GB1566379A - Method of refining steel - Google Patents

Description

(54) A METHOD OF REFINING STEEL (71) We, THYSSEN EDELSTAHLWERKE AKTIENGESELLSCHAFT, a German Company of Oberschlesienstrasse 16, 4150 Krefeld, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of refining steel, particularly stainless steel with a high chromium content, to provide low carbon contents less than 0.03% by blowing gaseous oxygen onto or into the steel melt, and in which a stirring action is exerted on the melt by blowing in an inert gas.

While both the melting and the refining of steel have in the past been effected in the melting furnace, for example an electric-arc furnace, some years ago it was suggested that the furnace should be used for melting down, and that the subsequent refining and alloying steps should be carried out in a following unit, for example a crucible or converter. This procedure had a significant economic advantage in that the melting furnace would be used for the purpose for which it was designed, and the time was reduced to about half. Thus almost twice the amount of charge could be melted in the furnace compared with the previous practice.

Various methods have been proposed for refining the melt in a separate vessel. The so-called AOD method (argon-oxygen-decarburization) is described for example in the German standard method DT-AS 1 508 280 and consists in blowing a mixture of argon and oxygen into and through the steel melt transferred from the melting furnace into a converter. The purpose of the inert argon gas is to reduce the oxygen partial pressure in the melt, so as to increase the affinity of the oxygen for carbon preferentially to chromium contained in the melt. In this manner the risk can be reduced of the chromium content of the melt becoming oxidized or slagged when oxygen is blown onto or into the melt and having to be replaced at the end of the refining operation.

Other refining methods were developed wherein a steel melt produced in a melting furnace, for example a hot-blast cupola, was pre-refined in a basic oxygen converter and then fully refined in a ladle degassing installation under vacuum by blowing on gaseous oxygen (cf "Stahl und Eisen" 88 (1968), pages 153 to 159). By this known method decarburizing of the steel melt to a carbon content less than 0.01% substantially without loss of chromium, was possible.

In the procedure of DT-OS 1 458 901, the blowing of oxygen or a mixture of oxygen and argon onto the surface of a steel melt present in a crucible is effected by blowing oxygen or a mixture of argon and oxygen through the melt. Instead of using argon, steam or methane (DT-Os 2 308 469) or oil (DT OS 2 033 975) have also been proposed.

An early proposal to allow the refining process to take place under reduced pressure (DT-PS 676 565) for the production of ferro-chrome, has recently become of interest (cf United States Patent specification 3,854,932 which describes the production of steel containing chromium). In these two processes gaseous oxygen is blown onto the surface of a melt present in a converter, and in the latter process inert gas is allowed to bubble through the melt to exert a stirring action and to activate the refining.

It was an object of such previously known methods for refining steel melts containing chromium, to prevent oxidation of the chromium present in the melt. Generally however it was necessary to replace the proportion of chromium which was slagged by the addition of chromium in metallic form or as a preliminary alloy, for example ferrochrome.

Furthermore the apparatus required was relatively expensive particularly using a process of pre-refining in a crucible and final refining under vacuum in a ladle degassing installation, and to the refining under vacuum. With the AOD process the high consumption of argon is undesirable on economic grounds.

The present invention enables the refining to be carried out in an economical manner using comparatively simple apparatus.

The invention provides a method of refining steel containing chromium to provide a carbon content less than 0.03% by blowing gaseous oxygen onto or into the steel melt in a vessel and in which a stirring action is exerted by blowing in inert gas, characterised in that a first refining step is effected using gaseous oxygen at atmospheric pressure until the carbon has dropped to from 0.2% to 0.4%, and a second refining step is effected in the said vessel by continuing the agitation of the melt by inert gas but with interruption of the supply of gaseous oxygen, with continuous reduction of the pressure above the melting bath to or below 10 torr, whereby the metal oxides formed during the first refining step are reduced until the carbon content of the melt is further lowered to the required value.

When carrying out the method according to the invention, the steel melt containing chromium which has been melted in a furnace is transferred into a refining vessel, for example a converter, and is first pre-refined therein in known manner at atmospheric pressure by blowing oxygen onto or into the melt until the carbon has dropped to a value of from 0.2% to 0.4%. Then, in the same vessel and after interruption of the supply of oxygen but while maintaining the bubbling through the melt of inert gas to stir it, the pressure is continuously reduced until it has reached 10 torr. At this pressure the steel melt is left to boil until the required final carbon content is reached.In the process of the invention it has surprisingly been found that the chromium and manganese oxides inevitably formed during the refining, are reduced during the continued boiling under reduced pressure so that the melt again becomes alloyed with the chromium and possibly manganese initially present.

An addition of these metals as a replacement for the proportion slagged during refining is therefore not necessary to the extent required in earlier processes and not at all if the initial steel analysis has been adjusted before the refining. The reduction of chromium and manganese oxides occurring when the melt is left to boil under reduced pressure is indicated in the accompanying Table: TABLE Composition of After refining before After treatment steel bath use of reduced pressure under reduced pressure C 0.25% 0.013% Mn 0.99% 1.11% Cr 16.34% 16.73% 2 650 ppm 420 ppm Fe rest rest Temperature 17800C. 1724"C.

Thus the chromium content rises from 16.34cue before the treatment under reduced pressure to 16.73ago and the manganese content from 0.99% to 1.11%. Since chromium and manganese have not been added during the vacuum treatment, this increase can only be attributed to reduction of chromium and manganese oxides out of the slag above the steel bath. The oxygen released during the reduction of chromium leads to a further reduction in the carbon content in the melt.

If the carbon content has to be reduced below 0.02aXc. it is an advantage in the method according to the invention that oxides preferably in solid form, for example in the form of pitticite, be added during the second stage of the refining, or the time occupied in the said second stage extended. Through reduction of these oxides under vacum, oxygen becomes free which encourages the decarburizing action by binding the carbon.

The second stage of refining in the process according to the invention may be effected at a temperature above 1700"C. for a time of about 15 minutes, the loss of temperature occurring during this time being about 60"C.

In comparison with known refining processes using a vacuum, the method according to the invention has the following advantages: (a) Refining is possible without spitting. Thus splashes may not occur which cake at the opening of the converter, as happens when refining under vacuum. A vacuum covering hood can therefore be fitted with satisfactory sealing.

(b) The vacuum pump used to obtain the reduced pressure can be smaller than those used in corresponding units which are necessary for carrying out the process taking place entirely under vacuum, as less gas has to be pumped off when producing the vacuum at the end of the refining process than is necessary while oxygen is being blown on during the refining.

(c) The possibility of the reduction of the chromium and manganese slagged during the refining is not possible with known methods to the same extent as when carrying out the method according to the invention. As a result, the expense of providing comparatively expensive alloying or reduction metals can readily be reduced.

WHAT WE CLAIM IS: 1. A method of refining steel containing chromium to provide a carbon content less than 0.03% by blowing gaseous oxygen onto or into the steel melt in a vessel and in which a stirring action is exerted by blowing in inert gas, characterised in that a first refining step is effected using gaseous oxygen at atmospheric pressure until the carbon has dropped to from 0.2% to 0.4%, and a second refining step is effected in the said vessel by continuing the agitation of the melt by inert gas but with interruption of the supply of gaseous oxygen, with continuous reduction of the pressure above the melting bath to or below 10 torr, whereby the metal oxides formed during the first refining step are reduced until the carbon content of the melt is further lowered to the required value.

2. A method as claimed in Claim 1, wherein a metal oxide is added during the second stage.

3. A method as claimed in Claim 2, wherein pitticite is added during the said second stage.

4. A method as claimed in any of Claims 1 to 3, wherein a stainless steel is refined.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. (b) The vacuum pump used to obtain the reduced pressure can be smaller than those used in corresponding units which are necessary for carrying out the process taking place entirely under vacuum, as less gas has to be pumped off when producing the vacuum at the end of the refining process than is necessary while oxygen is being blown on during the refining. (c) The possibility of the reduction of the chromium and manganese slagged during the refining is not possible with known methods to the same extent as when carrying out the method according to the invention. As a result, the expense of providing comparatively expensive alloying or reduction metals can readily be reduced. WHAT WE CLAIM IS:

1. A method of refining steel containing chromium to provide a carbon content less than 0.03% by blowing gaseous oxygen onto or into the steel melt in a vessel and in which a stirring action is exerted by blowing in inert gas, characterised in that a first refining step is effected using gaseous oxygen at atmospheric pressure until the carbon has dropped to from 0.2% to 0.4%, and a second refining step is effected in the said vessel by continuing the agitation of the melt by inert gas but with interruption of the supply of gaseous oxygen, with continuous reduction of the pressure above the melting bath to or below 10 torr, whereby the metal oxides formed during the first refining step are reduced until the carbon content of the melt is further lowered to the required value.

2. A method as claimed in Claim 1, wherein a metal oxide is added during the second stage.

3. A method as claimed in Claim 2, wherein pitticite is added during the said second stage.

4. A method as claimed in any of Claims 1 to 3, wherein a stainless steel is refined.