GB2388847A

GB2388847A - A method of making ultra-low-carbon aluminium killed steel

Info

Publication number: GB2388847A
Application number: GB0309331A
Authority: GB
Inventors: Heinz-Peter Kaiser; Holger Friedrich Arnold
Original assignee: Mannesmannroehren Werke AG
Current assignee: Vodafone GmbH
Priority date: 2002-04-29
Filing date: 2003-04-24
Publication date: 2003-11-26
Anticipated expiration: 2023-04-24
Also published as: FR2838990A1; GB2388847B; FR2838990B1

Abstract

A method of producing an Al-killed steel by Bessemerisation of low sulphur pig iron into steel in a basic converter in a one-step LD process, tapping the steel in to a ladle, adding slag forming elements such as lime, gas treating at a quantity of 100-400 l/min and completely deoxidising with aluminium wire to give a steel comprising C: 0-0.05 % by weight, Si: 0-100 ppm, Mn: 0.05-0.7 % by weight, P: 0-0.02 % by weight, S: 0-150 ppm, Al: 0.005-0.05 % by weight, O 0-50 ppm, with the balance being iron and impurities. Alloying elements such as titanium and/or niobium may be added to the deoxidised steel. The deoxidised steel is then continuously cast.

Description

1 2388847

! METHOD FOR PRODUCING AN Al-KILLED STEEL The invention relates to a method for producing an Al-killed steel for continuous casting of slabs, bars or billets.

For certain requirements in the steel sphere, so-called ULC (ultra low carbon) products are increasingly being produced. These are suitable in particular for deep-drawing sheets. They are distinguished by the fact that the amounts of carbon and trace elements are as low as possible.

They are characterized by a low yield point coupled with high breaking elongation. A further feature relates to the oxygen content, i.e. these products are fully killed.

A method for continuous casting of Al-killed steel with an extremely low carbon content is known from EP 05 1 2 118 A 1. This steel is distinguished by a Ca content in the range of 6 - 20 ppm, an S content < 0.01% by weight and a low oxygen content of < 30 ppm. The temperature of superheating in the casting tank is intended to be higher than 16 C and the average flow rate of the molten steel in the straight part of the submerged spout is intended to be greater than 1.2 m/sec.

The specific use of Ca in order to avoid so-called "clogging" (clogging of the submerged spout) is disadvantageous in the case of this known steel.

This addition of Ca degrades the degree of purity of the steel.

An object of the invention is to provide a method for producing an Al-

killed steel which, without the addition of Ca, is suitable for casting as a continuous casting for a plurality of sequences and as a galvanisable deep-drawing sheet.

The invention provides a method for producing an Al-killed steel for continuous casting of slabs, bars or billets with the chemical composition:

2 l (in % by weight) C s 0.05 Si < 100 ppm | Mn 0.05 0.7 P max 0.02 S max 150 ppm Al 0.005 - 0.05 O < 50 ppm residue iron with impurities caused by melting, the method comprising: bessemerisation of low-sulphur pig iron into steel in a basic converter in a one-step LD process; unkilled lowslag tapping into a pouring ladle; addition of slag-forming constituents;; light circulation treatment with a circulation gas at a quantity of 100 400 1/mini complete deoxidation with Al in the form of wire; fine alloying; and I casting of the steel as a continuous casting.

The steel is distinguished by a very low Si content of c 100 ppm, preferably of < 50 ppm. The P and O contents are also set extremely low.

An addition of Ca is dispensed with. The low values of Si and P have a direct effect on the strength of thin sheets produced from the steel. A further advantage can be seen in the fact that the low Si content favourably influences the process of hot galvanizing. As fully galvanised bodies have increasingly become standard in the automobile field, the

demand for such sheets has increased accordingly.

During production of such a steel, low sulphur pig iron is bessemerised in a basic converter in a one-step LD process. There is effected thereafter an unkilled low-slag tapping into the pouring ladle. The remaining slag, which has particularly high contents of SiO2, iS inactivated by adding suitable slag-forming constituents. Lime is the preferred slag-forming constituent. Depending on the required C content, only a light circulation treatment with 100 - 400 1/min of gas is required during the complete deoxidation with A1 or a vacuum decarburisation must be performed. According to experience, this is required for a C content < 0.02% by weight.

The addition of the A1 is effected in the form of wire, preferably as continuously cast solid wire. Normally Me alloying follows the deoxidation. Ti and/or Nb is added in particular. This addition is effected preferably also in the form of wire.

Finally, the thus produced steel is cast in continuous casting.

Tests have shown that the inactivation of the residual slag of the LD process is a step in the desired direction of low content of strength-

increasing elements and also the avoidance of mixing of the slag with the steel bath due to too intensive bath movement.

A vigorous circulation movement can be dispensed with if A1 is supplied in the form of wire since the wire can be guided up to the pouring ladle base and a slight bath movement is adequate for the homogenization of the slag. If the bath movement is too vigorous, slag is introduced into the steel bath and the SiO2 iS reduced by A1 to Si. The Si increase leads in turn to an increase in strength which is undesirable. The preferred addition of Ti

and/or Nb in the form of wire also prevents entrainment of Si which when adding pieces is almost unavoidable.

The invention will now be further described with reference to an exemplary embodiment.

A slab of the dimension is produced as IF goods (IF = interstitial free) which is intended to be processed subsequently into a deep-drawn sheet.

The pig iron used is de-sulphurised in advance and has the following content (in % by weight): C Si Mn P S Al O 4.5 0.5 0.3 0.05 0.005 0.0001 not determined After bessemerising in the converter, a sample is withdrawn which has the following content: C Si Mn P S Al O 0.02!5 0.001 0.1 0.012 0.006 0.0001 0.1

The tapping from the converter into the ladle is effected with low slag.

After the tapping, lime is added. Thereafter a vacuum treatment in the sense of a further decarburisation is effected, the Vacuum Degassing method being used preferably.

During the vacuum treatment, argon circulation takes place in a quantity in the range of 100 - 400 l/mint After the vacuum treatment, the ladle sample has the following analysis: Ron Tore I 'AT TllB I ITI1lrT TO T] 11 1 -

( C Si Mn P S Al O 0.002 0.001 0.1 0.012 0.006 0.0001 0.05

The sharp drop in C content is characteristic for the VD treatment, likewise the decrease in oxygen content by approximately 50%.

For killing, Al in the form of wire is now added, again with argon circulation with a circulation gas quantity in the range of 100 - 400 l/mint At no point in time does this result in a breaking-up of the slag layer which would lead to an increase again in the Si content.

The third ladle sample now has the following content: C Si Mn P S Al O 0. 002 0.001 0.1 0.012 0.006 0.05 0.0035

Due to the killing, the oxygen content has dropped even further.

In a final step, titanium and Nb in the form of ferrotitanium and ferroniobium are added. This addition is also effected with moderate argon circulation.

The final sample is characterized by the following content: C Si Mn P S Al O 0.002 0.002 0.1 0.012 0.006 0.05 0.001

This steel produced in this manner is characterized by low contents of strength-increasing elements, such as Si, P and 0.

The desired value for P of maximum 0.01 was not in fact quite achieved but instead lower values that those sought for C with s 0.005 and Si < 50 ppm were clearly achieved. This also applies for the O content of < 20 ppm. The slight increase in the Si content of 0.001 to 0.002 can be explained by having dispensed with an addition of Ti and Nb in the form of wire. In this example, the addition was effected in the normal lump form Nevertheless, the desired low value for Si of < 50 ppm was able to be maintained reliably.

Claims

Method for producing an Al-killed steel for continuous casting of slabs, bars or billets with the chemical composition: (in % by weight) C s 0.05 Si s 100 ppm Mn 0.05 - 07 P max 0.02 S max 150 ppm Al 0.005 - 0.05 O s 50 ppm residue iron with impurities caused by melting, the method comprising: bessemerisation of low-sulphur pig iron into steel in a basic converter in a one-step LD process; unkilled low-slag tapping into a pouring ladle; addition of slag-forming constituents; light circulation treatment with a circulation gas at a quantity of 100 - 400 l/mini complete deoxidation with Al in the form of wire; fine alloying; and casting of the steel as a continuous casting.
2. Method according to claim 1, wherein the steel comprises (in % by weight) C s 0.0 1 Si s 60 ppm Mn 0.10 - 0.15 P max. 0.015 O max < 30 ppm.
3. Method according to claim 1, wherein the steel comprises (in % by weight) C c 0 005 Si < 50 ppm P max. 0.01 O < 20 ppm.
4. Method according to any preceding claim, wherein a vacuum decarburisation with light circulation treatment with a circulation gas quantity of 100 - 400 1/ min is carried out for a desired C content c 0. 02 % by weight before the Al deoxidation.
5. Method according to any preceding claim, wherein the slag-forming constituent primarily comprises lime.
6. Method according to any preceding claim, wherein a continuously cast Al solid wire is used.
7. Method according to any preceding claim, wherein Ti and/or Nb is added in the form of wire during fine alloying.
8. Method for producing an Al-killed steel for continuous casting of slabs, bars or billets substantially as hereinbefore described.