[0001] The present invention is directed to a hot rolled dual phase steel strip, having
features similar to those of a corresponding cold rolled dual phase steel strip.
[0002] Low carbon steel strips of the dual phase type (ferrite-martensite) are known, being
cold rolled, which have special geometrical, and metallurgical features, as well as
relating to planarity and deformability, so as to render the same particularly adapted
to the production of pressed or cut pieces requiring very strict tolerances, particularly
when designed for the car manufacturing industry with a thickness of more than 1.0
mm.
[0003] It is also known that the dual phase steel strip obtained by hot rolling, such as
according to the method disclosed in patents EP 0019193, EP 0072867, US 4790889 and
US 4561910, do not show features of quality, particularly relating to their cold workability,
that can be considered comparable with those of dual phase steel strips obtained by
cold rolling.
[0004] A basic feature for this product, especially when it is intended to form structural
portions in the car industry field, is in fact the tendency to be cold shaped, as
well as a good mechanical resistance being fit to absorb shocks as a consequence of
the crash tests recently developed in the car industry. It has been found that these
steels must show a microstructure mainly formed of ferrite and, as a slightest portion,
of martensite or bainite, i.e. a structure of high hardness that can be obtained by
suddenly cooling the steel from an intercritic temperature comprised between 700 and
800°C. This way the residual austenite enriched with carbon is converted into martensite
or bainite, giving rise to grains formed of very hard and brittle needle-shaped structures
which, when inserted in a much softer ferritic matrix allow cold shaping of pieces,
even of complex shape, being present in a very low percentage, never higher than 20%
(martensite) and 30% (bainite).
[0005] It is also known that this type of steel requires significant additions of chromium
and phosphorous, especially the first mentioned element in order to increase the steel
capacity of being hardened and to enhance the production of carbides, whereas the
second mentioned element is added to make ferrite harder and cause the yield point
to raise. Both elements have also the effect of increasing the tensile strength.
[0006] As already stated above, these products are generally derivating from cold rolled
and continuously annealed strips, while just during the cooling step after annealing
the desired dual phase structure is obtained to achieve the above-mentioned features.
On the other hand this type of processing, with cold rolling and subsequent annealing,
involves rather important burdens as far as the required costs and time are concerned,
whereby it is a steadily more and more felt need in this field that of obtaining a
hot rolled strip in dual phase steel which is provided with the same mechanical features
of the traditional cold rolled steel.
[0007] An object of the present invention is therefore that of providing a steel strip of
the above-mentioned type which, unlike the other cold rolled dual phase steels being
known so far, has the same features and may replace without problems a cold rolled
dual phase steel strip, in particular for cold pressed or cut pieces.
[0008] Another object of the present invention is that of providing a steel strip that,
even without important additions of chromium and phosphorous, is provided with the
same qualities as mentioned, which are peculiar of the steels wherein considerable
amounts of these two elements are present.
[0009] The strip according to the present invention is preferably, although not exclusively,
produced by means of in-line plants of the thin-slab type, as disclosed in EP 0415987
in the name of the present applicant and schematically illustrated in figure 1 and
is characterized, as set forth in claim 1, by a carbon content comprised between 0.06
and 0.15%, manganese between 1.0 and 2.0% with a chemical composition poorer than
that of the strip of this type according to the prior art, without important additions
of chromium and phosphorous, as well as by a constant geometrical profile along the
whole length, with low tolerances relating to the thickness, comparable with those
typical of a cold rolled strip.
[0010] Further objects, advantages and features of the dual phase steel strip according
to the present invention will be clearer from the following description with reference
to the annexed drawings in which:
Figure 1 schematically shows a casting and in-line rolling plant of the thin-slab type, particularly
suitable for manufacturing steel strips according to the invention;
Figure 2 shows a graph representing mechanical features, particularly relating to the cold
pressing, of a dual phase steel strip according to the invention when compared with
a cold rolled strip of the same thickness; and
Figure 3 shows a diagram of the variations, graphically obtained by points, of the frequency
with which the presence of certain dimensions of the ferritic grain is statistically
detected in a number of coils.
[0011] As stated in the foregoing, the dual phase steel strip according to the present invention
is preferably, although not exclusively, manufactured in thin-slab plants as schematically
shown in figure 1, where reference is particularly made to the plant being the object
of patent EP 0415987. The following processing steps can be distinguish therein, downstream
of the continuous casting step: a) liquid core reduction; b) roughing step directly
adjoining to the continuous casting; c) heating in an induction furnace; d) keeping
temperature in a furnace provided with internal mandrel; e) finishing rolling; f)
compact controlled cooling; and g) coiling on a reel. It has been found in fact that
the particular working conditions, typical of this plant, give the final product a
particularly thin and homogeneous structure with positive consequences on the chemical-physical
characteristics of the final product itself.
[0012] The features that, as set forth in claim 1, should be shown by the product, i.e.
the hot rolled low carbon steel strip with a dual phase structure (formed of either
ferrite and martensite or ferrite and bainite), are basically: a thickness ≥ 1.0 mm
with tolerances comprised between ± 0.06 mm and ± 0.12 mm up to thicknesses ≤ 8.0
mm, a parallelism < 0.05 mm and a structure with grain fineness better than grade
10 of the ASTM E 112 standard.
[0013] In the following table there are indicated, for various thicknesses from 1.5 to 8
mm, the corresponding standard tolerances, respectively for the usual hot coils, the
cold rolled strips (distinguished between standard and strict tolerances) and the
tolerances pertaining to a dual phase strip according to the invention. In the last
column there are also indicated the crown or convexity values, i.e. corresponding
to the differences between the values of thickness measured centrally and on the side
edges of the strip.
Thickness |
Standard Tolerances |
Tolerance of the strip of the invention |
EN 10051 |
EN 10031 Cold Strips |
Hot Tolerances |
Max Crown |
Hot Coils |
Standard |
Strict |
≥1.50 |
+/- 0.17 |
+/-0.11 |
+/-0.08 |
+/-0.06 |
0.03 |
1.51 - 2.00 |
+/- 0.17 |
+/-0.13 |
+/-0.09 |
+/-0.07 |
0.04 |
2.01 - 2.50 |
+/- 0.18 |
+/-0.15 |
+/-0.11 |
+/-0.10 |
0.04 |
2.51 - 3.00 |
+/- 0.20 |
+/-0.17 |
+/-0.12 |
+/-0.11 |
0.05 |
3.01 - 4.00 |
+/- 0.22 |
|
|
+/-0.12 |
0.06 |
4.01 - 5.00 |
+/- 0.24 |
|
|
+/-0.12 |
0.06 |
5.01 - 6.00 |
+/- 0.26 |
|
|
+/-0.12 |
0.07 |
6.01 - 8.00 |
+/-0.29 |
|
|
+/-0.15 |
0.07 |
[0014] It is easy to see that the tolerances, as detected for the hot rolled steel strip
according to the present invention not only correspond on average to less than one
half of the tolerances relating to the traditional hot rolled strips, but are even
lower than the strict tolerances of the cold strips having the same thickness.
[0015] Furthermore with reference to figure 3, it can be observed from a microcrystalline
analysis of the structure of a steel strip according to the invention that more than
80% of the grains, detected on average at various positions on the strip and statistically
for a number of strips, has lower dimensions than those corresponding to grade 10
of the ASTM E112 standard, and consequently a better fineness than that grade.
[0016] These features, together with a breaking strain > 20%, make this type of hot rolled
strip particularly suitable for fine shearing and hole formation by punching, as well
as cold stamping of complex shapes. In particular it has been practically proved that
with strips according to the invention it has been possible to form bends at right
angles and 180° with a radius ≤ 3 times the strip thickness for thicknesses ≤ 3.0
mm and ≤ 5 times the thickness for strips having thickness ≥ 3.1 mm without giving
rise to defects in the region of maximum stress, this confirming the good plasticity
of the material. It is clear that these results have been made possible thanks to
the fine grain microstructure with homogeneous development of the grain in every direction,
or of the polygonal type, with complete separation of the iron carbides from the ferritic
grains. Such a structure eliminates any resilient recovery of the material upon shaping,
thus allowing to meet in this way very strict tolerances.
[0017] Experimental tests of forming capability have been carried out by comparison with
cold rolled strips of the same thickness. From these tests it appears, as resulting
from figure 2, that FLD lines of the Forming Limit Diagram relating to two different
steel strips can be overlapped, thus confirming that the strip according to the invention
can suitably replace a cold rolled one. The tests of forming capability which have
brought to the graphs of figure 2 have been carried out on a strip having thickness
of 1.0 mm, at room temperature with a mould having diameter of 100 mm and a stamping
speed of 1 mm/s.
[0018] Homogeneity and fineness of the microcrystalline structure therefore appear to be
the reasons of the particular deformability shown by this type of strip.
[0019] Finally a typical example of chemical analysis relating to the steel strip according
to the invention is reported in the following, while bearing in mind that it is not
the case of a binding composition except for the low carbon and manganese content,
without important additions of chromium and phosphorous, contrary to the situation
in the known dual phase steels: C 0.06-0.15%, Mn 1.0-2.0%, Si ≤ 0.80%, P ≤ 0.010%,
S ≤ 0.005%, Cr < 0.30%, Ni ≤ 0.30%, Mo ≤ 0.03%, Al 0.030 ÷ 0.050%.
[0020] It should be noted that in the case of the present invention the percentage at which
the chromium and phosphorous elements are present can be limited to the values stated,
without any necessity of high amounts of these elements being added, although the
same good qualities are maintained, thanks to the fact that the temperature of slab,
pre-strip and rolled strip never goes below the critical values beyond which the chromium
carbides precipitate and phosphorous is separated from the solid solution.
1. A hot rolled, low carbon dual phase steel strip, with a structure composed of ferrite
and martensite or ferrite and bainite, having a thickness ≥ 1.0, particularly suitable
for producing cold pressed and cut pieces requiring mechanical features of forming
capability and a very small resilient recovery, characterized by consisting of peritectic steel with chemical analysis without any important addition
of chromium and phosphorous, having a constant geometrical profile on the whole length
and thickness tolerances of less than 0.05 mm, with a crown between the strip centre
and side edge lower than 0.07 mm, being provided with a homogeneous microcrystalline
structure with fineness better than grade 10 of ASTM E 112 standard at a percentage
higher than 80% of the whole structure.
2. A dual phase steel strip according to claim 1, characterized by having a coefficient of breaking strain > 20%.
3. A dual phase steel strip according to claim 1, having the following composition: C
0.06-0.15%, Mn 1.0-2.0%, Si ≤ 0.80%, P ≤ 0.010%, S ≤ 0.005%, Cr < 0.30%, Ni ≤ 0.30%,
Mo ≤ 0.03%, Al 0.030 ÷ 0.050%.
4. A dual phase steel according to claim 1, resulting from an in-line thin-slab plant
comprising, downstream of the continuous casting step, a first liquid core reduction,
a roughing step, heating in an induction furnace and subsequent keeping of the temperature
in a furnace with internal mandrel before finishing rolling, as well as compact controlled
cooling and final coiling on a reel.