JPH0796685B2 - Method for producing Cr-Ni series stainless steel thin plate with excellent surface quality and material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention casts a slab of a size close to the product thickness by a so-called synchronous continuous casting method in which there is no relative speed difference between the slab and the inner wall surface of the mold. To produce a Cr-Ni-based stainless steel sheet.

[Conventional technology]

Conventionally, in order to manufacture a stainless steel thin plate using a continuous casting method, casting is performed on a slab having a thickness of 100 mm or less while vibrating the mold according to the casting method, and the surface of the obtained slab is cared for in a heating furnace. After heating to 1000 ° C. or higher, hot rolling was performed by a hot strip mill including a row of rough rolling mills and finishing rolling mills to obtain hot strips having a thickness of several mm.

When cold-rolling the hot strip thus obtained, the shape (flatness), material,
In order to secure the surface texture, hot-rolled sheet annealing is performed to soften the hot strip that has undergone strong hot working, and the surface scale and the like are removed by grinding after the pickling step. In this conventional process, a large amount of energy is required for heating and processing the material in a long hot rolling facility, and it is difficult to say that the manufacturing process is excellent in terms of productivity. In addition, the final product has a lot of restrictions in use, such as a texture developed, and it is necessary for the user to take the anisotropy into consideration when applying press working or the like.

Therefore, in order to solve the problem of requiring a long hot rolling facility, enormous energy, and rolling power in order to roll a slab with a thickness of 100 mm or more into hot strip, recently in the process of continuous casting. Research on a process for obtaining a slab (thin band) having a thickness equal to or close to that of a hot strip is under way. For example, "Iron and Steel"'85, A197 to A2
A paper featured at 56 discloses a process for directly obtaining hot strip by continuous casting.
In such a continuous casting process, the twin drum method is used when the gauge of the slab to be obtained is in the level of 1 to 10 mm, and the twin drum method is used when the gauge of the slab is in the range of 20 to 50 mm. A belt system is being considered.

[Problems to be Solved by the Invention]

Twin drum casting, SUS304 manufactured by one-time cold rolling process
It is known that the thin plate product has a fine grain structure and lower elongation than the thin plate product manufactured by the conventional process. For example, it was reported in a paper featured in "CAMP ISIJ" vol.1 1988, 1670-1705.
It is described that the slab is annealed to eliminate the δ ferrite remaining in the slab.

As a result of a detailed study by the inventors of the present invention on a process for producing a Cr-Ni-based stainless steel thin plate by continuous strip casting, it is clear that there are the following three factors as factors that suppress the growth of recrystallized grains during cold rolling and annealing. Became.

(1) δ ferrite remaining in the slab (2) MnS finely precipitated in the final annealing after cold rolling (3) Small inclusions in the slab For δ ferrite to be eliminated by reheating and annealing the slab Is possible. However, the δ-ferrite remaining in the slab does not transform into the γ-phase sufficiently by the short-time annealing that is performed in the conventional austenitic stainless steel hot-rolled steel strip. It is necessary to transform the to the γ phase.

It was also revealed for the first time that finely precipitated MnS strongly suppressed grain growth in the final annealing after cold rolling. Therefore, it is necessary to make the MnS coarse enough to render it harmless at the stage of the slab. However, the method of reheating and annealing a cast piece requires heat treatment at a high temperature for a long time, and a method of efficiently performing the high temperature and long time heat treatment to facilitate grain growth is desired.

Further, minute inclusions in the cast slabs can also be rendered harmless by controlling the components such as deoxidizing elements for the inclusions that hinder the grain growth of the cold rolled annealed sheet.

Twin drum structure, SUS304 manufactured by one-time cold rolling process
Another problem of thin plate products is surface problem (roping). The roping is a phenomenon that occurs because the γ grain size of the slab is coarse, and it is effective to quench the slab immediately after casting in order to reduce the γ grain size of the slab and suppress the roping. However, a large amount of δ-ferrite remains in the quenched slab and precipitation of MnS in the slab is suppressed, which causes deterioration of the material as described above.

[Means for Solving the Problems]

The present inventors have studied the conditions for achieving both material quality (elongation) and surface quality (roping) of the thin plate, and as a result, casting, the slab γ grains are refined by controlling the solidification atmosphere,
Furthermore, it was clarified that the slab γ grain size became finer by adjusting the main component, and by maintaining the slab at a high temperature, it is possible to achieve both the thin plate material (elongation) and surface quality (roping). I found it.

That is, the gist of the present invention is to cast a thin-plate cast piece having a plate thickness of 6 mm or less from Cr-Ni-based stainless steel represented by 18% Cr-8% Ni steel and directly perform cold rolling without hot rolling. In the process of manufacturing a thin plate product by casting, the casting and solidifying atmosphere is mainly N ₂ or He, and δ-Fe c
al (%) = 3 (Cr + 1.5Si + Mo + Nb + Ti) -2.8 (Ni + 0.
5Mn + 0.5Cu) -84 (C + N) -19.8 (%) defined as δ-Fe cal (%) of 0 to 10% to make the primary crystal of solidification as δ phase and crystallization of γ phase. The precipitation start temperature is lowered to suppress the growth of γ grains during solidification and after solidification, and then the temperature is maintained in the temperature range of 800 ° C or higher and 1250 ° C or lower.
This is a method for producing a Cr-Ni-based stainless steel thin plate in which nS is precipitated and δ ferrite is reduced, and thereafter cold rolling is performed as usual and final annealing is performed.

[Work]

In the SUS304 thin plate manufacturing process by the strip continuous casting method, in order to secure the surface quality and material of the product, a method of refining the γ grains of the cast piece and reducing the δ ferrite remaining in the cast piece and sufficiently precipitating MnS coarsely It was necessary to develop an efficient heat treatment method to achieve this.

The present inventors have investigated the heat treatment conditions for reducing δ ferrite and coarse MnS precipitation, and
It was clarified that δ-ferrite disappears and MnS coarsely precipitates in a short time and efficiently by heat treatment in the temperature range of 1250 to 800 ℃. In the case of holding at 1200 to 1000 ° C, the precipitation of carbides is prevented by cooling the temperature range of 1000 to 550 ° C at a cooling rate of 50 ° C / sec or more, and the cast slab for solid solution of carbides is prevented. It is also possible to omit the heat treatment step.

In addition, when not cooling at a cooling rate as mentioned above, it manufactures by a normal continuous casting-hot rolling process manufacturing method. That is, after holding the thin cast piece in the temperature range of 800 ° C to 1250 ° C for 80 minutes or less,
Before cold rolling, the same annealing as the hot rolled sheet annealing performed by the above manufacturing method is performed. For example, the carbide is solution-treated by holding it in the temperature range of 1000 to 1150 ° C. for about 5 to 60 minutes.

Further, in order to refine the γ grain size of the slab, casting, by setting the solidification atmosphere to an atmosphere mainly containing N ₂ or He, fine chill crystals remain in the surface layer of the slab and the entire thickness of the slab is obtained. It was found that the slab γ grain size becomes finer than that of a slab cast in an Ar atmosphere.

FIG. 1 (a) is a photomicrograph of the microstructure of a cast piece obtained by casting molten steel having a composition of δ-Fe cal = 3.1% under an N ₂ atmosphere.
The figure (b) is a photomicrograph of the metallographic structure of a slab obtained by casting molten steel with a composition of δ-Fe cal = 3.5% in an Ar atmosphere. It is fine.

Further controlling the main component, δ-Fe cal = 3 (Cr + 1.5Si + M
o) -2.8 (Ni + 0.5Cu + 0.5Mn) -84 (C + N) -19.8 δ-Fe cal was set to 0-10%, and it became clear that the slab γ grain size became finer. . That is, in Fig. 1 (c), molten steel with the composition δ-Fe cal = -2.1% is
_{2 is} a photomicrograph of the microstructure of a cast piece cast in _two atmospheres. It can be seen that the γ grain size of the cast piece is larger than that of the cast piece cast in the same atmosphere as shown in FIG.

2 and 3 show JIS 304 stainless steel slabs (thickness) cast in a N ₂ atmosphere by a twin-drum type continuous casting machine.
2 mm) is a diagram showing the relationship between the holding conditions at 1300 to 800 ° C. immediately after casting and the elongation and roping of the final product.
By holding the slab at high temperature for a long time, δ ferrite decreases and MnS precipitates, so that the grains grow during cold rolling annealing and show good elongation. However, holding at a temperature higher than 1250 ° C causes γ grains of the slab to grow even for a short time, so that roping occurs during cold rolling. Therefore, in order to manufacture a thin plate product excellent in both surface quality and material, it is necessary to hold the slab for 80 minutes or less in the temperature range of 1250 to 800 ° C.

〔Example〕

Austenitic stainless steel with various components based on the 18% Cr-8% Ni steel shown in Table 1 was melted, and the internal water-cooled twin-drum caster was used in various atmospheres to obtain a thickness of 2 mm.
Was cast into a slab and held at a temperature of 800 to 1250 ° C. The slab was annealed, pickled, cold-rolled, annealed and temper-rolled to obtain a thin plate product, and surface quality and material evaluation were performed.

As a comparative example, heat treatment conditions immediately after casting, δ-Fe cal,
A thin plate product was similarly produced from a slab cast under conditions in which the casting atmosphere was outside the scope of the present invention, and surface quality and material evaluation were performed.

These evaluations are shown in Table 2. According to this table, the thin plate manufactured by the method of the present invention (No. 1 to 9) was excellent in material and surface quality was good, but the thin plate manufactured by the comparative method (No. 10 to 12) was made of material ( The elongation or surface quality (roping) was poor.

(Effects of the Invention) According to the present invention, it is possible to obtain an austenitic stainless steel thin plate having excellent surface properties by a simple process of continuously casting a thin strip-shaped slab having a thickness close to the product thickness into a product by direct cold rolling. it can. Therefore, its technical effect is extremely large in terms of economical efficiency and manufacturing purpose.

[Brief description of drawings]

FIG. 1 is a microscopic metallographic photograph of a thin cast piece obtained by a continuous casting method. In the figure, (a) is a metallographic photograph of a thin cast piece according to the method of the present invention, and (b) and (c) are comparative methods. 2 is a photograph of the metallographic structure of a thin slab according to the present invention, and FIG. 2 shows 700 to 130 immediately after casting the thin slab cast by the method of the present invention.
It is a figure which shows the state of elongation in the L direction when hold | maintaining for 5 to 80 minutes in the temperature range of 0 degreeC, and FIG. 3 hold | maintained the thin cast piece cast by the method of this invention on the same conditions as FIG. It is a figure which shows the state of roping at this time.

Claims (2)

[Claims] 1. A cold rolled thin plate product is manufactured by casting a thin cast piece having a plate thickness of 6 mm or less from Cr-Ni type stainless steel represented by 18% Cr-8% Ni steel and omitting hot rolling. In the method, δ-Fe cal (%) of the Cr-Ni system stainless steel component
Molten steel controlled to 0 to 10% in an atmosphere containing N ₂ or He as a main component, and then a thin slab obtained by the casting in a temperature range of 800 ° C. or higher and 1250 ° C. or lower for 80 minutes or less. Hold and
A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality and material, which is characterized by cold rolling and final annealing. However, δ-Fe cal (%) = 3 (Cr + 1.5Si + Mo + Nb + T
i) -2.8 (Ni + 0.5Mn + 0.5Cu) -84 (C + N) -19.8
(%) 2. The method according to claim 1, wherein after holding the thin cast piece in a temperature range of 800 ° C. or higher and 1250 ° C. or lower for 80 minutes or less, annealing, cold rolling and final annealing are performed.