WO2000003815A1

WO2000003815A1 - Continuous reduction of mill scale on hot rolled strip steel

Info

Publication number: WO2000003815A1
Application number: PCT/US1999/011211
Authority: WO
Inventors: Edgard A. Franco-Ferreira
Original assignee: Ut-Battelle, Llc
Priority date: 1998-07-14
Filing date: 1999-05-20
Publication date: 2000-01-27
Also published as: AU4309699A

Abstract

Mill scale is removed from strip steel (2) by passing the strip (2) into a chamber (7) containing a surface heater such as an induction coil, and contacting the mill scale within the chamber (7) with countercurrently passed reducing gas such as hydrogen. More than a stoichiometric amount of hydrogen is used; the exiting gas is chilled to remove water and the unused reducing gas is recycled.

Description

Continuous Reduction of Mill Scale on Hot Rolled Strip Steel

Technical Field

This invention relates to a continuous process and apparatus for the treatment of hot rolled strip steel to remove the oxide layer from the strip steel.

Government Rights

The United States Government has rights in this invention pursuant to Contract No. DE-AC0596OR22464 between the United States Department of Energy and Lockheed Martin Energy Research Corporation.

Background of the Invention

Steel mills commonly produce strip steel by hot rolling. The strip steel is typically prepared in the form of coils which are subsequently further processed by cold rolling, galvanizing, painting, and other treatment. Iron oxide scale, known as mill scale, is almost universally formed on the strip and must be removed in order for most such subsequent operations to be successful. The removal of the scale is almost universally accomplished by acid pickling, but this practice has been criticized because it generates large quantities of acidic waste. Continuous pickling lines are physically very long and expensive to install and maintain. A different method of removing mill scale, free of the above-mentioned problems, would be highly desirable for the industry.

Nevertheless, prior to the present invention, little has been done commercially to replace acid pickling of steel strip. Acid pickling is effective and well known, and less expensive processes are difficult to find and implement. Hydrogen has been used to reduce iron ore and for other reduction purposes in steel mills, but I am not aware of its use in conjunction with an induction coil to reduce mill scale.

Summary of the Invention

My invention is a process and apparatus for removing mill scale and/or other forms of iron oxide from strip steel. In the process, the strip steel is unrolled from a coil or obtained from another source, fed through an induction coil to heat the surfaces of the strip to about 1200°F (about 650°C) and exposed to a hydrogen atmosphere to reduce iron oxide on the surfaces of the strip. The process takes place in a reaction zone or chamber for containing the hydrogen. The induction coil is energized by a high frequency current which induces eddy currents in the strip, causing a rapid increase in temperature on the surfaces of the strip. The apparatus comprises a solenoid induction coil, preferably rectangular in shape, for surrounding the strip as it is continuously passed through the coil, a hydrogen supply, and a chamber or housing to contain and flow the hydrogen atmosphere.

The hydrogen gas is preferably passed through the apparatus countercurrently to the strip. It is not heated, and accordingly will tend to cool the strip as the strip exits the apparatus. As the hydrogen passes along the heated surfaces of the strip, it reacts with the oxides present to reduce them, combining with oxygen and forming water vapor, which is carried out with the unreacted hydrogen. The gases exiting the chamber or reaction zone are passed through a chiller which serves not only to reduce the temperature of the unreacted hydrogen for recycling, but also to condense and remove the water vapor. The strip leaves the process enclosure at surface temperatures preferably not greater than about 250°F (about 121 °C).

Broadly, my invention includes a method for continuously reducing oxides on the surface of strip steel by contact thereof with a reducing gas comprising continuously passing said strip steel to a heating zone, heating said surface of said strip steel to a temperature sufficient for said reducing gas to react wih said oxide to remove oxygen therefrom, and continuously contacting said surface of said strip steel with said reducing gas.

Brief Description of the Drawings

Figure 1 is a graph showing the more or less idealized hydrogen consumption per ton of steel per mil of scale thickness, as a function of strip thickness.

Figures 2a and 2b are more or less diagrammatic overhead and end views of the strip passing through the solenoid coil.

Figure 3 is a flow sheet for the overall process, including the chiller.

Figure 4 is a schematic of the equipment for an experiment in support of the concept.

Detailed Description of the Invention

As indicated above, the invention is a method and apparatus for removing mill scale from strip steel. Strip steel is conveniently continuously processed. For the present description, the strip steel may normally be unrolled from a coil, but it should be understood that the process could be applied as the strip is moving from a mill or otherwise from a source other than a coil.

Referring now to Figure 1, it should be understood that hydrogen consumption is assumed to be stoichiometric for the oxides available for reduction on the strip surface. That is, the consumption of hydrogen is assumed to be an essentially linear function with respect to the oxygen content of the mill scale, regardless of the thickness of the scale. The oxides are assumed to be in the form of Fe₃O₄ and on both sides of the strip. Given a constant mill scale thickness, a series of plots of hydrogen consumption per ton of steel will vary in slope with the thickness of the strip. Since the industry commonly calculates its cost in terms of tons of steel, Figure 1 plots calculated hydrogen consumption per ton of steel per mil of scale against strip thickness. Width of the strip is not a significant factor. Persons skilled in the art will recognize from Figure 1 that hydrogen consumption is economically attractive for a wide range of strip products.

For a 1/8 inch (about 3mm) thick strip 75 inches (about 2 meters) wide, having a typical oxide thickness of 0.0005 inch (about 0.0125 mm) traveling 2 feet (61 cm) per second, hydrogen consumption is about 66 cubic feet per ton, and power consumption for the induction coil will be about 13 Mw/hour. A continuous flow of about 300 cubic feet of hydrogen per ton of steel should be maintained; about 80 percent of this will normally be recycled. Generally, a flow of at least 0.0167 hydrogen must be maintained for each 0.0001 inch of scale thickness per square foot of strip steel, taking into account the surface on both sides, for stoichiometric reduction.

In Figure 2a, induction coil 1 surrounds the steel strip 2 as it passes (as depicted) from left to right. Coil 1 is connected by cables 4 to a suitable power supply 3, capable of providing 13 Megawatts of power. The steel strip may move at speeds from 1 to 5 feet per second through the coil. Figure 2b illustrates that the coil 1 may be advantageously quite close to the strip. Hydrogen will fill the entire space 5 between the coil 1 and strip 2. Referring again to Figure 2a, the hydrogen in space 5 passes from right to left, as depicted. As a rule of thumb, about 113 kwh are required to heat a ton of steel to 1200 degrees F. However, it should be kept in mind that we are only interested in heating the surfaces of the strip, and the heating process may be modified accordingly. For example, high intensity infra-red radiation may be used..

In Figure 3, the overall system is depicted as a flow sheet. Here, hydrogen from a source 8 is continuously pumped by blower 16 through conduit 9 and past seals 10 into housing 7, which contains the coil 1. The hydrogen passes from conduit 9 through housing 7 from right to left, as depicted, contacting the exiting end of strip 2 first. A slight positive pressure is maintained in housing 7 to prevent the entrance of air. Steel strip 2 passes into the housing 7 from left to right, as depicted, and is immediately heated by coil 1. Throughout its passage through housing 7, the strip is countercurrently contacted by hydrogen on both sides of strip 2. Flame curtains 11, which may comprise natural gas burners, guard against explosions from the small flow of exiting hydrogen where strip 2 enters and exits the housing 7. Housing 7 is also equipped with hoods 17 for evacuating combustion products from the flame curtains.

Gas exiting from housing 7 is collected in conduit 12 and passed to chiller 13, where it is cooled. As a consequence of cooling, water vapor condenses from the gas and is removed in pipe 14. The remaining gas after water removal is essentially unreacted hydrogen, which is recycled through conduit 15 for combination with make-up hydrogen in conduit 9.

Persons skilled in the art will recognize that the space 5 between the coil 1 and strip 2 is minimal to save energy. However, it is not considered desirable to approach the minimum stoichiometric amount of hydrogen to reduce the oxides in one pass, for two reasons - first, the water vapor (reacted hydrogen) obtained will tend to reduce the efficiency of the reduction process as it builds up, and, second, the cooling function of the hydrogen will be impaired if there is too little hydrogen available for the purpose. A high flow of hydrogen will accelerate the cooling of the strip, improve contact between surface oxides and unreacted hydrogen, and minimize the length of the enclosure. A preferred rate of hydrogen introduction is about 4 to about 6 times the stoichiometric amount (preferably about five times) required for theoretical stoichiometric reduction of the oxides being introduced to the process. But, in spite of the high flow, hydrogen consumption is limited to the amount actually used in the reduction reaction, plus the amount which is lost through leakage at the strip entrance and exit. Experimental Support

A proof of principle experiment was designed to test the concept described above. The setup for the experiment is shown schematically in Figure 4. This experiment involved static tests on typical hot-rolled steel in a small hydrogen- filled enclosure. An existing high frequency induction unit 18 including pancake coil 19 was used to provide the heating energy. This unit operates at a frequency of 450 KHz and a maximum power of lOKw. According to the manufacturer, the 450 KHz frequency will induce heating to a depth of 0.003 inch (about 0.075 mm). Data was obtained to measure oxide-reduction time (as determined by visual observation) as a function of induction unit power output and coupon thickness.

The pancake-type coil 19 was five inches in diameter (about 12.7 cm) and used in all the tests reported below.

Three initial test runs were performed in air to maximize the coupling of the power supply to the load. These runs were performed on a carbon steel plate 20 six inches by 6 inches by 1/8 inch (about 15.25 cm square and 3 mm thick) which had a thermocouple attached to the center of the side opposite that which faced the induction coil. A pancake coil does not surround the specimen, but is parallel to the specimen's surface.

For the three test runs, the instrumented plate was set up parallel to the pancake coil at spacings which varied from 1/8 inch (3 mm) to % inch (6 mm). A strip chart recorder was connected to the thermocouple and the time for the center of the plate back side to reach 600°C was measured. Optimized tuning of the power supply to the load resulted in a time of 30 seconds to reach 600°C. It was apparent from observing the incandescent heating pattern on the plate that the inductive coupling to the plate was not ideal. However, it was felt that the setup would be adequate for the purposes of the experiment. In order to provide an enclosure for the hydrogen for the subsequent tests, a 6 inch Tee 21 of Pyrex glass pipe was procured. The procedural steps for each of the hydrogen atmosphere test runs were as follows: 1. Remove one end plate from a leg of the Tee 21. 2. Place the 5 inch by 5 inch steel coupon 20 inside the Tee 21, parallel to the plane of the induction coil 19 with the coil-coupon spacing within a range of 1/8 inch to 3/8 inch.

3. Replace the Tee-leg end plate.

4. Evacuate the air from the Tee 21 with a mechanical vacuum pump through duct 22.

5. Valve off the vacuum pump and fill the Tee 21 to slightly above atmospheric pressure with hydrogen.

6. Open the valve on the gas outlet line and establish a continuous flow of hydrogen from entrance conduit 24 to exhaust line 23. 7. Energize the induction coil 19, and begin timing.

8. Visually observe the surface of the coupon 20. When the oxide layer disappears, de-energize the coil 19 and stop timing.

9. Close the gas inlet and outlet valves and allow the coupon to cool for about five minutes in a static hydrogen atmosphere. 10. Open the Tee and remove the coupon.

Seven tests were run on hot rolled carbon steel coupons with mill scale on both sides using the above procedure. Coupons of four different thicknesses were used. No difficulty was experienced in determining the time at which oxide reduction was complete by simple visual observation. Even though the heat input was from one side of the coupon, oxide removal occurred on both sides of all coupons. For each test, efforts were made to maximize the induction unit's power output to a point just short of overload cut-off. The results of the seven tests are tabulated in Table I, with "Reduction Time" being the dependant variable. Test 9, marked with an asterisk, was aborted because the power supply experienced an overload cut-off. Table I

These tests demonstrated that the mill scale normally encountered on hot rolled carbon steel can be reduced in a hydrogen atmosphere using high frequency induction as the heating source. Further, this oxide reduction can be accomplished in a time frame consistent with the steel traveling through a suitable induction coil at a speed of 2 to 3 feet (about 0.65 to 1 meter) per second. Thus, the stated purpose of the experiment has been met.

The efficiency of the reduction reaction may be improved by ionizing the hydrogen. This is done by placing corona discharge electrodes within the housing 7, preferably as close to the exit of the strip 2 from coil 1 as possible and in such a way as to ionize the hydrogen contacting the strip across its width.

Cracked ammonia may replace the hydrogen and be utilized otherwise in the same way as the hydrogen. Ammonia is cracked in a known manner to form hydrogen and nitrogen, which are introduced into housing 7, creating an excellent reducing atmosphere which can be used for the reduction reaction, chilled for the removal of water vapor after exiting housing 7, and recycled in the same manner as hydrogen as explained above, the hydrogen source 8 being replaced by a cracked ammonia source.

Claims

1. Method of reducing oxides on the surface of strip steel comprising heating the surface of said strip steel by an induction coil to at least 1200┬░F (650┬░C) and contacting said oxides with hydrogen.

2. Method of claim 1 wherein the contacting of said oxides with hydrogen takes place in an enclosed space.

3. Method of claim 1 wherein said induction coil is a substantially rectangular solenoid coil.

4. Method of claim 1 wherein said contacting of said strip steel with hydrogen is conducted by flowing said hydrogen in a direction countercurrent to the direction of movement of said strip steel.

5. Method of claim 2 wherein said induction coil is within said enclosed space.

6. Method of reducing oxides on the surface of strip steel comprising heating the oxides thereon to a temperature of at least 1200┬░F, and contacting the strip so heated in an enclosed space with a countercurrent flow of at least 0.0167 hydrogen for each 0.0001 inch thickness of mill scale per square foot of said strip steel.

7. Method of claim 6 wherein said hydrogen flow is about 4 to about 6 times the stoichiometric amount for reducing said oxides.

8. Method of claim 6 wherein said heating is accomplished by high intensity infra-red radiation.

9. Method of claim 6 wherein said heating is accomplished by a solenoid induction coil.

10. Method of claim 6 wherein said oxides are heated by heating the surface of said strip to a temperature of at least 1200┬░F to a depth of at least 0.005 inch.

11. Method of claim 7 wherein said strip steel is cooled by said hydrogen before said strip exits said enclosed space.

12. Method of continuously treating strip steel to reduce mill scale comprising (a) continuously passing said strip steel through an induction coil in an enclosed space to heat the surface thereof to at least 1200┬░F (b) continuously passing hydrogen into said enclosed space to contact said strip steel (c) removing gases from said enclosed space, cooling said gases to remove water vapor therefrom and to recycle the hydrogen therein to said enclosed space, and (d) continuously removing said strip steel from said enclosed space.

13. Apparatus for reducing mill scale on steel strip comprising an enclosure, means for passing said steel strip containing mill scale through said enclosure, means within said enclosure for heating the surfaces of said steel strip to about 1200┬░F, means for passing a reducing gas into said enclosure to contact said mill scale, means for continuously removing gas from said enclosure and removing water vapor from said gas, and means for recycling unreacted reducing gas to said enclosure.

14. Apparatus of claim 13 wherein said means for heating is an induction coil.

15. Apparatus of claim 13 wherein said means for heating is a source of infra-red radiation.

16. Apparatus of claim 13 wherein said reducing gas is hydrogen.

17. Apparatus of claim 13 wherein said reducing gas is ionized hydrogen.

18. Apparatus of claim 13 wherein said reducing gas is cracked ammonia.

19. Method of continuously reducing oxides on the surface of strip steel by contact thereof with a reducing gas comprising continuously passing said strip steel to a heating zone, heating said surface of said strip steel to a temperature sufficient for said reducing gas to react with said oxide to remove oxygen therefrom, and continuously contacting said surface of said strip steel with said reducing gas.

20. Method of claim 19 wherein said reducing gas comprises hydrogen.

21. Method of claim 19 wherein said heating is performed by an induction coil.

22. Method of claim 20 wherein said heating and said contacting with reducing gas takes place by flowing said reducing gas through an enclosure, water is removed from said gas exiting said enclosure, and unreacted reducing gas is recycled to said enclosure.

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