FIELD OF THE INVENTION

The invention relates to an hot-rolled enamelling steel sheet which is not susceptible to the occurrence of fish-scale defects. The term "enamelling steel sheet" used in the present invention represents a steel sheet material prior to the application of an enamel layer, which steel sheet is hot-rolled only, not cold-rolled. The invention further relates to a method of manufacturing a hot-rolled enamelling steel sheet. The hot-rolled enamelling steel sheet can be used for enamelled industrial silos and tanks, enamelled shower trays and other sanitary wares, and for enamelled building facades and building cladding.

BACKGROUND TO THE INVENTION

It is known when the enamelling steel sheet is enamelled, various surface defects, such as fish-scales, foams and firing strains may appear. The so-called fish-scales in particular appear during a period of several days to several months after the firing of the enamel layer, and once they appear, they are very hard, if not at all impossible, to eliminate. Considerable costs and labour are required for the remedy, if feasible.

Therefore, there is a demand for an enamelling steel sheet free from the occurrence of the fish-scale defects.

It has been generally accepted in the art that the fish-scales are caused by a large amount of atomic hydrogen being absorbed into the steel sheet from the enamel glaze and the moisture within the firing furnace during the firing operation of the enamel layer at high temperatures. Thus, as the temperature of the enamelled steel sheet is lowered, the solubility of the hydrogen in the steel sheet decreases and the hydrogen collects together in the interlayer between the enamel layer and the steel sheet in the form of molecular hydrogen under a high pressure, thus breaking the enamel film formed by the firing process and exploding out of the enamelled steel sheet to cause the fish-scales on the sheet surface.

Various studies have been made from the aspects as mentioned above, and enamelling steel sheets, mostly in cold rolled state but also in hot-rolled state having resistance to the occurrence of fish-scales have been developed.

Normally, hot-rolled steel strip does not contain enough hydrogen traps (mainly in the form of precipitates) to capture and store the hydrogen effectively which leads to the surface defects in the enamel layer known as fish-scaling, where internal pressure of hydrogen builds up after the enamelling process and breaks chips off the glass layer. Since the hydrogen solubility in steel steeply decreases during cooling, hydrogen moves to the steel-enamel

SUBSTITUTE SHEET (RULE 26) interface. At the interface hydrogen builds up pressure, which can result in breaking out of small particles of the enamel layer.

Patent document W02014/170315-A1 discloses a cold reduced enamelling steel sheet comprising (in weight ppm unless otherwise indicated): 5 < C < 90, 0.10 < Mn < 0.50 (wt.%), Alas— 300 (acid soluble Al), O < 35, S < 350, 30 < N < 110, B _m in<B<B _m ax, wherein B _m in= N x 0.80x 10.8/14 and B _max = N x 10.8/14 + 144/6; 50 < P < 160, in combination with Cu _min s Cu < Cu _ma x, wherein Cu _m in= P x 1.00x63.6/31 and Cu _ma x= P x 2.00x63.6/31, and optionally Si < 190, the balance being Fe and unintentional and/or inevitable impurities, wherein the steel sheet has been continuous annealed, and wherein the steel sheet has a thickness between 0.10 mm and 0.36 mm.

Patent document EP2145971-A1 discloses a hot-rolled and enamelled steel sheet free of fish scale defects, comprising, in wt.%: C: 0.02-0.1 %, Mn : 0.3-1%, Al: 0.1 -0.4%, Si : 0-0.3%, B: 0.002-0.01%, Nb: 0-0.1%, V: 0-0.08%, Ni : 0-0.35%, Cu: 0-0.25%, N: < 0.04%, P: < 0.03%, S: < 0.03%, the balance being iron and unavoidable impurities due to smelting, said steel sheet having a thickness Th and having being coiled at a temperature Tiling, after being hot rolled, said composition moreover comprising a boron content B above or equal to B _min defined according to the following relationship: B _m in (ppm) = 60 - (12 x Th (mm)) + (T _CO iiin _B (°C) 1 18). It discloses also a method of manufacturing a hot-rolled and enamelled steel sheet consisting in: smelting as steel of defined composition; casting said steel composition into a slab and hot rolling said slab into a hot rolled steel sheet presenting a thickness under 10 mm; coiling said hot rolled steel sheet at a temperature comprised between 380°C and 650°C; degreasing and/or shot blasting said hot rolled steel sheet; depositing at least an enamel layer under liquid or powdery form on at least one side of said hot rolled sheet; optionally drying said enamel layer; and firing the said enamel layer.

DESCRIPTION OF THE INVENTION

As will be appreciated herein, for any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated in weight ppm.

As used herein, the term "about" when used to describe a compositional range or amount of an alloying addition means that the actual amount of the alloying addition may vary from the nominal intended amount due to factors such as standard processing variations as understood by those skilled in the art. The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.10% Si may include a steel having no Si.

It is an object of the invention to provide an improved hot-rolled steel sheet that is very suitable for enamelling and especially for double-sided (or two-sided) enamelling purposes without generating fish-scales defects after enamelling.

It is another object of the invention to provide a method of manufacturing an enamelling hot-rolled steel sheet.

These and other objects and further advantages are met or exceeded by the present invention providing a hot-rolled and batch-annealed enamelling steel sheet wherein the steel has a composition comprising, in weight % unless indicated in weight ppm:

C up to about 0.006%,

Mn 0.20% - 0.390%,

Al 0.01% - 0.08%,

S 0.0005% - 0.030%,

P 0.0005% - 0.030%,

N 70 - 120 in weight ppm,

B 50 - 100 in weight ppm,

Si up to about 0.1%,

Cu up to about 0.25%, the balance being made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process, wherein the steel sheet suitable for enamelling has a thickness in a range of 0.30 mm to 4.50 mm, and is in a hot-rolled, batch-annealed and optionally temper-rolled condition and has not undergone any subsequent cold-rolling operation.

According to the present invention, there is provided a hot-rolled enamelling steel sheet intended for enamelling, especially for double-sided enamelling, showing a very excellent resistance to the occurrence of fish-scale defects, which is a result of various extensive studies made by the inventor from both aspects of the steel composition and a production method for developing the superior hot-rolled enamelling steel sheet and not having undergone any cold rolling operation.

An important aspect of the invention is that compared to the state of the art a key difference is that the hot-rolled steel sheet to be used for enamelling has undergone a batch annealing operation. As a consequence of the batch annealing BN has formed, which has a beneficial anti-fish-scale effect.

Compared to cold rolled enamelling steel sheets, the steel sheets according to the invention have the advantage that their production is more cost effective, as the cold rolling step is avoided which frees up cold rolling capacity at the steel sheet manufacturer.

Carbon (C) tends to increase the firing strain during the firing operation of the applied enamel layer and deteriorate the press-formability of the enamelled steel sheet product. The carbon content should not be too high to avoid carbon boiling during enamelling at typical temperatures in a range of about 800°C to 840°C. Therefore, in the present invention the carbon content is limited to up to 0.006 wt.%, and preferably up to about 0.004 wt.%. In a preferred embodiment the C content does not exceed 0.003 wt.%. In an embodiment the C content is at least about 0.001 wt.%, and preferably at least about 0.0015 wt.%.

The steel has between about 70 ppm to 120 ppm of nitrogen, and boron is present in an amount of between about 50 ppm to 100 ppm. Boron combines with nitrogen to form boron nitrogen (BN) in the steel during processing of the molten steel. It improves the resistance to the occurrence of fish-scales of the steel sheet when present in the defined ranges. In an embodiment the nitrogen is between 87 and 112 ppm. In a preferred embodiment the nitrogen is in a range of 95 to 109 ppm. In an embodiment the boron is present in a range of 68 to 86 ppm, and preferably in a range of 70 to 85 ppm.

In a preferred embodiment of the steel the B to N ratio is in a range of 0.70 to 0.85, and preferably in a range of 0.75 to 0.79, and provides an optimal anti fish-scale behaviour.

Aluminium is used as a deoxidizer in the melting step, fixes the oxygen in the steel to enhance the effect of the boron addition and simultaneously fixes the nitrogen in the steel to improve the press-formability and the non-aging property of the resultant steel sheet. A suitable amount for Al is in a range of about 0.01 to 0.08 wt.%. In an embodiment the Al content does not exceed about 0.035 wt.%, and more preferably does not exceed about 0.030 wt.%. As is common practice in the art, reference is made to the amount of metallic aluminium in the steel and does not include the amount of aluminium bound in aluminium-oxides. The amount of metallic aluminium is commonly also expressed as Al _as or acid soluble Al, and is determined as known in the art using a 22% HNO ₃ acidic solution for 1 hour at room temperature.

Manganese (Mn) is present in amounts ranging from about 0.20 wt.% to 0.390 wt.%. In an embodiment the Mn content is at least about 0.250 wt.%, and preferably at least about 0.275 wt.%. In an embodiment the Mn-content does not exceed 0.365 wt.%, and more preferably does not exceed about 0.340 wt.%. Sulphur (S) can be present in an amount of up to about 0.030 wt.%, and is commonly present in an amount of at least about 0.0005 wt.%. In an embodiment sulphur is present in an amount of about 0.010 wt.% to 0.019 wt.%, and preferably in a range of about 0.012 wt.% to 0.019 wt.%.

Phosphorus (P) can be present in an amount of up to about 0.030 wt.%, and is commonly present in an amount of at least about 0.0005 wt.%, as an impurity from the smelting. In an embodiment phosphorus is present in an amount of about 0.004 wt.% to 0.017 wt.%, and preferably in a range of about 0.008 wt.% to 0.017 wt.%.

Silicon (Si) can be present in the steel in an amount of up to about 0.1 wt.%, preferably up to 0.10 wt.%, which acts as a deoxidizing element in the same way as aluminium. In an embodiment the Si is present up to 0.09 wt.%.

Copper (Cu) can be present in the steel in an amount of up to about 0.25 wt.% to enhance, in particular in conjunction with the phosphorus, the adhesion of the enamelling coating, and more preferably in a range of about 0.10 wt.% to 0.25 wt.%. In an embodiment Cu is not purposively added to the steel and can be present at regular impurity levels, typically up to about 0.080 wt.%., and preferably up to 0.050 wt.%, and more preferably up to 0.030 wt.%.

The balance being made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process. Elements like Ni, V, Nb, Mo, and Cr are not purposively added to the steel and may be present at regular impurity levels.

In an embodiment of the steel sheet it has a composition consisting of, in weight % unless indicated in weight ppm:

C up to about 0.006%,

Mn 0.20% - 0.390%,

Al 0.01% - 0.08%,

S 0.0005% - 0.030%,

P 0.0005% - 0.030%,

N 70 - 120 in weight ppm,

B 50 - 100 in weight ppm,

Si up to about 0.1%,

Cu up to about 0.25%, the balance being made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and with more preferred ranges as herein disclosed and claimed. ln a preferred embodiment of the steel sheet it has a composition consisting of, in weight % unless indicated in weight ppm:

C 0.001% - 0.003%, (targeted at about 0.002%),

Mn 0.275% - 0.365%, (targeted at about 0.33%),

Al 0.015% - 0.030%, (targeted at about 0.022%),

S 0.014% - 0.017%, (targeted at about 0.015%),

P 0.006% - 0.015%, (targeted at about 0.013%),

N 90-110 in weight ppm, (targeted at about 102 ppm),

B 70-85 in weight ppm, (targeted at about 77 ppm),

Si up to 0.09%,

Cu up to 0.25%, and preferably up to 0.050%, the balance being made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and with more preferred ranges as herein disclosed and claimed.

In a preferred embodiment of the steel sheet it has a composition consisting of, in weight % unless indicated in weight ppm:

C 0.0015% - 0.0025%, (targeted at about 0.002%),

Mn 0.290% - 0.340%,

Al 0.020% - 0.024%, (targeted at about 0.022%),

S 0.014% - 0.017%, (targeted at about 0.015%),

P 0.006% - 0.015%, (targeted at about 0.013%),

N 95-109 in weight ppm, (targeted at about 102 ppm),

B 72-83 in weight ppm, (targeted at about 77 ppm),

Si up to 0.09%,

Cu up to 0.25%, and preferably up to 0.050%, the balance being made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process, and with more preferred ranges as herein disclosed and claimed.

The hot-rolled steel sheet according to the invention and ideally suitable for enamelling, has a final thickness in a range of 0.30 mm to 4.50 mm, and is in a hot-rolled, batch-annealed and optionally temper-rolled condition about without having undergone any subsequent coldrolling operation. In an embodiment the thickness of the steel sheet is at least 0.50 mm, and preferably at least 1 mm, and more preferably at least 1.5 mm. In an embodiment the thickness of the steel sheet is not more than 4.0 mm. In an embodiment the thickness of the steel sheet is not more than about 3.5 mm.

The invention further relates to a method of manufacturing a hot-rolled and batch- annealed enamelling steel sheet, the method comprising the steps of, in that order: smelting a steel which a composition according to the invention; casting a steel composition into a rolling slab; hot rolling said rolling slab into a hot-rolled steel sheet having a thickness of up to 4.50 mm, finishing said hot rolling at a finish rolling temperature between about 840°C to 940°C, and preferably between about 890°C and 930°C. accelerated cooling of the hot-rolled steel strip on a run-out table. In an embodiment the cooling rate at the run-out table is between about 20-250°C/s, and preferably between 40- 200°C/s; coiling said hot-rolled steel sheet at a temperature in a range of about 665°C to 710°C, and allowing the coiled hot-rolled steel strip to further cool to ambient temperature; optionally pickling of the hot-rolled steel sheet, typically at ambient temperature. The oxides (scale) on the hot-rolled steel sheet is removed either by pickling in an acid solution (e.g. HCI) at warm temperatures (about 80-120°C) or by a combination of pickling and mechanical brushing of the steel sheet surface. This step is necessary for rendering the steel strip surface suitable for making it amenable to a coating process by enamelling. batch-annealing of the coiled hot-rolled steel sheet; optionally applying a temper rolling permanent elongation of the hot-rolled steel sheet of less than about 1.8%, preferably of less than about 1.5%, and more preferably between about 1.0 and 1.4%. The temper rolling can be performed either before or after the batch-annealing operation, but is preferably performed after the batch-annealing operation. Next the steel sheet is coiled and stored until further processing. and wherein the hot-rolled sheet and batch-annealed steel sheet is not subjected to any subsequent cold rolling operation.

The method enables the production of hot-rolled steel sheet that is very suitable for enamelling, and especially for double-sided (or two-sided) enamelling purposes, without generating fish-scales defects after enamelling.

An important aspect of the invention is that compared to the state of the art the key difference is that the hot-rolled steel sheet to be used for enamelling has undergone a batch annealing operation. Another important aspect is that the hot-rolled and batch-annealed steel sheet is not subjected to any subsequent cold rolling operation other than the optional temperrolling operation.

The invention is not limited by the casting method. The steel can be cast as a conventional thick-slab having a cast thickness of between 150 mm and 350 mm, and typically of 225 mm to 250 mm, as well as a thin-slab having a cast thickness of between 50 mm and 150 mm in a direct strip plant. For conventional thick-slab casting, reheating of the slab is necessary to reheat the slab from ambient temperatures (usually the thick cast slabs have cooled down from the casting temperature to ambient temperatures in a slab yard) and to homogenise the slab with respect to composition, and therefore the reheating temperature should be in a range of 1000°C to 1300°C also to dissolve any precipitates when microalloying elements are present and to bring the slab to such a temperature that the final hot rolling in the finishing mill can still be performed at FRT>Ar3. Often this requires a (slab) reheating temperature of between 1050°C up to about 1260°C. For thin-slab casting the cast slab is subjected to a homogenisation treatment in a homogenising furnace immediately after casting the thin slab wherein the homogenisation temperature should be above about 1050°C, and is typically about 1100°C to 1160°C. This would also prevent any precipitates from forming when microalloying elements, if any, are present and also bring the thin slab to such a temperature that the final hot rolling in the finishing mill can still be performed at a temperature between 840°C to 940°C, and preferably between 890°C and 920°C.

The steel sheet after being hot rolled is coiled at a temperature of at least 665°C and at most 710°C. This has the effect that the optimal condition for the nucleation of BN particles is obtained.

In a preferred embodiment the method is characterised in that the steel sheet is coiled at a temperature of at least 680°C, and preferably at most 695°C, and more preferably the coiling is performed at around 690°C. The inventor has found that an optimal enamelling steel sheet can be produced if such coiling temperatures are used.

In another preferred embodiment the method is characterised in that the coiling temperature profile of the hot-rolled steel sheet is controlled according to a flat profile. Contrary to expectation, if the temperature profile was set at a ‘belly profile”, the resulting enamelling steel sheet had better properties than if it was set at a “flat profile”. This ‘belly profile” includes a higher temperature of the head and tail of the coil during the hot rolling process, so as to realise a uniform temperature in the coiled material.

It is an important aspect of the invention that the steel sheet after being hot rolled is batch annealed. Preferably the batch annealing is at a temperature of at most 680°C. The batch annealing temperature should not be lower than 600°C (top temperature). This achieves the effect that the formed BN nuclei during the hot rolling can grow to an optimal preferred size and distribution for an optimal effect to capture and store the hydrogen atoms.

In a preferred embodiment the batch annealing is performed at a temperature between about 620°C and 680°C, preferably of around 630°C, and the steel sheet is afterwards temper rolled. The optimal heating cycle is shown in Figure 1.

Regarding the batch-annealing method, a tight-coil or open-coil type box-annealing may be applied. In the open-coil type box-annealing, the steel may be decarburized by using a wet hydrogen atmosphere. During the batch annealing cycle, the formation of precipitates takes place. The second phases for precipitate formation are dissolved during the preheat of the slabs and stay partially in solid solution during the hot rolling operation. The batch annealing is done on a specific alloy depending temperature, which for this steel alloy is not lower than 600°C.

The temper rolling achieves the effect that the optimal surface roughness is applied to the steel sheet. The surface roughness average Ra of the steel sheet is preferably in a range of 3 pm to 5 pm, and with an target surface roughness average Ra of about 4 pm. The surface roughness average Ra of the steel sheet is according to NEN-EN 10049:2013, section 3.11. Good results are achieved with temper rolling elongations between about 1.0 % and 1.4 %. Lower elongations lead to too low surface roughness and higher elongations lead to too high surface roughness.

In an embodiment of the method the steel sheet is tension levelled before pickling. This achieves the effect that extra nucleation sites are created for the formation of BN particles.

In an embodiment the steel sheet is temper rolled before batch annealing. This achieves the effect that a higher amount of nucleation sites is created for the formation of BN particles.

The method according to the invention may have the following further processing steps, in that order: forming the steel sheet in a forming operation into a three-dimensional formed product. Preferably such a forming operation is a cold forming operation, and more preferably the cold forming operation includes at least a deep drawing step or a bending step. pre-treating of the surface of the steel sheet via a degreasing operation, and preferably by means of anodic cleaning in an alkaline solution. Such a treatment has a clear effect of the cleanliness of the surface. This improves the adherence of the enamel to the metal substrate. depositing at least an enamel layer under liquid or powdery form on at least one face of the hot-rolled and batch-annealed steel sheet, and preferably on both sides of the steel sheet; optionally drying said enamel layer(s); and firing said enamel layer(s) on one or both faces of the steel sheet.

The invention is also embodied in the use of the hot-rolled and batch-annealed enamelling steel sheet according to the invention and provided on one or both of its faces, and preferably on both faces, with an enamelling layer for the production of an enamelled product, preferably for a shower tray, wash basin or other sanitary ware. It can be used also for the production of enamelled industrial silos and tanks, and for enamelled building facades and building cladding.

The invention will now be described in more detail using the results of investigations and a figure, in which

Fig. 1 shows the heat-up, soaking and cooling-cycle of the applied batch-annealing heattreatment applied in accordance with the invention to the samples of the example.

EXAMPLE

By way of illustration only, the invention will be described in greater detail and certain specific examples set out.

During the preheat prior to the hot rolling process, the elements B, Al and N dissolve into solid solution. During hot rolling and subsequent coiling the temperature of the material reduces. This leads to first and limited precipitation of Boron Nitrides (BN) and Aluminium Nitrides (AIN) during hot rolling and coiling. The major part of the precipitation of BN and AIN takes place as a result of the heating during batch annealing. The heating causes the BN to grow to the optimal dimensions and distribution to prevent fish scaling defects.

Steel samples having the composition of the Table 1 were produced. For these steels the Cu content is at tolerable impurity levels. The balance is made by Fe and inevitable impurities resulting from the ironmaking and steelmaking process. The samples were hot rolled into slabs of 20 mm in thickness, heated to the temperature range of from 1100°C to 1300°C, then again hot rolled into sheets of 4.5 mm in thickness with a finishing temperature of about 900°C. The hot-rolled steel sheets were coiled at a temperature of either 660°C (outside the invention), 690°C, or 710°C, and subsequently cooled to room temperature.

The hot-rolled steel sheets were cut into specimens of 60mmx1 10mm and subjected to pickling, subsequently batch annealed either at 630°C or not batch annealed. The average tensile properties for these samples in the transverse-direction are given in Table 2. These results are for the material according to the invention in a hot-rolled, coiled at 690°C and batch annealed at 630°C.

Rp02 denotes the tensile yield strength, Rm denotes the ultimate tensile strength, Ag denotes the yield strength elongation, and A80 denotes the uniform elongation, all measured according to EN-ISO 6892-1.

Table 1 : Steel composition, all elements in mwt. %, except C, B and N in ppm.

Table 2: Tensile properties of hot-rolled enamelling steel sheet in the T-direction (average of three samples).

From the results of Table 2 it can be seen that the hot-rolled and batch-annealed steel sheet according to the invention has a favourable set of mechanical properties rendering it suitable for a brought range of enamelling applications. Next sample sheets taken from hot-rolled steel sheet having a composition according to Table 1, sample 4 of various manufacturing routes were subsequently subjected to a double enamel firing operation and evaluated for the occurrence the fish-scales on both sides of the enamelled samples. Two enamelled sheets from each batch have been evaluated, viz. test 1 and test 2. The results are shown in Table 3. Prior to enamelling the samples have been pre-treated either by blasting or an alkaline treatment. The latter is common practice in the art. The alkaline pre-treatment included alkalic cleaning in a bath involving anodic cleaning in an alkaline solution having 27 g/l NaOH and 8.1 ml/l Unisurfa KB 35 using a current density of 1.5 A/dm ² .

Table 3: Fish-scale sensitivity.

From the results of Table 3 it can be seen that a blasting pre-treatment of the steel sheet prior to enamelling renders (sample 4I) the material very susceptible to the formation of fish scale defects. Sample which may have been hot rolled only and not being subjected to a batchannealing may also still be susceptible to the formation of fish scale defects after enamelling. Samples which have been coiled at 660°C may also still suffer from fish-scale defects, irrespective of the treatment (alkaline or blasting).

From these tests it can be concluded that hot-rolled and batch annealed steel sheet according to the invention is not susceptible to so-called fish-scale surface defects after enamelling and can be used for manufacturing a brought range of enamelled products, e.g., industrial silos and tanks, enamelled shower trays (e.g., having a thickness of about 2.7 mm) and other sanitary wares, and for enamelled building facades and building cladding.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described.