JP2002224801A - Method for preventing development of uneven pickling on cast slab surface in stainless steel strip casting and cast slab and hot-rolled steel plate produced by the method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent uneven acid pickling caused by entrapment of non-metallic inclusion (scum) floated on molten steel surface during casting, in the production of a Cr-Ni base austenite stainless steel cast strip. SOLUTION: In a method for preventing the development of the uneven pickling in the stainless steel strip casting, is performed, by which in the method for casting the stainless steel strip, when last refining of the molten steel is performed just before casting, the basicity [(CaO)/(SiO2)] of slag existing on the molten steel is regulated to 1.0-1.8 and the total oxygen quantity contained in the molten steel is controlled to 20-45 ppm and further, the oxygen concentration in the atmosphere in the molten steel pool part during casting, is regulated to <=0.01% in order to prevent the surface of the cast slab from uneven acid pickling in the production of stainless steel cast strip.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to the production of a strip of a Cr-Ni austenitic stainless steel strip produced in a twin-drum continuous casting of a non-metallic inclusion (floating on the surface of molten steel during casting). The present invention relates to a casting method for preventing the occurrence of pickling unevenness caused by unevenness of the structure caused by non-uniformity of initial solidification due to entrainment of scum, and a slab or a hot-rolled steel sheet manufactured by the method.

[0002]

2. Description of the Related Art In twin-drum continuous strip casting, during casting, the surface of the molten metal in the pool of molten steel is oxidized by oxygen in the casting atmosphere and foreign substances usually called scum due to floating of inclusions in the molten metal. Floats. This scum is
As the cooling drum rotates, it is caught below the surface of the solidified shell along the outer peripheral surface of the cooling drum, and enters between the outer peripheral surface of the cooling drum and the solidified shell of the slab, resulting in flaws. In particular, scum that exists stably on the surface of the high-temperature molten metal has a very high melting point, so solidification starts on the outer surface of the cooling drum before the solidified shell of the slab is formed, and the scum adheres to the slab surface after casting. To remain. Further, the scum is generated quite randomly due to the above-mentioned generation factors, and due to its high melting point, it floats as uneven spots on the surface of the pool. As a result, the entrained scum also adheres and remains as uneven spots on the surface of the slab, causing uneven solidification.

[0003] Such uneven scum entrainment causes uneven coagulation.
As a method for preventing solidification, for example, Japanese Patent Application Laid-Open No.
As proposed in Publication No. 1, the constituent oxides of scum
A compound that lowers the melting point of_Two O_Three , K_Two 
O, Na_Two O, LiF, SiO _Two , CaCl_Two , CaF
_Two For adding at least one of the above to the molten metal
There is. However, even with the above proposals,
Cannot completely prevent non-uniform coagulation.
You. As a measure for preventing scum generation, Japanese Patent Application Laid-Open No.
Molten steel pool as proposed in Japanese Patent No. 30749
Molten steel pool so that the atmosphere in the
Inert gas is supplied to the air to replace it, and the residual oxygen concentration becomes lower than the limit.
There is a method of casting after reducing it below, but this residual oxygen
Smoke generation is still complete with only concentration reduction
There is a situation that cannot be prevented.

The reason why uneven solidification due to entrainment of scum cannot be prevented is that scum flowing from the pool of molten steel into the surface layer of the slab flows unevenly in the width and longitudinal directions of the slab. This is also because the inflow of scum cannot be prevented over the entire slab. From the observation of the slab under the cooling drum during casting by the present inventors and the examination of the slab structure flaws after casting, the above-mentioned uneven flow of the scum shows that the cooling rate at each part of the slab during the slab cooling It was clarified that the uneven cooling caused by the difference in the structure was caused by the difference in the structure during the solidification of the slab. That is,
The difference between the coarse structure and the fine structure in each part of the slab results in uneven pickling, and the uneven pickling appears as a quality defect such as uneven gloss in the product.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to prevent pickling unevenness due to uneven structure generated in stainless steel ribbon slabs produced in twin drum type continuous ribbon casting.
The first task is to set the physical properties of the scum, especially the melting point, to the optimum value so that the uneven inflow of the scum floating on the surface of the molten steel pool flows uniformly. A second object of the present invention is to provide a method for preventing the occurrence of pickling unevenness on the surface of a slab and obtaining a slab having a sound surface property without uneven gloss in a product. is there.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems. In a method of casting a thin strip of stainless steel, a base of slag existing on molten steel at the time of final refining of molten steel immediately before casting. The degree [(CaO) / (SiO ₂ )] is controlled to 1.0 or more and 1.8 or less, the total oxygen content in the molten steel is controlled to 20 to 45 ppm, and the atmosphere of the molten steel pool during casting is further controlled. Is a method for preventing the occurrence of uneven pickling on the surface of a slab in stainless steel strip casting in which the oxygen concentration of the slab is made 0.01% or less. 45 gloss at each part
° A slab or a hot-rolled steel sheet whose difference in reflection intensity in glossiness is less than 30

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have found that components of nonmetallic inclusions (scum) floating on the surface of a molten steel pool are SiO ₂ ,
It has been found that it is composed of MnO, Al ₂ O ₃ , CaO and the like. Therefore, the basicity of the ladle slag considered to be related to these compositions was investigated, and the relationship between this basicity and the pickling unevenness was investigated. As shown in FIG. 1, the basicity [(CaO)
/ (SiO ₂ )] was 1.0 or more and 1.8 or less. Then, it was found that there was a relationship between T [O] greatly affected by the basicity and the pickling unevenness, and T [O] in the molten steel was 20 to 45 p.
The best improvement at pm was evident from many field experiments.

In general, the relationship between basicity and T [O] is inversely proportional. When the basicity is low, T [O] tends to increase, and conversely, the basicity and the melting point of slag or scum are proportional. When the basicity is low, the melting point also tends to decrease, but when the basicity is very low, [Cr] and [O] contained in the molten steel react and a large amount of Cr ₂ O ₃ is generated, resulting in a high melting point. . The results of this study show that by adjusting the basicity and T [O], the scum lowers its melting point, lowering its viscosity and reducing T [O], thereby reducing the amount of scum generated from molten steel. This is due to the effect obtained. That is, when the melting point of the scum is high, or when the amount is large, the scum flows unevenly and thickly between the cooling drum and the slab, but when it is low,
Alternatively, when the amount is small, the scum flows uniformly and thinly between the cooling drum and the slab, and as a result, the cooling of the slab becomes uniform and the structure does not become dense and tight.

[0009] If there is a difference in the structure of the cast slab, the difference in the structure is promoted by abrasion of the surface layer by pickling, and the plate surface after pickling becomes uneven. Observation of the uneven portion in detail revealed that the portion into which the scum had flowed had a large particle size, and because of the large number of flat portions in the grain, it was found that the gloss appeared to be good by visual observation. Then, such a pickling unevenness sample was quantified in advance by glossiness and then cold rolled in a lab to evaluate unevenness after product processing. If the glossiness was less than 30 °, unevenness after pickling was also slight. It was clear that this had no effect on cold rolled products.

Next, the present inventors have repeated experiments on a method for reducing the amount of scum generated itself. As described above, since the scum composition contains a large amount of SiO ₂ and MnO, it is considered that a large amount of scum is generated due to oxidation of the molten steel. Was discharged and replaced with nitrogen (N ₂ ) gas, and the amount of oxygen (O ₂ %) in the atmosphere was measured. The relationship between this amount of oxygen and the pickling unevenness was further investigated. as a result,
As shown in FIG. 2, the amount of oxygen (O ₂ %) in the atmosphere was 0.1%.
It has been found that when the content is less than 01%, the pickling unevenness is greatly improved. As a means for preventing the occurrence of uneven pickling on the slab surface, the basicity [(CaO) / (SiO ₂ )] of the slag existing on the molten steel during the final refining of the molten steel immediately before casting is as follows. 0
Not less than 1.8 and the total oxygen content: T [O] contained in the molten steel is controlled to 20 to 45 ppm.
Oxygen concentration of molten steel pool atmosphere during casting is 0.01%
Casting as follows is extremely effective in improving pickling unevenness.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on embodiments. Cr-N
During the final refining of molten steel immediately before casting i-type austenitic stainless steel, the basicity of slag existing on the molten steel [(CaO) /
(SiO ₂ )] at each level of less than 1.8 and 1.8 or more
D refining was performed. As a result, as shown in Table 1, when the basicity was 2.0, 1.9, and 1.80, the T [O] in the molten steel was as low as 10 ppm, 14 ppm, and 18 ppm. Wash unevenness had deteriorated to an acceptable level. On the other hand, the basicity is 1.75 or 1.6 and 1.8 or less.
Assuming that 5, 1.5, and 1.4, T [O] in the molten steel is 25 ppm, 30 ppm, 35 p of 20 ppm or more.
pm, 40 ppm. As a result, the pickling unevenness was reduced and cast pieces having sound surface properties were obtained.

Further, when the basicity is less than 1.0, Cr ₂ O ₃ was observed in the scum, and as a result, scum flaws and accompanying uneven pickling were generated. Further, the difference in reflection intensity at 45 ° glossiness between the cast slab obtained by the present invention and the slab having a sound surface property, and the gloss at each part of the surface of the hot rolled steel sheet produced from the slab was 3 °.
An excellent product of less than 0 was obtained.

[0013]

[Table 1]

[0014]

As described above, according to the present invention, Cr
-In the production of Ni-based austenitic stainless steel strip slabs, it is possible to prevent the occurrence of pickling unevenness due to uneven structure caused by entrainment of nonmetallic inclusions (scum) floating on the surface of molten steel during casting. become.

[Brief description of the drawings]

FIG. 1 is a view showing the relationship between basicity of slag [(CaO) / (SiO ₂ )], T [O], and pickling unevenness.

FIG. 2 is a diagram showing a relationship between an oxygen amount (O ₂ %) in a molten steel pool atmosphere and pickling unevenness.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/00 302 C22C 38/00 302Z // C22B 9/10 101 C22B 9/10 101 (72) Inventor Ryuuji Nakao 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (72) Inventor Hideaki Kobayashi 3434, Hikari-shi, Hikari-shi, Yamaguchi Prefecture Nippon Steel Hikari Works (72) Inventor Katsumi Kirihara 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (72) Inventor Takashi Arai 3434 Shimada, Hikari-shi, Hikari City Yamaguchi Prefecture Nippon Steel Hikari Works (72) Invention Shunji Shoda 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works F-term (reference) 4E004 DA13 SB07 4K001 KA02 KA06 4K013 AA02 AA09 BA1 4 EA03 EA04 FA06

Claims (2)

[Claims] In the method of casting a thin strip of stainless steel, the basicity [(CaO) / (SiO ₂ )] of slag existing on the molten steel at the time of final refining of the molten steel immediately before casting is 1.0 or more and 1.8 or less. age,
And the total oxygen content in molten steel is 20-45p
pm and the oxygen concentration in the atmosphere of the molten steel pool during casting is reduced to 0.01% or less. 2. The pickled slab or hot-rolled steel sheet produced by the method according to claim 1 has a gloss of 45 ° at each site.
A cast slab or a hot-rolled steel sheet, wherein a difference in reflection intensity in glossiness is less than 30.