JPH0219426A - Manufacture of cr-ni stainless steel sheet having excellent quality and surface property - Google Patents

Abstract

PURPOSE:To obtain the title sheet at the time of manufacturing a Cr-Ni stainless steel sheet by subjecting it to continuous casting, cooling, coiling. pre-cold rolling and annealing under specific conditions. CONSTITUTION:A Cr-Ni stainless steel represented by an 18% Cr-8% Ni steel is subjected to continuous casting by synchronizing and moving the wall face of a mold to a cast slab and the solidifycation cooling speed is regulated to >=100 deg.C/sec to form a sheet-shaped cast slab having <=10mm thickness. The cooling of the cast slab is started preferably from the high temp. range; the cast slab is cooled at >=100 deg.C/sec cooling speed by 1,100 deg.C while preventing the recuperation in the cast slab to suppress the gamma growth, is cooled at >=50 deg.C/sec cooling speed in the temp. range of 900 to 550 deg.C and is coiled at <=650 deg.C. The cast slab is then pickled without annealed, is subjected to pre-cold rolling at <=60% draft, is annealed in the temp. range of >=850 deg.C, is recrystallized to regulated the average grain size of the gamma grains to <=50mm is thereafter pickled. The cast slab is furthermore subjected to cold rolling into the final plate thickness; after that, the cold rolled plate is subjected to final annealing and is subjected to pickling or bright annealing.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention uses 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, so that the thickness of the slab can be reduced to a size close to the product thickness. The present invention relates to a method for manufacturing a Cr-Ni stainless steel thin plate having excellent surface properties by refining the structure from the slab stage.

(Prior art) Conventionally, in order to manufacture thin stainless steel sheets using the continuous casting method, a slab of thickness 100M or more is cast while the mold is vibrated in the casting direction, and the surface of the resulting slab is treated. conduct,
After heating to 1000° C. or higher in a heating furnace, hot rolling was performed in a hot strip mill consisting of a row of rough rolling mills and a row of finishing rolling mills to form a hot strip with several thicknesses.

When cold rolling the hot strip obtained in this way, the shape (flatness), material, and
In order to secure the surface quality, hot-rolled sheets were annealed to soften the hot strips that had undergone intense hot working, and scales and the like on the surface were removed by grinding after the pickling process.

This conventional process requires a large amount of energy to heat and process the material in a long hot rolling facility, and cannot be said to be an excellent manufacturing process in terms of productivity. In addition, because the final product is manufactured from a cast slab with a thickness of 100 plates or more and subjected to many processes, the texture develops, and when the user applies press processing etc. to the product, its anisotropy There were many restrictions on use, such as the need to take into consideration the following.

Recently, in order to solve the problem that rolling slabs with a thickness of 100 m or more into hot strips requires a long hot rolling equipment and a large amount of energy and rolling power, a continuous casting process has been recently developed. Research is underway on a process to obtain slabs (thin strips) with a thickness equal to or close to that of hot strip.

For example, "Tetsu to Hagane" 85, A197~°85, A25
The article featured in No. 6 discloses a process for obtaining hot strip directly by continuous casting.

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

However, in these continuous casting processes, there are still problems at the casting stage, and the problems regarding the material and surface quality of the product have not yet been resolved.

(Problem B to be solved by the invention) In a process that is being developed as a new process and is based on the premise of obtaining a slab (thin strip) with a thickness equal to or close to that of hot strip by continuous casting. Because the process from casting to products is simplified,
The surface properties of stainless steel products are sensitively affected by the properties of the slab. That is, in order to obtain a product with excellent surface properties, it is necessary to obtain an excellent slab.

The present invention has been made with the object of providing a simple manufacturing process capable of obtaining an austenitic stainless steel thin plate free of surface defects called gloss unevenness and roping phenomenon that are characteristic of stainless steel thin plate products.

(Means for Solving the Problems) The gist of the present invention is to cast austenitic stainless steel represented by 18% Cr-8% Ni1il during solidification using a continuous casting machine in which the mold wall surface moves in synchronization with the slab. Continuously cast into thin strip slabs with a thickness of 10 mm or less at a cooling rate of 100 ° C / sec or more, and start cooling the obtained slab from the high temperature range to prevent reheating of the slab. Tsutsu 1100
℃ at a cooling rate of 100℃/sec or more to suppress the growth of γ grains, and then cooled in the temperature range of 900 to 550℃ for 50℃.
After cooling at a cooling rate of ℃/sec or more, 650"C
It is rolled up in the following temperature range, then pickled without annealing, then preliminary cold rolled with a rolling reduction of 60% or less, and then annealed in a temperature range of 850°C or higher and recrystallized. After the average grain size of each grain is reduced to 50 μm or less, the sheet is pickled, then cold rolled to give the final thickness, the obtained cold rolled sheet is subjected to final annealing, and then pickled or bright annealed. The present invention provides a method for producing a thin Cr-Ni stainless steel plate with excellent surface quality and material quality.

The present invention will be explained in detail below.

Molten steel containing 5US304 steel as a basic component was cast into a thin ribbon with a thickness of 2 to 4 mm using an internal water-cooled twin-roll (twin-drum) continuous casting machine, which was cooled and wound up.

The thus obtained slab (thin ribbon) was descaled, directly cold rolled, finally annealed, and pickled to obtain a 2B product. The surface quality of these products can be improved by conventional continuous casting of molten steel to obtain slabs with a thickness of 100 mm or more, which are then reheated, hot rolled in a hot strip mill, and then cold rolled. A detailed comparison was made with the surface properties of the products.

As a result, the molten steel was cast into a thin strip with a thickness of 2 to 4 mm using an internal water-cooled twin-roll (twin-drum) continuous casting machine, which was cooled and wound, which was then descaled and cold-rolled. It has been found that the following surface defects may occur in the final annealed and pickled 2B product.

(1) Roping and orange peel: fine irregularities occur on the surface during cold rolling or product processing.

(2) Unevenness in gloss: Unevenness in gloss occurs due to the sensitization of the structure of the material during winding of the slab (thin ribbon), grain boundary oxidation, or coarsening of γ grains.

Problems regarding the surface properties of these products are unique to the process, which does not occur in conventional processes, and involves the process of obtaining thin slabs (thin strips) directly by continuous casting.

As a result of a detailed study of the causes of these surface defects, the inventors found that these defects occur when the γ grain size of the material before cold rolling is large or when the slab is insufficiently cooled in the Cr carbide precipitation temperature range. It was revealed that surface defects occur significantly.

Thus, as a countermeasure against roping, it is necessary to make the γ grain size of the material before cold rolling to a grain size of Nα6 or more, that is, 50 μm or less, and as a countermeasure against gloss unevenness, it is necessary to control the cooling of the slab in the high temperature range. It has been shown that it is desirable to employ a process that involves obtaining slabs directly by continuous casting.

Below, these measures will be explained in detail.

There are various ideas as follows as means for producing a material for cold rolling with a γ grain size of 50 μm or less. That is, 1) reducing the size of each grain of the thin slab itself, 2) hot working the thin slab after casting to recrystallize it into fine grains, 3) cold working the thin slab, It is annealed and recrystallized to make the grain finer.

The present invention particularly aims to produce stainless steel by combining the above-mentioned methods (1) reducing the γ grains of the slab itself and (3) cold working the slab, annealing and recrystallization, and refining the grains. The present invention relates to a method for manufacturing stainless steel that prevents roping from occurring on the surface and has excellent surface quality without uneven gloss.

First, when obtaining a thin slab using a continuous casting process such as the twin roll method, in which the mold wall moves in synchronization with the slab,
In order to reduce the size of each grain of a thin slab, it is important to reduce the size of the γ grains from the time they are generated in the slab, and to cool the grains from a high temperature range in order to suppress their subsequent growth. It is also extremely effective to subject these slabs to cold working and annealing them to recrystallize and refine the grains. Based on the above idea, the present inventors have developed 18C of various compositions.
R-8Ni steel was cast using a small twin roll in a laboratory, and the relationship with the surface properties of the product was investigated by changing the conditions of quenching immediately after casting, cold rolling, and annealing, and by changing the grain size of the slab before main cold rolling. was clarified.

In the process of casting molten steel into a thin strip (thin slab) using a type of continuous casting machine in which the mold wall surface moves in synchronization with the slab, such as a twin-roll continuous casting machine, as a means to reduce the γ grains in the slab, In the present invention, casting is performed at a cooling rate of 100°C/sec or more during solidification of the slab,
For the obtained slab (strip) with a thickness of 10 mm or less, cooling was started from the highest possible temperature immediately below the casting machine in order to prevent heat recovery of the slab at the exit of the continuous casting machine.
℃ at a cooling rate of 100℃/sec or more,
It is essential to inhibit grain growth.

Furthermore, the temperature range from 900 to 550℃ is controlled at '50℃/sec.
It is necessary to cool the slab at the above average cooling rate and to coil it in a temperature range of 650° C. or lower. Otherwise, carbides will precipitate at the grain boundaries of the slab, leading to intergranular corrosion when the strip is pickled, which will impair the gloss of the final product.

By the means described above, the growth of γ grains in the slab is suppressed, and furthermore, the precipitation of carbides at the 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 finer.

Figure 1 shows the main cold rolling reduction ratio when performing preliminary cold rolling on a slab, short-time annealing at 1 oso"c, and then cold rolling to the final plate thickness (main cold rolling). The relationship between roping height and product roping height is shown for each rolling reduction level in preliminary cold rolling.

When the gamma grains of the slab are fine, for example when continuously casting thin slabs such as 2 casts, and directly under the casting machine, 13
Cooling of the slab in the temperature range of 00 to 1100"C
If the rolling strength is 0°C/sec or more, recrystallization will be sufficient even if the reduction rate in preliminary cold rolling is as low as 10% or more, and the average γ grain size can be refined to 50 chains or less. The roping height can be lowered.

On the other hand, when the grain size of the slab is large, for example, the slab with a thickness of 4 mm is placed directly under the casting machine using a cooling means with a low cooling rate.
For example, if one grain of a slab grows due to air cooling, the average γ grain size of the material can be reduced to 50% by rolling with a high reduction rate of 50% or more in preliminary cold rolling and then annealing.
Fine particles of less than 0 μm can be obtained, thus improving the roping of the product.

As mentioned above, the thickness of the slab directly under the casting machine is 1300 to 11
The higher the cooling rate of 100°C/sec or more in the 00°C temperature range, the more difficult the preliminary cold working (rolling, etc.) will be.
It is possible to recrystallize even at a low rolling reduction rate, and the average γ grain size after recrystallization can be made finer to 50 μm or less, so the roping of the product can be reduced and a product with excellent surface properties without uneven gloss can be obtained. be able to.

EXAMPLE A Cr-Ni stainless steel, typically 18Cr8Ni, produced by a conventional method was cast by an internal water-cooled twin drum method to obtain slabs of 3 and 4.5 cylinders. In addition to air cooling, spray cooling is performed directly below the exit of the twin drum casting machine, with an average cooling rate of 100°C/s up to 1100°C.
Aim for cooling above EC, further water cooling below 900°C, and cooling at an average rate of 70°C/sec or above up to 550°C.
It was wound up between 50 and 600°C.

It was.

Thereafter, the slab was descaled by mechanical descaling and pickling, and preliminarily cold rolled by cold rolling. 3 Cabinet Atsushi,
Both slabs with a thickness of 4.5 mm were preliminarily cold rolled in the range of 10 to 40%, annealed at 1000"C or higher for 20 seconds or less, and then rapidly cooled. In this way, the slabs were recrystallized and the γ grains were formed. diameter to 5
The thickness was set to 0 μm or less.

After that, 30%, 50%, 80% in main cold rolling.

Cold rolling of 95% and 95% or more was carried out, and final annealing was carried out in a conventional manner to prepare 2B and BA. Both the surface and mechanical properties of these products were very good.

In the comparative method, the same slab is directly cold rolled, and although the surface quality is improved by increasing the cold rolling reduction, fine surface roping remains and is unsatisfactory.

ratio comparison law It is a figure showing the relationship of the roping height.

(Effects of the Invention) According to the present invention, it is possible to obtain an austenitic stainless steel thin plate with excellent surface texture and material quality through a simple process of directly obtaining a thin strip with a thickness close to the product thickness by continuous casting. can.

[Brief explanation of the drawing]

Claims (2)

[Claims] (1) Austenitic stainless steel, such as 18% Cr-8% Ni steel, is cooled at a cooling rate of 100°C during solidification using a continuous casting machine in which the mold wall surface moves in synchronization with the slab.
/sec or more to continuously cast into a thin strip slab with a thickness of 10 mm or less, start cooling the obtained slab from the high temperature range as much as possible, and heat the slab at 100 °C to 1100 °C while preventing reheating of the slab.
Suppress the growth of γ grains by cooling at a cooling rate of /sec or more,
Next, after cooling in a temperature range of 900 to 550°C at a cooling rate of 50°C/sec or more, it is wound up in a temperature range of 650°C or less, and then pickled without annealing, and then 60%
Preliminary cold rolling is performed using the following rolling reduction ratio, and then annealing and recrystallization are carried out in a temperature range of 850°C or higher to reduce the average grain size of the γ grains to 50 μm or less, followed by pickling, and then cold rolling. A method for manufacturing a Cr--Ni stainless steel thin plate with excellent surface quality and material quality, which comprises adjusting the final plate thickness, subjecting the obtained cold-rolled plate to final annealing, and then subjecting it to pickling or bright annealing. (2) Select and combine the rolling reduction ratio of preliminary cold rolling and the rolling ratio of cold rolling performed after recrystallization annealing so that the roping height shown in Fig. 1 is 0.3 μm or less, and perform both cold rolling. 2. The method of manufacturing a Cr--Ni stainless steel thin plate with excellent surface quality and material quality according to claim 1.