DE2243839A1 - PROCESS FOR MANUFACTURING LOW CARBON HIGH CHROMIC FERRITIC STEEL - Google Patents

Description

K L Ö CK Ή E R - WEE KE Aktiengesellschaft, Duisburg

Process for making low carbon ferritic steels with high chromium content

The invention relates to a method for producing low-carbon, high-chromium ferritic steels, in particular ferritic stainless steel grades with chromium contents of 12-23 % and carbon contents of 0.01-0.08%. _:

The ferritic steel grades ipit chromium contents of 12 - 23% are mainly produced in an electric furnace. The chromium-containing scrap and the partial use of ferrochrome are liquefied in the electric arc furnace. The carbon is then freshened up to the desired content of 0.02-0.08% by blowing pure oxygen into the molten bath. In this refining process, which takes place in a temperature range of 1800 G ^0, a relatively large Chromverschlakkung can not be prevented ,. To get out of the -...-. To improve the chromium yield for economic reasons, the chromium oxide-containing slag is treated with reducing agents after the refining process. The nitrogen content present after the manufacturing process fluctuates between 0.010 and 0.025 %

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To keep the unavoidable when removing refining carbon, chromium-containing melts chromium slagging low and to refer to the slag reduction or performed only to a small extent, the carbon new method is removed in such a manner that observed in the refining about I700 ⁰ C low Kohlenoxidpartialdrücke will. Low carbon oxide partial pressures can be achieved either by lowering the total pressure, as in oxygen vacuum refining, or by adding inert gases to the oxygen flow, as in converter refining.

In the wind freshening process, but also when freshening oxygen-enriched wind, oxygen and water vapor, oxygen and carbonic acid as well as oxygen and argon, it is known that the carbon of the Thomas or steel iron is refreshed to a content of less than 0.01 % and that depending on the proportion of the inert gas, the nitrogen content of the steel can also be reduced.

The processes for the production of carbon, which have become known in the patent and specialist literature Substance poor ferritis rather steel good with under * - Different nitrogen contents are used vacuum fishing with pure oxygen or converter refining with oxygen-argon or oxygen-nitrogen mixtures. The steel melt to be refined under vacuum or in the converter is either in the Electric furnace pre-melted or in the LD-Kon-

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verter pretreated, with the chromium content between 12 and 23 % and the carbon. stop vary between 0.30 and 1.50 %.

All previously known processes are based on pig iron with a high chromium content and must be of relatively high carbon content with high chromium contents at the same time, which results in a mille of Difficulties with regard to the leaching of the chrome results.

The invention has set itself the task of shortening in particular the period in which the melt is refined with a reduced partial pressure of oxygen. She has also set herself the task of not subverting the final freshening of the melt having to perform reduced pressure.

The invention solves this problem in that pig iron that is low in phosphorus and chromium is first refined in a bottom-blowing converter. After the fresh process, there is a steel melt with 0.02-0.04% C, 0.010% P, 0.15% Mn, 0.015% S. and 0.003% H. before. The slag of the melt is completely removed in order to avoid any harmful back phosphorus. After skimming, the melt is heated by blowing pure oxygen gas at about ⁰ C under I7OO Zurgabe of lime and heating means. After this temperature has been reached, the melt is melted without the flow of oxygen being interrupted

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solid ferrochromium with 4-6 % carbon is continuously added in such a way that at a final chrome content of 12-25 % the carbon content of the bath does not exceed 0.50% and the temperature does not fall below 1700 ° C. Refining with pure building material is continued until the carbon content of the bath is 0.25-0.30 % . ., Then the oxygen flow is continuously and increasingly inert gas added in order with decreasing carbon content and the ^{Kohlenoxidpartial-:} - 85% by volume increased to a final carbon content of 0 to 80 to lower ⁱ "■ Pressure The proportion of inert gas is λ I 02% and less, the unavoidable chromium burn-off in this fresh process fluctuates between 10 and 20%, based on the residual chromium content of the melt. In order to largely reduce the chromium oxide from the slag, carbon-free reducing agents are added to the melt after the fresh process Reduction of the chromium oxide-rich slag is completed in less than three minutes as a result of the intimate mixing of bath, slag and reducing agents by blowing in oxygen-free inert gases such as argon or nitrogen. The chromium yield, based on the added amount of chromium, is over 95 %

After deslagging, the missing alloying elements are used to stop the analysis added and the bath briefly treated with inert gases for homogenization.

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When using argon as an inert gas and ever According to the nitrogen content of the ferrochrome, steel grades with below 0.010% N can be generated. Higher nitrogen levels are made by using nitrogen or argon-nitrogen mixtures achieved ... .

The procedure is characterized by its flexibility and productivity. It. is easy to use. The heat balance or the temperature the melt can easily. by adding heating agents, but also by the temporal Oxygen flow or the oxygen-inert gas mixture be regulated. Depending on the operational conditions, the bottom blower Converter for the production of ferritic chromium steel goods a liquid chromium-free and low-carbon feed from a large-scale konyerter obtain. -

The following is an example of the Melting sequence of low-nitrogen ferritic chromium steel melts given :.

If you go to the manufacture of ferritic Chrome steel, made from low-phosphorus pig iron, see above in the first process step, the low-phosphorus Pig iron with the addition of the appropriate quantities of lime and cooling scrap with pure oxygen to a low-phosphorus and low-carbon one Steel refurbished. The decrease in carbon and phosphorus over time takes place accordingly the charring curves shown in the figure. The batch is then fully

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constantly deslagged to avoid back phosphorus. '^;

During the deslagging, the temperature of the melt drops from about 1580 Ms 1600 ⁰ C to 1530 to 15 ^ 0 ⁰ C. This is followed by the addition of about 20 kg fine lime / t, 5 kg Al / t, and 5 kg FeSi / t for heating the batch and for slag formation. After a blowing time of about 2 minutes, the continuous addition of carbonaceous ferrochroia is started. The addition takes place so quickly that an average carbon content of about 0.3 % is maintained in the bath at the set oxygen rate. In the example given, the chromium content is set to around 15%. In between, either additional heating agents (CaSi, FeSi or Al) or Kühlsch: pött can be added to correct the temperature. After the intended ^{amount of chromium 5 has been} charged, increasing amounts of argon are added to the oxygen flow at carbon contents of about 0.3% and the carbon in the example given is reduced from 0.3% to $ 0.015. The chromium content decreases from 15% to around 13%, the Ar content is around 90% at the end of the fresh season.

The final stage of the process is by adding carbon-free reducing agents the Cr content of the slag is largely reduced. This increases the Cr content of the steel Measure back to over 15% »being in this Process step only Ar is blown in through the bottom nozzles.

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If the process sequence is based on pre-freshened, slag-free LD steel, the first two process stages fall out
Pre-freshening and deslagging continue, so that the heating of the melt and the addition of Cr can be started immediately. The rest of the process - freshening with pure oxygen, freshening with oxygen-argon mixtures, reducing and deoxidizing - proceeds in a similar way.

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Claims (1)

Expectations : as ssassizs 1.) Process for the production of low-carbon, high-chromium ferritic steels, in particular ferritic stainless steel grades with chromium contents of 12-23% and carbon contents of 0.01-0.08% in a basic ■ bottom-blowing converter using gaseous oxygen and inert gases from argon or nitrogen , characterized in that low-phosphorus pig iron is first refined in a bottom-blowing converter with pure oxygen to a soft, low-nitrogen steel, the slag is removed and then the melt is heated to over 170 ° C. with the addition of lime and heating agents and then continuously without interrupting the flow of oxygen with ferrochrome in alloyed in such a way that the carbon of the melt does not exceed 0.50% at a final chromium content of 12-23% and the refining with pure oxygen is ended at a carbon content of 0.25 0.30% and then the oxygen stream is continuously inert gas, and although argon. or an argon-nitrogen mixture, or a nitrogen, is supplied in such a way that the carbon oxide partial pressure does not exceed 76 torr at final carbon contents which are 0.02% or less. - 2 409813/0052 2.) Method sur production of low-carbon , high-chromium ferritic steels according to claim 1, - characterized in that the soft converter steel. with 0.02-0.04 ° / ö C, 0.010 ° / o P, 0.01.5% S and 0.003% U contains silicon-containing alloys or aluminum for heating the melt. · ■ -. '■ 3-). Process for the production of low-carbon, high-chromium ferritic steels according to claims 1 and 2, characterized in that solid ferrochromium with 4-6% carbon is continuously added to the melt without interrupting the heating and mixing process. . 4 x) Yerfahren for producing low-carbon hochchromhaltiger ferritic steels according to claims 1 - 5 »characterized in that the chromhal-r term melt with 12 - 23% chromium Ms -from to a carbon content of 0.25 ■ * - -. '■ ■■ '.. O ₁ JO % freshened with pure oxygen':. becomes »- · ■ ί; ' 5.) Yerfaitren for the production of carbon; poor high-chromium ferritic.,. . Steels according to claims 1 '- "4" -. da> t ... by gefeeaaiaeiöhnet ^ that after areas with a carbon content of . 0.25 ..- .0, JO% the oxygen increasing amounts of inert gases in the form of argon or nitrogen - 3 - ■ ■ 409813/0052. ,: Jib substance or mixtures of argon and nitrogen to reduce the carbon content ialdruckes are added. 6.) Process for the production of low-carbon, high-chromium ferritic steels according to claims 1-5 » da- . characterized in that, in order to achieve a final carbon content of 0.02 % or less, the oxygen-inert gas · gasoline ratio is 1: 4 to 1: 4.5. 7.) Process for the production of low-carbon, high-chromium ferritic steels according to claims 1-6, characterized in that only argon is used as the inert gas to produce a low-nitrogen ferritic chromium steel. 8.) Process for the production of low-carbon, high-chromium ferritic steels according to claims 1-7 *, characterized in that after the end of the 3? Risch process, the chromium-containing slag with carbon-free reducing agents such as PeSi, CaSi, Al and by blowing oxygen-free inert gases consisting of argon or nitrogen or from mixtures of the two. 9.) Process for the production of low-carbon, high-chromium ferritic steels according to claims 1-8, characterized - Zf _ 409813/0052 net that before, during ode.r'after / 'the. ^. Slag reduction the missing alloying elements such as silicon, manganese and chromium are added to the bath to adjust the final analysis. 10.). Process for the production of low-carbon, high-chromium ferritic steels according to claim 1, characterized in that it is assumed that pre-freshened low-density steel is used to increase the melt with the addition of lime and heating agents to over 1,700. ⁰ C and then continuously alloyed with ferrochrome without interrupting the flow of oxygen in such a way that at final chrome contents of 12-27 )% of the carbon of the melt is 0.50 %. does not exceed and the refining with pure oxygen at a carbon content of 0.25-0.30 % is ended and then the oxygen stream is continuously supplied with inert gas, namely argon or argon-nitrogen mixture or pure nitrogen, in such a way that the Carbon oxide partial pressure does not exceed 76 torr at final carbon contents that are 0.0.2 % or less. ■ 4098.13 / 005.2 JNSPEGTEO