BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates broadly to the field of metal production, solidification and casting. More specifically, this invention relates to an improved mold for a continuous casting system that has a longer useful life, is safer and more reliable, improves the uniformity of heat removal, is more stable dimensionally and turns out a better product having enhanced surface quality and decreased stresses than conventional continuous casting molds do.

2. Description of the Related Technology

Metals such as steel are continuously cast into strands by pouring hot, molten metal into the upper end of a mold and continuously withdrawing a metal strand from the mold's bottom. As the molten metal passes through the mold, the surfaces of the metal that are next to the mold walls are cooled, solidified and hardened to form a casing or shell of solidified metal around the molten metal in the strand. After leaving the bottom of the mold, the metal continues to cool and the casing or shell of solidified metal around the molten core thickens until the whole strand section is solidified.

A conventional continuous casting mold includes a number of liner plates, usually made of copper or copper alloy, and outer support walls surrounding the liner plates. The liner plates define a portion of the mold that contacts the molten metal during the casting process. Parallel vertically extending water circulation slots or passageways

are provided between the outer walls and the liner plates to cool the liner plates. During operation, water is introduced to these slots, usually at the bottom end of the mold, from a water supply via an inlet plenum that is in communication with all of the slots in a liner plate. The cooling effect so achieved causes an outer skin of the molten metal to solidify as it passes through the mold. The solidification is then completed after the semi- solidified casting leaves the mold by spraying additional coolant, typically water, directly onto the casting. This method of metal production is highly efficient, and is in wide use in the United States and throughout the world.

In order ensure consistent product quality and to prevent avoidable failure of the casting machine, steelmakers must arrange for periodic maintenance to be performed on the casting mold, and in particular on the liner plates, which are susceptible to wear and cracking during the casting process. AG Industries Inc., the assignee of this invention, is the leading provider of mold maintenance services in the United States. In order to recondition and prepare the outer, steel contacting copper surface of the liner plate for service, it is typically machined and then finished smooth by sanding or buffing. Sometimes, the outer surface is further plated with a material such as alloys of nickel or chrome to provide additional protection from the extreme heat that is transmitted to the copper from the hot metal being cast and the corrosive effect of the mold fluxes that are used in the casting process. A chrome plating is very porous on the microscopic level, and it has a tendency to deteriorate and come off of the copper during service as a result of corrosion, particularly in the in the meniscus region of the mold liner. A nickel coating is more resistant to corrosion, but is very crack sensitive at the elevated temperatures that the mold surface is operated at. Even a thin nickel coating has a tendency to crack in the meniscus region of the mold liner, where the heat is the greatest. These cracks can penetrate through the plating interface and into the base copper material of the mold liner. This lessens the life of the mold liner, because the copper must be machined when reconditioned to a depth that is sufficient to remove all of the cracks. In addition, cracking presents a safety concern during operation. If a crack propagates through the mold liner into a coolant passage, water from the coolant passage can leak into contact with the molten metal, which can be catastrophic. It is clear that there are a number of important reasons why crack formation in the mold liner and/or in the coating that is applied to the mold liner should be prevented. To this date, however, the industry

h.as not developed a satisfactory way to suppress formation of such cracking during high speed mold operation, and it continues to pose a problem to mold operators and to the companies that recondition and maintain the molds.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved mold liner for a continuous casting machine, and/or a coating that is applied to an outer surface of such a mold liner, that is more resistant to cracking during operation.

It is further an object of the invention to provide a process for making a crack resistant mold face of the type referred to above. In order to achieve the above and other objects of the invention, an improved mold liner for use in a continuous casting machine includes, according to a first aspect of this invention, an inner surface that is constructed and arranged to be connected to structure for conducting heat away from the mold liner during operation; and an outer surface that forms a casting surface of the mold, said outer surface being compressively stressed substantially throughout so that cracks are unlikely to initiate or propagate at said outer surface, whereby the mold liner will exhibit increased lifespan and improved safety with respect to mold liners heretofore known.

According to a second aspect of the invention, a mold for a continuous casting machine includes a plurality of mold surfaces for guiding and cooling a strand of molten metal as it hardens and emerges from the mold as a casting, each of said mold surfaces having an outer surface, and wherein said outer surface is compressively stressed substantially throughout so that cracks are unlikely to initiate or propagate at said outer surface, whereby the mold will exhibit increased lifespan and improved safety with respect to molds heretofore known. According to a third aspect of the invention, a method of making a strand of continuously cast material includes steps of introducing molten metal into a mold that includes a plurality of mold surfaces, each of the mold surfaces having an outer surface that is compressively stressed substantially throughout to suppress and prevent cracking of the outer surface; cooling the molten metal by conducting heat away from the molten metal through the mold surfaces; and moving the cast strand out of the mold.

A method of preparing a mold surface for a continuous casting machine includes, according to a fourth aspect of the invention steps of: machining the mold surface to a substantially smooth surface; and work hardening the machined surface substantially throughout to impart a residual compressive stress to said surface. A method of preparing a mold surface for a continuous casting machine according to a fifth aspect of the invention includes steps of machining the mold surface to a substantially smooth surface; and applying a controlled shot peening process to the machined surface to impart a residual compressive stress to said surface.

These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a diagrammatical cross sectional view taken through a continuous casting mold that is constructed according to a preferred embodiment of the invention;

FIGURE 2 is a diagrammatical cross sectional view taken through a component of the mold that is depicted in FIGURE 1 ;

FIGURE 3 is a diagrammatical cross sectional view, similar to FIGURE 2, depicting a mold that is constructed according to a second preferred embodiment of the invention;

FIGURE 4 is a diagrammatical cross sectional view, similar to FIGURES 2 and 3, depicting a mold that is made according to a third embodiment of the invention;

FIGURE 5 is a graphical depiction of the compressive-tinsel strength of a metallic material that has been treated by a controlled shot peening process; and

FIGURES 6A-6E diagrammatical ly depict a process that is preformed according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIGURE 1, a mold assembly 10 for a continuous casting machine typically includes a plurality of mold liners 12, of which four are illustrated, each of which has an outer surface 14 that together define the mold's casting surface 16. Each of the mold liners 12 further has an inner surface 18 that is mounted to an outer wall 20 of the continuous casting mold 10 for conducting heat away from the mold liner 12 during operation. To further help to conduct heat away from the molten metal/casting during operation, each of the mold liners 12 has a plurality of interior coolant slots 22 defined therein, which are supplied a coolant, usually water, via a coolant supply pipe 24, which is also depicted in FIGURE 1.

One particularly advantageous feature of the invention is that the outer surface 14 of the mold liner 12 is compressively stressed substantially throughout so that cracks are unlikely to initiate or propagate at the outer surface 14. Referring to FIGURE 2, it will be seen that the outer surface 14 includes a layer 26 of such compressively stressed material. Ordinarily, the mold liner 12 is fabricated from a thermally conductive material, most often copper or a copper alloy, which can be coated with a corrosion resistant material such as nickel or chromium, alloys thereof, or other material, such as diamond or a refractory material. In a mold liner 32 that is constructed according to a second embodiment of the invention, shown in FIGURE 3, such a coating 30 is applied over the layer 26 of compressively stressed material. In a third embodiment of the invention shown in FIGURE 4, the coating 30 is again applied over the layer 26 of compressively stressed material, and than an outer layer of the coating 30 is itself compressively stressed so as to deter cracks in the coating 30 itself, and stop them from spreading if in fact they do occur. The layer 26 of compressively stressed material shown in FIGURE 2, and the layer 36 of compressively stressed coating material shown in FIGURE 4 are preferably given their residual compressive stress by a working hardening process, which is preferably a controlled shot peening process that is applied to the outer surface 14 of the mold liner 34, and the outer surface of the coating 30, respectively. FIGURE 5 is a graphical representation of the compressive stress that is created in a surface by the shot peening process, such as the outer surface 14 of the mold liner 12. It will be seen that shot peening produces a significant amount of residual compressive stress immediately beneath

the surface, which is reduced, and actually transitions to a net tensile stress at a significant depth beneath the surface.

Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts a tiny peening hammer, imparting to the surface a small indentation or dimple. In order for the dimple to be created, the surface fibers of the material must be yielded in tension. Beneath the surface, the fibers try to restore the surface to its original shape, thereby producing below the dimple a hemisphere of cold worked material that is highly stressed in compression. Overlapping dimples develop an even layer of metal in residual compressive stress. In other industries, such as aircraft manufacturing, it is well known that cracks will not initiate or propagate in a shot peened zone. However, to the inventors knowledge, this technique has never been utilized in the manufacture or refurbishing of continuous casting mold parts.

Referring now to FIGURES 6A through 6E, a process for reconditioning a moldface of a continuous casting mold will now be discussed. Typically, a number of cracks 38 and or imperfections will be found in the moldface surface after it has been in use for some time. The moldface must be machined to a depth that is sufficient to remove those cracks and imperfections, exposing a fresh relatively smooth surface of copper 40, as is shown FIGURE 6B. In the preferred embodiment of the invention shown in FIGURE 6C, this surface 40 of a main body 28 is then work hardened by bombarding it with small spherical particles at a controlled velocity as described above in a shot peening process, which creates the layer 26 of compressively stressed material that is shown in FIGURE 6C and is discussed above. Some molds might be buffed and polished to smooth out the outer surface of the compressively stress layer 26 at this point, and put back into service. Other molds, however, where additional coating is desired, will be coated with a layer 30 of a material such as nickel or chromium, or with a nonmetallic material such as refractory or diamond. In some molds, this material will be polished and the mold can be put into service. In other molds, where the coating 30 is metallic, it may be desirable to again use a shot peening process to form a work hardened compressively stressed layer on the outer surface of the metallic coating 30, as is shown in FIGURE 6E. As discussed above, this additional layer 36 of compressively stressed material will act as a further deterrent to the

initiation and propagate of cracks that might otherwise begin in the layer 30 of metallic material.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.