[Technical Field of the Invention]
[0001]
The present invention relates to a steel sheet.
This application claims the right of priority based on Japanese Patent
Application No. 2020-164884 filed with the Japan Patent Office on September 30,
2020, the content of which is incorporated herein by reference.
[Related Art]
[0002]
High strength steel sheets are used as steel sheets for a vehicle in order to
reduce the weight of a vehicle, improve fuel efficiency, reduce the amount of emission
of carbon dioxide gas, and secure the safety of passengers. Such steel sheets are also
required to have ductility to secure good workability, in addition to strength. Further,
in recent years, in order to sufficiently secure the corrosion resistance of a vehicle body
and parts, high strength hot-dip galvannealed steel sheets have also been used in
addition to high strength hot-dip galvanized steel sheets.
[0003]
However, when, for assembly of a vehicle body and/or parts, a high strength
hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet are spot-welded, or
a high strength cold-rolled steel sheet and a galvanized steel sheet are spot-welded,
cracking (LME cracking) due to liquid metal embrittlement (LME) may occur in a spotwelding
portion. The LME cracking is cracking that occurs when tensile stress acts on
a welding portion in a state where zinc in a galvanized layer melts due to heat that is
generated during spot welding and the molten zinc invades crystal grain boundaries of a
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steel sheet structure of the welding portion. The invasion of the molten zinc into the
crystal grain boundaries makes the crystal grain boundaries embrittle and easily causes
embrittlement cracking.
[0004]
LME remarkably occurs when a high strength TRIP steel sheet
(transformation-induced plasticity steel sheet) is spot-welded. The high strength TRIP
steel sheet is a steel sheet having excellent energy absorption power and press
formability by including residual austenite.
[0005]
Further, LME generally occurs during spot welding of high strength steel
sheets subjected to galvanizing. However, even in a high strength cold-rolled steel
sheet that is not subjected to galvanizing, when it is spot-welded to the galvanized steel
sheet, zinc melted in the galvanized steel sheet comes into contact with the high strength
cold-rolled steel sheet, so that there is a case where LME occurs.
[0006]
FIG. 1 schematically shows an aspect of LME occurred in a welding portion.
A steel sheet la, a steel sheet lb, and a steel sheet lc are superimposed and spot-welded
to form a nugget 2, so that the three steel sheets can be joined together. At this time, as
shown in FIG. 1, there is a case where an inner crack 3a occurs between the steel sheets,
an outer crack 3b occurs at a contact portion between the steel sheet and a spot welding
electrode, and an outer crack 3c occurs at a steel sheet portion that is not in direct
contact with the electrode.
[0007]
As described above, the zinc of the plating layer melted by the heat at the time
of welding invades the crystal grain boundaries of the welding portion structure, so that
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the grain boundaries become embrittle, and LME occurs due to stress that is generated
around the welding portion during welding. LME can occur not only in a case where
three steel sheets are superimposed and welded, as illustrated in FIG. 1, but also in a
case where two or four steel sheets are superimposed and spot-welded.
[0008]
As a high strength hot-dip galvanized steel sheet, for example, Patent
Document 1 discloses a hot-dip galvanized steel sheet having a composition including,
by a mass ratio, C: 0.05 to 0.30%, Mn: 0.8 to 3.00%, P: 0.003 to 0.100%, S: 0.010% or
less, Al: 0.10 to 2.50%, Cr: 0.03 to 0.500%, N: 0.007% or less, and a remainder
substantially including Fe and unavoidable impurities, in which the steel sheet includes
ferrite, residual austenite, and a low temperature transformation phase, a fraction of the
ferrite is 97% or less by a volume fraction, and the steel sheet has a structure in which
AlN is precipitated to a region from the steel sheet surface to 1 J.lm except for a plating
layer, and characteristics in which the tensile strength (TS) is 590 MPa or more and
fatigue strength (FL) in a state of having a punch fractured surface is 200 MPa or more.
[0009]
Patent Document 2 discloses a steel sheet in which a chemical composition of a
base metal includes, by mass%, C: 0.17 to 0.40%, Si: 0.10 to 2.50%, Mn: 1.00 to
10.00%, P: 0.001 to 0.03 %, S: 0.0001 to 0.02%, Al: 0.001 to 2.50%, N: 0.0001 to
0.010%, 0: 0.0001 to 0.010%, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, B: 0 to
0.010%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 2.00%, Ca: 0 to
0.50%, Mg: 0 to 0.50%, REM: 0 to 0.50%, and a remainder: Fe and impurities, an
internal oxidation layer in which at least a part of a crystal grain boundary is coated with
an oxide is provided from the surface of the base metal to a depth of 5.0 J.lm or more,
and grain boundary coverage of the oxide is 60% or more in a region from the surface
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of the base metal to the depth of 5.0 )lm. In Patent Document 2, it is described that the
invasion of molten zinc during welding is suppressed by covering the crystal grain
boundary in the surface layer of the base metal with an internal oxide in advance.
[0010]
Patent Document 3 discloses a method for manufacturing a high strength steel
sheet including a hot rolling step in which a steel slab having a predetermined
composition is heated to a temperature of ll00°C or higher and 1300°C or lower, is hotrolled
at a finish rolling outlet side temperature of 800°C or higher and 1000ac or
lower, and then coiled at an average coiling temperature of 200°C or higher and 500°C
or lower, thereby forming a hot-rolled steel sheet, a pickling step in which pickling is
performed on the hot-rolled steel sheet, and an annealing step in which the hot-rolled
steel sheet is held at a temperature of 740°C or higher and 840°C or lower for a time of
10 s or longer and 900 s or shorter, then cooled at an average cooling rate of 5°C/s or
faster and 50°C/s or slower to a cooling stop temperature of higher than 350°C and
550°C or lower, and held for a time of 10 s or longer at a temperature range of higher
than 35oac and 55oac or lower.

CLAIMS
Claim 1. A steel sheet comprising, as a chemical composition, by mass%:
C: 0.150% or more and 0.300% or less;
Si: 0.30% or more and 2.50% or less;
Mn: 1.50% or more and 4.00% or less;
Al: 0.30% or more and 2.00% or less;
P: 0% or more and 0.0400% or less;
S: 0% or more and 0.0100% or less;
N: 0.0025% or more and 0.0100% or less;
0: 0% or more and 0.0060% or less;
Cr: 0% or more and 0.500% or less;
Ni: 0% or more and 1.000% or less;
Cu: 0% or more and 1.000% or less;
Mo: 0% or more and 0.500% or less;
Ti: 0% or more and 0.200% or less;
Nb: 0% or more and 0.200% or less;
V: 0% or more and 0.500% or less;
B: 0% or more and 0.0100% or less;
W: 0% or more and 0.100% or less;
Ta: 0% or more and 0.100% or less;
Sn: 0% or more and 0.050% or less;
Co: 0% or more and 0.500% or less;
Sb: 0% or more and 0.050% or less;
As: 0% or more and 0.050% or less;
Mg: 0% or more and 0.050% or less;
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Ca: 0% or more and 0.040% or less;
Y: 0% or more and 0.050% or less;
La: 0% or more and 0.050% or less;
Ce: 0% or more and 0.050% or less;
Zr: 0% or more and 0.050% or less; and
a remainder consisting of Fe and impurities,
wherein a metallographic structure at a 114 thickness portion includes, by
volume percentage, a total of 50% or more of one or both of martensite and bainite and
8% or more of residual austenite, and a remainder being one or both of ferrite and
pearlite,
an average value of aspect ratios of prior austenite grains is 5.0 or more at the
114 thickness portion,
a number density of AIN is 3000 pieces/mm2 or more and less than 6000
pieces/mm2 at a depth position of 30 11m from a sheet surface,
an internal oxidation layer in which at least a part of a crystal grain boundary is
coated with an oxide is provided from the sheet surface to a depth of 5.0 11m or more,
a grain boundary coverage of the oxide is 60% or more in a region from the
sheet surface to a depth of 5.0 11m, and
a tensile strength is 980 MPa or more.
Claim 2. The steel sheet according to claim 1, wherein the steel sheet has a
hot-dip galvanizing plating or a hot-dip galvannealing plating.
Claim 3. The steel sheet according to claim 1 or 2, wherein a part or all of the
martensite at the 1/4 thickness portion is tempered martensite, and
a volume percentage of the tempered martensite at the 1/4 thickness portion is
5% or more.