The present invention relates to a steel substrate coated with a coating including nanographite, having a specific lateral size, and a binder, a method for the manufacture of this coated steel substrate. It is particularly well suited for steel industry.

In the steel route production, after the steel making step, the steel is casted in the continuous casting. Semi-products, such as slabs, billets or blooms, are thus obtained. Usually, the semi-products are reheated at high temperature in a reheating furnace to dissolve the precipitates formed during the continuous casting and to obtain a hot workability. They are then descaled and hot-rolled. However, during the reheating step, semi-products oxidized in a form of scale. A high proportion of scale is usually formed. Thus, a large amount of scale is removed during the descaling step resulting in an important weight loss of the steel product.

The patent application CN101696328 discloses a protective coating for a surface of a steel piece in order to prevent the surface from oxidation and decarburization at high temperature and, improve hardness and abrasion resistance and ultimately increase the overall service life of the steel workpiece, for the case of oxidation and decarburization of a surface (substrate) of a steel workpiece at high temperature, and the surface oxidation decarburization under the oxidizing atmosphere during heat treatment, forging, hot rolling, roll forming heating, particularly for the case that the steel workpiece is easy to be oxidized and decarbonized at high temperature in a heat treatment, leading to reduction in carbon atoms and carbon content, and the change in the surface (substrate) microstructure results in a reduced hardness, a reduced abrasion resistance and a short overall service life.

In this patent, the coating has a composition of: graphite, water glass and surface penetrant, in which a volume ratio of the graphite to sodium silicate is 1 : 3 to 1 : 7, and the surface penetrant constitutes 0.05% to 0.15% by volume of the coating. Nevertheless, the tests were performed with low carbon steels including 25 (carbon steel) and HT300 (cast iron) and with very high alloy steels including 32CrMo and Mn13. Thus, the purpose of the invention is to provide a hot steel product with a specific steel composition wherein the weight loss due to the oxidation of semi products during the reheating step is significantly reduced.

This is achieved by providing a coated steel substrate according to claim 1 . The coated steel substrate can also comprise any characteristic of claims 2 to 6.

The invention also covers a method for the manufacture of the coated steel substrate according to claims 7 to 15.

The invention also covers a method for manufacture of a Hot rolled steel product according to claims 16 to 19.

Finally, the invention covers the use of a hot rolled steel product according to claim 20.

To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figure:

Figure 1 illustrates an example of coated steel substrate according to the present invention.

Figure 2 illustrates an example of one nanographite according to the present invention.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

The invention relates to a coated steel substrate comprising a coating comprising nanographite having a lateral size between 1 and 60pm and a binder wherein the steel substrate has the following compositions in weight percent:

0.31 < C £ 1.2%,

0.1 < Si < 1.7%,

0.15 < Mn < 1.1 %,

P < 0.01 %,

S < 0.1 %,

Cr < 0.5%,

Ni < 0.5%,

Mo < 0.1 %,

and on a purely optional basis, one or more elements such as

Nb < 0.05 %,

B < 0.003%,

Ti < 0.06%, Cu < 0.1 %,

Co < 0.1 %,

N < 0.01 %,

V < 0.05%,

the remainder of the composition being made of iron and inevitable impurities resulting from the elaboration.

Without willing to be bound by any theory, it seems that a coating comprising nanographite having a lateral size between 1 and 60pm and a binder on a steel substrate having the above specific steel composition acts like a barrier to the oxidation and therefore to the scale formation during the reheating of the coated steel substrate. The inventors have found that not only the steel composition but also the nature of coating plays an important role on the reduction of steel oxidation during a heating treatment.

Additionally, as illustrated in Figure 1 , it is believed that in the coating (1 ) nanographite flakes (2) having this specific lateral size are well dispersed into the binder (3) in a form of tortuous path (4). Thus, it seems that the oxygen diffusion through the coating is very restricted allowing an important reduction of the scale formation and a significant weight gain of the steel substrate. Finally, it is believed that the use of nanographites having the lateral size between 1 and 60pm allows for a cluster including a large amount of nanographite flake resulting in a narrower space between each nanographite particle. Thus, the tortuous path is more difficult to cross reducing significantly the oxygen diffusion toward the steel substrate (5).

Regarding the chemical composition of the steel, preferably, the C amount is between 0.31 and 1.0% by weight.

Preferably, the Mn amount is between 0.15 and 0.7% by weight.

Advantageously, the amount of Cr is below or equal to 0.3% by weight.

Preferably, the amount of Ni is below or equal to 0.1 % by weight.

Advantageously, the amount of Mo is below or equal to 0.1 %.

Figure 2 illustrates an example of nanographite flake according to the present invention. In this example, the lateral size means the highest length of the nanoplatelet through the X axis and the thickness means the height of the nanoplatelet through the Z axis. The width of the nanoplatelet is illustrated through the Y axis.

Preferably, the lateral size of the platelet is between 20 and 55pm and more preferably between 30 and 55pm.

Preferably, the thickness of the coating is between 10 and 250pm. For example, the thickness of the coating is between 10 and 100pm or between 100 and 250pm.

Advantageously, the steel substrate is a, a slab, a billet or a bloom.

Preferably, the binder is sodium silicate or the binder includes aluminum sulfate and an additive being alumina. In this case, without willing to be bound by any theory, it seems that the coating according to the present invention better adheres on the steel substrate so that the steel substrate is even more protected. Thus, the risk of coating cracks and coating detachment, exposing the steel substrate to oxidation, is more prevented.

The invention also relates to a method for the manufacture of the coated steel substrate according to the present invention, comprising the successive following steps:

A. The provision of a steel substrate having the above steel composition,

B. The coating deposition using an aqueous mixture to form the coating,

C. The drying of the coated steel substrate obtained in step B).

Preferably, in step B), the deposition of the coating is performed by spin coating, spray coating, dip coating or brush coating.

Advantageously, in step B), the aqueous mixture comprises from 1 to 60g/L of nanographite and from 150 to 250g/L of binder. More preferably, the aqueous mixture comprises from 1 to 35g/L of nanographite.

Preferably, in step B), wherein the aqueous mixture comprises nanographite comprising above 95% and advantageously 99% by weight of C.

Advantageously, in step B), the ratio in weight of nanographite with respect to binder is below or equal to 0.3. The coating is dried in a step C). Without willing to be bound by any theory, it is believed that the drying step allows for an improvement of the coating adhesion. Indeed, since water evaporates, the binder becomes tackier and more viscous leading to a hardened condition. In a preferred embodiment, in step C), the drying is performed at a temperature between 50 and 150°C and preferably between 80 and 120°C.

Preferably, in step C), the drying step is performed with hot air.

Advantageously, in step C), the drying is performed during 5 to 60minutes and for example, between 15 and 45minutes.

The invention also relates to a method for manufacture of a Hot rolled steel product comprising the following successive steps:

I. The provision of the coated steel substrate according to the present invention,

II. The reheating of the coated steel substrate in a reheating furnace at a temperature between 750 and 1200°C,

III. The descaling of the reheated coated steel sheet obtained in step II) and

IV. The hot-rolling of the descaled steel product.

The reheating is performed at a temperature between 750 and 1200°C. Without willing to be bound by any theory, it is believed that above 1200°C, fayalite can be formed at the interface between the steel substrate and the coating. Preferably, in step II), the reheating is performed at a temperature between 750 and 900°C or between 900 and 1200°C.

Preferably, in step III), the descaling is performed using water under pressure. For example, the water pressure is between 100 and 150 bars. In another embodiment, the descaling is performed mechanically, for example, by scratching or brushing the scale layer.

With the method according to the present invention, a hot rolled steel product having a high weight mass is obtained compared to the prior art.

For example, after the hot-rolling, the hot product can be coiled, cold-rolled, annealed in an annealing furnace and also coated with a metallic coating. Finally, the invention relates to the use of a hot rolled steel product obtainable from the method according to the present invention for the manufacture of a part of an automotive vehicle, a rail, a wire or a spring.

The invention will now be explained in trials carried out for information only. They are not limiting.

Examples:

In Examples, steels substrates having the following steel composition in weight percent were used:

Trial 1 was casted in the form of slab and Trials 2 to 4 were casted in the form of billet.

Example 1 : Oxidation test

For T rials 1 , 3, 5, 7, 9, 11 , 13 and 15, steels 1 to 4 were coated by spraying an aqueous mixture comprising 30g/L of nanographite having a lateral size between 35-50pm and a binder onto the steel. Then, the coating was dried during 30 minutes at 100°C.

Then, uncoated steels (Trials 2,4,6,8,10,12,14 and 16) and coated steels (Trials 1 , 3, 5, 7, 9, 11 , 13 and 15) were reheated at 800°C, 1000°C and 1250°C. After the reheating, all the trials were weighted. For each Trial, the weight gain was determined by subtracting the weight after reheating from the weight before the reheating. The percentage of weight gain of the coated Trial was then calculated with the following formula:

gain (%) = 100 — ( weight of coated trial x 100\

weight

weight of uncoated trial ) ^'

The results are in the following Table 1 : Trials 7 to 12 according to the present invention show a significant increase of the percentage of weight gain. Indeed, the steel substrate having the specific steel composition according to the present invention is well protected with the aqueous mixture 1 and 6 during the reheating step.