U.S. patent application number 17/247358 was filed with the patent office on 2021-04-01 for method for producing a high strength coated steel sheet having improved strength and ductility and obtained sheet.
The applicant listed for this patent is ARCELORMITTAL. Invention is credited to Dongwei Fan, Hyun Jo Jun, Rashmi Ranjan Mohanty.
Application Number | 20210095357 17/247358 |
Document ID | / |
Family ID | 1000005266396 |
Filed Date | 2021-04-01 |
![](/patent/app/20210095357/US20210095357A1-20210401-D00001.png)
United States Patent
Application |
20210095357 |
Kind Code |
A1 |
Fan; Dongwei ; et
al. |
April 1, 2021 |
METHOD FOR PRODUCING A HIGH STRENGTH COATED STEEL SHEET HAVING
IMPROVED STRENGTH AND DUCTILITY AND OBTAINED SHEET
Abstract
A coated steel sheet includes a chemical composition including
in weight %: 0.13%.ltoreq.C .ltoreq.0.22%;
1.2%.ltoreq.Si.ltoreq.1.8%; 1.8%.ltoreq.Mn.ltoreq.2.2%;
0.10%.ltoreq.Mo.ltoreq.0.20%; Nb.ltoreq.0.05%; Al.ltoreq.0.5%;
Ti.ltoreq.0.05%; and a remainder being Fe and unavoidable
impurities. A structure of the steel sheet consists of, by volume
fraction, 3% to 15% of residual austenite and 85% to 97% of
martensite and bainite. The structure includes at least 65% of
martensite and does not including ferrite. At least one face of the
coated steel sheet includes a metallic coating. The steel sheet has
a yield strength of at least 800 MPa, a tensile strength of at
least 1180 MPa, a total elongation of at least 14% and a hole
expansion ratio HER of at least 30%.
Inventors: |
Fan; Dongwei; (Munster,
IN) ; Jun; Hyun Jo; (Clinton, NJ) ; Mohanty;
Rashmi Ranjan; (East Chicago, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ARCELORMITTAL |
Luxembourg |
|
LU |
|
|
Family ID: |
1000005266396 |
Appl. No.: |
17/247358 |
Filed: |
December 8, 2020 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15322722 |
Dec 28, 2016 |
|
|
|
PCT/IB2015/055035 |
Jul 3, 2015 |
|
|
|
17247358 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/06 20130101;
B32B 15/013 20130101; C21D 9/46 20130101; C22C 38/04 20130101; C21D
8/0247 20130101; C21D 1/19 20130101; C22C 38/12 20130101; C21D
2211/001 20130101; C22C 38/02 20130101; C21D 2211/002 20130101;
C21D 2211/008 20130101; C21D 8/0447 20130101; C22C 38/14 20130101;
C21D 6/005 20130101; C23C 2/40 20130101; C23C 2/28 20130101; C21D
8/0484 20130101; C23C 2/06 20130101; C23C 2/02 20130101; C21D 6/008
20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/04 20060101 C21D008/04; C21D 8/02 20060101
C21D008/02; C21D 1/19 20060101 C21D001/19; B32B 15/01 20060101
B32B015/01; C21D 6/00 20060101 C21D006/00; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/06 20060101
C22C038/06; C22C 38/12 20060101 C22C038/12; C22C 38/14 20060101
C22C038/14; C23C 2/02 20060101 C23C002/02; C23C 2/06 20060101
C23C002/06; C23C 2/28 20060101 C23C002/28; C23C 2/40 20060101
C23C002/40 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2014 |
IB |
PCT/IB2014/003249 |
Claims
1. A coated steel sheet comprising: a chemical composition of the
steel including in weight %: 13%.ltoreq.C.ltoreq.0.22%;
2%.ltoreq.Si.ltoreq.1.8%; 8%.ltoreq.Mn.ltoreq.2.2%
0.10%.ltoreq.Mo.ltoreq.0.20%; Nb.ltoreq.0.05%; Al.ltoreq.0.5%; and
Ti.ltoreq.0.05%; a remainder being Fe and unavoidable impurities; a
structure consisting of, by volume fraction, 3% to 15% of residual
austenite and 85% to 97% of martensite and bainite, the structure
including at least 65% of martensite, the structure not including
ferrite; at least one face of the coated steel sheet including a
metallic coating; and a yield strength of at least 800 MPa, a
tensile strength of at least 1180 MPa, a total elongation of at
least 14% and a hole expansion ratio HER of at least 30%.
2. The coated steel sheet according to claim 1, wherein the
chemical composition of the steel satisfies at least one of the
following conditions: 16%.ltoreq.C.ltoreq.0.20%;
3%.ltoreq.Si.ltoreq.1.6%; and 1.9%.ltoreq.Mn.ltoreq.2.1%.
3. The coated steel sheet according to claim 1, wherein the at
least one face including a metallic coating is galvanized.
4. The coated steel sheet according to claim 1, wherein the at
least one face including a metallic coating is galvannealed.
5. The coated steel sheet according to claim 1, wherein the
chemical composition of the steel satisfies
0.02%.ltoreq.Nb.ltoreq.0.05%.
Description
[0001] This is a Divisional of U.S. patent application Ser.
No.15/322,722, filed Dec. 28, 2016 which is a National Phase of
International Patent Application PCT/IB2015/055035, filed Jul. 3,
2015 claiming priority of International Patent Application
PCT/IB2014/003249, filed Jul. 3, 2014, the entire disclosures of
which are hereby incorporated by reference herein.
[0002] The present disclosure relates to a method for producing a
high strength coated steel sheet having improved strength,
ductility and formability and to the sheets obtained with the
method.
BACKGROUND
[0003] To manufacture various equipments such as parts of body
structural members and body panels for automotive vehicles, it is
usual to use galvanized or galvannealed sheets made of DP (dual
phase) steels or TRIP (transformation induced plasticity)
steels.
[0004] For example, such steels which include a martensitic
structure and/or some retained austenite and which contains about
0.2% of C, about 2% of Mn, about 1.7% of Si have a yield strength
of about 750 MPa, a tensile strength of about 980 MPa, a total
elongation of more than 8%. These sheets are produced on continuous
annealing line by quenching from an annealing temperature higher
than Ac.sub.3 transformation point, down to an overaging
temperature above Ms Transformation point and maintaining the sheet
at the temperature for a given time. Then the sheet is galvanized
or galvannealed.
SUMMARY
[0005] To reduce the weight of the automotive in order to improve
their fuel efficiency in view of the global environmental
conservation it is desirable to have sheets having improved yield
and tensile strength. But such sheets must also have a good
ductility and a good formability and more specifically a good
stretch flangeability.
[0006] In this respect, it is desirable to have sheets having a
yield strength YS of at least 800 MPa, a tensile strength TS of
about 1180 MPa, a total elongation of at least 14% and a hole
expansion ratio HER according to ISO standard 16630:2009 of more
than 25%. It must be emphasized that, due to differences in the
methods of measure, the values of hole expansion ration HER
according to the ISO standard are very different and not comparable
to the values of the hole expansion ratio .lamda., according to the
JFS T 1001 (Japan Iron and Steel Federation standard).
[0007] Therefore, the purpose of the present disclosure is to
provide such sheet and a method to produce it.
[0008] For this purpose, the present disclosure relates to a method
for producing a high strength coated steel sheet having an improved
strength and an improved formability, the sheet having a yield
strength YS of at least 800 MPa, a tensile strength TS of at least
1180 MPa, a total elongation of at least 14% and a hole expansion
ratio HER of at least 30%, by heat treating and coating a steel
sheet whose chemical composition of the steel contains in weight %:
[0009] 0.13%.ltoreq.C.ltoreq.0.22% [0010]
1.2%.ltoreq.Si.ltoreq.1.8% [0011] 1.8%.ltoreq.Mn.ltoreq.2.2% [0012]
0.10%.ltoreq.Mo.ltoreq.0.20% [0013] Nb.ltoreq.0.05% [0014]
A1.ltoreq.0.5% the remainder being Fe and unavoidable impurities.
The heat treatment and the coating comprise the following steps:
[0015] annealing the sheet at an annealing temperature TA higher
than Ac.sub.3 but less than 1000.degree. C. for a time of more than
30 s, [0016] quenching the sheet by cooling it down to a quenching
temperature QT between 325.degree. C. and 375.degree. C., at a
cooling speed sufficient to obtain a structure consisting of of
austenite and at least 60% of martensite, the austenite content
being such that the final structure i.e. after treatment, coating
and cooling to the room temperature, can contain between 3% and 15%
of residual austenite and between 85% and 97% of the sum of
martensite and bainite, without ferrite [0017] heating the sheet up
to a partitioning temperature PT between 430.degree. C. and
480.degree. C. and maintaining the sheet at this temperature for a
partitioning time Pt between 10 s and 90 s, [0018] hot dip coatings
the sheet and, [0019] cooling the sheet down to the room
temperature.
[0020] Preferably, the quenching temperature QT is between
350.degree. C. and 375.degree. C.
[0021] Preferably, the partitioning temperature PT is between
435.degree. C. and 465.degree. C.
[0022] The chemical composition of the steel can satisfy at least
one of the following conditions: [0023] 0.16%.ltoreq.C.ltoreq.0.20%
[0024] 1.3%.ltoreq.Si.ltoreq.1.6%, and [0025]
1.9%.ltoreq.Mn.ltoreq.2.1%
[0026] The hot dip coating step may be a galvanizing step.
[0027] The hot dip coating step may be a galvannealing step with an
alloying temperature TGA between 480.degree. C. and 510.degree. C.
In this case, the partitioning time PT is preferably between 50 s
and 70 s.
[0028] Preferably, after the sheet is quenched to the quenching
temperature QT and before the sheet is heated to the partitioning
temperature PT, the sheet is held at the quenching temperature QT
for a holding time comprised between 2 s and 8 s, preferably
between 3 s and 7 s.
[0029] The present disclosure, also, relates to a coated steel
sheet whose chemical composition of the steel contains in weight %:
[0030] 0.13%.ltoreq.C.ltoreq.0.22% [0031]
1.2%.ltoreq.Si.ltoreq.1.8% [0032] 1.8%.ltoreq.Mn.ltoreq.2.2% [0033]
0.10%.ltoreq.Mo.ltoreq.0.20% [0034] Nb.ltoreq.0.05% [0035]
Al.ltoreq.0.5% [0036] Ti.ltoreq.0.05% the remainder being Fe and
unavoidable impurities. The structure of the steel consists of 3%
to 15% of residual austenite and 85% to 97% of martensite and
bainite, without ferrite. At least one face of the sheet comprises
a metallic coating. The sheet has a yield strength of at least 800
MPa, a tensile strength of at least 1180 MPa, a total elongation of
at least 14% and a hole expansion ratio HER of at least 30%.
[0037] Optionally the chemical composition of the steel can satisfy
at least one of the following conditions: [0038]
0.16%.ltoreq.C.ltoreq.0.20% [0039] 1.3%.ltoreq.Si.ltoreq.1.6%, and
[0040] 1.9%.ltoreq.Mn.ltoreq.2.1%
[0041] The at least one coated face is, for example,
galvanized.
[0042] The at least one coated face is, for example,
galvannealed.
BRIEF SUMMARY OF THE DRAWING
[0043] The present disclosure will now be described in details but
without introducing limitations and illustrated by the FIGURE which
is a micrograph of an example of the present disclosure.
DETAILED DESCRIPTION
[0044] According to the present disclosure, the sheet is obtained
by hot rolling and optionally cold rolling of a semi product which
chemical composition contains, in weight %: [0045] 0.13% to 0.22%,
and preferably more than 0.16% preferably less than 0.20% of carbon
for ensuring a satisfactory strength and improving the stability of
the retained austenite which is necessary to obtain a sufficient
elongation. If carbon content is too high, the hot rolled sheet is
too hard to cold roll and the weldability is insufficient. [0046]
1.2% to 1.8%, preferably more than 1.3% and less than 1.6% of
silicon in order to stabilize the austenite, to provide a solid
solution strengthening and to delay the formation of carbides
during overaging without formation of silicon oxides at the surface
of the sheet which is detrimental to coatability. [0047] 1.8% to
2.2% and preferably more than 1.9% and preferably less than 2.1% of
manganese to have a sufficient hardenability in order to obtain a
structure containing at least 65% of martensite, tensile strength
of more than 1150 MPa and to avoid having segregation issues which
are detrimental for the ductility. [0048] 0.10% to 0.20% of
molybdenium to increase the hardenability and to stabilize the
retained austenitic in order to strongly reduce austenite
decomposition during overaging. [0049] up to 0.5% of aluminium
which is usually added to liquid steel for the purpose of
deoxidation, preferably, the Al content is limited to 0.05%. If the
content of Al is above 0.5%, the austenitizing temperature will be
too high to be easily reached and the steel will become
industrially difficult to process. [0050] Nb content and Ti content
are limited to 0.05% each because above such values numerous
precipitates will form and formability will decrease, making the
14% of total elongation more difficult to reach.
[0051] The remainder being iron and residual elements resulting
from the steelmaking. In this respect, Ni, Cr, Cu, V, B, S, P and N
at least are considered as residual elements which are unavoidable
impurities. Therefore, generally, their contents are less than
0.05% for Ni, 0.10% for Cr, 0.03 for Cu, 0.007% for V, 0.0010% for
B, 0.005% for S, 0.02% for P and 0.010% for N.
[0052] The sheet is prepared by hot rolling and optionally cold
rolling according to the methods known by those who are skilled in
the art.
[0053] After rolling the sheets are pickled or cleaned then heat
treated and hot dip coated.
[0054] The heat treatment which is made preferably on a combined
continuous annealing and hot dip coating line comprises the steps
of: [0055] annealing the sheet at an annealing temperature TA
higher than the Ac.sub.3 transformation point of the steel, and
preferably higher than Ac.sub.3+15.degree. C., in order to be sure
that the structure is completely austenitic, but less than
1000.degree. C. in order not to coarsen too much the austenitic
grains. Generally, a temperature higher than 865.degree. C. is
enough for the steel according to the present disclosure. The sheet
is maintained at the annealing temperature i.e. maintained between
TA-5.degree. C. and TA+10.degree. C., for a time sufficient to
homogenize the chemical composition. Preferably, the time is of
more than 30 s but does not need to be of more than 300 s. [0056]
quenching the sheet by cooling down to a quenching temperature QT
lower than the Ms transformation point at a cooling rate enough to
avoid ferrite and bainite formation. The quenching temperature is
between 325.degree. C. and 375.degree. C. and preferably between
350.degree. C. and 375.degree. C. in order to have, just after
quenching, a structure consisting of austenite and at least 60% of
martensite, the austenite content being such that the final
structure i.e. after treatment, coating and cooling to the room
temperature, can contain between 3% and 15% of residual austenite
and between 85 and 97% of the sum of martensite and bainite,
without ferrite. A cooling rate higher than 30.degree. C./s is
enough, [0057] reheating the sheet up to a partitioning temperature
PT between 430.degree. C. and 480.degree. C. and preferably between
435.degree. C. and 465.degree. C. For example, the partitioning
temperature can be equal to the temperature at which the sheet must
be heated in order to be hot dip coated, i.e. between 455.degree.
C. and 465.degree. C. The reheating rate can be high when the
reheating is made by induction heater, but that reheating rate had
no apparent effect on the final properties of the sheet.
Preferably, between the quenching step and the step of reheating
the sheet to the partitioning temperature PT, the sheet is held at
the quenching temperature for a holding time comprised between 2 s
and 8 s, preferably between 3 s and 7 s. [0058] maintaining the
sheet at the partitioning temperature PT for a partitioning time Pt
between 10 s and 90 s. Maintaining the sheet at the partitioning
temperature means that during partitioning the temperature of the
sheet remains between PT-20.degree. C. and PT+20.degree. C. [0059]
optionally, adjusting the temperature of the sheet by cooling or
heating in order to be equal to the temperature at which the sheet
has to be heated in order to be hot dip coated. [0060] hot dip
coating the sheet, the hot dip coating being, for example,
galvanizing or galvannealing, but all type of metallic hot dip
coating is possible provided that the temperatures at which the
sheet is brought to during coating remains less than 650.degree. C.
. When the sheet is galvanized, it is done with the usual
conditions. When the sheet is galvannealed, the temperature of
alloying TGA must not be too high to obtain good final mechanical
properties. This temperature is preferably between 500.degree. and
580.degree. C. Moreover, in this case, the partitioning time is
preferably between 50 s and 70 s. [0061] generally, after coating,
the sheet is processed according to the known art. In particular
the sheet is cooled to the room temperature.
[0062] With such treatment, coated sheets having a yield strength
YS of at least 800 MPa, a tensile strength of at least 1180 MPa, a
total elongation of at least 14% and a hole expansion ratio HER
according to the ISO standard 16630:2009 of at least 30% can be
obtained.
[0063] As an example a sheet of 1.2 mm in thickness having the
following composition:
[0064] C=0.18%, Si=1.5% Mn=2.0%, Nb=0.02%, Mo=0.15%, the remainder
being Fe and impurities, was manufactured by hot and cold rolling.
The theoretical Ms transformation point of this steel is
386.degree. C. and the Ac.sub.3 point is 849.degree. C.
[0065] Samples of the sheet were heat treated by annealing,
quenching and partitioning then galvanized or galvannealed, and the
mechanical properties were measured.
[0066] The conditions of treatment and the obtained properties are
reported at table I for the samples that were galvanized and at
table II for the samples that were galvannealed.
TABLE-US-00001 TABLE I TA QT PT Pt YS TS UE TE HER Sample .degree.
C. .degree. C. .degree. C. s MPa MPa % % % 1 900 300 460 60 1116
1207 7 12 2 900 350 460 30 952 1215 9 14 3 900 350 460 60 926 1199
8 14 31 4 900 350 460 90 909 1207 9 14 5 900 400 460 60 709 1187 10
15 6 900 460 460 60 685 1178 9 14
TABLE-US-00002 TABLE II TA QT PT Pt TGA YS TS UE TE HER Sample
.degree. C. .degree. C. .degree. C. s .degree. C. MPa MPa % % % 7
900 350 460 60 500 838 1185 9 14 34 8 900 350 460 60 520 854 1215 9
12 9 900 350 460 60 520 869 1167 8 12 -20 s- 10 900 350 460 60 570
898 1106 7 13
[0067] In these tables, TA is the annealing temperature, QT the
quenching temperature, PT the partitioning temperature, Pt the
maintaining time at the partitioning temperature, TGA the
temperature of alloying for the sheets that were galvannealed, YS
the yield strength, TS the tensile strength, UE the uniform
elongation, TE the total elongation and HER the hole expansion
ratio measured according to the ISO 16630:2009 standard.
[0068] For example 9, "520-20" (TGA) means that the steel has been
at the GA temperature of 520.degree. C. for 20 seconds, in the
other examples (7,8 and 10) once the GA temperature is reached,
then the temperature decreases slowly before the final cooling.
[0069] Examples 1 to 4 show that with a quenching temperature equal
or less than 350.degree. C., a partitioning at a temperature of
460.degree. C. with a partitioning time from 30 s to 90 s
galvanized sheets have a yield strength higher than 800 MPa, a
tensile strength higher than 1180 MPa, a total elongation of more
than or equal to 12% and a hole expansion ratio measured according
to ISO standard 16630: 2009 higher than 30%.
[0070] Examples for which the quenching temperature is higher than
Ms are comparative examples and/or according to the prior art. The
structure contains ferrite or bainite and austenite and the yield
strength is significantly less than 800 MPa.
[0071] The examples 7 to 10 show that, when the sheet is
galvannealed, the temperature of alloying has to be as low as
possible to obtain a total elongation of 14% and a hole expansion
ratio HER of more than 30%. Example 7, a micrograph of which is
shown at the figure, contains 7% of retained austenite and 96% of
the sum of martensite and bainite.
[0072] The conditions of treatment and the obtained properties are
reported at table I for the samples that were galvanized and at
table II for the samples that were galvannealed.
* * * * *