DE102004004038B3 - Process to manufacture fully-killed aluminium-steel by introduction of calcium carbonate - Google Patents

Abstract

In a process to produce fully-killed aluminum (Al)-steel with a low content of strengthening elements, pig iron virtually devoid of sulfur is converted in a blast furnace to steel with a carbon content of less than 0.01%. The molten steel is subsequently cast into a pan in which the sulfur content declines further by introduction of a calcium agent e.g. calcium carbonate.

Description

The The invention relates to a process for producing an Al-killed Steel with low contents of strength-increasing elements according to the preamble of patent claim 1 or patent claim 2.

In The thin sheet industry is increasingly demanding grades of steel with low strength values. These qualities are lowest in C, P, Si, Cu, Cr, Ni, Mo and lowest, respectively Strength values and also under the name ULC (Ultra Low Carbon) and IF (Interstitial Free). Especially the required extremely low carbon contents can be problems prepare because after the converter process in the subsequent secondary metallurgical Treatment carburizing has shown, depending on the degree, as a faulty melting had to be discarded.

task The invention is a process for producing an Al-calmed Indicate steel with low contents of strength-increasing elements, which avoids carburisation.

These The object is achieved with the features of claim 1 or 2. advantageous Further developments are the subject of dependent claims.

According to the teaching of the invention, in the secondary metallurgical treatment, in particular for further sulfur reduction, only those agents are added which are virtually free of limestone (CaCO ₃ ), for example pure CaO.

Closer investigations have shown that carbonate from the lime, which are preferably used in the secondary metallurgical treatment for sulfur reduction, are the source of the Kohlenstoffzubrand. The reactions that take place can be described as follows:
CaCO ₃ splits at high temperatures (> 1000 ° C) to CaO and CO ₂ . CaCO ₃ → (CaO) + {CO ₂ } (1)

The Al added for the deoxidation of the molten steel is able to split off carbon from CO and thus cause the carbon sequestration. In other words, the carbon monoxide is reduced by aluminum in the molten steel and the remaining carbon is dissolved in the steel. 2Al + 3CO ₂ → (Al ₂ O ₃ ) + 3 {CO} (2) 2Al + 3CO → (Al ₂ O ₃ ) + 3 [C] (3)

Neither the titanium present in the molten steel nor the manganese or silicon can cause a Kohlenstoffzubrand, because of the thermodynamics CO is more stable than MnO, SiO ₂ and Ti ₂ O ₃ . 2Ti + 3CO ₂ → (Ti ₂ O ₃ ) + 3 {CO} (4) Mn + CO ₂ → (MnO) + {CO} (5) Si + 2CO ₂ → (SiO ₂ ) + 2 {CO} (6)

Taking these relationships into account, care must be taken in the production of ULC and IF steels that, if possible, no limestone (CaCO ₃ ) is introduced during the secondary metallurgical treatment with Al-deoxidized molten steel, so that this does not lead to the reactions described above , This can be achieved, for example, by adding an almost pure CaO for the sulfur reduction.

The Obtaining an almost pure CaO and avoiding any limestone entry can be difficult and the cost of steelmaking in the Height.

Out This reason becomes the solution the problem proposed a second variant. This variant is characterized in that for the secondary metallurgical Treatment an unkilled molten steel is poured into the pan and for sulfur reduction commercial, d. H. with limestone contaminated CaO is added. Since no Al is present in the melt is, can the previously beschnebenen reactions do not expire. Only at the end the treatment when the CO is removed from the melt Al added for deoxidation.

Even with this procedure, the contamination with limestone will form CaO and CO ₂ . Nevertheless, the said reactions do not take place since there is no Al in the melt. However, before the final addition of Al, the CO largely escapes from the molten steel, so that the Al added can not split off C.

The The latter procedure has the advantage that on the expensive Addition of high purity CaO can be waived and only by modification the time of addition of Al for deoxidation a Kohlenstoffzubrand is prevented.

• – ein Kohlenstoffzubrand wird verhindert,
• – Mn-, Ti- und insbesondere Si-Gehalte werden werter abgesenkt,
• – Cr- und P-Gehalte werden etwas vermindert,
• – ein hoher Ausgangsgehalt an Kohlenstoff kann auf den für eine ULC-Güte abgesenkt werden,
• – ein niedriger Ausgangsgehalt an Kohlenstoff kann konstant gehalten werden.

With the proposed procedure, the following effects can be achieved:

• A carbon deposit is prevented
• - Mn, Ti and in particular Si contents are lowered,
• - Cr and P contents are slightly reduced,
• A high initial carbon content can be lowered to that for a ULC grade,
• - a low initial carbon content can be kept constant.

The The latter effect can be explained by the fact that at a low Initial content of carbon the activity is no longer sufficient to To run reactions of the type described above.

Of the Advantage of the proposed procedure is to be seen in that one part of anyway for receiving the deoxidation products and required for steel desulfurization softening lime amount not immediately when tapping in the pan, but only at the secondary metallurgical treatment added. Furthermore, the proposed procedure to do so used to be targeted by use of limestone in case of injury Upper limits of analysis of strength-enhancing elements the contents of these elements by burning down in the range of customer specifications bring to.

Based the following schematic diagrams of the inventive method is based An approximately 70kg trial melt explained in more detail. Show it:

1 Modification of the steel composition by adding CaCO ₃ to an Al-killed molten steel

2 Modification of the steel composition of an unkilled molten steel by the addition of CaCO ₃

3 as 2 but with low initial content of carbon in the molten steel

1 shows the change of the steel composition by adding CaCO ₃ to an Al-killed melt. The addition of CaCO ₃ and the addition of Al are indicated by arrows pointing vertically downwards.

As can be seen, the reactions begin according to equations 1 to 6 with the beginning of the addition. First, CaCO ₃ is split due to the high temperature in CaO and CO ₂ , followed by the reactions of the elements Al, Ti, Mn, Si with CO ₂ The clearly visible increase in carbon content of about 0.005 wt .-% to more than 0.015 wt .-% is due to the reaction according to equation 3. Al reduces carbon monoxide to carbon to form Al ₂ O ₃ . By the reaction according to Equations 4-6, the contents of Mn, Si, and Ti decrease.

These reactions would not proceed if one were to use high-purity CaO as a sulfur-lowering agent, so that no CO ₂ could form, which, in conjunction with Al, leads to the already mentioned carbon deposit.

A completely different picture shows 2 , In this case, CaCO _{3 is} added to an unfused melt. The carbon content has been increased to nearly 0.09 wt% by adding pure carbon. By adding Fe ₂ O ₃ , the oxygen content was adjusted to a usual level. With the addition of CaCO ₃ , the contents of Mn, Si, Ti and in particular C decrease. Instead of the previously observed increase in the carbon content (see 1 ) it sinks with an unkilled steel.

The after completion of treatment required deoxidation of the melt is effected by adding Al.

For the example in 3 An unused molten steel was also used and CaCO _{3 was} added in a known manner. For this example, an addition of C was omitted, so that the starting content for C is already very low at about 0.005 wt .-%, which corresponds to a ULC grade approximately.

As one can see from the presentation sink, as one would expect the contents Mn, Si, Ti due to the reaction according to equations 4-6, during the Carbon content remains nearly constant.

The Addition of Al for deoxidation of the molten steel takes place at the end the treatment.

Claims (4)

A process for producing an Al-killed steel with low levels of strength-increasing elements, in which desulphurised pig iron in a converter to steel with a C content <0.01 wt .-% and then blow the steel melt when emptying into a pan to further lowering the Sulfur content, a sulfur-binding, Ca-containing agent, and added for deoxidation Al is characterized in that in the secondary metallurgical treatment, a subset of the total sulfur desulphurization required sulfur scavenging agent is added after decarburization for slag formation and that this agent is almost free of lime stone (CaCO ₃ ). Process for producing an Al-killed steel with low levels of strength increasing elements, in which Pre-desulfurized pig iron in a converter to steel with one C content <0.01% by weight blown and then the molten steel during casting in a pan to further reduce the sulfur content of a sulfur-binding, Ca containing agent is added characterized that for the secondary metallurgical Treatment emptied an unkilled molten steel into the pan will and during the treatment is a subset of the total for steel desulfurization required sulfur scavenger added and last metallurgical treatment step the unkilled molten steel is deoxidized with Al. Method according to claim 1 characterized, that the secondary metallurgical Treatment under inert gas takes place. Method according to claims 1 and 2 characterized, that the secondary metallurgical Treatment is carried out under vacuum.