Patent Application
20200318213
The present invention relates to a steel substrate coated with a coating including nanographite, having a specific lateral size, and a binder, and to a method for the manufacture of this coated steel substrate. It is particularly well suited for the 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, especially having high carbon content, decarburized resulting in a change of mechanical properties. Indeed, during the reheating step, the semi-product can lose a large carbon content. For example, rails steels decarburize leading in a decrease the hardness of the steel. Consequently, mechanical properties at the rail head are low and mechanical properties of the whole rail head are heterogeneous. Thus, there is a need to find a way to prevent the decarburization of a steel substrate during the reheating.
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 application, 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.
An object of the present invention is to provide a hot steel product having a specific steel composition, wherein the decarburization is significantly reduced during the reheating step.
The present invention provides coated steel substrate comprising a coating comprising nanographite having a lateral size between 1 and 60 μm and a binder, wherein the steel substrate has the following compositions in weight percent:
Methods for the manufacture of a coated steel substrate and of a hot rolled steel product also are provided as is the use of a hot rolled steel product.
To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figure:
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 60 μm and a binder, wherein the steel substrate has the following compositions in weight percent:
Without willing to be bound by any theory, it seems that a coating comprising nanographite having a lateral size between 1 and 60 μm and a binder on a steel substrate having the above specific steel composition reduces the decarburization 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 or the elimination of steel decarburization during the heating treatment.
Indeed, it seems that there is a competition between oxidation and decarburization kinetics during the reheating. For the above specific steel substrate (5), the formation of iron of the steel into scale reduces the decarburized layer. Additionally, as illustrated in
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 0.9 and 2.5% and preferably between 1.1 and 2.0% 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%.
Preferably, the lateral size of the nanoparticles is between 20 and 55 μm and more preferably between 30 and 55 μm.
Preferably, the thickness of the coating is between 10 and 250 μm. For example, the thickness of the coating is between 10 and 100 μm or between 100 and 250 μm.
Advantageously, the steel substrate is 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 decarburization, is further prevented.
Preferably, the coating further comprises an organometallic compound. For example, the organometallic compound includes Dipropylene glycol monomethyl ether (CH3OC3H6OC3H6OH), 1,2-Ethanediol (HOCH2CH2OH) and 2-ethylhexanoic acid, manganese salt (C8H16MnO2). Indeed, without willing to be bound by any theory, it is believed that the organometallic compound allows for a fast curing of the coating avoiding a drying step at high temperature.
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:
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 60 g/L of nanographite and from 150 to 250 g/L of binder. More preferably, the aqueous mixture comprises from 1 to 35 g/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.
Preferably, in step B), the aqueous mixture comprises an organometallic compound. More preferably, the concentration of the organometallic compound is equal or below to 0.12 wt. %. Indeed, without willing to be bound by any theory, it is believed that this concentration allows for an optimized coating without any curing or with a curing at room temperature.
In a preferred embodiment, 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 room temperature or a temperature between 50 and 150° C. and preferably between 80 and 120° C.
In another preferred embodiment, no drying step is performed.
Preferably, in step C), when a drying is applied, the drying step is performed with hot air.
Advantageously, in step C), when a drying is applied, the drying is performed during 5 to 60 minutes and for example, between 15 and 45 minutes.
The invention also relates to a method for manufacture of a Hot rolled steel product comprising the following successive steps:
Preferably, in step I), the reheating is performed at a temperature between 800 and 1300° C., more preferably between 900 and 1300° C. and advantageously between 1100 and 1300° C.
Advantageously, in step II), 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 wherein the surface is mainly not decarburized is obtained.
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.
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 and 3 were casted in the form of bloom.
For some Trials, steels were coated by spraying an aqueous mixture comprising 30 g/L of nanographite having a lateral size between 35-50 μm, a binder being Na2SiO3 (sodium silicate) and optionally an organometallic compound being DriCAT® onto the steel. Then, optionally, the coating was dried at room temperature or during 30 minutes at 100° C.
Then, uncoated steels and coated steels were reheated at 1250° C. After the reheating, the trials were analyzed by optical microscopy (OM). 0 means that almost no decarburized areas are present at the trial surface, i.e. almost no decarburization happened, during the reheating and 1 means that a lot of decarburized areas are present at the surface of the trial.
The results are in the following Table 1:
For Trials according to the present invention, a very low amount of carbon was removed at the trial surface. On the contrary, for comparative Trials, a lot of decarburized areas were present allowing a change in the microstructure and therefore mechanical properties. Indeed, in the areas where there is a lot of carbon depletion, i.e. decarburized areas, ferrite is formed instead of pearlite.
In this case, after the reheating at 1250° C., some Trials were quenched in water to form martensite and the microhardness evolution from the hot steel product surface to a depth of 1500 μm was determined by microhardness measurements. Indeed, when martensite is formed, the carbon content of the martensite is directly proportional to the amount of carbon in the microstructure. Therefore, the higher the microhardness is, the higher the carbon content is.
The results are in the following Table 2:
280
420
600
700
380
640
820
900
The microhardness of Trials 4 and 9 clearly show that the decarburization was significantly reduced with the coated steel substrate according to the present invention compared to Trials 8 and 10.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017/058105 | Dec 2017 | IB | international |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2018/059873 | 12/11/2018 | WO | 00 |
