Method Of Hot Rolling Metal

Abstract

In the art of moving metal at hot forgeable temperature through and reducing such metal in a hot rolling mill, engaging the metal by a roll and applying to the roll coolant at a temperature below the boiling point of water so that the roll is subjected to repeated heating and cooling cycles, the roll having a surface contacting the metal which surface is of chromium alloy steel which has simultaneously exceptional resistance to thermal fatigue cracking and good resistance to wear at the temperatures produced by contact with metal at hot forgeable temperature and which has the following composition: the sum of the percentages of the chromium, molybdenum, tungsten and vanadium being between about 4 and about 9, the balance being substantially iron. The roll may comprise a ferrous metal body to which the metal contacting surface of chromium alloy steel is applied. The metal contacting surface may be and preferably is applied to the roll body by weld deposition. The metal moved through the rolling mill is preferably iron or steel above its critical temperature.

Parent Case Text

This application is a continuation of my copending application Ser. No. 656,365, filed July 27, 1967 now abandoned.

Description

This invention relates to a method of hot rolling metal and particularly to operations performed on metal in its hot forgeable temperature ranges, particularly in a hot rolling mill.

The invention is concerned with the drastic temperature extremes, necessarily cyclic in nature, to which certain working parts of the mill, especially rolls, are subjected and the damage which thermal shocks caused by the cyclic temperature changes do to the working parts of the mill and to improvement of the performance and efficiency of the mill which can be effected by importantly, in accordance with the invention, improving the resistance of such working parts of the mill to damage by thermal shock.

Contact with hot metal being rolled in a hot rolling mill is made principally by rolls. These may be the rolls which transport the hot metal and on which it may rest for a period while awaiting the next processing step or the work rolls which produce plastic deformation by applying squeezing pressure to the metal. During the engagement of a roll with hot metal heat is rapidly transferred to the surface of the roll by conduction and radiation. In some locations even convection may contribute to the transfer but in any case the roll surface receives an intense local heating. Immediately after engaging hot metal a roll surface is cooled to prevent too much build up of heat in the roll which would reduce its strength and cause the sticking of hot metal on subsequent contacts. Roll cooling is normally done by water sprays although there are a few locations in a mill where a blast of air at ambient temperature may be adequate.

The effect on a roll of the alternaTe heating by metal and cooling by water is to produce a deterioration of the roll surface known as "fire cracking" in which the surface develops cracks due to thermal fatigue in a pattern of small checks or alligatoring. These fire cracks steadily progress and finally render the roll useless by marking the hot metal which the roll engages. Normally a roll wears away at the same time that it fire cracks and depending upon the function performed either form of deterioration may predominate and force down time on the mill for roll replacement. A roll removed from the mill may be redressed. This is accomplished by metal removal which restores the roll contour and if possible establishes a new working surface below the fire crack damage. A roll can stand a limited number of redressings and more metal may be lost in dressing out fire cracks than is ever worn away in use.

The rate of formation and propagation of fire cracks in a mill roll varies with the hot working temperature of the metal being processed; steel is commonly handled above its critical temperature in the range of 1,450.degree. to 2,200.degree. F., nickel at about 1,200.degree. to 2,300.degree. F., copper at about 1,200.degree. to 1,600.degree. F. and aluminum at about 900.degree. to 950.degree. F. Because of the large tonnage rolled and the high-hot working temperature the rolling of steel is responsible for most fire crack damage to mill rolls. While the fire cracking of rolls cannot be stopped it can be slowed down as this invention teaches. The conduct of the rolling operation can thereby be greatly improved by reducing the rate at which fire cracking spreads into extensive and insupportable damage whereby the mill product becomes increasingly marked and the hazard of roll breakage increases with the deepening cracks which act as fatigue notches. At this point mill operation becomes a gamble of additional tonnage before a roll change against breakage in the mill. Roll failure by wear is bad enough but wear can be observed and followed and a replacement part scheduled for a time when a mill is down. Roll failure by breakage during mill operation is the most expensive type of failure and minimization of the risk of roll breakage is of the utmost importance to mill operations.

Through long years of experience the manufacturers of new mill rolls for hot work uses have settled on roll composition ranges which they offer as the optimum combination of mechanical properties and economy in service. These rolls are a high carbon and relatively low-alloy composition and are used in the dead soft annealed condition. The rolls must be at least soft enough for machining and although the dead soft annealing treatment is very expensive because of the weeks of furnace time and heavy losses of scaled and decarburized metal it does put the roll in condition to deliver reasonable wear while having maximum ductility which is known to be favorable to fire cracking resistance. While fire cracking of such rolls is suppressed somewhat there are many locations in a mill where rolls do crack and spall and become unfit for continued use long before wear can become a problem. Thus the operation of a rolling mill with the best rolls available has not been satisfactory.

There has also been a long and checkered history of efforts to rebuild by welding hot work rolls which have been repeatedly redressed after failures resulting from some combination of wear and thermal cracking. Individual cases have been cited as successful but the larger number of expensive failures have given the process the reputation of a losing gamble throughout the steel industry. The practice of reclamation by weld surfacing has been further retarded by the lack of weld deposits with high-thermal fatigue resistance and good resistance to wear at elevated temperatures. This combination of properties has not been attainable in a single material because those characteristics that promote wear resistance, for example, high hardness, are usually incompatible with the characteristics that favor fire crack resistance such as high ductility. Furthermore, the working surface of a welded roll must be used substantially as deposited since the finished bearings at either end will not tolerate exposure to a heat treatment such as the annealing treatment commonly used on a new roll body while still in the unfinished condition. Thus a roll produced by applying a weld deposit to a supporting structure such as a used roll body represents the more restricted situation and a material which performs satisfactorily in the weld deposited form without benefit of heat treatment can therefore also be employed as a new roll material where heat treatments can be added.

The problem of improving mill roll performance with a material resistant to fire cracking and also to wear directed attention to the tool steels, particularly the family of 5 percent chromium hot work die steels which satisfy one requirement in having good resistance to wear at elevated temperatures.

Since the wear resistance of materials generally changes as the hardness changes, and in the case of equipment operating at elevated temperatures the wear occurs at some high temperature, it is necessary to provide for wear resistance by utilizing materials of good high-temperature hardness and resistance to softening at elevated temperatures. Some help is available in the control of these properties because long experience with the performance of the family of 5 percent chromium tool steels used under hot working conditions has led to appreciation of the contributions to performance which various alloy additions can impart to such steels. For example, it has been proposed to add tungsten in an amount of about 1.3 to 2 percent as an aid to grain refinement, hot hardness and tempering resistance and it has been stated that when dissolved in the matrix tungsten is extremely reluctant to precipitate on tempering, and when it does, at temperatures of 950.degree. to 1,100.degree. F., probably in the form of W.sub.2 C, it accounts for the phenomenon of secondary hardness and, in the high-speed steel tools, for red hardness. It has also been said that because molybdenum has a smaller atomic weight than tungsten it will produce twice as many atoms for alloying in the steel; thus 1 percent molybdenum can be substituted for 1.6 to 2 percent tungsten. It has further been said that molybdenum also imparts high-temperature strength to steels when present in amounts as low as about 0.5 percent, that molybdenum when present alone, however, promotes decarburization and that small amounts of chromium minimize this effect. It has still further been said that vanadium present in virtually all the chromium-molybdenum steels is highly effective in promoting hot hardness at 1,100.degree. F. and that 0.5 percent vanadium is as effective as 1.5 percent vanadium.

The effects of chromium itself are well known. It imparts oxidation resistance to steel and greatly increases its hardenability. In conjunction with molybdenum an optimum increase in high-temperature strength is accomplished at about 2 percent chromium.

The relative effectiveness of various alloying elements in imparting hot hardness to tool and die steels has been said to be, in descending order of effectiveness, tungsten, molybdenum, cobalt (with tungsten or molybdenum), vanadium, chromium and manganese.

Such metallurgical conclusions and generalizations as those cited above have grown out of the very extensive study of tool steels, particularly those used for hot work applications. While there is a great difference between forged and heat-treated tool steels and cast hard facing weld deposits, the same basic metallurgical principles have been found to apply with regard to alloying elements and their effective ranges in regard to high-temperature properties. However, with regard to the vital property of thermal fatigue, no guidance has been available from any source.

Drawing upon the known metallurgical principles for managing high-temperature hardness and resistance to softening a number of chromium alloy steel weld deposit compositions were produced with contents of other alloy metals selected from the ranges known to be effective with respect to hot hardness and softening resistance. These weld deposits were then subjected to a test for evaluating resistance to thermal fatigue by which metal samples are subjected to alternate exposures to oxyacetylene flame impingement and cold water quench. This drastic test simulates the temperature cycle produced by contact with hot metal and roll cooling water. The number of cycles of heating and quenching required for visible crack initiation is used as a measure of the relative thermal fatigue resistance of a metal.

These thermal fatigue tests were carried out in collaboration with a principal manufacturer of mill rolls which has for some years done investigation work on the suitability of various steel compositions as materials resistant to the heat and wear encountered by steel mill rolls in a hot mill. This fire cracking test is so severe that not even the best of this manufacturer's roll materials had ever survived as many as 500 thermal cycles before showing fire cracking. Such thermal fatigue tests showed that while some of the chromium alloy steel analyses failed after as few as 16 cycles other analyses survived 500 cycles and still were satisfactory after enduring a second 500 cycle program. Table I lists a group of weld deposit analyses, all except No. 10 being chromium alloy steels, together with the number of cycles before failure in the fire cracking test.

Hot hardness tests and hardness measurements after exposure to elevated temperatures were made on many of the weld deposits and the results are shown in Table II. Alloy 13 was included in the tests as a steel of minimum alloy content. Its poor hot hardness eliminated it as an alloy of interest and its softening tendency and fire cracking resistance were not determined. The data in Table II confirm that in general the various alloying elements impart characteristics to weld deposits similar to those imparted to wrought tool steels. ##SPC1##

As shown for example by alloys No. 2, No. 3, No. 6 and No. 8 I have achieved in a single weld deposit both unprecedented resistance to fire cracking and a good level of the properties which produce resistance to wear at elevated temperatures.

While, as expected, the alloy content of my improved roll materials was found to be important to the development of hot hardness and hardness retention, I was not able to relate fire cracking resistance to such alloy content. Unexpectedly I discovered carbon to be the vital controlling factor. I developed through testing that whatever the metallic alloy analysis all compositions of exceptional resistance to thermal fatigue had a matrix carbon level of less than 0.2 percent. With carbon above 0.2 percent failure due to thermal fatigue always occurred within 500 test cycles (actually within 278 cycles) and in some instances failure was noted after as few as 10 cycles.

I have established the lower practical limit of carbon at about 0.03 percent by weight. Alloys below this carbon level may have excellent resistance to thermal fatigue but they are soft and their wear resistance is so seriously impaired that they are unsuitable for the uses intended such as the reclamation of steel mill rolls.

The data in Table I also indicate that care must be exercised in developing alloys for service in conditions of temperature cycling to prevent the formation of either low-melting eutectic-type phases or hard, brittle intermetallic grain boundary phases. These will cause premature failure by fire cracking due to a mechanism analogous to that which occurs in high-carbon alloys described previously. These phases are of the type that occur in a weld deposit when high concentrations of alloying elements such as molybdenum and nickel are present in the alloy. This condition is not present in wrought tool steels because of the hot work performed in the forging of the ingots and therefore imposes a limitation upon weld deposits only. Such an alloy is listed in Table I as alloy No. 10. It will be noted that although the carbon level is well below the threshold value of 0.2 percent, failure by fire cracking occurred after only 278 cycles. Microscopic examination showed that cracking had initiated at grain boundaries in a high-alloy intermetallic phase and then propagated in the usual manner. Consequently, limitations must be placed on the combination of alloying elements and the concentration of each as well as on the carbon level to maintain the resistance to fire cracking.

The data in Table II confirm that the information present in the literature concerning the effects of chromium, molybdenum, tungsten and vanadium on the hot hardness and softening resistance of wrought steels is generally applicable to weld deposits providing the amounts of alloying elements are kept low enough to prevent the formation of second phases. The presence of chromium is required for oxidation resistance and the data show that a 2 percent chromium steel, alloy No. 13, has insufficient hardenability and hot hardness to have utility in a hot mill roll. Increasing the chromium to about 3.5 percent without any other alloying addition, alloy No. 12, resulted in sufficient hardenability to obtain satisfactory hardness at room temperature and 800.degree. F. but the hardness at 1,000.degree. F. and 1,200.degree. F. and the softening resistance were too low. With as much as about 8 percent chromium and about 1 percent molybdenum, alloy No. 6, the softening resistance and the hardness at 1,200.degree. F. were marginal.

Alloy No. 11 illustrates the help which can be gained from use of low levels of vanadium in amounts up to 1 percent in a composition which otherwise would be unsatisfactory in hot hardness or softening resistance. Although in alloy No. 11 the addition of about 0.7 percent molybdenum and 0.3 percent vanadium increased the hardenability, hot hardness and softening resistance to an acceptable level in the presence of only 2 percent chromium, it is believed more conservative to take 3 percent chromium as the minimum which should be used.

Alloy No. 9, an alloy with about 7 percent chromium and about 1.5 percent molybdenum, has satisfactory hot hardness with marginal softening resistance. It may be noted that molybdenum above about 4 percent did cause premature failure by fire cracking due to a grain boundary phase. To insure against the possibility of a detrimental grain boundary condition, the upper limit for molybdenum is established at 2 percent.

All the chromium alloys that contained tungsten in amounts from about 1 percent to over 7 percent had satisfactory hot hardness and softening resistance. This was accomplished without lowering the fire cracking resistance by the formation of a grain boundary phase. Thus, an upper limit for tungsten is established at about 8 percent although this is becoming an expensive alloy. The alloys with the lower levels of tungsten were fortified by about 1.5 percent molybdenum with some benefit.

The data in Table II indicate that weld deposits containing chromium in amounts from about 3 to about 8 percent were satisfactory for use for hot mill rolls when fortified by additions of tungsten, vanadium and molybdenum or combinations of the three. In any case, however, a total of the chromium, molybdenum, tungsten and vanadium of 4 percent was the minimum acceptable level necessary for resistance to high temperature wear and fire cracking.

Since it has been established that a minimum of 3 percent chromium is required for oxidation resistance and hardenability, at least 1 additional alloy or a total of 4 percent is required to develop adequate high temperature properties.

Alloy No. 8, which had a chromium and tungsten total of about 9 percent, exhibits good fire cracking properties and excellent elevated temperature behavior. There is danger of the formation of intergranular phases in deposits with too high an alloy content as exemplified by alloy No. 10. Therefore, a value of 9 percent of the total of chromium, molybdenum, tungsten and vanadium is established as a maximum.

Good elevated temperature properties are achieved in weld deposits by the addition of chromium, molybdenum, tungsten and vanadium in the amount stated without the formation of any constituent that would be detrimental to the fire cracking resistance of the alloy. Good high-temperature properties may be obtained in weld deposits containing the above stated elements at percentages higher than those given. However, the contributions which those elements can confer are well developed within the limits stated and the use of higher concentrations would normally be considered as wasteful. In addition, the danger of impairing the fire cracking resistance of weld deposits by the use of higher percentages of alloying elements is substantial.

Certain residual elements normal to ferrous hard-facing deposits may also be present; the range of silicon is commonly from 0.2 percent to 2 percent and of manganese from 0.3 percent to 3 percent. Phosphorus and sulphur may be present in typically commercial amounts. The balance is substantially iron.

Thus I have discovered that a wear resistant hard-facing weld deposit resistant to fire cracking should contain

Broad Narrow Element Range (%) Range (%) Preferred (%) __________________________________________________________________________ carbon 0.03-0.2 0.05-0.2 about 0.15 chromium 3-8 4.5-6 about 5 molybdenum 0-2 0.5-1.5 about 1 tungsten 0-7 0.5-4 about 1.5 vanadium 0-1 0-0.5 about 0 manganese 0.3-3 0.3-2.5 about 2 silicon 0.2-2 0.2-2 about 1 __________________________________________________________________________

and with the sum of the percentages of chromium, molybdenum, tungsten and vanadium between about 4 and about 9, the balance being substantially iron.

In order to evaluate mill performance under actual operating conditions a roll weld surfaced with alloy No. 2 of Table I was installed and used for a full campaign in the universal structural mill of a leading steel producer. During the campaign the roll was teamed with a standard roll and both were employed in the primary stand of the mill while it was hot rolling heavy I-beam steel sections. Steel at a temperature of about 2,300.degree. F. was in contact with the roll during the rolling operation. Cold water at ambient temperature and in any event at a temperature below the boiling point of water to quench the heated surface after contact with the plastic hot steel is continuously sprayed on the roll. The roll was redressed at normal intervals and was visually examined prior to each redressing. Such examination evidenced a reduction of 75 percent in fire cracking as compared with the standard steel roll currently employed by the steel mill for the same purpose. The operating test proved conclusively that my composite roll has significantly superior resistance to thermal fatigue as compared with standard rolls while maintaining good resistance to wear.

Chromium steels with alloy metal contents equal to those of my chromium steels but with carbon contents over 0.20 percent and therefore unsuitable for rolling mill use have been used for many tool steels, for welding electrodes and also for steel mill rolls. Chromium steels with analyses which fall within the limits of my invention have been made previously. They have been shown to have properties and have been demonstrated as useful for purposes none of which remotely anticipates or foreshadows their useful employment as a working surface of a roll in the hot rolling of metal.

A steel within the analysis which I require for rolling mill use has been proposed as a valve surfacing material resistant to the corrosive attack of hot refinery oil. This is a very special type of corrosive attack by the sulfur and other compounds present in oil. In refinery use the service is continuous and the temperature is maintained constant by the hot oil. This service is not characterized by the rapid and extreme temperature cycles which by alternate heating and quenching produce fire cracking by fatigue in rolling mill rolls. The property of resistance to corrosion by hot oil teaches nothing about resistance to fire cracking under violent thermal cycles.

Since carbon content contributes to wear resistance in plastic molding dies but renders shaping of the die difficult a low carbon 5 percent chromium die steel made soft by an annealing heat treatment has been used for ease of shaping by cold hobbing. When shaping is finished the physical properties of the steel which prevailed up to this point now become a liability and before employment in the final and intended use as a plastic molding die a new set of properties must be produced. This is accomplished by a hardening heat treatment preceded by a carburization to introduce high-surface carbon and good wear resistance. The value of low-carbon content for ease of machining in no way points to the property of resistance to fire cracking under rolling mill use conditions upon which my invention depends.

While I have described certain present preferred methods of practicing the invention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously practiced within the scope of the following claims.

Claims

I claim:

1. A method for improving the performance and efficiency of a hot rolling mill which accepts metal at hot forging temperature, moves it through the mill and plastically reduces the metal, the method consisting of the steps of providing the mill with a roll having a metal contacting surface of chromium alloy steel having the following composition:

the sum of the percentages of chromium, molybdenum, tungsten and vanadium being between about 4 and about 9, the balance being substantially iron, whereby to simultaneously greatly improve the resistance to thermal fatigue cracking while maintaining a good level of resistance to wear of the roll at hot forging temperature, engaging the metal at hot forging temperature by such roll and applying to the roll a coolant at a temperature below the boiling point of water so that the roll is subjected to repeated heating and cooling cycles.

2. A method for improving the performance and efficiency of a hot rolling mill which accepts metal at hot forging temperature, moves it through the mill and plastically reduces the metal, the method consisting of the steps of providing the mill with a roll having a ferrous metal core and a metal contacting surface of chromium alloy steel having the following composition:

3. A method for improving the performance and efficiency of a hot rolling mill which accepts metal at hot forging temperature, moves it through the mill and plastically reduces the metal, the method consisting of the steps of providing the mill with a roll having a ferrous metal core and a weld deposited metal contacting surface of chromium alloy steel having the following composition:

the sum of the percentages of chromium, molybdenum, tungsten and vanadium being between about 4 and about 9, the balance being substantially iron, whereby to simultaneously greatly improve the resistance to thermal fatigue cracking while maintaining a good level of resistance to wear of the roll at hot forging temperature, engaging the metal at hot forging temperature by such roll and applying to the roll a coolant at a temperature below the boiling point of water so that the roll is subjected to repeated heating and cooling cycles.

4. A method as claimed in claim 1 in which the metal contacting surface has the following composition:

5. A method as claimed in claim 1 in which the metal contacting surface has the following composition:

6. A method as claimed in claim 1 in which the metal moved through the rolling mill is iron or steel about its critical temperature.