|
Patent Abstract
Hot-rolled steel with very high elasticity limit and mechanical
resistance usable in particular for auto parts production, characterized
by the following composition by weight: 0.08%<carbon<0.16%
1%<manganese<2% 0.02%<aluminum<0.1% silicon<0.5%
phosphorus<0.03% sulfur<0.01% vanadium<0.3% chromium<1%
nitrogen<0.015% molybdenum<0.6% [top to bottom] Temperature
(.degree. C.) rolling V (.degree. C./S) Cooling temperature Time(s)
2/2 [vertical] CONSTRAINT in MPa [horizontal] EXPANSION in %
Patent Claims
1. Hot-rolled steel with very high maximum elasticity and mechanical
resistance usable in particular for producing auto parts, characterized
by the following composition by weight: 0.08%<carbon<0.2%
1%<manganese<2% 0.02%<aluminum<0.1% silicon<0.5%
phosphorus<0.03% sulfur<0.01% vanadium<0.3% chromium<1%
nitrogen<0.015% molybdenum<0.6% the rest being of steel and
impurities inherent in processing.
2. Steel according to claim 1, characterized by the following composition
by weight: 0.1%<carbon<0.14% 1.4%<manganese<1.8% 0.02%<aluminum<0.08%
0.15%<silicon<0.3% phosphorus<0.03% sulfur<0.008% 0.1%<vanadium<0.3%
0.3%<chromium<0.6% nitrogen<0.012% 0.15<molybdenum<0.4
the rest being of iron and impurities inherent in processing.
3. Process for producing a hot-rolled steel sheet strip with very
high resistance usable in particular to produce auto parts and characterized
in that the steel has the following composition by weight: 0.08%<carbon<0.16%
1%<manganese<2% 0.02%<aluminum<0.1% silicon<0.5%
phosphorus<0.03% sulfur<0.01% vanadium<0.3% chromium<1%
nitrogen<0.015% molybdenum<0.6% the rest being of iron and
impurities inherent in processing is subjected to: rolling at a
temperature below 950.degree. C. and preferably below 880.degree.
C., cooling carried out at a rate of more than 20.degree. C. per
second and preferably at a rate ranging from 100.degree. C. to 200.degree.
C. per second up to a temperature ranging from 400.degree. C. to
600.degree. C., and preferably up to a temperature ranging from
450.degree. C. to 500.degree. C.
Patent Description
[0001] The invention relates to a hot-rolled steel with very high
elasticity limit and mechanical resistance usable in particular
for auto parts production.
[0002] In the field of hot-rolled steel sheet production, a steel
whose characteristics are obtained by controlled rolling, products
with high elasticity limits is known, i.e., 315 MPa to 700 MPa.
[0003] In the field of hot-rolled metal sheets produced from a
strip train, the operating performance of parts obtained by molding
from these sheets is an important criterion because it defines the
useful life of the molded parts, for example, by stamping, profiling
or hydroforming.
[0004] Operating performance in connection with performance with
fatigue defines the useful life for specific charges.
[0005] To improve the fatigue resistance of molded parts, one solution
consists of using very high-resistance steels with considerable
fatigue resistance properties, because at first glance, there is
a proportional relationship between maximum endurance and mechanical
resistance. Nevertheless, the steel must be able to be stamped.
In general, molding properties decrease as mechanical resistance
increases, thereby limiting the molding possibilities for parts
able to be manufactured from high-resistance steel.
[0006] Shock-resistance is also an important characteristic for
reasons of safety, namely in automobile applications, since shock-resistance
defines the resistance to sudden breakage of the parts. To improve
this characteristic of molded parts, a solution consists of using
very high-resistance steels with considerable fatigue resistance
properties, because at first glance, there is a linear relationship
between shock-resistance and maximum elasticity. However, molding
properties generally decrease as maximum elasticity increases.
[0007] In the range of common flat hot-rolled products, the mechanical
characteristics of which are obtained by controlled rolling on a
wide-strip train, in particular there are four primary categories
of steel with superior mechanical characteristics.
[0008] HLE steels with high maximum elasticity are micro-alloy
steels with maximum elasticity ranging from 315 MPa to 700 MPa,
but having limited moldability in particular because of a high Re/Rm
ratio greater than 0.85. These steels have a carbureted ferrite-phase
structure of the cementite kinds. The elasticity maximum is obtained
by controlled rolling and precipitation of the micro-alloying elements
such as niobium, vanadium and titanium during ferrite transformation.
[0009] Dual-phase steels are steels with a ferrite martensite structure
with noteworthy molding properties. Mechanical resistance levels
generally range from 550 MPa to 800 MPa. The highest level is obtained
by precipitation of micro-alloying elements during the ferrite transformation
that completes the hardening effect of martensite.
[0010] HR steels are steels referred to as high-resistance, with
carbon and manganese and undergoing relatively rapid cooling after
rolling, along with low-temperature coiling, to give them a ferrite-baintic
structure. Their intermediate moldability level is between that
HLE steels and that of dual-phase steels. Resistance levels range
from 450 MPa to 800 MPa.
[0011] Martensite steels have the highest resistance levels. These
steels have a martensite structure obtained by heat treatment after
rolling. It is difficult to produce this kind of structure on a
wide-strip train because of the fragility of martensite, which causes
the strip to break after rolling. Martensite steels make it possible
to achieve resistance levels above 1,000 MPa but with very low ductility
levels and expansions of less than 8%. In addition, a heat treatment
must be carried out after rolling.
[0012] Increasing the resistance level of all of the above-mentioned
steels entails an increase in rolling efforts, thereby limiting
the reduction in thickness and not allowing the full benefits of
alloying.
[0013] The goal of the invention is to present a hot-rolled steel
with very high maximum elasticity mechanical resistance and good
molding characteristics to produce parts by stamping, profiling
and hydroforming, namely for the auto industry.
[0014] The object of the invention is a hot-rolled steel with very
high maximum elasticity and mechanical resistance usable in particular
for producing auto parts, characterized by the following composition
by weight:
[0015] 0.08%<carbon<0.2%
[0016] 1%<manganese<2%
[0017] 0.02%<aluminum<0.1%
[0018] silicon<0.5%
[0019] phosphorus<0.03%
[0020] sulfur<0.01%
[0021] vanadium<0.3%
[0022] chromium<1%
[0023] nitrogen<0.015%
[0024] molybdenum<0.6%
[0025] the rest being of steel and impurities inherent in processing.
[0026] The steel is preferably characterized by the following composition
by weight:
[0027] 0.1%<carbon<0.14%
[0028] 1.4%<manganese<1.8%
[0029] 0.02%<aluminum<0.08%
[0030] 0.15%<silicon<0.3%
[0031] phosphorus<0.03%
[0032] sulfur<0.008%
[0033] 0.1%<vanadium<0.3%
[0034] 0.3%<chromium<0.6% nitrogen<0.012%
[0035] 0.15<molybdenum<0.4
[0036] the rest being of iron and impurities inherent in processing.
The invention also relates to a process for producing a hot-rolled
steel sheet strip with very high resistance usable in particular
to produce auto parts and characterized in that the steel has the
following composition by weight:
[0037] 0.08%<carbon<0.2%
[0038] 1%<manganese<2%
[0039] 0.02%<aluminum<0.1%
[0040] silicon<0.5%
[0041] phosphorus<0.03%
[0042] sulfur<0.01%
[0043] vanadium<0.3%
[0044] chromium<1%
[0045] nitrogen<0.015%
[0046] molybdenum<0.6%
[0047] the rest being of iron and impurities inherent in processing
is subjected to:
[0048] rolling at a temperature below 950.degree. C. and preferably
below 880.degree. C.,
[0049] cooling carried out at a rate of more than 20.degree. C.
per second and preferably at a rate ranging from 100.degree. C.
to 200.degree. C. per second up to a temperature ranging from 400.degree.
C. to 600.degree. C., and preferably up to a temperature ranging
from 450.degree. C. to 500.degree. C.
[0050] The following description and the attached figures, all
provided as non-limitative examples, will make the invention well
understood.
[0051] FIG. 1 is a schematic illustration showing the temperature
change as a function of time during cooling of the hot-rolled steel
strip.
[0052] FIG. 2 shows an expansion curve as a function of constraint
for steel according to the invention.
[0053] The steel according to the invention, with the following
composition by weight:
[0054] 0.08%<carbon<0.2%
[0055] 1%<manganese<2%
[0056] 0.02%<aluminum<0.1%
[0057] silicon<0.5%
[0058] phosphorus<0.03%
[0059] sulfur<0.01%
[0060] vanadium<0.3%
[0061] chromium<1%
[0062] nitrogen<0.015%
[0063] molybdenum<0.6%
[0064] the rest being of iron and impurities inherent in processing,
has an entirely bainite structure. In this form, resistance levels
of greater than 1,000 MPa with expansions exceeding 10% can be attained.
[0065] The steel molded from a hot-rolled strip according to the
invention is subjected to:
[0066] rolling at a temperature below 950.degree. C. and preferably
below 880.degree. C.,
[0067] cooling carried out at a rate of more than 20.degree. C.
per second and preferably at a rate ranging from 100.degree. C.
to 200.degree. C. per second up to a temperature of 400.degree.
C. to 600.degree. C., preferably up to a temperature of 450.degree.
C. to 500.degree. C.
[0068] As shown in the diagram of FIG. 1, the cooling cycle, starting
from a temperature of 400.degree. C. to 600.degree. C., preferably
up to a temperature of 450.degree. C. to 500.degree. C., is carried
out on coil.
[0069] From the perspective of the composition of the steel according
to the invention:
[0070] carbon limited to 0.2% ensures good welding while allowing
hardening by precipitation with the vanadium.
[0071] manganese makes it possible to lower the transformation
points AR3, Bs and Ms corresponding to the starting temperature
for ferrite transformation, the starting temperature for bainite
transformation, and the starting temperature for martensite transformation,
respectively. With this effect, it enables hardenability to be increased
while avoiding the forming of ferrite due to the high cooling speeds
and to obtain an entirely bainite structure. The lowered start of
bainite transformation allows the mechanical properties to be increased.
[0072] aluminum is used to calm the steel down.
[0073] silicon is kept at relatively low levels to benefit from
the hardenability in solid solution it provides without degrading
the surface condition after pickling, or the product's ability to
be coated on a continuous galvanizing or electro-zincing line. Silicon
is known to degrade the surface appearance of pickled products by
forming Fe.sub.2O.sub.3SiO.sub.4 on the one hand, and on the other
hand, degrading wettability and thus clinging to clothes.
[0074] molybdenum, due to its hardening effect, namely a reduction
of Bs, enables the mechanical properties to be increased by forming
a fully bainite structure.
[0075] vanadium is the element needed to form precipitate of nitride
and carbide type, which form at different temperatures during the
course of the heat treatment. These very hardening precipitates
allow to obtain the very high level of mechanical properties This
element makes this hardening possible by precipitation without increasing
hardness when hot. This effect runs contrary to the known effects
of micro-alloying elements which, by precipitation induced during
rolling, cause an increase in said hardness when hot. This finding
enabled the inventors, with the element vanadium contained in the
steel according to the invention, to roll thin sheets down to 1.4
mm thick without increasing the rolling efforts.
[0076] The examples below show the results obtained for an example
B applied according to the invention and supported by two comparative
examples, A and C, one comprising a low vanadium level and the other
with a high level of vanadium.
[0077] The compositions of the examples are shown in table 1 below:
1TABLE I Steel C Mn Cr Mo Si N V P A 0.11 1.58 0.51 0.33 0.2 0.0035
0 0.02 B 0.11 1.58 0.51 0.32 0.2 0.0040 0.2 0.02 C 0.11 1.58 0.51
0.34 0.2 0.0050 0.45 0.02
[0078] Table 2 below provides the conditions for heat treatment
after hot rolling.
2TABLE 2 Rolling Cooling Coiling Steel temperature .degree. C.
temperature .degree. C./s temperature .degree. C. A 900 65 450 B
885 40 450 C 890 50 450
[0079] Table 3 below shows the mechanical characteristics in mechanical
resistance Rm, maximum elasticity Re, and expansion A, of the three
forms of construction.
3 TABLE 3 Rm Re A Steel (MPa) (MPa) (%) A 790 670 14 B 1,090 990
10.4 C 1,125 1,015 8.9
[0080] It can be seen that the vanadium increases mechanical resistance
and reduces expansion. Vanadium is the necessary element in steel
with a bainite structure in order to produce a hardening effect,
something that was not expected since the micro-alloying elements
have a hardening effect by precipitation but this precipitation
ends at a higher temperature and must be carried out in the ferrite
domicile in order to be hardening. This effect cannot be obtained
by other micro-alloying elements such as titanium or niobium because
these elements cause an increase in hardness when hot, thus limiting
the hot-rolling reduction rates and thus the minimum thickness achievable
for this kind of sheet metal. It turns out that vanadium has no
effect on hardness when hot.
[0081] Other residual elements may be present and used according
to their known properties such as Cu and Ni. Added alloying elements
such as titanium or boron can be used to promote the precipitation
of vanadium carbides at the expense of vanadium nitrides. Titanium
and boron form nitrides at high temperature, which remain stable
during the subsequent heat treatment.
[0082] Industrial experiments were conducted based on analysis
B presented in table No. 4.
4TABLE NO. 4 C % Mn % Cr % N % V % Mo % Al % Si % 0.124 1.560 0.389
0.0051 0.189 0.280 0.031 0.185
[0083] An example of mechanical property obtained for a thickness
of 1.7 mm is shown in FIG. 2 by means of a traction curve.
[0084] The characteristics of the steel are 1,015 MPa mechanical
resistance, 880 MPa maximum elasticity and 12% expansion.
[0085] The final structure of the steel according to the invention
is a bainite structure. This structure makes it possible to achieve
maximum elasticity greater than 700 MPa, mechanical resistance greater
than 1,000 MPa and expansion greater than 10%. These values show
the good molding properties of the steel according to the invention.
[0086] The invention makes it possible to roll a steel 1.4 to 5
mm thick with high mechanical resistance, i.e., above 1,000 MP,
as well as noteworthy molding characteristics, thanks to an expansion
of more than 10%.
[0087] The flawless surface condition after pickling of the hot-rolled
steel is ensured by a silicon content in the steel's composition
of less than 0.5%.
[0088] The hot-rolled steel sheet strip of the invention is advantageous
in its use in sectors of activity such as the auto industry and
mechanical construction in general, for stamped, folded, profiled
or hydroformed parts, which can be expanded while ensuring fatigue
resistance, improved shock resistance and a combination of these
advantages. |