NOVELTY - Direct low-temperature growth (P1) of adhering coating of carbon nanotubes and/or carbon nanofibers (CNT/CNF) on surfaces of carbon steel/low alloy steel strip substrate involves providing steel substrate; growing CNT/CNF onto a surface of the substrate by using carbon source gas containing hydrogen through thermal chemical vapor deposition (CVD) process at 500-750 (preferably 600-750) degrees C, where no catalysts are added to catalyze the growing of CNT/CNF, and growing of the CNT/CNF is catalyzed by iron, nickel and/or chromium present in the substrate and/or metallic coating.    USE - For direct low-temperature growth of an adhering coating of carbon nanotubes and/or carbon nanofibers (CNT/CNF) on one or both surfaces of a carbon steel or low alloy steel strip substrate, which is used in corroding environments, in solar cell applications, fuel cell applications, in hydrogen storage, as catalytic supports, for radar capturing coatings or as an interfacial conductive layer or for use as anti-bacterial product; for the production of the electrode part of a battery such as Li-based batteries and/or alkali batteries; for the production of photovoltaic substrate for flexible back contact electrode; and in the production of aqueous dispersed nano-coolant or fluid for heat exchangers or in the production of nano-composite coatings (claimed).    ADVANTAGE - The method is simple and cheap method to produce carbon nanotubes and/or carbon nanofibers (CNT/CNF), which is used in electrical and mechanical applications. The polymer coating provides the CNT/CNF coated substrate with additional corrosion protection. The poly-imide based coating provides thermal stability, improved chemical resistance, excellent mechanical properties and insulating properties to the coated substrate. The chemical similarity between CNT/CNF and the polymer increases its wettability and compatibility, so that it prohibits microcrack formation and its propagation, thus provides excellent formability of the CNT/CNF-PI coated steel substrate and of the CNT/CNF-Polymer coated steel substrate.    DETAILED DESCRIPTION - Direct low-temperature growth (P1) of an adhering coating of carbon nanotubes and/or carbon nanofibers (CNT/CNF) on one or both surfaces of a carbon steel or low alloy steel strip substrate involves providing a steel substrate, optionally provided with a metallic coating; growing CNT/CNF onto a surface of the substrate by using a carbon source gas containing hydrogen through a thermal chemical vapor deposition (CVD) process at 500-750 (preferably 600-750) degrees C, where no catalysts are added to catalyze the growing of the CNT/CNF, and the growing of the CNT/CNF is catalyzed by iron, nickel and/or chromium present in the substrate and/or the metallic coating. INDEPENDENT CLAIMS are included for the following:    (1) producing (P2) a coating on a steel substrate by in-situ providing a layer of CNT/CNF on the substrate, and where the layer of CNT/CNF is coated with a polymer coating, such as a poly-imide based coating;    (2) producing CNT/CNF powder by producing a coating by the process (P1); and further involving removing the CNT/CNF from the surface of the steel substrate by mechanical means, e.g. by scraping, and collecting the CNT/CNF; and    (3) a steel substrate provided with the layer of CNT/CNF, optionally coated with a polymer coating, for use in corroding environments, in solar cell applications, fuel cell applications, in hydrogen storage, as catalytic supports, for radar capturing coatings or as an interfacial conductive layer or for use as anti-bacterial product.