US3488187A - Low-chromium alloy steel - Google Patents

Description

United States Patent 3,488,187 LOW-CHROMIUM ALLOY STEEL Rudolf Rinesch, Linz, Austria, assignor to Vereinigte Osterreichische Eisenund Stahlwerke Aktiengesellschaft, Linz, Austria, a company of Austria No Drawing. Continuation-impart of application Ser. No. 325,789, Nov. 22, 1963. This application Mar. 29, 1967, Ser. No. 626,702

Claims priority, application Austria, Nov. 28, 1962, A 9,330/62, A 9,331/62, A 9,332/62 Int. Cl. C22c 39/26, 39/44 US. Cl. 75126 1 Claim ABSTRACT OF THE DISCLOSURE A non-ageing soft rimming steel having a carbon content of 0.02 to 0.08%, a chromium content of 0.15 to 0.40%, and a nitrogen content of 0.001 to 0.005%, wherein more than half of the nitrogen in the steel is combined with chromium, forming a chromium nitride compound which is acid-insoluble.

This is a continuation-in-part of application Ser. No. 325,789 filed Nov. 22, 1963, now abandoned.

Rimming steels are steels which are cast without sinkhead in an ingot mould and form blowholes during their solidification. The gas consists mainly of CO. The blowholes are eliminated by welding when the ingot is being rolled. Rimming steels are substantially free of silicon and other deoxidizers. In some cases they may contain a maximum of 0.02% silicon. Rimming steels are known to lack resistance to ageing. Even a short-time storage will render the steel brittle whereby the workability of rimming steels is seriously affected.

In contrast to rimming steels, killed or deoxidized steels do not have the disadvantage of ageing. Killed steels are obtained by an addition of highly etfective deoxidizers, which are added into the ladle before or during teeming. The deoxidizer may consist, e.g. of aluminum alone or of silicon and aluminum together. These have a much higher affinity to oxygen than carbon. For this reason, such killed steels solidify without blowholes but form a deep pipe, and the upper part of the ingot (sinkhead) must be cut off before the further processing. The reduction in yield due to the removal of the sinkhead is 10 to 20%. Another drawback of killed steels is that their surface structure is deficient. The surface layer must be removed by flame scarfing before the rolling step, resulting in a further loss in material.

In steelmaking technology there is a trend to imparting to rimming steels an increased resistance to ageing with a view to providing a steel which, as compared to known killed steels, stands out for superior surface condition and increased output and which, in contrast to the known rimming steels, remains processable for a prolonged period, particularly regarding cold working and deep drawing applications.

This aim is achieved according to the invention b a low-chromium alloy rimming steel, consisting essentially of Percent Carbon 0.02-0.08 Manganese 0.10-05 Phosphorus 0005-0025 Sulphur 0.0100.06 Nitrogen 0.001-0.005 Chromium 0.15-0.40 Silicon 0.02

Balance essentially iron,

ice

wherein more than half of the nitrogen contained in the steel is fixed to chromium in an acid-insoluble nitride form.

Soft rimming steels containing chromium in an amount of more than 0.1% are not yet known. On the contarary, it has always been endeavoured in making rimming steels to achieve a finished product having a chromium content below 0.1%, preferably below 0.05%, because it was believed that higher chromium contents would render the teeming more difficult and would have an adverse efiiect on the technological properties of the steel, particularly on its workability.

This belief held in the art may have been due to the fact that chromium and phosphorus are highly similar in their metallurgical behavior. Steelworkers knowing the rule that a phosphorus content of about 0.1% cannot be tolerated, a corresponding chromium content has also been considered intolerable.

The steel composition according to the invention is based upon the recognition that in soft carbon steels chromium is an excellent means for converting nitrogen to an innocuous form. When chromium and nitrogen are present in a soft carbon steel in the mentioned percentual ranges, they will to a major extent combine to form acidinsoluble chromium nitrides, whereby the ageing tendency which is otherwise inherent in rimming steels is canceled. In the steels of the invention more than half of the analytically determined nitrogen content is insoluble, and only a minor portion, usuall about 30%, is present in an acid-soluble form. This low content of soluble nitrogen is harmless as regards the ageing resistance of the steel.

Optimal steel quality is achieved if the chromium is already present during the crude iron conversion operatio rather than add to the ladle, for a steel deriving its chromium content from the conversion process shows less non-metallic inclusions than a steel wherein the chromium originates from a ladle addition. This phenomenon is due to the fact that when chromium is added in such a late stage as in the ladle, complex chromium compounds such as spinels result, which will not or hardly separate.

Up to now, it has been difficult to convert chromium containing crude iron directly into steel by refining. Even such complicated methods as two-slag processes with over-refining of the steel or mixing chromium-containing crude iron with steelmaking iron in order to reduce the chromium content have not given satisfactory results. For this reason it has been endeavoured to find means and ways for removing the chromium from the crude iron before the carbon is removed in the Subsequent refining process. For instance, it has been suggested to subject the chromium-containing crude iron to a preliminary refining, which is carried out in the ladle and in which oxygen is blown by water-cooled immersed lances (depth of immersion 30 to 50 cm.) into the bath. Nozzle sets comprising a plurality of nozzles were used for this purpose, and oxygen was blown into the bath under high pressure. Chromium was removed until a chromium content below 0.12% had been reached. Brown smoke was formed as an undesired attendant phenomenon. As the oxidation of the accompanying elements is effected in the order: silicon, manganese, and chromium, depending on their aflinity to oxygen, these accompanying elements were necessarily also removed by the preliminary refining so that the heat content of the crude iron was adversely affected, because it is determined by the carriers of chemical heat. As a result, the crude iron which had been subjected to the preliminary refining lacked chemical heat in the actual refining process. In most cases the carbon content was also reduced to an undesirably high extent by the preliminary refining and amounted to only 2 to 2.5% when chromium values of 0.2 to 0.3% had been reached. Attempts to improve the effect of refining in the ladle by an addition of mill scale or the use of a mixture of oxygen and steam resulted only in gradual rather than decisive improvements. The intermediate product obtained by preliminary refining was then processed in the hearth furnace. Owing to the low contents of exothermic accompanying elements, the use of a refining process in the converter seemed to be possible only if additional carriers of chemical heat could be added to the starting product. This would obviously involve additional expense and labor.

Due to the notorious difficulties described, several important iron deposits of the world in which iron is associated in the ore with chromium have been insufliciently exploited. One of the best known chromium-containing iron ores is the Conakry ore, which has the following composition: 45-55% Fe, 0.1% Mn, 0.06% P, 2.5% SiO 9.8% A1 0.3% MgO, and 0.25 to 2.0% chromium in the form of chromium oxide.

If such an ore is smelted in a blast furnace as an important part of the burden rather than as an ore addition, a chromium-containing crude iron will be obtained which has a composition of 3.6 to 4% C, 0.2 to 1% Si, 0.5 to 1% Mn, 0.4 to 0.8% Cr, provided that the proportion of chromium-containing ore in the burden is 35 to 50%. If the burden contains even large proportions of chromium-containing ore, the chromium values of the crude iron will be correspondingly increased up to 1.7%.

It has now surprisingly been found that the abovementioned disadvantages and difiiculties can be avoided by the use of the oxygen top-blowing process for agonversion of chromium-containing crude iron into sitje'el.

Depending on the original chromium content of the crude iron to be refined, the steel may be made in singleor two-slag processes. In accordance therewith, the invention proposes, according to a first aspect, a process comprising providing a molten bath of chromium-containing crude iron having a chromium content of 0.9 to 1.1%, top-blowing oxygen onto said bath in the presence of a basic slag, continuing blowing without a slag change until a steel is obtained having a carbon content in the range between 0.02 and 0.08%, a manganese content of 0.10 to 0.5%, a phosphorus content of 0.005 to 0.025%, a sulphur content of 0.010 to 0.06%, a nitrogen content of 0.001 to 0.005%, and a chromium content of 0.15 to 0.40%, and teeming said steel under substantially rimming conditions. This is thus a one-slag method.

If a crude iron is to be processed which has a higher content of chromium, the modified process to be then applied according to the invention comprises providing a molten bath of chromium-containing crude iron having a chromium content of 1.1 to 1.7%, top-blowing oxygen onto said bath in the presence of a basic slag, removing said slag when a carbon content of 1.5 to 1.7% is reached, the chromium content simultaneously being 0.60 to 0.80%, forming a new basic slag, continuing blowing until a steel is obtained having a carbon content in the range of between 0.02 and 0.08%, a manganese content of 0.10 to 0.5%, a phosphorus content of 0.005 to 0.025%, a sulphur content of 0.010 to 0.06%, a nitrogen content of 0.001 to 0.005%, and a chromium content of 0.15 to 0.40%, and teeming said steel under substantially rimming conditions.

When chromium-containing crude iron is converted, according to the invention, with the aid of the oxygen top-blowing method, those difficulties which are mentioned hereinbefore and as a result of which steelmakers have felt bound to maintain the chromium content of steel below 0.1%, preferably below 0.05%, do not occur because, owing to the high temperature at the bath sur face -as is attained in an oxygen top-blowing process (as contrasted to other steelmaking processes), slag formation is accelerated and facilitated and, in particular,

chromium-containing slags can easily be maintained in fluid condition. Whereas in the previous practice involving a combination of a preliminary refining with a final refining (hearth furnace process), chromium was actually a great hindrance due to the formation of highly viscous slags, the top-blowing process enables a trouble free conversion of the chromium-containing crude iron into steel.

With regard to the object of the invention to make a steel of good properties in respect of resistance to ageing, it has further been found favorable to observe specific working conditions during the cold reduction and recrystallizing finish anneal of the steel. According to that aspect of the invention, optimum results are obtained if the degree of cold working is 30 to 70% and the temperature of the re-crystallizing finish anneal is between 600 and 660 C. A suitable mode of processing may reside, for instance, in hot-rolling the cast ingot and further reducing the bloom to give strip stock. Subsequently the hot rolled strip is cold rolled (degree of deformation in three passes being 30 to 70%), and finally the re-crystallizing finish anneal is carried out. The sheet materials obtained have a materially improved resistance to ageing. They may be used to advantage for all kinds of cold working and pressing applications, e.g. for the manufacture of pots, sinks, car body parts and other sheet metal pressings of all kinds.

The manufacture and composition of the steel of the invention is illustrated in more detail in the following Examples 1 to 4. Example 1 illustrates the making of steel from chromium-containing crude iron in an oxygen top-blowing process without intermediate deslagging. Example 2 relates to a process carried out with intermediate deslagging. Examples 3 and 4 illustrate the making of steel in a process in which ordinary steelmaking iron is first refined by top-blowing oxygen and the required chromium is then added in the form of ferro-chromium into the ladle. In the table following Examples 1 to 4 the technological values of steels according to the invention are given, which show that the changes in the mechanical properties in the yield point range which would occur with rimming steels after artificial ageing do not occur or occur only to a slight degree in the steels according to the invention. The table also shows the proportion of soluble nitrogen to insoluble nitrogen in the steel.

EXAMPLE 1 Single-slag process Charge: Kg. Crude iron 5300 Scrap 350 Fe from scarfing scale 24 Total (100.0%) 5674 Yield (87.8%) 2 4980 Course of process: I i Charging scrap and crude iron 5.00 Blowing period 15.70 Submerged temperature measurement,

sampling 9.00 Tapping 2.00

Total 31.70

Additions: Kg. Lime 330 Limestone Fine ore Bauxite 30 Quartz sand Fluorspar 30 Scarfing scale 40 Ladle addition.

Submerged Analyses percent: temperature, C Si Mn P S Cr N C.

Charging sample 4.12 0.60 1.69 0.160 0.040 0.95 1,230 Sample taken before tapping.-. 0.07 0.36 0.020 0.016 0.24 1,610 Finished sample- 0.07 0.35 0.016 0.017 0.23 0.0020

FeO M110 S10; CaO MgO P40 A110 C130.

Slag, percent 22.10 11.90 8.00 41.80 2.18 1.69 2.20 5.90

EXAMPLE 2 Two-slag process Charge:

Crude iron 5600 Perm-manganese 14 Fe from scarfing scale 30 Total (100.0%) 5644 Yield (86.1%) e 4860 Course of process: Min. Charging crude iron 3.00 First blowing period 11.20

Submerged temperature measurement,

sampling, intermediate deslagging 13.00 Second blowing period 6.30 Submerged temperature measurement,

sampling 7.00 Tapping 2.00

Total 42.50

Additions, kg.

Scarf- Llme- Fine Baux- Quartz Fluoring Lime stone ore ite sand spar scale First slag 150 50 50 Second slag 250 50 30 Ladle addition: 14 kg. FeMn (75.6%).

Submerged Analyses percent: temperature, C Si Mn P S Or N Charging sample. 4.18 0.55 1.51 0.128 0.041 1.59 1,280 Sample takenduring intermediate deslagglng 1.65 005 0.77 0.090 0.035 0.75 1,560 Sample taken before tapging.-- 0.04 0.20 0.020 0.019 0.23 1,620 Finis ed F90 MnO S101 0110 MgO P105 A110 CrzOa First slag,

percent..- 8.77 11.30 15.20 32.00 4.83 1.04 5.30 18.86 Second slag,

percent..- 23.42 7.85 6.52 41.90 3.50 1.50 4.55 10.80

EXAMPLE 3 Rimming steel melt having a low chromium content and obtained from crude iron without any Cr content. Addition of ferro-chromium into the ladle Course of process: Min. Charging scrap and crude iron 6.00 Blowing period 16.30 Submerged temperature measurement, sampling 8.00 Tapping 2.00

Total 32.30

Additions: Kg. Lime 330 Limestone Fine Ore Bauxite 30 Quartz sand Fluorspar 30 Scarfing scale 40 Ladle addition: 26 kg. FeCr (69.9%).

Submerged Analyses percent: temperature, o si Mn P 8 Cr N Charging sample 4.25 0.03 1.01 0.100 0.045 1,210 Sample taken before tapping.-. 0.05 0.31 0.011 0.014 1,025 Finished sample 0.05 0.20 0.010 0.015 0.29 0.0025

FeO MnO S101 CaO MgO P405 A; 0110;

Slag, percent 22.45 10.93 9.34 47.15 1.99 1.79 2.25

EXAMPLE 4 Rimming steel melt having a low chromium content and obtained in a 30-ton top-blowing converter from crude iron without any Cr content. Addition of ferro-chromium into the ladle Charge: Kg. Crude iron 28,540 Scrap 6,840 Ferro-chromium Fe from scarfing scale 100 Total (100.0%) 35,670

Yield (89.5%) 31,950

Course of process: Min. Charging scrap and crude iron 4.00 Blowing period 16.60 Submerged temperature measurement, sampling, deslagging 13.40 Tapping 2.00

Total 36.00

Additions: Kg. Lime 1500 Limestone 500 Fine ore Bauxite 50 Quartz sand Fluorspar Ladle addition:

190 kg. FeCr (69.9%) '3 kg. finely ground coke Submerged Analyses percent: temperature, C Si Mn P S Cr N 0.

Charging sample 4.18 0.73 1.99 0.160 0.040 1,200 Sample taken beiore taping..- 0.06 0.41 0.020 0.021 1,595 Finis ed sample 0.08 0.37 0. 019 0.022 0.33 0. 0034 F90 MnO S101 0110 MgO P40 A110 CrzO;

Slag, percent 18.76 12.32 12.45 45.55 3.26 1.57 0.89

TABLE TechnologicalvalueskgJsq. Thickness .mm. Percent of sheet, E, Example mm. State of specimen as IT]; d dos mm. After Sni es..." 18.0 32.3 39.7 6.0 11.3 Aged at 100 C. 17.8 32.6 39.7 0.0 11.2 1 v0.75. .1or5min..

Aged at 100 C. 21.4 32.7 36.1 0.4 10.0 for 2 hours. V A1terskinpass 24.0 36.1 37. 9- 0. 11.3 -Aged at 100 C. 24.2 36.7 36.6" 0.0 11.4 2 0.75 for-min. V

Aged at 100 C. 26.6 37.0 0.4 10. 7

for 2 hours. -Afterskin pass 20.7 36.5- 34.1 0.0 11.8 I Aged at 100 C. 21.3 37.0. 35.300 11.6 3-.. 1.00 fol-5min. 1 Aged at 100 C. 22.1 37.0 0.4 11.6 0.0" 11.8 0.0 11.7

4 in v.

- Aged at 100 C. 21.2 35.5 0.0 11.3

for 2 hours.

7 Chemical Composition (check enalysis) Percent v Example 0 Mn I s Cr N N' N Total. 2 Soluble. 3 Residual. o's=Y-ield point. a5 Ultimate tensile stress; d Rupture elongatl0u (80 mm. gauge length).

do's Yield point elongation. Tn =Erichsen cupping value.

What I claim is:

consisting essentially of t Percent Ge be Manganese 1 Phosphorus Sulphur Nitrogen Chromium; Silicon' Balance. essentially .1ron,..

vk'z hnein .iifmelre.il iain..1111.115.v 0ftheni1ir g3ritf con in dinthe steel is fixed to chromium -iu; an aci d insoluble nitride form.

Ram-sees enea' UNITED STATES PATENTS 214 351 1, 8 9 91 I Ma f m 5%126 2,532,117 11/1950 Newell -126 -7 5 126 -2 745,738 5/ 1956- Phillips-..

HY-LAND BIZOT, Primary Examiner