U.S. patent application number 14/343622 was filed with the patent office on 2014-07-24 for high strength hot dip galvanised steel strip.
This patent application is currently assigned to TATA STEEL IJMUIDEN B.V.. The applicant listed for this patent is Bernard Leo Ennis. Invention is credited to Bernard Leo Ennis.
Application Number | 20140205858 14/343622 |
Document ID | / |
Family ID | 46851934 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140205858 |
Kind Code |
A1 |
Ennis; Bernard Leo |
July 24, 2014 |
HIGH STRENGTH HOT DIP GALVANISED STEEL STRIP
Abstract
A high strength hot dip galvanised steel strip having, in mass
percent, the following elements: 0.13-0.19% C; 1.70-2.50% Mn; max
0.15% Si; 0.40-1.00% Al; 0.05-0.25% Cr; 0.01-0.05% Nb; max 0.10% P;
max 0.004% Ca; max 0.05% S; max 0.007% N; and optionally at least
one of the following elements: max 0.50% Ti, max 0.40% V, max 0.50%
Mo, max 0.50% Ni, max 0.50% Cu, max 0.005% B, the balance being Fe
and inevitable impurities; wherein 0.40%<Al+Si<1.05% and
Mn+Cr>1.90%. This steel offers improved formability at a high
strength, has a good weldability, and surface quality together with
a good producability and coatability.
Inventors: |
Ennis; Bernard Leo;
(Haarlem, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ennis; Bernard Leo |
Haarlem |
|
NL |
|
|
Assignee: |
TATA STEEL IJMUIDEN B.V.
Velsen-Noord
NL
|
Family ID: |
46851934 |
Appl. No.: |
14/343622 |
Filed: |
September 12, 2012 |
PCT Filed: |
September 12, 2012 |
PCT NO: |
PCT/EP2012/003823 |
371 Date: |
March 13, 2014 |
Current U.S.
Class: |
428/659 ;
148/533 |
Current CPC
Class: |
C21D 2211/008 20130101;
C23C 2/02 20130101; C21D 2211/005 20130101; C21D 8/0263 20130101;
C22C 38/04 20130101; C21D 1/20 20130101; C22C 38/26 20130101; Y10T
428/12799 20150115; C21D 2211/001 20130101; C21D 9/46 20130101;
C21D 2211/002 20130101; C22C 38/06 20130101 |
Class at
Publication: |
428/659 ;
148/533 |
International
Class: |
C23C 2/02 20060101
C23C002/02; C21D 8/02 20060101 C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2011 |
EP |
11007431.7 |
Claims
1. A high strength hot dip galvanised steel strip consisting, in
mass percent, of the following elements: 0.13-0.19% C 1.70-2.50% Mn
max 0.15% Si 0.40-1.00% Al 0.05-0.25% Cr 0.01-0.05% Nb max 0.10% P
max 0.004% Ca max 0.05% S max 0.007% N and optionally at least one
of the following elements: max 0.50% Ti max 0.40% V max 0.50% Mo
max 0.50% Ni max 0.50% Cu max 0.005% B the balance being Fe and
inevitable impurities, wherein 0.40%<Al+Si<1.05% and
Mn+Cr>1.90%, wherein the hot dip galvanised steel strip has a
microstructure containing 8-12% retained austenite, 10-20%
martensite, the remainder being a mixture of ferrite and bainite,
the hot dip galvanised steel strip containing not more than 10%
bainite, and wherein the hot dip galvanised steel strip has an
ultimate tensile strength Rm of at least 700 MPa, a 0.2% proof
strength Rp of at least 400 MPa and a total elongation of at least
18%.
2. The steel strip according to claim 1, wherein element C is
present in an amount of 0.13-0.16%.
3. The steel strip according to claim 1, wherein element Mn is
present in an amount of 1.95-2.40%.
4. The steel strip according to claim 1, wherein element Si is
present in an amount of 0.05-0.15%.
5. The steel strip according to claim 1, wherein element Al is
present in an amount of 0.60-0.80%.
6. The steel strip according to claim 1, wherein element Cr is
present in an amount of 0.10-0.25%.
7. The steel strip according to claim 1, wherein the element Nb is
present in an amount of 0.01-0.04%.
8. The steel strip according to claim 1, wherein the hot dip
galvanised steel strip has an ultimate tensile strength Rm of at
least 750 MPa.
9. The steel strip according to claim 1, wherein the hot dip
galvanised steel strip has an 0.2% proof strength Rp of at least
450 MPa.
10. The steel strip according to claim 1, wherein the hot dip
galvanised steel strip has a hole expansion coefficient of at least
35% when Rm is 750 MPa and Rp is 450 MPa.
11. The steel strip according to claim 1, wherein the hot dip
galvanised steel strip has an Erichsen cupping index of more than
10.5 mm when Rm is 750 MPa and Rp is 450 MPa.
12. The steel strip according to claim 1, wherein the hot dip
galvanised steel strip has an average grain size of at most 5
.mu.m.
13. A method for producing a high strength hot dip galvanised steel
strip according to claim 1, comprising: hot rolling and cold
rolling the cast steel to a strip having a desired thickness, after
said rolling to the desired thickness reheating the strip in an
annealing line to a temperature above the Ac1 temperature of the
steel, and fast cooling the reheated strip at a cooling rate to
avoid retransformation to ferrite, after which isothermal overaging
is applied to form bainite, and the strip is hot dip
galvanised.
14. The method according to claim 13, wherein the annealing is
applied at a temperature between 750.degree. C. and 850.degree.
C.
15. The method according to claim 13, wherein the overaging is
applied at a temperature between 360.degree. C. and 480.degree.
C.
16. The steel strip according to claim 1, wherein element Mn is
present in an amount of 1.95-2.30%.
17. The steel strip according to claim 1, wherein element Mn is
present in an amount of 2.00-2.20%.
18. The method according to claim 14, wherein the annealing is
applied at a temperature between 780.degree. C. and 820.degree.
C.
19. The method according to claim 14, wherein the strip is reheated
in the annealing line to a temperature between the Ac1 and the Ac3
temperature of the steel type.
Description
[0001] The invention relates to a high strength hot dip galvanised
steel strip having improved formability, such as used in the
automotive industry.
[0002] Such steel types are known and have been developed under the
name of dual phase steel types. Such steel types do not provide the
required formability as required in many applications for the
automotive industry. For this reason, TRIP assisted dual phase
steel types have been developed.
[0003] A document describing such steel types is EP 1 889 935 A1.
This document describes a high strength hot dip galvanised steel
sheet containing (in mass percent)
0.05-0.3% C
0.08-3% Mn
max 1.4% Si
0.1-2.5% Al
0.1-0.5% Cr
0.003-0.1% P
max 0.07% S
max 0.007% N
[0004] the balance being Fe and incidental impurities, and wherein
Si+Al.gtoreq.0.5%. Optionally a number of other elements can be
present. Thirty-one steel types have been tested at laboratory
scale, of which nineteen are deemed inventive compositions. These
examples show that a wide range of the amounts of especially Si and
Al are fulfilling the inventiveness requirements according to EP 1
889 935 A1.
[0005] Formability, however, is not the only requirement for a TRIP
assisted dual phase steel strip. The alloying elements should be
low in amount to make the cost of the steel as low as possible, it
should be as easy as possible to produce the steel strip and to
coat it, the steel strip has to have high strength, good
weldability and should also exhibit a good surface quality. These
requirements are especially important for industrially produced
TRIP assisted dual phase steel types, which have to be formed into
for instance automotive parts that will be spot welded into a body
in white.
[0006] It is thus an object of the invention to find a composition
of a high strength hot dip galvanised steel strip that strikes a
balance between the formability and the processability of the
strip.
[0007] It is a further object of the invention to provide a high
strength hot dip galvanised steel strip that has a good coatability
during the hot dip galvanising process.
[0008] It is a still further object of the invention to provide a
high strength hot dip galvanised steel strip that has a good
weldability.
[0009] It is another object of the invention to provide a high
strength hot dip galvanised steel strip that has a good surface
quality.
[0010] It is still another object of the invention to provide a
high strength hot dip galvanised steel strip having a cost price
that is as low as possible.
[0011] One or more of these objects are met according to the
invention by providing a high strength hot dip galvanised steel
strip consisting, in mass percent, of the following elements:
[0012] 0.13-0.19% C [0013] 1.70-2.50% Mn [0014] max 0.15% Si [0015]
0.40-1.00% Al [0016] 0.05-0.25% Cr [0017] 0.01-0.05% Nb [0018] max
0.10% P [0019] max 0.004% Ca [0020] max 0.05% S [0021] max 0.007%
N
[0022] and optionally at least one of the following elements:
[0023] max 0.50% Ti [0024] max 0.40% V [0025] max 0.50% Mo [0026]
max 0.50% Ni [0027] max 0.50% Cu [0028] max 0.005% B
[0029] the balance being Fe and inevitable impurities,
[0030] wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%.
[0031] The inventors have found that by a careful selection of the
amounts of the main constituting elements of the steel, being
carbon, manganese, silicon, aluminium and chromium, a high strength
hot dip galvanised steel strip can be produced that has the
required formability, processability, strength and elongation,
while at the same time providing a sufficient weldability,
coatability and surface quality. It has been found by the inventors
that none of the examples given in the state of the art provide all
these requirements at the same time.
[0032] Especially, the inventors have found that a relatively high
silicon content deteriorates the processability of the steel strip.
Si is traditionally used to effectuate the TRIP effect, due to
retardation of carbide formation in the presence of Si which leads
to carbon enrichment and, hence, stabilisation of austenite at room
temperature. The disadvantages of Si are that in very high
quantities (above 0.4 wt. %) it interferes with the wettability of
zinc, making galvanisation over tradition continuous annealing
lines impossible. It has also been shown in EP 1 889 935 A1 that Si
can be replaced by relatively high quantities of Al. The present
invention shows that the addition of Si can be omitted and Al kept
to a minimum by careful selection of the Cr content and with the
addition of Nb. In this case a strip is produced which has low
hot-rolling loads allowing a wider and thinner dimensional
capability. However, silicon is usually present in trace quantities
due to manganese addition. For this reason, the maximum silicon
content is set on 0.15% Si. Above this amount of silicon, the
rolling loads in the hot-strip mill are high. An amount of silicon
below 0.15% makes it possible to produce wide and thin steel
strip.
[0033] Moreover, the composition of the steel strip according to
the invention is such that the formability of the steel is good and
no necking occurs, and that the edge ductility of pressed parts is
such that no cracking occurs.
[0034] The reason for the amounts of the main constituting elements
is as follows.
[0035] C: 0.13-0.19 mass %. Carbon has to be present in an amount
that is high enough to ensure hardenability and the formation of
martensite at the cooling rates available in a conventional
annealing/galvanising line. Martensite is required to deliver
adequate strength. Free carbon also enables stabilisation of
austenite which delivers improved work hardening potential and good
formability for the resulting strength level. A lower limit of 0.13
mass % is desired for these reasons. A maximum level of 0.19 mass %
has been found to be essential to ensure good weldability.
[0036] Mn: 1.70-2.50 mass %. Manganese is added to increase
hardenability thus making the formation of martensite easier within
the cooling rate capability of a conventional continuous
annealing/galvanising line. Manganese also contributes to the solid
solution strengthening which increases the tensile strength and
strengthens the ferrite phase, thus helping to stabilise retained
austenite. Manganese lowers the transformation temperature range of
the dual phase steel, thus lowering the required annealing
temperature to levels that can be readily attained in a
conventional continuous annealing/galvanising line. A lower limit
of 1.70 mass % is needed for the above reasons. A maximum level of
2.50 mass % is imposed to ensure acceptable rolling forces in the
hot mill and to ensure acceptable rolling forces in the cold mill
by ensuring sufficient transformation of the dual phase steel to
soft transformation products (ferrite and pearlite). This maximum
level is also given in view of the stronger segregation during
casting and the forming of a band of martensite in the strip at
higher values.
[0037] Al: 0.40-1.00 mass %. Aluminium is added to liquid steel for
the purpose of de-oxidation. In the right quantity it also provides
an acceleration of the bainite transformation, thus enabling
bainite formation within the time constraints imposed by the
annealing section of a conventional continuous
annealing/galvanising line. Aluminium also retards the formation of
carbides thus keeping carbon in solution thus causing partitioning
to austenite during overaging, and promoting the stabilisation of
austenite. A lower level of 0.40 mass % is required for the above
reasons. A maximum level of 1.00 mass % is imposed for castability,
since high aluminium contents lead to poisoning of the casting
mould slag and consequently an increase in mould slag viscosity,
leading to incorrect heat transfer and lubrication during
casting.
[0038] Cr: 0.05-0.25 mass %. Chrome is added to increase
hardenability. Chrome forms ferrite and suppresses the formation of
carbides, thus enhancing the forming of retained austenite. A lower
level of 0.05 mass % is required for the above reasons. A maximum
level of 0.25 mass % is imposed to ensure satisfactory pickling of
the steel strip, and to keep the cost of the strip sufficiently
low.
[0039] Ca: max 0.004 mass %. The addition of calcium modifies the
morphology of manganese sulphide inclusions. When calcium is added
the inclusions get a globular rather than an elongated shape.
Elongated inclusions, also called stringers, may act as planes of
weakness along which lamellar tearing and delamination fracture can
occur. The avoidance of stringers is beneficial for forming
processes of steel sheets which entail the expansion of holes or
the stretching of flanges and promotes isotropic forming behaviour.
Calcium treatment also prevents the formation of hard, angular,
abrasive alumina inclusions in aluminium deoxidised steel types,
forming instead calcium aluminate inclusions which are softer and
globular at rolling temperatures, thereby improving the material's
processing characteristics. In continuous casting machines, some
inclusions occurring in molten steel have a tendency to block the
nozzle, resulting in lost output and increased costs. Calcium
treatment reduces the propensity for blockage by promoting the
formation of low melting point species which will not clog the
caster nozzles.
[0040] P: max 0.10 mass %. Phosphorus interferes with the formation
of carbides, and therefore some phosphorus in the steel is
advantageous. However, phosphorus can make steel brittle upon
welding, so the amount of phosphorus should be carefully
controlled, especially in combination with other embrittling
elements such as sulphur and nitrogen.
[0041] Sulphur and nitrogen are present in low amounts because
these elements are detrimental for weldability.
[0042] Niobium is added in an amount between 0.01 and 0.05 mass %
for grain refinement and formability. Niobium promotes
transformation on the runout table and thus provides a softer and
more homogeneous intermediate product. Niobium further suppresses
formation of martensite at isothermal overaging temperatures,
thereby promoting stabilisation of retained austenite.
[0043] The optional elements are mainly added to strengthen the
steel.
[0044] In addition the reasons given above, the ranges for
aluminium, chromium and manganese are chosen such that a correct
balance is found to deliver complete transformation on the runout
table to ensure a steel strip that can be cold rolled, and to
provide a starting structure enabling rapid dissolution of carbon
in the annealing line to promote hardenability and correct
ferritic/bainitic transformation behaviour. Moreover, because
aluminium accelerates and chromium decelerates the bainitic
transformation, the right balance between aluminium and chromium
has to be present to produce the right quantity of bainite within
the timescales permitted by a conventional hot dip galvanising line
with a restricted overage section.
[0045] Apart from the absolute contents of the elements as given
above, also the relative amounts of certain elements are of
importance.
[0046] Aluminium and silicon together should be maintained between
0.4 and 1.05 mass % to ensure suppression of carbides in the end
product and stabilisation of a sufficient amount of austenite, with
the correct composition, to provide a desirable extension of
formability.
[0047] Manganese and chromium together should be above 1.90 mass %
to ensure sufficient hardenability for formation of martensite and
thus achievement of strength in a conventional continuous annealing
line and hot dip galvanising line.
[0048] Preferably element C is present in an amount of 0.13-0.16%.
In this range the hardenability of the steel is optimal while the
weldability of the steel is enhanced.
[0049] According to a preferred embodiment element Mn is present in
an amount of 1.95-2.40%, preferably in an amount of 1.95-2.30%,
more preferably in an amount of 2.00-2.20%. A higher amount of
manganese provides steel with a higher strength, so it is
advantageous to raise the lower limit to 1.95 or even 2.00 mass %
manganese. On the other hand, hot rolling and cold rolling of the
steel is more difficult for higher amounts of manganese, so it is
advantageous to lower the upper limit to 2.40, 2.30 or even 2.20
mass % manganese.
[0050] Preferably element Si is present in an amount of 0.05-0.15%.
There is no objection to the presence of some Si in the steel,
since Si ensures a better retardation of carbides during overaging
which is advantageous for the formability of the steel.
[0051] According to a preferred embodiment element Al is present in
an amount of 0.60-0.80%. A raised lower level of aluminium has the
same effect as a higher amount of silicon, but also improves the
bainite formation. A lower upper limit of aluminium improves the
castability of the steel.
[0052] Preferably element Cr is present in an amount of 0.10-0.25%.
A raised lower level increases the hardenability of the steel.
[0053] According to a preferred embodiment element Nb is present in
an amount of 0.01-0.04%. As elucidated above, niobium improves the
homogeneity of the intermediate product. The upper limit is mainly
in consideration of the cost of niobium.
[0054] Preferably the steel has an ultimate tensile strength Rm of
at least 700 MPa, more preferably an ultimate tensile strength Rm
of at least 750 MPa. This strength can, due to the careful
selection of the amounts of the elements present in the steel, be
reached while the formability of a conventional 600 MPa dual phase
steel is maintained.
[0055] According to a preferred embodiment the hot dip galvanised
steel strip has an 0.2% proof strength Rp of at least 400 MPa,
preferably an 0.2% proof strength Rp of at least 450 MPa. Also this
strength can be reached due to the careful selection of the amounts
of the elements present in the steel.
[0056] Preferably, the hot dip galvanised steel strip has a total
elongation of at least 18%. This is a high elongation which is also
reached by the chosen presence of the elements in the steel.
[0057] According to a preferred embodiment the hot dip galvanised
steel strip has a hole expansion coefficient of at least 35% when
Rm is 750 MPa and Rp is 450 MPa. This is a good hole expansion
coefficient, as will be elucidated below. The hole expansion
coefficient decreases with increasing strength.
[0058] Preferably the hot dip galvanised steel strip has an
Erichsen cupping index of more than 10.5 mm when Rm is 750 MPa and
Rp is 450 MPa. This is satisfactory for the usability of the steel.
The Erichsen cupping index decreases with increasing strength.
[0059] According to a preferred embodiment the hot dip galvanised
steel strip has a dual phase structure containing 8-12% retained
austenite, 10-20% martensite, the remainder being a mixture of
ferrite and bainite, preferably the hot dip galvanised steel strip
containing not more than 10% bainite. With such microstructures, a
high elongation and a high strength will be reached.
[0060] According to a preferred embodiment the hot dip galvanised
steel strip has an average grain size of at most 5 .mu.m. This
small grain size helps to achieve the above mentioned mechanical
properties of the steel.
[0061] According to a second aspect of the invention there is
provided a method for producing a high strength hot dip galvanised
steel strip as defined above, wherein the cast steel is hot rolled
and cold rolled to a strip having a desired thickness, after which
the strip is reheated in an annealing line to a temperature above
the Ac1 temperature and preferably between the Ac1 and the Ac3
temperature of the steel type and fast cooled at a cooling rate
such as to avoid retransformation to ferrite, after which
isothermal overaging is applied to form bainite, and the strip is
hot dip galvanised.
[0062] In this method, the deformation schedule during hot rolling,
the finish rolling temperature and the subsequent cooling pattern
on a run-out table can be selected to achieve a microstructure in
the hot rolled product which is conducive to further reduction of
thickness in the cold mill. In particular attention can be paid to
limiting the strength of the hot rolled strip so as to minimise the
required cold rolling loads. The temperature in the annealing line
can be chosen such that the steel strip comprises both ferrite and
austenite. The cooling rate should be such that in principle no
ferrite is formed, and the isothermal overaging is applied to
promote the formation of bainite. Hot dip galvanising can be
performed in the usual manner. During this method the temperature
and duration of most steps is critical for the realisation of the
desired balance between strength and ductility in the final
product.
[0063] Preferably the annealing will be carried out a temperature
between 750.degree. C. and 850.degree. C. and more preferably at a
temperature between 780.degree. C. and 820.degree. C. At these
temperatures the steel strip comprises both ferrite and
austenite.
[0064] Preferably, the overaging is applied at a temperature
between 360.degree. C. and 480.degree. C.
[0065] As known to the skilled person, the iron-carbon eutectoid
system has a number of critical transformation temperatures as
defined below. These temperatures are dependent on chemistry and
processing conditions:
A1--temperature below which the microstructure is composed of a
mixture of ferrite (alpha-Fe) and Fe3C/pearlite; A2--Curie
temperature: temperature above which the material ceases to be
magnetic; A3--temperature above which the microstructure is
entirely composed of austenite. The suffixes c and r denote
transformations in the heating and cooling cycle respectively.
[0066] The invention will be elucidated hereinafter; a number of
compositions will be evaluated with regard to some well-known
formability parameters that are elucidated first.
[0067] n-value: The work hardening coefficient or n-value is
closely related to uniform elongation. In most sheet forming
processes the limit of formability is determined by the resistance
to local thinning or "necking" In uniaxial tensile testing necking
commences at the extent of uniform elongation. n-value and uniform
elongation derived from the tensile test can be taken as a measure
of the formability of sheet steels. When aiming to improve
formability of strip steels n-value and uniform elongation
represent the most suitable optimisation parameters.
[0068] Hole expansion coefficient (HEC): To be successfully applied
in industrial stamping operations, sheet metals must have a certain
ability to withstand stretching of their sheared edges. This is
tested in accordance with the international technical specification
ISO/TS16630. A hole having a diameter of 10 mm is made in the
centre of a test piece having the dimensions 90.times.90 mm. A cone
punch of 40 mm diameter with a 60.degree. apex is forced into the
hole while the piece is fixed with a die having an inner diameter
of 55 mm. The diameter of the hole is measured when a crack had
extended through the thickness of the test piece. The maximum HEC
was determined by: Max HEC %=((Dh-Do)/Do).times.100, wherein Do is
the original hole diameter and Dh is the diameter of the hole after
cracking Stretch flangeability is evaluated on the basis of the
maximum HEC and is deemed satisfactory when HEC>25%
[0069] Erichsen Index (EI): The Erichsen test describes the ability
of metals to undergo plastic deformation in stretch forming and is
tested in accordance with the international standard test ISO
20482:2003. A hemispherical punch is driven into a fully clamped
sheet. As lubrication graphite grease is used on top of the punch.
The punch travel is stopped when a through thickness crack is
detected. Due to friction the fracture is not on top of the punch
but to the side, so not in equi bi-axial strain but more towards
plane strain. The depth of the punch penetration is measured. The
value of the Erichsen cupping index (IE) is the average of a
minimum of three individual measurements, expressed in millimetres
and for the present invention is deemed satisfactory when EI>10
mm.
[0070] Weldability: Resistance spot welding is the major joining
technique used in the automotive industry, with an average car
containing around 2000-3000 spot welds. Traditionally spot welds
have always been a very cheap and reliable joint type, however
since the introduction of AHSS, this reliability has been
compromised. The weldability is measured by the ability of the
material to be spot-welded. Welding conditions were taken from
BS1140: 1993 which are standard for industry, although not
necessarily optimised for AHSS. Spot-weldability is measured by the
failure mode of the resultant spot-weld (plug). When a material
cannot be welded then the plug will split along the interface
between the two joining surfaces. In a fully welded material the
failure will be in the parent metal, outside of the plug and
preferably also outside the heat-affected zone. This is known as
full-plug failure, that is the full plug is pulled out of the
parent metal. Spot-weldability can be expressed on the scale
between full-interface failure and full-plug failure with the
former being deemed un-weldable.
[0071] One of the aims of the present invention is to provide a
high strength hot dip galvanised steel strip that has a formability
in the range of a 600 MPa AHSS hot dip galvanised steel strip, but
having a strength level of at least 700 MPa. This is achieved by
realising a suitable increase in the uniform elongation and
n-value.
[0072] During the development of the high strength hot dip
galvanised steel strip according to the invention a number of coils
of strip have been produced along with comparative examples. The
chemistry of the different alloys is given in Table 1 (in
10.sup.-3%). The processing conditions along with the resultant
mechanical properties are given in Table 2. In Table 2 RA indicates
retained austenite, M indicates martensite, and F indicates ferrite
plus bainite.
TABLE-US-00001 TABLE 1 Chemical composition of invented steel and
some comparative examples Alloy C Mn Al Cr Nb Si Notes A 154 1660
570 400 0 120 Example B 143 1944 680 103 21 72 Invention C 156 2054
729 105 21 60 Invention D 151 1730 580 110 0 410 Example E 150 1880
610 519 22 430 Example F 155 2027 707 92 20 57 Invention
TABLE-US-00002 TABLE 2 Processing conditions and resultant
mechanical properties of invented steel and some comparative
examples Iso- Anneal thermal temp holding % Rp Rm n- EI, Alloy
.degree. C. temp .degree. C. % F % M RA MPa MPa Ag % A80 % value
HEC % mm A 800 420 71 7 12 345 701 18.8 23.4 0.210 B 780 360 79 12
9 457 727 17.6 25.5 0.178 B 780 380 78 11 11 487 738 16.8 22.5
0.174 B 790 360 78 11 11 490 751 17.4 26.1 0.174 35 10.8 B 790 370
79 11 10 502 750 14.8 19.0 0.173 42 10.6 B 800 420 76 12 12 523 751
15.4 18.5 0.180 B 810 380 82 10 8 489 701 16.4 21.3 0.187 B 830 480
78 12 10 492 754 15.6 21.4 0.159 38 10.7 B 830 490 81 15 4 532 804
13.0 17.3 0.145 C 815 385 81 10 9 534 784 15.3 19.0 0.17 37 10.5 C
820 440 80 11 9 524 791 14.7 18.1 0.17 33 10.1 D 800 420 90 5 5 442
702 21.3 26.6 0.239 E 830 420 80 10 10 427 772 14.7 20.8 0.158 25
9.5 F 780 400 82 9 9 471 787 15.0 20.7 0.15 F 780 410 82 10 8 454
781 14.6 19.2 0.17 32 10.4 F 790 410 79 12 9 496 795 15.3 20.1
0.16
[0073] Table 1 shows six different alloys A to F, of which alloys
B, C and F have a composition according to the invention and alloys
A, D and E are comparative examples. For alloys A, D and E
processing conditions according to the invention have been applied
as shown in Table 2. The mechanical properties of the alloys A and
E are clearly outside the values as desired for according to the
invention, because for alloy A the Rp value is below 400 MPa, and
for alloy E the hole expansion coefficient is only 25% and the
Erichsen cupping index is less than 10. Alloy D has a composition
according to the invention, but the Si level is too high. Table 2
shows that the mechanical properties as measured fall within the
values as desired for according to the invention, though the Rm
value is only just above the lower limit of 700 MPa, and clearly
lower than the preferred value of at least 750 MPa. However, the
structure of alloy D is not in accordance with the desired
structure, because the amount of martensite and retained austenite
is too low.
[0074] Alloys D and E both have Si contents which are well outside
of the range of the invention which leads to high rolling loads
thereby reducing the available dimensions for the final
product.
[0075] Alloy B is an alloy according to the invention. For this
alloy, different processing conditions have been applied as shown
in Table 2. Different annealing temperatures and different
isothermal holding temperatures result in mechanical properties
that fulfil the desired values according to the invention as
regards the ultimate tensile strength and the 0.2% proof strength,
though desired total elongation is not always met. This occurs when
the overaging temperature is above the optimum bainite holding
temperature (i.e. above 480.degree. C.), thus reducing the amount
of retained austenite available.
[0076] Alloy C is also an alloy according to the invention. For
this alloy, two different processing conditions have been applied
as shown in Table 2. For both processing conditions, the ultimate
tensile strength Rm is more than 750 MPa and the total elongation
is more than 18%. The Erichsen Index is more than 10, the n-value
is 0.17 and the HEC is above 30.
[0077] Alloy F is another alloy according to the invention. Three
different processing conditions have been applied, as shown in
Table 2, and in all three cases the ultimate tensile strength is
more than 750 MPa, the total elongation is even more than 19%, and
the n-value is 0.15 or more. Where measured, the Erichsen Index is
more than 10 and the HEC is above 30.
[0078] Hole expansion coefficient HEC and Erichsen cupping index EI
are not always measured but where measured the value is
satisfactory. Also the work hardening coefficient or n-value is
good in all cases.
[0079] The TRIP assisted dual phase steel according to the
invention with low amounts of silicon thus shows to be a steel type
that gives satisfactory mechanical properties, contrary to the
expectation by the person skilled in the art that a certain amount
of added silicon is always needed.
* * * * *