Patent Application
20110024006
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 Ac3 (about 900° C.).
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.
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.
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.
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.
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 Ac3 (about 900° C.), then these steels loose their original strength. This situation occurs likewise also in similarly heat-treated zones after welding.
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.
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.
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 Ac3, if the cooldown is not controlled and intensified.
Another problem which may occur with high-strength steels is a significant increase in the solubility for hydrogen during heating above Ac3. The hydrogen then remains in the material structure during accelerated cooldown which may cause formation of cracks in the material.
For this reason, steels are in demand which produce a hard structure also during slow cooldown (for example, in still air).
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).
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.
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.
The reaction of water with the steel surface causes formation of atomic hydrogen which diffuses into the steel during the firing process.
After firing of the enamel at about 900° 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.
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.
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.
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:
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.
According to the teaching of the invention, this object is obtained with a steel having the following composition in % by mass:
with balance iron, including usual steel-accompanying elements.
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° 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.
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.
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.
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.
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.
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.
According to an advantageous embodiment of the invention, the steel has a low Si-content of ≦0.30% for zinc-plating, thereby ensuring suitability for zinc-plating, for example, for applications in the automotive industry.
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.
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.
After cold-forming or shaping, the temper-hardening conditions can then again be adjusted by way of a subsequent heat treatment.
The term cold-forming refers to the following process variants:
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:
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.
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.
For testing the fish scale resistance, a boiler test enamel was used in addition to the cold-band test frit Ferro 2290.
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.
The pickling removal for the two comparison steels was within the limits for the target values; however, there was no fish scale resistance.
The results of the fish scale test are illustrated in
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.
Reference is made here to
The advantages of the steel according to the invention can be summarized as follows:
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:
In the heat-treated state, for example after enameling or galvanizing above 900° C., the following exemplary mechanical parameters are attained:
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 058 222.8 | Dec 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE2008/001845 | 11/5/2008 | WO | 00 | 8/17/2010 |
