sheet is cooled rapidly to a temperature of 720°C or lower immediately after
being hot-rolled, and is held in a temperature range of 600 to 720°C for 2
seconds or longer, and the obtained hot-rolled steel sheet is subjected to cold
rolling and annealing.
Citation List
Patent Document
Patent Document 1 : JP 58- 123 823 A1
Patent Document 2: JP 59-2294 13 A1
Patent Document 3: JP 1 1- 152544 A1
Patent Document 4: JP 1 1-6 1326 A1
Patent Document 5: JP 2005- 179703 A1
Patent Document 6: JP 200 1- 192768 A1
Patent Document 7: W0200711554 1 A1
Summary of Invention
The above-described technique disclosed in Patent Document 7 is
excellent in that a cold-rolled steel sheet in which a fine grain structure is formed
and the workability and thermal stability are improved can be obtained by a
process in which after hot rolling has been finished, the work strain accumulated
in austenite is not released, and ferrite transformation is accomplished with the
work strain being used as a driving force.
However, due to needs for higher performance in recent years, a hot-dip
galvanized cold-rolled steel sheet provided with a high strength, good ductility,
excellent work hardenability, and excellent stretch flaneability at the same time
has been demanded.
The present invention has been made to meet such a demand.
Specifically, an objective of the present invention is to provide a high-tension
hot-dip galvanized cold-rolled steel sheet which has excellent ductility, work
hardenability and stretch flaneability, as well as a tensile strength of 750 MPa or
higher, and a method for producing the same.
Means for Solving the Problem
As a result of extensive examination on the effects of the chemical
compositions and production conditions on the mechanical properties of the
high-tension hot-dip galvanized cold-rolled steel sheet, the present inventors
have eventually obtained the following findings shown in (A) to (G).
(A) If the hot-rolled steel sheet, which is produced through a so-called
immediate rapid cooling process where rapid cooling is performed by water
cooling immediately after hot rolling, specifically, the hot-rolled steel sheet is
produced in such a way that the steel is rapidly cooled to the temperature region
of 720°C or lower within 0.40 second after the completion of hot rolling, is coldrolled
and annealed, the ductility and stretch flaneability of cold-rolled steel sheet
are improved with the rise in annealing temperature. However, if the annealing
temperature is too high, the austenite grains are coarsened, and the ductility and
stretch flaneability of annealed steel sheet may be deteriorated abruptly.
(B) When the final roll reduction of hot rolling is increased, the coarsening
of austenite grains, which may possibly occur when annealing is performed at a
high temperature after cold rolling, is restrained. Although the reason thereof is
not clear, it is presumably attributable to the facts that (a) as the final roll
reduction increases, the ferrite fraction increases and the ferrite grains are refined
in the structure of hot-rolled steel sheet, (b) as the final roll reduction increases, a
coarse low temperature transformation phase decreases in the structure of hotrolled
steel sheet, (c) since a ferrite grain boundary functions as a nucleation site
in the transformation from ferrite to austenite during annealing, as the amount of
fine ferrite increases, the frequency of nucleation increases and the austenite
grains are refined, and (d) a coarse low temperature transformation phase
transforms into a coarse austenite grain during annealing.
(C) When coiling temperature is increased in a coiling step after
immediate rapid cooling, the coarsening of austenite grains which may possibly
occur when annealing is performed at a high temperature after cold rolling is
restrained. Moreover, when a hot-rolled steel sheet which has been coiled at a
lowered coiling temperature in the coiling step after immediate rapid cooling is
annealed in a temperature range of 500°C or higher and Acl point or lower, and
thereafter is cold rolled and annealed at a high temperature, the coarsening of
austenite grains is restrained as well. Although the reason thereof is not clear, it
is presumably attributable to the facts that (a) since the grains of the hot-rolled
steel sheet are refined due to immediate rapid cooling, the amount of
precipitation of iron carbide in the hot-rolled steel sheet will remarkably increase
as the coiling temperature rises, or as a result of the coiling at a lower
temperature after immediate rapid cooling, fine martensitic structure is formed in
the structure, and as a result of the hot-rolled steel sheet being further annealed,
fine iron carbides precipitate into the structure, (b) since iron carbide acts as a
nucleation site in the transformation from ferrite to austenite during annealing, as
the amount of precipitation of iron carbide increases, the frequency of nucleation
increases, and the austenite grains are refined, and (c) since undissolved iron
carbide suppresses the grain growth of austenite, the austenite grains are refined.
(D) As the Si content in steel increases, the effect of preventing the
coarsening of austenite grains is enhanced. Although the reason thereof is not
clear, it is presumably attributable to the facts that (a) as the Si content increases,
the grain of iron carbide becomes fine and the number density thereof increases,
(b) as a result of this, the frequency of nucleation in the transformation from
ferrite to austenite hrther increases, and (c) the grain growth of austenite is
further restrained due to an increase in undissolved iron carbide, and the austenite
grains are further refined.
(E) If the steel sheet is soaked at a high temperature while the coarsening
of austenite grains is restrained and is cooled, a structure is obtained in which the
main phase is a fine low temperature transformation phase, the second phase
contains fine retained austenite.
(F) As a result of restraining the formation of coarse retained-austenite
grains whose grain size is 1.2 pm or more, the strech flangeability of a steel sheet
whose main phase is a low temperature transformation phase is improved.
Although the reason thereof is not clear, it is presumably attributable to the facts
that (a) although retained austenite is transformed into hard martensite by
machine working, if the retained-austenite grain is coarse, the martensite grain
also becomes coarse, causing an increase in stress concentration so that a void
readily occurs at an interface with the parent phase and acts as a starting point of
crack, and (b) since a coarse retained-austenite grain transforms into martensite
in an early stage of machine working, it is more likely to act as a starting point of
crack than a fine retained-austenite grain is.
(G) As annealing temperature increases, the fraction of low temperature
transformation phase increases and work hardenability tends to deteriorate;
however, by restraining the formation of coarse retained-austenite grains having
a grain size of 1.2 pm or more, it is possible to prevent the deterioration of work
hardenability in a steel sheet whose main phase is low temperature
transformation phase. Although the reason thereof is not clear, it is presumably
attributable to the facts that (a) since a coarse retained-austenite grain transforms
into martensite in an early stage of machine working in which strain is less than
5%, it seldom contributes to an increase in "n" value at strain of 5 to lo%, and
(b) when the formation of coarse retained-austenite grains is restrained, fine
retained-austenite grains, which transform into martensite in a high strain range
of 5% or more, increase.
From the results described so far, it has been found that by subjecting a
steel containing a fixed amount or more of Si to hot rolling at a raised final roll
reduction and thereafter to immediate rapid cooling, and either coiling it at a high
temperature or coiling it at a low temperature, subjecting it to hot-rolled sheet
annealing at a predetermined temperature and thereafter to cold rolling, and
further subjecting it to annealing at a high temperature and thereafter to cooling,
it is possible to obtain a hot-dip galvanized cold-rolled steel sheet which is
excellent in ductility, work hardenability, and stretch flangeability and which has
a structure in which a main phase is a low temperature transformation phase and
a second phase includes retained austenite, which has a small amount of coarse
retained-austenite grains having a grain size of 1.2 pm or more.
The present invention is a hot-dip galvanized cold-rolled steel sheet having
a hot-dip galvanized layer on a surface of a cold-rolled steel sheet, wherein
the cold-rolled steel sheet has: a chemical composition consisting, in mass
percent, of C: more than 0.10% and less than 0.25%, Si: more than 0.50% and
less than 2.0%, Mn: more than 1.50% and at most 3.0%, P: less than 0.050%, S:
at most 0.010%, sol. Al: at least 0% and at most 0.50%, N: at least 0.010%, Ti: at
least 0% and less than 0.040%, Nb: at least 0% and less than 0.030%, V: at least
s- 0% and at most 0.50%, Cr: at least 0% and at most 1.0%, Mo: at least 0% and
less than 0.20%, B: at least 0% and at most 0.010%, Ca: at least 0% and at most
0.010%, Mg: at least 0% and at most 0.010%, REM: at least 0% and at most
0.050%, Bi: at least 0% and at most 0.050%; and the remainder being Fe and
5 impurities and by having a structure in which a main phase is a low temperature
transformation phase and a second phase contains retained austenite, wherein
the retained austenite has a volume fraction of more than 4.0% to less than
25.0% with respect to the whole structure, and an average grain size of less than
0.80 pm, and in the retained austenite, a number density of retained austenite
10 grains having a grain size of 1.2 pm or more is 3.0 x 10'~lpmo~r less.
The above described chemical composition preferably contains at least one
element selected from the following groups (% is mass%):
(a) one or more types selected from a group consisting of Ti: at least
0.005% and less than 0.040%, Nb: at least 0.005% and less than 0.030%, and V:
15 at least 0.010% and at most 0.50%;
(b) one or more types selected from a group consisting of Cr: at least
0.20% and at most 1.0%, Mo: at least 0.05% and less than 0.20%, and B: at least
0.0010% and at most 0.010%, and
(c) one or more types selected from a group consisting of Ca: at least
20 0.0005% and at most 0.010%, Mg: at least 0.0005% and at most 0.010%, REM:
at least 0.0005% and at most 0.050%, and Bi: at least 0.0010% and at most
0.050%.
A hot-dip galvanized cold-rolled steel sheet using as a base material a
cold-rolled steel sheet having a structure in which a main phase is a low
2 5 temperature transformation phase and a second phase contains retained austenite,
relating to the present invention can be produced by either of the following
production method 1 or 2:
[Production method 11 A method including the following steps (A) to (D):
(A) a hot-rolling step in which a slab having the above described chemical
3 0 composition is subjected to hot rolling in which a reduction of final one pass is
more than 15% and rolling is completed in a temperature range of (AT3 point +
30°C) or higher, and higher than 880°C to form a hot-rolled steel sheet, and the
hot-rolled steel sheet is cooled to a temperature range of 720°C or lower within
0.40 seconds after the completion of the rolling, and is coiled in a temperature
range of higher than 400°C;
(B) a cold-rolling step in which the hot-rolled steel sheet is subjected to a
5 cold rolling to form a cold-rolled steel sheet;
(C) an annealing step in which the cold-rolled steel sheet is subjected to
soaking treatment in a temperature range of higher than Ac3 point, thereafter is
cooled to a temperature range of 450°C or lower and 340°C or higher, and is held
in the same temperature range for 15 seconds or more; and
10 (D) a hot-dip galvanizing step in which the cold-rolled steel sheet obtained
by the annealing step is subjected to hot-dip galvanizing.
[Production method 21 A method including the following steps (a) to (e):
(a) a hot-rolling step in which a slab having the above described chemical
composition is subjected to hot rolling in which a reduction of final one pass is
15 more than 15% and rolling is completed in a temperature range of (AT3 point +
30°C) or higher, and higher than 880°C to form a hot-rolled steel sheet, and the
hot-rolled steel sheet is cooled to a temperature range of 720°C or lower within
0.40 seconds after the completion of the rolling, and is coiled in a temperature
range of lower than 200°C;
20 (b) a hot-rolled sheet annealing step in which the hot-rolled steel sheet is
subjected to annealing in a temperature range of 500°C or higher, and lower than
Ac, point;
(c) a cold-rolling step in which the hot-rolled steel sheet obtained by the
hot-rolled sheet annealing step is subjected to cold rolling to forrn a cold-rolled
2 5 steel sheet;
(d) an annealing step in which the cold-rolled steel sheet is subjected to
soaking treatment in a temperature range of higher than Ac3 point, thereafter is
cooled to a temperature range of 450°C or lower and 340 "C or higher, and is
held in the same temperature range for 15 seconds or more; and
30 (e) a hot-dip galvanizing step in which the cold-rolled steel sheet obtained
by the annealing step is subjected to hot-dip galvanizing.
According to the present invention, a high strength hot-dip galvanized
cold-rolled steel sheet having sufficient ductility, work hardenability, and stretch
flangeability, which can be used for working such as press forming, can be
obtained. Therefore, the present invention can greatly contribute to the
5 development of industry. For example, the present invention can contribute to
the solution to global environment problems through the lightweight of
automotive vehicle body.
Description of Embodiments
The structure and chemical composition of a cold-rolled steel sheet in a
hot-dip galvanized cold-rolled steel sheet relating to the present invention, and
the rolling, annealing, and galvanizing conditions etc. in a production method
which allows effective, stable, and economical production of the cold-rolled steel
sheet and the hot-dip galvanized steel sheet will be described below in detail.
1. Structure
A cold-rolled steel sheet, which is the base material for plating of a hot-dip
galvanized cold-rolled steel sheet relating to the present invention, has a structure
in which a main phase is a low temperature transformation phase and a second
2 0 phase contains retained austenite, and in which the retained austenite has a
volume fraction of more than 4.0% and less than 25.0% with respect to the whole
structure, and an average grain size of less than 0.80 pm, and in the retained
austenite, a number density of retained austenite grains having a grain size of 1.2
pm or more is 3.0 x 10-~/pmo' r less.
2 5 The main phase means a phase or structure in which the volume fraction is
at the maximum, and the second phase means a phase or structure other than the
main phase.
The term "low temperature transformation phase" refers to a phase and
structure which is formed by low-temperature transformation such as those of
3 0 martensite and bainite. Other than those mentioned, examples of the low
temperature transformation phase include bainitic ferrite. Bainitic ferrite is
distinguished from polygonal ferrite from that a dislocation density is high, and
from bainite from that bainitic lath does not contain interlath or intralath iron
carbide and granular bainitic ferrite does not contain iron carbide insid. Bainitic
ferrite refers to a so-called lath or plate-like bainitic ferrite and granular bainitic
ferrite having a granular form. This low temperature transformation phase may
include phases and structures of two or more types, specifically martensite and
bainitic ferrite. When the low temperature transformation phase includes two or
more types of phases and structures, a total of volume fractions of these phases
and structures is assumed to represent the volume fraction of the low temperature
transformation phase.
The reason why the structure of the cold-rolled steel sheet which is the
base material for plating is limited as described above will be described next.
Here, a cold-rolled steel sheet implies both of the cold-rolled steel sheet which is
formed by cold-rolling a hot-rolled steel sheet obtained by hot-rolling, and an
annealed cold-rolled steel sheet which is thereafter subjected to annealing.
The reason why the inventive steel sheet is specified to have a structure in
which the main phase is a low temperature transformation phase and the second
phase contains retained austenite is that it is preferable for improving ductility,
work hardenability, and stretch flangeability while maintaining tensile strength.
If the main phase is polygonal ferrite which is not a low temperature
transformation phase, it becomes difficult to ensure the tensile strength and
strech flangeability.
The volume fraction of retained austenite with respect to the whole
structure is specified to be more than 4.0% and less than 25.0%. When the
volume fraction of retained austenite is 4.0% or less, ductility becomes
insufficient, and when it is 25.0% or more, strech flangeability remarkably
deteriorates. The volume fraction of retained austenite is preferably more than
6.0%. It is more preferably more than 8.0%, and particularly preferably more
than 10.0%. On the other hand, when the volume fraction of retained austenite
is excessive, the stretch flangeability will deteriorate. Therefore, the volume
fraction of retained austenite is preferably less than 18.0%. It is more
preferably less than 16.0%, and particularly preferably less than 14.0%.
The average grain size of retained austenite is let to be less than 0.80 pm.
In a hot-dip galvanized steel sheet using as a base material a cold-rolled steel
sheet having a structure in which the main phase is a low temperature
transformation phase and the second phase contains retained austenite, when the
average grain size of the retained austenite is 0.80 pm or more, the ductility,
work hardenability, and stretch flangeability thereof will remarkably deteriorate.
The average grain size of retained austenite is preferably less than 0.70 pm, and
more preferably less than 0.60 pm. Although the lower limit for the average
grain size of retained austenite will not be particularly limited, in order to obtain
fine grains of 0.15 pm or less, it is necessary to greatly increase the final
reduction for hot rolling, leading to a remarkable increase in the production load.
Therefore, the lower limit for the average grain size of retained austenite is
preferably more than 0.15 pm.
In a hot-dip galvanized steel sheet using as a base material a cold-rolled
steel sheet having a structure in which the main phase is a low temperature
transformation phase and the second phase contains retained austenite, when a
large amount of coarse retained-austenite grains having a grain size of 1.2 pm or
more are present, the work hardenability and stretch flaneability will be impaired
even if the average grain size of retained austenite is less than 0.80 pm.
Therefore, the number density of retained austenite grains having a grain size of
1.2 pm or more is let to be 3.0 x 10" lpm2 or less. The number density of
retained austenite grains having a grain size of 1.2 pm or more is preferably 2.0 x
loml2p m2 or less. The number density is more preferably 1.8 x 10'~I pm2 or
less, and is particularly preferably 1.6 x 1 pm2 or less.
To hrther improve the balance between ductility and stretch flaneability,
the average carbon concentration of retained austenite is preferably 0.80% or
more, and is more preferably 0.84% or more. On the other hand, when the
average carbon concentration of retained austenite becomes excessive, the stretch
flaneability will deteriorate. Therefore, the average carbon concentration of
retained austenite is preferably less than 1.7%. The average carbon
concentration is more preferably less than 1.6%, furthermore preferably less than
1.4%, and particularly preferably less than 1.2%.
% To further improve the ductility and work hardenability, the second phase
preferably contains polygonal ferrite besides retained austenite. The volume
fraction of polygonal ferrite with respect to the whole structure is preferably
more than 2.0%. On the other hand, when the volume fraction of polygonal
5 ferrite becomes excessive, the stretch flaneability will deteriorate. Therefore,
the volume fraction of polygonal ferrite is preferably less than 40.0%. The
volume fraction of polygonal ferrite is more preferably less than 30%, further
preferably less than 24.0%, particularly preferably less than 20.0%, and most
preferably less than 18.0%.
10 To improve tensile strength and work hardenability, the low temperature
transformation phase preferably contains martensite. In this case, the volume
fraction of rnartensite with respect to the whole structure is preferably more than
1.0%, and is further preferably more than 2.0%. On the other hand, when the
volume fraction of rnartensite becomes excessive, the stretch flaneability will
15 deteriorate. For this reason, the volume fraction occupied by rnartensite in the
whole structure is preferably less than 15.0%. The volume fraction of
martensite is more preferably less than 10.0%, particularly preferably less than
8.0%, and most preferably less than 6.0%.
The structure of a cold-rolled steel sheet, which is the base material for a
2 0 hot-dip galvanized cold-rolled steel sheet relating to the present invention, is
measured as follows. That is, the volume fractions of the low temperature
transformation phase and the polygonal ferrite are determined such that a
specimen is taken from a hot-dip galvanized steel sheet, a longitudinal cross
section in parallel with the rolling direction is polished and is subjected to Nital
2 5 etching, and thereafter the structure is observed using SEM at a position of a
depth of 114 sheet thickness from the surface of steel sheet (the interface between
the plated surface and the steel sheet as the base material, the same rule applies to
the following) to measure the area ratios of the low temperature transformation
phase and the polygonal ferrite by image processing and to determine respective
3 0 volume fractions assuming that the area ratio is equal to the volume fraction.
The volume fraction and the average carbon concentration of retained
austenite are determined such that a specimen is taken from a hot-dip galvanized
steel sheet, a rolled surface is chemically polished from the surface of steel sheet
to a point of a depth of 114 sheet thickness, and X-ray diffraction intensity and a
diffraction angle are respectively measured by using XRD.
The grain size of retained austenite and the average grain size of retained
austenite are measured as described below. A test specimen is sampled from
the hot-dip galvanized steel sheet, and the longitudinal cross sectional surface
thereof parallel to the rolling direction is electropolished. The structure is
observed at a position deep by one-fourth of thickness from the surface of steel
sheet by using a SEM equipped with an EBSP analyzer. A region that is
observed as a phase consisting of a face-centered cubic crystal structure (fcc
phase) and is surrounded by the parent phase is defined as one retained austenite
grain. By image processing, the number density (number of grains per unit
area) of retained austenite grains and the area fractions of individual retained
austenite grains are measured. From the areas occupied by individual retained
15 austenite grains in a visual field, the circle corresponding diameters of individual
retained austenite grains are determined, and the mean value thereof is defined as
the average grain size of retained austenite.
In the structure observation using the EBSP, in the region having a size of
50 pm or larger in the sheet thickness direction and 100 pm or larger in the
20 rolling direction, electron beams are applied at a pitch of 0.1 pm to make
judgment of phase. Among the obtained measured data, the data in which the
confidence index is 0.1 or more are used for grain size measurement as effective
data. Also, to prevent the grain size of retained austenite from being
undervalued by measurement noise, only the retained austenite grains each
2 5 having a circle corresponding diameter of 0.15 pm or larger is taken as effective
grains, whereby the average grain size is calculated.
In the present invention, the above-described structure is defined at a
position deep by one-fourth of thickness of steel sheet, which is a base material,
from the boundary between the base material steel sheet and a plating layer.
30 As mechanical properties which can be realized based on the
characteristics of the structure described so far, the hot-dip galvanized cold-rolled
steel sheet relating to the present invention has, to ensure shock absorbing
property, a tensile strength (TS) in a direction perpendicular to the rolling
direction of preferably 750 MPa or more, more preferably 850 MPa or more, and
particularly preferably 950 MPa or more. On the other hand, to ensure ductility,
the TS is preferably less than 1 180 MPa.
When the value obtained by converting the total elongation (Elo) in the
direction perpendicular to the rolling direction into a total elongation
corresponding to the sheet thickness of 1.2 mm based on formula (1) below is
taken as El, the work hardening index calculated by using the nominal strains of
two points of 5% and 10% with the strain range being made 5 to 10% in
conformity to Japanese Industrial Standards JIS 22253 and the test forces
corresponding to these strains is taken as n value, and the hole expanding ratio
measured in conformity to Japan Iron and Steel Federation Standards JFSTl 00 1
is taken as h, from the viewpoint of press formability, it is preferable that the
value of TS x El be 18,000 MPa% or higher, the value of TS x n value be 150
MPa or higher, the value of TS'.~x h be 4,500,000 or higher, and the
value of (TS x El) x 7 x 10' + (TS'.~x A) x 8 be 18 0 x 1o 6 or higher.
El = Elo x (1 .2/t0)0.2.. . (1)
in which Elo is the actually measured value of total elongation measured by using
JIS No. 5 tensile test specimen, to is the thickness of JIS No. 5 tensile test
specimen used for measurement, and El is the converted value of total elongation
corresponding to the case where the sheet thickness is 1.2 mm.
TS x El is an index for evaluating ductility from the balance between
strength and total elongation, TS x n value is an index for evaluating work
hardenability from the balance between strength and a work hardening
coefficient, and T S ~x .h~ is an index for evaluating hole expanding property from
the balance between strength and a hole expanding ratio. (TS x El) x 7 x 10' +
(TS'.~x h) x 8 is an index for evaluating formability which is a combined
property of elongation and hole expanding property, a so-called stretch
flaneability .
It is further preferable that the value of TS x El is 20000 MPa or more, the
value of TS x n value is 160 MPa or more, the value of TS'.~x h is 5500000
ma1.?%o r more, and the value of (TS x El) x 7 x 10' + (TS'.~x h) x 8 is 190 x
1 o6 or more. Particularly preferably, the value of (TS x El) x 7 x 10' + (TS'~ x
h) x 8 is 200 x lo6 or more
Since the strain occurring when an automotive part is press-formed is
about 5 to lo%, the work hardening index was expressed by n value for the strain
range of 5 to 10% in the tensile test. Even if the total elongation of steel sheet is
large, the strain propagating property in the press forming of automotive part is
insufficient when the n value is low, and defective forming such as a local
thickness decrease occurs easily. From the viewpoint of shape fixability, the
yield ratio is preferably lower than 80%, further preferably lower than 75%, and
still further preferably lower than 70%.
2. Chemical composition of steel
C: more than 0.10% and less than 0.25%
If the C content is 0.10% or less, it is difficult to obtain the abovedescribed
structure. Therefore, the C content is made more than 0.10%. The
C content is preferably more than 0.12%, further preferably more than 0.14%,
and still further preferably more than 0.16%. On the other hand, if the C
content is 0.25% or more, not only the stretch flaneability of steel sheet is
impaired, but also the weldability is deteriorated. Therefore, the C content is
made less than 0.25%. The C content is preferably 0.23% or less, further
preferably 0.21% or less, and still further preferably less than 0.19% or less.
Si: more than 0.50% and less than 2.0%
Silicon (Si) has a function of improving the ductility, work hardenability,
and stretch flaneability through the restraint of austenite grain growth during
annealing. Also, Si is an element that has a fbnction of enhancing the stability
of austenite and is effective in obtaining the above-described structure. If the Si
content is 0.50% or less, it is difficult to achieve the effect brought about by the
above-described function. Therefore, the Si content is made more than 0.50%.
The Si content is preferably more than 0.70%, further preferably more than
0.90%, and still further preferably more than 1.20%. On the other hand, if the
Si content is 2.0% or more, the surface properties of steel sheet are deteriorated.
Further, the platability is deteriorated remarkably. Therefore, the Si content is
made less than 2.0%. The Si content is preferably less than 1.8%, further
preferably less than 1.6%, and still further preferably less than 1.4%.
In the case where the later-described A1 is contained, the Si content and
the sol.Al content preferably satisfy formula (2) below, hrther preferably satisfy
formula (3) below, and still further preferably satis@ formula (4) below.
Si + sol.Al > 0.60 ... (2)
Si + sol.Al > 0.90 ... (3)
Si + sol.Al > 1.20 ... (4)
in which, Si represents the Si content (mass%) in the steel, and sol.Al represents
the content (mass%) of acid-soluble Al.
Mn: more than 1.50% and 3.0% or less
Manganese (Mn) is an element that has a function of improving the
hardenability of steel and is effective in obtaining the above-described structure.
If the Mn content is 1.50% or less, it is difficult to obtain the above-described
structure. Therefore, the Mn content is made more than 1 SO%. The Mn
content is preferably more than 1.60%, further preferably more than 1.80%, and
still further preferably more than 2.0%. If the Mn content becomes too high, in
the structure of hot-rolled steel sheet, a coarse low temperature transformation
phase elongating and expanding in the rolling direction is formed, coarse retained
austenite grains increase in the structure after cold rolling and annealing, and the
work hardenability and stretch flaneability are deteriorated. Therefore, the Mn
content is made 3.0% or less. The Mn content is preferably less than 2.70%,
further preferably less than 2.50%, and still further preferably less than 2.30%.
P: less than 0.050%
Phosphorus (P) is an element contained in the steel as an impurity, and
segregates at the grain boundaries and embrittles the steel. For this reason, the
P content is preferably as low as possible. Therefore, the P content is made less
than 0.050% or less. The P content is preferably less than 0.030%, further
preferably less than 0.020%, and still further preferably less than 0.01 5%.
$\ S:O.~l~%orless
Sulfur (S) is an element contained in the steel as an impurity, and forms
sulfide-base inclusions and deteriorates the stretch flangeability. For this reason,
the S content is preferably as low as possible. Therefore, the S content is made
5 0.0 10% or less. The S content is preferably less than 0.005%, further preferably
less than 0.003%, and still further preferably less than 0.002%.
sol.Al: 0.50% or less
Aluminum (Al) has a function of deoxidizing molten steel. In the present
10 invention, since Si having a deoxidizing fbnction like A1 is contained, A1 need
not necessarily be contained. That is, the sol.Al content may be impurity level.
In the case where sol.Al is contained for the purpose of promotion of deoxidation,
0.0050% or more of sol.Al is preferably contained. The sol.Al content is fbrther
preferably more than 0.020%. Also, like Si, A1 is an element that has a function
15 of enhancing the stability of austenite and is effective in obtaining the abovedescribed
structure. Therefore, A1 can be contained for this purpose. In this
case, the sol.Al content is preferably more than 0.040%, further preferably more
than 0.050%, and still further preferably more than 0.060%. On the other hand,
if the sol.Al content is too high, not only a surface flaw caused by alumina is
2 0 liable to occur, but also the transformation point rises greatly, so that it is
difficult to obtain a structure such that the main phase is a low temperature
transformation phase. Therefore, the sol.Al content is made 0.50% or less.
The sol.Al content is preferably less than 0.30%, Wher preferably less than
0.20%, and still further preferably less than 0.10%.
25
N: 0.010% or less
Nitrogen (N) is an element contained in the steel as an impurity, and
deteriorates the ductility. For this reason, the N content is preferably as low as
possible. Therefore, the N content is made 0.010% or less. The N content is
3 0 preferably 0.006% or less, fbrther preferably 0.005% or less, and still fbrther
preferably 0.003% or less.
The steel sheet relating to the present invention may contain elements
listed below as arbitrary elements.
One or more types selected from a group consisting of Ti: less than
0.040%, Nb: less than 0.030%, and V: 0.50% or less.
Ti, Nb, and V have effects of increasing work strain by suppressing
recrystallization in a hot rolling process, thereby fining the structure of the hotrolled
steel sheet. Moreover, they have an effect of precipitating as carbide or
nitride, thereby restraining the coarsening of austenite during annealing.
Therefore, one or more types of those elements may be contained. However,
even if those elements are excessively contained, effectiveness by the above
described effects will be saturated, which is uneconomical. Not only that, the
recrystallization temperature during annealing rises and thereby the structure
after annealing becomes non-uniform so that the stretch flaneability is impaired
as well. Further, the amount of the precipitation of carbide or nitride increases,
yield ratio increases, and shape freezing property deteriorates as well.
Therefore, it is decided that the Ti content is less than 0.040%, the Nb content is
less than 0.030%, and the V content is 0.50% or less. The Ti content is
preferably less than 0.030%, and more preferably less than 0.020%; the Nb
content is preferably less than 0.020%, and more preferably less than 0.012%;
and the V content is preferably 0.30% or less, and more preferably less than
0.050%. Further, the value of Nb + Ti x 0.2 is preferably less than 0.030%, and
more preferably less than 0.020%.
To surely achieve the effect brought about by the above-described function,
either of Ti: 0.005% or more, Nb: 0.005% or more, and V: 0.010% or more is
preferably satisfied. In the case where Ti is contained, the Ti content is further
preferably made 0.0 10% or more, in the case where Nb is contained, the Nb
content is further preferably made 0.010% or more, and in the case where V is
contained, the V content is further preferably made 0.020% or more.
One kind or two or more kinds selected from a group consisting of Cr: 1 .O% or
less, Mo: less than 0.20%, and B: 0.010% or less
Cr, Mo and B are elements that have a function of improving the
hardenability of steel and are effective in obtaining the above-described structure.
Therefore, one kind or two or more kinds of these elements may be contained.
However, even if these elements are contained excessively, the effect brought
about by the above-described hnction saturates, being uneconomical.
Therefore, the Cr content is made 1 .O% or less, the Mo content is made less than
0.20%, and the B content is made 0.010% or less. The Cr content is preferably
0.50% or less, the Mo content is preferably 0.10% or less, and the B content is
preferably 0.0030% or less. To more surely achieve the effect brought about by
the above-described function, either of Cr: 0.20% or more, Mo: 0.05% or more,
and B: 0.0010% or more is preferably satisfied.
One kind or two or more kinds selected from a group consisting of Ca: 0.0 10%
or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less
Ca, Mg and REM each have a function of improve the stretch flaneability
by means of the regulation of shapes of inclusions, and Bi also has a function of
improve the stretch flaneability by means of the refinement of solidified structure.
Therefore, one kind or two or more kinds of these elements may be contained.
However, even if these elements are contained excessively, the effect brought
about by the above-described function saturates, being uneconomical.
Therefore, the Ca content is made 0.0 10% or less, the Mg content is made
0.010% or less, the REM content is made 0.050% or less, and the Bi content is
made 0.050% or less. Preferably, the Ca content is 0.0020% or less, the Mg
content is 0.0020% or less, the REM content is 0.0020% or less, and the Bi
content is 0.0 10% or less. To more surely obtain above-described function,
either of Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005% or more,
and Bi: 0.0010% or more is preferably satisfied. The REM means rare earth
metals, and is a general term of a total of 17 elements of Sc, Y, and lanthanoids.
The REM content is the total content of these elements.
3. Hot-dip galvanized layer
+. Examples of the hot-dip galvanized layer include those formed by hot-dip
galvanizing, alloyed hot-dip galvanizing, hot-dip aluminum galvanizing, hot-dip
Zn-A1 alloy galvanizing, hot-dip Zn-Al-Mg alloy galvanizing, and hot-dip Zn-Al-
Mg-Si alloy galvanizing or the like. For example, when the galvanized layer is
5 formed by alloyed hot-dip galvanizing, the Fe concentration in the galvanized
film is 7% or more and 15% or less. Examples of the hot-dip Zn-A1 alloy
galvanizing include hot-dip Zn-5%Al alloy galvanizing and hot-dip Zn-55%A1
alloy galvanizing.
The mass of deposit of plating film is not particularly limited, and may be
10 the same as before. For example, it may be 25 g/m2 or more and 200 glm2 or
less per one side. When the plated layer is an alloyed hot-dip galvanized layer,
the mass of deposit of plating film is preferably 25 g/m2 or more and 60 g/m2 or
less per one side from the viewpoint of suppressing powdering.
For the purpose of further improving corrosion resistance and coatability,
15 post processing of single or multiple layers selected from chromic acid treatment,
phosphate treatment, silicate-type non-chromium chemical treatment, resin film
coating, and the like may be applied after plating.
4. Production method
20 First, a cold rolled steel sheet is produced, which has the above described
structure and chemical composition, and which is used as a base material.
Specifically, a steel having the above-described chemical composition is
melted by publicly-known means and thereafter is formed into an ingot by the
continuous casting process, or is formed into an ingot by an optional casting
2 5 process and thereafter is formed into a billet by a billeting process or the like.
In the continuous casting process, to suppress the occurrence of a surface defect
caused by inclusions, an external additional flow such as electromagnetic stirring
is preferably produced in the molten steel in the mold. Concerning the ingot or
billet, the ingot or billet that has been cooled once may be reheated and be
3 0 subjected to hot rolling. Alternatively, the ingot that is in a high-temperature
state after continuous casting or the billet that is in a high-temperature state after
billeting may be subjected to hot rolling as it is, or by retaining heat, or by
heating it auxiliarily. In this description, such an ingot and a billet are generally
called a "slab" as a raw material for hot rolling.
To prevent austenite from coarsening, the temperature of the slab that is to
be subjected to hot rolling is preferably made lower than 1250°C, Mher
preferably made lower than 1200°C. The lower limit of the temperature of slab
to be subjected to hot rolling need not be restricted specially, and may be any
temperature at which hot rolling can be finished in a temperature range of (Ar3
point + 30°C) or higher, and higher than 880°C as described later.
Hot-rolling is completed in a temperature range of (A3 point + 30°C) or
10 higher, and higher than 880°C to fine the structure of the hot-rolled steel sheet by
causing austenite to transform after the completion of rolling. When the
temperature at the completion of rolling is too low, a coarse low temperature
transformation phase which extends in the rolling direction occurs in the
structure of the hot-rolled steel sheet so that a coarse austenite grain increases in
15 the structure after cold rolling and annealing, and thereby work hardenability and
I stretch flaneability become more likely to deteriorate. For this reason, the
completion temperature of hot rolling is set to (Ar3 point + 30°C) or higher, and
higher than 880°C. The completion temperature is preferably (Ar3 point +
50°C) or higher, more preferably (AT3 point + 70°C) or higher, and particularly
2 0 preferably (Ar3 point + 90°C) or higher. On the other hand, when completion
temperature of rolling is too high, the accumulation of work strain becomes
insufficient, making it difficult to make the structure of the hot-rolled steel sheet
fine. For this reason, the completion temperature of hot rolling is preferably
lower than 950°C, and more preferably lower than 920°C. Moreover, to
2 5 mitigate the production load, it is preferable to increase the completion
temperature of hot rolling, thereby decreasing the rolling load. From this
viewpoint, the completion temperature of hot rolling is preferably (Ar3 point +
50°C) or higher and higher than 900°C.
In the case where the hot rolling consists of rough rolling and finish rolling,
3 0 to finish the finish rolling at the above-described temperature, the rough-rolled
material may be heated at the time between rough rolling and finish rolling. It
is desirable that by heating the rough-rolled material so that the temperature of
the rear end thereof is higher than that of the front end thereof, the fluctuations in
temperature throughout the overall length of the rough-rolled material at the start
time of finish rolling are restrained to 140°C or less. Thereby, the homogeneity
of product properties in a coil is improved.
The heating method of the rough-rolled material has only to be carried out
by using publicly-known means. For example, a solenoid type induction
heating apparatus is provided between a roughing mill and a finish rolling mill,
and the temperature rising amount in heating may be controlled based on, for
example, the temperature distribution in the lengthwise direction of the rough-
10 rolled material on the upstream side of the induction heating apparatus.
The reduction of hot rolling is set that the reduction of the final one pass is
more than 15% in a sheet-thickness reduction rate. This is for increasing the
amount of work strain to be introduced into austenite, thereby fining the structure
of hot-rolled steel sheet, restraining the formation of coarse retained-austenite
15 grains in the structure after cold-rolling and annealing, and fining polygonal
ferrite. The reduction of the final one pass is preferably more than 25%, more
preferably more than 30%, and particularly preferably more than 40%. When
the reduction becomes too high, the rolling load increases and rolling becomes
difficult. Therefore, the 'reduction of the final one pass is preferably less than
20 55%, and more preferably less than 50%. To decrease the rolling load, a socalled
lubricated rolling may be performed in which rolling is performed by
supplying rolling oil between the rolling-mill roll and the steel sheet to decrease
the friction coefficient.
After hot rolling, the steel sheet is rapidly cooled to a temperature range of
2 5 720°C or lower within 0.40 seconds after the completion of rolling. This is
done for the purpose of suppressing the release of work strain introduced into
austenite by rolling, making the austenite transform with work strain as a driving
force, fining the structure of the hot-rolled steel sheet, restraining the formation
of coarse retained-austenite grains in the structure after cold rolling and
3 0 annealing, and fining polygonal ferrite. The steel sheet is preferably rapidly
cooled to a temperature range of 720°C or lower within 0.30 seconds after the
completion of rolling, and more preferably rapidly cooled to a temperature range
of 720°C or lower within 0.20 seconds after the completion of rolling.
As the temperature at which rapid cooling stops is lower, the structure of
hot-rolled steel sheet is made finer. Therefore, it is preferable that the steel
5 sheet be rapidly cooled to the temperature region of 700°C or lower after the
completion of rolling. It is further preferable that the steel sheet be rapidly
cooled to the temperature region of 680°C or lower after the completion of
rolling. Also, as the average cooling rate during rapid cooling is higher, the
release of work strain is restrained. Therefore, the average cooling rate during
10 rapid cooling is made 400°C/s or higher. Thereby, the structure of hot-rolled
steel sheet can be made still finer. The average cooling rate during rapid
cooling is preferably made 600°Cls or higher, and fiuther preferably made
800°C/s or higher. The time from the completion of rolling to the start of rapid
cooling and the cooling rate during the time need not be defined specially.
15 The equipment for performing rapid cooling is not defined specially;
however, on the industrial basis, the use of a water spraying apparatus having a
high water amount density is suitable. A method is cited in which a water spray
header is arranged between rolled sheet conveying rollers, and high-pressure
water having a sufficient water amount density is sprayed from the upside and
2 0 downside of the rolled sheet.
After the stopping of rapid cooling, a hot-rolled steel sheet is obtained via
either of the following procedures:
(I) the steel sheet after the stopping of rapid cooling is coiled in a
temperature range of higher than 400°C; or
2 5 (2) the steel sheet after the stopping of rapid cooling is coiled in a
temperature range of lower than 200°C, and thereafter is annealed in a
temperature range of 500°C or higher, and lower than Acl point.
In the above described embodiment of (1), the reason why the steel sheet
is coiled in a temperature range of higher than 400°C is that when the coiling
3 0 temperature is 400°C or lower, iron carbides will not precipitate sufficiently in
the hot-rolled steel sheet so that coarse retained-austenite grains are formed and
polygonal ferrite is coarsened in the structure after cold rolling and annealing.
The coiling temperature is preferably higher than 500°C, more preferably higher
than 520°C, and particularly preferably higher than 550°C. On the other hand,
when the coiling temperature is too high, ferrite is coarsened in the hot-rolled
steel sheet, and coarse retained-austenite grains are formed in the structure after
the cold rolling and annealing. For this reason, the coiling temperature is
preferably lower than 650°C, and more preferably lower than 620°C.
In the case of the above described embodiment of (2), the reason why the
steel sheet is coiled in a temperature range of lower than 200°C, and the hotrolled
steel sheet is subjected to annealing in a temperature range of 500°C or
higher, and lower than Acl point is that when the coiling temperature is 200°C or
higher, the formation of martensite will become insufficient. When the
annealing temperature after the coiling is lower than 500°C, iron carbides will
not precipitate sufficiently, and when the temperature is Acl point or higher,
ferrite will be coarsened, and coarse retained-austenite grains will be formed in
15 the structure after cold rolling and annealing.
In the case of the above described embodiment of (2), the hot-rolled steel
sheet which has been hot-rolled and coiled is subjected to processing such as
degreasing according to a known method as needed, and thereafter is annealed.
The annealing applied to a hot-rolled steel sheet is referred to as hot-rolled sheet
2 0 annealing, and the steel sheet after the hot-rolled sheet annealing is referred to as
hot-rolled annealed steel sheet. Before hot-rolled sheet annealing, descaling
may be performed by acid pickling, etc. The holding time in the hot-rolled
sheet annealing does not need to be specifically limited. Since a hot-rolled steel
sheet produced via appropriate immediate rapid cooling process has a fine
2 5 structure, it does not need to be retained for long hours. Since as the holding
time becomes longer, the productivity deteriorates, the upper limit of the holding
time is preferably less than 20 hours. The holding time is more preferably less
than 10 hours, and particularly preferably less than 5 hours.
In either of the above described embodiments of (1) and (2), although
30 conditions from the stopping of rapid cooling to the coiling will not be
particularly specified, it is preferable that the steel sheet is retained in a
temperature range of 720 to 600°C for 1 second or more after the stopping of
* rapid cooling. Retaining for 2 seconds or more is more preferable, and retaining
for 5 seconds or more is particularly preferable. As a result of this, the
formation of fine ferrite is facilitated. On the other hand, since when the
holding time becomes too long, the productivity will be impaired, the upper limit
5 of the holding time in a temperature range of 720 to 600°C is preferably within
10 seconds. After the holding in the temperature range of 720 to 600°C, the
steel sheet is preferably cooled to the coiling temperature at a cooling rate of 20
"C/sec or higher to prevent the coarsening of ferrite that has been produced.
The hot-rolled steel sheet obtained through the procedure of (1) or (2) is
10 descaled by acid pickling, etc., and thereafter is subjected to cold rolling
I
I according to a common procedure. Cold-rolling is performed preferably at a
I cold reduction rate (the reduction in cold rolling) of 40% or higher to facilitate
I
recrystallization, thereby homogenizing the structure after cold rolling and
annealing, and further improving stretch flangeability. Since when the cold
15 reduction rate is too high, the rolling load increases making the rolling difficult,
the upper limit of cold reduction rate is preferably less than 70%, and more
preferably less than 60%.
The cold-rolled steel sheet which has been obtained in cold-rolling process
is subjected to processing such as degreasing as needed according to a known
2 0 method, and thereafter is annealed. The lower limit of soaking temperature in
annealing is set to higher than Ac3 point. This is for obtaining a structure in
which the main phase is a low temperature transformation phase and the second
phase contains retained austenite. However, when the soaking temperature
becomes too high, austenite becomes excessively coarse, and the ductility, work
2 5 hardenability, and stretch flaneability are likely to deteriorate. For this reason,
the upper limit of soaking temperature is preferably less than (Ac3 point + 100°C).
The upper limit is more preferably less than (Ac3 point + 50°C), and particularly
preferably less than (Ac3 point + 20°C).
Although the holding time (soaking time) at a soaking temperature does
3 0 not need to be particularly limited, it is preferably more than 15 seconds, and
more preferably more than 60 seconds to achieve stable mechanical properties.
On the other hand, when the holding time becomes too long, austenite becomes
'5; excessively coarse so that the ductility, work hardenability, and stretch
flaneability are likely to deteriorate. For this reason, the holding time is
preferably less than 150 seconds, and more preferably less than 120 seconds.
In a heating procedure in annealing, a heating rate from 700°C to a
5 soaking temperature is preferably less than 10.0 "C/sec to facilitate
recrystallization and homogenize the structure after annealing, further improving
the stretch flaneability. The heating rate is further preferably less than 8.0
"C/sec, and particularly preferably less than 5.0 "C/sec.
In a cooling procedure after soaking in annealing, cooling is preferably
10 performed at a cooling rate of 1 5"C/sec or higher through a temperature range of
650 to 500°C to achieve a structure in which the main phase is a low temperature
transformation phase. It is more preferable to perform cooling at a cooling rate
of 15"CIsec or higher through a temperature range of 650 to 450°C. Since the
volume fraction of low temperature transformation phase increases as the cooling
15 rate increases, the cooling rate is more preferably 20°C/sec or higher, and
particularly preferably 40°C/sec or higher. On the other hand, since when the
cooling rate is too high, the shape of steel sheet is impaired, the cooling rate in a
temperature range of 650 to 500°C is preferably 200°C/sec or lower. The
cooling rate is further preferably less than 150 "C/sec, and particularly preferably
2 0 less than 130 "Clsec.
When it is intended to facilitate the production of fine polygonal ferrite
and improve the ductility and work hardenability, the steel sheet is preferably
cooled by 50°C or more from the soaking temperature at a cooling rate of lower
than 5.0 "Clsec. The cooling rate after soaking is more preferably lower than
2 5 3.0 "Clsec. The cooling rate is particularly preferably lower than 2.0°C/sec.
Moreover, to further increase the volume fraction of polygonal ferrite, the steel
sheet is cooled preferably by 80°C or more, more preferably by 100°C or more,
and particularly preferably by 120°C or more from the soaking temperature at a
cooling rate of lower than 5.0°C/sec.
30 Moreover, to ensure the amount of retained austenite, the steel sheet is
retained in a temperature range of 450 to 340°C for 15 seconds or more. To
improve the stability of retained austenite, thereby further improving the ductility,
work hardenability, and stretch flaneability, the holding temperature range is
preferably 430 to 360°C. Moreover, since as the holding time increases, the
stability of retained austenite improves, the holding time is set to 30 seconds or
more. The holding time is preferably 40 seconds or more, and more preferably
5 50 seconds or more. Since when the holding time is excessively long, not only
the productivity is impaired, but also the stability of retained austenite rather
declines, the holding time is preferably 500 seconds or less. The holding time is
more preferably 400 seconds or less, particularly preferably 200 seconds or less,
and most preferably 100 seconds or less.
Thus produced cold-rolled steel sheet which has been annealed is
subjected to hot-dip galvanizing. In the hot-dip galvanizing, the cold-rolled
steel sheet is treated up to the annealing step in the above described manner, and
the steel sheet is reheated as needed, and thereafter is subjected to hot-dip
galvanizing. As for the conditions for hot-dip galvanizing, conditions
15 commonly applied depending on the kind of hot-dip galvanizing may be adopted.
When the hot-dip galvanizing is hot-dip galvanizing or hot-dip Zn-A1 alloy
galvanizing, the hot-dip galvanizing may be applied in a temperature range of
450°C or higher and 620°C or lower as with conditions performed in a common
hot-dip galvanizing line such that a hot-dip galvanized layer or a hot-dip Zn-A1
2 0 alloy galvanized layer is formed on the surface of steel sheet.
Moreover, after the hot-dip galvanizing treatment, alloying treatment for
alloying the hot-dip galvanized layer may be applied. In this occasion, the A1
concentration in the plating bath is preferably controlled to be 0.08 to 0.15%.
There will be no problem even if the plating bath includes, besides Zn and Al,
25 0.1% or less of Fe, V, Mn, Ti, Nb, Ca, Cr, Ni, W, Cu, Pb, Sn, Cd, Sb, Si, and Mg.
Moreover, the galvannealing temperature is preferably 470°C or higher and
570°C or lower. This is because, when the galvannealing temperature is lower
than 470°C, the alloying rate will remarkably decline, and the time needed for the
alloying treatment increases, thereby leading to a decline of productivity.
3 0 Moreover, when the galvannealing temperature exceeds 570°C, the alloying rate
in the plated layer remarkably increases, which may lead to an embrittlement of
the alloyed hot-dip galvanized layer. The galvannealing temperature is more
preferably 550°C or lower. Since, after hot-dip galvanizing, mutual diffusion of
elements occurs between the steel material and the molten metal at the time of
dipping and cooling, the composition of the coated film on the surface of the
cooled steel sheet will have a slightly higher Fe concentration than the
composition of the plating bath. In the alloyed hot-dip galvanizing, which
actively exploits such mutual diffusion, Fe concentration in the coated film will
be 7 to 15%.
Although the mass of deposit of plating film is not particularly limited,
generally, 25 to 200 g/m2 per one side is preferable. In the case of alloyed hotdip
galvanizing, since there are concerns about powdering, the mass of deposit of
plating film is preferably 25 to 60 g/m2 per one side. Although hot-dip
galvanizing is typically performed on both sides, it can be performed on one side
as well.
Thus obtained hot-dip galvanized cold-rolled steel sheet may be subjected
to temper rolling according to a common procedure. However, since a high
elongation rate in temper rolling will lead to deterioration of ductility, the
elongation rate in temper rolling is preferably 1 .O% or less. More preferably,
the elongation rate is 0.5% or less.
The hot-dip galvanized cold-rolled steel sheet may be subjected to
chemical treatment which is well known to one skilled in the art to improve the
corrosion resistance thereof. The chemical treatment is preferably performed by
using a treatment solution which does not contain chromium. One example of
such chemical treatment includes one which forms a siliceous film.
Example
The present invention will be specifically described with reference to
examples.
By using an experimental vacuum melting furnace, steels each having the
chemical composition given in Table 1 were melted and cast. These ingots
were formed into 30-mm thick billets by hot forging. The billets were heated to
1200°C by using an electric heating furnace and held for 60 minutes, and
thereafter were hot-rolled under the conditions given in Table 2.
To be specific, an experimental hot-rolling mill was used to perform 6
passes of rolling in a temperature range of AT3 point + 30°C or higher, and higher
than 880°C so that the billet was finished into a thickness of 2 mm. The
reduction of the final one pass was set to 1 1 to 42% in thickness reduction rate.
After hot rolling, the steel was cooled to 650 to 720°C at various cooling
conditions by using a water spray, further allowed to cool for 5 to 10 seconds,
thereafter cooled to various temperatures at a cooling rate of 60°C/sec, and coiled
at the respective temperatures. Excepting those whose coiling temperature was
set to the room temperature, the steel was put into an electric heating furnace
which was held at the coiling temperature and held for 30 minutes, thereafter was
furnace cooled to the room temperature at a cooling rate of 20 "C/h, thereby
simulating slow cooling after coiling, to obtain a hot-rolled steel sheet.
Moreover, those whose coiling temperature were set to the room temperature
were, excepting some of them, heated fiom the room temperature to 600°C
which was a temperature range lower than Ac, point at a rate of temperature rise
of 50°C/h, and thereafter was subjected to hot-rolled sheet annealing in which
cooled to the room temperature at a cooling rate of 20°C/h.
The obtained hot-rolled steel sheet was subjected to acid pickling to be
used as a base metal for cold-rolling, which was subjected to cold-rolling at a
reduction of 50% to obtain a cold-rolled steel sheet having a thickness of 1.0 mm.
Using a continuous annealing simulator, the obtained cold-rolled steel sheet was
heated to 550°C at a heating rate of 10 "C/sec, and thereafter was heated to
various temperatures shown in Table 2 at a heating rate of 2 "C/sec to be soaked
for 95 seconds. Thereafter, the steel sheet was cooled to various primary
cooling stop temperatures shown in Table 2 at a cooling rate of 2 "Clsec; was
cooled to various secondary cooling stop temperatures shown in Table 2 at a
cooling rate of 40 "C/sec; next, was held at the secondary cooling stop
temperature for 60 to 330 seconds to perform heat treatment corresponding to an
annealing step, and thereafter was subjected to heat treatment corresponding to
dipping into a hot-dip galvanizing bath of 460°C and heat treatment
corresponding to alloying treatment at 500 to 520°C, and was cooled to the room
.3 -3 -
temperature to obtain an annealed steel sheet which has gone through heat
treatment corresponding to alloyed hot-dip galvanizing after annealing.
Note) Remarks: Symbol 0 indicates inventive example, symbol x indicates comparative example.
Symbol * indicates out of the scope of the present invention.

A test specimen for SEM observation was sampled from the annealed steel
sheet, and the longitudinal cross sectional surface thereof parallel to the rolling
direction was polished and was subjected to Nital etching. Thereafter, the
structure was observed at a position deep by one-fourth of thickness from the
surface of steel sheet, and by image processing, the volume fractions of low
temperature transformation phase and polygonal ferrite were measured. Also,
the average grain size (circle corresponding diameter) of polygonal ferrite was
determined by dividing the area occupied by the whole of polygonal ferrite by
the number of crystal grains of polygonal ferrite.
Moreover, a specimen for XRD measurement was taken from the annealed
steel sheet, the rolled surface thereof was chemically polished from the surface of
the steel sheet to a position at a depth of 114 sheet thickness, and thereafter
subjected to X-ray diffraction test to measure the volume fraction and average
carbon concentration of retained austenite. To be specific, RINT 2500
manufactured by Rigaku Corporation was used as the X-ray diffraction apparatus
to make Co-Ka rays incident on the specimen, and integrated intensities of (1 lo),
(200), and (2 1 1) diffraction peaks of a phase, and (1 1 l), (200), and (220)
diffraction peaks of y phase were measured to determine the volume fraction of
retained austenite. Further, a lattice constant dy (A) was determined from
diffraction angles of the (1 1 1), (200), and (220) diffraction peaks of y phase, and
an average carbon concentration Cy (mass%) of retained austenite was
determined from the following conversion formula.
Cy = (dy - 3.572 + 0.00157 x Si - 0.0012 x Mn) 1 0.033
Furthermore, a test specimen for EBSP measurement was sampled from
the annealed steel sheet, and the longitudinal cross sectional surface thereof
parallel to the rolling direction was electropolished. Thereafter, the structure
was observed at a position deep by one-fourth of thickness from the surface of
steel sheet, and by image analysis, the grain size distribution of retained austenite
and the average grain size of retained austenite were measured. Specifically, as
an EBSP measuring device, OIM5 manufactured by TSL Corporation was used,
electron beams were applied at a pitch of 0.1 pm in a region having a size of 50
pm in the sheet thickness direction and 100 pm in the rolling direction, and
among the obtained data, the data in which the reliability index was 0.1 or more
was used as effective data to make judgment of fcc phase. With a region that
was observed as the fcc phase and was surrounded by a parent phase being made
one retained austenite grain, the circle corresponding diameter of individual
retained austenite grain was determined. The average grain size of retained
austenite was calculated as the mean value of circle corresponding diameters of
individual effective retained austenite grains, the effective retained austenite
grains being retained austenite grains each having a circle corresponding
diameter of 0.15 pm or larger. Also, the number density (Nd per unit area of
retained austenite grains each having a grain size of 1.2 pm or larger was
determined.
The yield stress (YS) and tensile strength (TS) were determined by
sampling a JIS No. 5 tensile test specimen along the direction perpendicular to
the rolling direction from the annealed steel sheet, and by conducting a tensile
test at a crosshead speed of 10 mrnlmin. The total elongation (El) was
determined as follows: a tensile test was conducted by using a JIS No. 5 tensile
test specimen sampled along the direction perpendicular to the rolling direction,
and by using the obtained actually measured value (Elo), the converted value of
total elongation corresponding to the case where the sheet thickness is 1.2 mm
was determined based on formula (1) above. The work hardening index (n
value) was calculated with the strain range being 5 to 10% by conducting a
tensile test by using a JIS No. 5 tensile test specimen sampled along the direction
perpendicular to the rolling direction. Specifically, the n value was calculated
by the two point method by using test forces with respect to nominal strains of
5% and 10%.
The stretch flaneability was evaluated by performing the Hole Expanding
Test specified by the Japan Iron and Steel Federation standard JFSTlOOl and
measuring a hole expanding ratio (h). A square element sheet of 100 mm
square was taken from an annealed steel sheet, a punch hole having a diameter of
10 mm was provided at a clearance of 12.5%, and the punch hole was expanded
from a punched side with a conical punch of a point angle of 60" to measure an
-&-
expansion ratio of the hole when a crack extended through the sheet thickness so
that the expansion ratio was adopted as the hole expanding ratio.
Table 3 gives the structure observation results and the performance
evaluation results of the cold-rolled steel sheet after being annealed. In Tables
5 1 to 3, mark "*" attached to a symbol or numeral indicates that the symbol or
numeral is out of the range of the present invention.
Table 31
Any of the test results (Test Nos. 1 to 27) of steel sheets which were
within the scope of the present invention showed a value of TS x El of 18000
MPa or more, a value of TS x n value of 150 or more, a value of TS'.~x h of
4500000 M..al.'% or more, and a value of (TS x El) x 7 x lo3+ (TS'.~x h) x 8
5 of 180 x lo6 or more, thus exhibiting excellent ductility, work hardenability, and
stretch flangeability .
The test results (Test Nos. 28 to 33) of steel sheets whose structures were
out of the scope specified by the present invention showed poor performance in
at least one of ductility, work hardenability, and stretch flangeability.

1. A hot-dip galvanized cold-rolled steel sheet having a hot-dip galvanized
layer on a surface of a cold-rolled steel sheet, characterized by having a chemical
5 composition comprising, in mass percent, C: more than 0.10% and less than
0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and at
most 3.0%, P: less than 0.050%, S: at most 0.010%, sol. Al: at least 0% and at
most 0.50%, N: at least 0.010%, Ti: at least 0% and less than 0.040%, Nb: at
least 0% and less than 0.030%, V: at least 0% and at most 0.50%, Cr: at least 0%
10 and at most 1.0%, Mo: at least 0% and less than 0.20%, B: at least 0% and at
most 0.010%, Ca: at least 0% and at most 0.010%, Mg: at least 0% and at most
0.010%, REM: at least 0% and at most 0.050%, Bi: at least 0% and at most
0.050%, and the remainder being Fe and impurities, and
by having a structure in which a main phase is a low temperature
I. 5 transformation phase and a second phase contains retained austenite, wherein
the retained austenite has a volume fraction of more than 4.0% to less than
25.0% with respect to a whole structure, and an average grain size of less than
0.80 pm, and in the retained austenite, a number density of retained austenite
grains having a grain size of 1.2 pm or more is 3.0 x lPm2 or less.
2 0
2. The hot-dip galvanized cold-rolled steel sheet as set forth in claim 1,
wherein the chemical composition contains, in mass percent, one kind or two or
more kinds selected from a group consisting of Ti: at least 0.005% and less than
0.040%, Nb: at least 0.005% and less than 0.030%, and V: at least 0.010% and at
2 5 most 0.50%.
3. The hot-dip galvanized cold-rolled steel sheet as set forth in claim 1 or 2,
wherein the chemical composition contains, in mass percent, one kind or two or
more kinds selected from a group consisting of Cr: at least 0.20% and at most
3 0 1.0%, Mo: at least 0.05% and less than 0.20%, and B: at least 0.0010% and at
most 0.010%.
4. The hot-dip galvanized cold-rolled steel sheet as set forth in any one of
claims 1 to 3, wherein the chemical composition contains, in mass percent, one
kind or two or more kinds selected from a group consisting of Ca: at least
0.0005% and at most 0.010%, Mg: at least 0.0005% and at most 0.010%, REM:
at least 0.0005% and at most 0.050%, and Bi: at least 0.0010% and at most
0.050%.
5. A method for manufacuring a hot-dip galvanized cold-rolled steel sheet
using as a base material a cold-rolled steel sheet characterized by having a
structure in which a main phase is a low temperature transformation phase and a
second phase contains retained austenite, comprising,
(A) a hot-rolling step in which a slab having the chemical composition as
set forth in any of claims 1 - 4 is subjected to hot rolling in which a reduction of
final one pass is more than 15% and rolling is completed in a temperature range
of (Ar3 point + 30°C) or higher, and higher than 880°C to form a hot-rolled steel
sheet, and the hot-rolled steel sheet is cooled to a temperature range of 720°C or
lower within 0.40 seconds after the completion of the rolling, and is coiled in a
temperature range of higher than 400°C;
(B) a cold-rolling step in which the hot-rolled steel sheet is subjected to a
cold rolling to form a cold-rolled steel sheet;
(C) an annealing step in which the cold-rolled steel sheet is subjected to
soaking treatment in a temperature range of higher than Ac3 point, thereafter is
cooled to a temperature range of 450°C or lower and 340°C or higher, and is held
in the same temperature range for 15 seconds or more; and
(D) a hot-dip galvanizing step in which the cold-rolled steel sheet obtained
by the annealing step is subjected to hot-dip galvanizing.
6. A method for producing a hot-dip galvanized cold-rolled steel sheet
using as a base material a cold-rolled steel sheet characterized by having a
structure in which a main phase is a low temperature transformation phase and a
second phase contains retained austenite, comprising the following steps (a) to
(el:
+- (a) a hot-rolling step in which a slab having the chemical composition as
set forth in any of claims 1 - 4 is subjected to hot rolling in which a reduction of
final one pass is more than 15% and rolling is completed in a temperature range
of (Ar3 point + 30°C) or higher, and higher than 880°C to form a hot-rolled steel
5 sheet, and the hot-rolled steel sheet is cooled to a temperature range of 720°C or
lower within 0.40 seconds after the completion of the rolling, and is coiled in a
temperature range of lower than 200°C;
(b) a hot-rolled sheet annealing step in which the hot-rolled steel sheet is
subjected to annealing in a temperature range of 500°C or higher, and lower than
10 Acl point;
(c) a cold-rolling step in which the hot-rolled steel sheet obtained by the
hot-rolled sheet annealing step is subjected to cold rolling to form a cold-rolled
steel sheet;
(d) an annealing step in which the cold-rolled steel sheet is subjected to
15 soaking treatment in a temperature range of higher than Ac3 point, thereafter is
cooled to a temperature range of 450°C or lower and 340°C or higher, and is held
in the same temperature range for 15 seconds or more; and
(e) a hot-dip galvanizing step in which the cold-rolled steel sheet obtained
by the annealing step is subjected to hot-dip galvanizing.
Dated this 13" day of January, 20 14.
Nippon Steel & Sum' mo Metal Corporation
DW&&+ I (Dev Robinson)
of Amarchand & Mangaldas &
Suresh A. Shroff & Co.
Attorneys for the Applicant