JPH0730405B2 - Method for producing Cr-Ni type stainless steel thin plate with excellent surface quality - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention uses a so-called synchronous continuous casting process in which there is no relative speed difference between a slab and an inner wall surface of a mold, and the slab thickness is close to the product thickness. The present invention also relates to a method for producing a Cr-Ni-based stainless steel sheet, which is a method for producing a Cr-Ni-based stainless steel sheet having excellent surface properties by refining the structure from the cast piece stage.

(Prior art) Conventionally, in order to produce a stainless steel thin plate using a continuous casting method, a mold having a thickness of 100 mm or more is cast while vibrating the mold in the casting direction, and surface treatment of the obtained slab is performed. After heating to 1000 ° C. or higher in a heating furnace, hot rolling was performed using a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row to obtain a hot strip having 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. This conventional process requires a large amount of energy for heating and processing the material in a long hot rolling facility, and it is hard to say that the manufacturing process is excellent in terms of productivity. In addition, the final product develops a texture because many processes are made from a slab with a thickness of 100 mm or more, and the product develops its anisotropy when the user presses it. There were also many restrictions on use, which required consideration.

In order to roll a slab with a thickness of 100 mm or more into a hot strip, a long hot rolling facility and a large amount of energy,
Recently, in order to solve the problem of requiring rolling power,
Research on a process for obtaining a slab (thin band) having a thickness equal to or close to that of a hot strip in the process of continuous casting is under way. For example, "Iron and Steel"'85, A197 to '85, A256
In the article featured in, 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 at a level of 1 to 10 mm, the twin durham system is used, and when the gauge of the slab is at a level of 20 to 50 mm. Twin belt method is under consideration.

However, it is said that there is still a problem in the casting stage in these continuous casting processes, and it has not reached the stage where the problems regarding the product, the material and the surface property are solved.

(Problems to be Solved by the Invention) In a process that is being developed as a new process and is premised on obtaining a slab (thin strip) having a thickness equal to or close to that of a hot strip by continuous casting, Since the process from casting to product is simplified,
The surface characteristics of stainless steel products will be sensitively affected by the slab properties. That is, in order to obtain a product having excellent surface properties, it is necessary to obtain an excellent slab.

The present invention has been made for the purpose of providing a simple manufacturing process capable of obtaining a Cr—Ni-based stainless steel sheet without the surface defects called uneven luster and roping phenomenon peculiar to a stainless steel sheet product.

(Means for Solving the Problems) The gist of the present invention is Cr-Ni represented by 18% Cr-8% Ni steel.
Series stainless steel was obtained by continuously casting into a strip-shaped slab with a thickness of 10 mm or less at a cooling rate during solidification of 100 ° C / sec or more by a continuous casting machine in which the mold wall surface moves in synchronization with the slab. Start cooling the slab from as high a temperature range as possible, and prevent the reheating of the slab to 1100 ℃ at a cooling rate of 100 ℃ / sec or more to suppress the growth of γ grains, then 900 ~ 550 After cooling the temperature range of ℃ at a cooling rate of 50 ℃ / sec or more, winding, followed by pickling without annealing, then pre-cold rolling applying a reduction of 60% or less, then The average grain size of γ grains is 50μ by annealing and recrystallizing in the temperature range of 850 ℃ or more.
The surface quality is excellent in that it is subjected to pickling after being made m or less, and then cold-rolled to a final thickness, and the obtained cold-rolled sheet is subjected to final annealing and then pickled or bright annealed. This is a method for manufacturing a Cr-Ni-based stainless steel thin plate.

Hereinafter, the present invention will be described in detail.

Molten steel containing SUS304 steel as a basic component was cast by an internal water-cooling twin-roll (twin-drum) continuous casting tester into a ribbon having a thickness of 2 to 4 mm, cooled, and wound.

The cast piece (thin band) thus obtained was descaled, then directly cold-rolled, finally annealed, and pickled to obtain a 2B product. The surface texture of these products was obtained by continuous casting of molten steel into a slab with a thickness of 100 mm or more, reheating, hot rolling with a hot strip mill, and cold rolling. The surface properties of the products were compared and examined in detail.

As a result, the molten steel was cast by an internal water-cooled twin-roll continuous casting tester into a ribbon of 2 to 4 mm in thickness, which was cooled and wound, followed by descaling and cold rolling, It was found that the following surface defects may occur in the 2B product obtained by final annealing and pickling.

(1) Roping or orange peel ... Fine irregularities are generated on the surface during cold rolling or during product processing.

(2) Uneven luster: Uneven luster occurs due to sensitization of the structure of the material during winding of the slab (thin band), oxidation of grain boundaries, or coarsening of γ grains.

The problem regarding the surface texture of these products is a process-specific problem that includes a process of directly obtaining a thin ingot (strip) by continuous casting, which is not found in a conventional process.

The inventors, as a result of detailed examination of the causes of these surface defects, when the γ grain size of the material before cold rolling is large, or when the cooling of the Cr carbide precipitation temperature range of the slab is insufficient, these It was clarified that the surface defects are remarkable.

Thus, as a roping measure, the γ grain size of the material before cold rolling should be set to a grain size No. 6 or more, that is, 50 μm or less. , It was clarified that it is desirable when a process including a process of directly obtaining a thin slab by continuous casting is adopted.

Hereinafter, these measures will be described in detail.

As a material for cold rolling, there are various ideas as follows as means for making the material having a γ particle diameter of 50 μm or less. That is, 1) reduce the γ grain of the thin slab itself, 2) hot work the thin slab continuously after casting, and recrystallize the fine grain, 3) cold work the thin slab, Annealing to recrystallize the grains, and so on.

In particular, the present invention is a combination of the above (1) reducing the γ grains of the slab itself and (3) cold working, annealing recrystallization, and refining the slab to obtain stainless steel. The present invention relates to a method for preventing roping that occurs on the surface and also producing stainless steel with excellent surface quality without uneven gloss.

First, when obtaining a thin slab by a continuous casting process such as a twin roll method in which the mold wall surface moves in synchronization with the slab,
As a method for reducing the γ grains themselves of the thin slab, it is important to reduce the γ grains in the slab from the time of generation of the γ grains, and to cool the slab from a high temperature range in order to suppress subsequent growth. It is also very effective to subject these slabs to cold working and anneal to recrystallize the slabs to form fine particles. Based on the above idea, the present inventors
−8Ni steel was cast with small twin rolls in the laboratory, and the conditions of rapid cooling immediately below casting, cold rolling, and annealing conditions were changed, and the grain size of the slab before this cold rolling was changed to show the relationship with the surface texture of the product. Clarified.

In a process of casting molten steel into a thin strip (thin slab) by a continuous casting machine of a type in which the mold wall surface moves in synchronization with the slab, for example, a twin roll row casting machine, as a means for reducing γ grains in the slab In the present invention, the cooling rate at the time of solidification of the slab is cast at 100 ° C./sec or more, and the resulting slab (strip) having a thickness of 10 mm or less is reheated at the outlet of the continuous casting machine. To prevent this, start cooling from as high a temperature range as possible directly under the casting machine, and keep the temperature up to 1100 ° C for 10
It is important to suppress the growth of γ grains by cooling at a cooling rate of 0 ° C / sec or more.

Furthermore, it is necessary to cool the slab in the temperature range of 900 to 550 ° C with an average cooling rate of 50 ° C / sec or more. Otherwise, carbide precipitates on the grain boundaries of the slab, causing slab (strip).
When it is pickled, it causes intergranular corrosion and impairs the gloss of the final product.

By the means described above, the growth of γ grains of the cast slab is suppressed, and further, the precipitation of carbides at grain boundaries is prevented. The slab thus obtained is subjected to preliminary cold working, for example, cold rolling, and then annealed at a high temperature for a short time to recrystallize the slab.

In this way, the γ grains of the material can be made fine.

Fig. 1 shows the cold rolling reduction ratio of the slab after preliminary cold rolling, short-time annealing at 1080 ° C, and cold rolling (main cold rolling) to the final plate thickness. The relation of the roping height in the product is shown for each reduction ratio level in the preliminary cold rolling.

If the γ grain of the slab is fine, for example when continuously casting a thin slab of 2 mm, and directly below the casting machine, 1300
When the cooling of the slab in the temperature range of ~ 1100 ° C is made strong at 100 ° C / sec or more, even if the reduction ratio in pre-cold rolling is 10% or more, it is sufficiently recrystallized and the average γ grain size is
Since the size can be reduced to 50 μm or less, the roping height of the product can be reduced.

On the other hand, when the γ particle size of the slab is large, for example, a slab having a thickness of 4 mm, a cooling means at a low cooling rate immediately below the casting machine,
For example, if the γ grains of the slab have grown by air cooling, the average γ grain size of the material is 50μ by annealing after rolling with a reduction rate of 50% or more in the preliminary cold rolling.
Fine particles of m or less can be obtained, thus improving the roping of the product.

As described above, the more the cooling rate of the slab directly below the casting machine in the temperature range of 1300 to 1100 ° C is performed at a high cooling rate of 100 ° C / sec or more, the lower the reduction rate in the preliminary cold working (rolling, etc.) is. Can be recrystallized with the average γ after recrystallization.
Since the particle size can be reduced to 50 μm or less, roping of the product can be reduced, and a product having excellent surface properties without uneven gloss can be obtained.

Example A Cr-Ni system stainless steel represented by 18Cr-8Ni melted by a conventional method was cast by a twin-drum method of an internal water cooling system, and 3 m
m and 4.5 mm slabs were used. Immediately below the outlet of the twin-drum casting machine, spray cooling is performed in addition to air cooling, and cooling is performed at an average cooling rate of up to 1100 ° C, aiming at 100 ° C / sec or more, and further 900 ° C.
The following was water-cooled, and was cooled up to 550 ° C at an average of 70 ° C / sec or more and wound up.

Thereafter, the slab was descaled by mechanical descaling and pickling, and pre-cold rolled by cold rolling. 3mm thickness,
For 4.5 mm thick slabs, pre-cold rolling in the range of 10-40%
It was annealed at 0 ° C or higher for 20 seconds or less and then rapidly cooled. Thus, the slab was recrystallized so that the γ grain size was 50 μm or less.

After that, 30%, 50%, 80%, 95%, and 95% or more cold rolling was performed in the final cold rolling, and the final annealing was performed in the usual manner.
Made as a product. The surface properties and mechanical properties of these products were very good.

In the comparative method, the same slab is directly cold-rolled, and the surface quality is improved by increasing the cold-rolling reduction ratio, but fine surface roping remains and is insufficient.

(Effects of the Invention) According to the present invention, a Cr-Ni-based stainless steel thin plate having excellent surface quality can be obtained by a simple process of directly obtaining a thin strip having a thickness close to the product thickness by continuous casting. it can.

[Brief description of drawings]

FIG. 1 shows that, in the process of the present invention, after pre-cold rolling (cold working) of a slab (thin strip) under the application of various reduction ratios, it was subjected to short-time annealing at 1080 ° C. The reduction rate when cold rolling (main cold rolling) to the final plate thickness after crystallization,
It is a figure which shows the relationship of the roping height in a product surface.

Claims (2)

[Claims] Claims: 1. A Cr-Ni type stainless steel represented by 18% Cr-8% Ni steel is cooled by a continuous casting machine in which the wall surface of the mold moves in synchronism with the slab and the cooling rate during solidification is 100 ° C / Continuously cast into strip-shaped slabs with a thickness of 10 mm or less for sec or more, start cooling the obtained slab from the highest possible temperature range and prevent reheating of the slab to 100 ° C up to 1100 ° C. Suppress the growth of γ grains by cooling at a cooling rate of sec or more, and then increase the temperature range from 900 to 550 ° C by 50
After cooling at a cooling rate of ℃ / sec or more, winding, then pickling without annealing, and then pre-cold rolling applying a rolling reduction of 60% or less, then a temperature range of 850 ° C or more Annealing and recrystallization to reduce the average grain size of γ grains to 50 μm or less, then pickling, and then cold rolling to the final thickness,
A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality, which comprises subjecting the obtained cold-rolled sheet to final annealing, followed by pickling or bright annealing. 2. Both cold rolling are performed by selecting and combining the reduction ratio of pre-cold rolling and the reduction ratio of cold rolling performed after recrystallization annealing so that the roping height becomes 0.3 μm or less. The method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality according to claim 1.