JPH0796684B2 - 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 so that the slab thickness is close to the product thickness. As a size, it relates to 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 scales and flaws are pickled and removed by grinding.

In the conventional process, a large hot rolling facility requires a large amount of energy to heat and enable the material, 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, a large amount of energy, and rolling power 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 ~ 'A256 and "CAMP ISIJ"
A paper featured in vol.1, 1988, 1670-1705 discloses a process for directly obtaining hot strip by continuous casting. In such a continuous casting process, the gauge of the slab (strip) to be obtained is 1
The twin drum system is considered when the level is ~ 10 mm, and the twin belt system is considered when the gauge of the slab is 20 to 50 mm.

(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
Describes the material problem of Cr-Ni system stainless steel sheet and the ridging phenomenon of Cr system stainless steel sheet. 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. Of course, if cracks are present in the slab, even if it is extremely fine, it remains in the product, so it is a necessary condition that the slab does not crack.

In the experiment, molten steel containing SUS 304 steel as a basic component was cast by an internal water-cooled twin-roll type 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, and after hot heating, it is hot-rolled by a hot strip mill into a thin strip having a thickness of 3 to 6 mm and 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
It has been found that the following surface defects occur in the.

(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.

(3) Fine cracks ... Scattered on the product surface.

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 casting a thin-walled slab close to the final shape from austenitic stainless molten steel and cold rolling, and 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 was clarified that these surface defects occur when the thickness is larger than the above, when the thin cast piece is insufficiently cooled in the temperature range where the Cr-based carbide is precipitated, and when the crack of the cast piece remains.

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
The present invention has invented a method for producing a Cr-Ni-based stainless steel thin plate that does not precipitate Cr-based carbides and prevents cracking of cast slabs to obtain good surface properties.

For example, after solidification, cooling to 1200 ° C. at a cooling rate of 100 ° C./sec or more and adjusting the components to adjust δ-Fe _cal to −2 to 10% [Japanese Patent Application No. 63-221471 (JP-A-2-133528). Gazette)], and 0.01 to 1 mol% of a crystal grain refining element is added.

However, the cooling condition from 1400 to 1200 ℃ is 100 ℃ / sec.
As above, cooling is started from the highest temperature immediately after solidification and the growth of γ grains is suppressed.Therefore, when the slab plate thickness is large or the plate width is wide, it is possible to obtain sufficient uniform cooling in terms of equipment. Industrially difficult.

Therefore, the present invention, by controlling the cooling of the cast slab and adjusting the composition utilizing δ-Fe, reduce the γ grain size and utilize the cooling process and the temper rolling process after the final annealing, and it is stable and excellent. (EN) A method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality in which molten steel components, particularly harmful impurity components are limited in order to obtain excellent surface quality and prevent fine cracking of cast slabs.

(Means for Solving the Problems) The gist of the present invention is C: 0.1% or less by weight%, Si: 1.0% or less, Mn: 3% or less, Cr: 18-21%, Ni: 6-11%, P: 0.05%
Below, S: 0.006% or less, B: 0.0020% or less, O: 0.010% or less, N: 0.30% or less, and the balance is substantially Fe and unavoidable impurities, and δ−Fe _cal (%) = 3 (Cr + 1 .5Si) -2.8 (N
i + 0.5Mn) -84 (C + N) -19 defined as δ-Fe _cal
Molten steel of Cr-Ni stainless steel with (%) of -2 to 10% was continuously cast into a strip-shaped cast piece having a thickness of 10 mm or less by a continuous casting machine in which the mold wall surface moves in synchronization with the cast piece. , Starting the cooling of the obtained slab from the highest possible temperature below the solidification temperature, while suppressing the recuperation of the slab at a cooling rate of 50 ℃ / sec or more
Cool to 1200 ° C to suppress the growth of γ grains in the slab, then cool the temperature range from 1200 ° C to 600 ° C at a cooling rate of 10 ° C / sec or higher and wind up, pickling the slab. After that, hot rolling,
One or two types of unrolling rolling are applied and annealed / pickled or bright annealed, and the elongation of the rolling is controlled in the range of 0.3 to 2.5% in the temper rolling process to improve the gloss of the surface and improve the surface irregularities. A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality, which is characterized by reducing roping and preventing fine cracks and flaws.

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

In thin-wall continuous casting, the composition is adjusted so that δ-Fe _cal is -2 to 10%, and the γ grains are refined by setting the cooling rate from solidification of the cast to 1200 ° C to 100 ° C / sec or more. Is extremely effective. However, industrially, whether or not cooling equipment can be adequately and uniformly cooled in response to fluctuations in plate thickness and plate width depends on whether or not appropriate cooling equipment can be developed. A technology that reduces the required degree of cooling is desired from the viewpoint of equipment.

The present inventors have already studied the roping reduction in detail, and in combination with the technology for refining the γ grains during the production of the slab described above,
As a result of a detailed study in order to improve the surface quality in the cooling step, it was found that, in addition to the means previously filed, temper rolling at a predetermined rolling reduction after the final annealing significantly improves roping.

In this way, it was revealed that there are many possibilities for surface quality, which is a new issue of SUS 304 system by twin roll casting / direct cooling method which is a new process, especially as a countermeasure for roping. In order to ensure stable and excellent surface quality, it was necessary to combine these improvements.

That is, the reason for limiting each manufacturing condition is as follows.

The reason why δ-Fe _cal (%) is set to -2 or more is that the γ grains are made fine by the δ / γ transformation and the effect of suppressing the growth of γ grains by δ ferrite. If δ−F _cal is less than −2, γ single phase solidification occurs and γ grains are coarsened. The reason why the content is 10% or less is that the δ-ferrite is stabilized and the amount of δ-ferrite remaining in the product increases as the δ-Fe _cal increases. If it is 10% or less, δ ferrite hardly remains in the product.

The reason for setting the cooling rate of the slab to 1200 ° C to 50 ° C or higher is
This is because the cooling rate is the limit at which a thick slab having a slab thickness of about 10 mm can be cooled industrially stably without using water-water cooling and can be uniformly cooled over the entire width. If the cooling rate is slow, the slab γ grains grow more, which is not desirable. Although faster cooling rate is possible by using steam cooling, steam cooling immediately after casting of thin wall continuous casting is not desirable for safety.

The reason for setting the cooling rate from 1200 ° C to 600 ° C to 10 ° C / sec or more is that if slow cooling is performed during this period, Cr carbide precipitates and the surface properties after pickling deteriorate, so the purpose of preventing precipitation of Cr carbide Set the lower limit to the cooling rate.

The reason why the elongation of the temper rolling is set to 0.3 to 1.9% is to correct the roping generated in the cold rolled sheet. Raising the elongation of temper rolling reduces roping, and the effect saturates when the elongation exceeds 1.9%. When δ-Fe _cal is set to -2 to 10% and the cooling rate up to 1200 ° C is set to 50 ° C / sec or more, in order to correct the roping that occurs in the cold-rolled sheet, the elongation rate must be at least 0.3% or more. Since temper rolling is required, the lower limit was made 0.3%. However, the upper limit was set to 1.9% because the effect of improving roping saturates even if the elongation rate was made larger than 1.9%.

Next, the fine cracks in the slab will be described.

Regarding the cracking of the slab during solidification, the effect of the components in the 18Cr-8Ni system was examined by the method of applying a thermal cycle of melt solidification and conducting a tensile test. As a result, the components that deteriorate the ductility immediately after solidification in the 18Cr-8Ni system are S, B, Si as shown in FIG.
Met. This test method has a cooling rate of 100 ° C / sec during solidification.
Although it does not directly correspond to the crack limit in thin-wall casting in which the cooling rate during solidification is fast, it is considered that there is a similar tendency in thin-wall casting.

The reason why S is 0.006% or less, B is 0.002% or less, and Si is 1.0% or less is that fine cracks did not occur in the thin cast piece in this range.

In addition, C, Mn, P, O, and N are set within the range normally added in Cr-Ni system stainless steel.

(Examples) Austenitic stainless steel with various components based on 18Cr-8Ni steel shown in Table 1 was melted. Especially, S, B, and Si were lowered for the purpose of preventing cracking.

Molten steel with δ-Fe _cal (%) in the range of -2 to 10% was continuously cast into a slab 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. The slab solidified from the outlet of the casting machine was rapidly cooled by pressing it against a water-cooled roll. In this case, the average cooling rate up to 1200 ° C was 70 to 300 ° C / sec or more even in the minimum part.

It was cooled at 1200 to 600 ° C at 10 ° C / sec or more and wound up at 600 ° C or less. After that, these slabs were descaled as usual,
Cold rolling was carried out as usual. The cold rolling reduction is 70%,
After that, these cold-rolled sheets were brightly annealed at 1000 ° C. or higher and finished to 0.3 to 2.0 mm as usual.

Then, the elongation of temper rolling was changed to 0.3 to 1.9% and the surface quality was examined.

The results are shown in Table 1.

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

As a comparative material, a slab containing a large amount of S, B, and Si had excellent roping of the product surface quality and had no gloss unevenness, but small cracks were scattered.

(Effects of the Invention) According to the present invention, a thin strip-shaped slab having a thickness close to the product thickness is obtained by continuous casting, and the structure of the slab is refined and refined by a simple process of directly commercializing by cold rolling. It is possible to obtain an austenitic stainless steel thin plate having an excellent surface shape by selecting the conditions for quality rolling.

[Brief description of drawings]

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

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/54

Claims (1)

[Claims] [Claim 1] C: 0.1% or less, Si: 1.0% or less, Mn: 3% or less, Cr: 18-21%, Ni: 6-11%, P: 0.05% or less, S: 0.006% by weight Below, B: 0.0020% or less, O: 0.010% or less, N: 0.30% or less, and the balance consists essentially of Fe and inevitable impurities.
_cal (%) = 3 (Cr + 1.5Si) -2.8 (Ni + 0.5Mn) -84
Δ-Fe _cal (%) defined by (C + N) -19 is -2 to 1
Using a continuous casting machine in which the wall surface of the mold moves in synchronism with the slab, the molten steel of 0% of Cr-Ni system stainless steel is 10 mm thick.
Continuous casting into the following strip-shaped slab, start cooling the obtained slab from the highest possible temperature below the solidification temperature, while suppressing the reheat of the slab at a cooling rate of 50 ℃ / sec or more Cool to 1200 ℃ to suppress the growth of γ grains in the slab, and then increase the temperature from 1200 ℃ to 600 ℃.
The temperature range up to ℃ is cooled at a cooling rate of 10 ℃ / sec or more and wound up. After pickling the cast slab, one or two kinds of hot rolling and unwinding rolling are applied to annealing, pickling or brightening. It is characterized by controlling the elongation of rolling in the range of 0.3 to 1.9% in the annealing and temper rolling process, improving the gloss of the surface and improving the surface irregularities to reduce roping and prevent fine cracks and flaws. A method for manufacturing Cr-Ni stainless steel thin plates with excellent surface quality.