Abstract:
A method of producing steel with reduced internal stress concentrations is disclosed. In an embodiment, hot steel is shaped by a rolling mill. The resultant steel product is bundled as soon as practicable and the bundle is allowed to cool. Vibration energy is applied to the bundle of steel product so that internal stress concentrations within the steel product are relieved. In an embodiment, a plurality of bundles are stored on a rack and the rack is vibrated, the vibrations being transmitted to the plurality of bundles so that undesired internal stress concentrations within the steel products are relieved. Thus, improved steel is produced as well as improved steel that can be produced more rapidly than known techniques.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/526,243, filed Dec. 2, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the field of production of steel, more specifically to a method of making steel with reduced internal stress concentrations.  
         [0004]     2. Description of Related Art  
         [0005]     Methods for ferrous metallurgy are known, perhaps the most common method being the production of steel. Typically, iron ore and other various raw materials such as coke, limestone and dolomite are heated in a blast furnace to a sufficient temperature to melt the raw materials and allow them to mix. Slag is separated from the mixture and the remaining molten metal is transferred to a steel melting shop where further refining is done. The resultant crude steel can then be further refined with the addition of alloys that give the particular steel the desired properties. As is known, some of the above processes can be supplemented with the inclusion of scrap steel or iron. The resultant product is typically continuously cast into billets, blooms or slabs, sometimes referred to as “semis”, and these semis are then processed to form the final product. In some plants the product is cast directly into strip on strip casters. In others, the semis can be beam blanks or near-net-shapes to reduce rolling requirements.  
         [0006]     During the processing of semis, the semis are typically heated to a temperature sufficient to allow the semis to be worked, a typical such temperature being 1200 degrees Celsius. The semis are then processed by a rolling mill, the design of the rolling mill dependent on the desired shape of the finished product. The rolling mill, through the application of heat and pressure, forms the steel product. Thus, significant energy is used to shape the semis into the steel product.  
         [0007]     Steel product, in a final form, can be a variety of shapes and configurations. Steel product includes, for example, flat rolled steel, steel strip, bars, beams, wires, rods, sheets, plates, bands, channels, tubes, pipes, tracks, and rails. If the steel product is a bar or a beam, for example, it may be stored in bundles. When steel product is shaped into flat rolled steel, for example, it is often rolled into round coils. Steel product, when shaped into wire or rod, for example, is also often typically rolled into round coils. For ease of reference, coils of steel product will also be referred to as bundles unless otherwise noted.  
         [0008]     In general, there is a significant desire that the steel being produced have relatively constant dimensional straightness. Thus, significant resources are exerted in controlling the rolling mill process so that the finished product has the correct dimensions and straightness. Steel product with poor dimensional straightness control must be either sold at a lower cost, be reworked, or be reprocessed. The designation for out of tolerance straightness is referred to in the trade as camber or sweep; herein it will be called warp or warpage. Part of the process of producing steel product involves cooling the hot shaped steel to a temperature where the steel is dimensionally stable and/or can be stored. As is known, the rate at which steel cools has a significant affect on the properties of the steel due to, in part, the affect the rate of cooling has on the grain structure of the resultant steel product. Uneven cooling tends to produce stresses in the steel and such stresses may cause the steel product to warp or crack or otherwise suffer damage. When some coils are produced, it is necessary to retard the rate of cooling to prevent damage from stress. Special furnaces or other devices such as covers are used to control the rate of heat loss and temperature reduction.  
         [0009]     Therefore, substantial resources are devoted to ensuring the hot shaped steel cools at a desired rate. Often the hot shaped steel is controllably cooled on a cooling bed. Cooling beds, depending on the dimensions of the steel product, and the desired rate of cooling, can be quite long and can add significant cost to the production of steel because of the upfront capital expenditures required to create the necessary facilities. Sometimes the size of the cooling bed is a limiting factor in determining the rate at which the steel production facility can operate. In addition, the time needed to cool the steel increases the amount of work in process. Naturally, increasing the amount of work in process increases the necessary level of inventory, which in turn decreases the efficiency of the plant operation. In addition, higher levels of inventory make the steel production facility less flexible and potentially less able to respond quickly to variations in the quality of the steel product. Thus, a decrease in the level of inventory would tend to make a steel production facility more profitable while potentially increasing the quality of the steel product produced.  
         [0010]     For example, as is known in the art, when the steel product is a steel bar, the steel bars are first sufficiently cooled and then bundled together via straps and removed from the production line and typically placed in a storage facility until the steel product is transported to the customer. If the steel bars are bundled too soon, the interior portion of the bundle will cool at a slower rate than the exterior portion of the bundle. Also, the portion of the steel bar that is exposed to the outside air will cool more rapidly than the portion of the steel bar that is in contact with other bars. Thus, the exterior steel bars of the bundle will have internal stresses as a result of the disparate cooling rates. These stresses can cause the steel bars to warp once the straps holding the bundle together are removed, potentially making the steel bars unusable.  
         [0011]     Longer cooling beds relieve this problem but, as discussed above, are costly and inefficient to implement. As can be appreciated, general storage facilities are somewhat less costly to install and maintain as compared to cooling beds. And the storage facilities are usually a necessary requirement anyway. Thus, storing the steel in a storage area while the steel cools would be less costly from a facility investment perspective and this decreased cost could significantly benefit the profitability of the steel production facility. Therefore, it would be beneficial to be able to bundle the steel bars sooner (i.e., while still quite hot) without having to later rework the steel bars due to warpage caused by internal stress concentrations affecting the dimensional straightness of the steel bars.  
         [0012]     Once the steel product is delivered to the customer, the steel product is typically further processed to make finished goods. The processing can include machining the steel, drilling, punching, grinding, cutting, welding, cold working the steel, and various other known methods of processing steel into finished goods. During this process of working the steel, the initial internal forces are often unbalanced in the steel product. These forces tend to create localized stress concentrations in the finished good. As can be appreciated, a particular grade of steel can only withstand a particular level of stress before the steel deforms in an undesirable plastic manner. Thus, it is undesirable to have excessive internal stresses in the steel product prior to the steel product being processed into the finished good, because this additional processing can cause the internal stresses to distort the final product.  
         [0013]     Depending on the desired properties, even the localized stresses created by the processing of the steel product into the finished good may be undesirable. Therefore, various methods of relieving the stresses of finished goods are known. One method is to let the finished good sit for a substantial time so that the excessive internal stress concentrations have time to relax. Another method is to heat the finished good so that the internal stress concentrations can more quickly be relieved. Another method is to vibrate the finished good in a known manner, the vibrations providing energy that allows the stress concentrations to more quickly dissipate. While these methods of reducing the resultant stresses in the finished product are sometimes necessary, it is undesirable for significant variations in the stress concentrations to exist prior to the processing of the steel. Therefore, it would be advantageous to ensure the steel product, before being further processed, is essentially free of internal stresses or at least has a relatively constant internal stress level throughout the steel product.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     In an embodiment, the process of making steel bars includes the shaping of semis with a rolling mill. The bars, after being shaped by the rolling mill, are directed via a conveyor to a shearing, straightening and bundling station. The bars are then bundled while still at an elevated temperature. In an embodiment, the cut-to-length and bundled bars can be removed from the bundling station and allowed to cool in a separate location such as in a storage facility. Once sufficiently cooled, bundled bars are then vibrated to reduce internal stresses. The bars can then be unbundled without concern that internal stresses will cause the bars to warp. Thus, it is possible to reduce the size of the cooling bed so that the cost of building a steel production facility can be reduced.  
         [0015]     With the invention, in an embodiment, the rate of production through an existing steel production facility is greatly increased by allowing steel to move more quickly across the existing cooling bed because the requirement to wait for cooling to take place is reduced or eliminated by ignoring the stresses and then relieving those stresses at a later time. In this way, the cooling bed capacity is not the limiting factor on the rate of steel production, as is sometimes the case.  
         [0016]     With the invention, in yet another embodiment, the coils of steel strip are allowed to cool and then vibrated so that stresses are relieved. These stresses ordinarily cause the edges of the coiled strip to cool and contract more than the center of the strip, thus the edges can crack or the center of the strip can tend to bulge, when the coil is opened. Coils are often slowly cooled or even annealed and slow-cooled to help alleviate this situation.  
         [0017]     In still another embodiment the coils are vibrated while cooling to dissipate stresses that would otherwise form. This allows for a faster cooling rate.  
         [0018]     With the invention, in the case of coiled steel rod or wire, the coils are cooled in a series of loose loops as they pass along a cooling conveyor and sometimes through a quench tank of liquid coolant. The loose loops are then coiled on a mandrel and wire tied or strapped together. These coils also have stress concentrations where the loops are resting on each other as they move along the cooling conveyor line. The stresses can be relieved by vibration techniques and methods of the invention as described herein, after cooling. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which:  
         [0020]      FIG. 1  depicts a block diagram of a typical production of steel product.  
         [0021]      FIG. 2   a  illustrates a side view of a bundle of steel bars.  
         [0022]      FIG. 2   b  illustrates an end view of the bundle of steel bars depicted in  FIG. 2   a.    
         [0023]      FIG. 3  illustrates a side view of the bundle of steel bars as shown in  FIG. 2   a , depicting an embodiment of a method of vibrating a bundle of steel bars.  
         [0024]      FIG. 4  illustrates an alternative embodiment for vibrating a bundle of steel bars.  
         [0025]      FIG. 5   a  illustrates a side view of another alternative embodiment of a method for vibrating a bundle of steel bars.  
         [0026]      FIG. 5   b  illustrates a front view of the embodiment depicted in  FIG. 5   a.    
         [0027]      FIG. 6  depicts yet another alternative embodiment of a method for vibrating a steel product. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     The production of steel is a costly endeavor involving significant capital investment. Therefore, the amount of steel produced by a production plant needs to be quite large for the return on the capital investment to be positive. Thus, significant effort has been exerted to make the production of steel as streamlined and cost efficient as possible. It should be noted that while steel production is likely to enjoy the largest benefit from this invention given the volume of steel being produced, the production of other materials, including non-ferrous materials, having similar stress concentration issues could likewise benefit from this invention.  
         [0029]     Turning to  FIG. 1 , a block diagram of a typical steel production facility is depicted. In step  10 , the liquid steel is refined and the various other additives are introduced so as to produce the desired alloy. As is known, steel can have a varied composition. For example, stainless steel typically requires the addition of nickel and chromium.  
         [0030]     Once the liquid steel is ready, it can be cast into semis, such as billets, in step  20 . In step  30 , the hot semis, are shaped by a rolling mill. Typically, the semis are reheated in a reheat furnace and a series of inline rolling mills are used to form the steel product. In an exemplary embodiment, the semis are shaped into long lengths of shaped bars. The bars can be in the shape of an angle, channel, beam, round, flat, oval, railroad track rails or any other suitable specialized shape for use in a final end product.  
         [0031]     After being shaped, the hot shaped steel passes through a cooling bed in step  40 ; the cooling bed typically includes notched walking beams called rakes. The notched rakes help confine the bars to keep them from warping as they cool. Forced air or water can be used to increase the rate of cooling, with necessary attention given to metallurgical properties that may be altered by cooling.  
         [0032]     In step  50 , the long lengths of shaped bars are cut to the desired length and then run through a straightening machine to ensure the steel product is not warped. The steel product is then bundled is step  60 . In step  70 , the bundles are placed in storage until needed. Finally, in step  80 , the bundles are transported, often to the customer. Transportation can be over short or long distances. Common means of transporting steel product over long distances include trucks, trains, and ships.  
         [0033]     As discussed above, the term “bundle,” is not limited to bundles of bars of steel product but also encompasses other shapes such as rolled coils of steel product and also stacks of plates and sheets. In general, the term “bundle” is used to reference an amount of shaped steel that can be conveniently held together. As used herein, the term “steel product” includes any bar, rod, strip, sheet, plate, band, hot-band, beam, channel, tube, pipe, track, rail, wire, and structural and special shapes (such as, bed rails, window frames, fence posts, and so forth), of any shape and configuration, and made of any type of metal.  
         [0034]      FIG. 2   a  depicts an exemplary embodiment of a bundle  100  of steel bars.  FIG. 2   b  illustrates a close up end view of bundle  100 . As can be readily appreciated, if the bars are still hot, the exterior bars along the outer edge  105  of the bundle  100  will cool quicker than the interior area  110 . Thus, the bars on the outer edge  105  of bundle  100  will be especially likely to have localized stress concentrations. In addition, the act of rolling the bars will tend to create internal stress concentrations within the bars. Thus, bars created via a rolling mill are quite likely to have unwanted localized stress concentrations.  
         [0035]      FIG. 3  depicts an exemplary embodiment of the invention and includes the step of vibrating a bundle  100 . A support frame  120  is mounted to the bundle  100  via a clamp  125 . Connected to the frame  120  is a vibration generating device  130 . The bundle is supported by a plurality of support blocks  140 .  
         [0036]     As depicted in  FIG. 3 , the vibration generating device  130  is portable. Thus, the system can be moved from bundle to bundle as desired.  
         [0037]     Turning next to  FIG. 4 , an alternative exemplary embodiment of the present invention is depicted. A bundle  200  travels down a conveyer system  202 . The bundle travels over a plurality of rollers  245  that are mounted on a conveyer roll support  240 . As the bundle travels along, the bundle passes through a conveyer vibration section  212 .  
         [0038]     The vibration section  212  acts to vibrate the bundle while the bundle passes through the vibration section  212  so as to aid in reducing the internal stresses in the bars that make up the bundle. As depicted, the vibration section consists of a vibration isolator  215  that supports a support frame  220 . Mounted on the support frame  220  is a force cylinder  225 . The force cylinder  225  exerts a force on the movable support frame  250  that in operation exerts a force on a roller  245 . In turn, the roller  245  mounted to the movable support frame  250  prevents independent vertical movement of the bundle  200  by restraining the bundle  200  between two opposing rollers  245 . Mounted to the frame  220  is a vibration generating device  230 . The vibration generating device  230  provides a vibration energy that is transmitted through the support frame  220  and the rollers  245  into the bundle  200 .  
         [0039]     As can be appreciated, the time it takes the bundle  200  to travel through the vibration section, along with the amount of vibration energy supplied by the vibration device  230  determines the effectiveness of relieving internal stress concentrations.  
         [0040]     Another exemplary embodiment of the present invention is depicted in  FIG. 5   a  and  FIG. 5   b . As depicted, a plurality of bundles  300  is held in a storage rack  305 . The rack includes a frame portion  340 . The frame portion  340  is supported by vibration isolators  315 . As depicted, mounted to the frame portion  340  is a plurality of vibration generators  330 , each having the capability of providing different vibration forces or energy to the rack, or that the vibration force of one generator is not ordinarily aligned with the vibration force of a second generator, unless it is desired to augment the vibrations from the second. In between the plurality of bundles  300  are support blocks  345 . Support blocks  345  facilitate the addition and removal of bundles  300  and also serve to transfer vibration energy between adjacent bundles  300 .  
         [0041]     In an embodiment, a plurality of bundles of hot steel product is placed on the rack. The bundles are then cooled. The cooling can be via application of a cool liquid or a blast of air. In an alternative embodiment, the bundles can be cooled by allowing them to reach near ambient temperature through conventional heat transfer between the hot bundles and the cooler ambient air and surroundings. Vibrations are then applied to the frame portion  340  via the vibration generators  330 . In an embodiment, the level of vibration being applied to the frame portion  340  is lower than the vibration energy being applied during the conveyer method. Metal castings, for example, typically are allowed to age for an extended period of time so that the stress concentrations have time to be naturally relieved by seasonal changes in temperature and the like. The above embodiment allows for similar stress relief but on a much faster scale, such as within hours or days instead of months or a year.  
         [0042]      FIG. 6  depicts another exemplary embodiment of the present invention. As depicted, a bundle  400  is supported by a crane  420  via a cable  424  or chain or rigid member. A vibration generating device  430  supports the cable  424 . The vibration generating device is supported by a cable  425  which is in turn supported by crane  420 . A vibration isolator, similar to the vibration isolators described above, is located between the crane and the vibration generation device to protect the crane from unwanted vibration. Thus, the vibration generating device  430  can be used to vibrate the bundle  400  while the bundle  400  is being transported. In this manner, the bundle  400  can experience stress relief without the need to separately vibrate bundle  400  at some other location. Naturally, when vibrating the bundle  400  during transportation between a first and a second location, it is preferable that the bundle  400  be sufficiently cooled so as to avoid further accumulation of internal stress as a result of later cooling. Other types of cranes or mobile carriers would use a similar arrangement to that shown, including cranes such as overhead traveling cranes, or specialized mobile carriers as typically used in steel mills and steel warehouses. Vibrators and isolators would be suitably mounted to the transporter to allow the bundles to be vibrated in transit.  
         [0043]     The present invention has been described in terms of preferred and exemplary embodiments thereof. Numerous other embodiments, modifications and variations within the scope and spirit of the appended claims will occur to persons of ordinary skill in the art from a review of this disclosure.