U.S. patent application number 12/745798 was filed with the patent office on 2011-02-03 for steel for high-strength components made of bands, sheets or tubes having excellent formability and particular suitability for high-temperature coating processes.
This patent application is currently assigned to SALZGITTER FLACHSTAHL GMBH. Invention is credited to Volker Flaxa, Joachim Schotter.
Application Number | 20110024006 12/745798 |
Document ID | / |
Family ID | 40419155 |
Filed Date | 2011-02-03 |
United States Patent
Application |
20110024006 |
Kind Code |
A1 |
Schotter; Joachim ; et
al. |
February 3, 2011 |
STEEL FOR HIGH-STRENGTH COMPONENTS MADE OF BANDS, SHEETS OR TUBES
HAVING EXCELLENT FORMABILITY AND PARTICULAR SUITABILITY FOR
HIGH-TEMPERATURE COATING PROCESSES
Abstract
A steel for high-strength components including bands, sheets or
pipes having excellent formability and particular suitability for
high-temperature coating processes above Ac.sub.3 (about
900.degree. C.) is disclosed. The steel includes the following
elements (contents in % by mass): C 0.07 to .ltoreq.0.15,
Al.ltoreq.0.05, Si.ltoreq.0.80, Mn 1.60 to .ltoreq.2.10,
P.ltoreq.0.020, S.ltoreq.0.010, Cr 0.50 to .ltoreq.1.0, Mo 0.10 to
.ltoreq.0.30, Ti.sub.min 48/14.times.[N], V 0.03 to .ltoreq.0.12, B
0.0015 to .ltoreq.0.0050, with the balance iron including usual
steel-accompanying elements.
Inventors: |
Schotter; Joachim;
(Braunschweig, DE) ; Flaxa; Volker; (Salzgitter,
DE) |
Correspondence
Address: |
HENRY M FEIEREISEN, LLC;HENRY M FEIEREISEN
708 THIRD AVENUE, SUITE 1501
NEW YORK
NY
10017
US
|
Assignee: |
SALZGITTER FLACHSTAHL GMBH
Salzgitter
DE
|
Family ID: |
40419155 |
Appl. No.: |
12/745798 |
Filed: |
November 5, 2008 |
PCT Filed: |
November 5, 2008 |
PCT NO: |
PCT/DE2008/001845 |
371 Date: |
August 17, 2010 |
Current U.S.
Class: |
148/648 ;
148/330; 420/106 |
Current CPC
Class: |
C22C 38/32 20130101;
C22C 38/22 20130101; C22C 38/38 20130101; C22C 38/24 20130101 |
Class at
Publication: |
148/648 ;
420/106; 148/330 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/32 20060101 C22C038/32; C22C 38/22 20060101
C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2007 |
DE |
10 2007 058 222.8 |
Claims
1.-13. (canceled)
14. A steel for high-strength components of bands, sheets or pipes,
comprising by mass percent: TABLE-US-00007 C 0.07 to .ltoreq.0.15
Al .ltoreq.0.05 Si .ltoreq.0.80 Mn 1.60 to .ltoreq.2.10 P
.ltoreq.0.020 S .ltoreq.0.010 Cr 0.50 to .ltoreq.1.0 Mo 0.10 to
.ltoreq.0.30 Ti.sub.min 48/14 .times. [N] V 0.03 to .ltoreq.0.12 B
0.0015 to .ltoreq.0.0050,
with balance iron, including usual steel-accompanying elements.
15. The steel of claim 14, having a C-content of 0.08 to
.ltoreq.0.10%.
16. The steel of claim 14, having a Si-content of
.ltoreq.0.30%.
17. The steel of claim 14, having a Mn-content of 1.80 to
.ltoreq.2.0%.
18. The steel of claim 14, having a Cr-content of 0.70 to
.ltoreq.0.80%.
19. The steel of claim 14, having a Mo-content of 0.15 to
.ltoreq.0.25%.
20. The steel of claim 14, having a Ti-content of 0.02 to
.ltoreq.0.03%.
21. The steel of claim 14, having a V-content of 0.05 to
.ltoreq.0.10%.
22. The steel of claim 14, having a B-content of 0.0025 to
.ltoreq.0.0035%.
23. The steel of claim 14 for application in a high-temperature
coating process above Ac.sub.3 (about 900.degree. C.).
24. A component formed from a readily formable band, sheet or tube
made of a steel comprising, in mass percent: TABLE-US-00008 C 0.07
to .ltoreq.0.15 Al .ltoreq.0.05 Si .ltoreq.0.80 Mn 1.60 to
.ltoreq.2.10 P .ltoreq.0.020 S .ltoreq.0.010 Cr 0.50 to .ltoreq.1.0
Mo 0.10 to .ltoreq.0.30 Ti.sub.min 48/14 .times. [N] V 0.03 to
.ltoreq.0.12 B 0.0015 to .ltoreq.0.0050,
with balance iron, including usual steel-accompanying elements,
wherein the component is, after forming, heat-treated at a
temperature above Ac.sub.3 (about 900.degree. C.) and has after
cooldown a minimum yield strength of 450 MPa.
25. The component of claim 24, wherein the heat treatment includes
enameling with one or more firings.
26. The component of claim 24, wherein the heat treatment includes
a metallic coating.
27. The component of claim 26, wherein the coating is a
zinc-plating.
28. A method of making a component, comprising the steps of:
forming a structure selected from the group consisting of band,
sheet, and tube into a shape; heat-treating the structure at a
temperature above Ac.sub.3 (about 900.degree. C.); and allowing the
structure to cool down to produce a component with a minimum yield
strength of 450 MPa.
Description
[0001] The invention relates to steel for high-strength components
made of bands, sheets or pipes having excellent formability and
particular suitability for high-temperature coating processes
according to claim 1. The term high temperature in this context
indicates temperatures above A.sub.c3 (about 900.degree. C.).
[0002] Modern lightweight construction of components made of steel
intended to have the greatest possible resource utilization by way
of maximum weight savings increasingly requires the use of
high-strength steels.
[0003] This applies, for example, to the tinplate or sanitary
industry, the construction of chemical equipment, the power plant
technology and more particularly the automobile industry with the
goal to reduce the fleet fuel consumption.
[0004] Components made of high-strength steels employed in the
automobile industry are typically coated with corrosion-inhibiting
coatings, predominantly made of zinc. In other of the
aforementioned application fields, enamel coatings are also used in
addition to corrosion-inhibiting coatings.
[0005] Semi-finished goods, such as bands or sheets made of
conventional high-strength steels for these application fields are
predominantly produced by thermo-mechanical rolling. This requires
that the steels are not subjected to additional heat treatment in
subsequent processing steps, because the mechanical properties
obtained with the thermo-mechanical treatment would otherwise be
lost.
[0006] If the steels are subjected to a subsequent thermal
treatment where, for example, a corrosion-inhibiting layer in form
of enamel or metallic coatings made of zinc, aluminum or their
alloys is applied at treatment temperatures reaching the values
higher than A.sub.c3 (about 900.degree. C.), then these steels
loose their original strength. This situation occurs likewise also
in similarly heat-treated zones after welding.
[0007] This phenomenon is repeated if multiple heat treatments are
preformed, for example with thermal coating methods, with
intersecting weld seams in the respective heat-treated region, as
well during repeated enamel firings typically performed during
enameling, causing the material to continuously loose strength.
[0008] The following Table 1 shows this phenomenon on the example
of the steel grade S-420 in 3.0 mm and 8.0 mm, respectively, with a
minimum yield strength of 420 MPa.
TABLE-US-00001 TABLE 1 Change in the mechanical properties of
sheets made of S-420 after 1 and 2 enamel firings, respectively.
Thickness, Sample R.sub.p0.2 - mm orientation State MPa R.sub.m -
MPa A.sub.80 - % 3.0 mm Longitudinal Initial state 444 569 21.3
after 1 anneal 430 521 25.1 after 2 anneals 405 522 25.9 Transverse
Initial state 502 592 20.9 after 1 anneal 453 537 25.1 after 2
anneals 439 537 23.0 8.0 mm Longitudinal Initial state 426 523 29.8
after 1 anneal 391 498 31.0 after 2 anneals 385 494 31.5 Transverse
Initial state 437 538 26.7 after 1 anneal 401 505 29.7 after 2
anneals 395 503 29.2
[0009] This loss in strength after corresponding heat treatment is
even more pronounced in high-strength multiphase steels, because
the original martensitic phase fraction disappears during heating
above the transition temperature A.sub.c3, if the cooldown is not
controlled and intensified.
[0010] Another problem which may occur with high-strength steels is
a significant increase in the solubility for hydrogen during
heating above A.sub.c3. The hydrogen then remains in the material
structure during accelerated cooldown which may cause formation of
cracks in the material.
[0011] For this reason, steels are in demand which produce a hard
structure also during slow cooldown (for example, in still
air).
[0012] These steels may have another problem in that egression of
hydrogen from the material is hindered by a thick protective layer,
such as enamel. If this is the case, then the coating may be at
risk of spalling (fish scales).
[0013] Fish scales indicate defects in the enamel, which no longer
guarantee a continuous protection of the steel substrate. A high
resistance of the enamel component against fish scales is therefore
important when enameling steel.
[0014] It is generally assumed that the occurrence of fish scales
is caused by contact of the steel surface with humidity from the
furnace atmosphere and from the enamel slurry during the enameling
process.
[0015] The reaction of water with the steel surface causes
formation of atomic hydrogen which diffuses into the steel during
the firing process.
[0016] After firing of the enamel at about 900.degree. C. and
subsequent cooldown, the solubility of hydrogen in steel decreases,
and the hydrogen is driven out of the steel and recombines at the
material boundary steel/enamel to form molecular hydrogen.
[0017] This reaction is accompanied by an increase in volume,
wherein locally a high pressure can be generated which finally
becomes so large that the yield strength of the composite
enamel/steel is exceeded and half-moon-shaped enamel splinters
(fish scales) occur on the enamel that has meanwhile
solidified.
[0018] For cold-rolled or hot-rolled sheets, a number of
conventional steels are known which are resistant to fish scale
formation. Because of the frequently required particular
deep-drawing properties, these steels are typically designed as
low-strength IF steels (e.g., EP 0 386 758 B1) and are based on
alloy concepts where the fish scale resistance is produced by
cementite precipitates broken down by cold-rolling at the grain
boundaries, with the atomic hydrogen accumulating at the cementite
precipitates and hence rendered harmless with respect to fish scale
formation.
[0019] High-strength, readily formable steels suitable for
high-temperature treatments, for example during the enameling, are
not known up till now. The requirements for a high-strength steel,
which need not always be satisfied at the same time, can be
summarized for the aforedescribed application fields as follows:
[0020] High material strength of the component after forming also
after heat treatment at temperatures above 900.degree. C., [0021]
Fish scale resistance after enameling. [0022] Good formability,
[0023] Generally good weldability, [0024] Good high-frequency
induction (HFI) and laser weldability in the production of pipes,
[0025] Suitable for zinc-plating of the component.
[0026] The invention is based on the invention to produce a
low-cost steel for high-strength components made of bands, sheets
or pipes, which has excellent formability and suitablility for
high-temperature coating methods, while simultaneously ensuring
general weldability and, more particularly, HFI weldability.
[0027] According to the teaching of the invention, this object is
obtained with a steel having the following composition in % by
mass:
TABLE-US-00002 C 0.07 to .ltoreq.0.15 Al .ltoreq.0.05 Si
.ltoreq.0.80 Mn 1.60 to .ltoreq.2.10 P .ltoreq.0.020 S
.ltoreq.0.010 Cr 0.50 to .ltoreq.1.0 Mo 0.10 to .ltoreq.0.30
Ti.sub.min 48/14 .times. [N] V 0.03 to .ltoreq.0.12 B 0.0015 to
.ltoreq.0.0050
with balance iron, including usual steel-accompanying elements.
[0028] The high-strength steel according to the invention is
designed as heat-treated steel which can be hardened in air or in a
medium with comparable cooldown gradients. The steel is
particularly suited for high-temperature coating methods, for
example in enameling or zinc-plating, even at treatment
temperatures above 900.degree. C., and is distinguished in that it
does not lose strength during cooldown after coating, but even
becomes stronger as a result of the heat treatment. It was
surprising to persons skilled in the art to observe in extensive
test series, that for the first time a steel could be provided with
the alloy composition of the invention, which has both an excellent
enameling ability and fish scale resistance, while attaining at the
same time a high strength as a result of the heat treatment during
enamel firing or during zinc-plating.
[0029] This comparatively very cost-effective alloying concept, in
particular the low carbon content, also produces excellent
cold-forming properties in the initial state "soft", which is of
particular importance for use with deep-drawn parts, for example in
sanitary installations for hot water heaters, in boiler
construction, in the construction of chemical equipment or in the
construction of automobile chassis.
[0030] The relatively low carbon equivalent furthermore ensures
excellent general weldability. Weldability is excellent, in
particular, with high-frequency induction welding (HFI welding), as
used for example in the production of pipes, because the chromium
content in the weld seam, which prevents unwanted chromium carbide
precipitates, is relatively small.
[0031] The fish scale resistance of the steel is attained with the
invention through addition of chromium and vanadium, wherein finely
dispersed precipitates of chromium and vanadium carbides or carbon
nitrites and titanium nitrites form hydrogen traps in the hard
structure of the steel, with the atomic hydrogen formed during
enameling accumulating at the hydrogen traps without damaging the
enamel.
[0032] The alloy concept based on Mn, Cr, Mo, V and B enables
temper-hardening of the steel already with a cooldown gradient that
is comparable to cooldown in air through an advantageous shift in
the relevant transformation points.
[0033] This presumes that, according to the present invention, the
existing nitrogen in the steel is completely bound in form of
titanium nitrites through addition of titanium, in order to prevent
boron nitrite precipitates and to thereby ensure the effectiveness
of the added boron. Accordingly, according to the invention, at
least a stoichiometric addition of titanium relative to the
nitrogen content must be maintained.
[0034] According to an advantageous embodiment of the invention,
the steel has a low Si-content of .ltoreq.0.30% for zinc-plating,
thereby ensuring suitability for zinc-plating, for example, for
applications in the automotive industry.
[0035] Conventional temper-hardened steels are known where
hardening is attained solely by cooldown of the steel in air, for
example after heat treatment of the component, in order to realize
the required material properties.
[0036] If the steel cools down after hot-rolling at least partially
in air so fast that the air hardening effect sets in, then
cold-formability can be attained by way of a subsequent
soft-annealing process, for example in a hood-type annealing
furnace, or by homogenizing annealing. Alternatively, the
cold-formability after hot-rolling can also be maintained by slowly
cooling a suitably tightly wound coil, optionally in a special
insulated hood.
[0037] After cold-forming or shaping, the temper-hardening
conditions can then again be adjusted by way of a subsequent heat
treatment.
[0038] The term cold-forming refers to the following process
variants: [0039] a) The direct production of corresponding
components from hot-band by deep drawing and the like with
subsequent optional heat treatment. [0040] b) Further processing
into pipes using suitable drawing and annealing processes. The
pipes themselves are subsequently made into components, for example
by bending, internal high-pressure forming (IHU) and the like, and
subsequently temper-hardened. [0041] c) Further processing of the
hot-band into cold-band with subsequent annealing and shaping
process. The cold-band is subsequently processed by deep-drawing
and the like, as described under a) or b).
[0042] The following Table 2 lists parameters measured on samples
of the steel according to the invention for hot-rolled and
cold-rolled sheets or bands, as well as pipes produced
therefrom:
TABLE-US-00003 TABLE 2 Change in the mechanical parameters of the
steel of the invention after enameling. R.sub.p0.2 [MPa] R.sub.m
[MPa] A.sub.5 [%] Cold band 1.5 mm Delivery state soft 339 494 35.1
After enameling 490 770 12.1 Hot band 4.6 mm Delivery state soft
336 528 33.4 After enameling 475 740 12.2
[0043] The pickling removal and the fish scale resistance of sheets
made of the steel of the invention as well as of three comparative
steels with higher strengths were tested with respect to their
suitability for enameling.
[0044] The test results for the suitability of the steel of the
invention for enameling in comparison to other higher-strengths
types of steels are summarized in the following Table 3. The tests
for pickling removal and fish scale resistance of the sheets were
performed according to the standard EN 10209.
[0045] For testing the fish scale resistance, a boiler test enamel
was used in addition to the cold-band test frit Ferro 2290.
TABLE-US-00004 TABLE 3 Comparison of the enameling results Steel
Comparison Comparison Comparison according to steel steel steel the
invention H420LAD MS1200 TRIP HXT800 Sheet metal Sheet metal Sheet
metal Sheet metal Test Nominal value thickness 1.5 mm thickness 2.5
mm thickness 1.5 mm thickness 1.0 mm Pickling 20-50 g/m.sup.2 45 25
49 212 removal Fish scale test No result No result More than 30
More than 30 Test not Boiler test (see FIG. 1a) (see FIG. 1b) (see
FIG. 1c) possible enamel Fish scale test No result No result More
than 30 More than 30 Test not Ferro RTU possible 2290
[0046] The test results show that the comparison steel TRIP HXT800
has a pickling removal which is significantly higher than the
allowed value, so that fish scale resistance could not be
tested.
[0047] The pickling removal for the two comparison steels was
within the limits for the target values; however, there was no fish
scale resistance.
[0048] The results of the fish scale test are illustrated in FIGS.
1a to 1c.
[0049] The change of the mechanical parameters of the steel of the
invention during enameling compared to other higher-strength steels
is illustrated in the following Figures. No values after enameling
could be determined for the comparison steel HXT800, because the
steel cannot be enameled due to excessive pickling removal.
[0050] Reference is made here to FIGS. 2 and 3.
[0051] The advantages of the steel according to the invention can
be summarized as follows: [0052] High material strength also after
heat treatment above 900.degree. C., [0053] Fish scale resistance
after enameling of the component, [0054] Significantly increased
strength on the finished component and therefore possibility for
lightweight construction by reducing the thickness compared to
conventional enameled steels, [0055] Very good weldability of the
steel, in particular also with HFI welding in the context of pipe
production, [0056] Excellent cold formability of the steel in
non-temper-hardened state and therefore possibility for producing
complex components, [0057] The steel can be zinc-plated, [0058]
Suitability for non-metallic protective layers.
[0059] The following typical parameters for hot-rolled or
cold-rolled sheets and pipes in a soft-annealed state are listed
below for the steel of the invention:
TABLE-US-00005 R.sub.el and/or R.sub.p0.2 310-430 [MPa] R.sub.m
450-570 [MPa] A.sub.5 .gtoreq.23 [%]
[0060] In the heat-treated state, for example after enameling or
galvanizing above 900.degree. C., the following exemplary
mechanical parameters are attained:
TABLE-US-00006 R.sub.el and/or R.sub.p0.2 450-600 [MPa] R.sub.m
700-850 [MPa] A.sub.5 .gtoreq.12 [%]
[0061] The steel according to the invention can be used in many
applications in form of a band, sheet, hot- or cold-rolled, or for
welded and seamless pipes.
[0062] For cold-rolled or cold-formed products, the thickness range
or wall-thickness range may be, for example, 0.5-4 mm. The
corresponding values for hot-rolled or hot-formed products are
about 1.5-8 mm.
* * * * *