JPS61157627A - Manufacture of austenitic stainless steel - Google Patents

Abstract

PURPOSE:To manufacture an austenitic stainless steel with little spills by restricting the concn. of Si in a steel and subjecting a slab of the steel to shot blasting at a proper projection density before it is put in a heating furnace. CONSTITUTION:When an austenitic stainless steel is manufactured, the concn. of Si in a steel is regulated to 0.40-0.50% to improve the strippability of scale, and a slab of the steel is subjected to shot blasting at 350-500kg/m<2> projection density before it is put in a heating furnace prior to hot rolling. By shot blasting, the surface of the slab is smoothened, stress is left in the surface layer, and the structure is made fine to increase the strength. Thus, the occurrence of spills is prevented during hot rolling and cold rolling, and the yield of steel strips is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a method for producing austenitic stainless steel.

``Conventional technology'' Many measures have been proposed to improve the quality of austenitic stainless steel, but these include adjusting the amount of δ ferrite to an optimal value by controlling the composition and preventing cracking during hot rolling. There are two main types of technology: technology that reduces oscillation marks by reducing the number of motions in the casting machine, and technology that promotes the flotation and separation of inclusions through high-temperature casting. Ru.

"Problems to be Solved by the Invention" The present inventor has conducted various experiments based on the above-mentioned prior art.
These conventional techniques alone cannot prevent sludge-like defects in hot-rolled steel strips and cold-rolled steel strips, which are thought to be caused by poor descaling of scale generated in a heating furnace during hot rolling or insufficient surface strength of slabs. It was found that it is difficult to prevent such occurrence.

"Means for Solving the Problems" In order to solve the problems of the conventional AFL, the present invention aims to reduce the Si concentration in the steel to a range of 0.40 to 0.50% when manufacturing austenitic stainless steel. 350 to 500 ky/W before charging the slab into the heating furnace. The gist is to carry out shot blasting at a projection density of . .

In the process of investigating the concentration optimization of deoxidizing light -fis1, which is the most important among the steelmaking conditions for austenitic stainless steel, the present inventor discovered that '1 Si≦0.4.0% and sl〉o, 5o Under both conditions of %,
It was confirmed that the incidence of henge-like defects was on the rise.

As a result, the target range of Si concentration in steel is O,ln-05
It has been found that 0% is optimal.

In addition, the treatment of the surface of slabs manufactured in continuous casting is
Shot blasting from 350 to 500 #/n? with or without surface care? real &11 [i
L: Smooth the surface and at the same time leave stress in the surface layer.

After maintaining the inside and surface of the slab under certain conditions in this way, the heating conditions in the heating furnace are set to 1200 to 125
The heating time is controlled at 0° C. and the heating time is 210 to 270 minutes, and then hot rolling is performed.

For information on how to deoxidize austenitic stainless steel,
There have been many reported examples in the past, and when puff polishability on the surface of a steel strip is required, there are many cases in which AJ3z 03-based micro-inclusions are disliked, and of course U cannot be used as a deoxidizing agent.

From this point of view, it is important to efficiently deoxidize Si.

Now, when deoxidizing is performed using only Si without using farts, the target values are firstly to reduce the oxygen concentration in the steel to an appropriate value of about 80 PPM or less, and secondly to reduce the form of inclusions. Instead of using 5102 or 03 alone, which has a high melting point, MnO-5
It can be said that in many cases, the decision was made taking into consideration two points: the formation of low melting point inclusions such as i02, MnO-5i02-OA, and O.

Of course, these two points are important, but in addition to this, the present inventor has discovered that there is a deep relationship between the scale formation conditions in the hot rolling heating furnace and the Si concentration.
This was confirmed by observing the scale formation state through experiments.

180r-, a typical austenitic stainless steel
In 8Ni steel, due to its heat resistance, only a small scale of about 300 microns at most is generated in a hot rolling heating furnace, but the peelability of the scale changes greatly depending on the change in the Si concentration in the steel.

FIG. 1 shows the relationship between the scale removability index and the Si concentration in steel.

Scale with poor peelability is incompletely peeled off during scale breaking after rough rolling, and the remaining scale becomes the starting point and causes a difference in the plastic flow of the surface layer of the steel during the finish rolling process, resulting in sagging. It is easy to leave scale that causes defects and cracks.

Shot blasting in adjusting the slab surface condition is naturally effective in smoothing the slab surface, but it also works to refine the structure directly beneath the slab surface and improve its strength. have.

FIG. 2 shows the hardness distribution from the surface of the slab immediately below it after shot blasting. By performing shot blasting, the surface layer is reliably hardened and the flaking that causes cracks during hot rolling is reduced.

This is considered to be because the processing strain imparted to the surface layer of the slab facilitated recrystallization after heating and improved hot workability.

Also slab (C shot blast 17″1 winter)
Therefore, it is desirable that the projection density is 350 kg/i or more, in order to form a hardened surface layer extending 0.5 mm below the surface of the slab.

Even when the shot blasting density is increased to 500 kp/i or higher, there is almost no change in both surface roughness and surface hardness.
Therefore, increasing the projection density to 500, %/i or more,
It is meaningless in terms of quality, energy saving, and cost saving.

"Example" SU, the most common austenitic stainless steel
The quality of cold-rolled steel strip was investigated by changing the Si concentration and slab shot blasting conditions for S304.

When conducting experiments, 1. In order to keep the casting conditions constant, the typical temperature ΔT for continuous casting is in the range of 45 to 55°C.
And the oscillation conditions are stroke 3mm, S
= X100 (where S is c negative strip rate, vc is casting speed, and VM is average descending speed of the mold) Casting was performed for 200 cycles with a negative strip rate of 40%.

In addition, in the experiment, the slab care (loss) was reduced to -1.0 to 1.
．． It was carried out within a range of 5%.

Furthermore, the heating conditions for slabs that have the greatest effect on quality are: heating temperature in the range of 1200 to 1250°C, heating time in the range of 210 to 270 minutes, and oxygen concentration in the heating furnace of 10 to 1.
．． It was controlled within a range of 5%.

Furthermore, the thickness of the cast slab is 200mm%
The thickness of the hot-rolled sheet is 4 mm. The thickness of the cold-rolled steel strip is 0.8 mm.
mm, and the quality was determined by observing the degree of occurrence of jagged hege defects in this cold-rolled steel strip coil.

The experimental results are shown in Table 1 below.

Table 1 n, 45 n, 99
n, 005 50 1.
5. As is clear from Table 1 above, it was difficult to obtain Palanque quality in either case of controlling only the S1 concentration or performing only shot blasting.

In particular, the influence of SI1 degree is remarkable; at 0.5% or more, it becomes difficult to obtain palanque quality, and at 07%,
Unless the heating conditions are changed or a certain l/AG cold-rolled steel strip coil is subjected to coil grinder treatment, all of the coils will be ranked C.

In addition, regarding shot blasting treatment, 350ky/W
? It was found that if the above steps were carried out, the same quality could be obtained in all cases.

"Effects of the Invention" As stated above, by implementing the method of the present invention when manufacturing austenitic stainless steel, it is possible to stably achieve a pass rate of 95% or more for cold-rolled steel strip coils. , it is possible to reduce the coil grinder implementation rate in the cold rolling process, and it is also possible to contribute to improving the yield.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the scale removability index and the Si concentration in steel, and FIG. 2 is a graph showing the hardness distribution immediately below the slab surface after shot blasting at various projection densities. Chain S Ha Toyohiko <7. )

Claims (1)

[Claims] When manufacturing austenitic stainless steel, S in the steel is
Austenite characterized by adjusting the i concentration to a range of 0.40 to 0.50% and subjecting the slab to shot blasting at a blast density of 350 to 500 kg/m^2 before charging it to a heating furnace. A method for manufacturing stainless steel.