removes less material for the same mean depth of grind and increases the life of the grinding wheel.

The development of continuous casting to produce semi-finished products is now so far advanced that almost any grade of steel can be continuously cast, and in the most appropriate cross section for further shaping.

Many steel grades may show surface defects after casting such as cracks or other surface defects. In such cases the semi-finished products must be conditioned before rolling. The removal of defects is either performed selectively by removing the specific defect, or by a complete surface treatment. High quality finished products can only be produced by using defect free slabs, blooms or billet.

The necessary surface treatment of semis can be achieved by flame scarfing or high pressure grinding. Flame scarfing, the quickest and most economical way of surface treatment, is performed on steel grades, which, after this intense localised heating, are not susceptible to defects caused by such treatment. Other steel grades must be treated by grinding out the defects using high-pressure wheel grinders. Such steels are classified into grades that are either ground in the cold condition or others which, because of their compositions, require grinding while still hot (between 300 and 500°C). Grinding slab at a temperature of 700 to 900° (shortly after casting) is also possible, should the material flow or metallurgical reasons necessitate it.

There are differently concepts for high pressure grinding machines on the market. The main difference in the design of the machines is the method of feed, which is executed either by the movement of the work piece or by moving the tool.

If the cast semi is moved, the length of the grinding machine has to be double that for when the tool is moved over the semi. There are several advantages of moving the tool rather than the work piece. In addition to the space saving, the removal of swarf by suction is easier as the equipment is stationary and also deadening of the noise of grinding can be more easily accomplished. Another advantage is that, because of its smaller size, the tool-feed machine can be moved to a new location if necessary without difficulty since no foundation is needed.

Grinding angle
The finish after grinding is greatly influenced by the angle of the grinding head to the longitudinal axis of the semi. Grinding at 90° or 45° has prevailed, with the current trend within the industry leaning towards 45° . All existing machines on the market offer both angles(1)

Surface defects can be easily recognised within the grinding marks at either angle. The wheel at 45°, however, offers technical advantages in grinding and for later rolling. These are illustrated in Figs 1 & 2 . In essence, at 45°, the grinding wheel is operated in back and forth strokes on the contact area resulting from the line contact of the grinding wheel and the depth of the grind. In contrast, at 90° the grind wheel theoretically grinds in a back and forth movement using in turn the respective leading edge of the grinding wheel. The contact arc depends on the depth ground. In practice, the grinding stone adjusts to the wear of the two edges, so that as it operates in the back and forth movement it grinds only approximately half the theoretical width at each pass.

It can be seen in Fig 2 that each pass when grinding at 90° results in a circular indentation on the surface of the semi, equal to the radius of the grinding wheel. In contrast, with the wheel at 45°, a smoother flatter grind is achieved. For the same depth for inspection, the 45° grind will remove less material and result in a finer surface texture.

Another effect of a 45° grinding head is a reduction in noise due to better contact between the grinding wheel and the work piece.

The advantage of a 45° grind in subsequent rolling is obvious when viewing the grinding marks. Fig 3 demonstrates this. While the grinding marks from the 90° grind are stretched parallel after rolling, the marks from a 45° grind extend more in the rolling direction. This lessens the risk of cracks caused by the grinding marks.

Machines
The manufacturers of high pressure grinding machines supply the market with machines, capable of realising the individual customer's needs. Fully automatic special machines for certain purposes are available eg machines which simultaneously grind the broad and narrow sides of a slab. They are predominately designed with the main feed executed by the work piece.

Grinding wheel suppliers offer a wide range of wheels for hot and cold grinding as well as for the different steel grades. They also provide detailed information on grind stone performance for different grinding conditions. Generally, consumption of grinding stones for hot grinding is higher than for cold grinding, and grinding at 90° uses more grind stones than at 45°.

Egon Evertz KG, a service company which has been working for the steel industry for more than 45 years, has chosen a policy which entails the design and building of purpose built grinders which it then operates itself in the steelworks providing a subcontracting service. This philosophy differs from most other builders of grinding equipment who sell their machines to the steelworks or third party service providers. Evertz believe they can provide a grinding service more economic to the steelmaker in this way. In 1973, Evertz developed and operated the first high pressure-grinding machine for semi-finished products in a steel plant. Even then, the stipulation was ease of mobility, no foundations, and universal use and ease of operation of the machine. Consequently, it was decided to have the main feed executed by the grinding wheel. This basic principle has also been adhered to throughout further developments for higher performance machines. Even Evertz's semi-automatic machines, with their potential for multi-machine operation, follow this principle.

Evertz's maxim from the start, and still valid today, is: 'We do not sell machines, we design and construct them for use in the steel industry under our responsibility'. The Evertz company now works as a service contractor with its own machines and equipment in many steel works worldwide.

For example, the high pressure-grinding machine, type RSM 690/90, is a universal machine capable of hot and cold grinding in either 90° or 45° grinding direction. It can also be modified to any required grinding angle by changing the grinding arm. The grinding power is 90kW, the feed is executed mechanically and the contact pressure between the wheel and the semi is controlled hydraulically. The grinding level is also regulated hydraulically, a necessity for grinding narrow and broad slab sides at their appropriate levels as well to treat unfinished material of various dimensions on the grinding tables. The angle of the grinding arm can be regulated in the horizontal position from -8° to +8°. This compensates for the approximately 400mm deviation in the height of slabs, billets and blooms as well as for wear of the grinding wheel. The standard model is designed for a grinding stroke up to 2500mm. Since the grinding head is moving on rails, the length of the unfinished material can vary. The rate of removal of material is between 200 and 500kg per hour depending on grinding conditions and requirements. The latter rate normally achieves a surface roughness of 90 to 200µm depending on the number of grinding passes (1, 2 or 3). Since the contact pressure is infinitely and sensitively variable, surface finishes of 30µm can be met in exceptional cases.

The machine is equipped with a control system to enable automatic grinding of predetermined surfaces. This facilitates multi-machine handling by a single operator.

Since these machines create swarf and dust along the entire working length, a dust exhaust plant running parallel to the machine is provided. The swarf is collected in containers behind the machine or directly removed by a continuously operating transport device.

However, not only is the high pressure grinder an important factor for the economic surface treatment of semis, but also the handling of slabs, billets and blooms plays a very important part in the economies of the operation. Separate turnover devices facilitate the handling of the semis. Fig 4 shows a tilting device for the grinding of narrow slab sides. Evertz also designs isolating units and turn-over devices for billets as well as the necessary rotating devices for round, multiple and square sections.

With multi-machine use, these handling devices, including the necessary crane, can be controlled from any of the grinding machines.

Should a large quantity of narrow slab sides need to be ground, the tilting device can be replaced with an edge grinder unit on the grinding machine. This enables both edges to be simultaneously ground as well as the broad sides.

The machines and devices are especially suitable for:

    the grinding of billets, all over or partially on defects;
    the grinding of slabs, all over grinding or partially on defects;
    deburring of torch slit slabs;
    test line grinding;
    grinding of round, polygon or square sections.

 

This variable system offers a favourable profit ratio with respect to capital expense and machine performance. A single high pressure-grinding machine with a turnover and a tilting device can be economically operated in a single shift operation. When the deburring work is completed, individual defects can be selectively treated or test lines can be ground. Fig. 5 shows such a machine deburring a slit slab. Alternatively, when high throughput is required, several machines can be operated simultaneously. Fig 6 shows part of a multi-machine operation. As an example, should a yearly quantity of 500kt of slabs each 200mm thick, need to be hot ground, then six Evertz high pressure grinding machines with two turn-over devices would be sufficient to perform this work. The grinding machines will be either in a single line or placed in two parallel rows, the latter configuration having the advantage that crane movement is minimised Fig 7 The use of banks of machines is particularly important when hot grinding. If only a single machine is used, any break down of this will result in the slabs cooling and having to be reheated. If a number of machines are operated, breakdown of one unit can be overcome by reassigning slabs to the other machines so avoiding the need to reheat. Similarly, if one of the turnover devices breaks down, the other can take over its workload. The slab turning time is calculated to complete more than 12 turns per hour although a somewhat longer crane transport distance for the slabs normally turned by the failed machine will be necessary. Universal & special grinders Both types of high pressure grinding machines, the special machines and the universal machines, have advantages and disadvantages. Both work economically when grinding large quantities of semi-finished products. A single special machine has the grinding capacity of three universal machines, but the depreciation cost to buy and install three universal machines along with a manipulator is less than that for the single special machine with its integrated transport and turnover devices. In grinding shops for semi-finished products, the universal machines are generally best suited for single-machine-operation with the option for installing more machines later. This is advantageous in cases where preliminary surface grinding tests on unfinished materials are to be performed. For problems that could eventually occur, Evertz can provide the know-how, gained from its many years of experience. Evertz's principle 'Construction and operation in the same hand' has made it possible to accumulate, combine and evaluate the experience acquired throughout 45 years in machine design and grinding. Swarf handling When a slab weighing, say, 20t has 2% of its material removed in grinding, 400kg of swarf results. This is re melted in the steel plant, but the steel industry knows of the difficulties in handling loose swarf. Evertz recently introduced a service to handle this swarf by briquetting it. The swarf is pressed without any additives under high pressure to form blocks of a firm consistency, which can be charged to the steelmaking furnace without difficulty In conclusion, it must be noted that in addition to other activities, Evertz designs and operates scarfing machines for the treatment of semi-finished products as well as flame cutting machines for the slitting and cross-cutting of slabs. Steel-Times International September 2002 Dr. Engineer Rolf Seybold, a member of the Egon Evertz KG advisory council