Coated steel with improved oxidative resistance at high temperature and method of manufacturing

Abstract

A coated steel substrate comprising a surface; the steel composition comprising: 0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 0-80 ppm boron; less than 0.1 wt.% phosphorus, less than 0.1 wt. %sulfur; less than 1 wt. % silicon; and less than 5 wt.% of other transition metals; a remainder iron and unavoidable impurities; a remainder iron and unavoidable impurities; and a layer comprising metal particulate and a base polymer adjacent the surface is provided. The coated steel substrate is hot formable with the layer providing high oxidation resistance and sufficient electrical conductivity for welding. In one example, the steel substrate is suitable for automotive applications.

Description

COATED STEEL WITH IMPROVED OXIDATIVE RESISTANCE AT HIGH TEMPERATURE AND METHOD OF MANUFACTURINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 565,965 filed on March 15, 2014, the entirety of which is incorporated herein by reference. Technical Field

[0002] This disclosure is directed to coated steel with improved high temperature oxidation resistance. In one aspect, the coating comprises metal particulate and a base polymer.BACKGROUND

[0003] Since the early 2000's, the usage of PHS in the manufacture of lightweight vehicles has significantly increased. A great advantage of the press-hardening technology (also referred to as "hot stamping" or "hot forming" or "hot press forming") is its ability to produce complicated structural components with ultrahigh strength while avoiding issues caused by cold forming high-strength steels, such as inadequate ductility, increased springback and dimensional inaccuracy.

[0004] Current requirements for PHS are that its surface coating be able to prevent the steel from oxidation and decarburization during hot stamping / hot press forming, enable parts to be spot welded to other parts and able to provide press-hardened parts with corrosion protection in service. Various coatings and coating systems have been developed for PHS applications to meet these requirements. Of these coatings, an aluminum or aluminized coating is used, which consists of either pure aluminum (Type II) or aluminum with 5 wt. %-ll wt. % Si (Type I). However, such aluminized coatings are only able to provide a steel substrate with barrier protection. A shortcoming of this type of protection is that once the barrier coating is damaged or develops pores, the exposed areas of the steel substrate could be attacked with no further protection. After hot stamping / hot press forming, coating cracks and break-offs are often observed in such aluminized coatings due to the coating brittleness. Weldability and paintability of aluminized coatings after hot stamping are often concerns for automotive applications.

[0005] Galvanized (Gl) and galvannealed (GA) coatings have long been an excellent corrosion protection choice for automotive steel parts. These zinc-based coatings are ableto offer cathodic protection to the steel substrate, thus possessing a great advantage in cutedge protection. Moreover, conventional Gl and GA coatings that are produced on continuous galvanizing lines (CGL) can readily retain the capability of cathodic protection even after experiencing a high temperature stamping process, however, zinc-coated PHS is generally limited to the indirect press-hardening or high temperature stamping process to mitigate the cracking problems. However, this process is more time-consuming and requires additional equipment, thus increasing costs.SUMMARY

[0006] In one example, a coated steel substrate is provided, the coated steel substrate comprising: a surface; 0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 0-80 ppm boron; less than 0.1 wt.% phosphorus, less than 0.1 wt. %sulfur; less than 1 wt. % silicon; and less than 5 wt.% of other transition metals; a remainder iron and unavoidable impurities; and a layer comprising metal particulate and a base polymer adjacent the surface.

[0007] In one aspect, alone or in combination with any one of the previous aspects, further comprising a galvanizing coating on the steel substrate. In one aspect, alone or in combination with any one of the previous aspects, the galvanizing coating is annealed. In one aspect, alone or in combination with any one of the previous aspects, the galvanizing coating is adjacent the layer.

[0008] In one aspect, alone or in combination with any one of the previous aspects, the galvanizing coating is directly adjacent the surface and the layer is directly adjacent the galvanizing coating.

[0009] In one aspect, alone or in combination with any one of the previous aspects, the layer is cured and / or crosslinked.

[0010] In one aspect, alone or in combination with any one of the previous aspects, the steel substrate is a sheet, slab, or rebar. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate is hot rolled or cold rolled. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate is a transformation-delayed steel. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate is a TRIP steel. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate is a dual phase steel. In one aspect,alone or in combination with any one of the previous aspects, the steel substrate is a multiphase steel.

[0011] In one aspect, alone or in combination with any one of the previous aspects, the steel substrate has a pre-hot formed tensile strength of at least 200 MPa. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate has a posthot formed tensile strength of at least 500 MPa.

[0012] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises at least one of iron, titanium, nickel, cadmium, tantalum, zinc, zirconium, manganese, magnesium, bismuth, molybdenum, chromium, aluminum, and oxides thereof.

[0013] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of at least two of iron, titanium, tantalum, zinc, zirconium, manganese, aluminum and oxides thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of at least two of aluminum, titanium, tantalum, zinc, and oxides thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises aluminum and / or aluminum oxides, or titanium and / or titanium oxides, zinc and / or zinc oxides, and combinations thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of AL / MG; AL / FE aluminum, zinc, and oxides thereof.

[0014] In one aspect, alone or in combination with any one of the previous aspects, the weight percent of the metal particulate is greater than the weight percent of the base polymer in the layer. In one aspect, alone or in combination with any one of the previous aspects, the weight percent of the metal particulate is less than or equal to the weight percent of the base polymer in the layer.

[0015] In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises at least one of polysilazanes, polysiloxazanes, polysiloxanes, and blends thereof. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysilazanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysiloxazanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprisespolysilazanes and polysiloxazanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysilazanes and polysiloxanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysiloxazanes and polysiloxanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises linear or cyclic oligomers of silazanes, siloxazanes, siloxanes, or mixtures thereof.

[0016] In another example, a method of continuously coating at least a portion of a steel surface is provided, the method comprising the steps of: (i) continuously agitating a non-dispersed mixture comprising metal particulate and a base polymer so as to provide a dispersed high solids mixture suspension dispersions of the metal particulate and the base polymer; (ii) continuously contacting a steel surface with the dispersed mixture and providing a coated steel substrate; (iii) drying and / or curing; and (iv) providing a coated steel substrate.

[0017] In one aspect, where, prior to step (i) or (ii), the method further comprises applying a galvanizing coating to the coated steel substrate. In one aspect, alone or in combination with any one of the previous aspects, the method further comprises annealing the galvanizing coating.

[0018] In one aspect, alone or in combination with any one of the previous aspects, the non-dispersed high solids mixture is solventless. In one aspect, alone or in combination with any one of the previous aspects, the non-dispersed high solids mixture comprises an organic solvent.

[0019] In one aspect, alone or in combination with any one of the previous aspects, the non-dispersed mixture comprises silicon oxide. In one aspect, alone or in combination with any one of the previous aspects, the non-dispersed mixture comprises a curing agent.

[0020] In one aspect, alone or in combination with any one of the previous aspects, the continuous contacting comprises blade coating, bar coating, solvent dispensing, roll coating, dip coating, single or dual spray coating, vapor coating or combinations thereof. In one aspect, alone or in combination with any one of the previous aspects, the continuous contacting is carried out at a speed of between 0.5 m / min to 50 m / min.

[0021] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate is present at 40 weight percent to 90 weight percent of the total weight ofthe mixture. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate is present at 50 weight percent to 90 weight percent of the total weight of the mixture. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate is present at 60 weight percent to 90 weight percent of the total weight of the mixture.

[0022] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises at least one of iron, titanium, nickel, cadmium, tantalum, zinc, zirconium, manganese, magnesium, bismuth, molybdenum, chromium, aluminum, and oxides thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises aluminum or aluminum oxides.

[0023] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of at least two of aluminum, titanium, tantalum, zinc, and oxides thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of aluminum, zinc, and oxides thereof.

[0024] In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises at least one of polysilazanes, polysiloxazanes, polysiloxanes, and blends thereof. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysilazanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysiloxazanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysilazanes and polysiloxazanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysilazanes and polysiloxanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises polysiloxazanes and polysiloxanes. In one aspect, alone or in combination with any one of the previous aspects, the base polymer comprises linear or cyclic oligomers of silazanes, siloxazanes, siloxanes, or mixtures thereof.

[0025] In one aspect, alone or in combination with any one of the previous aspects, In one aspect, alone or in combination with any one of the previous aspects, the substrate is hot rolled or cold rolled. In one aspect, alone or in combination with any one of the previous aspects, the substrate is a transformation-delayed steel. In one aspect, alone or incombination with any one of the previous aspects, the substrate is a TRIP steel. In one aspect, alone or in combination with any one of the previous aspects, the substrate is a dual phase steel. In one aspect, alone or in combination with any one of the previous aspects, the substrate is a multiphase steel.

[0026] In one aspect, alone or in combination with any one of the previous aspects, the steel substrate comprises 0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 0-80 ppm boron; less than 0.1 wt.% phosphorus, less than 0.1 wt. %sulfur; less than 1 wt. % silicon; and less than 5 wt.% of other transition metals

[0027] In one aspect, alone or in combination with any one of the previous aspects, the steel substrate comprises 0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 0-80 ppm boron; less than 0.05 wt.% phosphorus, less than 0.05 wt. % sulfur; 0.05-0.5 wt. % silicon; and 0-5 wt.% of other transition metals. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate comprises 0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 5-80 ppm boron; less than 0.1 wt.% phosphorus, 0.0001-0.5 wt. % sulfur; 0.01- 0.5 wt. % silicon; and 0-3 wt.% of other transition metals.

[0028] In one aspect, alone or in combination with any one of the previous aspects, the steel has a pre-hot formed tensile strength of at least 200 MPas. In one aspect, alone or in combination with any one of the previous aspects, the steel has a pre-hot formed tensile strength of at least 400 MPas. In one aspect, alone or in combination with any one of the previous aspects, the steel has a post-hot formed tensile strength of at least 500 MPas. In one aspect, alone or in combination with any one of the previous aspects, the steel has a post-hot formed tensile strength of at least 900 MPas. In one aspect, alone or in combination with any one of the previous aspects, the steel has a post-hot formed tensile strength of at least 1200 MPas. In one aspect, alone or in combination with any one of the previous aspects, the steel has a post-hot formed tensile strength of at least 1600 MPas. In one aspect, alone or in combination with any one of the previous aspects, the steel substrate is a hardenable steel, for example, press or air hardenable, in one aspect, alone or in combination with any one of the previous aspects, the steel substrate is a heat treatable steel.

[0029] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises at least one of titanium, tantalum, zinc, zirconium, manganese,magnesium, aluminum and oxides thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises aluminum or aluminum oxides.

[0030] In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of at least two of aluminum, titanium, tantalum, zinc, and oxides thereof. In one aspect, alone or in combination with any one of the previous aspects, the metal particulate comprises a combination of aluminum, zinc, and oxides thereof.

[0031] In one aspect, alone or in combination with any one of the previous aspects, the drying comprises infrared light, heat, air knife or combination thereof. In one aspect, alone or in combination with any one of the previous aspects, the drying comprises curing and / or crosslinking the base polymer. In one aspect, alone or in combination with any one of the previous aspects, the drying comprises heating at a temperature of between 50 °C to 500 °C. In one aspect, alone or in combination with any one of the previous aspects, the drying comprises heating at a temperature of between 100 °C to 500 °C for a time between 20 seconds to 400 seconds.

[0032] In one aspect, alone or in combination with any one of the previous aspects, the method further comprises coiling the sheet.

[0033] In one aspect, alone or in combination with any one of the previous aspects, the method further comprises the step of hot-forming / hot stamping the coated steel substrate.

[0034] In another example, a hot-formed part is provided, the hot-formed part made by any of the previous method aspects. In one aspect, the hot formed part comprises a surface coating having high temp oxidation resistance and electrical conductivity sufficient for welding the surface to at least one other metal surface. In one aspect, alone or in combination with any one of the previous aspects, the hot formed part passes automotive component weld testing criteria.

[0035] In one aspect, alone or in combination with any one of the previous aspects, the steel substrate, is weldable by spot welding, laser welding, or gas metal arc welding, in one aspect, alone or in combination with any one of the previous aspects, the steel substrate after coating, is weldable by spot welding, laser welding, or gas metal arc welding, in one aspect, alone or in combination with any one of the previous aspects, the steel substrate after coating and a heat history, for example air hardening, heat treated, press hardening, isweldable by spot welding, laser welding, or gas metal arc welding, in one aspect, alone or in combination with any one of the previous aspects, the hot formed part is continuously welded or weldable by spot welded. In one aspect, alone or in combination with any one of the previous aspects, the hot formed part is spot weldable at a current of 0.5 KA to 10 KA.

[0036] In one aspect, alone or in combination with any one of the previous aspects, the hot formed part is laser weldable without edge ablation or removal of the surface coating about the weld area. In one aspect, alone or in combination with any one of the previous aspects, the hot formed part is laser cut without edge ablation or removal of the surface coating about the weld area.

[0037] In one example, an internal combustion engine (ICE) vehicle or electric vehicle comprising the hot-formed part of any one of the previous aspects is provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to understand and to see how the present disclosure may be carried out in practice, examples will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

[0039] FIG. 1 is a diagrammatical side view of a portion of a continuous coating process line showing the continuous production of steel sheet with a layer of metal particulate and a base polymer according to the present disclosure.

[0040] FIGs. 2A and 2B are micrographs of welded steel samples after hot forming steel samples with a layer of metal particulate and a base polymer according to the present disclosure.

[0041] FIGs. 3A and 3B depict a welding current waveform and current range, respectively, of welded steel samples after hot forming steel samples with a layer of metal particulate and a base polymer according to the present disclosure.

[0042] FIG. 4 depicts scribe creep data of steel samples after hot forming steel samples with a layer of metal particulate and a base polymer according to the present disclosure.DETAILED DESCRIPTION

[0043] A high solids coating is a single- or two-component coating formulated to have higher concentrations (at least 65%) of solid components (binders, pigments and additives) than a conventional coating but still maintain satisfactory coating or application propertiesin spite of the lower percentage of volatile organic compounds (VOCs). A high solids coating can be solvent- or water-based.

[0044] A coated, steel is presently disclosed that provides for improved oxidation resistance and electrical conductivity or impedance and excellent weldability after hot- forming during stamping. Methods of coating a steel substrate are provided. Hot formed parts configured for welding are also disclosed. In some aspects, the coated steel substrate is a hardenable steel, suitable for automotive applications.

[0045] As used herein the phrase "base polymer" or "polymer base" is inclusive of cyclic and / or linear oligomeric or polymeric molecules with silazane repeating units -[SIR2-NR'-] also referred to as polysilazanes. The phrase "base polymer" or "polymer base" is also inclusive of cyclic and / or linear oligomeric or polymeric molecules with siloxane repeating units -[R'R2Si-O-] where R' and R2 are independently hydrogen or an organic moiety, and / or blends or copolymers of polysilazanes and polysiloxanes. As used herein the phrase "base polymer" or "polymer base" is also inclusive of any linear and / or cyclic oligomeric or polymeric composition comprising a plurality of Si— N repeat units, as well as a plurality of Si— 0 repeat units, commonly referred to as polysiloxazanes. The phrase "base polymer" or "polymer base" is also inclusive of blends or copolymers of polysilazanes and polysiloxazanes. The phrase "base polymer" or "polymer base" is also inclusive of blends or copolymers of polysiloxanes and polysiloxazanes. The phrase "base polymer" or "polymer base" is also inclusive of blends or copolymers of polysilazanes, polysiloxanes, and polysiloxazanes.

[0046] Polysilazanes, polysiloxazanes, and polysiloxanes can be crosslinked by hydrolysis, for example, by reaction with moisture from the air. This leads to an increasing molecular weight and to a solidification or curing of the material. Thus, as used herein, the terms "curing" and "crosslinking" and the corresponding verbs "cure" and "crosslink" are used interchangeably herein when referred to the base polymers e.g. polysilazanes, polysiloxazanes, polysiloxanes, and blends thereof.

[0047] In one example, the base polymer comprises a polysiloxazane having at least two silyl amine groups, or at least two terminal and primary silyl amine groups as part of the polymer molecule, or the presence of at least two Si— N bonds within the polymer molecule.

[0048] In one example, organic or inorganic powders / particulates are included with the base polymer, alone or with solvent, for example, to increase the viscosity or solids content of the coating solutions to enable preparation of a paste, or a relatively thick solution so that it may be "painted" onto a substrate, or to impart specific performance characteristics to the coating. In one example, a non-dispersed high solids mixture comprises an organic solvent, inorganic powders / particulates and the base polymer is provided. In one example, the non-dispersed high solids mixture comprising an organic solvent, inorganic powders / particulates and the base polymer further includes a curing agent or catalyst and other coating aids.

[0049] In one example, metal powders or particulate, such as iron, titanium, nickel, cadmium, tantalum, zinc, zirconium, manganese, magnesium, bismuth, molybdenum, chromium, aluminum, and oxides thereof may be admixed with base polymer priorto coating. In one example, metal powders or particulate, such zirconium, titanium, tungsten, molybdenum and hafnium powders are used. Combination of any of the above powders can be used.

[0050] In one example, the metal particulate comprises at least one iron, titanium, nickel, cadmium, tantalum, zinc, zirconium, manganese, magnesium, bismuth, molybdenum, chromium, aluminum, and oxides thereof. In one example, the metal particulate comprises at least two of iron, titanium, nickel, cadmium, tantalum, zinc, zirconium, manganese, magnesium, bismuth, molybdenum , chromium, aluminum, and oxides thereof.Combinations of the above metals and / or their oxides can be used. In one example the metal particulate has an average size distribution of about 1-100 micron, 2-75 micron, or 5- 50 micron.

[0051] In one example, the weight percent solids of the mixture of metal particulate in the base polymer is about 40%, 50%, 60%, 70%, 80%, or 90%. In one example, the mixture of metal particulate and the base polymer is solventless.

[0052] In one example the metal particulate is provided as an organometallic compound that decomposes to provide the metal or metal particulate, for example, metalorganic coordination complex, (e.g., metal acetylacetonates, metal amines), metal porphyrins, metal alkyls, metal aryls, and the like.Steel Substrate

[0053] Steel substrates include, for example, steel, galvanized steel, annealed steel, galvannealed steel, hot rolled steel, or cold rolled steel. The steel can be a TRIP steel, a dual phase steel, a multiphase steel, a transformation-delayed steel, or an OEM automotive grade steel. In one example, the steel composition comprises 0.15 to 0.3 wt.% carbon; 0.8- 2.0 wt.% manganese; 5-50 ppm boron; less than 0.02 wt.% phosphorus, less than 0.01 wt. % sulfur; 0.05-0.5 wt.% silicon; and 0.1-2 wt.% of other transition metals; a remainder of iron and unavoidable impurities.

[0054] In one example, the steel substrate has a pre-hot formed tensile strength of at least 200 MPa, at least 300 MPa, at least 400 MPa, or at least 500 MPa.

[0055] In one example, the presently disclosed coating can be used in order to avoid liquid metal embrittlement that otherwise may occur during hot forming with galvanized coatings. Advantageously, the presently disclosed coated steel can be used alone on a transformation-delayed steel, or, a metallic coated transformation-delayed steel coated with the presently disclosed coating applied to the metallic coating.

[0056] The present disclosure provides for coating a wide variety of steel sheet substrates employing the polymeric compositions and methods disclosed herein. The steel sheet substrates include dual phase, multiphase, TRIP, high strength, ultrahigh strength steels, for example. In one example, the substrate is a steel sheet meeting OEM automotive requirements, such as a boron containing high strength steel.

[0057] In one example, the substrate is a steel comprising 0.15 to 0.3 wt.% carbon; 0.8- 2.0 wt.% manganese; 0-80 ppm boron; 0.01-0.1 wt.% phosphorus, sulfur; 0.1-1 wt. % silicon; and 0-1 wt.% of other transition metals, e.g., chromium, nickel, aluminum, copper and / or other metals typically present in scrap steel or purposefully added, for example vanadium, niobium, tantalum. In one example, the substrate is a steel comprising 0.15 to 0.3 wt.% carbon; 0.8-2.0 wt.% manganese; 5-50 ppm boron; less than 0.02 wt.% phosphorus, less than 0.01 sulfur; less than 0.5wt. % silicon; and 0.1 -1.5 wt.% of other transition metals; a remainder iron and unavoidable impurities. In one example, the steel substrate is hot rolled only or hot rolled and cold rolled. In one example, the steel substrate is a steel sheet, a pillar, rebar, or other form. In one example, the steel substrate is a non-boron containing steel sheet intended for direct or indirect hot forming or press forming.

[0058] The resultant pre-hot formed cold rolled, annealed, coated steel of the present disclosure can have high yield strength, and / or high tensile strength, and can have a complex metallographic structure, or multi-phase structure ,or a substantial single ferrite phase. The multi-phase microstructure may comprise bainite martensite, pearlite, and / or have a remainder volume being essentially ferrite and / or no or low retained austenite.

[0059] In one example, the present disclosed steel with a layer comprising metal particulate and a base polymer has one or more of a property, before or after hot-forming, of excellent weldability and, excellent surface and shape quality. By improved weldability, it is meant that the present disclosed steel with a layer comprising metal particulate and a base polymer weldability is superior to the weldability of conventional painted steel.

[0060] Various means of introducing the coating to the steel sheet can be used, such as blade coating, bar coating, solvent dispensing, roll coating, dip coating, single or dual spray coating, vapor coating, and the like. In one example, both sides of a continuous steel sheet are coated with the presently disclosed composition. In other examples, one side of the a continuous steel sheet is coated with the presently disclosed composition.Continuous Coating Process

[0061] In one example, the steel sheet, metal alloy coated, galvanized and / or annealed steel sheet is continuously coated with the presently disclosed dispersed mixture comprising metal particulate and a base polymer. In one example, the steel sheet, metal alloy coated, galvanized and / or annealed steel sheet may pass through a cleaning station with a rinse bath prior to being introduced to the dispersed mixture comprising metal particulate and a base polymer.

[0062] In one example, the continuous coating of the steel sheet, metal alloy coated, galvanized and / or annealed steel sheet with the presently disclosed dispersed mixture comprising metal particulate and a base polymer is performed using a continuous coating system that includes a continuous dispersing or agitating system, a coating station, a heating zone, a drying or curing zone, and a cooling zone. In one example, the steel may be air cooled by traveling through an air cooling tower or other cooling system.

[0063] FIG. 1 depicts an exemplary continuous coating process where coiled steel sheet is uncoiled using an uncoiler (with optional butt-welding capability) and fed into a accumulator. Optional pretreatment station receives the coil from the accumulator andthen the coil is coated on one or both sides and introduced to a finishing oven. In one example, the coil is continuously coated at a speed of about 0.5 meters per minute to about 25 meters per minute. Dried and / or cured coated coil is taken up by exit accumulator and recoiled.

[0064] In one example, the continuous dispersing or agitating system is configured to receive a continuous, semi-continuous, or batch quantity of a non-dispersed mixture comprising metal particulate and a base polymer and the mixture is dispersed using any conventional dispersing or mixing equipment designed to mix / agitate / disperse such compositions and / or high-solids mixtures so as to provide a dispersed mixture comprising metal particulate and a base polymer suitable for introduction to the steel sheet.

[0065] In one example, the application of the mixture comprising metal particulate and a base polymer of the present disclosure take places at a suitable part of the rolled sheet steel production process or after metal alloy, galvanizing, galvannealing. In one example, the mixture comprising metal particulate and a base polymer, after suitable dispersion / agitation is introduced to the steel using any coating application method known in the prior art, such as bar coating, air-knife coating, roll coating, spray coating, and continuous dip coating. In one example, the mixture comprising metal particulate and a base polymer, after suitable continuous dispersion / agitation is continuously introduced to the steel using any continuous coating application method known in the prior art, such as bar coating, air-knife coating, roll coating, spray coating and dip coating.

[0066] In one example, a surface treatment of the steel sheet, metal alloy coated, galvanized and / or annealed steel sheet is used prior to introduction of the dispersed mixture comprising metal particulate and a base polymer. In one example, the surface of the steel sheet is prepared using organic solvents to remove processing oils. In one example, the surface of the steel sheet is prepared using either abrasive pads or a stiff brush to remove all loose adherent materials. This can be followed by a hot detergent wash and rinsing. In one example, the substrate is pretreated, by means for example, pickling and / or sand / CO2 blasting.

[0067] After introduction of the dispersed mixture comprising metal particulate and a base polymer to the steel surface, the coated steel can be dried and / or cured by flashing off at room temperature or at an elevated temperature.

[0068] In one example, the layer on the steel substrate is dried and / or cured at a temperature of between 50-500 °C for 30min, 20min, lOmin, 5min or less.

[0069] In one example, the layer on the steel substrate is heated to an elevated temperature of above about 100 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, or more, at which temperatures the base polymer undergoes at least some pyrolysis and forms a ceramic coating comprising, among other things, silicon nitrides, silicon carbides, silicon oxides, and / or binary / ternary silicon-metal compounds. During the above heating at elevated temperature, e.g., above 850 °C, the steel substrate can be heat formed and / or hot stamped.

[0070] In one example, thin metal strips of steel with or without a metal alloy or zinc coating, are continuously coil coated, where the previously dispersed mixture comprising metal particulate and a base polymer is introduced to the steel surface, via rolls or by spraying to one or both sides of the metal strip ("coil"), heat dried and / or cured in a drying section, and then subsequently rolled up. In one example, rapid heating resulting in drying and / or curing at high temperatures is used, so that the strips can be processed at high speed through the coil coating installations.

[0071] The presently disclosed steel sheets (with or without a metal alloy or zinc coating) with a layer of the mixture comprising metal particulate and a base polymer to the steel surface after drying and / or curing demonstrate very good adhesion, are rigidly flexible. Hot-Forming

[0072] The presently disclosed steel sheet with a layer comprising metal particulate and a base polymer is suitable for direct hot stamping and indirect hot stamping. In the direct hot stamping process, a blank is heated up in a furnace, transferred to a die on a press, formed immediately, and quenched in the closed die. In the indirect hot stamping process, the part is prestamped at room temperature, heated up to austenitization temperature or above, and subjected to quenching.

[0073] Hot-forming experiments were carried out using a press-hardenable steel sheets as described above using a temperature range of about AC3+ (20°C to 200°C) and a soaking time of 60-600 seconds and subjected to non-isothermal plastic deformation at AC3+ (0°C to 60°C), and finally quenched to below 300°C with a cooling rate of about 10-100 °C / s. Tensile properties of the sheets were evaluated for their final mechanical properties andmicrohardness, with three replicates for each condition tested and the average yield strength, tensile strength, and microhardness values. The mechanical properties of the presently disclosed post-hot-formed coated press-hardenable steel were within performance specifications for hot-stamped parts of at least one OEM automaker. Post-Hot Forming Welding

[0074] The weldability of the presently disclosed coated press-hardenable steel was evaluated, using 600° Coupons with dimensions of 50mmxl50mm extracted from hot- formed parts produced with an optimized furnace residence time as determined above. The results were found to fall within industry reference guidelines for resistance spot welding (RSW) for coupons in a 2T homogeneous stack up configuration.

[0075] For a material thickness of 0.9 mm, the minimum required weld size diameter was determined to be 3.7mm, using 16x20mm ISO electrodes. The weld current range methodology consisted in determining the minimum current intensity (Imin.) for which the minimum weld size diameter (Dmin) of 3.7mm is obtained, and the maximum current intensity (Imax.) before expulsion occurs that produces the maximum weld size diameter (Dmax).

[0076] FIGs. 2A and 2B depict results demonstrating that a 2.0kA current range can be obtained for the presently disclosed coated press-hardenable steel hot formed coupons in a 2T homogeneous stack up configuration, using a welding current waveform with two prepulses of 5ms and 200ms at 2.5kA, followed by a weld pause of about 400 milliseconds.

[0077] An example of the welding current waveform is presented in Figure 3A. Figure 3B presents the current range obtained for the 2T homogeneous stack up configuration using an 8mm diameter electrode-face and 400ms welding time. It was observed that the minimum weld size diameter (Dmin) of 3.8mm was obtained at a minimum current (Imin) of 3.3kA, while the maximum weld size diameter (Dmax) of 6.1mm was obtained at a maximum current (Imax) of 5.3kA resulting in a 2.0kA current range.

[0078] Mechanical properties, Shear Tension and Cross Tension tests, of the welds of the presently disclosed coated press-hardenable steel samples were conducted on specimens produced using a welding schedule developed for the minimum weld size diameter (MWS). The average peak load obtained from the Shear Tension tests was 7.15KN, while the average peak load obtained from the Cross Tension tests was 1.61KN.

[0079] Tables 1A, IB. Mechanical properties, Shear Tension and Cross Tension tests, of the welds of the presently disclosed coated press-hardenable steel samples.

[0080] Compared to a control, presently disclosed coated press-hardenable steel with a thickness of 0.9mm aluminum, a current range of 1.2kA to 1.4kA was observed. The weldability performance of the presently disclosed hot stamped coated press-hardenable steel was observed to depend on the furnace residence time. Adequate RSW results were obtained using a waveform welding current with two pre-pulses of 5ms and 200ms at 2.5kA, respectively.Post Hot Stamped Corrosion Testing

[0081] The presently disclosed coated press-hardenable steel hot formed parts, produced with 2 minutes and 5 minutes of furnace residence time had no observed surface defects. The corrosion resistance performance of the presently disclosed coated press- hardenable steel hot stamped parts with 2 minutes and 5 minutes of furnace residence time was evaluated based on exemplary OEM Scribe Creep Test using an OEM specification. The exemplary OEM Scribe Creep Test involved scribing a diagonal line on each test panel and subjecting each panel to an accelerated cyclic corrosion testing for up to twelve weeks. The scribe creep was measured at six and twelve weeks.

[0082] An exemplary OEM Scribe Creep Testing specification for low alloyed steels and iron castings requires that the average of the scribe creep measured at 6 weeks to be less than 10mm.

[0083] FIG. 4 depicts results for the indicates that the average scribe creep value measured after 6 weeks, respective 12 weeks of accelerated cyclic corrosion increases with an increase in the furnace residence time, and in case of the presently disclosed hot stamped coated press-hardenable steel produced with 5 minutes furnace residence time, the scribe creep measurements after 6 weeks were observed to be approximately 3.5mm, but vary with coating thickness. By comparison, an aluminized coating is about 30-50 microns, whereas the presently disclosed coating target is about 8-20 microns, notwithstanding variations of the chemical composition, inclusion content, and processing paths of the aluminum coated control and that of the presently disclosed hot stamped coated press-hardenable steel. Nonetheless, the results of corrosion testing of the presently disclosed post hot stamped coated press-hardenable steel indicate applicability for interior structural parts located in dry areas, as well as exterior structural parts.

[0084] While certain embodiments of the present disclosure have been illustrated with reference to specific combinations of elements, various other combinations may also be provided without departing from the teachings of the present disclosure. Thus, the present disclosure should not be construed as being limited to the particular exemplary embodiments described herein and illustrated in the Figures, but may also encompass combinations of elements of the various illustrated embodiments and aspects thereof.

Claims

WE CLAIM:

1. A steel substrate comprising: a surface;0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 0-80 ppm boron; less than 0.1 wt.% phosphorus, less than 0.1 wt. %sulfur; less than 1 wt. % silicon; and less than 5 wt.% of other transition metals; a remainder iron and unavoidable impurities; and a layer comprising metal particulate and a base polymer adjacent the surface.

2. The steel substrate of claim 1, further comprising a galvanizing coating.

3. The steel substrate of claim 2, wherein the galvanizing coating is annealed.

4. The steel substrate of claim 2, wherein the galvanizing coating is adjacent the layer.

5. The steel substrate of claim 2, wherein the galvanizing coating is directly adjacent the surface and the layer is directly adjacent the galvanizing coating.

6. The steel substrate of any one of the previous clams, wherein the layer is cured and / or crosslinked.

7. The steel substrate of any one of the previous clams, wherein the steel substrate is a sheet, slab, or rebar.

8. The steel substrate of any one of the previous claims, wherein the steel substrate is hot rolled or cold rolled.

9. The steel substrate of any one of the previous claims, wherein the steel substrate is a transformation-delayed steel.

10. The steel substrate of any one of the previous claims, wherein the steel substrate is a TRIP steel.

11. The steel substrate of any one of the previous claims, wherein the steel substrate is a dual phase steel.

12. The steel substrate of any one of the previous claims, wherein the steel substrate is a multiphase steel.

13. The steel substrate of any one of the previous claims, wherein the steel substrate has a pre-hot formed tensile strength of at least 200 MPa.

14. The steel substrate of any one of the previous claims, wherein the steel substrate has a post-hot formed tensile strength of at least 500 MPa.

15. The steel substrate of any one of the previous claims, wherein the metal particulate comprises at least one of iron, titanium, nickel, cadmium, tantalum, zinc, zirconium, manganese, bismuth, molybdenum , chromium, aluminum, and oxides thereof.

16. The steel substrate of any one of the previous claims, wherein the metal particulate comprises a combination of at least two of iron, titanium, tantalum, zinc, zirconium, manganese, aluminum and oxides thereof.

17. The steel substrate of any one of the previous claims, wherein the metal particulate comprises a combination of at least two of aluminum, titanium, tantalum, zinc, and oxides thereof.

18. The steel substrate of any one of the previous claims, wherein the metal particulate comprises aluminum and / or aluminum oxides, or titanium and / or titanium oxides, zinc and / or zinc oxides, and combinations thereof.

19. The steel substrate of any one of the previous claims, wherein the metal particulate comprises a combination of aluminum, zinc, and oxides thereof.

20. The steel substrate of any one of the previous claims, wherein the weight percent of the metal particulate is greater than the weight percent of the base polymer in the layer.

21. The steel substrate of any one of the previous claims, wherein the weight percent of the metal particulate is less than or equal to the weight percent of the base polymer in the layer.

22. The steel substrate of any one of the previous claims, wherein the base polymer comprises at least one of polysilazanes, polysiloxazanes, polysiloxanes, and blends thereof.

23. The steel substrate of any one of the previous claims, wherein the base polymer comprises polysilazanes.

24. The steel substrate of any one of the previous claims, wherein the base polymer comprises polysiloxazanes.

25. The steel substrate of any one of the previous claims, wherein the base polymer comprises polysilazanes and polysiloxazanes.

26. The steel substrate of any one of the previous claims, wherein the base polymer comprises polysilazanes and polysiloxanes.

27. The steel substrate of any one of the previous claims, wherein the base polymer comprises polysiloxazanes and polysiloxanes.

28. The steel substrate of any one of the previous claims, wherein the base polymer comprises linear or cyclic oligomers of silazanes, siloxazanes, siloxanes, or mixtures thereof.

29. A method of continuously coating at least a portion of a steel surface, the method comprising the steps of:(i) continuously agitating a non-dispersed mixture comprising metal particulate and a base polymer so as to provide a dispersed high solids mixture suspension dispersions of the metal particulate and the base polymer;(ii) continuously contacting a steel surface with the dispersed mixture and providing a coated steel substrate;(iii) drying and / or curing; and(iv) providing a coated steel substrate.

30. The method of claim 29, further comprising annealing the galvanizing coating.

31. The method of claim 29, wherein, prior to step (i) or (ii), applying a galvanizing coating to the coated steel substrate.

32. The method of claim 31, further comprising annealing the galvanizing coating.

33. The method of any one of claim 29-32, wherein the non-dispersed high solids mixture is solventless.

34. The method of any one of claim 29-33, wherein the non-dispersed high solids mixture comprises an organic solvent.

35. The method of any one of claim 29-34, wherein the non-dispersed mixture comprises silicon oxide.

36. The method of any one of claim 29-35, wherein the non-dispersed mixture comprises a curing agent.

37. The method of any one of claim 29-36, wherein the continuous contacting comprises blade coating, bar coating, solvent dispensing, roll coating, dip coating, single or dual spray coating, vapor coating or combinations thereof.

38. The method of any one of claim 29-37, wherein the continuous contacting is carried out at a speed of between 0.5 m / min to 25 m / min.

39. The method of any one of claim 29-38, wherein the metal particulate is present at 40 weight percent to 90 weight percent of the total weight of the mixture.

40. The method of any one of claim 29-39, wherein the metal particulate is present at 50 weight percent to 90 weight percent of the total weight of the mixture.

41. The method of any one of claim 29-40, wherein the metal particulate is present at 60 weight percent to 90 weight percent of the total weight of the mixture.

42. The method of any one of claim 29-41, wherein the metal particulate comprises at least one of titanium, tantalum, zinc, zirconium, manganese, aluminum and oxides thereof.

43. The method of any one of claim 29-42, wherein the metal particulate comprises aluminum or aluminum oxides.

44. The method of any one of claim 29-43, wherein the metal particulate comprises a combination of at least two of aluminum, titanium, tantalum, zinc, and oxides thereof.

45. The method of any one of claim 29-44, wherein the metal particulate comprises a combination of aluminum, zinc, and oxides thereof.

46. The method of any one of claim 29-45, wherein the base polymer comprises at least one of polysilazanes, polysiloxazanes, polysiloxanes, and blends thereof.

47. The method of any one of claim 29-46, wherein the base polymer comprises polysilazanes.

48. The method of any one of claim 29-47, wherein the base polymer comprises polysiloxazanes.

49. The method of any one of claim 29-48, wherein the base polymer comprises polysilazanes and polysiloxazanes.

50. The method of any one of claim 29-49, wherein the base polymer comprises polysilazanes and polysiloxanes.

51. The method of any one of claim 29-50, wherein the base polymer comprises polysiloxazanes and polysiloxanes.

52. The method of any one of claim 29-51, wherein the base polymer comprises linear or cyclic oligomers of silazanes, siloxazanes, siloxanes, or mixtures thereof.

53. The method of any one of claim 29-52, wherein the substrate is hot rolled or cold rolled.

54. The method of any one of claim 29-53, wherein the substrate is a transformation- delayed steel.

55. The method of any one of claim 29-54, wherein the substrate is a TRIP steel.

56. The method of any one of claim 29-55, wherein the substrate is a dual phase steel.

57. The method of any one of claim 29-56, wherein the substrate is a multiphase steel.

58. The method of any one of claim 29-57, wherein the steel has a pre-hot formed tensile strength of at least 600 psi.

59. The method of any one of claim 29-58, wherein the steel has a post-hot formed tensile strength of at least 1500 psi.

60. The method of any one of claim 29-59, wherein the steel substrate is a hardenable steel.

61. The method of any one of claim 29-60, wherein the metal particulate comprises at least one of titanium, tantalum, zinc, zirconium, manganese, aluminum and oxides thereof.

62. The method of any one of claim 29-61, wherein the metal particulate comprises aluminum or aluminum oxides.

63. The method of any one of claim 29-62, wherein the metal particulate comprises a combination of at least two of aluminum, titanium, tantalum, zinc, and oxides thereof.

64. The method of any one of claim 29-63, wherein the metal particulate comprises a combination of aluminum, zinc, and oxides thereof.

65. The method of any one of claim 29-64, wherein the steel substrate comprises 0.05 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 0-80 ppm boron; less than 0.1 wt.% phosphorus, less than 0.1 wt. %sulfur; less than 1 wt. % silicon; and less than 5 wt.% of other transition metals; a remainder iron and unavoidable impurities.

66. The method of any one of claim 29-65, wherein the steel substrate comprises 0.5 to 0.5 wt.% carbon; 0.5-3.0 wt.% manganese; 2-80 ppm boron; less than 0.1 wt.% phosphorus, 0.001-0.5 wt. % sulfur; 0.1-2 wt. % silicon; and 0-5 wt.% of other transition metals.

67. The method of any one of claim 29-66, wherein the drying comprises infrared light, heat, air knife or combination thereof.

68. The method of any one of claim 29-67 wherein the drying comprises curing and / or crosslinking the base polymer.

69. The method of any one of claim 29-68, wherein the drying comprises heating at a temperature of between 50° C to 500° C.

70. The method of any one of claim 29-69, wherein the drying comprises heating at a temperature of between 100° C to 500° C for a time between 30 seconds to 400 seconds.

71. The method of any one of claim 29-70, further comprising coiling the sheet.

72. The method of claim 29, further comprising the step of hot-forming the coated steel substrate.

73. A hot-formed part made by the method of any one of claims 29-72.

74. The hot-formed part of claim 73, wherein the hot formed part comprises a surface coating having high temp oxidation resistance and electrical conductivity sufficient for welding the surface to at least one other metal surface.

75. The hot-formed part of claim 73, wherein the hot formed part passes automotive component weld testing criteria.

76. The hot-formed part of claim 73, wherein the hot formed part is weldable by spot welding, laser welding, or gas metal arc welding.

77. The hot-formed part of claim 73, wherein the hot formed part is continuously welded or weldable by spot welded.

78. The hot-formed part of claim 73, wherein the hot formed part is spot weldable at a current of 0.5 KA to 10 KA.

79. The hot-formed part of claim 73, wherein the hot formed part is laser weldable without edge ablation or removal of the surface coating about the weld area.

80. The hot-formed part of claim 73, wherein the hot formed part is laser cut without edge ablation or removal of the surface coating about the weld area.

81. A coated steel sheet made by the method of any one of claims 29-72.

82. The coated steel sheet of claim 81, wherein the coated steel sheet comprises a surface coating having high temp oxidation resistance and electrical conductivity sufficient for welding the surface to at least one other metal surface.

83. An internal combustion engine (ICE) vehicle, electric vehicle (EV), train, aerospace vehicle, submarine, ship, boat, structure, or building comprising the hot-formed part of claim 73 or 81.

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