JPH0735550B2 - Method for producing Cr-Ni type stainless steel thin plate with excellent surface quality - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention casts a slab of a size close to the product thickness by a so-called synchronous continuous casting process in which there is no relative speed difference between the slab and the inner wall surface of the mold. And a method for producing a Cr-Ni-based stainless steel thin plate.

(Prior Art) Conventionally, in order to produce a stainless steel thin plate by using a continuous casting method, a casting having a thickness of 100 mm or more is cast while vibrating the mold in the casting direction, and the surface of the obtained casting is cleaned. Done,
After heating to 1000 ° C. or higher in a heating furnace, hot rolling was performed by a hot strip mill consisting of a row of rough rolling mills and finishing rolling mills to obtain hot strips with a thickness of several mm.

When cold-rolling the hot strip thus obtained, the shape (flatness), material,
In order to secure the surface quality, hot-rolled sheet annealing is performed to soften the hot strip that has been subjected to strong hot working, and the surface scale and the like are removed by grinding after the pickling step.

In the conventional process, a large hot rolling facility requires a large amount of energy for heating and processing the material, and it cannot be said 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 pressing or the like.

Therefore, in order to solve the problem that a long hot rolling facility, enormous energy, and rolling power are required to roll a slab with a thickness of 100 mm or more into hot slip, 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 ~ 'A256 and "CAMP ISIJ" v
No. 1,1988,1670 to 1705, a paper disclosing a process for directly obtaining hot strip by continuous casting is disclosed.

In such a continuous casting process, when the gauge of the slab (strip) to be obtained is in the level of 1 to 10 mm, the twin drum system is used, and the gauge of the slab is 20 to 50 mm.
The twin-belt method is being considered when the level is.

(Problems to be Solved by the Invention) In a continuous casting process of this type, a slab having a final shape is manufactured, and intermediate steps such as a hot rolling process and a heat treatment process are omitted or reduced. Therefore, it is known that the structure and surface condition of the slab have a great influence on the material and surface properties of the product.

That is, the aforementioned "CAMP ISIJ" vol.1,1988,1670-1705
In the Cr-Ni system stainless steel sheet,
The ridging phenomenon of stainless steel guide plates is described.
However, the surface quality of Cr-Ni-based stainless steel sheets has not been particularly problematic.

The present inventors, as a result of detailed study of the Cr-Ni-based stainless steel thin plate manufacturing process by strip continuous casting, surface defects and uneven luster called roping occur in the product as specifically shown below. There was found.

That is, molten steel containing SUS304 steel as a basic component was cast by an internal water-cooled twin-roll (twin-drum) continuous casting tester and wound as a thin strip having a thickness of 1 to 4 mm. The slab (thin strip) thus obtained was descaled, then directly cold-rolled, finally annealed, and pickled to obtain a product A having a thickness of 1 to 0.4 mm.

On the other hand, a conventional molten steel is continuously cast into a slab having a thickness of 100 mm or more, which is reheated and then hot-rolled by a hot strip mill to form a thin strip having a thickness of 3 to 6 mm, which is cooled and wound. The obtained product was cold-rolled after descaling, finally annealed, and pickled to obtain a product B having a thickness of 1 to 0.4 mm.

Comparing the surface properties of product A and product B, product A
Has been found to have the following surface defects.

(1) Roping: Fine irregularities are generated on the surface during cold rolling.

(2) Uneven gloss: Uneven gloss appears on the surface after cold rolling, annealing, and pickling.

On the other hand, the product B has no such defects.
Therefore, the issues regarding the surface texture of these products are:
This is a peculiar problem that occurs when cold-rolling is performed by producing a thin-walled slab close to the final shape from molten austenitic stainless steel, which is an essential drawback of NNS casting.

As a result of detailed investigation of the causes of these problems relating to the surface texture, the inventors have found that the γ grains of the material before cold rolling are 50 μm.
It has been clarified that these surface defects occur when the thickness is larger than the above or when the thin cast piece is insufficiently cooled in the temperature range where the Cr-based carbide is precipitated.

In order to prevent these surface defects, the molten steel composition and cooling conditions are improved in the process of solidifying and cooling the molten steel so that the average γ grain size before cold rolling is 50 μm or less, and
Good surface quality of products is obtained without depositing Cr-based carbides
A method for producing a Cr-Ni-based stainless steel thin plate was invented.

For example, by the method and composition adjustment to cool at 100 ° C. / sec or more cooling rate until 1200 ° C. After coagulation, the method to -2～10% of δ-Fe _cal (Japanese Patent Application Sho 63-221471), and further the crystal grain This is a method of adding 0.01 to 1 mol% of a refining element.

However, the cooling conditions from 1400 to 1200 ° C are set to 100 ° C / sec or more to suppress the growth of γ grains, so it is difficult to perform sufficient cooling when the slab plate thickness is large, and When the width was wide, the entire width could not be sufficiently cooled and γ particles of 50 μm or less could not be stably obtained.

Therefore, the present invention proposes a method of stably producing a slab having a γ particle size of 50 μm or less and a plate thickness of 1 to 10 mm by controlling the cooling of the slab and adjusting the composition thereof to obtain a product having excellent surface properties. To do.

(Means for Solving the Problem) The gist of the present invention is to use Cr-Ni stainless steel as δ-Fe.
_cal (%) = 3 (Cr + 1.5Si + Mo + 2Ti + Nb) -2.8 (Ni +
0.5Mn + 0.5Cu) -84 (C + N) -19.8 (each component is wt%)
Δ-Fe _cal defined by is -2 to 10%, and 0.01%
Al, Ti, Nb, Zr, La, Ce, Nd above or Y, Ca, Mg above 0.001%
Or a total of 0.001 to 1% of molten steel in a composition range of 0.001 to 1% by a continuous casting machine in which the mold wall surface moves in synchronization with the slab, and the cooling rate at the time of solidification is 100 ° C / sec or more. Continuously cast into a strip-shaped slab of 10 mm or less, and start cooling the obtained slab from the highest possible temperature below the solidification temperature,
1,200 at a cooling rate of 50 ° C / sec or more while suppressing the reheat of the slab
After cooling to ℃, the γ grain size of the slab is refined to an average of 50μm or less, and then the temperature range from 1200 ℃ to 600 ℃ is 10 ℃ / sec.
Cooled at the above cooling rate and wound up, after pickling the slab,
A Cr-Ni-based stainless steel sheet with excellent surface quality characterized by being subjected to one or two types of warm rolling and cold rolling, annealing / pickling or bright annealing, and temper rolling to obtain a product. Is a manufacturing method.

(Operation) The present invention will be described in detail below.

In thin-wall continuous casting, the method of refining γ grains by cooling the solidification of the slab to 1200 ° C at a cooling rate of 100 ° C / sec or more,
It is difficult to carry out when the thickness of the slab is too thick to remove heat sufficiently, or when it is difficult to uniformly cool the entire width of the slab with a wide width.

Therefore, in the present invention, in the thin wall continuous casting in which the solidification cooling rate is 100 ° C./sec or more, the cooling rate up to 1200 ° C. after solidification is
If the temperature is 50 ° C / sec or more, the γ grain should be 50 μm or less.
It is characterized in that the molten steel composition is adjusted so that δ-Fe _cal is −2 to 10% and that the grain refining element 0.001 to 1% is added.

The crystal grain size of Cr-Ni stainless steel slabs solidified at a cooling rate of 100 ° C / sec or more by thin wall continuous casting increases as the slab thickness increases. Has a particle size of 50 μm or less. After solidification, the grain growth proceeds, but the grain growth is faster at higher temperatures and faster in a single phase.

In the present invention, δ-Fe _cal is set to −2 to 10% to divide γ grains with the δ ferrite phase as a primary crystal, and fine ferrite is dispersed in the γ grain boundary (Photo 1). N
By adding crystal grain refining elements such as b, Zr, La, Ce, Nd, Y, Ca, and Mg, fine oxides, nitrides, sulfides, and carbides precipitated in molten steel or during solidification or after solidification It is characterized by being uniformly dispersed within the grain and at the γ grain boundary (Photo 2).
The growth of γ grains is significantly suppressed by the δ ferrite and the precipitate.

FIG. 1 shows these γ grain refinement effects.

When cooling from solidification temperature to 1200 ℃ at 50 ℃ / sec or more,
Due to the effect of refining δ ferrite, the growth of γ grains is greatly suppressed. However, if the amount of δ is large, it becomes difficult to identify γ grains. By adding δ-ferrite and adding grain refining elements in this way, the average γ grain size can be reduced to 50 μm or less by cooling from the solidification temperature up to 1200 ° C. at 50 ° C./sec or more. As shown in, roping is greatly reduced.

The grain growth suppression effect of δ ferrite and precipitates depends on its amount. If δ-Fe _cal (%) is less than -2%, the effect of δ solidification is insufficient, and the more δ-Fe _cal (%) is, the more effective it is. But 1
If it exceeds 0%, the effect is saturated.

As grain refinement elements, Al, Ti, Nb, Zr, La, Ce, Nd, Y, Mg,
Ca or the like is effective, and the grain growth is remarkably suppressed by the oxides, nitrides, sulfides, and carbides of the above elements finely precipitated in the cast piece. The effect of the grain refinement element increases as the amount of addition increases, but if it exceeds 1%, the effect of refinement becomes saturated. Furthermore, since a large amount of addition increases the inclusions, which deteriorates the material of the product plate and also leads to an increase in cost, the addition amount must be 1% or less.

(Example) Austenitic stainless steel with various components based on 18Cr-8Ni steel shown in Table 1 was melted.

δ−Fe _cal (%) is changed in the range of −2 to 10%, and Al, Ti, Nb, Zr, La, Ce, Nd, Y, Mg, Ca are used as grain refinement elements.
Was added in the range of 0.001 to 0.9% in the total amount of one kind or two kinds or more.

These molten steels were continuously cast into slabs having a thickness of 1 to 6 mm and a width of 1000 mm by an internal water-cooled twin roll continuous casting machine, and the slabs solidified from the twin roll outlet were rapidly cooled by a method of pressing them on a water cooling drum. In this case, the average cooling rate up to 1200 ° C was at least 50 ° C / sec or more.

After that, it was cooled in a so-called secondary cooling zone between 1200 and 600 ° C, cooled at 10 ° C / sec or more, and wound at 600 ° C or less. After that, descaling is performed in the usual way, cold rolling of 50 to 85% is performed, annealing at 1050 to 1200 ° C for 30 seconds and then pickling or bright annealing to a thin sheet of 0.1 to 2.0 mm and temper rolling. Was carried out to obtain a product.

In the comparative example, the average cooling rate up to 1200 ° C. was 50 ° C./sec or more, and the refined component or δ-Fe _cal was outside the range of the present invention.

The slab thus obtained was cold-rolled by 50% to obtain a product, and the roping height was evaluated as the surface quality.

The results are shown in Table 2.

The roping height of the product of the present invention was 0.2 μm or less, which was good, but as with the comparative material, δ−Fe _cal was −2.
% Or less than the specified amount of the refined component, the roping height was 0.2 μm or more, which was unsatisfactory.

(Effect of the invention) According to the present invention, a simple process for producing a thin strip-shaped slab having a thickness close to the product thickness by continuous casting-direct cold rolling is used.
It is possible to obtain an austenitic stainless steel thin plate having excellent surface properties.

[Brief description of drawings]

Fig. 1 is a chart showing the relationship between γ grain size and δ-Fe _cal .
The figure is a table showing the relationship between roping height and δ-Fe _cal during 50% cold rolling.

Claims (1)

[Claims] 1. Cr-Ni system stainless steel δ-Fe _cal (%)
= 3 (Cr + 1.5Si + Mo + 2Ti + Nb) -2.8 (Ni + 0.5Mn + 0.
5Cu) -84 (C + N) -19.8 ( each component and -2～10% of [delta]-Fe _cal defined in wt%), more preferably 0.01% or more Al,
Ti, Nb, Zr, La, Ce, Nd or 0.001% or more of Y, Ca, Mg alone, or molten steel with a composition range of 0.001 to 1% in the total amount, the mold wall surface is synchronized with the slab. With a moving continuous casting machine, the cooling rate during solidification is 100 ° C / sec or more and the thickness is 1
Continuously cast into a strip-shaped slab of 0 mm or less, cooling the obtained slab starting from the highest possible temperature below the solidification temperature, while suppressing recuperation of the slab, cooling rate of 50 ° C / sec or more At 1200 ° C to reduce the γ grain size of the slab to an average of 50 μm or less, then cool the temperature range from 1200 ° C to 600 ° C at a cooling rate of 10 ° C / sec or more and wind it up. After pickling the piece, it is subjected to one or two kinds of warm rolling and cold rolling and annealed / pickled or bright annealed and temper-rolled to obtain a product with excellent surface quality. -A method for manufacturing a Ni-based stainless steel thin plate.