JPH0342151A - Production of cr-ni stainless steel sheet having excellent surface quality - Google Patents

Abstract

PURPOSE:To obtain the sheet having excellent surface quality b casting a molten Cr-Ni stainless steel having a specific compsn. to a thin strip-like ingot of a specific thickness at a specific solidifying and cooling rate, cooling the ingot at a specific cooling rate after the solidification, and then cooling the ingot at a specific cooling rate down to a specific temp. range. CONSTITUTION:The molten Cr-Ni stainless steel having the compsn. contg. <=0.090% C+N and having 30 to 60 deg.C Md30 is cast by a continuous casting machine in which the wall surfaces of a casting mold move in synchronization with the ingot. Namely, the molten steel is cast to the thin strip-like ingot of <=10mm thickness at >=100 deg.C/sec solidification cooling rate. The ingot is cooled from the highest possible temp. from below the solidification temp. at >=50 deg.C/sec cooling rate down to 1,200 deg.C and at >= 10 deg.C/sec down to 1,200 to 550 deg.C, following which the ingot is subjected to descaling, cold rolling and final annealing. The Cr-Ni stainless steel sheet having the excellent surface quality is obtd. in this way.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention produces a cast 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. A piece is cast and made of Cr-Ni stainless steel tFJ.
The present invention relates to a method of manufacturing a Wi board.

[Conventional technology]

Conventionally, in order to manufacture stainless steel thin plates using the continuous casting method, the mold is vibrated in the casting direction and the thickness is 100 mm.
The above slab is cast, the surface of the obtained slab is treated, heated to 1000°C or higher in a heating furnace, and then hot rolled by a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row, The hot strip had a thickness of several meters.

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.

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. Is it equivalent to a hot strip?
Research is underway on a process to obtain a slab (FilJ band) with a thickness close to that.

For example, "Tetsu to Hagane J '85. Al97~" A256
Yar CAMPTSIJJvo#, 1.1988.1
670-1705 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 to be obtained is 20 mm.
At the level of ~50 mm, a twin belt system is being considered.

[Problem 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.

As a result of detailed research by the present inventors on the manufacturing process of Cr-Ni stainless steel iILME plates by continuous strip casting, we found that surface defects called rovings and uneven gloss occur in the product as specifically shown below. There was found.

(1) Roving: 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.

Problems related to the surface properties of these products are unique problems that arise when thin slabs of nearly final shape are produced from molten austenitic stainless steel and then cold rolled without hot rolling.

As a result of a detailed study of the causes of these surface quality problems, the inventors found that the
It has been found that these surface defects occur when the grains are larger than 50 μm or when the thin slab is insufficiently cooled in the temperature range where Cr-based carbides precipitate.

In order to prevent these surface defects, we improved the molten steel composition and cooling conditions during the process of solidifying and cooling the molten steel, reducing the average γ grain size before cold rolling to 501M or less, and precipitating Cr-based carbides. We have invented a method for manufacturing Cr--Ni stainless steel thin plates that can produce products with good surface properties without any problems.

In other words, he discovered that it is effective to cool the slab at a rate of 100°C/sec or more in the temperature range from 1400 to 1200'C after it solidifies, and to control the amount of delta ferrite. -169096.

However, it is difficult to uniformly rapidly cool the entire width of the slab.
It has been particularly difficult to rapidly cool thick slabs. In addition, there were also problems such as the i'' species restriction occurring when the amount of delta ferrite was defined.

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

The present invention alleviates the quenching conditions, eliminates the restriction due to the amount of delta ferrite, and without adding grain refining elements.
Cr-Ni stainless steel with excellent surface qualityff1lFJ
The purpose of the present invention is to provide a method for manufacturing temporary products.

[Means to solve the problem]

The above object, according to the present invention, is such that C+N is 0.090%.
The following and Md. Cr-Ni stainless steel having a composition of 30 to 60°C is solidified and cooled at a rate of 100°C/sec or more using a continuous casting machine in which the mold wall surface moves in synchronization with the slab. Cast into slabs, and after solidifying, heat from the highest possible temperature to 1200℃ at 50℃
After cooling at a rate of 10°C/sec or more from 1200°C to 550°C, descaling, cold rolling, and final annealing are performed.The surface quality is deteriorated. Cr-Ni
This is achieved by a method for manufacturing thin stainless steel sheets.

M d 30 is 30 as used in -i
This is the temperature at which 50% or more of the M weave becomes martensite when subjected to % cold working.

The present inventor has determined that by setting M d 30 to 30°C or higher and generating a large amount of deformation-induced martensite during cold rolling, if the γ grain size is 100° or less, roving will not occur and the Cooling rate of slab to 50°
(The present invention was completed by discovering that the particle size of 7 particles becomes 100-0 or less when the particle diameter is set to 7/sec or more.

M d 30 needs to be 30° C. or higher for the above-mentioned reasons, but if it is too high, the cold workability of the product thin plate deteriorates, so it is limited to 60° C. or lower.

C+N promotes aging cracking, so it is set to 0.090% or less.

In the present invention, cooling to 1200'C after solidification is performed at a rate of 50°C/sec or more, and the particle size is reduced to 100°C.
control to below p. Therefore, the aforementioned earlier application (Japanese Patent Application No. 63)
-169096) since the cooling conditions are relaxed compared to 100° (/sec), making manufacturing easier.

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

Ru.

After cooling, descaling, cold rolling, and final annealing are performed in the usual manner. After the final annealing, temper rolling is performed in the usual manner if necessary.

〔Example〕

Cr with Md30 changed to 2 levels as the composition shown in Table 1 in backyard history
-Ni-based stainless steel @ (sus304m) is cast into a thin strip slab with a thickness of 2.4 cm using a twin-roll continuous casting machine, and after solidification, it is heated from 1400'C to 1200°C for 20 minutes.
The particles were cooled at a rate of ~700°C/sec to obtain various γ particle sizes, and cooled at a rate of 20°C/sec from 1200°C/sec to room temperature. After that, pickling and cold rolling (total reduction rate 5
0%) and final annealing to obtain a sheet product. After pickling in this state, the roving on the product surface was measured and an age cracking test was conducted.

The following margin results in Md, according to the present invention, as shown in FIG. -37.5℃ (Please correct as shown in Table 1)
For sample A, if the particle size of 7 is set to 100- or less,
The roving height was within the permissible limit (0.2 μn or less), but in the comparative example (sample B) where M d was 30-23.2°C, it was necessary to make the roving height 0.2 mark or less. It was necessary to make the γ grain size 501 rm or less.

Cr-Ni stainless steel with the composition shown in Table 2 is cast into a thin strip slab with a thickness of 2 marks using a twin-roll continuous casting machine, and after solidification, it is heated at 60°C from 1400°C to 1200°C. °C/
sec cooling from 1200'C to conventional method.
Cooling was performed at 0° C./sec. Thereafter, pickling, cold rolling (total rolling reduction of 50%), and final annealing were performed to obtain a sheet product. After pickling was carried out in this state, roving and age cracking on the product surface was observed.

Table 3 As a result, as shown in Table 3, Md. In the steels of the present invention (samples 1 to 5) with a temperature of 30°C or higher, roving was within the allowable range even with 7 grain sizes of 70 to 96p, but Md,
In the comparative example (sample 6.7) in which lo was less than 30°C, roping failure occurred at the same particle size of 7. All samples had good aging cracking properties.

Effects of the present invention] As explained above, according to the present invention, from solidification to 120
Even if the cooling to 0°C is relatively relaxed, the occurrence of roving can be substantially prevented, so Cr-Ni with excellent surface quality
This makes it easier to manufacture thin sheets of stainless steel.

[Brief explanation of drawings]

Figure 1 shows the relationship between γ grain size and roving height for two levels of M
d. This is a graph showing a comparison of the following.

Claims (1)

[Claims] 1. C+N is 0.090% or less and Md_3_0 is 3
Molten Cr-Ni stainless steel having a composition of 0 to 60°C is turned into a thin strip with a thickness of 10 mm or less at a solidification cooling rate of 100°C/sec or more using a continuous casting machine in which the mold wall surface moves in synchronization with the slab. It is cast into slabs, and after solidification, it is cooled from the highest possible temperature to 1200°C at a cooling rate of 50°C/sec or more, and from 1200°C to 550°C at a cooling rate of 10°C/sec.
A method for manufacturing a Cr-Ni stainless steel thin plate with excellent surface quality, which comprises cooling at a cooling rate of c or more, followed by descaling, cold rolling, and final annealing.