JPH07292438A - Austenitic stainless cold rolled steel sheet excellent in oxidation resistance and its production - Google Patents

Abstract

PURPOSE:To produce an austenitic stainless cold rolled steel sheet excellent in oxidation resistance and rusting resistance by recuperating the surface of a thin band-shaped slab subjected to continuous casting without being subjected to forced cooling, cooling the same and executing cold rolling. CONSTITUTION:The slab of a thin band-shaped austenitic stainless steel having about <=6mm thickness is cast by using a continuous casting machine in which the wall face of a mold is moved synchronism with the slab. The surface of the slab is recuperated without forcedly cooling the slab, and it is held for at least >=2sec in a temp. range of 1300 to 1200 deg.C. After that, it is cooled and coiled and is successively subjected to cold rolling, air annealing and descaling or bright annealing. Thus, the austenitic stainless steel sheet in which the Ni distribution in the surface of the cold rolled face is regulated to 0.9 to <1.0 by the Ni segregating degree A calculated by the formula; Ni segregating degree A/Nimin/Niave and excellent in oxidation resistance can be produced.

Description

Detailed Description of the Invention

[0001]

The present invention relates to acid resistance (particularly acid resistance limit).
The present invention relates to the production of excellent austenitic stainless cold rolled steel sheet.

[0002]

2. Description of the Related Art Austenitic stainless steel has good rust resistance and is widely used as a corrosion resistant material. Austenitic stainless steel is also used in a dilute acid environment depending on the application, and not only rust resistance but also acid resistance is one of the necessary characteristics.

As means for improving the rust resistance of stainless steel,
Methods for controlling the distribution of inclusions are known. This is because rusting is likely to occur starting from inclusions. For example, Japanese Patent Application Laid-Open No. 4-202628 discloses a method of improving rust resistance by finely dispersing the inclusions so that the passive film can be repaired even if inclusions become the starting point of rusting. on the other hand,
It is said that the acid resistance is determined exclusively by the components of the base metal, and high-acid-resistant steel grades with addition of components such as Cr, Mo, Cu, and Ni that improve the acid resistance have been developed. However, this method cannot avoid an increase in cost due to soaring raw material costs and lower manufacturability such as workability.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a cold rolled steel sheet having excellent acid resistance in an austenitic stainless steel cold rolled steel sheet and a method for producing the same.

[0005]

The gist of the present invention is as follows. 1. A cold-rolled sheet which is cast from an austenitic stainless molten steel into a strip-shaped slab by a continuous casting machine in which the mold wall surface moves in synchronization with the slab, and is further cold-rolled, and the Ni on the surface of the cold-rolled sheet is An austenitic stainless cold-rolled steel sheet having a distribution of 0.9 or more and less than 1.0 with a Ni segregation degree A calculated by the following formula (1) and excellent in acid resistance. Ni segregation degree A = Nimin / Niave (1) where Nimin: 1 in an arbitrary 500 μm square region
Ni minimum value (%) measured in 0 μm square unit Niave: Ni average value (%) in the region where Nimin was calculated. Continuous casting of austenitic stainless molten steel into a strip-shaped cast having a thickness of 6 mm or less by a continuous casting machine in which the mold wall surface moves in synchronization with the cast, and the cast obtained without forcibly cooling the cast. Reheat the surface of 1300-120
Hold in the temperature range between 0 ° C for at least 2 seconds, then cool and wind, then cold-roll, followed by atmospheric annealing /
A method for producing an austenitic stainless cold-rolled steel sheet, characterized by performing descaling or bright annealing.

[0006]

The present invention will be described in detail below. The austenitic stainless hot rolled sheet manufactured by the conventional slab-hot rolling process has a width of several tens to 100 μm on the outermost surface.
It has a degree of Ni segregation zone. Since this segregation cannot be eliminated by cold rolling or annealing, it remains in the final cold-rolled steel sheet product.

For example, in the SUS304 cold rolled sheet manufactured by the conventional slab-hot rolling-cold rolling process, any 5
The distribution of Ni content in the area of 00 μm square was measured by EPMA (El
ect-ron Probe X-ray Microanalyzer: electron beam diameter;
When measured in a unit of 10 μm square using 1 μm, accelerating voltage; 25 kV), the Ni segregation degree A obtained by dividing the minimum value of Ni amount by the average value of Ni amount was about 0.8. Therefore, in the cold-rolled sheet manufactured by the conventional method, the acid resistance limit is determined by the minimum Ni content of the material, so that the Ni content needs to be increased excessively.

The present inventors have investigated the relationship between the acid resistance limit of austenitic stainless steel and the Ni segregation degree A in detail, and have found that the acid resistance limit sharply improves when the Ni segregation degree A is 0.9 or more. . Further, as a means for increasing the Ni segregation degree A to 0.9 or more, the present inventors have found that the solidification structure of the surface of the slab rapidly quenched and solidified by the synchronous continuous casting process or the like has a dendrite arm interval (Ni segregation zone Note that the spacing is much smaller than the conventional slab,
The present invention has been completed by earnestly researching an appropriate diffusion treatment for increasing the i-segregation degree A to 0.9 or more.

The synchronous continuous casting process is a slab (thin slab) having a thickness equal to or close to that of the hot strip.
Is a new process for obtaining “iron and steel” '85, A
197-A256. This new process can omit the existing hot rolling process, does not require a huge hot rolling facility and a large amount of energy for heating and processing the material, and is a manufacturing process excellent in terms of productivity.
In order to obtain a cost reduction effect without imposing a burden on the cold rolling process, the thickness of the cast piece should be 6.0 mm or less.

SUS3 obtained by cold rolling a slab cast by a synchronous continuous casting process and a hot rolled sheet produced by a conventional process
The relationship between the Ni segregation degree of the 04 and SUS316 cold-rolled sheets and the acid resistance limit of HCl is shown in FIG. The acid resistance limit was determined by the following method. A sample obtained by lightly polishing the surface with # 500 emery paper was immersed in boiling dilute hydrochloric acid whose concentration was changed in the range of 0.1 to 1.0% for 6 hours to obtain the corrosion weight loss. The results are shown in FIG. Although it hardly dissolves in a dilute solution, the dissolution rate rapidly increases from a certain concentration. Almost no dissolution (dissolution rate 0.03 g /
The maximum concentration of m ² hr or less) hydrochloric acid was acid limitation. From this figure, it can be seen that the acid resistance limit sharply improves when the Ni segregation degree becomes 0.90 or more. Incidentally, in the definition of the Ni segregation degree, in the actual material, the segregation degree cannot be 1.0 or more. Further, the method of measuring the Ni segregation degree is not limited to EPMA.
S (Auger Elect-ron Spectroscopy) may be used.

As a result of intensive research on the reason why the Ni segregation of the cold-rolled sheet manufactured by using the synchronous continuous casting process is small, the following has been clarified. Since the thin slab produced by the synchronous continuous casting process is rapidly solidified, the dendrite arm interval is smaller than that of the slab-hot rolling process material and the Ni segregation interval is smaller, but the Ni segregation immediately after solidification is slab-hot rolled. It is equivalent to the process material. Therefore, the Ni segregation of the thin slab due to the synchronous continuous casting process is small because the Ni segregation once generated is homogenized by diffusion after solidification, and at that time, since the dendritic arm interval of the slab is small, Ni easily diffuses. As a result, Ni segregation is reduced.

Therefore, in order to set the Ni segregation degree to 0.90 or more, 1300 to 1300 after solidification as a means for promoting Ni diffusion.
Hold in the temperature range of 1200 ° C. for an appropriate time. Since the diffusion rate of Ni decreases exponentially as the temperature decreases, the holding temperature must be as high as possible in order to perform the necessary diffusion in a short time. However, SUS30
In the case of 4, when the temperature exceeds 1300 ° C., the δ phase precipitates and hinders diffusion. If it is less than 1200 ° C, it hardly diffuses in the industrially acceptable time. Therefore, 1300 to 1
It was decided to maintain the temperature range of 200 ° C.

FIG. 3 shows the relationship between the holding time between 1300 and 1200 ° C. and the segregation degree A of the cold-rolled sheet surface. When kept for 2 seconds or more, the segregation degree becomes 0.90 or more, and a cold-rolled sheet having an improved acid resistance limit can be manufactured. This required time is almost constant when the cast piece thickness is 6.0 mm or less.

As a means for maintaining the temperature range of 1300 to 1200 ° C. for a proper time after solidification, first, after the solidification of the rapidly cooled and solidified slab is completed, the surface of the slab is reheated without being forcibly cooled. At that time, by reducing the solid fraction of the slab, the recuperative ability is enhanced, and it becomes easy to maintain the specified holding time regardless of the variations in the width direction and the longitudinal direction of the slab. Further, the heat retention time can be guaranteed by installing an appropriate heat retention cover just below the drum. Further, in some cases, the heat insulating cover may be provided with heating equipment such as a burner.

[0015]

EXAMPLES Examples of the present invention and conventional examples will be described below. SUS304 and SUS having the composition shown in Table 1
316 was cast into thin slabs on a twin roll continuous caster. Table 1 shows the compositions of thin slabs produced by the twin roll continuous casting process and 3.0 mm thick SUS304 and SUS316 hot rolled sheets produced by the slab-hot rolling process. The thin slab and the scale of the hot-rolled sheet were removed by dipping in acid pickling with hydrofluoric acid, cold-rolled at a rolling reduction of 80%, and bright annealed to obtain a thin-plate product. The surface Ni content of this material is EPMA (electron beam diameter;
1 μm, accelerating voltage; 25 kV) measured in 10 μm square units, dividing the Ni amount at the Ni minimum point by the average Ni amount in the same area N
i The degree of segregation was calculated. The surface of this material was lightly polished with # 500 emery paper to give a sample with a concentration of 0.1-1.0.
% Boiling dilute hydrochloric acid for 6 hours to measure the corrosion weight loss,
The acid resistance limit was determined by the method described above.

Table 2 shows the Ni segregation degree of each cold rolled sheet. The Ni segregation degree of the comparative example is 0.75 to 0.87, whereas the cold rolled sheet of the present invention example is 0.9 or more. Table 2 shows the acid resistance limit in boiling hydrochloric acid, and Fig. 1 shows the relationship between the Ni segregation degree and the acid resistance limit. While the current material SUS304 has an acid resistance limit of 0.1% and SUS316 of 0.3%, the materials of the present invention have SUS304 of 0.2% and SUS316 of 0.4%, thus improving the acid resistance limit. .

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

EFFECTS OF THE INVENTION According to the present invention, a stainless cold-rolled steel sheet having better acid resistance and better acid resistance limit than the current material can be manufactured at a low cost without adding Ni or the like in the manufacture of an austenitic stainless steel thin plate. be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Ni segregation degree and an acid resistance limit of a cold rolled austenitic stainless steel.

FIG. 2 is a diagram showing a method for calculating an acid resistance limit in HCl.

FIG. 3 is a diagram showing a relationship between a holding time of 1300 to 1200 ° C. of a thin cast piece and a Ni segregation degree of a cold rolled sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C22C 38/44 (72) Inventor Takahashi Tohashi Yamaguchi Prefecture Hikari City 3434 Shimada, Nippon Steel Co., Ltd. Inside the Hikari Steel Works

Claims (2)

[Claims] 1. A cold-rolled sheet which is cast from austenitic stainless molten steel into a strip-shaped cast piece by a continuous casting machine in which the mold wall surface moves in synchronism with the cast piece, and is further cold-rolled. An austenitic stainless cold-rolled steel sheet having excellent acid resistance, wherein the Ni distribution on the surface of the plate has a Ni segregation degree A calculated by the following formula (1) of 0.9 or more and less than 1.0. Ni segregation degree A = Nimin / Niave (1) where Nimin: 10 μm in arbitrary 500 μm square area
Ni minimum value (%) measured in square units Niave: Ni average value (%) in the area where Nimin was calculated 2. A continuous casting machine in which the wall surface of the mold moves austenitic stainless molten steel in synchronism with the slab to obtain a thickness of 6
Continuously cast into strip-shaped slabs of mm or less, and re-heat the surface of the slabs without forcibly cooling the obtained slabs to 1300
Hold for at least 2 seconds in the temperature range between ~ 1200 ° C,
A method for producing an austenitic stainless cold-rolled steel sheet excellent in acid resistance, which comprises cooling and winding, followed by cold rolling, followed by atmospheric annealing / descaling or bright annealing.