JPS5856012B2 - Manufacturing method of ferrite single-phase stainless steel sheet that does not cause ridging - Google Patents

Description

[Detailed description of the invention] The present invention has less occurrence of gluing, and has excellent workability and corrosion resistance.
This invention relates to a method for manufacturing excellent ferritic stainless steel sheets.

Improvements in steel manufacturing technology have made it possible to extremely reduce impurity elements such as CN in stainless steel.

Therefore, the material %flE of ferritic stainless steel has been greatly improved and it is being widely used in place of the expensive austenitic stainless steel.

Such ferritic stainless steels contain Ti and Nb for the purpose of having properties that are comparable to austenitic stainless steels in various applications, especially corrosion resistance, weldability, workability, etc.

Contains an appropriate amount of elements such as Mo.

In particular, it is well known that Ti or Nb is advantageous in improving workability, mechanical properties of welded parts, and corrosion resistance.

In this way, C and N can be lowered and the residual C and N can be reduced by Ti.
Or, even if steel stabilized with Nb is kept at high temperatures of 900°C or higher, only ferrite phase remains, and 5US430 and SUS
The two-phase region present in steels such as 434 is absent.

Therefore, even in the blooming or hot rolling process, the ferrite is a single phase.

Such steel is hereinafter referred to as ferritic single phase steel.

The present inventor conducted various experiments regarding gluing and discovered the following facts.

The ferritic single phase steel gluing mentioned above is SUS 430.
The cause of the occurrence is different from that of ridging, and in terms of metallographic structure, coarse ferrite grains present in the center of the thickness of the hot-rolled sheet have an effect even after cold rolling and annealing, resulting in a single crystal orientation group. is formed, and large ripples occur due to plastic anisotropy with adjacent crystal orientation groups.

On the other hand, according to the known literature and research conducted by the present inventors to date, the cause of ridging in 5US430 is the striped structure consisting of a ferrite phase and a martensitic phase formed in the hot-rolled sheet, and each Ridging occurs to create unique crystal orientation groups.

Therefore, known methods for eliminating ridging focus on destroying this striped structure. For example, as shown in Japanese Patent Publication No. 51-30008, a hot-rolled sheet is heated to a temperature in the two-phase region, and an austenite phase appears. It is said that ridging can be eliminated by causing martensitic transformation during cooling, performing cold rolling in a mixed structure state of ferrite and martensitic phases, and dividing the striped structure.

However, in this case, since it contains a martensitic phase, the strength of the cold-rolled sheet is high and its ductility is low, so the edge of the coil will break during cold rolling and the coil will break, making cold rolling unsuitable for industrial use. It becomes possible.

Furthermore, such a method cannot be applied to ferritic single-phase steel since there is no martensitic phase, and it is impossible to refine the coarse ferrite grains present in the center of the sheet thickness.

Therefore, among ferritic stainless steels, the present inventors
In order to industrially eliminate ferritic single-phase steel welding, we conducted various technical studies and found that coarse ferrite grains present in the center of the thickness of hot-rolled sheets can be removed by cooling immediately after hot-rolling and by hot-rolling. It has been discovered that by combining sheet heating, it is possible to make the sheet finer, and that the custom-made gluing of cold-rolled steel sheets can be greatly improved.

Immediately after hot rolling is completed, a rapid cooling treatment such as water cooling or forced air cooling treatment is performed to reduce the winding temperature to 400° C. or lower.

By rapidly heating this hot-rolled sheet to 900°C or higher, the coarsened ferrite grains of the hot-rolled sheet can be eliminated, and fine equiaxed grains can be obtained, which greatly improves the sludge of the cold-rolled steel sheet during cold-rolled recrystallization annealing. I discovered a fact that can be improved.

In other words, among ferritic stainless steels, the r-phase does not precipitate even at high temperatures of 900°C or higher, and steels that have a ferrite single phase structure during hot rolling can be heated to temperatures below 400°C by rapid cooling immediately after hot rolling. By increasing the residual internal strain of the hot-rolled sheet and rapidly heating the hot-rolled sheet to a temperature of 900°C or higher, it is possible to refine the giant ferrite grains present in the center of the thickness of the hot-rolled sheet. As a result, the custom-made gluing of cold rolled steel sheets obtained through cold rolling and recrystallization annealing processes is greatly improved.

According to the present invention, a slag or steel ingot of ferritic stainless steel, which has only a ferrite phase even at high temperatures of 900°C or higher, is rapidly cooled immediately after hot rolling and rolled up at 400°C or lower, and this hot-rolled sheet is Provided is a method for producing a ferritic stainless steel sheet without occurrence of writhing, which is characterized by rapidly heating the steel sheet to C or higher, soaking for 5 minutes or less, cooling, and performing cold rolling recrystallization annealing.

The present invention is generally applied to ferritic stainless steels that are single-phase ferritic at temperatures above 900°C, as noted above.

And such steel can be easily manufactured by those skilled in the art by adjusting the balance between Ni equivalent 1 and Cr equivalent, and contains Ti, Nb,
Individual characteristics can be imparted by appropriately adding Mo or the like.

However, more specifically, it is applied to ferritic single phase stainless steel containing 16 to 19% Cr.

In the above manufacturing method, the reason why the winding temperature is limited to 400°C or less is that the winding temperature of ordinary ferritic stainless steel is 550° to 750°C, but this hot-rolled sheet is subjected to a similar rapid heating treatment. This is because, although the coarsened ferrite grains in the hot rolled sheet disappear when this is applied, recrystallization does not result in equiaxed grains but in elongated grains, and therefore the effect of improving ridging is small.

In other words, when the winding temperature is high, the internal energy such as thermal strain and processing strain stored in the hot-rolled sheet is released by the heat of the hot-rolled sheet itself after winding, and is recovered in the next annealing process. , the energy driving force for recrystallization decreases.

However, by rapidly cooling the hot-rolled sheet to 400° C. or lower, these strains remain in the hot-rolled sheet without being released.

Therefore, there are many recrystallization nuclei during annealing of the hot rolled sheet,
The refinement of crystal grains and uniform grain refinement become remarkable.

In addition, when the annealing temperature is 900°C or lower, recrystallization is not completed even with industrially possible short-time annealing, and the structure of the hot-rolled sheet, that is, coarsened ferrite grains, remains, and the effect of improving ridging is small. .

Furthermore, the upper limit temperature may be as high as industrially possible, but if it exceeds 1100°C, the crystal grains will coarsen and the oxide scale will increase sharply, causing crystals to fall off during the pickling process, which is an advantage of stainless steel. It is industrially disadvantageous as it significantly damages the surface texture.

If the soaking time is 900℃ or higher, recrystallization will be completed even if it is instantaneous, and if the soaking time is longer than 5 minutes, the crystal grains will become coarse. Optimal.

Instant soaking is a method in which the furnace temperature is set higher than the target temperature, and the steel plate is cooled immediately after it is heated to the target temperature.

The reason why the steel in the present invention is limited to steel that has a single ferrite phase at high temperatures of 900°C or higher is that S[JS430
This is because when the heat treatment method of the present invention is applied to such steels, a martensitic phase appears and the strength of the steel increases, making industrial production impossible.

In addition, martensite phase does not appear when annealing at a relatively low temperature of about 900°C for a short time, but after annealing, the martensite phase with a striped structure that exists in the hot rolled sheet decomposes into a ferrite phase and chromium carbide. Chromium carbide remains in stripes.

This prevents recrystallized grains from becoming uniformly fine and makes them elongated grains, so the effect of improving ridging percentage is small.

The present invention will be explained in detail below according to examples.

Example 1 A 40-ton steel having the chemical composition shown in Table 1 was melted in an electric furnace, and a slab manufactured by the continuous casting method was water-cooled immediately after hot-rolling, and the heat was rolled up at 400°C or less. The rolled steel strip is divided into two, one of the hot-rolled plates is heated to 950°C, and then immediately cooled.
The other was subjected to normal box annealing, and both were manufactured once by a cold rolling method from 3.8 mm to 0.8 mrn, and recrystallization annealing was performed at 900°C.

A comparison was made regarding the gluing characteristics of these two cold rolled steel sheets.

Exterior photographs of both are shown in FIGS. 1 and 2, and when the hot-rolled sheets were annealed at 950° C., there was very little ridging, and the ridging evaluation was also B rank.

On the other hand, when box annealing is performed, the ridging is large and the ridging evaluation is also D rank.

Note that the ridging evaluation is carried out based on the standards shown in Table 2, and it is considered that the plain molded product has good ridging if it is ranked B or higher.

These results show that by annealing the hot-rolled sheet at a high temperature, the jings obtained by the conventional low-temperature, long-time box annealing method can be greatly improved.

The reason for this is that when comparing the microstructures of hot-rolled sheets after heat treatment, equiaxed grains are obtained when annealing is performed at 950°C, whereas expanded ferrite grains are present in the center of the sheet thickness when box annealing is performed. It's for a reason.

Example 2 In Example 1, we learned that the ridging properties vary greatly depending on the heat treatment method for hot-rolled sheets, so next we will discuss how the winding temperature and heat treatment temperature of hot-rolled sheets differ from custom-made product soldering. In order to clarify whether there is a relationship, an investigation was conducted using a steel slab having the same composition as in Example 1.

Regarding the process after hot rolling, cold rolling is performed from 3.8 mm to 16 mm, intermediate annealing is performed at 900°C, and after further cold rolling from 1.6 mm to 0.7 mm, recrystallization annealing is performed at 900°C. I did it.

The results are shown in Table 3. The lower the winding temperature, the better the ridging evaluation, but the structure after heat treatment did not become equiaxed grains in the samples that were not subjected to the rapid cooling treatment according to the method of the present invention.
The grains become elongated and the ridging evaluation is also poor.

That is, the fact confirmed in Example 1 is due to the synergistic effect of performing a specific treatment immediately after hot rolling and high temperature annealing of the hot rolled sheet.

That is, the quenching treatment method for a hot rolled sheet according to the method of the present invention and the 900
In combination with the annealing method at temperatures higher than .degree. C., the percent ridging ratio is greatly improved.

Further, when the heat treatment temperature of the hot rolled sheet is as low as 850° C., the ridging evaluation is quite poor because giant ferrite grains, which are the structure of the hot rolled sheet, remain.

In this example, a rapid cooling treatment is performed immediately after hot rolling to reduce the winding temperature to 400°C or less, and this hot rolled sheet is
It has been confirmed that ridging can be significantly improved by combining rapid heating annealing to a high temperature of ℃ or higher.

In Example 1, the manufacturing method according to the present invention had a rating of B.
On the other hand, in Example 2, all of the ridging evaluations are A, because in Example 2, intermediate annealing is performed once, and generally, the ridging evaluation when intermediate annealing is performed is about one rank better. This is known.

Example 3 Next, the method of the present invention was carried out on steel J having the chemical composition shown in Table 4.

The hot-rolled sheet was heat-treated at 950° C., and the subsequent steps were two-time cold rolling and two-time annealing.

The results are shown in Table 5, and except for 5US430, all other steels had an A rank ridging evaluation.

A common property of these steels is that all hot-rolled sheets have a ferrite single-phase structure, whereas the hot-rolled SUS 430 sheet has a two-phase structure of a ferrite phase and a martensitic phase.

In other words, the hot-rolled sheet of 5US430 has a striped structure consisting of a ferrite phase and a martensite phase, and during annealing, the martensite phase decomposes into a ferrite phase and chromium carbide, but the chromium carbide has a layered or linear structure. remains.

This chromium carbide prevents the formation of equiaxed grains, and the structure after annealing is all extended grains, so the effect of improving ridging is small.

[Brief explanation of drawings]

FIG. 1 is a photographic sketch of a steel sheet gluing test piece obtained by rapidly cooling to below 400° C. immediately after hot rolling, soaking at 950° C., and then cold rolling according to the method of the present invention. FIG. 2 is a photograph of a steel plate gluing test piece obtained by box-annealing and cold-rolling a hot-rolled plate.

Claims (1)

[Claims] A slab or steel ingot of ferritic stainless steel, which only has a ferrite phase even at high temperatures of 1900°C or higher, is hot-rolled, rapidly cooled immediately after hot rolling, rolled up at 400°C or lower, and the hot-rolled sheet is rapidly heated to 900°C or higher. After heating and soaking for less than 5 minutes,
A method for manufacturing a ferritic stainless steel sheet without occurrence of writhing, characterized by cooling, cold rolling, and recrystallization annealing.