PROCESS FOR PRODUCING A STEEL STRIP

Abstract

A process for producing a strip-cast, low-carbon, partly Mn/Si killed steel strip which is largely free of cracks and surface defects from a steel melt having a sulfur content of between 20 and 300 ppm and an Mn/Si ratio ≧3.5, using a roll separating force of between 2 and 50 kN/m.

Description

The invention relates to a process for the continuous production of a steel strip using at least two casting rolls and, if appropriate, laterally arranged side plates, wherein a casting reservoir, from which liquid steel melt can be introduced to the casting rolls, can be formed between the casting rolls and the side plates during operation.

BACKGROUND OF THE INVENTION

During the production of a steel strip from a low-carbon, partly Mn/Si killed steel melt, the steel strip produced, when the two-roll casting process known from the prior art is used, has many cracks and surface defects which significantly reduce the quality of the steel strip produced.

It is known from WO03024644 and US2005145304 to prevent or at least reduce the number of cracks and surface defects by selecting the composition of a steel melt in such a way that liquid non-metallic inclusions are produced in the steel melt, and these remain liquid during the solidification of the steel shell and permit a homogeneous heat flow and therefore a homogeneous cooling effect to be achieved by forming a liquid film on the surface of the casting rolls.

• During melting operation on an industrial scale, the MnO/SiO₂ ratios actually present in a partly Mn/Si killed steel melt are often substantially lower than those calculated theoretically due to operational reasons. The melting temperature of the non-metallic inclusions in partly Mn/Si killed steel melts is very sensitive to changes in the steel composition and to associated changes in the MnO/SiO₂ ratio in the composition of said inclusions. When observing the metallurgical rules specified in the prior art for producing the liquid, non-metallic inclusions, it is therefore not possible to assume, during melting operation on an industrial scale, that each treated ladle has a composition which ensures that liquid, non-metallic inclusions are present during the casting process. Cracks and surface defects can therefore reappear.

OBJECT OF THE INVENTION

The object of the present invention is to avoid these known drawbacks of the prior art, and to provide a process for producing a steel strip, which is largely free of cracks and surface defects and has a homogeneous surface, from a low-carbon, partly Mn/Si killed steel melt. In this process, the tolerance of the melting temperature of non-metallic inclusions to deviations from a desired value of the steel composition should be sufficient to ensure that liquid, non-metallic inclusions are present in each treated ladle during the casting process during melting operation on an industrial scale.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the object of the invention is achieved by means of a process in which a steel melt with a particular Mn/Si content ratio and with a particular sulfur content is processed, during normal operation, using a particular roll separating force (RSF).

The invention therefore relates to a process for producing a strip-cast, low-carbon, partly Mn/Si killed steel strip, wherein a steel melt is introduced from a melt reservoir between at least two casting rolls, that are cooled and move together with a steel strip, and at least partly solidifies on the casting rolls to form the steel strip, characterized in that the steel melt has a sulfur content of between 20 and 300 ppm and an Mn/Si ratio ≧3.5 and, during normal operation, the roll separating force is between 2 and 50 kN/m.

A steel strip produced in this way is unexpectedly largely free of cracks and surface defects and has a homogeneous surface.

A low-carbon steel strip is to be understood as meaning a steel strip with a carbon content of less than 0.1% by weight.

The composition of the steel melt according to the invention ensures that the non-metallic inclusions have a low melting temperature. The low melting temperature has the effect that the non-metallic inclusions are present in a liquid state during the solidification of the steel shell on the casting rolls during the casting process. The tolerance of the melting temperature of non-metallic inclusions to deviations from a desired value of the steel composition is increased by the broadening of the composition range in which liquid, non-metallic inclusions are present in the multiphase system. This broadened composition range ensures that the steel melt has a composition which guarantees liquid, non-metallic inclusions during the casting process even when the desired value for a particular steel composition is not exactly met during melting operation on an industrial scale.

During the preparation of steel, oxidic or sulfidic non-metallic inclusions are produced in a steel melt. The main components of the non-metallic inclusions in partly Mn/Si killed steel melts are MnO and SiO₂.

The setting of the sulfur content to values of between 20 and 300 ppm and of the Mn/Si ratio to values ≧3.5, in accordance with the invention, has the effect that the non-metallic inclusions are principally composed of a multiphase system having the main components MnO—SiO₂—MnS. If the MnS content of this multiphase system is less than 37% by weight MnS, the melting temperature of the multiphase system is less than the melting temperature of a multiphase system composed of the main components MnO and SiO₂. The 3-phase system MnO—SiO₂—MnS has a ternary eutectic at approximately 1130° C.

The modeling of the 3-phase system MnO—SiO₂—MnS in FIG. 1 shows that the liquidus range meets the binary boundary system MnO—SiO₂ at the eutectic temperature of 1251° C. of the latter at the eutectic point, and expands during the transition to a 3-phase system with an increasing MnS content. At lower temperatures, the liquidus range moved away from the boundary system and can still be found only above certain minimum MnS contents.

Typical operating points, which simultaneously have a low melting temperature of the non-metallic inclusions and a tolerance of the melting temperature to fluctuations in the MnS content which is sufficient during melting operation on an industrial scale, are approximately 15% by weight MnS in the case of the composition of the steel melt according to the invention.

The simulation of the solidification conditions in a thin-strip casting installation using immersion tests at inert gas, contact time and overheating levels corresponding to strip casting and with sulfur contents of the steel melt of between 150 and 500 ppm resulted in mean MnS contents of the liquid, non-metallic inclusions of between 7 and 40% by weight. Increased sulfur contents of partly Mn/Si killed steel melts lead to increased MnS contents of the non-metallic inclusions.

FIG. 2 shows the influence of the sulfur content of a low-carbon, partly Mn/Si killed steel melt (0.05% by weight C; 0.7% by weight Mn; 0.2% by weight Si) with an Mn/Si ratio ≧3.5 on the tendency to cracking, expressed by the frequency of cracking or by the width of the melting interval of the steel melt, in relation to the composition of non-metallic inclusions and in relation to the melting temperatures (liquidus temperatures) of the non-metallic inclusions. The measured data in FIG. 2 were obtained from the above-mentioned immersion tests.

Below a sulfur content of the melt which leads to an MnS content of the non-metallic inclusions which corresponds to the ternary eutectic at approximately 1130° C., the melting temperature of the non-metallic inclusions decreases with an increasing sulfur content.

• Above a sulfur content of the melt which leads to an MnS content of the non-metallic inclusions which corresponds to the ternary eutectic at approximately 1130° C., the melting temperatures of the non-metallic inclusions and also the frequency of cracking increase rapidly.
• The width of the melting interval increases up to a sulfur content of approximately 300 ppm, and then remains approximately constant.

FIG. 2 shows the relative behavior between increasing tendency to hot cracking and decreasing melting temperature of the non-metallic inclusions. The sulfur content which is recommended according to the invention and at which sufficiently low melting temperatures of the non-metallic inclusions and simultaneously a tolerable tendency to hot cracking are achieved can therefore be derived from FIG. 2. The presence of sulfur in a steel alloy expands the solid/liquid 2-phase area, i.e. the melting interval, of the steel alloy while simultaneously reducing the solidus temperature thereof, and this expands the temperature range in which hot cracks are produced between the liquid impenetration temperature (LIT) and the zero ductility temperature (ZDT).

The width of the 2-phase area increases approximately linearly up to approximately 45° C. at a sulfur content of up to 300 ppm in the steel melt. Above this sulfur content, the width of the 2-phase area remains approximately constant because MnS precipitates during the solidification with an increasing sulfur content. These MnS precipitates are deposited in solid form on the surfaces of the casting rolls and thereby prevent a homogeneous heat flow or a homogeneous cooling effect, and this encourages the formation of surface defects and cracks. An increasing sulfur content of the steel melt leads to increasing quantities of MnS precipitates and therefore to an increase in the number of surface defects and cracks.

• According to the invention, the maximum sulfur content is therefore limited to 300 ppm.

At a sulfur content of the steel melt of less than 20 ppm, the lowering of the melting temperature of the liquid, non-metallic inclusions compared to multiphase systems composed of the main components MnO and SiO₂ is not large enough to ensure that liquid, non-metallic inclusions are present during the solidification of the steel shell on the casting rolls during the casting process.

In addition, at a sulfur content of less than 20 ppm, the width of the composition range in which liquid, non-metallic inclusions are present in the multiphase system is not large enough to ensure that there is a sufficient tolerance to deviations from a desired value of the steel composition during melting operation on an industrial scale.

The sulfur content is preferably at least 50 ppm, particularly preferably at least 70 ppm. The upper limit of the sulfur content is preferably 250 ppm, particularly preferably 200 ppm. The sulfur content of the steel melt can be adjusted to the desired level by desulfurization or by the controlled addition of sulfur or of sulfur compounds.

At an Mn/Si ratio of less than 3.5 in the steel melt, no multiphase system which is composed of the main components MnO—SiO₂—MnS and has a sufficient reduction of the melting temperature of the liquid, non-metallic inclusions to values below the melting temperature of the steel mixture compared to a multiphase system composed of the main components MnO and SiO₂ is formed. According to the invention, the Mn/Si ratio therefore needs to be greater than or equal to 3.5.

The roll separating force is the force with which the casting rolls are pressed against one another during the casting process, based on the width of the steel strip. The roll separating force influences the presence of cracks and surface defects in a strip-cast steel strip.

The greater the roll separating force, the more temperature inhomogeneities that occur at the kissing point of the steel shells. Temperature inhomogeneities of this type result in non-uniform cooling of the steel strip, and this can result in surface cracks. In addition, large roll separating forces mean that stresses are built up in the strip-cast steel strip, and these stresses can also result in cracks and impaired mechanical properties.

The use of a small roll separating force avoids these problems and additionally affords the advantage that the casting apparatus is subjected to less mechanical stress. However, the selection of a small roll separating force may adversely affect the stability of the casting process since, in the case of a small roll separating force, there is the risk that the metal shells solidified on the casting rolls are insufficiently pressed together owing to inhomogeneities during the solidification and the steel strip cracks under its own weight, that the steel shells remain adhered to parts or over the whole width of the casting roll, and that cracks occur in the steel shell.

• In processes according to the prior art, the magnitude of the roll separating force, during normal operation, is between 5 and 250 kN/m.

According to the invention, the roll separating force is less than 50 kN/m. Since the composition of the steel melt according to the invention minimizes the occurrence of inhomogeneities during the solidification of the steel shells owing to the fact that it ensures the occurrence of liquid, non-metallic inclusions, such a small roll separating force can be used without risking the stability of the casting process.

The frequency of cracking increases with an increasing roll separating force. When roll separating forces of more than 50 kN/m are used, it is not possible to ensure the production of a homogeneous surface of the steel strip which is largely free of cracks and surface defects.

According to the invention, the lower limit for the roll separating force is 2 kN/m. Sufficient stability of the casting process is not ensured below this value.

• The roll separating force is preferably at least 5 kN/m. The upper limit thereof is preferably 30 kN/m.
• The stated values for the roll separating force refer to the steady-state normal operation of a casting installation, but not to the conditions when the installation is starting up or when extraordinary load effects temporarily occur.

According to a further preferred embodiment of the process according to the invention, the non-metallic inclusions in the steel melt have a mass fraction of Al₂O₃ of less than 45% by weight. The resultant multiphase system with the main components MnO—SiO₂—MnS—Al₂O₃ has a melting temperature which is less than the melting temperature of a multiphase system composed of the main components MnO and SiO₂. In addition, the composition range in which liquid, non-metallic inclusions are present is broader in the multiphase system with the main components MnO—SiO₂—MnS—Al₂O₃ than in the multiphase system composed of the main components MnO and SiO₂. The Al₂O₃ content is set by selecting the starting materials for producing the steel melt and, if appropriate, by the targeted addition of Al or Al compounds.

Claims

1. A process for producing a strip-cast, low-carbon, partly Mn/Si killed steel strip, comprising

introducing a steel melt between at least two casting rolls that are cooled and that move together with a steel strip, such that the steel strip at least partly solidifies on the casting rolls to form the steel strip, and

selecting the steel melt with a sulfur content of between 20 and 300 ppm and an Mn/Si ratio ≧3.5 and, during normal operation, applying a roll separating force at the casting rolls of between 2 and 50 kN/m.

2. The process as claimed in claim 1, wherein the steel melt has a sulfur content of between 50 and 250 ppm.

3. The process as claimed in claim 1, wherein the roll separating force is between 5 and 30 kN/m.

4. The process as claimed in claim 1, wherein the steel melt contains non-metallic inclusions with a mass fraction Al2O3 of less than 45% by weight.

5. The process as claimed in claim 1, wherein the steel melt has a sulfur content of between 70 and 200 ppm.