US2811436A

US2811436A - Process of producing steel

Info

Publication number: US2811436A
Application number: US408806A
Authority: US
Inventors: Heuer Russell Pearce
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-02-08
Filing date: 1954-02-08
Publication date: 1957-10-29
Anticipated expiration: 1974-10-29

Description

Oct. 29, 1957 R. P. HEUER PROCESS OF PRODUCING STEEL Filed Feb. 8, 1954 INVENTOR. fimssef/ 1. nswer United States Patent PROCESS OF PRODUCING STEEL Russell Pearce Heuer, Villanova, Pa.

Application February 8, 1954, Serial No. 408,806

Claims. (Cl. 75-52) The present invention relates to the manufacture of steel which has improved properties, particularly improved ductility which renders the steel especially suitable for drawing and cold forming.

A purpose of the invention is to secure a steel for cold drawing and forming which has a maximum degree of uniformity and freedom from physical imperfections, so that drawing and forming operations of greater severity can be performed and operations involving larger areas in the sheet undergoing mechanical work, with fewer imperfections in the product.

A further purposes of the present invention is to manufacture steel suitable for improved drawing and cold forming from pig iron containing less than 0.013 percent sulphur by weight, using a combustion furnace in the absence of external sulphur in the furnace to obtain a steel having an unusually low sulphur content, thereby avoiding the objectionable segregation of sulphur compound which occurs in steel which is made in conventional combustion furnaces and also avoiding the adverse effect of such segregation on the deep drawing and cold forming properties of the steel.

A further purpose is to desulphurize molten pig iron after it has been produced in the blast furnace and has left the blast furnace, and to convert the pig iron thus produced into steel under conditions freefrorn contamination with sulphur by using a converter in. which metalloids in the steel are oxidized with relatively pure oxygen gas (containing substantially more oxygen than air) which is brought into contact with the carbon, silicon. and other oxidizable constituents of the iron to yield the necessary heat.

A further purpose is to produce steel from pig iron by desulphurizing molten pig iron external 'to the blast furnace inv a closed space under reducing conditions with exclusion of air. and in the presence of a desulphurizing agent until the sulphur. content is below 0.013 percent by weight, then to convert the desulphurized iron into steel in the presence of a basic slag and in the absence of external sulphur-bearing fuel by blowing with oxygen containing substantially less nitrogen than air and thus producing combustion until 85 to 95 percent or more by weight of the carbon present is removed as required, and then to separate the slag from the steel. I

A further purpose is to convert the ironto steel using oxygen containing less than 4 percent of impurities by volume and to continue the blowing until the carbon content is less than 0.10 percent by weight. g

A further purpose is to desulphurize by introducing lime blown in with nitrogen gas.. 7

A further. purpose. is to blow oxygen on the surface of the iron from a tuyere. under a pressure of 50 to 175 p. s. i. gage in the converter.

A further purpose is to desulphurize until the iron contains less than 0.005 percent sulphur by weight.

A further purpose is to produce a steel containing less than 0.010 percent sulphur by weight and preferably less than 0.004 percent sulphur by weight.

"ice

Further purposes appear in the specification and in the claims.

The drawings illustrate mechanism which may be used in carrying out the invention.

Figure 1 is a central vertical diagrammatic section of a converter which may be employed in the invention.

Figure 2 is a section on the line 22 of Figure 1.

The conventional practice in the manufacture of steel which is required to have highest ductility and especially of steel intended to be drawn or cold formed is to use the basic open hearth process.

It is preferred over the acid open hearth, the Bessemer and the Thomas processes because the product has a lower content of sulphur and nitrogen. The sulphur content is especially critical for sheet steel which is to be deep drawn. Even sheet from the basic open hearth process leaves much to be desired in this respect.

In basic open hearth steel sheet the sulphur compounds are not distributed uniformly during solidification of the ingot, and the soaking pit and rolling operations fail to overcome the difliculty due to segregation. The nonuniform distribution of sulphur compounds in the final sheet causes many difficulties in forming. As an example, in the forming of automobile fenders, the presence of one objectionable imperfection on the surface of the sheet or one local failure of the steel to withstand the forming operation. causes rejection of the entire product, even though the imperfection may apply to a very small part of the entire piece, for example, one-ten-thousandth of the entire piece.

The limitations of the steel sheet furthermore greatly restrict the severity of the drawing operations and necessitate that more expensive and prolonged drawing operations be used in order not to exceed the limited ability of the steel sheet to withstand forming.

One ofthe great advantages of the present invention is that steel sheet can be obtained which has a lower total sulphur content and less pronounced segregation of sulphur. Thus in accordance with the invention it is readily possible to produce steel sheet having a sulphur content of less than 0.013 percent sulphur by weight, and in many cases the sulphur content is as low as 0.004 percent sulphur by weight or less.

In accordance with the present invention, molten pig iron after it leaves the blast furnace, normally containing about 0.025 to 0.050 percent sulphur by weight, or even more, is placed in a suitable desulphurizing vessel With exclusion of air, and in the presence of a desulphurizing agent, preferably lime. It is best to desulphurize in an unheated vessel, decreasing the activity of oxygen in the vessel by blowing a small quantity of an inert gas such as nitrogen through the molten iron to remove the carbon monoxide present as taught in Wynne U. S. Patent No. 2,587,573, granted February 26, 1952, for Desulphuriziug Process. p

The desulphurizing agent may be for'example a molten slag whose primary active ingredient is lime, with suitable amounts of silica and/ or alumina and fiuorspar. desulphurizing agent may also be used, for example lime powder in an unfused state, and preferably blown into the molten iron by a stream of nitrogen gas. The use of nitrogenin desulphurizing will have no adverse effect on the quality of the steel ultimately obtained by the present process.

Powerful reducing conditions are built up during the desulphurization by thecarbon content of the pig iron when air is excluded, and in the presence of a desulphurizing agent as already explained, the sulphur of the pig iron can be reduced to less than half the initial content, so that the molten pig ironv after desulphurization will contain less than 0.013 percent sulphur by weight and preferably less than 0.005 percent sulphur by weight, and

A solid' most desirably of the order of 0.004 percent sulphur by weight or less.

From the ladle or other desulphurizing vessel, the desulphurized iron is transferred to a basic lined converter. Figures 1' and 2 show a suitable converter 20 having a casing 21 and an interior basic refractory lining 22 which may consist of tar-dolomite brick or magnesite brick. The converter has a lower chamber 23 and is reduced in cross section at the neck 24, and open at a blowing opening 25 at the top. The converter will be suitably adjusted for tilting as well known. A charge of molten pig iron 26 is brought into the converter from the desulphurizing step, and. the charge is blown with relatively pure oxygen.

The oxygen content of the blowing gas should be substantially greater than that of air, although it may be obtained by enriching air in oxygen. Advantage from the invention will be obtained by using more than 50 percent of oxygen by volume in the blowing gas, the balance being inert impurities such as nitrogen, although great advantage is obtained by using substantially pure blowing gas, containing at least, 96 percent of oxygen by volume and preferably from 98 to 99.5 percent of oxygen by volume, the balance being inert impurities such as nitrogen. The use of the relatively ure oxygen is advantageous not only because. of the high calorific intensity of the reaction obtained, but also because of the low nitrogen content which tends to produce a steel unusually low in nitrogen.

The effect of the blowing is to remove the silicon from the pig iron and reduce the carbon content to a. suitably low figure, for example less than 0.10 percent by weight and usually to 0.05'percent carbon by weight or less. At least 85 percent and preferably at least95 percent by weight of the carbon present in the original pig iron is removed at this stage- The phosphorus is removed to about 0.040 percent or less.

The desirable technique is not to blow the oxygen containing gas through the molten metal bath from bottom tuyeres as in the conventional Thomas process. Best results according to the present invention are obtained by blowing the oxygen containing gas upon the surface of the iron suitably from an external vertical tuyere 27 shown in Figures 1 and 2. The vigorous gas flow is important, and it is recommended that the pressure of the oxygen containing gas be between 50 and 175 p. s. i. gage at the time the gas. is released by the tuyere.

The slag which is on the molten iron at 2 8 is made basic by the addition of sufiicient lime to facilitate re.- moval of phosphorus. The ratio of lime to silica in this slag 28 is from about 2 to about 3.5 to 1, and the total content of lime plus silica will be 50 percent or more by weight. The total content of FeO plus MnO in the slag will be between about 25 and 40 percent by weight. a

If desired limited amounts of scrap may be added tothe converter charge if the sulphur content of the scrap is suitably low. The preferred scrap is mill and fabricating scrap arising from the working of the steel produced by the process of the present invention.

One great advantage of heating by the oxygen reaction. with the metalloids in the pig iron of the converter is that; by this procedure sulphur pick-up from combustion gases usually encountered in ordinary combustion furnaces like the open hearth furnace is avoided, and also the excessive nitrogen content commonly present inconverter steel from the Thomas or Bessemer processes are avoided.

It will be evident that the low sulphur content is obtained by a combination of a metallic charge in the converter having a very low sulphur content and avoidance of sulphur pick-up from the furnace gases. or other mate-. rials used in the converter. The slag in the converter is not. relied upon as the principal cause. of the low sulphur contentthe finished steel. In. fact he cqnve rter slag at the end of the operation of steel making may contain lessthan 0:1" percent sulphur by weight.

The steel produced in the present invention has a sulphur content which is not more than 0.010 percent sulphur by weight and preferably as little as 0.004 percent sulphur by weight.

The nitrogen content of the finished steel will depend upon the nitrogen content of the oxygen blown by the tuyere 27. By using oxygen which contains less than 3 percent nitrogen and preferably less than 1 percent nitrogen by volume, thesteel product obtained; contains about 0.005 percent nitrogen by. weight or less. In some cases the nitrogen content of the converter steel of the present invention may somewhat exceed the nitrogen content obtained in conventional open hearth steel, but the combined effect of the unusually low sulphur content as compared With open hearth steel and the moderate nitrogen content makes the product oi the present invention exceptionally valuable, especially for deep drawing and cold forming.

It will be evident that the end of the blow may be determined by any suitable means, either by visual inspection, automatic control, or analysis as well known. After the bl'ow'is complete, the molten steel is poured from the converter to a ladle or the like, while the slag is preferably retained in the converter. The composition of the steel may be adjusted in the ladle by desired additions of ingredients well known in the art, such as carbon, manganese, silicon, and. other alloys, with or without ingredients which control oxide content such as aluminum, as required in normal practice to make killed steel, semi-killed, capping or rimming grade. The process of the. Present invention is especially advantageous for making capping or rimming steels intended for deep drawing where a minimum of croppingis practiced to remove the segregated cap of the ingot, since in accordance with the present invention segregation of sulphur in the ingot is practically eliminated.

It will be evident that by using low sulphur raw materials, and refining the steel in a combustion furnace in the absence of sulphur, while avoiding excessive nitrogen, I obtain a steel of maximum uniformity which is remarkably free from physical imperfections, and which has unusual ductility and is, capable of most eflicient cold 7 forming with minimum waste and rejection.

In view of my invention and disclosure, variations and modifications to meet, individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the, process shown, and I therefore claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention, what I claim as new. and desire to secure by Letters Patent is:

1. The process of producing. steel of improved ductility, which. comprises desulphurizing moltenv pig iron external to the. blast furnace, in a closed space. under reducing conditions intensified by an inertnitrogen atmosphere and in the presence of a desulphurizing agent until the sulphur content. is below 0.0.13 percent'by weight, converting the desulphurized iron into steel in the presence of a basic slag and, in the absence of sulphur-bearing fuel by blowing with oxygen containing substantially less nitrogen than that in the air and thus oxidizing impurities liberating voluminous carbon. monoxide and, thereby removing nitrogen, the. sulphur. content at the end of blowing being still less than 0.013 percent by weight, and separating the slag from the molten steel.

2. The process of claim 1,. which comprises blowing the pig iron with nitrogen during desulphurizing.

.3. The process of claim 2, which comprises blowing in line with thenitrogcn dutingdesulphurizing.

4. The process of claim, 1, in which the oxygen con- 3 talu less han. 4- nercent. of impurities. by vol mee p ss'ofi cla m 1.; in-which he. pig iron s;

5 desulphurized until it contains less than 0.005 percent sulphur by weight, and a sulphur content below this limit is maintained at the end of blowing with oxygen.

6. The process of claim 1, in which the iron is contained in a basic lining during conversion into steel, and the oxygen is blown upon the surface of the iron from a tuyere under a pressure of 50 to 175 p. s. i. gage.

7. The process of claim 1, in which the steel contains less than 0.010 percent sulphur by weight.

8. The process of claim 7, in which the steel contains less than 0.004 percent sulphur by weight.

9. The process of claim 1, in which the steel contains not more than 0.005 percent nitrogen by weight.

10. The process of claim 1,' in which the charge for conversion into steel is partly liquid metal and partly solid metal.

6 References Cited in the file of this patent UNITED STATES PATENTS 1,032,653 Brassert July 16, 1912 2,290,961 Heuer July 28, 1942 2,644,746 Hauttmann July 7, 1953 2,741,555 Cuscolcca et al Apr. 10, 1956 FOREIGN PATENTS 19,727 Great Britain of 1892 OTHER REFERENCES Metal Progress, vol. 64, No. 1, July 1953, pages 142, 144, and 176. (The foregoing publication is a digest of Blowing Thomas Pig Iron With Pure Oxygen, published in Stahl and Eisen, vol. 73, Jan. 1, 1953,) Date relied on, Jan. 1, 1953.

Claims

1. THE PROCESS OF PRODUCING STEEL IMPROVED DUCTILITY, WHICH COMPRISES DESULPHURIZING MOLTEN PIG IRON EXTERNAL TO THE BLAST FURNACE IN A CLOSED SPACE UNDER REDUCING CONDITIONS INTENSIFIED BY AN INERT NITROGEN ATMOSPHERE AND IN THE PRESENCE OF A DESULPHURIZING AGENT UNTIL THE SULPHUR CONTENT IS BELOW 0.013 PERCENT BY WEIGHT, CONVERTING THE DESULPHURIZED IRON INTO STEEL IN THE PRESENCE OF A BASIC SLAG AND IN THE ABSENCE OF SULPHUR-BEARING FUEL BY BLOWING WITH OXYGEN CONTAINING SUBSTANTIALLY LESS NITROGEN THAN THAT IN THE AIR AND THUS OXIDIZING IMPURITIES LIBERATING VOLUMINOUS CARBON MONOXIDE AND THEREBY REMOVING NITROGEN, THE SULPHUR CONTENT AT THE END OF BLOWING BEING STILL LESS THAN 0.013 PERCENT BY WEIGHT, AND SEPARATING THE SLAG FROM THE MOLTEN STEEL.