JPS63421A - Novel production of thin austenitic stainless steel sheet having excellent surface characteristic and material quality - Google Patents

Abstract

PURPOSE:To simplify a hot rolling stage, to omit an annealing stage for a hot rolled sheet, to simplify a grinding stage for a coil and to obtain the titled steel sheet based on the hot rolled coil by executing casting in the shape approximate to a product size as far as possible by a synchronous type continuous casting method. CONSTITUTION:The titled steel is cast at >=30 deg./C cooling rate in the solidification state to a thin slab having <=100mm thickness by the synchronous type continuous casting method and the slab is continuously hot rolled as it is at >=800 deg.C and <=50% draft and is then cooled. The rolled sheet is coiled at <=650 deg.C. Such hot rolled coil is used as a starting material for the cooling state. The hot rolled coil is subjected to descaling without annealing at used and is thereafter subjected to a process for processing the thin cold rolled sheet in accordance with the conventional method. The cold rolling in the above-mentioned process for producing the thin cold rolled sheet is preferably tandem cold rolling by large-diameter rolls having the roll diameter exceeding about 100mphi and is further the reverse rolling with small-diameter rolls having about <=100mmphi roll diameter after the above-mentioned tandem cold rolling.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention uses the so-called synchronous continuous casting method in which there is no relative speed difference between the slab and the mold. The present invention relates to a method of manufacturing a thin steel plate in a simplified manner.

[Prior art] In conventional continuous casting, thick continuous casting slabs with a thickness of 100 mm or more are produced while oscillating the mold, and after surface treatment and heating to 1000°C or more in a heating furnace, they are cast from multiple stands. This hot rolling process has been carried out using a group of hot rolling mills to produce thin steel sheet materials. From the perspective of mass production, the method of using large continuous slabs and hot rolling them with a group of large, highly efficient hot rolling mills can be considered as a technological system, but there are This method can be said to be expensive in terms of cost due to heating fuel, electricity, etc. In addition, as a result of intense hot rolling, problems arise, and as will be described later, hot rolled sheet annealing is required and a coil grinding process is required.

The situation is exactly the same in austenitic stainless steels of which 18 Cr-8Ni is a representative example.

These are also hot-rolled thinly using a hot rolling mill using a thick CC slab as a starting material, and then wound to form a hot-rolled steel strip. In the cold rolling process of stainless steel using these hot-rolled steel strips as starting material, not only the dimensions and shape required for the final product, but also the
In addition to the material, excellent surface quality is required. In order to meet these demands, the hot-rolled copper strip that has been strongly hot-rolled is hard and needs to be annealed and softened to make it easier to cold-roll in the next step of cold-rolling. In addition, scale flaws and the like that occur during hot rolling due to intense hot rolling must be removed by grinding after pickling. In this way, in the conventional technology of large-scale continuous casting and high-intensity hot rolling mill method, the degree of hot rolling is too large in the method of manufacturing stainless steel thin plates.
Additional processes such as a hot-rolled plate annealing process and a coil grinding process are required.

[Problems to be solved by the present invention] As a new technology that is fundamentally different from conventional methods.

If it is possible to cast as close to the product size as possible at the continuous casting stage, the hot rolling process can be greatly simplified, and it is thought that this technology could revolutionize the steel manufacturing process.

Intensive research is being conducted in various countries regarding the focus on thin-sen piece manufacturing technology, but in Japan, ``Tetsu to Hagane J 85'
It is featured in special features in A197-85'A256 and is described in detail. In this way, Twin Roll
When the thickness is about 20 to 50 mm, Tνin Ba
Progress has been made in the 1-ton synchronous continuous casting method. Regarding stainless steel, these studies are also progressing in various fields. In the future, the remaining tasks are to expand the rlJ of these thin slabs and improve productivity, and to develop a consistent manufacturing technology for various steel materials using these slabs.

If the manufacturing method of these thin slabs advances, it will be possible to solve the problems faced by the current methods at once by applying it to the manufacturing method of stainless steel thin plates. In other words, we used a thin CC43 piece that was as close to the size of the product as possible.

In addition to simplifying hot rolling as much as possible, hot rolling sheet annealing can be omitted and the coil grinding process can also be omitted or greatly reduced, making it an extremely desirable process for thin stainless steel sheets. To do this, we will use actual thin CC.
It is necessary to manufacture thin stainless steel plates using the ideal process described above and confirm their material and surface properties, but there have been no studies on the specific manufacturing process, materials and surface properties of wide, thin-walled CC slabs. I can't find it. In place of the conventional method, the present invention discloses a method for manufacturing thin plates of austenitic stainless steel with excellent surface properties and material quality, using thin slabs as raw materials.

[Means for Solving the Problems] The present invention (1) produces austenitic stainless steel, typically 18Cr-8Ni, during solidification by a so-called synchronous continuous spinning method in which there is no relative speed difference between the slab and the mold. A thin cast piece with a thickness of 10 mm or less is cast at a cooling rate of 30 ° C / s or more, and the thin cast piece is continuously hot rolled as it is at a temperature of 800 ° C or more with a reduction rate of 50% or less, Cool to 650
Using a hot-rolled coil that is characterized by being wound at temperatures below ℃,
This is a new method for producing austenitic stainless steel thin sheets with excellent surface properties and material quality, and (2) the hot-rolled coil is descaled without annealing, and thereafter subjected to conventional cold-rolled thin sheet manufacturing processing. This is a new manufacturing method for patent-free steel thin sheets, which is characterized by:
This is a new manufacturing method for austenitic stainless steel thin sheets with excellent surface properties and material quality, which involves tandem cold rolling with large-diameter rolls exceeding u+φ. After rolling, the roll diameter is 10 mm after tandem cold rolling with large diameter rolls with a roll diameter of over 100 mmφ.
This is a new manufacturing method for austenitic stainless steel thin sheets with excellent surface properties and material quality, which involves reverse rolling with small diameter rolls of 0+smφ or less.

[Operations and Examples] The present inventors used thin CC slabs manufactured by the twin roll method as the basic material, and researched the manufacturing process of thin austenitic stainless steel sheets, typified by 1 8 Cr-8 Ni, and published this book. Completed the invention.

In the case of 18Cr-8Ni thin sheets, which are made by hot-rolling conventional hot-rolled steel strips from thick CCB pieces, omitting annealing, and then cold-rolling and final annealing, the material of the product is in the rolling direction of the sheet.

It exhibits so-called anisotropy, which differs greatly depending on the direction perpendicular to the rolling direction and at 45°. Thus, anisotropy of the product has been a problem in thin austenitic stainless steel sheets that have excellent workability. The present inventors have researched a method for reducing anisotropy due to the omission of hot-rolled plate annealing in the conventional method of hot rolling using a thick slab as a starting material, and have developed a method for reducing anisotropy by using a powerful hot rolling mill. A method of regulating hot rolling conditions (Japanese Patent Application No. 56-130910) or a method of regulating steel composition (
(Japanese Patent Application No. 56-120068) Furthermore, it has been revealed that this can be achieved by a method such as cold rolling (Japanese Patent Application No. 56-120067). This time, the present inventors used a twin-roll thin slab of austenitic stainless steel, typically 18Cr-8Ni, as a starting material, and in a method to simplify hot rolling as much as possible,
We will reveal a method that does not cause anisotropy in the product sheet even if hot-rolled sheet annealing is omitted. In this case, the thin-walled slab is, for example, cast using an internally cooled twin copper roll while being pushed, and has an average cooling rate of 10'C/s in the solidification temperature range.
A study was conducted using the slab cooled above.

After casting, the slab was placed in a hot rolling machine installed at the outlet of the casting machine, where it was hot rolled, cooled, and then wound. During hot rolling, reduction was applied from 0% to approximately 70% in one or two passes. The rolling temperature was about 1100'C to 800'C. Thereafter, the hot-rolled coil was pickled without annealing, cold-rolled, and finally annealed, and the material of the thin plate was investigated. The conditions under which anisotropy was not significant even if hot-rolled plate annealing was omitted were that the slab thickness was less than 101IllI and the hot-rolling reduction ratio was 50% or less.

As the slab thickness increased and the hot rolling reduction ratio increased, the anisotropy deteriorated. In this way, when a slab of less than 10ml1 is hot-rolled cold or directly at a rolling rate of 30°C/s or more with a reduction rate of 50% or less using the twin-roll method, anisotropy is noticeable even if hot-rolled sheet annealing is omitted. isn't it. There are no other properties that particularly require hot rolling, and there is no lower limit to the hot rolling reduction rate from the viewpoint of the material of the austenitic stainless steel thin plate. However, when the width is increased by 1 to 10 mmE [9 pieces] using the twin roll method, the thickness changes in the width direction due to changes in the solidification phenomenon in the width direction, so hot rolling is completely omitted and the material for cold rolling is used. It is not a good idea to do so. In this way, aiming at the effect of dimensional correction, it is effective to apply hot reduction to the slab at a reduction rate of 50% or less. Surface quality plays an extremely important role in stainless steel sheet products. In addition to the inherent characteristics of corrosion resistance and rust resistance, the appearance is glossy, beautiful,
Furthermore, it has abrasiveness, etc. It is known that these defects are caused by microscopic defects on the surface of stainless steel, but the present inventors' research revealed that in the current manufacturing method, the contribution of irregularities on the surface of hot-rolled steel strips. turned out to be large. In other words, current thick CC slabs are heated in a heating furnace at a high temperature of 1000'C or more for a long time and then hot rolled. It is pushed into the surface of the steel plate during rolling.

Creates local unevenness on the coil surface. These irregularities have a maximum depth of about 20 μm before cold rolling, and have conventionally been removed by grinding the surface in a coil grinding process. The present inventors have been conducting research on the unevenness existing on the surface of these hot-rolled stainless steel strips. It has been found that these irregularities are not observed at all in the inter-rolled coil. In particular, the depth of surface irregularities was compared with that of the current cold-rolled material when the surface of the cast slab pressed by twin rolls was hot-rolled to 50% or less without subsequent heating, and hot-rolled plate annealing was omitted. . The current material is a 5US304 CC slab with a thickness of 150 mm heated at 1200°C or higher for 2 hours, hot rolled to 3 mm, annealed and pickled, and the depth of the irregularities on the coil surface was determined using an optical microscope. 100
The average of the points was found to be 18μ. On the other hand, the one from the twin roll method had a depth of 6μ, which was actually 1/3 of the depth. This level of unevenness can be repaired by adjusting the cold rolling reduction in the cold rolling process. In particular, excellent surface gloss can be obtained not only by the Sendzimir method using small diameter rolls, but also by the tandem cold rolling method using large diameter rolls.

The thin cast slab of stainless steel mainly made of 5US304 produced by the twin roll method is hot-rolled in one pass at a rolling reduction of 50% or less without heating, cooled, rolled up at 650'C or less, and pickled and descaled. After cold rolling, final annealing and pickling in the usual manner to produce 2B products, and bright annealed B
Regarding product A.

The complete resistance was large, the gloss was good, and the polishability was good. If coiling is performed at a temperature of 650'C or higher, carbides will precipitate at grain boundaries during coiling, causing intergranular corrosion during pickling and degrading the surface gloss of the cold-rolled product.

In slabs whose cooling rate during solidification is less than 30°C/s, anisotropy increases due to omission of hot-rolled plate annealing. Also, the thickness is 10mm1
The cooling rate during solidification is less than 30'C/s for super cast slabs, and the anisotropy increases due to the omission of hot-rolled plate annealing. It becomes more than a stand and is uneconomical.

Hot rolling of the thin slab is carried out directly and continuously in a hot rolling mill installed on the exit side of the casting machine, and the reduction ratio of the hot rolling is preferably 50% or less. If it exceeds 50%, there is no particular advantage in terms of material quality, and it only increases the anisotropy of the product and increases the size of the hot rolling mill. In addition to the material properties, hot rolling has the effect of crimping internal defects during casting, improving the width and longitudinal dimensions, shape, and microscopic irregularities of the slab, and slightly reducing rolling for correction purposes. It is effective to add

The hot rolling temperature is preferably 800°C or higher. This is because if the temperature is lower than 800°C, the load on the hot rolling rolls will be increased and the precipitation of carbides will be accelerated. The winding temperature must be 650° C. or lower, because if the winding temperature exceeds 650° C., carbides will precipitate due to reheating, causing intergranular corrosion in the subsequent pickling process.

As described above, the hot-rolled coil is hot-rolled without heating the thin slab, wound at 650°C or below, and descaled.The hot-rolled coil has no surface irregularities caused by local scale, and has a small surface roughness. It can be made into a coil.

Next, examples of the present invention will be described.

Table 1 shows the steel components used as examples of the present invention. Table 2 shows the slab manufacturing conditions, hot rolling conditions, and coiling conditions for each steel type, and Table 3 shows the properties of these thin sheets. This time, the current thick CC shown as a comparative method
Thin CC is produced from a slab by heating it to 1200℃ or higher and giving a hot rolling rate of 95% or higher in 11 baths by omitting hot-rolled plate annealing and also omitting the coil grinding process. The comparison was made with a product manufactured using a reduced process.

As is clear from Table 3, starting from thin CC, omitting long-time heating, hot rolling with less than 50% hot rolling, coiling at less than 650°C, omitting hot-rolled plate annealing, and coil grinding process. The cold rolling process can be performed not only by the conventional small-diameter reverse method such as Sendzimya Mill, but also by the tandem cold-rolling method using large-diameter rolls used for ordinary steel cold-rolling, or by using small-diameter rolls after tandem cold rolling. Excellent surface properties can also be obtained by combining the reverse method.

The reason for this is that, as shown in Table 4, the unevenness of the surface of the material before cold rolling is significantly better in the method of the present invention than in the conventional method.

[Effects of the present invention] According to the present invention, the hot rolling process can be simplified by using a slab with a shape as close as possible to the product shape, and the hot rolled sheet annealing process can be omitted, and the coil grinding process can be omitted. This is a revolutionary method for producing austenitic stainless steel, as it is not only simple but also allows the production of cold-rolled products with excellent material and surface properties.

Its industrial effects are extremely large.

Claims (1)

[Claims] (1) Austenitic stainless steel, typically 18Cr-8Ni, is produced by a so-called synchronous continuous casting method in which there is no relative speed difference between the slab and the mold, so that the cooling rate during solidification is 30°C/s.
The above-cast thin slab with a thickness of 10 mm or less is then continuously hot rolled at a temperature of 800°C or higher with a reduction rate of 50% or less, cooled, and then rolled at a temperature of 650°C or lower. A new manufacturing method for thin austenitic stainless steel sheets with excellent surface properties and material quality using a hot-rolled coil characterized by winding. Cast with a thickness of 10°C using the so-called synchronous continuous casting method at a cooling rate of 30°C/s or more during solidification.
mm or less, the thin slab is continuously hot-rolled as it is at a temperature of 800°C or higher with a rolling reduction of 50% or less, cooled and wound at 650°C or lower to form a hot-rolled coil, A new manufacturing method for austenitic stainless steel thin sheets with excellent surface properties and material quality, characterized by descaling the hot-rolled coil without annealing, and then subjecting it to a conventional cold-rolled thin sheet manufacturing process (3). ) In a conventional cold-rolled sheet manufacturing process, the cold rolling is tandem cold rolling with a large-diameter roll having a roll diameter exceeding 100 mmφ. A new manufacturing method for excellent austenitic stainless steel thin sheets (4) In the conventional cold rolling thin sheet manufacturing process, cold rolling is performed by tandem cold rolling with large diameter rolls with a roll diameter of over 100 mmφ, and then the roll diameter is 100 mmφ. A new method for manufacturing an austenitic stainless steel thin plate with excellent surface properties and material quality, as set forth in claim 2, which is reverse rolling using the following small diameter rolls.