ENAMELABLE STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of German Patent Application No. 10 2024 102 000.8 filed on Jan. 24, 2024, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The invention relates to an enamelable, cold-rolled and finally annealed steel sheet and a method of manufacturing an enamelable, cold-rolled and finally annealed steel sheet. The invention also relates to a cold-rolled and finally annealed steel sheet which is enameled.

BACKGROUND

Enamelable steels are used in container construction (boilers, tanks, etc.), for example, as the weight and/or used material can be reduced by enameling.

However, enamelable steels usually have rather low strength and yield strengths (proof strengths), which are further reduced during enamel firing.

High-strength, enamelable steels are characterized by the fact that the loss of strength during enamel firing is lower. The yield strength should not fall below a minimum value in order to ensure sufficient formability of the steel.

Another important property of enamelable steels is their resistance to fish scales. After the enameling process, fish scale-like defects can occur in the enamel layer, which are caused by outgassing of hydrogen. These should be avoided. However, common alloy concepts with titanium and/or niobium for cold-rolled, enamelable steels often do not exhibit good fish scale resistance. In addition, it is often not possible to achieve a higher yield strength after enameling with these concepts due to carbide coarsening and the associated grain coarsening during enamel firing.

SUMMARY

An object of the invention may be seen in creating an enamelable, cold-rolled and finally annealed steel sheet which, after the enameling process, has a high strength and at the same time a sufficiently high yield strength for formability. The steel sheet should also be resistant to fish scales.

According to one aspect of the disclosure, an enamelable, cold-rolled and finally annealed steel sheet comprises (in % by weight): C: 0.05-0.09%, Mn: 1.0-2.0%, V: 0.02-0.1%, Nb: 0-0.3%, Ti: 0-0.3%, Si: <0.3%, Al: <0.1%, Ni: <0.35%, Co: <0.2%, N: <0.04%, S: <0.04%, P: <0.1%, Mo: <0.3%, Ca: <0.2%, the balance iron and unavoidable impurities.

According to another aspect of the disclosure, an enameled, cold-rolled and finally annealed steel sheet can have the composition of the enamelable steel sheet in relation to the steel sheet as well as a yield strength Rp0.2>300 MPa. It also contains an enamel layer (as it is enameled).

According to still another aspect of the disclosure, a method of manufacturing an enamelable, cold-rolled and finally annealed steel sheet comprises melting a steel melt having the above-mentioned composition of the enamelable steel sheet is described. The method further comprises casting the molten steel into a preliminary product, hot rolling the preliminary product into a hot strip, cold rolling the hot strip into a cold-rolled steel sheet, and final annealing the cold-rolled steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples and possible embodiments of the invention are explained in more detail below with reference to the drawings. In the drawings, FIG. 1 shows a schematic representation of an exemplary process sequence for manufacturing an enamelable, cold-rolled steel sheet according to the present disclosure.

DETAILED DESCRIPTION

Referring to the enamelable, cold-rolled and finally annealed steel sheet, the addition of vanadium ensures that the increase in strength of the enamelable steel achieved by the manganese microalloy is not lost during the enameling process, or only to a lesser extent than with conventional microalloys. This means that the vanadium prevents the strength gained through microalloying from decreasing too much during the enameling process. This is achieved by the fact that vanadium goes into solution during firing (enameling) and has a grain-refining effect during cooling (from the enameling process) due to precipitation. A finer grain results in a higher strength (i.e. counteracts the loss of strength that always occurs during enameling and thus maintains a higher strength). At the same time, a high yield strength can be achieved and maintained even after enameling.

Further, the combination of manganese and vanadium means that titanium and/or niobium, which are usually used as microalloys to increase the strength of a steel suitable for enameling, can be partially or even completely dispensed with. This increases the fish scale resistance of the steel. For example, niobium contents of less than 0.1% or 0.05% or 0.04% or 0.03% or 0.02% or 0.01% can be provided (here and in the following, all percentages relating to alloying elements are in % by weight). Alternatively or in combination, titanium contents below 0.1% or 0.05% or 0.04% or 0.03% or 0.02% or 0.01% may also be provided, for example.

Phosphorus has a strength-enhancing effect and can be added in comparatively high concentrations, e.g. equal to or greater or less than 0.05% or 0.01%. Low-phosphorus steel sheets (with P<0.005%) are also possible.

All elements for which no lower limit is specified are optional elements that may not be included (i.e. having a content of 0%) in the steel composition

Referring to the enameled, cold-rolled and finally annealed steel, the enameled steel sheet can have a yield strength Rp0.2>300 MPa (in the longitudinal direction of the steel sheet) after a single or double enamel firing (baking) process.

As already mentioned, vanadium prevents excessive loss of strength of the enameled steel sheet (by re-dissolving in the steel structure during enameling). For example, a vanadium content of between 0.025% and 0.07%, in particular between 0.03% and 0.05% or 0.06%, may be advantageous.

The manganese content can, for example, be between 1.1% and 1.8%, in particular between 1.2% and 1.7%. Further preferred range limits can be found in Table 1. This ensures a high strength of the steel sheet. With the values in Table 1, the steel sheet consists of the remaining iron and unavoidable impurities in addition to the alloying elements listed.

In particular, the method described in more detail below can be used to manufacture steel sheets with a high thickness. In addition to the usual lower thicknesses, a comparatively high thickness in the range from 1.6 mm to 4 mm, in particular from 2 mm to 3 mm, can also be realized.

Referring to the method of manufacturing an enamelable, cold-rolled and finally annealed steel sheet, an end temperature of hot-rolling (also referred to in the art as finish hot-rolling temperature) can be between 890° C. and 950° C., for example. In particular, a hot-rolling end temperature can be between 920° C. and 940° C., for example.

A high hot rolling end temperature can be advantageous as it promotes the formation of coarse (globular) cementite/perlite. This is advantageous as it is “broken up” during cold rolling and thus forms pores for hydrogen absorption. The more pores there are for hydrogen absorption, the lower the risk of fish scale formation during enameling.

The higher the hot rolling end temperature, the higher the coiling temperature can be set if coiling of the hot strip is planned.

The cold rolling of the hot strip to form the cold-rolled steel sheet can be carried out in one or more stages (runs). For example, a total cold rolling degree of at least 50% or 60% or 65% can be realized. Higher total cold rolling degrees of over 75% and even 80% are also possible. The higher the total cold rolling degree, the more effective the generation of pores for hydrogen absorption (i.e. the fish scale resistance can increase).

The final annealing of the cold-rolled steel sheet can be carried out in the temperature range between 550° C. and 700° C., in particular 550° C. and 650° C. or 550° C. and 620° C. or 550° C. and 600° C., for example. The lower the final annealing temperature, the lower the loss of strength of the cold-rolled steel sheet. However, the final annealing temperature must be above the recrystallization temperature of the steel sheet in order to ensure the desired formability.

While continuous annealing plants are usually used for the final annealing of enamelable steels, the final annealing of the cold-rolled steel sheet can be carried out here in particular by means of a batch annealing process. With batch annealing (also referred to as bell or box annealing), the annealing atmosphere can be set in a targeted manner and relatively long annealing times and uniform temperature distribution can be achieved in a cost-effect way. This enables lower final annealing temperatures and therefore lower strength losses.

For example, the batch annealing of the cold-rolled steel sheet can be carried out in a batch annealing furnace with a total annealing time whose lower limit is, for example, 20 or 25 or 30 hours and whose upper limit is, for example, 40 hours. Longer total annealing times can lead to loss of strength.

However, as an alternative to batch annealing, the final annealing can also be carried out as a continuous annealing process in a continuous annealing furnace. The annealing time in the continuous annealing furnace can be between 500-1000 s depending on the thickness and width of the steel sheet. The holding time at the set maximum annealing temperature of e.g. 650-750° C., preferably 690-725° C., can be between 20 s and 500 s, in particular 50 s and 300 s, whereby longer holding times of e.g. over 200 s, 300 s or 400 s can be used to set a specific solidification in the desired range.

The method can also include coiling the hot-rolled steel sheet. Especially at high hot rolling end temperatures, high coiling temperatures of 600° C.-750° C., in particular 650° C.-750° C., can advantageously be achieved. A high coiling temperature can promote the fish scale resistance of the cold-rolled steel sheet.

Referring to FIG. 1, the process stages explained below are merely examples and can be replaced or supplemented by other or similar process steps. In particular, further processes may be provided between the process stages described below, which are not discussed in detail in this description.

The starting point for steel production is a furnace process 1, in which molten steel is melted.

After post-treatment of the steel (secondary metallurgy), which is not shown in FIG. 1, the molten steel has a composition within the ranges specified above.

The steel is then cast 2, which is used to produce preliminary products such as e.g. rolling slabs.

The preliminary products produced during casting 2 of the molten steel (e.g. continuous casting) are then hot-rolled in a rolling station 3. Hot rolling takes place at a hot rolling end temperature of between 890-950° C., preferably 920-950° C., in order to enable high coiling temperatures.

After hot rolling, the hot strip is optionally coiled into a coil in station 4. The coiling temperature can vary over a wide range, for example from about 600° C. to about 750° C. Preferably, coiling temperatures are set above 650° C., e.g. equal to or greater than 675° C. or 700° C. or 725° C. Since coiling temperatures above about 650° C. increase the fish scale resistance, high coiling temperatures can be advantageous.

In the next stage of the process, the hot strip is cold-rolled in a rolling station 5. The total degree of cold rolling can be at least 50% or higher, e.g. equal to or greater than 55%, 60%, 65% or even 70%, 75% or 80%. The higher the degree of cold rolling, the more coarse cementite/perlite is crushed during cold rolling, which generates more pores for H absorption. These increase the fish scale resistance.

After cold rolling, the cold-rolled steel sheet is annealed at a final annealing temperature (e.g. temperature of an annealing furnace chamber) between 550° C. and 700° C., in particular 550° C. and 650° C. or 550° C. and 620° C. or 550° C. and 600° C. The final annealing is carried out in a final annealing station 6, for example a continuous annealing furnace or a batch annealing furnace. The annealing time (total annealing time) in a batch-type annealing furnace can in particular be equal to or greater than 20 or 25 or 30 hours and can, for example, have an upper limit of 40 hours. The final annealing, which is also referred to as recrystallization annealing (as the final annealing step causes recrystallization of the steel sheet), guarantees the formability of the steel and can in all cases achieve a yield strength of Rp0.2>300 MPa. As the strength losses increase with increasing final annealing temperature, low final annealing temperatures (which must be above the recrystallization temperature) are preferred.

The enameling process can take place at the customer's premises, for example. It has been shown that during the subsequent enameling process, which can take place at 800-850° C. in a firing (baking) station 7, for example, a large proportion of the vanadium goes back into solution. During cooling, it has a grain refining effect and therefore increases strength. At the same time, a high fish scale resistance can be achieved, for example, through a high coiling temperature and/or a high degree of cold rolling as well as through microalloy-related precipitation.

A enamelable steel sheet product can, for example, comprise a container (boiler, tank, silo, etc.).

In the following, the limit values of the alloying elements and their preferred ranges are summarized in a table (UL: upper limit; PUL: preferred upper limit; SPUL: specifically preferred upper limit; SPLL: specifically preferred lower limit; PLL: preferred lower limit; LL: lower limit).

EXAMPLES

Table 2 shows steel compositions (alloys: AL) no. 1 to 9. Alloys no. 3 to 9 are alloys according to the invention, while alloys no. 1 and 2 are not according to the invention due to too low V contents and too low Mn contents (in relation to alloy no. 1). The residual content (balance) of all alloys consists in all cases of iron and the unavoidable impurities. Furthermore, the necessary additional properties (fish scale resistance, adhesion) in relation to the enamel layer are specified in the table.

It can be seen that alloy no. 2, which has a high manganese content but too low a vanadium content, does not provide a sufficient quality of the enamel coating.

Table 3 shows the test results for alloys no. 1 to 9 at different final batch annealing temperatures. The annealing time (total annealing time) was set so that the final annealing temperature was reached for all areas of the steel strip (holding time e.g. approx. 0.5-1 hour). The total annealing time for the coil was around 35 hours (longer total annealing times can also be used). Enamel firing was always carried out at 830° C. For steel samples of alloys no. 1 to 5, the mechanical characteristics were also determined for double enamel firing. The yield strength at 0.2 plastic deformation (Rp.2) in MPa and the elongation at break (A30) in percent were determined as the mechanical characteristic values of tensile tests on the single and double enameled steel samples. The product of the yield strength and the elongation at break is also given.

Alloy no. 2 without vanadium shows a significant loss of yield strength (Rp0.2 value) after firing. The Rp0.2 values are already below 300 MPa after a single enamel firing, with an even greater decrease after a double enamel firing. The steel samples according to the invention always showed an Rp0.2 value above 300 MPa even after double firing, sometimes significantly higher (e.g. above 320 MPa, 330 MPa, 340 MPa and occasionally over 350 MPa (alloy no. 5)).

In other words, the compositions no. 3 to 9 according to the invention with higher vanadium contents show a significant improvement in the Rp0.2 values, so that even after double enamel firing, values above 300 MPa can still be guaranteed.

Table 4 shows the test results for alloy no. 3 at different final annealing temperatures. The mechanical characteristics were determined before enamel firing (0 burn-in processes), after a single enamel firing (1 burn-in process) and after a double enamel firing (2 burn-in processes). The tensile tests were carried out in the longitudinal direction (L) and in the transverse direction (Q). In addition to the mechanical characteristics already mentioned, the tensile strength (Rm) in MPa, the uniform elongation (Ag) in percent, the modulus of elasticity (EMODUL) in kN/mm² and the Vickers hardness (HV5: test force 5 kp=49.03N) in HV (Vickers hardness) were determined. Here, the enamel firing was always carried out at 800° C.

Table 4 shows that enameling could be carried out with practically no loss of hardness. In addition, a high tensile strength Rm of the steel sheet could be maintained even after enameling once or twice.

By using Mn-V in combination, the loss of strength between single and double enamel firing could be limited to <15 MPa. Depending on the selected final annealing temperature, the loss of strength between the final annealed material and the once-fired material was limited to less than 150 MPa. The higher the final annealing temperature, the lower the loss of strength after a single enamel firing.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.