JPH04254522A - Manufacture of austenitic stainless thin steel sheet - Google Patents

Abstract

PURPOSE:To offer a method for improving the surface properties of an austenitic stainless steel sheet manufactured by a thin sheet direct casting process. CONSTITUTION:At the time of manufacturing an austenitic stainless thin sheet by a twin roll type thin sheet direct casting process, at first, a thin strip having <=10mm thickness is cast and is subjected to heat treatment (homogenizing treatment) for the purpose of rolling at a low draft and diffusion before normal working such as cold rolling to uniformize the thin strip cast structure. After that, normal working and heat treatment according to the conventional method are executed into a product. In this way, the austenitic stainless thin steel sheet free from surface defects such as irregularity of luster and having good surface properties can be manufactured without causing the deterioration in internal quality.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a thin austenitic stainless steel sheet having excellent surface properties by pre-treating a thin strip cast by a twin-roll thin sheet direct casting process using a combination of light reduction and diffusion heat treatment. The present invention relates to a method of manufacturing a steel plate.

0002

[Prior Art] Conventionally, stainless thin steel sheets are produced by first hot rolling a slab of 100 mm or more in thickness obtained by continuous casting into a hot strip several mm thick, and then cold rolling it. was. On the other hand, recently, such stainless steel sheets have been produced by directly casting molten steel into a thin strip with a thickness equal to or close to that of the above-mentioned hot strip, and then cold rolling the cast thin strip. Manufacturing technology has been developed, and it has come to be widely adopted due to its advantages such as simplification of the process and associated cost reduction.

However, in the case of this new development method,
Compared to previously known methods, the hot rolling process is omitted from the process from casting to finished product.
It has been difficult to obtain thin steel sheets with excellent quality because the internal quality and surface quality of the product are not sufficiently controlled. Even so, it has been recognized that the internal quality of thin steel sheets has been considerably improved, and that the quality is equal to or higher than that of products made through the conventional continuous casting and hot rolling process. However, since the surface of the thin steel sheet is directly transferred from the casting surface, there are still many problems, and improvements have been desired.

[0004] Namely, undesirable surface defects found on the surface of a thin steel sheet include uneven gloss and fine waviness (roping). These surface defects are completely absent in thin steel sheets manufactured by conventional methods, and are a phenomenon unique only to the so-called direct casting process, and must be eliminated in order to commercialize the product.

[0005] In view of such problems, conventionally,
JP-A-240218 and JP-A-2-240219 disclose a method for reducing surface defects, such as uneven gloss and roping, for example, by solution treatment of a thin strip cast to a thickness of 10 mm or less at a temperature of 1050°C or higher. 900
It is cooled in a temperature range of ~550°C at a cooling rate of 10°C/sec or more, then descaled, cold rolled, and finally annealed. After that, the growth rate is 0.
A method of performing temper rolling of 5 to 2.5% is proposed.

[0006]

[Problems to be Solved by the Invention] However, in these conventional techniques, since the reduction ratio during initial cold rolling exceeds 50%, colonies caused by the non-uniformity of the solidified structure of the ribbon are stretched. On the contrary, a problem arose in that unevenness in gloss became clear. In addition, although skin pass rolling is performed in the final step, there is a problem that elongation and formability are slightly impaired, and it is necessary to solve these problems.

[0007] An object of the present invention is to eliminate problems such as gloss unevenness without requiring skin pass rolling when manufacturing a thin steel plate by cold rolling a thin strip cast by a twin-roll direct thin plate casting process. The objective is to establish a thin steel sheet manufacturing technology that can completely eliminate surface defects.

[0008]

[Means for Solving the Problems] As a result of intensive research to achieve the above object, the present inventors have newly discovered that the occurrence of the above-mentioned surface defects is caused by the non-uniform solidification structure of the cast ribbon. After discovering this idea, we investigated various casting ribbon processing methods that are effective in eliminating this non-uniform solidified structure, and came up with the manufacturing method described below.

That is, the present invention first casts an austenitic stainless steel ribbon with a thickness of 10 mm or less using a twin-roll thin plate direct casting machine, descales this ribbon, and then applies a rolling reduction of 5 to 20 mm. %, and then subjected to α-γ transition temperature of 1200°C or higher for 3 to 3 minutes.
The second step is to cast an austenitic stainless steel ribbon with a thickness of 10 mm or less using a twin-roll thin plate direct casting machine, and then process and heat treat it according to conventional methods to produce a product. α-
This is a method for producing an austenitic stainless thin steel sheet, which is characterized by maintaining the product at a gamma transition temperature for 30 minutes to 2 hours, and then subjecting it to processing and heat treatment according to conventional methods to obtain a product.

0010

[Operation] The present inventors investigated the causes of surface defects such as uneven gloss observed in thin steel sheets produced by cold rolling austenitic stainless steel ribbons cast using a twin-roll direct sheet casting machine. We have largely elucidated the phenomenon and clarified the solution, which we will describe below.

Among the surface defects, it has been found that uneven gloss in particular is caused by the non-uniformity of the solidified structure of the ribbon during casting. In other words, when a cast ribbon obtained by the direct casting method is closely observed, the solidified structure on its surface is divided into two types: a δ-ferrite-rich structure in which dendrites are predominant, and an austenite-rich structure in which dendrites are indistinctly visible. It was found that two coagulated tissues were present. The δ-ferrite-rich structure, which is dominated by dendrites, remains even after cold rolling and annealing, forming a fine ditch-like structure on the pickled surface and exhibiting a dull luster. The austenite-rich structure in which dendrite was indistinctly appeared had a rough ditch-like structure with no remaining δ-ferrite, and exhibited strong luster.

Therefore, in the present invention, such a non-uniform solidified structure is treated by subjecting the thin ribbon after casting to preliminary treatment by a combination of cold rolling with light reduction and annealing for the purpose of diffusion (diffusion heat treatment). The aim was to eliminate this problem and, in turn, prevent the formation of surface defects such as uneven gloss.

That is, the present invention eliminates the dendrite structure and converts the residual δ-ferrite structure into an austenite single-phase structure by immediately subjecting the cast ribbon to cold rolling with light reduction and/or diffusion heat treatment. This eliminated the cause of uneven gloss.

[0014] According to such a method, since the rolling reduction during initial cold rolling is as small as 5 to 20%, it is possible to overcome the drawbacks of slightly impairing elongation and formability, which were problems with conventional techniques. . The manufacturing method of the present invention will be explained in more detail below.

[0015] In the present invention, a ribbon is first cast.
This ribbon must be less than 10 mm thick. When a thin strip with a thickness exceeding 10 mm is cast using a twin-roll type direct sheet casting machine, the austenite grains (γ grains) become significantly coarser after solidification, resulting in a phenomenon called orange peel in the cold-rolled product. This is because it causes minute skin irritation, which is undesirable. Note that the diameter of the γ grains in this ribbon is preferably 50 μm or less in terms of average grain size.

Next, in the present invention, as a first method, such a ribbon is first subjected to cold rolling. The purpose of this cold rolling is light reduction, and the reduction ratio must be 5 to 20%. When this is done at a large rolling reduction, the colonies (delta-ferrite-rich structure dominated by dendrites) caused by the non-uniformity of the solidified structure mentioned above are elongated and elongated.
This is because it becomes difficult to eliminate it even in subsequent steps.

In this regard, stretching of the colonies can be suppressed by restricting the initial processing applied to the ribbon to a small rolling reduction of 5 to 20%.

In this method, a diffusion heat treatment is then performed in which the material is held at an α-γ transition temperature of 1200° C. or higher for 3 to 30 minutes. This treatment promotes the γ-transformation of δ-ferrite and completely eliminates colonies caused by the heterogeneity of the coagulation structure that was initially observed. In this case,
The strain introduced near the surface by the reduction is δ-
It is thought that this is the driving force for the γ transformation of ferrite.

[0019] Here, the reason why the rolling reduction rate during the first cold rolling applied to the ribbon is set in the range of 5 to 20% is that the rolling reduction rate is 5%.
If it is less than δ, the strain introduced near the surface is small, and δ
- This is because the γ transformation of ferrite is not sufficiently carried out,
If it exceeds 20%, the problem of colony elongation due to non-uniformity of the coagulation structure will occur, and it will not be effective in improving surface gloss unevenness.

Further, the reason why the temperature of the diffusion heat treatment is set to 1200° C. or higher is to complete the diffusion treatment of component segregation. The reason for limiting the holding time of diffusion heat treatment to 3 to 30 minutes is that if the retention time is less than 3 minutes, the γ of δ-ferrite
This is because if the transformation is not completed and it takes more than 30 minutes, fine rough skin called orange peel will occur due to coarsening of the γ grains.

Next, in the second method of the present invention, the cast ribbon is immediately heated to an α-γ transition temperature of 1200°C or higher for 3
Diffusion heat treatment is performed for 0 minutes to 2 hours. By this treatment, colonies caused by non-uniform coagulation structure can be eliminated and uneven gloss can be improved.

[0022] Here, the retention time of the diffusion heat treatment is 30 minutes to
The reason for setting it to 2 hours is that if the time is less than 30 minutes, the strain introduced near the surface by processing in the first method will not be introduced in this method, so the γ transformation of δ-ferrite will be insufficient. This is because if it exceeds 50%, the γ grains will become coarser and a fine skin roughness called orange peel will occur when the product is manufactured.

[0023] In this method, since the cast ribbon is immediately placed in a heat insulating furnace, there is little loss of energy and cost, and furthermore, it is possible to promote the solutionization of Cr carbide.

[0024]

[Example] Austenitic stainless steels having the three types of compositions shown in Table 1 were cast using a twin-roll thin plate direct casting machine to obtain ribbons with a thickness of 2 mm.

[0025] This continuously cast ribbon is subjected to initial cold rolling and diffusion heat treatment satisfying the conditions specified in the present invention (1, 2).
), a method of immediately performing diffusion heat treatment (3, 4), and a comparative method of performing initial cold rolling or diffusion heat treatment that deviates from the conditions limited in the present invention, and then main processing and annealing under the same conditions. A thin steel sheet product was manufactured by applying the same method.

[0026] Table 2 shows the results of uneven gloss and occurrence of orange peel on the pickled skin of thin steel sheet products manufactured by variously changing the initial cold rolling conditions and heat treatment conditions under each of the above methods. show. In Table 2, the presence or absence of uneven gloss and orange peel was determined by visual observation.

[0027]

[Table 1] C ~ N:
wt% O: ppm

0028
]

[Table 2]

As is clear from the results shown in Table 2, all of methods 1 to 4 according to the present invention exhibited good surface properties without uneven gloss or orange peel. In addition, the method of the present invention performs preliminary treatment to make the structure uniform even after skin pass rolling and main processing, so elongation and formability were not impaired.

On the other hand, in Comparative Methods 5 to 7, strong unevenness in gloss occurred, and in some cases, rough skin of the orange peel also occurred. In other words, in the case of comparison methods 5 and 6,
Because the initial cold rolling reduction ratio is as high as 50%, the structure caused by uneven gloss is stretched and does not disappear even after subsequent heat treatment.
Uneven gloss occurred. In the case of Comparative Method 6, the initial cold rolling reduction was as high as 50% and the heat treatment time exceeded 30 minutes, resulting in coarsening of the γ grains and rough skin of the orange peel. In the case of Comparative Method 7, since the heat treatment temperature was lower than 1200° C., the diffusion treatment for component segregation was insufficient, the structure caused by uneven gloss did not disappear, and uneven gloss occurred.

[0031]

[Effects of the Invention] As explained above, according to the present invention,
When the ribbon obtained by twin-roll direct sheet casting is cold-rolled into a product, light reduction rolling and diffusion heat treatment are performed in advance to make the ribbon casting structure uniform.
It is possible to produce an austenitic stainless thin steel sheet having good surface properties without uneven gloss without causing deterioration in internal quality.

Furthermore, according to the present invention, by keeping the initial cold rolling reduction low and omitting temper rolling in the final step, there is also the effect of improving elongation and formability.

Claims (2)

[Claims] Claim 1: After casting an austenitic stainless steel ribbon with a thickness of 10 mm or less using a twin-roll type thin plate direct casting machine, and descaling the ribbon, the reduction rate is 5 to 2.
It is characterized by cold rolling under 0% light reduction, then holding at an α-γ transition temperature of 1200°C or higher for 3 to 30 minutes, and then processing and heat treating according to conventional methods to obtain a product. A method for producing austenitic stainless steel sheet. 2. An austenitic stainless steel ribbon with a thickness of 10 mm or less is cast using a twin-roll thin plate direct casting machine, and the ribbon is heated to an α-γ transition temperature of 1200°C or higher for 30 minutes.
A method for manufacturing an austenitic stainless thin steel sheet, which comprises holding for 0 minutes to 2 hours, and then processing and heat treating according to conventional methods to obtain a product.