Treatment Process for Bars

Abstract

Treatment process for stainless steel bars, in particular a solution quenching, to be performed directly in-line downstream of the rolling mill which makes it possible to obtain a material devoid of intergranular corrosion and with microstructural characteristics suitable for subsequent uses. Advantageously, said process also makes it possible to improve the productivity of the entire rolling plant. The treatment is suitable to be performed on austenitic, ferritic, or austeno-ferritic stainless steel bars, Al—Cu alloy bars, Nickel alloy bars, and all other alloys requiring rapid cooling in order to prevent undesired phase precipitations. Prevention of intergranular corrosion obtained with the treatment process of the invention makes it possible to prevent problems, and relative costs, during surface treatment of the bars and those that could occur in final use.

Description

FIELD OF THE INVENTION[0001]The present invention relates to an in-line treatment process of rolled bars, in particular to a solution quenching treatment, at the outlet of the rolling train, of bars in stainless steel, nickel alloys or other alloys that require rapid cooling after rolling.STATE OF THE ART[0002]The structure of austenitic steels is practically composed only of austenite stable at ambient temperature; the chromium carbides, which may be held in supersaturated solid solution in the austenite, are capable of precipitating, at the austenite grain boundary, when the steel is heated in the interval ranging from 450 to 850° C. or cooled slowly through this interval. The considerable carbide-forming power of the Cr causes disproportionation of the alloy at the grain boundary, while in the centre of the grains the Cr content remains practically unchanged. In the case in which the content of free Cr, that is, not bonded to form carbides, drops locally below the limit of stainl...

AI Technical Summary

Benefits of technology

[0017]The present invention makes it possible to perform the solution quenching on bars with round, square, hexagonal, rectangular section, directly at the outlet of the rolling mill, and to obtain a high quality product that does not require straightening and is free of scoring and surface defects. The process is performed in a compact plant, without specific maintenance requirements, which allows to eliminate all costs related to intermediate storage of the material, as well as those related to reheatings in furnace, which are necessary for normal off-line treatments. This highly flexible system makes it possible to pass from producing one shape to another one without having to perform mechanical intervention with the related loss of time.

Problems solved by technology

In the case in which the content of free Cr, that is, not bonded to form carbides, drops locally below the limit of stainlessness (12%), the steel becomes susceptible to inter-granular attack.

Attack by intergranular corrosion can cause disbonding of a grain with respect to the others, with a considerable decrease in mechanical resistance and to impacts, notwithstanding a negligible loss in mass.

Higher temperatures and longer holding times can, in fact, cause more pronounced softening and, depending on the parameters adopted, also promote undesirable enlargement of the grains, which is needed for certain specific applications.

These tanks are usually small in size and allow the treatment of limited length bars, drastically reducing the yield of the plant and overall productivity.

If the treatment is performed outside the line, it has the disadvantage of requiring intermediate storage and re-heating from ambient temperature to a temperature of around 1100 to 1200° C., with considerable energy consumption as well as consumption of material and maintenance.

All this requires considerable overall dimensions of the plant, high maintenance and leads to low productivity.

All this requires considerable overall dimensions of the plant, high maintenance and leads to low productivity.

Furthermore, it is not always possible to guarantee that the temperatures at the beginning of the treatment are the optimal temperatures, both for obtaining a material without carbides, and for obtaining a homogeneous microstructure in terms of size of the austenitic grain.

Images