JP3098109B2 - Method for producing thin Cr-Ni stainless steel sheet with excellent elongation properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Cr-Ni stainless steel sheet by a twin-roll continuous casting method. In the twin-roll continuous casting method, molten metal is poured into a pool formed by a pair of cooling rolls arranged in parallel and side weirs sealing both end surfaces thereof, and solidified shells are formed on the circumferential surfaces of both cooling rolls. This is a continuous casting method in which solidified shells are united near the nearest position (so-called “kissing point”) of both rotating cooling rolls and are sent out as an integrated thin strip slab.

[0002] A strip-shaped slab cast by a twin-roll continuous casting method has a thickness of several mm (usually about 1 to 6 mm), and is subjected to cold rolling without hot rolling to produce a thin sheet product. can do. Therefore, a manufacturing method of casting a slab as a slab for hot rolling of several 100 mm square by continuous casting using a vibration mold or the like, hot-rolling the slab, and then cold-rolling (slab slab / hot rolling process) ), The production efficiency and cost are significantly more advantageous.

[0003]

2. Description of the Related Art Cold-rolled Cr-Ni stainless steel sheets are subjected to various cold forming processes and have high added value which are widely used as corrosion-resistant structural materials and exterior materials for industrial and household use. It is a product, and it is expected that high economic effects can be obtained by applying the twin-roll continuous casting method to its production.

[0004] The casting of a ribbon-shaped slab by twin-roll type continuous casting results in remarkably rapid solidification as compared with the continuous casting of a slab slab having a large cross section, so that the solidification structure is fine and many inclusions are also finely precipitated. . In order to secure an elongation value that directly affects the cold formability of a thin plate, it is necessary that the crystal grains are sufficiently coarse. However, particularly when MnS-based inclusions and supersaturated S are present in a large number, the growth of crystal grains during recrystallization when the strip-shaped slab is cold-rolled and annealed is suppressed,
There has been a problem that the elongation of the cold-rolled product has a large decrease and variation is large.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for stably producing a Cr-Ni stainless steel sheet having excellent elongation properties by a twin-roll continuous casting method.

[0006]

SUMMARY OF THE INVENTION According to the present invention, there is provided a method for producing a steel, comprising:
003% or less, and the content of Al ₂ O _{3 in the} molten steel is 3
CaO-A mainly composed of MnO-SiO ₂ 0% or less
C containing _{l 2 O 3 -MnO-SiO 2} -MgO inclusions
Production of a Cr-Ni stainless steel sheet having excellent elongation characteristics, characterized in that molten steel of an r-Ni stainless steel is cast into a thin strip slab by a twin roll continuous casting method, and the slab is cold rolled. Achieved by the method.

The method of the present invention is defined by the following equation 1.
Cr-Ni having a composition in which Md30 is in the range of 0 to 50 ° C
It is particularly advantageous when applied to stainless steel. Md30 = 413-462 (C + N) -9.2Si-8.1Mn-13.7Cr-18.5Mo-9.1 (Ni + Cu) ... Equation 1

[0008]

When a thin strip of Cr-Ni stainless steel is cast by a twin-roll continuous casting method, the cooling rate is 2000 ° C /
Since the speed is as fast as about sec, fine MnS precipitates over the entire slab or S exists in supersaturation. It was found that this hindered the growth of recrystallized γ grains during cold rolling and annealing. Therefore, it is effective to preferentially coarsely precipitate MnS in the slab to reduce the number of fine MnS that hinders the growth of recrystallized grains.

On the other hand, it was found that MnS in the cast slab was precipitated with Mn silicate as a nucleus. Therefore, it is effective to ensure the presence of Mn silicate in the slab in order to facilitate the coarse precipitation of MnS. Furthermore, it was found that the composition of the inclusions could be optimized by adjusting the basicity during refining. Here, the basicity is determined based on the weight percentage of CaO in slag and SiO ₂
% By weight (% CaO /% SiO ₂ ). At that time, it was found that it was necessary to set the molten steel S concentration to 0.003% or less and the molten steel Al concentration to 0.003% or less. When the S concentration is more than 0.003%, coarse MnS does not sufficiently precipitate during casting, and fine MnS increases, and elongation decreases. When the Al concentration is more than 0.003%, the composition of the inclusions are Al ₂ O ₃ is more than 30% CaO-
Al ₂ O ₃ —MnO—SiO ₂ —MgO system or M
becomes gO-Al ₂ O ₃ system, it is insufficient as a precipitation nucleus of MnS.

Conventionally, twin roll type continuous casting of Cr-Ni
-Based stainless steel is produced by electric furnace melting-AOD refining (E
F-AOD method) or melting by vacuum induction furnace (VIM
Law). In the case of the EF-AOD method, the basicity at the time of refining is as low as about 1.6 to 2, and the inclusion is Al.
₂ O ₃ but concentration is _{CaO-Al 2 O 3 -MnO-} SiO 2 -MgO system composition as a main component MnO-SiO ₂ 30% or less, the molten steel S concentration is relatively high and 50～80Ppm. In this case, a large number of Mn silicates serving as precipitation nuclei of MnS are present, but since the S concentration is high, a large number of MnS are precipitated in the end, and the elongation of the cold rolled product is reduced.

On the other hand, in the case of the VIM method, high basicity refining of about 3 to 4 is possible, and the S concentration can be reduced to 10 to 30 ppm, but the inclusions have an Al ₂ O ₃ —MgO system composition. In this case, although the S concentration is low, since there is no Mn silicate that can be a nucleus for MnS precipitation, the fine Mn
Since S is present in S and supersaturation, the growth of crystal grains during recrystallization due to the difference between cold rolling and annealing is suppressed, and the elongation of the cold rolled product is reduced.

In order to realize the S and Al concentrations in molten steel and the composition of inclusions specified in the present invention, the above-mentioned conventional smelting method is not sufficient. For example, in the case of the VIM method, a desulfurization treatment is first performed at a high basicity, and then the waste is discharged. The basicity is adjusted to a lower value of 2.5 or less rather than the conventional high basicity of 3 to 4. Thus, the inclusion composition of the present invention can be obtained. In the case of the EF-AOD method, EF-PIM-AOD in which powder injection refining (PIM) is performed after EF and before AOD, or S is set to 0.00 by using double slag in AOD.
It is necessary to reduce it to 3% or less.

Hereinafter, the present invention will be described in more detail by way of examples.

[0014]

EXAMPLE A molten steel of Cr-Ni stainless steel having the composition shown in Table 1 was cast into a 3 mm-thick strip-shaped slab by a twin roll continuous casting method, and this was cold rolled and bright annealed to a thickness of 0 mm. A 0.6 mm sheet product was produced. Measurement of elongation of each product plate by tensile test and CMA (Computer-aided)
Micro Analyzer: element mapping device) was used to measure the MnS precipitation distribution.

Table 2 shows the formula 1 based on the smelting method and composition after EF.
Shows the Md30 value, the slag basicity, the composition of inclusions, and the propriety as a MnS nucleus (〇, × unsuitable), and the elongation (%) of the product thin plate.

[0016]

[Table 1]

[0017]

[Table 2]

In Tables 1 and 2, A to H are examples according to the present invention, and I to M are comparative examples. Comparative material I is S
Amount / inclusion composition, comparison material J is inclusion composition, comparison material K is S
The amount of the comparative material L, the amount of S and the amount of Al, and the composition of the inclusions in the comparative material L, and the amount of S and the amount of Al in the comparative material M did not satisfy the scope of the present invention. FIG.
The following shows the relationship between the Md30 value and elongation. At the same Md30 value, it can be seen that the elongation of the thin plate according to the present invention is improved by 3 to 4% as compared with the comparative material according to the conventional method.

FIGS. 2A and 2B show the MMA precipitation distribution by CMA for the H material (elongation: 52%) of the present invention and the J material (elongation: 48.2%) of the comparative example. CMA can detect MnS having a size of a circle of 0.5 μm or more. The H material of the present invention has a composition (Md30 value) substantially the same as the J material of the comparative example, but has a large amount of coarse MnS and a small amount of fine MnS that hinders grain growth when viewed from the opposite side. This is because the inclusion composition of the H material is Mn silicate-based, while the inclusion composition of the J material is Al ₂ O ₃ -based.

As shown in FIG. 3, it can be seen that MnS in steel is precipitated with Mn silicate as a nucleus. The composition of each part A, B, and C is based on thin film analysis using a scanning electron microscope. FIG. 4 shows the effect of slag basicity (CaO
/ SiO ₂ ratio). Using this relationship, the inclusion composition can be controlled by adjusting the basicity. When the basicity is in the range of 0.5 to 2.5, a suitable inclusion composition can be obtained as MnS precipitation nuclei. If the basicity is less than 0.5, the oxygen concentration in the molten steel increases, the cleanliness of the slab deteriorates, and the plating property and rust resistance deteriorate due to inclusions. If the basicity is greater than 2.5, inclusions will be Al
_The composition becomes ₂ O ₃ —MgO based and becomes unsuitable as MnS precipitation nuclei.

FIG. 5 shows a casting speed of 40 m / min and 105
The temperature transition of the slab surface at m / min is shown. MnS
It is considered that the precipitation of is more advantageous as the time at 900 ° C. or higher is longer.

[0022]

As described above, according to the present invention,
Compared to the conventional method in which the Cd and N are reduced and the Md30 of the outline is adjusted and cast, MnS is reduced even during rapid solidification in twin-roll continuous casting by lowering S and optimizing the inclusion composition.
By coarsely precipitating, a Cr-Ni stainless steel thin plate having excellent elongation characteristics as compared with the conventional method can be stably manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between Md30 value and elongation for a material of the present invention and a comparative material.

FIG. 2 is a CMA measurement result showing the MnS precipitation distribution of the inventive material (a) and the comparative material (b).

FIG. 3 is an electron micrograph showing the form of precipitation of MnS in steel.

FIG. 4 is a graph showing a relationship between slag basicity and inclusion composition.

FIG. 5 is a graph showing a temperature transition on a slab surface.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Katsumi Kirihara 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Inside Nippon Steel Corporation Hikari Works (56) References JP-A-56-146862 (JP, A) JP-A-3-42151 (JP, A) JP-A-57-118846 (JP, A) JP-A-5-230538 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11 / 06 330 B22D 11/00

Claims (2)

(57) [Claims] 1. A molten steel having an S concentration of 0.003% or less, an Al concentration of 0.003% or less, an Al ₂ O ₃ content of 30% or less in the molten steel, and MnO and SiO ₂ as main components.
Cr-Ni stainless steel molten steel containing O-Al ₂ O ₃ -MnO-SiO ₂ -MgO-based inclusions is cast into a thin strip by twin-roll continuous casting, and the cast is cold-rolled. A method for producing a Cr-Ni-based stainless steel sheet having excellent elongation characteristics, characterized by: 2. The method according to claim 1, wherein the Cr—Ni-based stainless steel has a composition in which Md30 defined by the following formula 1 is in the range of 0 to 50 ° C. Md30 = 413-462 (C + N) -9.2Si-8.1Mn-13.7Cr-18.5Mo-9.1 (Ni + Cu) ... Equation 1