US2186328A - Making ferrous alloys - Google Patents

Description

Patented Jan. 9, 1940 UNITED STATES MAKING FERROUS ALLOYS Fred C. T. Daniels, Bridgeville, Pa., assignor to Mackintosh-Hemphill Company, a corporation of Delaware No Drawing.

Application April 8, 1938,

Serial No. 200,891

4 Claims.

This invention relates to the manufacture of chromium-containing steels.

Steel makers have encountered difficulty in the making and working of steels containing substantial percentages of chromium. Chromium is well recognized as a useful alloying ingredient in steels, primarily because of its efieot upon the carbon of the steel, in that it tends to produce thorough dispersion of carbides throughout the body of the steel. The use of chromium in steel has, however,

heretofore proven uncertain and in many in- 'stances unsatisfactory. In standard steel making practice the chromium when added to a furnace charge of the steel does not always serve uniformly to distribute through the body of the steel a proportionally high content of persistent chromium carbide.

Resultantly different heats of chromium-containing steel made in accordance with identical specifications vary in the hardness which they are capable of acquiring upon heat treatment. This is so marked that there is frequently a variance of as much as 15% from the hardness which it is attempted to obtain by the analysis of the steel and theconditions of the heat treatment. Such variation in hardness causes substantial difference in the life and performance of rolls made of the steel, and an even greater difference in the life of roller bearings, wear plates and the like made of the steel. This variation is due chiefly to variation in the extent to which the chromium included in the analysis of the steel is ultimately present in the form of chromium carbides. Also, if added to a charge of steel in accordance with previous practice, the addition of the chromium tends to the production of a steel which contains non-metallic inclusions which detract from the physical qualities of the body of the steel, and the steel tends to form cracks and seams under both hot and cold working. Attempts to correct these disadvantageous characteristics by any modern method of grain size control are not uniformly successful. In spite of any such method, clean steel is not invariably produced, and heats of steel frequently fail to respond to heat treatment in.

the manner theoretically to be expected.

I have invented a process which so avoids the difficulties in the manufacture of a'steel containing a substantial quantity of chromium that I am able to produce a clean steel of such sort, which does not possess the usual tendency to form seams and cracks;- and a steel which, as the most strikingadvantage of my invention, presents remarkable uniformity in the ability of heats of like analysis uniformly to respond to the hardening effects of heat treatments.

Primarily my invention consists in the discovery that I may make steel invariably containing the advantages theoretically to be anticipated from a substantial chromium content, and without the disadvantageous effects previously associated with such content; by making my steelin two charges, one of which consists fundamentally of a ferrous solution of chromium carbide, and contains a substantial proportion of the chromium which is to I be included in the total body of steel. My preferred manner of preparing the charge forming a ferrous solution of chromium carbide'is to melt a charge of ferrous metal and chromium in a cupola. Since under the normal conditions of cupola practice any ferrous. metal becomes approximately saturated with carbon, the ferrous metal of the charge need not consist of pig iron of initially high carbon content, but may equivalently be a mixture of scrap steel and pig iron, or even a charge consisting wholly of scrap steel with the addition of the normal deoxidizing agents such as ferro-silicon and ferro-manganese. By melting the chromium with a saturated ferrous solution of carbon, initially present as such, or produced in the practice, the chromium is thoroughly dispersed through the ferrous'metal of the solution, and is given opportunity to combine to its maximum capacity with carbon in forming highly persistent chromium carbide.

It may be here explained that a chromium carbide content formed in the making of steel inaccordance with previous practice does not invariablypersist preponderantly as such in the steel, and to this fact there may be attributed the marked variability in the response of a chromiumcontaining steel to subsequent heat treatment. Thus when in previous standard practice chromium is added in a furnace to ferrous metal con- 40 taining merely the percentage of carbon which -is to appear in final analysis of the steel, there is a tendency for the chromium in a ferrous carbon solution of such dilution to be ultimately only partially combined with the carbon of the ferrous solution, and to the extent to which the chromium fails so to be, or remain. combined, the steel will fail to respond to its theoretically anticipated characteristics. In my method this tendency is .avoided by initially including a sub- 6 stantial proporton of the total chromum in a ferrous solution of carbon of such concentration that it becomes, and remains, present to a maximum extent in the form of persistent chromium carbide. 4 6b There are various alternative operating procedures by which a ferrous solution high in chromium carbide may be prepared. For example, I may take direct blast furnace metal, or mixer metal, naturally high in carbon, and may add the chromium to such ferrous carbon solution in a suitable furnace. Regardless of the exact procedure followed, the ferrous solution should contain' a suflicient percentage of carbon fully to develop the capacity of the chromium persistently to combine therewith. It may be noted also that the ferrous metal itself may be wholly or partially a scrap containing a large percentage of chromium, and as melted in the cupola, chromium need then be added only in the quantity desired to provide the final chromium percentage in the total body of steel.

Since my method, as so far described, presup poses primarily the preparation'of a charge of ferrous metal containing a carbon content substantially higher than would usually be desired in the finished steel, in accordance with my preferred practice I prepare another charge of steel containing a lower percentage of carbon than ultimately desired. In preparing this portion of my total charge I prepare a charge of molten steel in a furnace, which is desirably an openhearth furnace or electric furnace, but which may be if desired their substantial equivalent a Bessemer converter, or other suitable agency for working down a charge of molten steel. This treatment I continue until the melt has been brought to the high order of purity rendered possible by its. proportionally low carbon content. That is, the furnace is operated normally, in bringing the bath of steel to its best condition, until the boiling action in the bath dies down and the bath and slag are relatively free from oxides. I then add the normal deoxidizing agents, such as silicon and manganese, unless-the proportional inclusionpf these in the other charge is so high as to render their inclusion in the furnace unnecessary; and when the steel-making treatment is to be considered as substantially finished, I teem together the carburizing and chromium-impregnating charge, the preparation of which has been above described, and the charge of. steel proportionally low in carbon and chromium. If the low carbon charge has been made in a furnace such as an open-hearth furnace or electric furnace, I prefer to mix the carburizing and impregnating charge in the furnace. If this portion of the steel is made in a Bessemer converter, the mixture may best be made in a ladle.

After the carburizing and chromium-impregnating charge and the steel low in carbon and chromium have been mixed, the total metal may be teemed into ingot molds for subsequent mechanical working, or may be immediately used for casting rolls or other bodies of specific shape.

By commingling the charge low in carbon and the charge high in chromium carbide while they are molten and in fluid conduit, I obtain a fine and uniform distribution of chromium carbide through the entire body of the steel, and the persistence of the chromium in the form of its carbide having been established by its inclusion in a charge saturated with carbon, there is no substantial tendency for the chromium carbide to be disassociated.

The following table gives actual typical heats which I have made, and this table will illustrate the principles followed in practicing my inven- Heat A Percent by weight Mn 01' Low carbon charge 75 l5 38 46 Chromium carbide solution. 25 3. 88 60 4 40 Result 100 1. 06 .42 l 35 Heat B Percent by weight 0 Mn 01' Low carbon charge -i 95 09 49 20 Chromium carbide solution 6 3. 12 76 i2. 41 Result 100 24 62 75 Heat 0 Percent by weight 0 Mn Cr Low carbon charge 60 l6 35 0 0 Chromium carbide solution 40 3.10 65 3 50 Result 100 1,32 .36 l 32 Heat D Percent by weight 0 Mn or Low carbon charge 60 2.12 52 45 Chromium carbide solution 40 3. 50 68 2. 92 Result 100 2. 44 5s 1.

Heat E Percent by weight 0 Mn Cr Low carbon charge 89 06 85 3 l4 Chromium carbide solution ll 3. l6 68 ll 05 Result 100 .38 76 4 10 The total alloying content of the heats above It will be observed that the exemplary heats, above given, vary widely as to the proportion of the total chromium content included in the steel by means of the impregnating charge. Heat B approximately 80% of the total chromium is supplied by the impregnating charge, and in Heat C 100% of the total chromium is supplied by the impregnating charge. .In Heat E but slightly more than 20% of the total chromium is supplied by the impregnating charge. This last-named example is given to illustrate the fact that advantage is derived by following to any extent the principles of my invention; that is, by including any proportion of the total chromium in an approximately carbonsaturated ferrous solution, to that extent persistence of chromium carbide as such is assured. Specifically, the steel of Heat E is a steel purposed for use as an anticreep steel in petroleum refining stills, and the like, in which use hard- 'ness of a high order is not requisite. The steel Thus in of Heat A, for instance, is a steel suitable for use in roller bearings.

I have found that the best practice in accordance with my method is to, introduce as much of the total chromium as possible by means of the impregnating charge. In the above examples, it may be understood that the scrap used in every heat, save Heat. C, contained chromium, and that some chromium content was therefore present in the furnace charge.

Aside from the percentage inclusion of carbon and chromium in the two charges it is a matter steel to'come to a high state of purity. When when they are of steel made in accordance with V my above described method, instruments such as rolls, roller bearings and the like may give pre- I mium-containing steels show a particularly uniincluded in the final analysis of the steel, vanadium is desirably added in the ladle in accordance with the usual practice. When the total metal is in the higher carbon ranges, I may by including in the ferrous metal 0. either or both the charges graphitizing agents, such as silicon, manganese, nickel, or titanium, sumcient so to overbalance the total chromium content as to.

,cause some graphite precipitation, cause the ferrous product of my method to be a cast iron rather than a steel.

My method presents a two-fold advantage, resulting in the production of steel, containing a substantial chromium content, of particularly desirable characteristics. Thus, by making one of the subsequently combined charges lower in car-' bon than ultimately desired, I obtain the advantage of a high order of cleanliness in that charge, which cleanliness is not destroyed by the addition of the inherently clean ferrous solution of chromium carbide. Additionally, a substantial proportion of the chromium carbide is included and finely distributed in the entire body of steel in such manner that it persists in its combined form throughout the cooling, and any subsequent working or treatment of the steel. thus inherently both harder and tougher than chromium-containing steels of corresponding analysis made in accordance with standard practice.

Standard practice in making chromium containing steels has proven undependable in the response of the steel to heat treatment, due apparently to variation in the content and character of carbides of chromium in the steel whether cast as a shaped casting, or as an ingot. Thehardness of steels of like composition made in accordance with my method vary at most no more than 2% or 3% from the predicted hardness which they should theoretically acquire in response to heat treatments. This means 1 that dictable performance and have an accurately predictable lifev Microphotographs of my chroform grain formation, as well as a particularly uniform chromium carbide distribution; and it The steel is should be emphasized that with any analysis of such steels in accordance with good general practice, my method renders the results invariably predictable.

I claim as my invention:

1. The herein described method of making chromium steel of dependable heat-hardening properties containing a substantial percentage of chromium below that normally included in stainless steels and which is approximately all in the form of carbides, which comprises making a major bath charge of ferrous metal containing lower percentages of carbon and chromium than ultimately desired, making separately a minor impregnating charge of ferrous metal containing higher percentages of carbon and chromium than the major charge and in which the carbon is included in a quantity to give at least one part of carbon to each fourparts of chromium, and mixing the charges while-molten in a proportional quantity of the impregnating charge to the major charge to give a total body of metal in which the chromium content is supplied preponderantly by the impregnating charge.

2. The herein described method of making chromium steel of dependable heat-hardening properties containing a substantial percentage of chromium below that included in stainless steels.

which is preponderantly in the form of persistent chromium carbide which comprises making a major bath charge of ferrous metal low in chromium, separately making a minor charge of ferrous metal containing from about 3% to about 4% carbon and from about 3% to about 12% chromium, and mixing the charges while molten in a proportion of from about 5% to about 40% of the minor charge to from 60% to 95% of the major charge to give a total body of ferrous metal containing from about 0.75% chromium to about 1.35% chromium.

3. Theherein described method of making a chromium-carbide steel suitable for roller bearings and'having carbon in the order of about 1.00% and chromium in the order of about 1.35% which consists in making a major bath charge of ferrous metal low-in carbon and lowin chromium, separately making a minor charge of ferrous metal containing more than 3% carbon andmore than 4% chromium, and in which the carbon is included in a proportional quantity with the chromium to give at least one part of carbon to each four parts of chromium and mixing the charges while molten. in a proportion to give a total body of steel in which the above ultimately desired approximate percentages of carbon and chromium are approximated.

4. The herein described method of making a chromium-carbide steel having about 1% to about 2.50% carbon and from about 1% to about a mnncunnanme