American Society for Metals


11-12 September 1974

Chatham Center, Pittsburgh




Motoya Fujii

Nippon Steel Corporation


Abstract @

Since the No.2 continuous casting machine was installed in 1968 for casting of blooms for austenitic stainless steel tubes, research and development efforts in the casting and extrusion techniques aiming at the improvement of the quality and product yield of austenitic stainless steel tubes have produced a number of substantial results. Thus, a method for stabilized manufacture of stainless steel tubes from continuously-cast blooms has been established. This report discusses workability, surface conditions of extruded tubes, inclusions and segregation, which are particularly important determinants of the quality of stainless steel tubes made from continuously-cast blooms.

1)      Workability
As hot workability is greatly affected by the amount delta-ferrite, workability comparable to that from steel ingot material can be obtained by soaking continuously-cast blooms prior to rolling so as to reduce the amount of delta-ferrite.  There is no difference whatsoever in cold workability between continuously-cast bloom and ingot-bloom

2)      Surface conditions of extruded tube
Surface longitudinal scores unique to austenitic steel tubes made by extrusion of as-cast structures can be eliminated by providing a draft percentage of bloom of more than 15%.  Also, inside-structure defects, attributable to center cavities and internal cracks, may be eliminated by determination of the optimum casting conditions and the application of induction stirring.

3)      Macro-inclusions
The major cause of macro-inclusions in continuously-cast blooms is air oxidation products occurring during pouring. These products have been eliminated by the establishing of the oxidation less casting practice, which involves the use of the immersed nozzle and powder casting. It has thus become possible to obtain better quality than in the case of ingot-blooms.

4)      Segregation
Perhaps the most serious flaw in continuously-cast blooms is the striped structure. This is derived from delta-ferrite and solidification segregation. It is possible to eliminate them, as with ingot-blooms, however, through performing the homogenizing heat-treatment or solution heat-treatment on steel tubes.

On the strength of the improved quality of blooms and by employing the production process suited to continuously-cast blooms, product quality of continuously-cast blooms has now become stabilized and fully comparable to that of the conventional ingot-blooms.


Lecture manuscript

Mr. Chairman, thank you for your kind introduction.

Ladies and gentlemen, my name is Motoya Fujii and Ifm engaged in the hot extrusion plant at Hikari works of Nippon Steel Corporation.

Today Ifm very honored to present a report titled gQualities of austenitic stainless steel tubes from continuously cast bloomsh.

Well, the austenitic stainless steel tubes are indispensable for the construction of chemical plants and power plants, and their demand has been steadily on the increase with the remarkable industrial progress of recent years.

In response to this demand, in 1968, Hikari Works installed a continuous bloom casting machine for the austenitic stainless steel tubes. Since then, many studies and technical development concerning casting and extrusion technology have been undertaken to improve the qualities and to increase the yields of austenitic stainless steel tubes.

Through these studies and technical developments, we have established a process for stable manufacturing of high-quality austenitic stainless steel tubes from continuously cast blooms.

And, I would like to explain the outline of my report with the aid of slides.


n       Slide 1 --
Now, this slide shows the manufacturing process of austenitic stainless steel tubes from continuously cast blooms. The clean molten steel refined by EF and AOD is continuously cast into blooms. These blooms are rolled and worked into billets, and then it goes through heating, extrusion, and cold-working process, and are finished into the final products.

n       Slide 2 --
At present the grades of steel which is produced from continuously cast blooms are four austenitic stainless steels, as shown in this table, that is, AISI304, 304L, 316, and 321, and most of austenitic stainless steel tubes are produced from continuous casting.
And now I would like to explain some important items which are concerned with those factors determining the qualities of stainless steel tubes, especially, the workability, the surface conditions, the macro-inclusions, and the segregation.

n       Slide 3--
First of all I will discuss the workability.
As shown in this figure, we find that the hot workability of AISI304 is improved markedly with the decrease of delta-ferrite content.

n       Slide 4--
By the way, this slide shows the delta-ferrite content of as-cast blooms produced from continuous casting which is higher than that of ingot blooms by a small percentage. But by soaking, the delta-ferrite content of continuously cast blooms is decreased to the same level as that of ordinary ingot blooms, so there is no difference in the hot workability between tow blooms.

n       Slide 5--
I will discuss the surface conditions of austenitic stainless steel tubes.
The principal problems of the surface conditions are surface longitudinal scores and inside surface defects.
With respect to surface longitudinal scores of tubes, we carried out various tests for the purpose of complete elimination of the surface longitudinal scores. And consequently, we have discovered that reduction in excess of 15 percent is needed to eliminate the longitudinal scores on the surface of austenitic stainless steel tubes from continuously cast blooms.
This figure shows the influence of bloom reduction on surface longitudinal scorefs depth. If the bloom reduction is more than 15 percent, the surface longitudinal scores are about 20 microns in depth and become insignificant.

n       Slide 6--
These are the photographs of the tube surface. This is at zero percent reduction, and this is at 15 percent reduction. We can not find any problematic scores.

n       Slide 7--
Now I will discuss the inside surface defects of tubes. Defects like this shown in the photograph appear on the inside surface of tubes. This is an example of inside surface defects, and this is a micrograph of transversal section of defects. These surface defects are caused mainly by center cavities and internal cracks of continuously cast blooms. However, we have succeeded in removing these inside surface defects by obtaining satisfactory sound blooms, which are obtained by a determination of optimum casting conditions and by a special casting practice.

n       Slide 8--
Next I will discuss the non-metallic macro-inclusions. This photograph is an example of surface defects on austenitic stainless steel tubes, which are caused by non-metallic inclusions.

n       Slide 9—
As a result of investigations on the surface defects and the macro-inclusions in continuously cast blooms, we have found, as shown in this slide, that such defects are caused by the air oxidation products developed in the molten metal during pouring, that is, Mn-Si, TiO2 and TiN.
At present, those problems are perfectly solved by the oxidation-free casting practice with an immersed long nozzle, by a method of refining up low N content, and by a special casting practice.

n       Slide 10--
The results of investigations are shown in this figure. This is in the case of open cast blooms, and this is in the case of oxidation-free powder cast blooms.

n       Slide 11--
Now I will discuss the segregation. There will be a problem in macro-segregation and micro-segregation.
This photograph shows the changes in macrostructure from continuously cast bloom to finished tubes of AISI304. When billets are rolled into 175 mm round with 45 percent reduction, the cast structure is not yet completely destroyed. Finished product tubes, however, have a completely homogeneous macro-structure, so there is no problems of macro-segregation. It is the same for AISI316 and AISI321.

n       Slide 12--
Now I will explain the micro-segregation. This photograph shows typical micro-segregation of tubes from continuously cast blooms, Which are called band structures.
Such band structures can also be seen in tubes from ingot blooms. In the same working and heat treatment process up to the billet stages, band structure of tubes from continuously cast bloom much more than those from ingot blooms.
@However, these band structures of tubes from continuously cast blooms can be reduced to the same level as ingot blooms, when thay are sufficiently heated for the purpose of decreasing delta-ferrite and solidification segregation.

n       Slide 13--
This photograph shows the micro-structure of tubes from continuously cast blooms, which are sufficiently heated, and this shows the micro-structure of tubes from ingot blooms. There is almost no difference between these two blooms. This can applied to both AISI316 and AISI321 also.

n       Slide 14--
This is the last slide and table-3 summarizes the mechanical properties of austenitic stainless steel tubes from continuously cast blooms.
This shows that the quality of tubes from continuously cast blooms is quite equal to that of ingot blooms.

Now Ifd like to summarize my report. We have discussed the hot extrusion process of austenitic stainless steel tubes from continuously cast blooms and the quality of the blooms obtained. In producing austenitic stainless steel tubes from continuously cast blooms, it is essential to use sound continuously cast blooms and to develop rolling and heat-treatment processes that are adapted to continuously cast blooms if the qualities of the required product, especially tube surface conditions, inclusions. And homogeneity, are to be secured.

The use of continuously cast blooms for the production of stainless steel tubes has just began, and the scope of application will be more and more increasingly expanded through the refining of clean stainless steel by the AOD process and through the rapid improvement of continuously cast blooms by the special casting practice. We intend to continue tests on the continuous casting.

I hope my report will help you in someway with your business. And if you would like to know the minute details, please refer to the report at your hand.

Thank you very much for your kind attention