JPH02267225A - Production of cr-ni stainless steel sheet excellent in surface quality - Google Patents

Abstract

PURPOSE:To refine the structure in the stage of cast slab and to improve surface characteristics by subjecting a cast strip-like slab prepared by continuously casting a Cr-Ni stainless steel and having a thickness close to product thickness to rolling, etc., under prescribed conditions. CONSTITUTION:A Cr-Ni stainless steel having a composition consisting of, by weight, <=0.1% C, <=1% Si, <=3% Mn, <=0.005% P, <=0.006% S, <=0.002% B, <=0.01% O, <=0.3% N, and the balance Fe is refined. This molten steel is continuously cast into a cast strip-like slab of <=10mm thickness. Subsequently, this cast slab is cooled down to 1200 deg.C at <=50 deg.C/sec cooling rate, by which the growth of gamma-grains in the cast slab is inhibited. Then, cooling is applied to the above cast slab at >=10 deg.C/sec cooling rate from 1200 deg.C down to 600 deg.C. After pickling, the above cast slab is subjected to hot rolling, to cold rolling, and to bright annealing, followed by temper rolling at 0.3-2.5% elongation. By this method, surface luster can be improved and also ruggedness can be improved and roping can be reduced, and further, fine cracking can be prevented.

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 of the mold, so that the thickness of the slab can be made close to the product thickness. The present invention relates to a method for producing a Cr-Ni stainless steel thin plate as steel.

(Prior art) Conventionally, in order to manufacture stainless steel thin plates using the continuous casting method, the mold is vibrated in the casting direction and cast into a slab with a thickness of 100 mm or more, and the surface of the obtained slab is treated. 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 hot strips of several thicknesses.

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

In the conventional process, a large amount of energy is required to heat and process the material using 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 had a developed texture, and the user had to take into account its anisotropy when applying press processing, etc., and there were many restrictions on use.

Therefore, in order to solve the problem that rolling slabs with a thickness of 100 mm or more into hot slips requires a long hot rolling equipment and a large amount of energy and rolling power, we have recently developed a continuous casting process. 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 J '85. A197~'A25
G and rcAMP l5IJJ vo, Q, l, 198
8.1 [470-1705]
A process for obtaining hot strip directly by continuous casting is disclosed.

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 to be obtained is 20 mm.
At the level of ~50 mm, a twin belt system is being considered.

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

That is, the aforementioned rcAMP ISI month vof1.1.
1988, 1870-1705, the material problems of Cr--Ni stainless steel thin plates and the phenomenon of Cr-based stainless steel thin plate sliding are discussed. However, no particular problem has been raised regarding the surface quality of the Cr--Ni stainless steel thin plate.

As a result of detailed research by the present inventors on the manufacturing process of Cr-Ni stainless steel thin sheets by continuous strip casting, we found that surface defects called roping and uneven gloss occur in the products as specifically shown below. There was found. Of course, if there are any cracks in the slab, even if they are extremely minute, they will remain in the product, so it is a necessary condition that there be no cracks in the slab.

In an experiment, molten steel whose basic component was 5US304 steel,
It was cast using a twin-roll continuous casting machine with internal water cooling and wound into a ribbon having a thickness of 1 to 4 mm. The thus obtained slab (thin ribbon) was descaled, directly cold rolled, finally annealed, and pickled to obtain a product A having a thickness of 1 to 0.4 m+s.

On the other hand, conventional molten steel is continuously cast into a slab with a thickness of 100 mm or more, which is then reheated and hot rolled into a thin strip with a thickness of 3 to 6 m11 using a hot strip mill, which is then cooled and rolled. After descaling, the sample is cold rolled, final annealed, and pickled to produce product B with a thickness of 1 to 4 mm.
And so.

Comparing the surface properties of product A and product B, product A
It was found that the following surface defects occur.

O) Roping: Fine irregularities are created on the surface during cold rolling.

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

(3) ? Fine cracks...scattered on the product surface.

On the other hand, product B has no such defects.

Therefore, issues regarding the surface properties of these products are
This is a unique problem that occurs when thin-walled slabs close to the final shape are cast and cold-rolled from molten austenitic stainless steel, and is an essential drawback of N, N, and S casting.

As a result of a detailed study of the causes of these surface quality problems, the inventors found that the γ grains of the material before cold rolling were 50 μm.
It has been found that these surface defects occur when the thickness of the slab is larger than s, when the cooling of the thin slab is insufficient in the temperature range where Cr-based carbides precipitate, and when cracks in the slab remain.

In order to prevent these surface defects, improvements were made to the molten steel composition and cooling conditions during the process of solidifying and cooling the molten steel, so that the average γ grain size before cold rolling was 50 tlIM or less, and Cr-based carbides were precipitated. We have invented a method for manufacturing Cr--Ni stainless steel thin plates that prevents cracking of cast slabs and provides good surface quality.

For example, by cooling to 1200°C after solidification at a cooling rate of 100°C/sec or more and adjusting the components, δ-F e o
,, a method in which p is -2 to 10% (Japanese Patent Application No. 632214)
No. 71), and furthermore, a method of adding 0.01 to 1 mol % of grain refining elements.

However, the cooling conditions from 1400 to 1200℃ are
Cooling starts at the highest possible temperature immediately after solidification, at 00°C/sec or higher, to suppress the growth of γ grains, so when the slab is thick or wide, the equipment allows for sufficient uniform cooling. is industrially difficult to obtain.

Therefore, the present invention aims to reduce the γ grain size by controlling the cooling of the slab and adjusting the composition using δFc, and by utilizing the cooling process and the skin pass rolling process after final annealing, a stable and excellent surface can be achieved. The present invention provides a method for producing a thin Cr--Ni stainless steel sheet with excellent surface quality by limiting molten steel components, especially harmful impurity components, in order to obtain high quality and prevent microcracks in slabs.

(Means for Solving the Problems) The gist of the present invention is that C: 0.1% or less in weight%, sJ: 1
．． 0% or less, Mn: 3% or less, P: 0.05
% or less, s: 0.006% or less, B: 0.002
0% or less, 0:O, 010% or less, N: 0.30
%, the remainder consists essentially of Fe and unavoidable impurities, reducing the cracking susceptibility during solidification of Cr-Ni stainless steel,
)-2,8(NI+0.5Mn)-84(C+N)-
δ-Pe defined in 19, (%) -2 to 10%
The molten steel is continuously cast into thin strip slabs with a thickness of 10 mm or less using a continuous casting machine in which the mold wall moves in synchronization with the slab, and the resulting slab is heated to as high a temperature as possible below the solidification temperature. Start cooling to 50℃/SC while suppressing reheating of the slab.
By cooling the slab to 1200℃ at a cooling rate of C or higher, the γ
Grain growth is suppressed, and then the slab is cooled at a cooling rate of 10°C/sec or more in a temperature range of 1200°C to 600°C and rolled up. After pickling, the slab is subjected to two steps of warm rolling and cold rolling. The first or second type is subjected to annealing, pickling, or bright annealing, and the rolling elongation rate is controlled in the range of 0.3 to 2.5% in the temper rolling process to improve surface gloss and surface unevenness. This is a method for manufacturing a Cr--Ni stainless steel thin plate with excellent surface quality, which is characterized by reducing roping and preventing minute cracks.

(for production) The present invention will be explained in detail below.

In thin wall continuous casting, it is extremely effective to refine the γ grains by increasing the cooling rate from the solidification of the slab to 1200°C to 100°C/sec or more. However, in the industry, whether or not it is possible to sufficiently uniformly cool the plate in response to changes in plate thickness and width depends on whether or not appropriate cooling equipment can be developed. There is. From an equipment standpoint, a technology that reduces the degree of required cooling is desired.

The present inventors have already studied the roping phenomenon in detail, and combined it with the already mentioned technology of refining γ grains during slab production.
As a result of a detailed study to improve the surface quality in the cold rolling process, in addition to the previously applied measures, we found that the improvement in roping was noticeable in the hardening of the cold rolling rolls and the J3 quality rolling process after final annealing. It turns out that there is something.

In this way, many possibilities for surface quality, especially roping countermeasures and L2, which are new issues for the SUS 304 series using the new process of twin-roll casting and direct cold rolling, were found, but the casting plate thickness and plate width In order to ensure stable and excellent surface quality, including variations in surface quality, it was necessary to combine these improvement effects.

Next, we will discuss microcracks in slabs.

Regarding cracking during solidification of slabs, the effect of component elements in the 18cr-8Nl system was investigated by applying a thermal cycle of melting and solidification and performing a tensile test.

As a result, in the 18cr-8Ni system, the components that degrade the ductility immediately after solidification are S and B, followed by St and Ni, which also degrade the ductility in large amounts.

As shown in Figure 1, S is 0.006% or less, B is 0.006% or less, and B is 0.006% or less.
0.002% or less, and preferably 0.8% or less for Si.

(Example) Austenitic stainless steels of various compositions based on 18cr-8Nl steel shown in Table 1 were melted. Especially S
, B, and Sl were made low for the purpose of preventing cracking.

Molten steel with δ-F e call) (%) in the range of -2 to 10% is cast into slabs with a width of 1000 mm and a thickness of 1.0 to 6.0 mm using an internal water-cooled twin roll casting machine. Continuous casting was performed, and the solidified slab was rapidly cooled from the outlet of a twin-roll casting machine by pressing it against water-cooled rolls. In this case 12
As for the uniform cooling rate of jP up to 00°C, even the smallest part should be cooled at 10°C/rec or more between 1200 and 600°C,
It was wound up at a temperature below 00°C. Thereafter, these slabs were descaled in a conventional manner and cold rolled in a conventional manner.

The rolling reduction ratio in cold rolling was 70%, and then these cold-rolled sheets were brightly annealed at 1000°C or higher as usual to achieve a rolling reduction of 0.3 to 2.0.
I immediately went up one rank.

Thereafter, the elongation rate during temper rolling was changed to 0.3 to 9%, and the surface quality was examined.

These results are shown in Table 1.

In the case of the method of the present invention, the surface quality of the product was excellent in roping, and there was no uneven gloss and no microcracks were observed.

As comparative materials, slabs containing a large amount of S, B, and Sl have excellent product surface quality with roping and no uneven gloss. Through a simple process of casting and direct cold rolling, it is possible to refine the structure at the slab stage and select the temper rolling conditions to obtain austenitic stainless steel sheets with excellent surface properties. .

[Brief explanation of drawings]

FIG. 1 is a chart showing the relationship between tensile test temperature and cross-sectional shrinkage rate.

Claims (1)

[Claims] In weight percent, C: 0.1% or less, Si: 1.0% or less, Mn: 3% or less, P: 0.05% or less, S: 0.006% or less, B: 0 .0020% or less, 0: 0.010% or less, N: 0.30% or less, the remainder being Cr-N consisting essentially of Fe and unavoidable impurities.
By reducing the cracking susceptibility of i-series stainless steel during solidification, δ
−Fe_c_a_l(%)=3(Cr+1.5Si)−
2.8(Ni+0.5Mn)-84(C+N)-19 δ-Fe_c_a_l (%) is -2 to 10%
The molten steel is continuously cast into thin strip slabs with a thickness of 10 mm or less using a continuous casting machine in which the mold wall moves in synchronization with the slab, and the resulting slab is heated to as high a temperature as possible below the solidification temperature. Start cooling to 50℃/se while suppressing reheating of the slab.
By cooling to 1200℃ at a cooling rate of c or more, the γ of the slab is
Grain growth is suppressed, and then the slab is cooled at a cooling rate of 10°C/sec or more in a temperature range of 1200°C to 600°C and rolled up. After pickling, the slab is subjected to two steps of warm rolling and cold rolling. The first or second type is subjected to annealing, pickling, or bright annealing, and the rolling elongation rate is controlled in the range of 0.3 to 2.5% in the temper rolling process to improve surface gloss and surface unevenness. A method for manufacturing a Cr-Ni stainless steel thin plate with excellent surface quality, characterized by reducing roping and preventing minute cracks.