TITLE:
A method for producing a Tin coated substrate.
FIELD OF THE INVENTION:
The present invention relates to a method for producing a Tin coated substrate.
BACKGROUND OF THE INVENTION:
Tinplated steels are essentially low carbon thin gauged steels which are further plated with tin. Tinplating can be done by two ways; hot dipping and electrodeposition process. Electro deposited tin plated surface produces a dull surface finish, called matte finish. The characteristic bright lustre only comes after the tin-electroplated steel goes through a flow brightening treatment where the tin melts and covers the entire surface of the substrate steel. It has been well established that lower is the roughness of the surface of the final product, higher is the brightness or lustre.
The surface roughness is controlled by the amount of tin deposition and the flatness of the substrate steel. It has been observed that higher tin coating results in better surface roughness and higher lustre. However, when the amount of tin coating comes down, the surface roughness becomes dependant on the flatness of the substrate steel as well. Because of the thin coated layer, the underneath roughness of the substrate steels gets reflected in the final roughness of the tin coated products. This is mostly observed coating below 5 g/m2 and in particular below 3g/m2 tin coated plates.

Traditionally, the surface roughness of the substrate steel and thereof final coated ones is manipulated through temper milling process. A predefined roughness of the working roll is maintained which subsequently pass on to the steel substrate. Keeping the working roll roughness to the lower side, the roughness of the steel substrate as well as the final product is kept toward the lower side. This results in brighter surface finish of tin coated plates. Controlling steel substrate roughness becomes more important for thinner coating as already discussed, in this cases the steel roughness gets directly reflected in the final roughness of the material. Harry S Schutte [US PATENT US2434290A] adopted a shot blasting of temper rolls to incorporate a brighter surface. Fred et al [US4565609A] added various additives and thus improved the brightness of the tin plated steels. Asano and Hiromae [US3912601A] controlled the tin bath Ph to more than 3 and this way obtained brighter surface finish. As most of the above knowledge in known in the industrial circuit, not much literatures or patents are available in the open on roughness control of the tin plated steels.
Nevertheless, the existing knowledge and prior art to control the final roughness of the Sn coated steels deals with external means such as temper milling process, higher Sn coating (<5 g/m2), etc. In the present invention, a novel method is proposed to control the final roughness of the tin coated steels and gives a uniform brightness to the tin plated steels with tin coating <= 3g/m2. Unlike prior art, this method controls the roughness of the final products by controlling the structure of the steel substrate itself.

OBJECTS OF THE INVENTION:
An object of the present invention is to prepare a method for producing a tin coated substrate.
Another object of the present invention is to propose a method to produce tin plated low carbon steels with high and uniform surface brightness.
Still another object of the present invention is to prepare a method for producing tin coated low carbon steel with high and uniform surface brightness from a high rough steel substrate.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for producing a Tin coated substrate, the method comprising: casting a steel slab with composition C: 0.04-0.07%, Mn: 0.2-0.6%, N: 0.025-0.1%, Al: 0.02-0.06 (all in wt%); reheating the steel slab at a temperature 1100 to 1200°C for 1.0 to 2.5 hours; hot rolling the steel slab to a steel sheet of a thickness of range 2 - 2.2 mm, a finish rolling temp. (FRT) being maintained above Ae3 (~870°C) and coiling temperature between 550-600°C are maintained. cold rolling the steel sheetto a thickness of 0.25 to 0.3 mm; and batch annealing the steel sheet at a temperature of 570 to 600 °C for 20-25 hrs.

FIG. 1 shows the Microstructure and crystallographic texture information in form of ODFs of the steels.
DETAILED DESCRIPTION OF THE INVENTION:
In the present invention the roughness of the final Sn coated products is controlled by internal means, i.e. by controlling the internal structure of the substrate steel. Sn when gets molten during flow brightening treatment forms FeSn2 intermetallic layer at the Fe and Sn interface. Controlling the growth rate of FeSn2 formation and thus height, the roughness of final product is controlled.
To provide uniform rate of growth to the FeSn2 crystals, the grain orientations of the substrate steels are engineered in such a manner that most of the grains have similar grain orientations. Because of the similar grain orientations of most the grains vis-à-vis most area of coating, the rate of growth of FeSn2 layer is also similar. As a result, the molten Sn when flows over it gives a smoother surface finish than those steel plates whose surface grains are more randomly oriented.
According to the present invention, it is possible to produce a low roughness Sn coated steel products by controlling the substrate steel texture. The roughness of the Sn coating is controlled during the flow brightening treatment after the Sn electrodeposition

process on steel. During flow brightening process, liquid Sn flow uniformly over the surface and upon solidification gives a smooth surface finish. However, during flow brightening process, not only Sn melts but a chemical reaction also starts just before Sn gets molten and continues till it gets solidified. In this reaction, Sn atoms react with Fe atoms and they form an alloy of Fe-Sn. Primarily FeSn2 crystals forms which has a very specific cuboidal crystal shapes. The lengths of the cuboidal axis are found to be different.
The present invention also reveals that these cuboidal shaped FeSn2 crystals project out from the steel surface at different directions. Biber and Harter2 have clearly showed that the orientations of this FeSn2 particles and their growth rate on steel substrate are different on different planes of Fe-crystals. As a result of this the height of this layer after a specified time differs from each other. These literature findings also explain why a highly textured steel substrate results in lower overall roughness of the sheet. A uniform texture means the Fe-Sn layer has close to a uniform growth which ultimately results in lower roughness of the Sn coated steel.
The present invention relates to a Sn coated low carbon steel which has a specific alloying composition and is manufactured with a precise control of the casting, hot rolling, cold rolling and annealing, temper rolling and Sn coating parameters in order to produce the target roughness in the final products.

Alloying additions: The addition of each alloying element and the limitations imposed on each element are essential for achieving the target microstructure, texture and properties.
C: 0.04-0.07%: Carbon is one of the most important elements in steel. Depending on its quantity, the property of steels differs significantly. In the present invention Carbon is kept around 0.4 to 0.07 % in order to achieve the required strength of the products. However, in order to achieve the desired texture, carbon is kept at the lower side of the range and preferably within 0.05 ±0.005 wt. %.
Mn: 0.2-0.6%: Manganese imparts solid solution strengthening to the ferrite but it also affect {111} texture formation adversely. Thus Mn level is kept around lower level.
N: 0.025-0.1%: N is kept within the specified range. N forms AlN and controls the recrystallization microstructure and texture.
Al: 0.02-0.06: Al forms AlN and controls recrystallization texture. Fine AlN enhances the growth of {111} oriented grains at the expense of recrystallization annealing.
Production process: The method of manufacturing the low surface roughness Sn coated low carbon steel starts from the close control of casting, hot rolling, cold rolling

and annealing, temper rolling, Sn plating and flow brightening treatment. The various processing steps are described in their respective order below:
Casting: In the present invention, the steel of the specified composition is first continuously cast either in a conventional continuous caster or a thin slab caster.
Hot Rolling: The slabs are first reheated to a temperature between 1100 to 1200°C for 1.0 to 2.5 hours. Then the slab is rolled from an initial thickness of 210 mm to a final thickness of 2 to 2.2 mm by roughing and finishing operations. A Finish rolling temperature above Ae3 (~870°C) and coiling temperature between 550-600°C are maintained.
Cold Rolling and Annealing: The hot rolled steel is further cold rolled in a 6 high 5 pass cold rolling mill to a thickness of 0.25 to 0.3 mm and subsequently batch annealed at a temperature between 570 to 600 °C for 20-25 hrs. Care should be taken that the cold rolling reduction is always at least 85%, preferably close to 87-88%. During annealing soaking temperature, heating time, soaking time, cooling with heating hood of the batch annealed furnace are very important. The soaking temperature should be close to 580-590°C is desirable. For a 70 kg stack weight, the heating up time should be 285 minutes and more, soaking time should be 840 minutes and more and cooling with heating hood should be 75 minutes and more. This is to ensure full recrystallization and intense and uniform {111} texture components on the surface of the steels plate.

Temper Rolling: The cold rolled and batch annealed coils are further passed through a temper mill and an extension between 1.1 to 1.45% is given to them.
Tin Plating: Then it is directly passed through a tin bath kept at room temperature where electrodeposition of Tin takes place. Sn purity was maintained at 99%. The sheet then continuously passed vertically through a furnace which is kept between 240°C to 265°C, just above tin melting temperature (232°C). A residence time of 10-20 seconds inside furnace is maintained and once the sheet is out of the furnace, it is quenched in water and tin solidified. The temperature of water is kept within 50-80°C.
Examples
Three steel compositions within the specified limit have been chosen. The composition of the steels is given in Table 1. These steels are cast, hot rolled in a similar fashion. Further these steels are cold rolled by 86-87% and annealed in batch annealing furnace. The batch annealing parameters are given in Table 2a. The skin pass mill (temper mill) tin coating parameters are listed in table 2b. Apart from Steel 3, the other two steels annealing parameters are varied from the ideal parameters proposed for the current invention. The measured gloss, annealing parameter and crystallographic texture study given in Table 3, while Figure 1 depicts microstructures and orientation distribution function from XRD crystallographic study .

As can be seen from Table 3, the measured gloss is highest in Steel 3 while Steel 1 has the lowest gloss whereas steel 2 has a value in between. The crystallographic texture study by XRD reveals highly intense texture for Steel 3 while steel 1 has a poor texture. Again Steel 2 shows something in between. More importantly, Steel 2 shows more inhomogeneous textural distribution from one place to another. On the contrary, the crystallographic texture is very intense and more homogeneous in case of Steel 3. This is manifested by the calculation of volume fraction of texture components from different areas of the studied steels (i.e. region 1 (R1) and region 2 (R2) which in case of Steel 2 varies significantly.
Steel 1 shows more random kind of texture as can be seen from the volume fraction calculation of (100), (111) fiber and (RC(Rotated Cube)+RG(Rotated Goss texture component)+G(Goss texture component)). All of them show significant volume fraction, in other words more random kind of texture. Because of less intense texture, the adjacent grains are oriented differently on the surface of Steel 1 which results in different growth rate of FeSn2 crystals and difference in final height. This results in rougher surface and finally high roughness in the top Sn layer as well.

In case of Steel 2, although the volume fraction of (100) fibre is comparatively less but fraction of (RC(Rotated Cube)+RG(Rotated Goss texture component)+G(Goss texture component)) is higher. Apart from that, as already stated, the texture components volume fractions vary appreciably from one region to another. This inhomogeneous distribution in grain orientation across the steel substrate surface again results in non-uniform growth rate of FeSn2 crystals from one area to the adjacent area, resulting in higher roughness in alloy layer as well as in the top Sn layer.
In contrast, in Steel 3 the structure is more intensely textured and moreover homogeneously distributed all over the surface of the tin plated steels. This results in uniform growth of FeSn2 crystals and smoother surface of the top Tin layer as well.
This above desired process of imparting target texture is possible by closely controlling the annealing cycle as described in the previous section.

The invention as per the current invention provides a method of producing a bright finish tin coated (<3 g/m2) low carbon steel with lower roughness by controlling the steel substrate structure. Further, the process of the current invention can be applied to any coating specification of Sn coasted steel to improve surface roughness and brightness.

WE CLAIM:
1. A method for producing a Tin coated substrate, the method comprising:
casting a steel slab with composition C: 0.04-0.07%, Mn: 0.2-0.6%, N: 0.025-0.1%, Al:
0.02-0.06 (all in wt%);
reheating the steel slab at a temperature 1100 to 1200°C for 1.0 to 2.5 hours;
hot rolling the steel slab to a steel sheet of a thickness of range 2 - 2.2 mm, a finish rolling
temp. (FRT) being maintained above Ae3 (~870°C) and coiling temperature between 550-
600°C are maintained.
cold rolling the steel sheet to a thickness of 0.25 to 0.3 mm; and
batch annealing the steel sheet at a temperature of 570 to 600 °C for 20-25 hrs.
2. The method as claimed in claim 1, wherein the steel sheet is temper rolled.
3. The method as claimed in claim 2, wherein the steel sheet is tin plated after temper rolled.
4. The method as claimed in claim 3, wherein the tin plating is done by an electrodeposition.
5. The method as claimed in claim 4, wherein the electrodeposition is done at a room temperature.
6. The method as claimed in claim 3 and 4, wherein the tin purity is 99%.

6 The method as claimed in claim 3 and 4, wherein the tin purity is 99%.
7 The method as claimed in claim 3, wherein the sheet then continuously passed vertically through a furnace.

8. The method as claimed in claim 7, wherein temperature in the furnace is kept between 240°C to 265°C.
9. The method as claimed in claims7 and 8, wherein a residence time in the furnace is 10-20 seconds.
10.The method as claimed in claim 8, wherein the steel sheet is quenched in water for tin to be solidified on the steel sheet.
11. A Tin coated substrate, comprising:
a composition of C: 0 04-0.07%, Mn: 0.2-0.6%, N: 0.025-0.1%, Al: 0.02-0.06 (all in wt %), with brightness >110gu and roughness <0 3 pm.