JPH0747778B2 - Method for producing Cr-Ni type stainless steel thin plate with excellent surface quality - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [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 of the mold. And a method for producing a Cr-Ni-based stainless steel thin plate.

[Conventional technology]

Conventionally, in order to produce a stainless steel thin plate using a continuous casting method, while casting the mold in a casting direction, cast into a slab having a thickness of 100 mm or more, and perform surface maintenance of the obtained slab,
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 huge 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 "CAMPISIJ" vo
The paper featured in l.1,1988,1670-1705 discloses a process for obtaining hot strip directly by continuous casting.

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 the 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.

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

(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.

The problem regarding the surface properties of these products is a peculiar problem that arises when a thin cast piece close to the final shape is produced from molten austenitic stainless steel and cold rolled without hot rolling.

The present inventors have studied in detail the causes of these problems regarding the surface properties, and as a result, the γ of the material before cold rolling was examined.
It was clarified that these surface defects occur when the grain size is larger than 50 μm 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
Cr that does not precipitate Cr-based carbides and provides good product surface properties
-Invented a method for producing a Ni-based stainless steel sheet.

That is, the temperature range from 1400 to 1200 ° C after the slab has solidified
We invented that it is effective to control the amount of delta ferrite while cooling at a cooling rate of 100 ° C./sec or more, and filed as Japanese Patent Application No. 63-169096 (Japanese Patent Application Laid-Open No. 2-133529).

However, it is difficult to uniformly quench the entire width of the slab,
In particular, it was difficult to rapidly cool a thick slab. In addition, when the amount of delta ferrite is specified, there are problems such as steel type restrictions.

There is also a method of adding a crystal grain refining element (Japanese Patent Application No. 1-84790 (Japanese Patent Application Laid-Open No. 2-263929)), but this leads to an increase in cost and an increase in the amount of inclusions.

The present invention relaxes the quenching conditions, eliminates the constraint due to the amount of delta ferrite, and without the addition of grain refinement elements,
An object of the present invention is to provide a method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality.

[Means for Solving the Problems]

According to the present invention, the above object is to move a Cr-Ni-based stainless steel molten steel having a composition of C + N of 0.090% or less and Md _{30 of 30} to 60 ° C in such a manner that a mold wall surface is synchronized with a slab. Thickness of 1 at a solidification cooling rate of 100 ℃ / sec or more by a continuous casting machine
Cast into thin strips of 0 mm or less, and after solidification, cool from the highest possible temperature to 1200 ° C at a cooling rate of 50 ° C / sec or more,
After cooling from ℃ to 550 ℃ at a cooling rate of 10 ℃ / sec or more, descaling, cold rolling, and final annealing are performed. Achieved by

As is generally used, Md ₃₀ is the temperature at which 50% or more of the structure becomes martensite when 30% cold working is performed.

The present inventors set Md ₃₀ to 30 ° C. or higher and generated a large amount of work-induced martensite during cold rolling, so that if the γ grain size was 100 μm or less, roping did not occur, and the cast piece after solidification The present invention has been completed by finding that the γ particle diameter becomes 100 μm or less when the cooling rate of 50 ° C./sec or more is obtained.

Md ₃₀ needs to be 30 ℃ or higher for the above reason, but if it is too high, the cold workability of the product sheet deteriorates.
Limit below ℃.

C + N is 0.090% or less to promote age cracking.

In the present invention, cooling from 1200 ° C after solidification to 50 ° C
The cooling rate is not less than / sec, and the γ particle size is controlled to 100 μm or less. Therefore, the earlier application (Japanese Patent Application No. 63-169096)
Since the cooling conditions are relaxed compared to 100 ° C./sec, the manufacturing can be performed more easily.

From 1200 ° C to 550 ° C, cooling is performed at a cooling rate of 10 ° C / sec or more to prevent precipitation of Cr-based carbides.

After cooling, descaling, cold rolling, and final annealing are performed as usual. After the final annealing, temper rolling according to the usual method is carried out if necessary.

〔Example〕

Example 1 Cr-Ni-based stainless steel (SUS304 steel) having the composition shown in Table 1 in which Md ₃₀ was changed to two levels was cast into a strip-shaped slab having a thickness of 2.4 mm by a twin roll type continuous casting machine, 1400 after solidification
℃ to 1200 ℃ at a cooling rate of 20 ~ 700 ℃ / sec to obtain various γ particle size, 1200 ℃ / sec to room temperature 20 ℃ / sec
It cooled at the cooling rate of. Then, pickling, cold rolling (total reduction of 50%), and final annealing were performed to obtain a thin plate product. After pickling in this state, the roping of the product surface was measured and an age cracking test was conducted.

As a result, according to the present invention, as shown in FIG.
For sample A with Md ₃₀ = 37.5 ° C (correct as in Table 1), if the γ particle size was 100 μm or less, the roping height was within the allowable limit (0.2 μm or less). ₃₀
In the comparative example (Sample B) at 23.2 ° C., the γ particle size needs to be 50 μm or less in order to set the roping height to 0.2 μm or less.

Example 2 A Cr-Ni stainless steel having the composition shown in Table 2 was cast into a strip-shaped slab having a thickness of 2 mm by a twin roll type continuous casting machine, and after solidification, it was 60 ° C / sec from 1400 ° C to 1200 ° C. Was cooled at a cooling rate of 1,200 ° C. to a conventional method at a cooling rate of 20 ° C./sec. Then, pickling, cold rolling (total reduction of 50%), and final annealing were performed to obtain a thin plate product. After pickling in this state, roping and age cracking of the product surface were observed.

As a result, as shown in Table 3, the present invention steel with a Md ₃₀ and 30 ° C. or more (samples 1-5) is roping was within the allowable range at γ grain size of 70~96μm, Md ₃₀ In Comparative Examples (Samples 6 and 7) in which the temperature was less than 30 ° C, roping failure occurred with the same γ particle size. Age cracking was good for all samples.

〔The invention's effect〕

As described above, according to the present invention, 1200 ° C from solidification
Since the occurrence of roping can be substantially prevented even if the cooling up to is relatively relaxed, a Cr-Ni-based stainless steel thin plate having excellent surface quality can be manufactured more easily.

[Brief description of drawings]

Figure 1 shows the relationship between the γ particle size and the roping height at two levels of Md.
It is a graph compared and shown about ₃₀ .

Claims (1)

[Claims] 1. C + N is 0.090% or less and Md ₃₀ is ₃₀ to 60.
The Cr-Ni system stainless steel molten steel having a composition of ℃ is used by a continuous casting machine in which the mold wall surface moves in synchronization with the slab.
Cast into thin strips with a thickness of 10 mm or less at a solidification cooling rate of 0 ° C / sec or more, and after solidification 50 ° C from the highest possible temperature to 1200 ° C
Cool at a cooling rate of at least / sec, from 1200 ° C to 550 ° C in 1
A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality, which comprises performing descaling, cold rolling, and final annealing after cooling at a cooling rate of 0 ° C / sec or more.