U.S. patent application number 16/072606 was filed with the patent office on 2019-01-31 for high-yield-ratio high-strength galvanized steel sheet and method for producing the same.
This patent application is currently assigned to JFE Steel Corpration. The applicant listed for this patent is JFE Steel Corpration. Invention is credited to Takuya Hirashima, Masaki Koba, Hiroyuki Masuoka, Tatsuya Nakagaito, Yasuhiro Nishimura, Seisuke Tsuda, Hiromi Yoshitomi.
Application Number | 20190032185 16/072606 |
Document ID | / |
Family ID | 59503978 |
Filed Date | 2019-01-31 |
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United States Patent
Application |
20190032185 |
Kind Code |
A1 |
Yoshitomi; Hiromi ; et
al. |
January 31, 2019 |
HIGH-YIELD-RATIO HIGH-STRENGTH GALVANIZED STEEL SHEET AND METHOD
FOR PRODUCING THE SAME
Abstract
Provided are a high-yield-ratio high-strength galvanized steel
sheet that includes a Mn-containing steel sheet serving as a base
material, and a production method therefor. The high-yield-ratio
high-strength galvanized steel sheet includes a steel sheet having
a specific composition and a metallographic structure including, by
area percentage, 20% or less ferrite, 40% or more in total of
bainite and tempered martensite, and 60% or less as-quenched
martensite, the bainite having an average grain size of 6.0 .mu.m
or less, and a coated layer arranged on the steel sheet, the coated
layer having a coating weight per side of 20 to 120 g/m.sup.2 and
having a Mn content of 0.05 g/m.sup.2 or less, in which the
high-yield-ratio high-strength galvanized steel sheet has a yield
ratio of 65% or more and a tensile strength of 950 MPa or more.
Inventors: |
Yoshitomi; Hiromi;
(Chiyoda-ku, Tokyo, JP) ; Hirashima; Takuya;
(Chiyoda-ku, Tokyo, JP) ; Masuoka; Hiroyuki;
(Chiyoda-ku, Tokyo, JP) ; Tsuda; Seisuke;
(Chiyoda-ku, Tokyo, JP) ; Koba; Masaki;
(Chiyoda-ku, Tokyo, JP) ; Nishimura; Yasuhiro;
(Chiyoda-ku, Tokyo, JP) ; Nakagaito; Tatsuya;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corpration |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corpration
Tokyo
JP
|
Family ID: |
59503978 |
Appl. No.: |
16/072606 |
Filed: |
January 26, 2017 |
PCT Filed: |
January 26, 2017 |
PCT NO: |
PCT/JP2017/002616 |
371 Date: |
July 25, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/58 20130101;
C21D 2211/008 20130101; C23C 2/26 20130101; C22C 38/002 20130101;
C23C 2/06 20130101; C22C 38/001 20130101; C22C 38/04 20130101; C22C
38/06 20130101; C22C 38/14 20130101; C23C 2/28 20130101; C22C 38/02
20130101; C21D 9/561 20130101; C21D 2211/005 20130101; C22C 18/04
20130101; C21D 8/0442 20130101; C22C 38/08 20130101; C22C 38/48
20130101; C22C 38/50 20130101; C21D 8/0226 20130101; B32B 15/013
20130101; C21D 8/0247 20130101; C22C 38/00 20130101; C21D 8/0236
20130101; C21D 9/46 20130101; C22C 38/16 20130101; C21D 2211/002
20130101; C22C 38/54 20130101; C22C 38/60 20130101; C23C 2/40
20130101; C22C 38/12 20130101; C23C 2/02 20130101 |
International
Class: |
C23C 2/02 20060101
C23C002/02; C22C 38/60 20060101 C22C038/60; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 8/02 20060101
C21D008/02; C23C 2/06 20060101 C23C002/06; C23C 2/40 20060101
C23C002/40; C23C 2/26 20060101 C23C002/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 27, 2016 |
JP |
2016-013205 |
Jan 23, 2017 |
JP |
2017-009277 |
Claims
1. A high-yield-ratio high-strength galvanized steel sheet
comprising: a steel sheet including: a composition containing, on a
percent by mass basis: C: 0.12% or more and 0.25% or less; Si: less
than 1.0%; Mn: 2.0% or more and 3% or less; P: 0.05% or less; S:
0.005% or less; Al: 0.1% or less; N: 0.008% or less; Ca: 0.0003% or
less; and 0.01% to 0.1% in total of one or more of Ti, Nb, V, and
Zr, the balance being Fe and unavoidable impurities, and a
metallographic structure containing, by area percentage, 20% or
less of ferrite, 40% or more in total of bainite and tempered
martensite, and 60% or less of as-quenched martensite, the bainite
having an average grain size of 6.0 .mu.m or less; and a coated
layer arranged on the steel sheet, the coated layer having a
coating weight per side of 20 to 120 g/m.sup.2 and having a Mn
content of 0.05 g/m.sup.2 or less; and a yield ratio of 65% or more
and a tensile strength of 950 MPa or more.
2. The high-yield-ratio high-strength galvanized steel sheet
according to claim 1, wherein the composition further contains, on
a percent by mass basis, 0.1% to 0.5% in total of one or more of
Mo, Cr, Cu, and Ni, and/or B: 0.0003% to 0.005%.
3. The high-yield-ratio high-strength galvanized steel sheet
according to claim 1, wherein the composition further contains, on
a percent by mass basis, Sb: 0.001% to 0.05%.
4. A method for producing a high-yield-ratio high-strength
galvanized steel sheet, comprising: a hot-rolling step of heating a
steel material having the composition according to claim 1 to
1,100.degree. C. or higher and 1,350.degree. C. or lower,
hot-rolling the steel material at a finish rolling temperature of
800.degree. C. or higher and 950.degree. C. or lower, and coiling
the resulting hot-rolled steel sheet at a temperature of
450.degree. C. or higher and 700.degree. C. or lower; a
cold-rolling step of cold-rolling the hot-rolled steel sheet
obtained in the hot-rolling step; an annealing step of annealing
the cold-rolled steel sheet obtained in the cold-rolling step under
conditions that a heating temperature T is in a temperature range
of an Ac3 point to 950.degree. C., an annealing furnace atmosphere
has a temperature in the temperature range and a hydrogen
concentration H of 5% or more by volume, a dew point D in the
annealing furnace having a temperature in the temperature range
satisfies expression (1) described below, a heating time in the
temperature range of the Ac3 point to 950.degree. C. is 60 seconds
or less, and a retention time in a temperature range of 450.degree.
C. to 550.degree. C. is five seconds or more; a galvanizing step of
subjecting the annealed steel sheet obtained in the annealing step
to coating treatment and cooling the resulting coated steel sheet
to 50.degree. C. or lower at an average cooling rate of 5.degree.
C./s or more to form a coating having a coating weight per side of
20 to 120 g/m.sup.2; and a skin-pass rolling step of subjecting the
coated steel sheet obtained in the galvanizing step to skin-pass
rolling at an elongation percentage of 0.1% or more,
-40.ltoreq.D.ltoreq.(T-1137.5)/7.5 (1) where in expression (1), D
represents the dew point (.degree. C.) in the annealing furnace,
and T represents a temperature (.degree. C.) in the annealing
furnace.
5. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 4, wherein the coating
treatment is hot-dip galvanizing treatment or treatment in which
hot-dip galvanizing is performed and then alloying is performed at
a temperature of 450.degree. C. or higher and 600.degree. C. or
lower.
6. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 4, wherein the
composition further contains, on a percent by mass basis, 0.1% to
0.5% in total of one or more of Mo, Cr, Cu, and Ni, and/or B:
0.0003% to 0.005%.
7. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 4, wherein the
composition further contains, on a percent by mass basis, Sb:
0.001% to 0.05%.
8. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 6, wherein the
composition further contains, on a percent by mass basis, Sb:
0.001% to 0.05%.
9. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 6, wherein the coating
treatment is hot-dip galvanizing treatment or treatment in which
hot-dip galvanizing is performed and then alloying is performed at
a temperature of 450.degree. C. or higher and 600.degree. C. or
lower.
10. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 7, wherein the coating
treatment is hot-dip galvanizing treatment or treatment in which
hot-dip galvanizing is performed and then alloying is performed at
a temperature of 450.degree. C. or higher and 600.degree. C. or
lower.
11. The method for producing a high-yield-ratio high-strength
galvanized steel sheet according to claim 8, wherein the coating
treatment is hot-dip galvanizing treatment or treatment in which
hot-dip galvanizing is performed and then alloying is performed at
a temperature of 450.degree. C. or higher and 600.degree. C. or
lower.
12. The high-yield-ratio high-strength galvanized steel sheet
according to claim 2, wherein the composition further contains, on
a percent by mass basis, Sb: 0.001% to 0.05%.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of
PCT/JP2017/002616, filed Jan. 26, 2017, which claims priority to
Japanese Patent Application No. 2016-013205, filed Jan. 27, 2016
and Japanese Patent Application No. 2017-009277, filed Jan. 23,
2017, the disclosures of each of these applications being
incorporated herein by reference in their entireties for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to a high-yield-ratio
high-strength galvanized steel sheet including a Mn-containing
high-strength steel sheet as a base material and having high yield
strength, good coating appearance, good corrosion resistance after
forming, and good exfoliation resistance during severe forming, and
to a method for producing the high-yield-ratio high-strength
galvanized steel sheet.
BACKGROUND OF THE INVENTION
[0003] In recent years, surface-treated steel sheets obtained by
imparting rusting resistance to base steel sheets, in particular,
hot-dip galvanized steel sheets and galvannealed steel sheets, have
been widely used in fields such as automobiles, household
appliances, and construction materials. From the viewpoint of
improving the fuel economy and the crashworthiness of automobiles,
there has been a growing demand for lighter, higher-strength
automobile bodies obtained by the use of higher-strength automobile
body materials having a smaller thickness. This accelerates the use
of high-strength steel sheets for automobiles. In particular, a
high-strength steel sheet having a high yield ratio (YR:
YR=YS/TS.times.100(%), YS: yield strength, TS: tensile strength) is
required.
[0004] A galvanized steel sheet is commonly produced as follows: A
thin steel sheet obtained by subjecting a slab to hot rolling and
cold rolling is used as a base material. The base material is
subjected to annealing in an annealing furnace of a continuous
hot-dip galvanizing line (hereinafter, referred to as a "CGL") and
hot-dip galvanizing treatment. In the case of the production of a
galvannealed steel sheet, alloying treatment is performed after the
hot-dip galvanizing treatment.
[0005] Examples of the types of heating furnaces used as the
annealing furnace of the CGL include a direct-fired-furnace type
(DFF type), a non-oxidation-furnace type (NOF type), and an
all-radiant-tube type (ART type). In recent years, the construction
of a CGL equipped with an all-radiant-tube-type heating furnace has
been increased because of, for example, an easy operation and
because a low-cost high-quality coated steel sheet can be produced
owing to the fact that pickup marks are less likely to be formed.
However, unlike the direct-fired-furnace type (DFF type) or
non-oxidation-furnace type (NOF type), since in the all radiant
tube type heating furnace, there is no oxidation step immediately
before annealing, a steel sheet containing an easily oxidizable
element such as Mn is disadvantageous in coatability.
[0006] As a method for producing a galvanized steel sheet including
a high-strength steel sheet that contains a large amount of Mn and
that serves as a base material, Patent Literatures 1 and 2 each
disclose a technique in which a heating temperature in a reducing
furnace is specified by an expression using water vapor partial
pressure and in which surface layers of base steel are subjected to
internal oxidation by increasing a dew point.
[0007] Patent Literature 3 discloses a technique in which the
concentration of CO.sub.2 as well as H.sub.2O and O.sub.2, which
are oxidizing gases, is specified and surface layers of base steel
immediately before coating are subjected to internal oxidation to
inhibit external oxidation, improving coating appearance.
[0008] Patent Literature 4 discloses a technique in which by
setting the dew point of an atmosphere in an annealing furnace to
-45.degree. C. or lower in the temperature ranges of 820.degree. C.
to 1,000.degree. C. in a soaking process and 750.degree. C. or
higher in a cooling process in annealing, oxygen potential in the
atmosphere is lowered to reduce a segregated oxide on surfaces
without causing internal oxidation, thereby improving coating
appearance.
PATENT LITERATURE
[0009] PTL 1: Japanese Unexamined Patent Application Publication
No. 2004-323970 [0010] PTL 2: Japanese Unexamined Patent
Application Publication No. 2004-315960 [0011] PTL 3: Japanese
Unexamined Patent Application Publication No. 2006-233333 [0012]
PTL 4: Japanese Unexamined Patent Application Publication No.
2010-255106
SUMMARY OF THE INVENTION
[0013] The techniques described in Patent Literatures 1 and 2 are
based on the assumption that a zone in which the dew point is
controlled is the whole interior of the furnace; thus, it is
difficult to control the dew point and to stably perform an
operation. If a galvannealed steel sheet is produced while the dew
point is unstably controlled, variations in the distribution of
internal oxides formed in a base steel sheet may occur to cause
defects such as variations in coating wettability and nonuniform
alloying in the longitudinal and width directions of the steel
sheet.
[0014] In the technique described in Patent Literature 3, similarly
to Patent Literatures 1 and 2, the presence of internal oxides
easily causes cracking during forming to degrade exfoliation
resistance. The corrosion resistance is also degraded. Furthermore,
CO.sub.2 may cause problems such as furnace contamination and the
carburization of the surfaces of the steel sheet to change
mechanical properties.
[0015] The technique described in Patent Literature 4 has problems
that it is very difficult to control water in the annealing furnace
atmosphere to stably maintain the dew point of an atmosphere in the
soaking process at -45.degree. C. or lower and that huge facility
costs and operating costs are required.
[0016] Recently, high-strength galvanized steel sheets and
high-strength galvannealed steel sheets have been increasingly used
for portions to be severely formed. Thus, importance is placed on
exfoliation resistance during severe forming. Specifically, the
inhibition of the peeling of the coating in a formed portion is
required when a coated steel sheet is subjected to bending at an
angle of more than 90.degree. and bent at a more acute angle or
when a coated steel sheet is formed by impact.
[0017] To provide a high-strength high-yield-ratio steel sheet
having good exfoliation resistance properties during severe
forming, performing high-temperature annealing alone is not enough
because a desired steel sheet metallographic structure is obtained
by an addition of a large amount of Mn to steel. In the
conventional art, a galvanized steel sheet having high yield
strength and good corrosion resistance after forming cannot be
produced by the use of a Mn-containing high-strength steel sheet
serving as a base material using a CGL equipped with an
all-radiant-tube-type heating furnace serving as an annealing
furnace.
[0018] The present invention has been accomplished in light of the
foregoing circumstances and aims to provide a high-yield-ratio
high-strength galvanized steel sheet that includes a Mn-containing
steel sheet serving as a base material and that has high yield
strength, good coating appearance, good corrosion resistance, and
good exfoliation resistance during severe forming, and to a
production method therefor.
[0019] To achieve good coating appearance free from a bare spot,
conventionally, the interior of a steel sheet is intentionally
oxidized. However, the corrosion resistance and formability are
deteriorated by the internal oxidation.
[0020] Specifically, until now, there have not been many attempts
to perform the hot-dip galvanizing of a Mn-containing high-strength
steel sheet after annealing in an atmosphere at a heating
temperature equal to or higher than an Ac3 point in a soaking
process in view of the degradation of appearance. The reason for
this is that persons skilled in the art have common knowledge that
the surface concentration of Mn increases with increasing heating
temperature to degrade coating appearance and thus to form bare
spots in an industrially easily operable annealing furnace
atmosphere having a dew point of -40.degree. C. or higher and
-20.degree. C. or lower and a hydrogen concentration of 5% or more
by volume. Accordingly, there has been almost no attempt to subject
a Mn-containing steel sheet to zinc coating after annealing at a
temperature equal to or higher than the Ac3 point in an environment
having a dew point of -40.degree. C. or higher and -20.degree. C.
or lower and a hydrogen concentration of 5% or more by volume.
[0021] However, the inventors have dared to study a heating
temperature region equal to or higher than the Ac3 point and have
accomplished the present invention.
[0022] Specifically, the inventors have investigated a new
unconventional method to solve the foregoing problems and have
found that a high-yield-ratio high-strength galvanized steel sheet
having high yield strength, good coating appearance, good corrosion
resistance, and good exfoliation resistance is produced by
appropriately controlling the dew point of an atmosphere and a
heating temperature in an annealing step to form a specific
metallographic structure and by adjusting the Mn content of a
coated layer to a specific range. This is presumably because the
occurrence of internal oxidation and surface concentration can be
inhibited in surface layer portions of the steel sheet directly
below the coated layer. More specifically, the inventors have found
that a specific metallographic structure is obtained by performing
heat treatment including a soaking process in which the temperature
range of a heating temperature T is Ac3 point or higher and
950.degree. C. or lower, the hydrogen concentration is 5% or more
by volume in the temperature range, and the dew point D in a
furnace in the temperature range satisfies expression (1) described
below, and that the Mn content of the coated layer is adjusted to a
specific range to inhibit the selective surface oxidation reaction
(hereinafter, referred to as "surface concentration") of an
oxidizable element such as Mn on surfaces of the steel sheet,
-40.ltoreq.D.ltoreq.(T-1137.5)/7.5 (1)
[0023] Although a mechanism of the inhibition of the surface
concentration of Mn in a high-temperature region equal to or higher
than the Ac3 point is not clear at this time, a possible mechanism
is described below.
[0024] From the Mn/MnO equilibrium diagram, since Mn approaches
from an oxidation region to a reduction region in high temperatures
equal to or higher than the Ac3 point, substantially no surface
concentration of Mn seems to occur. It is conceivable that, by
using specific production conditions to form a specific
metallographic structure and by adjusting the Mn content of a
coated layer to a specific range, the surface concentration of Mn
may be inhibited without causing internal oxidation in surface
portions of a steel sheet, and a high-yield-ratio high-strength
galvanized steel sheet having, for example, high yield-strength and
good corrosion resistance after forming may be provided. Mn
concentration on surfaces during annealing in a furnace is taken in
the coated layer when a steel sheet reacts with a coating bath in a
coating process; thus, the amount of Mn concentration on the
surfaces can be estimated from the Mn content in the coating.
[0025] It has been revealed that appropriate control of the Mn
content of the coated layer and the area percentages and grain
sizes in the metallographic structure by adjustment of a
composition and production conditions is important to solve the
foregoing problems.
[0026] The present invention according to exemplary embodiments is
based on the aforementioned findings and has features as listed
below.
[1] A high-yield-ratio high-strength galvanized steel sheet having
a steel sheet including a composition containing, on a percent by
mass basis, C: 0.12% or more and 0.25% or less, Si: less than 1.0%,
Mn: 2.0% or more and 3% or less, P: 0.05% or less, S: 0.005% or
less, Al: 0.1% or less, N: 0.008% or less, Ca: 0.0003% or less, and
0.01% to 0.1% in total of one or more of Ti, Nb, V, and Zr, the
balance being Fe and unavoidable impurities, and a metallographic
structure containing, by area percentage, 20% or less of ferrite,
40% or more in total of bainite and tempered martensite, and 60% or
less of as-quenched martensite, the bainite having an average grain
size of 6.0 .mu.m or less; and a coated layer arranged on the steel
sheet, the coated layer having a coating weight per side of 20 to
120 g/m.sup.2 and having a Mn content of 0.05 g/m.sup.2 or less, in
which the high-yield-ratio high-strength galvanized steel sheet has
a yield ratio of 65% or more and a tensile strength of 950 MPa or
more. [2] In the high-yield-ratio high-strength galvanized steel
sheet described in [1], the composition further contains, on a
percent by mass basis, 0.1% to 0.5% in total of one or more of Mo,
Cr, Cu, and Ni, and/or B: 0.0003% to 0.005%. [3] In the
high-yield-ratio high-strength galvanized steel sheet described in
[1] or [2], the composition further contains, on a percent by mass
basis, Sb: 0.001% to 0.05%. [4] A method for producing a
high-yield-ratio high-strength galvanized steel sheet includes a
hot-rolling step of heating a steel material having the composition
described in any one of [1] to [3] to 1,100.degree. C. or higher
and 1,350.degree. C. or lower, hot-rolling the steel material at a
finish rolling temperature of 800.degree. C. or higher and
950.degree. C. or lower, and coiling the resulting hot-rolled steel
sheet at a temperature of 450.degree. C. or higher and 700.degree.
C. or lower, a cold-rolling step of cold-rolling the hot-rolled
steel sheet obtained in the hot-rolling step, an annealing step of
annealing the cold-rolled steel sheet obtained in the cold-rolling
step under conditions that a heating temperature T is in a
temperature range of an Ac3 point to 950.degree. C., an annealing
furnace atmosphere has a temperature in the temperature range and a
hydrogen concentration H of 5% or more by volume, a dew point D in
the annealing furnace having a temperature in the temperature range
satisfies expression (1) described below, a heating time in the
temperature range of the Ac3 point to 950.degree. C. is 60 seconds
or less, and a retention time in a temperature range of 450.degree.
C. to 550.degree. C. is five seconds or more, a galvanizing step of
subjecting the annealed steel sheet obtained in the annealing step
to coating treatment and cooling the resulting coated steel sheet
to 50.degree. C. or lower at an average cooling rate of 5.degree.
C./s or more to form a coating having a coating weight per side of
20 to 120 g/m.sup.2, and a skin-pass rolling step of subjecting the
coated steel sheet obtained in the galvanizing step to skin-pass
rolling at an elongation percentage of 0.1% or more,
-40.ltoreq.D.ltoreq.(T-1137.5)/7.5 (1)
where in expression (1), D represents the dew point (.degree. C.)
in the annealing furnace, and T represents a temperature (.degree.
C.) in the annealing furnace. [5] In the method for producing a
high-yield-ratio high-strength galvanized steel sheet described in
[4], the coating treatment is hot-dip galvanizing treatment or
treatment in which hot-dip galvanizing is performed and then
alloying is performed at a temperature of 450.degree. C. or higher
and 600.degree. C. or lower.
[0027] According to embodiments of the present invention, the
high-yield-ratio high-strength galvanized steel sheet having high
yield strength, good coating appearance, good corrosion resistance,
and good exfoliation resistance during severe forming is
provided.
[0028] The use of the high-yield-ratio high-strength galvanized
steel sheet according to the present invention for frameworks of
automobile bodies can greatly contribute to an improvement in
crashworthiness and a reduction in weight.
BRIEF DESCRIPTION OF DRAWINGS
[0029] The FIGURE illustrates an example of an image obtained by
microstructural observation.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0030] Embodiments of the present invention will be described
below. The present invention is not limited to these embodiments
described below.
<High-Yield-Ratio High-Strength Galvanized Steel Sheet>
[0031] A high-yield-ratio high-strength galvanized steel sheet has
a composition containing, on a percent by mass basis, C: 0.12% or
more and 0.25% or less, Si: less than 1.0%, Mn: 2.0% or more and 3%
or less, P: 0.05% or less, S: 0.005% or less, Al: 0.1% or less, N:
0.008% or less, Ca: 0.0003% or less, and 0.01% to 0.1% in total of
one or more of Ti, Nb, V, and Zr, the balance being Fe and
unavoidable impurities.
[0032] The composition may further contain, on a percent by mass
basis, 0.1% to 0.5% in total of one or more of Mo, Cr, Cu, and Ni,
and/or B: 0.0003% to 0.005%.
[0033] The composition may further contain, on a percent by mass
basis, Sb: 0.001% to 0.05%.
[0034] Each of the components will be described below. In the
following description, the symbol "%" that expresses the content of
a component refers to "% by mass".
C: 0.12% or more and 0.25% or less
[0035] C is an element effective in strengthening the steel sheet
and contributes to an increase in strength by the formation of
martensite supersaturated with C. Furthermore, C contributes to the
increase in strength by the formation of fine alloy compounds or
alloy carbonitrides with carbide-forming elements such as Nb, Ti,
V, and Zr. To provide these effects, the C content needs to be
0.12% or more. A C content of more than 0.25% results in the
degradation of spot weldability of the steel sheet. The increase of
martensite tends to increase the hardness of the steel sheet and
decrease YR and bending workability. Accordingly, the C content is
0.12% or more and 0.25% or less. In view of properties, the C
content is preferably 0.23% or less. The lower limit is preferably
in the range of 0.13% or more.
Si: less than 1.0%
[0036] Si is an element that contributes to strengthening mainly by
solid-solution hardening. Since a relatively small decrease in
ductility is observed with respect to an increase in strength, Si
contributes to an improvement not only in strength but also a
balance between strength and ductility. Meanwhile, Si is liable to
form a Si-based oxide on surfaces of the steel sheet and thus can
cause the formation of bare spots. Accordingly, Si may be added
only in an amount required for enhancing strength. In view of
coatability, the upper limit of the Si content is less than 1.0%,
preferably 0.8% or less. The Si content is preferably 0.01% or
more, more preferably 0.05% or more.
Mn: 2.0% or More and 3% or Less
[0037] Mn is an element that contributes to strengthening by
solid-solution hardening and the formation of martensite. To
provide this effect, the Mn content needs to be 2.0% or more,
preferably 2.1% or more, more preferably 2.2% or more. Meanwhile, a
Mn content of more than 3% leads to spot weld cracking and is
liable to cause variations of the metallographic structure due to,
for example, the segregation of Mn, thereby degrading workability.
Mn is also liable to concentrate on surfaces of the steel sheet as
an oxide or composite oxide to possibly cause the formation of bare
spots. Accordingly, the Mn content is 3% or less, preferably 2.8%
or less, more preferably 2.7% or less.
P: 0.05% or Less
[0038] P is an element that contributes to the strengthening of the
steel sheet by solid-solution hardening. However, a P content of
more than 0.05% results in the degradation of workability such as
weldability and stretch-flangeability. Thus, the P content is
desirably 0.03% or less. Although the lower limit of the P content
is not particularly specified, the P content is preferably 0.001%
or more because a P content of less than 0.001% leads to a decrease
in production efficiency in the production process and an increase
in phosphorus removal cost. At a P content of 0.001% or more, the
effect of increasing strength is provided.
S: 0.005% or Less
[0039] S is a harmful element that causes hot shortness and that is
present in the form of sulfide inclusions in steel to degrade the
workability such as bendability of the steel sheet. Thus, the S
content is preferably minimized as much as possible. In the present
invention, a S content of up to 0.005% can be allowable.
Preferably, the S content is 0.002% or less. Although the lower
limit is not particularly specified, a S content of less than
0.0001% leads to a decrease in production efficiency in the
production process and an increase in cost. Accordingly, the S
content is preferably 0.0001% or more.
Al: 0.1% or Less
[0040] Al is added as a deoxidizer. When the effect needs to be
provided, the Al content is preferably 0.01% or more, more
preferably 0.02% or more. At an Al content of more than 0.1%, the
material cost is increased, and excess Al causes an induction of
surface defects of the steel sheet. Accordingly, the Al content is
0.1% or less, preferably 0.08% or less.
N: 0.008% or Less
[0041] A N content of more than 0.008% results in the formation of
an excess of a nitride in steel, thereby possibly decreasing
ductility and toughness and degrading the surface properties of the
steel sheet. Accordingly, the N content is 0.008% or less,
preferably 0.006% or less. From the viewpoint of improving
ductility owing to higher cleanliness of ferrite, the N content is
minimized as much as possible. However, an excessive reduction in N
content leads to a decrease in production efficiency in the
production process and an increase in cost. Accordingly, the N
content is preferably 0.0001% or more.
Ca: 0.0003% or Less
[0042] Ca forms a sulfide and an oxide in steel to decrease the
workability of the steel sheet. Accordingly, the Ca content is
0.0003% or less, preferably 0.0001% or less. A lower Ca content is
more preferred and may be 0%.
One or More of Ti, Nb, V, and Zr: 0.01% to 0.1% in Total
[0043] Ti, Nb, V, and Zr form precipitates by forming carbides and
nitrides (sometimes carbonitrides) with C and N. Fine precipitates
contribute to the strengthening of the steel sheet. These elements
have the effect of refining the metallographic structure of a
hot-rolled coil and also refine the metallographic structure after
the subsequent cold rolling and annealing to contribute to an
increase in strength and an improvement in workability such as
bendability. Accordingly, the total content of these elements is
0.01% or more, preferably 0.02% or more. However, the excessive
addition thereof increases deformation resistance in the cold
rolling to hinder productivity. Furthermore, the presence of
excessive or coarse precipitates decreases ductility of ferrite to
decrease the workability, such as ductility, bendability, and/or
stretch-flangeability of the steel sheet. Accordingly, the total
content of these elements is 0.1% or less, preferably 0.08% or
less.
[0044] The balance other than the elements described above is
composed of Fe and unavoidable impurities. The composition of the
steel sheet may contain components described below.
One or More of Mo, Cr, Cu, and Ni: 0.1% to 0.5% in Total, and/or B:
0.0003% to 0.005%
[0045] These elements enhance hardenability to facilitate the
formation of martensite and thus contribute to strengthening. In
order to provide these effects, the total content of one or more of
Mo, Cr, Cu, and Ni is preferably 0.1% or more. The excessive
addition of Mo, Cr, Cu, and Ni leads to a saturation of the effects
and an increase in cost. Furthermore, Cu induces cracking in the
hot rolling to cause surface flaws. Thus, the total content is 0.5%
or less. Ni has the effect of inhibiting the occurrence of surface
flaws due to the addition of Cu. Thus, when Cu is added, Ni is
desirably added together with Cu. The Ni content is preferably 1/2
or more of the Cu content. As described above, B also enhances
hardenability to contribute to strengthening. The lower limit of
the B content is specified from the viewpoint of providing the
effect of inhibiting the formation of ferrite in the course of
cooling in annealing and the viewpoint of improving hardenability.
Specifically, the lower limit thereof is preferably 0.0003% or
more, more preferably 0.0005% or more. Regarding the excessive
addition thereof, the upper limit is specified because of the
saturation of the effect. Specifically, the upper limit is
preferably 0.005% or less, more preferably 0.002% or less.
Excessive hardenability disadvantageously causes weld cracking
during welding.
Sb: 0.001% to 0.05%
[0046] Sb is an element effective in inhibiting, for example,
decarbonization, denitrization, and deboronization to inhibit a
decrease in the strength of the steel sheet. Furthermore, Sb is
effective in inhibiting spot weld cracking. Thus, the Sb content is
0.001% or more, more preferably 0.002% or more. However, the
excessive addition of Sb deteriorates workability such as
stretch-flangeability of the steel sheet. Thus, the Sb content is
preferably 0.05% or less, more preferably 0.02% or less.
[0047] When the optional components described above are contained
in amounts of less than the lower limits, the effects of the
present invention are not impaired. Thus, the incorporation of the
optional components in amounts of less than the lower limits is
considered to be regarded as the incorporation of the optional
components serving as unavoidable impurities.
[0048] The metallographic structure of the high-yield-ratio
high-strength galvanized steel sheet will be described below.
[0049] The metallographic structure of the high-yield-ratio
high-strength galvanized steel sheet has, by area percentage, 20%
or less ferrite, 40% or more in total of bainite and tempered
martensite, and 60% or less as-quenched martensite. The bainite has
an average grain size of 6.0 .mu.m or less.
Ferrite: 20% or Less
[0050] Although the presence of ferrite is not preferred in view of
the strength of the steel sheet, a ferrite content of up to 20% is
allowable in embodiments of the present invention. The ferrite
content is preferably 15% or less. The ferrite content may be 0%.
As the area percentage, a value measured by a method described in
an example is used. Here, carbide-free bainite formed at a
relatively high temperature is not distinguished from ferrite by
observation with a scanning electron microscope described in an
example and is regarded as ferrite.
Bainite and Tempered Martensite: 40% or More in Total
[0051] In order to achieve both high tensile strength and a high
yield ratio, the total area percentage of bainite (this bainite
refers to carbide-containing bainite because the carbide-free
bainite is regarded as ferrite as described above) and tempered
martensite is 40% or more. In particular, in order to obtain high
YS, this bainite and tempered martensite are important in
embodiments of the present invention. To obtain high YS, the total
area percentage needs to be 40% or more, preferably 45% or more,
more preferably 50% or more, even more preferably 55% or more.
Although the upper limit thereof is not particularly limited, it is
preferably 90% or less, more preferably 80% or less, even more
preferably 70% or less, in view of the balance between strength and
ductility. As the area percentage, a value measured by a method
described in an example is used.
As-Quenched Martensite: 60% or Less
[0052] As-quenched martensite is hard and effective in
strengthening the steel sheet. To provide the effect of increasing
TS, the area percentage of as-quenched martensite is preferably 20%
or more, more preferably 25% or more, even more preferably 30% or
more. As-quenched martensite, which is hard, decreases YR; thus,
the upper limit thereof is 60% or less, preferably 50% or less,
more preferably 40% or less, even more preferably 30% or less. As
the area percentage, a value measured by a method described in an
example is used.
Average Grain Size of Bainite: 6.0 .mu.m or Less
[0053] As described above, bainite and tempered martensite are
important in embodiments of the present invention in order to
obtain high YS. It is important that the bainite have an average
grain size of 6.0 .mu.m or less in addition to the area percentage
being in the range described above. The bainite preferably has an
average grain size of 5 .mu.m or less. Although the lower limit
thereof is not particularly limited, it is preferable that the
lower limit thereof is substantially 1.0 .mu.m or more, more
preferably 2.0 .mu.m or more. As the average grain size, a value
measured by a method described in an example is used.
[0054] Technical significance of the components of the
metallographic structure is as described above. A high-yield-ratio
high-strength galvanized steel sheet having high yield strength,
good coating appearance, good corrosion resistance, and good
exfoliation resistance is provided by a production method described
below. The reason for this is presumably that in the production
method described below, in the case where the Mn content of the
coated layer is adjusted to a specific range and where the
metallographic structure is adjusted to the foregoing
metallographic structure, the formation of internal oxidation and
surface concentration can be inhibited in surface layer portions of
the steel sheet directly below the coated layer.
[0055] The foregoing metallographic structure may contain a
component other than ferrite, bainite, tempered martensite, and/or
as-quenched martensite. Examples of the other component include
pearlite and retained austenite. The area percentage of the other
component is preferably 10% or less.
[0056] The coated layer will be described below. The coated layer
has a coating weight per side of 20 to 120 g/m.sup.2. A coating
weight of 20 g/m.sup.2 or more is required to ensure corrosion
resistance. The coating weight is preferably 30 g/m.sup.2 or more.
A coating weight of 120 g/m.sup.2 or less is required to provide
good exfoliation resistance. The coating weight is preferably 90
g/m.sup.2 or less.
Mn Content of Coated Layer: 0.05 g/m.sup.2 or Less
[0057] A Mn oxide formed in a heat-treatment process before coating
is taken in the coated layer when a coating bath reacts with a
material steel sheet to form a FeAl or FeZn alloy phase. In the
case of an excessive amount of the oxide, the oxide is left at
coating/base steel boundaries to degrade the adhesion of the
coating. Thus, the lower limit of the amount of the Mn oxide formed
in the heat-treatment process is not specified. A smaller amount
thereof is more preferred. When the Mn content of the coated layer
is more than 0.05 g/m.sup.2, the reaction to form the FeAl or FeZn
alloy phase is insufficient, leading to the formation of bare spots
and a decrease in exfoliation resistance. Here, the amount of the
Mn oxide formed in the heat-treatment process can be quantitatively
determined from the Mn content of the coated layer after a coating
step. The measurement thereof is performed by a method described in
an example. Accordingly, by measuring "the Mn content of the coated
layer", the presence or absence and the degree of the foregoing
problem due to the Mn oxide can be evaluated.
[0058] The coated layer is preferably a zinc coated layer. The zinc
coated layer may be a zinc alloy coated layer that has been
subjected to alloying treatment.
<Method for Producing High-Yield-Ratio High-Strength Galvanized
Steel Sheet>
[0059] A production method of the present invention includes a
hot-rolling step, a cold-rolling step, an annealing step, a
galvanizing step, and a skin-pass rolling step.
[0060] The hot-rolling step is a step of heating a steel having the
composition described above to 1,100.degree. C. or higher and
1,350.degree. C. or lower, hot-rolling the steel at a finish
rolling temperature of 800.degree. C. or higher and 950.degree. C.
or lower, and coiling the resulting steel sheet at a temperature of
450.degree. C. or higher and 700.degree. C. or lower. In the
following description, the temperature refers to the surface
temperature of the steel sheet.
Production of Slab (Cast Slab (Steel))
[0061] The steel used in the production method according to
embodiments of the present invention is one generally called as a
slab produced by a continuous casting process. To prevent the
macrosegregation of alloy components, the continuous casting
process is employed. The steel may be produced by, for example, an
ingot-making process or a thin slab casting process.
[0062] Any of the following processes may be employed: a
conventional process in which a steel slab is produced, temporarily
cooled to a room temperature, and reheated may be employed; a
process in which a hot steel slab is transferred into a heating
furnace without cooling down to about room temperature, and
hot-rolled; a process in which a steel slab is slightly heated and
immediately hot-rolled; and a process in which after casting, a
steel slab is hot-rolled while maintained at a high
temperature.
[0063] Conditions of the hot rolling are as follows: the steel
having the composition described above is heated at a temperature
of 1,100.degree. C. or higher and 1,350.degree. C. or lower,
hot-rolled at a finish rolling temperature of 800.degree. C. or
higher and 950.degree. C. or lower, and coiled at a temperature of
450.degree. C. or higher and 700.degree. C. or lower.
Heating Temperature of Slab
[0064] The heating temperature of the steel slab is in the range of
1,100.degree. C. or higher and 1,350.degree. C. or lower. The use
of a heating temperature outside the range of the upper limit
temperature easily coarsens precipitates present in the steel slab.
This can be disadvantageous when strength is enhanced by
precipitation hardening. Furthermore, when the heating temperature
is outside the range of the upper limit temperature, coarse
precipitates can act as nuclei to adversely affect the formation of
a metallographic structure in the subsequent annealing step. In
contrast, appropriate heating allows voids, defects, and so forth
on surfaces of the slab to scale off to reduce cracks and
irregularities on surfaces of the steel sheet, thereby beneficially
providing smooth surfaces of the steel sheet. To provide the
effect, the heating temperature needs to be 1,100.degree. C. or
higher. A heating temperature of higher than 1,350.degree. C. can
result in the coarsening of austenite grains and the coarsening of
the metallographic structure of a final product to cause decreases
in the strength and workability, such as bendability and
stretch-flangeability, of the steel sheet.
Hot Rolling
[0065] The steel slab obtained as described above is subjected to
hot rolling including rough rolling and finish rolling. In general,
a steel slab is formed by rough rolling into a sheet bar, and the
sheet bar is formed by finish rolling and coiling into a hot-rolled
coil (hot-rolled steel sheet). Depending on the ability of a mill,
it suffices to provide a hot-rolled steel sheet having a
predetermined size without regard to such a division. The hot
rolling needs to be performed under conditions described below.
Finish Rolling Temperature: 800.degree. C. or Higher and
950.degree. C. or Lower
[0066] The use of a finish rolling temperature of 800.degree. C. or
higher enables the formation of a uniform metallographic structure
of the hot-rolled coil. The uniform metallographic structure at
this stage contributes to provide a final product having a uniform
metallographic structure. A nonuniform metallographic structure
decreases workability such as ductility, bendability, and
stretch-flangeability. Meanwhile, a finish rolling temperature of
higher than 950.degree. C. results in a large amount of oxide
(scale) formed to roughen the boundary between the oxide and the
base steel, thereby degrading surface quality after pickling and
cold rolling. Furthermore, a finish rolling temperature of higher
than 950.degree. C. can result in an increase in the grain size of
the metallographic structure to cause a decrease in the strength
and workability, such as bendability and stretch-flangeability, of
the steel sheet, similarly to the steel slab.
[0067] After the hot rolling is completed, preferably, cooling is
initiated in three seconds after the completion of the finish
rolling, and the cooling is performed in a temperature range of
[finish rolling temperature].degree. C. to [finish rolling
temperature-100].degree. C. at an average cooling rate of 10 to
250.degree. C./s, in order to refine and uniformize the
metallographic structure.
Coiling Temperature: 450.degree. C. to 700.degree. C.
[0068] The coiling temperature, which is a temperature immediately
before the coiling of the hot-rolled steel sheet, needs to be
450.degree. C. or higher in view of the fine precipitation of, for
example, NbC. The coiling temperature needs to be 700.degree. C. or
lower so as not to allow precipitates to coarsen excessively. The
lower limit, thereof is preferably 500.degree. C. or higher. The
upper limit thereof is preferably 680.degree. C. or lower.
[0069] Subsequently, the cold-rolling step is performed. The
cold-rolling step is a step of cold-rolling the hot-rolled sheet
(hot-rolled steel sheet) obtained in the hot-rolling step. Usually,
after the scales are removed by pickling, the cold rolling is
performed to provide a cold-rolled coil. The pickling is performed
on an as-needed basis.
[0070] The cold rolling is preferably performed at a rolling
reduction of 20% or more in order to provide a uniform fine
metallographic structure (metallographic structure) in the
subsequent annealing. At a rolling reduction of less than 20%, if
coarse grains are easily formed in the annealing, a nonuniform
metallographic structure can be obtained, in some cases. As
described above, the strength and workability of the final product
sheet can be decreased. Although the upper limit of the rolling
reduction is not particularly specified, because of the
high-strength steel sheet, a high rolling reduction can decrease
the productivity due to rolling load and can cause an odd shape.
The rolling reduction is preferably 90% or less.
[0071] The cold-rolled steel sheet after the cold-rolling step is
annealed (annealing step) under conditions that a heating
temperature T is in the temperature range of an Ac3 point to
950.degree. C., an annealing furnace atmosphere has a temperature
in the temperature range and a hydrogen concentration H of 5% or
more by volume, a dew point D in the annealing furnace having a
temperature in the temperature range satisfies expression (1)
described below, a heating time in the temperature range of the Ac3
point to 950.degree. C. is 60 seconds or less, and a retention time
in the temperature range of 450.degree. C. to 550.degree. C. is
five seconds or more.
Heating Temperature T: Ac3 Point to 950.degree. C.
[0072] When the heating temperature (annealing temperature) is
lower than the Ac3 point or higher than 950.degree. C., a
predetermined metallographic structure is not obtained, failing to
provide high yield strength.
Ac3=937-477C+56Si-20Mn-16Cu-27Ni-5Cr+38Mo+125V+136Ti+35Zr-19Nb+-
198Al+3315B. In this expression, the element symbols refer to the
contents of the elements. The content of a composition that is not
contained is defined as zero.
Hydrogen Concentration H in Temperature Range of Ac3 to 950.degree.
C.: 5% or More by Volume
[0073] When the volume percentage of hydrogen gas in an atmosphere
is less than 5% by volume, an activation effect by reduction is not
obtained, and result in degrading coating appearance. Although the
upper limit thereof is not particularly specified, a volume
percentage of the hydrogen gas of more than 20% by volume leads to
the saturation of the effect of improving the coating appearance
and an increase in cost. Accordingly, the volume percentage of
hydrogen gas is preferably 5% or more by volume and 20% or less by
volume. Gas components in the furnace are composed of nitrogen gas
and incidental impurity gases other than the hydrogen gas. Any gas
component may be contained as long as the effect of the present
invention is not impaired. The hot-dip galvanizing treatment can be
performed in the usual manner. Outside the temperature range, the
hydrogen concentration need not be in the range of 5% or more by
volume.
Dew Point D in Temperature Range of Ac3 to 950.degree. C.: Range
Given by Expression (1)
[0074] To achieve good coating appearance, good corrosion
resistance, and exfoliation resistance during severe forming while
the foregoing heating-temperature range is used, the dew point
needs to be appropriately controlled to the range given by
expression (1). Annealing in an atmosphere having a dew point that
does not satisfy expression (1) causes the formation of bare spots,
resulting in degraded coating appearance. Specifically, at a dew
point D of more than the upper limit, an alloying element such as
Si is easily picked up in operation in embodiments of the present
invention. Although the lower limit of the dew point is not
particularly specified, a difficulty lies in controlling the dew
point to lower than -40.degree. C.; thus, huge facility costs and
operating costs are disadvantageously required.
-40.ltoreq.D.ltoreq.(T-1137.5)/7.5 (1)
where in expression (1), D represents the dew point (.degree. C.)
in the furnace, and T represents the heating temperature (.degree.
C.).
Heating Time in Temperature Range of Ac3 Point to 950.degree. C.:
60 Seconds or Less
[0075] The heating time in the temperature range of the Ac3 point
to 950.degree. C. is controlled to 60 seconds or less. A heating
time of more than 60 seconds results in the surface concentration
of the oxides or composite oxides of Si and Mn formed on the
surfaces of the steel sheet to cause poor appearance. The heating
time is preferably 20 seconds or less. The heating time in this
temperature range is preferably three seconds or more from the
viewpoint of stabilizing the material (uniformizing the
metallographic structure (metallographic structure)).
Retention Time in Temperature Range of 450.degree. C. to
550.degree. C.: Five Seconds or More
[0076] The use of a retention time of five seconds or more in the
temperature range of 450.degree. C. to 550.degree. C. before the
coating step stably provides the metallographic structure, in
particular, bainite, and stabilizes the temperature of the sheet
before immersion in the coating bath. To achieve a desired
metallographic structure and good coating quality, the use of a
retention time of five seconds or more in the predetermined
temperature range provides high YS and stabilizes the temperature
of the sheet before immersion in the coating bath. If the
temperature is lower than 450.degree. C., because the temperature
for the retention time is usually equal to a cooling stop
temperature, coating is performed at a temperature of lower than
450.degree. C., thereby degrading the quality of the coating bath
in the galvanizing step. Accordingly, the lower limit of the
temperature range is 450.degree. C. If the temperature of the sheet
before immersion in the bath is higher than 550.degree. C., the
temperature of the coating bath is increased to easily cause the
formation of dross. The dross adheres to the surfaces to cause poor
appearance. Accordingly, the upper limit of the temperature range
is 550.degree. C. Cooling from the heating temperature to this
temperature range is preferably performed at a cooling rate
(average cooling rate) of 3.degree. C./s or more from the viewpoint
of achieving desired properties. The upper limit is not
particularly specified. The cooling stop temperature may be
450.degree. C. to 550.degree. C. as described above. The steel
sheet may be temporarily cooled to a temperature equal to or lower
than the cooling stop temperature, reheated, and retained in the
temperature range of 450.degree. C. to 550.degree. C.
[0077] To soften the coil and so forth, a heating step may be
performed before the annealing described above.
[0078] Next, the galvanizing step is performed. The galvanizing
step is a step of subjecting the annealed sheet obtained by
annealing to coating treatment and cooling the resulting sheet to
50.degree. C. or lower at an average cooling rate of 5.degree. C./s
or more.
[0079] The coating treatment may be performed at a coating weight
per side of 20 to 120 g/m.sup.2. The coating weight needs to be 20
g/m.sup.2 or more in order to enhance corrosion resistance and
needs to be 120 g/m.sup.2 or less in order to achieve good
exfoliation resistance.
[0080] Other conditions of the coating treatment are not
particularly limited. For example, this step is a step of forming a
coated layer on surfaces of the annealed sheet obtained by the
foregoing method, the coated layer containing, on a percent by mass
basis, Fe: 0.1% to 18.0%, Al: 0.001% to 1.0%, and 0% to 30% in
total of one or two or more selected from Pb, Sb, Si, Sn, Mg, Mn,
Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, the balance being Zn
and unavoidable impurities. A method of the coating treatment is
galvanizing. Conditions thereof may be appropriately set. After the
galvanizing, alloying treatment may be performed by heating. For
example, the alloying treatment is a treatment in which the
galvanized sheet is held in a temperature range of 450.degree. C.
to 600.degree. C. for about 1 to about 60 seconds. In the case of
performing the alloying treatment, the coated layer preferably has
a Fe content of 7.0% to 15.0% by mass. A Fe content of less than 7%
by mass can lead to the occurrence of nonuniform alloying and the
degradation of flaking properties. Meanwhile, a Fe content of more
than 15% by mass can lead to the degradation of the exfoliation
resistance.
[0081] After the coating treatment (or after the alloying treatment
when the alloying treatment is performed), the resulting steel
sheet is cooled to 50.degree. C. or lower at an average cooling
rate of 5.degree. C./s or more in order to provide martensite,
which is essential for strengthening. An average cooling rate of
less than 5.degree. C./s makes it difficult to form martensite
required for strength. If cooling is terminated at a temperature
higher than 50.degree. C., martensite is excessively self-tempered
into tempered martensite, making it difficult to obtain strength
required. The average cooling rate is preferably 30.degree. C./s or
less in order to form appropriately tempered martensite for the
purpose of providing a high YR.
[0082] Subsequently, the skin-pass rolling step is performed. The
skin-pass rolling step is a step of subjecting the coated sheet
after the galvanizing step to skin-pass rolling at an elongation
percentage of 0.1% or more. The coated sheet is subjected to
skin-pass rolling at an elongation percentage of 0.1% or more in
order to stably provide high YS in addition to shape correction and
the adjustment of the surface roughness. Regarding the shape
correction and the adjustment of the surface roughness, a levelling
process may be performed in place of the skin-pass rolling.
Excessive skin-pass rolling imparts excessive strain to the
surfaces of the steel sheet to decrease the evaluation values of
bendability and stretch-flangeability. Furthermore, excessive
skin-pass rolling decreases ductility and imposes a high load on
the facility because of the high-strength steel sheet. Accordingly,
the rolling reduction in the skin-pass rolling is preferably 3% or
less.
Examples
[0083] Molten steels having compositions given in Table 1 were made
in a converter and formed into slabs with a continuous casting
apparatus. Subsequently, the slabs were subjected to hot rolling,
cold rolling, annealing, coating treatment, and skin-pass rolling
(SKP) under various conditions given in Table 2 to produce
high-yield-ratio high-strength galvanized steel sheets (product
sheets). In the annealing, cold-rolled steel sheets were charged
into a CGL equipped with an all-radiant-tube-type heating furnace
serving as an annealing furnace. Table 2 lists specific conditions.
Gas components in an atmosphere are composed of nitrogen gas,
hydrogen gas, and incidental impurity gases. The dew point was
controlled by absorbing and removing water in the atmosphere. Table
2 lists the hydrogen concentration. GA indicates that a Zn bath
containing 0.14% by mass Al was used. GI indicates that a Zn bath
containing 0.18% by mass Al was used. The coating weight was
adjusted by gas wiping. GA indicates that alloying treatment was
performed. In cooling (cooling after the coating treatment), the
steel sheets were cooled to 50.degree. C. or lower by being passed
through a water tank having a water temperature of 40.degree.
C.
TABLE-US-00001 TABLE 1 Steel Composition % by mass No. C Si Mn P S
N Al Ti Nb V Zr A 0.120 0.20 2.80 0.031 0.0010 0.0040 0.035 0.015
0.042 B 0.140 0.10 2.45 0.008 0.0008 0.0038 0.030 0.022 0.020 C
0.160 0.06 2.30 0.010 0.0009 0.0039 0.035 0.025 0.020 D 0.180 0.02
2.22 0.010 0.0009 0.0056 0.035 0.025 0.020 E 0.230 0.60 2.05 0.001
0.0015 0.0040 0.035 0.025 F 0.160 0.10 1.85 0.010 0.0010 0.0040
0.030 0.018 0.023 G 0.160 1.20 2.30 0.010 0.0009 0.0039 0.030 0.025
0.020 H 0.160 0.08 2.30 0.010 0.0009 0.0040 0.035 I 0.190 0.50 2.05
0.001 0.0015 0.0040 0.035 0.031 J 0.190 0.50 2.05 0.001 0.0015
0.0040 0.035 0.051 K 0.190 0.50 2.05 0.001 0.0015 0.0040 0.035
0.028 L 0.160 0.06 2.30 0.010 0.0009 0.0039 0.035 0.025 0.020 M
0.160 0.06 2.30 0.010 0.0009 0.0039 0.035 0.025 0.020 N 0.160 0.06
2.30 0.010 0.0009 0.0039 0.035 0.025 0.020 Steel Composition % by
mass Ac3 No. B Mo Cr Cu Ni Sb Ca .degree. C. Remarks A 0.0001 843
Example B 0.0010 0.10 842 Example C 0.12 0.0001 833 Example D 0.12
0.0001 822 Example E 0.0010 830 Example F 0.0010 0.10 0.1 0.0001
844 Comparative example G 0.12 895 Comparative example H 0.10
0.0001 830 Comparative example I 0.0010 848 Example J 847 Example K
841 Example L 0.18 827 Example M 0.10 0.05 825 Example N 0.004 828
Example * Values outside the range of the present invention are
underlined.
TABLE-US-00002 TABLE 2 Hot rolling Annealing Slab Finish Cold
rolling Dew heating rolling Coiling Cold-rolling Temperature
Hydrogen point Steel temperature temperature temperature reduction
T concentration D No. No. (.degree. C.) (.degree. C.) (.degree. C.)
(%) (.degree. C.) (vol. %) (.degree. C.) 1 A 1280 920 520 60 870 6
-37 2 B 1150 840 580 50 870 12 -39 3 C 1200 880 550 50 850 8 -40 4
D 1230 900 600 40 860 10 -38 5 E 1250 860 620 30 860 15 -38 6 F
1200 880 560 50 850 8 -39 7 G 1220 890 560 50 910 8 -39 8 H 1180
890 560 50 850 8 -39 9 C 1200 880 550 50 865 8 -37 10 C 1200 880
550 50 850 3 -39 11 C 1200 880 550 50 850 8 -25 12 C 1200 880 550
50 850 8 -39 13 C 1200 880 550 50 850 8 -40 14 C 1200 880 550 50
850 8 -40 15 C 1200 880 550 50 760 8 *3 16 C 1200 880 550 50 910 8
-40 17 C 1200 880 550 50 850 8 -40 18 E 1200 860 600 50 860 15 -38
19 I 1300 830 600 50 860 15 -38 20 J 1200 900 600 50 860 15 -38 21
K 1260 850 600 50 860 15 -38 22 L 1200 880 550 50 850 8 -40 23 M
1200 880 550 50 850 8 -40 24 L 1200 880 550 50 850 8 -40 After
coating Annealing Average SKP Heating Retention cooling Coating
Elongation time time*2 rate weight percentage No. (s) (s) (.degree.
C./s)*1 Type (g/m.sup.2) (%) Remarks 1 15 7 8 GA 60 0.15 Example 2
15 12 10 GI 46 0.2 Example 3 17 15 10 GI 45 0.3 Example 4 15 15 20
GI 47 0.3 Example 5 15 10 10 GI 48 0.3 Example 6 15 15 10 GI 45 0.3
Comparative example 7 15 15 10 GI 40 0.3 Comparative example 8 15
15 10 GI 46 0.3 Comparative example 9 13 15 10 GA 48 0.3 Example 10
13 15 10 GI 15 0.3 Comparative example 11 15 15 10 GI 25 0.3
Comparative example 12 70 15 10 GI 15 0.3 Comparative example 13 15
1 10 GI 45 0.3 Comparative example 14 5 5 25 GI 45 0.3 Example 15
15 15 10 GI 45 0.3 Comparative example 16 55 15 10 GI 20 0.3
Example 17 17 15 150 GI 45 0.3 Example 18 15 10 1 GI 48 0.3
Comparative example 19 15 10 10 GI 48 0.3 Example 20 15 10 10 GI 48
0.3 Example 21 15 10 10 GI 48 0.45 Example 22 17 15 10 GI 45 0.3
Example 23 17 15 10 GI 45 0.3 Example 24 17 15 10 GI 45 0.3 Example
* Values outside the range of the present invention are underlined.
*1Regarding the average cooling rate after coating, the temperature
range was from 450.degree. C. or lower to 50.degree. C. after a
steel sheet was passed through a final cooling zone, and the steel
sheet was passed through a water tank having a water temperature of
40.degree. C. in the last cooling stage so as to adjust the
temperature to 50.degree. C. or lower. *2This indicates a retention
time in the temperature range of 450.degree. C. to 550.degree. C.
*3 This indicates that the dew point is outside the range.
[0084] Samples of the galvanized steel sheets produced as described
above were taken and subjected to a tensile test by a method
described below to measure and calculate the yield strength (YS),
the tensile strength (TS), and the yield ratios
(YR=YS/TS.times.100%). The coatability (surface properties) was
evaluated by visual observation of appearance. The corrosion
resistance after forming and the exfoliation resistance during
forming were evaluated. Evaluation methods are described below.
Tensile Test
[0085] A JIS No. 5 test piece for a tensile test (JIS Z 2201) was
taken from each of the galvanized steel sheets in a direction
perpendicular to a rolling direction. A tensile test was performed
at a constant cross-head speed of 10 mm/min. The yield strength
(YS) was defined as a value obtained by reading 0.2% proof stress
from a slope for an elastic region at a stress of 100 to 200 MPa.
The tensile strength was defined as a value obtained by dividing a
maximum load by the initial cross-sectional area of the parallel
portion of the test piece in the tensile test. Regarding the
thickness used for the calculation of the cross-sectional area of
the parallel portion, a thickness value including the thickness of
the coating was used.
Surface Properties (Appearance)
[0086] The appearance after coating was visually observed. A sample
in which no bare-spot defects occurred was denoted by "O". A sample
in which bare-spot defects occurred was denoted by "x". A sample in
which no bare-spot defects occurred and nonuniform coating
appearance was obtained was denoted by ".DELTA.". A ripple pattern
is acceptable for interior panel components of automobiles. The
bare-spot defects refer to regions where no coating is present and
the steel sheet is exposed, each of the regions having a size of
about several micrometers to about several millimeter.
Exfoliation Resistance
[0087] Regarding the exfoliation resistance during severe forming,
in the case of GA (steel sheet that was subjected to the alloying
treatment), when GA is bent at an acute angle of more than
90.degree., the peeling of the coating in a bent portion needs to
be inhibited. In this example, a cellophane tape was pressed
against the bent portion at an angle of 120.degree.. Separated
matter was transferred to the cellophane tape. The amount of the
separated matter on the cellophane tape was determined in terms of
the number of counts for Zn measured by X-ray fluorescence
spectrometry. Here, a mask had a diameter of 30 mm. In the
fluorescent X-ray spectrometry, the acceleration voltage was 50 kV,
the acceleration current was 50 mA, and the measurement time was 20
seconds. According to the following criteria, samples rated ranks 1
and 2 were evaluated as having good exfoliation resistance (symbol
"O", and samples rated ranks 3 or more were evaluated as having
poor exfoliation resistance (symbol "x"). [0088] Number of counts
for Zn measured by Rank [0089] fluorescent X-ray spectrometry
[0090] 0 to less than 500: 1 (good) [0091] 500 or more to less than
1,000: 2 [0092] 1,000 or more and less than 2,000: 3 [0093] 2,000
or more and less than 3,000: 4 [0094] 3,000 or more: 5 (poor)
[0095] GI (steel sheet that was not subjected to the alloying
treatment) needs to have good exfoliation resistance in an impact
test. A ball impact test was performed. A formed portion
(corresponding to the formed portion obtained by severe forming)
was subjected to tape peeling. The presence or absence of peeling
of the coated layer was visually determined. Regarding ball impact
conditions, the weight of a ball is 1,000 g, and the drop height is
100 cm.
O (Good): the coated layer was not peeled. x (NG): the coated layer
was peeled. Corrosion Resistance after Forming
[0096] Test pieces that were subjected to forming in the same way
as in the exfoliation resistance test and that were not subjected
to tape peeling were provided. Chemical conversion treatment was
performed with a degreasing agent FC-E2011, a surface conditioner
PL-X, and a chemical conversion coating agent Palbond PB-L3065,
available from Nihon Parkerizing Co., Ltd., under standard
conditions described below in such a manner that the chemical
conversion coating weight was 1.7 to 3.0 g/m.sup.2.
<Standard Conditions>
[0097] Degreasing step: treatment temperature: 40.degree. C.,
treatment time: 120 seconds [0098] Spray degreasing and surface
conditioning step: pH: 9.5, treatment temperature: room
temperature, treatment time: 20 seconds [0099] Chemical conversion
treatment step: temperature of chemical conversion coating liquid:
35.degree. C., treatment time: 120 seconds
[0100] The surfaces of each of the test pieces that had been
subjected to the chemical conversion treatment were subjected to
electrodeposition coating with an electrodeposition paint V-50,
available from Nippon Paint Co., Ltd., in such a manner that the
coating thickness was 25 .mu.m. The resulting test piece was
subjected to a corrosion test described below.
<Salt Spray Test (SST)>
[0101] Regarding the test pieces that had been subjected to the
chemical conversion treatment and electrodeposition coating, a
surface of the bent portion of GA and a portion of GI that had been
subjected to impact by the ball were scored with a cutter in such a
manner that the resulting cut lines reached the zinc coating. The
test pieces were subjected to a salt spray test for 240 hours with
an aqueous solution containing 5% by mass NaCl in conformity with a
neutral salt spray test specified in JIS Z 2371:2000. Then, each of
the cross-cut portions was subjected to a tape peeling test to
measure the maximum peeling full width including both sides of the
cut line portion. When the maximum peeling full width is 2.0 mm or
less, the corrosion resistance in the salt spray test can be
evaluated as good.
O (Good): the maximum bulging full width from each cut line was 2.0
mm or less x (NG): the maximum bulging full width from each cut
line was more than 2.0 mm
[0102] Table 3 lists the results.
Observation of Metallographic Structure
[0103] A test specimen for microstructural observation was taken
from each of the galvanized steel sheet. An L section (cross
section parallel to the rolling direction) was polished and etched
with nital. Three or more fields-of-view of positions about 1/4t (t
represents the thickness of the sheet) from the surfaces were
observed with a scanning electron microscope (SEM) at a
magnification of .times.3,000, and images thereof were analyzed
(area percentages in the fields of view were measured, and the
average area percentage was calculated). The average grain size of
bainite was calculated using the images by a method in which grain
diameters in two directions perpendicular to each other (the width
direction and the longitudinal direction of the paper plane) were
averaged arithmetically. The FIGURE illustrates an example of the
images.
Mn Content of Zinc Coated Layer
[0104] The Mn content of the zinc coated layer was determined by
dissolving the coated layer in dilute hydrochloric acid containing
an inhibitor and measuring the Mn content using ICP emission
spectrometry.
TABLE-US-00003 TABLE 3 Metallographic structure Bainite Average
Surface and As- grain Mechanical Amount Exfoliation Corrosion
tempered quenched size of properties of Mn in resistance resistance
Steel Ferrite martensite martensite bainite TS YS YR coating
Appear- during after No. No. % % % .mu.m MPa MPa % g/m.sup.2 ance
forming forming Remarks 1 A 15 55 30 4.0 960 660 69 0.04 .DELTA.
.smallcircle. .smallcircle. Example 2 B 10 65 30 3.8 990 721 73
0.03 .smallcircle. .smallcircle. .smallcircle. Example 3 C 5 70 25
4.5 1010 750 74 0.04 .smallcircle. .smallcircle. .smallcircle.
Example 4 D 3 70 25 3.3 1005 730 73 0.03 .smallcircle.
.smallcircle. .smallcircle. Example 5 E 5 45 50 3.2 1150 775 67
0.03 .smallcircle. .smallcircle. .smallcircle. Example 6 F 5 65 30
5.0 945 630 67 0.04 .smallcircle. .smallcircle. .smallcircle.
Comparative example 7 G 10 60 30 3.2 960 630 66 0.02 x x x
Comparative example 8 H 5 65 30 3.4 955 600 63 0.04 .smallcircle.
.smallcircle. .smallcircle. Comparative example 9 C 5 70 25 4.5 995
730 73 0.04 .smallcircle. .smallcircle. .smallcircle. Example 10 C
5 70 25 4.5 1000 715 72 0.04 .DELTA. .smallcircle. x Comparative
example 11 C 5 70 25 4.5 1000 740 74 0.12 x x x Comparative example
12 C 5 65 30 5.5 990 750 76 0.09 x x x Comparative example 13 C 2
33 65 2.0 985 620 63 0.04 .DELTA. .smallcircle. .smallcircle.
Comparative example 14 C 5 70 25 2.1 1220 790 65 0.03 .smallcircle.
.smallcircle. .smallcircle. Example 15 C 45 30 20 2.1 790 480 61
0.03 .smallcircle. .smallcircle. .smallcircle. Comparative example
16 C 5 65 30 5.0 985 705 72 0.02 .smallcircle. .smallcircle.
.smallcircle. Example 17 C 0 40 60 5.5 1325 860 65 0.03
.smallcircle. .smallcircle. .smallcircle. Example 18 E 30 60 10 3.4
800 700 88 0.04 .smallcircle. .smallcircle. .smallcircle.
Comparative example 19 I 5 70 25 3.2 995 725 73 0.04 .smallcircle.
.smallcircle. .smallcircle. Example 20 J 5 70 25 3.5 980 730 74
0.04 .smallcircle. .smallcircle. .smallcircle. Example 21 K 5 70 25
3.4 985 750 76 0.04 .smallcircle. .smallcircle. .smallcircle.
Example 22 L 5 60 35 4.6 995 715 72 0.03 .smallcircle.
.smallcircle. .smallcircle. Example 23 M 5 70 25 5.1 1000 730 73
0.03 .smallcircle. .smallcircle. .smallcircle. Example 24 N 5 70 25
5.2 1010 750 74 0.03 .smallcircle. .smallcircle. .smallcircle.
Example * Values outside the range of the present invention are
underlined.
[0105] The steel sheets that contain the components and that are
produced under the production conditions within the scope of the
present invention have a predetermined coating quality,
TS.gtoreq.950 MPa, and YR.gtoreq.65%.
[0106] The galvanized steel sheet according to embodiments of the
present invention has high yield strength and good surface
properties as well as high tensile strength. In the case where the
galvanized steel sheet is used for frameworks of automobile bodies,
in particular, mainly for cabins and their peripheries that affect
crashworthiness, the safety performance is improved. Furthermore,
the use of the higher-strength galvanized steel sheet having a
smaller thickness is effective in reducing the weight of automobile
bodies and offers the environmental advantages such as low CO.sub.2
emission. Because of good surface properties and good coating
quality, the galvanized steel sheet can also be positively used for
portions such as underbodies, which can be corroded by rain or
snow; thus, rusting resistance and corrosion resistance of
automobile bodies should be improved. Because of these properties,
the galvanized steel sheet is effectively used in the fields of
civil engineering, construction, and household appliances as well
as automobile parts.
* * * * *