WO1998018562A1

WO1998018562A1 - Manufacture of composite-metal slabs and flat-rolled product

Info

Publication number: WO1998018562A1
Application number: PCT/US1997/019623
Authority: WO
Inventors: Craig T. Costello; Leonard A. Jenkins
Original assignee: Weirton Steel Corp
Current assignee: Weirton Steel Corp
Priority date: 1996-10-31
Filing date: 1997-10-30
Publication date: 1998-05-07
Anticipated expiration: 1999-04-30
Also published as: ZA979819B; AU5154598A

Abstract

Melt-phase spray-coating methods and apparatus for melt-phase spray-coating of a plain-carbon steel slab (79) of selected carbon content in the range of .02 % C to .12 % C, with corrosion-resistant metal selected to have thermal expansion and contraction values compatible with those of the steel selected for such slab (79), for production of composite-metal product. Atomized molten metal of selected size is interspersed with solid particulate of selected particulate size to enable substantially uniform spray-coating of a prepared slab surface. Control of the temperature of slab (79), atomized metal and particulate metal, and hot reduction processing facilitate desired metal diffusion, desired bonding with the steel substrate (79) at the interface and adhesion during cooling and thickness reduction.

Description

MANUFACTURE OF COMPOSITE-METAL SLABS AND FLAT-ROLLED PRODUCT

RELATED APPLICATION

This application is based on Provisional Patent Application U.S. Serial No. 60/019,187, filed October 31, 1996. INTRODUCTION

This invention relates to processing plain-carbon steel slabs for surface protection of resulting product. More specifically, this invention is concerned with controlled processing of low-carbon steel slabs for melt- temperature spray-coating of corrosion-resistant metal and subsequent thickness reduction. PRIOR PRACTICE

In the past, selected non-ferrous corrosion-resistant metals, such as tin or zinc, have been applied to low- carbon steel substrate after the substrate had been cold- rolled to substantially its flat-rolled finish thickness gauge . SUMMARY OF THE INVENTION

The present invention departs from such prior practice by selective treatment and melt-phase spray-coating of an elongated low-carbon steel slab. In a specific embodiment, a continuously-cast elongated low-carbon steel slab surface is prepared and thermally-conditioned for melt-temperature spray-coating with a molten corrosion-resistant metal (such as austenitic or ferritic stainless steel) to form a composite-metal slab. Temperature control continues during initial thickness reductions and hot rolling to produce composite-metal sheet product having a low-carbon steel substrate and a metallurgically-bonded corrosion-resistant exterior finish metal surface. Specific procedures and contributions of the invention for achieving spray-coating and thickness gauge reductions are set forth in more detail in describing processes, apparatus and product with references to the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general arrangement box diagram for reference during description of procedures of the invention;

FIG. 2 is a schematic elevational view, partially in cross section, of apparatus for continuous casting of low- carbon steel slabs for carrying out the procedures of the invention;

FIG. 3 is a schematic elevational view, partially in cross section, and FIG. 4 is a schematic plan view, with portions in cross section, each for describing slab surface preparation in accordance with the invention;

FIG. 5 is a schematic elevational cross-sectional view for describing thermal conditioning of a low-carbon steel slab for melt-temperature metal spray-coating in accordance with the invention;

FIG. 6 is a schematic elevational view, partially in cross section, for describing melt-phase metal spray- coating of the invention;

FIG. 7 is a perspective view of a composite-metal slab of the invention;

FIG. 8 is a schematic elevational view, partially in cross section, for describing hot-reduction of a composite- metal slab in accordance with the invention;

FIG. 9 is a schematic elevational view, partially in cross section, for describing cold-reduction processing of flat-rolled composite-metal in accordance with the invention, and

FIG. 10 is an enlarged cross-sectional view for describing cold-rolled composite-metal product of the invention.

DETAILED DESCRIPTION OF THE INVENTION The general arrangement box diagram of FIG. 1 is referred to during description of procedures combined by the invention from slab production to a finish-gauge flat- rolled product.

A plain-carbon steel slab, indicated at stage 20 of FIG. 1, is produced by continuous casting apparatus as shown in FIG. 2.

The "plain-carbon" terminology is used to distinguish from stainless steel, and is referred to as low-carbon or mild steel in the present application. Low-carbon steel has a carbon content, by ladle analysis, in the range of about point zero two percent (.02%C) to about point twelve percent (.12%C); plus low percentages of elements, such as silicon, phosphorous and sulphur, and low percentages of certain metals, such as aluminum, manganese and, possibly, impurity level percentages of other metals.

The predominant physical and mechanical properties for the composite-metal product of the present invention are largely determined by selection and processing of the low- carbon steel substrate.

Low-carbon steel is preferably cast into a continuous- length strand (FIG. 2) with thickness capabilities of about two to about twelve inches and width between about twelve and about sixty inches; such strand is cut, after solidification, into elongated slabs having a length of about five to about forty feet. Continuous casting of the low-carbon steel is described more fully in copending, co- owned U.S. Patent Application Serial No. 08/639,524, which is incorporated herein by reference. Continuous casting is preferably correlated with slab preparation procedures of the invention, where permitted by available facilities, to conserve casting heat for thermal conditioning of the slab for spray-coating as taught herein. FIG. 2. schematically depicts continuous casting apparatus for steel used for manufacture of various flat- rolled products used in construction, household appliances, transportation car parts, etc. Molten steel 22, from ladle 24, is discharged into tundish 26 for continuous uniform delivery to cooled casting mold 28. The surface of molten steel 22 in the ladle and the molten steel 30 in the tundish are protected from atmospheric elements by slag cover 32 in the ladle and slag cover 34 in the tundish. Molten steel is delivered from the ladle and the tundish within a protective shroud at 38 and 40, respectively. The shrouds are purged with an inert gas to facilitate casting of clean steel . Casting powder 42 forms a liquid slag to protect the upper surface of molten metal 44 in casting mold 28 and, also, to decrease adhesion of cast metal to cooled mold surfaces as solidified metal 45 is formed. As the continuously cast steel strand 46 is withdrawn from the casting mold, exterior surfaces of solidified metal 45 are supported by rollers such as 47. Solidified metal 45, while enclosing remaining molten steel 48, increases in thickness as heat is removed from the strand by coolant sprays (not shown) . As cross-sectional solidification of cast metal nears completion (contiguous to location 49) , the strand is guided by rollers such as 50, on each surface, into a horizontal orientation for subsequent cutting into elongated individual slabs, such as 52.

One purpose of slab surface preparation, carried out at station 53 of FIG. 1, is to enhance surface bonding of melt -phase corrosion-resistant metal with the low-carbon steel slab. Thermal conditioning of the slab takes place at station 54 of FIG. 1 prior to melt-phase spray-coating. Corrosion-resistant metal for melt-phase spray-coating refers to a single metal, more than one metal, or an alloy selected to have a melt temperature significantly higher then a thermal conditioning temperature selected for the steel slab. Such corrosion-resistant metal and thermal conditioning temperatures of the slab, before and after spray-coating, are selected to enhance adhesion during solidification and cooling of the melt-phase spray-coating metal, and to facilitate desired proportional decrease in spray-coating metal thickness as the low-carbon steel slab is simultaneously decreased in thickness.

Slab surface preparation apparatus for molten metal spray-coating is shown schematically in the two views of FIGS. 3 and 4. Surface shot-blasting is preferably used (apparatus 55, FIG. 3) to remove surface oxide or other metallic compounds from a slab surface, or surfaces, to be spray-coated so as to present a clean surface. Surface grinding, or other abrasion means, can also be used to augment surface preparation. Controlled atmosphere, inert or slightly reducing, is maintained in enclosure 56 by means of gas inlets 57, 58. Surface debris is removed pneumatically through outlet means 59, 60 (FIG. 4) during surface preparation, which is preferably carried out prior to thermal conditioning of the slab for coating. Referring to the general arrangement view of FIG. 1, thermal conditioning is carried out at station 54 to provide a uniform temperature throughout the cross section of the slab as it is being spray-coated. Metal for spray- coating is provided at station 66 and such metal is prepared for melt-phase spray-coating at station 67.

An added contribution of the invention comprises combining pulverant solid metal with atomized molten metal during spray-coating. Preferably, such solid pulverized metal, provided at 68, is the same as the atomized metal for melt-phase spray-coating. However, use of solid particulate provides an opportunity for selecting a possible second metal, and offers other advantages. The solid particulate metal is prepared within a selected range of mesh sizes with temperature control at station 69 of FIG. 1.

Molten metal prepared for spray-coating is pneumatically atomized for melt-phase spraying onto a surface, or surfaces, of a temperature-controlled low- carbon steel slab. In specific embodiments described, mild steel slabs (carbon content of point zero two percent [.02%C] to about point twelve percent [.12%C]) are preferably cast with a thickness of about two to about ten inches. Spray-coating thickness on a slab surface is correlated with the selected slab thickness and desired surface protection which takes into account subsequent slab thickness reduction and flat-rolled steel processing.

When spray coating with atomized molten-metal and solid particulate metal, a range of particle sizes is selected so as to have substantially no solid particulate larger than the average atomized metal size. Heating of the solid particulate; and the temperature for slab reduction and coiling the hot-rolled product provide a type of sintering action for the coated metal. And, also, enhance metallurgical diffusion and bonding at the slab interface. Each helps to achieve adhesion and proportional thickness reduction of the steel substrate and the selected thickness spray-coated metal.

A desired corrosion-resistant finish coating for flat- rolled low-carbon steel stock can be achieved by spray- coating with a coating thickness on each surface of about .01" to about .05". To accomplish the same corrosion- resistant finish for a slab of about ten-inch thickness requires a spray-coating thickness of between about .05" and .25" per surface; the above-described temperature control steps increase metal diffusion and bonding at the coated interface.

In preferred spray-coating practice of the invention, a low-carbon steel slab is heated uniformly to a temperature which is selected in relation to the melt temperature of a melt -phase spray-coating metal. For example, when spray-coating ferritic or austenitic stainless steels, a melt temperature of about 2500°F to about 3000°F is used and the steel slab is thermally conditioned for melt-phase spray-coating to a uniform temperature of about 1500°F throughout the cross section of the slab as being coated.

Surface preparation and thermal conditioning of a slab are preferably coordinated for melt-phase spray-coating of a corrosion-resistant metal. Each is important for purposes of enhanced metal diffusion at the spray-coating metal interface and facilitating desired surface adhesion cooling and thickness-reduction processing; and, each contributes to uniform and complete surface coverage by the coating. Removal of iron oxide and other surface contaminants from a slab surface to be coated is carried out in a controlled atmosphere enclosure which prevents re-oxidation as surface debris is removed. A pristine steel slab surface, substantially free of iron oxide or other metallic compounds, and non-metallic molding powders or any non- ferrous material, augments diffusion at the interface of the melt-phase sprayed metal with the low-carbon steel slab surface. Cleansed slab surface (s) are preserved by a controlled atmosphere, which can be mildly reducing, during surface preparation (FIGS. 3 and 4) and during thermal conditioning of slab 52 in enclosure 70 of FIG. 5.

Thermal conditioning of the slab provides a uniformly controlled temperature extending through the cross section of the slab, as the coating is taking place. In practice, that temperature is established in an elongated chamber along the full length for the slab surface coating operation. Preventing surface oxidation and providing a uniform temperature throughout the cross section where the slab is being coated are important to adhesion. The thermal conditioning temperature for the slab is correlated with melt temperature for the selected melt-phase spray- coating metal. In a specific embodiment, melt-phase austenitic or ferritic stainless steel spray-coating is maintained while a mild steel slab temperature is provided which avoids too rapid cooling of sprayed metal; above 1500°F to about 2000°F is preferred. The coefficient of contraction of the metals during cool-down should be correlated; and, the temperature of the slab selected to function as a heat sink with major heat transfer from the melt-phase spray-coated metal being directed into the slab. Such directional heat transfer is augmented by the direct contact between the melt-phase spray-coating metal, at its elevated temperature, and the slab; and by control of the temperature differential between such sprayed metal and the slab.

Concurrent with such production and preparation of the steel slab, the corrosion-resistant metal is provided and prepared, as shown in the general arrangement of FIG. 1.

Molten spray-coating metal can be in a temperature range above 2000 °F to about 3000°F; for example, copper

(depending on percentage of zinc, manganese or aluminum) has a melt temperature from about 1500°F to 1980°F, nickel and nickel alloy from about 2350°F to about 2650°F, titanium alloy at about 3000°F, and austenitic or ferritic stainless steels have a molten metal temperature range of 2500°F to about 2800°F; also, physical properties, such as thermal expansion and contraction during cooling, and during reduction of the thickness of the substrate and coating metal, should be correlated with properties of the steel substrate; that is, stainless steel and other alloys selected should be about the same thermal coefficient of expansion as the steel being coated.

Molten corrosion-resistant metal is atomized for spraying in the melt-phase. The invention facilitates control of the consistency of the surface-applied coating by admixing temperature-controlled solid particulate metal as the molten metal is being sprayed. The molten corrosion-resistant metal is atomized for slab surface coverage, with prepared solid particulate metal being interspersed with the molten as atomized and as substantially concurrently spray-coated.

Average particle sizes of atomized particles and solid particulate are correlated for moderating the viscosity of the melt-phase spray-coating metal. The solid particulate is added so as to be interspersed with the melt-phase metal contiguous to the point of application. An objective is to facilitate achieving desired heavy thicknesses of spray- coated metal as applied, particularly when the length of the spray coating chamber is limited by available space in a processing line, and for heavier thickness slabs. Correlating the temperature of heated solid particulate metal helps to sustain desired slab conditioning temperature range during melt-phase spray metal coating as a portion of the heat of solidification is absorbed by the solid particulate.

Such preparation, selective introduction of solid particulate of the coating metal into the molten spray metal, and overspray recovery are carried out at stations 68, 69, 71 and 72 of the general arrangement view of FIG. 1. Quantitative, pulverant size and temperature control of added solid particulate are carried out contiguous to the spray-coating site to facilitate obtaining a uniform coat of increased thickness, while helping to avoid or diminish over-spray beyond the edges of the slab surface being coated. Melt-phase time and melt-phase temperature are important for enhanced diffusion of sprayed metal coating and slab metal. Such metal diffusion at the surface of the composite-metal slab helps to provide the desired metallurgical bonding, with melt-phase spraying helping to achieve coverage of the entire surface area, free of surface voids.

Solid particulate temperature, when introducing the same metal as that atomized, is selected below the melt temperature for the atomized metal; for example when spray- coating stainless steel, the solid particulate can be selected at about 500°F below that of the molten metal, and can extend to about one thousand degrees F below a 3000°F melt temperature for the metal being atomized while avoiding too rapid cool-down of the spray-coated metal in relation to the slab. Atomized particle size, solid pulverant size, and the relative quantity of solid particulate are correlated with the spray-coating to provide a desired consistency, avoiding splashing or running of molten metal while maintaining a temperature to achieve a desired adhesion, uniform thickness of the spray- coated metal, and a desired directional heat transfer to augment metal diffusion at a substrate interface. Preferably, desired coating thickness is accomplished in a single pass through a coating enclosure; the use of solid particulate helps to achieve that objective. The particulate metal temperature can be about 250°F to about 500 °F less than the molten metal when spray-coating copper and its alloys.

Melt-phase metal spray-coating onto a prepared surface of a thermally conditioned steel slab is indicated at 71 in the general arrangement of FIG. 1. Apparatus for melt- phase and solid particulate spray-coating are shown schematically in FIG. 6; such apparatus is maintained within a controlled-atmosphere enclosure 73 providing inert, or reducing, gas shielding for the prepared surface and the coating operation. Heated molten-metal crucible 74 can be located externally of enclosure 73. Crucible 74 delivers coating metal to tundish 75, which can also be located externally of enclosure 73, provided surface metal of such tundish is protected from contamination. Coating metal tundish 75 delivers molten spray-coating metal at a controlled rate for pneumatic atomizing. Atomizing gas is provided by means 76 to impinge on molten spray-coating metal. The consistency of the atomized melt- phase metal, as applied, is also selected so as to achieve complete melt-phase contact over the surface texture. A solid particulate metal injector 77 is positioned to deliver pulverant metal with a particle size selected to enhance interspersing with atomized molten sprayed metal. The solid particulate temperature is controlled by apparatus 78.

The following data indicate ranges of average atomized particle sizes, average solid pulverant sizes, and achievable coating thicknesses. TABLE I

Controlled atmosphere enclosure 73 protects the prepared surface (s) of low-carbon steel slab 79 for melt- phase spray-coating of metal as the slab is moved by slab support table rollers, such as 80. The melt-phase metal impacts the heated slab and adheres, as indicated at 81. Selected atomizing size for the molten metal, with added solid pulverant of selected size and temperature, help to establish the coating thickness desired for slab thickness reduction processing and to provide corrosion protection for the rolled product. Any overspray at edges of slab 79 is collected and removed at 84 (FIG. 6) .

Apparatus for atomizing molten metal can be obtained from Osprey Metals, Ltd., West Glamorgan, SA11 1NJ, U.K, and others. Molten metal atomizing apparatus, developed for purposes other than as taught herein, has been adapted by the invention for present corrosion-resistant spray- coating purposes. The admixing of temperature-prepared and selected size range solid pulverant metal into the molten metal, as atomized, facilitates achieving desired melt- phase coating thickness and uniform spray-coating; splashing or running of molten metal on the spray-coated surface is avoided.

Overspray accumulation collected at 84 (FIG. 6) can be reprocessed for reuse; the controlled atmosphere can be withdrawn for temperature control purposes, filtered and, by augmentation of protective gas and temperature control, can be prepared for reuse. In a preferred embodiment of the invention, shown in FIG. 7, low-carbon steel slab 86 is coated with stainless steel on each respective widthwise surface (coating 87, 88) . In such embodiment, a slab having a thickness of about nine inches and a stainless steel spray-coating thickness gauge of about one and three eighths inches per surface, after slab thickness reduction at an elevated temperature, and hot rolling followed by finish cold- rolling (as illustrated in later figures) , presents a composite-metal of about point zero two five inch (.025") total thickness; the corrosion-protective stainless steel coating thickness on each surface is about three thousands inch (.003") .

Referring to the general arrangement diagram of FIG. 1, a composite-metal slab can be cooled at station 90, without concern for corrosion of its coated surface, and shipped as a commercial product.

For heat conservation purposes, and preferred in-house steel mill practice, where in-line holding stations are made available, a slab as spray-coated at 71, FIG. 1, can preferably be thermally conditioned at stage 94 and hot reduced at 96 while minimizing unnecessary interim cooling by use of intermediate holding stations. Holding a coated slab at an elevated temperature improves metallurgical bonding at each coating interface; and can have a sintering effect on melt-phase metal as spray-coated with solid particulate metal .

An initial roughing-stand pass utilizing large- diameter rolls is preferably used to augment compaction and metallurgical bonding at each surface. Thickness reduction of the composite metal slab can then be carried out in multiple roughing-stand passes. An initial temperature for slab thickness reduction is selected at about 1500°F to 2000°F for a stainless steel coated slab. During subsequent hot rolling, as depicted schematically in FIG. 8, stainless steel composite-metal 98, after slab thickness reduction, is further reduced in thickness in a series of hot rolling mill stands, with rolls such as 100. The hot- rolled composite-metal strip 102 is progressively decreased to a thickness (about .1" to .2") capable of being wound into a composite-metal coil 104, as its temperature decreases to about 1250°F to about 1500°F.

A hot-rolled composite metal coil can be cooled at stage 106 of FIG. 1 (for shipment at 108 as commercial product) . Preferably, both the composite-metal slab product and the hot-rolled product are gradually cooled. The surface is cleansed at station 110 for cold-rolling at station 112 to a flat-rolled composite-metal finish gauge.

Cold-rolling of the composite-metal strip is depicted schematically in FIG. 9. Hot-rolled composite-metal 102 is unwound from hot-rolled coil 104 for cold-rolling in mill stands with rolls such as 113. Finished flat-rolled composite-metal product 114 is wound to form coil 116 for shipment (118 of FIG.l).

Cold mill processing can utilize a series of cold- rolling stands (tandem mill) while spraying with water emulsified rolling oil. Whether heat treatment subsequent to cold rolling is carried out depends on substrate, coating composition and desired properties for the finished composite-metal product. As taught herein, mechanical properties, such as ductility, are selected and established by means of cold rolling selection and heat treatment for the low-carbon steel substrate, which enables selection from a substantial range of low-carbon steel properties; while the coating metal provides desired corrosion- resistant properties for the surface.

The following data refer to substrate and coating thicknesses for a range of slab and spray metal coating thicknesses for a range of reductions.

TABLE II

The schematic cross-sectional view of flat-rolled composite metal of FIG. 10 depicts a preferred embodiment in which cold-rolled low-carbon steel substrate 120 has a corrosion-resistant coating on each surface (122, 124). Spray-coating of the invention enables uniform one-side coating, uniform coating of both surfaces, or differential coating thicknesses to be selected for the surfaces; differing surface coating characteristics can be established, by the above-described coating procedures, and by selection of characteristics for the coating metal for each surface .

Surface spray-coating of molten stainless steel has been described in a preferred embodiment which utilizes solid particulate to facilitate achieving uniform corrosion-resistant metal spray-coating of mild steel while controlling temperature of the slab, the coating metal (s) and hot thickness reduction procedures.

Combinations of apparatus for melt -phase spray-coating metal, other than as shown schematically in FIG. 6, are adaptable to the procedures taught; and, other types of rolling mills than those specifically named can be used in carrying out the invention without departing from its precepts .

Metal combinations, other than those specifically named, can be adapted to composite-metal production and processing, in light of the above teachings on relating physical properties of the steel and metal being spray- coated during combined cooling and thickness reduction. Also, selecting temperatures for melt-phase spraying, thermal conditioning of the slab and for thickness reduction processing can be adapted to other metals in view of specific embodiment teachings set forth above, Therefore, for purposes of evaluating useful applications of the principles and precepts of the present invention, reference shall be had to the language of the following claims in combination with the above description.

Claims

WHAT IS CLAIMED IS: 1. Manufacture of composite-metal, comprising (A) producing an elongated low-carbon steel slab of preselected length, width and thickness dimensions; (B) selecting and preparing corrosion-resistant metal in a melt -phase for thermal application to at least one surface of such slab; (C) preparing such slab surface by (i) surface cleansing, and (ii) selectively establishing and controlling slab temperature for correlated application of such melt-phase corrosion-resistant metal to such slab surface; (D) atomizing such melt-phase corrosion-resistant metal; (E) applying such atomized metal to such slab surface, and (F) solidifying such melt-phase metal on such prepared slab surface with such controlled-temperature steel slab functioning as a heat sink so as to remove heat from such atomized melt-phase metal as applied to such slab surface.

2. The method of claim 1, including selecting such corrosion-resistant metal to have a melt temperature in the range of about 1500 °F to about 3000°F; and with a coefficient of expansion which correlates with that of the steel slab for combined cooling and thickness reduction.

3. The method of claim 2, including heating such corrosion-resistant metal to at least melt temperature, and atomizing such corrosion-resistant metal within a selected range of atomized molten metal particle sizes, for directing, as atomized, toward such prepared slab surface .

4. The method of claim 3, further including selecting a solid pulverant metal selecting a particulate size range for such solid pulverant metal, heating such solid pulverant metal to a temperature less than melt temperature for such selected solid metal and less than such atomized metal, and introducing such heated solid pulverant metal so as to be interspersed with such atomized melt-temperature corrosion-resistant metal for directing toward such slab surface to facilitate spray-coating a uniform thickness coating on such slab surface at an elevated melt-phase spray metal temperature.

5. The method of claim 3 or 4 , including providing a low-carbon steel slab having: a carbon content in a range of about .02%C to about 0.12%C, with a thickness selected in the range of about two to about ten inches, a width selected in the range of about twelve to about seventy-two inches, and a length selected in the range of about five feet to about twenty-five feet; controlling temperature across such slab cross section, as such slab surface is being coated, selecting such slab temperature in the range of 250°F to 500°F less than such melt-phase spray coating metal; and subsequent to solidification of such atomized spray-coated metal, further including controlling temperature of such composite-metal slab such that the steel substrate has a temperature of about 1250°F to about 1500°F, and, at such controlled temperature: compacting such spray-coated metal, and decreasing thickness of such temperature controlled composite-metal slab.

6. The method of claim 5, in which decreasing thickness of such temperature controlled slab is followed by: hot-rolling to produce a flat-rolled composite- metal product of predetermined thickness gauge in the range of above about .025" to about .25".

7. The method of claim 6, further including cold-rolling such hot-rolled composite-metal product to a thickness gauge in the range of about .011" to about .025", with such corrosion-resistant metal thickness, per coated surface being in the range of about .0002" to about .003".

8. Metal coating process, comprising atomizing molten corrosion-protective metal for melt -phase application to a low-carbon steel slab surface, providing solid particulate metal, within a predetermined particle size range selected for interspersing with such atomized corrosion-protective metal, heating such particulate metal in a range above ambient temperature to less than such melt temperature for such atomized protective metal, and interspersing such heated solid particulate metal with such atomized molten metal for application of an admixed melt-phase spray-coating metal.

9. The process of Claim 8, including selecting such corrosion-protective metal from the group consisting of copper, copper alloy, nickel, nickel alloy, austenitic stainless steel, ferritic stainless steel, and titanium alloy for correlated thermal expansion and contracting values with those of the steel slab.

10. The process of Claim 9, including selecting such temperature-controlled solid pulverant metal from the group consisting of copper, copper alloy, nickel, nickel alloy, austenitic stainless steel, ferritic stainless steel, and titanium alloy.

11. As a new article of manufacture, an elongated low-carbon steel slab having a thickness in the range of about two to about ten inches and a width in the range of about twelve to about seventy-two inches, with a widthwise slab surface spray-coated by application of an atomized melt -phase corrosion-protective metal interspersed with heated sold-particulate compatible metal to provide a metal coating thickness in the range of .025" to about 1.375" per coated surface.

12. The product of manufacture of Claim 11, including selecting such atomized metal and solid particulate metal for thermal expansion and contraction values compatible with those of the slab steel selected from the group consisting of copper, copper alloy, nickel, nickel alloy, austenitic stainless steel, ferritic stainless steel and titanium alloy.

13. As a new article of manufacture, a low-carbon steel slab, with surface spray-coated with melt-phase corrosion- resistant metal selected for thermal expansion and contraction values which correspond to those of the low- carbon steel slab.

14. The article of manufacture of claim 13, in which such low-carbon steel slab, coated with melt- phase corrosion-resistant metal, is reduced in thickness and hot -rolled within a temperature range of about 1000°F to about 1500°F, and then cold rolled to a selected finish thickness gauge , with such a corrosion-resistant metal surface coating having a substantially uniform thickness in the range gauge of about .0002" to about .003" on such surface.

15. The article of manufacture of claim 13 , produced by thermally-controlling such low-carbon steel slab to have a uniform temperature between about 1250 °F and about 1500°F, atomizing melt-phase corrosion-resistant metal, interspersing such atomized metal with solid particulate metal, with each selected from the group consisting of copper, copper alloy, nickel, nickel alloy, austenitic stainless steel, ferritic stainless steel and titanium alloy, applying such atomized metal and interspersed solid particulate metal to such thermally-controlled slab by selecting from the group consisting of (i) one widthwise slab surface only coated to a substantially-uniform thickness gauge, (ii) both widthwise surfaces coated to uniform thickness gauge, and (iii) each surface coated to uniform thickness gauge per surface, with a differing thickness gauge being selected for each such widthwise slab surface.

16. The product of manufacture of claim 15, in which such low-carbon steel slab is selected with a thickness of about two to about ten inches, and such corrosion-resistant metal slab coating thickness gauge is selected in a thickness range of about point zero two inch (.02") to about one point four inches (1.4") for each such slab surface selected for such melt- phase spray-coating, so as to provide, after spray-coated slab thickness reduction, hot rolling, and cold roll processing, a coated steel substrate thickness gauge of about .01" to about .018" .

17. Apparatus for manufacturing a composite-metal slab for producing flat -rolled sheet metal, comprising (A) means for producing an elongated low-carbon steel slab of predetermined thickness, length and width dimensions, (B) means for surface conditioning of such elongated slab for corrosion-protective metal spray-coating, (C) means for thermally-conditioning such elongated slab uniformly across its cross section for such protective coating, (D) means for atomizing molten corrosion-protective metals for melt-phase spray-coating surface application to such slab, with such slab surface application being selected from the group consisting of (i) uniform thickness gauge coating on one such surface, (ii) uniform thickness gauge coating on both widthwise slab surfaces, and (iii) coating of each widthwise slab surface, with differing spray-coated metal.

18. The apparatus of claim 17, further including means for thermally-conditioning of such slab, prior to spray-coating, to a temperature in a range of about 1250°F to about 1500°F; means for decreasing thickness of such composite- metal slab at a temperature of about 1250 °F, and means for hot rolling such decreased thickness composite-metal to produce spray-coated flat-rolled steel of about .02 inch to about .25 inch thickness gauge 19. The apparatus of claim 17, further including means for cold-rolling such hot-rolled composite- metal to produce : a low-carbon steel substrate composite-metal with a thickness gauge in the range of about .01 inch to about .018 inch, and a corrosion-protective metal surface of uniform thickness gauge on at least one widthwise surface, in which such corrosion-resistant metal thickness gauge is selected in a range between about .0002 inch and about .002 inch. 20. Melt-phase spray-coating apparatus, comprising means for atomizing molten corrosion-resistant spray- coating metal, means for heating solid particulate corrosion- resistant metal to a temperature of about 1250°F, and means for interspersing such atomized metal with such heated solid particulate for melt-phase spray-coating of a solid metal substrate surface.